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

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

Nov 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002a%26a...394..561s&link_type=abstract

Astronomy and Astrophysics, v.394, p.561-583 (2002)

Astronomy and Astrophysics

Astrophysics

29

Ism: Clouds, Ism: Individual Objects: Afgl 490, Ism: Jets And Outflows, Ism: Molecules, Stars: Formation

Scientific paper

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 x 103 Lsun ). 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->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 x 6000 AU size with a position angle of ~-45o. 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 vlsr = -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 Msun of the disk which is comparable to the stellar mass of about 8 Msun. 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. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands and the UK) with the participation of ISAS and NASA.

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