Prestellar and protostellar cores in Ori B9

TL;DR

This study maps and analyzes dense cores in Ori B9, revealing their properties, evolutionary stages, and dynamical timescales, and finds a balanced ratio of pre- and protostellar cores indicating similar durations for these phases.

Contribution

It provides the first detailed characterization of dense cores in Ori B9, including new Class 0 candidates, and compares core properties with more active regions, highlighting the role of turbulence and chemical evolution.

Findings

Equal number of pre- and protostellar cores suggests similar prestellar and protostellar phase durations.

Discovery of two new Class 0 protostar candidates in Ori B9.

Core properties are consistent with turbulent fragmentation models.

Abstract

The aims of this study are to determine the properties and spatial distribution of dense cores in Ori B9, and to estimate their ages and dynamical timescales. The cloud was mapped in the 870 micron continuum with APEX/LABOCA, and selected positions were observed in the lines of N2H+ and N2D+ using IRAM-30m. These were used together with our previous H2D+ observations. Moreover, archival FIR Spitzer/MIPS maps were combined with the LABOCA map to distinguish between pre- and protostellar cores, and to estimate the evolutionary stages of protostars. Twelve dense cores were detected at 870 micron in the Ori B9 cloud. The submm cores constitute ~4% of the total mass of the Ori B9 region. There is an equal number of pre- and protostellar cores. Two of the submm sources, which we call SMM 3 and SMM 4, are previously unknown Class 0 candidates. We found a moderate degree of deuteration in N2H+ (0.03-0.04). There is, furthermore, evidence for N2H+ depletion in the core SMM 4. We derive a relatively high degree of ionization (~10^-7) in the clump associated with IRAS 05405-0117. The ambipolar diffusion timescales for two of the cores are ~70-100 times longer than the free-fall time. The distribution and masses of dense cores in Ori B9 are similar to those observed in more active regions in Orion, where the statistical core properties have been explained by turbulent fragmentation. The 50/50 proportions of pre- and protostellar cores suggest that duration of the prestellar phase is comparable to the free-fall time. This timescale can be questioned, however, on the basis of chemical data on the IRAS 05405-0117 region. A possible explanation is that this survey samples only the densest, i.e., dynamically most advanced cores.