Volume 625, May 2019
|Number of page(s)||6|
|Section||Letters to the Editor|
|Published online||07 May 2019|
Letter to the Editor
Direct estimation of electron density in the Orion Bar PDR from mm-wave carbon recombination lines⋆
Instituto de Física Fundamental (IFF-CSIC), Calle Serrano 121-123, 28006 Madrid, Spain
2 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
3 Centro de Astrobiología (CSIC-INTA), Ctra. de Torrejón a Ajalvir, km 4, 28850, Torrejón de Ardoz Madrid, Spain
Accepted: 16 April 2019
Context. A significant fraction of the molecular gas in star-forming regions is irradiated by stellar UV photons. In these environments, the electron density (ne) plays a critical role in the gas dynamics, chemistry, and collisional excitation of certain molecules.
Aims. We determine ne 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 [13C II] hyperfine line observations.
Methods. We detect 12 mmCRLs (including α, β, and γ transitions) observed with the IRAM 30 m 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.
Results. 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 H II region. This is readily seen from their narrow line profiles (Δv = 2.6 ± 0.4 km s−1) and line peak velocities (vLSR = +10.7 ± 0.2 km s−1). Optically thin [13C II] 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 [13C II] and mmCRLs and derive ne = 60–100 cm−3 and Te = 500–600 K toward the DF.
Conclusions. 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 Pth ≥ (2−4) × 108 cm−3 K assuming that the electron abundance is equal to 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.
Key words: astrochemistry / photon-dominated region / HII regions / ISM: clouds
© ESO 2019
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