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 (firstname.lastname@example.org)
Subjects: Optical, Nova, Transient, Variables
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.