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Magnetospheric accretion models predict that matter from protoplanetary disks accretes onto the star via funnel flows which follow the stellar field lines and shock on the stellar surface leaving a hot spot with a density gradient. Previous work has inferred different densities in the hot spot, but has not been sensitive to the radial density distribution. Attempts have been made to measure this with X-ray observations, but X-ray emission only traces a fraction of the hot spot and also coronal emission. Here we report periodic ultraviolet and optical light curves of the accreting star GM Aur that display a time lag of about 1 day between their peaks. The periodicity arises as the source of the ultraviolet and optical emission moves into and out of view as it rotates along with the star. The time-lag indicates a difference in the spatial distribution of ultraviolet and optical brightness over the stellar surface. Within the framework of the magnetospheric accretion model, this indicates a radial density gradient in a hot spot on the stellar surface since different density parts of the hot spot are expected to emit radiation at different wavelengths. These results are the first observational confirmation of the magnetospheric accretion model's prediction of a density gradient in the hot spot and demonstrate the insights gained from focusing on the wavelengths where the bulk of the accretion energy can be observed., published in Nature; open access link: https://www.nature.com/articles/s41586-021-03751-5

We present the complete sample of protoplanetary disks from the Gemini- Large Imaging with GPI Herbig/T-tauri Survey (Gemini-LIGHTS) which observed bright Herbig Ae/Be stars and T-Tauri stars in near-infrared polarized light to search for signatures of disk evolution and ongoing planet formation. The 44 targets were chosen based on their near- and mid-infrared colors, with roughly equal numbers of transitional, pre-transitional, and full disks. Our approach explicitly did not favor well-known, "famous" disks or those observed by ALMA, resulting in a less-biased sample suitable to probe the major stages of disk evolution during planet formation. Our optimized data reduction allowed polarized flux as low as 0.002% of the stellar light to be detected, and we report polarized scattered light around 80% of our targets. We detected point-like companions for 47% of the targets, including 3 brown dwarfs (2 confirmed, 1 new), and a new super-Jupiter mass candidate around V1295 Aql. We searched for correlations between the polarized flux and system parameters, finding a few clear trends: presence of a companion drastically reduces the polarized flux levels, far-IR excess correlates with polarized flux for non-binary systems, and systems hosting disks with ring structures have stellar masses $100$ au) scattered light around each, signs of extreme youth for these enigmatic systems. Science-ready images are publicly available through multiple distribution channels using a new FITS file standard jointly developed with members of the VLT/SPHERE team., 51 pages, 31 figures, 7 tables, accepted to AJ

The evolution of young stars and disks is driven by the interplay of several processes, notably accretion and ejection of material. Critical to correctly describe the conditions of planet formation, these processes are best probed spectroscopically. About five-hundred orbits of the Hubble Space Telescope (HST) are being devoted in 2020-2022 to the ULLYSES public survey of about 70 low-mass (M, Comment: Accepted for publication on Astronomy & Astrophysics. 15 pages + appendix, language edited version

We present the largest sample of brown dwarf (BD) protoplanetary disk spectral energy distributions modeled to date. We compile 49 objects with ALMA observations from four star-forming regions: $\rho$ Ophiuchus, Taurus, Lupus, and Upper Scorpius. Studying multiple regions with various ages enables us to probe disk evolution over time. Specifically, from our models we obtain values for dust grain sizes, dust settling, and disk mass; we compare how each of these parameters vary between the regions. We find that disk mass decreases with age. We also find evidence of disk evolution (i.e., grain growth and significant dust settling) in all four regions, indicating that planet formation and disk evolution may begin to occur at earlier stages. We generally find these disks contain too little mass to form planetary companions, though we cannot rule out that planet formation may have already occurred. Finally, we examine the disk mass -- host mass relationship and find that BD disks are largely consistent with previously-determined relationships for disks around T Tauri stars., Comment: preprint includes 31 pages, 15 figures. Accepted to ApJ June 28, 2021

We present sensitive ALMA observations of TWA 3, a nearby, young ($\sim$10 Myr) hierarchical system composed of three pre-main sequence M3--M4.5 stars. For the first time, we detected ${}^{12}$CO and ${}^{13}$CO $J$=2-1 emission from the circumbinary protoplanetary disk around TWA 3A. We jointly fit the protoplanetary disk velocity field, stellar astrometric positions, and stellar radial velocities to infer the architecture of the system. The Aa and Ab stars ($0.29\pm0.01\,M_\odot$ and $0.24\pm0.01\,M_\odot$, respectively) comprising the tight ($P=35$ days) eccentric ($e=0.63\pm0.01$) spectroscopic binary are coplanar with their circumbinary disk (misalignment $< 6^{\circ}$ with 68% confidence), similar to other short-period binary systems. From models of the spectral energy distribution, we found the inner radius of the circumbinary disk ($r_\mathrm{inner} = 0.50 - 0.75$ au) to be consistent with theoretical predictions of dynamical truncation $r_\mathrm{cav}/a_\mathrm{inner} \approx 3$. The outer orbit of the tertiary star B ($0.40\pm0.28\,M_\odot$, $a\sim65 \pm 18$ au, $e=0.3\pm0.2$) is not as well constrained as the inner orbit, however, orbits coplanar with the A system are still preferred (misalignment $ < 20^{\circ}$). To better understand the influence of the B orbit on the TWA 3A circumbinary disk, we performed SPH simulations of the system and found that the outer edge of the gas disk ($r_\mathrm{outer}=8.5\pm0.2$ au) is most consistent with truncation from a coplanar, circular or moderately eccentric orbit, supporting the preference from the joint orbital fit., 19 pages, 9 figures, accepted to ApJ

Photometric observations of accreting, low-mass, pre-main-sequence stars (i.e., Classical T Tauri stars; CTTS) have revealed different categories of variability. Several of these classifications have been linked to changes in $\dot{M}$. To test how accretion variability conditions lead to different light-curve morphologies, we used 1D hydrodynamic simulations of accretion along a magnetic field line coupled with radiative transfer models and a simple treatment of rotation to generate synthetic light curves. We adopted previously developed metrics in order to classify observations to facilitate comparisons between observations and our models. We found that stellar mass, magnetic field geometry, corotation radius, inclination, and turbulence all play roles in producing the observed light curves and that no single parameter is entirely dominant in controlling the observed variability. While the periodic behavior of the light curve is most strongly affected by the inclination, it is also a function of the magnetic field geometry and inner disk turbulence. Objects with either pure dipole fields, strong aligned octupole components, or high turbulence in the inner disk all tend to display accretion bursts. Objects with anti-aligned octupole components or aligned, weaker octupole components tend to show light curves with slightly fewer bursts. We did not find clear monotonic trends between the stellar mass and empirical classification. This work establishes the groundwork for more detailed characterization of well-studied targets as more light curves of CTTS become available through missions such as the Transiting Exoplanet Survey Satellite (TESS)., Comment: 18 pages, 11 figures

