Spectroscopy and photometry of the nova M31N 2015-01a

ATel #6985; S. Fabrika, E. A. Barsukova, A. F. Valeev, A. Vinokurov, O. Sholukhova (Special Astrophysical Observatory), V. P. Goranskij (SAI, Moscow University), K. Hornoch (Astronomical Institute, Ondrejov), M. Henze (ESA/ESAC, Spain), A. W. Shafter (SDSU)
on 24 Jan 2015; 17:43 UT
Credential Certification: Sergei Fabrika (fabrika@sao.ru)

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We report optical spectroscopy and photometry of the recent M31 nova M31N 2015-01a first detected by MASTER (ATel #6911) and also observed spectrally (ATels #6924, #6941, #6952) and photometrically (MASTER, ATel #6951). Our first data were obtained on 2015 Jan. 15.80 UT, when the nova was at its pre-maximum state, the second observations were on 2015 Jan. 21.71 UT, when the nova presumably was about at maximum (ATel #6951). We used the Russian BTA telescope equipped with the SCORPIO spectrograph. The spectra were taken with a spectral resolution of 5 A in the spectral range of 4050 - 5850 A. Photometry of the nova obtained shortly prior to the spectroscopy gave B = 17.06 ± 0.04, V = 16.40 ± 0.02, R = 15.99 ± 0.02 on Jan. 15 and B = 15.94 ± 0.03, V = 15.41 ± 0.01, R = 15.09 ± 0.01 on Jan. 21.

The pre-maximum spectrum has shown a forest of absorption lines: hydrogen, Fe II, Ti II, Cr II, Mg II, Fe I. The hydrogen line widths (corrected for spectral resolution) are 360 km/s (H-gamma) and 540 km/s (H-delta), they are notably broader than metal lines. Their equivalent widths are 1.2 A (H-gamma) and 0.8 A (H-delta). The radial velocity of the both lines is -450 km/s (all velocities were reduced to the heliocentric frame). The strongest metal lines have the same velocity. The H-beta line has an emission component which partly fills the absorption profile. Taking into account the radial velocity map of M31 from 21-cm measurements (Corbelli et al., 2010, Astron. Astrophys. 511, A89), we find the M31 radial velocity for the nova location is -370 ± 50 km/s. Therefore the expansion velocity of the nova at that stage is ~ 80 km/s. We have correlated the absorption line spectrum with a library of stellar spectra (Jacoby et al., 1984, ApJSS, 56, 257) in the spectral range 4350 - 4830 A and found a good correlation with F5I supergiant star.

Our second spectrum has changed in two positions: (i) it became notably bluer and (ii) the expansion velocity increased by 85 km/s (now it is 165 km/s). Otherwise the absorption line spectrum practically did not change. A type of this likely nova may be established after the envelope becomes transparent.