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ISO-LWS observations of Herbig Ae/Be stars *
II. Molecular lines
T. Giannini 1,2,3,
D. Lorenzetti 2,
E. Tommasi 4,
B. Nisini 3,
M. Benedettini 3,
S. Pezzuto 3,
F. Strafella 5,
M. Barlow 6,
P.E. Clegg 7,
M. Cohen 8,
A.M. DiGiorgio 3,
R. Liseau 9,
S. Molinari 10,
F. Palla 11,
P. Saraceno 3,
H.A. Smith 12,
L. Spinoglio 3 and
G.J. White 7
Received 25 November 1998 / Accepted 11 March 1999
We present the first ISO-LWS observations of the molecular FIR lines in 3 out of a sample of 11 Herbig Ae/Be stars (HAEBE), namely IRAS12496-7650, RCrA and LkH234. High-J rotational CO lines (from = 14 to = 19) have been observed in all the spectra, while two (at 79 µm and 84 µm) and three OH lines (at 71 µm, 79 µm and 84 µm) were detected in LkH234 and RCrA respectively.
For all sources the molecular emission has been consistently fitted with a Large Velocity Gradient (LVG) model and it results originated in a warm (T 200 K) and dense ( 105 cm-3) gas located in very compact regions having diameters of few hundreds of AU.
These three sources are those with the highest density among the stars of the sample; this suggests that the molecular emission arises in regions showing density peaks.
By comparing the observed cooling ratios with model predictions, we find that the FUV radiation from the central source (or from a more embedded companion) is the most likely responsible for the line excitation. At least for the sources where OH has been observed, the contribution of shocks to the line emission can be reasonably ruled out because of the absence in the spectra of any water vapour lines, in contrast with the predictions for molecular emission coming from warm shocked environments.
Key words: stars: circumstellar matter stars: pre-main sequence infrared: ISM: lines and bands infrared: stars
Send offprint requests to: T. Giannini (firstname.lastname@example.org)
Online publication: May 21, 1999