Direct estimation of electron density in the Orion Bar PDR from mm-wave carbon recombination lines
| Autor: | Javier R. Goicoechea, A. G. G. M. Tielens, José Cernicharo, A. Báez-Rubio, S. Cuadrado, P. Salas |
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| Přispěvatelé: | Ministerio de Ciencia, Innovación y Universidades (España), European Commission, Netherlands Organization for Scientific Research |
| Rok vydání: | 2019 |
| Předmět: |
Electron density
HII regions Astrochemistry FOS: Physical sciences Electron Astrophysics Photodissociation region 01 natural sciences 7. Clean energy Molecular physics ISM: clouds Article Ion 0103 physical sciences 010303 astronomy & astrophysics Collisional excitation Hyperfine structure Physics 010308 nuclear & particles physics Astronomy and Astrophysics Astrophysics - Astrophysics of Galaxies 3. Good health 13. Climate action Space and Planetary Science Astrophysics of Galaxies (astro-ph.GA) Photon-dominated region clouds [ISM] Excitation |
| Zdroj: | Astronomy and Astrophysics (0004-6361) Digital.CSIC. Repositorio Institucional del CSIC instname Astronomy and Astrophysics (0004-6361), 625, L3 |
| DOI: | 10.48550/arxiv.1904.10356 |
| Popis: | 6 pags., 3 figs., 1 app. 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 [13Cii] hyperfine line observations. Methods. We detect 12 mmCRLs (including α, β, and γ 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. 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 Hii 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 [13Cii] and mmCRLs and derive ne = 60-100 cm-3 and Te = 500-600K 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)×10 cm 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. We thank the Spanish MICIU for funding support under grant AYA2017-85111-P and the ERC for support under grant ERC-2013-Syg610256-NANOCOSMOS. A.B.-R. also acknowledges support by the MICIU and FEDER funding under grants ESP2015-65597-C4-1-R and ESP2017-86582-C4-1-R. P.S. and A.G.G.M.T. acknowledge financial support from the Dutch Science Organisation through TOP grant 614.001.351. |
| Databáze: | OpenAIRE |
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