Imaging galactic diffuse clouds: CO emission, reddening and turbulent flow in the gas around Zeta Oph

📝 Abstract

Methods: 12CO emission is imaged in position and position-velocity space analyzed statistically, and then compared with maps of total reddening and with models of the C+ - CO transition in H2-bearing diffuse clouds. Results: Around Zeta Oph, 12CO emission appears in two distinct intervals of reddening centered near EBV = 0.4 and 0.65 mag, of which < 0.2 mag is background material. Within either interval, the integrated 12CO intensity varies up to 6-12 K-km/s compared to 1.5 K-km/s toward Zeta Oph. Nearly 80% of the individual profiles have velocity dispersions < 0.6 km/s, which are subsonic at the kinetic temperature derived from H2 toward Zeta Oph, 55 K. Partly as a result, 12CO emission exposes the internal, turbulent, supersonic (1-3 km/s) gas flows with especial clarity in the cores of strong lines. The flows are manifested as resolved velocity gradients in narrow, subsonically-broadened line cores. Conclusions: The scatter between N(CO) and EBV in global, CO absorption line surveys toward bright stars is present in the gas seen around Zeta Oph, reflecting the extreme sensitivity of N(12CO) to ambient conditions. The two-component nature of the optical absorption toward Zeta Oph is coincidental and the star is occulted by a single body of gas with a complex internal structure, not by two distinct clouds. The very bright 12CO lines in diffuse gas arise at N(H2) ~ 10^21/cm^2 in regions of modest density n(H) ~ 200-500/cc and somewhat more complete C+-CO conversion. Given the variety of structure in the foreground gas, it is apparent that only large surveys of absorption sightlines can hope to capture the intrinsic behavior of diffuse gas.

💡 Analysis

Methods: 12CO emission is imaged in position and position-velocity space analyzed statistically, and then compared with maps of total reddening and with models of the C+ - CO transition in H2-bearing diffuse clouds. Results: Around Zeta Oph, 12CO emission appears in two distinct intervals of reddening centered near EBV = 0.4 and 0.65 mag, of which < 0.2 mag is background material. Within either interval, the integrated 12CO intensity varies up to 6-12 K-km/s compared to 1.5 K-km/s toward Zeta Oph. Nearly 80% of the individual profiles have velocity dispersions < 0.6 km/s, which are subsonic at the kinetic temperature derived from H2 toward Zeta Oph, 55 K. Partly as a result, 12CO emission exposes the internal, turbulent, supersonic (1-3 km/s) gas flows with especial clarity in the cores of strong lines. The flows are manifested as resolved velocity gradients in narrow, subsonically-broadened line cores. Conclusions: The scatter between N(CO) and EBV in global, CO absorption line surveys toward bright stars is present in the gas seen around Zeta Oph, reflecting the extreme sensitivity of N(12CO) to ambient conditions. The two-component nature of the optical absorption toward Zeta Oph is coincidental and the star is occulted by a single body of gas with a complex internal structure, not by two distinct clouds. The very bright 12CO lines in diffuse gas arise at N(H2) ~ 10^21/cm^2 in regions of modest density n(H) ~ 200-500/cc and somewhat more complete C+-CO conversion. Given the variety of structure in the foreground gas, it is apparent that only large surveys of absorption sightlines can hope to capture the intrinsic behavior of diffuse gas.

