An unbiased search for the signatures of protostars in the ρ  Ophiuchi molecular cloud

II. Millimetre continuum observations

¹ Max-Planck-Institut für Radioastronomie Bonn, Auf dem Hügel 69, 53121 Bonn, Germany
² Institute for Astronomy, University of Hawai'i, 2680 Woodlawn Drive, Honolulu, Hawai'i 96822, USA e-mail: stanke@ifa.hawaii.edu
³ Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland, UK e-mail: mds@arm.ac.uk
⁴ Centre for Astrophysics and Planetary Science, School of Physical Sciences, University of Kent CT2 7NR
⁵ Max-Planck-Institut für Astronomie, Königsstuhl 17, 69117 Heidelberg, Germany e-mail: gredel@caha.es,khtig@mpia-hd.mpg.de
⁶ Centro de Astrofísica da Universidade do Porto, rua das Estrelas, 4150-726 Porto, Portugal

Received: 21 May 2004
Accepted: 21 October 2005

Abstract

The dense cores which conceive and cradle young stars can be explored through continuum emission from associated dust grains. We have performed a wide field survey for dust sources at 1.2 millimetres in the ρ  Ophiuchi molecular cloud, covering more than 1 square degree in an unbiased fashion. We detect a number of previously unknown sources, ranging from extended cores over compact, starless cores to envelopes surrounding young stellar objects of Class 0, Class I, and Class II type. We analyse the mass distribution, spatial distribution and the potential equilibrium of the cores. For the inner regions, the survey results are consistent with the findings of previous narrower surveys. The core mass function resembles the stellar initial mass function, with the core mass function shifted by a factor of two to higher masses (for the chosen opacity and temperature). In addition, we find no statistical variation in the core mass function between the crowded inner regions and those in more isolated fields except for the absence of the most massive cores in the extended cloud. The inner region contains compacter cores. This is interpreted as due to a medium of higher mean pressure although strong pressure variations are evident in each region. The cores display a hierarchical spatial distribution with no preferred separation scale length. However, the frequency distribution of nearest neighbours displays two peaks, one of which at 5000 AU can be the result of core fragmentation. The orientations of the major axes of cores are consistent with an isotropic distribution. In contrast, the relative orientations of core pairs are preferentially in the NW-SE direction on all separation scales. These results are consistent with core production and evolution in a turbulent environment. Finally, we report the discovery of a new, low-mass Class 0 object candidate and its CO outflow.