Evolution of protoplanetary disks from their taxonomy in scattered light: Group I vs. Group II
Antonio Garufi, Gwendolyn Meeus, Myriam Benisty, Sascha Quanz, Andrea Banzatti, Mihkel Kama, Hector Canovas, Carlos Eiroa, Hans Martin Schmid, Tomas Stolker, Adriana Pohl, Elisabetta Rigliaco, Francois Menard, Micheal Meyer, Roy van Boekel, Carsten Dominik
AAstronomy & Astrophysics manuscript no. aa c (cid:13)
ESO 2017July 6, 2017
Evolution of protoplanetary disks from their taxonomyin scattered light: Group I vs Group II (cid:63)
A. Garufi , , G. Meeus , M. Benisty , S.P. Quanz , A. Banzatti , , M. Kama , H. Canovas , C. Eiroa , H.M. Schmid ,T. Stolker , A. Pohl , E. Rigliaco , F. Ménard , M. R. Meyer , , R. van Boekel , and C. Dominik Universidad Autonónoma de Madrid, Dpto. Física Teórica, Módulo 15, Facultad de Ciencias, Campus de Cantoblanco, E-28049Madrid, Spain. e-mail: [email protected] Institute for Astronomy, ETH Zurich, Wolfgang-Pauli-Strasse 27, CH-8093 Zurich, Switzerland Univ. Grenoble Alpes, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG, UMR 5274), F-38000 Grenoble, France Lunar and Planetary Laboratory, The University of Arizona, Tucson, AZ 85721, USA Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA Institute of Astronomy, Madingley Rd, Cambridge, CB3 0HA, UK Astronomical Institute Anton Pannekoek, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany INAF - Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy University of Michigan, Department of Astronomy, 1085 S. University, Ann Arbor, MI 48109Received - / Accepted -
ABSTRACT
Context.
High-resolution imaging reveals a large morphological variety of protoplanetary disks. To date, no constraints on their globalevolution have been found from this census. An evolutionary classification of disks was proposed based on their IR spectral energydistribution, with the Group I sources showing a prominent cold component ascribed to an earlier stage of evolution than Group II.
Aims.
Disk evolution can be constrained from the comparison of disks with di ff erent properties. A first attempt at disk taxonomy isnow possible thanks to the increasing number of high-resolution images of Herbig Ae / Be stars becoming available.
Methods.
Near-IR images of six Group II disks in scattered light were obtained with VLT / NACO in Polarimetric Di ff erential Imaging,which is the most e ffi cient technique for imaging the light scattered by the disk material close to the stars. We compare the stellar / diskproperties of this sample with those of well-studied Group I sources available from the literature. Results.
Three Group II disks are detected. The brightness distribution in the disk of HD163296 indicates the presence of a persistentring-like structure with a possible connection with the CO snowline. A rather compact ( <
100 AU) disk is detected around HD142666and AK Sco. A taxonomic analysis of 17 Herbig Ae / Be sources reveals that the di ff erence between Group I and Group II is due to thepresence or absence of a large disk cavity ( (cid:38) Conclusions.
Group II disks are not evolved versions of the Group I disks. Within the Group II disks, very di ff erent geometries exist(both self-shadowed and compact). HD163296 could be the primordial version of a typical Group I disk. Other Group II disks, likeAK Sco and HD142666, could be smaller counterparts of Group I unable to open cavities as large as those of Group I. Key words. stars: pre-main sequence – planetary systems: protoplanetary disks – ISM: individual object: HD144432, HD163296,HD152404, AK Sco, HD144668, HD142666, HD150193, HD145263, HD34282, HD135344B, SAO206462, HD169142, HD97048,HD100453, HD142527, HD139614, HD36112, MWC758, HD100546, HD179218 – Techniques: polarimetric
1. Introduction
Recent space missions and high-contrast imagers have revealed arich variety of planetary systems and protoplanetary disks. How-ever, our incomplete knowledge of the processes of planet for-mation is limiting our ability to establish a link between thetwo. In particular, the morphological evolution of protoplane-tary disks is not fully understood. While the transition from a fullgas-rich disk to a planetary system associated with a debris disk(e.g., Williams & Cieza 2011) is a corroborated theory, there isno general consensus on the intermediate stages. The taxonomyof protoplanetary disks is the key to obtaining new insights intotheir evolutionary paths. (cid:63)
Based on observations collected at the European Organisation forAstronomical Research in the Southern Hemisphere, Chile, under pro-gram number 095.C-0658(A)
A classification reflecting the radial evolution of disks wasproposed by Strom et al. (1989). They noticed that some disksshow a diminished near- to mid-IR flux and ascribed it to therapid dissipation of the inner dusty material. A more recent clas-sification by Meeus et al. (2001) involves the vertical evolutionof disks. They showed that the mid- / far-IR excess in the spec-tral energy distribution (SED) of intermediate-mass stars can ei-ther be fit by a power-law continuum or requires an additionalcold blackbody. These two categories, named Group II and I,respectively (hereafter GII and GI), were originally thought torepresent two distinct disk geometries, flat and flared disks, dueto di ff erent stages of the dust grain growth and consequent set-tling toward the disk mid-plane (see, e.g., Dullemond & Do-minik 2004). This process would act to decrease the illuminatedsurface resulting in a smaller amount of reprocessed light andthus in the transition from GI to GII. Article number, page 1 of 16 a r X i v : . [ a s t r o - ph . E P ] J u l & A proofs: manuscript no. aa
Table 1.
Properties of Group I and II disks from di ff erent observational techniques. Technique Group I Group II Illustrative referenceNear-IR scattered light Routinely detected Mostly undetected Garufi et al. (2014)Near-IR ro-vibrational lines Hot, from larger radii Cold, from smaller radii van der Plas et al. (2015)Mid-IR continuum Routinely resolved Typically unresolved Mariñas et al. (2011)Silicate emission features Often detected Always detected Meeus et al. (2001)Far-IR emission lines Routinely detected Mostly undetected Meeus et al. (2012, 2013)Millimeter continuum Routinely resolved Mostly unresolved Mannings & Sargent (1997)Stellar abundances Fe, Mg, Si subsolar Fe, Mg, Si solar-like Kama et al. (2015)These radial and vertical classifications may be more corre-lated than was initially thought. In fact, the powerful imagingtechniques of the last decade have highlighted the high recur-rence rates of large cavities in GI disks (e.g., Honda et al. 2015).Conversely, no GII disk has shown the presence of a large (tensof AU) inner gap (whereas the presence of small gap appearspossible; see Menu et al. 2015). Also, Kama et al. (2015) founda systematic depletion of refractory elements only in the pho-tosphere of stars hosting a GI disk, and they linked this to thelarge-dust trap exerted by giant planets in a disk cavity. Anotherdissimilarity is the significantly larger emitting radius of the COro-vibrational lines from the inner ∼
10 AU in GI compared toGII (Banzatti & Pontoppidan 2015; van der Plas et al. 2015),which points toward the presence of a cavity in the moleculargas as well. This dichotomy could also explain why all GII showsilicate features, whereas a large fraction of GI do not. In fact,Maaskant et al. (2013) argued that the lack of silicate features isdue to an intrinsic depletion of dust material where this emissionoriginates.GII disks are also more elusive than GI. The mid-IR emis-sion of GII is not resolved, contrary to GI (Mariñas et al. 2011;Honda et al. 2015). The same consideration may apply at mil-limeter wavelengths since no resolved observations of a GII havebeen seen so far (with one possible exception, HD163296, whichis discussed in this work). Furthermore, Garufi et al. (2014) haveshown that GI are routinely detected in scattered light, whereasGII are only marginally detected or not detected. Far-IR emis-sion lines of CO and OH are strong in GI and remain mostlyundetected in GII (Meeus et al. 2012, 2013). All these obser-vations may suggest that GII disks are self-shadowed, i.e., theinner disk intercepts most of the stellar light and casts a shadowfar out (e.g., Dullemond et al. 2001). This scenario is supportedby the anti-correlation between the brightness of the inner andouter disk (e.g., Acke et al. 2009).All this said, it is clear that the idea of an evolution fromGI to GII has to be revised. Currie (2010) and Maaskant et al.(2013) proposed that the two groups represent di ff erent tracks inthe evolution of disks, where the most recognizable geometricalevolution is either gap formation or disk settling. Table 1 summa-rizes the properties of GI and GII disks from various techniques.In this paper, we present new near-IR observations of someGII in scattered light. The sample is described in Sect. 2. Theobservations are performed in polarimetric di ff erential imaging(PDI), a technique that allows a substantial fraction of the scat-tered light from the ∼ µ m-sized dust grains in the disk surface tobe imaged (e.g., Apai et al. 2004; Quanz et al. 2011). The PDItechnique is based on the simple and powerful principle that stel- lar light is mostly unpolarized, whereas the scattered light fromthe disk is to a large extent polarized. The observational setupand data reduction are described in Sect. 3, and the results of thenew dataset in Sect. 4. The new observations of GII are then usedto perform a taxonomic analysis of a larger sample of protoplan-etary disks in Sect. 5. A consequent discussion on the nature ofGI and GII is finally given in Sect. 6 and Sect. 7.
