Astron. Astrophys. 333, 205-218 (1998)

1. Introduction

HD 185510 (= V1379 Aql) is a binary system composed of a red giant star (K0 III) and an evolved hot subluminous star (sdB). The first indication of chromospheric activity on the giant star came from the detection of Ca II H & K emission (Bidelman & MacConnell 1973). A strong emission in the cores of the Ca II H & K lines, comparable to the more active RS CVn binaries, and an extremely weak H line has been reported by Fekel & Simon (1985).

Photometric variations with an amplitude of about were first observed by Henry et al. (1982). The behavior of the photometric wave was better defined by Lloyd Evans & Koen (1987) while Balona et al. (1987) discovered the eclipse of the subdwarf from the variation of about .12 in the U-B color index.

The system is asynchronous, the rotational period of 25.4 days found by Balona et al. (1987) being longer than the orbital period (20.66 days) determined from radial velocity measurements (Balona 1987, Fekel et al. 1993). Hooten & Hall (1990) determined a photometric period of about 26 days with a variation amplitude of - in the V band. In some seasons, they found that a smaller amplitude () light curve with a period of 13 days would better represent the data, but they suggested that such light curve results from a configuration of two-spot groups laying on opposite hemispheres of the active star.

The presence of a hot companion was noticed by Fekel & Simon (1985) in ultraviolet IUE spectra. From the flux distribution they derived an effective temperature of 20 000-30 000 ^oK for the hot star that was classified as a B type subdwarf. Jeffery et al. (1992) measured the radial velocity of the hot component in two high resolution IUE spectra taken at quadratures. Combining the radial velocity curves of both stars, they deduced a mass ratio , and estimated masses of 2.3-2.8  for the K star and 0.31-0.37  for the hot star. On the basis of the eclipse light curve and the spectral energy distribution from the UV to the red, they derived effective temperatures of , and for the hotter and cooler component respectively. The mass of 0.31-0.37 , which is lower than the typical value of sdB (Heber et al. 1984), and its higher gravity leads Jeffery et al. (1992) to conclude that HD 185510B is not a true sdB, but, presumably, a star in the lower part of the Helium main sequence, or it is becoming a Helium white dwarf.

From a new radial velocity curve for the cool component, Fekel et al. (1993) significantly improved the orbital elements and the spectroscopic ephemeris. They also give a value of 15 2 Km s^-1 for the of the cool star, constraining the radius to 7.5-8 . In addition, Fekel et al. (1993) discussed the characteristics of HD 185510 and other chromospherically active systems with hot compact companions in the context of the barium star scenario. They conclude that due to the short orbital period (small orbit size) the mass transfer in HD 185510 occurred before the mass donor reached the phase where the s -process elements could be transferred to the surface, consistent with the lack of abundance anomalies in the cool component of the system.

While this paper was nearly completed, a paper on the analysis of the eclipse based on UV observations with IUE was published by Jeffery & Simon (1997).

In order to reconcile the results from the eclipse solution and the gravity derived from the Ly  profile of the subdwarf component, they claim that the ingress/egress profile is affected by eclipse from the cool star atmosphere. They deduced and from the Ly  profile and the CII,III and SiII,III ionization equilibrium, while the radius inferred from the eclipse analysis leads to .

The scale height of the optical depth they deduce from the solution of the light curve at Å is significantly larger than that of the supergiant HR 6902A ( Aurigae) and Arcturus, both of (Schröder et al. 1996). Since HD 185510A has , the atmosphere height should be coherently smaller. Moreover the shorter ingress observed at Å and the smaller scale height of only 0.012  they derived at this wavelength is inconsistent with the expected optical depth at the two observation bands. In fact the linear absorption coefficient at 1800 Å is smaller than at 1400 Å at least by a factor of ten (Travis & Matsushima 1968, Dragon & Mutschlecner 1980) and therefore the optical depth scale height should be ten times larger implying a longer duration of the atmospheric eclipse.

The resolution time of Jeffery & Simon (1997) observations (19 min) is very close to ingress/egress duration so that, with observations of only one eclipse, they can hardly define the real eclipse duration and the light curve profile, therefore their argumentation on the atmospheric eclipse should be taken with some caution.

We have observed 4 eclipses ingresses and egresses in the U band with an average time resolution of 40 sec. With these data we should be able to settle the problem of the atmospheric eclipse and accurately determine the radius and therefore the gravity of the secondary hot component.

In the following we present photometric and spectroscopic H observations of HD 185510 and discuss them in terms of activity at the surface of the cool giant component. On the grounds of the new light curve solution we will discuss a possible scenario for the evolutionary stage of the system.

© European Southern Observatory (ESO) 1998

Online publication: April 15, 1998
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