Dr Anastasia Ponomareva

Abstract:

V341 Ara was recently recognized as one of the closest (d ≃ 150 pc) and brightest (V ≃ 10) nova-like cataclysmic variables. This unique system is surrounded by a bright emission nebula, likely to be the remnant of a recent nova eruption. Embedded within this nebula is a prominent bow shock, where the system’s accretion disc wind runs into its own nova shell. In order to establish its fundamental properties, we present the first comprehensive multiwavelength study of the system. Long-term photometry reveals quasi-periodic, super-orbital variations with a characteristic time-scale of 10–16 d and typical amplitude of ≃1 mag. High-cadence photometry from theTransiting Exoplanet Survey Satellite (TESS) reveals for the first time both the orbital period and a ‘negative superhump’ period. The latter is usually interpreted as the signature of a tilted accretion disc. We propose a recently developed disc instability model as a plausible explanation for the photometric behaviour. In our spectroscopic data, we clearly detect antiphased absorption and emission-line components. Their radial velocities suggest a high mass ratio, which in turn implies an unusually low white-dwarf mass. We also constrain the wind mass-loss rate of the system from the spatially resolved [O III] emission produced in the bow shock; this can be used to test and calibrate accretion disc wind models. We suggest a possible association between V341 Ara and a ‘guest star’ mentioned in Chinese historical records in AD 1240. If this marks the date of the system’s nova eruption, V341 Ara would be the oldest recovered nova of its class and an excellent laboratory for testing nova theory.

Abstract:

The mass-to-light ratio (M/L) is a key parameter in decomposing galactic rotation curves into contributions from the baryonic components and the dark halo of a galaxy. One direct observational method to determine the disc M/L is by calculating the surface mass density of the disc from the stellar vertical velocity dispersion and the scale height of the disc. Usually, the scale height is obtained from near-IR studies of edge-on galaxies and pertains to the older, kinematically hotter stars in the disc, while the vertical velocity dispersion of stars is measured in the optical band and refers to stars of all ages (up to ∼ 10 Gyr) and velocity dispersions. This mismatch between the scale height and the velocity dispersion can lead to underestimates of the disc surface density and a misleading conclusion of the sub-maximality of galaxy discs. In this paper we present the study of the stellar velocity dispersion of the disc galaxy NGC 6946 using integrated star light and individual planetary nebulae as dynamical tracers. We demonstrate the presence of two kinematically distinct populations of tracers which contribute to the total stellar velocity dispersion. Thus, we are able to use the dispersion and the scale height of the same dynamical population to derive the surface mass density of the disc over a radial extent. We find the disc of NGC 6946 to be closer to maximal with the baryonic component contributing most of the radial gravitational field in the inner parts of the galaxy (⁠Vmax(bar)=0.76(±0.14)Vmax⁠).