Violent environment of the inner disk of RW Aurigae A probed by the 2010 and 2015 dimming events

Abstract

Aims. The young binary system RW Aur shows strong signatures of a recent tidal encounter between the circumprimary disk and the secondary star. The primary star has recently undergone two major dimming events (Δmag ≈ 2 in V band) whose origin is still under debate. To shed light on the mechanism leading to the dimming events, we study the extinction properties, accretion variability, and gas kinematics using absorption lines from the material that is obscuring RW Aur A. Methods. We compared our moderate-resolution X-shooter spectra of the dim state of RW Aur A with other spectral observations. In particular, we analysed archival high-resolution UVES spectra obtained during the bright state of the system to track the evolution of the spectral properties across the second dimming event. Since the X-shooter spectrum is flux calibrated, we provide new synthetic photometry of RW Aur A during the dim state. Results. The spectrum obtained during the dim state shows narrow absorption lines in the Na and K optical doublets, where the former is saturated. With a velocity of - 60 km s-1, these lines indicate that during the dim state the disk wind is either enhanced or significantly displaced into the line of sight. The photometric evolution across the dimming event shows a grey extinction, and is correlated with a significant reduction of the EW of all photospheric lines. Emission lines that trace accretion do not vary significantly across the dimming. Conclusions. From comparing our observations with complementary results from the last years, we conclude that the dimming event is related to a major perturbation on the inner disk. We suggest that the inner disk is occulting (most of) the star and thus its photosphere, but does not occult the accretion regions within a few stellar radii. Since observations of the outer disk indicate that the disk is modestly inclined (45-60°), we propose that the inner disk might be warped by an as yet unseen (sub-) stellar companion, which may also explain the 2.77 day periodic variability of the spectral lines.