3.1. The CO outflow
Fig. 1 shows the channel map of the J = 2-1 CO emission, centered (as well as the other ones presented in this paper) at = , = +56o 25´ 32", close to the position of the 1-1300 µm continuum source (hereafter CB3-mm, pointed out by a black star), which is located at (, ). It is worth noting that Launhardt et al. (1998) have recently presented a deep NIR (K band) image showing a number of very red stars located near the position of CB3-mm. Moreover, NIR images of CB3 reported by Yun & Clemens (1995) show a source (designated as CB3-NIR, pointed out by a filled triangle) located at about 32", while IRAS00259+5625 is located in an intermediate position with respect to the NIR and mm objects (18" east of CB3-mm), maybe collecting contributions from both sources at IRAS wavelengths.
The ambient LSR velocity emission (see Sect. 3.2) is centered around the -38.5 km s-1 panel, underlined by the thicker box: the strong self-absorption in the CO spectra (see e.g. Fig. 2) does not allow to define the molecular structure at this velocity. On the other hand, a CO outflow is clearly detected, confirming the result of Yun & Clemens (1992, 1994). From Fig. 1, it is possible to see that: (i) high-velocity blueshifted emission is located in a jet-like structure located along the north-south direction with CB3-mm at one of its extremes, (ii) some redshifted emission which seems emanating from CB3-mm is also detected, (iii) there is a significant spatial overlap between the blue and red emissions. We conclude that, among the two different objects detected in the CB3 globule, CB3-mm is the driving source of the CO outflow .
The outflow structure is clearly detectable already at low velocity 1-2 km s-1 with respect to the ambient one, suggesting that the bulk of the CO emission is due to gas interacting with the mass loss from the central object. The CO outflow is about 1 pc length with a rough estimate of the collimation factor (ratio between length and width) of 2-3. In order to definitely characterize the main outflow structure, this has been carefully sampled using measurements spaced every 5".
The CO channel map reveals different clumps along the main axis of the outflow, suggesting that episodic increases of the mass loss process from the central object have occurred. In particular, the southern lobe clearly shows two clumps, one at (0", -30"), designated as S1, detectable at velocities bluer than -47 km s-1 and in the -37, -33 km s-1 range and another one, S2, at (0", -10") and about -40 km s-1 with some red emission around -35 km s-1. S1 is thought to be older than S2, since it is more distant from the central star. The northern CO lobe has a clump (hereafter N2) at (0",+10") at about -40 km s-1 and -34 km s-1. One could expect to find another clump in the northern lobe, i.e. a sort of symmetric emission with respect to S1 and the central object. Actually, although there is only a hint of such a compact emission in Fig. 1 (which could be attributed to the confusion due to the interaction of the flowing gas with the ambient medium), we detect the N1 clump in the SiO and SO channel maps (see Sects. 3.3 and 3.4). In summary, we find four compact structures (pointed out in the channel maps): S1 (0", -30"), S2 (0", -10"), N2 (0",+10") and N1 (0",+40").
From Fig. 1 it is also possible to infer for the outflow geometry an intermediate situation between a small inclination to the plane of the sky (which would lead to a definite spatial separation between blue and red emissions) and a complete location of the main axis along the line-of-sight (which would imply a complete overlap between the two lobes). For this reason, we assume a value of 30o as the angle between the direction of the outflow and the plane of the sky.
Fig. 2 reports examples of molecular line profiles observed towards positions in the CB3 outflow and towards a position offset from the main outflow axis. Comparing the CO J = 1-0 and J = 2-1 spectra, plausible estimates of opacity and excitation temperature can be obtained. Two positions in the blue and the red lobes have been considered and, to allow the comparison between the spectra, the different HPBWs of the two transitions have been taken into account, convolving the J = 2-1 spectra to the spatial resolution of the J = 1-0 transition (HPBW = 21"). Indeed, the convolved J = 2-1 spectra show a weaker peak emission, according to dilution, since the high-velocity clumps are smaller than the beam (see Fig. 1). On the other hand, the temperature of the self-absorption feature remains the same, indicating that the cool gas responsible of the absorption is located in a quite extended region.
