Vla observations of carbon radio recombination lines toward the H II region complex S88B
TL;DR: In this paper, high angular resolution VLA observations of the C92α, C110α, and C166α radio recombination lines of carbon from the region of massive star formation known as S88B were presented.
Abstract: We present high angular resolution VLA observations of the C92α, C110α, and C166α radio recombination lines of carbon from the region of massive star formation known as S88B. The observations reveal that the carbon emission arises from two distinct components that are intimately associated with the compact (S88B2) and cometary (S88B1) regions of ionized gas within the complex. The brighter carbon component has an angular size of ~66, an average line-center velocity of 21.0 ± 0.5 km s-1, and an average line width of 5.1 ± 1.0 km s-1; it is associated with the compact H II region. The second component has an angular size of ~16'' and is found projected toward the head of the cometary-like H II region. The average center velocity and width of the carbon line emission are 21.1 ± 0.7 km s-1 and 5.1 ± 1.7 km s-1, respectively. The spatial location and velocity of both carbon regions suggest that the emission arises in layers of photodissociated gas at the interface between the molecular cloud and the regions of ionized gas that are undergoing a champagne phase. From a model analysis of the dependence of the recombination line intensity with principal quantum number, we conclude that the carbon emission originates in warm photodissociated regions. The electron temperatures and electron densities of the photodissociated gas range between 400 and 600 K and between 40 and 80 cm-3, respectively. Further, we find that stimulated amplification of the background H II region continuum radiation contributes significantly to the carbon emission in both components. We also detected emission in sulfur radio recombination lines from both components. We find that the ratios of sulfur to carbon line intensities are considerable larger than the [S/C] cosmic abundance ratio and that they vary with principal quantum number, with values in the range between 0.3 and 0.6. We attribute the large values of the intensity ratios to depletion of carbon in the gas phase by a factor of ~5 and the variations with principal quantum number to stimulated emission effects in a region of low electron density (ne ~ 3 cm-3) and low temperature (Te ~ 50 K) that surrounds the C+ region.
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TL;DR: In this article, a multiline UV pumping model was proposed to compare the effect of self-shielding on the overall fluorescent efficiency of the photodissociation front, including the effects of line overlap.
Abstract: The structure of stationary photodissociation fronts is revisited. H_2 self- shielding is discussed, including the effects of line overlap. We find that line overlap is important for N(H_2) > 10^{20} cm^{-2}. We compute multiline UV pumping models, and compare these with simple analytic approximations for the effects of self-shielding.
The overall fluorescent efficiency of the photodissociation front is obtained for different ratios of chi/n_H (where chi characterizes the intensity of the incident UV) and different dust extinction laws. The dust optical depth tau_{pdr} to the point where 50% of the H is molecular is found to be a simple function of a dimensionless quantity phi_0 depending on chi/n_H, the rate coefficient for H_2 formation on grains, and the UV dust opacity. The fluorescent efficiency of the PDR also depends primarily on phi_0 for chi 10^4K, but shows some sensitivity to the v-J distribution of newly-formed H_2. The 1-0S(1)/2-1S(1) and 2-1S(1)/6-4Q(1) intensity ratios, the ortho/para ratio, and the rotational temperature in the $v$=1 and $v$=2 levels are computed as functions of the temperature and density, for different values of chi and n_H.
We apply our models to the reflection nebula NGC 2023. We are best able to reproduce the observations with models having chi=5000, n_H=10^5 cm^{-3}.
548 citations
TL;DR: The different forms of cosmic carbon are reviewed ranging from carbon atoms and carbon-bearing molecules to complex, solid-state, carbonaceous structures.
Abstract: Carbon is a major player in the evolutionary scheme of the universe because of its abundance and its ability to form complex species It is also a key element in the evolution of prebiotic molecules The different forms of cosmic carbon are reviewed ranging from carbon atoms and carbon-bearing molecules to complex, solid-state, carbonaceous structures The current state of knowledge is assessed on the observational and laboratory fronts Fundamental astrophysical implications are examined as well as the impact of these studies on the hitherto poorly understood physical and chemical properties of carbon materials in space
348 citations
TL;DR: In this paper, the authors report the results of their second survey for Galactic H2CO maser emission toward young stellar objects, including detection of H 2CO absorption features toward 48 sources, detection of the H110α recombination line toward 29 sources, and detection (including tentative detections) of the carbon recombination lines C110α toward 14 sources, subarcsecond angular resolution images of 6-cm continuum emission toward five sources.