2. Sample
The classification of GI and GII is not univocal. Contrary tothe original Meeus classification, authors have defined multiplemethods based solely on photometry which leave the partition ofsources substantially unaltered. For example, van Boekel et al.(2003) used the m − m color in relation to five near-IR mag-nitudes. A more direct approach is to use the [30 µ m / . µ m]ratio since at these wavelengths the impact of spectral featuresof silicates and hydrocarbons is minimized (Acke et al. 2009). Astrong correlation with the Meeus classification exists (Maaskantet al. 2013) and, in this context, the transition between GI andGII may lie at [30 / . = . / ff ected by two disk parameters,i.e., the flaring angle and the inner radius. This degeneracy is dis-cussed in Sect. 5.We used the [30 / / – HD144668 (or HR5999, [30 / . = .
0) is an A5 star (vanden Ancker et al. 1997) forming a visual binary with a T Taustar at 1.4 (cid:48)(cid:48) (Stecklum et al. 1995). Optical emission linessuggest that the disk is close to edge-on (Perez et al. 1993).The mid-IR emission from the source is mostly unresolved,even though a faint extended signal is detected up to 95 AUto the N and S by Mariñas et al. (2011). – HD142666 ([30 / . = .
5) is an A3 / A8 star (Blondel &Djie 2006) that is thought to be isolated. The circumstellardisk has an inner radius slightly larger than the dust subli-mation radius at sub-AU scale (Schegerer et al. 2013; Menuet al. 2015). – HD144432 ([30 / . = .
8) is the primary A8 star(Sylvester et al. 1996) of a triple system (Müller et al. 2011).Sub-mm observations of the CO emission lines reveal a ∼ Article number, page 2 of 16arufi et al. 2017: The evolution of protoplanetary disks, Group I vs Group II
AK Sco Q Φ U Φ ″ Detected
HD163296 Q Φ U Φ ″ Detected
HD142666 Q Φ U Φ ″ Detected
HD144432 Q Φ U Φ ″ Undetected
HD144668 Q Φ U Φ ″ Undetected
HD145263 Q Φ U Φ ″ Undetected
Fig. 1.
Polarized light imagery of the sample. For each object, the Q φ image is shown to the left and the U φ to the right. All pairs of images havethe same linear color stretch and are not scaled by the squared distance. The main stars are at the center of the ∼ (cid:48)(cid:48) green circles. When stellarcompanions are present, they are displayed with their I image in an inset circle. North is up, east is left. AU disk which is ∼ ◦ inclined (Dent et al. 2005). Simi-larly to HD142666, near-IR interferometry (Chen et al. 2012)hints at the presence of a small-scale cavity. – HD163296 ([30 / . = .
0) is the best-known object ofthe sample. The disk around the isolated A1 star (Moraet al. 2001) has been systemically imaged at (sub-)mm (e.g.,Isella et al. 2007) and near-IR wavelengths (e.g., Grady et al.2000). ALMA observations (Guidi et al. 2016) revealed anexcess in the mm emission in proximity to the radial loca-tion of the CO snowline (Qi et al. 2015) and to a ring visiblein scattered light (Garufi et al. 2014). This excess may be dueto an increase in dust density caused by a local dust trapping. – HD145263 ([30 / . = .
0) is an F0 star (Smith et al. 2008)with a disk exhibiting an IR excess of intermediate amountbetween a young and a debris disk (Honda et al. 2004). – HD152404 (or AK Sco, [30 / . = .
3) is a F5 + F5 closebinary (with a separation of 0.16 AU, Anthonioz et al. 2015).It is classified as a GII source based on its far-IR excess.However, according to the mid-IR criterion it is a GI sourcewhose [30 /
3. Observations and data reduction
The six new sources of this work were observed over twonights (22-23 July 2015) with the Adaptive Optics(AO)-assistedNAOS / CONICA instrument (NACO, Lenzen et al. 2003; Rous-set et al. 2003) at the Very Large Telescope (VLT) in polari-metric di ff erential mode (PDI). The observing strategy followedthat used in the works with NACO by Quanz et al. (2013), Garufiet al. (2013), and Avenhaus et al. (2014b). In PDI with NACO,the stellar light is split into two beams containing orthogonalpolarization states by a Wollaston prism. A rotatable half-waveplate provides a full cycle of polarization state at 0 ◦ , 45 ◦ , 90 ◦ ,and 135 ◦ . At the time of the observations, NACO was fixed in theS13 objective. The small scale of this camera (13.27 mas / pixel)and the high background level caused by insu ffi cient shieldingin the instrument impeded us from performing optimal observa-tions. Furthermore, one of the camera quadrants that we partlyused showed a non-static noise across the detector rows thatcould not be completely removed during the data reduction. Article number, page 3 of 16 & A proofs: manuscript no. aa
Table 2.
Summary of observations. Columns are: night number (seetext), source name, detector integration time (sec) multiplied by numberof integrations and by number of exposures, total integration time (sec),and DIMM seeing during the observation. We note that t exp = DIT × NDIT × NE × Night Source DIT(s) × NDIT × NE t exp (s) Seeing1 HD144432 0.3454 × ×
20 4,090 1.0 (cid:48)(cid:48) -1.6 (cid:48)(cid:48) × ×
20 4,090 1.1 (cid:48)(cid:48) -2.5 (cid:48)(cid:48) × ×
24 7,680 1.2 (cid:48)(cid:48) -2.4 (cid:48)(cid:48) × ×
24 4,310 0.8 (cid:48)(cid:48) -1.3 (cid:48)(cid:48) × ×
20 4,480 0.8 (cid:48)(cid:48) -1.3 (cid:48)(cid:48) × ×
20 4,000 0.8 (cid:48)(cid:48) -1.4 (cid:48)(cid:48)
All targets were observed in the K S band for a total time of1.12 to 2.13 hours per source (see Table 2). The sources had anaverage airmass of ∼ ff ected by a highly variable seeing(0 . (cid:48)(cid:48) − . (cid:48)(cid:48) ) with the intermittent passage of thin cirrus.The data reduction was performed following the method out-lined by Avenhaus et al. (2014b). Apart from the standard cos-metic steps (dark current subtraction, flat fielding, bad pixel cor-rection), this method consists in upscaling the image before de-termining the star position to achieve a sub-pixel accuracy, ex-tracting the two beams with perpendicular polarization states,and equalizing the flux contained in annuli from the two beamsto attenuate the instrumental polarization. The final images areproduced by combining the beams from the full polarimetric cy-cle into the parameters Q φ , U φ , P , and I (see Schmid et al. 2006,for details) . The images shown in the paper are binned by 1.5with respect to the original pixel size to reduce the shot-noise andare smoothed with a Gaussian kernel of 0.04 (cid:48)(cid:48) (approximatelyhalf of the instrument PSF).
4. New polarized images
The final PDI images of the six sources are shown in Fig. 1. Eventhough these observations are non-coronagraphic, we considerthe inner ∼ . (cid:48)(cid:48) around the central star unreliable and thus wemasked out the region. This is motivated by both the smearinge ff ect due to the finite PSF resolution (see Avenhaus et al. 2014a)and by the suboptimal AO correction of these observations. AK Scorpii.
The disk around AK Sco is clearly detected inthe Q φ image. Two bright lobes are seen along the NE-SW di-rection and their signal can be detected inward down to the in-nermost reliable distance from the star ( ∼ . (cid:48)(cid:48) ). In the NWquadrant, the signal is detected in the form of an arch connect-ing the two lobes. No significant signal is detected in the SEquadrant. The U φ also shows a strong signal along the NE axisin correspondence with one Q φ lobe. The pair of parameters ( Q φ , U φ ) is referred to as ( Q r , U r ) in the ref-erence paper. Other authors have also used the nomenclature ( P ⊥ , P (cid:107) )and ( Q T , U T ).
50 AU ! (d=122 pc) HD163296
PDI 2012 ! CO snowline ! Millimeter continuum
Fig. 2.
PDI image of HD163296 from this paper compared to otherworks. The cyan dashed line indicates the peak intensity of the ring de-tected in scattered light by Garufi et al. (2014) and shown in the insetimage. The violet solid line lies at the CO snowline (Qi et al. 2015) tak-ing into account the disk inclination and keeping the star in the center.The green dotted lines are obtained similarly from the peak intensity ofthe two innermost rings revealed at 1.3 mm by Isella et al. (2016). Thestar is at the center of the gray circle. North is up, east is left.
HD163296.
