A&A 453, 911-922 (2006)
DOI: 10.1051/0004-6361:20054646
M. Felli1 - F. Massi1 - M. Robberto2 - R. Cesaroni1
1 - INAF-Osservatorio Astrofisico di Arcetri,
Largo E. Fermi 5, 50125 Firenze, Italy
2 -
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
Received 5 December 2005 / Accepted 15 February 2006
Abstract
Aims. We report on new aspects of the star-forming region S235AB revealed through high-resolution observations at radio and mid-infrared wavelengths.
Methods. Using the Very Large Array, we carried out sensitive observations of S235AB in the cm continuum (6, 3.6, 1.3, and 0.7) and in the 22 GHz water maser line. These were complemented with Spitzer Space Telescope Infrared Array Camera archive data to clarify the correspondence between radio and IR sources. We made also use of newly presented data from the Medicina water maser patrol, started in 1987, to study the variability of the water masers found in the region.
Results. S235A is a classical HII region whose structure is now well resolved. To the south, no radio continuum emission is detected either from the compact molecular core or from the jet-like structure observed at 3.3 mm, suggesting emission from dust in both cases. We find two new compact radio continuum sources (VLA-1 and VLA-2) and three separate maser spots. VLA-1 coincides with one of the maser spots and with a previously identified IR source (M 1). VLA-2 lies towards S235B and represents the first radio detection from this peculiar nebula that may represent an ionized wind from a more evolved star. The two other maser spots coincide with an elongated structure previously observed within the molecular core in the C34S line. This structure is perpendicular to a bipolar molecular outflow observed in HCO+(1-0) and may trace the associated equatorial disk. The Spitzer images reveal a red object towards the molecular core. This is the most viable candidate for the embedded source originating the outflow and maser phenomenology.
Conclusions. The picture emerging from these and previous data shows the extreme complexity of a small (0.5 pc) star-forming region where widely different stages of stellar evolution are present.
Key words: stars: formation - ISM: individual objects: S235A-B - ISM: jets and outflows - radio continuum: ISM - masers
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Figure 1: Overview of the S235A-B star forming complex Left: overlay of the single-dish HCN map (contours) from Cesaroni et al. (1999) with the K-band image of the S235A-B region from Felli et al. (1997). Right: overlay of the interferometric maps of the 3.3 mm continuum emission (thick contours and grey scale) with the HCO+(1-0) outflows (thin solid line: V < -20.9 km s-1, dashed line: V> -15.5 km s-1) from Felli et al. (2004). In both, a cross marks the location of the H2O maser at -61.2 km s-1 from Tofani et al. (1995). The infrared source with the largest near-IR excess detected in previous works, M 1, is also indicated. The offsets between M 1, the water maser, the HCN peak, and the mm continuum peak are all larger than the position uncertainties. |
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This paper continues the study of the star-forming complex S235A-B (see Felli et al. 2004, and references therein), focussing in particular on a deeply embedded Young Stellar Object (YSO) found between S235A and S235B, close to a water maser. The presence of the YSO is implied from the typical signposts of early stellar evolution, including two molecular outflows, a hot molecular core, a sub-millimeter peak, and a water maser. The YSO represents the youngest object in the star-forming complex.
The site morphology derived from all previous observations is summarised in Fig. 1. S235A is a small optical nebulosity coinciding with a compact, but well-resolved, HII region. It appears as a less-evolved region, with respect to the more extended and diffuse HII region S235 located further north, and probably is unrelated to the S235A-B complex. S235A lies at the northern edge of a molecular clump that represents the brightest peak of a more extended molecular cloud (Evans & Blair 1981; Nakano & Yoshida 1986; Cesaroni et al. 1999).
Lying
south of S235A (0.35 pc at the assumed distance of 1.8 kpc, Nakano & Yoshida 1986), at the SW edge of the molecular
core, S235B is a smaller diffuse nebulosity detected both in the optical and
near-IR, exhibiting a near-IR excess and
intense emission in optical and IR hydrogen lines
(H
,
Br
), but without a radio continuum counterpart,
making it
a rather
peculiar object.
In H
,
it consists of an unresolved peak superimposed
on a circular nebula that is
in diameter (Krassner et al. 1982;
Alvarez et al. 2004). S235B appears to be a young star with an expanding
ionized envelope surrounded by a diffuse nebulosity (Felli
et al. 1997).
A large-scale molecular outflow was first found in 12CO(1-0) by
Nakano & Yoshida (1986) with a resolution of
;
it was
centred at S235B and aligned in a NE-SW direction, about 35
(0.3 pc) in
length.
Felli et al. (1997)
confirmed the blue lobe of the outflow in 13CO(2-1) with a
resolution of
,
but failed to detect the red lobe.
Near-IR images revealed a highly obscured stellar cluster
between S235A and S235B, i.e. centred on the water maser,
with several sources with IR excess
(Felli et al. 1997), in particular source M 1, which exhibits the largest near-IR
excess. This source has been suggested to be the
candidate YSO supplying energy to the -60 km s-1 water maser,
but with great uncertainty since it lies more than 5
to the south
(see also Fig. 1).
The above picture was clarified
by Felli et al. (2004), who presented high-resolution (between
2
and 4
) mm line (HCO+, C34S, H2CS,
SO2, and CH3CN) and continuum observations, together with far-IR
observations.
