Toward a robust physical and chemical characterization of heterogeneous lines of sight: The case of the Horsehead nebula

¹ IRAM, 300 rue de la Piscine, 38406 Saint Martin d’Hères, France
² Université de Toulon, Aix Marseille Univ, CNRS, IM2NP, Toulon, France
³ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, 75014 Paris, France
⁴ Instituto de Física Fundamental (CSIC), Calle Serrano 121, 28006, Madrid, Spain
⁵ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁶ Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, Allée Geoffroy Saint-Hilaire, 33615 Pessac, France
⁷ Univ. Grenoble Alpes, Inria, CNRS, Grenoble INP, GIPSA-Lab, Grenoble 38000, France
⁸ Univ. Lille, CNRS, Centrale Lille, UMR 9189 – CRIStAL, 59651 Villeneuve d’Ascq, France
⁹ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, 92190 Meudon, France
¹⁰ Institut de Recherche en Astrophysique et Planétologie (IRAP), Université Paul Sabatier, Toulouse cedex 4, France
¹¹ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
¹² Laboratoire de Physique de l’Ecole normale supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université de Paris, Sorbonne Paris Cité, Paris, France
¹³ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹⁴ Department of Earth, Environment, and Physics, Worcester State University, Worcester, MA 01602, USA
¹⁵ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA, 02138, USA
¹⁶ School of Physics and Astronomy, Cardiff University, Queen’s buildings, Cardiff CF24 3AA, UK
¹⁷ Department of Astronomy, University of Florida, PO Box 112055, Gainesville, FL 32611, USA

^★ Corresponding author; antoine.roueff@univ-tln.fr

Received: 18 July 2024
Accepted: 23 September 2024

Abstract

Context. Dense and cold molecular cores and 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 and cold one because their intensities result from a complex interplay of radiative transfer and chemical properties of these heterogeneous lines of sight (LoSs).

Aims. We extend our previous single-zone modeling with a more realistic approach that introduces multiple layers to take account of possibly varying conditions along the LoS. We used the IRAM-30m data from the ORION-B large program toward the Horsehead nebula in order to demonstrate our method’s capability and effectiveness.

Methods. We propose a cloud model composed of three homogeneous slabs of gas along each LoS, representing an outer envelope and a more shielded inner layer. We used the non-LTE radiative transfer code RADEX to model the line profiles from the kinetic temperature (T_kin), the volume density (n_H2), kinematics, and chemical properties of the different layers. We then used a fast and robust maximum likelihood estimator to simultaneously fit the observed lines of the CO and HCO⁺ isotopologues. To limit the variance on the estimates, we propose a simple chemical model by constraining the column densities.

Results. A single-layer model cannot reproduce the spectral line asymmetries that result from a combination of different radial velocities and absorption effects among layers. A minimal heterogeneous model (three layers only) is sufficient for the Horsehead application, as it provides good fits of the seven fitted lines over a large part of the studied field of view. The decomposition of the intensity into three layers allowed us to discuss the distribution of the estimated physical or chemical properties along the LoS. About 80% of the ¹²CO integrated intensity comes from the outer envelope, while ~55% of the integrated intensity of the (1 − 0) and (2 − 1) lines of C¹⁸O comes from the inner layer. For the lines of the ¹³CO and the HCO⁺ isotopologues, integrated intensities are more equally distributed over the cloud layers. The estimated column density ratio N(¹³CO)/N(C¹⁸O) in the envelope increases with decreasing visual extinction, and it reaches 25 in the pillar outskirts. While the inferred T_kin of the envelope varies from 25 to 40 K, that of the inner layer drops to ~15 K in the western dense core. The estimated n_H2 in the inner layer is ~3 × 10⁴ cm⁻³ toward the filament, and it increases by a factor of ten toward dense cores.

Conclusions. Our proposed method correctly retrieves the physical and chemical properties of the Horsehead nebula. It also offers promising prospects for less supervised model fits of wider-field datasets.