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Register a new userA Disk-based Dynamical Constraint on the Mass of the Young Binary DQ Tau
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We present new ALMA observations of CO $J$=2$-$1 line emission from the DQ Tau circumbinary disk. These data are used to tomographically reconstruct the Keplerian disk velocity field in a forward-modeling inference framework, and thereby provide a dynamical constraint on the mass of the DQ Tau binary of $M_ast = 1.27_{-0.27}^{+0.46} ,M_odot$. Those results are compared with an updated and improved orbital solution for this double-lined system based on long-term monitoring of its stellar radial velocities. Both of these independent dynamical constraints on the binary mass are in excellent agreement: taken together, they demonstrate that the DQ Tau system mass is $1.21pm0.26,M_odot$ and that the disk and binary orbital planes are aligned within $3^circ$ (at 3$sigma$ confidence). The predictions of various theoretical models for pre-main sequence stellar evolution are also consistent with these masses, although more detailed comparisons are difficult due to lingering uncertainties in the photospheric properties of the individual components. DQ Tau is the third nearly equal-mass double-lined spectroscopic binary with a circumbinary disk that has been dynamically weighed with these two independent techniques: all show consistent results, validating the overall accuracy of the disk-based approach and demonstrating that it can be robustly applied to large samples of young, single stars as ALMA ramps up to operations at full capacity.
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We present sensitive, arcsecond-resolution Submillimeter Array observations of the 12CO J=2-1 line emission from the circumstellar disk orbiting the double-lined spectroscopic binary star V4046 Sgr. Based on a simple model of the disk structure, we use a novel Monte Carlo Markov Chain technique to extract the Keplerian velocity field of the disk from these data and estimate the total mass of the central binary. Assuming the distance inferred from kinematic parallax measurements in the literature (d is approximately 73 pc), we determine a total stellar mass M_star = 1.75^{+0.09}_{-0.06} solar masses and a disk inclination i_d = 33.5^{+0.7}_{-1.4} degrees from face-on. These measurements are in excellent agreement with independent dynamical constraints made from multi-epoch monitoring of the stellar radial velocities, confirming the absolute accuracy of this precise (~ few percent uncertainties) disk-based method for estimating stellar masses and reaffirming previous assertions that the disk and binary orbital planes are well aligned (with |i_d - i_star| approx 0.1pm1 degree). Using these results as a reference, we demonstrate that various pre-main sequence evolution models make consistent and accurate predictions for the masses of the individual components of the binary, and uniformly imply an advanced age of ~5-30 Myr. Taken together, these results verify that V4046 Sgr is one of the precious few nearby and relatively evolved pre-main sequence systems that still hosts a gas-rich accretion disk.
The young high-eccentricity binary DQ Tau exhibits powerful recurring millimeter-band (mm) flaring attributed to collisions between the two stellar magnetospheres near periastron, when the stars are separated by only ~8Rstar. These magnetospheric interactions are expected to have scales and magnetic field strengths comparable to those of large X-ray flares from single pre-main-sequence (PMS) stars observed in the Chandra Orion Ultradeep Project (COUP). To search for X-rays arising from processes associated with colliding magnetospheres, we performed simultaneous X-ray and mm observations of DQ Tau near periastron phase. We report here several results. 1) As anticipated, DQ Tau was caught in a flare state in both mm and X-rays. A single long X-ray flare spanned the entire 16.5 hour Chandra exposure. 2) The inferred morphology, duration, and plasma temperature of the X-ray flare are typical of those of large flares from COUP stars. 3) However, our study provides three lines of evidence that this X-ray flare likely arises from colliding magnetospheres: the chance of capturing a large COUP-like flare within the span of our observation is small; the relative timing of the X-ray and mm flares indicates the Neupert effect and is consistent with a common coronal structure; the size of the emitting coronal structure (4-5Rstar) inferred from our analysis (which is admittedly model-dependent and should be considered with caution) is comparable to half the binary separation. 4) The peak flare X-ray luminosity is in agreement with an estimate of the power dissipated by magnetic reconnection within the framework of a simple model of interacting magnetospheres.
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 starting near apastron orbital phase. We further obtained five epochs of spectroscopy using IRTF SpeX, with a wavelength range of 0.8 to 5 microns, and derived spectra of the infrared excess emission. The shape and strength of the excess varies with time, with cooler and weaker characteristic dust emission (T ~ 1100-1300 K) over most of the binary orbit, and stronger/warmer dust emission (T ~ 1600 K, indicative of dust sublimation) just before periastron passage. We suggest our results are broadly consistent with predictions of simulations of disk structure and accretion flows around close binaries, with the varying dust emission possibly tracing the evolution of accretion streams falling inwards through a circumbinary disk cavity and feeding the accretion pulses traced by the optical photometry and NIR emission lines. However, our results also show more complicated behavior that is not fully explained by this simple picture, and will require further observations and modeling to fully interpret.
We report our analyses of the multi-epoch (2015-2017) ALMA archival data of the Class II binary system XZ Tau at Bands 3, 4 and 6. The millimeter dust continuum images show compact, unresolved (r <~ 15 au) circumstellar disks (CSDs) around the individual binary stars; XZ Tau A and B, with a projected separation of ~ 39 au. The 12CO (2-1) emission associated with those CSDs traces the Keplerian rotations, whose rotational axes are misaligned with each other (P.A. ~ -5 deg for XZ Tau A and ~ 130 deg for XZ Tau B). The similar systemic velocities of the two CSDs (VLSR ~ 6.0 km s-1) suggest that the orbital plane of the binary stars is close to the plane of the sky. From the multi-epoch ALMA data, we have also identified the relative orbital motion of the binary. Along with the previous NIR data, we found that the elliptical orbit (e = 0.742+0.025-0.034, a = 0.172+0.002-0.003, and {omega} = -54.2+2.0-4.7 deg) is preferable to the circular orbit. Our results suggest that the two CSDs and the orbital plane of the XZ Tau system are all misaligned with each other, and possible mechanisms to produce such a configuration are discussed. Our analyses of the multi-epoch ALMA archival data demonstrate the feasibility of time-domain science with ALMA.
The first long-baseline ALMA campaign resolved the disk around the young star HL Tau into a number of axisymmetric bright and dark rings. Despite the very young age of HL Tau these structures have been interpreted as signatures for the presence of (proto)planets. The ALMA images triggered numerous theoretical studies based on disk-planet interactions, magnetically driven disk structures, and grain evolution. Of special interest are the inner parts of disks, where terrestrial planets are expected to form. However, the emission from these regions in HL Tau turned out to be optically thick at all ALMA wavelengths, preventing the derivation of surface density profiles and grain size distributions. Here, we present the most sensitive images of HL Tau obtained to date with the Karl G. Jansky Very Large Array at 7.0 mm wavelength with a spatial resolution comparable to the ALMA images. At this long wavelength the dust emission from HL Tau is optically thin, allowing a comprehensive study of the inner disk. We obtain a total disk dust mass of 0.001 - 0.003 Msun, depending on the assumed opacity and disk temperature. Our optically thin data also indicate fast grain growth, fragmentation, and formation of dense clumps in the inner densest parts of the disk. Our results suggest that the HL Tau disk may be actually in a very early stage of planetary formation, with planets not already formed in the gaps but in the process of future formation in the bright rings.
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