Issue |
A&A
Volume 507, Number 2, November IV 2009
|
|
---|---|---|
Page(s) | 795 - 802 | |
Section | Galactic structure, stellar clusters, and populations | |
DOI | https://doi.org/10.1051/0004-6361/200912608 | |
Published online | 15 September 2009 |
A&A 507, 795-802 (2009)
The RMS survey
H2O masers towards a sample
of southern hemisphere massive YSO candidates and ultra compact HII
regions![[*]](/icons/foot_motif.png)
J. S. Urquhart1,2 - M. G. Hoare1 - S. L. Lumsden1 - R. D. Oudmaijer1 - T. J. T. Moore3 - P. R. Brook1 - J. C. Mottram1,4 - B. Davies1 - J. J. Stead1
1 - School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK
2 - Australia Telescope National Facility, CSIRO, Sydney, NSW 2052, Australia
3
- Astrophysics Research Institute, Liverpool John Moores University,
Twelve Quays House, Egerton Wharf, Birkenhead, CH41 1LD, UK
4 - School of Physics, University of Exeter, Exeter, EX7 4QL, UK
Received 1 June 2009 / Accepted 25 August 2009
Abstract
Context. The red MSX source (RMS) survey has identified a
large sample of candidate massive young stellar objects (MYSOs) and
ultra compact (UC) HII regions from a sample of 2000 MSX and 2MASS colour selected sources.
Aims. To search for H2O masers towards a large sample of young high mass stars and to investigate the statistical correlation of H2O masers with the earliest stages of massive star formation.
Methods. We have used the Mopra Radio telescope to make position-switched observations towards 500 UCHII regions and MYSOs candidates identified from the RMS survey and located between 190
< l < 30
.
These observations have a 4
sensitivity of
1 Jy and a velocity resolution of
0.4 km s-1.
Results. We have detected 161 H2O masers,
approximately 75% of which were previously unknown. Comparing the maser
velocities with the velocities of the RMS sources, determined from 13CO observations, we have identified 135 RMS-H2O maser associations, which corresponds to a detection rate of 27%.
Taking into account the differences in sensitivity and source selection
we find our detection rate is in general agreement with previously
reported surveys.
Conclusions. We find similar detection rates for UCHII regions
and MYSOs candidates, suggesting that the conditions needed for maser
activity are equally likely in these two stages of the star formation
process. Looking at the detection rate as a function of distance from
the Galactic centre we find it significantly enhanced within the solar
circle, peaking at 37%
between 6-7 kpc, which is consistent with previous surveys of UC
HII regions, possibly indicating the presence of a high proportion
of more luminous YSOs and HII regions.
Key words: stars: formation - stars: early-type - stars: pre-main sequence
1 Introduction
Massive young stellar objects (hereafter MYSOs) are an early phase in the life of OB stars when fusion has most likely started in the core, but when they have not yet begun to ionize their surroundings to form an HII region. The embedded mid-infrared point source usually possesses a strong ionized stellar wind (e.g., Bunn et al. 1995) and drives bipolar molecular outflows (e.g., Lada 1985; Bronfman et al. 2008, and reference therein). This brief (104-5 yr) phase is clearly crucial to our understanding of how these massive young stars form since during this time any ongoing accretion will be halted and the final mass of the star is set. In addition, the winds, outflows and eventual HII regions have an important feedback role in determining the fate of the rest of the molecular cloud.
The main difficulty in studying MYSOs is their relative rarity. Most
current work still relies on samples drawn from the IRAS Point Source
Catalogue, but whose selection criteria bias them away from complex
regions by requiring them to be away from HII regions identified in
single-dish radio surveys (e.g., Sridharan et al. 2002; Molinari et al. 1996).
Additionally, the known samples are too small to test many aspects of
massive star formation theories, and are probably unrepresentative of
the class as a whole. In an effort to address these issues we have
conducted a new galaxy-wide search for MYSOs starting from the MSX
mid-infrared survey of the Galactic Plane (Price et al. 2001).
