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Astron. Astrophys. 320, 594-604 (1997)

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3. Results

3.1. The molecular clump

Fig. 2 shows a contour map of C34 S(J=3-2) superimposed on our K band image. The center of the C34 S molecular core is unresolved and the peak coincides with the H2 O maser. S 235 B and S 235 A are located at the edges of the molecular core, [FORMULA] and [FORMULA] away, respectively. The peak main beam brightness temperature is 4.3 K and the peak velocity -17.1 km s-1.

[FIGURE] Fig. 2. Contour map of integrated C34 S (J=3-2) emission overlayed on the K band gray-scale image. The cross marks the position of the H2 O maser ([FORMULA], [FORMULA], Tofani et al. 1995). Contour levels are from 2.9 to 9.9 K km s-1 in steps of 1 K km s-1, the thicker contour represents the half power level and it is at 5.4 K km s-1. The HPBW is [FORMULA]. Also indicated are the three infrared sources (M1, M2, and M3) close to the maser.

Fig. 3 shows the profiles of [FORMULA] around the H2 O maser position. The peak main beam brightness temperature (43 K) occurs [FORMULA] to the north of the maser, at a velocity of -17.1 km s-1. The velocity integrated intensity distribution (not shown) is extended and similar to that found by NY in 12 CO, i.e. peaking between the maser and S 235 B. From Fig. 3 we clearly see that a new component at velocity [FORMULA] -15 km s-1 becomes prominent in the south-west part of the map, close to the position of S 235 B.

[FIGURE] Fig. 3. 13 CO(J=2-1) line profiles over the observed region. The center of each spectrum is offset in steps of [FORMULA] in [FORMULA] with respect to the H2 O maser position. The velocity of each box is from -28 to -6 km s-1, the intensity scale from -5.3 to 44.5 K

As far as the CO outflow reported by NY, the blue-shifted lobe, from -24 to -20 [FORMULA], can be barely seen also in the [FORMULA] data (see Fig. 4) and it is located between the H2 O maser and S 235 B (as also in NY). However, we do not detect the red-shifted lobe observed in [FORMULA] from -13 to -6 km s-1 (which is also weaker in NY). The red wing of the 12 CO profile may be affected by a superposition of different velocity components south of S 235 B and has been questioned by Snell et al. (1990) and by NY themselves. In conclusion, we confirm the presence of a blue-shifted lobe (at velocities up to 7 km s-1 from the rest velocity of the cloud) between the H2 O maser and S 235 B, but we cannot support S 235 B as the unique source of the outflow. With the present result the possibility that the origin of the blue outflow coincides with the H2 O maser should also be retained.

[FIGURE] Fig. 4. Map of the 13 CO(J=2-1) blue lobe. The velocity interval is from -24 to -20 km s-1. The contour unit is 2,3,4,5 K km s-1. The position of the H2 O maser (open triangle), of the three infrared sources M1, M2 and M3, and of S 235 B are also indicated.

The comparison between the very high peak [FORMULA] brightness temperature (43 K) with the peak temperature in the [FORMULA] line (8 K) and with the peak temperature in the [FORMULA] line (25 K) observed with lower resolution by NY can give us some indications on the molecular core density.

In the hypothesis of a constant temperature cloud we shall consider two possibilities: 1) the [FORMULA] line is optically thick, 2) the [FORMULA] line is optically thin. In the first case the (1-0) and (2-1) [FORMULA] temperatures can be reconciled only by a beam filling factor effect and implies a very small source size. A beam filling factor [FORMULA] is necessary, which is compatible with the ratio of the two beam areas, [FORMULA], and implies a source size less than [FORMULA].

In the second case, since the (2-1) optical depth is higher than the (1-0), an excitation temperature much higher than 43 K must be invoked.

The comparison between [FORMULA] and [FORMULA] temperatures also requires a very large optical depth, even after correcting the 12 CO observations for the source size.

However, if we allow for a temperature gradient in the cloud, an intrinsic 12 CO temperature lower than the observed 13 CO is not unplausible. In fact, we can hypothesize a cooler surrounding envelope at 25 K which is optically thick at 12 CO and optically thin at 13 CO and a hotter inner core at a temperature [FORMULA] 43 K which is optically thick at 13 CO.