A compact molecular core (hereafter, the mm core) was found
both in the mm continuum (hot dust emission,
K) and in the
molecular lines, peaking close to the water maser position
and well-separated from S235A and S235B.
Two molecular outflows were found in HCO+(1-0)
centred on the mm core. One of them (hereafter, the NE-SW outflow) is
aligned along the same NE-SW direction of the large-scale outflow detected by
Nakano & Yoshida (1986). It spans
0.4 pc,
and has an estimated mass of 9
and a mechanical luminosity >
.
The other (hereafter, the NNW-SSE outflow) is more compact and aligned in a
NNW-SSE direction. It spans
0.3 pc and has
an estimated mass of 4
and a mechanical luminosity of
.
Felli et al. (2004) derived an upper limit of
for the bolometric luminosity of the mm core and
suggested the presence of an embedded intermediate-mass YSO
driving the NE-SW outflow and supplying the energy for the -60 km s-1 water maser.
Studying the high velocity red and blue emission of C34S(5-4) towards the mm core, they also found a compact structure with a
velocity gradient perpendicular to
the NE-SW outflow that might represent the signature of a circumstellar disk.
An elongated structure (called a "jet'') protruding from the mm core and
coinciding with the blue lobe of the NNW-SSE outflow
was also detected in the continuum at 3.3 mm (see Fig. 1, right)
and, with a lower signal-to-noise ratio, also at 1.2 mm.
The spectral index of the jet,
(defined as
)
is rather uncertain, but different from that of the mm core,
,
suggesting that the emission
might arise in an ionized wind rather than being due to dust.
Radio jets
are often observed at the base of molecular outflows (Rodríguez
1997; Anglada et al. 1998; Beltrán et al. 2001) in both low-mass
and high-mass
star-forming regions (e.g., Rodríguez 1996). They are characterized
by spectral indices in the range -0.1 to
1 and are elongated in
the outflow direction.
Expanding ionized envelopes also have a spectral index
(Panagia & Felli 1975).
Water masers are one of the most reliable signposts of early phases in star formation (see e.g. Tofani et al. 1995) since they provide the best indication of the position of the required powering source, i.e. the YSO. They occur both in low-mass (see, e.g., Furuya et al. 2001, 2003) and in high-mass (see Churchwell 2002, and references therein) star-forming regions and are often found to be associated with outflowing matter. Sometimes they are also found in close association with radio jets (e.g. Gómez et al. 1995).
The presence of a water maser in this region had been known since the observations of Henkel et al. (1986) and Comoretto et al. (1990), but with insufficient spatial resolution to properly locate it in the region. Only with Very Large Array (VLA) cm line observations (Tofani et al. 1995) and interferometric mm observations of the continuum sources (Felli et al. 2004), was the location of the water maser in-between S235A and S235B, almost coincident with the mm core, firmly established. This proved that the water maser is not associated with either of the two nebulosities and that a local early type star, presumably the YSO within the mm core, is needed for its excitation. In the VLA observations, maser emission was only searched for in a limited velocity range, around -60 km s-1, since at the time this was the only component detected by single-dish observations.
Table 1: Summary of VLA observations.
The water maser in S235A-B has been monitored with the Medicina radio telescope since 1987, with coverage
The S235A-B region contains other masers, namely methanol
(CH3OH) and SiO (Nakano & Yoshida 1986; Haschick et al. 1990;
Harju et al. 1998). Kurtz et al. (2004) included S235A-B in their recent VLA survey of the CH3OH maser
line at 44 GHz. This is a class I methanol
source,
and it is believed to trace outflow activity. These authors found
a cluster of 6 CH3OH masers spread over an area of
a
few square arcsec around the water maser spot at -60 km s-1.
To clarify the nature of the embedded YSO and its relation with the outflow found in the S235A-B region, we have performed an extensive observational program using the VLA and the Medicina radio telescopes, complemented by archival Spitzer data.
The two primary goals of the new VLA continuum observations
were: 1) to clarify the nature of the "jet'' and to derive its spectrum over
a larger frequency interval and 2) to search further for cm emission from
ionized hydrogen in the mm core. VLA observations in the water maser line
together with the Medicina patrol can give indications on the
location and activity of the maser in the star-forming region.
Finally, for a better understanding of the precise correspondence
between
IR and radio sources,
in particular the precise role of M 1 and the
possibility of detecting IR emission from the mm core,
archive Spitzer-IRAC observations of the S235A-B region in the four
wavelengths (3.6, 4.5, 5.8, and 8 m) were retrieved and
analyzed.
In Sect. 2 we describe the observations, and in Sect. 3 we present the results, while in Sect. 4 we discuss our findings and how they enrich our current understanding of the S235A-B region. In Sect. 5, the main results are summarised. The reader can refer to Fig. 16 for a comprehensive sketch of the S235A-B star forming-region, including the latest data.
The observations carried out with the VLA of the
National Radio Astronomy Observatory
(NRAO) were made with the VLA in the C configuration
on February 26 and on March 7, 2004.
