Scattered Light from Dust in the Cavity of the V4046 Sgr Transition Disk
Valerie A. Rapson, Joel H. Kastner, Sean M. Andrews, Dean C. Hines, Bruce Macintosh, Max Millar-Blanchaer, Motohide Tamura
SScattered Light from Dust in the Cavity of the V4046 Sgr Transition Disk
Valerie A. Rapson , Joel H. Kastner , Sean M. Andrews , Dean C. Hines , Bruce Macintosh ,Max Millar-Blanchaer , Motohide Tamura [email protected] ABSTRACT
We report the presence of scattered light from dust grains located in the giant planetformation region of the circumbinary disk orbiting the ∼ ∼ ∼ ∼ ∼
10 AU) ring ofpolarized near-infrared flux whose brightness peaks at ∼
14 AU. This ∼
14 AU radiusring is surrounded by a fainter outer halo of scattered light extending to ∼
45 AU,which coincides with previously detected mm-wave thermal dust emission. The presenceof small grains that efficiently scatter starlight well inside the mm-wavelength diskcavity supports current models of planet formation that suggest planet-disk interactionscan generate pressure traps that impose strong radial variations in the particle sizedistribution throughout the disk.
Subject headings: circumstellar matter, polarization, stars: pre-main sequence, stars:individual (V4046 Sgr)
1. Introduction
According to the core accretion model of gas giant planet formation, such planets are formed incircumstellar disks via agglomeration of dust particles into a planetary core, followed by accretion School of Physics and Astronomy, Rochester Institute of Technology, 1 Lomb Memorial Drive, Rochester, NY14623-5603, USA Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA Space Telescope Science Institute, Baltimore, MD, USA Physics Department, Stanford University, Stanford, CA 94305, USA Department of Astronomy and Astrophysics, University of Toronto, ON, M5S 3H4, Canada National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan a r X i v : . [ a s t r o - ph . S R ] M a r <
10 Myr), low-mass stars in optical and near-infrared scatteredlight images and sub-mm thermal emission (Hughes et al. 2007; Andrews et al. 2009; Isella et al.2010; Andrews et al. 2011; Garufi et al. 2013). These same systems show depletions of cm- andmm-sized dust grains within the central regions of their disks. Such central disk “holes” have beeninterpreted as evidence of formation of giant planets with orbital semimajor axes of (cid:46)
30 AU, i.e.,well interior to the sub-mm bright rings (Pinilla et al. 2012; Zhu et al. 2012; Garufi et al. 2013; Zhuet al. 2014; Owen 2014).Thanks to its proximity (D ∼
73 pc; Torres et al. 2008) and advanced age ( ∼
23 Myr; Mamajek& Bell 2014), the circumbinary disk orbiting the close (2.4 day period) binary system V4046 SgrAB represents an excellent subject for the study of such late-stage planet-building processes. TheV4046 Sgr AB system consists of two nearly equal mass components (0.9 and 0.85 M (cid:12) ; Rosenfeldet al. 2012) with spectral types K5Ve and K7Ve (Stempels & Gahm 2004) separated by only ∼ R (cid:12) (0.045 AU). Since the recognition that V4046 Sgr AB possesses a large circumbinary dust mass(Jensen & Mathieu 1997) and is orbited by (and actively accreting from) a gaseous disk (Stempels& Gahm 2004; G¨unther et al. 2006; Kastner et al. 2008), this system has been extensively studiedfrom X-ray to sub-mm wavelengths (Rodriguez et al. 2010; Donati et al. 2011; ¨Oberg et al. 2011;Argiroffi et al. 2012; Rosenfeld et al. 2012, 2013; Kastner et al. 2014). Radio interferometricimaging of the V4046 Sgr disk in CO emission reveals that the circumbinary molecular disk isinclined at 33.5 ◦ (Rosenfeld et al. 2012; Rodriguez et al. 2010) and extends to ∼