We can estimate a lower limit for the column density and the extinction, from our [FORMULA] data using optically thin approximation:

[EQUATION]

(from Wootten et al. 1983). From the measurements of Evans & Blair (1981) of the [FORMULA] and [FORMULA] lines we can estimate [FORMULA] for that transition to be approximately equal to 1. With [FORMULA]  K, and [FORMULA], we obtain [FORMULA]. Assuming the standard abundance of [FORMULA], [FORMULA], then the total column density becomes greater than [FORMULA]. Using the standard relation [FORMULA] (Bertoldi & McKee, 1992), this implies a visual extinction [FORMULA] magnitudes (thus the extimate of 9 magnitudes made by Tokunaga & Thompson 1979 should be considered as a lower limit). With a size of the clump of the order of [FORMULA] the mean density in the molecular clump is [FORMULA].

The molecular hydrogen density can be also obtained with a completely independent method from the three C34 S line intensities by using the statistical equilibrium model of Cesaroni et al (1991). In this model the statistical equilibrium equations for the level populations are solved in the Sobolev approximation, with a constant velocity gradient assumed equal to the ratio between the line width and the source size. The input parameters to the model are the kinetic temperature, the H2 density and the C34 S abundance, and are varied until a best fit with the observed intensities (1.5, 4.4 and 2 K for the (2-1), (3-2) and (5-4) transitions, respectively) is obtained. In Fig. 5 the profiles of the three C34 S lines at the position of the peak (corresponding to the position of the H2 O maser) are shown. Due to the low signal to noise of the observations, only a crude estimate of the parameters can be obtained. We obtain a resonable agreement between the model and the observed data for T [FORMULA]  K, [FORMULA], and [FORMULA].

[FIGURE] Fig. 5. Profiles of the C34 S lines observed at the position of the H2 O maser.

In summary, all the indications point to a very high density (n(H2) [FORMULA] 106 cm-3), small and very hot ([FORMULA] 50 K) core , probably hotter than the surrounding molecular gas, located at the position of the maser.

The mass of the hot core, assuming a constant density of [FORMULA] (compatible with the two estimates given above) within a spherical source with a diameter of [FORMULA], derived as the diameter of the circle with an area equal to that inside the 50% peak intensity contour of the C34 S(3-2) (see Fig,  2) and deconvolved for a beam FWHM of [FORMULA], is of the order of 450 [FORMULA].

3.2. H2 O maser variability

H2 O maser emission at 22 GHz from this region has been known for a long time and is highly variable: in 1974 only one component at -22.3 km s-1 (Lo et al. 1975) is present, in 1976 a high-velocity (-4 km s-1) and a low-velocity (-59 km s-1) feature appear (Blair et al. 1978), in 1978-79 a group of components at -60 km s-1 and two components (at -17 and 4 km s-1, respectively) are present, which steadily decrease and completely disappear in 1979 (Rodriguez et al. 1980), and, finally, only one broad component between -61 and -57 km s-1 is found in 1983 (Henkel et al. 1986). In our Medicina 2 data the source has been observed since 1989 (see Persi et al. 1994 for a recent summary of the results).

The light curves of the four velocity components observed at Medicina since 1986, including more recent observations after Persi et al. (1994), are reported in Fig. 6. The two brightest ones (partly blended) are at -58 and -60 km s-1, as in the 1978-79 and 1983 spectra. The -60 km s-1 component reached a peak of 111 Jy on 25/01/1993. The components at 0 and -70 km s-1 are much weaker and only occasionally rise above the mean noise of [FORMULA] 5 Jy (3 [FORMULA]). After 1995 the maser is quiescent (i.e. below 3 Jy). The peak maser luminosity is 1.8 10-5 [FORMULA], and during quiescent period is less than 2 10-7 [FORMULA].

[FIGURE] Fig. 6. Light curves of the 4 velocity components found in the H2 O spectra in the Medicina observations. Day 0 corresponds to 23/jun/1987.

Accurate maser position estimates were made with the VLA by Tofani et al. (1995), at the time of the peak of the -60 km s-1 component. The -70, -60 and -58 km s-1 components were not spatially resolved. The position of the 0 km s-1 component remains unsettled because it was quiescent at the time of the VLA observations. It should also be noted that the velocity difference between the -60 km s-1 maser and the peak of the molecular emission ([FORMULA] -17 km s-1) is quite large (43 km s-1), much larger than the 10 km s-1 half width dispersion of the velocity difference over a large sample of CO outflows associated with H2 O masers (Felli et al., 1992; Codella and Felli, 1995; Anglada et al., 1996). Only in the 1978-79 observations there were components at the same velocity as the molecular cloud.

