5. Summary

We have conducted a sensitive, multiwavelength submillimeter survey of 153 Taurus-Auriga YSOs in an effort to analyze properties of the outer regions of circumstellar dust disks. Some of the key results from this survey are summarized here:

• The disk mass (or submillimeter flux density) distribution function is well matched with a log-normal distribution centered around {tex}5\times10^{-3}M_\odot{/tex} with a large dispersion (0.5 dex). The vast majority of disks in Taurus-Auriga have substantially lower masses than is thought to be required for giant planet formation. However, a significant fraction of the disk mass could be stored in large grains or planetesimals which do not contribute to the submillimeter emission.

• We provide the largest set of submillimeter continuum slope measurements of YSOs to date. The empirical behavior of the continuum from 350 µm to 1.3mm is well-described by {tex}F_v\propto v^{2.0\pm 0.5}{/tex}, which is much flatter than for the interstellar medium. The low observed slope values are probably due to a combination of optical depth effects (Beckwith & Sargent 1991) and an inherently shallow opacity function from the top-heavy grain size distribution produced by collisional agglomeration of material in the disk.

• There do not appear to be any links between stellar and disk properties in the sample, although the stellar masses and ages span a relatively limited range. The median disk to star mass ratio is ~0.5%.

• Submillimeter flux densities and disk masses are statistically lower for stars with close companions (projected semimajor axes less than ~100AU) than for wider binaries or single stars. However, multiple star systems often still contain disks, regardless of their projected separations. Multiple star systems with wider separations have flux densities and disk masses comparable to single stars.

• In general, the standard signatures of the inner disk (e.g., accretion diagnostics or infrared excess emission) are accurate predictors of a disk mass greater than {tex}\sim10^{-4}M_\odot{/tex}.

• Statistically significant changes in the distribution functions of submillimeter flux densities, disk masses, and continuum slopes are found for the progressive stages of YSO evolution inferred from inner disk observations. These measured outer disk properties decrease from Class I → II → III objects as well as for CTTSs → WTTSs. The implication is that the inner and outer disk develop along a similar evolutionary sequence. A multiwavelength interferometric survey of Class I objects would be very useful for determining the relative contributions of a disk and inner envelope in these systems for a more sophisticated comparison with their presumably more evolved counterparts (Class II and III objects).

• Only a small fraction of objects (< 10%) which have no inner disk signatures were detected in the submillimeter, suggesting that both infrared and submillimeter disk emission disappear on a similar timescale (within ~10⁵ years of each other). There are two timescales in operation for disk evolution: (1) the relatively long (~5 to 10Myr) lifetime of Class II/CTTS disks, and (2) the rapid (a few × 10⁵ years) transition period to Class III/WTTS disks. Understanding the mechanisms responsible for these timescales, particularly the trigger for the transition stage, remain key problems in disk evolution. Some possible explanations for the essentially radially independent disk dissipation timescale include viscous accretion with photoevaporation by the central star (e.g., Clarke, Gendrin, & Sotomayor 2001) or rapid grain growth in the early stages of planet formation (e.g., Weidenschilling & Cuzzi 1993).

We acknowledge useful conversations, suggestions, and advice from Michael Liu, Lee Hartmann, Ted Simon, and Alan Boss. We would like to thank the JCMT and CSO support staffs, and in particular Colin Borys, for their assistance. An anonymous referee provided valuable criticism which helped improve this paper. This work was supported by NSF grant AST-0324328. This research has made use of the JCMT archive at the Canadian Astronomy Data Center, which is operated by the Dominion Astrophysical Observatory for the National Research Council of Canada’s Herzberg Institute of Astrophysics and the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.