CIRCUMSTELLAR DUST DISKS IN TAURUS-AURIGA: THE SUBMILLIMETER PERSPECTIVESean M. Andrews and Jonathan P. WilliamsInstitute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822; [email protected], [email protected] 2005 April 11; accepted 2005 June 8ABSTRACTWe present a sensitive, multiwavelength submillimeter continuum survey of 153 young stellar objects in theTaurus-Auriga star formation region. The submillimeter detection rate is 61% to a completeness limit of 10 mJy(3 )at850m. The inferred circumstellar disk masses are lognormally distributed with a mean mass of 5 ; 103M and a large dispersion (0.5 dex). Roughly one-third of the submillimeter sources have disk masses larger than the mini-mal nebula from which the sola r system formed. The median disk-to-star mass ratio is 0.5%. The empirical behaviorof the submillimeter continuum is best described as F / 2:00:5between 350 m and 1.3 mm, which we argue isdue to the combined effects of the fraction of optically thick emission and a flatter frequency behavior of the opacitycompared to the interstellar medium. The latter effect could be due to a substantial population of large d ust grains,which presumably wo uld have grown through collisional agglomeration. In this sample, the only stellar propertythat is correlated with the outer disk is the presence of a compa nion. We find evidence f or significant decreases insubmillimeter flux densities, disk masses, and submillimeter continuum slopes along the canonical infrared spectralenergy distribution evolution sequence for young stellar objects. The fraction of objects detected in the submilli-meter is essentially identical t o the fraction with excess near-infrared emission , su gges ting th at dust in the inner andouter disks is removed nearly simultaneously.Subject headinggs: circumstellar ma tter — planetary systems: protoplanetary disks — solar system: formation —stars: pre–main-sequenceOnline material: machine-readable table1. INTRODUCTIONThe formation and early evolution of stars are intimatelycoupled to the properties of their ac compa nying circumstellardisks of gas and dust. These disks also provide the material reser-voirs for the assembly of planetary systems. Angular momentumconservation dictates that a collapsing molecular cloud core withsome initial rotation will result in both a central p rotostar and aflattened circumstellar disk (e.g., Terebey et al. 1984). Indirect ob-servations indicate that disks are essentially ubiquitous in youngstar clusters, while optical images in silhouette (O’Dell & Wen1994) and millimeter spectral line confirmations of Keplerianrotation (e. g., Simon et al. 2000) provide mo re direct evidencein specific cases. Comparisons of infrared observations with phys-ical models of young stellar objects (YSOs; here taken to mean ayoung star and its associated circumstellar material) h ave led toa seque nce of evolutionary stage s that occur before the start ofthe main sequence (Lada & Wilking 1984; Adams & Shu 1986;Adams et al. 1987). In the Class I stage, an ex tended circum-stellar env elope is rapidly dumpin g material o nto a c entral pro-tostar and a massive accretion disk. After the supply of envelopematerial is dissipated, the YSO becomes a Class II object, with adisk that is actively accreting material onto a central, opticallyvisible star. In the final Class III stage, at least the inner part ofthe circumstellar disk has been evacuated, although the domi-nant physical mechanism for this pr ocess remain s in debate (s eeHollenbach et al. 2000). The most interesting possibility, atleast from a cosmogonical viewpoint, is that the gas and dust inthe disk have agglomerated into larger objects in a developingplanetary system.Observations of the morphology of the b roadb and spectralenergy distribution (SED) and various diagnostics of accretioncan be used to trace the evolution of a YSO. Longward of 1 m,the SED of a YSO is composed of a continuum of the rmalspectra from the radially distributed circumstellar dust, modi-fied by the radiative transfer properties of the grains. Changes inthe SED th rough the evolutionary seque nce are indicative of theloss of circumstellar components in the system: first the enve-lope and then the dis k. The slope of t he infrared SED is deter-mined by the radial temperature distribution of the circumstellardust (e.g., Ada ms et al. 1987; Beckwith et al. 1990). Therefo re,measurements of infrare d colors provide a relative ly simple ob-servational c onstrai nt on the temperature structure of a disk.However, more detailed physical interpretations of the infraredSED are challenging, due to the strong depend ence o n the rel-atively u nknown radiative transfe r properties of the grains anddetailed disk structure (e.g., the inner disk radius or vertical scaleheight).In the early evolution stages (Class I and II), material fromthe inner disk is dragged in magnetosph eric funn el flows tothe stellar surface, with a n accretion shock resulting on impact(see the review by Najita et al. 2000). This process is respon-sible for the obser ved continuum excesses (Calvet & Gullbring1998; Johns-Krull & Valenti 2001; Muzerolle et al. 2003) andthe shapes an d s trengths of emission lines in YSOs (Hartmannet al.1994; Muzerol le et al. 1998, 2001). The most common ob-servational measurement providing a breakdo wn of objects asaccreting or n onaccreting is the equ ivalen t width (W )oftheH emission line. Although a standard division at W ¼ 10 8was set by historical instrument limitations rather than a phys-ical motivation, this criterion provides an effective discrimi-nant as many properties of weak-line T Taur i stars (WTTSs;W 10 8; nonaccreting) and classica l T Tauri stars (CTTSs;W > 10 8; a ccreting) are remarkably different (e.g., Ghez et al.1993; Osterloh & Beckwith 1995 ; Chiang e t al. 1 996; Ste lzer& Neuha¨user 2001).Millimeter and submillimeter observations of circumstellardisks can provide unique information. These observations probeA1134The Astrophysical Journal, 631:1134–1160, 2005 October 1# 2005. The American Astronomical Society. All rights reserved. Printed in U.S.A.the cool, outer parts of the disk, where giant planets are expectedto form and contamination from th e stellar photosphere is neg-ligible. The low submillimeter opacities in disks can be used toextrapolate the surface density of the outer disk into the inner,optically
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