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Register a new userAbundances and Behavior of 12CO, 13CO, and C2 in Translucent Sight Lines
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P. Sonnentrucker
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Using UV spectra obtained with FUSE, HST, and/or IUE, we determine interstellar column densities of 12CO, 13CO, and/or C_2 for ten Galactic sight lines with 0.37<E(B-V)<0.72. The N(CO)/N(H_2) ratio varies over a factor of 100 in this sample, due primarily to differences in N(CO). For a given N(H_2), published models of diffuse and translucent clouds predict less CO than is observed. The J=1-3 rotational levels of 12CO are sub-thermally populated in these sight lines, with T_ex typically between 3 and 7 K. In general, there is no significant difference between the excitation temperatures of 12CO and 13CO. Fits to the higher resolution CO line profiles suggest that CO (like CN) is concentrated in relatively cold, dense gas. We obtain C_2 column densities from the F-X (1-0) and (0-0) bands (1314 and 1341 A), the D-X (0-0) band (2313 A), and the A-X (3-0) and (2-0) bands (7719 and 8757 A). Comparisons among those N(C_2) yield a set of mutually consistent f-values for the UV and optical C_2 bands, but also reveal some apparent anomalies within the F-X (0-0) band. Both the kinetic temperature inferred from the C_2 rotational populations (up to J=18) and the excitation temperature T_02(C_2) are generally smaller than the corresponding T_01(H_2). Incorporating additional data for K I, HD, CH, C_2, C_3, CN, and CO from the literature (for a total sample of 74 sight lines), we find that (1) CO is most tightly correlated with CN; (2) the ratios 12CO/H_2 and 13CO/H_2 both are fairly tightly correlated with the density indicator CN/CH (but C_2/H_2 is not); and (3) the ratio 12CO/13CO is somewhat anti-correlated with both CN/CH and N(CO). Sight lines with 12CO/13CO below the average local Galactic value of 12C/13C appear to sample colder, denser gas in which isotope exchange reactions have enhanced 13CO, relative to 12CO.
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The dust properties inferred from the analysis of Planck observations in total and polarized emission challenge current dust models. We propose new dust models compatible with polarized and unpolarized data in extinction and emission for translucent lines of sight ($0.5 < A_V < 2.5$). We amended the DustEM tool to model polarized extinction and emission. We fit the spectral dependence of the mean extinction, polarized extinction, SED and polarized SED with PAHs, astrosilicates and amorphous carbon (a-C). The astrosilicate population is aligned along the magnetic field lines, while the a-C population may be aligned or not. With their current optical properties, oblate astrosilicate grains are not emissive enough to reproduce the emission to extinction polarization ratio $P_{353}/p_V$ derived with Planck data. Models using prolate astrosilicate grains with an elongation $a/b=3$ and an inclusion of 20% of porosity succeed. The spectral dependence of the polarized SED is steeper in our models than in the data. Models perform slightly better when a-C grains are aligned. A small (6%) volume inclusion of a-C in the astrosilicate matrix removes the need for porosity and perfect grain alignment, and improves the fit to the polarized SED. Dust models based on astrosilicates can be reconciled with Planck data by adapting the shape of grains and adding inclusions of porosity or a-C in the astrosilicate matrix.
We report total abundances and related parameters for the full sample of the FUSE survey of molecular hydrogen in 38 translucent lines of sight. New results are presented for the second half of the survey involving 15 lines of sight to supplement data for the first 23 lines of sight already published. We assess the correlations between molecular hydrogen and various extinction parameters in the full sample, which covers a broader range of conditions than the initial sample. In particular, we are now able to confirm that many, but not all, lines of sight with shallow far-UV extinction curves and large values of the total-to-selective extinction ratio, $R_V$ = $A_V$ / $E(B-V)$ -- characteristic of larger than average dust grains -- are associated with particularly low hydrogen molecular fractions ($f_{rm H2}$). In the lines of sight with large $R_V$, there is in fact a wide range in molecular fractions, despite the expectation that the larger grains should lead to less H$_2$ formation. However, we see specific evidence that the molecular fractions in this sub-sample are inversely related to the estimated strength of the UV radiation field and thus the latter factor is more important in this regime. We have provided an update to previous values of the gas-to-dust ratio, $N$(H$_{rm tot}$)/$E(B-V)$, based on direct measurements of $N$(H$_2$) and $N$(H I). Although our value is nearly identical to that found with Copernicus data, it extends the relationship by a factor of 2 in reddening. Finally, as the new lines of sight generally show low to moderate molecular fractions, we still find little evidence for single monolithic translucent clouds with $f_{rm H2}$ $sim$ 1.
