arXiv subjects

Paola D'Alessio

Publications and source records attributed to Paola D'Alessio.

At least 55 records · Page 3Linked to original sources

arXiv2005

Effects of Dust Growth and Settling in T Tauri Disks

We present self-consistent disk models for T Tauri stars which include a parameterized treatment of dust settling and grain growth, building on techniques developed in a series of papers by D'Alessio etal. The models incorporate depleted distributions of dust in upper disk layers along with larger-sized particles near the disk midplane, as expected theoretically and as we suggested earlier is necessary to account for mm-wave emission, SEDs, scattered light images, and silicate emission features simultaneously. By comparing the models with recent mid- and near-IR observations, we find that the dust to gas mass ratio of small grains at the upper layers should be < 10 % of the standard value. The grains that have disappeared from the upper layers increase the dust to gas mass ratio of the disk interior; if those grains grow to maximum sizes of the order of mm during the settling process, then both the millimeter-wave fluxes and spectral slopes can be consistently explained. Depletion and growth of grains can also enhance the ionization of upper layers, enhancing the possibility of the magnetorotational instability for driving disk accretion.

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arXiv2005

IRAC Observations of Taurus Pre-Main Sequence Stars

We present infrared photometry obtained with the IRAC camera on the Spitzer Space Telescope of a sample of 82 pre-main sequence stars and brown dwarfs in the Taurus star-forming region. We find a clear separation in some IRAC color-color diagrams between objects with and without disks. A few ``transition'' objects are noted, which correspond to systems in which the inner disk has been evacuated of small dust. Separating pure disk systems from objects with remnant protostellar envelopes is more difficult at IRAC wavelengths, especially for objects with infall at low rates and large angular momenta. Our results generally confirm the IRAC color classification scheme used in previous papers by Allen et al. and Megeath et al. to distinguish between protostars, T Tauri stars with disks, and young stars without (inner) disks. The observed IRAC colors are in good agreement with recent improved disk models, and in general accord with models for protostellar envelopes derived from analyzing a larger wavelength region. We also comment on a few Taurus objects of special interest. Our results should be useful for interpreting IRAC results in other, less well-studied star-forming regions.

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arXiv2005

Spitzer Identification of the Least Massive Known Brown Dwarf with a Circumstellar Disk

Using the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope, we have obtained mid-infrared photometry of the least massive known brown dwarf in the Chamaeleon I star-forming region. For this young brown dwarf, OTS 44, we have constructed a spectral energy distribution (SED) from 0.8-8 um by combining the measurements at 3.6, 4.5, 5.8, and 8.0 um from IRAC with ground-based photometry at I, J, H, and K. The resulting SED for OTS 44 exhibits significant excess emission longward of 3 um relative to the SED expected from the photosphere of the brown dwarf. We have successfully modeled the source of this excess emission in terms of an irradiated viscous accretion disk with M'<=10^-10 M_sun/year. With a spectral type of M9.5 and a mass of ~15 M_Jup, OTS 44 is now the coolest and least massive brown dwarf observed to have a circumstellar disk. These measurements demonstrate that disks exist around brown dwarfs even down to the deuterium burning mass limit and the approximate upper mass limit of extrasolar planetary companions.

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arXiv2005

IRAS 16293-2442B: A Compact, Possibly Isolated Protoplanetary Disk in a Class 0 Object

Theoretical arguments suggest that protoplanetary disks around young stars should start small and grow with the addition of high angular momentum material to reach the radii of several hundred AUs that characterize the disks around optically visible T Tauri stars. Examples of much more compact disks, with radii much less than 100 AU, have been found around some very young stars, but in all cases tidal truncation from a near binary companion provides a ready explanation for the small disk size. We report here an example of a compact, possibly isolated disk around the class 0 object IRAS16293-2422B, which is thought to be among the youngest protostars known. This disk has a Gaussian half power radius of only $\sim$8 AU, and a detailed, self-consistent, accretion disk model indicates an outer radius of only 26 AU. This discovery supports the notion that protoplanetary disks start small and grow with time, although other explanations for the compact size cannot be ruled out, including gravitational instability in its outer parts and tidal truncation from the close approach of a now distant stellar companion.