Young stars are surrounded by a circumstellar disk of gas and dust, within which planet formation can occur. Gravitational forces in multiple star systems can disrupt the disk. Theoretical models predict that if the disk is misaligned with the orbital plane of the stars, the disk should warp and break into precessing rings, a phenomenon known as disk tearing. We present observations of the triple star system GWOrionis, finding evidence for disk tearing. Our images show an eccentric ring that is misaligned with the orbital planes and the outer disk. The ring casts shadows on a strongly warped intermediate region of the disk. If planets can form within the warped disk, disk tearing could provide a mechanism for forming wide-separation planets on oblique orbits., 63 pages, 4+13 Figures, 6 Tables, published at Science

We model the spectral energy distributions (SEDs) of 23 protoplanetary disks in the Taurus-Auriga star-forming region using detailed disk models and a Bayesian approach. This is made possible by combining these models with artificial neural networks to drastically speed up their performance. Such a setup allows us to confront $\alpha$-disk models with observations while accounting for several uncertainties and degeneracies. Our results yield high viscosities and accretion rates for many sources, which is not consistent with recent measurements of low turbulence levels in disks. This inconsistency could imply that viscosity is not the main mechanism for angular momentum transport in disks, and that alternatives such as disk winds play an important role in this process. We also find that our SED-derived disk masses are systematically higher than those obtained solely from (sub)mm fluxes, suggesting that part of the disk emission could still be optically thick at (sub)mm wavelengths. This effect is particularly relevant for disk population studies and alleviates previous observational tensions between the masses of protoplanetary disks and exoplanetary systems., Comment: 24 pages (including appendices), 13 figures. Accepted for publication in Astronomy and Astrophysics

Snowlines of major volatiles regulate the gas and solid C/N/O ratios in the planet-forming midplanes of protoplanetary disks. Snow surfaces are the 2D extensions of snowlines in the outer disk regions, where radiative heating results in a decreasing temperature with disk height. CO and N$_2$ are two of the most abundant carriers of C, N and O. N$_2$H$^+$ can be used to probe the snow surfaces of both molecules, because it is destroyed by CO and formed from N$_2$. Here we present Atacama Large Millimeter/submillimeter Array (ALMA) observations of N$_2$H$^+$ at 0.2$''$-0.4$''$ resolution in the disks around LkCa 15, GM Aur, DM Tau, V4046 Sgr, AS 209, and IM Lup. We find two distinctive emission morphologies: N$_2$H$^+$ is either present in a bright, narrow ring surrounded by extended tenuous emission, or in a broad ring. These emission patterns can be explained by two different kinds of vertical temperature structures. Bright, narrow N$_2$H$^+$ rings are expected in disks with a thick Vertically Isothermal Region above the Midplane (VIRaM) layer (LkCa 15, GM Aur, DM Tau) where the N$_2$H$^+$ emission peaks between the CO and N$_2$ snowlines. Broad N$_2$H$^+$ rings come from disks with a thin VIRaM layer (V4046 Sgr, AS 209, IM Lup). We use a simple model to extract the first sets of CO and N$_2$ snowline pairs and corresponding freeze-out temperatures towards the disks with a thick VIRaM layer. The results reveal a range of N$_2$ and CO snowline radii towards stars of similar spectral type, demonstrating the need for empirically determined snowlines in disks., Accepted for publication in ApJ

Measuring the properties of protoplanetary disks around brown dwarfs is central to understanding the formation of brown dwarfs and their planetary companions. We present modeling of CFHT Tau 4 and 2M0444, two brown dwarfs with protoplanetary disks in the Taurus Molecular Cloud. By combining modeling of the spectral energy distributions and ALMA images, we obtain disk radii and masses for these objects; these parameters can be used to constrain brown dwarf formation and planet formation, respectively. We find that the disk around CFHT Tau 4 has a mass of 0.42 M$_{Jup}$ and a radius of 80 au; we find 2M0444's disk to have a mass of 2.05 M$_{Jup}$ and a radius of 100 au. These radii are more consistent with those predicted by theoretical simulations of brown dwarf formation via undisturbed condensation from a mass reservoir than those predicted by ejection from the formation region. Furthermore, the disk mass of 2M0444 suggests that planet formation may be possible in this disk, although the disk of 2M0444 is likely not massive enough to form planets. The disk properties measured here provide constraints to theoretical models of brown dwarf formation and the formation of their planetary companions., 16 pages, 6 figures. Accepted to ApJ on May 10, 2019

Variability is a defining characteristic of young low-mass stars that are still accreting material from their primordial protoplanetary disk. Here we present the largest \textit{HST} variability study of Classical T Tauri stars (CTTS) to date. For 5 of these objects, we obtained a total of 25 spectra with the Space Telescope Imaging Spectrograph (STIS). Mass accretion rates and the fraction of the star covered by accretion columns (i.e., filling factors) were inferred using 1D NLTE physical models whose parameters were fit within a Bayesian framework. On week long timescales, typical changes in the mass accretion rates range up to a factor of $\sim2$, while changes of up to a factor of $\sim5$ are inferred for the filling factors. In addition to this, we observed a possible accretion burst in the transitional disk system GM Aur, and an incident we interpret as a chance alignment of an accretion column and the undisturbed photosphere along our line of sight in the full disk system VW Cha. We also measure correlations between mass accretion rate and line luminosities for use as secondary tracers of accretion. We place our objects in context with recent high-cadence photometric surveys of low-mass star formation regions and highlight the need for more broad-wavelength, contemporaneous data to better understand the physical mechanisms behind accretion variability in CTTS., 27 pages, 10 figures, 7 tables

We present ALMA observations of 101 protoplanetary disks within the star-forming region Lynds 1641 in the Orion Molecular Cloud A. Our observations include 1.33 mm continuum emission and spectral windows covering the J=2-1 transition of $^{12}$CO, $^{13}$CO, and C$^{18}$O. We detect 89 protoplanetary disks in the dust continuum at the 4$\sigma$ level ($\sim$88% detection rate) and 31 in $^{12}$CO, 13 in $^{13}$CO, and 4 in C$^{18}$O. Our sample contains 23 transitional disks, 20 of which are detected in the continuum. We target infrared-bright Class II objects, which biases our sample towards massive disks. We determine dust masses or upper limits for all sources in our sample and compare our sample to protostars in this region. We find a decrease in dust mass with evolutionary state. We also compare this sample to other regions surveyed in the (sub-)millimeter and find that Lynds 1641 has a relatively massive dust disk population compared to regions of similar and older ages, with a median dust mass of 11.1$^{+32.9}_{-4.6}$ $M_\oplus$ and 27% with dust masses equal to or greater than the minimum solar nebula dust mass value of $\sim$30 $M_\oplus$. We analyze the disk mass-accretion rate relationship in this sample and find that the viscous disk lifetimes are similar to the age of the region, however with a large spread. One object, [MGM2012] 512, shows large-scale ($>$5000 AU) structure in both the dust continuum and the three gas lines. We discuss potential origins for this emission, including an accretion streamer with large dust grains., Comment: 21 pages, 15 figures, 3 tables