The top right panel of Fig. 1 shows the detectionof the ring-structure around HD163296. The emission is maxi-mized to the north, while it is very marginal to the south. Twosymmetric minima are seen along the SE-NW direction. Sincethese minima correspond to the strong AO spots of these ob-servations, they should not be trusted. Inside the ring, we cannotreveal any coherent disk structures. Similarly to AK Sco, the sig-nal in the U φ is very strong, with a positive branch on one sideof the major axis and a negative branch on the other. HD142666 . The Q φ of this star reveals a relatively strongsignal to the north and to the south that is persistent across indi-vidual frames. This notion and the di ff erent brightness and dis-tribution of the U φ image suggest that the signal detected in the Q φ is actually a signal from the circumstellar disk. The signalfrom Q φ extends inward at least down to the innermost reliableradius. HD144432 . The binary companions of HD144432 are eas-ily detected in the intensity image to the north of the mainstar. In particular, the center of the binary system lies at r = . (cid:48)(cid:48) ± . (cid:48)(cid:48) with P.A. = . ◦ ± . ◦ . Thus, the system hasmoved counterclockwise from 2005 to 2015 by 1 . ◦ (based onthe astrometric analysis by Müller et al. 2011). Also, the pair ofcompanions has moved relative to each other by ∆ r = − . (cid:48)(cid:48) and ∆ P.A. = − . ◦ . The Q φ and U φ are comparable in bothmorphology and intensity. An extended feature is visible from Q φ across the SE-NW axis. This is persistent in many individualframes and absent in the U φ image. However, the vertices of thefeature match the location of the AO spots in the intensity image.Thus, we consider the signal as spurious. HD144668 . The intensity image of HD144668 also revealsthe presence of a companion, at r = . (cid:48)(cid:48) ± . (cid:48)(cid:48) and P.A. = . ◦ ± . ◦ . Thus, the orbital motion from 1992 ( r = . (cid:48)(cid:48) and P.A. = . ◦ , Stecklum et al. 1995) is still very marginal.A very uneven signal is seen in both the Q φ and the U φ caused Article number, page 4 of 16arufi et al. 2017: The evolution of protoplanetary disks, Group I vs Group II by a very variable PSF across the observations, and the presenceof scattered light cannot be inferred.
HD145263 . Both the Q φ and the U φ show only a verymarginal signal close to the star with comparable brightness.Thus, no polarized light is detected from this object. Three of the six disks in the sample are detected. The strongestsignal is revealed around AK Sco, which is the source with thehighest [30 / ff erential imaging (ADI). The only significant di ff erence is thepresence in the PDI images of the NW arch, which may representthe far side of a full ring that remained undetected in the ADI im-age. In fact, Garufi et al. (2016) showed that the process of ADIacts to damp the disk emission from the minor axis and thus thatan azimuthally symmetric feature can be seen as a double-wingstructure aligned with the major axis. We defer further consider-ations on the disk geometry to a forthcoming SPHERE paper.The presence of signal in the U φ image of both AK Sco andHD163296 is qualitatively consistent with the deviation fromtangential scattering from inclined disks (these disks are ∼ ◦ and ∼ ◦ inclined), which acts to redirect part of the polar-ized signal from the Q φ to the U φ image (Canovas et al. 2015).We emphasize that the signal close to the star in U φ may appearstronger than from other works partly because these images, forconsistency with the rest of the dataset, are not scaled with thesquared distance from the star. In any case, a partly instrumentalcontribution cannot be ruled out because of the NACO cross-talke ff ect between the Stokes parameters (Witzel et al. 2010).In Fig. 2 we show the polarized image of HD163296 andcompare it to that obtained with the same mode in 2012 (seeinset image) by Garufi et al. (2014). The evident di ff erence be-tween the two is largely due to the spurious signal present inboth datasets. For example, the northern region (the brightest inthe 2015 dataset) was mostly unaccessible in 2012 because ofthe presence of a strong artifact. In any case, the radial locationand the apparent flattening of the ring remains to first order un-changed between the two epochs (see cyan dashed lines). Thisdemonstrates that the ring in scattered light is due to a persis-tent disk morphology rather than a transient shadow from theinner disk (as proposed by Garufi et al. 2014). This finding re-inforces the spatial connection with the excess in the continuumemission at 850 µ m shown by Guidi et al. (2016) and with thelocation of the CO snowline (see violet solid line) as inferred byQi et al. (2015). Recent ALMA images (Isella et al. 2016) haverevealed the presence of three rings in the millimeter continuumthat are indicated in Fig. 2 by the green dotted lines. Guidi et al.(2016) discussed a scenario where dust trapping is favored at theCO iceline and results in an increased dust surface density. Thisshould also have an impact on the disk surface to allow the de-tection of scattered light from a disk elsewhere undetected. Inparticular, the PDI ring is nearly co-spatial with the innermostALMA millimeter ring at ∼
80 AU.Finally, the signal detected around HD142666 points to-ward an inclined disk with the major axis oriented in the north-south direction. This geometry is consistent with what wasinferred for the inner disk by Vural et al. (2014), i.e., i ∼ ◦ and P . A . ∼ ◦ . The presence of a signal as close to thecenter as ∼ . (cid:48)(cid:48) rules out the existence of any cavity largerthan ∼
10 AU. More importantly, the abrupt outward decreasein signal at 0 . (cid:48)(cid:48) is most likely not due to the sensitivity. Thus, these observations are consistent with a rather compact disk of ∼
60 AU in size.The non-detection of the other disks can be due to manydi ff erent factors (e.g., self-shadowing, deficit of scattering par-ticles, dust properties). These possibilities are explored in thebroader context of the dichotomy between GI and GII through-out the paper.
5. Taxonomy of Group I and Group II
In this section, we investigate the brightness of a number of GIand GII disks in scattered light and explore connections withthe disk properties known from the literature. The sample con-sists of ten GI and seven GII disks that have been observed innear-IR PDI over the last five years with either VLT / NACO orVLT / SPHERE. The full sample is described in Appendix A.Measuring the amount of scattered light from a sample ofdisks in a consistent way is not an easy task. In fact, the stellarbrightness, the distance of the source, and the disk inclinationsignificantly alter the intrinsic amount of light that we detect.However, it is possible to compute the polarized-to-stellar lightcontrast along the disk major axis to elude their influence. Bydoing this, the polarized-to-stellar light contrast (referred to ascontrast hereafter) is a direct measurement of the capability ofthe disk surface to scatter photons, which in turn depends on thedisk geometry and the dust properties. A detailed description onthe calculation of the contrast can be found in Appendix B.The measured contrast for all the targets in the sample isshown in Fig. 3. From the plot, it is clear that GI disks are sys-tematically brighter than GII disks in scattered light. With fewexceptions, all GI disks have comparable contrast. It is also ev-ident that the dichotomy between GI and GII can be expressedin terms of the presence or absence of a disk inner cavity. In theplot, we also show the cavity sizes constrained by millimeter im-ages (where available), PDI images (when the cavity is detected),or SED fitting (see Appendix A for details). We defer the discus-sion on discrepant estimates from these techniques to Sect. 6.1.Interestingly, the disks with larger cavities ( R (cid:38)
15 AU) havehigher [30 / R (cid:46)
15 AU,[30 / <
4) a possible trend is seen in the diagram since boththe ratio and the contrast are primarily a ff ected by the flaring an-gle. Three of the four non-detections (from this work and Garufiet al. 2014) are inconsistent with the trend. For sources with cavi-ties larger than ∼
15 AU, the relation is no longer present possiblybecause the ratio is mostly a ff ected by the deficit of inner mate-rial. This idea is supported by the notion that the four sourcesin the plot with the highest [30 / / > Article number, page 5 of 16 & A proofs: manuscript no. aa
F(30 μ m) / F(13.5 μ m) P o l a r i z ed li gh t c on t r a s t ( · ⁻ ³ ) Group IGroup IIGroup II (upper limit)Flat 30-13 SED
Group IGroup II
HD142527 HD34282HD97048HD100453HD100546 HD139614 HD169142HD179218 MWC758 HD135344BAK ScoHD142666 HD144432HD144668 HD145263HD150193HD163296
Cavity size (AU)
140 30 10
Fig. 3.
Polarized-to-stellar light contrast for all the sources in the sample (see Appendix A) compared with the flux ratio at 30 µ m and 13.5 µ m. GIdisks are plotted in green, GII in purple. The disk cavity, where known and as taken from di ff erent datasets (see text), is indicated by a gap in thesymbol, proportional to the cavity size with dynamic range from 5 AU to 140 AU. The dashed line indicates the ratio corresponding to a flat SED,obtained from 30 ÷ . = .
2. The ratios are from Acke et al. (2010), while the contrasts are from this work, as explained in Appendix B.
To investigate whether the polarized contrast is related to thestellar properties, we compare our contrasts with the e ff ectivetemperature of the stars (see Fig. 4a). It turned out that GI andGII disks in the sample are uniformly distributed across stellartemperature and mass. There is an accidental selection valleyin the sample: 6 stars are warmer than 9,000 K and 11 colderthan 8,000 K (note the discontinuity in the x-axis). In the fig-ure, we also label those disks that show peculiar structures inscattered light, namely rings or spirals . The ring-like disks inthe sample are found predominantly around B and early-A stars( T e ff > T e ff < ∼
340 AU vs ∼
170 AU).There is no correlation between the cavity size and the stellartype. Two of the three GII disks detected are significantly smallerthan the GI disks. The only large GII is HD163296, whichmay di ff er in many other aspects from the other GII disks (seeSect. 6.3). Interestingly, three of the four non-detections have acompanion at a projected distance of 100-200 AU. The only twoGI disks with an outer companion are those with smaller de-tected extent, suggesting that the outer disk truncation may playan important role (see Sect. 6.3). For most disks, this classification is obvious. Two cases are subjectto interpretation: HD100546, showing wrapped arms that may resem-ble rings (Garufi et al. 2016), and HD142527, showing a disk wall withmultiple spiral arms outward of it (Canovas et al. 2013). We do notclassify these objects here because their natures are di ff erent from thenominal ring-like disks (e.g., HD97048, Ginski et al. 2016) or symmet-ric spiral-like disks (e.g., HD135344B, Garufi et al. 2013) Spatially unresolved information on the outer disk structure canbe obtained from the SED at wavelengths from the mid-IR to themillimeter regime. We compare some of these constraints withour contrast in Fig. 5.