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arXiv2004

WWW Database of Models of Accretion Disks Irradiated by The Central Star

We announce the release of a catalog of physical models of irradiated accretion disks around young stars based on the modelling techniques by D'Alessio et al. The WWW catalog includes ~ 3000 disk models for different central stars, disk sizes, inclinations, dust contents and mass accretion rates. For any of them, radial profiles of disk physical parameters and synthetic spectral energy distributions can be browsed and downloaded to compare with observations. It can be accessed at http://www-cfa.harvard.edu/youngstars/dalessio/ (US), http://www.astrosmo.unam.mx/~dalessio/ (Mexico), and at http://www.laeff.esa.es/models/dalessio/ (Spain).

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arXiv2004

The Truncated Disk of CoKu Tau/4

We present a model of a dusty disk with an inner hole which accounts for the Spitzer Space Telescope Infrared Spectrograph observations of the low-mass pre-main sequence star CoKu Tau/4. We have modeled the mid-IR spectrum (between 8 and 25 mic) as arising from the inner wall of a disk. Our model disk has an evacuated inner zone of radius ~ 10 AU, with a dusty inner ``wall'', of half-height ~ 2 AU, that is illuminated at normal incidence by the central star. The radiative equilibrium temperature decreases from the inner disk edge outward through the optically-thick disk; this temperature gradient is responsible for the emission of the silicate bands at 10 and 20 mic. The observed spectrum is consistent with being produced by Fe-Mg amorphous glassy olivine and/or pyroxene, with no evidence of a crystalline component. The mid-infrared spectrum of CoKu Tau/4 is reminiscent of that of the much older star TW Hya, where it has been suggested that the significant clearing of its inner disk is due to planet formation. However, no inner disk remains in CoKu Tau/4, consistent with the star being a weak-emission (non-accreting) T Tauri star. The relative youth of CoKu Tau/4 (~ 1 Myr) may indicate much more rapid planet formation than typically assumed.

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arXiv2004

Disk evolution in the Ori OB1 association

We analyze multi-band photometry of a subsample of low mass stars in the associations Ori OB1a and 1b discovered during the CIDA Orion Variability Survey, which have ages of 7 - 10 Myr and 3 - 5 Myr, respectively. We obtained UBVRcIc photometry at Mt. Hopkins for 6 Classical T Tauri stars (CTTS) and 26 Weak T Tauri stars (WTTS) in Ori OB1a, and for 21 CTTS and 2 WTTS in Ori OB1b. We also obtained L band photometry for 14 CTTS at Mt. Hopkins, and 10um and 18um photometry with OSCIR at Gemini for 6 CTTS; of these, all 6 were detected at 10um while only one was detected at 18um. We estimate mass accretion rates from the excess luminosity at U, and find that they are consistent with determinations for a number of other associations, with or without high mass star formation. The observed decrease of mass accretion rate with age is qualitatively consistent with predictions of viscous evolution of accretion disks. We find an overall decrease of disk emission from Taurus to Ori OB1b to Ori OB1a. This decrease implies that significant grain growth and settling towards the midplane has taken place in the inner disks of Ori OB1. We compare the SED of the star detected at both 10um and 18um with disk models for similar stellar and accretion parameters. We find that the low <= 18 um fluxes of this Ori OB1b star cannot be due to the smaller disk radius expected from viscous evolution in the presence of the FUV radiation fields from the OB stars in the association. Instead, we find that the disk of this star is essentially a flat disk, with little if any flaring, indicating a a significant degree of dust settling towards the midplane, as expected from dust evolution in protoplanetary disks.

