System V773 Tau Research Articles

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 SURPRISING DYNAMICAL MASS FOR V773 Tau B

We report on new high-resolution imaging and spectroscopy on the multiple T Tauri star system V773 Tau over the 2003 -- 2009 period. With these data we derive relative astrometry, photometry between the A and B components, and radial velocity (RV) of the A-subsystem components. Combining these new data with previously published astrometry and RVs, we update the relative A-B orbit model. This updated orbit model, the known system distance, and A subsystem parameters yields a dynamical mass for the B component for the first time. Remarkably the derived B dynamical mass is in the range of 1.7 -- 3.0 M$_\sun$. This is much higher than previous estimates, and suggests that like A, B is also a multiple stellar system. Among these data, spatially-resolved spectroscopy provide new insight into the nature of the B component. Similar to A, these near-IR spectra indicate that the dominant source in B is of mid-K spectral type. If B is in fact a multiple star system as suggested by the dynamical mass estimate, the simplest assumption is that B is composed of similar $\sim$ 1.2 M$_\sun$ PMS stars in a close ($<$ 1 AU) binary system. This inference is supported by line-shape changes in near-IR spectroscopy of B, tentatively interpreted as changing RV among components in V773 Tau B. Relative photometry indicate that B is highly variable in the near-IR. The most likely explanation for this variability is circum-B material resulting in variable line-of-sight extinction. The distribution of this material must be significantly affected by both the putative B multiplicity, and the A-B orbit.

VLBA DETERMINATION OF THE DISTANCE TO NEARBY STAR-FORMING REGIONS. V. DYNAMICAL MASS, DISTANCE, AND RADIO STRUCTURE OF V773 Tau A

(ABRIDGED) We present multi-epoch Very Long Baseline Array (VLBA) observations of V773 Tau A, the 51-day binary subsystem in the multiple young stellar system V773 Tau. Combined with previous interferometric and radial velocity measurements, these new data enable us to improve the characterization of the physical orbit of the A subsystem. In particular, we infer updated dynamical masses for the primary and the secondary components of 1.55 pm 0.11 Msun, and 1.293 pm 0.068 Msun, respectively, and an updated orbital parallax distance to the system of 135.7 pm 3.2 pc, all consistent with previous estimates. Using the improved orbit, we can calculate the absolute coordinates of the barycenter of V773 Tau A at each epoch of our VLBA observations, and fit for its trigonometric parallax and proper motion. This provides a direct measurement of the distance to the system almost entirely independent of the orbit modeling. The best fit yields a distance of 129.9 pm 3.2 pc, in good agreement (i.e. within 1 sigma) with the distance estimate based on the orbital fit. Taking the mean value of the orbital and trigonometric parallaxes, we conclude that V773 Tau is located at d =132.8 pm 2.3 pc. The accuracy of this determination is nearly one order of magnitude better than that of previous estimates. In projection, V773 Tau and two other young stars (Hubble 4 and HDE 283572) recently observed with the VLBA are located toward the dark cloud Lynds 1495, in the central region of Taurus. These three stars appear to have similar trigonometric parallaxes, radial velocities, and proper motions, and we argue that the weighted mean and dispersion of their distances (d = 131.4 pc and sigma_d = 2.4 pc) provide a good estimate of the distance to and depth of Lynds 1495 and its associated stellar population.

Interacting coronae of two T Tauri stars: first observational evidence for solar-like helmet streamers

Context. The young binary system V773 Tau A exhibits a persistent radio flaring activity that gradually increases from a level of a few mJy at apoastron to more than 100 mJy at periastron. Interbinary collisions between very large (>15 R∗) magnetic structures anchored on the two rotating stars of the system have been proposed to be the origin of these periodic radio flares. Magnetic structures extended over tens of stellar radii, that can also account for the observed fast decay of the radio flares, seem to correspond to the typical solar semi-open quite extended magnetic configurations called helmet streamers. Aims. We aim to find direct observational evidence for the postulated, solar-like, coronal topologies. Methods. We performed seven-consecutive-day VLBI observations at 8.4 GHz using an array consisting of the VLBA and the 100-m Effelsberg telescope. V773 Tau A was phase-referenced to QSO B0400+258. Results. Two distinctive structures appear in the radio images here presented. They happen to be associated with the primary and secondary stars of the V773 Tau A system. In one image (Fig. 2B) the two features are extended up to 18 R∗ each and are nearly parallel revealing the presence of two interacting helmet streamers. One image (Fig. 2E) taken a few hours after a flare monitored by the 100-m Effelsberg telescope shows one elongated fading structure substantially rotated with respect to those seen in the B run. The same decay scenario is seen in Fig. 2G for the helmet streamer associated with the other star. Conclusions. This is the very first direct evidence revealing that even if the flare origin is magnetic reconnection due to interbinary collision, both stars independently emit in the radio range with structures of their own. These structures are helmet streamers, observed for the first time in stars other than the Sun. The complete extent of each helmet streamer above the stellar surface is about 24 R∗ which implies that they can practically interact throughout the whole orbit, even rather close to apoastron where the stellar separation is 52 R∗. However, the radio flares become stronger when the stars approach. Around periastron the stellar separation is only 30 R∗, nearly covered by a single streamer: the two streamers overlap producing the observed giant flares.

