A young protoplanet candidate embedded in the circumstellar disk of HD100546
Sascha P. Quanz, Adam Amara, Michael R. Meyer, Matthew A. Kenworthy, Markus Kasper, Julien H. Girard
aa r X i v : . [ a s t r o - ph . GA ] F e b HD100546
Published by ApJ Letters online February 28, 2013
Preprint typeset using L A TEX style emulateapj v. 12/14/05
A YOUNG PROTOPLANET CANDIDATE EMBEDDED IN THE CIRCUMSTELLAR DISK OF HD100546
Sascha P. Quanz , , Adam Amara , Michael R. Meyer , Matthew A. Kenworthy , Markus Kasper , and JulienH. Girard Published by ApJ Letters online February 28, 2013
ABSTRACTWe present high-contrast observations of the circumstellar environment of the Herbig Ae/Be starHD100546. The final 3.8 µm image reveals an emission source at a projected separation of 0.48 ′′ ± ′′ (corresponding to ∼ ± ◦ ± ◦ . The emission appears slightly extendedwith a point source component with an apparent magnitude of 13 . ± . ∼ ◦ thede-projected separation of the object is ∼
68 AU. Assessing the likelihood of various scenarios we favoran interpretation of the available high-contrast data with a planet in the process of forming. Follow-up observations in the coming years can easily distinguish between the different possible scenariosempirically. If confirmed, HD100546 “b” would be a unique laboratory to study the formation processof a new planetary system, with one giant planet currently forming in the disk and a second planetpossibly orbiting in the disk gap at smaller separations.
Subject headings: stars: formation — planets and satellites: formation — protoplanetary disks —planet-disk interactions — stars: individual (HD100546) INTRODUCTION
To possibly extend the ongoing census of exoplanetdemographics from our solar neighborhood to the en-tire Milky Way we need to understand planet formationand its dependence on initial physical and chemical con-ditions. A key step is to directly detect and charac-terize forming planets in their natal environment. Re-cently, based on sparse aperture masking observations, afew low-mass companion candidates have been revealedin the gap of their host star’s transitional disks (e.g.,LkCa15 b, T Cha b; Kraus & Ireland 2012; Hu´elamoet al. 2011), but in some cases scattered light from thedisk rim or other disk structures may be a valid expla-nation for the observed features (e.g., Cieza et al. 2013).Until now no protoplanet has yet been found embeddedin the optically thick gas and dust disk of its host star.Here we present high contrast imaging data revealing aprotoplanet candidate embedded in the disk around theHerbig Ae/Be star HD100546.HD100546 (see Table 1 for stellar properties) has acomplex circumstellar environment consisting of an innerdisk from ∼ ∼ ∼
13 AU out to a few hundred AU(e.g., Benisty et al. 2010; Tatulli et al. 2011). A massiveplanet was suggested to be orbiting in the gap (Bouwmanet al. 2003). Based on asymmetries in the line profile of
Electronic address: [email protected] Based on observations collected at the European Organisationfor Astronomical Research in the Southern Hemisphere, Chile, un-der program number 087.C-0701(A). Institute for Astronomy, ETH Zurich, Wolfgang-Pauli-Strasse27, 8093 Zurich, Switzerland Sterrewacht Leiden, P.O. Box 9513, Niels Bohrweg 2, 2300 RALeiden, The Netherlands European Southern Observatory, Karl Schwarzschild Strasse,2, 85748 Garching bei M¨unchen, Germany European Southern Observatory, Alonso de C´ordova 3107, Vi-tacura, Cassilla 19001, Santiago, Chile
TABLE 1Basic parameters of HD100546.
Parameter HD100546 Reference a RA (J2000) 11 h m s .44 (1)DEC (J2000) -70 ◦ ′ ′′ .24 (1) J ± .
02 mag (2) H ± .
03 mag (2) K s ± .