350 AU (Rodriguezet al. 2010), with an estimated total gas+dust mass of ∼ M (cid:12) (Rosenfeld et al. 2013). Sucha large and massive disk is unexpected, given that the V4046 Sgr system is a factor ∼
10 olderthan the vast majority of known, actively accreting stars with circumstellar disks (e.g., Inglebyet al. 2014). The V4046 Sgr disk also displays more compact, distinctly ring-like mm continuumemission, suggesting a depletion of mm-sized dust interior to ∼
29 AU due to dust particle growthand particle size segregation associated with recent or ongoing planet formation (Rosenfeld et al.2013).Polarimetric near-infrared imaging of light scattered off dust particles is useful for determiningthe radial and azimuthal distribution of micron-sized dust within the planet forming regions ofcircumstellar disks (e.g. Avenhaus et al. 2014a; Hashimoto et al. 2011). These observations serveas a powerful complement to mm-wave interferometric imaging in establishing radial gradients indust particle size (e.g. Dong et al. 2012). Here, we present coronagraphic/polarimetric images ofthe circumbinary disk around V4046 Sgr obtained with the Gemini Planet Imager (GPI; Macintoshet al. 2008, 2014) on Gemini South. These images probe closer to the central star(s) than previously 3 –achieved for a gas-rich, protoplanetary disk (R ∼ ∼
2. Observations and Data Reduction
Early Science coronagraphic/polarimetric images of V4046 Sgr were obtained with Gemini/GPIthrough J (1.24 µ m) and K2 (2.27 µ m) band filters and 0.184 and 0.306 (cid:48)(cid:48) diameter coronagraphicspots on April 23 and 24, 2014, respectively. Four sets of J (K2) band images were obtained atwaveplate position angles of 0, 22, 45 and 68 ◦ with exposure times of 30 s (60 s) through airmassesranging from 1.24-1.97 (1.00-1.01) and DIMM seeing ∼ ∼ ∼ .
05” ( ∼ I =9.11 mag) was the faintest target successfully observed with GPI and hence served as a test of theuseful domain of the GPI instrument in coronagraphic/polarimetric mode.Images in each filter were reduced and combined using the GPI pipeline v1.2.1 (Maire et al.2010; Perrin et al. 2014), following methods similar to those outlined in Perrin et al. (2015). Thesebasic reduction steps include image background subtraction, removal of correlated noise due todetector readout electronics and microphonics, and interpolation over bad pixels. Calibration spotgrids, which define the location of each polarization spot pair produced by the lenslet array, wereused to extract the data from each raw image and produce a pair of orthogonally polarized images.Satellite spots on each J-band image were used to determine the location of the (unresolved) binarystar behind the coronagraph. The binary was insufficiently bright in the K2 images for accuratedetermination of its location behind the occulting spot via this technique, so its location wasassumed to be at the center of the apparent coronagraph spot in each image. To avoid positive biasin the polarized intensity image, the radial and tangential Stokes parameters P (cid:107) and P ⊥ (Avenhauset al. 2014b) were computed via the pipeline; we assume that all polarized flux is in the tangentialcomponent. Procedures for subtraction of the total intensity PSF for extended objects in GPI’spolarization mode are still under development. We therefore focus our analysis on the P ⊥ images.
