Astron. Astrophys. 325, 542-550 (1997)

3. Individual regions

In this section we discuss four individual regions in detail because there we have detected new mm-continuum sources. Descriptions of the other regions in Table 1 can be found in Reipurth (1994).

3.1. HH 114

The molecular shell surrounding the young massive star Orionis is a region rich in newborn stars and associated Herbig-Haro objects. Among the outflows is a giant HH complex, HH 114-115, comprising two bow shocks with a separation of 2.4 pc. The two HH objects are located on either side of a Class I FIR source IRAS 051555+0707 (Reipurth et al. 1997).

We have mapped the source region at 1300 µm, and detected IRAS 05155+0707, see Fig. 1. In addition, we have discovered another, hitherto unknown source 1.5 , to the north-west, which we here call HH 114 MMS. As discussed in Sect. 2, this new source is not detected by IRAS, and suspecting that we here have a protostellar candidate, we obtained the submm photometry from the JCMT listed in Table 2. In the following we discuss the nature of HH 114 MMS based on these data.

Fig. 1. 1300 µm continuum map of the region around HH 114, displaying the IRAS source and new mm source. Contour levels are -80, 80 to 400 by 80, 560, 720 mJy. Coordinates are epoch 1950.

The spectral energy distribution of HH 114 MMS is shown in Fig. 2. As a first guess one may assume that the emission arises from dust at a single temperature. In this case we can fit the spectrum by a greybody of the form

where is the solid angle of the emitting region, is the Planck function and is the optical depth approximated by ; denotes the critical frequency where the optical depth is 1. The best fit to the data is shown by the solid curve in Fig. 2 and corresponds to and = 28 K. The -value is rather well determined by the data at longer wavelengths whereas the short submm wavelengths together with the IRAS 60 µm limit constrain the temperature to a certain degree. This extraordinary flat spectrum may originate from dust at a range of temperatures where part of the emission is optically thick and/or from dust grains with unusual optical properties. In any case the spectral properties of this source are very similar to the protostellar condensation HH 24 MMS (Chini et al. 1993, Ward-Thompson et al. 1995).

Fig. 2. Spectral energy distribution of HH 114 MMS. The solid curve is a greybody fit with a dust temperature K and .

In order to determine the evolutionary stage of HH 114 MMS we calculate the bolometric luminosity as the energy output from 12 to 2000 µm; the submm luminosity is the energy emitted at µm. As suggested by André et al. (1993), a ratio is characteristic for Class 0 sources. In the case of HH 114 MMS we can only derive upper limits for and because of the upper limits from IRAS. But even taking the numbers in Table 2 at their face value we obtain L and L and thus , indicating that the source must be still in an early protostellar phase.

Under the assumption of optically thin emission we may estimate the total amount of mass associated with HH 114 MMS from the flux density at 1300 µm according to

As outlined by Ward-Thompson et al. (1995) the dust properties in protostellar sources are likely to be different than those in the diffuse interstellar medium, resulting in larger-than-normal fluffy grains. We therefore use an enhanced mass absorption cross section cm² per gram of interstellar matter. The total mass is then 5.5 M at an assumed distance of 460 pc.

Following our discovery of HH 114 MMS, we mapped the region at 3.6 cm at the VLA, and in addition to detecting IRAS 05155+0707, we found a source with a flux of 0.13 mJy only 4.4 , from our mm position of HH 114 MMS, which is within the positional errors (Rodriguez & Reipurth 1996). The VLA position is 05:15:33.22 = 07:08:55.6. So HH 114 MMS can be added to the growing list of Class 0 sources, which are detectable at centimeter wavelengths. The 3.6 cm flux lies a factor of 4 above the extrapolation of the fit in Fig. 2. At present it is impossible to distinguish whether this means a flattening of the dust emission spectrum at mm and cm wavelengths or simply involves a new emission component originating from a shock front.

The question arises which of the two sources, HH 114 IRAS or HH 114 MMS, is the driving source of the giant HH 114-115 complex. Unfortunately, the two sources lie more or less on a northwest-southeast line, which is also the direction of the HH flow, so on geometric grounds we cannot favor one over the other.

3.2. The HH 1-2 region

Since their discovery by Herbig (1951) and Haro (1952), the Herbig-Haro objects HH 1 and HH 2 have been extensively studied and have emerged as the prototypical bipolar HH flow (see, e.g. Solf et al. 1989 and references therein). They are driven by a deeply embedded source, VLA 1, detected by Pravdo et al. (1985) and with a far-infrared luminosity of about 50 L (Harvey et al. 1986). Recently, a new collimated HH flow, HH 144, was discovered emerging at a large angle to the HH 1-2 flow from a source, VLA 2, displaced by only 3 , from VLA 1, suggesting that they form a young binary (Reipurth et al. 1993b).

The region around the VLA 1/2 sources is rich in young objects. The central star is an optically visible T Tauri star (Cohen & Schwartz 1979), an H₂ O maser was found south-west of HH 1 (Lo et al. 1975, Haschick et al. 1983), an infrared source is associated with HH 3 (Roth et al. 1989), and a number of IRAS sources were found by Pravdo & Chester (1987).

