Results of the ROTOR-program. I. The long-term photometric variability of classical T Tauri stars

We present a unique, homogeneous database of photometric measurements for Classical T Tauri stars extending up to 20 years. The database contains more than 21,000 UBVR observations of 72 CTTs. All the data were collected within the framework of the ROTOR-program at Mount Maidanak Observatory (Uzbekistan) and together they constitute the longest homogeneous, accurate record of TTS variability ever assembled. We characterize the long term photometric variations of 49 CTTs with sufficient data to allow a robust statistical analysis and propose an empirical classification scheme. Several patterns of long term photometric variability are identified. The most common pattern, exhibited by a group of 15 stars which includes T Tau itself, consists of low level variability (Delta(V)<=0.4mag) with no significant changes occurring from season to season over many years. A related subgroup of 22 stars exhibits a similar stable long term variability pattern, though with larger amplitudes (up to Delta(V)~1.6 mag). Besides these representative groups, we identify three smaller groups of 3-5 stars each which have distinctive photometric properties. The long term variability of most CTTs is fairly stable and merely reflects shorter term variability due to cold and hot surface spots. Only a small fraction of CTTs undergo significant brightness changes on the long term (months, years), which probably arise from slowly varying circumstellar extinction.Comment: 16 pages, 11 figures. Astron. Astrophys., in pres

Another deep dimming of the classical T Tauri star RW Aur A

Context. RW Aur A is a classical T Tauri star (CTTS) with an unusually rich emission line spectrum. In 2014 the star faded by ~ 3 magnitudes in the V band and went into a long-lasting minimum. In 2010 the star suffered from a similar fading, although less deep. These events in RW Aur A are very unusual among the CTTS, and have been attributed to occultations by passing dust clouds. Aims. We want to find out if any spectral changes took place after the last fading of RW Aur A with the intention to gather more information on the occulting body and the cause of the phenomenon. Methods. We collected spectra of the two components of RW Aur. Photometry was made before and during the minimum. Results. The overall spectral signatures reflecting emission from accretion flows from disk to star did not change after the fading. However, blue-shifted absorption components related to the stellar wind had increased in strength in certain resonance lines, and the profiles and strengths, but not fluxes, of forbidden lines had become drastically different. Conclusions. The extinction through the obscuring cloud is grey indicating the presence of large dust grains. At the same time, there are no traces of related absorbing gas. The cloud occults the star and the interior part of the stellar wind, but not the wind/jet further out. The dimming in 2014 was not accompanied by changes in the accretion flows at the stellar surface. There is evidence that the structure and velocity pattern of the stellar wind did change significantly. The dimmings could be related to passing condensations in a tidally disrupted disk, as proposed earlier, but we also speculate that large dust grains have been stirred up from the inclined disk into the line-of-sight through the interaction with an enhanced wind.Comment: 5 pages, 5 figures, Accepted for publication in A&

The Stellar Composition of the Star Formation Region CMa R1. II. Spectroscopic and Photometric Observations of 9 Young Stars

We present new high and low resolution spectroscopic and photometric data of nine members of the young association CMa R1. All the stars have circumstellar dust at some distance as could be expected from their association with reflection nebulosity. Four stars (HD 52721, HD 53367, LkHalpha 220 and LkHalpha 218) show Halpha emission and we argue that they are Herbig Be stars with discs. Our photometric and spectroscopic observations on these stars reveal new characteristics of their variability. We present first interpretations of the variability of HD 52721, HD 53367 and the two LkHalpha stars in terms of a partially eclipsing binary, a magnetic activity cycle and circumstellar dust variations, respectively. The remaining five stars show no clear indications of Halpha emission in their spectra, although their spectral types and ages are comparable with those of HD 52721 and HD 53367. This indicates that the presence of a disc around a star in CMa R1 may depend on the environment of the star. In particular we find that all Halpha emission stars are located at or outside the arc-shaped border of the H II region, which suggests that the stars inside the arc have lost their discs through evaporation by UV photons from nearby O stars, or from the nearby (< 25 pc) supernova, about 1 Myr ago.Comment: 17 pages, 13 figures, accepted by MNRA

