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. The optical depth of (23-12) component toward peak intensity positions of 10 sources is ~ 0.2-1. In total, 47 clumps have been revealed in 26 sources. Integrated intensity maps with aspect ratios < 2 have been fitted with a power-law radial distribution $r^{-p}$ convolved with the telescope beam. Mean power-law index is close to unity corresponding to the $\sim r^{-2}$ density profile provided N2H+ excitation conditions do not vary inside these regions. Line widths of the cores either decrease or stay constant with distance from the center. The ratio of rotational to gravitational energy is too low for rotation to play a significant role in the dynamics of the cores. A correlation between mean line widths and sizes of clumps has been found.Comment: 17 pages, Late

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. We observed five prominent sites of high mass star formation in HCN, HNC, HCO+, their isotopes, C18O, C34S 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 N2H+ we derive molecular abundances and physical parameters of the sources (mass, density, ionization fraction, etc.). The kinetic temperature is estimated from CH3C2H observations. Then we analyze 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}, ionization fraction ~ 10^{-7}. In most cases the ionization fraction slightly (a few times) increases towards the embedded 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 N2H+ strongly vary in these objects. They anti-correlate with the ionization fraction and as a result decrease towards the embedded YSOs. For N2H+ this can be explained by dissociative recombination to be the dominant destroying process. N2H+, HCO+, and HNC are valuable indicators of massive protostars.Comment: 15 pages, 8 figure

Oxygen abundance distributions in six late-type galaxies based on SALT spectra of HII regions

Spectra of 34 H II regions in the late-type galaxies NGC1087, NGC2967, NGC3023, NGC4030, NGC4123, and NGC4517A were observed with the South African Large Telescope (SALT). In all 34 H II regions, oxygen abundances were determined through the "counterpart" method (C method). Additionally, in two H II regions in which the auroral lines were detected oxygen abundances were measured through the classic Te method. We also estimated the abundances in our H II regions using the O3N2 and N2 calibrations and compared those with the C-based abundances. With these data we examined the radial abundance distributions in the disks of our target galaxies. We derived surface-brightness profiles and other characteristics of the disks (the surface brightness at the disk center and the disk scale length) in three photometric bands for each galaxy using publicly available photometric imaging data. The radial distributions of the oxygen abundances predicted by the relation between abundance and disk surface brightness in the W1 band obtained for spiral galaxies in our previous study are close to the radial distributions of the oxygen abundances determined from the analysis of the emission line spectra for four galaxies where this relation is applicable. Hence, when the surface-brightness profile of a late-type galaxy is known, this parametric relation can be used to estimate the likely present-day oxygen abundance in its disk.Comment: 15 pages, 11 figures; Accepted for publication in Astronomy & Astrophysic

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.Comment: 23 pages including 4 figures and 4 table