The location of the water maser at -60 km s-1 was used as phase centre;
its coordinates are:
Since the forthcoming analysis is based on a comparison of the position of the
radio sources with those
of near- and mid-IR sources, we checked
whether the radio
coordinate system is consistent with the near-IR coordinate system.
To this end, we searched the 2MASS point source catalogue
for a near-IR counterpart of our phase calibrator, finding an object at
the same position as the calibrator within 0
1.
This gives us confidence that the two coordinate systems are consistent
with each other within this limit.
Spitzer-IRAC observations of a large area around the S235A-B region in the four
wavelengths (3.6, 4.5, 5.8, and 8 m) were extracted from the
Spitzer public archive.
The observations are part of the GTO program 201 "The Role of
Photodissociation Regions in High Mass Star
Formation'' (Principal Investigator G. Fazio). Integration time is 12 s at
all filters.
The positional accuracy is better than
.
Point-source FWHM resolutions range from
at 3.6
m
to
at 8.0
m.
The IRAC bands are large and may contain various features,
depending on the environment being observed. Among these, the
most important for our case are polycyclic aromatic
hydrocarbon (PAH) features (3.6, 5.8, and 8.0 bands) and the
Br
line (4.5 band).
An overview of IRAC is given by Fazio et al. (2004a).
A colour coded image of the region covering S235A-B, obtained by combining 4.5, 5.8 and 8.0 m observations,
is shown in Fig. 2.
The two diffuse nebulosities S235A and S235B clearly dominate the extended emission.
Most of the point sources detected in the K-band and shown in
Fig. 1 are also present here.
Two
sources deserve more attention
in view of the present work:
1) M 1,
which
is detected at all bands; and 2) a source
(hereafter, S235AB-MIR)
detected only at 4.5, 5.8, and 8.0 m, which
is present in the area of the mm core. Both
sources are marked in Figs. 2 and 3 where
we show an overlay of the mm core (at 1.2 mm) with the 5.8
m IRAC image. S235AB-MIR lies
to the
south of the peak of the mm core.
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Figure 2:
Three colour (4.5 blue, 5.8 green, and 8.0 ![]() |
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Figure 3:
The 1.2 mm core (contours)
from Felli et al. (2004) overlaid on the
5.8 ![]() |
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We performed aperture photometry on the IRAC sources
found near to the mm-core by using DAOPHOT in IRAF.
For all four bands, we selected
a radius of 2 pix (1 FWHM; 1 pix
)
and a 2-pix wide annulus with an
inner radius of 4 pix, to account for the highly
variable background. We applied aperture corrections as estimated
from the IRAC PSFs retrieved from the Spitzer Web Page
(http://ssc.spitzer.caltech.edu/obs/). To derive the
photometric zero points,
we used the zero-magnitude fluxes given by Fazio et al. (2004b).
Photometry was done only on the most relevant sources present in the area of the S235A-B cluster, which are indicated in Fig. 2. Positions and flux densities in the four IRAC bands of the sources labelled in Fig. 2 are given in Table 2
Table 2: MIR fluxes for the Spitzer-IRAC sources towards the mm core.
Colour-colour plots are shown in Fig. 4. In the
[5.8]-[8.0]/[3.6]-[4.5] plot, the region occupied by Class I and Class II
objects (Allen et al. 2004) is indicated.
M 1 has colours typical of a Class I object, while S235AB-MIR
is located in
a part of the plot redward of that
occupied by YSOs of mass
,
according to Whitney et al. (2004), i.e. the region corresponding to
[5.8]-[8.0
and [3.6]-[4.5
.
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Figure 4: Colour-colour plots of the most relevant sources in the area of the S235A-B cluster. The identifying numbers and symbols are the same as those used in Table 2 and Fig. 2. In the bottom box, the regions occupied by Class I and Class II sources are enclosed with dotted and dashed lines, respectively. When only an upper limit to the flux density could be estimated in one of the bands, the corresponding point in the plot is marked by an arrow. |
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The VLA radio continuum maps
are dominated by the emission from the
compact HII region S235A.
We calculated the integrated flux at all frequencies
over the area within the 3 contour of the emission at 6 cm (i.e. at the lowest resolution);
the obtained values are
listed in Table 3. The integrated flux at 6 cm is in very
good agreement
with previous measurements, yielding
mJy. Israel & Felli (1978)
found
mJy at 21 cm and suggested a partially thick emission
at 6 cm, but the
ratio of fluxes at 6 and 3.6 cm that we derive from our data is in agreement
with an optically thin emission. The presence of an ionizing star of
spectral type B0.5 derived from the radio fluxes in previous
works (e.g. Felli et al. 1997) is therefore confirmed.
In Fig. 5, we show the maps of S235A at 6, 3.6, and 1.3 cm.
The 3.6 cm map is overlaid on
the 5.8
m Spitzer-IRAC image and
shows a well-resolved spherical shape. The Spitzer-IRAC
image reveals the ionizing star S235A* at the centre of the nebula, previously
detected in the K band (Felli et al. 1997).
The main feature of the radio maps is the asymmetry of the isophotal contours in the
SE-NW direction. In the IRAC images, the morphology clearly indicates
the presence of a brighter ridge SE of S235A*.
The contours at 1.3 cm outline the brightest parts of the radio ridge.
In the original maps, the emission is more fragmented because of the
high resolution and low surface brightness.
