V711 Tau Research Articles

Spectral variations of RS CVn binary system (V711 Tau) in the ultraviolet

Spectral variations of RS CVn binary system (V711 Tau) in the ultraviolet

Time-lapse Very Long Baseline Interferometry Imaging of the Close Active Binary HR 1099

We report multiepoch astrometric very long baseline interferometry observations of the chromospherically active binary HR 1099 (V711 Tau, HD 22468) at six epochs over 63 days using the Very Long Baseline Array at 22.2 GHz. We determined hourly radio positions at each epoch with a positional uncertainty significantly smaller than the component separation. The aggregate radio positions at all epochs define an ellipse in the comoving reference frame with an inclination i=39.°5−3.°5+3.°6 and longitude of ascending node Ω = 212° ± 22°. The ellipse center is offset from the Third Gaia Celestial Reference Frame position by Δα = −0.81−0.37+0.25 , Δδ = 0.45−0.25+0.23 mas. All radio centroids are well displaced from the binary center of mass at all epochs, ruling out emission from the interbinary region. We examined the motion of the radio centroids within each epoch by comparing hourly positions over several hours. The measured speeds were not statistically significant for five of the six epochs, with 2σ upper limits in the range 200–1000 km s−1. However, for one flaring epoch, there was a 3σ detection v ⊥ = 228 ± 85 km s−1. This speed is comparable to the mean speed of observed coronal mass ejections on the Sun.

Magnetic activity on two contact binaries: VW Boo and V1128 Tau from TESS data

Using the continuous TESS data, we found small intrinsic variations in the light curves of VW Boo and V1128 Tau. We selected five sets of light curves and derived the photometric solutions by using the Wilson-Devinney code. The results indicate that the intrinsic light curve variations in VW Boo are attributed to the evolution of a cool spot. In the case of V1128 Tau, the disappearance or reappearance of a cool spot might account for the variation in three sets of light curves. The obtained solutions also reveal that these two systems are W-type shallow-contact binaries (f≤15%). Period analyses for these two systems were carried out based on all available times of minimum. The period analysis of VW Boo shows that its period is decreasing at a rate of dP/dt=−1.52×10−7d yr−1, together with a cyclic oscillation with a period of 27.17 yr and a semi-amplitude of 0.0063 d. Similarly, the orbital period of V1128 Tau also shows a secular decrease (dP/dt=−1.40×10−7d yr−1), accompanied by a cyclic oscillation (P3= 16.11 yr and K3=0.0011 d). The secular decrease might be caused by mass transfer, while the cyclic oscillation might result from the presence of a third body or magnetic activity.

A search for thermal gyro-synchrotron emission from hot stellar coronae

ABSTRACT We searched for thermal gyro-synchrotron radio emission from a sample of five radio-loud stars whose X-ray coronae contain a hot ($T_e\gt 10^7\,$K) thermal component. We used the JVLA to measure Stokes I and V/I spectral energy distributions (SEDs) over the frequency range 15 – 45 GHz, determining the best-fitting model parameters using power-law and thermal gyro-synchrotron emission models. The SEDs of the three chromospherically active binaries (Algol, UX Arietis, HR 1099) were well-fit by a power-law gyro-synchrotron model, with no evidence for a thermal component. However, the SEDs of the two weak-lined T Tauri stars (V410 Tau, HD 283572) had a circularly polarized enhancement above 30 GHz that was inconsistent with a pure power-law distribution. These spectra were well-fit by summing the emission from an extended coronal volume of power-law gyro-synchrotron emission and a smaller region with thermal plasma and a much stronger magnetic field emitting thermal gyro-synchrotron radiation. We used Bayesian inference to estimate the physical plasma parameters of the emission regions (characteristic size, electron density, temperature, power-law index, and magnetic field strength and direction) using independently measured radio sizes, X-ray luminosities, and magnetic field strengths as priors, where available. The derived parameters were well-constrained but somewhat degenerate. The power-law and thermal volumes in the pre-main-sequence stars are probably not co-spatial, and we speculate they may arise from two distinct regions: a tangled-field magnetosphere where reconnection occurs and a recently discovered low-latitude poloidal magnetic field, respectively.

