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

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

Corresponding author: K. Schreyer, martin@astro.uni-jena.de

Received: 16 January 2002
Accepted: 5 August 2002

Abstract

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  10³ ). 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 = 21 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  6000 AU size with a position angle of ≈–45°. This bar represents the flattened inner envelope surrounding a disk-like structure (radius ≤500 AU) for which we find evidence very close to the young B star. Due to strong (self-)absorption in the velocity range v_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  of the disk which is comparable to the stellar mass of about 8 . This configuration is unstable and will disappear in 10³–10⁴ 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 H₂O, CO, and CO₂. The chemical results, such as the relatively low gas-to-solid ratios and the low CH₃OH 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.