In Fig. 5, the map has been smoothed to a
resolution of
.
At 1.3 and 0.7 cm, the radio fluxes are lower than expected
from an optically thin emission, as was also found by Felli et al. (2004) at 3.3 mm.
We attribute this to an instrumental effect caused by the filtering
of extended structures in the interferometric observations
(see Table 1).
At none of the 4 VLA wavelengths
were we
able to
detect emission from the mm core where the presence of an intermediate-mass YSO
is suggested by the mm observations.
This excludes any thermal emission from a UCHII region
associated with the YSO with a flux density above the noise level
given in Table 1
and implies
that emission from the core is dominated by dust.
At the same time, our non-detection is not in contradiction
with the existence of
a dust core.
In fact,
extrapolating at 0.7 cm, the flux density measured at 3.3 mm (20 mJy)
with a spectral index of 2.5
(Felli et al. 2004), we obtain 3 mJy for the flux expected from the core.
Felli et al. (2004) estimate that the core diameter in the continuum
at the highest resolution (1.2 mm) is
;
hence, assuming that
all the emission is uniformly distributed in a circle of
in radius
and using the synthesized beam size at 0.7 cm given in Table 1,
we obtain an expected flux of
0.12 mJy/beam, i.e. <
(see Table 1). Hence, dust emission at 0.7 cm
could be present below the sensitivity limit of our observations.
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Figure 5:
Maps of the continuum radio emission from S235A at 6, 3.6, and 1.3 cm (contours). Levels are from 2 to 14 mJy/beam by 2 mJy/beam for 6 cm, from 1 to 5 mJy/beam by 0.5 mJy/beam for 3.6 cm, and 0.1 to 0.5 mJy/beam by 0.1 mJy/beam for 1.3 cm.
The 1.3 cm map has been smoothed to a lower resolution to increase
the S/N ratio. The 3.6 cm map is
overlaid with the 5.8 ![]() |
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Similarly, no extended emission
from the jet was observed at any of our four frequencies.
While this could be an effect of over-resolution
and low surface brightness of the jet at the shortest wavelengths,
it definitely rules out the hypothesis
of an ionized jet at 6 and 3.6 cm. In fact, at 6 cm, the flux density
per beam area
extrapolated from the 3.3 mm flux in the hypothesis of an ionized
jet (i.e. using a spectral index
)
would be a factor of 3 higher
than the upper limit quoted in Table 3.
At 1.3 and 0.7 cm, where the resolution is higher,
two nearly unresolved sources are present.
They have been named VLA-1 (or S235AB-VLA-1) and
VLA-2 (or S235AB-VLA-2) and are indicated in Fig. 6,
where we show the VLA
map at 1.3 cm (contours) overlaid on the Plateau de Bure map at 1.2 mm
(grey scale) from Felli et al. (2004).
We derived the integrated fluxes of the two sources within the level
on the maps obtained with natural weighting.
At 3.6 and 6 cm, the two sources fall within the sidelobes of S235A. This results in a noise higher than that predicted from the
total integration time (see Table 1).
Using different weightings
to partially filter out the extended emission does not yield any significant
improvement in the
measurements. The flux densities are listed in Table 3; the
upper limits at 6 cm (for both) and at 3.6 cm (for VLA-2) refer
to a point source.
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Figure 6:
Map of the continuum radio emission at 1.3 cm of the area
around the water masers (contours)
overlaid with the map of the continuum 1.2 mm emission
(grey scale) from Felli et al. (2004).
Levels are from 0.12 (![]() ![]() |
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Table 3:
Radio continuum fluxes. Upper limits are
noise levels.
VLA-2 lies within the boundary of the
S235B nebulosity. VLA-2 represents
the first radio detection of this peculiar region.
Our flux densities of 0.5 mJy (see Table 3)
are not in conflict with the previously derived upper limits of 5 mJy at 6 cm
(
beam, Israel & Felli 1978)
and 0.3 mJy at 3.6 cm (
beam, Tofani et al. 1995).
We have checked the probability that VLA-1 and VLA-2 are
background sources. The expected number of extragalactic sources
at 1.3 cm in the field of view of Fig. 6 (1 square arcmin)
based on Eq. (A11) of Anglada et al. (1998) is
,
making this
possibility very unlikely.
In the selected velocity range (roughly from -70 to 10 km s-1),
three maser spots were detected
above the noise.
We have determined the flux densities and positions of the maser spots
by 2-dimensional Gaussian fits in each channel. All velocity components
in the same spot are spatially unresolved.
Coordinates of the three maser spots and
flux density for each velocity peak are listed in Table 4;
their relations to the other features present in the area are shown
in Figs. 6 and 13.
It is important to note that the three maser spots cover different velocity ranges, as shown in Fig. 7, so that there is no velocity overlap among the three spatial components.
One of them (S235AB-H2O/3)
coincides, within
,
with that found with the VLA at
-60 km s-1 by Tofani et al. (1995) and emits in the
same velocity range.
The other two (S235AB-H2O/1 and S235AB-H2O/2) occur at radial
velocities that had not been searched for in the previous
VLA observation because at that time they were not detectable in single-dish
observations, but they have since been revealed in the Medicina patrol.
