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Astron. Astrophys. 318, 60-72 (1997)

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2. ER Vulpeculae

ER Vulpeculae is an eclipsing binary of the short-period RS CVn type with an orbital period of 0.698 days and consists of G0 and G5 main-sequence stars both of radii 1.07 [FORMULA] at a separation of about 2.01 [FORMULA]. ER Vul was first identified as an eclipsing double-lined spectroscopic binary by Northcott & Bakos (1956). The radial velocity variations and a preliminary photometric light curve solution were presented by Northcott & Bakos (1967) based on these earlier data supplemented with additional observations. The photometric solution given by Abrami & Cester (1963) differed from Northcott & Bakos (1967); this coupled with the observation of Ca II H and K emission by Bond (1970) led Hall (1976) to classify the system as a short-period RS CVn system. The system is obviously detached with an inclination near [FORMULA]. Mennella (1990) recently gave a good account of the changes in the light curve of ER Vul and derived a new ephemeris for moments of eclipses. The paper by Hill, Fisher & Holmgren (1990) provides an excellent discussion of both photometric and spectroscopic results for this system.

A spectroscopic orbit for ER Vul was presented by McLean (1982) based on cross-correlation velocities. Analysis of these data gave an estimate of the mass ratio of the system and suggested the presence of circumstellar material near the primary component. This was seen as a variation in the strength of the primary component spectral lines which, if caused by surface star-spots, would require star-spots much larger than those imposed by the photometric constraints. It is interesting to note that Northcott & Bakos (1967) suggested the presence of a gaseous cloud at the inner Lagrangian point extending to one side. This was required to explain the difference in the appearance of spectral lines at different phases. More recently Arevalo, Lazaro & Fuensalida (1988) suggested on the basis of their photometric variations the existence of a gas stream between the two components at a temperature of 1-5 104 K which would also account for the observed IR and UV excesses. The evidence for such material is not found spectroscopically. Newmark (1990) found no evidence for extended material around ER Vul in optical spectra while line ratios indicated that the chromospheric activity arises primarily in plage-like structures. The photometric variations have been shown to be broadly consistent with a star-spot distribution on the primary component (Budding & Zeilik 1987).

ER Vul has also been studied in wave-bands other than the optical. Budding, Kadouri & Gimenez (1982) demonstrated that both components show emission in the Mg II lines measured with IUE. UV line fluxes were also measured by Rucinski & Vilhu (1983). Vilhu & Rucinski (1983) also measured the various chromospheric and transition region lines in IUE spectra and concluded that the UV line fluxes lie on the borderline between those expected for detached and contact systems based on a UV period-activity relation. The division between these two classes is thought to be due to a saturation limit for magnetic flux generation which implies that ER Vul has an extensively active chromosphere. ER Vul was detected in the X-ray by the Einstein satellite with a luminosity of 1.6 1030 erg s-1 (Walter & Bowyer 1981). A more comprehensive treatment of the system's X-ray emission was achieved by White et al. (1987) using EXOSAT observations in the 0.05-6 keV band. They found that the X-ray spectrum could be fitted well with a two-component thermal emission model with temperatures of 6 106 K and 4 107 K. The lack of modulation in the X-ray light curve suggested that for both plasma regimes the loop heights are larger than the stellar radius or a multitude of loops are uniformly distributed with longitude. In the radio domain Drake, Simon & Linsky (1986) found that ER Vul was the strongest source (4.97 mJy) detected in their 6-cm survey of short-period active binaries. Morris & Mutel (1988) also detected ER Vul with a flux of 2.7 mJy. A comprehensive study of the 3.6-cm and 6-cm radio light curve using the VLA by Rucinski (1992) revealed complex variability. This suggested the presence of a highly structured radio corona giving rise to a slow variability component with a period somewhat longer than the orbital period. Rucinski (1992) suggests this may be evidence of magnetic structures dragging behind the rotation frame in a spiral pattern. Thus the complex geometry of active regions on ER Vul may preclude any conclusions based on the temporal behaviour of radio and X-ray emissions.

Overall ER Vul is an interesting system that appears to be highly active both in terms of surface features and the extent of its environment. However the presence of extended material around ER Vul which is suggested by some studies is yet to be confirmed spectroscopically.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998