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

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

Multi-frequency Studies of Massive Cores with Complex Spatial and Kinematic Structures

Five regions of massive star formation have been observed in various molecular lines in the frequency range $\sim 85-89$ GHz. The studied regions possess dense cores, which host young stellar objects. The physical parameters of the cores are estimated, including kinetic temperatures ($\sim 20-40$ K), sizes of the emitting regions ($\sim 0.1-0.6$ pc), and virial masses ($\sim 40-500 M_{\odot}$). Column densities and abundances of various molecules are calculated in the local thermodynamical equilibrium approximation. The core in 99.982+4.17, associated with the weakest IRAS source, is characterized by reduced molecular abundances. Molecular line widths decrease with increasing distance from the core centers ($b$). For b\ga 0.1~pc, the dependences $\Delta V(b)$ are close to power laws ($\propto b^{-p}$), where $p$ varies from $\sim 0.2$ to $\sim 0.5$, depending on the object. In four cores, the asymmetries of the optically thick HCN(1--0) and HCO$^+$(1--0) lines indicate systematic motions along the line of sight: collapse in two cores and expansion in two others. Approximate estimates of the accretion rates in the collapsing cores indicate that the forming stars have masses exceeding the solar mass.Comment: 18 pages, 7 figures, 6 table

Relations between abundance characteristics and rotation velocity for star-forming MaNGA galaxies

We derive rotation curves, surface brightness profiles, and oxygen abundance distributions for 147 late-type galaxies using the publicly available spectroscopy obtained by the MaNGA survey. Changes of the central oxygen abundance (O/H)_0, the abundance at the optical radius (O/H)_R25, and the abundance gradient with rotation velocity V_rot are examined for galaxies with rotation velocities from 90 km/s to 350 km/s. We found that each relation shows a break at V_rot^* ~200 km/s. The central (O/H)_0 abundance increases with rising V_rot and the slope of the (O/H)_0 - V_rot relation is steeper for galaxies with V_rot < V_rot^*. The mean scatter of the central abundances around this relation is 0.053 dex. The relation between the abundance at the optical radius of a galaxy and its rotation velocity is similar; the mean scatter in abundances around this relation is 0.081 dex. The radial abundance gradient expressed in dex/kpc flattens with the increase of the rotation velocity. The slope of the relation is very low for galaxies with V_rot > V_rot^*. The abundance gradient expressed in dex/R25 is rougly constant for galaxies with V_rot < V_rot^*, flattens towards V_rot^*, and then again is roughly constant for galaxies with V_rot > V_rot^*. The change of the gradient expressed in terms of dex/h_d (where h_d is the disc scale length) with rotation velocity is similar to that for gradient in dex/R25. The relations between abundance characteristics and other basic parameters (stellar mass, luminosity, and radius) are also considered.Comment: Accepted for publication in the Astronomy and Astrophysic

Breaks in surface brightness profiles and radial abundance gradients in the discs of spiral galaxies

We examine the relation between breaks in the surface brightness profiles and radial abundance gradients within the optical radius in the discs of 134 spiral galaxies from the CALIFA survey. The distribution of the radial abundance (in logarithmic scale) in each galaxy was fitted by simple and broken linear relations. The surface brightness profile was fitted assuming pure and broken exponents for the disc. We find that the maximum absolute difference between the abundances in a disc given by broken and pure linear relations is less than 0.05 dex in the majority of our galaxies and exceeds the scatter in abundances for 26 out of 134 galaxies considered. The scatter in abundances around the broken linear relation is close (within a few percent) to that around the pure linear relation. The breaks in the surface brightness profiles are more prominent. The scatter around the broken exponent in a number of galaxies is lower by a factor of two or more than that around the pure exponent. The shapes of the abundance gradients and surface brightness profiles within the optical radius in a galaxy may be different. A pure exponential surface brightness profile may be accompanied by a broken abundance gradient and vise versa. There is no correlation between the break radii of the abundance gradients and surface brightness profiles. Thus, a break in the surface brightness profile does not need to be accompanied by a break in the abundance gradient.Comment: 18 pages, 17 figures, accepted for publication in A&