The ratio R between the observed brightness temperatures of the CO J = 2-1 and J = 1-0 transitions gives values of 0.6-0.7 for the optically thick emission at low velocity, indicating an excitation temperature of 10 K (see e.g. Fig. 12 of Levreault 1988). As already found for well known molecular outflows, the value of R increases monotonically with velocity. This can be due to a variation of the excitation temperature with the velocity or, more probably, it can be an indication that the CO emission is thinner at higher velocities, where a smaller amount of flowing gas is usually observed. At the highest CO velocity, R is about 1.3-1.4. These values can be explained by an optically thin emission and a 10 K, but, taking into account the thick emission at low velocity, it seems more realistic to assume for the high-velocity gas a typical excitation temperature of 15-20 K (e.g. Levreault 1988) and consequently get an optical depth of 1.5.
Using the standard procedure reported e.g. by Lada (1985), the dynamical parameters of the CB3 outflow have been obtained. Following our results mentioned above, we assume an excitation temperature of 20 K, an optical depth of 1.5 and a relative CO abundance of 10-4 (and an inclination of 30o to the plane of the sky). A total mass of M = 4.0 has been calculated by integrating over the velocity and the entire spatial extent of the outflow (2.4 for the blue lobe and 1.6 for the red one). The momentum results P = 37.2 km s-1 and the kinetic energy = 5.5 1045 ergs. A dynamical time scale of the flow of about 5 103-1 104 yr has been deduced. This has allowed to estimate the force required to drive the outflow, F = 5.3 10-3 km s-1 yr- 1, the mass loss rate, M_ = 3.2 10-4 yr-1, and the mechanical luminosity, = 5.6 . These values show a discrepancy with those calculated by Yun & Clemens (1994) which can be attributed to the fact that the authors adopted a distance of 600 pc, an optical depth of 3 and applied no correction for the effect of the orientation of the outflow relative to the line of sight.
Fig. 3 reports the brightness distribution with the velocity from the central source in each of the lobes, which can be considered a good estimate of the distribution of flow mass in the case of optically thin emission. In the case of a constant optical depth along the wing emission, thicker emission would affect the distribution just moving down the points in the vertical axis by a same quantity, leaving unchanged the plot shape. Fig. 3 shows that for velocities in the range 1-10 km s-1 the distribution appears to be well described by a power-law with -1.6, while at higher velocities the slope becomes steeper ( -7.0). This result is well in agreement with the velocity profiles found for other outflows like, e.g., L1448, OrionA, MonR2 and NGC2264G (Tafalla 1993, Lada & Fich 1996, Bachiller & Tafalla 1999and references therein), where a break-point in the brightness-velocity distribution is clearly present. This could reflect the occurrence of high-velocity gas slowing down because the interaction with the ambient medium and consequently producing a deflection in the power-law slope. In this case, we expect younger outflows to have faster gas than more evolved ones, and, thus, to have the change of the slope located at lower velocities (see Fig. 10 of Bachiller & Tafalla 1999). In other words, the spectra should evolve in time due to the increase of the relative fraction of slow material with respect to the fast moving gas. In the case of CB3: (i) the distribution looks very similar to that of the NGC2071 outflow and (ii) the red lobe presents the break-point at a slightly lower velocity (5-6 km s-1) with respect to that of the blue emission (10-12 km s-1), suggesting that the blue lobe is strongly interacting with the ambient gas. This is in agreement with the CO channel map, where the red component is confined in a region smaller than that of the blue one. Moreover, the comparison between the two distribution (red and blue) of Fig. 3 suggests that the red emission is slightly thicker than the blue one. It is worth noting that no correction due to the outflow geometry has been taken into account drawing Fig. 3. However, this correction can affect the distribution moving all its points of a small factor ( 1.3, due to the sine of the inclination) in the logarithmic velocity scale, without altering the considerations reported above.
