CO Emission from the Inner Disk around Intermediate-Mass Stars

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To investigate the interaction between young stars and their circumstellar material we conducted a spectral survey of Herbig Ae/Be stars in the K-band. Non-photospheric Br Gamma emission is observed in the spectra from most stars, indicating a similar Br Gamma flux as the cooler intermediate mass T-Tauri stars. We find that the emission is probably caused by winds or by hot disk gas. To study different methods to estimate accretion luminosity we compared our Br Gamma fluxes with other accretion indicators. The mismatch between the estimated accretion luminosities suggests that using Br Gamma could overestimate the Lacc by several orders of magnitude.

We also observed CO overtone emission, but in a few objects only, perhaps because hot, dense gas is not common around these stars. We obtained high resolution spectra of the objects with observable CO overtone emission. These spectra allow us to establish constraints on the geometric distribution and physical characteristics of the emitting gas. We implemented a model of a hot gas disk to fit to our observations. Our results agree with previous conclusions, that 51 Oph is most likely a classical Be star, surrounded by a massive disk of hot gas. Our new findings include that the disk is probably seen at high inclination (i>83deg) though a lower inclination (i<36deg) is also possible. The vibrational excitation temperature of the gas is close to 3500K at the inner edge and falls off as expected from comparison to various disk models. The excitation temperature of the v=2->0 rotational bands is higher than for the vibrational transitions and indicates a source of energy beyond radiative equilibrium in the inner regions of the disk. The quality of our data allows us to fit the emission line width. We find that on average, turbulence and geometrical broadening in the disk contribute ~3km/s to the emission line width.

The other two stars from which we observed CO overtone emission are HD58647 and HD36917. HD58647 is probably a classical Be star like 51 Oph. Combining our high-resolution spectra with our model indicates that this star is surrounded by a disk of hot gas which is optically thin and not in radiative equilibrium with the radiation of the star. The star HD36917 is the only Herbig Ae/Be star for which we have observed CO overtone emission at high resolution. The emission appears to be caused by hot gas in a directed flow. We determined an upper limit for the mass flow rate of about 5x10^-6 Msun/yr, which would be rather high for such a star.

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Spectra; Observations; Star Formation; Infrared


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bibid: 6475912

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