Variability of eclipse timing: the case of V471 Tauri

ABSTRACT The post-common envelope binary V471 Tauri has been an object of interest for decades. V471 Tau shows various phenomena due to its evolutionary state and unique properties, e.g. its magnetic accretion and eclipse timing variation (ETV). Previous authors explained the ETVs by different sometimes contradictory theories. In this paper, we present and analyse the variability of the eclipse timing of this star. We observed V471 Tauri over the last 10 yr and covered the second cycle of its period variation. Based on our analysis of the presented data, we assess the possible existence of a brown dwarf in this system and derive its orbital parameters. We compare the results of our dynamical modelling to the solution predicted by Applegate-mechanism theories, which have been developed in recent studies. We found that the observed ETV cannot be explained only by the presence of additional components to the binary.

Eclipse of the V773 Tau B circumbinary disc

Context. Young multiple stellar systems can host both circumstellar and circumbinary discs composed of gas and dust, and the orientations of circumbinary discs can be sculpted by the orientation and eccentricity of the central binaries. Studying multiple binary systems and their associated discs enables our understanding of the size and distribution of the planetary systems that subsequently form around them. Aims. A deep (~70%) and extended (~150 days) eclipse was seen towards the young multiple stellar system V773 Tau in 2010. We interpret it as being due to the passage of a circumbinary disc around the B components moving in front of the A components. Our aim is to characterise the orientation and structure of the disc, to refine the orbits of the sub-components, and to predict when the next eclipse will occur. Methods. We combined the photometry from several ground-based surveys, constructed a model for the light curve of the eclipse, and used high angular resolution imaging to refine the orbits of the three spatially resolved components of the system: A, B, and C. A frequency analysis of the light curves, including from the TESS satellite, enabled the characterisation of the rotational periods of the Aa and Ab stars. Results. A toy model of the circumbinary disc shows that it extends out to approximately 5 au around the B binary and has an inclination of 73° with respect to the orbital plane of AB, where the lower bound of the radius of the disc is constrained by the geometry of the AB orbit and the upper bound is set by the stability of the disc. We identify several frequencies in the photometric data that we attribute to rotational modulation of the Aa and Ab stellar companions. We produced the first determination of the orbit of the more distant C component around the AB system and limited its inclination to 93°. Conclusions. The high inclination and large diameter of the disc, together with the expected inclination of the disc from theory, suggest that B is an almost equal-mass, moderately eccentric binary. We identify the rotational periods of the Aa and Ab stars and a third frequency in the light curve that we attribute to the orbital period of the stars in the B binary. We predict that the next eclipse will occur around 2037, during which both detailed photometric and spectroscopic monitoring will characterise the disc in greater detail.

A TESS search for donor-star pulsations in high-mass X-ray binaries

ABSTRACT Ground-based optical photometry of the counterparts of high-mass X-ray binaries (HMXBs) has revealed the presence of periodic modulations on time-scales of ∼0.3–0.5 d. More recent space-based observations (CoRoT and TESS) of OB and Be stars have shown that pulsations caused by p and g modes are common in early-type stars. We have therefore undertaken a systematic search for variability in the optical counterparts of 23 HMXBs (mostly neutron star systems, but including one black hole, Cyg X-1) using TESS data primarily in 2 min cadence mode. After removing the orbital period modulation in four systems, we find that all 23 sources show evidence for quasi-periodic variability on periods shorter than ∼1 d. We compare their power spectra with those from observations of other OB- and Be-type stars. In two systems, V725 Tau and HD 249179 (which may not be an HMXB), we find evidence for an outburst, the former being simultaneous with an X-ray flare. We search for changes in the power spectra over the outburst duration and compare them with outbursts seen in other Be systems.