3. Results
Figure 1 shows the total intensity, P ⊥ , and scaled P ⊥ images of the disk around V4046 Sgr atJ and K2. The orientation of the polarization (pseudo-)vectors are elliptically symmetric around 4 –the occulted central binary and the polarization fraction is much greater than the instrumentalpolarization of ∼ ⊥ and scaled P ⊥ images in Figure 1 show a relatively narrow,bright central ring that peaks in brightness at ∼
14 AU and is surrounded by a fainter, outer halodetected at ≥ σ out to ∼
45 AU. The scaled P ⊥ images, which account for dilution of incidentstarlight (e.g. Garufi et al. 2014) highlight this ring/halo structure. The structure is detected atboth J and K2, but is most clearly seen in the former because this shorter near-infrared wavelengthprobes dust that — in addition to being illuminated by a brighter incident stellar radiation field —likely has a larger scattering efficiency. The inner ring also shows radial dark features at J, whichcould be shadowing from dust within or interior to the bright ring.Fig. 1.— Left : Total intensity J (top) and K2 (bottom) images with polarization degree (p=P/I)(pseudo-)vectors overlaid in green for pixels where the total polarized intensity is greater than 40counts. Middle : J (top) and K2 (bottom) polarized intensity (P ⊥ ) images. Right: P ⊥ scaled by r ,where r is the distance in pixels from the central binary, corrected for projection effects. All imagesare shown on a linear scale. The coronagraph is represented by the black filled circles and imagesare oriented with north up and east to the left. A small artifact from slight telescope mispointingduring acquisition of the J-band image sequence can be seen to the west of the coronagraph in theP ⊥ images.The ring structures seen in the P ⊥ images cannot be attributed to the point spread function 5 –of the occulted central star system, given that similarly bright stars imaged by GPI in coron-agraphic/polarimetric mode show no such features (e.g. Perrin et al. 2015). Furthermore, theinclination, position angle, and north/south brightness contrast of the disk dust ring system areconsistent with the outer ( ∼ CO observations of V4046 Sgr (Rosenfeld et al. 2012).To better characterize the double ring structure that is evident in the polarized intensityimages, we constructed radial brightness profiles from the J band image in the East-West andSouth-North directions, after rotating to account for the disk equatorial plane position angle of76 ◦ (Fig. 2). Both radial profiles show a distinct inner ring that peaks in brightness at ∼
14 AU,surrounded by a ”shoulder” and a fainter halo extending out to ∼
45 AU. The inner ring, whichhas a FWHM of ∼
10 AU (as measured using the eastern-directed radial profile in Fig. 2), blendssmoothly with the outer halo in the North, but a distinct break is evident at ∼
20 AU in the East,West and South.In Figure 3 we present surface brightness profiles at J and K2 created by averaging concentricelliptical annuli at 0.014” intervals (the angular size of one pixel) with minor:major axis ratios of0.84 (approximating a circular disk with inclination of 33.5 ◦ ) oriented at a position angle of 76 ◦ (Rosenfeld et al. 2012). Both the J and K2 surface brightness profiles display a roughly power-lawdependence on radius (i.e., surface brightness ∝ r − n ) between ∼
14 AU and ∼
45 AU, which is theapparent outer limit of scattered light detected in the radial profiles. In both bands, there is a clearbreak in the power-law index at ∼
28 AU, wherein n ≈ . ∼
14 - 28 AU and n ≈ .
4. Discussion4.1. Comparison of GPI and SMA imaging of V4046 Sgr
Rosenfeld et al. (2013) modeled 1.3 mm CO and continuum interferometric imaging data forV4046 Sgr in terms of a ring of large (mm-sized) dust grains following a Gaussian ring surfacedensity profile with a mean radius of 37 AU and FWHM of 16 AU, surrounded by an extended( ∼
350 AU radius) halo of molecular gas and small grains. In this model, the region interior to ∼
29 AU is depleted of large grains, but contains smaller ( µ m-sized) dust. This model also wellreproduces mid- to far-infrared spectrophotometry of V4046 Sgr (Rapson et al. 2014, submitted).Figure 4 compares the GPI total linear polarized intensity with the SMA 1.3mm continuum emissionmap (Rosenfeld et al. 2013). The fainter, outer scattered light halo in the GPI polarized intensityimages, which extends to ∼