In conclusion, the velocity profile and the comparison of the obtained dynamical flow parameters with the values taken from the literature (e.g. Cabrit & Bertout 1992and references therein), place the CB3 outflow close to others driven by 102-103 sources like, e.g., HH7-11 or NGC2071.
3.2. H13CO+ and CS emission
The H13CO+ spectra (see Fig. 2) show narrow lines located at about -40 km s-1, and hence they can be used as a tracer of the high-density ambient gas. Unfortunately, because of the poor velocity resolution (3.4 km s-1), the present H13CO+ spectra add no further dynamical information about the clump neighbouring CB3-mm. Fig. 4 shows the map of the integrated H13CO+ J = 1-0 flux: a quite rounded clump roughly centered around CB3-mm is clearly present, in agreement with the C18O map of Wang et al. (1995). Its size is of about 40" (FWHP) which corresponds at 0.5 pc, and should represent the remnant of the high-density clump where the star forming process has occurred.
The CS molecule is another well known tracer of high-density gas, which can be used as a diagnostic of the occurrence of infall motion around a YSO (e.g. Zhou et al. 1993, Zhou 1995, Mardones et al. 1997, Tafalla et al. 1998). We observed the J = 3-2 line trying to get information from the high excitation inner part of the clump.
The CS profiles observed towards positions slightly offset from the central one (and from the outflow, in order to avoid confusion due to self-absorption and to the presence of high-velocity gas, like e.g. in Fig. 2), an ambient LSR velocity between -38.4 and -38.2 km s-1 (depending on the map position) can be derived. These values are in agreement with the LSR velocities obtained using C18O (Wang et al. 1995) and NH3 (Lemme et al. 1996) data.
The CS spectra reported in Fig. 2 show that there exists a self-absorption feature which always shows a brighter blue peak and a fainter red peak and which is spatially concentrated around the YSO, consistent with the presence of infall motions. However, when a molecular outflow is present, like in CB3, such profiles cannot be unambiguosly attributed to infall because the spectra can be contaminated by asymmetries or secondary peaks produced by high-velocity emission. Nevertheless, the present CS results make of CB3-mm a good candidate for collapse and for being a Class 0 source, in agreement with the strong 1-1300 µm emission (Launhardt et al. 1997), confirming the suggestion of Wang et al. (1995) based on H2CO observations and calling for high resolution observations in order to further verify this assumption.
Fig. 5 reports the channel map of the J = 3-2 CS emission. At velocities near the ambient one, a CS clump is located close to CB3-mm. Its size is unresolved and therefore 0.19 pc. Moreover, it is possible to see that in CB3 the CS emission is not coming just from the ambient medium, but also from higher velocity gas. In particular, Fig. 5 shows that CS mainly maps the southern lobe, tracing the outflow motion at low velocities: 6 km s-1. This justifies the cautions used discussing the CS spectra profiles looking for infall signatures and shows that in CB3 CS can also be used to get information about the molecular outflow, at least of the slower component, in agreement with the result about the L1157 outflow reported by Bachiller & Pérez Gutiérrez (1997b). In any case, CS results unsuitable to investigate the highest velocity component of the molecular outflow, calling for the analysis of standard tracers as CH3OH and SiO.
