Small-scale Structure Traced by Neutral Hydrogen Absorption in the Direction of Multiple-component Radio Continuum Sources

Rybarczyk, Daniel R.; Stanimirović, Snežana; Zweibel, Ellen G.; Murray, Claire E.; Dickey, John M.; Babler, B.; Heiles, Carl

doi:10.3847/1538-4357/ab83f7

Cited by 6 publications

(1 citation statement)

References 62 publications

(164 reference statements)

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“…One incentive to explore this issue is a quest to see if the C I results support findings on the existence of tiny scale atomic structures (TSAS) with extraordinarily high densities. Stanimirović & Zweibel (2018) have presented a review of various investigations on the nature of TSAS, most of which arise from observations of spatial and temporal variations of 21-cm absorption (see also Rybarczyk et al (2020)). These observations have the shortcoming that the strengths of such absorptions depend on temperatures, which are often poorly known.…”

Section: Possible Origins For the High Pressure Gasmentioning

confidence: 99%

Thermal Pressures in the Interstellar Medium away from Stellar Environments

Jenkins,

Tripp

2021

Preprint

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Interstellar thermal pressures can be measured using C I absorption lines that probe the pressure-sensitive populations of the fine-structure levels of its ground state. In a survey of C I absorption toward Galactic hot stars, Jenkins & Tripp (2011) found evidence of small amounts (∼ 0.05%) of gas at high pressures (p/k 10 4 cm −3 K) mixed with a more general presence of lower pressure material exhibiting a log normal distribution that spanned the range 10 3 p/k 10 4 cm −3 K. In this paper, we study Milky Way C I lines in the spectra of extragalactic sources instead of Galactic stars and thus measure the pressures without being influenced by regions where stellar mass loss and H II region expansions could create localized pressure elevations. We find that the distribution of low pressures in the current sample favors slightly higher pressures than the earlier survey, and the fraction of gaseous material at extremely high pressures is about the same as that found earlier. Thus we conclude that the earlier survey was not appreciably influenced by the stellar environments, and the small amounts of high pressure gas indeed exist within the general interstellar medium.

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Section: Possible Origins For the High Pressure Gasmentioning

confidence: 99%

Thermal Pressures in the Interstellar Medium away from Stellar Environments

Jenkins,

Tripp

2021

Preprint

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The temperature of the neutral interstellar medium in the Galaxy

Patra,

Roy

2024

Monthly Notices of the Royal Astronomical Society

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Atomic Hydrogen-21 cm transition (H i) is an excellent tracer to study and understand the properties of the atomic gas in the Galaxy. Using the Westerbork Synthesis Radio Telescope (WSRT), we observed 12 quasar sightlines to detect galactic H i in absorption. We achieve an optical depth RMS of ∼1 − 2 × 10−3, essential to detect the Warm Neutral Medium (WNM). We detect H i absorption in all our sightlines except along 1006+349, for which we set a strict upper limit on the spin temperature as 〈Ts〉 > 570 K. We find around 50% of our sightlines have 〈Ts〉 > 500 K, indicating a WNM dominance. Further, we calculate an upper limit of the CNM fraction along our sightlines and find a median CNM fraction of ∼0.12. With our observations, we reconfirm the existence of a threshold column density of ∼2 × 1020 $\rm atoms cm^{-2}$to form CNM in the ISM. Using a two-temperature model of the H i disk, we explore the distribution of spin temperature in the Galactic ISM. We find that a simple fixed axisymmetric two-temperature model could not produce either the observed column density or the integral optical depth. This indicates the existence of a more complex distribution of spin temperatures in the Galaxy.

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The Transition from Diffuse Molecular Gas to Molecular Cloud Material in Taurus

Federman

Rice

Ritchey

et al. 2021

ApJ

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We study four lines of sight that probe the transition from diffuse molecular gas to molecular cloud material in Taurus. Measurements of atomic and molecular absorption are used to infer the distribution of species and the physical conditions toward stars behind the Taurus Molecular Cloud (TMC). New high-resolution spectra at visible and near-IR wavelengths of interstellar Ca ii, Ca i, K i, CH, CH+, C2, CN, and CO toward HD 28975 and HD 29647 are combined with data at visible wavelengths and published CO results from ultraviolet measurements for HD 27778 and HD 30122. Gas densities and temperatures are inferred from C2, CN, and CO excitation and CN chemistry. Our results for HD 29647 are noteworthy because the CO column density is 1018 cm−2 while C2 and CO excitation reveals a temperature of 10 K and a density of ∼1000 cm−3, more like conditions found in dark molecular clouds. Similar results arise from our chemical analysis for CN through reactions involving observations of CH, C2, and NH. Enhanced potassium depletion and a reduced CH/H2 column density ratio also suggest the presence of a dark cloud. The directions toward HD 27778 and HD 30122 probe molecule-rich diffuse clouds, which can be considered CO-dark gas, while the sight line toward HD 28975 represents an intermediate case. Maps of dust temperature help refine the description of the material along the four sight lines and provide an estimate of the distance between HD 29647 and a clump in the TMC. An appendix provides results for the direction toward HD 26571; this star also probes diffuse molecular gas.

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Small-scale Structure Traced by Neutral Hydrogen Absorption in the Direction of Multiple-component Radio Continuum Sources

Cited by 6 publications

References 62 publications

Thermal Pressures in the Interstellar Medium away from Stellar Environments

Thermal Pressures in the Interstellar Medium away from Stellar Environments

The temperature of the neutral interstellar medium in the Galaxy

The Transition from Diffuse Molecular Gas to Molecular Cloud Material in Taurus

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