A high-resolution study of the CO-H2 conversion factor in the diffuse cloud MBM 40

Cotten, David L.; Magnani, L.

doi:10.1093/mnras/stt1646

Cited by 20 publications

(15 citation statements)

References 29 publications

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“…We found the X CO factor significantly varies within the Perseus cloud, X CO ∼ (0.3-2.0) × 10 20 cm −2 K −1 km −1 s with a typical uncertainty of 10-50%. The X CO variation within molecular clouds is discussed in some previous studies (e.g., Magnani et al 1998;Cotten & Magnani 2013; Lee et al 2014), and the present result agrees with these results. We also found the average value is X CO ∼ 1.0 × 10 20 cm −2 K −1 km −1 s (Figure 11(e)), which is consistent with the empirical and typical value of the Galaxy, (1-2) × 10 20 cm −2 K −1 km −1 s. On the other hand, Lee et al (2012Lee et al ( , 2014) also estimated a spatial distribution the X CO factor in the Perseus molecular cloud.…”

Section: Co Estimationsupporting

confidence: 93%

“…This anti-correlation reflects that in the diffuse (low-W CO ) region CO molecule is photo-dissociated more effectively than H 2 , and therefore X CO increases. This trend is consistent with the result in Cotten & Magnani (2013); Schultheis et al (2014), and we can say that the spatial distribution of X CO can be well calculated by using the present method.…”

Section: 87±004supporting

confidence: 92%

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H i, CO, and Dust in the Perseus Cloud

Okamoto

Yamamoto

Tachihara

et al. 2017

ApJ

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Comparison analyses between the gas emission data (H i 21 cm line and CO 2.6 mm line) and the Planck /IRAS dust emission data (optical depth at 353 GHz τ 353 and dust temperature T d ) allow us to estimate the amount and distribution of the hydrogen gas more accurately, and our previous studies revealed the existence of a large amount of optically-thick H i gas in the solar neighborhood. Referring to this, we discuss the neutral hydrogen gas around the Perseus cloud in the present paper. By using the J-band extinction data, we found that τ 353 increases as a function of the 1.3-th power of column number density of the total hydrogen (N H ), and this implies dust evolution in high density regions. This calibrated τ 353 -N H relationship shows that the amount of the H i gas can be underestimated to be ∼60% if the optically-thin H i method is used. Based on this relationship, we calculated optical depth of the 21 cm line (τ H i ), and found that τ H i ∼ 0.92 around the molecular cloud. The effect of τ H i is still significant even if we take into account the dust evolution. We also estimated a spatial distribution of the CO-to-H 2 conversion factor (X CO ), and we found its average value is X CO ∼ 1.0 × 10 20 cm −2 K −1 km −1 s. Although these results are inconsistent with some previous studies, these discrepancies can be well explained by the difference of the data and analyses methods.

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Section: Co Estimationsupporting

confidence: 93%

Section: 87±004supporting

confidence: 92%

H i, CO, and Dust in the Perseus Cloud

Okamoto

Yamamoto

Tachihara

et al. 2017

ApJ

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“…Our average X CO is higher than the average value found in Perseus , which may suggest that the molecular fraction of the dark gas in Perseus is significantly lower than 1 or cosmic rays do not penetrate deeply into the CO-bright regions of the cloud. Our X CO for every cloud is consistent with that found in the high latitude cloud MBM 40 which found an average X CO = 1.3×10 20 cm −2 (K km s −1 ) −1 (Cotten & Magnani 2013). Figure 10 shows X CO as a function of Galactocentric distance.…”

Section: X-factorssupporting

confidence: 86%

High-Latitude, Transluscent Molecular Clouds as Probes of Local Cosmic Rays

Abrahams

2015

ApJ

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We analyze the gamma-ray emission from 9 high latitude, translucent molecular clouds taken with the Fermi Large Area Telescope (LAT) between 250 MeV and 10 GeV. Observations of gamma-rays allow us to probe the density and spectrum of cosmic rays in the solar neighborhood. The clouds studied lie within ∼ 270 pc from the Sun and are selected from the Planck all-sky CO map. Gammarays in this energy range mostly result from cosmic ray interactions with the interstellar medium, which is traced with three components: H I , CO, and dark gas. Every cloud is detected and shows significant, extended gamma-ray emission from molecular gas. The gamma-ray emission is dominated by the CO-emitting gas in some clouds, but by the CO-dark gas in others. The average emissivity and gamma-ray power law index from H I above 1 GeV shows no evidence of a systematic variation. The CO-to-H 2 conversion factor shows no variation between clouds over this small spatial range, but shows significant variations within each cloud. The average CO-to-H 2 conversion factor suggests that the CO-dark gas is molecular as opposed to optically thick H I .

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“…The X CO becomes smaller in the MBM 53-55 region and larger in the Perseus region. According to recent studies of X CO in the local interstellar clouds, X CO in high-density regions tends to be lower (e.g., Cotten & Magnani 2013;Schultheis et al 2014). A comparison with the W CO (peak) follows this tendency.…”

Section: Comparison With Other Molecular Cloud Regionsmentioning

confidence: 99%

Gas and Dust Properties in the Chamaeleon Molecular Cloud Complex Based on the Optically Thick H i

Hayashi

Okamoto²,

Yamamoto³

et al. 2019

ApJ

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Gas and dust properties in the Chamaeleon molecular cloud complex have been investigated with emission lines from atomic hydrogen (H i) and 12 CO molecule, dust optical depth at 353 GHz (τ 353 ), and J-band infrared extinction (A J ). We have found a scatter correlation between the H i integrated intensity (W H i ) and τ 353 in the Chamaeleon region. The scattering has been examined in terms of possible large optical depth in H i emission (τ H i ) using a total column density (N H ) model based on τ 353 . A nonlinear relation of τ 353 with the ∼1.2 power of A J has been found in opaque regions (A J 0.3 mag), which may indicate dust evolution effect. If we apply this nonlinear relation to the N H model (i.e., N H ∝ τ 1/1.2 353 ) allowing arbitrary τ H i , the model curve reproduces well the W H i -τ 353 scatter correlation, suggesting optically thick H i (τ H i ∼1.3) extended around the molecular clouds.Based on the correlations between the CO integrated intensity and the N H model, we have then derived the CO-to-H 2 conversion factor (X CO ) on ∼1.5 • scales (corresponding to ∼4 persec) and found spatial variations of X CO ∼(0.5-3)×10 20 cm −2 K −1 km −1 s across the cloud complex, possibly depending on the radiation field inside or surrounding the molecular clouds. These gas properties found in the Chamaeleon region are discussed through a comparison with other local molecular cloud complexes.

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A high-resolution study of the CO-H2 conversion factor in the diffuse cloud MBM 40

Cited by 20 publications

References 29 publications

H i, CO, and Dust in the Perseus Cloud

H i, CO, and Dust in the Perseus Cloud

High-Latitude, Transluscent Molecular Clouds as Probes of Local Cosmic Rays

Gas and Dust Properties in the Chamaeleon Molecular Cloud Complex Based on the Optically Thick H i

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