Near-IR photometry of the proposed stellar merger M31N 2015-01a

ATel #7595; J. Harmanen (NOT/Univ. of Turku), B. McCollum (CUA), S. Laine (IPAC/Caltech), L. Rottler (PARI), F. C. Bruhweiler (CUA)
on 5 Jun 2015; 17:48 UT
Credential Certification: Bruce McCollum (mccollub@cua.edu)

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We report JHK photometry of the 2015 stellar merger candidate Nova M31N 2015-01a = MASTER J004207.99+405501.1 (Shumkov et al. 2015, ATel #6885) on 2015 May 29.2 UT. Our observations were carried out with the the 2.5m Nordic Optical Telescope (NOT) at the Observatorio Roque de Los Muchachos (ORM), La Palma, using the NOTCam instrument. The observations were performed shortly before sunrise at an airmass of 1.8.

We find magnitudes of J = 17.4 ± 0.1, H = 16.1 ± 0.1, and K = 14.7 ± 0.1, uncorrected for extinction. The H magnitude is the same, within the uncertainty, as the magnitude reported by Steele et al. (ATel #7555) on 23 May 2015. These magnitudes show a large near-IR color change from the previously reported values of J = 14.05, H = 13.40 and K = 13.0 observed on 6 March 2015 by Srivastava et al. (ATel #7236). As noted by Srivastava et al., the J, H, and K colors were consistent at that time with those of a K star. Spitzer observations of this object on April 18.65 and 23.69 (Adams et al., ATel #7485) also showed IR magnitudes consistent with a normal stellar flux distribution of a late type star.

Bersier et al. (ATel #7537) reported that the I magnitude observed on May 15.21 had declined greatly in the previous few weeks, implying either a decline of luminosity or the formation of enough dust to produce ~8 magnitudes of extinction. Pessev et al. (Atel #7552) confirmed this on 27 May with more I-band observations, and found a decline of over four magnitudes in R (giving new limit R > 21.0) since their March observations (ATel #7272).

Our data show that J-H = 1.3 and H-K = 1.4, which is much redder than those of a late-type star with typical ISM reddening. Comparing these values with those plotted in Messineo et al. (2012), we find that these colors are much redder in H-K than those of, e.g., red supergiants or SiO stars, and are at the extreme edge of the H-K location of OH/IR AGB stars in that sample. Thus the current near-IR colors appear to confirm the recent formation of a large amount of circumstellar dust, with a much larger proportion of flux now being emitted in the K band than was observed in March (ATel #7236).