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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 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

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

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

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).

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 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

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

Quasi-periodic oscillations and X-ray spectroscopy are powerful probes of black hole masses and accretion disks, and here we apply these diagnostics to an ultraluminous X-ray source (ULX) in the spiral galaxy NGC628 (M74). This object was observed four times over two years with the Chandra X-ray Observatory and XMM-Newton, with three long observations showing dramatic variability, distinguished by a series of outbursts with a quasi-period (QPO) of 4,000-7,000 seconds. This is unique behavior among both ULXs and Galactic X-ray binaries due to the combination of its burst-like peaks and deep troughs, its long quasi-periods, its high variation amplitudes of $>90$%, and its substantial variability between observations. The X-ray spectra is fitted by an absorbed accretion disk plus a power-law component, suggesting the ULX was in a spectral state analogous to the Low Hard state or the Very High state of Galactic black hole X-ray binaries. A black hole mass of $\sim2$--$20\times10^3 M_\odot$ is estimated from the $f_b$--$M_\bullet$ scaling relation found in the Galactic X-ray binaries and active galactic nuclei.

We analyze 3 epochs of ultraviolet (UV), optical and near-infrared (NIR) observations of the Taurus transitional disk GM Aur using the Hubble Space Telescope Imaging Spectrograph (STIS) and the Infrared Telescope Facility SpeX spectrograph. Observations were separated by one week and 3 months in order to study variability over multiple timescales. We calculate accretion rates for each epoch of observations using the STIS spectra and find that those separated by one week had similar accretion rates (~1E-8 solar masses/yr) while the epoch obtained 3 months later had a substantially lower accretion rate (~4E-9 solar masses/yr). We find that the decline in accretion rate is caused by lower densities of material in the accretion flows, as opposed to a lower surface coverage of the accretion columns. During the low accretion rate epoch we also observe lower fluxes at both far UV (FUV) and IR wavelengths, which trace molecular gas and dust in the disk, respectively. We find that this can be explained by a lower dust and gas mass in the inner disk. We attribute the observed variability to inhomogeneities in the inner disk, near the corotation radius, where gas and dust may co-exist near the footprints of the magnetospheric flows. These FUV--NIR data offer a new perspective on the structure of the inner disk, the stellar magnetosphere, and their interaction., Comment: 22 pages, 5 figures, Accepted to ApJ

We report photometry of the helium-dominated cataclysmic variable HP Librae during 1995-2001. The main photometric signal varies between 1118.89 and 1119.14 s, on a timescale of a few years, and displays a waveform characteristic of "superhumps." After subtracting the main signal, we found a weak residual signal at 1102.70±0.05 s, which we interpret as the underlying orbital period of the binary. The full amplitude of this putative orbital variation is just 0.005 mag, the weakest orbital signal yet found in a cataclysmic variable (CV). The 1119 s signal of HP Lib is a superb match to the well-studied 1051 s superhump of AM CVn, the "mother of all helium CVs." The superhump shows no change in amplitude or waveform on any timescale and no essential change in period on timescales shorter than ~3000 cycles. Such great stability makes the star a promising test case for detailed studies of the underlying spiral structure in the disk, the likely cause of superhumps. Comparison of orbital and superhump periods for the family of AM CVn stars supplies evidence that these stars are evolving toward longer orbital period.

Accretion onto Classical T Tauri stars is thought to take place through the action of magnetospheric processes, with gas in the inner disk being channeled onto the star's surface by the stellar magnetic field lines. Young stars are known to accrete material in a time-variable manner and the source of this variability remains an open problem, particularly on the shortest (~ day) timescales. Using one-dimensional time-dependent numerical simulations that follow the field line geometry, we find that for plausibly realistic young stars, steady-state transonic accretion occurs naturally in the absence of any other source of variability. However, we show that if the density in the inner disk varies smoothly in time with ~ day long time-scales (e.g., due to turbulence) this complication can lead to the development of shocks in the accretion column. These shocks propagate along the accretion column and ultimately hit the star, leading to rapid, large amplitude changes in the accretion rate. We argue that when these shocks hit the star the observed time-dependence will be a rapid increase in accretion luminosity followed by a slower decline and could be an explanation for some of the short period variability observed in accreting young stars. Our one-dimensional approach bridges previous analytic work to more complicated, multi-dimensional simulations, and observations., 16 Pages, 12 figures