This has significantly better spatial resolution than IRAS and so does
not have the same confusion problems. We have used the MSX Point Source
Catalogue (Egan et al. 2003) to colour-select a large sample of candidate MYSOs (Lumsden et al. 2002).
This initial sample was further refined using 2MASS data to eliminate
blue objects and from visually inspecting the MSX images to remove more
extended sources. Our colour-selection and subsequent filtering
produced a sample of 2000 MYSO candidates (Lumsden et al. 2002).
The red MSX source (RMS) survey is a multi-wavelength programme of
follow-up observations designed to identify genuine MYSOs and ultra
compact (UC) HII regions from the other kinds of embedded or dusty
objects such as planetary nebulae (PNe), evolved stars and nearby
low-mass YSOs (Urquhart et al. 2008b; Mottram et al. 2006; Hoare et al. 2005).
The main aim of the project is to produce a large sample of MYSOs and
UCHII regions, and to compile a database of complementary
multi-wavelength data with which to study their properties. We have used arcsecond resolution mid-infrared imaging from the Spitzer GLIMPSE survey (Benjamin et al. 2003) or our own ground-based imaging (e.g., Mottram et al. 2007)
to reveal multiple and/or extended sources within the MSX beam, as well
as MYSOs in close proximity to existing HII regions. We have obtained
arcsecond resolution radio continuum data with ATCA and the VLA (Urquhart et al. 2009,2007a) to identify UCHII regions and PNe, whilst observations of 13CO transitions (Urquhart et al. 2007b,2008a) deliver kinematic velocities. Finally we have obtained near-infrared spectroscopy (e.g., Clarke et al. 2006) which allows us to identify the more pathological evolved stars.
The kinematic velocities can be used in conjunction with a Galactic rotation model (e.g., Clemens 1985; Alvarez et al. 1990; Brand & Blitz 1993)
to derive kinematic distances and luminosities, which allow us to
distinguish between nearby low- and intermediate-mass YSOs and genuine
MYSOs. However, the velocity of sources located within the solar circle
- which accounts for 80%
of the sample - results in two possible kinematic distances equally
spaced on either side of the tangent position; these are referred to as
the near and far distances. This distance ambiguity needs to be solved
before luminosities can be calculated and our sample of MYSO candidates
can be turned into a sample of bona fide MYSOs. We have solved the
distance ambiguities towards a sample of MYSOs using the HI
self-absorption (see Busfield et al. 2006, for details and a description of the technique) but distances to the whole sample are not currently available.
By combining these observational data sets it is possible to identify
the different source populations and remove the contaminates. With the
classification effectively complete we are now moving into the
exploitation phase of the RMS survey. The first step is to examine the
global characteristics of this galaxy-wide sample of massive young
stars as a prelude to detailed studies of sub-samples. Part of this
involves determining the physical and chemical nature of the
environment as a way of gauging their evolutionary status. We have used
the Mopra telescope and the large bandwidth available with the
broadband (8 GHz)
MOPS spectrometer to survey a sub-sample of MYSO candidates and UCHII
regions at 12 mm. The primary focus is to study water maser
emission and hot ammonia, but also class II methanol maser
emission and other serendipitous molecular transitions found in the
range.
In this paper we present the results of our search for water masers made towards 500 MYSO
candidates and UCHII regions observable by Mopra. Water masers are the
most likely masing species to be found associated with the mid-infrared
bright MYSO phase. VLBI proper motion measurements show that they arise
in the main from the base of bipolar outflows and jets (Moscadelli et al. 2005; Goddi et al. 2005)
and as such trace an important part of the physics of massive star
formation. Although the spatial resolution is not sufficient to warrant
a detailed study of individual sources, these single-dish observations
allow us to look at the statistical occurrence of water masers towards
massive star forming regions, as well as paving the way for future
interferometric follow-up observations.
