Properties of the Youngest Protostars in Perseus, Serpens, and Ophiuchus

Abstract

We present an unbiased census of deeply embedded protostars in Perseus, Serpens, and Ophiuchus, assembled by combining large-scale 1.1 mm Bolocam continuum and Spitzer Legacy surveys. We identify protostellar candidates based on their mid-infrared properties, correlate their positions with 1.1 mm core positions from Enoch et al. (2006), and Enoch et al. (2007, and construct well-sampled SEDs using our extensive wavelength coverage (λ = 1.25 − 1100 µm). Source classification based on the bolometric temperature yields a total of 39 Class 0 and 89 Class I sources in the three cloud sample. We compare to protostellar evolutionary models using the bolometric temperature-luminosity diagram, finding a population of low luminosity Class I sources that are inconsistent with constant or monotonically decreasing mass accretion rates. This result argues strongly for episodic accretion during the Class I phase, with more than 50% of sources in a "sub-Shu" (dM/dt < 10 −6 M ⊙ yr −1 ) accretion state. Average spectra are compared to protostellar radiative transfer models, which match the observed spectra fairly well in Stage 0, but predict too much near-IR and too little mid-IR flux in Stage I. Finally, the relative number of Class 0 and Class I sources are used to estimate the lifetime of the Class 0 phase; the three cloud average yields a Class 0 lifetime of 1.7 ± 0.3 × 10 5 yr, ruling out an extremely rapid early accretion phase. Correcting photometry for extinction results in a somewhat shorter lifetime (1.1 × 10 5 yr). In Ophiuchus, however, we find very few Class 0 sources (N Class 0 /N Class I ∼ 0.1 − 0.2), similar to previous studies of that cloud. The observations suggest a consistent picture of nearly constant average accretion rate through the entire embedded phase, with accretion becoming episodic by at least the Class I stage, and possibly earlier.

The JCMT Gould Belt Survey: first results from the SCUBA-2 observations of the Ophiuchus molecular cloud and a virial analysis of its prestellar core population

“…Enoch et al (2009) list four embedded Class I protostars in Oph B: none in Oph B1; one in Oph B1B2; and three in Oph B2, of which one is the outflow-driving source IRS 47 (White et al 2015). Cores in Oph B1 and B2 typically show similar behaviour, although the ratio of gravitational to pressure energy is consistently in the range 0.3-0.4 in B1, and more varied in B2.…”

“…However, as shown in Figure 10, cores in the densest regions of Oph A do not appear to be dominated by external pressure. Enoch et al (2009) list only one protostar embedded in Oph A: the Class 0 protostar VLA 1623 (the only Class 0 source in L1688). This is consistent with star formation in this dense clump being in its early stages.…”

Submillimetre Studies of Prestellar and Starless Cores in the Ophiuchus, Taurus and Cepheus Molecular Clouds

“…Class 0 systems are relatively short-lived; Barsony (1994) At this stage, the majority of the emission from the system is from the envelope surrounding the protostar, and the bolometric temperature of the system is less than 70 K. This means that Class 0 sources cannot be distinguished from Class I sources using their infrared spectral indices, as the majority of the envelope emission is at longer (submillimetre) wavelengths (e.g. Enoch et al 2009). The original observational definition of a Class 0 protostar was an object whose submillimetre luminosity (at wavelengths λ > 350µm) contributed greater than 0.5% to its total bolometric luminosity, i.e.…”

“…This suggests that the local environment has a significant effect on the nature of the starless cores. Enoch et al (2009) provide a catalogue of deeply embedded Class 0 and Class I protostars in L1688…”

“…Enoch et al (2009) list four embedded Class I protostars in L1689S; one Class 0 source in L1689N; and two Class I sources in L1709. The low number of cores relative to L1688, the low ratio of embedded sources to starless cores, and the presence of the Class 0 source IRAS 16293-2422 suggests that L1689 and L1709 are likely to be less evolved than, or forming stars less efficiently than, L1688.…”