Infall in Herbig Ae/Be stars: what NA D lines tell us.

Details Infall in Herbig Ae/Be stars: what NA D lines tell us. Infall in Herbig Ae/Be stars: what NA D lines tell us.

May 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...309..155s&link_type=abstract

Astronomy and Astrophysics, v.309, p.155-162

Astronomy and Astrophysics

Astrophysics

41

Herbig Ae/Be Stars, Circumstellar Matter, Stars: Pre-Main Sequence, Stars: Formation

Scientific paper

This paper discusses the origin of the redshifted absorption components observed in the Na D lines of some Herbig Ae/Be stars. We have computed non-LTE models of the thermal and ionization structure of gas clouds of different density, column density and chemical composition, from the solar one to that typical of CI-chondrites. The redshifted absorption lines can only form in small, dense, infalling gas clumps at distances <~10R_*_ from the star. If the gas has solar chemical composition, then the clump size must be L~10^11^cm (about R_*_) and the density n_H_>3x10^12^cm^-3^. These conditions can be produced at the base of a column of gas falling into the star from a circumstellar accretion disk along magnetic lines. In this case, an accretion rate ˙(M)_acc_>3x10^-7^Msun_ yr^-1^ and a stellar magnetic field of about 600 Gauss are required. As the gas metallicity increases, less dense clouds are required to fit the Na D observations. In the extreme case of a gas cloud resulting from the evaporation of CI-chondrite meteorites, it is n_H_> 5x10^8^ cm^-3^, L~10^11^ cm. The mass of the cloud is therefore of the order of 10^20^gr, and the parental body must have a radius of 20km at least. These results show that both scenarios, i.e, magnetospheric accretion and evaporation of star-grazing planetesimal bodies, are in principle possible.

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