The Earliest Phases of Star Formation (EPoS): aHerschelkey project

Abstract

Context. The temperature and density structure of molecular cloud cores are the most important physical quantities that determine the course of the protostellar collapse and the properties of the stars they form. Nevertheless, density profiles often rely either on the simplifying assumption of isothermality or on observationally poorly constrained model temperature profiles. The instruments of the Herschel satellite provide us for the first time with both the spectral coverage and the spatial resolution that is needed to directly measure the dust temperature structure of nearby molecular cloud cores. Aims. With the aim of better constraining the initial physical conditions in molecular cloud cores at the onset of protostellar collapse, in particular of measuring their temperature structure, we initiated the guaranteed time key project (GTKP) "The Earliest Phases of Star Formation" (EPoS) with the Herschel satellite. This paper gives an overview of the low-mass sources in the EPoS project, the Herschel and complementary ground-based observations, our analysis method, and the initial results of the survey. Methods. We study the thermal dust emission of 12 previously well-characterized, isolated, nearby globules using FIR and submm continuum maps at up to eight wavelengths between 100 μm and 1.2 mm. Our sample contains both globules with starless cores and embedded protostars at different early evolutionary stages. The dust emission maps are used to extract spatially resolved SEDs, which are then fit independently with modified blackbody curves to obtain line-of-sight-averaged dust temperature and column density maps. Results. We find that the thermal structure of all globules (mean mass 7 M ) is dominated by external heating from the interstellar radiation field and moderate shielding by thin extended halos. All globules have warm outer envelopes (14-20 K) and colder dense interiors (8-12 K) with column densities of a few 10 22 cm −2 . The protostars embedded in some of the globules raise the local temperature of the dense cores only within radii out to about 5000 AU, but do not significantly affect the overall thermal balance of the globules. Five out of the six starless cores in the sample are gravitationally bound and approximately thermally stabilized. The starless core in CB 244 is found to be supercritical and is speculated to be on the verge of collapse. For the first time, we can now also include externally heated starless cores in the L smm /L bol vs. T bol diagram and find that T bol < 25 K seems to be a robust criterion to distinguish starless from protostellar cores, including those that only have an embedded very low-luminosity object.

SABOCA 350-μm and LABOCA 870-μm dust continuum imaging of IRAS 05399-0121: mapping the dust properties of a pre- and protostellar core system

“…This is not a unique case in this sense; a similar situation holds for the double-core globules CB 26 and 244 recently studied by Launhardt et al (2013). Also, the filamentary clump associated with the Class 0 protostar IRAS 05405-0117 in Orion B9 shows starless substructure in the N 2 H + (1−0) map by Caselli & Myers (1994) and in submm dust emission maps (Papers I and III).…”

Radio and infrared study of the star-forming region IRAS 20286+4105

“…Following the procedure outlined in Battersby et al (2011), Nielbock et al (2012, Launhardt et al (2013), andMallick et al (2015), a pixel-by-pixel modeling of this dust emission with a gray body/modified blackbody is adopted in order to generate temperature and column density maps of the region of our interest. For the preliminary steps, the Herschel data compatible software HIPE 5 is used.…”

“…The background fluxes in the bands are estimated by fitting a Gaussian function to the distribution of individual pixel values in the selected region. The fitting is done iteratively by rejecting the pixel values outside ±2σ, until the fit converges (Launhardt et al 2013;Battersby et al 2011;Mallick et al 2015). The resultant background flux levels at 70, 160, 250, 350 and 500 µm are -1.0, -1.5, 1.1, 0.7 and 0.3 Jy/pixel respectively.…”

“…The non-linear least square Levenberg-Marquardt fitting algorithm is used. We include 15% uncertainty in the background subtracted Hi-GAL fluxes (Launhardt et al 2013). Dust temperature and column density are taken as free parameters in the model.…”

Dust and gas environment of the young embedded cluster IRAS 18511+0146

“…The best fits were obtained using nonlinear least squares Marquardt-Levenberg algorithm, considering N(H 2 ) and T D as free parameters. Based on other studies, we assumed flux uncertainties of the order ∼ 15% in all the bands (Launhardt et al 2013).…”