Extremely red stellar objects revealed by IPHAS

TL;DR

This study identifies and analyzes a large sample of extremely red stellar objects from IPHAS, revealing their nature as highly-reddened evolved stars, particularly S-type stars, and introduces a new optical diagnostic for their identification.

Contribution

First large-scale detection of evolved low-mass stars in optical wavelengths, linking optical and infrared data, and proposing a new method to identify S-type stars using IPHAS colours.

Findings

Most objects are highly-reddened AGB stars with circumstellar material.

IPHAS (r' - H-alpha) colour effectively distinguishes S-type stars.

Potential to double the known S-type stars in the galactic plane.

Abstract

We present photometric analysis and follow-up spectroscopy for a population of extremely red stellar objects extracted from the point-source catalogue of the INT Photometric H-Alpha Survey (IPHAS) of the northern galactic plane. The vast majority of these objects have no previous identification. Analysis of optical, near- and mid-infrared photometry reveals that they are mostly highly-reddened asymptotic giant branch stars, with significant levels of circumstellar material. We show that the distribution of these objects traces galactic extinction, their highly reddened colours being a product of both interstellar and circumstellar reddening. This is the first time that such a large sample of evolved low-mass stars has been detected in the visual and allows optical counterparts to be associated with sources from recent infrared surveys. Follow-up spectroscopy on some of the most interesting objects in the sample has found significant numbers of S-type stars which can be clearly separated from oxygen-rich objects in the IPHAS colour-colour diagram. We show that this is due to the positions of different molecular bands relative to the narrow-band H-alpha filter used for IPHAS observations. The IPHAS (r' - H-alpha) colour offers a valuable diagnostic for identifying S-type stars. A selection method for identifying S-type stars in the galactic plane is briefly discussed and we estimate that over a thousand new objects of this type may be discovered, potentially doubling the number of known objects in this short but important evolutionary phase.