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Astron. Astrophys. 324, 699-703 (1997)

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The Paschen decrement as a density indicator in MWC349 *

C. Thum and A. Greve

Institut de Radio Astronomie Millimetrique, Domaine Universitaire de Grenoble, F-38406 St. Martin-d'Hères, France

Received 10 October 1996 / Accepted 7 February 1997


We present new observations of the Paschen series hydrogen recombination lines in the circumstellar shell of MWC349. We find the Paschen decrement to be characteristic of a very high density plasma, [FORMULA] cm-3, indicating the origin of the higher quantum number Paschen lines in the dense corona of the circumstellar disk.

The strength of the masing hydrogen recombination [FORMULA] -lines detected previously in the high density plasma of this source is strongly dependent on [FORMULA], since the [FORMULA] for maximum amplification increases systematically for decreasing quantum numbers n. The absence of gas with [FORMULA] significantly higher than [FORMULA] cm-3 implies that the strength of recombination [FORMULA] -line masers may peak near [FORMULA] ([FORMULA] m) and that the shorter wavelength transitions may mase only at reduced efficiency.

The good agreement over a large range of n between the observed and the theoretical Paschen decrement shows that any departure from standard recombination theory, possibly due to the strong IR radiation field of the circumstellar disk, must be small.

Key words: recombination lines – stellar envelopes – young stellar objects – circumstellar disks – stars: individual: MWC349

* Based on observations collected at ESO, La Silla, Chile.

Send offprint requests to: C. Thum

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© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998