1. Introduction

The formation and early evolution of stars are intimately coupled to the properties of their accompanying circumstellar disks of gas and dust. These disks also provide the material reservoirs for the assembly of planetary systems. Angular momentum conservation dictates that a collapsing molecular cloud core with some initial rotation will result in both a central protostar and a flattened circumstellar disk (e.g., Terebey, Shu, & Cassen 1984). Indirect observations indicate that disks are essentially ubiquitous in young star clusters, while optical images in silhouette (O’Dell &Wen 1994) and millimeter spectral line confirmations of Keplerian rotation (e.g., Simon, Dutrey, & Guilloteau 2001) provide more direct evidence in specific cases. Comparisons of infrared observations with physical models of young stellar objects (YSOs; here taken to mean a young star and its associated circumstellar material) have led to a sequence of evolutionary stages which occur before the start of the main-sequence (Lada & Wilking 1984; Adams & Shu 1986; Adams, Lada, & Shu 1987). In the Class I stage, an extended circumstellar envelope is rapidly dumping material onto a central protostar and a massive accretion disk. After the supply of envelope material is dissipated, the YSO becomes a Class II object, with a disk that is actively accreting material onto a central, optically visible star. In the final Class III stage, at least the inner part of the circumstellar disk has been evacuated, although the dominant physical mechanism for this process remains in debate (see Hollenbach, Yorke, & Johnstone 2000). The most interesting possibility, at least from a cosmogonical viewpoint, is that the gas and dust in the disk have agglomerated into larger objects in a developing planetary system.

Observations of the morphology of the broadband spectral energy distribution (SED) and various diagnostics of accretion can be used to trace the evolution of a YSO. Longward of <1 µm, the SED of a YSO is composed of a continuum of thermal spectra from the radially distributed circumstellar dust, modified by the radiative transfer properties of the grains. Changes in the SED through the evolutionary sequence are indicative of the loss of circumstellar components in the system; first the envelope and then the disk. The slope of the infrared SED is determined by the radial temperature distribution of the circumstellar dust (e.g., Adams, Lada, & Shu 1987; Beckwith et al. 1990). Therefore, measurements of infrared colors provide a relatively simple observational constraint on the temperature structure of a disk. However, more detailed physical interpretations of the infrared SED are challenging, due to the strong dependence on the relatively unknown radiative transfer properties of the grains and detailed disk structure (e.g., the inner disk radius or vertical scale height).

In the early evolution stages (Class I and II), material from the inner disk is dragged in magnetospheric funnel flows to the stellar surface, with an accretion shock resulting upon impact (see the review by Najita et al. 2000). This process is responsible for the observed continuum excesses (Calvet & Gullbring 1998; Johns-Krull & Valenti 2001; Muzerolle et al. 2003) and the shapes and strengths of emission lines in YSOs (Hartmann, Hewett, & Calvet 1994; Muzerolle, Hartmann, & Calvet 1998; Muzerolle, Calvet, & Hartmann 2001). The most common observational measurement providing a breakdown of objects as accreting or non-accreting is the equivalent width (W) of the H emission line. Although a standard division at W = 10Å was set by historical instrument limitations rather than a physical motivation, this criterion provides an effective discriminant as many properties of weak-line (WTTSs; W {tex}\leq{/tex} 10Å; non-accreting) and classical (CTTSs; W > 10 A; accreting) T Tauri stars are remarkably different (e.g., Ghez, Neugebauer, & Matthews 1993; Osterloh & Beckwith 1995; Chiang, Phillips, & Lonsdale 1996; Stelzer & Neuhäuser 2001).