2 Observations and source selection
2.1 Source selection
To date our ongoing programme of classification has led to the
identification of approximately 500 MYSO candidates and a further
600 UC HII regions. In addition to these categories we have two
further classifications that are relevant to this study; these are the
HII/YSO and Young/old classifications. The first of these is reserved
for RMS sources where both an HII region and YSOs are found within the
MSX beam (i.e., <18
). We have identified
200
of these, which are interesting in that they may indicate sequential
star formation. The second of these classes contains sources that
appear
isolated in near-infrared images and which would normally indicate an
evolved star classification, however, they also display strong CO
towards
them and/or lie on dark filaments seen at 8
m.
They could be luminous evolved background stars seen through a
molecular cloud or be genuine YSOs forming in the cloud. These cannot
be unambiguously classified until infrared spectra have been obtained
and so remain within this class until a reliable classification can be
attributed.
Approximately 900 RMS sources classified as one of the four
types mentioned in the previous paragraph are observable by the Mopra
telescope (i.e., 190 < l < 30
).
However, due to a combination of limited observing time and density of
sources towards the Galactic centre we were only able to observe a
little over half this number. Since the overall aim of the RMS project
is to identify a large sample of massive YSOs, priority was given to
sources identified as either YSOs or HII/YSOs with
80% and
90% of these types observed respectively. A high priority was also given to the Young/old class of sources, with
80%
being observed, as a significant number of genuine YSOs may still be
included in this catagory. Many of the sources excluded from our
observations of these three classes of objects were on the grounds that
water masers have previously been reported in the literature, or
because they are located within a region of the Galactic Plane which is
already the focus of a blind 19-27 GHz survey currently underway
(HOPS; Walsh et al. 2008). HII regions
were considered a lower priority and were observed when time and RA
range became available; only approximately 30% of the available
HII regions have been observed.
Table 1: Summary of sources observed, positions and sensitivities.
2.2 Observations and data reduction
The observations were made using the Mopra 22 m Radiotelescope
in April and September 2008 towards 499 massive star forming
regions. Mopra is located near Coonabarabran, New South Wales, Australia. The telescope is situated at an elevation of 866 metres above sea level, and at a latitude of 31 degrees south.
The telescope is equipped with a 12 mm receiver with a frequency
range of 16 to 27.5 GHz. The UNSW Mopra spectrometer (MOPS)
is made up of four 2.2 GHz bands which overlap slightly to provide
a total of 8 GHz continuous bandwidth. Up to four zoom windows can
be placed within each 2.2 GHz band allowing up to 16 spectral
lines to be observed simultaneously. Each zoom window provides a
bandwidth of 137 MHz with 4096 channels, which at 12 mm
corresponds to a total velocity range of 2000 km s-1 with a velocity resolution of
0.4 km s-1 per channel.
The 8 GHz bandpass was centred at 23.5 GHz providing complete
coverage over the 19.5-27.5 GHz range with a corresponding
resolution of 2.7-1.8
respectively. Individual zoom windows were deployed to cover: the water maser at 22.235 GHz, the NH3 (1, 1) and (2, 2) inversion lines at 23.694 and 23.722 GHz, two more covering the NH3
(3, 3) and (4, 4) lines located at 23.870 and 24.139 GHz
respectively. Additional zoom windows were placed to cover the thermal
methanol lines around 24.9 GHz, and at 23.121 GHz to cover
the rare methanol maser transition (Cragg et al. 2004). The
remaining windows were used to search for the more common
19.97 GHz methanol maser line (Ellingsen et al. 2004) and HC5N
(7-6) transition at 18.64 GHz. In this paper we will concentrate
on the detection statistics of water masers and postpone the analysis
of the other transitions to a subsequent publication.
The observations were conducted in position-switched mode towards 499 RMS sources (see Table 1
for positions). Each source was observed for a total of ten minutes of
on-source integration, split into a number of separate scans consisting
of 1 min on- and 1 min off-source. Reference positions were
offset from the MSX positions by 1 degree in a direction
perpendicular to the Galactic plane. Given that the size of the beam at
12 mm (2
)
is much larger than the possible deviations from the global pointing
model no pointing checks were performed. System temperatures were
between 65-100 K depending on weather conditions and telescope
elevation, resulting in typical rms values of
25 mK per channel (see Fig. 1 for distribution).
