The Search for Rotational Modulation of T Tauri Stars in the Ophiuchus Dark Clouds (original) (raw)
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Results of the ROTOR-program-II. The long-term photometric variability of weak-line T Tauri stars
Context. T Tauri stars exhibit variability on all timescales, whose origin is still debated. On WTTS the variability is fairly simple and attributed to long-lived, ubiquitous cool spots. Aims. We investigate the long term variability of WTTS, extending up to 20 years in some cases, characterize it statistically and discuss its implications for our understanding of these stars. Methods. We have obtained a unique, homogeneous database of photometric measurements for WTTS extending up to 20 years. It contains more than 9 000 UBV R observations of 48 WTTS. All the data were collected at Mount Maidanak Observatory (Uzbekistan) and they constitute the longest homogeneous record of accurate WTTS photometry ever assembled. Results. Definitive rotation periods for 35 of the 48 stars are obtained. Phased light curves over 5 to 20 seasons are now available for analysis. Light curve shapes, amplitudes and colour variations are obtained for this sample and various behaviors exhibited, discussed and interpreted. Conclusions. Our main conclusion is that most WTTS have very stable long term variability with relatively small changes of amplitude or mean light level. The long term variability seen reflects modulation in the cold spot distributions. Photometric periods are stable over many years, and the phase of minimum light can be stable as well for several years. On the long term, spot properties do change in subtle ways, leading to secular variations in the shape and amplitudes of the light curves.
Results of the ROTOR-program. I. The long-term photometric variability of classical T Tauri stars
Astronomy and Astrophysics
Context. T Tauri stars exhibit variability on all timescales, whose origin is still debated. Aims. We investigate the long term variability of CTTs over up to 20 years, characterize it from a set of statistical parameters and discuss its origin. Methods. 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 U BV R 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. Results. 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 (∆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 ∆V ≃1.6 mag). Besides these representative groups, we identify three smaller groups of 3-5 stars each which have distinctive photometric properties. Conclusions. 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. Key words. stars: activity of stars -pre-main-sequence stars -variables: general 2 K.N. Grankin et al.: Results of the ROTOR-program. I. The long-term photometric variability of CTTs
Photometric variability of the T Tauri star TW Hya on time-scales of hours to years ★
Monthly Notices of the Royal Astronomical Society, 2008
MOST (Microvariability & Oscillations of STars) and ASAS (All Sky Automated Survey) observations have been used to characterize photometric variability of TW Hya on time scales from a fraction of a day to 7.5 weeks and from a few days to 8 years, respectively. The two data sets have very different uncertainties and temporal coverage properties and cannot be directly combined, nevertheless, they suggests a global variability spectrum with "flicker noise" properties, i.e. with amplitudes a ∝ 1/ √ f , over > 4 decades in frequency, in the range f = 0.0003 to 10 cycles per day (c/d). A 3.7 d period is clearly present in the continuous 11 day, 0.07 d time resolution, observations by MOST in 2007. Brightness extrema coincide with zero-velocity crossings in periodic (3.56 d) radial velocity variability detected in contemporaneous spectroscopic observations of Setiawan et al. (2008) and interpreted as caused by a planet. The 3.56/3.7 d periodicity was entirely absent in the second, four times longer MOST run in 2008, casting doubt on the planetary explanation. Instead, a spectrum of unstable single periods within the range of 2-9 days was observed; the tendency of the periods to progressively shorten was well traced using the wavelet analysis. The evolving periodicities and the overall flicker-noise characteristics of the TW Hya variability suggest a combination of several mechanisms, with the dominant ones probably related to the accretion processes from the disk around the star.
Rotation periods of Post-T Tauri stars in Lindroos systems
Astronomy and Astrophysics, 2004
We present a rotational study of Post-T Tauri stars (PTTSs) in Lindroos systems, defined as binaries with early type primaries on the main-sequence (MS) and late-type secondaries on the pre-main-sequence (PMS) phase. The importance of this study in comparison with previous ones is that the Lindroos sample is not X-ray selected so we avoid a possible bias towards fast rotators. In this preliminary study we have monitored eleven stars in the UBVRI bands during two campaigns of ten consecutive nights each. Eight of the observed PTTSs show periodic modulations in their lightcurves and the derived periods range from 1.9 d to 8.0 d. The comparison of these results with theoretical rotational tracks based on disk-star locking theory shows that star-disk decoupling times of 1−20 Myr could reproduce the rotational properties of the targets, assuming an initial rotation period of ∼8 d and a mass of 1 M. We have studied the rotation-activity relations of Lindroos PTTSs and compared them with those found in other groups of PMS and zero-age main-sequence (ZAMS) ∼1 M stars. The Lindroos sample displays activity-rotation relations very similar to those found in TTSs. It contains a mixture of very active stars, with L X /L bol ratios close to the saturation level of-3, and less active (unsaturated) stars. This could be the result of different star-disk decoupling times. Future monitoring of a larger and unbiased sample of PTTS will be important to confirm the significance of these results.
