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

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.

Detection of the C(_3^+) Hydrocarbon Cation in the Horsehead Nebula: Implications for Interstellar Chemistry

The paper provides an in-depth study of the chemical composition of the Horsehead Nebula's photo-dissociation region (PDR), focusing on the millimeter-wave observations of the elusive C(_3^+) cation. Utilizing the IRAM-30m telescope, the researchers conducted a comprehensive spectral survey of the nebula, revealing eight distinct lines that have been attributed to the C(_3^+) hydrocarbon cation. This detection represents a significant advancement in understanding interstellar medium (ISM) chemistry, particularly the presence and role of small hydrocarbon cations.

Key Findings

• Spectral Observations: The survey detected eight unidentified lines toward the Horsehead PDR, with six lines showing a signal-to-noise ratio between 6 and 19. These lines were absent in the nearby dense core, emphasizing a distinct spatial distribution.

• Rotational Analysis: The authors fitted the observed spectral lines using a linear rotor model, allowing them to calculate the rotational and centrifugal distortion constants. The derived rotational constant (B = 11244.9512 \pm 0.0015) MHz closely aligns with theoretical values for the C(_3^+) cation, supporting the identification.

• Chemical Modeling: A comparison with the Meudon PDR photochemical model highlights discrepancies between observed and predicted abundances of small hydrocarbons in the PDR. While models fail to match the detected abundance of C(_3^+), they corroborate its presence, hinting at unknown chemical processes in PDR environments.

Implications for Astrochemistry

The detection of C(_3^+) facilitates a deeper understanding of hydrocarbon chemistry in the ISM. As hydrocarbon ions are crucial intermediates in ion-molecule reactions, these findings underscore the necessity to refine chemical models to account for photo-erosion of larger carbon species or alternative formation routes of hydrocarbons in PDRs. The study suggests that existing models, particularly at the UV-illuminated edges of molecular clouds, require modifications to include mechanisms beyond pure gas-phase reactions.

Future Directions

This research sets a foundation for further astrochemical investigations, particularly:

• Laboratory Spectroscopy: Confirmatory laboratory spectroscopy of C(_3^+) is imperative to refine its spectral characteristics.

• Advanced Modeling: Enhanced models incorporating surface chemistry and grain interactions may reconcile observed and theoretical abundances.

• High-Resolution Imaging: Interferometric observations with facilities like ALMA could provide high-resolution chemical maps, offering insights into the distribution and formation pathways of C(_3^+) and related hydrocarbons in the ISM.

Conclusion

The identification of the C(_3^+) cation in the Horsehead Nebula's PDR not only enriches our understanding of hydrocarbon chemistry but also challenges existing paradigms. This work highlights the dynamic and complex nature of interstellar chemistry and the necessity for integrating multi-faceted approaches to decipher the molecular processes driving ISM evolution.