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Register a new userChromospheric activity of ROSAT discovered weak-lined T Tauri stars
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We have started a high resolution optical observation program dedicated to the study of chromospheric activity in weak-lined T Tauri stars (WTTS) recently discovered by the ROSAT All-Sky Survey (RASS). It is our purpose to quantify the phenomenology of the chromospheric activity of each star determining stellar surface fluxes in the more important chromospheric activity indicators (Ca II H & K, H_beta, H_alpha, Ca II IRT) as well as obtain the Li I abundance, a better determination of the stellar parameters, spectral type, and possible binarity. A large number of WTTS have been discovered by the RASS in and around differen star formation clouds.Whether these stars are really WTTS, or post-TTS, or even young main sequence stars is a matter of ongoing debate. However, we have centered our study only on objects for which very recent studies, of Li I abundance (greater than Pleiads of the same spectral type) or radio properties, clearly confirmed their pre-main sequence (PMS) nature. In this contribution we present preliminary results of our January 1998 high resolution echelle spectroscopic observations at the 2.1m telescope of the McDonald Observatory. We have analysed, using the spectral subtraction technique, the H_alpha and Ca II IRT lines of six WTTS (RXJ0312.8-0414NW, SE; RXJ0333.1+1036; RXJ0348.5+0832; RXJ0512.0+1020; RXJ0444.9+2717) located in and around the Taurus-Auriga molecular clouds. A broad and variable double-picked H_alpha emission is observed in RXJ0444.9+2717. Emission above the continuum in H_alpha and Ca II IRT lines is detected in RXJ0333.1+1036 and a filling-in of these lines is present in the rest of the stars. Our spectral type and Li I EW deterninations confirm the PMS nature of these objects.
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We present the ROSAT PSPC pointed and ROSAT All-Sky Survey (RASS) observations and the results of our low and high spectral resolution optical follow-up observations of the T Tauri stars (TTS) and X-ray selected T Tauri star candidates in the region of the high galactic latitude dark cloud MBM12 (L1453-L1454, L1457, L1458). Previous observations have revealed 3 classical T Tauri stars and 1 weak-line T Tauri star along the line of sight to the cloud. Because of the proximity of the cloud to the sun, all of the previously known TTS along this line of sight were detected in the 25 ks ROSAT PSPC pointed observation of the cloud. We conducted follow-up optical spectroscopy at the 2.2-meter telescope at Calar Alto to look for signatures of youth in additional X-ray selected T Tauri star candidates. These observations allowed us to confirm the existence of 4 additional TTS associated with the cloud and at least 2 young main sequence stars that are not associated with the cloud and place an upper limit on the age of the TTS in MBM12 ~ 10 Myr. The distance to MBM12 has been revised from the previous estimate of 65+-5 pc to 65+-35 pc based on results of the Hipparcos satellite. At this distance MBM12 is the nearest known molecular cloud to the sun with recent star formation. We estimate a star-formation efficiency for the cloud of 2-24%. We have also identified a reddened G9 star behind the cloud with Av ~ 8.4-8.9 mag. Therefore, there are at least two lines of sight through the cloud that show larger extinctions (Av > 5 mag) than previously thought for this cloud. This higher extinction explains why MBM12 is capable of star-formation while most other high-latitude clouds are not.
We use Spitzer data to infer that the small infrared excess of V819 Tau, a weak-lined T Tauri star in Taurus, is real and not attributable to a companion 10 arcsec to the south. We do not confirm the mid-infrared excess in HBC 427 and V410 X-ray 3, which are also non-accreting T Tauri stars in the same region; instead, for the former object, the excess arises from a red companion 9 arcsec to the east. A single-temperature blackbody fit to the continuum excess of V819 Tau implies a dust temperature of 143 K; however, a better fit is achieved when the weak 10 and 20 micron silicate emission features are also included. We infer a disk of sub-micron silicate grains between about 1 AU and several 100 AU with a constant surface density distribution. The mid-infrared excess of V819 Tau can be successfully modeled with dust composed mostly of small amorphous olivine grains at a temperature of 85 K, and most of the excess emission is optically thin. The disk could still be primordial, but gas-poor and therefore short-lived, or already at the debris disk stage, which would make it one of the youngest debris disk systems known.
We present a detailed analysis of high-resolution spectropolarimetric observations of the weak-line T Tauri stars (wTTSs) TWA 9A and V1095 Sco as part of a wider sur- vey of magnetic properties and activity in weak-line T Tauri stars, called MaTYSSE (Magnetic Topologies of Young Stars and the Survival of close-in giant Exoplanets). Our targets have similar masses but differing ages which span the stage of radiative core formation in solar-mass stars. We use the intensity line profiles to reconstruct surface brightness maps for both stars. The reconstructed brightness maps for TWA 9A and V1095 Sco were used to model and subtract the activity-induced jitter, reducing the RMS in the radial velocity measurements of TWA 9A by a factor of $sim$7, and for V1095 Sco by a factor of $sim$3. We detect significant circular polarisation for both stars, but only acquired a high quality circular polarisation time-series for TWA 9A. Our reconstructed large-scale magnetic field distribution of TWA 9A indicates a strong, non-axisymmetric field. We also analyse the chromospheric activity of both stars by investigating their H$alpha$ emission, finding excess blue-ward emission for most observations of V1095 Sco, and symmetric, double-peaked emission for TWA 9A, with enhanced emission at one epoch likely indicating a flaring event.
We present an analysis of spectropolarimetric observations of the low-mass weak-line T Tauri stars TWA 25 and TWA 7. The large-scale surface magnetic fields have been reconstructed for both stars using the technique of Zeeman Doppler imaging. Our surface maps reveal predominantly toroidal and non-axisymmetric fields for both stars. These maps reinforce the wide range of surface magnetic fields that have been recovered, particularly in pre-main sequence stars that have stopped accreting from the (now depleted) central regions of their discs. We reconstruct the large scale surface brightness distributions for both stars, and use these reconstructions to filter out the activity-induced radial velocity jitter, reducing the RMS of the radial velocity variations from 495 m/s to 32 m/s for TWA 25, and from 127 m/s to 36 m/s for TWA 7, ruling out the presence of close-in giant planets for both stars. The TWA 7 radial velocities provide an example of a case where the activity-induced radial velocity variations mimic a Keplerian signal that is uncorrelated with the spectral activity indices. This shows the usefulness of longitudinal magnetic field measurements in identifying activity-induced radial velocity variations.
We present observations of the T Tauri stars BP Tau, DG Tau, DI Tau, GM Aur, LkCa 15, RW Aur and V830 Tau, using long baseline infrared interferometry at K band (2.2 microns) from the Keck Interferometer. The target sources have a range of mass accretion rates and excess near-infrared emission. The interferometer is most sensitive to extended emission on characteristic size scales of 1 to 5 millarcseconds. All sources show evidence for resolved K band emission on these scales, although a few of the sources are marginally consistent with being unresolved. We calculate the infrared excess based on fitting stellar photosphere models to the optical photometry and estimate the physical size of the emission region using simple geometric models for the sources with a significant infrared excess. Assuming that the K band resolved emission traces the inner edge of the dust disk, we compare the measured characteristic sizes to predicted dust sublimation radii and find that the models require a range of dust sublimation temperatures and possibly optical depths within the inner rim to match the measured radii.
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