Spectroscopic observations of the M31 optical transient, TCP J00403295+4034387
ATel #3806; A. W. Shafter, M. J. Darnley, M. F. Bode, S. C. Williams (Liverpool JMU, UK), K. Hornoch (Astronomical Institute, Ondrejov, Czech Republic), R. Ciardullo (PSU)
on 13 Dec 2011; 22:25 UT
Credential Certification: Allen W. Shafter (aws@nova.sdsu.edu)
Subjects: Optical, Nova, Transient
We report spectroscopic observations of the M31 transient
and nova candidate
TCP J00403295+4034387 discovered on 2011 Nov. 14.538 by
K. Nishiyama and F. Kabashima at m=18.3 (unfiltered).
At our request, a spectrum (410–900 nm)
was obtained by S. Odewahn on 2011 Nov. 30.23 UT with
the 9.2m Hobby-Eberly Telescope
(+ Marcario Low-Resolution Spectrograph).
The spectrum is characterized by an unusual energy distribution:
a blue continuum that rises shortward of λ 570 nm,
with a red component dominating at longer wavelengths.
In addition, the spectrum reveals narrow Balmer and
He II λ 468.6 nm emission lines
(FWHM Hα < ~1000 km/s; EW Hα ~ -6.0 nm),
along with TiO bands near λ 615.0, 705.4, and 766.6 nm.
The unusual nature of the spectrum led us to consider
the possibility that the source was composite,
with the red and blue components arising from
an unresolved pair of stars. The Massey et al.
(2006, AJ, 131, 2478)
catalog of stars in M31 lists a blue star,
LGGS J004032.97+403439.0 (V=20.498, B-V=-0.11, U-B = -1.15,
V-R = -.078, and R-I =0.269), very near the nominal
position of the transient. An examination of images
from Massey et al. (2006) shows that the source is indeed
a blend of two stars with an apparent separation of ~0.8".
This finding is confirmed by an examination of HST images
of the field (u9pi0401m, u9pi0402m, u9pi0403m, u9pi0404m),
which reveal the transient to be composed of two
stars, one red (B-I ~ 4.76) and one
blue (B-I ~ 0.28), clearly separated by ~0.5".
We tentatively identify the blue component in the
HST image with LGGS J004032.97+403439.0,
which is apparently an early-type, emission-line star.
In this case, the red component
(which was not reported in the Massey catalog) is
most likely a long-period red (Mira) variable,
whose variation is primarily responsible for the
pair of unresolved stars now being
identified as a transient source.
Photometry of the unresolved source with the
0.65-m telescope at Ondrejov reveals the following
R-band magnitudes for the transient:
UT Date R mag
--------------- --------------
Nov. 14.754 18.2 +/- 0.1
Nov. 28.705 19.0 +/- 0.2
Dec. 01.741 19.7 +/- 0.2
Dec. 08.747 18.9 +/- 0.3
Dec. 10.746 19.1 +/- 0.2
If we assume that the brightness of the blue source,
LGGS J004032.97+403439.0, has remained essentially
constant during the recent brightening, we deduce
the following R magnitudes for the variable red component:
UT Date R mag
--------------- --------------
Nov. 14.754 18.4 +/- 0.1
Nov. 28.705 19.3 +/- 0.2
Dec. 01.741 20.4 +/- 0.4
Dec. 08.747 19.2 +/- 0.4
Dec. 10.746 19.5 +/- 0.2
We thank K. Nishiyama and F. Kabashima for alerting us to the object.
A.W.S. thanks the NSF for support through AST-1009566.
HET Spectra of M31 Nova Candidates
