Oskari Miettinen

SiO and CH3CCH abundances and dust emission in high-mass star-forming cores ,

Aims. We determine the fractional SiO abundance in high-mass star-forming cores, and investigate its dependence on physical conditions, to provide constraints on the chemistry models of the formation of SiO in the gas phase or via grain mantle evaporation. The work addresses also CH3CCH chemistry, as the kinetic temperature is determined using this molecule. Methods. We estimate the physical conditions of 15 high-mass star-forming cores and derive the fractional SiO and CH3CCH abundances using spectral line and dust continuum observations with the SEST. Results. The kinetic temperatures as derived from CH3CCH range from 25 to 39 K, the average being 33 K. The average gas density in the cores is 4.5 × 10 6 cm −3 . The SiO emission regions are extended and typically half of the integrated line emission comes from the velocity range traced out by CH3CCH emission. The upper limit of SiO abundance in this “quiescent” gas component is ∼10 −10 . The average CH3CCH abundance is about 7 × 10 −9 . It shows a shallow, positive correlation with the temperature, whereas SiO shows the opposite tendency. Conclusions. We suggest that the high CH3CCH abundance and its possible increase when the clouds become warmer is related to the intensified desorption of the chemical precursors of the molecule from grain surfaces. In contrast, the observed tendency of SiO does not support the idea that the evaporation of Si-containing species from the grain mantles would be important, and it contradicts models where neutral reactions with activation barriers dominate SiO production. A possible explanation for the decrease is that warmer cores represent more evolved stages of core evolution with fewer high-velocity shocks and thus less efficient SiO replenishment.

Deuterium fractionation and the degree of ionisation in massive clumps within infrared dark clouds

Context. Massive clumps associated with infrared dark clouds (IRDCs) are promising targets for studying the earliest stages of highmass star and cluster formation. Aims. We aim to determine the degrees of CO depletion, deuterium fractionation, and ionisation in a sample of seven massive clumps associated with IRDCs. Methods. The APEX telescope was used to observe the C 17 O(2−1), H 13 CO + (3−2), DCO + (3−2), N2H + (3−2), and N2D + (3−2) transitions towards the clumps. The spectral line data were used in conjunction with the previously published and/or archival (sub)millimetre dust continuum observations of the sources. The data were used to derive the molecular column densities and fractional abundances for the analysis of deuterium fractionation and ionisation. Results. The CO molecules do not appear to be significantly depleted in the observed clumps. The DCO + /HCO + and N2D + /N2H + column density ratios are about 0.0002–0.014 and 0.002–0.028, respectively. The former ratio is found to decrease as a function of gas kinetic temperature. A simple chemical analysis suggests that the lower limit to the ionisation degree is in the range x(e) ∼ 10 −8 −10 −7 , whereas the estimated upper limits range from a few 10 −6 up to ∼10 −4 . Lower limits to x(e) imply that the cosmic-ray ionisation rate of H2 lies between ζH2 ∼ 10 −17 −10 −15 s −1 . These are the first estimates of x(e) and ζH2 towards massive IRDCs reported so far. Some additional molecular transitions, mostly around 216 and 231 GHz, were detected towards all sources. In particular, IRDC 18102-1800 MM1 and IRDC 18151-1208 MM2 show relatively line-rich spectra. Some of these transitions might be assigned to complex organic molecules, although the line blending hampers the identification. The C 18 O(2−1) transition is frequently seen in the image band. Conclusions. The finding that CO is not depleted in the observed sources conforms to the fact that they show evidence of star formation activity, which is believed to release CO from the icy grain mantles back into the gas phase. The observed degree of deuteration is lower than in low-mass starless cores and protostellar envelopes. Decreasing deuteration with increasing temperature is likely to reflect the clump evolution. On the other hand, the association with young high-mass stars could enhance ζH2 and x(e) above the levels usually found in low-mass star-forming regions. On the scale probed by our observations, ambipolar diffusion cannot be a main driver of clump evolution unless it occurs on timescales � 10 6 yr.

