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Title: THE FRACTIONAL IONIZATION OF THE WARM NEUTRAL INTERSTELLAR MEDIUM

Abstract

When the neutral interstellar medium is exposed to extreme-ultraviolet and soft X-ray radiation, the argon atoms in it are far more susceptible to being ionized than the hydrogen atoms. We make use of this fact to determine the level of ionization in the nearby warm neutral medium. By analyzing Far-Ultraviolet Spectroscopic Explorer observations of ultraviolet spectra of 44 hot subdwarf stars a few hundred parsecs away from the Sun, we can compare column densities of Ar I to those of O I, where the relative ionization of oxygen can be used as a proxy for that of hydrogen. The measured deficiency [Ar I/O I]=-0.427{+-}0.11 dex below the expectation for a fully neutral medium implies that the electron density n(e) Almost-Equal-To 0.04 cm{sup -3} if n(H) = 0.5 cm{sup -3}. This amount of ionization is considerably larger than what we expect from primary photoionizations resulting from cosmic rays, the diffuse X-ray background, and X-ray emitting sources within the medium, along with the additional ionizations caused by energetic secondary photoelectrons, Auger electrons, and photons from helium recombinations. We favor an explanation that bursts of radiation created by previous, nearby supernova remnants that have faded by now may have elevated the ionization, andmore » the gas has not yet recombined to a quiescent level. A different alternative is that the low-energy portion of the soft X-ray background is poorly shielded by the H I because it is frothy and has internal pockets of very hot, X-ray emitting gases.« less

Authors:
 [1]
  1. Princeton University Observatory, Princeton, NJ 08544-1001 (United States)
Publication Date:
OSTI Identifier:
22167077
Resource Type:
Journal Article
Journal Name:
Astrophysical Journal
Additional Journal Information:
Journal Volume: 764; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0004-637X
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ARGON; ATOMS; COMPARATIVE EVALUATIONS; COSMIC RADIATION; ELECTRON DENSITY; ELECTRONS; EXTREME ULTRAVIOLET RADIATION; FAR ULTRAVIOLET RADIATION; HELIUM; HYDROGEN; MATTER; OXYGEN; PHOTOIONIZATION; PHOTONS; SOFT X RADIATION; SUN; SUPERNOVA REMNANTS; ULTRAVIOLET SPECTRA

Citation Formats

Jenkins, Edward B., E-mail: ebj@astro.princeton.edu. THE FRACTIONAL IONIZATION OF THE WARM NEUTRAL INTERSTELLAR MEDIUM. United States: N. p., 2013. Web. doi:10.1088/0004-637X/764/1/25.
Jenkins, Edward B., E-mail: ebj@astro.princeton.edu. THE FRACTIONAL IONIZATION OF THE WARM NEUTRAL INTERSTELLAR MEDIUM. United States. https://doi.org/10.1088/0004-637X/764/1/25
Jenkins, Edward B., E-mail: ebj@astro.princeton.edu. 2013. "THE FRACTIONAL IONIZATION OF THE WARM NEUTRAL INTERSTELLAR MEDIUM". United States. https://doi.org/10.1088/0004-637X/764/1/25.
@article{osti_22167077,
title = {THE FRACTIONAL IONIZATION OF THE WARM NEUTRAL INTERSTELLAR MEDIUM},
author = {Jenkins, Edward B., E-mail: ebj@astro.princeton.edu},
abstractNote = {When the neutral interstellar medium is exposed to extreme-ultraviolet and soft X-ray radiation, the argon atoms in it are far more susceptible to being ionized than the hydrogen atoms. We make use of this fact to determine the level of ionization in the nearby warm neutral medium. By analyzing Far-Ultraviolet Spectroscopic Explorer observations of ultraviolet spectra of 44 hot subdwarf stars a few hundred parsecs away from the Sun, we can compare column densities of Ar I to those of O I, where the relative ionization of oxygen can be used as a proxy for that of hydrogen. The measured deficiency [Ar I/O I]=-0.427{+-}0.11 dex below the expectation for a fully neutral medium implies that the electron density n(e) Almost-Equal-To 0.04 cm{sup -3} if n(H) = 0.5 cm{sup -3}. This amount of ionization is considerably larger than what we expect from primary photoionizations resulting from cosmic rays, the diffuse X-ray background, and X-ray emitting sources within the medium, along with the additional ionizations caused by energetic secondary photoelectrons, Auger electrons, and photons from helium recombinations. We favor an explanation that bursts of radiation created by previous, nearby supernova remnants that have faded by now may have elevated the ionization, and the gas has not yet recombined to a quiescent level. A different alternative is that the low-energy portion of the soft X-ray background is poorly shielded by the H I because it is frothy and has internal pockets of very hot, X-ray emitting gases.},
doi = {10.1088/0004-637X/764/1/25},
url = {https://www.osti.gov/biblio/22167077}, journal = {Astrophysical Journal},
issn = {0004-637X},
number = 1,
volume = 764,
place = {United States},
year = {Sun Feb 10 00:00:00 EST 2013},
month = {Sun Feb 10 00:00:00 EST 2013}
}