Issue |
A&A
Volume 510, February 2010
|
|
---|---|---|
Article Number | A96 | |
Number of page(s) | 7 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/200912722 | |
Published online | 17 February 2010 |
A multiwavelength study of the star forming region IRAS 18544+0112
M. E. Ortega1 - S. Paron1 - S. Cichowolski1 - M. Rubio2 - G. Castelletti1 - G. Dubner1
1 - Instituto de Astronomía y Física del Espacio (IAFE),
CC 67, Suc. 28, 1428 Buenos Aires, Argentina
2 - Departamento de Astronomía, Universidad de Chile, Casilla 36-D,
Santiago, Chile
Received 18 June 2009 / Accepted 6 November 2009
Abstract
Aims. This work aims at investigating the molecular and
infrared components in the massive young stellar object (MYSO)
candidate IRAS 18544+0112. The purpose is to determine the nature
and the origin of this infrared source.
Methods. To analyze the molecular gas towards IRAS 18544+0112, we have carried out observations in a 90
90
region around
69,
65, using the Atacama Submillimeter Telescope Experiment (ASTE) in the 12CO J = 3-2, 13CO J = 3-2, HCO+J = 4-3 and CS J = 7-6 lines with an angular resolution of 22
.
The infrared emission in the area has been analyzed using 2MASS and Spitzer public data.
Results. From the molecular analysis, we find self-absorbed 12CO J = 3-2
profiles, which are typical in star forming regions, but we do not find
any evidence of outflow activity. Moreover, we do not detect either HCO+J = 4-3 or CS J = 7-6 in the region, which are species normally enhanced in molecular outflows and high density envelopes. The 12CO J = 3-2
emission profile suggests the presence of expanding gas in the region.
The Spitzer images reveal that the infrared source has a conspicuous
extended emission bright at 8 m with an evident shell-like morphology of
1
5 in size (
1.4 pc at the proposed distance of 3 kpc) that encircles the 24
m
emission. The non-detection of ionized gas related to
IRAS 18544+0112 together with the fact that it is still
embedded in a molecular clump suggest that IRAS 18544+0112
has not reached the UCHII region stage yet. Based on near infrared
photometry we search for YSO candidates in the region and propose that
2MASS 18565878+0116233 is the infrared point source associated with
IRAS 18544+0112. Finally, we suggest that the expansion of a
larger nearby HII region, G034.8-0.7, might be related to the formation
of IRAS 18544+0112.
Key words: ISM: molecules - HII regions - stars: formation
1 Introduction
Star formation processes start when a pressure-bounded, self-gravitating molecular clump becomes gravitationally unstable. As summarized by Whitworth et al. (1994b,a), the action of an expanding nebula can produce gravitationally unstable shocked layers of interstellar gas. For example, the expansion of an HII region can sweep up the surrounding molecular gas into a dense shell, which then fragments and forms new massive stars (``collect and collapse'' model; see Elmegreen & Lada 1977). Several recent works support this model (e.g. Comerón et al. 2005; Zavagno et al. 2006; Pomarès et al. 2009; Deharveng et al. 2005). On the other hand, shockwaves from expanding wind- and/or supernova-driven superbubbles can also trigger cloud collapse and star formation, but at larger scales. Numerical studies (Melioli et al. 2006; Vanhala & Cameron 1998) demonstrated that the effect caused by a passing shockwave mainly depends on the shock-type: close to the supernova remnant (SNR) the shockwave disrupts the ambient molecular clouds and thus terminates the star formation process; however, a little further away from the SNR the shock velocity decreases, and cloud collapse is possible if the right circumstances were given.
This work is part of a systematic study towards IR sources embedded in
molecular condensations with evidence of being affected by SNRs shock
fronts or expanding HII regions. In a previous work, Paron et al. (2009)
studied the infrared (IR) source IRAS 18542+0114 located near the
border of the SNR W44. They discovered that this source is probably a
massive young stellar object (MYSO) located at the border of the
HII region G034.8-0.7 which is evolving within a molecular cloud.
In
this work, we present a study of the neighboring source
IRAS 18544+0112, another IR source embedded in the same molecular
cloud. It is important to remark that the molecular cloud, the HII
region G034.8-0.7 and the SNR W44 are not only located in the same
region in the plane of the sky but also at the same distance from the
Sun, at about 3 kpc (corresponding to the kinematic velocity of
km s-1, Paron et al. 2009, and references therein).