3.3. CH3OH and SiO emission
The maps of the integrated CH3OH fluxes (Fig. 6) shows that methanol has been detected only towards the region associated with the molecular outflow and it is distributed in a structure elongated along the main axis. As the angular resolution improves, moving from 3 to 1mm, the structure assumes a jet-like configuration containing clumps in its interior. The map of the 50-40A+ emissions, i.e. of the strongest component of the 1mm group of transitions (investigated with a HPBW of 10"), is also shown in Fig. 6. Since the outflow gas is already detectable at quite low velocity (see Sect. 3.1), in order to plot the red and blue emissions, the velocity profile has been divided in two, at = -38.3 km s-1. The 50-40A+ map clearly shows the outflow, pointing out the S1, S2 and N2 clumps, with a collimation factor of 3, i.e. similar to that derived from the CO J = 2-1 emission. It is worth noting that the - emission (and in particular, the 50-40A+ one) shows that the CH3OH jet-like structure slightly bends to the east direction (by an amount of 10", i.e. one HPBW) where the S2 emission is present. This effect will be discussed in Sect. 5, once shown the results regarding all the molecular species.
The methanol emission is very weak at positions offset from the outflow (Fig. 2). On the opposite, the emission is spectacularly enhanced along the outflow, showing very bright and broad lines ( 10 km s-1, FWHM), and confirming that the CB3-mm outflow belongs to the class of the chemically active ones (Bachiller et al. 1998, Bachiller & Tafalla 1999)).
Fig. 6 reports also the emission due to the three observed SiO transitions. Even in this case, the outflow structure is clearly detectable. Like CH3OH, SiO is present only along its main axis, whose emission is represented by broad profiles ( 8-10 km s-1; Fig. 2). Thus, also for SiO, the present data confirm its enhancement in young molecular outflows and its validity as a tracer of the YSO mass loss (e.g. Martín-Pintado et al. 1992, Bachiller 1996, Codella et al. 1999). The collimation factor in SiO seems to be higher than that given by methanol emission, suggesting that SiO and CH3OH can trace gas with slightly different physical conditions (see Sect. 5).
Also in this case, the S1, S2 and N2 clumps are detected, but, on the contrary of CH3OH, Fig. 6 shows a weak emission also from the N1 clump. In order to investigate this effect and the outflow kinematics, we present in Fig. 7 the velocity channel maps of the SiO J = 5-4 emission (i.e. that investigated with the best spectral and angular resolution). These show that the SiO emission extends in velocity up to -10 km s-1 (in the blue part) and +3 km s-1 (in the red one). Comparing these velocities with those of CO (Fig. 1) and CS (Fig. 4), we see that: (i) for the blueshifted emission, the terminal SiO velocity is slightly higher than that of CS and lower than the CO one; (ii) for the redshifted one, the highest SiO velocity is surprisingly smaller than that of CS (and CO). Also these different behaviour in the velocity patterns will be discussed in Sect. 5. It is worth noting that also Fig. 7 clearly demonstrates that CB3-mm is the driving source of the molecular outflow and that episodic mass loss has occurred. Moreover, the SiO channel maps allow to emphasize the high-velocity jet-like structure, pointing out the four (N1, N2, S2, S1) clumps, whose size is, for this molecule, less than 0.1 pc. The estimate of the collimation factor from the SiO channel map leads to a value 7 (a length of about 0.85 pc and a width less than 0.12 pc) confirming the indication of Fig. 6 and showing that SiO, with respect to CO, traces the inner part of the outflow (see Sect. 5).
By comparing the positions of the different clumps located along the main axis of the outflow and assuming that the material traveled from the center to its present location with a typical velocity of 2 km s-1, it is possible to give a rough estimate of their age (corrected for the projection effect): 3 104 yr for the N2-S2 emissions and 105 yr for the N1-S1 ones. The latter value give an estimate of the age of the SiO outflow, while the time elapsed between the two ejections of clumps is consequently thought to be about 7 104 yr. Taking into account the large uncertainties given by the assumptions made before (above all, the geometry and the selection of a constant velocity for each clump), this age estimate reasonably fits with that derived from CO, even if the latter yields an outflow younger by a factor 4 with respect to the SiO analysis, probably due to the lower velocity spectra of the silicon monoxide.
