{
  "abstract": "The HyGAL Stratospheric Observatory for Infrared Astronomy (SOFIA) legacy program aims to characterize the interstellar medium (ISM) in the Milky Way using hydrides, [C II], and [O I] absorption lines with the 2.7 m SOFIA telescope toward twenty-five submillimeter-bright Galactic star-forming regions. As part of HyGAL, we investigated correlations among the known H 2 tracers -CH and OH from SOFIA observations, and HCO + and CCH from ancillary absorption line data from ground-based telescopes. We also examined the abundance variation of neutral atomic oxygen, [O I], observed in absorption. CH, OH, HCO + , and CCH all exhibit strong mutual correlations. OH in particular shows tight correlations with HCO + and CCH, reflecting their linked chemical and physical pathways. Column density ratios among these H 2 tracers are consistent with previous measurements in local diffuse clouds and remain uniform across Galactic environments and velocity intervals. The gas phase oxygen abundance relative to total hydrogen, ⟨X(O)⟩ = N(O)/N(H total ), is (3.09 ± 0.64) × 10 -4 , which is slightly below the elemental solar value but consistent with the previous observations measuring gas-phase abundances. We also find that N(H I) decreases toward the regions where the molecular fraction exceeds f N H2 ∼ 0.5, marking the onset of the molecular phase. While the atomic oxygen abundance remains roughly constant, the abundances of OH, HCO + , and CCH increase with the molecular fraction. Gas traced by the HCO + absorption corresponds to higher molecular fractions than that traced by H I and hydride ions, highlighting density variations in the diffuse-to-translucent ISM along different lines of sight.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": null,
  "author_total": 20,
  "authors": [
    {
      "name": "W-J Kim"
    },
    {
      "name": "A M Jacob"
    },
    {
      "name": "D A Neufeld"
    },
    {
      "name": "P Schilke"
    },
    {
      "name": "H Wiesemeyer"
    },
    {
      "name": "M Gerin"
    },
    {
      "name": "M G Wolfire"
    },
    {
      "name": "V Ossenkopf-Okada"
    },
    {
      "name": "V Valdivia"
    },
    {
      "name": "E Falgarone"
    },
    {
      "name": "D C Lis"
    },
    {
      "name": "S Bialy"
    },
    {
      "name": "M R Rugel"
    },
    {
      "name": "Á Sánchez-Monge"
    },
    {
      "name": "M Busch"
    },
    {
      "name": "T Möller"
    },
    {
      "name": "F Wyrowski"
    },
    {
      "name": "D Seifried"
    },
    {
      "name": "K M Menten"
    },
    {
      "name": "A Saintonge"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/a4ajpwg0twmb2.bib",
  "cited_by": 0,
  "doi": "10.1051/0004-6361/202658863",
  "fulltext_url": null,
  "id": "a4ajpwg0twmb2",
  "issns": [
    "0004-6361",
    "1432-0746"
  ],
  "landing_url": "https://hal.science/hal-05753723v1",
  "language": "en",
  "licence": "CC-BY-4.0",
  "licence_label": "CC BY 4.0",
  "licence_source": "article",
  "licence_url": "https://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": null,
  "native_subjects": [
    [
      "hal",
      "0.chim"
    ],
    [
      "hal",
      "0.phys"
    ],
    [
      "hal",
      "1.chim.theo"
    ],
    [
      "hal",
      "1.phys.astr"
    ],
    [
      "hal",
      "2.phys.astr.ga"
    ],
    [
      "openalex",
      "3103|Astrophysics and Star Formation Studies"
    ]
  ],
  "openalex": "W7161690944",
  "pdf_url": "https://hal.science/hal-05753723/document",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-17",
  "publisher": "EDP Sciences",
  "ris": "https://research.draconitic.com/paper/a4ajpwg0twmb2.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:17:36Z",
      "native_id": "hal-05753723",
      "source": "hal",
      "url": "https://hal.science/hal-05753723v1"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "hal:2.phys.astr.ga",
      "subfield": "galaxies"
    },
    {
      "field": "chemistry",
      "origin": "hal:1.chim.theo",
      "subfield": "theoretical"
    },
    {
      "field": "physics",
      "origin": "hal:0.phys",
      "subfield": "general"
    }
  ],
  "title": "HyGAL: Characterizing the Galactic ISM with observations of hydrides and other small molecules",
  "title_text": "HyGAL: Characterizing the Galactic ISM with observations of hydrides and other small molecules",
  "type": "peer-reviewed",
  "updated_at": "2026-10-30T12:17:36Z",
  "url": "https://research.draconitic.com/paper/a4ajpwg0twmb2",
  "venue": "Astronomy & Astrophysics - A&A",
  "versions": []
}