The source investigated in this Paper, IRAS 18544+0112, was cataloged as a MYSO and a high mass protostellar object (HMPO) by Molinari et al. (1996) and Kumar & Grave (2007), respectively. On the basis of the analysis of new molecular data and near- and mid-IR data, we present a multiwavelength study of the IR source aiming to discern its origin and evolutionary stage.
![]() |
Figure 1:
Two-color image of the HII region G034.8-0.7 containing
IRAS 18542+0114 and IRAS 18544+0112. Green is the Spitzer-IRAC 8
|
Open with DEXTER |
2 IRAS 18544+0112 and its environment
Figure 1 shows a two-color IR image of the HII region
G034.8-0.7. We show in green the Spitzer-IRAC 8 m
emission and in red the Spitzer-MIPSGAL 24
m emission.
Yellow corresponds to regions where both emissions overlap. The white
contours represent the 13CO J = 1-0 line emission distribution as
extracted from the Galactic Ring Survey (GRS;
Jackson et al. 2006) averaged over the velocity interval from 36 to 45 km s-1.
The 8 m emission, which arises mainly from the polycyclic
aromatic hydrocarbons molecules (PAHs; Leger & Puget 1984) and an
underlying continuum attributed to very small grains, is observed
enclosing the 24
m emission that is originated in the heated dust
of the HII region G034.8-0.7. These molecules cannot survive inside
HII regions and are located over the photodissociation regions (PDRs)
that encircle the ionized gas (Cesarsky et al. 1996). The PDRs are the
interphase zone between the ionized and molecular gas, and its
presence evidences the interaction between them. A well defined 8
m arc-like structure is observed bordering the HII region
G034.8-0.7 towards lower Galactic latitudes. It is important to
note that the ionized region is partially bordered by two molecular
clouds which are part of the giant molecular complex (GMC)
G34.8-0.6. The morphology of the molecular clouds, as seen in the 13CO
J = 1-0 line, suggests that they are shaped by the action of the HII region.
On the other hand, IRAS 18544+0112, located at (l, b) = (34
69,
-0
65), appears as a bright yellow knot. From Fig. 1
it can be noticed that this IR source is located in a bulge shaped
molecular gas condensation which is observed in the velocity interval from 36
to 45 km s-1.
Figure 2 shows a two-color image of IRAS 18544+0112 , where green and
red are the 8 m and 24
m emission, respectively. The
observed 8
m emission distribution shows a shell-like structure
of about 1
5 in size (
1.4 pc at a distance of 3 kpc),
bright filaments and diffuse emission. The emission at 24
m is
observed mainly inside the shell-like structure.
![]() |
Figure 2:
The green is the Spitzer-IRAC 8 |
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Since most HMPOs present molecular outflows, we now analyze whether this phenomenon is taking place in the region.
![]() |
Figure 3:
Spitzer-IRAC three-color image (3.5 |
Open with DEXTER |
The IRAC 4.5 m band contains lines that may be excited by high
velocity shocks, such as those expected when protostellar outflows
crash into the ambient ISM (Cyganowski et al. 2008) or when a SNR interacts with
a molecular cloud. We thus inspected the IRAC 4.5
m emission
distribution in the area.
Figure 3 shows a Spitzer-IRAC three-color
image of a square region of 22 in size. The three IR bands presented
are: 3.6
m (in blue), 4.5
m (in green) and 8
m (in
red). The white contours represent the radio continuum emission at
1.4 GHz as extracted from the VGPS (VLA Galactic Plane Survey;
Stil et al. 2006), which depict the SE border of the
SNR W44 and the border of the radio continuum emission associated with
the HII region G034.8-0.7 encompassing the PDR.
An inspection of Fig. 3 shows that the source
IRAS 18542+0114 studied by Paron et al. (2009), appears slightly extended
in the 4.5 m emission (green). Such a characteristic is suggestive
of a YSO origin for the emission, a condition that was confirmed by
Paron et al. (2009) after discovering associated molecular
outflows. Figure 3 also shows some extended
diffuse filaments in this IRAC band, which are probably illuminating
molecular gas shocked by the SNR. In the case of the source studied
in this work, IRAS 18544+0112, it does not present significant
emission in the 4.5
m band. Indeed, this source is clearly
brighter at 8
m than at 4.5
m, suggesting that there is no
outflow activity in the region.
