JWST: Deuterated PAHs, PAH Nitriles, and PAH Overtone and Combination Bands. I. Program Description and First Look

Abstract

A first look is taken at the NIRSpec 1–5 μm observations from James Webb Space Telescope program 1591 that targets seven objects along the low-mass stellar life cycle with polycyclic aromatic hydrocarbon (PAH) emission. Spectra extracted from a 1.″5 radius circular aperture are explored, showing a wealth of features, including the 3 μm PAH complex, the PAH continuum, and atomic and molecular emission lines from H i, He, H2, and other species. CO2- and H2O-ice absorption and CO emission is also seen. Focusing on the bright-PDR position in M17, the PAH CH stretch falls at 3.29 μm (FWHM = 0.04 μm). Signs of its 1.68 μm overtone are confused by line emission in all targets. Multicomponent decomposition reveals a possible aliphatic deuterated PAH feature centered at 4.65 μm (FWHM = 0.02 μm), giving [D/H]alip. = 31% ± 12.7%. However, there is little sign of its aromatic counterpart between 4.36 and 4.43 μm. There is also little sign of PAH nitrile emission between 4.34 and 4.39 μm. A PAH continuum rises from ∼1 to 3.2 μm, after which it jumps by about a factor of 2.5 at 3.6 μm, with bumps at 3.8, 4.04, and 4.34 μm adding structure. The CO2 absorption band in M17 is matched with 10:1 H2O:CO2 ice at 10 K. The v = 0 pure rotational molecular hydrogen population diagram reveals >2200 K UV-pumped gas. The hydrogen Pfund series runs from levels 10 to >30. Considering Brα/Brβ = 0.381 ± 0.01966 and Case B recombination results in A V ≃ 8. CO emission in IRAS 21282+5050 originates from 258 K gas. In-depth spectral–spatial analysis of all features and targets is planned for a series of forthcoming papers.

Mid-infrared spectroscopy of 1-cyanonaphthalene cation for astrochemical consideration

“…Observations from the James Webb Space Telescope (JWST) are now facilitating higher-resolution measurements in the 1–5 μm region. 52 There is evidence of emission features in the 4.51–4.52 μm region in observation toward young planetary nebulae BD+303639 and NGC7027. 52 In previous studies, some strong emission lines in this region were observed from M17 and from Orion Bar.…”

“…52 There is evidence of emission features in the 4.51–4.52 μm region in observation toward young planetary nebulae BD+303639 and NGC7027. 52 In previous studies, some strong emission lines in this region were observed from M17 and from Orion Bar. 34,53 It should be noted that our measurement concern the absorption of cold 1-CNN + –He, and therefore differences between the band profiles of this complex and emission of UV pumped 1-CNN + are expected.…”

“…This is in contrast to the location of the CH-stretch of other PAHs which has been located at 3.29 μm. 52 Previous theoretical studies comparing neutral and cationic PAHs have shown changes in both the intensity and peak positions of the CH-stretch. 57 In the laboratory spectrum, the CH-stretches of 1-CNN + appear as very weak features that would be difficult to distinguish in an observational data set and are probably not worth investigating further for detection.…”

“…49–51 In the case of large PAHs containing 35–45 C atoms, with a singly-substituted CN group, the CN-stretch has been proposed to fall in the 4.34–4.39 μm range. 52…”

“…53 Observation of a 4.65 μm band in M17 is attributed to CD-stretch, but interestingly no companion CD-stretch that has been predicted to be at 4.40 μm is seen. 34,52 PAHs are predicted to become enriched with deuterium via H-exchange reactions in UV-rich environments due to the lower zero-point energy of C–D bonds compared to C–H bonds. 55 However, in CSE and PNe regions, deuterium will be in low abundance since it is readily consumed by stars during nucleosynthesis.…”