I. Zinchenko

N2H+(1–0) survey of massive molecular cloud cores

We present the results of N2H + (1-0) observations of 35 dense molecular cloud cores from the northern and southern hemispheres where massive stars and star clusters are formed. Line emission has been detected in 33 sources, for 28 sources detailed maps have been obtained. Peak N2H + column densities lie in the range: 3:6 10 12 1:5 10 14 cm 2 . Intensity ratios of (01-12) to (23-12) hyperfine components are slightly higher than the LTE value. The optical depth of (23-12) component toward peak intensity positions of 10 sources is0:2 1. In many cases the cores have elongated or more complex structures with several emission peaks. In total, 47 clumps have been revealed in 26 sources. Their sizes lie in the range 0.3-2.1 pc, the range of virial masses is30 3000 M. Mean N2H + abundance for 36 clumps is 5 10 10 . Integrated intensity maps with axial ratios<2 have been fitted with a power-law radial distribution r p convolved with the telescope beam. Mean power-law index for 25 clumps is close to 1.3. For reduced maps where positions of low intensity are rejected mean power-law index is close to unity corresponding to ther 2 density profile provided N 2H + excitation conditions do not vary inside these regions. In those cases where we have relatively extensive and high quality maps, line widths of the cores either decrease or stay constant with distance from the center, implying an enhanced dynamical activity in the center. There is a correlation between total velocity gradient direction and elongation angle of the cores. However, the ratio of rotational to gravitational energy is too low (4 10 4 - 7:1 10 2 ) for rotation to play a significant role in the dynamics of the cores. A correlation between mean line widths and sizes

Chemistry in infrared dark clouds

Context. Massive stars play an important role in shaping the structure of galaxies. Infrared dark clouds (IRDCs), with their low temperatures and high densities, have been identified as the potential birthplaces of massive stars. In order to understand the formation processes of massive stars, the physical and chemical conditions in infrared dark clouds have to be characterized. Aims. The goal of this paper is to investigate the chemical composition of a sample of southern infrared dark clouds. One important aspect of the observations is to check, whether the molecular abundances in IRDCs are similar to the low-mass pre-stellar cores, or if they show signatures of more evolved evolutionary stages. Methods. We performed observations toward 15 IRDCs in the frequency range between 86 and 93 GHz using the 22-m Mopra radio telescope. In total, 13 molecular species comprising N 2 H + , 13 CS, CH 3 CN, HC 3 N, HNC, HCO + , HCN, HNCO, C 2 H, SiO, H 13 CO + , H 13 CN, and CH 3 C 2 H were observed for all targets. Hence, we included in general species appropriate for elevated densities, where some of them trace the more quiescent gas, while others are sensitive to more dynamical processes. Results. We detect HNC, HCO + , and HNC emission in all clouds and N 2 H + in all IRDCs except one. In some clouds we detect SiO emission. Complicated shapes of the HCO + emission line profile are found in all IRDCs. Both signatures indicate infall and outflow motions and the beginning of star-formation activity, at least in some parts of the IRDCs. Where possible, we calculate molecular abundances and make a comparison with previously obtained values for low-mass pre-stellar cores and high-mass protostellar objects (HMPOs). We show a tendency for IRDCs to have molecular abundances similar to low-mass pre-stellar cores rather than to HMPOs abundances on the scale of our single-dish observations.

Chemical differentiation in regions of high-mass star formation – II. Molecular multiline and dust continuum studies of selected objects

