Mass accretion from the circumstellar disk onto the protostar is a fundamental process during star formation. Measuring the mass accretion rate is particularly challenging for stars belonging to binary systems, because it is often difficult to discriminate which component is accreting. DQ~Tau is an almost equal-mass spectroscopic binary system where the components orbit each other every 15.8~days. The system is known to display pulsed accretion, i.e., the periodic modulation of the accretion by the components on eccentric orbit. We present multiepoch ESO/VLT X-Shooter observations of DQ~Tau, with the aim to determine which component of this system is the main accreting source. We use the absorption lines in the spectra to determine the radial velocity of the two components, and measure the continuum veiling as a function of wavelength and time. We fit the observed spectra with non-accreting templates to correct for the photospheric and chromospheric contribution. In the corrected spectra we study in details the profiles of the emission lines and calculate mass accretion rates for the system as a function of orbital phase. In accordance with previous findings, we detect elevated accretion close to periastron. We measure that the accretion rate varies between $10^{-8.5}$ and $10^{-7.3}$ Msun/yr. The emission line profiles suggest that both stars are actively accreting and the dominant accretor is not always the same component, varying in few orbits.
Binary star-formation theory predicts that close binaries (a<100 AU) will experience periodic pulsed accretion events as streams of material form at the inner edge of a circumbinary disk, cross a dynamically cleared gap, and feed circumstellar disks or accrete directly onto the stars. The archetype for the pulsed-accretion theory is the eccentric, short-period, classical T Tauri binary DQ Tau. Low-cadence (~daily) broadband photometry has shown brightening events near most periastron passages, just as numerical simulations would predict for an eccentric binary. Magnetic reconnection events (fl…
We report here results of spectropolarimetric observations of the classical T Tauri star DN Tau carried out (at 2 epochs) with ESPaDOnS at the Canada-France-Hawaii Telescope within the `Magnetic Protostars and Planets' programme. We infer that DN Tau, with a photospheric temperature of 3,950+-50 K, a luminosity of 0.8+-0.2 Lsun and a rotation period of 6.32 d, is a ~2Myr-old fully-convective 0.65+-0.05 Msun star with a radius of 1.9+-0.2 Dsun, viewed at an inclination of 35+-10degr. Clear circularly-polarized Zeeman signatures are detected in both photospheric and accretion-powered emission li…
In the framework of the GIARPS@TNG High-resolution Observations of T Tauri stars (GHOsT) project, we study the accretion properties of 37 Classical T Tauri Stars of the Taurus-Auriga star forming region (SFR) with the aim of characterizing their relation with the properties of the central star, of jets and disk winds, and of the global disk structure, in synergy with complementary ALMA millimiter observations. We derive stellar parameters, optical veiling, accretion luminosity ($\rm L_{acc}$) and mass accretion rate ($\rm \dot M_{acc}$) in a homogeneous and self-consistent way using high-resol…
Transition disc systems are young stars that appear to be on the verge of dispersing their protoplanetary discs. We explore the nature of these systems by comparing the stellar accretion rates and disc masses of transition discs and normal T Tauri stars in Taurus and Ophiuchus. After controlling for the known dependencies of stellar accretion rate and disc mass and on age, stellar accretion rate on stellar mass, and disc mass on the presence of stellar or sub-stellar companions, we find that the normal T Tauri stars show a trend of stellar accretion rate increasing with disc mass. The transiti…
Interpreting the short-timescale variability of the accreting, young, low-mass stars known as Classical T Tauri stars remains an open task. Month-long, continuous light curves from the Transiting Exoplanet Survey Satellite (\textit{TESS}) have become available for hundreds of T Tauri stars. With this vast data set, identifying connections between the variability observed by \TESS and short-timescale accretion variability is valuable for characterizing the accretion process. To this end, we obtained short-cadence \TESS observations of 14 T Tauri stars in the Taurus star-formation region along w…