Our 1300 µm continuum map is shown in Fig. 3. It reveals the presence of a number of sources; their positions and fluxes are listed in Table 1. Reipurth et al. (1993a) detected a strong mm/submm source (S₈₇₀ = 1672 mJy, S₁₃₀₀ = 645 mJy) at the position of the VLA source. It is seen in Fig. 3 as a slightly elongated source, MMS1, and is the brightest one in the map. In addition to this, the map shows a ridge, 17 , SSW of MMS1. At our spatial resolution of 11 , MMS1 and the ridge are not fully separated but being present in each of our individual maps, the reality of the ridge is beyond doubt. MMS1 and the ridge are embedded in a faint extended halo.

Fig. 3. 1300 µm continuum map of the region around HH 1-2. Contour levels are -30, 30 to 270 by 30, 270 to 450 by 60 mJy. Coordinates are epoch 1950.

The position of MMS1 is 6 , SSE of the VLA 1 radio position. The internal accuracy between our individual maps is 1.5 . Likewise, maps of the region from previous observing runs show a similar offset so that we have no evidence to attribute this difference to a pointing error. Nevertheless, taking into account our pointing accuracy we cannot exclude that MMS1 and VLA 1 (plus VLA 2) are the same object.

MMS1 and its SSW ridge are located in a dense cloud core detected in NH₃ by Torrelles et al. (1985), in CS by Cernicharo (1991) and in H₂ CO by Davis et al. (1990). More detailed observations of the NH₃ (1,1) and (2,2) transitions show an intricate morphology with an overall elongated shape perpendicular to the HH 1-2 flow axis and a complex kinematics (Martin-Pintado & Cernicharo 1987, Marcaide et al. 1988, Rodriguez et al. 1990a).

MMS1 is situated centrally in the dense elongated molecular core in what appears to be an evacuated region. The SSW ridge, on the other hand, is found towards one of the densest parts of the south-western ammonia lobe. The halo seen in the continuum at 1300 µm coincides rather well with this south-western lobe. It thus appears that the ridge could represent a secondary star formation event in the dense material left over from the formation of the VLA 1 and 2 system. Whereas both of these VLA sources actively drive HH flows, there is no evidence for HH objects driven by sources in the ridge (Reipurth et al. 1993b).

A two-dimensional Gaussian fit to MMS1, after subtraction of the ridge, shows that it is marginally resolved, with major and minor axes of 17.4 , and 15.1 , respectively, and a position angle of the major axis of 298 ^{Paper I}. At 460 pc distance, this corresponds to a major axis of 8000 AU. If real, this suggests that MMS1 still has an extended dust envelope surrounding it, presumably a vestige of the collapsing cloud fragment which formed the young stars VLA 1 and 2. The spectral energy distribution measured by Reipurth et al. (1993a) suggests a dust temperature of K and a of ; the dust envelope is thus warmer than expected for a true protostar. We also note that the position angle of the minor axis is close to the position angle of 148 degrees for the HH 1-2 flow, which leads us to speculate that the dust envelope is either flattened, or the "top and bottom" have been blown off by the collimated outflow.

In addition to MMS1, Fig. 3 shows another source (MMS2) about 80 , to the northwest. In this region, Lo et al. (1975) and Haschick et al. (1983) found an H₂ O maser. VLA observations have revealed two 6 cm radio continuum sources here, one is coincident with the H₂ O maser (Pravdo et al. 1985), and the other, only 5 , further southwest, shows pronounced time variability (Rodriguez et al. 1990b). A near-, mid- and far-infrared source detected by Harvey et al. (1986), with a total luminosity of 70 L , is coincident with the H₂ O maser source, and was imaged in the near-infrared by Roth et al. (1989). Our continuum source, MMS2, is displaced by about 5 , to the SE from the two VLA sources, similar to MMS1 and VLA 1. The NH₃ map of Rodriguez et al. (1990a) and the CS observations of Cernicharo (1991) show that MMS2 is located close to a region of high density. Indeed, the more detailed NH₃ map in the (2,2) transition of Torrelles et al. (1993) shows both sources at the center of a small dense cloud core.

A slightly fainter source, MMS3, is situated 20 , SSW of MMS2, in the center of the extended NH₃ cloud observed by Rodriguez et al. (1990a). There is no previous evidence for star formation in this region.

In the upper NE corner of Fig. 3 we find another 1300 µm source. In this region there is a faint HH object, HH 147, (Eislöffel et al. 1994), which moves away from a faint T Tauri star (No. 3 of Strom et al. 1985). This star coincides precisely with our 1300 µm source, here called HH 147 MMS.