The aim of this study is to investigate systematic chemical differentiation of molecules in regions of high-mass star formation (HMSF). We observed five prominent sites of HMSF in HCN, HNC, HCO + , their isotopes, C 18 O, C 34 S and some other molecular lines, for some sources both at 3 and 1.3 mm and in continuum at 1.3 mm. Taking into account earlier obtained data for N 2 H + , we derive molecular abundances and physical parameters of the sources (mass, density, ionization fraction, etc.). The kinetic temperature is estimated from CH 3 C 2 H observations. Then, we analyse correlations between molecular abundances and physical parameters and discuss chemical models applicable to these species. The typical physical parameters for the sources in our sample are the following: kinetic temperature in the range ∼30-50 K (it is systematically higher than that obtained from ammonia observations and is rather close to dust temperature), masses from tens to hundreds solar masses, gas densities ∼10 5 cm ―3 and ionization fraction ∼10 ―7 . In most cases, the ionization fraction slightly (a few times) increases towards the embedded young stellar objects (YSOs). The observed clumps are close to gravitational equilibrium. There are systematic differences in distributions of various molecules. The abundances of CO, CS and HCN are more or less constant. There is no sign of CO and/or CS depletion as in cold cores. At the same time, the abundances of HCO + , HNC and especially N 2 H + strongly vary in these objects. They anticorrelate with the ionization fraction and as a result decrease towards the embedded YSOs. For N 2 H + this can be explained by dissociative recombination to be the dominant destroying process. N 2 H + , HCO + and HNC are valuable indicators of massive protostars.

Molecular abundance ratios as a tracer of accelerated collapse in regions of high-mass star formation

Recent observations suggest that the behaviour of tracer species such as N_2H+ and CS is significantly different in regions of high and low mass star formation. In the latter, N_2H+ is a good tracer of mass, while CS is not. Observations show the reverse to be true in high-mass star formation regions. We use a computational chemical model to show that the abundances of these and other species may be significantly altered by a period of accelerated collapse in high mass star forming regions. We suggest these results provide a potential explanation of the observations, and make predictions for the behaviour of other species.

The disk-outflow system in the S255IR area of high-mass star formation

We report the results of our observations of the S255IR area with the Submillimeter Array (SMA) at 1.3 mm in the very extended configuration and at 0.8 mm in the compact configuration as well as with the IRAM 30 m at 0.8 mm. The best achieved angular resolution is about 0.4 arcsec. The dust continuum emission and several tens of molecular spectral lines are observed. The majority of the lines is detected only toward the S255IR-SMA1 clump, which represents a rotating structure (probably a disk) around the young massive star. The achieved angular resolution is still insufficient to make any conclusions about the Keplerian or non-Keplerian character of the rotation. The temperature of the molecular gas reaches 130-180 K. The size of the clump is about 500 AU. The clump is strongly fragmented as follows from the low beam-filling factor. The mass of the hot gas is significantly lower than the mass of the central star. A strong DCN emission near the center of the hot core most probably indicates a presence of a relatively cold (less than or similar to 80 K) and rather massive clump there. High-velocity emission is observed in the CO line as well as in lines of high-density tracers HCN, HCO+, CS and other molecules. The outflow morphology obtained from a combination of the SMA and IRAM 30 m data is significantly different from that derived from the SMA data alone. The CO emission detected with the SMA traces only one boundary of the outflow. The outflow is most probably driven by jet bow shocks created by episodic ejections from the center. We detected a dense high velocity clump associated apparently with one of the bow shocks. The outflow strongly affects the chemical composition of the surrounding medium.

NGC 7538: multiwavelength study of stellar cluster regions associated with IRS 1–3 and IRS 9 sources

We present deep and high-resolution (FWHM ∼ 0.4 arcsec) near-infrared (NIR) imaging observations of the NGC 7538 IRS 1–3 region (in JHK bands), and IRS 9 region (in HK bands) using the 8.2 m Subaru telescope. The NIR analysis is complemented with Giant Metrewave Radio Telescope (GMRT) low-frequency observations at 325, 610, and 1280 MHz, molecular line observations of H 13 CO + (J=1–0), and archival Chandra X-ray observations. Using the ‘J − H/H − K’ diagram, 144 Class II and 24 Class I young stellar object (YSO) candidates are identified in the IRS 1–3 region. Further analysis using ‘K/H − K’ diagram yields 145 and 96 red sources in the IRS 1–3 and IRS 9 regions, respectively. A total of 27 sources are found to have X-ray counterparts. The YSO mass function (MF), constructed using a theoretical mass–luminosity relation, shows peaks at substellar (∼0.08–0.18 M ⊙ ) and intermediate (∼1–1.78 M ⊙ ) mass ranges for the IRS 1–3 region. The MF can be fitted by a power law in the low-mass regime with a slope of Γ ∼ 0.54–0.75, which is much shallower than the Salpeter value of 1.35. An upper limit of 10.2 is obtained for the star to brown dwarf ratio in the IRS 1–3 region. GMRT maps show a compact H ii region associated with the IRS 1–3 sources, whose spectral index of 0.87 ± 0.11 suggests optical thickness. This compact region is resolved into three separate peaks in higher resolution 1280 MHz map, and the ‘east’ subpeak coincides with the IRS 2 source. H 13 CO + (J=1–0) emission reveals peaks in both IRS 1–3 and IRS 9 regions, none of which are coincident with visible nebular emission, suggesting the presence of dense cloud nearby. The virial masses are approximately of the order of 1000 and 500 M ⊙ for the clumps in IRS 1–3 and IRS 9 regions, respectively.