Depending on whether a T Tauri star accretes material from its circumstellar disk or not, different X-ray emission properties can be found. The accretion shocks produce cool heating of the plasma, contributing to the soft X-ray emission from the star. Using X-ray data from the Chandra Orion Ultra-deep Project and accretion rates that were obtained with the Hubble Space Telescope/WFPC2 photometric measurements in the Orion Nebula Cluster, we studied the relation between the accretion processes and the X-ray emissions of a coherent sample of T Tauri sources in the region. We performed regression…
TWA 3A is the most recent addition to a small group of young binary systems that both actively accrete from a circumbinary disk and have spectroscopic orbital solutions. As such, it provides a unique opportunity to test binary accretion theory in a well-constrained setting. To examine TWA 3A's time-variable accretion behavior, we have conducted a two-year, optical photometric monitoring campaign, obtaining dense orbital phase coverage (~20 observations per orbit) for ~15 orbital periods. From U-band measurements we derive the time-dependent binary mass accretion rate, finding bursts of accreti…
Context. Classical T Tauri stars (cTTs) are pre-main sequence stars surrounded by an accretion disk. They host a strong magnetic field, and both magnetospheric accretion and ejection processes develop as the young magnetic star interacts with its disk. Studying this interaction is a major goal toward understanding the properties of young stars and their evolution. Aims. The goal of this study is to investigate the accretion process in the young stellar system HQ Tau, an intermediate-mass T Tauri star (1.9 M$_{\odot}$). Methods. The time variability of the system is investigated both photometri…
We analyze the accretion properties of 21 low mass T Tauri stars using a dataset of contemporaneous near ultraviolet (NUV) through optical observations obtained with the Hubble Space Telescope Imaging Spectrograph (STIS) and the ground based Small and Medium Aperture Research Telescope System (SMARTS), a unique dataset because of the nearly simultaneous broad wavelength coverage. Our dataset includes accreting T Tauri stars (CTTS) in Taurus, Chamaeleon I, $η$ Chamaeleon and the TW Hydra Association. For each source we calculate the accretion rate by fitting the NUV and optical excesses above t…
The magnetic field of the classical T Tauri star BP Tau can be approximated as a superposition of dipole and octupole moments with respective strengths of the polar magnetic fields of 1.2 kG and 1.6 kG (Donati et al. 2008). We adopt the measured properties of BP Tau and model the disc accretion onto the star. We observed in simulations that the disc is disrupted by the dipole component and matter flows towards the star in two funnel streams which form two accretion spots below the dipole magnetic poles. The octupolar component becomes dynamically important very close to the star and it redirec…
FU Tau belongs to a rare class of young, wide brown dwarf binaries. We have resolved the system in a Chandra X-ray observation and detected only the primary, FU Tau A. Hard X-ray emission, presumably from a corona, is present but, unexpectedly, we detect also a strong and unusually soft component from FU Tau A. Its X-ray properties, so far unique among brown dwarfs, are very similar to those of the T Tauri star TW Hya. The analogy with TW Hya suggests that the dominating soft X-ray component can be explained by emission from accretion shocks. However, the typical free-fall velocities of a brow…
We present a deep XMM-Newton observation of DN Tau, a young M0-type accreting CTTS and investigate its X-ray properties and X-ray generating mechanisms. Specifically we examine the presence of X-ray emission from magnetic activity and accretion shocks and compare our new X-ray data with UV data taken simultaneously and with X-ray/UV observations performed before. We find that the X-ray emission from DN Tau is dominated by coronal plasma generated via magnetic activity, but also clearly detect a contribution of the accretion shocks to the cool plasma component at about 2 MK as consistently infe…
