Volume 534, October 2011
|Number of page(s)||23|
|Section||Stellar structure and evolution|
|Published online||20 October 2011|
Appendix A: Other detected transitions
Besides the molecular species discussed in Sect. 5.1, the sources I18102 MM1, I18151 MM2, I18182 MM2, and I18223 MM3 show emission lines also from other species, some of which are complex organics. These include o-c-C3H2, C2D, C18O (seen in the image band), and possibly CNCHO (in the image band), CH3NH2 (blended with OCS), CH2CHC15N, and CH3COCH3. In this appendix we discuss each of these species (except C18O) and their derived column densities and abundances.
Cyclopropenylidene (c-C3H2). We have detected the ortho form of cyclic-C3H2 in four sources, and found the column densities and abundances in the range ~7.0 × 1012 − 7.7 × 1013 cm-2 and ~1.9 × 10-10 − 1.2 × 10-8, respectively. The c-C3H2 molecule has been detected in several different Galactic sources, but to our knowledge, this is the first reported detection of this molecule towards massive clumps associated with IRDCs. For comparison, the beam-averaged c-C3H2 column densities and abundances of ~4−9 × 1013 cm-2 and 3−7 × 10-11 have been found in Sgr B2 (Turner 1991; Nummelin et al. 2000). These column densities are comparable to those in our clumps, but the abundances appear lower in Sgr B2.
The major formation channel of c-C3H2 is ; (e.g., Park et al. 2006). The very low rotational excitation temperatures we derived, 3.1–4.8 K, suggest that c-C3H2 emission mainly comes from the cool and relatively low-density envelope (Turner 1991). Also, higher electron abundance in the outer layers can promote the formation of c-C3H2. This is consistent with the derived high fractional abundances.
Cyanoformaldehyde or formyl cyanide (CNCHO). The CNCHO transition is possibly seen in the image sideband towards I18102 MM1. Because of attenuated intensity, we cannot determine the column density of this molecule. Remijan et al. (2008) determined the values N(CNCHO) = 1−17 × 1014 cm-2 and x(CNCHO) = 0.7 − 11 × 10-9 towards Sgr B2(N). They suggested that CNCHO is likely formed in a neutral-radical reaction of formaldehyde (H2CO) and the cyanide (CN) radical.
Methylamine (CH3NH2). Rotational levels of CH3NH2 undergo the so-called A- and E-type transitions due to the internal rotation of the CH3-group. We have a possible detection of the E-type CH3NH2 transition, and we derived the values N(CH3NH2) = 5 × 1014 cm-2 and x(CH3NH2) = 5.1 ± 0.6 × 10-8. However, our CH3NH2 line is blended with OCS line, and therefore these values should be taken with caution. Beam-averaged column densities of N(CH3NH2) ~ 1 × 1014 − 8 × 1015 cm-2 have been found in Sgr B2 (Turner 1991; Nummelin et al. 2000). The origin of CH3NH2, and nitrogen-bearing organics in general, could be in the evaporation of ice mantles, indicative of a hot-core chemistry (e.g. Rodgers & Charnley 2001a).
Carbonyl sulfide (OCS). The high-J transition of OCS we have possibly detected (blended with CH3NH2) implies a column density and fractional abundance of 2.3 ± 0.8 × 1015 cm-2 and 2.4 ± 0.9 × 10-7, respectively. For example, Qin et al. (2010) detected OCS in the high-mass star-forming region G19.61-0.23. Assuming Trot = Eu/kB, they derived the beam-averaged column density and fractional abundance of 2.2 ± 0.1 × 1016 cm-2 and 2.7 ± 0.1 × 10-8, respectively. Note that Qin et al. achieved a much higher angular resolution with their SMA observations, and thus their high column density value could partly be caused by filtering out the extended envelope. Our coarser angular resolution probably causes a significant beam dilution. Sakai et al. (2010) derived OCS column densities of 1.2 − 5.5 × 1014 cm-2 for their sample of 20 massive clumps associated with IRDCs, i.e., lower than found here towards I18102 MM1.
The OCS molecules form on grain surfaces through the addition of S atom to CO, or via the O atom addition to CS (e.g., Charnley et al. 2004). Solid-state OCS has been detected towards high-mass star-forming regions by, e.g, Gibb et al. (2004). Sakai et al. (2010) suggested that OCS is released from the grains into the gas phase through protostellar shocks. This could well be the case in I18102 MM1 (Sect. B.1).
