* Astronomy

Title: Extremely red stellar objects revealed by IPHAS
Authors: N.J. Wright, R. Greimel, M.J. Barlow, J.E. Drew, M.-R.L. Cioni, A.A. Zijlstra, R.L.M. Corradi, E.A. González-Solares, P. Groot, J. Irwin, M.J. Irwin, A. Mampaso, R.A.H. Morris, D. Steeghs, Y.C. Unruh, N. Walton

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.

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