A (sub)millimetre study of dense cores in Orion B9

Context. Studies of dense molecular-cloud cores at (sub)millimetre wavelengths are needed to understand the early stages of star formation. Aims. We aim to further constrain the properties and evolutionary stages of dense cores in Orion B9. The prime objective of this study is to examine the dust emission of the cores near the peak of their spectral energy distributions, and to determine the degrees of CO depletion, deuterium fractionation, and ionisation. Methods. The central part of Orion B9 was mapped at 350 μm with APEX/SABOCA. A sample of nine cores in the region were observed in C 17 O(2−1), H 13 CO + (4−3) (towards 3 sources), DCO + (4−3), N2H + (3−2), and N2D + (3−2) with APEX/SHFI. These data are used in conjunction with our previous APEX/LABOCA 870-μm dust continuum data. Results. All the LABOCA cores in the region covered by our SABOCA map were detected at 350 μm. The strongest 350 μm emission is seen towards the Class 0 candidate SMM 3. Many of the LABOCA cores show evidence of substructure in the higher-resolution SABOCA image. In particular, we report on the discovery of multiple very low-mass condensations in the prestellar core SMM 6. Based on the 350-to-870 μm flux density ratios, we determine dust temperatures of Tdust � 7.9−10.8 K, and dust emissivity indices of β ∼ 0.5−1.8. The CO depletion factors are in the range fD ∼ 1.6−10.8. The degree of deuteration in N2H + is � 0.04−0.99, where the highest value (seen towards the prestellar core SMM 1) is, to our knowledge, the most extreme level of N2H + deuteration reported so far. The level of HCO + deuteration is about 1–2%. The fractional ionisation and cosmic-ray ionisation rate of H2 could be determined only towards two sources with the lower limits of ∼2−6 × 10 −8 and ∼2.6 × 10 −17 −4.8 × 10 −16 s −1 , respectively. We also detected D2CO towards two sources. Conclusions. The detected protostellar cores are classified as Class 0 objects, in agreement with our previous SED results. The detection of subcondensations within SMM 6 shows that core fragmentation can already take place during the prestellar phase. The origin of this substructure is likely caused by thermal Jeans fragmentation of the elongated parent core. Varying levels of fD and deuteration among the cores suggest that they are evolving chemically at different rates. A low fD value and the presence of gas-phase D2CO in SMM 1 suggest that the core chemistry is affected by the nearby outflow. The very high N2H + deuteration in SMM 1 is likely to be remnant of the earlier CO-depleted phase.

A molecular line study of the filamentary infrared dark cloud G304.74+01.32

Context. Infrared dark clouds (IRDCs) are promising sites to study the earliest formation stages of stellar clusters and high-mass stars, and the physics of molecular-cloud formation and fragmentation. Aims. We attempt to improve our understanding of the physical and chemical properties of the filamentary IRDC G304.74+01.32 (hereafter, G304.74). In particular, we investigate the kinematical and dynamical state of the cloud and clumps within it, and the amount of CO depletion. Methods. All of the submillimetre peak positions in the cloud identified from our previous LABOCA 870-μm map were observed in C 17 O(2−1) with APEX. These are the first line observations along the whole filament that have been made so far. Selected positions were also observed in the 13 CO(2−1), SiO(5−4), and CH3OH(5k−4k) transitions at ∼1 mm. Results. The C 17 O lines were detected towards all target positions at similar radial velocities. CO does not appear to be significantly depleted in the clumps, the largest depletion factors being only about 2. Two to three methanol 5k−4k lines near ∼241.8 GHz were detected towards all selected positions, whereas SiO(5−4) was seen in only one of these positions, namely SMM 3. In the band covering SiO(5−4), we also detected the DCN(3−2) line towards SMM 3. The 13 CO(2−1) lines display blue asymmetric profiles, which are indicative of large-scale infall motions. The clumps show transonic to supersonic non-thermal motions, and a virial-parameter analysis suggests that most of them are gravitationally bound. The external pressure may also play a non-negligible role in the dynamics. Our analysis suggests that the fragmentation of the filament into clumps is caused by a “sausage”-type instability, in agreement with results from other IRDCs. Conclusions. The uniform C 17 O radial velocities along the G304.74 cloud shows that it is a coherent filamentary structure. Although the clumps appear to be gravitationally bound, the ambient turbulent ram pressure may be an important factor in the cloud dynamics. This is qualitatively consistent with our earlier suggestion that the filament was formed by converging supersonic turbulent flows. The poloidal magnetic field could resist the radial cloud collapse, which conforms to the low infall velocites that we derived. The cloud may be unable to form high-mass stars based on the mass-size threshold. The star-formation activity in the cloud, such as outflows, is likely responsible for the release of CO from the icy grain mantles back into the gas phase. Shocks related to outflows may also have injected CH3OH, SiO, and DCN into the gas-phase in SMM 3.

A MALT90 study of the chemical properties of massive clumps and filaments of infrared dark clouds

We attempt to characterise the chemical properties of a sample of massive clumps of IRDCs through multi-molecular line observations. We also search for possible evolutionary trends among the derived chemical parameters. The clumps are studied using the MALT90 line survey data obtained with the Mopra telescope. The spectral-line data are used in concert with our previous LABOCA 870-

LABOCA mapping of the infrared dark cloud MSXDC G304.74+01.32

\mu

LABOCA 870 μm dust continuum mapping of selected infrared-dark cloud regions in the Galactic plane

m dust emission data. Most of the detected species (SiO, C

Prestellar and protostellar cores in Orion B9

_2

Physical properties of dense cores in Orion B9

H, HNCO, HCN, HCO

Detection of H 2 D + in a massive prestellar core in Orion B

^+