Results of the ROTOR-program. I. The long-term photometric variability of classical T Tauri stars (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.
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.
Study of the optical variability of T Tau in the period 1962–2003
Astronomy Letters, 2005
We present the results of our long-term U BV R observations of the star T Tauri performed at Mt. Maidanak Observatory from 1986 until 2003. These data, together with previous photoelectric observations of other authors, suggest that the long-term variations of the light curve are not periodic, but have a cycle with a time scale of 6-9 yr. The light curve also exhibits slower variations with time scales of ∼30-40 yr. We confirm the existence of periodic brightness variations with a period of P = 2 d .798 over many years; this process is peculiar in that the phase and shape of the phase curve change from season to season. We analyze the color behavior of the star. We found evidence of a strong flare occurred on October 5, 1999, when the brightness of the star reached 9 m . 22. This is the strongest flare recorded during its photoelectric observations. c 2005 Pleiades Publishing, Inc.
TIME VARIABILITY OF EMISSION LINES FOR FOUR ACTIVE T TAURI STARS. I. OCTOBER–DECEMBER IN 2010
The Astronomical Journal, 2013
We present optical spectrophotometric monitoring of four active T Tauri stars (DG Tau, RY Tau, XZ Tau, RW Aur A) at high spectral resolution (R 1 × 10 4 ), to investigate the correlation between time variable mass ejection seen in the jet/wind structure of the driving source and time variable mass accretion probed by optical emission lines. This may allow us to constrain the understanding of the jet/wind launching mechanism, the location of the launching region, and the physical link with magnetospheric mass accretion. In 2010, observations were made at six different epochs to investigate how daily and monthly variability might affect such a study. We perform comparisons between the line profiles we observed and those in the literature over a period of decades and confirm the presence of time variability separate from the daily and monthly variability during our observations. This is so far consistent with the idea that these line profiles have a long term variability (3-20 years) related to episodic mass ejection suggested by the structures in the extended flow components. We also investigate the correlations between equivalent widths and between luminosities for different lines. We find that these correlations are consistent with the present paradigm of steady magnetospheric mass accretion and emission line regions that are close to the star.
The Astrophysical Journal, 2001
We have collected low-resolution spectrophotometric data of the classical T Tauri star TW Hya in an effort to detect and to follow the excess continuum emission (veiling) and the line changes at < 5100 Å . The deveiled and calibrated flux distribution resembles that of a 30 Myr K7-M1 star of radius R ¼ 0:8 R , mass M ¼ 0:7 M , and log g ¼ 4:5. The anticorrelation between the veiling (in the B band) and the observed Balmer jump found by previous authors, based on large samples of classical T Tauri stars, is confirmed in TW Hya. The line emission luminosities of the H, Ca ii, and He i lines correlate with one another throughout the series, supporting the claims that the bulk of the line emission is formed in a single region or that their growth is controlled by a common mechanism. Surprisingly, the line emission fluxes do not correlate with the veiling at 4250 Å (B band). The line luminosities are, in general, less than 1% of the continuum luminosities. The veiling time series presents a cyclic behavior at 4:4 AE 0:4 days. We collect all of the archival photometric data and analyze the B-band observations using different algorithms. We found solutions at either the 4.4 day timescale or one-half of this value. The data sets presenting the 2.2 day periodicity yield double-peaked light curves when folded at the 4.4 day timescale. We interpret the 4.4 day solution as the rotation period of the star. The veiling and the line emission measurements yield accretion luminosities for the series. We model the impacted area in the photosphere by an isothermal gas of a given density, temperature, and size () whose parameters change as the star rotates. Estimates of the total spot area (), as a percentage of the stellar projected area, lie within the range 2:5 < < 6:0. The accretion luminosity of the impacted region does not remain constant throughout the series. The mass accretion rate ( _ M M acc ) that governs the luminosity varies within 1:0 Â 10 À9 M yr À1 < _ M M acc < 4:8 Â 10 À8 M yr À1 . The spot luminosity and the associated _ M M acc are tightly correlated to the projected spot area, , and change their absolute value as the star spins. If most of the accretion is channeled to a single spot, its colatitude will be larger than 70 , indicating that the magnetic dipole is largely inclined.
Astronomy Letters, 2005
We present the results of our long-term U BV R observations of the star T Tauri performed at Mt. Maidanak Observatory from 1986 until 2003. These data, together with previous photoelectric observations of other authors, suggest that the long-term variations of the light curve are not periodic, but have a cycle with a time scale of 6-9 yr. The light curve also exhibits slower variations with time scales of ∼30-40 yr. We confirm the existence of periodic brightness variations with a period of P = 2 d .798 over many years; this process is peculiar in that the phase and shape of the phase curve change from season to season. We analyze the color behavior of the star. We found evidence of a strong flare occurred on October 5, 1999, when the brightness of the star reached 9 m . 22. This is the strongest flare recorded during its photoelectric observations. c 2005 Pleiades Publishing, Inc.