With the purpose to analyze the small scale distribution and
dynamic of the molecular gas associated with IRAS 18544+0112 , we carried out
observations of the 12CO, 13CO J = 3-2, HCO+J = 4-3, and CS J = 7-6 lines towards
a region of 90
90
around this IR source using the Atacama
Submillimeter Telescope Experiment (ASTE; Ezawa et al. 2004).
3 New molecular observations
The molecular observations were performed on June 25, 2008 with
the 10 m ASTE Telescope. We used the CATS345 GHz band
receiver, which is a
two-single band SIS receiver remotely tunable in the LO frequency
range of 324-372 GHz. We simultaneously observed 12CO J = 3-2 at 345.796
GHz and HCO+ J = 4-3 at 356.734 GHz, mapping a region of 90
90
centered at the position of IRAS 18544+0112, (l, b) = (34
69, -0
65). The mapping grid spacing was 10
and the
integration time was 72 s. per pointing. Additionally, we observed
13CO J = 3-2 at 330.588 GHz and CS J = 7-6 at 342.883 GHz towards the
center of the region. All the observations were performed in
position-switching mode. The off-position was (l, b) = (34
87,
-0
14), which was checked to be free of emission.
We used the XF digital spectrometer with a bandwidth and spectral
resolution set to 128 MHz and 125 kHz, respectively. The velocity
resolution was 0.11 km s-1 and the half-power beamwidth (HPBW) was
22
at 345 GHz. The system temperature varied from
to 700 K. The typical rms noise (in units of
)
ranged
between 0.1 and 0.4 K, and the main beam efficiency was
.
The spectra were Hanning-smoothed to improve the signal-to-noise ratio, and only linear or/and some third order polinomia were used for baseline fitting. The spectra were processed using the XSpec software package developed at the Onsala Space Observatory.
4 Molecular analysis
As it was shown in Fig. 1, IRAS 18544+0112 appears embedded in a molecular clump, which is part of the GMC G34.8-0.6.
Figure 4 (left) shows the 12CO J = 3-2 spectra obtained
from a region of 90
90
centered at the position of IRAS 18544+0112
and Fig. 4 (right) presents a spectrum obtained
towards the center of the region. Figure 5 displays an
average spectrum of 13CO J = 3-2 obtained towards the center of the clump
at (l, b) = (34
69, -0
65). All the 12CO profiles in this
region present a dip at
km s-1. Such a narrow dip points to a
self-absorption origin instead of two 12CO emission components with very
close kinematical velocities. We notice that the
km s-1 dip
is very close to the peak velocity of the 13CO J = 3-2 line, which is an
optically thinner line. Such correspondence strongly suggests that the
dip in the 12CO profiles is in fact caused by self-absorption by less
excited gas (see for example Zhou et al. 1993).
Since we are searching for indicators of active star formation in this IR source, the presence of this dip is interesting because it is known that the 12CO J = 3-2 line has almost always been observed self-absorbed in star-forming regions (Johnstone et al. 2003).
![]() |
Figure 4:
Left: 12CO J = 3-2 spectra obtained towards IRAS
18544+0112. The velocity range of each spectrum is between 20 and 70 km s-1. The color scale corresponds to the emission at 8 |
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![]() |
Figure 5: 13CO J = 3-2 profile obtained towards the center of IRAS 18544+0112. |
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Additionally, we note that the 12CO J = 3-2 emission does not show any
evidence of outflow activity in the region, which agrees with the
faint 4.5 m emission mentioned in Sect. 2. The non-detection of
the HCO+J = 4-3 and CS J = 7-6, molecular species that are enhanced in
the interphase layer between the outflows and the surrounding
molecular core where a YSO is forming (Rawlings et al. 2004; Hogerheijde et al. 1998) points in
the same direction.
On the other hand, Fig. 4 clearly shows that most of the 12CO profiles in the region present a redshifted component brighter than the blueshifted one, suggesting that the molecular gas is expanding. This is because in an expanding cloud a line emission is composed by red and blueshifted photons. In the case of an optically thick line, such as the 12CO J = 3-2, the redshifted photons will encounter fewer absorbing material (which is expanding outward) than would blueshifted photons and hence have greater probabilities of escape (e.g. Zhou 1992; Leung 1978; Lehtinen 1997).