Millimeter and submillimeter observations of circumstellar disks can provide unique information.  These observations probe the cool, outer parts of the disk, where giant planets are expected to form and contamination from the stellar photosphere is negligible. The low submillimeter opacities in disks can be used to extrapolate the surface density of the outer disk into the inner, optically thick regions and therefore determine the total disk mass (Beckwith et al. 1990). Assuming the submillimeter emission arises in an optically thin, isothermal portion of the disk, the flux density ({tex}F_v{/tex}) and disk mass ({tex}M_d{/tex}) are directly proportional (Hildebrand 1983):

{tex}M_d = \frac{d^2F_v}{K_vB_v(T_c)}{/tex},

where d is the distance, {tex}K_v{/tex} is the opacity, and {tex}B_v(T_c){/tex} is the Planck function at a characteristic temperature {tex}T_c{/tex}.  Moreover, observations and theoretical models of the opacity in the submillimeter indicate that {tex}K_v{/tex} is well-matched by a simple power-law in frequency with index {tex}\beta{/tex}, although the proposed normalizations vary significantly (Hildebrand 1983; Wright 1987; Pollack et al. 1994; Henning & Stognienko 1996). With the same optically thin, isothermal disk assumptions, the submillimeter continuum emission should behave roughly as {tex}F_v \propto v^{2+\beta}{/tex}.  So, with major caveats (see §3 and the Appendix), a single submillimeter flux density can give the mass of a disk and {tex}\geq{/tex} 2 flux points can reveal the frequency dependence of the opacity. Assuming a uniform grain composition and shape, the frequency behavior of the opacity is set by the size distribution of the grains in the disk. A number of single-dish surveys with single-element (or small arrays of) bolometers have been conducted in the Taurus-Auriga star-forming region to address these issues, most of which were carried out at 1.3mm (Weintraub, Sandell, & Duncan 1989; Beckwith et al. 1990; Adams, Emerson, & Fuller 1990; Beckwith & Sargent 1991; Mannings & Emerson 1994; Osterloh & Beckwith 1995; Motte & André 2001). Current instrumentation provides the opportunity for significantly more sensitive observations of disks in the submillimeter.

High resolution observations with (sub-)millimeter interferometers have confirmed that circumstellar dust disks are geometrically thin with radii on the order of 100AU (e.g., Dutrey et al. 1996; Kitamura et al. 2002). Detailed studies of individual disks reveal molecular gas in Keplerian rotation around the central star (e.g., Weintraub, Masson, & Zuckerman 1989; Koerner, Sargent, & Beckwith 1993a,b; Dutrey, Guilloteau, & Simon 1994; Koerner & Sargent 1995; Mannings & Sargent 1997; Duvert et al. 1998; Guilloteau & Dutrey 1998; Simon, Dutrey, & Guilloteau 2001; Corder, Eisner, & Sargent 2005). While molecular gas is the primary reservoir of mass in a disk, it is difficult to directly determine {tex}M_d{/tex} from the high resolution spectral line data because the brightest, easily detectable lines (i.e., the rotational transitions of CO) are optically thick (Beckwith & Sargent 1993; Dutrey et al. 1996) and likely to be severely depleted (Dutrey, Guilloteau, & Simon 1994, 2003). Interpretation of these lines and those from trace molecular species require sophisticated models of the disk structure (e.g., Dartois, Dutrey, & Guilloteau 2003; Kamp & Dullemond 2004) and chemistry (e.g., van Zadelho et al. 2001, 2003; Aikawa et al. 2002; Qi et al. 2003). Despite the tremendous amount of information provided by these observations, our knowledge is still limited to a relatively few disks on account of the large amount of time which must be invested in an interferometric observation.

Multiwavelength submillimeter data could prove useful in placing observational constraints on the dominant mechanism of planet formation. By comparing with infrared SEDs and diagnostics of accretion, we can investigate the dissipation of disks as a function of radius and see if there is consistency with the timescales expected from the collisional growth of planetesimals. The functional form of the opacity may provide information on the mean grain size distribution in the disk, and therefore evidence for the growth of grains demanded by the standard models of planet formation (e.g., Beckwith, Henning, & Nakagawa 2000).

In this paper, we present a large catalog of such data for most of the known YSOs in the Taurus-Auriga star-forming region. The survey is uniform, sensitive, and provides the most multiwavelength measurements of the submillimeter continuum spectra of YSOs to date. In §2 we discuss the observations and data reduction procedures. In §3 we present a simple disk model and use it to derive circumstellar disk masses, place some new observational constraints on the submillimeter opacity properties of disks, and examine relationships between the disk properties and those of the central stars. The results are discussed in §4, and our conclusions are summarized in §5. A brief Appendix is included with a more in-depth discussion of the disk models we employ and comments on some particularly interesting sources.