What Infrared Spectroscopy of Carbon Monoxide Tells us about V838 Mon

We review our infrared spectroscopy of CO in V838 Mon during 2002-2006. Although the star superficially resembled a cool supergiant after both the first and third of its outbursts, its infrared “photosphere” consisted of very high velocity gas that must have been well detached from the star itself. A spectrum captured during the third outburst reveals a highly unusual composite spectrum of emission and absorption in the CO first overtone bands. At present a cool stellar photosphere is seen in CO, at a radial velocity redshifted by 15 km s−1 relative to the stellar velocity determined from SiO maser emission. V838 Mon is now truly a red giant, although its photosphere may be contracting and warming. Three shell components, corresponding to expansion velocities of 15, 85, and 145 km s, also are present in the spectrum. No lines of 13CO have yet been detected, but the current lower limit, 12C/13C >10, does not tightly constrain the evolutionary status of the progenitor.