We present Atacama Large Millimeter Array (ALMA) observations of TW Hya at 3.1 mm with $\sim50$ milliarcsecond resolution. These new data were combined with archival high angular resolution ALMA observations at 0.87 mm, 1.3 mm, and 2.1 mm. We analyze these multi-wavelength data to infer a disk radial profile of the dust surface density, maximum particle size, and slope of the particle size distribution. Most previously known annular substructures in the disk of TW Hya are resolved at the four wavelengths. Inside the inner 3 au cavity, the 2.1 mm and 3.1 mm images show a compact source of free-free emission, likely associated with an ionized jet. Our multi-wavelength analysis of the dust emission shows that the maximum particle size in the disk of TW Hya is $>1$ mm. The inner 20 au are completely optically thick at all four bands, which results in the data tracing different disk heights at different wavelengths. Coupled with the effects of dust settling, this prevents the derivation of accurate density and grain size estimates in these regions. At $r>20$ au, we find evidence of the accumulation of large dust particle at the position of the bright rings, indicating that these are working as dust traps. The total dust mass in the disk is between 250 and 330 $M_{\oplus}$, which represents a gas-to-dust mass ratio between 50 and 70. Our mass measurement is a factor of 4.5-5.9 higher than the mass that one would estimate using the typical assumptions of large demographic surveys. Our results indicate that the ring substructures in TW Hya are ideal locations to trigger the streaming instability and form new generations of planetesimals., 22 pages, 17 figures, accepted for publication in A&A. Language edited version

The high-energy radiation fields of T Tauri stars (TTS) should affect the surrounding circumstellar disk, having implications for disk transport and heating. Yet, observational evidence of the effect of high-energy fields on disks is scarce. Here we investigate the connection between X-ray emission and the innermost gas disk by leveraging the variability of TTS. We obtained multiple epochs of coordinated data (taken either simultaneously or within a few hours) of accreting TTS with the Hubble Space Telescope, the Neil Gehrels Swift Observatory, and the Chandra X-ray Observatory. We measured the far-ultraviolet (FUV) H2 bump feature at 1600 A, which traces gas, accepted to ApJ

International audience; We discuss science cases to support the development of sub-au imaging capabilities to image forming planets in the terrestrial region of nearby proto-planetary disks.

We present a detailed multiwavelength characterization of the multi-ring disk of HD 169142. We report new Atacama Large Millimeter/submillimeter Array (ALMA) observations at 3 mm and analyze them together with archival 0.89 and 1.3 mm data. Our observations resolve three out of the four rings in the disk previously seen in high-resolution ALMA data. A simple parametric model is used to estimate the radial profile of the dust optical depth, temperature, density, and particle size distribution. We find that the multiple ring features of the disk are produced by annular accumulations of large particles, probably associated with gas pressure bumps. Our model indicates that the maximum dust grain size in the rings is ∼1 cm, with slightly flatter power-law size distributions than the interstellar medium-like size distribution (p ∼ 3.5) found in the gaps. In particular, the inner ring (∼26 au) is associated with a strong and narrow buildup of dust particles that could harbor the necessary conditions to trigger the streaming instability. According to our analysis, the snowlines of the most important volatiles do not coincide with the observed substructures. We explore different ring formation mechanisms and find that planet-disk interactions are the most likely scenario to explain the main features of HD 169142. Overall, our multiwavelength analysis provides some of the first unambiguous evidence of the presence of radial dust traps in the rings of HD 169142. A similar analysis in a larger sample of disks could provide key insights on the impact that disk substructures have on the dust evolution and planet formation processes.© 2019. The American Astronomical Society. All rights reserved.., E.M., and C.C.E. acknowledge support from the National Science Foundation under CAREER grant No. AST-1455042 and the Sloan Foundation. M.O., G.A., J.M.T., and J.F.G. acknowledge financial support from the State Agency for Research of the Spanish MCIU through the AYA2017-84390-C2-1-R grant (co-funded by FEDER). M.O., G. A., and J.F.G. acknowledge support from the >Center of Excellence Severo Ochoa> award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709). M. F. and G.H.M.B. acknowledge support from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement no 757957).

We present a detailed study of the disk around the intermediate mass star SO 411, aiming to explain the spectral energy distribution of this star. We show that this is a transitional disk truncated at $\sim$11 au, with $\sim$0.03 lunar masses of optically thin dust inside the cavity. Gas also flows through the cavity, since we find that the disk is still accreting mass onto the star, at a rate of $\sim 5x10^{-9}$ Msun/yr. Until now, SO 411 has been thought to belong to the $\sim$3 Myr old {$\sigma$} Orionis cluster. However, we analyzed the second Gaia Data Release in combination with kinematic data previously reported, and found that SO 411 can be associated with an sparse stellar population located in front of the {$\sigma$} Orionis cluster. If this is the case, then SO 411 is older and even more peculiar, since primordial disks in this stellar mass range are scarce for ages $>$5 Myr. Analysis of the silicate 10$\mu$m feature of SO 411 indicates that the observed feature arises at the edge of the outer disk, and displays a very high crystallinity ratio of $\sim$0.5, with forsterite the most abundant silicate crystal. The high forsterite abundance points to crystal formation in non-equilibrium conditions. The PAH spectrum of SO 411 is consistent with this intermediate state between the hot and luminous Herbig Ae and the less massive and cooler T Tauri stars. Analysis of the 7.7$\mu$m PAH feature indicates that small PAHs still remain in the SO 411 disk., Comment: Accepted in the Astrophysical Journal (17 pages, 9 Figures)

We analyze Herschel Space Observatory observations of 104 young stellar objects with protoplanetary disks in the ~1.5 Myr star-forming region Lynds 1641 (L1641) within the Orion A Molecular Cloud. We present spectral energy distributions from the optical to the far-infrared including new photometry from the Herschel Photodetector Array Camera and Spectrometer (PACS) at 70 microns. Our sample, taken as part of the Herschel Orion Protostar Survey, contains 24 transitional disks, eight of which we identify for the first time in this work. We analyze the full disks with irradiated accretion disk models to infer dust settling properties. Using forward modeling to reproduce the observed nKS-[70] index for the full disk sample, we find the observed disk indices are consistent with models that have depletion of dust in the upper layers of the disk relative to the mid plane, indicating significant dust settling. We perform the same analysis on full disks in Taurus with Herschel data and find that Taurus is slightly more evolved, although both samples show signs of dust settling. These results add to the growing literature that significant dust evolution can occur in disks by ~1.5 Myr., 51 pages, 13 figures

