Abundance of Ammonia in Carbon-rich Asymptotic Giant Branch Stars as Observed by Herschel/HIFI—A Rotational Diagram Approach

This is the authors' abstract. We don't add key points for this paper.

The Herschel Heterodyne Instrument for the Far-Infrared (HIFI) obtained high-resolution spectroscopic observations of numerous Galactic and extragalactic sources, including 12 carbon-rich (C-rich) Galactic asymptotic giant branch (AGB) stars. The ground-state ortho-NH 3 emission at 572.498 GHz was detected toward eight sources (CW Leo, V384 Per, CIT 6, II Lup, V1417 Aql, V1418 Aql, V Cyg, and LP And), while BM Gem, V Hya, Y CVn, and S Cep were not detected. Previous estimates of NH 3 abundance in C-rich AGB stars were limited to CW Leo and V Cyg and relied on non-LTE radiative transfer modeling. In this study, we estimated the ammonia abundances using the rotational diagram approach for CW Leo and II Lup, and adopted representative excitation temperatures of 40, 100, and 150 K for the remaining sources, based on the values derived for these two stars. Gaia distances are incorporated where available. We find statistically significant correlations between both the NH 3 column density and fractional abundance and the circumstellar wind density ( M ̇ / V exp ). A notable outlier is V1417 Aql, which is often classified as a non-AGB carbon star. The observed increase in NH 3 column density with wind density cannot be explained solely by the larger amount of circumstellar material, but also requires an increase in the fractional abundance of ammonia. This finding suggests that ammonia is produced more efficiently in denser circumstellar envelopes. In consequence, because UV radiation penetrates less effectively into such environments, our results imply that UV-driven chemistry may play a less important role in ammonia formation than shock-induced chemistry.