Massive-Star Mergers and the Recent Transient in NGC4490: 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 an optical/IR transient
discovered in the nearby galaxy NGC4490, which appeared similar to these merger
events (unobscured progenitor, irregular multi-peaked light curve, increasingly
red color, 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 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
transient, indicating a transition to a self-obscured state qualitatively
similar to the evolution seen in other stellar mergers and in LBVs. 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.