We calculated the far-IR excess from 20 µ m to 450 µ m of allsources, following the method described by Pascual et al. (2016).This excess is plotted against the contrast in Fig. 5a. We founda clear trend between the two quantities. This relation is due tothe co-located origin of the far-IR thermal light and the near-IRscattered light, i.e., the disk surface at tens of AU. The followinglinear regression is found to fit the data: F (FIR) F ∗ = . × φ pol (1)In words, this trend says that on average the amount of fluxscattered (and polarized) by the disk in the near-IR is 2 . ≡ ÷ .
97) of the value of the thermal far-IR flux. In particu-lar, we found values spanning from 1.1% to 5.9%. These valuesare only upper limits of the real polarized scattered / thermal en-ergy budget since the far-IR is a global measurement (and thusa ff ected by the disk inclination), whereas our contrast is a localmeasurement (not a ff ected by the disk inclination). We did notfind any correlations for the contrast with any far-IR photometry(at 70 µ m, 100 µ m, and 160 µ m), indicating that the disk flaringangle cannot be properly estimated from a single waveband.There are three significant outliers to the faint wing of thedistribution. Even though HD150193 and HD144432 have a far-IR excess comparable to the other GII, their disks are not de-tected in scattered light (this work and Garufi et al. 2014), mak-ing the scattered / thermal flux ratio < .
6% and < . , respec-tively. A possible explanation is that the whole disk is less ex-tended than the inner working angle of the PDI observations Article number, page 6 of 16arufi et al. 2017: The evolution of protoplanetary disks, Group I vs Group II
Effective temperature (K) P o l a r i z ed li gh t c on t r a s t ( · ⁻ ³ ) B-type A-type F-type
Group I (rings)Group I (spirals)Group IGroup II (rings)Group IIGroup II (undetected)
Early ↔ Late .......... ∿ ∿ ∿ ∿ HD100546 HD97048 HD150193HD34282HD179218 HD163296HD144668MWC758HD169142HD142666HD139614 HD100453HD144432HD145263HD135344B HD142527AK Sco ◎ ) (a) Effective temperature (K) R ad i a l r ange o f d i sk de t e c t i on ( A U ) B-type A-type F-type
Early ↔ Late .......... (b)
Fig. 4.
Disk properties compared to stellar properties. (a) : Polarized-to-stellar light contrast compared to the stellar e ff ective temperature. Notethe discontinuity on the x-axis. The symbol size indicates the stellar mass with dynamic range between 1.6 M (cid:12) and 3.2 M (cid:12) . (b) : Radial range ofdetection in PDI, compared to the stellar e ff ective temperature. When continuum millimeter imaging reveals a larger radius, this is indicated bythe darker areas at the top of the bars. The white areas indicate disk cavities. The yellow symbols give the projected distance of the companions.When these symbols are at the base of the bars, they indicate a binary system surrounded by the disk. ( (cid:46)
15 AU). On the other hand, the non-detection of HD144668is still consistent with the trend.
The gaseous disk can be traced by the mid-IR emission of poly-cyclic aromatic hydrocarbons (PAH). Direct imaging of PAH hasshown that this emission can originate from the outer regions ofdisks (van Boekel et al. 2004; Lagage et al. 2006). The emissionfrom GI is typically stronger than from GII (e.g., Acke et al.2010). In Fig. 5b we show the PAH luminosity relative to thestar as obtained by Acke et al. (2010) and compare it with thecontrast.From the plot, the dichotomy for GI and GII is evident.Among the GI, the stellar temperature correlates with the PAHluminosity. All GI have prominent PAH emission. Among theGII, only three disks are detected and these are not sources withhigh stellar temperature. One GII, HD142666, even shows PAHbrightness comparable to the GI. This is the most significant de-parture from the expected correlation between the PAH strength(to the first order, tracing the gas) and the contrast (tracing thedust). A possible explanation to these departures may derivefrom the new view that GI are gapped disks and GII are not,which is discussed in Sect. 6.2.
The millimeter flux of the sources is compared to the contrastin Fig. 5c. The fluxes at 1.3 mm obtained by multiple authors(see Appendix A) have been scaled to a distance of 140 pcby means of the new GAIA measurements (Gaia Collabora-tion 2016), where available. Thus, the relative uncertainties aresmaller than in previous works. As can be seen from the figure,there is no clear trend between the millimeter flux and the polar-ized contrast. There is also no clear correlation with the stellarmass. Interestingly, all GII in the sample, except HD163296, arefainter in millimeter than the GI: the former group has an average F . (cid:39)
60 mJy and the latter F . (cid:39)
400 mJy.The emission at 1.3 mm from the outer regions of proto-planetary disks is typically optically thin. Therefore, this fluxis commonly used to estimate the dust mass M dust of the disk(e.g., Andrews et al. 2011). To convert flux into dust mass, as-sumptions on disk opacity and dust temperature T dust must betaken. This means that the fluxes shown in Fig. 5c do not neces- sarily reflect the dust mass of the targets and we cannot firmlyconclude that our GII are less massive than the GI. In fact, ina scenario where GII are flat disks and GI are flared disks, the T dust of GII can be smaller since a smaller disk height results ina lower e ffi ciency to heat the disk interior. We note that the es-timate on M dust only scales as T − . This means that to accountfor the above-mentioned factor of 7 di ff erence in flux betweenthe GI and GII in our sample, the T dust of GII should be approxi-mately as many times lower as that of GI. Also, as mentioned inthe Introduction the dichotomy between flared and flat disks isno longer obvious, and the di ff erent millimeter fluxes of the twogroups may actually support the view that GII are more compactthan GI (and thus have higher disk opacities) and / or that theyare less massive in dust than (most of) the GII, as discussed inSect. 6.3. In Fig. 5d we show the dust opacity index β mm compared withthe contrast. If the emission is optically thin, this index is relatedto the slope of the millimeter SED α mm , via α mm = β mm + α mm canbe significantly smaller than that of the ISM ( β mm ≈ . α mm ≈ . α mm or β mm from previous works (see Ap-pendix A) and made them uniform to β mm , as in Fig. 5d. Itturned out that the average β mm of the GI in our sample is onlymarginally larger than that of GII (1.21 against 1.03, with sin-gle uncertainties of ∼ β mm are those with large inner cavities. This can be a con-sequence of a pressure bump at the outer edge of a disk cavity,which can act to deplete the inner disk of millimeter grains andthus result in a higher β mm inside the cavity. This e ff ect was mod-eled and also seen in the possible trend between β mm and cavitysize of a large sample of disks by Pinilla et al. (2014). Alterna- We did not retrieve the errors for all measurements. We assumethat an average uncertainty for these relatively bright sources is ∼ & A proofs: manuscript no. aa
Polarized light contrast ( · ⁻³ ) F ( F I R ) / F ⭐ Group IGroup IIGroup II (undetected in PDI)FIR = Contrast ÷ 2.86%
HD150193 HD142527HD144668 HD163296HD142666 (a)
HD144432 AK Sco HD179218
Polarized light contrast ( · ⁻³ ) L ( PA H ) / L ⭐ ( · ⁻ ³ ) Group I Group IIGroup II (undetected in PDI)
HD142666HD144432 HD163296 AK Sco
10 6
Stellar T (1,000 K) (b)
Polarized light contrast ( · ⁻³ ) F l u x . mm a t c ( m Jy ) Group IGroup IIGroup II (undetected) HD163296 AK ScoHD142666 Stellar mass (M ◎ ) (c) Polarized light contrast ( · ⁻³ ) D u s t opa c i t y i nde x β Group IGroup IIGroup II (undetected)
ISM
50 10
Cavity size (AU) (d)
Fig. 5.
Outer disk properties of the sample compared with the contrast. (a) : Far-IR excess normalized to the stellar flux. (b) : PAH luminositynormalized to the stellar flux. The symbol size reflects the stellar temperature with dynamic range between 10470 K and 6450 K. (c) : Flux at 1.3mmnormalized at a distance of 140pc. The error on the y-axis reflects the uncertainty on the distance (typically smaller for GAIA measurements). Thesymbol size reflects the stellar mass with dynamic range between 1.6 M (cid:12) and 3.2 M (cid:12) . (d) : Dust opacity index β mm . The symbol size reflects thecavity size from the PDI images. Typical errors for this source type are indicated by the vertical bar to the right. tively, lower β mm values from disks without a large cavity can beexplained by the possible existence of optically thick central re-gions (as shown by, e.g., Isella et al. 2016). In fact, the resolved β mm values of these regions would be ∼ β mm is not a good tracer of theglobal grain growth, but may only reflect the di ff erent morphol-ogy of the two groups. In fact, if we compare the β mm of GII withthat of GI without a large cavity in scattered light ( R >
15 AU),we obtain similar values (1.03 against 1.07). Therefore, we pro-pose that the di ff erent β mm seen in larger sample of GI and GIImay only be the consequence of the dust grain di ff erentiationand / or the absence of an optically thick central region in gappeddisks rather than a real indication of di ff erent evolutionary stages(see Sect. 6.2). The spectral properties from the visible to the mid-IR constrainthe morphology of the inner disk and of the immediate surround-ing of the star. In this section and in Fig. 6 we compare somespectral properties with the contrast and the stellar properties.
