The IRAM-30m line survey of the Horsehead PDR: II. First detection of the l-C3H+ hydrocarbon cation

TL;DR

This paper reports the first detection of the l-C3H+ hydrocarbon cation in the interstellar medium, using millimeter line surveys of the Horsehead PDR, and discusses its implications for hydrocarbon chemistry.

Contribution

It provides the first observational identification of l-C3H+ in space, constraining its rotational constants and chemical abundance in the Horsehead PDR.

Findings

Detected eight lines consistent with l-C3H+ in the Horsehead PDR.

Derived the rotational constant close to that of l-C3H, confirming the identification.

Compared observed abundance with photochemical model predictions.

Abstract

We present the first detection of the l-C3H+ hydrocarbon in the interstellar medium. The Horsehead WHISPER project, a millimeter unbiased line survey at two positions, namely the photo-dissociation region (PDR) and the nearby shielded core, revealed a consistent set of eight unidentified lines toward the PDR position. Six of them are detected with a signal-to-noise ratio from 6 to 19, while the two last ones are tentatively detected. Mostly noise appears at the same frequency toward the dense core, located less than 40" away. We simultaneously fit 1) the rotational and centrifugal distortion constants of a linear rotor, and 2) the Gaussian line shapes located at the eight predicted frequencies. The observed lines can be accurately fitted with a linear rotor model, implying a 1Sigma ground electronic state. The deduced rotational constant value is Be= 11244.9512 +/- 0.0015 MHz, close to that of l-C3H. We thus associate the lines to the l-C3H+ hydrocarbon cation, which enables us to constrain the chemistry of small hydrocarbons. A rotational diagram is then used to infer the excitation temperature and the column density. We finally compare the abundance to the results of the Meudon PDR photochemical model.