Our goal is to relate the photometric and spectroscopic variability of classical T Tauri stars, of the star-forming cluster NGC 2264, to the physical processes acting in the stellar and circumstellar environment, within a few stellar radii from the star. NGC 2264 was the target of a multiwavelength observational campaign with CoRoT, MOST, Spitzer, and Chandra satellites and observations from the ground. We classified the CoRoT light curves of accreting systems according to their morphology and compared our classification to several accretion diagnostics and disk parameters. The morphology of t…
Monitoring the magnetospheric accretion of the classical T Tauri star DO Tau with SPIRou
Univ. de Toulouse, CNRS, IRAP（图卢兹大学）；Leiden Observatory, Leiden University（莱顿大学）；Univ. Grenoble Alpes, CNRS, IPAG（格勒诺布尔阿尔卑斯大学）；Departamento de Física – ICEx – UFMG（米纳斯吉拉斯联邦大学）；Department of Astronomy, University of Geneva（日内瓦大学）；Dublin Institute for Advanced Studies（都柏林高等研究院）
This paper summarizes several results concerning the comparison between accretion-related properties of cool (T Tauri; T < 7000 K, M < 1 Msun and hot (Herbig Ae/Be; 7000 < T(K) < 13000; 1 < M(Msun) < 6) pre-main sequence (PMS) stars. This comparison gives insight into the analogies/differences in the physics of the star-disk interaction and in the physical mechanisms driving disk dissipation. Several optical and near-IR line luminosities used for low-mass objects are also valid to estimate typical accretion rates for intermediate-mass stars under similar empirical expressions. In contrast, the…
This paper exploits spectropolarimetric data of the classical T Tauri star CI Tau collected with ESPaDOnS at the Canada-France-Hawaii Telescope, with the aims of detecting and characterizing the large-scale magnetic field that the star hosts, and of investigating how the star interacts with the inner regions of its accretion disc through this field. Our data unambiguously show that CI Tau has a rotation period of 9.0d, and that it hosts a strong, mainly poloidal large-scale field. Accretion at the surface of the star concentrates within a bright high-latitude chromospheric region that spatiall…
Classical T Tauri stars are pre-main-sequence objects that undergo simultaneous accretion, wind outflow, and coronal X-ray emission. The impact of plasma on the stellar surface from magnetospheric accretion streams is likely to be a dominant source of energy and momentum in the upper atmospheres of these stars. This paper presents a set of models for the dynamics and heating of three distinct regions on T Tauri stars that are affected by accretion: (1) the shocked plasmas directly beneath the magnetospheric accretion streams, (2) stellar winds that are accelerated along open magnetic flux tube…
We report the detection of an anomalous accretion flare in the tight eccentric pre-main-sequence binary system DQ Tau. In a multi-epoch survey consisting of randomly acquired low to moderate resolution near-infrared spectra obtained over a period of almost ten years, we detect a significant and simultaneous brightening of four standard accretion indicators (CaII infrared triplet, the Paschen and Brackett series HI lines, and HeI 1.083 um), on back-to-back nights (phase = 0.372, 0.433) with the flare increasing in strength as the system approached apastron (phase = 0.5). The mass accretion rate…
FU Tau A is a young very low mass object in the Taurus star forming region which was previously found to have strong X-ray emission and to be anomalously bright for its spectral type. In this study we discuss these characteristics using new information from quasi-simultaneous photometric and spectroscopic monitoring. From photometric time series obtained with the 2.2m telescope on Calar Alto we measure a period of ~4d for FU Tau A, most likely the rotation period. The short-term variations over a few days are consistent with the rotational modulation of the flux by cool, magnetically induced s…
We present new 7 mm and archive 1.3 cm high angular resolution observations of the HL/XZ Tau region made with the VLA. At 7 mm, the emission from HL Tau seems to be arising in a clumpy disk with radius of order 25 AU. The 1.3 cm emission from XZ Tau shows the emission from a binary system with 0"3 (42 AU) separation, known from previous optical/IR observations. However, at 7 mm, the southern radio component resolves into a binary with 0"09 (13 AU) separation, suggesting that XZ Tau is actually a triple star system. We suggest that the remarkable ejection of gas from the XZ Tau system observed …