02 mag (2)Mass 2.4 ± ⊙ (3)Age 5... >
10 Myr (3),(4)Distance 97 +4 − pc (1)Sp. Type B9Vne (5) a (1) van Leeuwen (2007), (2) 2MASS pointsource catalog (Cutri et al. 2003), (3) vanden Ancker et al. (1997), (4) Guimar˜aes et al.(2006), (5) Houk & Cowley (1975). [OI] (Acke & van den Ancker 2006) and OH (Liskowskyet al. 2012) dynamic evidence for such an object wasfound. In the case of OH the emission is thought to arisefrom an eccentric inner rim of the outer disk with the ec-centricity being introduced by a planet. The outer diskhas been resolved at multiple wavelengths including scat-tered light (e.g., Augereau et al. 2001; Grady et al. 2001;Ardila et al. 2007) where it shows peculiar features suchas large-scale spiral arms. The remaining disk mass is es-timated to be 10 − – 10 − M ⊙ (e.g., Pani´c et al. 2010).Recently, polarimetric differential imaging (PDI) in thenear-infrared (NIR) revealed distinct sub-structures inthe innermost few tens of AU of the disk (Quanz et al.2011b). OBSERVATIONS AND DATA REDUCTION
HD100546 was observed with VLT/NACO (Lenzenet al. 2003; Rousset et al. 2003) and its Apodozing PhasePlate (APP) coronagraph (Kenworthy et al. 2010), which Quanz et al.
TABLE 2Summary of deep imaging observations in pupil tracking mode.
Parameter HD100546 HD100546Hemisphere 1 Hemisphere 2Date 2011-05-30 2011-07-13UT start 22h:48m:18s 23h:00m:49sUT end 00h:14m:58s 00h:26m:11sNDIT × DIT a × × b
130 126Parallactic angle start -17.33 ◦ ◦ Parallactic angle end 10.33 ◦ ◦ Airmass range 1.45. . . 1.43 1.54. . . 1.75Mean DIMM seeing [ λ =500 nm] 0.6 ′′ ′′ h τ i mean / h τ i min / h τ i maxc d -108.35 ◦ ◦ PA end d -80.86 ◦ ◦ a NDIT = Number of detector integration times (i.e., number of individualframes); DIT = Detector integration time (i.e., single frame exposure time). b NINT = Number of data cubes. c Average, minimum and maximum value of the coherence time of theatmosphere in data cube. Calculated by the Real Time Computer of theAO system. d Position angle of camera adaptor. was already used in earlier exoplanet imaging projects(Quanz et al. 2010, 2011a; Kenworthy et al. 2013). Weused the L27 camera ( ∼
27 mas pixel − ) with the L ′ filter ( λ c = 3 . µ m, ∆ λ = 0 . µ m) in angular differ-ential imaging mode (ADI; Marois et al. 2006). Twodatasets were taken on two different nights, referred toas“hemisphere 1” and “hemisphere 2”, with a 180 ◦ offsetin position angle of the APP to allow its high-contrasthalf to cover most of the circumstellar environment. Us-ing the “cube mode” option, all exposures, each 0.15 slong, were saved individually. The core of the stellarPSF was slightly saturated. Data cubes consisting of200 exposures were taken using a 3–point dither patternalong the detectors x-axis with roughly 7 ′′ separation.Table 2 summarizes the observations and the observingconditions.For photometric calibration we observed HD100546 inthe NB3.74 filter ( λ c = 3 . µ m, ∆ λ = 0 . µ m; cen-tered on the Pf γ line). The observing strategy was iden-tical to the deep science observations, but these expo-sures were in the linear detector regime (0.2 s exposuretime) and only 2 data cubes with 150 exposures eachwere recorded. No photometric or astrometric standardstar was observed.The basic data reduction steps (bad pixel cleaning, skysubtraction, image alignment) were done in a similar wayas described in Quanz et al. (2010). During the alignmentprocess the images were re-binned to twice their originalresolution. From each image in the stack of aligned ex-posures we created a 2 ′′ × ′′ sub-image centered on thestar. Individual images showing bad AO correction ornot covering the full size of the sub-images were disre-garded. In the end we had a stack of 16,117 images forhemisphere 1 and 18,916 images for hemisphere 2.The PSF-subtraction was done using the principlecomponent analysis based software package PynPoint (Amara & Quanz 2012), and the results were confirmedby using the LOCI algorithm (Lafreni`ere et al. 2007).For the final