In order to look for signs of on-going planet formation in young disks, we carried out the first J-band polarized emission imaging of the Herbig Ae/Be stars HD 150193, HD 163296, and HD 169142 using the Gemini Planet Imager (GPI), along with new H band observations of HD 144432. We confirm the complex "double ring" structure for the nearly face-on system HD 169142 first seen in H-band, finding the outer ring to be substantially redder than the inner one in polarized intensity. Using radiative transfer modeling, we developed a physical model that explains the full spectral energy distribution (SED) and J- and H-band surface brightness profiles, suggesting that the differential color of the two rings could come from reddened starlight traversing the inner wall and may not require differences in grain properties. In addition, we clearly detect an elongated, off-center ring in HD 163296 (MWC 275), locating the scattering surface to be 18 AU above the midplane at a radial distance of 77 AU, co-spatial with a ring seen at 1.3mm by ALMA linked to the CO snow line. Lastly, we report a weak tentative detection of scattered light for HD 150193 (MWC 863) and a non-detection for HD 144432; the stellar companion known for each of these targets has likely disrupted the material in the outer disk of the primary star. For HD 163296 and HD 169142, the prominent outer rings we detect could be evidence for giant planet formation in the outer disk or a manifestation of large-scale dust growth processes possibly related to snow-line chemistry., Accepted Astrophysical Journal

Transitional disks are objects whose inner disk regions have undergone substantial clearing. The Spitzer Space Telescope produced detailed spectral energy distributions (SEDs) of transitional disks that allowed us to infer their radial dust disk structure in some detail, revealing the diversity of this class of disks. The growing sample of transitional disks also opened up the possibility of demographic studies, which provided unique insights. There now exist (sub)millimeter and infrared images that confirm the presence of large clearings of dust in transitional disks. In addition, protoplanet candidates have been detected within some of these clearings. Transitional disks are thought to be a strong link to planet formation around young stars and are a key area to study if further progress is to be made on understanding the initial stages of planet formation. Here we provide a review and synthesis of transitional disk observations to date with the aim of providing timely direction to the field, which is about to undergo its next burst of growth as ALMA reaches its full potential. We discuss what we have learned about transitional disks from SEDs, color-color diagrams, and imaging in the (sub)mm and infrared. We then distill the observations into constraints for the main disk clearing mechanisms proposed to date (i.e., photoevaporation, grain growth, and companions) and explore how the expected observational signatures from these mechanisms, particularly planet-induced disk clearing, compare to actual observations. Lastly, we discuss future avenues of inquiry to be pursued with ALMA, JWST, and next generation of ground-based telescopes.

We present Herschel Space Observatory PACS spectra of T Tauri stars, in which we detect amorphous and crystalline water ice features. Using irradiated accretion disk models, we determine the disk structure and ice abundance in each of the systems. Combining a model-independent comparison of the ice feature strength and disk size with a detailed analysis of the model ice location, we estimate that the ice emitting region is at disk radii >30AU, consistent with a proto-Kuiper belt. Vertically, the ice emits most below the photodesorption zone, consistent with Herschel observations of cold water vapor. The presence of crystallized water ice at a disk location a) colder than its crystallization temperature and b) where it should have been re-amorphized in ~1 Myr suggests that localized generation is occurring; the most likely cause appears to be micrometeorite impact or planetesimal collisions. Based on simple tests with UV models and different ice distributions, we suggest that the SED shape from 20 to 50 micron may probe the location of the water ice snow line in the disk upper layers. This project represents one of the first extra-solar probes of the spatial structure of the cometary ice reservoir thought to deliver water to terrestrial planets., Accepted to ApJ on Nov. 20th 2014; 16 pages, including 11 figures and 4 tables