Constraints on the disk geometry of the T Tauri star AA Tau from linear polarimetry

We have simultaneously monitored the photometric and polarimetric variations of the Classical T Tauri star AA Tau during the fall of 2002. We combine these data with previously published polarimetric data covering two earlier epochs. The phase coverage is complete, although not contiguous. AA Tau clearly shows cyclic variations coupled with the rotation of the system. The star-disk system produces a repeatable polarisation curve where the polarisation increases with decreasing brightness. The data fit well with the model put forward by Bouvier et al. (1999) where AA Tau is viewed almost edge-on and its disk is actively dumping material onto the central star via magnetospheric accretion. The inner edge of the disk is deformed by its interaction with the tilted magnetosphere, producing eclipses as it rotates and occults the photosphere periodically. From the shape of the polarisation curve in the QU-Plane we confirm that the accretion disk is seen at a large inclination, almost edge-on, and predict that its position angle is PA~90 deg., i.e., that the disk's major axis is oriented in the East-West direction.Comment: Astron. Astrophys., in pres

Dynamo Processes in the T Tauri star V410 Tau

We present new brightness and magnetic images of the weak-line T Tauri star V410 Tau, made using data from the NARVAL spectropolarimeter at Telescope Bernard Lyot (TBL). The brightness image shows a large polar spot and significant spot coverage at lower latitudes. The magnetic maps show a field that is predominantly dipolar and non-axisymmetric with a strong azimuthal component. The field is 50% poloidal and 50% toroidal, and there is very little differential rotation apparent from the magnetic images. A photometric monitoring campaign on this star has previously revealed V-band variability of up to 0.6 magnitudes but in 2009 the lightcurve is much flatter. The Doppler image presented here is consistent with this low variability. Calculating the flux predicted by the mapped spot distribution gives an peak-to-peak variability of 0.04 magnitudes. The reduction in the amplitude of the lightcurve, compared with previous observations, appears to be related to a change in the distribution of the spots, rather than the number or area. This paper is the first from a Zeeman-Doppler imaging campaign being carried out on V410 Tau between 2009-2012 at TBL. During this time it is expected that the lightcurve will return to a high amplitude state, allowing us to ascertain whether the photometric changes are accompanied by a change in the magnetic field topology.Comment: 12 pages, 11 figures, accepted by MNRA

Observation of enhanced X-ray emission from the CTTS AA Tau during a transit of an accretion funnel

AA Tau was observed for about 5h per XMM orbit (2 days) over 8 successive orbits, which covers two optical eclipse periods (8.2 days). The XMM optical/UV monitor simultaneously provided UV photometry with a ~15 min sampling rate. Some V-band photometry was also obtained from the ground during this period in order to determine the dates of the eclipses. Two X-ray and UV measurements were secured close to the center of the eclipse. The UV flux is the highest just before the eclipse starts and the lowest towards the end of it. We model the UV flux variations with a weekly modulation (inner disk eclipse), plus a daily modulation, which suggests a non-steady accretion. No eclipses are detected in X-rays. For one measurement, the X-ray count rate was nearly 50 times stronger than the minimum observed level, and the plasma temperature reached 60 MK, i.e., a factor of 2-3 higher than in the other observations. This X-ray event, observed close to the center of the optical eclipse, is interpreted as an X-ray flare. We identify the variable column density with the low-density accretion funnel flows blanketing the magnetosphere. The lack of X-ray eclipses indicates that X-ray emitting regions are located at high latitudes. Furthermore, the occurrence of a strong X-ray flare near the center of the optical eclipse suggests that the magnetically active areas are closely associated with the base of the high-density accretion funnel flow. We speculate that the impact of this free falling accretion flow onto the strong magnetic field of the stellar corona may boost the X-ray emission (abridged).Comment: 17 pages and 9 Figures. Accepted by A&