3.4. SO emission
The three observed SO lines show different profiles: while the = 43-32 and 65-54 transitions exhibit broad ( 7-10 km s-1) lines and high-velocity wings, the = 22-11 one is detected through lines with definitely smaller linewidths (1-2 km s-1). This indicates that the latter transition, with respect to the other ones, traces different physical conditions, and in particular gas at lower velocity. Moreover, it is worth noting that, as also shown in Fig. 2, while the SO 43-32 and 65-54 profiles as measured along the outflow are broad, those taken towards the positions offset from the main axis reports quite narrow linewidths ( 1 km s-1). Fig. 8 reports the contour maps of the integrated SO emissions. The 22-11 panel shows a clump located near CB3-mm which looks similar to that traced by H13CO+ and which is anyway definitely different from the CH3OH and SiO structures. In other words, the 22-11 SO transition results to be more a tracer of the high-density medium around the YSO rather than a tool to investigate high-velocity gas connected with the mass loss. On the other hand, the 43-32 and 65-54 ones are able to trace the outflow, revealing its compact structures. In particular, Fig. 8 suggests that the three observed lines of SO trace different part of the star forming region: from the ambient (at 3mm) to the outflow (with the highest excitation transition, at 1mm), passing through a complex distribution at 2mm probably due to a superposition between a quite extended structure spreading over the north-west direction, like in the CO maps, and the molecular outflow.
In order to investigate the outflow kinematics as traced by SO, we present in Fig. 9 the velocity channel maps of the 65-54 emission, which, for the spectral and spatial resolution, can be directly compared with the SiO 5-4 distribution. Fig. 9 shows that: (i) also in SO the jet-like structure driven by CB3-mm and the four clumps are clearly detected, (ii) the outflow structure bends to the east direction, where the S2 emission is present, in agreement with the methanol maps (but not with the SiO ones), (iii) in the blueshifted lobe the terminal velocity is at about -10 km s-1 like for SiO, while in the red part, SO extends up to -7 km s-1, i.e. at velocities higher than those got from the SiO maps (see the discussion in Sect. 5).
The collimation factor from the SO channel map is 4.5, i.e. a lower value compared to that of the SiO jet-like structure ( 7). Moreover, it is worth noting that (i) the estimation of clump size obtained through SO gives a value ( 0.15 pc) larger than that obtained by SiO data and that (ii) even for the 65-54 SO emission, there is the indication of a weak extended structure offset from the outflow axis. Finally, the comparison between the SO and SiO channel maps allows to see that the N1 emission comes from different positions depending on the observed molecule: (+10",+40") for SO and (-10",+40") from SiO. All these indications suggest that the 65-54 SO and the 5-4 SiO transitions trace different part of the jet-like structure, with the SO probably coming from a more extended region at lower excitation with respect to SiO.
3.5. Other molecular species
The spectra of the other observed S-bearing molecules (SO2, H2S and OCS) reported in Fig. 2 show that the emission due to these species is definitely enhanced in the outflow. In particular, OCS has been detected only towards two positions near the S2 emission. Fig. 8 reports the maps of the integrated SO2 313-202 and H2S 101-001 emissions, obtained sampling the main outflow axis and the region in the noth-east direction, where, as the previous results have shown, is located gas at ambient velocity. In these maps, the molecular outflow is clearly detected. Moreover, the comparison between the SO2 and H2S distributions shows that the latter molecule is mainly located near CB3-mm, around the N2-S2 emissions, while SO2 is enhanced also at the position of the S1 clump. Finally, Figs. 2 and 8 shows that also H2CO is spectacularly enhanced along the outflow axis and that it allows to trace its structure, specially pointing out the S1 structure. The different behaviour of the three observed sulphuretted species and of formaldehyde suggests that along the main outflow axis there are different chemical situations, maybe reflecting the evolutionary stage of the molecular outflow (see discussion in Sect. 5).
© European Southern Observatory (ESO) 1999
Online publication: October 4, 1999