Spots, Flares, Accretion, and Obscuration in the Pre-main-sequence Binary DQ Tau

Abstract

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 3.017 d. In our model this is caused by extended stellar spots 400 K colder than the stellar effective temperature. During our 80-day-long monitoring we detected 40 stellar flares with energies up to 1.2×10 35 erg and duration of a few hours. The flare profiles closely resemble those in older late-type stars, and their occurrence does not correlate with either the rotational or the orbital period. We observe elevated accretion rate up to 5×10 −8 M ⊙ yr −1 around each periastron. Our Spitzer data suggests that the increased accretion luminosity heats up the inner part of the circumbinary disk temporarily by about 100 K. We found an inner disk radius of 0.13 au, significantly smaller than expected from dynamical modeling of circumbinary disks. Interestingly, the inner edge of the disk is in corotation with the binary's orbit. DQ Tau also shows short dips of <0.1 mag in its light curve, reminiscent of the well-known "dipper phenomenon" observed in many low-mass young stars.

The accretion process in the DQ Tau binary system

“…Our results that the accretion depends on the orbital phase and is highest nearby periastron support previous observational and numerical results in the literature for DQ Tau and for eccentric binary systems in general (e.g. Mathieu et al 1997;Günther & Kley 2002a;Salter et al 2010;D'Orazio et al 2013;Farris et al 2014a;Muñoz & Lai 2016;Tofflemire et al 2017;Kóspál et al 2018;Muzerolle et al 2019). This is in agreement with the hypothesis that the accretion flow of DQ Tau can be explained by the "pulsed accretion model" (Artymowicz & Lubow 1996), according to which the accretion is highly modulated by the binaries orbital motion, peaking during periastron passages.…”

“…This interpretation has been confirmed in the following DQ Tau accretion studies (e.g. Salter et al 2010;Tofflemire et al 2017;Kóspál et al 2018;Muzerolle et al 2019). However, while the on-average results confirm the model predictions, Tofflemire et al (2017) found a complex variability from epoch to epoch, suggesting that the material accretes from the inner edge of the CBD in a more complex way than predicted by models.…”

“…Tofflemire et al 2017), and with our spectral typing. We adopt this latter value as the interstellar extinction and A V = 0 for the circumstellar extinction, because in Kóspál et al (2018), the dips due to circumstellar extinction only cause very small changes, and only for 4.85 days in total, which corresponds to 6% of the time, which suggests all the extinction of DQ Tau is basically due to the interstellar medium. The radial velocity (RV) of DQ Tau components was measured using SAPHIRES code 4 , which determines the relative broadening functions (BFs).…”

“…DQ Tau is composed by two low-mass M0−M1 type CTTs that orbit each other with an orbital period of P = 15.80158 ± 0.00066 days and eccentricity of 0.568 (Czekala et al 2016). The rotational period of the stars is T = 3.017 ± 0.004 days (Kóspál et al 2018). The most recent and accurate determination of the stellar mass was given by Czekala et al (2016), by combining the orbital solution fitted to radial velocity (RV) data and a disk model fitted to ALMA CO observations.…”

Gap, shadows, spirals, and streamers: SPHERE observations of binary-disk interactions in GG Tauri A

“…Such a phase-dependent pulsed accretion process has been seen in numerous theoretical A62, page 10 of 15 studies (e.g., Günther & Kley 2002;Dunhill et al 2015;Duffell et al 2019). Time-variable spectroscopic signatures of accretion activity (e.g., through hydrogen recombination lines) correlated with the binary orbital phase have been observed in some tight (spectroscopic) binary systems (e.g., Mathieu et al 1997;Kóspál et al 2018). While the detection of pulsed accretion is usually restricted to very tight systems (because of the restricted time base), the detection of periodic streamers in GG Tau A, if confirmed, would be the first indication of such processes in a wider system.…”