To estimate the physical parameters of the molecular clump where
IRAS 18544+0112 is embedded, we determine the line parameters of
the 13CO J = 3-2 line from a Gaussian fitting. We obtain:
(
) K,
(
) km s-1,
(
) km s-1 and I
(
)
K km s-1, where
represents the peak brightness temperature,
the central velocity referring to the local standard of rest,
the line width and I the integrated
line intensity. Errors are a formal 1
value for the model of the
Gaussian line shape.
Using the 13CO J = 3-2 and J = 1-0 lines we calculated the ratio of the
integrated line intensities (13
R3-2/1-0). The 13CO J = 1-0 line
was extracted from the GRS and the J = 3-2 line was convolved to the
J = 1-0 beam. We obtained 13
.
Assuming LTE conditions and a beam filling factor of 1, which of
course may not be completely true but allows us to make an initial
guess, we use the 12CO J = 3-2 emission towards the center of the
analyzed region to estimate an excitation temperature,
.
As noticed above, this emission appears self-absorbed, showing a dip
between the blue and red emission components. Thus, we use an average
between the
of both components to obtain
15 K. Using this factor and the parameters obtained for the 13CO J = 3-2,
we derive an optical depth for this line of
0.2 and a 13CO column density of N(13CO)
cm-2. Adopting the 12CO and 13CO
relationships of N(H2)/N(12CO) = 105 and
N(12CO)/N(13CO) = 89 (Guan et al. 2008) and taking into account
that 13
R3-2/1-0 = 1 as estimated above, we obtain an H2column density of N(H2)
2.0
1021 cm-2.
Finally, assuming a spherical geometry for the clump as seen in the 13CO J = 1-0
line (Fig. 1-left) with a radius of
0.7 pc, we
estimate a mass and a volume density of
1.7
102
,
and
3.2
103 cm-3, respectively.
5 Is IRAS 18544+0112 an ultracompact HII region?
In view of the lack of outflow activity in the area, a possible explanation is that IRAS 18544+0112 is an evolved high-massive protostellar object that has finished its accretion phase. Since the next step in the life of these objects is to develop an ultracompact HII (UCHII) region we looked for the presence of ionized gas. The fact that this source lies in the same region of the sky than the SNR W44 and the HII region G 34.7-0.8, makes it difficult to discern whether there is radio emission related to IRAS 18544+0112. An inspection of the 1420 MHz image obtained from the VGPS does shows the presence of emission at the position of IRAS 18544+0112, but its extended morphology suggests that it is most probably related to G 34.7-0.8. To avoid the contamination of extended radio sources, we have inspected the 1420 MHz radio continuum image obtained from the NRAO VLA Sky Survey (NVSS; Condon et al. 1998). No radio continuum emission is detected at the position of IRAS 18544+0112. This agrees with the null detection reported by Hughes & MacLeod (1994) in the direction of this source.
From the cataloged IRAS fluxes of IRAS 18544+0112 and assuming a
distance of 3 kpc, we estimated the corresponding IR luminosity
and dust temperature
.
The IR luminosity was
estimated on the basis of the four-bands IRAS measurements (12, 25, 60
and 100
m) following Chan & Fich (1995) as
(
) = 1.58
(Jy) D2 (kpc), where
is the integrated flux given
by
(
S12+S25) + 0.7(
S25+S60) + 0.2(
S60+S100)
and Si is the flux density in the IRAS band i expressed in Jy. We
obtained
(7.9
1.6)
103
.
Adopting standard parameters for dust grains (Draine & Lee 1984), the dust
temperature can be derived from the relation (K) = (95.94/ln
Bn), where
Bn = 1.6673+n
S100/S60 is the modified
Planck function, and n is a parameter related to the absorption
efficiency of the dust (
). We obtained
(29.0
5.0) K for the adopted value
n=1. The
estimated dust temperature of about 29 K for IRAS 18544+0112 is quite
low for a UCHII region, which typically has temperatures of about 200 K (Ball et al. 1996), while it agrees with the ones derived for
high mass protostellar candidates by Sridharan et al. (2002).
In summary, from the observed characteristics we suggest that IRAS 18544+0112 is not yet a UCHII region.
Table 1: 2MASS sources towards IRAS 18544+0112.