The maser luminosity for each
spot was obtained by integrating the line emission
within the respective velocity ranges.
The results are listed in Table 5,
along with the corresponding velocity range.
The H2O luminosity is typical of masers associated with
far-Infrared (FIR) sources
of
103-104
(see Palagi et al. 1993). This is
consistent with the
upper limit of the bolometric luminosity inferred for the mm core
by Felli et al. (2004).
Table 4: Water masers: fluxes and positions.
From Figs. 6 and 13,
S235AB-H2O/2 clearly coincides with VLA-1 and M 1
and does not seem to be
directly related to the mm core.
The other two spots (S235AB-H2O/1 and S235AB-H2O/3)
are very close to the mm core and are perpendicularly aligned
to the NE-SW outflow,
apart from each other.
The possibility that they might be tracing a disk or torus
perpendicular to the NE-SW outflow will be examined in Sect. 4.
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Figure 7:
H2O VLA spectra averaged on a circle 1
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In Fig. 9, we show the time-velocity-intensity plot from the Medicina
patrol. An indicative value of the noise level in these observations throughout
the entire period is of the order of 1-2 Jy.
The starting date corresponds to March 31, 1987. The patrol is
sparser at the beginning. After 1992, there are about 4 observations
every year. Following the separation in velocity of the three maser spots,
each velocity component (at -60,
-25, and
0 km s-1)
is labelled in Fig. 9 with the corresponding VLA name.
The dates of the two VLA observations are indicated.
The systemic velocity of the
molecular cloud (-17 km s-1) is also indicated.
Only S235AB-H2O/2 occurs at a
velocity close to that of the thermal molecular lines; the other two components
(S235AB-H2O/1 and S235AB-H2O/3) emit well outside the width of the
thermal molecular lines.
The biggest flare occurred from the component S235AB-H2O/3 at the time of the first VLA observation. The source then disappeared below the noise and came up again just shortly before the second VLA observation.
Component S235AB-H2O/2 was undetectable for most of the time and appeared above the noise only after 2000.
Component S235AB-H2O/1, although always rather weak, is present
most of the time. Its most noteworthy aspect is that the velocity
changes up to 5 km s-1 around a mean value of
0 km s-1.
To better illustrate this effect, the time-velocity-intensity plot for the
velocity range from -10 to 10 km s-1 is shown in Fig. 10.
To make sure that the change in velocity is a real effect and not an instrumental one and to provide an independent estimate of the accuracy on the velocity, we have examined another water maser (G32.74-0.08), that is also included in the Medicina patrol, and that was chosen because it is characterized by a single, narrow, and intense velocity component. For this maser, the velocity of the peak displays a maximum deviation from the mean value over the entire period of <0.1 km s-1, a factor of 50 smaller than the velocity spread observed in S235AB-H2O/1.
In Fig. 5, the overlay of the 3.6 cm map
with the 5.8 m Spitzer image shows that the peak of
the IR emission occurs in a shell outside the boundary of the radio
emission. This proves that PAH and thermal dust emissions are mostly
located beyond the ionization front in the Photodissociation Region.
In Fig. 11, we show the overlay of the HCO+ integrated
emission around -17 km s-1 with the 5.8 m Spitzer-IRAC image.
The overlay indicates that the lower
contours of the molecular emission closely follow the outer boundary of
S235A, suggesting that the HII region and the molecular cloud are
interacting. It is also worth noting that the mm core is just outside the S235A
boundary. This situation resembles that of some well-known cometary-shaped
UCHII regions such as G29.96-0.02 and G34.26+0.15 (Reid & Ho 1985;
Wood & Churchwell 1989; Fey et al. 1995), which face a density peak of
the molecular clump enshrouding them (Maxia et al. 2001; Gibb et al. 2004;
Watt & Mundy 1999). In a number of cases, it has been found that such
a peak coincides with a hot molecular core, where
massive star formation is going on (Cesaroni et al. 1998;
Heaton et al. 1989; Garay & Rodríguez 1990). In our case, the
temperature of the molecular core is
30 K (see Felli et al. 2004), well below the typical temperature
of hot molecular cores (see Kurtz et al. 2000),
and the HII region S235A has an asymmetric rather than a cometary shape;
nevertheless, the interaction
between the ionized gas and the molecular cloud, as traced by
the overall structure
of the region, suggests that in this case also, one is observing an active
burst of star formation where different evolutionary phases (from cores to
evolved HII regions) co-exist. Whether the star formation episode in
the molecular core has been triggered by the expansion of the
nearby HII region S235A remains an open issue.
Table 5: Water masers: line luminosity.
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Figure 8: Upper envelope of all water maser spectra observed with the Medicina radio telescope towards S235A-B, until July 2005. The three corresponding maser spots detected with the VLA observations are indicated. The vertical line defines the velocity of the molecular cloud. |
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Our radio continuum observations were unable
to detect the elongated structure that had previously been found
at 3.3 mm, which also coincides with the blue lobe
of the NNW-SSE HCO+(1-0) outflow. Instead,
a compact radio source, VLA-1, was found coincident with
a small blob of emission in the central part of the 1.2 mm "jet'',
close (3
east) to the weaker molecular peak at -19 km s-1,
called C 19
(Felli et al. 2004). More importantly, VLA-1 coincides with the newly found
water maser S235AB-H2O/2 and with M 1, as shown in
Fig. 12, where the 1.3 cm VLA map is
overlaid on the Spitzer-IRAC 3.6
m image.