Dynamical Masses for Pre–Main‐Sequence Stars: A Preliminary Physical Orbit for V773 Tau A

We report on interferometric and radial velocity observations of the double-lined 51 day period binary (A) component of the quadruple pre-main-sequence (PMS) system V773 Tau. With these observations we have estimated preliminary visual and physical orbits of the V773 Tau A subsystem. Among other parameters, our orbit model includes an inclination of 66.0° ± 2.4° and allows us to infer the component dynamical masses and system distance. In particular, we find component masses of 1.54 ± 0.14 and 1.332 ± 0.097 M☉ for the Aa (primary) and Ab (secondary) components, respectively. Our modeling of the subsystem component spectral energy distributions finds temperatures and luminosities consistent with previous studies and, coupled with the component mass estimates, allows for comparison with PMS stellar models in the intermediate-mass range. We compare V773 Tau A component properties with several popular solar composition models for intermediate-mass PMS stars. All models predict masses consistent to within 2 σ of the dynamically determined values, although some models predict values that are more consistent than others.

Synchrotron emission from the T Tauri binary system V773 Tauri A

The pre-main sequence binary system V773 Tau A shows remarkable flaring activity around periastron passage. Here, we present the observation of such a flare at a wavelength of 3 mm (90 GHz) performed with the Plateau de Bure Interferometer. We examine different possible causes for the energy losses responsible for the e-folding time of h of that flare. We exclude synchrotron, collisional, and inverse Compton losses because they are not consistent with observational constraints, and we propose that the fading of the emission is due to the leakage of electrons themselves at each reflection between the two mirror points of the magnetic structure partially trapping them. The magnetic structure compatible with both our leakage model and previous observations is that of a helmet streamer that, as in the solar case, can occur at the top of the X-ray-emitting, stellar-sized coronal loops of one of the stars. The streamer may extend up to ~20 and interact with the corona of the other star at periastron passage, causing recurring flares. The inferred magnetic field strength at the two mirror points of the helmet streamer is in the range 0.12-125 G, and the corresponding Lorentz factor, γ, of the partially trapped electrons is in the range . We therefore rule out that the emission could be of gyro-synchrotron nature: the derived high Lorentz factor proves that the nature of the emission at 90 GHz from this pre-main binary system is synchrotron radiation.

Preliminary Orbits and System Masses for Five Binary T Tauri Stars

First visual orbits for the binary T Tauri stars V773 Tau, FO Tau, and FS Tau and revised orbits for DF Tau and GG Tau are presented. The orbits are determined from a compilation of previous high spatial resolution imaging measurements and new astrometric measurements from the Calar Alto 3.5 m telescope. The orbital solutions, which must be considered preliminary because of the short orbital arcs observed, are used in combination with distance estimates to determine the dynamical masses for all five systems. The total mass of the triple system V773 Tau (whose primary is a K2+K5 spectroscopic binary) is 3.20 ± 0.71 M☉, and the masses of the four binary pairs range from 0.77 to 1.22 M☉. The orbital solutions for these systems predict projected relative orbital velocities that range from 0.1 to 13.4 km s-1 (2002 epoch); the majority are spectroscopically measurable if spatially resolved. Of particular interest is the GG Tau binary, which has a mass that is consistent with the value determined from circumbinary disk kinematics and an orbit that is coplanar with this disk. The stellar properties of the components of these five systems are determined from spatially resolved optical measurements, assuming dwarflike colors and a dwarflike temperature scale. With luminosity and temperature estimates, theoretical masses are determined from the predictions of four pre-main-sequence evolutionary models and compared with the dynamically determined values as a test of these models. Of the models tested, those of Palla & Stahler and Siess, Dufour, & Forestini predict masses that are the most consistent (within 1 σ) with the dynamical values over the component mass range of ~0.3-1.5 M☉. All models support coeval formation of these binaries.