Deuterated ethynyl (C2D). We have made the first detection of C2D towards IRDCs. The column density and abundance estimated from the N = 3 − 2 transition are ~1.5 × 1013 cm-2 and 4.2 ± 0.5 × 10-10, respectively. Sakai et al. (2010) detected the normal isotopologue C2H(N = 1 − 0) towards I18151 MM2. The C2H column density they derived, ~2.2 × 1014 cm-2, together with N(C2D) derived by us, suggest a deuteration degree of ~0.07 in C2H. Vrtilek et al. (1985) detected the C2D(N = 2 − 1) transition near the Orion-KL position, and obtained the column density ~1.8 × 1013 cm-2. Moreover, they derived the N(C2D)/N(C2H) ratio of 0.05. These are very similar to what we have found. Quite similarly, Parise et al. (2009) found upper limits of N(C2D) < 2.5 × 1013 cm-2 and x(C2D) < 2 × 10-10 in a clump associated with the Orion Bar.
The formation of C2D is believed to take place in the gas phase through the route (see, e.g., Parise et al. 2009). The N(C2D)/N(C2H) ratio we have obtained is comparable with those predicted by the low-metal abundance model by Roueff et al. (2007) at temperature around 30–40 K. We note that the C2H abundance, ~6 × 10-9, calculated from the observed deuteration degree (0.07) is comparable to the values x(C2H) = 2.5 × 10-9 − 5.3 × 10-8 recently found by Vasyunina et al. (2011) towards IRDCs.
Acrylonitrile or vinyl cyanide, 15N isotopologue (CH2CHC15N). The Weeds modelling suggests that there is a CH2CHC15N transition blended with the hf group of the detected C2D line. Thus, reliable column density estimate cannot be performed. Assuming that the detected line is completely due to CH2CHC15N emission, we derive very high values of 2.5 × 1016 cm-2 and ~7 × 10-7 for the column density and fractional abundance. For comparison, a column-density upper limit of 3 × 1015 cm-2 towards Sgr B2(N) was derived by Müller et al. (2008).
If present, this molecule would indicate a hot-core chemistry. The main isotopologue C2H3CN is expected to form through gas-phase reactions after the ethyl cyanide (C2H5CN), forming on dust grains, evaporates into the gas phase (Caselli et al. 1993).
Acetone (CH3COCH3). The first detection of CH3COCH3 in the interstellar medium was made by Combes et al. (1987) towards Sgr B2. They found the column density and fractional abundance of this molecule to be 5 × 1013 cm-2 and 5 × 10-11. These are significantly lower than we have estimated towards I18182 MM2 (2 × 1015 cm-2 and ~4 × 10-7). Combes et al. (1987) found that the CH3COCH3 abundance is about 1/15 of its precursor molecule CH3CHO (acetaldehyde), and suggested the formation route of acetone to be . However, Herbst et al. (1990) showed that this radiative association reaction is likely too slow to be consistent with the observed abundance in Sgr B2. The chemistry behind the formation of acetone is not clear. It could be caused by some other gas-phase ion-molecule reactions, or caused by grain chemistry (Herbst et al. 1990).
Friedel et al. (2005) found that acetone in Orion BN/KL is concentrated towards the hot core. They derived the beam-averaged column densities of ~2−8 × 1016 cm-2. More recently, Goddi et al. (2009) found the column density N(CH3COCH3) = 5.5 × 1016 cm-2 in Orion BN/KL, in agreement with the Friedel et al. results. The very high column densities and abundances of CH3COCH3 found in Orion BN/KL and in I18182 MM2 in the present work require some other reaction pathway(s) than the above radiative association reaction to be efficient. Grain surface and hot-core gas-phase chemistry may both play critical roles (Garrod et al. 2008).
The detection of complex molecules in clumps associated with IRDCs, indicating the presence of hot cores, supports the scenario that high-mass star formation can take place in these objects. For example, Rathborne et al. (2007, 2008) found that the clumps G024.33+00.11 MM1 and G034.43+00.24 MM1 are both likely to contain a hot molecular core. On the other hand, complex organics could also be ejected from grain mantles through shocks, and cosmic rays heating up the dust can also have some effect (e.g., Requena-Torres et al. 2008, and references therein).