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arXiv2004

A New Probe of the Planet-Forming Region in T Tauri Disks

We present new observations of the FUV (1100-2200 Angstrom) radiation field and the near- to mid-IR (3--13.5 micron) spectral energy distribution (SED) of a sample of T Tauri stars selected on the basis of bright molecular disks (GM Aur, DM Tau, LkCa15). In each source we find evidence for Ly alpha induced H2 fluorescence and an additional source of FUV continuum emission below 1700 Angstroms. Comparison of the FUV spectra to a model of H2 excitation suggests that the strong continuum emission is due to electron impact excitation of H2. The ultimate source of this excitation is likely X-ray irradiation which creates hot photo-electrons mixed in the molecular layer. Analysis of the SED of each object finds the presence of inner disk gaps with sizes of a few AU in each of these young (~1 Myr) stellar systems. We propose that the presence of strong H2 continuum emission and inner disk clearing are related by the increased penetration power of high energy photons in gas rich regions with low grain opacity.

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arXiv2004

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 (Werner et al. 2004, Fazio et al. 2004). 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: S 140, S 171, NGC 7129, and Cep C. The IRAC color-color diagram ([3.6]-[4.5] vs. [5.8]-[8.0]) can be used to help to distinguish between young stars with only disk emission and protostars with circumstellar envelopes.

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arXiv2003

Molecular Line Emission from Accretion Disks Around YSOs

In this work we model the expected molecular emission from protoplanetary disks, modifying different physical parameters, such as dust grain size, mass accretion rate, viscosity, and disk radius, to obtain observational signatures in these sources. Having in mind possible future observations, we study correlations between physical parameters and observational characteristics. Our aim is to determine the kind of observations that will allow us to extract information about the physical parameters of disks. We also present prospects for molecular line observations of protoplanetary disks, using millimeter and submillimeter interferometers (e.g., SMA or ALMA), based on our results.

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arXiv2003

Unveiling the Inner Disk Structure of T Tauri Stars

We present near-infrared spectra of the excess continuum emission from the innermost regions of classical T Tauri disks. In almost all cases, the shape of the excess is consistent with that of a single-temperature blackbody with T ~ 1400 K, similar to the expected dust sublimation temperature for typical dust compositions. The amount of excess flux roughly correlates with the accretion luminosity in objects with similar stellar properties. We compare our observations with the predictions of simple disk models having an inner rim located at the dust sublimation radius, including irradiation heating of the dust from both the stellar and accretion luminosities. The models yield inner rim radii in the range 0.07-0.54 AU, increasing with higher stellar and accretion luminosities. Using typical parameters which fit our observed sample, we predict a rim radius ~ 0.2 AU for the T Tauri star DG Tau, which agrees with recent Keck near-infrared interferometric measurements. For large mass accretion rates, the inner rim lies beyond the corotation radius at (or within) which magnetospheric accretion flows are launched, which implies that pure gaseous disks must extend inside the dust rim. Thus, for a significant fraction of young stars, dust cannot exist in the innermost disk, calling into question theories in which solid particles are ejected by a wind originating at the magnetospheric radius.

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arXiv2003

Models of Accretion Disks around Young Stars

We discuss the importance of accretion in calculating disk models for young stellar objects. In particular, we show that a disk inner rim, irradiated by both the star and the accretion shocks at the stellar surface, can naturally explain recent observations of DG Tau with the Keck interferometer. We present models for two objects, with mass accretion rates differing by almost two orders of magnitude, to illustrate the effects of accretion on the overall disk structure and emission.

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arXiv2003

The Effects of UV Continuum and Lyman alpha Radiation on the Chemical Equilibrium of T Tauri Disks

We show in this Letter that the spectral details of the FUV radiation fields have a large impact on the chemistry of protoplanetary disks surrounding T Tauri stars. We show that the strength of a realistic stellar FUV field is significantly lower than typically assumed in chemical calculations and that the radiation field is dominated by strong line emission, most notably Lyman alpha radiation. The effects of the strong Lyman alpha emission on the chemical equilibrium in protoplanetary disks has previously been unrecognized. We discuss the impact of this radiation on molecular observations in the context of a radiative transfer model that includes both direct attenuation and scattering. In particular, Lyman alpha radiation will directly dissociate water vapor and may contribute to the observed enhancements of CN/HCN in disks.