6 A search of MYSO candidates in IRAS 18544+0112
To search for MYSO candidates associated with IRAS 18544+0112 we performed
a near-infrared photometric analysis of all the sources that are
enclosed within the borders of the IR source as seen in the 8 m
band (see e.g. Fig. 2). We used the 2MASS All-Sky Point
Source Catalogue (Skrutskie et al. 2006) in bands J (1.25
m),
H (1.65
m) and
(2.17
m), selecting only
the sources detected in at least two bands. We found 13 sources in
this region which are listed in Table 1 and shown in
Figs. 6a-c. In Table 1 we present
the source numbers (same as in the following figures), the Galactic
coordinates in degrees, the Two Micron All Sky Survey (2MASS)
designation and the J, H and
magnitudes in
Cols. 1 to 7, respectively. Figures 6a-c
display the spatial location of these sources superimposed over: the
8
m emission (a), the near infrared JHK three-color image
extracted from the 2MASS (b), and the optical emission as extracted
from the 2nd Digitized Sky Survey Blue (DSS2-B) (c). The dashed circle
represents the area in which we searched for the mentioned sources.
Figures 7 and 8 display the (H-Ks) versus (J-H) color-color (CC) diagram and the (H-Ks) versus Ks color-magnitude (CM) diagram, respectively, of the 13 selected 2MASS sources.
![]() |
Figure 6:
Spatial location of the 2MASS sources found towards
IRAS 18544+0112 over the 8 |
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![]() |
Figure 7: Color-color diagram of the 13 2MASS sources found towards IRAS 18544+0112. The two solid curves represent the location of the main sequence (thin line) and the giant stars (thicker line) derived from Bessell & Brett (1988). The parallel dashed lines are reddening vectors with the crosses placed at intervals corresponding to five magnitudes of visual extinction. We have assumed the interstellar reddening law of Rieke & Lebofsky (1985) ( AJ/AV=0.282; AH/AV=0.175 and AK/AV=0.112). The plot is classified into three regions: cool giants, normally reddened stars and infrared excess sources. The numbers correspond to the numbered sources of Figs. 6a-c. |
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![]() |
Figure 8: Color-magnitude diagram of the 13 2MASS infrared sources towards IRAS 18544+0112. The solid curve represents the position of the main sequence at a distance of 3 kpc. The reddening vector for an O3 star, with the squares placed at intervals corresponding to five magnitudes of visual extinction, is shown with a dashed line. The numbers correspond to the numbered sources of Figs. 6a-c. |
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Figure 7 shows that sources one to four, seven and twelve lie in the region of sources with infrared excess. In particular sources two and three appear as the most reddened ones. Sources six, nine, eleven and thirteen correspond to normally reddened main sequence stars and sources eight and ten, the most conspicuos ones in the optical image (Fig. 6c), are probably no reddened foreground stars.
According to the CM diagram (Fig. 8), sources two, three, and nine are the earliest spectral type stars in the region. Sources six and, thirteen lie in the region of the giant stars, and sources one, four, five, seven, eight, ten, eleven, and twelve correspond to spectral-type stars later than B3. Moreover, as Fig. 6c shows, sources one, five, eight, and ten are detected in the DSS2-B optical band which, given the high obscuration of the region, suggests that they are probably foreground stars.
Thus, based on both diagrams, we conclude that the most likely
high mass protostars related to IRAS 18544+0112 would be sources two and
three. The fact that source three lies towards the geometrical center of
the IR nebula and that it is placed on a maximun of the 24 m
emission while source two is located onto the nebulas's border,
suggests that source three is the main one responsible for the observed
infrared nebula.
![]() |
Figure 9:
Left: Two-color Spitzer image of the HII region G034.8-0.7
(8 |
Open with DEXTER |
7 Possible scenario
In this section we discuss a possible formation scenario for IRAS 18544+0112.
As mentioned in Sect. 2, the presence of a PDR bordering the HII region G034.8-0.7 together with the observed morphology of its associated molecular cloud, are clear evidence of the fact that the HII region perturbs its enviroment.
The IR source IRAS 18544+0112 is seen in projection inside the HII region G034.8-0.7, while IRAS 18542+0114 (Paron et al. 2009) is located upon its border (see Fig. 9). Based on an infrared and molecular study, Paron et al. (2009) found that IRAS 18542+0114 is a MYSO with molecular outflow activity and whose formation was probably triggered by the expansion of G034.8-0.7 onto the molecular cloud. In this context, we suggest that the HII region G034.8-0.7 has also triggered the formation of IRAS 18544+0112.
