A Class 0 Protostars and Other YSO Stages
1. InfraredYSOClasses. Inthenear/mid-infrared,threebroadclasses of YSOs can be distinguished based on the slopes of their SEDs between 2.2 ¡m and 10-25 ¡m, aIR = d log(AFA)/d log(A), which are interpreted in terms of an evolutionary sequence (Lada and Wilking 1984; Lada 1987). Going backward in time, Class III (aIR < -1.5) and Class II (-1.5 < aIR < 0) sources correspond to pre-main-sequence (PMS) stars ("weak" and "classical" T Tauri stars, respectively), surrounded by a circumstel-lar disk (optically thin in Class III and optically thick in Class II at As 10 ¡m), but lacking a dense circumstellar envelope (see Andre and Mont-merle 1994). The youngest YSOs detected at 2 ¡m are the Class I sources, which are characterized by aIR > 0 (e.g., Wilking et al. 1989) and the close association with dense molecular gas (e.g., Myers et al. 1987). Class I objects are now interpreted as relatively evolved protostars with typical ages ~1-2 X 105 yr (e.g., Barsony and Kenyon 1992; Greene et al. 1994; Kenyon and Hartmann 1995), surrounded by both a disk and a diffuse circumstellar envelope of substellar (S0.1-0.3 M©) mass (Whitney and Hartmann 1993; Kenyon et al. 1993b; Andre and Montmerle 1994; Lucas and Roche 1997). Their SEDs are successfully modeled in the framework of the "standard" theory of isolated protostars (e.g., Adams et al. 1987; Kenyon et al. 1993a), in agreement with the idea that they derive a substantial fraction of their luminosity from accretion (see also Greene and Lada 1996 and Kenyon et al. 1998).
2. Class 0 Protostars. Several condensations detected in submillimeter dust continuum maps of molecular clouds (such as those described in section II.E) appear to be associated with formed, hydrostatic YSOs and have been designated "Class 0" protostars (Andre et al. 1993). Specifically, Class 0 objects are defined by the following observational properties (Andre et al. 1993):
(i) Indirect evidence for a central YSO, as indicated by, e.g., the detection of a compact centimeter radio continuum source, a collimated CO outflow, or an internal heating source.
(ii) Centrally peaked but extended submillimeter continuum emission tracing the presence of a spheroidal circumstellar dust envelope (as opposed to just a disk).
(iii) High ratio of submillimeter to bolometric luminosity, suggesting that the envelope mass exceeds the central stellar mass: Lsmm/Lbol > 0.5%, where Lsmm is measured longward of 350 ¡m. In practice, this often means an SED resembling a single-temperature blackbody at T ~ 15-30 K (see Fig. 2).
Property (i) distinguishes Class 0 objects from the prestellar cores and condensations discussed in section II. In particular, deep VLA observations reveal no compact radio continuum sources in the centers of prestellar cores (Bontemps 1996; Yun et al. 1996). Properties (ii) and (iii) distinguish Class 0 objects from more evolved (Class I and Class II) YSOs. As shown by Andre et al. (1993), the Lsmm/Lbol ratio should roughly track the ratio Menv/M* of envelope to stellar mass and may be used as an evolutionary indicator (decreasing with time) for low-luminosity (Lbol s 50 L©) embedded YSOs. Criterion (iii) approximately selects objects that have Menv/M* > 1, assuming plausible relations between Lbol and M* on the one hand and between Lsmm and Menv on the other (see Andre et al. 1993; Andre and Montmerle 1994). [A roughly equivalent criterion is Menv/Lbol > 0.1 M©/L©.] Class 0 objects are therefore excellent candidates for being very young accreting protostars in which a hydrostatic core has formed but not yet accumulated the majority of its final mass. In practice, most of the confirmed Class 0 objects listed in Table I have LSmm/Lbol >> 0.5% and are likely to be at the beginning of the main accretion phase with Menv >> M* (see Fig. 6b).
3. Evolutionary Diagrams for Embedded YSOs. Combining infrared and submillimeter data, it is therefore possible to define a complete, empirical evolutionary sequence (Class 0 y Class I y Class II y Class III) for low-mass YSOs, which likely correspond to conceptually different stages of evolution: (early) main accretion phase, late accretion phase, PMS stars with protoplanetary disks, PMS stars with debris disks (see Andre and Montmerle 1994). This sequence is quasicontinuous and may be parameterized by the "bolometric temperature," Tbol, defined by Myers and Ladd (1993) as the temperature of a blackbody having the same mean frequency as the observed YSO SED. Myers and Ladd proposed to use the Lbol-Tbol diagram for embedded YSOs as a direct analog to the HR diagram for optically visible stars. As shown by Chen et al. (1995, 1997), YSOs with known classes have distinct ranges of Tbol: <70 K for Class 0, 70-650 K for Class I, 650-2880 K for Class II, and >2880 K for Class III (e.g., Fig. 6a). The evolution of Tbol and Lbol from the Class 0 stage to the zero-age main sequence (ZAMS) has been modeled in the context of various envelope dissipation scenarios by Myers et al. (1998).
A perhaps more direct approach to tracking the circumstellar evolution of YSOs is to use the circumstellar mass Mc * derived from (submillimeter continuum measurements of optically thin dust emission. Such measurements show that Mc* (= Menv + Mdisk) is generally dominated by Menv in Class 0/Class I sources (e.g., Terebey et al. 1993) and decreases by a factor ~5-10 on average from one YSO class to the next (Andre and Montmerle 1994). In the spirit of the Lsmm/Lbol evolutionary indicator of Andre et al. (1993), Saraceno et al. (1996a) proposed the Lsmm-Lbol (or equivalently Menv-Lbol) diagram as an alternative evolutionary diagram for self-embedded YSOs. While Lsmm and Menv are well correlated with Lbol for the majority of embedded YSOs (e.g., Reipurth et al. 1993),
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