arXiv subjects

I. Zinchenko

Publications and source records attributed to I. Zinchenko.

28 records · Page 2Linked to original sources

arXiv2013

Molecular line and continuum study of the W40 cloud

The dense cloud associated with W40, one of the nearby H II regions, has been studied in millimeter-wave molecular lines and in 1.2 mm continuum. Besides, 1280 MHz and 610 MHz interferometric observations have been done. The cloud has complex morphological and kinematical structure, including a clumpy dust ring and an extended dense core. The ring is probably formed by the "collect and collapse" process due to the expansion of neighboring H II region. Nine dust clumps in the ring have been deconvolved. Their sizes, masses and peak hydrogen column densities are: $\sim 0.02-0.11$ pc, $\sim 0.4-8.1 M_{\odot}$ and $\sim (2.5-11)\times 10^{22}$ cm$^{-2}$, respectively. Molecular lines are observed at two different velocities and have different spatial distributions implying strong chemical differentiation over the region. The CS abundance is enhanced towards the eastern dust clump 2, while the NH$_3$, N$_2$H$^+$, and H$^{13}$CO$^+$ abundances are enhanced towards the western clumps. HCN and HCO$^+$ do not correlate with the dust probably tracing the surrounding gas. Number densities derived towards selected positions are: $\sim (0.3-3.2)\times 10^6$ cm$^{-3}$. Two western clumps have kinetic temperatures 21 K and 16 K and are close to virial equilibrium. The eastern clumps 2 and 3 are more massive, have higher extent of turbulence and are probably more evolved than the western ones. They show asymmetric CS(2--1) line profiles due to infalling motions which is confirmed by model calculations. An interaction between ionized and neutral material is taking place in the vicinity of the eastern branch of the ring and probably trigger star formation.

astro-ph.GA↗

arXiv2012

A Multi-Wavelength High Resolution Study of the S255 Star Forming Region. General structure and kinematics

We present observational data for two main components (S255IR and S255N) of the S255 high mass star forming region in continuum and molecular lines obtained at 1.3 mm and 1.1 mm with the SMA, at 1.3 cm with the VLA and at 23 and 50 cm with the GMRT. The angular resolution was from ~ 2" to ~ 5" for all instruments. With the SMA we detected a total of about 50 spectral lines of 20 different molecules (including isotopologues). About half of the lines and half of the species (in particular N2H+, SiO, C34S, DCN, DNC, DCO+, HC3N, H2CO, H2CS, SO2) have not been previously reported in S255IR and partly in S255N at high angular resolution. Our data reveal several new clumps in the S255IR and S255N areas by their millimeter wave continuum emission. Masses of these clumps are estimated at a few solar masses. The line widths greatly exceed expected thermal widths. These clumps have practically no association with NIR or radio continuum sources, implying a very early stage of evolution. At the same time, our SiO data indicate the presence of high-velocity outflows related to some of these clumps. In some cases, strong molecular emission at velocities of the quiescent gas has no detectable counterpart in the continuum. We discuss the main features of the distribution of NH3, N2H+, and deuterated molecules. We estimate properties of decimeter wave radio continuum sources and their relationship with the molecular material.

astro-ph.GA↗

arXiv2011

Star Formation Activity in the Galactic HII Complex S255-S257

We present results on the star-formation activity of an optically obscured region containing an embedded cluster (S255-IR) and molecular gas between two evolved HII regions S255 and S257. We have studied the complex using optical, near-infrared (NIR) imaging, optical spectroscopy and radio continnum mapping at 15 GHz, along with Spitzer-IRAC results. It is found that the main exciting sources of the evolved HII regions S255 and S257 and the compact HII regions associated with S255-IR are of O9.5 - B3 V nature, consistent with previous observations. Our NIR observations reveal 109 likely young stellar object (YSO) candidates in an area of ~ 4'.9 x 4'.9 centered on S255-IR, which include 69 new YSO candidates. Our observations increased the number of previously identified YSOs in this region by 32%. To see the global star formation, we constructed the V-I/V diagram for 51 optically identified IRAC YSOs in an area of ~ 13' x 13' centered on S255-IR. We suggest that these YSOs have an approximate age between 0.1 - 4 Myr, indicating a non-coeval star formation. Using spectral energy distribution models, we constrained physical properties and evolutionary status of 31 and 16 YSO candidates outside and inside the gas ridge, respectively. The models suggest that the sources associated within the gas ridge are of younger population (mean age ~ 1.2 Myr) than the sources outside the gas ridge (mean age ~ 2.5 Myr). The positions of the young sources inside the gas ridge at the interface of the HII regions S255 and S257, favor a site of induced star formation.