The observational differences found between both IRAS sources as well as their relative location with respect to the center of the HII region G034.8-0.7, suggest that the associated shockfront reached first the molecular gas where IRAS 18544+0112 was formed, thus explaining their different evolutionary stages. Kobayashi et al. (2008) proposed a similar scenario for the star forming region in Digel's Cloud 2 , where an expanding shell has perturbed different parts of a molecular cloud during its evolution, producing different star generations.
To test the proposed scenario we estimate the age of the HII region at
the moment it reached the location of each IRAS source. Using a simple
model described by Dyson & Williams (1980) we calculated the age of the HII region at a given radius R as
![\begin{displaymath}t(R)=\frac{4~R_{\rm s}}{7~c_{\rm s}}\left[\left(\frac{R}{R_{\rm s}}\right)^{7/4}-1\right]\end{displaymath}](/articles/aa/full_html/2010/02/aa12722-09/img46.png)
where






The number
of UV ionizing photons needed to keep an
HII region ionized is given by (see, e.g. Chaisson 1976)
,
where
is the
electron temperature in units of 104 K,
the distance
in kpc,
the frequency in GHz, and
the
measured total flux density in Jy. To calculate the radio flux
density and in order to overcome the problem of superposition of the
thermal radiation of the HII region and the non-thermal flux of the
SNR that occurs in the western half of the HII region, we assumed
the HII region to consist of two identical halves; we therefore
integrated the flux density over the ``free'' eastern half of the
ionized sphere and assumed that for the complete region it is just
twice this value. Using the 1.4 GHz VLA image we estimated
3 Jy, which agrees with
previous estimations,
Jy (Kuchar & Clark 1997),
and
Jy (Paladini et al. 2003) for a spectral
index
= -0.1 (
)
typical for the
optically thin regime of HII regions. For an HII region with
T= 104 K placed at a distance of 3 kpc, the total amount of
ionizing photons needed to keep the source ionized turns out to be
about
.
Based on
the ionizing fluxes for massive stars given by Schaerer & de Koter (1997), we
infer that the ionizing star cannot be later than O9.5 V. However,
this is only a coarse limit. The exciting star is probably earlier
than O9.5 since if the observed infrared emission originates in dust
heated by stellar radiation, part of the UV radiation is dissipated
in this way.
To estimate the dynamical age of the HII region it is important to
determine the center from which the ionized gas is expanding. Based
on the infrared and radio continuum emission distribution we identify
an almost circular morphology for the HII region G034.8-0.7.
Figure 9 (left) shows that infrared emission at 8 m
(green) clearly delineates the southeastern border of the HII region,
while the border where IRAS 18542+0114 is located is diffuse and
fainter. To better delineate this border we inspected a tomographic
map traced between 74 and 324 MHz towards W44 (see
Fig. 9 (right)). In a radio study of SNR W44,
Castelletti et al. (2007) found a curious spectral index inversion on the
southeastern limb of W44 that appears as an indentation in the SNR
boundary. There, the spectrum changes from a negative value
corresponding to the SNR synchrotron radiation (spectral index
)
to
.
In Fig. 9 (right)
darker
regions correspond to a negative spectral index. We propose that the
spectral inversion must be the result of free-free absorption produced
by thermal ionized gas naturally explained by the presence of the
HII region G034.8-0.7 located between us and the SNR. In this way,
we
can delineate the northwestern border of the HII region where
IRAS 18542+0114 is located, thus determining the probable center
of the
expanding HII region.
By adopting the radii R1=3 pc and R2=4 pc for IRAS 18544+0112 and
IRAS 18542+0114, respectively, an ambient density in the region of less
than 103 cm-3 and as the exciting agent for the HII region an O9.5 star, we derived dynamical ages of about
and
yr for the HII region at the position of
IRAS 18544+0112 and IRAS 18542+0114, respectively. An age difference of
yr is compatible with the observed different evolutive
stages of both IR sources.
The remaining question is whether the expansion of the SNR W44 had
some influence in the processes observed in this molecular complex.