3.3. The S 235 A-B stellar cluster

Fig. 1 clearly shows that the S 235 A and B nebulosities host stellar sources (S 235 A [FORMULA] and B [FORMULA]) that are the brightest members of a stellar cluster visible at K band. The cluster was found by Hodapp (1994) in K', who discovered more than 300 sources around S 235 B, down to a magnitude of about [FORMULA]. In the same region we detect 144 sources in our K image, down to a magnitude of [FORMULA]. The large difference in the number of sources detected is primarily due to the different sensitivities of the two data sets. Our observations are unable to reveal the low luminosity tail of the K band luminosity function, but are in good agreement for the high luminosity part of the distribution, after correcting for the difference between K' and K.

In Fig. 7 the (J-H,H-K) colour-colour diagram is plotted for the sources detected by us in the cluster area defined by Hodapp. The solid line, labelled MS, marks the position of unreddened main sequence stars. The two dashed lines show the position of the reddening belt for main sequence stars. A dotted line [FORMULA] is drawn parallel to the reddening law. This line takes into account the error in the colours, and is used to separate sources with infrared excess (to the right of the line and represented as crosses in Fig. 7) from reddened MS stars.

[FIGURE] Fig. 7. The (J-H,H-K) colour-colour diagram of the total (43) sources with detection in all the three bands in the S 235 A-B complex region. The exciting stars of S 235 A and B are marked as well as the K-band sources (M1,M2 and M3) near the H2 O maser. The dotted line is used to separate the sources with IR excess from reddened MS stars allowing for errors on the colours.

Out of the 91 sources detected in all the three bands, 20 (22%) show near infrared excess and are probably young stars, the other are reddened MS stars. An important point that can be settled with our multi-band observations, is that most of the sources with infrared excess are located near the maser position between S 235 A and B. This implies that the H2 O maser (and its powering source, see Sect.  3.7) is at the center of the embedded cluster, and not S 235 B, as supposed by Hodapp (1994). Moreover, since the stars with IR excess are believed to be the youngest, this also suggests a gradient in the star formation process, in which the stars near the centre of the cluster and around the maser are the least evolved, while S 235 A [FORMULA] and B [FORMULA] represent more evolved stages.

A few sources which will be discussed in more detail are labelled in Fig. 7, with observed parameters given in Table 3.


[TABLE]

Table 3. Positions and magnitudes of some near-infrared point sources.


3.4. S 235 A

S 235 A is classical HII region with flux density of 270 mJy at 5 GHz, size [FORMULA] (0.17 pc) and electron density [FORMULA] (Israel & Felli 1978). The central star is a B0.5 type star with luminosity of [FORMULA] (Thompson et al. 1983).

The centre of the radio continuum emission (which is representative of the ionized gas distribution over the entire HII region) and the stellar near IR source approximately coincide, as expected for a constant density spherical HII region ionized by a central star. However, the H [FORMULA] and diffuse K band emission are offset from the position of S 235 A [FORMULA]. In Fig. 8 we have overlayed the central part of the K map (full contours) on the H [FORMULA] photo of Krassner et al. (1982). It can be clearly seen that S 235 A [FORMULA] coincides approximately with the centre of the radio continuum emission (whose outer boundaries are outlined by the dashed line in Fig. 8), but lies to the west of the diffuse H [FORMULA] nebulosity. This offset suggests that the star is still surrounded by a thick dust envelope on all sides except to the east, where ionized gas is less obscured and becomes visible in H [FORMULA] and in diffuse K band emission (most probably reflected star-light). About [FORMULA] south of S 235 A [FORMULA] there is another H [FORMULA] nebulosity (or perhaps a fainter extension of the upper one). Also in this case there is a K band point source at its edge (this time the eastern edge of the H [FORMULA] emission), which has possibly a similar explanation. However, in this case no radio continuum peak is found at the same position, which is located towards the southern boundary of the radio HII region. Presumably, the spectral type of this star must be later and the luminosity smaller than that of S 235 A [FORMULA].

[FIGURE] Fig. 8. Overlay of the K band emission (full contours) on the H [FORMULA] photo (gray-scale) of Krassner et al. (1982). The matching between the two images is obtained with the three stars present in both. The cross is the H2 O maser position. The dashed line is the lowest contour of the 6 cm radio continuum emission from Israel & Felli (1978). The full triangle marks the position of the radio continuum peak. The three crosses mark the positions of M1, M2 and M3. M2 and M3 are too faint to be well represented in the contour map.

