Near-infrared images of galactic masers I. Association between infrared sources and masers

We present the first results of an extensive near infrared survey of galactic H_2_O and OH masers in high-luminosity star forming regions (SFR) aimed to see if there are near-infrared (NIR) sources directly associated with the masers. Seventeen fields for which accurate VLA positions of the masers were available have been imaged in the three J, H, and K NIR broad band filters with pixel resolution of 1.34 arcsec/pixel and a field of view of roughly 3'x3'. All observed fields show a high density of K-band sources, completely undetected in previous surveys, probably stellar clusters located in the SFR. From numerical simulations we find that the distributions of the observed first-neighbour K-band source to the maser is very unlikely due to chance coincidence with uniformly distributed field sources. For this reason, the infrared source nearest to the maser (<~10 arcseconds) is considered to be associated with the maser. All these sources have distinctive characteristics: they are weak and detected only in K, or if the H magnitude is measurable, they show an H-K colour index greater than 2. Although not in all sources there are high sensitivity-high resolution radio continuum observations, only few of the K-band/maser sources are closely associated with known ultracompact (UC) HII regions. After considering several plausible alternatives we find that the observed NIR emission is produced by a young stellar object (YSO) surrounded by a dusty circumstellar envelope. In the evolutionary scheme of SFR this result places the NIR/maser sources in a stage preceding that of UCHII regions, in which the radio continuum from ionized gas is undetectable with present sensitivities either because so much reduced by self-absorption or by dust absorption of stellar UV photons in the very dense envelope of the YSO or intrinsically weak due to low UV photon fluxes.