Using NASA IRTF SpeX data from 0.8 to 4.5 $\mu$m, we determine self-consistently the stellar properties and excess emission above the photosphere for a sample of classical T Tauri stars (CTTS) in the Taurus molecular cloud with varying degrees of accretion. This process uses a combination of techniques from the recent literature as well as observations of weak-line T Tauri stars (WTTS) to account for the differences in surface gravity and chromospheric activity between the TTS and dwarfs, which are typically used as photospheric templates for CTTS. Our improved veiling and extinction estimates for our targets allow us to extract flux-calibrated spectra of the excess in the near-infrared. We find that we are able to produce an acceptable parametric fit to the near-infrared excesses using a combination of up to three blackbodies. In half of our sample, two blackbodies at temperatures of 8000 K and 1600 K suffice. These temperatures and the corresponding solid angles are consistent with emission from the accretion shock on the stellar surface and the inner dust sublimation rim of the disk, respectively. In contrast, the other half requires three blackbodies at 8000, 1800, and 800 K, to describe the excess. We interpret the combined two cooler blackbodies as the dust sublimation wall with either a contribution from the disk surface beyond the wall or curvature of the wall itself, neither of which should have single-temperature blackbody emission. In these fits, we find no evidence of a contribution from optically thick gas inside the inner dust rim., Comment: Accepted to ApJ, 22 pages, 2 appendices, 12 figures, 7 tables

Transitional disks are protoplanetary disks around young stars, with inner holes or gaps which are surrounded by optically thick outer, and often inner, disks. Here we present observations of 62 new transitional disks in the Orion A star-forming region. These were identified using the \textit{Spitzer Space Telescope}'s Infrared Spectrograph and followed up with determinations of stellar and accretion parameters using the Infrared Telescope Facility's SpeX. We combine these new observations with our previous results on transitional disks in Taurus, Chamaeleon I, Ophiuchus and Perseus, and with archival X-ray observations. This produces a sample of 105 transitional disks of "cluster" age 3 Myr or less, by far the largest hitherto assembled. We use this sample to search for trends between the radial structure in the disks and many other system properties, in order to place constraints on the possible origins of transitional disks. We see a clear progression of host star accretion rate and the different disk morphologies. We confirm that transitional disks with complete central clearings have median accretion rates an order of magnitude smaller than radially continuous disks of the same population. Pre-transitional disks --- those objects with gaps that separate inner and outer disks --- have median accretion rates intermediate between the two. Our results from the search for statistically significant trends, especially related to $\dot{M}$, strongly support that in both cases the gaps are far more likely to be due to the gravitational influence of Jovian planets or brown dwarfs orbiting within the gaps, than to any of the photoevaporative, turbulent or grain-growth processes that can lead to disk dissipation. We also find that the fraction of Class II YSOs which are transitional disks is large, 0.1-0.2, especially in the youngest associations., Comment: 96 pages, 25 figures, resubmitted to ApJ

Context. Transitional discs are a class of circumstellar discs around young stars with extensive clearing of dusty material within their inner regions on 10s of au scales. One of the primary candidates for this kind of clearing is the formation of planet(s) within the disc that then accrete or clear their immediate area as they migrate through the disc. Aims. The goal of this survey was to search for asymmetries in the brightness distribution around a selection of transitional disc targets. We then aimed to determine whether these asymmetries trace dynamically-induced structures in the disc or the gap-opening planets themselves. Methods. Our sample included eight transitional discs. Using the Keck/NIRC2 instrument we utilised the Sparse Aperture Masking (SAM) interferometry technique to search for asymmetries indicative of ongoing planet formation. We searched for close-in companions using both model fitting and interferometric image reconstruction techniques. Using simulated data, we derived diagnostics that helped us to distinguish between point sources and extended asymmetric disc emission. In addition, we investigated the degeneracy between the contrast and separation that appear for marginally resolved companions. Results. We found FP Tau to contain a previously unseen disc wall, and DM Tau, LkH α 330, and TW Hya to contain an asymmetric signal indicative of point source-like emission. We placed upper limits on the contrast of a companion in RXJ 1842.9-3532 and V2246 Oph. We ruled the asymmetry signal in RXJ 1615.3-3255 and V2062 Oph to be false positives. In the cases where our data indicated a potential companion we computed estimates for the value of M c Ṁ c and found values in the range of 10 −5 −10 −3 M 2 J yr −1 . Conclusions. We found significant asymmetries in four targets. Of these, three were consistent with companions. We resolved a previously unseen gap in the disc of FP Tau extending inwards from approximately 10 au.