The rotational evolution of accreting pre-main-sequence stars is influenced by its magnetic interaction with its surrounding circumstellar disk. Using the PLUTO code, we perform 2.5D magnetohydrodynamic, axisymmetric, time-dependent simulations of star-disk interaction---with an initial dipolar magnetic field structure, and a viscous and resistive accretion disk---in order to model the three mechanisms that contribute to the net stellar torque: accretion flow, stellar wind, and magnetospheric ejections (periodic inflation and reconnection events). We investigate how changes in the stellar magn…
(Abridged) Optical spectra of classical T Tauri stars (cTTS) are rich in emission lines of low-excitation species that are composed of narrow and broad components, related to two regions with different kinematics, densities, and temperatures. The photospheric spectrum is often veiled by an excess continuous emission. This veiling is usually attributed to radiation from a heated region beneath the accretion shock. The aim of this research is to clarify the nature of the veiling, and whether the narrow chromospheric lines of Fe I and other metals represent a standard chromosphere of a late-type …
The YY Ori stars are T Tauri stars with prominent time-variable redshifted absorption components that flank certain emission lines. One of the brightest in this class is S CrA, a visual double star. We have obtained a series of high-resolution spectra of the two components during four nights with the UVES spectrograph at the Very Large Telescope. We followed the spectral changes occurring in S CrA to derive the physical structure of the accreting gas. We found that both stars are very similar with regard to surface temperature, radius, and mass. Variable redshifted absorption components are pa…
In classical T Tauri stars, X-rays are produced by two plasma components: a hot low-density plasma, with frequent flaring activity, and a high-density lower temperature plasma. The former is coronal plasma related to the stellar magnetic activity. The latter component, never observed in non-accreting stars, could be plasma heated by the shock formed by the accretion process. However its nature is still being debated. Our aim is to probe the soft X-ray emission from the high-density plasma component in classical T Tauri stars to check whether this is plasma heated in the accretion shock or whet…
Classical T Tauri stars with ages of less than 10 Myr possess accretion discs. Magnetohydrodynamic processes at the boundary between the disc and the stellar magnetosphere control the accretion and ejections gas flows. We carried out a long series of simultaneous spectroscopic and photometric observations of the classical T Tauri stars RY Tau and SU Aur with the aim to quantify the accretion and outflow dynamics at time scales from days to years. It is shown that dust in the disc wind is the main source of photometric variability of these stars. In RY Tau we observed a new effect: during event…
Recent spectropolarimetric observations suggest that young low-mass stars such as classical T Tauri stars (CTTSs) possess relatively strong (~kG) magnetic field. This supports a scenario in which the final accretion onto the stellar surface proceeds through a magnetosphere, and the winds are formed in magnetohydrodynamics (MHD) processes. We examine recent numerical simulations of magnetospheric accretions via an inclined dipole and a complex magnetic fields. The difference between a stable accretion regime, in which accretion occurs in ordered funnel streams, and an unstable regime, in which …
We investigate the properties of X-ray emission from accretion shocks in classical T Tauri stars (CTTSs), generated where the infalling material impacts the stellar surface. Both observations and models of the accretion process reveal several aspects that are still unclear: the observed X-ray luminosity in accretion shocks is below the predicted value, and the density versus temperature structure of the shocked plasma, with increasing densities at higher temperature, deduced from the observations, is at odds with that proposed in the current picture of accretion shocks. To address these open i…
The high-energy radiation fields of T Tauri stars (TTS) should affect the surrounding circumstellar disk, having implications for disk transport and heating. Yet, observational evidence of the effect of high-energy fields on disks is scarce. Here we investigate the connection between X-ray emission and the innermost gas disk by leveraging the variability of TTS. We obtained multiple epochs of coordinated data (taken either simultaneously or within a few hours) of accreting TTS with the Hubble Space Telescope, the Neil Gehrels Swift Observatory, and the Chandra X-ray Observatory. We measured th…