PynPoint images we used 80 PCA coeffi-cients and kept only the best 12000 images in terms oftotal residuals over the whole image frame. After PSF subtraction, each image was de-rotated to the same field-orientation and we computed the mean image of the im-age stack clipping data points that were beyond 2.5 σ ofthe mean. The results shown below are robust againstall of these numbers (see next section).For LOCI we median combined 20 consecutive expo-sures into a single image in those data cubes where thiswas possible, resulting in 734 stacked images. The LOCIalgorithm was then applied to this stack of images usingthe following LOCI parameters: FWHM=8 px, N δ =0.75, dr =8, N A =500. The choice of these values reflect the factthat the images have been re-binned to twice their origi-nal resolution (see above). All final images (see, Figure 1)were smoothed with a circular gaussian with a width of3 pixels. RESULTS AND ANALYSIS
Detection of an emission source
In Figure 1 an emission source is revealed north of thecentral star in the hemisphere 1 dataset. To examine therobustness of this detection we did a series of tests: • We varied the number of PCA coefficients used in
PynPoint (between 20 and 200). • We split the dataset in half in different ways in-cluding random selections of ∼
50% of the images. • We applied the LOCI algorithm as independent re-duction approach.In all cases we found a bright feature at the same loca-tion. In addition, we analyzed the hemisphere 2 datasetin exactly the same way without finding a persistentsource at any location in the final image.Using the approach described in Quanz et al. (2011a)and a 8-pixels wide aperture ( ∼ · FWHM), the sourcehas a signal-to-noise of ∼
15 in the final LOCI image.The detected emission appears slightly elongated in thenorthern direction (Figure 1). To estimate the photome-try and astrometry of the source we did a detailed anal-ysis inserting fake negative planets (details see below). protoplanet candidate embedded in the circumstellar disk of HD100546 3
Fig. 1.—
NACO/APP L ′ images of the circumstellar environment of HD100546. From left to right: Final PynPoint images of hemisphere1 and hemisphere 2 and final LOCI image of hemisphere 1. An emission source is clearly detected in left and right panel. The shaded areaindicates the region that was only covered by the low sensitivity hemisphere of the APP. The images are scaled with respect to their peakflux.
It showed that the observed emission can be explainedwith a point source plus some extended component. Forthe point source the projected separation amounts to ∼ ′′ ± . ′′ ( ∼ ± ∼ ◦ ± ◦ . This error excludesany systemic error in the orientation of the camera withrespect to the true celestial north, which is estimated tobe . ◦ based on calibration data from an ongoing largeimaging program (PI: J.-L. Beuzit).To estimate the brightness of the point source we in-serted artificial negative planets in the individual expo-sures and re-ran PynPoint . For the fake objects weused an unsaturated PSF of HD100546 from one of thephotometric calibration datasets. To scale the flux ofthese objects, the difference in exposure time betweenthe science and the calibration images had to be con-sidered as well as the transmission curves of the twodifferent filters . Using published L-band spectra forHD100546 from ISO and VLT/ISAAC (Geers et al. 2007,and references therein) we derived a throughput fractionof ∼ . ± .
002 for the narrow band filter compared tothe broadband L ′ filter. The error arises from changesin the Pf γ line emission in HD100546 between the twodatasets suggesting that the NB3.74 filter traces variableaccretion activity. Also the whole NIR and MIR contin-uum varies with an offset of a factor of ∼ ′ = 9 . ± . ′ = 8 . ± . ′ ≈ . vicinity of the object – not at the object’s location itself– if we cancel out its peak flux completely. For the restof the analyses and discussion we use ∆L ′ ≈ . ′ ≈ . . . . . ± .
06 mag reported in de Winter et al. (2001).Hence we derive an apparent magnitude of L ′ =13.2 ± ∼ γ line flux and the intrinsic errorin our photometric observations are negligible. Estimating the minimum luminosity
Assuming that the flux of the point source peaks inthe L ′ filter we can estimate its blackbody temperatureusing Wien’s law. We can then derive a lower limit onthe object’s luminosity by taking into account its appar-ent L ′ magnitude and its distance. Integrating over allfrequencies this exercise yields a minimum luminosity of L & · − L ⊙ . Interaction with the circumstellar disk?