Magnetic field evolution of the K2 dwarf V471 Tau

ABSTRACT Observations of the eclipsing binary system V471 Tau show that the time of the primary eclipses varies in an apparent periodic way. With growing evidence that the magnetically active K2 dwarf component might be responsible for driving the eclipse timing variations (ETVs), it is necessary to monitor the star throughout the predicted ∼35 yr activity cycle that putatively fuels the observed ETVs. We contribute to this goal with this paper by analysing spectropolarimetric data obtained with ESPaDOnS at the Canada–France–Hawaii Telescope in 2014 December and 2015 January. Using Zeeman–Doppler Imaging, we reconstruct the distribution of brightness inhomogeneities and large-scale magnetic field at the surface of the K2 dwarf. Compared to previous tomographic reconstructions of the star carried out with the same code, we probe a new phase of the ETVs cycle, offering new constraints for future works exploring whether a magnetic mechanism operating in the K2 dwarf star is indeed able to induce the observed ETVs of V471 Tau.

Revised Stellar Parameters for V471 Tau, A Post-common Envelope Binary in the Hyades

Abstract V471 Tau is a post-common-envelope binary consisting of an eclipsing DA white dwarf and a K-type main-sequence star in the Hyades star cluster. We analyzed publicly available photometry and spectroscopy of V471 Tau to revise the stellar and orbital parameters of the system. We used archival K2 photometry, archival Hubble Space Telescope spectroscopy, and published radial-velocity measurements of the K-type star. Employing Gaussian processes to fit for rotational modulation of the system flux by the main-sequence star, we recovered the transits of the white dwarf in front of the main-sequence star for the first time. The transits are shallower than would be expected from purely geometric occultations owing to gravitational microlensing during transit, which places an additional constraint on the white-dwarf mass. Our revised mass and radius for the main-sequence star is consistent with single-star evolutionary models given the age and metallicity of the Hyades. However, as noted previously in the literature, the white dwarf is too massive and too hot to be the result of single-star evolution given the age of the Hyades, and may be the product of a merger scenario. We independently estimate the conditions of the system at the time of common envelope that would result in the measured orbital parameters today.

Light curve analysis of the eclipsing binary system V781 Tau

V781 Tau is one of W UMa eclipsing binary systems whose orbital period is 0.34 days. The 0.7-meter telescope with CCD photometric system in B and V filters was conducted at the Regional Observatory for the Public, Chachoengsao, Thailand during December 2018, UT. The Wilson-Devinney Technique was used for calculating the physical properties of V781 Tau. The results showed the inclination of their orbital is 66.140°±0.14. The effective temperature of the primary and secondary star is 6,060 and 5,881 K, respectively and the degree of contact is 4.38 %

T Tauri stars: magnetic fields and planets

In this short review we present the results of a study of the large-scale magnetic topologies of T Tauri stars (TTS). A small spectropolarimetric survey of 8 young stars was carried out within two international projects MaPP (Magnetic Protostars and Planets) and MaTYSSE (Magnetic Topologies of Young Stars and the Survival of massive close-in Exoplanets) between 2009 and 2016. For each of our targets we reconstructed the brightness map and the magnetic field topology using Zeeman–Doppler imaging (ZDI). This review contains a brief description of spectropolarimetricdata, the ZDI method, one example of the reconstruction of brightness and magnetic maps, and the properties of magnetic fields of 8 TTS. Our results suggest that AA Tau and LkCa 15 interact with their disks in the propeller mode when their rotation is actively slowed by the star/disk magnetic coupling. We find that magnetic fields of some TTS are variable on a time scale of a few years and are thus intrinsically nonstationary. We report on the detection of a giant exoplanet around V830 Tau and TAP 26. These two new detections suggest that the type II disk migration is efficient at generating newborn hot Jupiters (hJs) around young TTS. The result of our survey is compared to the global picture of magnetic field properties of twenty TTS in the Hertzsprung–Russell diagram. The comparison shows that WTTS exhibit a wider range of field topologies as compared to CTTS, and that magnetic fields of all TTS (CTTS and WTTS as a whole) are mostly poloidal and axisymmetric when they are mostly convective and cooler than 4300 K. This needs to be confirmed with a larger sample of stars.