The Planet Formation Imager (PFI) is a near- and mid-infrared interferometer project with the driving science goal of imaging directly the key stages of planet formation, including the young proto-planets themselves. Here, we will present an update on the work of the Science Working Group (SWG), including new simulations of dust structures during the assembly phase of planet formation and quantitative detection efficiencies for accreting and non-accreting young exoplanets as a function of mass and age. We use these results to motivate two reference PFI designs consisting of a) twelve 3m telescopes with a maximum baseline of 1.2km focused on young exoplanet imaging and b) twelve 8m telescopes optimized for a wider range of young exoplanets and protoplanetary disk imaging out to the 150K H2O ice line. Armed with 4x8m telescopes, the ESO/VLTI can already detect young exoplanets in principle and projects such as MATISSE, Hi-5 and Heimdallr are important PFI pathfinders to make this possible. We also discuss the state of technology development needed to make PFI more affordable, including progress towards new designs for inexpensive, small field-of-view, large aperture telescopes and prospects for Cubesat-based space interferometry., Comment: Presented at 2018 SPIE Astronomical Telescopes + Instrumentation, Austin, Texas, USA. See www.planetformationimager.org for more information

We present results of our large scale, optical, multi-epoch photometric survey of ~180 square degrees across the Orion OB1 association, complemented with extensive follow up spectroscopy. We map and characterize in an uniform way the off-cloud, low-mass, pre-main sequence populations. We report 2064, mostly K and M-type, confirmed T Tauri members. Most (59%) are located in the OB1a subassociation, 27% in OB1b, and 14% within the confines of the A and B molecular clouds. There is significant structure in the spatial distribution of the young stars. We characterize two new clusterings of T Tauri stars, HD 35762 and HR 1833 in the OB1a subassociation, and two stellar overdensities in OB1b. There is indication of two populations of young stars in the OB1b region, located at two different distances, which may be due to the OB1a subassociation overlapping on front of OB1b. The various groups and regions can be ordered in an age sequence that agrees with the long standing picture of star formation starting in Orion OB1a some 10-15 Myr ago. We define a new type of T Tauri star, the C/W class, objects we propose may be nearing the end of their accretion phase. We detect the observational signature of Li depletion in young K and M stars with a timescale of 8.5 Myr. The decline of the accretion fraction from ~2 - 10 Myr, implies an accretion e-folding timescale of 2.1 Myr. Finally, the median amplitude of the V-band variability shows the decline of stellar activity, from accreting Classical T Tauri stars to the least active field dwarfs., 41 pages, 30 figures, submitted to the Astronomical Journal on May 02, 2018. Abridged abstract

Herbig Ae/Be (HAeBe) stars have been classified into Group I or Group II, which were initially thought to be flared and flat disks, respectively. Several Group I sources have been shown to have large gaps, suggesting ongoing planet formation, while no large gaps have been found in the disks of Group II sources. We analyzed the disk around the Group II source, HD 142666, using irradiated accretion disk modeling of the broad-band spectral energy distribution along with the 1.3 millimeter spatial brightness distribution traced by Atacama Large Millimeter and Submillimeter Array (ALMA) observations. Our model reproduces the available data, predicting a high degree of dust settling in the disk, which is consistent with the Group II classification of HD 142666. In addition, the observed visibilities and synthesized image could only be reproduced when including a depletion of large grains out to ~16 au in our disk model, although the ALMA observations did not have enough angular resolution to fully resolve the inner parts of the disk. These results may suggest that some disks around Group II HAeBe stars have cavities of large grains as well. Further ALMA observations of Group II sources are needed to discern how commonly cavities occur in this class of objects, as well as to reveal their possible origins., 11 pages, 3 figures, accepted for publication by The Astrophysical Journal

We present \emph{Herschel} PACS observations of 8 Classical T Tauri Stars in the $\sim 7-10$ Myr old OB1a and the $\sim 4-5$ Myr old OB1b Orion sub-asscociations. Detailed modeling of the broadband spectral energy distributions, particularly the strong silicate emission at 10 $\mu$m, shows that these objects are (pre)transitional disks with some amount of small optically thin dust inside their cavities, ranging from $\sim 4$ AU to $\sim 90$ AU in size. We analyzed \emph{Spitzer} IRS spectra for two objects in the sample: CVSO-107 and CVSO-109. The IRS spectrum of CVSO-107 indicates the presence of crystalline material inside its gap while the silicate feature of CVSO-109 is characterized by a pristine profile produced by amorphous silicates; the mechanisms creating the optically thin dust seem to depend on disk local conditions. Using millimeter photometry we estimated dust disk masses for CVSO-107 and CVSO-109 lower than the minimum mass of solids needed to form the planets in our Solar System, which suggests that giant planet formation should be over in these disks. We speculate that the presence and maintenance of optically thick material in the inner regions of these pre-transitional disks might point to low-mass planet formation., Comment: 20 pages, 10 figures, 8 tables

Our understanding of protoplanetary disks is rapidly departing from the classical view of a smooth, axisymmetric disk. This is in part thanks to the high angular resolution that (sub)millimeter observations can provide. Here, we present the combined results of Atacama Large Millimeter/submillimeter Array (ALMA) (0.9 mm) and Very Large Array (VLA) (7 mm) dust continuum observations toward the protoplanetary disk around the solar analog GM Aur. Both images clearly resolve the ∼35 au inner cavity. The ALMA observations also reveal a fainter disk that extends up to ∼250 au. We model our observations using two approaches: an analytical fit to the observed deprojected visibilities, and a physical disk model that fits the spectral energy distribution as well as the VLA and ALMA observations. Despite not being evident in the deconvolved images, the VLA and ALMA visibilities can only be fitted with two bright rings of radii ∼40 and ∼80 au. Our physical model indicates that this morphology is the result of an accumulation or trapping of large dust grains, probably due to the presence of two pressure bumps in the disk. Even though alternative mechanisms cannot be discarded, the multiple rings suggest that forming planets may have cleared at least two gaps in the disk. Finally, our analysis suggests that the inner cavity might display different sizes at 0.9 and 7 mm. This discrepancy could be caused by the presence of free-free emission close to the star at 7 mm, or by a more compact accumulation of the large dust grains at the edge of the cavity.© 2018. The American Astronomical Society. All rights reserved., E.M., C.C.E., and A.R. acknowledge support from the National Science Foundation under CAREER grant Number AST-1455042 and the Sloan Foundation. G.A., M.O., and J.F.G. are supported by the MINECO (Spain) AYA2014-57369-C3 and AYA2017-84390-C2 grants (co-funded by FEDER).

