Lev Pirogov

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

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.

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.

Analysis of Spatial Temperature Variations in Regions of Massive Star Formation

Using the 20-m Onsala Observatory telescope (Sweden), we performed observations of the CH3C2H(6-5) line toward several regions of massive star formation to estimate the kinetic temperature of the gas and study its variations over the sources. Intense lines were detected in five objects. For these, we estimated the kinetic temperature of the gas near the CS and N2H+ molecular emission peaks by the method of population diagrams. A significant temperature difference between these peaks is noticeable only in W3 and, to a lesser degree, in DR 21. In the remaining cases, it is insignificant. This indicates that the chemical differentiation of the molecules in these regions cannot be associated with temperature variations. The kinetic temperature determined from methyl acetylene observations is usually slightly higher than the temperature estimated from ammonia observations. This is probably because the methyl acetylene emission originates in denser, i.e., deeper and hotter layers of the cloud.

HC 3 N observations of the outer galaxy dense cores

Results of HC 3 N observations of 15 dense molecular cloud cores, associated with bright IRAS sources in the outer Galaxy, are reported. HC 3 N column densities and emission region sizes have been calculated. The J =10-9 line analysis have been performed, along with the J =12-11 and J =5-4 data in the framework of the microturbulent isothermal model. The logarithms of densities found for 4 objects lie in the range: 5.5-5.9 being ∼(0.5-1) orders of magnitude higher than mean densities. Possible models for these objects should incorporate inhomogeneous density structure.

Physical and chemical structure of dense cores in regions of high mass star formation

We found that in regions of high mass star formation the CS emission correlates well with the dust continuum emission and is therefore a good tracer of the total mass while the N

J = 1–0 HCN towards bright far-infrared sources: Observational data and results of modelling

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Spatial structure of five star forming regions: 121.28+0.65, 34.403+0.233, 77.462+1.759, 99.982+4.17 and 37.427+1.518

H

Multifrequency Studies of Massive Cores with Complex Spatial and Kinematic Structures

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The region of triggered star formation W40: Observations and model

distribution is frequently very different. This is opposite to their typical behavior in low-mass cores. The behavior of other high density tracers varies from source to source but most of them are closer to CS. Radial density profiles in massive cores are fitted by power laws with indices about −1.6, as derived from the dust continuum emission. The radial temperature dependence on intermediate scales is close to the theoretically expected one for a centrally heated optically thin cloud. The velocity dispersion either remains constant or decreases from the core center to the edge. Several cores including those without known embedded IR sources show signs of infall motions. They can represent the earliest phases of massive protostars. There are implicit arguments in favor of small-scale clumpiness in the cores.