45 AU, appears to merge into the peaks in SMA-detected flux. Figure3 (right) shows the surface brightness profiles at J and K2 with the surface density of small ( µ m- 6 – Inner ring “Shoulder” “Shoulder” “Halo” “Halo”
Fig. 2.— Radial profile extracted from the J band P ⊥ image binned 2x2 along the 76 ◦ positionangle showing the East-West (black) and South-North (orange) brightness profile of the disk. Thehorizontal red lines show the location of the coronagraph. The uncertainties in each bin of the P ⊥ profiles, determined from P (cid:107) , at both J and K2 is ∼ − fit from ∼
14 - 28 AU and black solidline represents r − . fit from ∼
28 - 45 AU. Right: Background subtracted surface brightness curvesmultiplied by r with the surface density of small ( µ m-sized; black dashed) and large (mm-sized;black solid) dust grains from the Rosenfeld et al. (2013) model overlaid. The surface density ofboth the small and large dust grains has been scaled up by 10 .sized) and large (mm-sized) dust grains from the Rosenfeld et al. (2013) model overlaid. We clearlysee that the dust in our GPI images fills the gap interior to ∼
29 AU, and extends into the ring ofmm-sized dust that peaks at 37 AU. The Rosenfeld et al. (2013) model also predicts the existenceof µ m-sized grains interior to ∼ The foregoing comparison between GPI and SMA imaging demonstrates that the inner ∼
45 AUof the disk has undergone significant dust particle growth and particle size segregation. Such particlegrowth and migration processes are expected to accompany an epoch of giant planet formation.Specifically, theories of giant planet formation in dusty disks with embedded, nascent planets predictthe generation of radially localized pressure maxima as a consequence of planet-disk dynamicalinteractions (Rice et al. 2006; Zhu et al. 2012). These pressure gradients trap larger (mm- tocm-sized) particles outside the planet-forming regions of the disk, whereas smaller (micron-sized)grains freely pass through the pressure traps, resulting in strong dust particle size gradients.Interferometric imaging of dusty protoplanetary disks often reveal central disk clearings whoseinner radii are significantly larger than the orbits of the Jovian planets in our solar system (e.g.Andrews et al. 2011). This dichotomy in disk size scales has left open the potential connectionbetween inner disk clearings and giant planet building. Modeling by Pinilla et al. (2012) showsthat pressure bumps in the disk due to gaps opened by orbiting planets trap mm-sized dust grainsinto a ring that may be located at radii greater than twice a planet’s orbital radius, depending onthe mass of the planet. Smaller ( (cid:46) µ m-sized) dust particles drift inward and create a ring betweenthe planet-induced gap and the mm dust ring.Such an interpretation has been applied to mm-wave and near-IR imaging of the disk orbitingthe intermediate-mass star SAO 206462 (Garufi et al. 2013). The surface brightness profile fromnear-infrared scattered light off the disk around SAO 206462 shows a similar power law dependenceon scales about twice that determined here for V4046 Sgr, despite the fact that the SAO 206462disk exhibits spiral structure, rather than rings, and the scattered light lies exterior to the sub-mmhole. Thus, the dust ring structure and radial profiles seen in both V4046 Sgr and SAO 206462suggest that dust segregation and planet formation may be occurring within these disks. It is possible that the depletion in scattered light at R (cid:46)
14 AU in our GPI images may be dueto giant planet formation. This cavity cannot be due purely to dynamical effects from the binarysystem, as the tight ( ∼ R (cid:12) ) binary can only truncate the disk out to ∼ ad hoc ) inner disk geometry as less likely than that of an opening carved out by a giantplanet. It is also possible, but unlikely, that photoevaporation has caused this depletion of dustwithin ∼
14 AU, since models predict that micron-sized dust is rapidly destroyed by X-ray/UVradiation (Gorti et al. 2009; Owen et al. 2012), and the GPI images demonstrate that a significantmass of micron-sized dust is still present at disk radii between 14 to 45 AU after ∼