Star formation in the filament of S254-S258 OB complex: a cluster in the process of being created

Infrared Dark Clouds (IRDCs) are ideal laboratories to study the initial processes of high-mass star and star cluster formation. We investigated star formation activity of an unexplored filamentary dark cloud (size ∼ 5.7 pc × 1.9 pc), which itself is part of a large filament (∼ 20 pc) located in the S254-S258 OB complex at a distance of 2.5 kpc. Using Multi-band Imaging Photometer (MIPS) Spitzer 24 µm data, we uncover 49 sources with signal-to-noise ratio greater than 5. We identified 45 sources as candidate young stellar objects (YSOs) of Class I, Flat-spectrum, and Class II nature. Additional 17 candidate YSOs (9 Class I and 8 Class II) are also identified using JHK and Wide-field Infrared Survey Explorer (WISE) photometry. We find that the protostar to Class II sources ratio (∼ 2) and the protostar fraction (∼ 70 %) of the region are high. When the protostar fraction compared to other young clusters, it suggests that the star formation in the dark cloud was possibly started only 1 Myr ago. Combining the near-infrared photometry of the YSO candidates with the theoretical evolutionary models, we infer that most of the candidate YSOs formed in the dark cloud are low-mass (< 2 M) in nature. We examine the spatial distribution of the YSOs and find that majority of them are linearly aligned along the highest column density line (N(H 2)∼ 1 × 10 22 cm −2) of the dark cloud along its long axis at mean nearest neighbor separation of ∼ 0.2 pc. Using observed properties of the YSOs, physical conditions of the cloud and a simple cylindrical model, we explore the possible star formation process of this filamentary dark cloud and suggest that gravitational fragmentation within the filament should have played a dominant role in the formation of the YSOs. From the total mass of the YSOs, gaseous mass associated with the dark cloud, and surrounding environment, we infer that the region is presently forming stars at an efficiency ∼ 3% and a rate ∼ 30 M Myr −1 , and may emerge to a richer cluster.