On the nature of variations in the T Tauri star WY Arietis (LkHα264)
Astronomy & Astrophysics, 2002
We report optical spectroscopic and photometric results from our long-term study of the T Tauri star WY Arietis (LkHα 264). The data gathered show different types of variability: variations in the continuum level, in the emission line fluxes and line profiles. The timescales associated with these variations appear quite diverse. The correlation found between the variations observed in the veiling and in the continuum flux strongly suggest that an extra continuum source veiling the stellar photospheric spectrum is the cause driving the continuum variability. The present work also unveils the presence of an accretion flow onto the star, as revealed by the O I λ7773Å and λ8446Å line profiles, which is the first unambiguous model-independent detection of such an event in this star. Our photometric data allowed us to find a period of 3.04 days for this star, somewhat in tune with Fernandez & Eiroa (1996). However, due to the poor time sampling our finding should be taken as tentative. A detailed analysis of the broad and narrow components of the He I line profiles indicates the presence of a hot wind during the November 1993 observation while in October 1999 a wind is only revealed by the blue wing asymmetry of the observed Balmer and CaII infrared triplet line profiles. The correlation between the strength of the hot wind and the amount of flux in the emission lines led also to the conclusion that this type of wind provides a significant contribution to the hydrogen and metal emission lines. We have also witnessed an exceptional activity during one of the nights which may be attributed to an increase in LkHα 264's accretion rate or to a flare-like event. Although it is not possible to clearly distinguish between these possibilities, the available data set points towards variable accretion as being responsible for the observed event.
The Astrophysical Journal, 2002
We present the analysis of 42 spectra of the Classical T Tauri star TW Hya observed with the FEROS echelle spectrograph over 2 yr. We determined the rotational and radial velocities of TW Hya, obtaining v sin i ¼ 5 AE 2 km s À1 and v rad ¼ 12:5 AE 0:5 km s À1 . The star exhibits strong emission lines that show substantial variety and variability in their profile shapes. Emission lines such as H, H, and He i show both outflow and infall signatures, which change on different timescales. The system displays periodic variations in line and veiling intensities, but the stellar rotation period remains uncertain. We see evidence of a variation in the mass accretion rate over a 1 yr period from the Na D line profiles that are well fitted by magnetospheric accretion models with moderate mass accretion rates (10 À9 up to 10 À8 M yr À1 ). The lower values inferred from the models are close to the average mass accretion rate obtained from the veiling estimates (~2 Â 10 À9 M yr À1 ), but the veiling results are consistent with a constant mass accretion rate within the errors of the calculations. The H, He i, Na D, and H emission-line equivalent widths corrected from veiling correlate well with each other and are correlated with the veiling, indicating the same mechanism should be powering them and suggesting an origin related to the accretion process. The wings of the main emission lines are generally correlated, except when the Balmer lines exhibit properties suggesting a strong contribution from a wind. The blueward absorption components of the Balmer lines, most likely from a wind, are not correlated with veiling. The spectroscopic analysis allows us to infer the inclination of the stellar rotation axis (i ¼ 18 AE 10 ) that matches the current estimations of the disk orientation (0 < i < 15 ). A magnetospheric dipole axis that is misaligned with the stellar/disk rotation axis could produce the observed photometric variability, and we tend to favor a low inclination but not a totally face-on geometry for the system. TW Hya exhibits typical spectral characteristics of many classical T Tauri stars in Taurus despite its older age, indicating that active accretion disks can readily survive up to 10 Myr.
