Methanol Masers – Tracing the Earliest Stages of
High-mass Star Formation?
Simon Ellingsen – University of Tasmania
Introduction :
The strongest class II methanol maser transition (6.7 GHz) was discovered by Menten (1991) and in the period since then more than 500 sites have been found in
targeted and blind searches throughout the Galaxy (see Pestalozzi et al. 2005). Both classes of methanol maser are associated with star forming regions, class II masers
are closely associated with infrared sources and other maser species such as OH. There is growing evidence that the class II methanol masers exclusively trace high-
mass star formation and that many are associated with the phase prior to the formation of an ultracompact HII region. In particular :
•Blind searches have found no evidence for association with any other class of astrophysical object other than high-mass star formation (Ellingsen et al. 1996 ;
Szymczak et al. 2002)
•Sensitive searches for class II methanol masers towards low-mass star forming regions haven’t detected any (Minier et al. 2003).
•The majority of class II methanol masers are not associated with ultracompact HII regions (Phillips et al. 1998 ; Walsh et al. 1998).
•They are associated with cold dust emission detected in millimetre and sub-millimetre continuum observations (Pestalozzi et al. 2002 ; Walsh et al. 2003).
If class II methanol masers do trace a pre-HII region phase of high-mass star formation, then Figure 1 : Colour-colour diagram of GLIMPSE
it makes them a powerful signpost for finding and studying the earliest stages of the point sources within 30” of l=326°, b=0° (black
process. In particular we want information on the physical conditions in the region near the dots) and class II methanol masers in the region
l=325-335° (red squares). A total of 21226 of
high-mass young stellar objects. Two ways in which we can obtain this information are the 109053 GLIMPSE point sources in this
region have information for the 3.6, 5.8 and 8.0
•Studies of the young stellar objects that drive the maser process.
m bands (for the methanol masers it is 19 of
36). Only 34 of the 21226 sources in the
•Modelling of the masers to obtain estimates of the physical conditions in the masing gas. comparison field have [3.6]-[8.0] > 4.0 which
is characteristic of the sources with associated
methanol masers.
Young Stellar Objects associated with Methanol Masers :
The majority of young stellar objects that have associated methanol masers are only detectable in the millimetre through to mid-infrared region of the spectrum, a region
where high resolution, sensitive observations are very challenging. A number of searches for class II methanol masers have been targeted towards IRAS sources with
colours consistent with ultracompact HII regions (e.g. Walsh et al. 1997). However, the modest spatial resolution of IRAS, confusion in the Galactic plane and a number of
other issues has meant that it is not possible to infer anything about the infrared properties of the objects associated with the masers. The MSX mission made observations
in the mid-infrared with higher resolution and positional accuracy than IRAS (2”). However, recent studies have shown that only a small percentage of methanol masers are
associated with MSX point sources and approximately 50% are not projected against 21m emission in MSX images (Ellingsen 2005).
The recently completed GLIMPSE survey of the inner Galactic Plane (Spitzer Space Telescope) has for the first time provided high-resolution (1.2” pixels), sensitive
observations of large regions in four mid-infrared bands (Benjamin et al. 2003). The positional accuracy (0.4”) and resolution is for the first time allowing us to
unambiguously identify the young stellar objects that drive the class II methanol masers. Using the first public release of the GLIMPSE catalogue in the fourth quadrant
of the Galactic Plane, I have investigated the properties of a statistically complete sample of methanol masers from the region l=325-335°, |b|<0.53° associated with
GLIMPSE point sources. Two-thirds of the methanol masers have an associated GLIMPSE point source within 2” (in most cases the offset is smaller) and approximately
85% are projected against emission in the 8.0 m band. Colour-colour and colour-magnitude diagrams of the GLIMPSE point sources associated with methanol masers
show that they are significantly redder than the vast majority of sources in comparison fields (see Figure 1). In fact the methanol masers are associated with the reddest
sources in the GLIMPSE point source catalogue, adding weight to the argument that they are associated with a very early phase of high-mass star formation.