The dust cannot survive closer to the star from the point where a grain reaches a temperature equal to the sublimation temperature. The boundary between a dust-free and a dusty region defines the sublimation wall. In the literature two models for the structure of the wall are used: a wall with a fixed sublimation temperature and a wall with a density-dependent sublimation temperature. In the former, the wall is vertical and in the latter, the wall is curved. We find important differences between these models SEDs in the wavelength range from 3 to 8\mu m, being the emission of the former larger than that of the latter model. We quantify the differences in IRAC colors when these models are used to explain the observations. In the IRAC color-color diagram ([3.6]-[4.5] vs. [5.8]-[8.0]), the models are located in specific regions, either depending on the inclination, the mass accretion rate, or which model is used., Comment: 17 pages, 6 figures, accepted for publication in Revista Mexicana de Astronomia y Astrofisica

This is the second in a series of papers based on data from DISCS, a Submillimeter Array observing program aimed at spatially and spectrally resolving the chemical composition of 12 protoplanetary disks. We present data on six Southern sky sources - IM Lup, SAO 206462 (HD 135344b), HD 142527, AS 209, AS 205 and V4046 Sgr - which complement the six sources in the Taurus star forming region reported previously. CO 2-1 and HCO+ 3-2 emission are detected and resolved in all disks and show velocity patterns consistent with Keplerian rotation. Where detected, the emission from DCO+ 3-2, N2H+ 3-2, H2CO 3-2 and 4-3,HCN 3-2 and CN 2-1 are also generally spatially resolved. The detection rates are highest toward the M and K stars, while the F star SAO 206462 has only weak CN and HCN emission, and H2CO alone is detected toward HD 142527. These findings together with the statistics from the previous Taurus disks, support the hypothesis that high detection rates of many small molecules depend on the presence of a cold and protected disk midplane, which is less common around F and A stars compared to M and K stars. Disk-averaged variations in the proposed radiation tracer CN/HCN are found to be small, despite two orders of magnitude range of spectral types and accretion rates. In contrast, the resolved images suggest that the CN/HCN emission ratio varies with disk radius in at least two of the systems. There are no clear observational differences in the disk chemistry between the classical/full T Tauri disks and transitional disks. Furthermore, the observed line emission does not depend on measured accretion luminosities or the number of infrared lines detected, which suggests that the chemistry outside of 100 AU is not coupled to the physical processes that drive the chemistry in the innermost few AU., Comment: accepted for publication in ApJ, 41 pages including 7 figures

We report on the {\lambda} = 5-36{\mu}m Spitzer Infrared Spectrograph spectra of 79 young stellar objects in the very young nearby cluster NGC 1333. NGC 1333's youth enables the study of early protoplanetary disk properties, such as the degree of settling as well as the formation of gaps and clearings. We construct spectral energy distributions (SEDs) using our IRS data as well as published photometry and classify our sample into SED classes. Using "extinction-free" spectral indices, we determine whether the disk, envelope, or photosphere dominates the spectrum. We analyze the dereddened spectra of objects which show disk dominated emission using spectral indices and properties of silicate features in order to study the vertical and radial structure of protoplanetary disks in NGC 1333. At least nine objects in our sample of NGC 1333 show signs of large (several AU) radial gaps or clearings in their inner disk. Disks with radial gaps in NGC 1333 show more-nearly pristine silicate dust than their radially continuous counterparts. We compare properties of disks in NGC 1333 to those in three other well studied regions, Taurus-Auriga, Ophiuchus and Chamaeleon I, and find no difference in their degree of sedimentation and dust processing., Comment: 67 pages, 20 figures, accepted to The Astrophysical Journal Supplement Series

We present high resolution (0.3" = 16 AU), high signal-to-noise ratio Submillimeter Array observations of the 870 microns (345 GHz) continuum and CO J=3--2 line emission from the protoplanetary disk around TW Hya. Using continuum and line radiative transfer calculations, those data and the multiwavelength spectral energy distribution are analyzed together in the context of simple two-dimensional parametric disk structure models. Under the assumptions of a radially invariant dust population and (vertically integrated) gas-to-dust mass ratio, we are unable to simultaneously reproduce the CO and dust observations with model structures that employ either a single, distinct outer boundary or a smooth (exponential) taper at large radii. Instead, we find that the distribution of millimeter-sized dust grains in the TW Hya disk has a relatively sharp edge near 60 AU, contrary to the CO emission (and optical/infrared scattered light) that extends to a much larger radius of at least 215 AU. We discuss some possible explanations for the observed radial distribution of millimeter-sized dust grains and the apparent CO-dust size discrepancy, and suggest that they may be hallmarks of substructure in the dust disk or natural signatures of the growth and radial drift of solids that might be expected for disks around older pre-main sequence stars like TW Hya., Comment: ApJ, in press (fixed typo in Equation 4)

