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-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.

18 pages, 20 figures, published in MNRAS. Minor text changes and Figure 17 replaced

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• The VST Photometric Halpha Survey of the Southern Galactic Plane and Bulge (VPHAS+)
• The Second Data Release of the INT Photometric H-Alpha Survey of the Northern Galactic Plane (IPHAS DR2)
• High Spatial Resolution Galactic 3D Extinction Mapping with IPHAS
• A 3D Photoionization Model of the Extreme Planetary Nebula NGC 6302
• K-band spectroscopy of IGR J16358-4726 and IGR J16393-4643: two new symbiotic X-ray binaries
• The protoplanetary disks in the nearby massive star forming region Cygnus OB2
• IRAS 20050+2720: Anatomy of a young stellar cluster
• Near-IR spectra of IPHAS extremely red Galactic AGB stars
• Probing infrared excess connection with Li enhancement among red clump giants
• UV-excess sources with a red/IR-counterpart: low-mass companions, debris disks and QSO selection
• Candidate Hα emission and absorption line sources in the Galactic Bulge Survey
• Pomeron and Odderon Contributions at LHC Energies
• H-alpha as a Luminosity Class Diagnostic for K- and M-type Stars
• Galactic AGB stars from the IPHAS survey
• Galactic Plane H$\alpha$ Surveys: IPHAS & VPHAS+