Cross scans of Jupiter were made to determine the telescope main beam efficiency during our observations which was found to be
0.55 at 23 GHz. The corrected antenna temperatures (
)
were put on the main beam temperature scale by dividing by the telescope main beam efficiency (
). Finally, the main beam temperatures were converted to flux density using a conversion factor of 6.41 Jy K-1 (Eq. (7.19); Rohlfs & Wilson 2004). Taking the conversion from
to Jy into account the nominal 4
detection limit is
1 Jy per channel. Hereafter all quoted intensities will be given in Janskys.
![]() |
Figure 1: Histogram of the rms noise level in the different observations. The bin size is 5 mK channel-1. |
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![]() |
Figure 2: Spectra of detected H2O masers. Only a small portion of the plots are provided here, the full figure is only available in electronic form at the CDS. |
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The data were reduced using the ATNF spectral line reducing software (ASAP). The reduction steps consisted of dividing the individual on-off scans to remove sky emission (these scans were then inspected and poor scans were removed), fitting a low-order polynomial to the baseline. Finally, the scans were averaged together to produce a single spectrum for each source.
3 Results and analysis
3.1 Detection statistics and general properties
We detect maser emission towards 151 of the 499 star forming
regions observed. In ten of these cases two distinct groups of maser
emission were observed separated in velocity by >30 km s-1.
The emission structure and velocity separation makes it likely that the
emission seen in these cases is from two distinct sites of maser
emission along the same line of sight, within the 2 arcmin
beam. We have therefore classified these as separate detections and
indicate these in the plots presented in Fig. 2 and Table 2 by appending an ``_n'' to the MSX name (where n
= 1 or 2). In all of these cases at least one of the line of sight
components is found at a similar velocity as a molecular tracer and is
therefore likely to be associated with an RMS source (see
Sect. 4.1 for details). However, the origin of these other masers
detected along each line of sight is unclear since the
majority (8) have velocities outside the velocity range usually
associated with molecular gas and so were not covered by our
CO observations (see discussion in Sect. 4.3). Only two of
these masers are found within the velocity range of our
CO observations. One of which is found to have a similar velocity
as a weak CO component, whilst no CO emission is seen around
the velocity of the other, however, given the difference in resolution
this is perhaps not surprising (CO observations have a resolution
of 34
; Urquhart et al. 2007b).
Table 2: Parameters of detected masers.
Taking these cases into account, we have detected a total of 161 masers above a 4
noise level (
1 Jy), which
corresponds to an overall detection rate of
32%.
We conducted a literature search using SIMBAD and found that
approximately 75% of these masers were previously unknown. In
Fig. 2 we present
plots of all the detected masers. Where two masers have been detected
along the same line of sight we present a separate plot for each. To
avoid confusion the velocity range for a number of these multiple
detections has been reduced. As can be seen from the spectra presented
in this figure, the maser emission generally consists of either a
single component or a group of emission peaks spread over a clearly
identifiable velocity range. In order to include all emission
components associated with each maser the plots presented in Fig. 2 cover a velocity range of
100 km s-1 and are centred on the velocity of the strongest component.
In Fig. 3
we present histograms of the distribution of the peak flux density and
maser group velocity dispersion (defined as a number of discrete
components within 30 km s-1 of each other). The distribution of peak flux densities spans a range of
4 orders of magnitude from 0.5 Jy
up to a couple of thousand Jy, however, more typical values are between
1-10 Jy. The histogram of the distribution of maser velocity
dispersion, shown in the right panel of Fig. 3,
illustrates the large spread in velocity of maser emission. Maser
groups consisting of a single component have an intrinsic width of as
little as a few km s-1 with multiple component groups having velocity spreads of up to 90 km s-1. However, these are the extremes with the majority of water masers having velocity ranges of 10-15 km s-1.
