EDP Sciences
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Volume 394, Number 2, November I 2002
Page(s) 561 - 583
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20021160

A&A 394, 561-583 (2002)
DOI: 10.1051/0004-6361:20021160

The young intermediate-mass stellar object AFGL 490 - A disk surrounded by a cold envelope

K. Schreyer1, Th. Henning2, F. F. S. van der Tak3, A. M. S. Boonman4 and E.F. van Dishoeck4

1  Astrophysikalisches Institut und Universitäts-Sternwarte (AIU), Schillergäßchen 2-3, 07745 Jena, Germany
2  Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
3  Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
4  Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands

(Received 16 January 2002 / Accepted 5 August 2002)

AFGL 490 is a key target of the class of deeply embedded intermediate-mass young stellar objects in a transition stage to Herbig Be stars ( L = 2.2-4.0  $\times$ 10 3 $L_{\odot}$). In this paper, we present a comprehensive set of single-dish line data which characterize the envelope of the source. In addition, observations of CS J = 2 $\rightarrow$1 and the corresponding continuum at 97.98 GHz have been obtained with the Plateau de Bure (PdB) interferometer, which are sensitive to the small-scale structure around the stellar source. The PdB line data show a bar-like elongated gas structure of 22 000 AU  $\times$ 6000 AU size with a position angle of $\approx$-45°. This bar represents the flattened inner envelope surrounding a disk-like structure (radius $\le$500 AU) for which we find evidence very close to the young B star. Due to strong (self-)absorption in the velocity range $v_{\rm lsr}$ = -12.5 ... -15 km s -1, only the outer line wings can be used to study the gas motion. Maps of the integrated red and blue line wing emission show two well-separated gas blobs around AFGL 490, which are interpreted as a disk. The 3 mm continuum interferometer map shows a point source at the position of AFGL 490 with a flux of 240 mJy. This flux translates into a total mass of 3-6  $M_{\odot}$ of the disk which is comparable to the stellar mass of about 8  $M_{\odot}$. This configuration is unstable and will disappear in 10 3-10 4 years due to gravitational instabilities. Photometric data from ISOPHOT and spectroscopic data from ISO-SWS have been obtained. Together with submillimetre continuum data a very complete spectral energy distribution of the envelope could be compiled. Analysis of the data shows that the central region of AFGL 490 has a steeper density gradient compared with the outer molecular envelope. All data clearly point to a low temperature (25-35 K) of this envelope. To determine the chemical state of the object, we determined the abundances of 13 molecules towards AFGL 490. The molecular line and ISO-SWS data are used to derive the gas-solid abundance ratios for H 2O, CO, and CO 2. The chemical results, such as the relatively low gas-to-solid ratios and the low CH 3OH excitation, emphasize the presence of a cold molecular envelope. We found evidence for other outflow systems in the envelope around AFGL 490. Red-shifted and blue-shifted gas blobs with a separation of about 20 000 AU were detected. Their centre is located roughly 3 '' to the south of AFGL 490, and their morphology implies that a deeply embedded low-mass object drives a jet which enters the denser envelope material at such a large distance. Two further outflow systems in the close neighbourhood of AFGL 490 could be identified. All these data point to the formation of a group of low-mass stars around AFGL 490. It is very remarkable that these outflows do not influence the global physical and chemical structure of the envelope.

Key words: ISM: clouds -- ISM: individual objects: AFGL 490 -- ISM: jets and outflows -- ISM: molecules -- stars: formation

Offprint request: K. Schreyer, martin@astro.uni-jena.de

© ESO 2002