Radiative transfer modelling offers a powerful tool for understanding the enigmatic hydrogen emission lines from T Tauri stars. This work compares optical and near-IR spectroscopy of 29 T Tauri stars with our grid of synthetic line profiles. The archival spectra, obtained with VLT's X-Shooter, provide simultaneous coverage of many optical and infrared hydrogen lines. The observations exhibit similar morphologies of line profiles seen in other studies. We used the radiative transfer code TORUS to create synthetic H$α$, Pa$β$, Pa$γ$, and Br$γ$ emission lines for a fiducial T Tauri model that inc…
We conjecture that observed protoplanetary disc accretion rates may be explained with low viscosities which could be the result of hydrodynamic turbulence. We show that viscosities parameterized in the usual way with $α\gtrsim 10^{-4}$, comparable to values suggested for hydrodynamic turbulence, can explain the observed accretion rates and lifetimes with plausible inner disc surface densities. Our models are also in better agreement with surface density estimates of the minimum mass solar nebula than models with rapid transport for a given mass accretion rate, such as recent models of accretio…
We present spectra of an M-type, binary star system (LDS 5606) that belongs to the nearby ~20 Myr old beta Pictoris moving group. Both stars are very dusty; the dustier member displays optical emission lines from eight elements indicative of ongoing mass accretion. The spectra of both stars contain oxygen forbidden line emission at 6302 and 5579 A, consistent with a recent model of far ultraviolet photodissociation of OH molecules in a circumstellar disk. These are the oldest dwarf stars presently known to display such a phenomenon. The spectral energy distribution of the dustier star indicate…
We used the TNG/GIANO instrument to obtain near-IR high-resolution spectra (R~50000) of XZ Tau and DR Tau, two actively accreting T Tauri stars classified as EXors. The analysis of the observed features provides information on the properties of the inner disk, the accretion columns, and the winds. Both sources display composite HI line profiles, with contributions from both accreting gas and high-velocity winds. These lines become more symmetric and narrower with increasing upper energy, which may be interpreted in terms of two components with different decrements or imputed to self-absorption…
DQ Tau is a young low-mass spectroscopic binary, consisting of two almost equal-mass stars on a 15.8 d period surrounded by a circumbinary disk. Here, we analyze DQ Tau's light curves obtained by Kepler K2, the Spitzer Space Telescope, and ground-based facilities. We observed variability phenomena, including rotational modulation by stellar spots, brief brightening events due to stellar flares, long brightening events around periastron due to increased accretion, and short dips due to brief circumstellar obscuration. The rotational modulation appears as sinusoidal variation with a period of 3.…
Understanding Balmer Decrements in T Tauri stars in terms of Multiflow Magnetospheric Accretion
Institute for Astrophysical Research, Department of Astronomy, Boston University（波士顿大学天体物理研究所）；Department of Astronomy, University of Michigan（密歇根大学天文学系）；Centro de Investigaciones de Astronomía (CIDA)（天文学研究中心）；Department of Physics, Faculty of Science, Chulalongkorn University（朱拉隆功大学理学院物理学系）
Variability in emission lines is a characteristic feature in young stars and can be used as a tool to study the physics of the accretion process. Here we present a study of Hα variability in 15 T Tauri and Herbig Ae stars (K7-B2) over a wide range of time windows, from minutes, to hours, to days, and years. We assess the variability using linewidth measurements and the time series of line profiles. All objects show gradual, slow profile changes on time-scales of days. In addition, in three cases there is evidence for rapid variations in Hα with typical time-scales of 10 min, which occurs in 10…
We present multi-epoch optical and near-infrared (NIR) photometry and spectroscopy of the spectroscopic binary T Tauri star DQ Tau. The photometric monitoring, obtained using SMARTS ANDICAM, recovers the previously-seen correlation between optical flux and the 15.8-day binary orbital period, with blue flux peaks occurring close to most observed periastron passages. For the first time, we find an even more consistent correlation between orbital period and NIR brightness and color. The onset of pulse events in the NIR on average precedes those in the optical by a few days, with the rise usually …