This large velocity dispersion is rather typical for water masers and
is one of the reasons a connection with molecular outflows is often
inferred (e.g., Elitzur et al. 1989; Menten 1996).
In Table 2 we present the parameters of the detected H2O masers; we give the MSX name and J2000 co-ordinates in Columns 1-3. In Cols. 4 and 5 we give the peak flux density and the velocity of the peak emission. We give the minimum and maximum velocity range of the emission in Cols. 6 and 7.
4 Discussion
4.1 RMS-maser associations
We have detected and identified 161 water masers towards RMS sources. Not all will necessarily be associated, however, as some are likely to be chance alignments along the line of sight. Given the spatial resolution of these observations, we are unable to confirm associations but we can identify likely RMS-H2O associations by considering the velocities of the maser and the RMS sources (determined from CO observations, see Urquhart et al. 2007b,2008a for details) and look for a velocity correlation.
In an effort to identify likely RMS-H2O maser
associations we compared the peak maser velocity with the velocity of
the RMS source along each line of sight. The distribution shown in
Fig. 4
illustrates the excellent correlation between the velocity of the peak
intensity component in each maser group and the velocity of the
molecular cloud within which the RMS source resides. The distribution
is strongly peaked at 2 km s-1, and falls off steadily until reaching a background level at
14 km s-1.
The shape of the histogram corresponds to that of a one-sided Gaussian
and we have therefore determined the standard deviation from a Gaussian
fit to the profile.
![]() |
Figure 3: Histograms of the H2O maser parameters. In the left panel we present the histogram of the peak intensity distribution (the bin size is 0.5 dex) and in the right panel we present a histogram of the H2O maser groups dispersions (binned using a value of 5 km s-1). |
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We consider an RMS source and H2O maser to be associated if the difference in their velocities is less than 3,
where
is the standard deviation of the distribution
5 km s-1 (cf. a median difference of 4.5 km s-1; Kurtz & Hofner 2005). Applying this criterion (i.e.,
< 15 km s-1) we find 131 RMS-H2O
maser associations. No velocity has been assigned to five RMS sources
due to the presence of multiple CO components in the spectra,
which makes it difficult to determine a unique velocity (Urquhart et al. 2007b).
However, it is still possible that the masers detected towards these
sources are associated. To investigate these possible associations we
compared the velocities of the peak maser components to the velocities
of the detected CO components seen towards all five RMS sources.
We found the peak component velocity of the maser within
15 km s-1 of at least two of the CO peaks
present in four of the five cases, and therefore consider these to be
possible associations. Unfortunately, we were not able to use the
presence of the masers to identify the CO component associated
with the RMS source in these cases. Including the 131 RMS sources
already found to have an associated water maser this brings the total
to 135. The remaining 27 masers are likely to be associated
with other star forming regions that happen to be located along the
same line of sight as the RMS source.
![]() |
Figure 4: Histogram of the differences between the velocity of the detected H2O maser and the 13CO velocity of the RMS source within the beam. The bin size is 2 km s-1. |
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4.2 Comparison of RMS-H2O detection rate
Our detection rate for finding water masers associated with RMS sources is

All of these surveys where made towards IRAS sources that satisfied the Wood & Churchwell (1989) colour selection criteria for identification of UCHII regions (i.e.,
and
). However, most of the Churchwell et al. (1990) sources, and all of the Kurtz & Hofner (2005) sources, have IRAS 100
m > 1000 Jy and IRAS 60
m > 100 Jy and are therefore much more biased towards brighter sources than the Palla et al. (1991) and Codella et al. (1995) surveys. Additionally, the Churchwell et al. (1990) and Kurtz & Hofner (2005)
surveys are approximately 10 times more sensitive than the other two
surveys. If a similar sensitivity to that of Palla and Codella were
applied to the Churchwell et al. (1990) and Kurtz & Hofner (2005) surveys, their detection rates would be reduced
to 35% and 36%, respectively (see Kurtz & Hofner 2005,
for more details). Given the differences in sensitivity, source
selection of these surveys, and statistical noise we find our results
are in good agreement with all of the surveys mentioned.
