The molecular environment of massive star forming cores associated with
  Class II methanol maser emission
Astrophysical masers and their environments
Proceedings IAU Symposium No. 242, 2007
A.C. Editor, B.D. Editor & C.E. Editor, eds.
c© 2007 International Astronomical Union
DOI: 00.0000/X000000000000000X
The molecular environment of massive star
forming cores associated with Class II
methanol maser emission
S. N. Longmore1,2†, M. G. Burton1, P. J. Barnes3, T. Wong1,2,5,
C. R. Purcell1,4, J. Ott2,6
1School of Physics, University of New South Wales, Kensington, NSW 2052, Sydney, Australia
2Australia Telescope National Facility, CSIRO, PO Box 76, Epping, NSW 1710, Australia
3School of Physics A28, University of Sydney, NSW 2006, Australia
4University of Manchester, Jodrell Bank Observatory, Macclesfield, Cheshire SK11 9DL, UK
5Department of Astronomy, University of Illinois, Urbana IL 61801, USA
6National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
Abstract. Methanol maser emission has proven to be an excellent signpost of regions under-
going massive star formation (MSF). To investigate their role as an evolutionary tracer, we
have recently completed a large observing program with the ATCA to derive the dynamical
and physical properties of molecular/ionised gas towards a sample of MSF regions traced by
6.7GHz methanol maser emission. We find that the molecular gas in many of these regions
breaks up into multiple sub-clumps which we separate into groups based on their association
with/without methanol maser and cm continuum emission. The temperature and dynamic state
of the molecular gas is markedly different between the groups. Based on these differences, we
attempt to assess the evolutionary state of the cores in the groups and thus investigate the role
of class II methanol masers as a tracer of MSF.
Keywords. stars: formation, ISM: evolution, ISM: molecules, line: profiles, masers, molecular
data, stars: early-type, radio continuum: stars
1. Introduction
In terms of luminosity, energetics and chemical enrichment, massive stars exert a dis-
proportionate influence compared to their number on Galactic evolution. However, the
collective effects of their rarity, short formation timescales and heavy obscuration due to
dust, make it difficult to find large samples of massive young sources at well constrained
evolutionary stages needed to develop an understanding of their formation mechanism.
The 51→ 60 A
+ class II methanol (CH3OH) maser transition at 6.7GHz is one of the
most readily observable signposts of MSF [Menten 1991].
The specific conditions required for the masers to exist makes them a powerful probe
of the region’s evolutionary stage. While masers probe spatial scales much smaller than
their natal cores, the numerous feedback processes from newly formed stars [e.g. turbu-
lence injection from jets/outflows, ionisation (stars M>8M⊙) and heating from radiation
etc.] must significantly alter the physical conditions of the surrounding region. We have
completed an observing program with the Australia Telescope Compact Array (ATCA)
to derive properties of molecular and ionised gas towards MSF regions traced by 6.7GHz
methanol maser emission [Longmore et al. (2007)]. In this contribution, we use these re-
† E-mail:snl@phys.unsw.edu.au
http://arxiv.org/abs/0704.1684v1
120 S. N. Longmore et al.
sults to investigate the use of class II methanol masers as a diagnostic of the evolutionary
stage of MSF.
2. Observations
Observations of NH3(1,1), (2,2), (4,4) & (5,5) and 24GHz continuum were carried out
using the ATCA towards 21 MSF regions traced by 6.7GHz methanol maser emission
[selected from a similar sample to Hill et al. (this volume)]. The H168 [NH3(1,1)/(2,2)]
and H214 [NH3(4,4)/(5,5)] antenna configurations with both East-West and North-South
baselines, were used to allow for snapshot imaging. Primary and characteristic synthesised
beam sizes were ∼2.′2 and ∼8 − 11′′ respectively. Each source was observed for 4×15
minute cuts in each transition separated over 8 hours to ensure the best possible sampling
of the uv-plane. The data were reduced using the MIRIAD (see Sault et al. 1995) package.
Characteristic spectra were extracted at every transition for each core at the position of
the peak NH3(1,1) emission, baseline subtracted and fit using the gauss and nh3(1,1)
methods in CLASS (see http://www.iram.fr/IRAMFR/GILDAS/). Continuum source
fluxes and angular sizes were calculated in both the image domain and directly from the
uv data.
