Issue A&A Volume 498, Number 1, April IV 2009 Page(s) 167 - 194 Section Interstellar and circumstellar matter DOI http://dx.doi.org/10.1051/0004-6361/200810445 Published online 05 March 2009

A&A 498, 167-194 (2009)
DOI: 10.1051/0004-6361/200810445

The nature of the Class I population in Ophiuchus as revealed through gas and dust mapping

T. A. van Kempen^{1, 2}, E. F. van Dishoeck^{1, 3}, D. M. Salter¹, M. R. Hogerheijde¹, J. K. Jørgensen⁴, and A. C. A. Boogert<sup>5

1</sup>  Leiden University, Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
²  Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
    e-mail: tvankempen@cfa.harvard.edu
³  Max-Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstr. 1, 85748 Garching, Germany
⁴  Argelander-Institut für Astronomie, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁵  Infrared Processing and Analysis Center (IPAC), NASA Herschel Science Center, Mail Code 100-22, California Institute of Technology, Pasadena, CA 91125, USA

Received 23 June 2008 / Accepted 22 February 2009

Abstract
Context. The Ophiuchus clouds, in particular L 1688, are an excellent region to study the embedded phases of star formation, due to the relatively large number of protostars. However, the standard method of finding and characterizing embedded young stellar objects (YSOs) through just their infrared spectral slope does not yield a reliable sample. This may affect the age determinations, often derived from the statistics on the total number of embedded YSOs and pre-main sequence stars within a cloud.
Aims. Our aim is to characterize the structure of protostellar envelopes on an individual basis and to correctly identify the embedded YSO population of L 1688.
Methods. Spectral maps of the HCO⁺ J= 4-3 and C¹⁸O J= 3-2 lines, using the HARP-B array on the James Clerk Maxwell Telescope and SCUBA 850 m dust maps, are obtained of all sources in the L 1688 region with infrared spectral slopes consistent with, or close to, that of embedded YSOs. Selected 350 m maps obtained with the Caltech Submillimeter Observatory are presented as well. The properties, extent and variation of dense gas, column density and dust up to 1' (~7500 AU) are probed at 15'' resolution. Using the spatial variation of the gas and dust, together with the intensity of the HCO⁺ J= 4-3 line, we are able to accurately identify the truly embedded YSOs and determine their properties.
Results. The protostellar envelopes range from 0.05 to 0.5  in mass. The concentration of HCO⁺ emission (~0.5 to 0.9) is generally higher than that of the dust concentration. Combined with absolute intensities, HCO⁺ proves to be a better tracer of protostellar envelopes than dust, which can contain disk and cloud contributions. Our total sample of 45 sources, including all previously classified Class I sources, several flat-spectrum sources and some known disks, was re-classified using the molecular emission. Of these, only 17 sources are definitely embedded YSOs. Four of these embedded YSOs have little (0.1-0.2 ) envelope material remaining and are likely at the interesting transitional stage from embedded YSO to T Tauri star. About half of the flat-spectrum sources are found to be embedded YSOs and about half are disks.
Conclusions. The presented classification method is successful in separating embedded YSOs from edge-on disks and confused sources. The total embedded population of the Ophiuchus L 1688 cloud is found almost exclusively in Oph-A, Oph-B2 and the Ophiuchus ridge with only three embedded YSOs not related to these regions. The detailed characterization presented will be necessary to interpret deep interferometric ALMA and future Herschel observations.

Key words: astrochemistry -- circumstellar matter -- stars: formation -- submillimeter -- ISM: clouds -- ISM: molecules

© ESO 2009

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