Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.mrao.cam.ac.uk/yerac/codella/codella.ps Äàòà èçìåíåíèÿ: Wed Feb 22 19:36:48 1995 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 02:37:33 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: massive stars

A water maser search towards
colour­selected IRAS point sources
By C l a ud i o C o d e l l ay AND F r a n c e s c o Pa l l az
e­mail: codella@arcetri.astro.it
y Dipartimento di Astronomia e Scienza dello Spazio, Universit`a degli Studi di Firenze, Largo
E. Fermi 5, I--50125, Firenze, ITALY
z Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I--50125, Firenze, ITALY
The nature of 160 IRAS sources has been investigated through a search for water maser emission
at 22.2 GHz. These IRAS sources have been selected using the Wood & Churchwell (1989) colour
criteria to identify high­mass star forming regions. Out of the whole sample 11 water masers
have been detected, 2 for the first time. Therefore, our detection rate is very low: 7%. There
is a strong dependence of the detection rate on the intensity at IRAS 60­micron flux density: it
decreases from 24% for sources brighter than 100 Jy to 1% for weaker sources. If one considers
only the bright sources, the detection rate does not depend on the IRAS colours. However, there
is a net distinction for the weak sources: the detection rate is 11% for sources with [60 \Gamma 25]
micron colour greater than 0.61 and 1% for the rest. These results, combined with those found
in previous surveys, indicate that it is very unlikely that the population of weak IRAS sources
with shallow far­infrared continuum spectra is associated with high­mass star forming regions
As a consequence, the population of O­type stars estimated from the number of IRAS sources
located inside the Wood & Churchwell (1989) colour box may be overestimated.
1. Introduction
It is well established that 22.2 GHz water masers are associated with star forming
regions (SFRs). H 2 O masers are considered good tracers of optically obscured molecular
cores in which a star forming process is occurring. Particularly, H 2 O maser emission has
been detected where other phenomena typical of high­mass star formation are observed:
ultracompact (UC) Hii regions, molecular outflows, far­infrared (FIR) sources. This
indicates that water masers are primarily associated with high­mass star forming regions.
Wood & Churchwell (1989) (hereafter WC) have proposed criteria to identify UC
Hii regions based on FIR flux density distribution (and hence the colours) of sources
extracted from the IRAS Point Source Catalogue (IRAS (1985)). More recently, several
works have pointed out that the population of IRAS PS selected through the WC criteria
may contain not only massive O­type stars, but also lower mass protostars (White et al.
(1991), Hughes & MacLeod (1994), Kurtz et al. (1994)). The possibility of answering
this question comes from the investigation of the occurrence of water masers in the WC
sample. Palla et al. (1991) and Palla et al. (1993) (hereafter P1 and P2 respectively)
have performed a survey of H 2 O masers towards a sample of IRAS PS which satisfy
both the WC and the Richards et al. (1987) criteria, which select compact molecular
clouds with high densities (nH2 – 10 5 cm \Gamma3 ). P1 and P2 found that the IRAS PS with
60 micron flux density greater than 100 Jy are UC Hii region candidates, while fainter
sources may be connected with lower mass B­type stars. The aim of this work is to
verify if the maser properties of the IRAS PS located inside the WC colour box have any
dependence on the flux density distribution. In order to do this, we have performed a
survey of maser emission towards the IRAS PS located inside the WC box, but which
do not satisfy the Richards criteria. The survey has been carried out with the 32­m
1

