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تسجيل مستخدم جديدClumpy photon-dominated regions in Carina. I. [CI] and mid-J CO lines in two 4x4 fields
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The Carina region is an excellent astrophysical laboratory for studying the feedback mechanisms of newly born, very massive stars within their natal giant molecular clouds (GMCs) at only 2.35 kpc distance. We use a clumpy PDR model to analyse the observed intensities of atomic carbon and CO and to derive the excitation conditions of the gas. The NANTEN2-4m submillimeter telescope was used to map the [CI] 3P1-3P0, 3P2-3P1 and CO 4-3, 7-6 lines in two 4x4 regions of Carina where molecular material interfaces with radiation from the massive star clusters. One region is the northern molecular cloud near the compact OB cluster Tr14, and the second region is in the molecular cloud south of etaCar and Tr16. These data were combined with 13CO SEST spectra, HIRES/IRAS 60um and 100um maps of the FIR continuum, and maps of 8um IRAC/Spitzer and MSX emission. We used the HIRES far-infrared dust data to create a map of the FUV field heating the gas. The northern region shows an FUV field of a few 1000 in Draine units while the field of the southern region is about a factor 10 weaker. We constructed models consisting of an ensemble of small spherically symmetric PDR clumps within the 38 beam (0.43pc), which follow canonical power-law mass and mass-size distributions. We find that an average local clump density of 2x10**5 cm-3 is needed to reproduce the observed line emission at two selected interface positions. Stationary, clumpy PDR models reproduce the observed cooling lines of atomic carbon and CO at two positions in the Carina Nebula.
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Aims: We aim at deriving the excitation conditions of the interstellar gas as well as the local FUV intensities in the molecular cloud surrounding NGC 3603 to get a coherent picture of how the gas is energized by the central stars. Methods: The NANTE
N2-4m submillimeter antenna is used to map the [CI] 1-0, 2-1 and CO 4-3, 7-6 lines in a 2 x 2 region around the young OB cluster NGC 3603 YC. These data are combined with C18O 2-1 data, HIRES-processed IRAS 60 and 100 micron maps of the FIR continuum, and Spitzer/IRAC maps. Results: The NANTEN2 observations show the presence of two molecular clumps located south-east and south-west of the cluster and confirm the overall structure already found by previous CS and C18O observations. We find a slight position offset of the peak intensity of CO and [CI], and the atomic carbon appears to be further extended compared to the molecular material. We used the HIRES far-infrared dust data to derive a map of the FUV field heating the dust. We constrain the FUV field to values of chi = 3 - 6 times 10^3 in units of the Draine field across the clouds. Approximately 0.2 to 0.3 % of the total FUV energy is re-emitted in the [CII] 158 {mu}m cooling line observed by ISO. Applying LTE and escape probability calculations, we derive temperatures (TMM1 = 43 K, TMM2 = 47 K), column densities (N(MM1) = 0.9 times 10^22 cm^-2, N(MM2) = 2.5 times 10^22 cm^-2) and densities (n(MM1) = 3 times 10^3 cm^-3, n(MM2) = 10^3 -10^4 cm^-3) for the two observed molecular clumps MM1 and MM2. Conclusions: The cluster is strongly interacting with the ambient molecular cloud, governing its structure and physical conditions. A stability analysis shows the existence of gravitationally collapsing gas clumps which should lead to star formation. Embedded IR sources have already been observed in the outskirts of the molecular cloud and seem to support our conclusions.
