Molecular globules in the Veil bubble of Orion

IRAM 30 m ¹²CO, ¹³CO, and C¹⁸O (2–1) expanded maps of Orion A^★

¹ Instituto de Física Fundamental (CSIC), Calle Serrano 121-123, 28006 Madrid, Spain
e-mail: javier.r.goicoechea@csic.es
² Leiden Observatory, Leiden University, Leiden, The Netherlands
³ I. Physikalisches Institut der Universität zu Köln, Cologne, Germany
⁴ IRAP, Université de Toulouse, CNRS, CNES, Université Paul Sabatier, Toulouse, France
⁵ Institut de Radioastronomie Millimétrique (IRAM), Grenoble, France
⁶ Max Planck Institute for Astronomy, Heidelberg, Germany
⁷ Telespazio Vega UK Ltd. for ESA/ESAC, Madrid, Spain
⁸ University of Maryland, Astronomy Department, College Park, MD, USA

Received: 7 January 2020
Accepted: 27 April 2020

Abstract

Strong winds and ultraviolet (UV) radiation from O-type stars disrupt and ionize their molecular core birthplaces, sweeping up material into parsec-size shells. Owing to dissociation by starlight, the thinnest shells are expected to host low molecular abundances and therefore little star formation. Here, we expand previous maps made with observations using the IRAM 30 m telescope (at 11″ ≃ 4500 AU resolution) and present square-degree ¹²CO and ¹³CO (J = 2–1) maps of the wind-driven “Veil bubble” that surrounds the Trapezium cluster and its natal Orion molecular core (OMC). Although widespread and extended CO emission is largely absent from the Veil, we show that several CO “globules” exist that are blueshifted in velocity with respect to OMC and are embedded in the [C II] 158 μm-bright shell that confines the bubble. This includes the first detection of quiescent CO at negative local standard of rest velocities in Orion. Given the harsh UV irradiation conditions in this translucent material, the detection of CO globules is surprising. These globules are small (R_g = 7100 AU), not massive (M_g = 0.3 M_⊙), and are moderately dense: n_H = 4 × 10⁴ cm⁻³ (median values). They are confined by the external pressure of the shell, P_ext∕k ≳ 10⁷ cm⁻³ K, and are likely magnetically supported. They are either transient objects formed by instabilities or have detached from pre-existing molecular structures, sculpted by the passing shock associated with the expanding shell and by UV radiation from the Trapezium. Some represent the first stages in the formation of small pillars, others of isolated small globules. Although their masses (M_g <M_Jeans) do not suggest they will form stars, one globule matches the position of a known young stellar object. The lack of extended CO in the “Veil shell” demonstrates that feedback from massive stars expels, agitates, and reprocesses most of the disrupted molecular cloud gas, thereby limiting the star-formation rate in the region. The presence of molecular globules is a result of this feedback.