Abstract: We report the results of our second survey for Galactic H2CO maser emission toward young stellar objects. Using the GBT and the VLA in the A configuration we observed 58 star-forming regions and discovered the fifth H2CO 6 cm maser region in the Galaxy (G23.71-0.20). We have discussed the detection toward G23.71-0.20 in a previous paper. Here we present all the other results from our survey, including detection of H2CO absorption features toward 48 sources, detection of the H110α recombination line toward 29 sources, detection (including tentative detections) of the carbon recombination line C110α toward 14 sources, subarcsecond angular resolution images of 6 cm continuum emission toward five sources, and observations of the H2CO masers in IRAS 18566+0408 and NGC 7538. In the case of NGC 7538, we detected the two main H2CO maser components, and our observations confirm variability of the blueshifted component recently reported by Hoffman et al. The variability of both maser components in NGC 7538 could be caused by a shock wave that reached the redshifted component approximately 14 yr before reaching the blueshifted component. If that were the case, we would expect to detect an increase in the flux density rate of change of the blueshifted component sometime after the year 2009. Our data also support the use of H2CO/H110α observations as a tool to resolve the kinematic distance ambiguity of massive star-forming regions in the Galaxy.
49 citations
TL;DR: In this article, the authors used the VLA and the BIMA array to obtain high-resolution (10-20") observations of C+, traced by the C91 alpha recombination line at 86 GHz, and the dense molecular gas, traced by HCN and HCO+ associated with the reflection nebula NGC 2023.
Abstract: The VLA and the BIMA array were used to obtain high-resolution (10"-20") observations of C+, traced by the C91 alpha recombination line at 86 GHz, and the dense molecular gas, traced by HCN and HCO+(1-0), of the photon-dominated region (PDR) associated with the reflection nebula NGC 2023 Using the VLA, continuum emission is detected at 86 GHz from a faint H II region associated with HD 37903 The C91 alpha emission originates from a 04 pc long filament, extending from the east to the south of the exciting star HD 37903 Within the filament three C91 alpha clumps can be distinguished, each associated with filamentary vibrationally excited H-2 emission in the direction toward HD 37903 The HCO+ emission has a clumpy appearance superimposed on a more extended component C91 alpha is, in general, closer to the exciting star than HCO+ emission as expected from PDR models The morphologies of HCO+ and HCN are quite similar Based on the C91 alpha line width toward one of the clumps, a limit of 170 K on the kinetic temperature in the ionized carbon layer can be derived This value is consistent with PDR models with H, densities of about 10(5) cm(-3) However, this result suggests surprisingly low limits on the turbulence in the PDR We detected a compact 3 mm continuum source in the PDR, which appears to be a cold "core" of density 10(7) cm(-3), 003 pc diameter, and 6 M-circle dot We conclude that it may have formed within the PDR In an appendix, observations of the C91 alpha recombination line toward five additional PDRs using the Efflelsberg 100 m telescope are described
37 citations
TL;DR: In this paper, the authors presented the observed carbon line parameters in the high-resolution survey and a study of the galactic distribution and angular extent of the line emission observed in the surveys.
Abstract: In earlier papers (Roshi & Anantharamaiah 2000, 2001a), we presented extensive surveys (angular resolution - 2 2 and 2 6 0 ) of radio recombination lines (RRLs) near 327 MHz in the longitude range l = 332 ! 89 using the Ooty Radio Telescope. These surveys have detected carbon lines mostly between l = 358! 20 and in a few positions at other longitudes. This paper presents the observed carbon line parameters in the high-resolution survey and a study of the galactic distribution and angular extent of the line emission observed in the surveys. The carbon lines detected in the surveys arise in "diuse" Cii regions. The l-v diagram and radial distribution constructed from our carbon line data shows similarity with that obtained from hydrogen recombination lines at 3 cm from Hii regions indicating that the distribution of the diuse Cii regions in the inner Galaxy resembles the distribution of the star-forming regions. We estimated the (Cii) 158m emission from diuse Cii regions and find that upto 95% of the total observed (Cii) 158m emission can arise in diuse Cii regions if the temperature of the latter80 K. Our high-resolution survey data shows that the carbon line emitting regions have structures on angular scale 6 0 . Analysis of the dual-resolution observations toward a 2 wide field centered at l= 13:9 and toward the longitude range l= 1:75 to 6:75 shows the presence of narrow (V 15 km s 1 ) carbon line emitting regions extending over several degrees in l and b. The physical size perpendicular to the line-of-sight of an individual diuse Cii region in these directions is>200 pc.
22 citations
Cites background from "Vla observations of carbon radio re..."
...Garay et al. 1998, Wyrowski et al. 2000)....
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TL;DR: In this article, a theoretical parameter study of the temperature and chemical structure of dense photodissociation regions and their resultant spectrum is presented, which is relevant not only to the dust and gas between molecular clouds and H II regions, but also apply to any neutral cloud illuminated by intense FUV fluxes.
