Paola D’Alessio

Evidence for a Developing Gap in a 10 Myr Old Protoplanetary Disk

We have developed a physically self-consistent model of the disk around the nearby 10 Myr old star TW Hya that matches the observed spectral energy distribution and 7 mm images of the disk. The model requires both significant dust-size evolution and a partially evacuated inner disk region, as predicted by theories of planet formation. The outer disk, which extends to at least 140 AU in radius, is very optically thick at infrared wavelengths and quite massive (~0.06 M☉) for the relatively advanced age of this T Tauri star. This implies long viscous and dust evolution timescales, although dust must have grown to sizes of the order of ~1 cm to explain the submillimeter and millimeter spectral slopes. In contrast, the negligible near-infrared excess emission of this system requires that the disk be optically thin inside 4 AU. This inner region cannot be completely evacuated; we need ~0.5 lunar mass of ~1 μm particles remaining to produce the observed 10 μm silicate emission. Our model requires a distinct transition in disk properties at ~4 AU separating the inner and outer disks. The inner edge of the optically thick outer disk must be heated almost frontally by the star to account for the excess flux at mid-infrared wavelengths. We speculate that this truncation of the outer disk may be the signpost of a developing gap due to the effects of a growing protoplanet; the gap is still presumably evolving because material still resides in it, as indicated by the silicate emission, the molecular hydrogen emission, and the continued accretion onto the central star (albeit at a much lower rate than typical of younger T Tauri stars). Thus, TW Hya may become the Rosetta stone for our understanding of the evolution and dissipation of protoplanetary disks.

Accretion Disks around Young Objects. III. Grain Growth

We present detailed models of irradiated T Tauri disks including dust grain growth with power-law size distributions. The models assume complete mixing between dust and gas and solve for the vertical disk structure self-consistently including the heating effects of stellar irradiation as well as local viscous heating. For a given total dust mass, grain growth is found to decrease the vertical height of the surface where the optical depth to the stellar radiation becomes unity and thus the local irradiation heating, while increasing the disk emission at mm and submillimeter wavelengths. The resulting disk models are less geometrically thick than our previous models assuming interstellar medium dust, and agree better with observed spectral energy distributions and images of edge-on disks, like HK Tau/c and HH 30. The implications of models with grain growth for determining disk masses from long-wavelength emission are considered.

Accretion Disks Around Young Objects. II. Tests of Well-Mixed Models with Ism Dust

We construct detailed vertical structure models of irradiated accretion disks around T Tauri stars with interstellar medium dust uniformly mixed with gas. The dependence of the structure and emission properties on mass accretion rate, viscosity parameter, and disk radius is explored using these models. The theoretical spectral energy distributions (SEDs) and images for all inclinations are compared with observations of the entire population of classical T Tauri stars (CTTSs) and class I objects in Taurus. In particular, we find that the median near-infrared fluxes can be explained within the errors with the most recent values for the median accretion rates for CTTSs. We further show that the majority of the class I sources in Taurus cannot be class II sources viewed edge-on because they are too luminous and their colors would be consistent with disks seen only in a narrow range of inclinations. Our models appear to be too geometrically thick at large radii, as suggested by (1) larger far-infrared disk emission than in the typical SEDs of T Tauri stars, (2) wider dark dust lanes in the model images than in the images of HH 30 and HK Tau/c, and (3) a larger predicted number of stars extincted by edge-on disks than consistent with current surveys. The large thickness of the model is a consequence of the assumption that dust and gas are well mixed, suggesting that some degree of dust settling may be required to explain the observations.