We calculated the near-IR excess from 1.2 µ m to 4.6 µ m of allsources, following the method described by Pascual et al. (2016).This excess is shown in Fig. 6a. Three clusters of datapointsstand out from the plot: GII disks with mid to high near-IR flux,half of the GI with low near-IR flux, and the other half with highnear-IR flux. In the plot, we also indicate the presence of fea- tures on the disk surface to highlight that three of the four GIwith high near-IR flux have a double-arm spiral structure. Thefourth member of this cluster, HD142527, also shows multiplespirals, but with di ff erent opening angles and at larger radii.The near-IR flux of the GII in the diagram is intermediatebetween that of the two clusters of GI (with the exception ofHD144668). Therefore, on average the thermal emission of hotdust from GI and GII is to first order comparable, contrary tothe far-IR and the millimeter flux. All GII show a relatively highnear-IR excess indicating a recurrently large contribution to theSED from hot particles (see Sect. 6.4). In Fig. 6b we show the mass accretion rate calculated from theUV excess by Fairlamb et al. (2015) for some of the sources inour sample. The highest rates are found around more massivestars. There is no significant di ff erence between GI and GII; theformer group has an average value of 0 . · − M (cid:12) / year (6 / . · − M (cid:12) / year (4 / ff erent behavior for the accretionrate of Herbig and T Tau stars. Article number, page 8 of 16arufi et al. 2017: The evolution of protoplanetary disks, Group I vs Group II
CO ro-vibrational lines in the near- and mid-IR trace the hot gasin the very inner disk. From these lines, a characteristic emittingradius for the hot CO can be measured (e.g., Banzatti & Pontop-pidan 2015; Banzatti et al. 2017). These radii for some of oursources are shown in Fig. 6c. Similarly to the PAH strength, thisproperty is linked to the stellar temperature in GI, with the threeearly stars in our sample showing emitting radii as large as 10AU or more. On the other hand, the emitting radii of the latestars lie between 1.7 and 2.6 AU.The three GII shown in the diagram all have slightly smallerCO radii (from 0.8 to 1.4 AU) than all GI, even though the starsare warmer or comparable to the late stars of the GI.
In massive stars, which have long convective mixing times, thephotospheric abundance of refractory elements show a correla-tion with the structure of the inner disk (Kama et al. 2015). Thisis likely because the depletion of large grains in the inner regionsof GI and a few GII disks leads to an increased gas-to-dust ra-tio in the material accreting onto the star, which may in turn beconnected to the trapping of large dust grains by substellar com-panions. In Fig. 6d we show the stellar photospheric abundanceof the iron relative to that of the hydrogen for the stars in oursample.Similarly to both Fig. 6a (the near-IR flux) and Fig. 6c (theCO radius), three clusters of sources are visible. The GII andfour of the GI show a solar abundance of iron or slightly lower.The other five GI in the diagram show a significantly depletedabundance. Of particular interest is that the four GI with a solarabundance of iron are the same four GI with high near-IR excess,and the three of them present in the CO diagram all show smallCO radius. Thus, these three diagrams seem to reveal a physicalconnection between the stellar photospheric abundance of heavyelements, the near-IR excess, and the emitting radius of hot CO.In this regard, the GII of our sample are similar to roughly halfof the GI, namely with high near-IR, high [Fe / H], and small COemitting radius.
6. Discussion
Keeping in mind the small number of objects, the following re-sults on the taxonomy of GI and GII disks are to be considered:(a) What gives rise to the observed features defining GI and GIIis the presence or absence of a disk cavity ( (cid:38) few AU large).(b) Most sources (but not all of them, see HD150193) have po-larized contrast scaling with the far-IR excess.(c) Most non-detected GIIs have a stellar companion at 100sAU. The GIs with a companion are the smallest disks in ex-tent.(d) GIIs typically have weaker millimeter fluxes. However, oneGII (HD163296) has the third highest flux in the sample.(e) If disks with large cavities ( R (cid:38)
30 AU) are not considered,GIIs have on average the same opacity index as GIs.(f) GIs and GIIs are indistinguishable in terms of mass accretionrate. There is no relation for this rate with the cavity size.(g) GIIs show high near-IR excess, IR CO emission on smallradii, and solar photospheric abundance of iron. Four (out ofnine) GIs have the same properties (and their outer disks allshow spirals), whereas the other GIs have low near-IR ex-cess, CO from larger radii, and depleted abundance of iron. The implications of these seven major results are discussedin this section with the aim of providing an explanation for theelusiveness of GII disks in scattered light, as well as insight intothe nature of the GI-GII dichotomy.
Figure 3 confirms what was proposed in previous works (e.g.,Currie 2010; Maaskant et al. 2013; Menu et al. 2015), namelythat GI sources are gapped disks whereas GII sources are contin-uous disks. In fact, all GI disks in our sample show the presenceof a large cavity ( R (cid:38) small cavities ( R (cid:46) ≡ gapped and GII ≡ continuous disksis established, it is clear that the study of the observational prop-erties di ff ering in the two groups must be revised in the con-text of the new dichotomy. For example, treating GI as gappeddisks may also explain the incongruous PAH brightness in GIsand GIIs raised by Dullemond et al. (2007). In fact, their modelsshow that if GIIs are the result of dust sedimentation occurringin GIs, their PAH brightness should be enhanced because of thereduced opacity, and thus increased UV radiation, in the envi-ronment where the PAH luminosity originates. However, the ob-servations show the opposite trend (Acke et al. 2010, and Fig. 5bof this work). We speculate that, if instead the GIs are gappeddisks and GIIs are not, an increased amount of UV-exposed PAHmolecules is to be expected in GIs, reconciling theory and obser-vations. This hypothesis has an intriguing consequence relatedto the weak detection of PAH emission from three GIIs only.In fact, HD142666 and HD144432 are the GIIs in our samplewhere a small disk cavity has been claimed (Chen et al. 2012;Menu et al. 2015). The third, AK Sco, is composed of binarystars separated by 0.16 AU and an intrinsically larger inner cav-ity is thus to be expected. Therefore, our dataset suggests thatthe PAH emission may be intimately related to the presence ofan inner cavity and the reason is an increased UV radiation indisks with lower optical depth at small radii.More generally, it is not obvious whether some disk prop-erties (see Table 1) are ( i ) the result of the geometry providedby the presence / absence of a cavity, or that ( ii ) they trace thedisk conditions that allow or not the formation of a cavity. Forexample, is the lower scattered light of GII disks due to self-shadowing in continuous disks ( i ) or does it reflect a di ff erentgeometry for disks (maybe smaller or less massive) that cannotopen large cavities ( ii )? The former explanation points towardGI and GII being di ff erent evolutionary stages , whereas the latterpoints toward them being di ff erent evolutionary tracks in the disklifetime. These two scenarios are discussed in Sect. 6.2. Here westress that the answer to this question is intimately related tothe origin of disk cavities, which is in turn a longstanding de-bate in the disk community. Interactions with orbiting compan-ions (Rice et al. 2003), photoevaporation (Alexander et al. 2006),and dust grain growth (Dullemond & Dominik 2005) are onlysome of the proposed explanations for the disk cavities. Withspecific focus on the objects of our sample, the literature indi-cates an increasing consensus on the interaction with (forming)planets as the most probable cause. Also, two GIs in our sam- Article number, page 9 of 16 & A proofs: manuscript no. aa
Polarized light contrast ( · ⁻³ ) F ( N I R ) / F ⭐ Group I (rings)Group I (spirals)Group IGroup IIGroup II (undetected in PDI) ∿ ∿ ∿ ∿ HD142527HD135344BMWC758 HD100453HD144668 (a)
Polarized light contrast ( · ⁻³ ) -9-8.5-8-7.5-7-6.5-6 A cc r e t i on r a t e ( l og ( M ⦿ / y r)) Group IGroup IIGroup II (undetected in PDI) Stellar mass (M ◎ ) (b) Polarized light contrast ( · ⁻³ ) I R C O e m i tt i ng r ad i u s ( A U ) Group IGroup IIGroup II (undetected in PDI)
HD142527HD135344BHD163296HD144432HD150193 HD179218 HD100546HD97048 MWC758HD139614
10 6
Stellar T (1,000 K) (c)
Polarized light contrast ( · ⁻³ ) -5.8-5.6-5.4-5.2-5-4.8-4.6-4.4-4.2 l og ( F e / H ) ⭐ Group I (rings)Group I (spirals)Group IGroup II (rings)Group II (Fe/H) ⦿ ∿∿ ∿ ∿ HD142527HD135344BMWC758 HD100453HD163296HD142666 (d)
Fig. 6.