Aims. We seek to understand how the 4.8 GHz formaldehyde absorption line is distributed in the MON R2, S156, DR17/L906, and M17/M18 regions. More specifically, we look for the relationship among the H2CO, 12CO, and 13CO spectral lines. Methods. The four regions of MON R2 (60x90), S156 (50x70), DR17/L906 (40x60), and M17 /M18 (70x80)were observed for H2CO (beam 10), H110a recombination (beam 10), 6 cm continuum (beam 10), 12CO (beam 1), and 13CO (beam 1). We compared the H2CO,12CO,13CO, and continuum distributions, and also the spectra line parameters of H2CO,12CO, and 13CO. Column densities of H2CO,13CO, and H2 were also estimated. Results. We found out that the H2CO distribution is similar to the 12CO and the 13CO distributions on a large scale. The correlation between the 13 CO and the H2CO distributions is better than between the 12CO and H2CO distributions. The H2CO and the 13CO tracers systematically provide consistent views of the dense regions. T heir maps have similar shapes, sizes, peak positions, and molecular spectra and present similar centr al velocities and line widths. Such good agreement indicates that the H2CO and the 13CO arise from similar regions.
We present fully sampled ~3 resolution images of the 12CO(J=2-1), 13CO(J=2-1), and C18O(J=2-1) emission taken with the newly developed 1.85-m mm-submm telescope toward the entire area of the Orion A and B giant molecular clouds. The data were compared with the J=1-0 of the 12CO, 13CO, and C18O data taken with the Nagoya 4-m telescope and the NANTEN telescope at the same angular resolution to derive the spatial distributions of the physical properties of the molecular gas. We explore the large velocity gradient formalism to determine the gas density and temperature by using the line combinations of 12CO(J=2-1), 13CO(J=2-1), and 13CO(J=1-0) assuming uniform velocity gradient and abundance ratio of CO. The derived gas density is in the range of 500 to 5000 cm-3, and the derived gas temperature is mostly in the range of 20 to 50 K along the cloud ridge with a temperature gradient depending on the distance from the star forming region. We found the high-temperature region at the cloud edge facing to the HII region, indicating that the molecular gas is interacting with the stellar wind and radiation from the massive stars. In addition, we compared the derived gas properties with the Young Stellar Objects distribution obtained with the Spitzer telescope to investigate the relationship between the gas properties and the star formation activity therein. We found that the gas density and star formation efficiency are well positively correlated, indicating that stars form effectively in the dense gas region.
After 30 Doradus, N11 is the second largest and brightest nebula in the LMC. This large nebula has several OB associations with bright nebulae at its surroundings. N11 was previously mapped at the lowest rotational transitions of $^{12}$CO (J=1--0 and 2--1), and in some particular regions pointings of the $^{13}$CO J=1--0 and 2--1 lines were also performed. Using ASTE we mapped the whole extension of the N11 nebula in the $^{12}$CO J=3--2 line, and three sub-regions in the $^{13}$CO J=3--2 line. The regions mapped in the $^{13}$CO J=3--2 were selected based on that they may be exposed to the radiation at different ways: a region lying over the nebula related to the OB association LH10 (N11B), another one that it is associated with the southern part of the nebula related to the OB association LH13 (N11D), and finally a farther area at the southwest without any embedded OB association (N11I). We found that the morphology of the molecular clouds lying in each region shows some signatures that could be explained by the expansion of the nebulae and the action of the radiation. Fragmentation generated in a molecular shell due to the expansion of the N11 nebula is suggested. The integrated line ratios $^{12}$CO/$^{13}$CO show evidences of selective photodissociation of the $^{13}$CO, and probably other mechanisms such as chemical fractionation. The CO contribution to the continuum at 870 $mu$m was directly derived. The distribution of the integrated line ratios $^{12}$CO J=3--2/2--1 show hints of stellar feedback in N11B and N11D. The ratio between the virial and LTE mass (M$_{rm vir}$/M$_{rm LTE}$) is higher than unity in all analyzed molecular clumps, which suggests that the clumps are not gravitationally bounded and may be supported by external pressure. A non-LTE analysis suggests that we are mapping gas with densities about a few 10$^{3}$ cm$^{-3}$.
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