Spitzer Observations of the 2015 M31 Stellar Merger

ATel #7468; S. Adams (OSU), C. S. Kochanek (OSU), Subo Dong (KIAA-PKU), R. M Wagner (LBTO)
on 1 May 2015; 20:34 UT
Credential Certification: Krzysztof Stanek (stanek.32@osu.edu)

Tweet

We report Spitzer observations of the 2015 M31 stellar merger ("Nova" M31N 2015-01a = MASTER J004207.99+405501.1, Shumkov et al. 2015) on April 6.87 and 13.12 when the source was not otherwise observable from Earth. As seen in Figure 1, there is a bright new source at the position of the transient (shown by a 4" radius circle) compared to earlier observations in 2005 by Barmby et al. (2006).

We find preliminary Vega aperture magnitudes of [3.6]=12.79+/-0.07 and [4.5]=12.80+/-0.09 mag for the first epoch and [3.6]=12.82+/-0.07 and [4.5]=12.91+/-0.09 mag for the second epoch. The near zero [3.6]-[4.5] color means that the spectral slope is consistent with the Rayleigh-Jeans tail of a stellar atmosphere, indicating that no dust formation has occurred at this point.

Figure 2 shows the recent evolution of the SED assuming an M31 distance of 0.8 Mpc (Stanek & Garnavich 1998) and (only) Galactic extinction of E(B-V)=0.05 mag, adding recent optical (Pessev et al. 2015, Williams et al. 2015) and near-IR (Srivastava et al. 2015) data. The current luminosity of log(L/Lsun)~5.3 is roughly 500 times brighter than the progenitor star identified by Dong et al. (2015) (also see Williams et al. 2015) and it appears to be slowly fading.

Dust can form if adequately dense material has reached 5e14/T3^2 cm given this luminosity and an effective black body temperature of Tbb=1000T3 K for dust formation (see, e.g., Kochanek 2015). Reaching this radius by the present epoch would require a velocity v>700/T3^2 km/s that is either comparable (800-900 km/s FWHM, Kurtenkov et al. 2015, Williams et al. 2015) or significantly larger (165 km/s expansion, Fabrika et al. 2015) then reported velocities. However, dust formation is likely by the next Spitzer visibility window (Oct/Nov) and near-IR observations will be a crucial complement to the on-going Spitzer observations as the transient rises.

We thank Director B.T. Soifer for approving this Spitzer Director's Discretionary time program and the Spitzer team for implementing these observations so rapidly.