M 1, which had been previously assumed to be associated with
the -60 km s-1 water maser (Felli et al. 1997),
is instead associated with a a radio continuum source and a new, separate
water maser at
-30 km s-1.
Although the errors on the flux densities are large, the values given
in Table 3 for VLA-1 are consistent with
a spectral index
.
This value is
typical of partially optically thick free-free emission.
Since the present morphology no longer supports an ionized
jet interpretation, we have to consider the alternative
possibility that VLA-1 is an independent UCHII region, a hypothesis
that should also be considered, in light
of the precise association of the water maser with M 1. A lower limit
to the total number
of ionizing photons can be obtained from the
1.3 cm radio flux (the one with the highest signal-to-noise ratio) by
assuming that the HII regions are optically thin (Mezger 1978). We
obtain
s-1, typical of B2-B3 ZAMS stars
(Panagia 1973).
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Figure 9: Time-velocity-intensity plot of the water maser emission from S235A-B observed with the Medicina radio telescope. The starting date is March 31, 1987. The dates of the first VLA observation by Tofani et al. (1995) and of the present observations are indicated (month/year) with an arrow and their velocity ranges are enclosed within a long rectangle. The three maser spots are indicated by bracketing the velocity ranges with vertical dashed lines. The vertical solid line defines the velocity of the molecular cloud. The black areas are time-velocity regions with no observations. |
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Figure 10: Time-velocity-intensity plot of the water maser emission from S235A-B observed with the Medicina radio telescope over the velocity range from -10 to +10 km s-1, which corresponds to the maser spot S235AB-H2O/1. |
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Figure 11:
The HCO+ cloud at -17 km s-1 (contours)
overlaid on the 5.8 ![]() |
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What may have caused the apparent disagreement between the cm radio observations
which postulate the presence of a UCHII region and the mm observation that
had suggested a jet?
Extrapolating the radio flux of VLA-1 according to
(
), we derive
0.8 mJy
at 1.2 mm and
0.6 mJy at 3.3 mm. The 1.2 mm flux is below
(
1 mJy) in the Plateau de Bure map, so it could
not have been detected as a point source. The 1.2 mm
flux of the "jet'', 13 mJy, was integrated over a much larger
area defined by the 3.3 mm map.
At 3.3 mm, the extrapolated flux of VLA-1 is at a
level,
again barely detectable as a point source, and in any case
difficult to see since it would lie within the elongated 3.3 mm structure
(about
in the elongated direction and unresolved in the perpendicular
direction), with 7 mJy of integrated flux.
These contradictory aspects can be reconciled if
the emission from the elongated 3.3 mm structure comes
from dust, perhaps that associated with
the NNW-SSE blue outflow lobe or with the C 19 molecular peak.
The spectral index
between 3.3 and 1.2 mm could be the result
of the simultaneous presence of a dusty jet with a steep spectral index
and a weak UCHII region showing up only at lower frequencies, where
dust emission is negligible.
Is VLA-1/M 1 the driving source of the NNW-SSE outflow? Due to the overlap with the NE-SW outflow, the centre of the NNW-SSE outflow is ill-defined and any argument based on the position of VLA-1 with respect to the centre of the NNW-SSE outflow is inconclusive. However, we can exclude any association of VLA-1 with the NE-SW outflow, given its offset with respect to its axis.
Finally, we note that one
of the methanol masers (CH3OH/4) lies close (2
southwest) to VLA-1 and the water maser
S235AB-H2O/2 (see Fig. 13) and has velocities in a
similar range (from -16 to -21 km s-1, see Kurtz et al. 2004), suggesting a common origin.
The velocities of the water and methanol masers
are very close to that of the molecular core, indicating that in this
case, the component of the motions along the line of sight
of the masers with respect to the molecular core is negligible.
![]() |
Figure 12:
Overlay of the 1.3 cm VLA map (full contours) with the
Spitzer-IRAC 3.6 ![]() |
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![]() |
Figure 13:
Position of the three water masers (H20/1-3 +) and of
the six methanol masers (CH3OH/1-6 ![]() |
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The overlay of Fig. 12 shows that VLA-2 lies at the centre
of the S235B bright diffuse nebula (saturated in all IRAC bands).
The Spitzer-IRAC images, in particular
those at longer wavelengths,
indicate that this nebula is composed of two very close components
(see Fig. 2).
VLA-2 lies at the centre of the southern and more extended one.
Overall, the size of the mid-IR nebula is about 10
,
similar to that
observed in H
,
so that the optical-IR
morphology is more reminiscent of S235A (resolved HII region)
than that of VLA-1 (IR and radio unresolved).
Besides being detected in H,
strong Br
emission from
an unresolved source (i.e. <3
)
coincident with the
K-band point source in S235B had been reported
by Krassner et al. (1982) and Felli et al. (1997), with an
integrated line
flux
of F(Br
erg s-1 cm-2.
