Extended CO emission in the field of the light echo of V838 Monocerotis

Context. V838 Mon erupted at the beginning of 2002, becoming an extremely luminous star with L = 106 $L_{\odot}$. The outburst was followed by a spectacular light echo that revealed that the star is immersed in a diffuse and dusty medium, plausibly interstellar in nature. Low angular-resolution observations of the star and its closest vicinity in the lowest CO rotational transitions revealed a molecular emission from the direction of V838 Mon. The origin of this CO emission has not been estalished. Aims. The main aim of this paper is to better constrain the nature of the CO emission. In particular, we investigate the idea that the molecular emission originates in the material responsible for the optical light echo. Methods. We performed observations of 13 positions within the light echo in the two lowest rotational transitions of 12CO using the IRAM 30 m telescope. Results. Emission in CO J = 1-0 and J = 2-1 was detected in three positions. In three other positions only weak J = 1-0 lines were found. The lines appear at two different velocities $V_{\rm LSR}$ = 53.3 km s-1 and VLSR = 48.5 km s-1, and both components are very narrow with FWHM ≈ 1 km s-1. Conclusions. The molecular emission from the direction of V838 Mon is extended and has a complex distribution. We identify the emission as arising from diffuse interstellar clouds. A rough estimate of the mass of the molecular matter in those regions gives a few tens of solar masses. The radial velocity of the emission at 53.3 km s-1 suggests that the CO-bearing gas and the echoing dust are collocated in the same interstellar cloud