A Search for Small-Scale Clumpiness in Dense Cores of Molecular Clouds

We have analyzed HCN(1-0) and CS(2-1) line profiles obtained with high signal-to-noise ratios toward distinct positions in three selected objects in order to search for small-scale structure in molecular cloud cores associated with regions of high-mass star formation. In some cases, ripples were detected in the line profiles, which could be due to the presence of a large number of unresolved small clumps in the telescope beam. The number of clumps for regions with linear scales of ~0.2-0.5 pc is determined using an analytical model and detailed calculations for a clumpy cloud model; this number varies in the range: ~2 10^4-3 10^5, depending on the source. The clump densities range from ~3 10^5-10^6 cm^{-3}, and the sizes and volume filling factors of the clumps are ~(1-3) 10^{-3} pc and ~0.03-0.12. The clumps are surrounded by inter-clump gas with densities not lower than ~(2-7) 10^4 cm^{-3}. The internal thermal energy of the gas in the model clumps is much higher than their gravitational energy. Their mean lifetimes can depend on the inter-clump collisional rates, and vary in the range ~10^4-10^5 yr. These structures are probably connected with density fluctuations due to turbulence in high-mass star-forming regions.We have analyzed HCN(1-0) and CS(2-1) line profiles obtained with high signal-to-noise ratios toward distinct positions in three selected objects in order to search for small-scale structure in molecular cloud cores associated with regions of high-mass star formation. In some cases, ripples were detected in the line profiles, which could be due to the presence of a large number of unresolved small clumps in the telescope beam. The number of clumps for regions with linear scales of ∼0.2–0.5 pc is determined using an analytical model and detailed calculations for a clumpy cloud model; this number varies in the range: ∼2 × 104–3 × 105, depending on the source. The clump densities range from ∼3 × 105–106 cm−3, and the sizes and volume filling factors of the clumps are ∼(1–3) × 10−3 pc and ∼0.03–0.12. The clumps are surrounded by inter-clump gas with densities not lower than ∼(2–7) × 104 cm−3. The internal thermal energy of the gas in the model clumps is much higher than their gravitational energy. Their mean lifetimes can depend on the inter-clump collisional rates, and vary in the range ∼104–105 yr. These structures are probably connected with density fluctuations due to turbulence in high-mass star-forming regions.

MULTIWAVELENGTH STUDY OF THE STAR FORMATION IN THE S237 H ii REGION

We present a detailed multiwavelength study of observations from X-ray, near-infrared, and centimeter wavelengths to probe the star formation processes in the S237 region. Multiwavelength images trace an almost sphere-like shell morphology of the region, which is filled with the 0.5–2 keV X-ray emission. The region contains two distinct environments—a bell-shaped cavity-like structure containing the peak of 1.4 GHz emission at center, and elongated filamentary features without any radio detection at edges of the sphere-like shell—where Herschel clumps are detected. Using the 1.4 GHz continuum and 12 CO line data, the S237 region is found to be excited by a radio spectral type of B0.5V star and is associated with an expanding H II region. The photoionized gas appears to be responsible for the origin of the bell-shaped structure. The majority of molecular gas is distributed toward a massive Herschel clump (M clump ∼ 260 M ⊙ ), which contains the filamentary features and has a noticeable velocity gradient. The photometric analysis traces the clusters of young stellar objects (YSOs) mainly toward the bell-shaped structure and the filamentary features. Considering the lower dynamical age of the H II region (i.e., 0.2–0.8 Myr), these clusters are unlikely to be formed by the expansion of the H II region. Our results also show the existence of a cluster of YSOs and a massive clump at the intersection of filamentary features, indicating that the collisions of these features may have triggered cluster formation, similar to those found in the Serpens South region.

STAR-FORMATION ACTIVITY IN THE NEIGHBORHOOD OF W–R 1503-160L STAR IN THE MID-INFRARED BUBBLE N46

In order to investigate star-formation (SF) processes in extreme environments, we have carried out a multi-wavelength analysis of the mid-infrared bubble N46, which hosts a WN7 Wolf–Rayet (W–R) star. We have used 13 CO line data to trace an expanding shell surrounding the W–R star containing about five condensations within the molecular cloud associated with the bubble. The W–R star is associated with a powerful stellar wind having a mechanical luminosity of ~4 × 10 37 erg s −1 . A deviation of the H-band starlight mean polarization angles around the bubble has also been traced, indicating the impact of stellar wind on the surroundings. The Herschel temperature map shows a temperature range of ~18–24 K toward the five molecular condensations. The photometric analysis reveals that these condensations are associated with the identified clusters of young stellar objects, revealing ongoing SF process. The densest among these five condensations (peak N(H 2 ) ~9.2 × 10 22 cm −2 and A V ~ 98 mag) is associated with a 6.7 GHz methanol maser, an infrared dark cloud, and the CO outflow, tracing active massive SF within it. At least five compact radio sources (CRSs) are physically linked with the edges of the bubble, and each of them is consistent with the radio spectral class of a B0V–B0.5V-type star. The ages of the individual infrared counterparts of three CRSs (~1–2 Myr) and a typical age of WN7 W–R star (~4 Myr) indicate that the SF activities around the bubble are influenced by the feedback of the W–R star.