Article

Extreme Ultraviolet Explorer optical identification campaign. IV. A northern hemisphere sample of active late-type stars and typical EUV sources

We present new optical identifications of previously unidentified faint extreme ultraviolet sources. Our total sample of 30 identified sources, of which 22 are new identifications, includes 24 late-type stars, three white dwarfs, two cataclysmic variables (CVs), and one active galactic nucleus. These sources are joint detections of the faint sources from the all-sky surveys of the Extreme Ultraviolet Explorer (EUVE) in the 58-174 A (0.071-0.214 keV) EUV band and of the ROSAT Position Sensitive Proportional Counter in the 5-120 A (0.1-2.4 keV) X-ray band. We obtained medium-resolution spectra of the possible optical counterparts with the Shane 3 m telescope at Lick Observatory using the Kast double spectrograph covering a bandpass of 3600-7500 A. Our sample of active late-type stars is dominated by K and M stars showing strong Balmer and Ca II emission lines. The white dwarfs are fairly typical for those detected in the EUVE survey with Teff and logg ranging from 35 to 53 kK and 7.6 to 8.7, respectively. We found strong H and He emission lines typical of cataclysmic variables (CVs) for EUVE J0854+390 and EUVE J1802+180. EUVE J0854+390 is a newly identified cataclysmic variable showing radial velocity shifts to the red as large as ~400 km s-1. We associate EUVE J1802+180 with the previously identified CV, V884 Her (RX J1802.1+1804). Including the present work (22 new identifications), EUVE optical identification campaigns have identified ~28% of the presently cataloged NOID sources.

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