The expected radio flux
density
from an optically thin HII region at 6 cm
would have been greater than 200 mJy, where the lower limit accounts
for the fact that the
Br
flux was not corrected for extinction. This is at odds
with lower limits to the radio flux found in all previous
radio continuum observations, as well as with the present detection.
In the past, this forced
abandon of
the hypothesis of a classical HII region (unless
extremely optically thick)
suggested
that the Br
emission originates from an ionized expanding envelope around an early type
star (Felli et al. 1997), in which case the ratio of radio-to-IR line
emission
would be
reduced by about two orders of magnitude (Simon et al. 1983).
Our measured fluxes represent the first detection of the radio
continuum from an underlying unresolved source.
The radio flux density at 3.6 cm
expected from a fully ionized envelope using
F(Br)/F(Br
and Eq. (20) of Simon et al. (1983)
is 0.46 mJy. While this agreement with the observed value
might be fortuitous in view of the many unknown parameters involved
(velocity of the
wind, correction for extinction, etc.) it clearly points out that an
ionized envelope around a lower luminosity, more-evolved star remains
the best interpretation. However, it must be noted that
the observed spectral index is smaller than the expected 0.6 value.
Our flux densities are fully
consistent with the previous upper limits at radio wavelengths and with
the unresolved nature of S235B.
The implied mass loss using Eq. (14) of
Felli & Panagia (1981) is
yr-1, quite large
and indicative of a luminous star.
Future radio recombination line observations
with sufficiently high sensitivity may
permit measurement of
the line width to determine the velocity of the ionized wind.
There are no masers, molecular peaks, outflows, or mm
peaks associated with S235B, all of which suggest
that S235B-VLA-2 is more evolved than the mm core and VLA-1.
The diffuse H
and IR emission from S235B may be attributed to reflected light from ionized stellar envelopes.
Finally,
what
is the luminosity and mass of the star
embedded in S235B? We cannot use
the Spitzer-IRAC observations because they are saturated.
Instead, we used
the J, H, and K magnitudes from Felli et al. (1997) and the MSX fluxes
(see Egan et al. 1999).
The integral of these values gives 410 ,
which must be considered to be a lower limit
because the FIR part of the spectrum is not taken into
account in our calculation.
As was already pointed out, an important result of the cm radio continuum
observations is the lack of emission
from the mm core.
In Felli et al. (2004), the luminosity of the embedded YSO had
been estimated to be 103 .
If this comes from a ZAMS star of spectral type B3 or earlier,
to make the radio
free-free
emission undetectable, the
radius of the associated HII region must be less than
6.5 AU due to
confinement by a density larger than
cm-3. In any case, the
detection of pure thermal dust emission from the core indicates a very early
evolutionary phase of the embedded YSO.
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Figure 14:
HCO+(1-0) emission from the NE-SW outflow (thin contours)
overlaid with the blue- and red-shifted emission of
C34S(5-4) (thick contours) from Felli et al. (2004).
The "+'' symbols mark
the location of the water maser spots H2O/1 (![]() ![]() ![]() |
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It is possible to check whether the S235AB-MIR fluxes
between
m are consistent with the spectral energy distribution
(SED) of a heavily embedded
(proto-)star of 103
.
Felli et al. (2004) show that the non-detection of such a source in
the K band towards the mm core implies
mag, in agreement with the derived H2 column density. Using the extinction
law found by Indebetouw et al. (2005), we derived upper limits
for the intrinsic fluxes (or upper limits) of S235AB-MIR in the four
IRAC bands.
Of course, S235AB-MIR
being
a Class I source, most of the emission at these wavelengths
arises from circumstellar matter rather than from the (proto-)star
photosphere. For this reason, we compared our results to the SEDs
models for intermediate-mass Class I sources (star plus disk and envelope,
Whitney et al. 2003; Whitney et al. 2004).
We found that, within the limits of
the many parameters of their
models,
the MIR SED of
S235AB-MIR is consistent with a central star later than B3.
The two northernmost water masers, S235AB-H2O/1 and S235AB-H2O/3,
are located close to the mm core and emit over velocity ranges
quite different from
those of the other water and methanol masers,
as well as those of the molecular
cloud (-17 km s-1).
Figure 14 (adapted
from Fig. 21 of Felli et al. 2004) shows the NE-SW HCO+ outflow
with the contours of a perpendicular bipolar
structure traced by the wings
of C34S(5-4).
This was interpreted by Felli et al. (2004) as the signature of a rotating
disk around the YSO driving the outflow. Strikingly enough, the
two water masers are aligned in the same direction and very close to the
C34S(5-4) structures: the blue-shifted
maser (S235AB-H2O/3 at
-60 km s-1) lying towards
the blue C34S(5-4) lobe
(integrated from -21 to -19 km s-1)
and the red-shifted maser (S235AB-H2O/1 at
7 km s-1)
lying towards the red C34S(5-4) lobe (from -16 to -14 km s-1).
However, in both cases the velocity of the water masers are more red or blue
shifted than the corresponding C34S(5-4) lobes.
A simple calculation based on the maser velocities
shows that they cannot simply belong to
a disk in Keplerian rotation around the YSO. From the difference between
the most extreme maser velocities (7 and -68 km s-1),
we can infer a lower limit to the rotation velocity of 37 km s-1.