The VLT/NACO PDI data presented in Quanz et al.(2011b) have sufficient spatial resolution and inner work-ing angle to probe the disk surface on scales relevant forthe APP dataset. Those NIR observations revealed sub-structures in the disk in the inner few tens of AU. Inparticular the existence of a disk “hole” was suggestedas both the final polarization intensity images as well asthe polarization fraction images in H and K s revealed alocal flux deficit at the same location.In Figure 2 we show the large scale disk environmentrevealed by HST/ACS (Ardila et al. 2007) and then,zooming in the inner disk regions, the polarization frac-tion image of the PDI study. We overplot in red thecontours of the object detected here. The disk is in-clined by ∼ ◦ ± ◦ and the position angle of the diskmajor axis is ∼ ◦ ± ◦ (Quanz et al. 2011b). If the Quanz et al. Fig. 2.—
The HD100546 disk on different scales. In the HST/ACS image obtained in the F814W filter (left) the circumsteller diskaround HD100546 can be traced out to a few hundred AU in scattered light (Ardila et al. 2007). The inner disk regions ( ∼ ′′ in radius)are hidden behind the coronagraph or suffer from PSF subtraction residuals. The polarization fraction image (left) obtained at the VLTin PDI mode in the H band (Quanz et al. 2011b) probes regions very close to the star, enabling the detection of disk asymmetries notaccessible with other imaging techniques. The position of the planet candidate is overlaid in the PDI image. North is up and east to theleft in both images. disk surface was smooth and azimuthally symmetric, thedisk image shown in Figure 2 should be mirror symmet-ric with respect to the disk minor axis running with aposition angle of ∼ ◦ through the image center (Quanzet al. 2011b). However, there are clear asymmetries inform of a deficit in polarization fraction in northern di-rection, i.e., along the position of the detected object.Based on Figure 2 the disk “hole” extends to larger sep-arations and appears more like a “wedge”. As discussedin Quanz et al. (2011b) the underlying physical reasonfor this feature is not clear at the moment (e.g., dropin surface density, disk surface geometry, changing dustproperties). However, finding an asymmetry at this spe-cific location renders plausible a physical link betweenthose structures and the source detected here. DISCUSSION
The image of an embedded exoplanet?
Based on the object’s position angle, the disk incli-nation and the distance to HD100546, the object’s de-projected separation from the central star is ∼ ±
10 AU,i.e., within the large circumstellar disk. Different scenar-ios to explain both the L band emission and the observeddisk structure can be assessed:
Background source:
A background source would be ob-served through the HD100546 disk. Based on the diskmodel presented in Mulders et al. (2011) backgroundflux in the L band should be attenuated by a factor of ∼ · − ≈ . ∼
70 AU . Takingthis factor into account we used the Besancon galacticmodel (Robin et al. 2003) to estimate the number of ob-jects in the apparent magnitude range 7 mag ≤ L ≤ ∼
330 objects in a 2 square degreepatch on the sky centered around HD100546. This num- This factor does not include that the object is seen through aninclined disk which would yield an even higher optical depth. ber translates into a probability of having such a physi-cally unrelated source in a 1 ′′ × ′′ field of view aroundthe star of p = 1 . · − . Furthermore, the fact that theL band emission appears to be extended argues againsta background object. Disk feature:
The observed L ′ brightness and mini-mum luminosity are difficult to explain with disk-internalprocesses alone as the expected temperature in the diskmid-plane at the location of the source is only ∼
50 K(Mulders et al. 2011). Furthermore, we are not awareof shock-processes that act only locally and might leadto the observed luminosity in a disk that appears to benot very massive. If it was scattered light that we see,one would expect that also in the NIR a maximum inscattered light would be seen. Using the PDI images astracer for scattered light we find a local minimum hereas described above.
Photospheric emission:
If the observed point sourceflux arose solely from the photosphere of a young objectthe COND and DUSTY models suggest masses between ∼
15 – 20 M
Jupiter for an age of 5 – 10 Myr (Baraffeet al. 2003; Chabrier et al. 2000). Models with lowerspecific entropy in the initial conditions for the formationprocess predict even higher masses (cf. Spiegel & Burrows2012). Classical binary formation via core fragmentationor formation via disk instability when the disk was stillmassive would be the preferred formation mechanismsfor an object of this mass. In this case the object formedroughly coeval with the star and would have had timeto significantly alter the structure of the main disk, e.g.,dynamically clearing a large azimuthal gap, which hasnot been observed.
