Massive star mergers and the recent transient in NGC 4490: a more massive cousin of V838 Mon and V1309 Sco

The Galactic transient V1309 Sco was the result of a merger in a low-mass star system, while V838 Mon was thought to be a similar merger event from a more massive B-type progenitor. In this paper, we study a recent optical and infrared (IR) transient discovered in the nearby galaxy NGC4490 named NGC4490-OT2011 (NGC 4490-OT hereafter), which appeared similar to these merger events (unobscured progenitor, irregular multi-peaked light curve, increasingly red colour, similar optical spectrum, IR excess at late times), but which had a higher peak luminosity and longer duration in outburst. NGC4490-OT has less in common with the class of SN 2008S-like transients. A progenitor detected in pre-eruption Hubble Space Telescope (HST) images, combined with upper limits in the IR, requires a luminous and blue progenitor that has faded in late-time HST images. The same source was detected by Spitzer and ground-based data as a luminous IR (2-5 mu m) transient, indicating a transition to a self-obscured state qualitatively similar to the evolution seen in other stellar mergers and in luminous blue variables. The post-outburst dust-obscured source is too luminous and too warm at late times to be explained with an IR echo, suggesting that the object survived the event. The luminosity of the enshrouded IR source is similar to that of the progenitor. Compared to proposed merger events, the more massive progenitor of NGC4490-OT seems to extend a correlation between stellar mass and peak luminosity, and may suggest that both of these correlate with duration. We show that spectra of NGC4490-OT and V838 Mon also resemble light-echo spectra of eta Car, prompting us to speculate that eta Car may be an extreme extension of this phenomenon.Some of the data reported here were obtained at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution. We thank the staffs at Lick and MMT Observatories for their assistance with the observations. We also appreciate the help of Jeff Silverman for some of the Lick observations. Data from Steward Observatory facilities were obtained as part of the observing programme AZTEC: Arizona Transient Exploration and Characterization. Lindsey Kabot assisted with early stages of the MMT spectral data reduction. The work presented here is based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. These are based in part on observations associated with programme #13364 (Legacy ExtraGalactic UV Survey, LEGUS). This paper has made use of the higher level data products provided by the LEGUS team.; NS and JEA received partial support from National Science Foundation (NSF) grants AST-1210599 and AST-1312221. MMK acknowledges support from the Carnegie-Princeton fellowship. Funding for this effort was provided in part by the Spitzer SPIRITS Cycles 10-12 exploration science programme. The supernova research of AVF's group at U.C. Berkeley presented here is supported by Gary & Cynthia Bengier, the Christopher R. Redlich Fund, the TABASGO Foundation, and NSF grant AST-1211916. KAIT and its ongoing operation were made possible by donations from Sun Microsystems, Inc., the Hewlett-Packard Company, AutoScope Corporation, Lick Observatory, the NSF, the University of California, the Sylvia & Jim Katzman Foundation, and the TABASGO Foundation. Research at Lick Observatory is partially supported by a generous gift from Google. JJ is supported by an NSF Graduate Research Fellowship under Grant No. DGE-1144469.This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at repository@u.library.arizona.edu