We use 2D hydrodynamic simulations of viscous disks to examine whether dynamically-interacting multiple giant planets can explain the large gaps (spanning over one order of magnitude in radius) inferred for the transitional and pre-transitional disks around T Tauri stars. In the absence of inner disk dust depletion, we find that it requires three to four giant planets to open up large enough gaps to be consistent with inferences from spectral energy distributions, because the gap width is limited by the tendency of the planets to be driven together into 2:1 resonances. With very strong tidal torques and/or rapid planetary accretion, fewer planets can also generate a large cavity interior to the locally formed gap(s) by preventing outer disk material from moving in. In these cases, however, the reduction of surface density produces a corresponding reduction in the inner disk accretion rate onto the star; this makes it difficult to explain the observed accretion rates of the pre/transitional disks. We find that even with four planets in disks, additional substantial dust depletion is required to explain observed disk gaps/holes. Substantial dust settling and growth, with consequent significant reductions in optical depths, is inferred for typical T Tauri disks in any case, and an earlier history of dust growth is consistent with the hypothesis that pre/transitional disks are explained by the presence of giant planets. We conclude that the depths and widths of gaps, and disk accretion rates in pre/transitional disks cannot be reproduced by a planet-induced gap opening scenario alone. Significant dust depletion is also required within the gaps/holes. Order of magnitude estimates suggest the mass of small dust particles (, Accepted for publication in ApJ, 8 pages, 9 figures, and 2 tables

The emission of FU Orionis objects in outburst has been identified as arising in rapidly accreting protoplanetary disks, based on a number of observational properties. A fundamental test of the accretion disk scenario is that the differentially rotating disk spectrum should produce a variation of rotational velocity with the wavelength of observation, as spectra taken at longer wavelengths probe outer, more slowly rotating disk regions. Previous observations of FU Ori have shown smaller rotation at near-infrared (~ 2.2 micron) wavelengths than observed at optical (~ 0.6 micron) wavelengths consistent with the assumption of Keplerian rotation. Here we report a spectrum from the Phoenix instrument on Gemini South which shows that differential (slower) rotation continues to be observed out to ~ 5 micron. The observed spectrum is well matched by the prediction of our accretion disk model previously constructed to match the observed spectral energy distribution and the differential rotation at wavelengths < 2.2 micron. This kinematic result allows us to confirm our previous inference of a large outer radius (~ 1 AU) for the rapidly accreting region of the FU Ori disk, which presents difficulties for outburst models relying purely on thermal instability. While some optical spectra have been interpreted to pose problems for the disk interpretation of FU Ori, we show that the adjustment of the maximum effective temperature of the disk model, proposed in a previous paper, greatly reduces these difficulties., Accepted by ApJL

Recently, analysis of near-infrared broad-band photometry and Spitzer IRS spectra has led to the identification of a new "pre-transitional disk" class whose members have an inner optically thick disk separated from an outer optically thick disk by an optically thin gap. This is in contrast to the "transitional disks" which have inner disk holes (i.e. large reductions of small dust from the star out to an outer optically thick wall). In LkCa 15, one of these proposed pre-transitional disks, detailed modeling showed that although the near-infrared fluxes could be understood in terms of optically thick material at the dust sublimation radius, an alternative model of emission from optically thin dust over a wide range of radii could explain the observations as well. To unveil the true nature of LkCa 15's inner disk we obtained a medium-resolution near-infrared spectrum spanning the wavelength range 2-5 microns using SpeX at the NASA Infrared Telescope Facility. We report that the excess near-infrared emission above the photosphere of LkCa 15 is a black-body continuum which can only be due to optically thick material in an inner disk around the star. When this confirmation of a primordial inner disk is combined with earlier observations of an inner edge to LkCa 15's outer disk it reveals a gapped structure. Forming planets emerge as the most likely mechanism for clearing the gap we detect in this evolving disk., Comment: accepted ApJL

