Keeping an Eye on the Young: Monitoring T Tauri Stars

Transcription

1 Keeping an Eye on the Young: Monitoring T Tauri Stars

2 T Tauri stars are young (0.5-5 Myr), low mass (0.1-1 solar mass) stars. They are typically found in close association with the gas & dust clouds out of which they have recently formed. They tend to come in groups ranging in size from loose associations of only a few to dense clusters of thousands. Some well known local associations of TTS accessible to northern hemisphere observers include the following: Tau/Aur Orion Nebula Cluster Ori OB Ia, Ib and Ic NGC 2264 IC 348 NGC 1333

3 High resolution images of young stars reveal disks and jets around many. When viewed edge-on the stars themselves may not be visible optically. In the Orion Nebula Cluster HST images reveal many disks in silhouette against the bright nebular background.

4 The Basic Properties of T Tauri Stars were determined primarily by George Herbig following the pioneering studies of Alfred Joy. A partial modern list includes the following traits: -- G,K or M spectral class with emission lines of H, CaII and often other elements, sometimes including forbidden emission lines. -- Location on the HR diagram above the main sequence -- Li I absorption -- Photometric variability which is typically at the level of 10% and sometimes reaches several magnitudes! -- Strong X-ray emission typically at a level of 10-4 Lbol -- IR excess emission in a sub-set of stars now known as CLASSICAL T Tauri stars (CTTS). These stars also have stronger H emission lines, with EW typically exceeding 10 Å. The other stars are often called WEAK LINED T Tauri stars (WTTS). Classical T Tauri Star Jet Weak-lined T Tauri Star Gas Flow No Accretion Accretion Disk Planetesimal Disk Accretion Disk Magnetic Axis Spin Axis Planetesimal Disk Magnetic Axis Spin Axis Jet

5 Photometric Variations of TTS arise from: Type I: Rotation of photospheres with an asymmetric distribution of large, cool spots. This is the most common form of variability found in young clusters and is the only form exhibited by WTTS. Type II: Hot spots (or zones ), presumably heated by magnetically channeled accretion but possibly also by flares, which come and go on time scales of hours to days. This is the most common cause of variability in CTTS. Type IIp: Rotation of unusually long-lived hot spots yielding periodic variations similar to Type I variations but of larger amplitude, particularly at short wavelengths. Type III (UXors): Variable obscuration caused by the relative motions of stars and circumstellar dust concentrations (UXors) FUors: Rapid accretion phase leading to eruptive brightness increase (after FU Orionis) EXors: Smaller scale version of rapid accretion phase (after EX Lupi) KH 15D-like: Occultations by circumstellar matter in a disk

6 The study of photometric variations of T Tauri stars yields information on, among other things: -- the angular momentum evolution of young stars -- accretion rates, processes and their evolution -- disk structure possibly related to planet formation -- magnetic field configurations, evolution and star spots -- fundamental properties of stars including masses and radii -- association membership and evolution of star forming regions It is a project well suited to small telescopes because stars are bright

7 Perkin telescope image of IC 348

8 Typical light curve in one season Periodogram Phased Light Curve Harmonic Periods for ONC stars measured by Stassun et al. (2001) or at Wesleyan compared with those measured at ESO (Herbst et al. 2002). Note that 85% of the periods agree and disagreements are, in all cases but one, either beat periods or harmonics

9 HMW 12 (P=2.24 days) Example Type I variables over five years: HMW 16 (P=5.22 days) HMW 40 (P=8.38 days)

10 Periods are very stable, supporting the hypothesis that this is the rotation period of the star. Little evidence for period changes associated with differential rotation. There is nothing we know more accurately about PMS stars than their rotation rates. Angular velocities, accurate to ~1%, have been measured by the photometric monitoring technique for ~1,700 stars in Taurus/Auriga, Orion, NGC 2264, IC 348, NGC 1333, Rho Oph, and other extremely young clusters and associations.

11 During PMS Stars Spin faster as they age. Also, lower mass PMS stars spin faster than higher mass ones. P ~ R 2 ~ t -(2/3) Bimodal Period Distribution Higher mass Lower mass Older Younger

12 Stars with disks are kept from spinning up as fast as they should (to conserve angular momentum), a phenomenon known as disk locking David Amrhein is working on NGC2264 data obtained with MPIA/ESO 2.2m at Cerro Tololo to investigate link between rotation and accretion... more later.

13 Summary of Surface Rotation vs age for Low Mass Stars -- Contraction to P ~1-2 d occurs during first 10 5 yr (maybe!) -- Creation of wide range of rotation rates (P~1-20 days) by magnetic effects ( disk-locking ) between yrs. -- Spin-up due to contraction between yrs -- Long Spin-down due to stellar winds between yrs

14 Type II and III Variables In collaboration with Matt Templeton and Arne Hendon of the AAVSO we received one month of time on the MOST satellite to monitor the ONC last year. MOST (Microvariability and Oscillations of Stars) Canadian Satellite, also known as the Humble Space Telescope has a 0.15 m telescope. Provides continuous coverage for up to a month! Diana Windemuth is working on BM Ori. Light Curve of the Type III variable T Ori over a month.

15 KH 15D CENSORED Holly Capelo will discuss this star... later...

16 HMW 15 in IC 348 (P=4.7 years!)