Ionization toward the high-mass star-forming region NGC 6334 I

Abstract

Context. Ionization plays a central role in the gas-phase chemistry of molecular clouds. Since ions are coupled with magnetic fields, which can in turn counteract gravitational collapse, it is of paramount importance to measure their abundance in star-forming regions. Aims. We use spectral line observations of the high-mass star-forming region NGC 6334 I to derive the abundance of two of the most abundant molecular ions, HCO + and N 2 H + , and consequently, the cosmic ray ionization rate. In addition, the line profiles provide information about the kinematics of this region. Methods. We present high-resolution spectral line observations conducted with the HIFI instrument on board the Herschel Space Observatory of the rotational transitions with J up ≥ 5 of the molecular species C 17 O, C 18 O, HCO + , H 13 CO + , and N 2 H + . Results. The HCO + and N 2 H + line profiles display a redshifted asymmetry consistent with a region of expanding gas. We identify two emission components in the spectra, each with a different excitation, associated with the envelope of NGC 6334 I. The physical parameters obtained for the envelope are in agreement with previous models of the radial structure of NGC 6334 I based on submillimeter continuum observations. Based on our new Herschel/HIFI observations, combined with the predictions from a chemical model, we derive a cosmic ray ionization rate that is an order of magnitude higher than the canonical value of 10 −17 s −1 . Conclusions. We find evidence of an expansion of the envelope surrounding the hot core of NGC 6334 I, which is mainly driven by thermal pressure from the hot ionized gas in the region. The ionization rate seems to be dominated by cosmic rays originating from outside the source, although X-ray emission from the NGC 6334 I core could contribute to the ionization in the inner part of the envelope.

Water in star-forming regions withHerschel(WISH)

“…Morales Ortiz et al (2014) report optically thick HCO + (6-5) emission for NGC 6334 I and a size of 40 ± 6 for the outer envelope. This suggests that the HCO + (6-5) emission originates predominantly from an optically thick surface (see also Appendix D).…”

Cosmic rays in molecular clouds probed by H2 rovibrational lines

“…(Caselli et al 1998), empty hexagons (Bialy et al 2022), an empty circle (Maret & Bergin 2007), and an empty pentagon (Fuente et al 2016). Observational estimates in high-mass star-forming regions are shown as stars (Sabatini et al 2020), solid diamonds (de Boisanger et al 1996), empty diamonds (van der Tak et al 2000, empty thin diamonds (Hezareh et al 2008), and solid thin diamonds (Morales Ortiz et al 2014). Observational estimates in circumstellar discs are shown as solid squares (Ceccarelli et al 2004) and in massive hot cores as empty squares (Barger & Garrod 2020).…”

On the chemistry of the young massive protostellar core NGC 2264 CMM3

“…However, since ζ was recorded to have wide range (0.8 × 10 −17 < ζ(in s −1 ) < 2 × 10 −16 ) in different dense cores around massive protostellar objects (e.g. van der Tak and van Dishoeck 2000; Doty et al 2002;Morales Ortiz et al 2014;Kaźmierczak-Barthel et al 2015), we performed some calculations with higher rates, ζ CMM3 ∼ (5 -10) ×ζ ISM , as described in the context below.…”

“…Several studies showed that the CR ionisation rate, ζ, in massive star-forming regions are much higher than the standard value, ζ ISM = 1.3 × 10 −17 s −1 , by 5 -500 times with a preference values (∼ 5 -10)×ζ ISM (e.g. van der Tak and van Dishoeck 2000; Doty et al 2002;Morales Ortiz et al 2014;Kaźmierczak-Barthel et al 2015). In order to investigate the impact of increasing ζ on the chemistry of CMM3, we run models with higher ionisation rates.…”