Direct estimation of electron density in the Orion Bar PDR from mm-wave carbon recombination lines
Abstract
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.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- May 2019
- DOI:
- 10.1051/0004-6361/201935556
- arXiv:
- arXiv:1904.10356
- Bibcode:
- 2019A&A...625L...3C
- Keywords:
-
- astrochemistry;
- photon-dominated region;
- HII regions;
- ISM: clouds;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- Accepted for publication in A&