A Long-term Photometric Variability and Spectroscopic Study of Luminous Blue Variable AF And in M31

Abstract

Abstract We present a photometric and spectroscopic analysis of the Hubble–Sandage variable star AF And in M31. The data have been taken under the Nainital Microlensing Survey during 1998–2002, and follow-up observations were carried out until 2011. During this period, photometric observations in Cousins R and I bands were obtained for 169 nights spanning about 5000 days. AF And showed a prominent outburst around 1999 mid-January, followed by a gradual decrease in brightness of about 1.5 mag in the next 3 yr with a declining rate of ∼0.0015 mag day−1, leading to a quiescent phase at the end of 2001. After lying low for about 9 yr, AF And again went through a secondary outburst phase in late 2010 with an amplitude of 0.44 mag, where it lasted for one year before fading back to its quiescent phase. Spectroscopic observations of AF And show prominent Balmer and He i emission lines along with the comparatively weaker Fe ii and [Fe ii] emissions. Asymmetric emission line profiles in its spectrum imply a mass-loss rate of about 2.2 × 10−4 M ⊙ yr−1 through the stellar winds in the photosphere. Using spectral energy distribution fitting, we find a photospheric temperature of 33,000 ± 3000 K during the visual minimum. Using a weak P Cygni profile of the He i emission line, the wind terminal velocity for AF And is found to be around 280–300 km s−1.