CI Tau is a young (~2 Myr) classical T Tauri star located in the Taurus star forming region. Radial velocity observations indicate it hosts a Jupiter-sized planet with an orbital period of approximately 9 days. In this work, we analyze time series of CI Tau's photometric variability as seen by K2. The lightcurve reveals the stellar rotation period to be ~6.6 d. Although there is no evidence that CI Tau b transits the host star, a ~9 d signature is also present in the lightcurve. We believe this is most likely caused by planet-disk interactions which perturb the accretion flow onto the star, re…
We present high spatial resolution maps of ro-vibrational molecular hydrogen emission from the environment of the GG Tau A binary component in the GG Tau quadruple system. The H2 v= 1-0 S(1) emission is spatially resolved and encompasses the inner binary, with emission detected at locations that should be dynamically cleared on several hundred-year timescales. Extensions of H2 gas emission are seen to ~100 AU distances from the central stars. The v = 2-1 S(1) emission at 2.24 microns is also detected at ~30 AU from the central stars, with a line ratio of 0.05 +/- 0.01 with respect to the v = 1…
Young close binaries open central gaps in the surrounding circumbinary accretion disc, but the stellar components may still gain mass from gas crossing through the gap. It is not well understood how this process operates and how the stellar components are affected by such inflows. Our main goal is to investigate how gas accretion takes place and evolves in close T Tauri binary systems. In particular, we model the accretion flows around two close T Tauri binaries, V4046 Sgr and DQ Tau, both showing periodic changes in emission lines, although their orbital characteristics are very different. In…
Results of measurements of the longitudinal magnetic field in a hot accretion spot in three classical T Tauri stars (CTTS) are presented. The magnetic field in the formation region of the narrow component of the emission line HeI 5876 Å was found for each star in our sample at a level of more than 2σ. In case of DS Tau we have found the field in the narrow components of NaI D lines, which was equal to +0.8 \pm 0.3 kG, i.e. it was equal to the field measured on the narrow component of HeI 5876 Å. Our results indicate that the magnetic field in the hot spots can be studied for CTTS down to 13^m …
Spectroscopic results of HU Tauri that I presented in my Ph.D thesis (Parthasarathy 1979) are published in this paper. The H-alpha line of the secondary is detected. The detection of H-alpha line of the secondary is confirmed based on the high resolution Coude Reticon spectra obtained with the 2.1-m telescope of the McDonald observatory.
The mass-accretion rate, Macc, is a crucial parameter for the study of the evolution of accretion disks around young low-mass stellar objects (YSOs) and for planet formation studies. The Taurus star forming region (SFR) is rich in pre-main sequence (PMS) stars, most of them of the T Tauri class. A variety of methodologies have been used in the past to measure mass accretion in samples of YSOs in Taurus, but despite being a general benchmark for star formation studies, a comprehensive and systematic analysis of the Taurus T Tauri population, where the stellar and accretion properties are derive…
We look for trends relating the mass accretion rate (Macc) and the stellar ages (t), spectral energy distributions (SEDs), and disk masses (Mdisk) for a sample of 38 HAeBe stars, comparing them to analogous correlations found for classical T Tauri stars. Our goal is to shed light on the timescale and physical processes that drive evolution of intermediate-mass pre-main sequence objects. Macc shows a dissipation timescale τ= 1.3^{+1.0}_{-0.5} Myr from an exponential law fit, while a power law yields Macc(t) \propto t^{-η}, with η= 1.8^{+1.4}_{-0.7}. This result is based on our whole HAeBe sampl…
