Near infrared images of galactic masers:
I. Association between infrared sources and masers.
L. Testi 1, M. Felli 2, P. Persi 3, and M. Roth 4
1 Dipartimento di Astronomia e
Scienza dello Spazio, Universitá di Firenze, Largo E. Fermi, 5, I-50125 Firenze, Italy
2 Osservatorio Astrofisico di Arcetri,
Largo Enrico Fermi 5, I-50125 Firenze, Italy
3 C.N.R., I.A.S., C.P. 67, I-00044 Frascati, Italy
4 Las Campanas Observatory, Casilla 601, La Serena, Chile
We present the first results of an extensive near infrared survey of
galactic 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 and a field of view
of roughly .
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 HK 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
is produced by a young stellar object (YSO) surrounded by a dusty
In the evolutionary scheme of SFR this result places the NIR/maser
sources in a stage preceding that of UCHii regions,
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.