Abstract: A theoretical parameter study of the temperature and chemical structure of dense photodissociation regions and their resultant spectrum is presented. Models are discussed which are relevant not only to the dust and gas between molecular clouds and H II regions, but also apply to any neutral cloud illuminated by intense FUV fluxes. The models relate observed line and continuum emission from these regions to physical parameters such as the gas density and temperature, the elemental and chemical abundances, the local radiation field, and the grain properties. The results are applied to observational data from the OMC-1 region. The model shows that the observed high brightness temperature of the C I 609 microns line can be explained by emission from the C(+)/C/CO transition region. This difference with previous chemical models is due to a higher gas phase elemental abundance of carbon, to the charge exchange reactions of C(+) with S and SiO, and to carbon self-shielding.
1,234 citations
TL;DR: The present status of abundance information for elements in meteorites and in the Sun is reviewed, and a new table of abundances of the elements, which should be characteristic of the primitive solar nebula, is compiled and presented.
Abstract: The present status of abundance information for elements in meteorites and in the Sun is reviewed, and a new table of abundances of the elements, which should be characteristic of the primitive solar nebula, is compiled and presented. Special attention is called to the elemental abundances in the silicon-to-calcium region, where many of the abundances are rather poorly determined, and where these abundances have an impact on theories of nucleosynthesis of the elements. To each elemental isotope is assigned a mechanism of nucleosynthesis which may have been responsible for production of most of that isotope, and brief comments are made concerning the present status of understanding of the different mechanisms of nucleosynthesis.
825 citations
"Vla observations of carbon radio re..." refers background in this paper
...…of emission in a sulfur radio recombination line toward the S88B complex was Ðrst reported by who found that the ratio ofSilverglate (1984), integrated 166a line intensities of sulfur and carbon is 0.39^ 0.06, about 9 times larger than the cosmic abundance ratio of 0.042 et al.(Cameron 1973)....
[...]
...Assuming (1) ionization of all carbon within the PDR, (2) a carbon-to-hydrogen cosmic abundance ratio of 3.7 ] 10~4 and (3) a depletion factor, f, of 5Èrequired(Cameron 1973), to explain the observed sulfur-to-carbon line ratios (see discussion below)Èthen the number density of C ions, is cm~3....
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TL;DR: In this paper, a multiline UV pumping model was proposed to compare the effect of self-shielding on the overall fluorescent efficiency of the photodissociation front, including the effects of line overlap.
Abstract: The structure of stationary photodissociation fronts is revisited. H_2 self- shielding is discussed, including the effects of line overlap. We find that line overlap is important for N(H_2) > 10^{20} cm^{-2}. We compute multiline UV pumping models, and compare these with simple analytic approximations for the effects of self-shielding.
The overall fluorescent efficiency of the photodissociation front is obtained for different ratios of chi/n_H (where chi characterizes the intensity of the incident UV) and different dust extinction laws. The dust optical depth tau_{pdr} to the point where 50% of the H is molecular is found to be a simple function of a dimensionless quantity phi_0 depending on chi/n_H, the rate coefficient for H_2 formation on grains, and the UV dust opacity. The fluorescent efficiency of the PDR also depends primarily on phi_0 for chi 10^4K, but shows some sensitivity to the v-J distribution of newly-formed H_2. The 1-0S(1)/2-1S(1) and 2-1S(1)/6-4Q(1) intensity ratios, the ortho/para ratio, and the rotational temperature in the $v$=1 and $v$=2 levels are computed as functions of the temperature and density, for different values of chi and n_H.
We apply our models to the reflection nebula NGC 2023. We are best able to reproduce the observations with models having chi=5000, n_H=10^5 cm^{-3}.
766 citations
Book•
19 Jul 1974
TL;DR: In this paper, the authors present a review of radio astronomy beyond the solar system, presenting the latest observations and interpretations as well as an extensive view of the literature (as of 1988) including galactic continuum emission, HII regions, the diffuse interstellar medium, interstellar molecules, astronomical masers, neutral hydrogen, radio stars, radio galaxies and quasars, the microwave background and cosmological radio sources.
Abstract: This text, designed for graduate students and professional astronomers, covers all aspects of radio astronomy beyond the solar system Each chapter is written by an expert in the field and contains a review of a particular area of radio astronomy, presenting the latest observations and interpretations as well as an extensive view of the literature (as of 1988) Topics covered include: galactic continuum emission, HII regions, the diffuse interstellar medium, interstellar molecules, astronomical masers, neutral hydrogen, the galactic centre, radio stars, supernova remnants, pulsars, extragalactic hydrogen, radio galaxies and quasars, the microwave background and cosmological radio sources
564 citations