Far-infrared and (sub)millimeter fluxes can be used to study dust in protoplanetary disks, the building blocks of planets. Here, we combine observations from the Herschel Space Observatory with ancillary data of 284 protoplanetary disks in the Taurus, Chamaeleon I, and Ophiuchus star-forming regions, covering from the optical to mm/cm wavelengths. We analyze their spectral indices as a function of wavelength and determine their (sub)millimeter slopes when possible. Most disks display observational evidence of grain growth, in agreement with previous studies. No correlation is found between other tracers of disk evolution and the millimeter spectral indices. A simple disk model is used to fit these sources, and we derive posterior distributions for the optical depth at 1.3 mm and 10 au, the disk temperature at this same radius, and the dust opacity spectral index. We find the fluxes at 70 microns to correlate strongly with disk temperatures at 10 au, as derived from these simple models. We find tentative evidence for spectral indices in Chamaeleon I being steeper than those of disks in Taurus/Ophiuchus, although more millimeter observations are needed to confirm this trend and identify its possible origin. Additionally, we determine the median spectral energy distribution of each region and find them to be similar across the entire wavelength range studied, possibly due to the large scatter in disk properties and morphologies., Comment: ApJ, accepted. 38 pages, 11 figures, 16 tables, 3 appendices. The associated data and tables can be found in the Zenodo repository https://doi.org/10.5281/zenodo.889053

We report the discovery of eclipses by circumstellar disc material associated with the young star PDS 110 in the Ori OB1a association using the SuperWASP and KELT surveys. PDS 110 (HD 290380, IRAS 05209-0107) is a rare Fe/Ge-type star, a ~10 Myr-old accreting intermediate-mass star showing strong infrared excess (L$_{\rm IR}$/L$_{\rm bol}$ ~ 0.25). Two extremely similar eclipses with a depth of ~30\% and duration ~25 days were observed in November 2008 and January 2011. We interpret the eclipses as caused by the same structure with an orbital period of $808\pm2$ days. Shearing over a single orbit rules out diffuse dust clumps as the cause, favouring the hypothesis of a companion at ~2AU. The characteristics of the eclipses are consistent with transits by an unseen low-mass (1.8-70M$_{Jup}$) planet or brown dwarf with a circum-secondary disc of diameter ~0.3 AU. The next eclipse event is predicted to take place in September 2017 and could be monitored by amateur and professional observatories across the world., Comment: 10 pages, 6 figures, submitted to MNRAS 9th Feb 2017. Accepted 18th May 2017

We are undertaking a large survey of over thirty disks using the Gemini Planet Imager (GPI) to see whether the observed dust structures match spectral energy distribution (SED) predictions and have any correlation with stellar properties. GPI can observe near-infrared light scattered from dust in circumstellar environments using high-resolution Polarimetric Differential Imaging (PDI) with coronagraphy and adaptive optics. The data have been taken in J and H bands over two years, with inner working angles of 0.08'' and 0.11'' respectively. Ahead of the release of the complete survey results, here we present five objects with extended and irregular dust structures within 2'' of the central star. These objects are: FU Ori; MWC 789; HD 45677; Hen 3-365; and HD 139614. The observed structures are consistent with each object being a pre-main-sequence star with protoplanetary dust. The five objects' circumstellar environments could result from extreme youth and complex initial conditions, from asymmetric scattering patterns due to shadows cast by misaligned disks, or in some cases from interactions with companions. We see complex U_phi structures in most objects that could indicate multiple scattering or result from the illumination of companions. Specific key findings include the first high-contrast observation of MWC 789 revealing a newly-discovered companion candidate and arc, and two faint companion candidates around Hen 3-365. These two objects should be observed further to confirm whether the companion candidates are co-moving. Further observations and modeling are required to determine the causes of the structures.

Understanding disk dissipation is essential for studying how planets form. Disk gaps and holes, which almost correspond to dust-free regions, are inferred from infrared observations of T Tauri stars (TTS), indicating the existence of a transitional phase between thick accreting disks and debris disks. Transition disks are usually referred to as candidates for newly formed planets. We searched for transition disk candidates belonging to NGC 2264. We characterized accretion, disk, and stellar properties of transition disk candidates and compared them to systems with a full disk and diskless stars We modeled the spectral energy distribution (SED) of a sample of 401 TTS, with Hyperion SED fitting code using photometric data from the U band to the MIPS band. We used the SED modeling to distinguish transition disk candidates, full disk systems, and diskless stars. We classified $52\%$ of the sample as full disk systems, $41\%$ as diskless stars, and $7\%$ of the systems as transition disk candidates, among which seven systems are new transition disk candidates belonging to the NGC 2264 cluster. The sample of transition disk candidates present dust in the inner disk similar to anemic disks, according to the $\alpha_{IRAC}$ classification, which shows that anemic disk systems can be candidate transition disks. We show that the presence of a dust hole in the inner disk does not stop the accretion process since $82\%$ of transition disk candidates accrete and show $H\alpha$, UV excess, and mass accretion rates at the same level as full disk systems. We estimate the inner hole sizes, ranging from 0.1 to $78AU$, for the sample of transition disk candidates. In only $18\%$ of the transition disk candidates, the hole size could be explained by X-ray photoevaporation from stellar radiation., Comment: Accepted for publication in A&A

Here we analyze the first simultaneous X-ray, ultraviolet, optical, infrared, and centimeter observations of a T Tauri star (TTS). We present three epochs of simultaneous Spitzer and VLA data of GM Aur separated by ~1 wk. These data are compared to previously published HST and Chandra observations from which mass accretion rates ($\dot M$) and X-ray luminosities, respectively, were measured. The mid-infrared emission increases along with $\dot M$, and we conclude that this is due to an increase in the mass in the inner disk. The cm emission, which probes the jet, also appears to increase as $\dot M$ increases, and the changes in the cm flux are consistent with the variability in $\dot M$ assuming the mass-loss rate is ~10% $\dot M$. The 3 cm emission morphology also appears changed compared with observations taken three years previously, suggesting that for the first time, we may be tracking changes in the jet morphology of a TTS. The X-ray luminosity is constant throughout the three epochs, ruling out variable high-energy stellar radiation as the cause for the increases in the mid-infrared or cm emission. Tying together the multiwavelength variability observed, we conclude that an increase in the surface density in the inner disk resulted in more mass loading onto the star and therefore a higher $\dot M$, which led to a higher mass-loss rate in the jet. These results stress the importance of coordinated multiwavelength work to better understand the star-disk-jet connection., accepted to ApJL