While Churchwell et al. (1990) and Kurtz & Hofner (2005) concentrated on the UCHII stage (identified from their radio continuum emission), the Palla et al. (1991) and the Codella et al. (1995) samples are likely to contain a mixture of nearby low-mass YSOs and more distant HII regions and massive YSOs. Having a well selected sample of bona fide YSOs and UCHII regions associated with water masers we can now go a step further and look at how the association rates vary between the different evolutionary stages. In Table 3 we present a breakdown of the number of RMS-H2O maser associations as a function of source classification (the errors are calculated assuming a binomial distribution). We can immediately see that the detection rates of H2O masers for all UCHII regions and YSOs are very similar. This would suggest that the conditions necessary for maser activity are equally likely in these two stages of the star formation process. The lower detection rate seen towards the Young/old class of sources is probably a reflection of the number of evolved stars within this group as discussed in Sect. 2.1, however, given the small number of statistics this may not be all that significant.
Table 3: Summary of number of sources observed, maser detections and detections rates by source classification.
It is somewhat surprising to find the detection rate for UCHII regions and YSOs is effectively the same, given that one might expect the environments to be very different. Water masers are widely thought to be associated with molecular outflows and/or accretion, however, both of these phenomenon are expected to be disrupted and eventually halted altogether once the HII region begins to form. We compared the measured parameters (e.g., maser velocity dispersions, peak intensity) for the UCHII regions and MYSO candidates to try and identify any differences that might give an insight into their nature, however, we found no significant differences - Kolmogorov-Smirnov (KS) tests of these parameters are consistent with the masers being drawn from the same overall population.
The detection rate found towards YSOs and UCHII regions is double the rate reported by Wang et al. (2006) (12%)
from observations of 140 compact cores found within a sample of
infrared dark clouds (IRDCs). These observations were made with the VLA
and had a sensitivity of
0.1 Jy beam-1 channel-1,
which is a factor of a few times more sensitive than the observations
presented here. The rate of detection of water masers is therefore
significantly higher for star forming cores than for cores found in
IRDCs, which are presumably at an earlier stage in their evolution, and
many of which, are likely to be of relatively low luminosity.
4.3 Galactic distribution
Comparing the Galactic longitude and latitude distributions we find no
significant differences between the RMS sources associated with masers
and those without. However, we do find a difference in the proximity to
the Galactic centre between the two samples, with a much higher
proportion of those associated with H2O masers being located at smaller Galactocentric radii than those that are not associated. In Fig. 5
we present a histogram of the distribution of Galactocentric radii for
the whole sample (outlined by the solid line) and the RMS-H2O
maser associated sources (filled histogram). The distributions of the
two populations are similar with both showing the same features. The
two most interesting features are the two peaks located at 6-7 kpc
and 8-9 kpc. These distributions are very similar to the radial
distribution of an IRAS selected sample of UCHII regions reported by Bronfman et al. (2000) from CS observations.
The 6-7 kpc peak correlates with a peak in the radial
distribution of a sample of southern infrared dark clouds (IRDCs) reported by Jackson et al. (2008). This peak is at a larger radial distance from the Galactic centre than found for a sample of northern IRDCs which peaks at 5 kpc. The difference galactocentric distributions between the Galactic first and fouth quadrants led Jackson et al. (2008)
to conclude that these features are more likely to be associated with
the Scutum-Centaurus arm in a two arm Galactic model (e.g., Drimmel 2000; Drimmel & Spergel 2001) than part of a molecular ring of material located at
5 kpc (see Jackson et al. 2006
and references therein). The coincidence of the peak in the
distribution of our sample of UCHII regions and MYSO candidates, the
sample of IRAS selected UCHII regions (Bronfman et al. 2000) and IRDCs (Jackson et al. 2008)
with the Scutum-Centaurus arm is consistent with observations of
external spiral galaxies where the formation of OB stars is seen to be
exclusively associated with the spiral arms.
