The Spitzer Survey of Interstellar Clouds in the Gould Belt. V. Ophiuchus North Observed with IRAC and MIPS

We present Spitzer IRAC (2.1 sq. deg.) and MIPS (6.5 sq. deg.) observations of star formation in the Ophiuchus North molecular clouds. This fragmentary cloud complex lies on the edge of the Sco-Cen OB association, several degrees to the north of the well-known rho Oph star-forming region, at an approximate distance of 130 pc. The Ophiuchus North clouds were mapped as part of the Spitzer Gould Belt project under the working name `Scorpius. In the regions mapped, selected to encompass all the cloud with visual extinction AV>3, eleven Young Stellar Object (YSO) candidates are identified, eight from IRAC/MIPS colour-based selection and three from 2MASS K/MIPS colours. Adding to one source previously identified in L43 (Chen et al. 2009), this increases the number of YSOcs identified in Oph N to twelve. During the selection process, four colour-based YSO candidates were rejected as probable AGB stars and one as a known galaxy. The sources span the full range of YSOc classifications from Class 0/1 to Class III, and starless cores are also present. Twelve high-extinction (AV>10) cores are identified with a total mass of approx. 100 solar masses. These results confirm that there is little ongoing star formation in this region (instantaneous star formation efficiency <0.34%) and that the bottleneck lies in the formation of dense cores. The influence of the nearby Upper Sco OB association, including the 09V star zeta Oph, is seen in dynamical interactions and raised dust temperatures but has not enhanced levels of star formation in Ophiuchus North.

Evolutionary Signatures in the Formation of Low-Mass Protostars. II. Towards Reconciling Models and Observations

A long-standing problem in low-mass star formation is the luminosity problem, whereby protostars are underluminous compared to the accretion luminosity expected both from theoretical collapse calculations and arguments based on the minimum accretion rate necessary to form a star within the embedded phase duration. Motivated by this luminosity problem, we present a set of evolutionary models describing the collapse of low-mass, dense cores into protostars, using the Young & Evans (2005) model as our starting point. We calculate the radiative transfer of the collapsing cores throughout the full duration of the collapse in two dimensions. From the resulting spectral energy distributions, we calculate standard observational signatures to directly compare to observations. We incorporate several modifications and additions to the original Young & Evans model in an effort to better match observations with model predictions. We find that scattering, 2-D geometry, mass-loss, and outflow cavities all affect the model predictions, as expected, but none resolve the luminosity problem. A cycle of episodic mass accretion, however, can resolve this problem and bring the model predictions into better agreement with observations. Standard assumptions about the interplay between mass accretion and mass loss in our model give star formation efficiencies consistent with recent observations that compare the core mass function (CMF) and stellar initial mass function (IMF). The combination of outflow cavities and episodic mass accretion reduce the connection between observational Class and physical Stage to the point where neither of the two common observational signatures (bolometric temperature and ratio of bolometric to submillimeter luminosity) can be considered reliable indicators of physical Stage.