Progenitor, Precursor and Evolution of the Dusty Remnant of the Stellar Merger M31-LRN-2015

Creators: Blagorodnova, N.; Karambelkar, V.; Adams, S. M.; Kasliwal, M. M.; Kochanek, C. S.; Dong, S.; Campbell, H.; Hodgkin, S.; Jencson, J. E.; Johansson, J.; Kozlowski, S.; Laher, R. R.; Masci, F.; Nugent, P.; Rebbapragada, U.

Abstract

M31-2015-LRN is a likely stellar merger discovered in the Andromeda Galaxy in 2015. We present new optical to mid-infrared photometry and optical spectroscopy for this event. The transient brightened by ∼3 mag as compared to its progenitor. The complex precursor emission, which started ∼2 years before the nova event, may be explained by the binary undergoing Roche-lobe overflow. The dynamical mass loss from the outer Lagrange point L2 creates an optically thick outflow to power the observed brightening of the system. We find two possible periods of 16±0.3 and 28.1±1.4 days at different phases of the precursor lightcurve, possibly related to the geometry of the mass-loss from the binary. Although the progenitor spectral energy distribution shows no evidence of pre-existing warm dust in system, the remnant forms an optically thick dust shell 2−4 months after the outburst peak. The optical depth of the shell increases after 1.5 years, suggesting the existence of shocks that enhance the dust formation process. We propose that the merger remnant is likely an inflated giant obscured by a cooling shell of gas with mass ∼0.2 M⊙ ejected at the onset of the common envelope phase.

Additional Information

© 2020 The Authors. Last updated 13 April 2020; in original form 13 April 2020. N. B. would like to thank O. Pejcha and M. MacLeod for useful discussions, A. Kurtenkov and S. C. Williams for making available the spectra of M31-LRN-2015, A. Pastorello for data on AT2017fjs, and T. Kaminski for the spectrum of V1309Sco. To M. Fraser, D. Perley, and R. M. Wagner for observations and data reduction, and to T. Szalai for the dust emission models. This work is part of the research programme VENI, with project number 016.192.277, which is (partly) financed by the Netherlands Organisation for Scientific Research (NWO). SK acknowledges the financial support of the Polish National Science Center (NCN) through the OPUS grant 2018/31/B/ST9/00334. P.E.N. acknowledges support from the DOE under grant DEAC02-05CH11231, Analytical Modeling for Extreme-Scale Computing Environments. CSK is supported by NSF grants AST-1908570 and AST-1814440. The Intermediate Palomar Transient Factory project is a scientific collaboration among the California Institute of Technology, Los Alamos National Laboratory, the University of Wisconsin, Milwaukee, the Oskar Klein Center, the Weizmann Institute of Science, the TANGO Program of the University System of Taiwan, and the Kavli Institute for the Physics and Mathematics of the Universe. The WHT spectrum was taken under program (2014B/P29). This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. This work is based in part on observations made with the Large Binocular Telescope. The LBT is an international collaboration among institutions in the United States, Italy and Germany. The LBT Corporation partners are: The University of Arizona on behalf of the Arizona university system; Istituto Nazionale di Astrofisica, Italy; LBT Beteiligungsge-sellschaft, Germany, representing the Max Planck Society, the Astrophysical Institute Potsdam, and Heidelberg University; The Ohio State University; The Research Corporation, on behalf of The University of Notre Dame, University of Minnesota and University of Virginia. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. We acknowledge Telescope Access Program (TAP) funded by the NAOC, CAS, and the Special Fund for Astronomy from the Ministry of Finance. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. This research made use of Astropy,4 a community-developed core Python package for Astronomy (Astropy Collaboration et al. 2013, 2018).

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