TY - JOUR

T1 - Infall in Herbig Ae/Be stars: What Na D lines tell us

AU - Sorelli, C.

AU - Grinin, V. P.

AU - Natta, A.

PY - 1996/5/1

Y1 - 1996/5/1

N2 - 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 ≲ 10 R, from the star. If the gas has solar chemical composition, then the clump size must be L ∼ 1011 cm (about R(Black star sign)) and the density nH ≳ 3 × 1012 c-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 Macc ≳ 3 × 10-7 M⊙ 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 nH ≳ 5 × 108 cm-3, L ∼ 1011 cm. The mass of the cloud is therefore of the order of 1020 gr, and the parental body must have a radius of 20 km at least. These results show that both scenarios, i.e, magnetospheric accretion and evaporation of star-grazing planetesimal bodies, are in principle possible.

AB - 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 ≲ 10 R, from the star. If the gas has solar chemical composition, then the clump size must be L ∼ 1011 cm (about R(Black star sign)) and the density nH ≳ 3 × 1012 c-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 Macc ≳ 3 × 10-7 M⊙ 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 nH ≳ 5 × 108 cm-3, L ∼ 1011 cm. The mass of the cloud is therefore of the order of 1020 gr, and the parental body must have a radius of 20 km at least. These results show that both scenarios, i.e, magnetospheric accretion and evaporation of star-grazing planetesimal bodies, are in principle possible.

KW - Circumstellar matter

KW - Herbig Ae/Be stars

KW - Stars: formation

KW - Stars: pre-main sequence

UR - http://www.scopus.com/inward/record.url?scp=3242871620&partnerID=8YFLogxK

M3 - Article

AN - SCOPUS:3242871620

VL - 309

SP - 155

EP - 162

JO - ASTRONOMY & ASTROPHYSICS

JF - ASTRONOMY & ASTROPHYSICS

SN - 0004-6361

IS - 1

ER -