Abstract
The synchronously rotating G stars in the detached, short-period (0.7 days), partially eclipsing binary ER Vul are the most chromospherically active solar-type stars known. We have monitored activity in the Ca II H and K reversals for almost an entire orbit. Rucinski's broadening function formalism allows the photospheric contribution to be objectively subtracted from the highly blended spectra. The power of the broadening function technique is also demonstrated by the good agreement of radial velocities with those measured by others from less crowded spectral regions. In addition to strong Ca II emission from the primary and secondary, there appears to be a high-velocity stream flowing onto the secondary, where it stimulates a large active region on the surface 30°–40° in advance of the subbinary longitude. A model light curve with a spot centered on the same longitude also gives the best fit to the observed light curve. A flare with ∼13% more power than at other phases was detected in one spectrum. We suggest that ER Vul may offer a magnified view of the more subtle chromospheric effects synchronized to planetary revolution seen in certain 51 Peg–type systems.
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