The T Tauri star V410 Tau in the eyes of SPIRou and TESS

ABSTRACT We report results of a spectropolarimetric and photometric monitoring of the weak-line T Tauri star V410 Tau based on data collected mostly with SPIRou, the near-infrared (NIR) spectropolarimeter recently installed at the Canada-France-Hawaii Telescope, as part of the SPIRou Legacy Survey large programme, and with TESS between October and December 2019. Using Zeeman–Doppler Imaging (ZDI), we obtained the first maps of photospheric brightness and large-scale magnetic field at the surface of this young star derived from NIR spectropolarimetric data. For the first time, ZDI is also simultaneously applied to high-resolution spectropolarimetric data and very-high-precision photometry. V410 Tau hosts both dark and bright surface features and magnetic regions similar to those previously imaged with ZDI from optical data, except for the absence of a prominent dark polar spot. The brightness distribution is significantly less contrasted than its optical equivalent, as expected from the difference in wavelength. The large-scale magnetic field (${\sim}410$ G), found to be mainly poloidal, features a dipole of ${\sim}390$ G, again compatible with previous studies at optical wavelengths. NIR data yield a surface differential rotation slightly weaker than that estimated in the optical at previous epochs. Finally, we measured the radial velocity of the star and filtered out the stellar activity jitter using both ZDI and Gaussian Process Regression down to a precision of ${\sim}0.15$ and 0.08 $\mathrm{km}\, \mathrm{s}^{-1}$ RMS, respectively, confirming the previously published upper limit on the mass of a potential close-in massive planet around V410 Tau.

Chandra X-Ray Observations of V830 Tau: A T Tauri Star Hosting an Evanescent Planet

Abstract The radial velocity study by Donati et al. (2016) reported the detection of a close-in giant planet in a 4.93 day orbit around the ∼2 Myr old weak-lined T Tauri star V830 Tau. Because of the stringent timescale constraints that a very young host star like V830 Tau would place on hot-Jupiter formation models and inward-migration mechanisms, independent confirmation of the planet’s existence is needed but so far has not been obtained. We present new Chandra X-ray observations of V830 Tau. The Chandra observations in combination with previous XMM-Newton observations reveal strong, variable X-ray emission with an X-ray luminosity spanning the range log L x = 30.10–30.87 erg s−1. Chandra High Energy Transmission Grating spectra show emission lines formed over a range of plasma temperatures from ∼4 MK (Ne ix) to ∼16 MK (S xv). At the separation of the reported planet (0.057 au) the X-ray flux is ∼106–107 times greater than the Sun’s X-ray flux at Jupiter. We provide estimates of the X-ray ionization and atmospheric heating rates at the planet’s separation and identify areas of uncertainty that will need to be addressed in any future atmospheric models.

A confined dynamo: Magnetic activity of the K-dwarf component in the pre-cataclysmic binary system V471 Tauri

