The Earliest Phases of Star Formation (EPoS): a Herschel key project

The thermal structure of low-mass molecular cloud cores^{⋆,⋆⋆,⋆⋆⋆}

¹ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
e-mail: rl@mpia.de
² Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany
³ ESA/ESTEC, Keplerlaan 1, Postbus 299, 2200 AG Noordwijk, The Netherlands
⁴ Laboratoire AIM Paris-Saclay, Service d’Astrophysique, CEA/IRFU – CNRS/INSU – Université Paris Diderot, Orme des Merisiers Bat. 709, 91191 Gif-sur-Yvette Cedex, France
⁵ Max-Planck-Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany
⁶ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
⁷ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands
⁸ Steward Observatory, 933 North Cherry Avenue, Tucson, AZ 85721, USA
⁹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹⁰ Institut de Planétologie et d’Astrophysique de Grenoble, Université de Grenoble, BP 53, 38041 Grenoble Cedex 9, France

Received: 1 October 2012
Accepted: 11 December 2012

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 10²² 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.