We present the first images of the transition disk around the close binary system HD 34700A in polarized scattered light using the Gemini Planet Imager instrument on Gemini South. The J and H band images reveal multiple spiral-arm structures outside a large (R = 0.49" = 175 au) cavity along with a bluish spiral structure inside the cavity. The cavity wall shows a strong discontinuity and we clearly see significant non-azimuthal polarization Uphi consistent with multiple scattering within a disk at an inferred inclination ~42deg. Radiative transfer modeling along with a new Gaia distance suggest HD 37400A is a young (~5 Myr) system consisting of two intermediate-mass (~2Msun) stars surrounded by a transitional disk and not a solar-mass binary with a debris disk as previously classified. Conventional assumptions of the dust-to-gas ratio would rule out a gravitational instability origin to the spirals while hydrodynamical models using the known external companion or a hypothetical massive protoplanet in the cavity both have trouble reproducing the relatively large spiral arm pitch angles (~30deg) without fine tuning of gas temperature. We explore the possibility that material surrounding a massive protoplanet could explain the rim discontinuity after also considering effects of shadowing by an inner disk. Analysis of archival Hubble Space Telescope data suggests the disk is rotating counterclockwise as expected from the spiral arm structure and revealed a new low-mass companion at 6.45" separation. We include an appendix which sets out clear definitions of Q, U, Qphi, Uphi, correcting some confusion and errors in the literature., Comment: Accepted by the Astrophysical Journal on Dec 15, 2018

The details of how protoplanetary disks evolve from initially well-mixed distributions of gas and dust to systems composed mostly of rocky planets and gas giants like our own solar system is a fundamental question in astronomy. It is widely accepted that the first step in planet formation is dust grain growth and settling to the disk midplane. This dust evolution in disks can be studied in greater detail with far-infrared and submillimeter wavelength observations, which offer us unique access to the outer disk's deeper layers. Here we present Herschel far-infrared and submillimeter spectra of GM Aur taken with PACS and SPIRE. GM Aur is a transitional disk, whose inner disk hole is proposed to have been cleared by yet unseen planets. By utilizing Herschel data, we can potentially link the properties of dust evolution in the outer disk to dust clearing in the inner disk. In particular, preliminary SED modeling presented here suggests that GM Aur may have a lower gas-to-dust mass ratio than typically assumed for disks, which may be linked to disk clearing by planets. With further study, such Herschel data may provide insight for theoretical modeling of dust evolution and planet formation.

The point at which a protoplanetary disk becomes a debris disk is difficult to identify. To better understand this, here we study the $\sim$40~AU separation binary T~54 in the Chamaeleon I cloud. We derive a K5 spectral type for T~54~A (which dominates the emission of the system) and an age of $\sim$2~Myr. However, the dust disk properties of T~54 are consistent with those of debris disks seen around older and earlier-type stars. At the same time, T~54 has evidence of gas remaining in the disk as indicated by [Ne II], [Ne III], and [O I] line detections. We model the spectral energy distribution of T~54 and estimate that $\sim$3$\times$10$^{-3}$ Earth-masses of small dust grains ($$10~$\mu$m) grains within a circumprimary disk. Assuming a solar-like mixture, we use Ne line luminosities to place a minimum limit on the gas mass of the disk ($\sim$3$\times$10$^{-4}$ Earth-masses) and derive a gas-to-dust mass ratio of $\sim$0.1. We do not detect substantial accretion, but we do see H$\alpha$ in emission in one epoch, suggestive that there may be intermittent dumping of small amounts of matter onto the star. Considering the low dust mass, the presence of gas, and young age of T~54, we conclude that this system is on the bridge between the protoplanetary and debris disk stages., Comment: 15 pages, 9 figures, accepted to ApJ

We present the Coordinated Synoptic Investigation of NGC 2264, a continuous 30-day multi-wavelength photometric monitoring campaign on more than 1000 young cluster members using 16 telescopes. The unprecedented combination of multi-wavelength, high-precision, high-cadence, and long-duration data opens a new window into the time domain behavior of young stellar objects. Here we provide an overview of the observations, focusing on results from Spitzer and CoRoT. The highlight of this work is detailed analysis of 162 classical T Tauri stars for which we can probe optical and mid-infrared flux variations to 1% amplitudes and sub-hour timescales. We present a morphological variability census and then use metrics of periodicity, stochasticity, and symmetry to statistically separate the light curves into seven distinct classes, which we suggest represent different physical processes and geometric effects. We provide distributions of the characteristic timescales and amplitudes, and assess the fractional representation within each class. The largest category (>20%) are optical "dippers" having discrete fading events lasting ~1-5 days. The degree of correlation between the optical and infrared light curves is positive but weak; notably, the independently assigned optical and infrared morphology classes tend to be different for the same object. Assessment of flux variation behavior with respect to (circum)stellar properties reveals correlations of variability parameters with H$\alpha$ emission and with effective temperature. Overall, our results point to multiple origins of young star variability, including circumstellar obscuration events, hot spots on the star and/or disk, accretion bursts, and rapid structural changes in the inner disk., Comment: Published in AJ. 59 pages; 4 tables; 49 figures, most of which are highly degraded to fit size limits. Author name typo corrected. For a better resolution version, please visit http://web.ipac.caltech.edu/staff/amc/codyetal2014.pdf

Photoevaporation is probably the main agent for gas dispersal during the last stages of protoplanetary disk evolution. However, the overall mass loss rate in the photoevaporative wind and its driving mechanism are still not well understood. Here we report multi-configuration Very Large Array observations at 0.7, 3, and 5 cm towards the transitional disk of GM Aur. Our radio continuum observations allow us to image and spatially resolve, for the first time, the three main components at work in this stage of the disk evolution: the disk of dust, the ionized radio jet perpendicular to it, and the photoevaporative wind arising from the disk. The mass loss rate inferred from the flux density of the radio jet is consistent with the ratio between ejection and accretion rates found in younger objects, suggesting that transitional disks can power collimated ejections of material apparently following the same physical mechanisms as much younger protostars. Our results indicate that extreme-UV (EUV) radiation is the main ionizing mechanism of the photoevaporative wind traced by the free-free emission. The required low EUV photon luminosity of $\sim6\times10^{40}$ s$^{-1}$ would produce a photoevaporation rate of only $\dot{M}_{\rm w,EUV}\simeq1.3\times10^{-10}~M_{\odot}$ yr$^{-1}$. Therefore, other mechanisms are required to disperse the disk in the timescale imposed by observations., 20 pages, 3 figures, accepted for publication in ApJ

