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

We report the findings of a comprehensive study of disk accretion and related phenomena in four of the nearest young stellar associations spanning 6-30 million years in age, an epoch that may coincide with the late stages of planet formation. We have obtained ~650 multiepoch high-resolution optical spectra of 100 low-mass stars that are likely members of the η Chamaeleontis (~6 Myr), TW Hydrae (~8 Myr), β Pictoris (~12 Myr), and Tucanae-Horologium (~30 Myr) groups. Our data were collected over 12 nights between 2004 December and 2005 July on the Magellan Clay 6.5 m telescope. Based on Hα line profiles, along with a variety of other emission lines, we find clear evidence of ongoing accretion in 3 out of 11 η Cha stars and 2 out of 32 TW Hydrae members. None of the 57 β Pic or Tuc-Hor members shows measurable signs of accretion. Together, these results imply significant evolution of the disk-accretion process within the first several Myr of a low-mass star's life. While a few disks can continue to accrete for up to ~10 Myr, our findings suggest that disks accreting for beyond that timescale are rather rare. This result provides an indirect constraint on the timescale for gas dissipation in inner disks and, in turn, on gas-giant planet formation. All accretors in our sample are slow rotators, whereas nonaccretors cover a large range in rotational velocities. This may hint at rotational braking by disks at ages up to ~8 Myr. Our multiepoch spectra confirm that emission-line variability is common even in somewhat older T Tauri stars, among which accretors tend to show particularly strong variations. Thus, our results indicate that accretion and wind activity undergo significant and sustained variations throughout the lifetime of accretion disks.