The Distribution of Metallicities in the Local Galactic Interstellar Medium

Ritchey, Adam M.; Jenkins, Edward B.; Shull, J. M.; Savage, Blair D.; Federman, S. R.; Lambert, David L.

doi:10.48550/arxiv.2301.09743

Cited by 3 publications

(1 citation statement)

References 49 publications

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“…For example, both ρ Oph A and ρ Oph D have anomalously low K I and Na I column densities relative to N(H tot ) (Welty & Hobbs 2001;Snow et al 2008). Furthermore, while both sight lines exhibit severe depletions of many different elements from the gas phase (e.g., Ritchey et al 2023), the molecular hydrogen fractions are unusually low. An enhanced UV radiation field may be responsible for the anomalously low column densities of Cl I, K I, Na I, and H 2 in these directions.…”

Section: Column Density Correlationsmentioning

confidence: 99%

A Reexamination of Phosphorus and Chlorine Depletions in the Diffuse Interstellar Medium*

Ritchey

Brown

Federman

et al. 2023

ApJ

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We present a comprehensive examination of interstellar P and Cl abundances based on an analysis of archival spectra acquired with the Space Telescope Imaging Spectrograph of the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer. Column densities of P ii, Cl i, and Cl ii are determined for a combined sample of 107 sight lines probing diffuse atomic and molecular gas in the local Galactic interstellar medium. We reevaluate the nearly linear relationship between the column densities of Cl i and H2, which arises from the rapid conversion of Cl+ to Cl0 in regions where H2 is abundant. Using the observed total gas-phase P and Cl abundances, we derive depletion parameters for these elements, adopting the methodology of Jenkins. We find that both P and Cl are essentially undepleted along sight lines showing the lowest overall depletions. Increasingly severe depletions of P are seen along molecule-rich sight lines. In contrast, gas-phase Cl abundances show no systematic variation with molecular hydrogen fraction. However, enhanced Cl (and P) depletion rates are found for a subset of sight lines showing elevated levels of Cl ionization. An analysis of neutral chlorine fractions yields estimates for the amount of atomic hydrogen associated with the H2-bearing gas in each direction. These results indicate that the molecular fraction in the H2-bearing gas is at least 10% for all sight lines with log N ( H 2 ) ≳ 18 and that the gas is essentially fully molecular at log N ( H 2 ) ≈ 21 .

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Section: Column Density Correlationsmentioning

confidence: 99%

A Reexamination of Phosphorus and Chlorine Depletions in the Diffuse Interstellar Medium*

Ritchey

Brown

Federman

et al. 2023

ApJ

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Constraining gas metal mixing strength in simulations using observations of the Milky Way’s disc

Sarrato-Alós,

Brook,

Di Cintio

2023

Monthly Notices of the Royal Astronomical Society

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This work explores the mixing rate of metals in the interstellar medium (ISM), comparing observational constraints from our solar neighbourhood to high resolution cosmological hydrodynamical simulations of Milky Way (MW)-like galaxies. The mixing rate, described by the coefficient C, is varied in simulations between 0 and 0.05, with resultant simulated galaxies compared to observations of metallicity dispersion in young star clusters, H ii regions and neutral gas in the disc of the MW. A value of C between 0.003125 and 0.0125 is found to self-consistently match a range of observables, with a best estimate of C = 0.0064 ± 0.0004. We demonstrate that the relationship between metal dispersion in young stars, H ii regions and neutral gas, versus the coefficient C, can be described by a power law. These constrained mixing rates infer a comparatively well-mixed ISM in the solar neighbourhood, at odds with some recent observations that have reported a highly inhomogeneous ISM. The degree of mixing suggested by this work is lower than what often employed in many hydrodynamical simulations. Our results have implications for studying the metallicity distribution of stars as well as of gas in the interstellar and circum-galactic media.

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Excitation of CO molecules in diffuse gas over cosmic history

Klimenko,

Balashev,

Noterdaeme

et al. 2024

Monthly Notices of the Royal Astronomical Society

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We studied the physical conditions in the local ISM and at high redshift by fitting observed column densities of CO and H2 rotational levels, and C i fine-structure levels using photo-dissociation models calculated with the Meudon PDR code. We analyzed CO absorption systems in 28 sight lines in the local ISM and 7 damped Lyman-α absorption systems at high redshift, covering N(H2) = 1019 to 1021.5 cm−2 and N(CO) = 1013 to 1018 cm−2. We constructed a method to accurately calculate CO excitation, incorporating the effects of photon trapping. Our findings indicate that in the local ISM, CO excitation is primarily driven by collisions and excitation due to the CMB radiation. We demonstrated that an increase in CO excitation, observed near N(CO) ≃ 1015 cm−2, is attributed to an increase in gas densities from ≃ 100 cm−3 to ≃ 300 − 1000 cm−3. CO absorption systems in the local ISM are characterized by a gas number density of about 10-1000 cm−3, a kinetic temperature of 10-100 K, and an intensity of external UV field ranging from 0.1 to 10 units of Mathis field. Compared to the average gas probed by C i absorption, the gas detected in CO is denser and colder, while the external UV field remains nearly constant. We observed a negative correlation between the kinetic temperature and both N(CO) and N(H2), with power-law slopes of −0.21 ± 0.02 and −0.65 ± 0.05, respectively. At the same time, the gas number density exhibits a positive correlation with N(CO) and N(H2) with slopes of 0.38 ± 0.02 and 1.15 ± 0.05, respectively.

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The Distribution of Metallicities in the Local Galactic Interstellar Medium

Cited by 3 publications

References 49 publications

A Reexamination of Phosphorus and Chlorine Depletions in the Diffuse Interstellar Medium*

A Reexamination of Phosphorus and Chlorine Depletions in the Diffuse Interstellar Medium*

Constraining gas metal mixing strength in simulations using observations of the Milky Way’s disc

Excitation of CO molecules in diffuse gas over cosmic history

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