Inner disk properties of the sample compared with the contrast. (a) : Near-IR excess normalized to the stellar flux. (b) : Mass accretion rate.The symbol size reflects the stellar mass with dynamic range between 1.6 M (cid:12) and 3.2 M (cid:12) . (c) : Hot CO emitting radius. The symbol size reflectsthe stellar temperature as in Fig. 5(b). (d) : Stellar photospheric abundance of iron relative to hydrogen. The dashed horizontal line indicates thesolar abundance. ple (HD100546 and HD169142) may have a detected substellarcompanion within the cavity (Brittain et al. 2014; Reggiani et al.2014; Biller et al. 2014).In this scenario, the di ff erent cavity sizes for µ m- and mm-sized dust grains are also fundamental constraints as they mayindicate a pressure bump at the outer edge of the cavity that fil-ters grains with di ff erent sizes (e.g., Pinilla et al. 2012; Garufiet al. 2013). In our sample, these di ff erences are varied. Twoextreme cases are HD97048 and MWC758, where millimeterimaging indicates cavities as large as 40 AU and 55 AU (vander Plas et al. 2017; Andrews et al. 2011), but PDI images trace µ m-sized grains at least as close to the star as ∼
15 AU (Ginskiet al. 2016; Benisty et al. 2015). We defer an in-depth analysis ofthese di ff erential cavity sizes to a specific work on gapped disks,and stress that the cavity sizes shown in Fig. 3 are not from ahomogenous observational techniques and should thus be takenfor qualitative consideration only. As noted in the introduction, GI disks were initially thought to beprecursors of GII disks in the framework of the vertical settlingof dust grains with time (e.g., Dullemond & Dominik 2004).However, the notion that all GIs are gapped disks discredits thisscenario.One of the observational pieces of evidence supporting theevolution from GI to GII was that GIs have on average smallergrains than GIIs (Acke et al. 2004). In Sect. 5.2.4 we confirm thistrend (see Fig. 5d), but also show that this is most likely entirelydue to the presence of a cavity, since comparing the dust opacityindex β mm of GII only to GI with small cavities results in compa-rable β mm values. This result can be explained by the presence of a pressure bump at the inner edge of gapped disks, which filterslarge grains and thus produces a large disk region populated bysmaller grains only (Pinilla et al. 2014). Alternatively, the dis-crepancy can be due to the presence of optically thick centralregions that contribute to lowering the β mm of sources without acentral cavity. In other words, GIs may show smaller β mm valuesthan GIIs because they have di ff erent disk morphologies and notbecause they are at an earlier stage of global dust grain growth.If GIIs are not evolved GIs, it can even be hypothesized thatthe disk evolution proceeds instead from GII to GI in a sce-nario where the formation of an increasingly large cavity actsto illuminate the outer disk. However, the low millimeter fluxes(and thus dust masses, see Fig. 5c) and the small radial extent(where detected, see Fig. 4b) of the GIIs in our sample (exceptHD163296) rules out this possibility. If the evolution in both di-rections is excluded, then GIs and GIIs are likely di ff erent evolu-tionary tracks, as proposed by Currie (2010) and Maaskant et al.(2013). Nonetheless, the scenario where HD163296 is a precur-sor of the GI cannot be ruled out and is discussed in Sect. 6.3.Owing to the large uncertainties on stellar ages, there has notbeen any conclusive evidence that GIIs are older than GIs. Thestellar ages of our sample vary enormously from work to workand thus we do not draw on this property. However, we note thatthe range of ages of GIIs is between 2 Myr and 6 Myr, whereasthat of GIs is between 1 Myr and 15 Myr with almost half ofthem aged ≥
10 Myr. Thus, the possibility that (some) GI disksmay be longer lasting structures should be cautiously considered(see also Kama et al. 2015). This longevity could be explained bythe possible presence of planetary bodies within the disk cavitythat prevent the rapid accretion of outer material.
Article number, page 10 of 16arufi et al. 2017: The evolution of protoplanetary disks, Group I vs Group II
The taxonomic analysis of Sect. 5 reveals that most propertiesof GII disks are comparable within the group, with dispersionslower than one order of magnitude for the entire sample. How-ever, the following properties strongly vary within the group, andmay indicate the need for a subclassification: – Disk extent. In scattered light, one source is large(HD163296), two are smaller (AK Sco and HD142666),while four are not detected. To date, the only resolved ob-servations in the millimeter were obtained for HD163296. – Millimeter flux. One source (HD163296) is very bright,whereas five are faint. Within the latter category, HD142666has a higher flux that is comparable to the fainter GIs. – PAH emission. In one case (HD142666) the emission is com-parable to the GIs around relatively late-type stars, whereasHD144432 and AK Sco show significantly lower emissionand four sources remain undetected. – Mass accretion rate. The sources span more than two ordersof magnitude, from the very high rate of HD144668 (whichis the most massive star in the sample) to the non-detectionsof AK Sco and HD142666.An evident dichotomy arises between HD163296 and the restof the GIIs. Keeping in mind the uncertainty of converting mil-limeter fluxes into dust masses, HD163296 is likely more mas-sive in dust than the other objects. Furthermore, it is known tohave a gaseous disk that is twice as large as the dusty disk (deGregorio-Monsalvo et al. 2013) and to host a prominent jet (e.g.,Ellerbroek et al. 2014). Scattered light images trace small dustgrains as far out as hundreds of AU (Grady et al. 2000). Allin all, the only properties that distinguish HD163296 from theother GI are those connected to the illumination of the outer disk(scattered light, far-IR, and PAH), which points toward a self-shadowed disk. Considering the absence of a large disk cavity(de Gregorio-Monsalvo et al. 2013) and the presence of a strongjet, it is possible that HD163296 is a precursor of the classicalGI. The existence of rings in both scattered light and millimetercontinuum images (this work and Isella et al. 2016) reinforcesthe analogy with the GIs, which often show these features (e.g.,Ginski et al. 2016).On the other hand, the detection in scattered light of the diskaround HD142666 and AK Sco (see Fig. 1) may constrain theirouter edge to a few tens of AU. Millimeter imaging of AK Scodetects signal on a slightly larger scale, i.e., up to ∼
100 AU(Czekala et al. 2015). Conversely, our outer edge for the diskof HD142666 is consistent with the cold CO distribution (tracedout to ∼
60 AU, Dent et al. 2005). Their millimeter fluxes arerespectively low ( ∼
105 mJy, if scaled at 140 pc) and very low( ∼
35 mJy). Even though it is not detected in scattered light,HD144432 may be a similar object, having CO traced as out as ∼
45 AU (Dent et al. 2005) and showing PAH emission. As com-mented in Sect. 6.1, the PAH detection is a possible consequenceof their small cavities at sub-AU scale. All this may suggest thatthese disks are slightly smaller counterparts of GIs, which wereunable to create a large disk cavity.Finally, the disk of HD145263 may be undergoing the finalstages of disk dissipation (Honda et al. 2004). The other threenon-detected GIIs all have a stellar companion at projected dis-tances of approximately 100 AU. They show very low millime-ter fluxes and two of them (HD144668 and HD150193) show noPAH emission. In the case of HD150193, we can infer that thenon-detection of the disk in scattered light (Garufi et al. 2014)is inconsistent with the amount of far-IR excess (Fig. 5a) and with the mid-IR [30 / ∼
15 AU, which is in agreement with works based onSED fitting (Dominik et al. 2003), and possibly with the non-detection of cold CO (Dent et al. 2005). For HD144668, wecannot infer the same inconsistency because the upper limit onthe disk detection (from this work) is higher. However, Preibischet al. (2006) showed that the mid-IR emission from this disk isconfined within 2.5 AU from the star, which is significantly lessthan for typical disks around Herbig Ae / Be stars.All the above considerations seem to indicate the existenceof a family of GII disks (like HD150193 and HD144668) withcompact disks, having an outer radius on the order of 10 AU oronly slightly more, and dust (and possibly gas) masses signifi-cantly lower than the GI disks. The most straightforward methodfor corroborating or rejecting this hypothesis is future millime-ter continuum imaging by ALMA. In any case, a relatively smalldisk is to be expected in HD150193, HD144432, and HD144668because of the presence of stellar companions. It is typically as-sumed that circumprimary disks are truncated at roughly 1 / Since the disks of all GIs are depleted in dust within at least 10AU of the central star, di ff erent optical / near-IR properties may beexpected from the GII disks. In Sect. 5.3, we show the existenceof three clusters of objects in terms of near-IR excess, emittingradius of CO, and stellar photospheric abundance of iron. To alarge extent, the objects in the three clusters are the same and theconnection between the properties is real. In particular, the near-IR excess is (cid:38)
10% of the stellar flux for all GIIs in our sample(Fig. 6a) and for four (out of nine) GIs: MWC758, HD100453,HD135344B, and HD142527. The outer disk of these stars allshow spiral-like features (e.g., Benisty et al. 2015, 2017; Garufiet al. 2013; Avenhaus et al. 2014b). All four of these sources arealso relatively late-type stars (see Fig. 4a). However, other late-type stars in the sample have low near-IR excess, whereas someearly-type stars have high near-IR excess. Thus, the dichotomycannot be explained by the stellar temperature alone.Why some Herbig Ae / Be stars have very high near-IR ex-cess is a longstanding debate. Hydrostatic disk models typicallyfail to reproduce it, indicating that the emission is partly due tomaterial uplifted from the disk by a wind (Bans & Königl 2012)or that the inner disk is composed of refractory elements thatare present at smaller radii than the sublimation radius for sili-cates. In particular, customized works have shown that the lat-ter case could be the explanation for the near-IR flux of the GIIHD163296 and HD144668 (Benisty et al. 2010, 2011). In thecase of uplifted and of inner material, it is not possible to infer
Article number, page 11 of 16 & A proofs: manuscript no. aa
Group II = shadowed disk (e.g. HD163296)
Group I = gapped disk (e.g. HD100546)
Group II = small disk (e.g. HD142666)
Group II : very small disk? (e.g. HD150193) x x x x x x x μ m) / F(13.5 μ m) Group IGroup IIGroup II (upper limit)
Group IGroup II
SPHERE inner working angle ALMA angular resolution time ? Scattered light: low
Far-IR: low
Millimeter flux: high
Sub-mm CO: extended
Near-IR: high
Stellar [Fe/H]: high
IR CO radius: smallScattered light: low
Far-IR: low
Millimeter flux: low
Sub-mm CO: compact
Near-IR: high
Stellar [Fe/H]: high
IR CO radius: small Scattered light: high
Far-IR: high
Millimeter flux: high
Sub-mm CO: extended
Near-IR: low/high
Stellar [Fe/H]: low/high
IR CO radius: small/largeScattered light: low
Far-IR: low
Millimeter flux: low
Sub-mm CO: undetected
Near-IR: high
Stellar [Fe/H]: high
IR CO radius: small
Fig. 7.