Appendix B: Discussion on individual sources
B.1. IRDC 18102-1800 MM1
The clump I18102 MM1 is the warmest (21.3 K) and most massive (~36 M⊙) source of our sample. It is associated with Spitzer point sources at 8 and 24 μm, and high-mass star formation is taking place within it as indicated by the presence of the 6.7 GHz Class II CH3OH maser (Beuther et al. 2002b). Among our sample, the lowest degree of deuteration in both HCO+ and N2H+ is found for I18102 MM1. On the other hand, the source shows the highest degree of ionisation.
Fuller et al. (2005) detected central dips and red asymmetries in the spectral lines HCO+(1 − 0), HCO+(4 − 3), N2H+(1 − 0), and H2CO(21,2 − 11,1). A similar line profile is seen in the J = 3 − 2 transition of N2H+ in the present study. These profiles indicate the presence of expanding or outflowing gas (see, e.g., Park et al. 2000 and references therein). Beuther & Sridharan (2007) detected very broad (42.2 km s-1 down to zero intensity) SiO(2−1) wings towards I18102 MM1, indicative of bipolar outflows. This source showed the second-broadest SiO line in the sample of Beuther & Sridharan (2007). Also, the SiO(2−1) line detected by SSH10 towards I18102 MM1 was very wide, Δv = 13 ± 1 km s-1. The SiO(6−5) line detected in the present study has Δv ≃ 4.7 km s-1, and the width down to zero intensity is ~10 km s-1. We have also detected the 13C isotopologue of CS in this source. Sakai et al. (2010) suggested that CS could originate in the shock evaporation of grain mantles or radiative heating.
Beuther & Sridharan (2007) detected CH3CN(6K − 5K) and CH3OH(5K − 4K) lines towards I18102 MM1, and derived the abundances of 3 × 10-11 and 4 × 10-11 for CH3CN and CH3OH, respectively. Sakai et al. (2008) detected CH3OH(7K − 6K) and HC3N(5−4) lines in this source, and SSH10 detected C2H(N = 1 − 0) and a hint of CH3OH(21 − 11) A−. We note that SSK08 and SSH10 did not detected the lines of CCS(43 − 32), SO(22 − 11), or OCS(8 − 7) in their surveys. The upper limit they derived for the OCS column density, ≲ 2.7 × 1014 cm-2, is about 8.5 times less than the N(OCS) value we obtained. The observational results cumulated so far indicate hot-core chemistry in I18102 MM1. This conforms to the fact that this clump is giving birth to a high-mass star(s), and possibly through disk accretion as indirectly suggested by the outflow signatures. The associated hot core is likely to be in its later stages of evolution because it is associated with a methanol maser (cf. Rathborne et al. 2008).
B.2. G015.05+00.07 MM1 and G015.31-00.16 MM3
G015.05 MM1 is the lowest mass (63 M⊙) and G015.31 MM3 is the second-coldest (13.7 K) clump of our sample. Both clumps are dark in the Spitzer 8 and 24 μm images. The highest deuteration degree in N2H+ (0.028) was found towards G015.05 MM1, whereas G015.31 MM3 shows the largest CO depletion factor (2.7) in our sample. G015.05 MM1 is associated with H2O maser (Wang et al. 2006), indicative of star-formation activity. Rathborne et al. (2010) derived the following properties for G015.05 MM1 from the broadband SED: Tdust = 11 − 36 K, L = 15.5 − 362 L⊙, and M = 35 − 158 M⊙ (scaled to the revised distance 2.6 kpc). The values Tkin = 17.2 K and M = 63 M⊙ derived in the present study lie at the low end of the above values of temperature and mass (at the derived clump densities, it is expected that Tkin ≃ Tdust, Goldsmith & Langer 1978).
Sakai et al. (2008) barely detected the HC3N(5−4), CH3OH(7K − 6K), and CCS(43 − 32) lines towards G015.05 MM1 and G015.31 MM3. Indeed, all the CH3OH detected objects in the survey by SSK08 are associated with the Spitzer 24-μm sources.
Both G015.05 MM1 and G015.31 MM3 are likely to be in a very early stage of evolution. Moreover, both sources are massive enough to allow high-mass star formation. G015.31 MM3 could represent or host the so-called high-mass prestellar core, whereas G015.05 MM1 could be slightly more evolved with H2O maser emission but still lacking IR emission at 8 and 24 μm. The small wave-coupling number of W ~ 2 for G015.31 MM3 suggests that “magnetic turbulence” is not able to fragment the clump into smaller pieces, strengthening the possibility that it is a massive prestellar “core”.