We present high‐speed photometry of three AM CVn stars discovered in the last few years (ES Cet, ‘2003aw’ and SDSS J1240). ES Cet has a very stable photometric modulation with a single period of 620.2117 (± 0.0002) s (Espaillat et al. 2005) which is identical to its spectroscopic period. The 2‐sigma upper limit on the period derivative (Ṗ) of ES Cet is < 1.5 × 10−11, based on a 3‐year observational baseline. Since the discovery of ‘2003aw’ in a supernova search, this AM CVn star has been observed during two outbursts and has a superhump modulation of Psh = 2041.5 ± 0.3 s. At quiescence, 2003aw is fairly faint (V ∼ 20.4 mag) and no orbital period has yet been derived for this system. Two of the AM CVn stars (V803 Cen and CR Boo) show rapid oscillations similar to the (longer‐period) dwarf nova oscillations and quasi‐periodic oscillations seen in hydrogen‐rich high‐Ṁ CVs (Warner 2004).

The rapid localization of GRB 021004 by the HETE-2 satellite allowed nearly continuous monitoring of its early optical afterglow decay, as well as high-quality optical spectra that determined a redshift of z3=2.328 for its host galaxy, an active starburst galaxy with strong Lyman-alpha emission and several absorption lines. Spectral observations show multiple absorbers at z3A=2.323, z3B= 2.317, and z3C= 2.293 blueshifted by 450, 990, and 3,155 km/s respectively relative to the host galaxy Lyman-alpha emission. We argue that these correspond to a fragmented shell nebula that has been radiatively accelerated by the gamma-ray burst (GRB) afterglow at a distance greater than 0.3 pc from a Wolf-Rayet star progenitor. The chemical abundance ratios indicate that the nebula is overabundant in carbon and silicon. The high level of carbon and silicon is consistent with a swept-up shell nebula gradually enriched by a WCL progenitor wind over the lifetime of the nebula prior to the GRB onset. The detection of statistically significant fluctuations and color changes about the jet-like optical decay further supports this interpretation since fluctuations must be present at some level due to inhomogeneities in a clumpy stellar wind medium or if the progenitor has undergone massive ejection prior to the GRB onset. This evidence suggests that the mass-loss process in a Wolf-Rayet star might lead naturally to an iron-core collapse with sufficient angular momentum that could serve as a suitable GRB progenitor., Comment: Replaced with version accepted by ApJ; 40 pages, 9 figures

For Classical T Tauri Stars (CTTSs), the resonance lines of N V, Si IV, and C IV, as well as the He II 1640 A line, act as diagnostics of the accretion process. Here we assemble a large high-resolution dataset of these lines in CTTSs and Weak T Tauri Stars (WTTSs). We present data for 35 stars: one Herbig Ae star, 28 CTTSs, and 6 WTTSs. We decompose the C IV and He II lines into broad and narrow Gaussian components (BC & NC). The most common (50 %) C IV line morphology in CTTSs is that of a low-velocity NC together with a redshifted BC. The velocity centroids of the BCs and NCs are such that V_BC > 4 * V_NC, consistent with the predictions of the accretion shock model, in at most 12 out of 22 CTTSs. We do not find evidence of the post-shock becoming buried in the stellar photosphere due to the pressure of the accretion flow. The He II CTTSs lines are generally symmetric and narrow, less redshifted than the CTTSs C IV lines, by ~10 km/sec. The flux in the BC of the He II line is small compared to that of the C IV line, consistent with models of the pre-shock column emission. The observations are consistent with the presence of multiple accretion columns with different densities or with accretion models that predict a slow-moving, low-density region in the periphery of the accretion column. For HN Tau A and RW Aur A, most of the C IV line is blueshifted suggesting that the C IV emission is produced by shocks within outflow jets. In our sample, the Herbig Ae star DX Cha is the only object for which we find a P-Cygni profile in the C IV line, which argues for the presence of a hot (10^5 K) wind. For the overall sample, the Si IV and N V line luminosities are correlated with the C IV line luminosities, although the relationship between Si IV and C IV shows large scatter about a linear relationship and suggests that TW Hya, V4046 Sgr, AA Tau, DF Tau, GM Aur, and V1190 Sco are silicon-poor., Accepted in the ApJSS. 52 pages, 9 tables, 20 figures