Ejected planet:
Another massive planet is thought tobe orbiting in the inner disk gap (e.g., Acke & van denAncker 2006; Tatulli et al. 2011) and we speculate thatdynamical interactions between multiple planets and thedisk could have led to an ejection event. The emission protoplanet candidate embedded in the circumstellar disk of HD100546 5we see in the L ′ images would then be a combinationof the planet’s intrinsic emission plus extra luminosityfrom disk material being heated from the planet movingthrough the disk. Assuming that the planet was initiallyorbiting at 10 AU its orbital period was ∼
20 years yield-ing an orbital velocity of ∼ − ( ∼ − ).If the ejection velocity is a few times that value it wouldhave taken the planet less then 20 years to reach its cur-rent location and within less than 100 years it would bebeyond the extent of the observable disk. Given the ageof the system, this timescale is extremely small and ob-serving the object exactly at the right time is unlikely.Adding further complexity to this scenario, the ejectionneeds to occur roughly in the plane of the disk to makea link to the observed disk structures. Forming planet:
In our view the best explanation forthe observed morphology of both the disk and the emis-sion source is the detection of a planet during its forma-tion process. The luminosity of the object is not comingfrom an isolated photosphere, but rather the planet isstill accreting material from the disk. Young, forminggas giants with masses between one and a few Jupitermasses are expected to have luminosities between 10 − –10 − L ⊙ during the first few hundred thousand years aftergas runaway accretion sets in (Mordasini et al. 2012), inagreement with our lower limit. Furthermore, an objectin this mass range is expected to affect the disk struc-ture much less and an azimuthal gap – if it exists – mightbe below our detection limits in the PDI data. A narrowgap is hard to see in an inclined disk. This scenario couldalso explain the extended component of the emission de-tected here with some disk material being heated in theaccretion process similar to the case of LkCa15 b(see Kraus & Ireland 2012, for possible mechanisms toheat the surrounding material during the accretion pro-cess). However, we acknowledge that from a theoreticalperspective the formation of a gas giant planet at thislocation is not readily explainable using first principles .For core accretion the timescales to assemble a massiverocky core seem to exceed the estimated age of the star,and given the observed disk parameters the disk does notseem to be gravitationally unstable. Observational tests to distinguish the scenarios
The different dynamics involved in the scenarios out-lined above and multi-wavelength photometry and/or spectra will eventually help us to confirm which hypoth-esis is correct. The proper and parallactic motion ofHD100546 will allow us to rule out a background sourcewith second epoch observations obtained as early as mid2013. To distinguish between the ejection scenario andthe “forming planet” scenario, the baseline for follow-upobservations needs to be a few years. While the objectis expected to orbit its star with a period of ∼
360 yearsin the latter case, it should move away quickly in radialdirection if it were ejected. Also, high spatial resolutionALMA observations will help to search for an azimuthalgap in the surface mass density (gas and/or dust) at theplanet’s location. Spatially resolved information aboutthe existence (or non-existence) of an azimuthal disk gapmay allow us to derive a dynamical mass estimate for theplanet (Lin & Papaloizou 1986; Bryden et al. 1999). CONCLUSION
We have presented observational evidence that a gasgiant planet could be forming in the circumstellar diskaround the Herbig Ae/Be star HD100546 at a separa-tion of ∼
68 AU. This scenario, among others that wediscussed, seems best capable of explaining most of theavailable data. However, some aspects remain qualita-tive and follow-up observations are required to validateour proposed interpretation. Together with LkCa15 b(Kraus & Ireland 2012), the object presented here is cur-rently the best candidate for a forming young gas giantplanet. Particularly interesting is that HD100546 “b”would be the first protoplanet that is still embedded ina circumstellar disk and that it forms at large orbitalseparations. If confirmed, HD100546 would be a uniquelaboratory to study planet formation and the interactionbetween forming planets and the disk directly.This research made use of the SIMBAD database, op-erated at CDS, Strasbourg, France, and of NASA’s As-trophysics Data System. V. Geers kindly provided uswith the ISO and ISAAC spectra. We are indebted to F.Meru, C. Dominik, H. M. Schmid and R. v. Boekel forhelpful discussions.
Facilities:
VLT:Yepun (NACO)
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