Molecular tracers of radiative feedback in Orion (OMC-1)

Abstract

Young massive stars regulate the physical conditions, ionization, and fate of their natal molecular cloud and surroundings. It is important to find tracers that quantify the stellar feedback processes that take place on different spatial scales. We present ~85 arcmin2 velocity-resolved maps of several submillimeter molecular lines, taken with Herschel/HIFI, toward the closest high-mass star-forming region, the Orion molecular cloud 1 core (OMC-1). The observed rotational lines include probes of warm and dense molecular gas that are difficult, if not impossible, to detect from ground-based telescopes: CH+ (J = 1–0), CO (J = 10–9), HCO+ (J = 6–5), HCN (J = 6–5), and CH (N, J = 1, 3/2–1, 1/2). These lines trace an extended but thin layer (AV ≃ 3–6 mag or ~1016 cm) of molecular gas at high thermal pressure, Pth = nH ⋅ Tk ≈ 107–109 cm−3 K, associated with the far-ultraviolet (FUV) irradiated surface of OMC-1. The intense FUV radiation field – emerging from massive stars in the Trapezium cluster – heats, compresses, and photoevaporates the cloud edge. It also triggers the formation of specific reactive molecules such as CH+. We find that the CH+ (J = 1–0) emission spatially correlates with the flux of FUV photons impinging the cloud: G0 from ~103 to ~105. This relationship is supported by constant-pressure photodissociation region (PDR) models in the parameter space Pth∕G0 ≈ [5 × 103 − 8 × 104] cm−3 K where many observed PDRs seem to lie. The CH+ (J = 1–0) emission also correlates with the extended infrared emission from vibrationally excited H2 (v ≥ 1), and with that of [C II] 158 μm and CO J = 10–9, all emerging from FUV-irradiated gas. These spatial correlations link the presence of CH+ to the availability of C+ ions and of FUV-pumped H2 (v ≥ 1) molecules. We conclude that the parsec-scale CH+ emission and narrow-line (Δv ≃ 3 km s−1) mid-J CO emission arises from extended PDR gas and not from fast shocks. PDR line tracers are the smoking gun of the stellar feedback from young massive stars. The PDR cloud surface component in OMC-1, with a mass density of 120–240 M⊙ pc−2, represents ~5–10% of the total gas mass; however, it dominates the emitted line luminosity, the average CO J = 10–9 surface luminosity in the mapped region being ~35 times brighter than that of CO J = 2–1. These results provide insights into the source of submillimeter CH+ and mid-J CO emission from distant star-forming galaxies.