Summary of the properties of the sources analyzed in this work. The proposed disk geometries are shown in logarithmic scale. The SPHEREinner working angle is imposed by the angular resolution of observations in the near-IR ( ∼
10 AU for sources at ∼
150 pc). The ALMA angularresolution of ∼ from the near-IR excess alone whether this material has the su ffi -cient optical depth to shadow the outer disk region. In any case, itmust be noted that the outer disk of three-quarters of the GI withhigh near-IR flux shows signs of shadows by an inclined innerdisk (Marino et al. 2015; Stolker et al. 2016; Benisty et al. 2017).Speculatively, it may even be possible to connect the presence ofthese shadows with that of spirals. In fact, both hydrodynamicalsimulations (Montesinos et al. 2016) and scattered light obser-vations (Wagner et al. 2015; Benisty et al. 2017) show a possibleconnection between these features, due to the reduced pressurein correspondence of the shadows that can excite spiral arms.The physical link between the near-IR flux, the radius of IRCO emission, and the stellar photospheric abundance of iron isnot straightforward, and will be discussed in depth in a dedicatedwork that is in preparation. The measurements included in thiswork show that whatever process is determining the dust and COgas emission from the inner disk also has an impact on the stel-lar photosphere. Kama et al. (2015) proposed that the depletedabundance of refractory elements in the stellar photosphere ofGI can be connected to the increased gas-to-dust ratio of the ma-terial flowing within a cavity because of the trapping of smalldust grains by substellar companions. Following this thinking,the decreased abundance of refractory elements of the inflow-ing material may result in a reduced near-IR excess, due to thelower sublimation temperature of silicates compared to refrac-tory elements. The distribution of CO in the inner few AU maybe also modified, either by the removal of volumes of gas orby a changed interplay with the local dust, or both. A decreasein small dust grains in the inner disk would in fact damp theIR pumping of the surrounding CO and, if the column densityof CO gas is reduced, would facilitate the excitation of CO byUV pumping at larger radii (as in HD179218, HD100546, andHD97048, see Fig. 6c and van der Plas et al. 2015).
7. Summary and conclusions
Since the beginning of the century, the most recognizable evolu-tionary track of protoplanetary disks around Herbig Ae / Be stars has been thought to be the dust settling that leads flared disks(Group I) to evolve into flat disks (Group II) (Meeus et al. 2001).In this work, we analyze VLT / NACO near-IR scattered light im-ages of six GIIs with the aim of complementing the availablesample of GIs. Even though the observations were carried outin suboptimal conditions, we detect a disk around half of thesources. In particular:1. The brightness distribution in the disk around HD163296 isspatially consistent with that by Garufi et al. (2014), indi-cating the persistency of a ring-like structure located slightlyinside the CO snowline (Qi et al. 2015; Guidi et al. 2016).2. A relatively small disk ( ∼ ∼
15 AU.We investigate the di ff erent nature of GI and GII disks bymeans of a taxonomic analysis of 17 sources (10 GIs and 7 GIIs)observed in polarized near-IR light and with stellar / disk prop-erties available from the literature. This sample represents morethan a half of all the polarimetric images of protoplanetary diskscurrently available. With specific regard to the analyzed sample,the main results are the following:3. All GI disks have a cavity larger than ∼ ∼ ff erent dust opacity index β mm , tracing the grain sizesof GIs and GIIs is likely due to the depletion of large grains Article number, page 12 of 16arufi et al. 2017: The evolution of protoplanetary disks, Group I vs Group II within the cavity. It does not necessarily reflect a more ad-vanced stage of global dust grain growth for GIIs.8. Keeping in mind the uncertainties on stellar ages, we find noGII older than 6 Myr, but half of the GIs are older than 10Myr.9. The PAH luminosity, tracing the volume of gas exposed toUV radiation, is high in all GIs while it is very low in fourout of seven GIIs. The peculiarity of the three GIIs withhigh PAH (HD142666, HD144432, and AK Sco) is that theyhost a small-scale cavity. Thus, an analogy between the PAHbrightness and the presence of a cavity emerges. This couldsolve the long-standing inconsistency for the PAH betweentheory and observations.10. We find a clear link between the amount of near-IR excess,the stellar photospheric abundance of iron, and the emittingradius of CO gas as probed in the IR. All GIIs and half of theGIs show respectively high, high, and small values, while theother half of the GIs show low, low, and large values.Point 3 in the above list indicates that the dichotomy of SEDsshown by Group I and Group II is due to the presence or absenceof a large inner cavity, and thus to the di ff erent illumination thatthe outer disk is subject to. Therefore, the evolution from GroupI (flared disks) to Group II (flat disks) as a result of dust set-tling must be revised. In fact, there is no property supporting thisevolutionary track (see Points 7 and 8).We also propose that the dichotomy between the millimeter-bright GII HD163296 and the other millimeter-faint GIIs(Point 5) is indicative of very di ff erent disk geometries. SomeGII disks may be smaller versions of the GI disks ( <
100 AU inextent and up to one order of magnitude less massive in dust)that are unable to form large cavities. The disks of HD142666and AK Sco (Point 2) are the prototypes of these relatively smallstructures. Our analysis also suggests the existence of very smalldisks, with R ∼
10 AU, which would be undetectable in scatteredlight and would thus explain the outlier of Point 4. These disksare possibly subject to truncation by stellar companions (Point 6)and can be currently only imaged by ALMA. On the other hand,HD163296 shows the same properties as the GI, with the excep-tion of those properties that are related to the disk illumination(far-IR, scattered light, PAH, see Point 9). It can therefore be aprimordial version of the GI, with a prominent jet and a contin-uous disk that e ffi ciently shadows its outer regions (Point 1 andGarufi et al. 2014). In Fig. 7, we show a sketch summarizing theproposed disk geometries.Finally, the dichotomy between sources with high and lownear-IR excess (Point 10) may provide new insight into the pro-cess of planet formation within the disk cavities. We hypothesizethat the amount of near-IR flux is related to the abundance of re-fractory elements in the inflowing material and that this also hasan imprint on the stellar photospheric abundance of elements. Apossible connection between the morphology of the inner andouter disk is also proposed, with those sources with high near-IRexcess also showing shadows and spirals in scattered light.Follow-up studies are needed to understand whether the con-clusions of this paper also apply to a larger sample of protoplan-etary disks. It is of particular importance to extend the study tolower mass stars (the T Tauri stars) and to sources with evidenceof primordial jets in order to obtain deeper insight into evolution-ary tracks and evolutionary stages of disks throughout the planetformation. Finally, ALMA observations of those disks that re-main undetected in scattered light are fundamental in order todisentangle their morphology and to provide a view of the vari-ety of protoplanetary disks that is less biased toward particularlybright and extended objects. Acknowledgements.
We thank L.B.F.M. Waters, I. Kamp, A. Carmona, andL. Klarmann for clarifying discussions. We acknowledge the referee for the in-teresting comments. We are grateful to the SPHERE consortium, and in par-ticular to C. Ginski and J. de Boer, for useful discussions and for making theSPHERE GTO data available for the paper. Part of this work has been carriedout within the framework of the National Centre for Competence in ResearchPlanetS supported by the Swiss National Science Foundation. S.P.Q. and H.M.S.acknowledge the financial support of the SNSF. H.C. and G.M. acknowledgesupport from the Spanish Ministerio de Economia y Competitividad under grantAYA 2014-55840-P. G.M. is funded by the Spanish grant RyC-2011-07920. Theauthors acknowledge the sta ff at VLT for their excellent support during the ob-servations. This research has made use of the SIMBAD database, operated atCDS, Strasbourg, France. References
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Appendix A: The sample
The sample studied in this work consists of 17 B, A, and F stars(10 GI and 7 GII) that were observed in near-IR PDI with eitherVLT / NACO or VLT / SPHERE from 2012 to 2016. Properties andreferences of all sources are shown in Table A.1.