Figure 2 : 8.0m GLIMPSE image of the
region surrounding the methanol maser (blue
cross) G329.469+0.502. The blue square
marks the location of the associated GLIMPSE
point source. The maser is associated with a
faint, very red point source embedded within
the dark cloud surrounding what appears to be
a more evolved star forming region.
Examination of the 8.0 m emission associated with the methanol masers shows that in
most cases the masers are associated with faint sources lying within dark clouds that are Figure 3 : (left) 8.0m GLIMPSE image of the region surrounding the methanol maser (blue crosses)
G329.031-0.198, G329.029-0.205, G329.066-0.308 & G329.183-0.314. The blue squares mark the
projected against the diffuse PAH background emission that dominates this band. In location of the associated GLIMPSE point source. All the masers lie along a long, narrow dark
some cases the masers appear to be associated with the first epoch of high mass star filament that is projected against diffuse background PAH emission. (right) A zoom in on the region
formation in that region. In other cases (e.g. Figure 2) the masers lie near the edge of of the filament containing the masers G329.031-0.198, G329.029-0.205.
what appear to be more evolved star forming regions, suggestive of a new epoch of
triggered star formation. Figure 3 shows a long, narrow dark filament along which four methanol masers are projected. The class II methanol masers associated with the
filament have similar velocities which lie in the range -35 to -60 kms-1 (near kinematic distance 2.5-3.5 kpc) with a general trend to more negative velocities for larger
Galactic Longitudes. The presence of a number of masers along this filament suggests that high-mass star formation can occur in such regions, as well as giant molecular
clouds.
References :
Theoretical Models of Methanol Masers : Benjamin, R.A., Churchwell, E.B., Babler, B. et al., 2003, PASP, 115, 953
Some class II methanol maser sources show emission from a number of different Cragg, D.M., Sobolev, A.M., Caswell, J.L, 2004, MNRAS, 351, 1327
transitions at the same velocity, suggesting that they arise from the same gas. Observations Ellingsen, S.P., 2005, MNRAS in press
of such sources can be used to constrain models of the maser emission and estimate the Ellingsen, S.P., von Bibra, M.L., McCulloch, P.M. et al., 1996, MNRAS, 280, 378
physical conditions in the masing gas. The models predict that the 6.7 and 12.2 GHz
Menten, K.M., 1991, ApJ, 380, L75
transitions are inverted over a wide range of physical conditions, consistent with them
being both strong and common. However, the majority of transitions are inverted over a Minier, V., Ellingsen, S.P., Norris, R.P., Booth, R.S., 2003, A&A, 403, 1095
much narrower range of physical conditions. This means that detection of a rare transition Pestalozzi, M., Humphreys, E.M.L., Booth, R.S., 2002, A&A, 384, L15
provides useful constraints on the physical conditions, conversely non-detection in the
Pestalozzi, M., Minier, V., Booth, R.S., 2005, A&A, 432, 737
majority of sources also rules out a region of parameter space. Searches for rarer
transitions that are favoured under different physical conditions can then be used to restrict Phillips, C.J., Norris, R.P., Ellingsen, S.P., et al., 1998, MNRAS, 300, 1131
the parameters in the majority of sources. This approach has been applied to the 23.1 and Szymczak, M., Kus, A.J., Hrynek, G. et al., 2002, A&A, 392, 277
107 GHz transitions by Cragg et al. (2004) and shows great promise.
Walsh, A.J., Hyland, A.R., Robinson, G. et al., 1997, MNRAS, 291, 261
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Combining information from theoretical models of maser pumping and sensitive, high Walsh, A.J., Macdonald, G.H., Alvey, N.D.S. et al., 2003, A&A, 410, 597
resolution observations of the sources that excite the maser emission in the millimetre
through to mid-infrared wavelength range shows great promise for advancing our
Acknowledgements : This poster makes use of data products from the
GLIMPSE survey, which is a legacy science program of the Spitzer Space
understanding of the high-mass star formation process. Telescope, funded by the National Aeronautics and Space Administration.