If one assumes the half-distance between the two maser
spots (
or 1800 AU) to be the orbital radius, then the mass needed
to maintain such a rotating disk should be >2500
,
much larger than the mass of the molecular core.
This fact proves that the water masers cannot trace rotation about an embedded YSO. One possibility is that they are instead the signature of the interaction with the C34S disk of high velocity outflowing material from the YSO. The higher (red and blue) velocities of the water maser with respect to C34S and their closer position to the mm core may suggest that the acceleration of the outflowing material occurs in the immediate surroundings of the YSO.
Another possibility is that the C34S emission is not tracing a disk, but rather an outflow, whose high-velocity component would be seen in the H2O maser lines. This scenario is more consistent with the common belief that water masers are strictly associated with jets powering molecular outflows (Felli et al. 1992), as observed, e.g., in the massive protostar IRAS 20126+4104 (Moscadelli et al. 2000, 2005). If this hypothesis is correct, it remains to be established whether the bipolar outflow seen in the C34S and H2O lines would be the same as the NNW-SSE outflow or a distinct one oriented approximately in the same direction, but originating from a different YSO.
Of the two hypotheses presented above, we believe that the outflow origin for the C34S and H2O emission is the most likely, given the tight association between outflows and water masers. However, at present it is impossible to rule out the possibility that the C34S emission is coming from a disk, as in the case of the Keplerian disk in IRAS 20126+4104 (Cesaroni et al. 2005). Class I methanol masers are believed to be excited in jets, so that a priori the 7 mm CH3OH masers imaged by Kurtz et al. (2004) in S235 could be used to establish the direction of the outflow and hence choose between the two hypotheses. As shown in Fig. 13, five of the maser spots cluster around the mm core suggesting that for them, too, the main source of energy is the YSO within the mm core. In particular, CH3OH/2 is very close to S235AB-H2O/3. However their velocities lie within a narrow range (from -15.9 to -21.0 km s-1) and the spots do not show a clear bipolarity, either in velocity or in their distribution. It is therefore difficult to associate the methanol maser emission to any precise geometry. The only conclusion is that they are unlikely to have the same dynamical origin as the two water masers.
Some indication of what occurs in the immediate surroundings of the YSO embedded within the mm core may come from the variability of the two associated water masers.
The emission at -60 km s-1(S235AB-H2O/3) reached its maximum (up to
110 Jy)
in 1992-1993, lasting at most 2 years, then disappearing for most of the Medicina patrol, and finally
reappearing just before the VLA observations.
Most noteworthy is
the emission from S235AB-H2O/1 (between -10 and 10 km s-1), not only
because of its long lasting presence and high variability, but also
because of its velocity shifts, 5 km s-1.
Figure 15 shows the LSR velocities
of S235AB-H2O/1 (obtained by Gaussian fitting)
in the period 1989-2005.
While it is possible that the velocity variations are simply due to the random flaring of spots at different velocities, we shall investigate more physically appealing interpretations.
To check whether the effect is due to a rotational modulation, we tried to fit all the observed velocities (the source is above the noise for about 60% of the time) with a sine function, as would be expected for a maser spot on a rotating disk viewed edge-on. No significant evidence was found.
The velocities in Fig. 15 are all redshifted
with respect to the systemic velocity,
but they are not all at random and seem to fan out at late periods.
We identify two groups of
points (labelled as 1 and 2 in
Fig. 15), which exhibit linear drifts of velocity
away from the systemic value.
A linear fit (dashed lines in Fig. 15) provides
velocity drifts of 0.93 and 0.98 km s-1 yr-1, respectively.
Velocity drifts of this amount have been observed in other water masers that are
stronger and less variable in intensity (Brand et al. 2003).
It is tempting to explain the velocity drifts
with shocked material that is accelerated from
a mean velocity 0 km s-1
by mass outflow from a central YSO.
The lifetime of the accelerated spots is
1000-2000 days
(
3-6 yrs) and could be related to the duration of
ejection events from
the YSO.
The maser outflows from S235AB-H2O/1 clearly deserve further, proper motion studies with VLBI techniques.
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Figure 15: Velocities of the peaks of S235AB-H2O/1 from Gaussian fits to the data from the Medicina radio telescope. We have tentatively outlined, with dashed lines, two components (labelled as 1 and 2) whose velocities might be drifting during the period of our observations. |
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We have presented new, more sensitive high-resolution VLA cm
radio observations of the S235A-B region, as well as the results
of the Medicina water maser patrol (started in 1987), and archive Spitzer-IRAC
observations. Several new aspects of this star-forming region emerge;
they are illustrated in Fig. 16 and summarised in the
following:
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Figure 16: Sketch (not to scale) of the star-forming region S235A-B in light of the new data presented in this paper. New and already-known sources are labelled, and their relationships are discussed in the text. |
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Acknowledgements
The Medicina observations are part of a long lasting project carried out by the Arcetri-INAF and IRA-INAF water maser group (see e.g. Brand et al. 2003, and references therein). This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. This research made use of data products from the Midcourse Space Experiment (MSX). Processing of the data was funded by the Ballistic Missile Defense Organization with additional support from the NASA Office of Space Science. We acknowledge G. Comoretto and F. Palagi for their help in the study of the variability of the water masers.