1. Toward robust physical and chemical characterization of heterogeneous lines of sight: The case of the Horsehead nebula
- Author
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Ségal, Léontine, Roueff, Antoine, Pety, Jérôme, Gerin, Maryvonne, Roueff, Evelyne, Goicoechea, R. Javier, Bešlic, Ivana, Coud'e, Simon, Einig, Lucas, Mazurek, Helena, Orkisz, H. Jan, Palud, Pierre, Santa-Maria, G. Miriam, Zakardjian, Antoine, Bardeau, S'ebastien, Bron, Emeric, Chainais, Pierre, Demyk, Karine, Magalhaes, Victor de Souza, Gratier, Pierre, Guzman, V. Viviana, Hughes, Annie, Languignon, David, Levrier, François, Bourlot, Jacques Le, Petit, Franck Le, Lis, C. Dariusz, Liszt, S. Harvey, Peretto, Nicolas, Sievers, Albrecht, and Thouvenin, Pierre-Antoine
- Subjects
Astrophysics - Astrophysics of Galaxies - Abstract
Dense cold molecular cores/filaments are surrounded by an envelope of translucent gas. Some of the low-J emission lines of CO and HCO$^+$ isotopologues are more sensitive to the conditions either in the translucent environment or in the dense cold one. We propose a cloud model composed of three homogeneous slabs of gas along each LoS, representing an envelope and a shielded inner layer. IRAM-30m data from the ORION-B large program toward the Horsehead nebula are used to demonstrate the method's capability. We use the non-LTE radiative transfer code RADEX to model the line profiles from the kinetic temperature $T_{kin}$, the volume density $n_{H_2}$, kinematics and chemical properties of the different layers. We then use a maximum likelihood estimator to simultaneously fit the lines of the CO and HCO$^+$ isotopologues. We constrain column density ratios to limit the variance on the estimates. This simple heterogeneous model provides good fits of the fitted lines over a large part of the cloud. The decomposition of the intensity into three layers allows to discuss the distribution of the estimated physical/chemical properties along the LoS. About 80$\%$ the CO integrated intensity comes from the envelope, while $\sim55\%$ of that of the (1-0) and (2-1) lines of C$^{18}$O comes from the inner layer. The $N(^{13}CO)/N(C^{18}O)$ in the envelope increases with decreasing $A_v$, and reaches $25$ in the pillar outskirts. The envelope $T_{kin}$ varies from 25 to 40 K, that of the inner layer drops to $\sim 15$ K in the western dense core. The inner layer $n_{H_2}$ is $\sim 3\times10^4\,\text{cm}^{-3}$ toward the filament and it increases by a factor $10$ toward dense cores. The proposed method correctly retrieves the physical/chemical properties of the Horsehead nebula and offers promising prospects for less supervised model fits of wider-field datasets.
- Published
- 2024