The conditions and evolution of protoplanetary disks in multiple systems can be considerably different from those around single stars, which may have important consequences for planet formation. We present Very Large Array (VLA) 8.8 mm (34 GHz) and 5 cm (6 GHz) observations of the quadruple system HD 98800, which consists of two spectroscopic binary systems (Aa-Ab, Ba-Bb). The Ba-Bb pair is surrounded by a circumbinary disk, usually assumed to be a debris disk given its $\sim$10 Myr age and lack of near infrared excess. The VLA 8.8 mm observations resolve the disk size (5-5.5 au) and its inner cavity ($\approx$3 au) for the first time, making it one of the smallest disks known. Its small size, large fractional luminosity, and millimeter spectral index consistent with blackbody emission support the idea that HD 98800 B is a massive, optically thick ring which may still retain significant amounts of gas. The disk detection at 5 cm is compatible with free-free emission from photoionized material. The diskless HD 98800 A component is also detected, showing partial polarization at 5 cm compatible with non-thermal chromospheric activity. We propose that tidal torques from Ba-Bb and A-B have stopped the viscous evolution of the inner and outer disk radii, and the disk is evolving via mass loss through photoevaporative winds. This scenario can explain the properties and longevity of HD 98800 B as well as the lack of a disk around HD 98800 A, suggesting that planet formation could have more time to proceed in multiple systems than around single stars in certain system configurations., Comment: 14 pages, 4 figures, 3 tables; Submitted to ApJ May 14 2018; Accepted to ApJ August 3 2018. This version fixes a mistake in the reported position angle. The order of the figures has been changed to match that of the references in the text

Chemistry plays an important role in the structure and evolution of protoplanetary disks, with implications for the composition of comets and planets. This is the first of a series of papers based on data from DISCS, a Submillimeter Array survey of the chemical composition of protoplanetary disks. The six Taurus sources in the program (DM Tau, AA Tau, LkCa 15, GM Aur, CQ Tau and MWC 480) range in stellar spectral type from M1 to A4 and offer an opportunity to test the effects of stellar luminosity on the disk chemistry. The disks were observed in 10 different lines at ~3" resolution and an rms of ~100 mJy beam-1 at ~0.5 km s-1. The four brightest lines are CO 2-1, HCO+ 3-2, CN 2_3-1_2 and HCN 3-2 and these are detected toward all sources (except for HCN toward CQ Tau). The weaker lines of CN 2_2-1_1, DCO+ 3-2, N2H+ 3-2, H2CO 3_03-2_02 and 4_14-3_13 are detected toward two to three disks each, and DCN 3-2 only toward LkCa 15. CH3OH 4_21-3_12 and c-C3H2 are not detected. There is no obvious difference between the T Tauri and Herbig Ae sources with regard to CN and HCN intensities. In contrast, DCO+, DCN, N2H+ and H2CO are detected only toward the T Tauri stars, suggesting that the disks around Herbig Ae stars lack cold regions for long enough timescales to allow for efficient deuterium chemistry, CO freeze-out, and grain chemistry.

Variations in molecular cloud environments have the potential to affect the composition and structure of the circumstellar disks therein. To this end, comparative analyses of nearby star-forming regions are essential to informing theoretical work. In particular, the Ophiuchus molecular clouds are ideal for comparison as they are more compact with much higher extinction than Taurus, the low-mass exemplar, and experience a moderate amount of external radiation. We have carried out a study of a collection of 136 young stellar objects in the

We present high angular resolution (down to 0.3" = 13 AU in diameter) Submillimeter Array observations of the 880 micron (340 GHz) thermal continuum emission from circumstellar dust disks in the nearby HD 98800 and Hen 3-600 multiple star systems. In both cases, the dust emission is resolved and localized around one stellar component - the HD 98800 B and Hen 3-600 A spectroscopic binaries - with no evidence for circum-system material. Using two-dimensional Monte Carlo radiative transfer calculations, we compare the SMA visibilities and broadband spectral energy distributions with truncated disk models to empirically locate the inner and outer edges of both disks. The HD 98800 B disk appears to be aligned with the spectroscopic binary orbit, is internally truncated at a radius of 3.5 AU, and extends to only 10-15 AU from the central stars. The Hen 3-600 A disk is slightly larger, with an inner edge at 1 AU and an outer radius of 15-25 AU. These inferred disk structures compare favorably with theoretical predictions of their truncation due to tidal interactions with the stellar companions., ApJ, in press

A few years ago, the mid-IR spectrum of a Weak Line T Tauri Star, CoKu Tau/4, was explained as emission from the inner wall of a circumstellar disk, with the inner disk truncated at ~10 AU. Based on the SED shape and the assumption that it was produced by a single star and its disk, CoKu Tau/4 was classified as a prototypical transitional disk, with a clean inner hole possibly carved out by a planet, some other orbiting body, or by photodissociation. However, recently it has been discovered that CoKu Tau/4 is a close binary system. This implies that the observed mid-IR SED is probably produced by the circumbinary disk. The aim of the present paper is to model the SED of CoKu Tau/4 as arising from the inner wall of a circumbinary disk, with parameters constrained by what is known about the central stars and by a dynamical model for the interaction between these stars and their surrounding disk. In order to fit the Spitzer IRS SED, the binary orbit should be almost circular, implying a small mid-IR variability (10%) related to the variable distances of the stars to the inner wall of the circumbinary disk. Our models suggest that the inner wall of CoKu Tau/4 is located at 1.7a, where a is the semi-major axis of the binary system (a~8AU). A small amount of optically thin dust in the hole (0, the model predicts mid-IR variability with periods similar to orbital timescales, assuming that thermal equilibrium is reached instantaneously., 42 pages, 15 Postscript figures

We analyze samples of Spitzer Infrared Spectrograph (IRS) spectra of T Tauri stars in the Ophiuchus, Taurus, and Chamaeleon I star-forming regions, whose median ages lie in the, 23 pages, 20 figures; accepted for publication in ApJ

We present 5-40 {mu}m Spitzer Infrared Spectrograph spectra of a collection of transitional disks, objects for which the spectral energy distribution (SED) indicates central clearings (holes) or gaps in the dust distribution, in the Chamaeleon I star-forming region. Like their counterparts in the Taurus-Auriga star-forming region that we have previously observed, the spectra of these young objects (1-3 Myr old) reveal that the central clearings or gaps are very sharp-edged, and are surrounded by optically thick dusty disks similar to those around other classical T Tauri stars in the Chamaeleon I association. Also like the Taurus transitional disks, the Chamaeleon I transitional disks have extremely large depletion factors for small dust grains in their gaps, compared to the full accretion disks whose SEDs are represented by the median SED of Class II objects in the region. We find that the fraction of transitional disks in the Chamaeleon I cloud is somewhat higher than that in the Taurus-Auriga cloud, possibly indicating that the frequency of transitional disks, on average, increases with cluster age. We also find a significant correlation between the stellar mass and the radius of the outer edge of the gap. We discuss the disk structures implied by themore » spectra and the constraints they place on gap-formation mechanisms in protoplanetary disks.« less