3. Core separation
NH3 detections within each region were separated into individual cores if offset by
more than a synthesised beam width spatially, or more than the FWHM in velocity if
at the same sky position. The same criteria were used to determine whether the NH3,
continuum and methanol maser emission in each of the regions were associated. In all
but three cases, these criteria were sufficient to both unambiguously separate cores and
determine their association with continuum and maser emission. We find 41 NH3(1,1)
cores (of which 3 are in absorption and 2 are separated in velocity) and 14 24GHz
continuum cores. Observationally the cores fall in to 4 groups: NH3 only (Group 1);
NH3 + methanol maser (Group 2); NH3 + methanol maser + 24GHz continuum (Group
3); NH3 + 24GHz continuum (Group 4). The cores were distributed with 16, 16, 6 and 2
cores in Groups 1 to 4, respectively. Based on this grouping, most of the NH3(1,1) cores
are coincident with methanol maser emission (Groups 2 & 3), but there are a substantial
fraction of NH3 cores with neither 24GHz continuum nor maser emission (Group 1).
Having separated the cores into these groups, we then considered observational biases
and selection effects which may affect their distribution. The biggest potential hindrance
was the difference in linear resolution and sensitivity caused by the factor of ∼5 varia-
tion in distance to the regions. Despite this, the NH3, continuum and methanol maser
observations have the same sensitivity limit towards all the regions: therefore, the rela-
tive flux densities of these tracers in a given region are directly comparable. In addition,
no correlation was found between a region’s distance and the number of cores toward
the region or their association with the different tracers. From this we conclude the dis-
tance variation does not affect the distribution of cores into separate groups. However,
it should be remembered that any conclusions drawn about the cores are limited by the
observational parameters used to define the groups.
4. Deriving physical properties
Properties of the molecular gas in each of the cores were derived from the NH3 obser-
vations. The core size was calculated from the extent of the integrated NH3(1,1) emission
http://www.iram.fr/IRAMFR/GILDAS/
The molecular environment associated with Class II methanol masers 121
after deconvolving the synthesised beam response. The dynamical state of the molecu-
lar gas was derived from the line profiles of the high spectral resolution (0.197kms−1)
NH3(1,1) observations after deconvolving the instrumental response. Preliminary gas
kinetic temperatures were calculated by fitting the measured column densities of the
multiple NH3 transitions to the LVG models described in Ott et al. (2005). Finally,
properties of the ionised gas were derived from the 24GHz continuum emission follow-
ing Mezger & Henderson (1967), assuming it was spherically symmetric and optically
thin at an electron temperature of 104K.
5. Results
In general the core properties are comparable to those derived from similar surveys
towards young MSF regions. Below we outline differences, in particular between the cores
in the different groups described in §3.
5.1. Molecular Gas
The measured NH3(1,1) linewidth varies significantly between the groups, increasing from
1.43, 2.43, 3.00 kms−1 for Groups 1 to 3 respectively. This shows the NH3-only cores are
more quiescent than those with methanol maser emission.
The NH3(1,1) spectra of some cores deviate significantly from the predicted sym-
metric inner and outer satellite brightness temperature ratios. These line profile asym-
metries are often seen toward star forming cores and are understood to be caused
by selective radiative trapping due to multiple NH3(1,1) sub-clumps within the beam
[see Stuzki & Winnewisser (1985) and references therein]. The NH3-only cores (Group
1) have by far the strongest asymmetries.
NH3(4,4) emission is detected toward the peak of 13 NH3(1,1) cores and 11 of these
also have coincident NH3(5,5) emission. The higher spatial resolution of the NH3(4,4)
and (5,5) images compared to the NH3(1,1) observations (8
′′ vs 11′′) provides a stronger
constraint to the criteria outlined in §3 as to whether this emission is associated with
either methanol or continuum emission. In every case, the NH3(4,4) and (5,5) emission
is unresolved, within a synthesised beam width of the methanol maser emission spatially
and within the FWHM in velocity. This shows the methanol masers form at the warmest
parts of the core. Significantly, no NH3(4,4) or (5,5) emission is detected toward NH3-only
sources.
As shown in Figure 1, cores with NH3 and methanol maser emission (Groups 2 and
3) are generally significantly warmer than those with only NH3 emission (Group 1).
However, there are also a small number of cores with methanol maser emission that
have very cool temperatures and quiescent gas, similar to the NH3-only cores. Modelling
shows pumping of 6.7GHz methanol masers requires local temperatures sufficient to
evaporate methanol from the dust grains (T&90K) and a highly luminous source of IR
photons [Cragg et al. (2005)] i.e. an internal powering source. It is therefore plausible
that the cold, quiescent sources with methanol maser emission are cores in which the
feedback from the powering sources have not had time to significantly alter the larger
scale properties of the gas in the cores.
