Fingerprinting the effects of hyperfine structure on CH and OH far infrared spectra using Wiener filter deconvolution

Abstract

Context. Despite being a commonly observed feature, the modification of the velocity structure in spectral line profiles by hyperfine structure complicates the interpretation of spectroscopic data. This is particularly true for observations of simple molecules such as CH and OH toward the inner Galaxy, which show a great deal of velocity crowding. Aims. In this paper, we investigate the influence of hyperfine splitting on complex spectral lines, with the aim of evaluating canonical abundances by decomposing their dependence on hyperfine structures. This is achieved from first principles through deconvolution. Methods. We present high spectral resolution observations of the rotational ground state transitions of CH near 2 THz seen in absorption toward the strong FIR-continuum sources AGAL010.62−00.384, AGAL034.258+00.154, AGAL327.293−00.579, AGAL330.954−00.182, AGAL332.826−00.549, AGAL351.581−00.352 and SgrB2(M). These were observed with the GREAT instrument on board SOFIA. The observed line profiles of CH were deconvolved from the imprint left by the lines' hyperfine structures using the Wiener filter deconvolution, an optimised kernel acting on direct deconvolution. Results. The quantitative analysis of the deconvolved spectra first entails the computation of CH column densities. Reliable N(CH) values are of importance owing to the status of CH as a powerful tracer for H 2 in the diffuse regions of the interstellar medium. The N(OH)/N(CH) column density ratio is found to vary within an order of magnitude with values ranging from one to 10, for the individual sources that are located outside the Galactic centre. Using CH as a surrogate for H 2 , we determined the abundance of the OH molecule to be X(OH) = 1.09 × 10 −7 with respect to H 2 . The radial distribution of CH column densities along the sightlines probed in this study, excluding SgrB2(M), showcase a dual peaked distribution peaking between 5 and 7 kpc. The similarity between the correspondingly derived column density profile of H 2 with that of the CO-dark H 2 gas traced by the cold neutral medium component of [CII] 158 µm emission across the Galactic plane, further emphasises the use of CH as a tracer for H 2 .

First detection of 13CH in the interstellar medium

“…where T l and T c represent the observed brightness temperatures of the line (prior to continuum subtraction) and the continuum, respectively. We determined the optical depth profile, that is, τ versus υ LSR , using the Wiener filter fitting technique as described in Jacob et al (2019). This fitting procedure first fits the observed spectral profile by minimising the mean square error between the model and observations and then deconvolves the hyperfine structure from the observed spectrum using the relative weights of the hfs components.…”

“…We assumed a 10% error in the continuum level calibration based on the instrumental performance (Kester et al 2017) and sideband dependence of the atmospheric transmission. The subsequently derived errors in the column densities (per velocity interval) were computed following the description presented in Jacob et al (2019) and scale with the deconvolved optical depths by a constant term comprised of the spectroscopic parameters that govern the transition and the excitation temperature. We compare the 13 CH column densities derived here, over velocity intervals associated with the different molecular clouds, with those of their corresponding N, J = 2, 3/2 → 1, 1/2 hfs transitions of CH near 2007 GHz discussed in Wiesemeyer et al (2018) and Jacob et al (2019).…”

“…The DSB-continuum level was then determined by accounting for contributions from the signal and image bands, which was added back to the spectra to obtain the correct line-to-continuum ratio. This calibration technique was applied not only to the 13 CH data presented in this work, but also to the previously published 12 CH data (Wiesemeyer et al 2018;Jacob et al 2019), which thus were re-calibrated for use in our analysis to avoid any inconsistencies. For the 12 CH spectra, we note that the continuum levels derived using the closure products are compatible with those derived using the standard calibration methods, except for W49(N).…”

“…The unwavering nature of the absorption feature at υ ∼ 64 km s −1 observed towards the envelope of Sgr B2(M) in each of the different IF frequency settings and the detection of its image band Λ-doublet counterpart near 2001 GHz with the absorption features displaying their expected relative intensities solidifies our detection of 13 CH near 1997.44 GHz at this velocity. We compare the 13 CH spectrum with that of CH towards Sgr B2(M) published in Jacob et al (2019). The CH spectrum has a true continuum of 15 K and is contaminated by C 3 absorption at 2004.833 GHz arising from the image band, as indicated by the blue box in Fig.…”

“…2. In particular, the hyperfine structure (hfs) components of the N, J = 1, 1/2 → 2, 3/2 transitions of CH near 2 THz have recently been observed in generally unsaturated absorption, which is ideal for column density determinations (Wiesemeyer et al 2018;Jacob et al 2019). Models and simulations by for photo-dissociation regions (PDRs) show that the fractionation present in CH is dominated by the fractionation of its parental species.…”