Context.Late-type stars in close binary systems can exhibit strong magnetic activity owing to rapid rotation supported by tidal locking. On the other hand, tidal coupling may suppress the differential rotation which is a key ingredient of the magnetic dynamo.Aims.We studied the red dwarf component in the eclipsing binary system V471 Tau in order to unravel the relations between the different activity layers, from the stellar surface through the chromosphere up to the corona. Our aim is to study how the magnetic dynamo in the late-type component is affected by the close white dwarf companion.Methods.We used space photometry, high-resolution spectroscopy, and X-ray observations from different space instruments to explore the main characteristics of magnetic activity. We applied a light curve synthesis program to extract the eclipsing binary model and to analyze the residual light variations. Photometric periods were obtained using a Fourier-based period search code. We searched for flares by applying an automated flare detection code. Spectral synthesis was used to derive or specify some of the astrophysical parameters. Doppler imaging was used to reconstruct surface temperature maps, which were cross-correlated to derive surface differential rotation. We applied different conversion techniques to make it possible to compare the X-ray emissions obtained from different space instruments.Results.From the K2 photometry we found that 5–10 per cent of the apparent surface of the red dwarf is covered by cool starspots. From seasonal photometric period changes we estimated a weak differential rotation. From the flare activity we derived a cumulative flare frequency diagram which suggests that frequent flaring could have a significant role in heating the corona. Using high-resolution spectroscopy we reconstructed four Doppler images for different epochs which reveal an active longitude, that is, a permanent dominant spot facing the white dwarf. From short term changes in the consecutive Doppler images we derived a weak solar-type surface differential rotation withαDR = 0.0026 shear coefficient, similar to that provided by photometry. The long-term evolution of X-ray luminosity reveals a possible activity cycle length of ≈12.7 yr, traces of which were also discovered in the Hαspectra.Conclusions.We conclude that the magnetic activity of the red dwarf component in V471 Tau is strongly influenced by the close white dwarf companion. We confirm the presence of a permanent dominant spot (active longitude) on the red dwarf facing the white dwarf. The weak differential rotation of the red dwarf is very likely the result of tidal confinement by the companion. We find that the periodic appearance of the inter-binary Hαemission from the vicinity of the inner Lagrangian point is correlated with the activity cycle.

Magnetic field and activity phenomena of the K2 dwarf V471 Tau

ABSTRACT We analyse spectropolarimetric data of the pre-cataclysmic variable binary system V471 Tau obtained with ESPaDOnS at the Canada–France–Hawaii Telescope in two observational campaigns (in 2004 November/December and 2005 December). Using Zeeman–Doppler imaging, we reconstruct the distribution of brightness map and large-scale magnetic field of the K2 dwarf at both epochs, as well as the amount of differential rotation by which surface maps are sheared. We detect significant fluctuations in the surface shear between the two campaigns. It goes from about twice the solar differential rotation rate to less than the solar value in a 1-yr interval. We conclude that the differential rotation fluctuations obtained for the K2 dwarf resemble those detected on the single-star analogue AB Dor, although even larger amplitudes of variation are seen in the K2 dwarf of V471 Tau. Finally, we show that the differential rotation results obtained in this work do not favour an Applegate mechanism operating in the V471 Tau system, at least in its standard form, but leave room for explaining the observed orbital period fluctuations with exotic forms of similar phenomena based on dynamo processes operating within the convective zone of the K2 star.

The Continuous Time Spectrum of Long-Term Activity of the Fast-Rotating K2 Dwarf V833 Tau

The Continuous Time Spectrum of Long-Term Activity of the Fast-Rotating K2 Dwarf V833 Tau

The weak-line T Tauri star V410 Tau in the eyes of SPIRou and TESS

The weak-line T Tauri star V410 Tau in the eyes of SPIRou and TESS

Magnetic activity & prominence behaviour of the binary system V471 Tau

Magnetic activity & prominence behaviour of the binary system V471 Tau

BRITE photometry and STELLA spectroscopy of bright stars in Auriga: Rotation, pulsation, orbits, and eclipses