📄 Content

Astronomy & Astrophysics manuscript no. 10905-xar c⃝ESO 2021 November 8, 2021 Imaging galactic diffuse clouds: CO emission, reddening and turbulent flow in the gas around ζ Oph H. S. Liszt1, J. Pety2,3, and K. Tachihara4 1 National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA, USA 22903-2475 2 Institut de Radioastronomie Millim´etrique, 300 Rue de la Piscine, F-38406 Saint Martin d’H`eres, France 3 Obs. de Paris, 61 av. de l’Observatoire, 75014, Paris, France 4 National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan received November 8, 2021 ABSTRACT Context. Most diffuse clouds are only known as kinematic features in absorption spectra, but those with appreciable H2 content may be visible in the emission of such small molecules as CH, OH, and CO. Aims. We interpret in greater detail the extensive observations of 12CO emission from diffuse gas seen around the archetypical line of sight to ζ Oph. Methods. The 12CO emission is imaged in position and position-velocity space, analyzed statistically, and then compared with maps of total reddening E∞ B−V and with models of the C+ - CO transition in H2-bearing diffuse clouds. Results. Around ζ Oph, 12CO emission appears in two distinct intervals of reddening centered near E∞ B−V ≈0.4 and 0.65 mag, of which <∼0.2 mag is background material. Within either interval, the integrated 12CO intensity varies up to 6-12 K km s−1, compared to 1.5 K km s−1 toward ζ Oph. Nearly 80% of the individual profiles have velocity dispersions σv < 0.6 km s−1, which are subsonic at the kinetic temperature derived from H2 toward ζ Oph, 55 K. Partly as a result, 12CO emission exposes the internal, turbulent, supersonic (1-3 km s−1) gas flows with especial clarity in the cores of strong lines. The flows are manifested as resolved velocity gradients in narrow, subsonically-broadened line cores. Conclusions. The scatter between N(CO) and EB−V in global, CO absorption line surveys toward bright stars is present in the gas seen around ζ Oph, reflecting the extreme sensitivity of N(12CO) to ambient conditions. The two-component nature of the optical absorption toward ζ Oph is coincidental and the star is occulted by a single body of gas with a complex internal structure, not by two distinct clouds. The very bright 12CO lines in diffuse gas arise at N(H2) ≈1021 cm−2 in regions of modest density n(H) ≈200 −500 cm−3 and somewhat more complete C+-CO conversion. Given the variety of structure in the foreground gas, it is apparent that only large surveys of absorption sightlines can hope to capture the intrinsic behavior of diffuse gas. Key words. interstellar medium – molecules

1. Introduction The line of sight to the nearby (140-160 pc) runaway O9.5 V star HD 149757, ζ Oph, has served as the archetype for detailed ob- servational studies of the internal composition of diffuse (AV<∼1 mag) clouds (Herbig, 1968; Morton, 1975), for optical/uv de- tection of new molecules in diffuse gas (Maier et al., 2001) and for theoretical models of molecular gas in diffuse clouds (Black & Dalgarno, 1977; Van Dishoeck & Black, 1986, 1988; Kopp et al., 1996). The H2-bearing portions of the gas occulting ζ Oph are dense enough to host appreciable column densities of carbon monoxide, N(12CO) ≈2.4 × 1015 cm−2 (Morton, 1975; Wannier et al., 1982; Lambert et al., 1994; Sonnentrucker et al., 2007), and these are readily detectable in mm-wave emission toward the star (Knapp & Jura, 1976; Liszt, 1979; Langer et al., 1987). They were very partially mapped in CO emission (Kopp et al., 1996; Liszt, 1997), as well as CH and OH (Crutcher, 1979; Liszt, 1997). CO J=1-0 emission around ζ Oph was imaged in much more complete fashion by Tachihara et al. (2000), who focused their discussion on the properties of the nearby dark cloud complex Send offprint requests to: H. S. Liszt Correspondence to: hliszt@nrao.edu L204 seen several degrees to the galactic South. L204 is clearly outlined against the Hα emission from the ionized gas in the star’s H II region (Gaustad et al., 2001). In this work, the 12CO datacube from Tachihara et al. (2000) is employed to study the diffuse gas at AV ≈1 mag seen nearer the star. We scrutinize the entire CO image of the absorption-line host whose overall prop- erties have so often been inferred from one microscopic absorp- tion sightline toward the star, and we inquire to what extent that line of sight faithfully represents the host gas. Moreover, large- scale maps of reddening and extinction have become available at comparable resolution (though only along the entire line of sight, see Schlegel et al. (1998) and Dobashi et al. (2005)), and we em- ploy these to control against possible confusion between diffuse and darker sightlines, a source of concern given the strong CO lines we see. The plan of this work is as follows. Section 2 summarizes what is known observationally of the line of sight toward the star and describes the pre-existing H I, CO, A∞ V and E∞ B−V da

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