2012
Context. The first million years of the pre-main-sequence (PMS) evolution of low-mass stars are characterized by magnetospheric accretion, a circumstellar proto-planetary disk, and the processes leading to its dispersal. Among these, photo-evaporation caused by strong X-ray emission from the central star is probably significant. Several aspects of the X-ray emission from coronae and accretion shocks remain mysterious, e.g., whether and how much accretion affects coronal emission. Aims. We studied the X-ray variability of ∼1Myr old low-mass PMS stars as a function of timescale, stellar rotation, and stellar characteristics to gain insights into the working of PMS coronae, their X-ray emission, and the circumstellar environment in which they are immersed. Methods. We have exploited the ∼850 ksec long Chandra observation of the Orion Nebula Cluster obtained by the COUP collaboration in January 2003, and statistically analyzed the X-ray lightcurves of low-mass stars in several subsamples. Our main focus was to characterize the different X-ray behavior of stars with and without circumstellar accretion disks, and to infer the physical mechanism responsible for the observed variability. Results. Accreting stars (classical T Tauri stars, CTTSs) are found to be more variable than non-accreting stars (weak-line T Tauri stars, WTTSs) at all timescales and in all X-ray energy bands considered. Variability is seen to increase with timescale, up to the longest probed, ∼10 days. Signatures of rotational modulation are observed for both CTTSs and WTTSs, and most clearly for CTTSs in the soft X-ray band. Lower mass stars are more variable than higher mass stars. Conclusions. We propose that the difference in variability between CTTSs and WTTSs may be explained assuming that the X-ray emission of CTTS is affected by time-variable absorption due to circumstellar structures, such as warps in the inner disk and/or accretion streams. This suggestion is appealing because, in the hypothesis that the coronae of CTTSs and WTTSs are similar, it may also explain why CTTSs have lower and more scattered X-ray emission levels compared to WTTSs.
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.
The Astronomical Journal, 2010
We provide an overview of stellar variability in the first quarter of data from the Kepler mission. The intent of this paper is to examine the entire sample of over 150,000 target stars for periodic behavior in their lightcurves, and relate this to stellar characteristics. These data constitute an unprecedented study of stellar variability given its great precision and complete time coverage (with a half hour cadence). Because the full Kepler pipeline is not currently suitable for a study of stellar variability of this sort, we describe our procedures for treating the "raw" pipeline data. About half of the total sample exhibits convincing periodic variability up to two weeks, with amplitudes ranging from differential intensity changes less than 10 −4 up to more than 10 percent. K and M dwarfs have a greater fraction of period behavior than G dwarfs. The giants in the sample have distinctive quasi-periodic behavior, but are not periodic in the way we define it. Not all periodicities are due to rotation, and the most significant period is not necessarily the rotation period. We discuss properties of the lightcurves, and in particular look at a sample of very clearly periodic G dwarfs. It is clear that a large number of them do vary because of rotation and starspots, but it will take further analysis to fully exploit this.
A long-term photometric study of V 1184 Tauri
Astronomy and Astrophysics, 2008
Aims. The main purpose of our investigation is to construct the long-time light curve of the PMS star V 1184 Tau. We consider the study of the photometrical variability of PMS stars as very important for understanding stellar evolution. The unusual photometric variability of V 1184 Tau was reported in previous studies as well, but the nature of the observed deep minima is still under discussion. Methods. We present recent data from CCD photometry and from archival photographic plates. The photometric BVRI data presented in this paper were collected from November 2005 to November 2007. To construct the historical light curve of V 1184 Tau, a search for archived photographic observations in the Wide-Field Plate Database (WFPDB) was made. As a result, 412 photographic plates were found containing the field of V 1184 Tau. A part of the plates were scanned at our request and a magnitude estimation was made of V 1184 Tau. Results. Our recent photometric data suggest that the star brightness is still near the minimum. Thus the period of strong light variations that started in 2003 continues up to the present, hence more than 4 years. Our data from the archival photographic plates suggest that an unknown minimum of brightness exists in the approximate period 1980−1985. Taking all available photometric and spectroscopic data into account we must reject the hypothesis that V 1184 Tau is an FU Orionis type object. V 1184 Tau is a G type low-mass star whose spectrum is similar to WTT stars, but its photometric behavior is typical of the UX Orionis variable stars. Assuming the obscuration from orbiting dust clouds as a reason for a deep minimum, we estimate the approximate period of obscurations and the interval between two deep minima. Our calculations give a 25−28 year period between the two minima and approximately 8−10 year duration of the minima.