Detection of Deuterated Hydrocarbon Nanoparticles in the Whirlpool Galaxy, M51

Abstract

Deuteration of hydrocarbon material, including polycyclic aromatic hydrocarbons (PAHs), has been proposed to account for the low gas-phase abundances of D in the interstellar medium. JWST spectra of four star-forming regions in M51 show an emission feature, with central wavelength $\sim$4.647$\mu$m and FWHM 0.0265$\mu$m, corresponding to the C-D stretching mode in aliphatic hydrocarbons. The emitting aliphatic material is estimated to have (D/H)${}_{\mathrm{aliph}}\approx 0.17\pm 0.02$ -- a factor $\sim$$10^4$ enrichment relative to the overall interstellar medium (ISM). On $\sim$$50$pc scales, deuteration levels toward four HII regions in M51 are 2-3 times higher than in the Orion Bar photodissociation region (PDR), with implications for the processes responsible for the formation and evolution of hydrocarbon nanoparticles, including PAHs. The deuteration of the aliphatic material is found to anticorrelate with helium ionization in the associated HII, suggesting that harsh FUV radiation may act to lower the deuteration of aliphatics in PDRs near massive stars. No evidence is found for deuteration of aromatic material, with (D/H)${}_{\mathrm{arom}}\lesssim 0.016$: deuteration of the aliphatic material exceeds that of the aromatic material by at least a factor 10. The observed levels of deuteration may account for the depletion of D observed in the Galactic interstellar medium. If so, the $4.65\mu$m feature may be detectable in absorption.

Revised. corresponding to published paper

References in corpus (22)

• A Unified Representation of Gas-Phase Element Depletions in the Interstellar Medium
• The Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope I. Overview of the instrument and its capabilities
• The James Webb Space Telescope Mission
• JWST MIRI flight performance: The Medium-Resolution Spectrometer
• The Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope III. Integral-field spectroscopy
• One Relation for All Wavelengths: The Far-Ultraviolet to Mid-Infrared Milky Way Spectroscopic R(V) Dependent Dust Extinction Relationship
• Excitation of Polycyclic Aromatic Hydrocarbon (PAH) Emission: Dependence on Size Distribution, Ionization, and Starlight Spectrum and Intensity
• Observational Constraints on the Physical Properties of Interstellar Dust in the Post-Planck Era
• PDRs4All III: JWST's NIR spectroscopic view of the Orion Bar
• The Impact of New d(p,γ)He3 Rates on Big Bang Nucleosynthesis
• All the PAHs: an AKARI-Spitzer Cross Archival Spectroscopic Survey of Aromatic Emission in Galaxies
• Deviations from He I Case B Recombination Theory and Extinction Corrections in the Orion Nebula
• JWST: Deuterated PAHs, PAH-nitriles, and PAH Overtone and Combination Bands I: Program Description and First Look
• H2, CO, and Dust Absorption through Cold Molecular Clouds
• PAH Spectroscopy from 1-5 $\mu$m
• Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups: Intensity Scaling for the C-H Stretching Modes and Astrophysical Implications
• Helium line emissivities for nebular astrophysics
• A High Precision Survey of the D/H Ratio in the Nearby Interstellar Medium
• Deuterated Polycyclic Aromatic Hydrocarbons in the Interstellar Medium: The C--D Band Strengths of Mono-Deuterated Species
• Reeling in the Whirlpool: the distance to M 51 clarified by Cepheids and the Type IIP SN 2005cs
• Near-infrared spectroscopy of a massive young stellar object in the direction toward the Galactic Center: XCN and aromatic C-D features
• Laboratory measurements of stretching band strengths of deuterated Quenched Carbonaceous Composites (D-QCC)

Cited by in corpus (1)

• PDRs4All XVI. Tracing aromatic infrared band characteristics in photodissociation region spectra with PAHFIT in the JWST era