Extremely red stellar objects revealed by IPHAS

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 Survey (IPHAS) of the northern galactic plane. The vast majority of these objects have no previous identification. Analysis of optical, near- and midinfrared 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 filter used for IPHAS observations.