Based on the age of the associated pulsar, Wolszczan et al. (1991) estimated an
age of about
yr for the SNR W44. Thus, the
possibility that the SNR W44 has triggered the star formation in this
region seems unlikely.
8 Summary
We present a molecular and infrared analysis of the IR source IRAS 18544+0112. The main results can be summarized as follows:
- (a)
- We find 12CO J = 3-2 self-absorbed profiles, which are typical of star forming regions. However, we do not detect any evidence of an outflow activity in IRAS 18544+0112 either in the molecular lines or in the infrared emission distribution.
- (b)
- The analysis of the 12CO J = 3-2 line suggests the presence of expanding molecular gas in the region.
- (c)
- Based on its morphology, infrared and molecular parameters, and the non detection of ionized gas, we suggest that IRAS 18544+0112 is an evolved high mass protostellar object which has not yet reached the ultracompact HII region stage.
- (d)
- From a near-infrared photometric analysis of the point sources observed towards IRAS 18544+0112 we propose that 2MASS 18565878+0116233 source (source three in the text) is the MYSO candidate most likely associated with IRAS 18544+0112.
- (e)
- Based on the observational evidence that the HII region G034.8-0.7 is perturbing the neighboring molecular clouds we suggest that it has triggered at least two star forming regions, IRAS 18542+0114 and IRAS 18544+0112 , during its expansion.
S.P. is grateful to the staff of ASTE for the support received during the observations, especially to Juan Cortés. S.P. acknowledges the support of Viviana Guzmán during the observations. M.O. is a doctoral fellow of CONICET, Argentina. S.P., S.C., G.C., and G.D. are members of the Carrera del Investigador Científico of CONICET, Argentina. This work was partially supported by Argentina grants awarded by CONICET, UBA and ANPCYT. M.R. is supported by the Chilean Center for Astrophysics FONDAP No. 15010003. M.R. and S.P. acknowledge support from FONDECYT N1080335. This work has made use of GLIMPSE and MIPSGAL data obtained with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract 1407. We also used data products from the Two Micron All Sky Survey, the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. We are grateful to the anonymous referee, whose comments and suggestions led to the improvement of this Paper.
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All Tables
Table 1: 2MASS sources towards IRAS 18544+0112.
All Figures
![]() |
Figure 1:
Two-color image of the HII region G034.8-0.7 containing
IRAS 18542+0114 and IRAS 18544+0112. Green is the Spitzer-IRAC 8
|
Open with DEXTER | |
In the text |
![]() |
Figure 2:
The green is the Spitzer-IRAC 8 |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Spitzer-IRAC three-color image (3.5 |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Left: 12CO J = 3-2 spectra obtained towards IRAS
18544+0112. The velocity range of each spectrum is between 20 and 70 km s-1. The color scale corresponds to the emission at 8 |
Open with DEXTER | |
In the text |
![]() |
Figure 5: 13CO J = 3-2 profile obtained towards the center of IRAS 18544+0112. |
Open with DEXTER | |
In the text |
![]() |
Figure 6:
Spatial location of the 2MASS sources found towards
IRAS 18544+0112 over the 8 |
Open with DEXTER | |
In the text |
![]() |
Figure 7: Color-color diagram of the 13 2MASS sources found towards IRAS 18544+0112. The two solid curves represent the location of the main sequence (thin line) and the giant stars (thicker line) derived from Bessell & Brett (1988). The parallel dashed lines are reddening vectors with the crosses placed at intervals corresponding to five magnitudes of visual extinction. We have assumed the interstellar reddening law of Rieke & Lebofsky (1985) ( AJ/AV=0.282; AH/AV=0.175 and AK/AV=0.112). The plot is classified into three regions: cool giants, normally reddened stars and infrared excess sources. The numbers correspond to the numbered sources of Figs. 6a-c. |
Open with DEXTER | |
In the text |
![]() |
Figure 8: Color-magnitude diagram of the 13 2MASS infrared sources towards IRAS 18544+0112. The solid curve represents the position of the main sequence at a distance of 3 kpc. The reddening vector for an O3 star, with the squares placed at intervals corresponding to five magnitudes of visual extinction, is shown with a dashed line. The numbers correspond to the numbered sources of Figs. 6a-c. |
Open with DEXTER | |
In the text |
![]() |
Figure 9:
Left: Two-color Spitzer image of the HII region G034.8-0.7
(8 |
Open with DEXTER | |
In the text |
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