Circumstellar ammonia in oxygen-rich evolved stars

Abstract

Context. The circumstellar ammonia (NH 3 ) chemistry in evolved stars is poorly understood. Previous observations and modelling showed that NH 3 abundance in oxygen-rich stars is several orders of magnitude above that predicted by equilibrium chemistry. Aims. We would like to characterise the spatial distribution and excitation of NH 3 in the oxygen-rich circumstellar envelopes (CSEs) of four diverse targets: IK Tau, VY CMa, OH 231.8+4.2, and IRC +10420. Methods. We observed NH 3 emission from the ground state in the inversion transitions near 1.3 cm with the Very Large Array (VLA) and submillimetre rotational transitions with the Heterodyne Instrument for the Far-Infrared (HIFI) aboard Herschel Space Observatory from all four targets. For IK Tau and VY CMa, we observed NH 3 rovibrational absorption lines in the ν 2 band near 10.5 µm with the Texas Echelon Cross Echelle Spectrograph (TEXES) at the NASA Infrared Telescope Facility (IRTF). We also attempted to search for the rotational transition within the excited vibrational state ( 2 = 1) near 2 mm with the IRAM 30m Telescope. Non-LTE radiative transfer modelling, including radiative pumping to the vibrational state, was carried out to derive the radial distribution of NH 3 in the CSEs of these targets. Results. We detected NH 3 inversion and rotational emission in all four targets. IK Tau and VY CMa show blueshifted absorption in the rovibrational spectra. We did not detect vibrationally excited rotational transition from IK Tau. Spatially resolved VLA images of IK Tau and IRC +10420 show clumpy emission structures; unresolved images of VY CMa and OH 231.8+4.2 indicate that the spatial-kinematic distribution of NH 3 is similar to that of assorted molecules, such as SO and SO 2 , that exhibit localised and clumpy emission. Our modelling shows that the NH 3 abundance relative to molecular hydrogen is generally of the order of 10 −7 , which is a few times lower than previous estimates that were made without considering radiative pumping and is at least 10 times higher than that in the carbon-rich CSE of IRC +10216. NH 3 in OH 231.8+4.2 and IRC +10420 is found to emit in gas denser than the ambient medium. Incidentally, we also derived a new period of IK Tau from its V-band light curve. Conclusions. NH 3 is again detected in very high abundance in evolved stars, especially the oxygen-rich ones. Its emission mainly arises from localised spatial-kinematic structures that are probably denser than the ambient gas. Circumstellar shocks in the accelerated wind may contribute to the production of NH 3 . Future mid-infrared spectroscopy and radio imaging studies are necessary to constrain the radii and physical conditions of the formation regions of NH 3 .

The Role of Internal Photons on the Chemistry of the Circumstellar Envelopes of AGB Stars

“…The results obtained for NH 3 are similar to those obtained in the C-rich outflows. It has been observed in the outflow of IK Tau with an abundance of 6 × 10 −7 relative to H 2 (Decin et al 2010a), with other outflows showing a similar abundance of about 10 −6 (Wong et al 2018). Such abundances are again only achieved in the two-component case for a solar-like or white dwarf companion.…”

Exploring the innermost dust formation region of the oxygen-rich AGB star IK Tauri with VLT/SPHERE-ZIMPOL and VLTI/AMBER

“…The Mira-type star IK Tau, also known as NML Tau, is one of the best studied oxygen-rich AGB-stars and often considered a reference of its class. IK Tau was discovered in 1965 by Neugebauer et al (1965) and is an extremely red Mira-type variable with brightness variations of ∆V ∼ 6 mag in the Vband over a period of ∼460 days (Wong et al 2018). Consequently, its spectral type varies from M8 to M11 (Kharchenko & Roeser 2009).…”

The spectra of evolved stars at 20--25\,GHz: tracing circumstellar chemistry during the asymptotic giant branch to planetary nebula transition

“…Recent observations show that circumstellar ammonia is much richer than theoretical predictions (i.e. Wong et al 2018). In the present surveys, ammonia is observed in IRC+10216 and CRL 2688 in emission, as well as in CRL 618 in absorption.…”