S 235 A [FORMULA] is the second brightest NIR source in the cluster. We find diffuse Br [FORMULA] emission around the source (already detected by Tokunaga & Thompson 1979), and hot molecular hydrogen emission in the belt-like photon dominated region (PDR) to the south of it (Fig. 9). The H2 is distributed around the ionized gas, preferentially at the interface between the HII region and the C34 S cloud core. This confirms that S 235 A [FORMULA] is interacting with the molecular core that hosts the maser and its powering source.

[FIGURE] Fig. 9. Full contours: H2 emission; dashed contours: Br [FORMULA] emission. Contour values are: [FORMULA] for the H2 ; [FORMULA] for the Br [FORMULA]. The positions of the K band point sources S 235 A [FORMULA], S 235 B [FORMULA], and M1 are marked with filled circles; the position of the H2 O maser is marked with a plus sign (note that the symbol is much larger than the position accuracy).

Evans et al. (1981) found far IR emission peaking close to S 235 A, and this was confirmed by the detection of IRAS05375+3540 at approximately the same position of S 235 A [FORMULA] (within [FORMULA]). The primary source for the far infrared emission seems to be the cool dust around the compact HII region S 235 A, which in turn is energized by S 235 A [FORMULA]. Possible contributions to the FIR emission from S 235 B [FORMULA] and from the exciting source of the H2 O maser must be weaker than that of S 235 A.

As already noted in Sect.  2.3, S 235 A is on the edge of the C34 S clump. One might speculate that star formation starts first from the outer edge of the molecular cloud and that star formation inside the molecular peak (as witnessed by the maser and the near IR source, see Sect.  3.7) is induced by the expansion of the S 235 A HII region. This sort of sequential (and possibly induced) star formation from the edges to the core of a molecular cloud has been observed also in other regions (see e.g. the case of the S155/Cepheus B interface, Testi et al. 1995).

According to Tofani et al. (1995), S 235 A [FORMULA] is the least probable candidate for the excitation of the maser. In fact, it is at a large distance from it, 0.29 pc, and the small solid angle subtended from S 235 A [FORMULA] would require a very large stellar luminosity. Also the H2 O maser is far beyond the molecular/HII region interface and beyond the hot molecular hydrogen emission.

3.5. S 235 B

From our observations, the near infrared spectrum of S 235 B [FORMULA] is consistent with a reddened stellar photosphere with a moderate NIR excess (see Fig. 7). The visual extinction appears to be in the range [FORMULA]  mag dependent upon the spectral type and assuming a standard extinction law. This estimate is in agreement with that of Evans & Blair (1981), who derived an extinction of [FORMULA]  mag from the [FORMULA] column density. The presence of the 8.6 and 11.3 µm emission features is also suggestive of hot dust (Krassner et al. 1982). Alternative interpretations of S 235 B have been discussed (Krassner et al. 1982, NY and Tofani et al. 1995), and could be: i) that it is an optically thick UCHII region self absorbed in the radio continuum, ii) or, more probably, that S 235 B is an ionized expanding envelope around a young star, which is optically thick in the radio continuum and moderately thin in the Brackett lines (Panagia & Felli 1975, Simon et al. 1981). Ionized envelopes of this type have also been found associated with low luminosity YSOs.

The overlay of the K image on the H [FORMULA] photo in Fig. 8 shows that the optical nebulosity has a sharp edge to the west and a diffuse tail on the opposite side. S 235 B [FORMULA] is located at the position of the sharp edge, and again suggests a configuration similar to that of S 235 A, in which the star is surrounded by a thick envelope on all sides except a small opening from which ionized gas can freely escape.

The Br [FORMULA] emission from this source is essentially unresolved in our images (i.e. [FORMULA], much smaller than the [FORMULA] size of the H [FORMULA] nebulosity). The integrated line emission is [FORMULA], which is in excellent agreement with the value given by Krassner et al. (1982). The peak of the Br [FORMULA] emission is coincident with the K-band point source.

In the region from which the H [FORMULA] radiation is observed no diffuse (down to 5 mJy at 5 GHz with a [FORMULA] beam, Israel & Felli 1978) nor compact radio emission (down to 0.3 mJy at 8.4 GHz with a [FORMULA] beam, Tofani et al. 1995) has been detected. This excludes the possibility that the H [FORMULA] emission is produced by an optically thin HII region.

