The Probable M31 Stellar Merger Now Has a Late K or Early M Spectrum

ATel #7208; R. M. Wagner (LBTO), S. G. Starrfield (ASU), A. Wilber (ASU), C. S. Kochanek (OSU), Subo Dong (KIAA-PKU), J.-L. Prieto (UDP), and S. Adams (OSU)
on 11 Mar 2015; 21:24 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Thomas Holoien (tholoien@astronomy.ohio-state.edu)

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We report a new optical spectrum of the probable stellar merger in M31 ("Nova" M31N 2015-01a = MASTER J004207.99+405501.1). The source was identified as a nova candidate (ATEL 6911) but the spectrum was increasingly dominated by late-type stellar photospheric features (F then K, ATELs 6985, 7150). A luminous variable progenitor was also identified (ATEL 7173).

On 2015 March 10.1144 UT, we obtained a 10 minute optical spectrum (range: 398-686 nm; resolution 0.3 nm) with the OSMOS spectrograph on the MDM Observatory 2.4 m Hiltner telescope on Kitt Peak (at airmass 2.6!). While not of high quality, the spectrum shows strong TiO absorption features indicating that the photospheric temperature has continued to decline compared to the report of weak TiO features in ATEL 6985. The new data is shown at bottom as compared to M3 III (top), K7 III (upper middle) and K4 III (lower middle) stellar atmospheres (Jacoby et al. 1984).

The source will be monitored ~weekly by Spitzer in April and bi-weekly in the Oct/Nov visibility window to watch for the onset of dust formation and to obtain some information about its evolution while unobservable from the ground. We encourage renewed photometric and spectroscopic monitoring of this transient once it rises in late May/early June.