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arXiv2002

Discovery of an Edge-On Disk in the MBM 12 Young Association

We report the discovery of a spatially-resolved edge-on protoplanetary disk in the ~2-Myr-old MBM 12 young association. Our near-infrared images of LkHa 263C (MBM 12A 3C), obtained with the Hokupa'a adaptive optics system on the Gemini North telescope, clearly show two elongated reflection nebulosities separated by a dark lane, a morphology well-matched by scattered light models of an optically thick (at near-infrared wavelengths) edge-on disk. An optical spectrum of the scattered light nebulosity obtained with the Keck II telescope exhibits a spectral type of M0 +/- 0.5 (T_eff = 3850 +/- 100 K) for the central star and contains H_alpha and forbidden emission lines, which may indicate the presence of a jet. The absence of a near-infrared point source implies A_K > 9.5 toward the unseen central star. The disk is flared and has a radius of ~150 AU (at a distance of 275 pc) and an inclination of 87 degrees. The aspect ratio of the model disk in the J-band is 0.72. There is possible evidence for dust settling to the disk midplane. LkHa 263C is 4.115" from the 0.415" binary LkHa 263 A and B (MBM 12A 3A and 3B), which is itself 15.5" from LkHa 262 (MBM 12A 2). Thus, LkHa 263C may be the first disk to be clearly resolved around an individual star in a young quadruple system. The detection of a faint edge-on disk near a bright star demonstrates both the high angular resolution and the high sensitivity that can be achieved with adaptive optics imaging on large telescopes.

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arXiv2002

Evidence for a developing gap in a 10 Myr old protoplanetary disk

We have developed a self-consistent model of the disk around the nearby 10 Myr old star TW Hya which matches the observed spectral energy distribution and 7mm images of the disk. The model requires a 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 IR wavelengths and quite massive ~0.06 Msun 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 order ~1cm to explain the sub-mm and mm 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 micron particles remaining to produce the observed 10 micron silicate emission. Our model requires a distinct transition in disk properties at ~4 AU, separating the inner and outer disk. 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-IR 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 by the continued accretion onto the central star (albeit at a much lower rate than typical of younger T Tauri stars). TW Hya thus may become the Rosetta stone for our understanding of the evolution and dissipation of protoplanetary disks.

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arXiv2001

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-consistentlyincluding 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 unit and thus the local irradiation heating, while increasing the disk emission at mm and sub-mm 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.

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arXiv1999

YSO disk structure and planetary signatures

Detailed radial/vertical structure modeling of observations of disks around Young Stellar Objects (YSOs) can provide information on the physical conditions and on the characteristics of the gas and dust in their interiors. We describe recent results of self-consistent modeling of spectral energy distributions, optical and infrared images, and millimeter fluxes of YSOs. We discuss observations and interpretations of the initial stages of planet formation, including indications of dust growth and settling in disks around young stars. We also discuss how the unprecedented resolution and sensitivity of ALMA may help us study the interior of the innermost disks, a region unaccessible with present day instrumentation, and witness the very first stages of planet formation.

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arXiv1999

Resolving Molecular Line Emission from Protoplanetary Disks: Observational Prospects for Disks Irradiated by Infalling Envelopes

Molecular line observations that could resolve protoplanetary disks of ~100 AU both spatially and kinematically would be a useful tool to unambiguously identify these disks and to determine their kinematical and physical characteristics. In this work we model the expected line emission from a protoplanetary disk irradiated by an infalling envelope, addressing the question of its detectability with subarcsecond resolution. We adopt a previously determined disk model structure that gives a continuum spectral energy distribution and a mm intensity spatial distribution that are consistent with observational constraints of HL Tau. An analysis of the capability of presently working and projected interferometers at mm and submm wavelengths shows that molecular transitions of moderate opacity at these wavelengths (e.g., C17O lines) are good candidates for detecting disk lines at subarcsecond resolution in the near future. We suggest that, in general, disks of typical Class I sources will be detectable.

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