3.3. HH 108-109

The two HH objects HH 108 and 109 are located at the edge of a highly opaque sharp-edged cloud in Serpens (Reipurth & Eiroa 1992). They each have some morphological features which suggest that they may be bow shocks facing away from a source to the north-east. In that direction one finds the Class I IRAS source 18331-0035, with a projected separation of only 0.12 pc and 0.21 pc from HH 109 and HH 108, respectively, at the assumed distance of 310 pc. The axis of the two HH objects is very well aligned with a line through the IRAS source, which led Reipurth & Eiroa (1992) to conclude that the IRAS source was a good candidate for the driving source of the two HH objects.

Our 1300 µm map of the HH 108-109 source region is shown in Fig. 4. Two well defined sources are visible, the brighter with a total flux of 528 mJy and the fainter with 228 mJy. The angular distance between the two sources is 71 , corresponding to a projected separation of only 0.11 pc, or 22000 AU. The center of the uncertainty ellipse of IRAS 18331-0035 coincides within a few arcseconds with the position of the brighter mm-source, and they are without doubt identical. The fainter object, which we here call HH 108 MMS, is a new source in the region. We do not have submm photometry of this new source, but the fact that it is not seen by IRAS, and does not even cause a slight shift in the IRAS position of the nearby mm source, suggests that the new object is much fainter at IRAS wavelengths, possibly because it is colder than the IRAS source. We speculate that the new source is similar to HH 24 MMS and HH 114 MMS and may be a protostellar object.

Fig. 4. 1300 µm continuum map of the region around HH 108-109 displaying the IRAS source and new mm source. Contour levels are -25, 25 to 150 by 25, 200, 250 mJy.

At present it is unclear which of these two sources is the more likely object to be driving the nearby HH objects because both lie close to the line defined by the two HH objects. Thus, further observations, for example in molecular hydrogen or in CO are required to identify the driving source.

3.4. HH 7-11

The Herbig-Haro objects 7-11 are among the best studied such objects (e.g. Solf & Böhm 1987). They are located in the NGC 1333 region, which is rich in young stars and HH objects (e.g. Strom et al. 1976, Aspin et al. 1994, Bally et al. 1996), and at a distance of about 350 pc (Herbig & Jones 1983). The young visible star SVS 13 has long been assumed to be the driving source. A high velocity molecular outflow also streams from the source (e.g Koo 1989). Far-infrared mapping by Cohen et al. (1985) showed an elongated circumstellar structure.

The region has been mapped in the mm continuum by several groups. Grossman et al. (1987) did 2.7 and 3.1 mm aperture synthesis observations and detected two sources, one approximately coincident with SVS 13, and another about 15 , further southwest, also known as SVS 13B. Some controversy has arisen over the reality of this second source, since Woody et al. (1989) failed to detect it with interferometer observations at 1.4 mm, and similarly Sandell et al. (1990) did not detect it with single dish observations at 800 and 1100 µm.

In Fig. 5 we show our 1300 µm map of the region. We clearly see two well defined sources, separated by about 12 , or 4200 AU in projection at 350 pc. These sources coincide within a few arcseconds with the NE and SW sources of Grossman et al. (1987), which is within the positional uncertainties, and are certainly identical with these two sources, thus confirming their reality. As doubt is emerging that SVS 13 can really drive the powerful optical and molecular outflows in the region, we prefer to call these two sources for HH 7-11 MMS1 and 2, rather than SVS 13 and SVS 13B as done previously. It seems to us possible, and even likely, that SVS 13 is simply one of the multitude of young already visible and therefore more evolved young stars in the region, while MMS1 and 2 are deeply embedded, possibly protostellar,sources.

Fig. 5. 1300 µm continuum map of the region around HH 7-11. Contour levels are -100, 100 to 400 by 100, 400 to 1200 by 200 mJy. Coordinates are epoch 1950.

In addition to MMS1 and 2, we find yet another, albeit weaker source further to the southwest, MMS3, which approximately coincides with the location of a 6 cm radio continuum source (, ) and an H₂ O maser (, ) (source B of Haschick et al. 1980). It would be of great interest to observe these three embedded sources at submm wavelengths to ascertain their evolutionary stages.

In a recent study, Rodríguez et al. (1997) have performed sensitive, high angular resolution VLA observations at 3.6 cm and 6 cm. Their source VLA 4 precisely coincides with SVS 13. About 6 , SW of SVS 13 they detect a new VLA source, VLA 3, which aligns better with the HH flow axis than SVS 13. VLA 3 is itself elongated along the flow axis. Altogether VLA 3 is a more probable source of HH 7-11 than SVS 13. Rodríguez et al. (1997) also detect (their VLA 2) the above mentioned VLA source first noted by Haschick et al. (1980).

With our 1300 µm beam size of 11 , and a pointing accuracy of about 3 , it is not easy to compare the new VLA sources with our map. Although none of the mm and VLA sources precisely coincide, it is suggestive that they are related: An offset of 3 , WNW of our map leads to a perfect alignement of VLA 2 with MMS3 and VLA 3 with MMS1. MMS2 has no VLA counterpart. The coincidence of a strong mm continuum source with VLA 3 supports the suggestion by Rodríguez et al. (1997) that iy is VLA 3, rahter than SVS 13 (= VLA 4) which drives the HH 7-11 flow.

© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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