2 C. Codella & F. Palla: A maser search towards IRAS point sources
Figure 1. [25 \Gamma 12] \Gamma [60 \Gamma 12] colour distribution of the IRAS PS observable at Medicina which
satisfy the WC criteria (continuous line). The crosses represent the sources with F60 ? 100 Jy,
while the open squares are for sources with F60 Ÿ 100 Jy. The dashed line bounds the region
given by the Richards criterion on [60\Gamma25] colour, while the dot­dashed line stands for F25 = F60 .
Medicina (Bologna, Italy) radiotelescope, used also by P1 and P2. Therefore, our results
are directly comparable with the conclusions of Palla and his collaborators.
2. The sample
In order to choose the sources for our sample, as a first step all the IRAS PS (1846
sources) which satisfy the WC colour criteria were selected. These criteria are: (i)
[25 \Gamma 12] – 0:57 (using the notation that [i \Gamma j] j log[F i =F j ]) and (ii) [60 \Gamma 12] – 1:30.
The IRAS PS without a detection at 25 and/or 60 micron bands have been rejected.
On the other hand, the IRAS PS with an upper limit at 12 micron have been accepted,
because they satisfy the WC limits automatically. The aim of our observations is to
investigate the nature of the IRAS PS located inside the WC box, but outside the region
delimited by the Richards criteria. These involve IRAS colours different from those used
by WC: (i) 0:61 Ÿ [60 \Gamma 25] Ÿ 1:74 and (ii) 0:087 Ÿ [100 \Gamma 60] Ÿ 0:52. We have projected
the Richards criterion on [60\Gamma25] colour into the [25\Gamma12]\Gamma[60\Gamma12] plane creating a region
(hereafter called the ``Richards strip'') that overlaps in a diagonal fashion the WC box. A
sample of 170 IRAS PS located inside the WC box, but outside the Richards strip, have
been selected. For this selection we have also considered only the sky region observable
with the Medicina radiotelescope with ffi – \Gamma30 ffi ). (No bias has been introduced by
considering only these sources.) Finally, 10 IRAS PS with F 25 ? F 60 have been rejected,
according to the classical IRAS spectrum of a SFR, rising towards longer wavelengths.
Thus, our final sample contains 160 IRAS PS (hereafter the ``NR'' sample). Figure 1
shows the distribution in the [25 \Gamma 12] \Gamma [60 \Gamma 12] colour plot of the IRAS PS (observable
at Medicina) which satisfy the WC criteria (continuous line). The dashed lines bound
the Richards strip, while the dot­dashed line stands for the points for which F 25 = F 60 ;
below this line are the rejected IRAS PS. In Figure 1 the IRAS PS are plotted with a
cross if F 60 ? 100 Jy (Sample A) or with an open square otherwise (Sample B). The
limit of 100 Jy at 60 micron have been chosen because:
(a) as pointed out in the Introduction, P1 and P2 have concluded that, among the
IRAS PS satisfying the Richards and the WC criteria, those with F 60 ? 100 Jy are

C. Codella & F. Palla: A maser search towards IRAS point sources 3
associated with UC Hii regions, while the faint sources may be connected with lower
mass (B­type) embedded stars;
(b) Codella et al. (1994) have shown that the reliability of the association between
IRAS PS and Hii regions is high (? 80%) only for the sources with F 60 ? 100 Jy.
The samples A and B have a different distribution in the colour plot of Figure 1. The
bulk (459 out 501 sources; 92%) of the objects from the sample A are inside the Richards
strip and the sources which extend out of this region are located very near its boundary.
On the other hand, sample B extends out of the strip delimited by the Richards criteria:
only the 75% (363/481) is inside this region, a smaller fraction than for sample A. This
is an indication that the A and B samples contain IRAS PS associated with objects of
different nature.
Figure 1 also shows the colour plot distribution of the NR sample. We have:
(a) 42 (26%) IRAS PS with F 60 ? 100 Jy (hereafter the ``NRA'' sample);
(b) 118 (74%) IRAS PS with F 60 Ÿ 100 Jy (hereafter the ``NRB'' sample).
Thus, the colour region investigated with our observations is primarily populated by
faint sources from sample B.
3. Observations
The observations were made with the 32­m radiotelescope at Medicina during several
runs between 1990 and 1994. The half­power beam width at the frequency of the water
maser line (6 16 ! 5 23 ; 22235.07985 MHz) is 1 0 :9. The antenna efficiency of the radi­
otelescope was 38% with a maximum gain of 0:11 K Jy \Gamma1 . The system temperature in
good weather conditions was 120 K. The calibration of the obtained spectra was made
using the continuum source DR21; the uncertainty of calibration is 20%. The spectra are
corrected for telescope gain changes with elevation. The pointing accuracy is 20 00 (rms).
The observations were made in beam switching mode; for each source the integration
time was 5 minutes (on and off source). The backend was an autocorrelation spectro­
meter with 1024 channels. A 25­MHz bandwidth was used, corresponding to a spectral
resolution of 0:330 km s \Gamma1 and a total velocity coverage of \Sigma160 km s \Gamma1 . The average
detection level (3oe) is 6 Jy.
4. Results
Out of the 160 IRAS PS of the NR sample, only 11 show maser emission; 2 are new
detections (IRAS 06190 + 1040 and IRAS 18361 \Gamma 0627). Therefore, the detection rate
is low: 7%. Considering the detection rate relative to the two samples NRA and NRB,
it can be noted that there is a strong dependence on the F 60 value. The detection rate
for the NRA sample is 24% (10 detections out 42 sources). On the other hand, out of
the 118 sources of the NRB sample, only one has been detected (1%).
Considering also the maser sources not detected with the Medicina radiotelescope due
to either variability or insufficient sensitivity, but known from the literature, we add 11
other masers (7 of the NRA sample and 4 of the NRB sample). Therefore, the total
number of known maser sources in the NR sample is 22. Table 1 compares the detection
rates of this work with those given by P1 and P2. Since all these results have been
obtained with the same detection limit, they are directly comparable. The comparison
shown in Table 1 indicates that if one considers only the bright sources (F 60 ? 100 Jy),
then the detection rate does not depend on the IRAS colours (24% vs. 26%). This
means that the NRA sources are of the same nature of the bright ones located in the
Richards strip: UC Hii regions, as confirmed by the high detection rates. Therefore,

