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 $\mu$m observations from JWST program 1591 that targets 7 objects along the low-mass stellar life cycle with PAH emission. Spectra extracted from a 1.5$^{\prime\prime}$ radius sized circular aperture are explored, showing a wealth of features, including the 3 $\mu$m PAH complex, the PAH-continuum, and atomic and molecular emission lines from HI, He, H$_{\rm 2}$, and other species. CO$_{\rm 2}$- and H$_{\rm 2}$O-ice absorption and CO emission is also seen. Focusing on the bright-PDR position in M17, the PAH CH stretch falls at 3.29 $\mu$m (FWHM=0.04 $\mu$m). Signs of its 1.68 $\mu$m overtone are confused by line emission in all targets. Multi-component decomposition reveals a possible aliphatic deuterated PAH feature centered at 4.65 $\mu$m (FWHM=0.02 $\mu$m), giving [D/H]$_{\rm alip.}$=31$\pm$12.7%. However, there is little sign of its aromatic counterpart between 4.36-4.43 $\mu$m. There is also little sign of PAH-nitrile emission between 4.34-4.39 $\mu$m. A PAH continuum rises from $\sim$1 to 3.2 $\mu$m, after which it jumps by about a factor of 2.5 at 3.6 $\mu$m, with bumps at 3.8, 4.04, and 4.34 $\mu$m adding structure. The CO$_{\rm 2}$ absorption band in M17 is matched with 10:1 H$_{\rm 2}$O:CO$_{\rm 2}$ 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 $>$30. Considering Br$\alpha$/Br$\beta$=0.381$\pm$0.01966 and Case B recombination results in A$_{\rm V}{\simeq}$8. CO emission in IRAS21282+5050 originates from 258 K gas. In-depth spectral-spatial analysis of all features and targets are planned for a series of forthcoming papers.