Accretion Disks around Young Stars: Lifetimes, Disk Locking, and Variability

We report the findings of a comprehensive study of disk accretion and relatedphenomena in four of the nearest young stellar associations spanning 6-30million years in age, an epoch that may coincide with the late stages of planetformation. We have obtained ~650 multi-epoch high-resolution optical spectra of100 low-mass stars that are likely members of the eta Chamaeleontis (~6 Myr),TW Hydrae (~8 Myr), beta Pictoris (~12 Myr) and Tucanae-Horologium (~30 Myr)groups. Our data were collected over 12 nights between 2004 December - 2005July on the Magellan Clay 6.5m telescope. Based on H$\alpha$ line profiles,along with a variety of other emission lines, we find clear evidence ofon-going accretion in three out of 11 eta Cha stars and two out of 32 TW Hydraemembers. None of the 57 beta Pic or Tuc-Hor members shows measurable signs ofaccretion. Together, these results imply significant evolution of the diskaccretion process within the first several Myr of a low-mass star's life. Whilea few disks can continue to accrete for up to ~10 Myr, our findings suggestthat disks accreting for beyond that timescale are rather rare. This resultprovides an indirect constraint on the timescale for gas dissipation in innerdisks and, in turn, on gas giant planet formation. All accretors in our sampleare slow rotators, whereas non-accretors cover a large range in rotationalvelocities. This may hint at rotational braking by disks at ages up to ~8 Myr.Our multi-epoch spectra confirm that emission-line variability is common evenin somewhat older T Tauri stars, among which accretors tend to showparticularly strong variations. Thus, our results indicate that accretion andwind activity undergo significant and sustained variations throughout thelifetime of accretion disks.Comment: to appear in The Astrophysical Journa