4 C. Codella & F. Palla: A maser search towards IRAS point sources
Sub­sample this work P1,P2
F60 ? 100 Jy 24% 26%
F60 Ÿ 100 Jy 1% 11%
Table 1. Results of water maser surveys
­10 ­5 0 5 10
0
5
10
15
20
Figure 2. Galactic latitude distribution of the observed samples.
it can be concluded that all the bright IRAS PS of the sample A are UC Hii regions
candidates. As a consequence, the lower limit of the Richards criteria on [60 \Gamma 25] colour
(– 0:61) does not select all high­mass protostars. In order to select these objects from
the sample of IRAS PS located in the WC box, a less restrictive limit on [60 \Gamma 25] should
be used. Figure 1 shows also the distribution of the IRAS PS associated with known
maser sources. As already reported, among the 22 masers of the NR sample, the majority
(17 sources; 77%) belong to the NRA sample. It can be noted that maser sources do
not populate in a homogeneous fashion the investigated colour region: they are located
near the boundary of the Richards strip. Even if this is primarily due to the fact that
the majority have high 60­micron flux densities and therefore reflect the distribution in
the colour plot of the NRA sample, it is also worth noting that the 5 faint IRAS PS in
Figure 1 are located near the Richards strip. The limit [60 \Gamma 25] = 0:37 can be used
for the selection of high­mass protostellar candidates, since it bounds almost all (except
one) of the maser sources.
The nature of the IRAS PS of the NRB sample remains an open question. Table 1
shows that there is a large difference between the two detection rates of the two faint
samples (F 60 Ÿ 100 Jy). This indicates that the lack of detections of the NRB sample
primarily reflects the different nature of this sample and not a sensitivity effect. Since

C. Codella & F. Palla: A maser search towards IRAS point sources 5
it is well known that maser emission is primarily associated with high­mass SFRs, the
lack of masers indicates that the NRB sample does not contain objects of this kind.
This is confirmed by the comparison between the galactic latitude distributions of the
observed samples (NRA and NRB) shown in Figure 1. The distribution of the NRB
sample is really larger than that of NRA sample, suggesting that the faint sources are
relatively nearby and not connected with high­mass stars. Therefore, it is confirmed that
the population of O­type stars estimated from the number of IRAS PS located inside the
WC box may be overestimated. In order to obtain an estimate of the fraction of IRAS
PS which, even if located inside the WC box, are not connected with an O­type star,
we have to extrapolate our result to the whole sample of 1846 IRAS PS (without the
instrumental restriction on declination). The estimated fraction is 11% (195 out 1846
sources). This value is a lower limit because it takes into account only the faint sources
outside the Richards strip. In fact, as already reported in Sect. 2, P1 and P2 have
concluded that also the faint IRAS PS of their sample (inside the Richards strip) may
be not contain only O­type stars.
5. Conclusions
We have studied the frequency of occurrence of 22.2 GHz H 2 O maser emission in a
sample of 160 IRAS sources selected using the WC colour criteria to identify high­mass
star forming regions. The aim was to verify if the maser properties of the IRAS PS located
inside the WC region have any dependence on the IRAS spectrum and, consequently, to
investigate the nature of the objects of the observed sample.
Out of the 160 IRAS PS, only 11 have been detected, 2 for the first time. Therefore,
the overall detection rate is low: 7%. There is a strong dependence of the detection rate
on the 60­micron flux. For the sources brighter than 100 Jy the detection rate is 24%,
while for the weaker sources it decreases to 1%. Considering our results and those given
by previous comparable surveys, it can be noted that the detection rate for the bright
IRAS PS does not depend on the IRAS colours. This indicates that all the bright IRAS
PS located inside the WC box are UC Hii regions candidates.
There is a net distinction for the weak sources (F 60 Ÿ 100 Jy): the detection rate
is 11% for sources with [60 \Gamma 25] greater than 0.61 and 1% for the rest. The lack of
masers indicates that the NRB sample does not contain high­mass star forming regions,
as confirmed by its galactic latitude distribution, much broader than that of UC Hii
region candidates. As a consequence, the population of O­type stars estimated from
the number of IRAS PS located inside the WC colour box may be overestimated. The
expected fraction of the WC sample not connected with O­type stars is at least 11%.
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