Appendix B: Polarized-to-stellar light contrast
To evaluate the amount of scattered light from a large dataset,we estimate the contrast of the polarized flux from the Q φ imagewith respect to the central star. Specifically, we perform a cutwith width equal to the resolution along the direction of the diskmajor axis (where known). The choice of the major axis is dic-tated by the minimized impact of the disk inclination, which cansignificantly alter the amount of scattered light along all the otherdirections. We multiply the extracted flux F pol by the squareddistance r from the star, to compensate for the dilution of thestellar radiation, and average radially. To remove the impact ofthe disk extent, the average is performed between two specificradii, r in and r out , di ff erent for each source and set by the diskinner edge and the outermost detectable signal. The value thusobtained is normalized by the stellar luminosity F ∗ , which is es-timated from the same dataset by means of the inner 1 (cid:48)(cid:48) of thetotal intensity I image. The I image is generated during the stan-dard data reduction from the sum of the two beams with orthog-onal polarization states (see Avenhaus et al. 2014b). In summary,the disk-to-star contrast used for the analysis can be expressed as φ pol = r out − r in · (cid:90) r out r in F pol ( r ) · π r F ∗ dr . (B.1)This quantity, sometimes referred to as geometric albedo,is the combination of both the intrinsic albedo (a ff ected by thespecific dust properties) and the disk geometry (corresponding,along the disk major axis, to the disk flaring angle). The pri-mary error on this estimate is computed from the Q φ image bymeans of the weighted standard deviation on the resolution el-ement around each datapoint, which is then propagated to φ pol .To define an upper limit of non-detections, we carried out thesame procedure with the cut on the Q φ image being obtainedfrom four averaged random directions. We found, nonetheless,that the error thus obtained is marginal compared to other sys-tematic uncertainties, which we discuss here.First of all, the stellar halo used to compute F ∗ also containsthe (unseen) disk contribution. We consider this e ff ect negligi-ble since the brightest disks in our sample (with the exceptionof HD142527) contributes to roughly 1% of the stellar bright-ness (assuming a conservative polarization fraction of 10%). InHerbig Ae / Be stars, a fraction of the near-IR flux may origi-nate in the hot inner disk rather than in the stellar photosphere.However, part of this near-IR emission may also contribute tothe illumination of the outer disk. Therefore, we do not correctfor it by means of the photometric excess. Some of our inten-sity images have their inner few pixels saturated or covered bythe coronagraph. In most of these cases, complementary frameswith shorter integrations and without the coronagraph are avail-able. Where these are not available, we estimate the missing in-ner photons by means of the dataset more similar in target, setup,and weather conditions. This procedure should not introduce anuncertainty larger than 10%. In the case of stellar companionsvisible from the image, we exclude the surrounding region andsubstitute it with the specular one.Secondly, a fraction of the scattered light from the disk maynot be registered in the Q φ images because of the deviations fromazimuthal scattering to be expected from inclined disks. In fact,these may act to transfer some signal from the Q φ to the U φ im-age. This may bias our estimates on inclined disks by an unpre-dictable but yet not dramatic fraction. Also, the smearing e ff ectdescribed by Avenhaus et al. (2014a) may damp the polarized Article number, page 14 of 16arufi et al. 2017: The evolution of protoplanetary disks, Group I vs Group II
HDA lt e r n a ti v e G r oup C on t r a s t d T e ff M ∗ [ / . ] F N I R / F ∗ F F I R / F ∗ F . mm β mm L P AH / L ∗ l og (˙ M ) R C O R g a p l og ( F e / H ) n a m e n a m e ( · − )( p c )( K )( M (cid:12) )( % )( % )( m J y )( · − )( M (cid:12) / y r)( AU )( AU ) I . ± . , . . . . . . - . p - . M W C I . ± . , . . . . . . . m - . I . ± . , . . . . . . < - . . m I . ± . , . . . . . . < - . p - . I . ± . , . . . . . . - . . p - . B S AO I . ± . , . . . . . . - . . m - . I . ± . , . . . . . . - . . s - . I . ± . , . . . . . . - . . p - . II . ± . , . . . . . . < - . -- . II < . , . . . . . . - . . -- . H R II < . , . . . . . -- . - II < . , . . -- II < . , . . . . . -- . . - AK S c o II . ± . , . . . . . . < - . - II . ± . , . . . . . -- . . -- . I . ± . , . . . . . . p - . I . ± . , . . . . . - . . s - . T a b l e A . . P r op e r ti e s o f t h e s a m p l e . C o l u m n s a r e : n a m e o f t h e s ou r ce ; a lt e r n a ti v e n a m ec o mm on l yu s e d i n t h e lit e r a t u r e ; G r oup ; po l a r i ze d - t o - s t e ll a r li gh t c on t r a s t , ca l c u l a t e d i n t h i s p a p e rfr o m t h e w o r k s li s t e db e l o w ; d i s t a n ce , fr o m G a i a C o ll a bo r a ti on ( ) , e x ce p t a nd163296 ( E S A ) a nd144668 ( v a n L ee u w e n e t a l . ) ; e ff ec ti v e t e m p e r a t u r e o f t h e m a i n s t a r , fr o m P a s c u a l e t a l . ( ) , e x ce p t fr o m F a i r l a m b e t a l . ( ) a nd144432 , , fr o m A c k ee t a l . ( ) ; s t e ll a r m a ss , i b i d . e x ce p t fr o m M ü ll e r e t a l . ( ) a nd145263 fr o m S y l v e s t e r & M a nn i ng s ( ) ; [ µ m / . µ m ]r a ti o , fr o m A c k ee t a l . ( ) ; n ea r-I R a nd f a r-I R e x ce ss no r m a li ze d t o t h e s t e ll a r fl ux , fr o m t h i s w o r k f o ll o w i ng t h e m e t hodby P a s c u a l e t a l . ( ) ; fl ux a t . mm , fr o m t h e r e f e r e n ce s b e l o w ; du s t op ac it y i nd e x , a d a p t e d fr o m P a s c u a l e t a l . ( ) , e x ce p t , , , fr o m A c k ee t a l . ( ) a nd36112 fr o m P i n ill ae t a l . ( ) ; P AH l u m i no s it yno r m a li ze d t o t h e s t e ll a r fl ux , fr o m A c k ee t a l . ( ) , e x ce p t fr o m M ee u s e t a l . ( ) ; m a ss acc r e ti on r a t e , fr o m F a i r l a m b e t a l . ( ) ; C O e m itti ng r a d i u s , fr o m B a n za tti & P on t opp i d a n ( ) a nd B a n za tti e t a l . ( ) , e x ce p t , fr o m v a nd e r P l a s e t a l . ( ) ; ca v it y s i ze fr o mm illi m e t e r c on ti nuu m ( m ) , P D I i m a g e ( p ) , o r S E D fi tti ng ( s ) a s fr o m b e l o w ; s t e ll a r pho t o s ph e r i ca bund a n ce o f i r on r e l a ti v e t ohyd r og e n , fr o m K a m ae t a l . ( ) . R e f e r e n ce s f o r t h e P D I i m a g e s u s e d f o r t h ec on t r a s t a nd f o r t h eca v it y s i ze ( w h e n t h i s i s fr o m t h e s a m e p a p e r , w e o m itt h e r e p e titi on ) : , SP H E R E c on s o r ti u m i np r e p . ; , B e n i s t y e t a l . ( ) , A nd r e w s e t a l . ( ) ; , G i n s k i e t a l . ( ) , v a nd e r P l a s e t a l . ( ) ; , B e n i s t y e t a l . ( ) ; , G a r u fi e t a l . ( ) ; B , G a r u fi e t a l . ( ) , A nd r e w s e t a l . ( ) ; , SP H E R E c on s o r ti u m i np r e p ., C a r m on ae t a l . ( ) , , A v e nh a u s e t a l . ( ) ; , , , t h i s w o r k ; , G a r u fi e t a l . ( ) ; , , t h i s w o r k ; , Q u a n ze t a l . ( ) , , SP H E R E c on s o r ti u m i np r e p ., F e d e l ee t a l . ( ) . R e f e r e n ce s f o r t h e m illi m e t e r fl ux e s : , N a tt ae t a l . ( ) ; , M a nn i ng s & S a r g e n t ( ) ; , H e nn i ng e t a l . ( ) ; , M ee u s e t a l . ( ) ; , H e nn i ng e t a l . ( ) ; B , S y l v e s t e r e t a l . ( ) ; , S y l v e s t e r e t a l . ( ) ; , W a l k e r & B u t n e r( ) ; , M ee u s e t a l . ( ) ; , W a l k e r & B u t n e r( ) ; , M ee u s e t a l . ( ) ; , S a nd e ll e t a l . ( ) ; , C ze k a l ae t a l . ( ) ; , M a nn i ng s & S a r g e n t ( ) ; , S y l v e s t e r e t a l . ( ) ; , M a nn i ng s & S a r g e n t ( ) . Article number, page 15 of 16 & A proofs: manuscript no. aa flux of the inner ∼ . (cid:48)(cid:48) and result in a small underestimate ofthe contrast.Finally, and more importantly, the outermost radius with de-tectable signal from the PDI image cannot be defined univocally.Here we chose the location where the polarized flux drops below3 σ , as calculated from the above-mentioned primary error on theimage. However, we note that changing this radius by a fractionof an arcsecond results in a significant change to the computedcontrast. The error bars listed in Table A.1 and used through thepaper take this uncertainty (which is by far the most significant)into account. On the other hand, the other uncertainties describedabove are not included and could account for an additional 20%systematic error on the contrast., as calculated from the above-mentioned primary error on theimage. However, we note that changing this radius by a fractionof an arcsecond results in a significant change to the computedcontrast. The error bars listed in Table A.1 and used through thepaper take this uncertainty (which is by far the most significant)into account. On the other hand, the other uncertainties describedabove are not included and could account for an additional 20%systematic error on the contrast.