B.3. IRDC 18151-1208 MM2
The I18151 MM2 clump is dark in the Spitzer 8-μm image (24 μm not available). It is associated with H2O (Beuther et al. 2002b) and Class I CH3OH masers (Marseille et al. 2010b). This clump has the highest volume-averaged H2 number density (~1.9 × 105 cm-3) among our sources.
Beuther & Sridharan (2007) detected the broadest SiO(2−1) wing emission (65 km s-1 down to zero intensity) towards I18151 MM2 in their sample. Also, the J = 2 − 1 and 3 − 2 SiO lines detected recently by López-Sepulcre et al. (2011) are very broad, i.e., FWZP = 84.3 and 103.1 km s-1, respectively. The SiO(6 − 5) line we detected is also very broad with the FWHM 46.8 km s-1. The outflow activity within the clump was confirmed by Marseille et al. (2008) who, for the first time, found that I18151 MM2 is driving a CO outflow and hosts a mid-IR-quiet, possibly a Class 0-like HMYSO (cf. Motte et al. 2007). Marseille et al. (2008) modelled the dust continuum emission (SED) of I18151 MM2 and found that the bolometric luminosity, mass, and the mean temperature of the source are Lbol = 2190 L⊙, Mgas = 373 ± 81 M⊙, and ⟨ T ⟩ = 19.4 ± 0.2 K (scaled to the revised distance 2.7 kpc). Within the errors, these mass and temperature values are comparable to the values derived in the present paper. Besides the Class I methanol maser tracing the molecular outflow, Marseille et al. (2010b) detected blue asymmetry in CH3OH(5 − 1,5 − 40,4) E, indicating infall motions. The 22 GHz H2O maser is probably excited in the outflow shocked gas (cf. Furuya et al. 2011).
Beuther & Sridharan (2007) also detected CH3CN(6K − 5K) and CH3OH(5K − 4K) lines towards I18151 MM2, which is a sign of hot-core chemistry, and they derived the abundances of 8 × 10-11 and 6 × 10-10 for CH3CN and CH3OH, respectively. SSK08 detected CH3OH(7K − 6K) lines towards I18151 MM2, but not CCS(43 − 32) or HC3N(5−4). In their line survey, SSH10 detected C2H(N = 1 − 0), but not SO(22 − 11), OCS(8 − 7), or CH3OH(21 − 11) A− lines. The C2H column density they derived, ~ 2.2 × 1014 cm-2, together with the value N(C2D) = 1.5 × 1013 cm-2 derived by us, suggest a deuteration degree of ~ 0.07 in C2H. Marseille et al. (2008) concluded from their modelling of CS transitions that CS is depleted in I18151 MM2. We derived only a small CO depletion factor of ~ 1.6 for this clump, and low degrees of deuteration in HCO+ and N2H+, namely 0.3% and 1%, respectively. Also, this source shows the lowest lower limit to ionisation degree, only 7 × 10-9.
B.4. IRDC 18182-1433 MM2
The filamentary clump I18182 MM2 is associated with Spitzer 8 and 24-μm sources. This source shows the highest degree of deuteration in HCO+ (0.014).
Beuther & Sridharan (2007) detected CH3OH(5K − 4K) lines towards I18182 MM2, and derived the CH3OH abundance of 2.1 × 10-10. Sakai et al. (2008) did not detect CCS(43 − 32), CH3OH(7K − 6K), or HC3N(5−4) lines towards this source in their survey. Our tentative detection of the O-bearing species CH3COCH3 indicates hot-core chemistry within the clump. Moreover, O-bearing species are sign of the early stage of chemical evolution (e.g., Shiao et al. 2010). This conforms to the presumable young age of the clump as it is associated with IRDC. Interestingly, the nearby clump I18182 MM1, which is associated with the HMYSO IRAS 18182-1433, also appears to contain a hot core (Beuther et al. 2006).
B.5. IRDC 18223-1243 MM3
I18223 MM3, part of a long filamentary IRDC, is a high-mass clump harbouring an embedded accreting low- to intermediate-mass protostar that could evolve to a high-mass star at some point in the future (see Beuther & Steinacker 2007; Beuther et al. 2010). Fallscheer et al. (2009) detected a molecular outflow in this source and found evidence for a large rotating structure, or toroid, perpendicular to the outflow. Evidence for outflow activity in this source was already found by Beuther et al. (2005c) and Beuther & Sridharan (2007), who found that there are 4.5 μm emission features at the clump edge and that the spectral lines of CO, CS, and SiO show broad wing emission. The line profile of SiO(6 − 5) detected in the present study also indicates outflowing gas.
