HD 62542: Probing the Bare, Dense Core of a Translucent Interstellar CloudD. E. Welty (dwelty[at]stsci.edu), P. Sonnentrucker, B. L. Rachford, T. P. Snow, and D. G. York
High spatial resolution maps of the emission from dust, neutral hydrogen, CO, and other species indicate that both the overall structure of the interstellar medium (ISM) and the internal structure of individual interstellar "clouds" can be quite complex. Filamentary and fractal-like structures are common, and clouds with appreciable fractions of hydrogen in molecular form have cold, dense, mostly molecular cores surrounded by more diffuse, primarily atomic gas (Wannier et al. 1999; Kalberla et al. 2016). High spectral resolution observations of the interstellar absorption from various atomic and molecular species indicate that many sight lines contain multiple narrow, closely blended velocity components, which can be characterized by wide ranges in relative abundances (Welty et al. 1996). This general spatial and spectral complexity has made it difficult to determine the properties of individual interstellar clouds. For the so-called "translucent" clouds—thought to be transitional objects between the more diffuse, primarily atomic clouds and the much denser molecular clouds associated with star formation—the expected high molecular fractions and severe depletions of various refractory elements into dust have thus far not been seen, due largely to the difficulty in separating the contributions of the denser molecular gas from those of the more diffuse atomic gas that is generally present (Rachford et al. 2009).
The sight line toward the moderately reddened B3-5 V star HD 62542 offers a rare opportunity to probe the relatively dense molecular core of a single translucent cloud. The interstellar absorption in this sight line is dominated by a strong, narrow "main" component/cloud containing nearly 90% of the total hydrogen (and >99% of the H2), with a molecular fraction f(H2) = 2H2/(H + 2H2) = 0.81; much of the usual associated atomic gas has apparently been stripped away by the winds and/or radiation from the nearby ζ Pup (O5 Iaf) and γ2 Vel (WC8+O7.5). High-resolution (FWHM ~ 2.7 km s-1), moderately high S/N ratio STIS echelle spectra of HD 62542 (Figs. 2 and 3) have yielded detections of many atomic and molecular species, which provide multiple diagnostics for assessing the physical conditions. Striking differences in the absorption-line profiles of the various species suggest significant variations in both abundances/depletions and physical conditions between the main and "other" components (Welty et al. 2019).
Analyses of the rotational excitation of H2, C2, CN, and CO in the main component toward HD 62542 can yield estimates for the local kinetic temperature and density (Sonnentrucker et al. 2007; Goldsmith 2013). For the homonuclear H2 and C2 (Fig. 3), the relative populations in the lowest rotational levels imply Tk = 40–43 K. The populations in the higher C2 levels (J = 8–18) and the high f(H2) = 0.81 then yield nH/INIR ~ 420 cm–3, where nH is the total hydrogen density (H + 2H2) and INIR gives the strength of the near-IR radiation field. The subthermal excitation temperatures obtained for the polar molecules CN (2.9 K), 12CO (11.7 K), and 13CO (7.7 K) reflect a minimum excitation due to the cosmic microwave background, with additional contributions from collisions and (for CO) from nearby CO line emission.
In predominantly neutral interstellar clouds, the relative populations in the three fine-structure levels in the ground electronic state of neutral carbon reflect a balance between collisional excitation, collisional de-excitation, and radiative decays—and thus can be used to estimate the thermal pressure p/k = nHTk (Jenkins & Tripp 2011). For the main component toward HD 62542, analysis of the STIS spectra of seven C Ⅰ multiplets yields nH ~ 1500 cm–3—much higher than the typical Galactic values, but consistent with the n(H2) = 720 cm–3 inferred from [C Ⅱ] emission toward HD 62542 (Velusamy et al. 2017), and also with the value obtained from C2 (if INIR ~ 3–4—i.e., if the radiation field is slightly enhanced).
Many neutral and singly ionized atomic species—exhibiting wide ranges in ionization and depletion behavior—are detected toward HD 62542 (Fig. 2). Figure 4 compares the abundances of three dominant ions—O, Mg+, Ti+ (which typically are mildly, moderately, and severely depleted, respectively)—in the relatively nearby Galactic ISM (crosses) and toward HD 62542 (open stars). The column densities are normalized by N(Htot) and compared with f(H2) (a proxy for the local density). For f(H2) < 0.1, the normalized abundances (and thus the depletions) of all three are relatively constant, at levels similar to those found for warm, diffuse gas. For f(H2) > 0.1, however, the normalized abundances decline—more steeply for Mg and (especially) Ti—suggestive of increasingly severe depletions in denser environments. For a given f(H2), the depletions of Mg and Ti are more severe toward a set of stars in the Sco-Oph region (filled blue squares); even more severe depletions are seen toward HD 62542—especially in the main translucent component (red star).
Figure 5 compares the depletions of 12 elements seen toward HD 62542 (open stars) with those found for other Galactic sight lines. For the whole sight line toward HD 62542 (blue stars), the depletions of most of the elements are within a factor ~2 of the representative ''cold cloud'' pattern—and are among the most severe known. In the main component (red stars), most of the depletions are even more severe—particularly for Mg, Si, Fe, Ni, and Cu—more like the newly defined ''molecular cloud'' pattern; the depletions of Cl and Kr, however, are quite mild. The overall pattern of depletions in the main component thus differs from those seen in the general Galactic ISM. The depletions in the other components (green stars) resemble the ''warm cloud'' pattern.
The combination of broad UV wavelength coverage, high spectral resolution, and high S/N ratio available with HST/STIS has been crucial for obtaining accurate abundances for a wide variety of atomic and molecular species in the various individual components/clouds toward HD 62542. Neither the very severe depletions (and the unusual depletion pattern) in the main translucent component nor the range in depletions among the components would have been discerned if only the integrated sight-line values had been available.
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Welty, D. E., Sonnentrucker, P., Rachford, B. L., Snow, T. P., & York, D. G. 2019, ApJ, submitted