Planets are thought to form at the early stage of stellar evolution when the mass accretion is still ongoing. RY Tau is a T Tauri type star at the age of a few Myr, with accretion disc seen at high inclination, so that line of sight crosses both the wind and the accretion gas flows. In a long series of spectroscopic monitoring of the star in 2013-2020, we detected variations in H-alpha and NaI D absorptions at radial velocities of infall (accretion) and outflow (wind) with a period of about 22 days. The absorptions in the infalling and the outflowing gas streams vary in anti-phase: an increase…
Photometric observations of accreting, low-mass, pre-main-sequence stars (i.e., Classical T Tauri stars; CTTS) have revealed different categories of variability. Several of these classifications have been linked to changes in $\dot{M}$. To test how accretion variability conditions lead to different light-curve morphologies, we used 1D hydrodynamic simulations of accretion along a magnetic field line coupled with radiative transfer models and a simple treatment of rotation to generate synthetic light curves. We adopted previously developed metrics in order to classify observations to facilitate…
Context: A large sample of T Tauri stars exhibits optical jets, approximately half of which rotate slowly, only at ten per cent of their breakup velocity. The disk-locking mechanism has been shown to be inefficient to explain this observational fact. Aims: We show that low mass accreting T Tauri stars may have a strong stellar jet component that can effectively brake the star to the observed rotation speed. Methods: By means of a nonlinear separation of the variables in the full set of the MHD equations we construct semi- analytical solutions describing the dynamics and topology of the stellar…
Planet-forming disks are not isolated systems. Their interaction with the surrounding medium affects their mass budget and chemical content. In the context of the ALMA-DOT program, we obtained high-resolution maps of assorted lines from six disks that are still partly embedded in their natal envelope. In this work, we examine the SO and SO$_2$ emission that is detected from four sources: DG Tau, HL Tau, IRAS 04302+2247, and T Tau. The comparison with CO, HCO$^+$, and CS maps reveals that the SO and SO$_2$ emission originates at the intersection between extended streamers and the planet-forming…
Results of high-cadence multi-colour observations of 121 pre-main sequence stars available from the northern hemisphere are presented. The aim of this survey was to detect transit-like signatures caused by occultation of these young stars and their accretion-induced hot spots by close-in planets and/or dusty clumps. Although none planetary transits were detected, our data allow to determine rotational periods for some T Tauri stars, characterise accretion processes operating in classical T Tauri-type stars in time scales ranging from a few minutes to days, as well as the large-scale dips cause…
The Kepler spacecraft observed a total of only four AM Herculis cataclysmic variable stars during its lifetime. We analyze the short-cadence K2 light curve of one of those systems, Tau 4 (RX J0502.8+1624), which underwent a serendipitous jump from a low-accretion state into a high state during the final days of the observation. Apart from one brief flare, there was no evidence of accretion during the 70 d of observations of the low state. As Tau 4 transitioned into a high state, the resumption of accretion was very gradual, taking approximately six days (~90 binary orbits). We supplement Tau 4…
Two sets of photometric observations of the system AH Tauri have been analyzed using the latest version of the Wilson-Devinney code. The results show that AH Tauri may classified as A-type of W-UMa eclipsing binary. The mass ratio of q = 0.81, an over-contact degree of f = 0.095, and a slightly temperature difference between the two components have been obtained. The asymmetry of its light curve explained by the presence of a dark spot on the massive component. The physical, geometrical, and absolute parameters have been derived and compared with previous work.
We present high-quality, medium resolution X-shooter/VLT spectra in the range 300-2500 nm for a sample of 12 very low-mass stars in the σOrionis cluster. The sample includes stars with masses ranging from 0.08 to 0.3 M$_\odot$. The aim of this first paper is to investigate the reliability of the many accretion tracers currently used to measure the mass accretion rate in low-mass, young stars. We use our spectra to measure the accretion luminosity from the continuum excess emission in the UV and visual; the derived mass accretion rates range from 10$^{-9}$ M$_{\odot}$ yr$^{-1}$ down to 5$\times…