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Figure 5: Histograms of the distance from the Galactic centre of all YSOs and HII regions (outlined by solid line) and those associated with H2O masers (filled histogram). The bin size is 1 kpc. |
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The second peak located 8-9 kpc is most likely due to a high number of sources found in the solar neighbourhood (
0.5 kpc from the Sun). The distribution of the RMS sources associated with H2O
masers shares some similarities with the distribution of the general
RMS population, however, comparing it with the distribution of
unassociated RMS sources reveals them to be significantly different. A
KS test of these samples results in only a 6% probability that
they are drawn from the same population.
Looking in a little more detail we find that the detection rate varies
as a function of distance from the Galactic centre. This is illustrated
in Fig. 6 which shows the detection rate to be approximately constant (27%) between 4-6 kpc, after which it increases to
37% between 6-7 kpc, before declining steadily to
24% at 11 kpc. After 11 kpc the detection rate falls off to
15%,
however, the low number statistics for distances greater than this
result in the spike between 12 and 13 kpc. A long term study of
water masers towards YSOs by Brand et al. (2003) found the detection rate, and maser stability, was significantly higher for more luminous young stars (
)
than for lower luminosity YSOs. The increase in detection rates for our
sample of young massive stars located within the solar circle probably
reflects the presence of a higher proportion of more luminous sources.
Since the majority of our sources are located within the solar circle,
and are thus subject to the distance ambiguity problem, reliable
distances are not yet available for many sources. We are therefore not
currently in a position to investigate this inference in more detail.
![]() |
Figure 6: Plot of the detection rate of H2O masers as a function of distance from the Galactic centre of all YSOs and HII regions. The bin size is 1 kpc. |
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In Fig. 7 we present a plot of the distribution of 12CO as a function of Galactic longitude and
(Dame et al. 2001).
We have over-plotted the velocities of all detected masers; we
distinguish between masers associated with an RMS source and the
unassociated masers by red and blue crosses respectively. The image
presented in Fig. 7 illustrate the excellent correlation of RMS-H2O maser associations with the molecular material in the Galaxy as traced by the integrated 12CO
emission. There is also a reasonable level of correlation between the
maser velocities and the spiral arm velocities as modelled by Taylor & Cordes (1993) and Cordes (2004).
In total 27 H2O masers were not associated with an RMS source (i.e., the velocity of their peak component is outside the 15 km s-1 criteria required for an association), this corresponds to 17%
of all the detected masers. These unassociated masers are the result of
chance alignments along the line of sight. The high number of
unassociated masers is due to the low resolution of these observations
and the density of star forming regions located in the inner Galaxy.
These masers fall into two catagories; those with velocities within the
velocity range covered by the molecular line observations (see Urquhart et al. 2007b,2008a,
for details), and those at velocities outside the range normally
associated with molecular gas in the Galaxy. We find weak
CO emission at similar velocities for a few of the masers that
have velocities within the Galactic range, however, for the majority no
such emission has been detected. This is probably due to the difference
in resolution of the CO observations and the maser observations
presented in this paper (which are
34
and
2
respectively), with the star forming regions associated with the maser lying outside the CO beam.
The second category include masers at velocities outside those
typically associated with the Galactic molecular gas. These can be
clearly seen in Fig. 7 to have significantly different velocities (e.g., l=10.4
and
= 72.7 km s-1 and l=16.8
and
= 70.4 km s-1). Since the conditions required to excite water maser emission are high densities (
)
and temperatures up to 400 K the lack of any large scale molecular
gas is puzzling. One possibility is that these masers are associated
with evolved stars, however, the nature of these masers remains
uncertain and further observations will be required to ascertain their
origin.
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Figure 7: Galactic longitude-velocity plot showing the velocities of all H2O masers detected as a function of Galactic longitude. Masers associated with an RMS source are shown in red, while unassociated H2O masers are shown in blue. The distribution of molecular material is shown in grey scale (Dame et al. 2001) for comparison. The location of the spiral arms taken from the model by Taylor & Cordes (1993) and updated by Cordes (2004) are over-plotted in yellow. |
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5 Summary and conclusions
We have presented the result of a set of water maser observations
towards a sample of 499 massive star forming regions. We detected a
total of 161 masers above a 4
sensitivity limit of
1 Jy,
which makes this one of the most sensitive water masers surveys so far
conducted. The majority of masers detected were previously unknown (
75%). We present plots for all detected H2O masers and tabulate their properties.
