The extremely sharp transition between molecular and ionized gas in the Horsehead nebula^★

¹ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
e-mail: chernandez@astro.puc.cl
² Núcleo Milenio de Formación Planetaria (NPF), Universidad de Valparaíso, Av. Gran Bretaña 1111, Valparaíso, Chile
³ Instituto de Física Fundamental (CSIC), Calle Serrano 121, 28006, Madrid, Spain
⁴ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, 92190 Meudon, France
⁵ IRAM, 300 rue de la Piscine, 38406 Saint-Martin-d’Hères, France
⁶ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, 75014 Paris, France
⁷ Institut d’Astrophysique Spatiale, Université Paris-Saclay, CNRS, Bâtiment 121, 91405 Orsay Cedex, France
⁸ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
⁹ Joint ALMA Observatory, Alonso de Cordova 3107 Vitacura, Santiago, Chile
¹⁰ Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, Allée Geoffroy Saint-Hilaire, 33615 Pessac, France
¹¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218-2463, USA
¹² Steward Observatory, University of Arizona, Tucson, AZ 85721, USA

Received: 15 June 2023
Accepted: 10 July 2023

Abstract

Massive stars can determine the evolution of molecular clouds by eroding and photo-evaporating their surfaces with strong ultraviolet (UV) radiation fields. Moreover, UV radiation is relevant in setting the thermal gas pressure in star-forming clouds, whose influence can extend across various spatial scales, from the rims of molecular clouds to entire star-forming galaxies. Probing the fundamental structure of nearby molecular clouds is therefore crucial to understand how massive stars shape their surrounding medium and how fast molecular clouds are destroyed, specifically at their UV-illuminated edges, where models predict an intermediate zone of neutral atomic gas between the molecular cloud and the surrounding ionized gas whose size is directly related to the exposed physical conditions. We present the highest angular resolution (~0.″5, corresponding to 207 au) and velocity-resolved images of the molecular gas emission in the Horsehead nebula, using CO J = 3–2 and HCO⁺ J = 4−3 observations with the Atacama Large Millimeter/submillimeter Array (ALMA). We find that CO and HCO⁺ are present at the edge of the cloud, very close to the ionization (H⁺/H) and dissociation fronts (H/H₂), suggesting a very thin layer of neutral atomic gas (<650 au) and a small amount of CO-dark gas (A_V = 0.006–0.26 mag) for stellar UV illumination conditions typical of molecular clouds in the Milky Way. The new ALMA observations reveal a web of molecular gas filaments with an estimated thermal gas pressure of P_th = (2.3 – 4.0) × 10⁶ K cm⁻³, and the presence of a steep density gradient at the cloud edge that can be well explained by stationary isobaric photo-dissociation region (PDR) models with pressures consistent with our estimations. However, in the H II region and PDR interface, we find P_th,PDR > P_th,H II suggesting the gas is slightly compressed. Therefore, dynamical effects cannot be completely ruled out and even higher angular observations will be needed to unveil their role.