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Astron. Astrophys. 318, L9-L12 (1997)

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4. The binary EUVE J0702+129

High-resolution optical spectroscopy offers two important insights into binary properties. Figure 4 shows the H [FORMULA] line in emission, indicating that the K0 star is chromospherically active. Narrow photospheric lines place an upper limit of [FORMULA] on the rotational velocity. Monitoring over time periods of hours, days, and months (Table 1) also shows no evidence of radial velocity (heliocentric) variations: [FORMULA]. A study of the Ca H&K lines and a comparison with other K0+DA binaries (HR 1608 and HD 18131) further constrain the properties of the K0 star (Figure 5). The K0 star in EUVE J0702+129 appears more active than its counterparts in the two other systems. Moreover, the Ca K full width at half maximum (FWHM) is [FORMULA] compared to [FORMULA] for both HR 1608 and HD 18131; application of the Wilson-Bappu relation for active stars (see Montes et al. 1994) implies an absolute magnitude [FORMULA] for EUVE J0702+129 and [FORMULA] for the two other systems. All three K0 stars are evolved; according to this relation, EUVE J0702+129 belongs to the luminosity class IV-V, and HR 1608 and HD 18131 to the class III-IV.

[FIGURE] Fig. 4. Lick Hamilton echelle spectrum of the H [FORMULA] emission line in the K0 star. Bad columns are excluded from the plot.
[FIGURE] Fig. 5. Lick Hamiton echelle spectra of Ca H&K lines of the K0 star in EUVE J0702+129 compared to two K0 stars paired with hot DA white dwarfs (HR 1608 or EUVE J0459-102, and HD 18131 or EUVE J0254-053). Application of the Wilson-Bappu relation shows that all three stars are above the main sequence: EUVE J0702+129 in the IV-V luminosity class and the two others in the III-IV range.

Voges et al. (1996) find a strong X-ray source near the position of EUVE J0702+129 ([FORMULA]) with a hardness ratio HR1 [FORMULA]. The hardness ratio implies a count rate of [FORMULA] in the 0.52-2.01 keV band. White dwarfs do not emit at high energy, their range extending to only about 0.25 keV; the hard PSPC band count rate is dominated by the late-type companion. Assuming a hydrogen column density of [FORMULA] and a distance of 130 pc, we generate an emission measure and a model spectrum using Monsignori-Fossi & Landini (1994) line emissivities. The model spectrum was then folded with the PSPC soft band (0.11-0.41 keV) effective areas, which predicts a count rate of approximately [FORMULA]. The soft PSPC band is therefore dominated by the white dwarf.

In summary, we find the K0 IV-V star in EUVE J0702+129 at a distance of 91-145 pc. The white dwarf and the K0 IV-V may form a physical pair, with the corollary that the white dwarf has a normal surface gravity ([FORMULA] [FORMULA]). Both objects are mingled in the LWP spatially resolved image implying a separation of less than [FORMULA]. The white dwarf possibly has a pure hydrogen atmosphere, in agreement with an identified abundance pattern in hot DA white dwarfs. The K0 IV-V star is active and displays Ca H&K and H [FORMULA] emission; ROSAT PSPC observations show a hard emission component, interpreted as coronal emission from the K0 star. If indeed the stars are associated, the absence of radial velocity variations on a timescale of one year would still imply a wide separation.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998