The clump shows 24 μm emission but is dark at the Spitzer IRAC wavelengths (3.6, 4.5, 5.8, and 8.0 μm). Beuther et al. (2010) derived an SED for this source between 24 μm and 1.2 mm, including the recent Herschel PACS and SPIRE data, and obtained the total luminosity of 539 L⊙ (scaled to d = 3.5 kpc). They also found that the ratio between the total and submm luminosity (integrated longward of 400 μm) is only 11, suggesting that the source is very young.
Beuther & Sridharan (2007) detected CH3CN(6K − 5K) and CH3OH(5K − 4K) lines towards I18223 MM3, indicative of
hot-core chemistry, and derived the abundances of 9 × 10-11 and 6.1 × 10-10 for CH3CN and CH3OH, respectively. Sakai et al. (2008) detected HC3N(5−4), only weak CH3OH(7K − 6K) lines, and no CCS(43 − 32) lines towards this source. Sakai et al. (2010) did not detect the lines SO(22 − 11), OCS(8 − 7), or CH3OH(21 − 11) A− in their survey; however, they detected the C2H(N = 1−0) line. Based on the column densities of different species (e.g., SiO and H13CO+), SSH10 suggested that I18223 MM3 is in early stage of evolution. This conforms to the fact that the second-highest value of RD(N2H+) in our sample (0.013) is found towards I18223 MM3.
B.6. ISOSS J18364-0221 SMM1
At the distance of ~2.5 kpc, the clump J18364 SMM1 is the nearest source in our sample. It is also the coldest (11.4 K) clump of our sample. The clump is dark at 8 μm but it is associated with the 24-μm point source. The second-highest value of RD(HCO+) in our sample, 0.012, is found towards this source. It also shows high cosmic-ray ionisation rate of H2 (~5 × 10-16 s-1). Birkmann et al. (2006) studied this clump through J = 3 − 2 transitions of HCO+ and H13CO+ (beam size ~ ). The former line showed blue asymmetric profile, indicating infall. They also found significant CO(2−1) line wings, indicating the presence of outflows. The H13CO+(3−2) line we observed shows an asymmetric profile with a central dip and slightly stronger red peak, contrary to that observed by Birkmann et al. (2006) in HCO+(3−2). This difference is probably caused by the larger beam size of our observations (24″), yielding a signature of outflowing gas motions.
More recently, J18364 SMM1 was studied in detail by Hennemann et al. (2009). They resolved this contracting clump in the interferometric mm continuum into two compact cores, named SMM1 North and South separated by (0.12 pc). Their positions are indicated in Fig. 1. The peak H2 column densities and dust temperatures were found to be 2.7 × 1023 cm-2 and 15 K for SMM1 North and, 2.4 × 1023 cm-2 and 22 K for SMM1 South. Using the revised distance 2.5 kpc, the radius, mass, and luminosity of the northern core are 0.06 pc, 19 M⊙, and 26 L⊙, whereas for the southern core these values are 0.05 pc, 13 M⊙ and 230 L⊙. Thus, the cores within the clump have comparable sizes and masses but the southern one is associated with the Spitzer 24 and 70 μm sources and is more luminous. Hennemann et al. (2009) found that SMM1 South drives an energetic molecular outflow and that the core centre is supersonically turbulent. On the other hand, the IR-dark core SMM1 North shows lower levels of turbulence, but it also drives an outflow. Both the outflows from SMM1 North and South are quite collimated and their estimated ages are < 104 yr. The HCN(1 − 0) modelling results by Hennemann et al. (2009) showed that the spectrum of SMM1 South can be explained with a collapse of the core. They obtained an infall velocity of 0.14 km s-1 and an estimated mass infall rate of ~ 3.4 × 10-5 M⊙ yr-1 (scaled to the revised distance).
In summary, the clump J18364 SMM1 is fragmented into two cores that both harbour protostellar seeds, possibly evolving into intermediate- to high-mass stars. As discussed in Sect. 5.4, MHD wave propagation could have played a role in fragmenting the parent clump. As the southern core harbours a 24 μm source, is highly turbulent in the central region, and shows jet features at large distance from the driving source, it appears to be more evolved than the northern core.
© ESO, 2011
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