5.2. Ionised Gas
Of the 14 continuum cores detected at 24GHz, 10 are within two synthesised beams of the
6.7GHz methanol maser emission. This is contrary to the results of Walsh et al. (1998),
who found the masers generally unassociated with 8GHz continuum emission. However,
six of the 24GHz continuum sources found at the site of the methanol maser emission
122 S. N. Longmore et al.
Figure 1. NH3(1,1) linewidth vs gas kinetic temperature. Cores with NH3 only (Group 1) are
shown as crosses while those with NH3 and methanol maser emission (Groups 2 and 3) are shown
as triangles. The dashed line shows the expected linewidth due to purely thermal motions.
have no 8GHz counterparts. A possible explanation for this may be that the continuum
emission is optically thick rather than optically thin between 8 and 24GHz and hence
has a flux density proportional to ν2 rather than ν−0.1. The seemingly low emission mea-
sures derived for the 24GHz continuum are unreliable due to the large beam size of the
observations. Alternatively, the 24GHz continuum sources may have been too extended
and resolved-out by the larger array configuration used at 8GHz by Walsh et al. (1998).
Further high spatial resolution observations at ν > 24GHz are required to derive reliable
emission measures and spectral indexes to unambiguously differentiate between the two
explanations.
6. Towards an evolutionary sequence
From the previous analysis, the core properties are seen to vary depending on their
association with methanol maser and continuum emission. Making the reasonable as-
sumption that cores heat up and becomes less quiescent with age, we now investigate
what these physical conditions tell us about their evolutionary state.
As the core sizes are similar, the measured linewidths can be reliably used to indi-
cate how quiescent the gas is, without worrying about its dependence on the core size
[Larson (1981)]. It then becomes obvious that the isolated NH3 cores (Group 1) con-
tain the most quiescent gas. However, from the linewidths alone it is not clear if these
cores will eventually form stars or if they are a transitory phenomenon. The fact that a
large number of these Group 1 cores contain many dense sub-clumps (as evidenced by the
NH3(1,1) asymmetries) suggests the former is likely for at least these cores. The linewidth
of sources with methanol maser emission (Groups 2 and 3) are significantly larger, and
hence contain less quiescent gas, than those of Group 1. The larger linewidths combined
with generally higher temperatures, suggests cores in Groups 2 and 3 are more evolved
than in Group 1.
The detection of continuum emission suggests a massive star is already ionising the
gas. With the current observations, the properties of the continuum sources are not well
enough constrained to further separate their evolutionary stages. However, as all (with
one possible exception) of the continuum sources only detected at 24GHz are associated
with dense molecular gas and masers, this would suggest they are younger than those
detected at both 8 + 24GHz, despite their seemingly small emission measures. In this
The molecular environment associated with Class II methanol masers 123
scenario, the cores with only 8 + 24GHz continuum and no NH3 emission, may be
sufficiently advanced for the UCHII region to have destroyed its natal molecular material.
From this evidence, the cores in the different groups do appear to be at different
evolutionary stages, going from most quiescent to most evolved according to the group
number.
7. Conclusions
What then, can we conclude about the role of methanol masers as a signpost of MSF?
The observations show that 6.7GHz methanol masers:
• are found at the warmest location within each core.
• generally highlight significantly warmer regions with less quiescent gas (i.e. more
evolved sources) than those with only NH3 emission.
• may also highlight regions in which the internal pumping source is sufficiently young
that it has not yet detectably altered the large scale core properties.
Methanol masers therefore trace regions at stages shortly after a suitable powering
source has formed right through to relatively evolved UCHII regions. While remaining
a good general tracer of young MSF regions, the presence of a methanol maser does not
single out any particular intermediate evolutionary stage.
Finally, these data confirm and strengthen the results of Hill et al. (this volume), that
the youngest MSF regions appear to be molecular cores with no detectable methanol
maser emission.
8. Acknowledgements
SNL is supported by a scholarship from the School of Physics at UNSW. We thank
Andrew Walsh for comments on the manuscript. We also thank the Australian Research
Council for funding support. The Australia Telescope is funded by the Commonwealth
of Australia for operation as a National Facility managed by CSIRO.
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	Introduction
	Observations
	Core separation
	Deriving physical properties
	Results
	Molecular Gas
	Ionised Gas
	Towards an evolutionary sequence
	Conclusions
	Acknowledgements