Rotation effects in classical T Tauri stars
Astronomy Letters, 2012
Surface temperature inhomogeneities in classical T Tauri stars (CTTS) induced by magnetic activity and mass accretion lead to rotational modulation of both photometric and spectroscopic parameters of these stars. Using the extended photometric catalogue by Grankin et al. , we have derived the periods and amplitudes of the rotational modulation of brightness and color for 31 CTTS; for six of them, the periods have been revealed for the first time. The inclinations of the rotation axis and equatorial rotational velocities of CTTS have been determined. We show that the known periods of brightness variations for some of the CTTS are not the axial rotation periods but are the Keplerian periods near the inner boundary of the dusty disk. We have found that the angular velocity of CTTS with a mass of 0.3-3 M ⊙ in the Taurus-Auriga complex remains constant in the age range 1-10 Myr. CTTS on radiative evolutionary tracks rotate faster than completely convective CTTS. The specific angular momentum of CTTS depends on the absolute luminosity in the Hα line.
Stable and unstable accretion in the classical T Tauri stars IM Lup and RU Lup as observed by MOST
Monthly Notices of the Royal Astronomical Society, 2016
Results of the time variability monitoring of the two classical T Tauri stars, RU Lup and IM Lup, are presented. Three photometric data sets were utilized: (1) simultaneous (same field) MOST satellite observations over four weeks in each of the years 2012 and 2013, (2) multicolour observations at the South African Astronomical Observatory in April-May of 2013, (3) archival V-filter All Sky Automated Survey (ASAS) data for nine seasons, 2001-2009. They were augmented by an analysis of high-resolution, public-domain VLT-UT2 Ultraviolet Visual Echelle Spectrograph spectra from the years 2000 to 2012. From the MOST observations, we infer that irregular light variations of RU Lup are caused by stochastic variability of hotspots induced by unstable accretion. In contrast, the MOST light curves of IM Lup are fairly regular and modulated with a period of about 7.19-7.58 d, which is in accord with ASAS observations showing a well-defined 7.247 ± 0.026 d periodicity. We propose that this is the rotational period of IM Lup and is due to the changing visibility of two antipodal hotspots created near the stellar magnetic poles during the stable process of accretion. Reanalysis of RU Lup high-resolution spectra with the broadening function approach reveals signs of a large polar coldspot, which is fairly stable over 13 years. As the star rotates, the spot-induced depression of intensity in the broadening function profiles changes cyclically with period 3.710 58 d, which was previously found by the spectral cross-correlation method.
COYOTES IV: the rotational periods of low-mass Post-T Tauri stars in Taurus
Astronomy & Astrophysics, 1997
We monitored the light variations of 58 weak-line T Tauri stars in Taurus, recently discovered in the X-ray wavelength range during the ROSAT All-Sky Survey. We derive photometric periods for 18 stars, all but one being ascribed to rotational modulation by stellar spots. The exception is a 37.6d period assigned to the orbital motion of a new pre-main sequence spectroscopic
The weak-line T Tauri star V410 Tau
Astronomy & Astrophysics, 2003
We present the results of an intensive coordinated monitoring campaign in the optical and X-ray wavelength ranges of the low-mass, pre-main sequence star V410 Tau carried out in November 2001. The aim of this project was to study the relation between various indicators for magnetic activity that probe different emitting regions and would allow us to obtain clues on the interplay of the different atmospheric layers: optical photometric star spot (rotation) cycle, chromospheric Hα emission, and coronal X-rays. Our optical photometric monitoring has allowed us to measure the time of the minimum of the lightcurve with high precision. Joining the result with previous data we provide a new estimate for the dominant periodicity of V410 Tau (1.871970± 0.000010 d). This updated value removes systematic offsets of the time of minimum observed in data taken over the last decade. The recurrence of the minimum in the optical lightcurve over such a long timescale emphasizes the extraordinary stability of the largest spot. This is confirmed by radial velocity measurements: data from 1993 and 2001 fit almost exactly onto each other when folded with the new period. The combination of the new data from November 2001 with published measurements taken during the last decade allows us to examine long-term changes in the mean light level of the photometry of V410 Tau. A variation on the timescale of 5.4 yr is suggested. Assuming that this behavior is truly cyclic V410 Tau is the first pre-main sequence star on which an activity cycle is detected. Two X-ray pointings were carried out with the Chandra satellite simultaneously with the optical observations, and centered near the maximum and minimum levels of the optical lightcurve. A relation of their different count levels to the rotation period of the dominating spot is not confirmed by a third Chandra observation carried out some months later, during another minimum of the 1.87 d cycle. Similarly we find no indications for a correlation of the Hα emission with the spots' rotational phase. The lack of detected rotational modulation in two important activity diagnostics seems to argue against a direct association of chromospheric and coronal emission with the spot distribution.