Aim: The aim of the paper is to understand the emission from the photon dominated regions in Cepheus B, estimate the column densities of neutral carbon in bulk of the gas in Cepheus B and to derive constraints on the factors which determine the abund
ance of neutral carbon relative to CO. Methods: This paper presents 15x15 fully sampled maps of CI at 492 GHz and 12CO 4-3 observed with KOSMA at 1 resolution. The new observations have been combined with the FCRAO 12CO 1-0, IRAM-30m 13CO 2-1 and C18O 1-0 data, and far-infrared continuum data from HIRES/IRAS. The KOSMA-tau spherical PDR model has been used to understand the CI and CO emission from the PDRs in Cepheus B and to explain the observed variation of the relative abundances of both C^0 and CO. Results: The emission from the PDR associated with Cepheus B is primarily at V_LSR between -14 and -11 km s^-1. We estimate about 23% of the observed CII emission from the molecular hotspot is due to the ionized gas in the HII region. Over bulk of the material the C^0 column density does not change significantly, (2.0+-1.4)x10^17 cm^-2, although the CO column density changes by an order of magnitude. The observed cbyco abundance ratio varies between 0.06 and 4 in Cepheus B. We find an anti-correlation of the observed C/CO abundance ratio with the observed hydrogen column density, which holds even when all previous observations providing C/CO ratios are included. Here we show that this observed variation of C/CO abundance with total column density can be explained only by clumpy PDRs consisting of an ensemble of clumps. At high H2 column densities high mass clumps, which exhibit low C/CO abundance, dominate, while at low column densities, low mass clumps with high C/CO abundance dominate.
We present CI 3P1-3P0 spectra at four spiral arm positions and the nuclei of the nearby galaxies M83 and M51 obtained at the JCMT. This data is complemented with maps of CO 1-0, 2-1, and 3-2, and ISO/LWS far-infrared data of CII (158 micron), OI (63
micron), and NII (122 micron) allowing for the investigation of a complete set of all major gas cooling lines. From the intensity of the NII line, we estimate that between 15% and 30% of the observed CII emission originate from the dense ionized phase of the ISM. The analysis indicates that emission from the diffuse ionized medium is negligible. In combination with the FIR dust continuum, we find gas heating efficiencies below ~0.21% in the nuclei, and between 0.25 and 0.36% at the outer positions. Comparison with models of photon-dominated regions (PDRs) of Kaufman et al. (1999) with the standard ratios OI(63)/CII_PDR and (OI(63)+CII_PDR) vs. TIR, the total infrared intensity, yields two solutions. The physically most plausible solution exhibits slightly lower densities and higher FUV fields than found when using a full set of line ratios, CII_PDR/CI(1-0), CI(1-0)/CO(3-2), CO(3-2)/CO(1-0), CII/CO(3-2), and, OI(63)/CII_PDR. The best fits to the latter ratios yield densities of 10^4 cm^-3 and FUV fields of ~G_0=20-30 times the average interstellar field without much variation. At the outer positions, the observed total infrared intensities are in perfect agreement with the derived best fitting FUV intensities. The ratio of the two intensities lies at 4-5 at the nuclei, indicating the presence of other mechanisms heating the dust.
We present a detailed theoretical study of the rotational excitation of CH$^+$ due to reactive and nonreactive collisions involving C$^+(^2P)$, H$_2$, CH$^+$, H and free electrons. Specifically, the formation of CH$^+$ proceeds through the reaction b
etween C$^+(^2P)$ and H$_2( u_{rm H_2}=1, 2)$, while the collisional (de)excitation and destruction of CH$^+$ is due to collisions with hydrogen atoms and free electrons. State-to-state and initial-state-specific rate coefficients are computed in the kinetic temperature range 10-3000~K for the inelastic, exchange, abstraction and dissociative recombination processes using accurate potential energy surfaces and the best scattering methods. Good agreement, within a factor of 2, is found between the experimental and theoretical thermal rate coefficients, except for the reaction of CH$^+$ with H atoms at kinetic temperatures below 50~K. The full set of collisional and chemical data are then implemented in a radiative transfer model. Our Non-LTE calculations confirm that the formation pumping due to vibrationally excited H$_2$ has a substantial effect on the excitation of CH$^+$ in photon-dominated regions. In addition, we are able to reproduce, within error bars, the far-infrared observations of CH$^+$ toward the Orion Bar and the planetary nebula NGC~7027. Our results further suggest that the population of $ u_{rm H_2}=2$ might be significant in the photon-dominated region of NGC~7027.
Large scale mapping observations of the 3P1-3P0 fine structure transition of atomic carbon (CI, 492 GHz) and the J=3-2 transition of CO (346 GHz) toward the Orion A molecular cloud have been carried out with the Mt. Fuji submillimeter-wave telescope.
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