If, instead, S 235 B were an ionized expanding envelope, we would expect a radio continuum flux given by:

[EQUATION]

(from Simon et al., 1983). Thus, even assuming [FORMULA], a radio continuum flux of 1.3 mJy at 8.5 GHz is expected. Since such radio emission is not detected, the most plausible explanation is that the expanding envelope is only partially ionized. The part of the envelope closer to the star (from which the optically thin Br [FORMULA] line radiation is produced) is ionized, the outer parts of the envelope, which would emit most of the radio continuum radiation, are neutral. Due to the high extinction toward the source, the ionized envelope is not observed directly in H [FORMULA]: the diffuse emission that we see in this line is probably scattered radiation, which escapes from a hole in the dust cocoon around S 235 B [FORMULA] toward the southeast and is reflected in our direction.

The weak FIR emission from this heavily obscured source implies that it is much less luminous than S 235 A [FORMULA]. Considering the distance from the H2 O maser, it would be difficult for this source to be the energy supply for the maser. In fact, Evans et al. (1981) claim that the FIR luminosity of S 235 B [FORMULA] is less than 10 times that of S 235 A [FORMULA], even though their measurements have been obtained with very large beams and could be contaminated by extended emission as well as by the contribution from a large number of faint sources.

Moreover, considering also its location at the edge of the C34 S cloud, we suggest that this source is in a rather evolved stage, and it is blowing away the parent molecular environment.

3.6. S 235 C

S 235 C is an optically thin HII region coinciding with an optical nebula. A faint star (probably the B0.5 star exciting the radio emission) is detected at the center of the nebula in the Palomar plates (Israel & Felli 1978). Our observations are completely consistent with this picture: we observe (Fig. 1) a point source inside a diffuse nebulosity. Unfortunately our Br [FORMULA] image does not extend so far to the south from the maser position. In Table 3 the NIR photometry of the source is reported, assuming a B0.5 spectral type, the NIR colours are consistent with a visual extinction of the order of 16 magnitudes.

3.7. The H2 O maser

In Fig. 2 a cross is plotted on the K band image at the maser position. There are three weak K band sources near the maser positions (all within [FORMULA]). The sources are reported in Table 3 and in Fig. 7 and are barely visible in the K-band grey-scales presented in Fig. 2 and also visible in the K' image of Hodapp (1994). They are labelled M1, M2, and M3, in order of increasing distance from the maser position (nearly coincident with the C34 S peak). The separation between the maser and the closest NIR source is [FORMULA], corresponding to 44 mpc.

While two of these sources (M2 and M3) show a moderate NIR excess in the colours, like many of the cluster stars, source M1, which is detected only in the K and H images, shows colours very different from all the sources in the field. This is evident from Fig. 7 when compared with the sources detected in all the three NIR bands, but it is also true if we compare M1 with the other sources detected in H and K only, none of which show such large value of the H-K colour index. Its characteristic colours resembles those of the NIR sources revealed in a large survey of NIR sources associated with H2 O masers (Testi et al. 1994; 1996), characterized by values of the H-K colour of the order or greater than 2 and distances between the NIR source and the maser spots of the order of [FORMULA]  mpc.

With the large [FORMULA] implied by the molecular observations the K band emission may be better explained by a hot dust envelope or disk close to the stellar source rather than by intrinsic stellar emission. In fact, if most of the stellar flux is absorbed by dust at high temperature (of the order of 103 K) near to the star and converted into near IR emission, the K band flux can be increased by several orders of magnitude with respect to the "naked" stellar emission.

The high density conditions existing in a cocoon (of the order of [FORMULA]  cm-3) may explain the lack of radio continuum emission because of self absorption effects.

That M1 represents a YSO in a very early phase, responsible for the maser excitation, is also supported by its coincidence with the molecular peak observed in C34 S and the high peak in brightness temperature observed in 13 CO. The first implies very high densities and the second may suggest an increase of temperature in the inner core of the cloud.

The large velocity differences of the maser components with respect to the molecular gas point to highly energetic winds/jets from the central source. Why then we do not see similar high velocities in the molecular gas? Perhaps in these early stages the acceleration occurs only very near to the central YSO and involves only a very small fraction of the molecular cloud gas, with the creation of high velocity and higly collimated jets with intrinsecally very little mass. In other words the transfer of momentum from the collapsing cloud to the surrounding molecular gas has just begun. The interaction of these high velocity jets with the surrounding molecular cloud may give rise to the masers. The high maser variability suggests that these jets are far from being a steady phenomenon (a wind) but are formed by recurrent episodic ejections of small duration.

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Online publication: June 30, 1998
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