We present 0.3 arcsec resolution observations of the disk around GM Aurigae with the Submillimeter Array (SMA) at a wavelength of 860 um and with the Plateau de Bure Interferometer at a wavelength of 1.3 mm. These observations probe the distribution of disk material on spatial scales commensurate with the size of the inner hole predicted by models of the spectral energy distribution. The data clearly indicate a sharp decrease in millimeter optical depth at the disk center, consistent with a deficit of material at distances less than ~20 AU from the star. We refine the accretion disk model of Calvet et al. (2005) based on the unresolved spectral energy distribution (SED) and demonstrate that it reproduces well the spatially resolved millimeter continuum data at both available wavelengths. We also present complementary SMA observations of CO J=3-2 and J=2-1 emission from the disk at 2" resolution. The observed CO morphology is consistent with the continuum model prediction, with two significant deviations: (1) the emission displays a larger CO J=3-2/J=2-1 line ratio than predicted, which may indicate additional heating of gas in the upper disk layers; and (2) the position angle of the kinematic rotation pattern differs by 11 +/- 2 degrees from that measured at smaller scales from the dust continuum, which may indicate the presence of a warp. We note that photoevaporation, grain growth, and binarity are unlikely mechanisms for inducing the observed sharp decrease in opacity or surface density at the disk center. The inner hole plausibly results from the dynamical influence of a planet on the disk material. Warping induced by a planet could also potentially explain the difference in position angle between the continuum and CO data sets., Comment: 12 pages, 6 figures, accepted for publication in ApJ

We present the Spitzer Infrared Spectrograph (IRS) spectrum of SR20, a 5--10 AU binary T Tauri system in the $\rho$ Ophiuchi star forming region. The spectrum has features consistent with the presence of a disk; however, the continuum slope is steeper than the $\lambda^{-4/3}$ slope of an infinite geometrically thin, optically thick disk, indicating that the disk is outwardly truncated. Comparison with photometry from the literature shows a large increase in the mid-infrared flux from 1993 to 1996. We model the spectral energy distribution and IRS spectrum with a wall + optically thick irradiated disk, yielding an outer radius of 0.39$_{+0.03}^{-0.01}$ AU, much smaller than predicted by models of binary orbits. Using a two temperature $\chi^2$ minimization model to fit the dust composition of the IRS spectrum, we find the disk has experienced significant grain growth: its spectrum is well-fit using opacities of grains larger than 1 $\mu$m. We conclude that the system experienced a significant gravitational perturbation in the 1990s., Comment: 4 pages, 4 figures, 1 table, accepted to ApJ Letters

Using SpeX at the NASA Infrared Telescope Facility and the Spitzer Infrared Spectrograph, we have obtained infrared spectra from 0.7 to 40 μm for three young brown dwarfs in the TW Hydra association (τ ~ 10 Myr), 2MASSW J1207334–393254, 2MASSW J1139511–315921, and SSSPM J1102–3431. The spectral energy distribution for 2MASSW J1139511–315921 is consistent with a stellar photosphere for the entire wavelength range of our data, whereas the other two objects exhibit significant excess emission at λ > 5μm. We are able to reproduce the excess emission from each brown dwarf using our models of irradiated accretion disks. According to our model fits, both disks have experienced a high degree of dust settling. We also find that silicate emission at 10 and 20 μm is absent from the spectra of these disks, indicating that grains in the upper disk layers have grown to sizes larger than ~5 μm. Both of these characteristics are consistent with previous observations of decreasing silicate emission with lower stellar masses and older ages. These trends suggest that either (1) the growth of dust grains, and perhaps planetesimal formation, occurs faster in disks around brown dwarfs than in disks around stars or (2) the radii of the mid-IR-emitting regions of disks are smaller for brown dwarfs than for stars, and grains grow faster at smaller disk radii. Finally, we note the possible detection of an unexplained emission feature near 14 μm in the spectra of both of the disk-bearing brown dwarfs.

Transitional disks are protoplanetary disks with opacity gaps/cavities in their dust distribution, a feature that may be linked to planet formation. We perform Bayesian modeling of the three transitional disks SZ Cha, CS Cha and T25 including photometry from the Herschel Space Observatory to quantify the improvements added by these new data. We find disk dust masses between 2x10^-5 and 4x10^-4 Msun, and gap radii in the range of 7-18 AU, with uncertainties of ~ one order of magnitude and ~ 4 AU, respectively. Our results show that adding Herschel data can significantly improve these estimates with respect to mid-infrared data alone, which have roughly twice as large uncertainties on both disk mass and gap radius. We also find weak evidence for different density profiles with respect to full disks. These results open exciting new possibilities to study the distribution of disk masses for large samples of disks., Comment: 10 pages, 3 figures. Accepted for publication in MNRAS

The recently recognized class of "transitional disk" systems consists of young starswith optically-thick outer disks but inner disks which are mostly devoid of small dust. Here we introduce a further class of "pre-transitional disks" with significant near-infrared excesses which indicate the presence of an optically thick inner disk separated from an optically thick outer disk; thus, the spectral energy distributions of pre-transitional disks suggest the incipient development of disk gaps rather than inner holes. In UX Tau A, our analysis of the Spitzer IRS spectrum finds that the near-infrared excess is produced by an inner optically thick disk and a gap of ~56 AU is present. The Spitzer IRS spectrum of LkCa 15 is suggestive of a gap of ~46 AU, confirming previous millimeter imaging. In addition, UX Tau A contains crystalline silicates in its disk at radii >~ 56 AU which poses a challenge to our understanding of the production of this crystalline material. In contrast, LkCa 15's silicates are amorphous and pristine. UX Tau A and LkCa 15 increase our knowledge of the diversity of dust clearing in low-mass star formation., 4 pages, accepted ApJL

We present a time-variability study of young stellar objects in the cluster IRAS 20050+2720, performed at 3.6 and 4.5 micron with the Spitzer Space Telescope; this study is part of the Young Stellar Object VARiability project (YSOVAR). We have collected light curves for 181 cluster members over 40 days. We find a high variability fraction among embedded cluster members of ca. 70%, whereas young stars without a detectable disk display variability less often (in ca. 50% of the cases) and with lower amplitudes. We detect periodic variability for 33 sources with periods primarily in the range of 2-6 days. Practically all embedded periodic sources display additional variability on top of their periodicity. Furthermore, we analyze the slopes of the tracks that our sources span in the color-magnitude diagram (CMD). We find that sources with long variability time scales tend to display CMD slopes that are at least partially influenced by accretion processes, while sources with short variability time scales tend to display extinction-dominated slopes. We find a tentative trend of X-ray detected cluster members to vary on longer time scales than the X-ray undetected members., Accepted for publication in The Astronomical Journal. Data from Table 2 is given as a machine-readable file in the source of this arXiv submission