20 Myr of diskevolution. 9 –Ruge et al. (2014) model the effects of giant planets in massive, dusty disks on the appearanceof gaps in scattered light images. They find that the formation of a giant planet results in adeficit of 1.3 mm emission at the location of the forming planet for disks more massive than2.67 × − M (cid:12) . Gaps in near-infrared scattered light are also evident for lower mass disks, butfor massive disks (M (cid:38) × − M (cid:12) ) the disk remains optically thick and thus a gap may notbe apparent in scattered light at near-infrared wavelengths. While the V4046 Sgr disk is massiveoverall (M ∼ (cid:12) ), modeling of the disk dust distribution (Rosenfeld et al. 2013), and the evidencefor dust segregation by size, suggests most of the dust mass resides beyond the ∼
29 AU sub-mm“edge”, thereby allowing for gaps formed by giant planets to be visible in scattered light imagesat near-infrared wavelengths. These models suggest that planets may be forming interior to ∼ ∼
20 AU break between the rings seen in scattered light (Figs. 2,3).Considering the depletion of sub-mm emission interior to ∼
29 AU (Rosenfeld et al. 2013), a giantplanet at ∼
20 AU would also be consistent with modeling predicting that sub-mm rings form at (cid:38) µ m and mm wavelengths at the locations of formingplanets. Planets interior to a sub-mm cavity can form multiple dust rings and gaps depending onthe planet’s size and location, just as we observe in our GPI images of V4046 Sgr (compare ourFigs. 1 and 3 with their Figs. 2,5 and 7). Overall, our GPI imaging of the V4046 Sgr disk henceprovides vivid evidence in support of so-called “dust filtration” models describing the structure ofprotoplanetary disks following giant planet formation.
5. Conclusions
GPI coronagraphic/polarimetric imaging of the V4046 Sgr AB circumbinary disk, combinedwith mm imaging, demonstrates dust segregation by size into rings, which may be caused bymultiple young giant planets orbiting the V4046 Sgr AB binary system at orbital semimajor axessimilar to those of the giant planets in our solar system. Polarized intensity images yield evidencethat the disk is well populated by relatively small (micron-sized or smaller) grains within its central, ∼
29 AU large-grain cavity and, furthermore, that there exists an interior (cid:46)
14 AU-radius region thatis devoid even of small grains. Comparison with models of protoplanet-disk interactions (Pinillaet al. 2012; Ruge et al. 2014; Dong et al. 2014) suggests that gas giant planets may be present, andactively carving gaps, at R (cid:46)
14 AU and at R ∼
20 AU in the V4046 Sgr disk.Further imaging with Gemini/GPI or similar instrumentation at higher signal to noise, as wellas high resolution imaging with ALMA, is necessary to better discern the structure of the ringsevident around V4046 Sgr. Modeling is also needed to determine the grain properties and dustmass within the disk, along with the location of possible forming planets. Further investigationsaimed at directly and indirectly detecting potential young giant planets orbiting V4046 Sgr AB will 10 –also provide essential constraints on simulations aimed at understanding the conditions in whichgiant planets might form in circumbinary orbits — a theoretical question that is presently of intenseinterest, given the Kepler Mission’s detection of circumbinary planets (Pierens & Nelson 2013).
Acknowledgements
This work is based on observations obtained at the Gemini Observatory, which is operated bythe Association of Universities for Research in Astronomy, Inc., under a cooperative agreement withthe NSF on behalf of the Gemini partnership: the National Science Foundation (United States), theNational Research Council (Canada), CONICYT (Chile), the Australian Research Council (Aus-tralia), Minist´erio da Ciˆencia, Tecnologia e Inova¸c˜ao (Brazil) and Ministerio de Ciencia, Tecnolog´ıae Innovaci´on Productiva (Argentina). Support is provided by the National Science Foundationgrant AST-1108950 to RIT.
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This preprint was prepared with the AAS L A TEX macros v5.2.
13 –Fig. 4.— Three color composite image comparing SMA 1.3mm continuum emission (blue withyellow contours overlaid; Rosenfeld et al. 2013) and GPI J (green) and K2 (red) total linear polarizedintensity. The SMA data has a beam size of 0 . × ..