Accretion rates in Herbig Ae stars

摘要

Aims. Accretion rates from disks around pre-main sequence stars are of importance for our understanding of planetary formation and disk evolution. We provide in this paper estimates of the mass accretion rates in the disks around a large sample of Herbig Ae stars. Methods. We obtained medium resolution 2 mu m spectra and used the results to compute values of. M-acc from the measured luminosity of the Br-gamma emission line, using a well established correlation between L(Br-gamma) and the accretion luminosity L-acc. Results. We find that 80 of the stars, all of which have evidence of an associated circumstellar disk, are accreting matter, with rates 3 x 10(-9) less than or similar to Macc less than or similar to 10(-6) M-circle dot/yr; for 7 objects, 6 of which are located on the ZAMS in the HR diagram, we do not detect any line emission. Few HAe stars (25) have M-acc > 10(-7) M-circle dot/yr. Conclusions. In most HAe stars the accretion rate is sufficiently low that the gas in the inner disk, inside the dust evaporation radius, is optically thin and does not prevent the formation of a puffed-up rim, where dust is directly exposed to the stellar radiation. When compared to the. M-acc values found for lower-mass stars in the star forming regions Taurus and Ophiuchus, HAe stars have on average higher accretion rates than solar-mass stars; however, there is a lack of very strong accretors among them, probably due to the fact that they are on average older.