Infrared Array Camera (IRAC) Colors of Young Stellar Objects

We compare the infrared colors predicted by theoretical models of protostellar envelopes and protoplanetary disks with initial observations of young stellar objects made with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. Disk and envelope models characterized by infall and/or accretion rates found in previous studies can quantitatively account for the range of IRAC colors found in four young embedded clusters: S140, S171, NGC 7129, and Cep C. The IRAC color-color diagram ([3.6]� [4.5] vs. [5.8]� [8.0]) can be used to help distinguish between young stars with only disk emission and protostars with circumstellar envelopes. Subject heading gs: infrared: stars — stars: formation — stars: pre–main-sequence

Disk Evolution in Cep OB2: Results from the Spitzer Space Telescope

We present the results of an infrared imaging survey of two clusters in the Cep OB2 Association, Tr 37 and NGC 7160, using the IRAC and MIPS instruments on board the Spitzer Space Telescope. Our observations cover the wavelengthrangefrom3.6to24 � m,allowingustodetectdiskemissionoveratypicalrangeofradii � 0.1to � 20AU from the central star. In Tr 37, with an age of about 4 Myr, about 48% of the low-mass stars exhibit detectable disk emission in the IRAC bands. Roughly 10% of the stars with disks may be ‘‘transition’’ objects, with essentially photospheric fluxes at wavelengths � 4.5 � m but with excesses at longer wavelengths, indicating an optically thin inner disk. The median optically thick disk emission in Tr 37 is lower than the corresponding median for stars in the youngerTaurusregion;thedecreaseininfraredexcessislargerat6–8 � mthanat24 � m,suggestingthatgraingrowth and/or dust settling has proceeded faster at smaller disk radii, as expected on general theoretical grounds. Only about 4% of the low-mass stars in the 10 Myr old cluster NGC 7160 show detectable infrared disk emission. We also find evidence for 24 � m excesses around a few intermediate-mass stars, which may represent so-called ‘‘debris disk’’ systems. Our observations provide new constraints on disk evolution through an important age range. Subject headingg accretion, accretion disks — planetary systems: protoplanetary disks — stars: pre–main-sequence

Magnetospheres and Disk Accretion in Herbig Ae/Be Stars

We present evidence of magnetically mediated disk accretion in Herbig Ae/Be stars. Magnetospheric accretion models of Balmer and sodium profiles calculated with appropriate stellar and rotational parameters are in qualitative agreement with the observed profiles of the Herbig Ae star UX Ori and yield a mass accretion rate of ~10-8?M??yr-1. If more recent indications of an extremely large rotation rate for this object are correct, the magnetic field geometry must deviate from that of a standard dipole in order to produce line emission consistent with observed flux levels. Models of the associated accretion shock qualitatively explain the observed distribution of excess fluxes in the Balmer discontinuity for a large ensemble of Herbig Ae/Be stars and imply typically small mass accretion rates, 10-7?M??yr-1. In order for accretion to proceed onto the star, significant amounts of gas must exist inside the dust destruction radius, which is potentially problematic for recently advocated scenarios of puffed inner dust wall geometries. However, our models of the inner gas disk show that for the typical accretion rates we have derived, the gas should generally be optically thin, thus allowing direct stellar irradiation of the inner dust edge of the disk.

A ∼ 0.2-solar-mass protostar with a Keplerian disk in the very young L1527 IRS system

In their earliest stages, protostars accrete mass from their surrounding envelopes through circumstellar disks. Until now, the smallest observed protostar-to-envelope mass ratio was about 2.1 (ref. 1). The protostar L1527 IRS is thought to be in the earliest stages of star formation. Its envelope contains about one solar mass of material within a radius of about 0.05 parsecs (refs 3, 4), and earlier observations suggested the presence of an edge-on disk. Here we report observations of dust continuum emission and 13CO (rotational quantum number J = 2 → 1) line emission from the disk around L1527 IRS, from which we determine a protostellar mass of 0.19 ± 0.04 solar masses and a protostar-to-envelope mass ratio of about 0.2. We conclude that most of the luminosity is generated through the accretion process, with an accretion rate of about 6.6 × 10−7 solar masses per year. If it has been accreting at that rate through much of its life, its age is approximately 300,000 years, although theory suggests larger accretion rates earlier, so it may be younger. The presence of a rotationally supported disk is confirmed, and significantly more mass may be added to its planet-forming region as well as to the protostar itself in the future.