Context. Knowing rotational and pulsational periods across the Hertzsprung-Russell diagram is of top priority for understanding stellar activity as a function of time. Aims. We aim to determine periods for bright stars in the Auriga field that are otherwise not easily accessible for ground-based photometry. Methods. Continuous photometry with up to three BRITE satellites was obtained for 12 targets and subjected to a period search. Contemporaneous high-resolution optical spectroscopy with STELLA was used to obtain radial velocities through cross correlation with template spectra as well as to determine astrophysical parameters through a comparison with model spectra. Results. The Capella red light curve was found to be constant over 176 days with a root mean square of 1 mmag, but the blue light curve showed a period of 10.1 ± 0.6 d, which we interpret to be the rotation period of the G0 component. From STELLA we obtained an improved orbital solution based on 9600 spectra from the previous 12.9 yr. We derive masses precise to ≈0.3% but 1% smaller than previously published. The BRITE light curve of the F0 supergiant ε Aur suggests 152 d as its main pulsation period, while the STELLA radial velocities reveal a clear 68 d period. An ingress of an eclipse of the ζ Aur binary system was covered with BRITE and a precise timing for its eclipse onset derived. A possible 70 d period fits the proposed tidal-induced, nonradial pulsations of this ellipsoidal K4 supergiant. η Aur is identified as a slowly pulsating B (SPB) star with a main period of 1.29 d and is among the brightest SPB stars discovered so far. The rotation period of the magnetic Ap star θ Aur is detected from photometry and spectroscopy with a period of 3.6189 d and 3.6177 d, respectively, likely the same within the errors. The radial velocities of this star show a striking non-sinusoidal shape with a large amplitude of 7 km s−1. Photometric rotation periods are also confirmed for the magnetic Ap star IQ Aur of 2.463 d and for the solar-type star κ1 Cet of 9.065 d, and also for the B7 HgMn giant β Tau of 2.74 d. Revised orbital solutions are derived for the eclipsing SB2 binary β Aur, which replaces the initial orbit dating from 1948 for the 27-year eclipsing SB1 ε Aur, and for the RS CVn binary V711 Tau, for which a spot-corrected orbital solution was achieved. The two stars ν Aur and ι Aur are found to be long-term, low-amplitude RV and brightness variables, but provisional orbital elements based on a period of 20 yr and an eccentricity of 0.7 could only be extracted for ν Aur. The variations of ι Aur are due to oscillations with a period of ≈4 yr.

The GAPS Programme at TNG

Context. Detecting and characterising exoworlds around very young stars (age ≤10 Myr) are key aspects of exoplanet demographic studies, especially for understanding the mechanisms and timescales of planet formation and migration. Any reliable theory for such physical phenomena requires a robust observational database to be tested. However, detection using the radial velocity method alone can be very challenging because the amplitude of the signals caused by the magnetic activity of such stars can be orders of magnitude larger than those induced even by massive planets. Aims. We observed the very young (~2 Myr) and very active star V830 Tau with the HARPS-N spectrograph between October 2017 and March 2020 to independently confirm and characterise the previously reported hot Jupiter V830 Tau b (Kb = 68 ± 11 m s−1; mb sin ib = 0.57 ± 0.10 MJup; Pb = 4.927 ± 0.008 d). Methods. Because of the observed ~1 km s−1 radial velocity scatter that can clearly be attributed to the magnetic activity of V830 Tau, we analysed radial velocities extracted with different pipelines and modelled them using several state-of-the-art tools. We devised injection-recovery simulations to support our results and characterise our detection limits. The analysis of the radial velocities was aided by a characterisation of the stellar activity using simultaneous photometric and spectroscopic diagnostics. Results. Despite the high quality of our HARPS-N data and the diversity of tests we performed, we were unable to detect the planet V830 Tau b in our data and cannot confirm its existence. Our simulations show that a statistically significant detection of the claimed planetary Doppler signal is very challenging. Conclusions. It is important to continue Doppler searches for planets around young stars, but utmost care must be taken in the attempt to overcome the technical difficulties to be faced in order to achieve their detection and characterisation. This point must be kept in mind when assessing their occurrence rate, formation mechanisms, and migration pathways, especially without evidence of their existence from photometric transits.