By comparing the peak intensity velocities of the detected H2O masers with the velocities of the RMS sources we have identified 135 RMS-H2O maser associations. The overall detection rate towards our sample of young massive star forming regions is 27%.
This is in agreement with other water maser surveys taking into account
their various selection criteria and sensitivities. We find similar
detection rates for MYSO candidates and UCHII regions suggesting that
maser activity is equally possible during these two stages of the star
formation process. The detection rate as a function of distance from
the Galactic centre is significantly enhanced within the solar circle,
peaking at
37%
6 kpc,
possibly indicating the presence of a high proportion of more luminous
YSOs and HII regions. Comparing our results with those reported by
previous surveys towards massive star forming regions we find them to
be consistent.
These observations are a first step in our programme of follow-up observations designed to examine the global characteristics of this galaxy-wide sample of massive young stars. Further observations are planned to follow up the H2O masers detected from the observations presented in this paper the results of which will be the focus of a subsequent paper.
The authors would like to thank the Director and staff of the Paul Wild Observatory for their assistance during the preparation of our observations. We would like to thank the referee for some very useful comments and suggestions. J.S.U. is supported by a STFC PDRA and CSIRO OCS fellowship. This research would not have been possible without the SIMBAD astronomical database service operated at CDS, Strasbourg, France and the NASA Astrophysics Data System Bibliographic Services.
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Footnotes
- ... regions
- Full Tables 1, 2 and full Fig. 2 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/507/795
- ... properties
- http://www.ast.leeds.ac.uk/RMS
- ... Australia
- Mopra is operated by the Australia Telescope National Facility, CSIRO.
All Tables
Table 1: Summary of sources observed, positions and sensitivities.
Table 2: Parameters of detected masers.
Table 3: Summary of number of sources observed, maser detections and detections rates by source classification.
All Figures
![]() |
Figure 1: Histogram of the rms noise level in the different observations. The bin size is 5 mK channel-1. |
Open with DEXTER | |
In the text |
![]() |
Figure 2: Spectra of detected H2O masers. Only a small portion of the plots are provided here, the full figure is only available in electronic form at the CDS. |
Open with DEXTER | |
In the text |
![]() |
Figure 3: Histograms of the H2O maser parameters. In the left panel we present the histogram of the peak intensity distribution (the bin size is 0.5 dex) and in the right panel we present a histogram of the H2O maser groups dispersions (binned using a value of 5 km s-1). |
Open with DEXTER | |
In the text |
![]() |
Figure 4: Histogram of the differences between the velocity of the detected H2O maser and the 13CO velocity of the RMS source within the beam. The bin size is 2 km s-1. |
Open with DEXTER | |
In the text |
![]() |
Figure 5: Histograms of the distance from the Galactic centre of all YSOs and HII regions (outlined by solid line) and those associated with H2O masers (filled histogram). The bin size is 1 kpc. |
Open with DEXTER | |
In the text |
![]() |
Figure 6: Plot of the detection rate of H2O masers as a function of distance from the Galactic centre of all YSOs and HII regions. The bin size is 1 kpc. |
Open with DEXTER | |
In the text |
![]() |
Figure 7: Galactic longitude-velocity plot showing the velocities of all H2O masers detected as a function of Galactic longitude. Masers associated with an RMS source are shown in red, while unassociated H2O masers are shown in blue. The distribution of molecular material is shown in grey scale (Dame et al. 2001) for comparison. The location of the spiral arms taken from the model by Taylor & Cordes (1993) and updated by Cordes (2004) are over-plotted in yellow. |
Open with DEXTER | |
In the text |
Copyright ESO 2009
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