The Probable M31 Stellar Merger: Identification and Variability of the Likely Progenitor System

ATel #7173; Subo Dong (KIAA-PKU), C. S. Kochanek (OSU), S. Adams (OSU), J.-L. Prieto (UdP, Chile)
on 4 Mar 2015; 01:41 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Subo Dong (dongsubo@pku.edu.cn)

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"Nova" M31N 2015-01a = MASTER J004207.99+405501.1 (see, primarily, ATELs 6911, 6985, 7150) was already identified as a likely stellar merger (aka "luminous red nova") based on its similarities to objects like V838 Mon or V1309 Sco. We have identified the likely progenitor in archival SDSS, CFHT, Local Group Survey and HST imaging data. The transient position is coincident with a g~20.8 mag source in CFHT images taken 24 October 2014 (Figure 1, upper left). The 2'' radius circle is centered on the reported position (00:42:07.99 40:55:01.1), and our progenitor candidate is the bright source inside the circle. Using two archival CFHT g-band mosaics from data collected between 27 Aug to 31 Oct 2005 and 19 Aug to 12 Sept 2009 as a reference image, we analyzed the three CFHT epochs and the SDSS data from 6 Oct 2002 using difference imaging. The upper middle and right panels of Figure 1, show that this source was considerably brighter (darker) on 24 October 2014 than in the earlier years. These panels are just two images from a full night monitoring this field for microlensing. The residuals from the archival SDSS data (Oct 2002) and CFHT mosaics collected in Aug to Oct 2005 (middle) and Aug to Sept 2009 (right) are shown in the lower panels. The estimated magnitude in this period (2002 to 2009) was g~22.6 mag. The source was also cataloged by Massey et al. (2006) at V~22.3 and V-I~1.3 mag in data taken in Sep 2002. The source also appears to be coincident with a V=23.2 +/- 0.03, I = 22.02 +/- 0.01 mag HST source (ACS images from 16 Aug 2004 analyzed with Dolphot), as shown in Figure 2. Crowding plays a role in making the HST magnitudes fainter, but they are in adequate agreement for present purposes. In Figure 2, the circle on the CFHT image has a radius of 2'' and is centered at the reported source position. The circle on the HST image has a radius of 0.3'' and is centered at the transformed position of our candidate for the progenitor. The scales of the two panels are the same.

The ~7 magnitude rise from I~22 to I~15 is slightly low for a main sequence merger, which suggests the progenitor is an evolved star (see Kochanek et al. 2014). Based on the magnitude and the peak magnitude/progenitor mass correlation in Kochanek et al. (2014), the transient properties suggest that the progenitor star was 3-7 msun. Modeling the HST magnitudes using a distance of 0.8 Mpc (Stanek & Garnavich 1998) and Galactic extinction of E(B-V)=0.05 mag with no additional extinction gives a luminosity of Log(L/Lsun)~2.6 and T~4700K. Assuming one star dominates these fluxes and matching to the Marigo et al. (2008) isochrones, this corresponds to an evolved ~2-4 msun star, broadly consistent with the inferences from the transient.

The present luminosity is comparable to the 1989 M31 RV (Rich et al. 1989). The estimated rate of mergers of this luminosity is roughly once every 10-30 years (Kochanek et al. 2014), so it is not surprising for another one to have occurred 26 years later.