Discovery of a TiO emission band in the infrared spectrum of the S star NP Aurigae

TL;DR

This paper reports the first detection of a TiO emission band in the infrared spectrum of the S star NP Aurigae, revealing details about the molecular environment and conditions around the star.

Contribution

First observation of TiO emission in an S star, with modeling of molecular temperature, column density, and spatial extent, and identification of potential stellar characteristics linked to this emission.

Findings

TiO emission observed at approximately 600 K

Emission detected in NP Aurigae and two other candidates

Stars with TiO emission show large pulsations, s-process enrichment, and low C/O ratio

Abstract

We report on the discovery of an infrared emission band in the Spitzer spectrum of the S-type AGB star NP Aurigae that is caused by TiO molecules in the circumstellar environment. We modelled the observed emission to derive the temperature of the TiO molecules (\approx 600 K), an upper limit on the column density (\approx 10^17.25 cm^{-2}) and a lower limit on the spatial extent of the layer that contains these molecules. (\approx 4.6 stellar radii). This is the first time that this TiO emission band is observed. A search for similar emission features in the sample of S-type stars yielded two additional candidates. However, owing to the additional dust emission, the identification is less stringent. By comparing the stellar characteristics of NP Aur to those of the other stars in our sample, we find that all stars with TiO emission show large-amplitude pulsations, s-process enrichment, and a low C/O ratio. These characteristics might be necessary requirements for a star to show TiO in emission, but they are not sufficient.