astro-ph.GA↗

arXiv2011

A search for HeH+ and CH in a high-redshift QSO

We performed a search for the HeH+ J=1-0 line (ν_{rest} = 2010.183873 GHz) and simultaneously for the CH ^2Π_{3/2}(F_2)J=3/2 - ^2Π_{1/2}(F_2)J=1/2 lines (ν_{rest}\approx 2006.8 and 2010.8 GHz) toward one of the highest-redshift quasars known, SDSS J114816.64+525150.3 (z= 6.4189). No clearly visible line was detected after obtaining an rms noise level of ~ 0.4 mK (~ 3 mJy) in 16 MHz (18 km s^{-1}) channels. At a level of 2.9sigma, however, there is a tentative emission feature shifted by about 100 km s^{-1} from the expected frequency of the HeH+ line. This shift is well within the width of the line profiles for CO and C+. The putative feature is about four times narrower than the previously detected CO and C+ lines. The difference in velocities as well as in the line widths could be explained by quite different conditions required for formation and excitation of HeH+ with respect to CO and C+. The HeH+ emission, if real, could probably arise in the dense ionized gas of this QSO. The velocity integrated flux in this tentative feature is 0.62 +/- 0.21 Jy km s^{-1} which corresponds to a total luminosity of L(HeH+)\approx 7.1 x 10^8 L_{sun}. As long as there is no independent confirmation, these values should be considered rather as upper limits.

astro-ph.CO↗

arXiv2010

Chemistry in Infrared Dark Clouds

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. 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, if the molecular abuncances in IRDCs are similar to the low-mass pre-stellar cores, or whether they show signatures of more evolved evolutionary stages. We performed observations toward 15 IRDCs in the frequency range between 86 and 93 GHz using the 22-m Mopra radio telescope. 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 indicates the presence of infall and outflow motions and 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.

astro-ph.SR↗

arXiv2009

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.

astro-ph.GA↗

arXiv2006

Chemical differentiation in regions of high-mass star formation I. CS, dust and N2H^+ in southern sources

Aims. Our goals are to compare the CS, N2H+ and dust distributions in a representative sample of high-mass star forming dense cores and to determine the physical and chemical properties of these cores. Methods. We compare the results of CS(5-4) and 1.2 mm continuum mapping of twelve dense cores from the southern hemisphere presented in this work, in combination with our previous N2H+(1-0) and CS(2-1) data. We use numerical modeling of molecular excitation to estimate physical parameters of the cores. Results. Most of the maps have several emission peaks (clumps). We derive basic physical parameters of the clumps and estimate CS and N2H+ abundances. Masses calculated from LVG densities are higher than CS virial masses and masses derived from continuum data, implying small-scale clumpiness of the cores. For most of the objects, the CS and continuum peaks are close to the IRAS point source positions. The CS(5-4) intensities correlate with continuum fluxes per beam in all cases, but only in five cases with the N2H+(1-0) intensities. The study of spatial variations of molecular integrated intensity ratios to continuum fluxes reveals that I(N2H+)/F{1.2} ratios drop towards the CS peaks for most of the sources, which can be due to a N2H+ abundance decrease. For CS(5-4), the I(CS)/F{1.2} ratios show no clear trends with distance from the CS peaks, while for CS(2-1) such ratios drop towards these peaks. Possible explanations of these results are considered. The analysis of normalized velocity differences between CS and N2H+ lines has not revealed indications of systematic motions towards CS peaks.

astro-ph↗

arXiv2004

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.

astro-ph↗

arXiv2003

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.

astro-ph↗

arXiv2000

HNCO in massive galactic dense cores

We surveyed 81 dense molecular cores associated with regions of massive star formation and Sgr A in the 5_{05}-4_{04} and 10_{010}-9_{09} lines of HNCO. Line emission was detected towards 57 objects. Selected subsamples were also observed in other HNCO lines covering a frequency range from 22 to 461 GHz. HNCO lines from the K_{-1} = 2,3 ladders were detected in several sources. Towards Orion-KL, K_{-1} = 5 transitions with upper state energies E_u/k ~ 1100 and 1300 K could be observed. Five HNCO cores were mapped. The sources remain spatially unresolved at 220 and 461 GHz with beam sizes of 24" and 18", respectively. Typical HNCO abundances relative to H_2 as derived from a population diagram analysis are ~ 10^{-9}. The rotational temperatures reach ~ 500 K. The gas densities in regions of HNCO $K_{-1}=0$ emission should be n > 10^6 cm^{-3} and in regions of K_{-1}>0 emission about an order of magnitude higher even for radiative excitation. HNCO abundances are found to be enhanced in high-velocity gas. HNCO integrated line intensities correlate well with those of thermal SiO emission. This indicates a spatial coexistence of the two species and may hint at a common production mechanism, presumably based on shock chemistry.

astro-ph↗