Direct estimation of electron density in the Orion Bar PDR from mm-wave carbon recombination lines

Abstract

A significant fraction of the molecular gas in star-forming regions is irradiated by stellar UV photons. In these environments, the electron density (n_e) plays a critical role in the gas dynamics, chemistry, and collisional excitation of certain molecules. We determine n_e in the prototypical strongly irradiated photodissociation region (PDR), the Orion Bar, from the detection of new millimeter-wave carbon recombination lines (mmCRLs) and existing far-IR [13CII] hyperfine line observations. We detect 12 mmCRLs (including alpha, beta, and gamma transitions) observed with the IRAM 30m telescope, at ~25'' angular resolution, toward the H/H2 dissociation front (DF) of the Bar. We also present a mmCRL emission cut across the PDR. These lines trace the C+/C/CO gas transition layer. As the much lower frequency carbon radio recombination lines, mmCRLs arise from neutral PDR gas and not from ionized gas in the adjacent HII region. This is readily seen from their narrow line profiles (dv=2.6+/-0.4 km/s) and line peak LSR velocities (v_LSR=+10.7+/-0.2 km/s). Optically thin [13CII] hyperfine lines and molecular lines - emitted close to the DF by trace species such as reactive ions CO+ and HOC+ - show the same line profiles. We use non-LTE excitation models of [13CII] and mmCRLs and derive n_e = 60-100 cm^-3 and T_e = 500-600 K toward the DF. The inferred electron densities are high, up to an order of magnitude higher than previously thought. They provide a lower limit to the gas thermal pressure at the PDR edge without using molecular tracers. We obtain P_th > (2-4)x10^8 cm^-3 K assuming that the electron abundance is equal or lower than the gas-phase elemental abundance of carbon. Such elevated thermal pressures leave little room for magnetic pressure support and agree with a scenario in which the PDR photoevaporates.

Accepted for publication in A&A Letters (includes language editor corrections)

References in corpus (10)

• The anatomy of the Orion B Giant Molecular Cloud: A local template for studies of nearby galaxies
• Compression and ablation of the photo-irradiated cloud the Orion Bar
• The chemistry and spatial distribution of small hydrocarbons in UV-irradiated molecular clouds: the Orion Bar PDR
• The chemistry of ions in the Orion Bar I. - CH+, SH+, and CF+: The effect of high electron density and vibrationally excited H2 in a warm PDR surface
• Electron Excitation of High Dipole Moment Molecules Reexamined
• The Ionization Fraction of Barnard 68: Implications for Star and Planet Formation
• Complex organic molecules in strongly UV-irradiated gas
• Trans-cis molecular photoswitching in interstellar Space
• The Orion HII Region and the Orion Bar in the Mid-Infrared
• Low Frequency Carbon Radio Recombination Lines I: Calculations of Departure Coefficients

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