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Astron. Astrophys. 350, 659-671 (1999)

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5. Discussion

The previous results indicate that CB3 is a prototype of globule hosting an intermediate-mass star forming process. Previous observations reveal that in CB3 at least two YSOs are located: CB3-mm and CB3-NIR. Moreover, the image obtained in the K band presented by Launhardt et al. (1998) indicates the presence of several young stars in CB3. In this respect, it is worth noting that a recent work of Testi et al. (1999) based on a large sample of NIR images confirms the tendency of early intermediate-mass stars (like for CB3) to be surrounded by dense stellar clusters and that the higher the mass of the newly born stars is, the denser is the associated cluster. In addition, the present observations indicate the CO clouds has a sharp boundary towards south-east which could be an indication of the presence of massive stars in such direction. Among the YSOs in CB3, the youngest is CB3-mm, probably a Class 0 object, who drives a powerful outflow, able to strongly interact with the surrounding ambient material. Actually, assuming a homogeneous spherical clump with density 105 cm-3 (Sect. 4.2) and size 40" (see the H13CO+ results in Sect. 3.2), we derive a total mass of [FORMULA] 550 [FORMULA] and a gravitational selfbinding energy of [FORMULA] = 2.1 1046 ergs. Comparing this value with the kinetic energy of the outflow (5.5 1045 ergs; Sect. 3.1), we can see that [FORMULA]/[FORMULA][FORMULA] 0.26, indicating that the outflow is able to affect the structure of CB3, clearing a significant amount of gas of the clump where the star forming process has occurred.

All the molecular species investigated have been detected through remarkable emission in several transitions, indicating that the CB3 outflow belongs to the class of the chemically active ones like e.g. L1157, following the classification reported by Bachiller & Tafalla (1999). The maps reported in the previous sections show that different molecular species trace different high-density components of the system composed by the molecular core hosting the star forming process and the high-velocity gas of the outflow. The SiO molecule points out the highest velocity jet-like structure closely connected with the mass loss process. On the other hand, SO and CH3OH seem to be associated with more extended regions produced by the interaction of the mass loss itself with the molecular clump. Thus, the present observational results converge to the picture where SO and CH3OH seem to play an intermediate role between SiO and CO: (i) the SiO collimation factor derived from the channel maps is definitely higher than that got from SO data, which, in turn, better reveal the outflow with respect to CO; (ii) while the SiO structure is well aligned along the main axis of the outflow, SO and CH3OH present a bending towards the S2 emission, which shows a maximum displaced to the east direction; (iii) comparing the emission from the S1 clumps (at the end of the blueshifted lobe) due to the highest transitions, the SiO terminal velocity is lower with respect to that got from SO, in agreement with a geometrical effect due to the outflow inclination combined with the exclusive location of silicon monoxide on a jet-like flow.

Comparing the total column densities derived in Sect. 4, we can note that:

  1. SO/H2S [FORMULA] SO2/H2S [FORMULA] 1 (up to a factor 6);

  2. SO/SO2 [FORMULA] 1;

  3. OCS has been detected only towards two positions, where OCS/H2S [FORMULA] 1.

  4. SO/SiO [FORMULA] 1;

These results can be used in order to discuss the evolutionary stage of the CB3 outflow, considering the recent time-dependent model of Charnley (1997, see his Fig. 4), where some abundance ratio such as SO/H2S and SO/SO2 are used as a crude molecular clock. According to his model, SO/SO2 [FORMULA] 1 can occur at [FORMULA] 104 yr as well as definitely later, at about 5 105- 106 yr. It is believed that sulphur is released from dust grains in the form of H2S, which is successively converted in SO and SO2 in a few 104 yr. At the same time, the OCS abundance achieves a maximum in a late phase, at about 105 yr. Thus, the column density ratios found for the CB3 outflow can be explained considering a quite evolved phase, [FORMULA] 106 yr according the Charnley (1997) model, when the bulk of H2S has been already converted in SO and SO2 and the OCS abundance has definitely decreased after its maximum. This result is supported by the low SO/SiO ratio, suggesting that a considerable fraction of SiO has been already removed from the gas-phase by direct accretion onto grains or by converting in SiO2 (e.g. Bergin et al. 1998, Pineau des For[FORMULA]ts et al. 1997). We stress that the present indication about the outflow evolution is exclusively based on chemical considerations and it represents an independent measure of its evolved state, in agreement with the age estimations based on outflow dynamics. In particular, the CB3 outflow seems to be more evolved than the L1157 one, for which Bachiller & Peréz Gutiérrez (1997a) have found definitely lower SO/SiO, SO/H2S and SO2/H2S ratios.

Trying to make a more quantitative discussion, it is worth noting that our age estimations based on CO, SiO and CS data yield values around 104-105 yr, producing a disagreement with the age provided by the column density ratios. The apparent discrepancy would smooth away if the conversion of H2S in SO and SO2 would happen in a time scale shorter than that reported in the model. In other words, our observations and the consequent age estimates suggest that the H2S [FORMULA] SO [FORMULA] SO2 formation sequence proceeds more rapidly by a factor of about one order of magnitude with respect the Charnley (1997) model. This is supported also by the fact that OCS has been detected only at the (0,0) and (0, -10") positions (see Fig. 10), i.e. towards the second generation of clumps (N1, S1), for which the SiO data give an age estimation of about 104 yr; this value corresponds to the age of the OCS abundance maximum in the Charnley (1997) model, once applied the proposed correction.

Finally, it is worth noting that for some molecular species such as SO, SO2, H2S and CH3OH, while the linewidths are large ([FORMULA] 10 km s-1) towards the outflow, the profile becomes quite narrow ([FORMULA] 1 km s-1) towards positions offset from the outflow itself (Fig. 2). This effect has been already found for the SiO molecule in a sample of star forming regions by Codella et al. (1999). The authors discussed the observed linewidths in the context of the outflow evolution: the SiO molecules, whose formation is thought to be associated with shock-chemistry, once produced at high velocities could be slowed down by interacting with the ambient gas before they stick onto the dust grains. Regarding SO, SO2, H2S and CH3OH, it is not well understood the relative importance of the chemistries leading to the their formation (ion-molecule reactions, grain surface reactions, neutral-neutral reactions in shock waves). However, it is interesting to note that if their formation would be a consequence of a shock chemistry, narrow linewidths could be caused by an evolutionary effect and the narrow emission could be a tracer of old shocked gas, dispersing in the neighbourhood of the outflow.

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999
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