Imaging of the CO Snow Line in a Solar Nebula Analog

Solar Snow Lines Models of the formation of our solar system have suggested that condensation lines, or snow lines—the distance from a star beyond which a gas or a liquid can condense into the solid phase—are favorable locations for planet formation. Taking advantage of the increase of N2H+ abundance in cold regions where CO condenses out of the gas phase, Qi et al. (p. 630, published online 18 July) used the Atacama Large Millimeter/Submillimeter Array to image the CO snow line in the disk around TW Hya, an analog of the solar nebula from which the solar system formed. This disks snow line corresponds to Neptunes orbit in our solar system. Millimeter-wavelength observations locate the carbon monoxide condensation line within the disk around a young planet-forming star. Planets form in the disks around young stars. Their formation efficiency and composition are intimately linked to the protoplanetary disk locations of “snow lines” of abundant volatiles. We present chemical imaging of the carbon monoxide (CO) snow line in the disk around TW Hya, an analog of the solar nebula, using high spatial and spectral resolution Atacama Large Millimeter/Submillimeter Array observations of diazenylium (N2H+), a reactive ion present in large abundance only where CO is frozen out. The N2H+ emission is distributed in a large ring, with an inner radius that matches CO snow line model predictions. The extracted CO snow line radius of ∼30 astronomical units helps to assess models of the formation dynamics of the solar system, when combined with measurements of the bulk composition of planets and comets.

THE THERMAL REGULATION OF GRAVITATIONAL INSTABILITIES IN PROTOPLANETARY DISKS. III. SIMULATIONS WITH RADIATIVE COOLING AND REALISTIC OPACITIES

This paper presents a fully three-dimensional radiative hydrodymanics simulation with realistic opacities for a gravitationally unstable 0.07 Mdisk around a 0.5 Mstar. We address the following aspects of disk evolution: the strengthofgravitationalinstabilitiesunderrealisticcooling,masstransportinthediskthatarisesfromGIs,comparisons betweenthegravitationalandReynoldsstressesmeasuredinthediskandthoseexpectedinan � -disk,andcomparisons between the SED derived for the disk and SEDs derived from observationally determined parameters. The mass transport in this disk is dominated by global modes, and the cooling times are too long to permit fragmentation for all radii. Moreover, our results suggest a plausible explanation for the FU Ori outburst phenomenon. Subject headingg accretion, accretion disks — convection — hydrodynamics — instabilities — solar system: formation

Crystalline silicates and dust processing in the protoplanetary disks of the taurus young cluster

We characterize the crystalline-silicate content and spatial distribution of small dust grains in a large sample of protoplanetary disks in the Taurus-Auriga young cluster, using the Spitzer Space Telescope mid-IR spectra. In turn we use the results to analyze the evolution of structure and composition of these 1-2 Myr old disks around Solar- and later-type young stars, and test the standard models of dust processing which result in the conversion of originally amorphous dust into minerals. We find strong evidence of evolution of the dust-crystalline mass fraction in parallel with that of the structure of the disks, in the sense that increasing crystalline mass fraction is strongly linked to dust settling to the disk midplane. We also confirm that the crystalline silicates are confined to small radii, r 10 AU. However, we see no significant correlation of crystalline mass fraction with stellar mass or luminosity, stellar-accretion rate, disk mass, or disk/star mass ratio, as would be expected in the standard models of dust processing based upon photoevaporation and condensation close to the central star, accretion-heating-driven annealing at r 1 AU, or spiral-shock heating at r 10 AU, with or without effective large-scale radial mixing mechanisms. Either another grain-crystallizing mechanism dominates over these, or another process must be at work within the disks to erase the correlations they produce. We propose one of each sort that seems to be worth further investigation, namely X-ray heating and annealing of dust grains, and modulation of disk structure by giant-planetary formation and migration.