VVV-WIT-01: highly obscured classical nova or protostellar collision?

A search of the first Data Release of the VISTA Variables in the Via Lactea (VVV) Survey discovered the exceptionally red transient VVV-WIT-01 (H - K-s = 5.2). It peaked before March 2010, then faded by similar to 9.5 mag over the following 2 yr. The 1.6-22 mu m spectral energy distribution in March 2010 was well fit by a highly obscured blackbody with T similar to 1000 K and A(Ks) similar to 6.6 snag. The source is projected against the Infrared Dark Cloud (IRDC) SDC G331.062-0.294. The chance projection probability is small for any single event (p approximate to 0.01-0.02), which suggests a physical association, e.g. a collision between low mass protostars. However, blackbody emission at T similar to 1000 K is common in classical novae (especially CO novae) at the infrared peak in the light curve due to condensation of dust similar to 30-60 d after the explosion. Radio follow-up with the Australia Telescope Compact Array detected a fading continuum source with properties consistent with a classical nova but probably inconsistent with colliding protostars. Considering all VVV transients that could have been projected against a catalogued IRDC raises the probability of a chance association to p = 0.13-0.24. After weighing several options, it appears likely that VVV-WIT-01 was a classical nova event located behind an IRDC.