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Astron. Astrophys. 337, 253-260 (1998)

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5. Peculiar motions. The case of A 35

5.1. Orbital parameters of A 35

The binary central star of A 35 is composed of a bright G-star, BD-[FORMULA], which is the HIPPARCOS target HIC 62905, and of a hot star which ionizes the nebula, detected in IUE spectra by Grewing and Bianchi (1989). Thévenin and Jasniewicz (1997) have estimated for BD-[FORMULA] a surface gravity [FORMULA]=3.7[FORMULA]0.5 and an effective temperature [FORMULA] K, which are typical of a G8IV spectral type. They deduced a distance of [FORMULA]pc for Abell 35. The HIPPARCOS distance of [FORMULA]pc confirms their spectroscopic analysis: it implies for the G-star an absolute magnitude of MV=[FORMULA] in agreement with a luminosity IV and, according to the Popper's (1980) empirical radius-luminosity relation a radius of [FORMULA] [FORMULA] in agreement with [FORMULA]=3.7.

The origin of the Abell 35 system is discussed in Thévenin and Jasniewicz (1997). The high equatorial rotational velocity of BD-[FORMULA], about [FORMULA] km s-1, can be explained by two scenarios: spin-up during a common envelope interaction (Bond 1993) or during a wind-accretion phase (Jeffries and Stevens 1996). Both scenarios could explain the observed contamination of the companion by the s-process-overabundant envelope of the former AGB star (Thévenin and Jasniewicz, 1997). But in the first case, the actual binary should be a close binary whilst in the second case the binary could be a wide one.

The value of the goodness-of-fit of the astrometric solution to the accepted data (column H30) is 3.14 for BD-[FORMULA], indicating a bad fit to the data. However the star was not detected as binary by HIPPARCOS (column H59 is blank). If we assume that the standard deviation [FORMULA] (about 2[FORMULA]10-3 arcsec) of astrometric measurements with regard to the best fit is entirely due to orbital motion, we can estimate an upper value of [FORMULA]=6[FORMULA]10-3 arcsec for the angular distance corresponding to the semi-major axis a. In assuming a total mass of 1.5[FORMULA] for the binary system (0.9 [FORMULA] for the G8IV star and 0.6[FORMULA] for the white dwarf), we derive from the Kepler's third law an upper value of 0.6 year for the orbital period. Interferometric observations are planned in order to detect the companion which is about 4 magnitudes fainter than the bright G8IV star.

5.2. Asymmetric morphology

In the case of A35, Jacoby (1981) and Hollis et al. (1996) have emphasized the existence of two kinds of features in the nebula: a bow shock, best seen predominantly in [OIII], and two prominent parallel nebular (pipes) features seen at 6cm continuum and in H[FORMULA], [O II], and [N II]. These authors suggest that these features are the consequence of an interaction, respectively between the nebula and the central binary star, and between the nebular material and the interstellar medium.

From the HIPPARCOS proper motions, we deduce a transverse velocity vT=40[FORMULA]10km[FORMULA]s-1 and a proper motion position angle of 256[FORMULA][FORMULA], in agreement with the the results of Hollis et al. (1996): the [FORMULA] position angle of the symmetry axis of the bow shock and the 253[FORMULA][FORMULA] position angle of the pipes (see Fig. 3).

[FIGURE] Fig. 3. Transverse velocity direction (arbitrary scale) and asymmetric morphology of some PN. The maps are taken from Balick (1987) for NGC 40, from Miranda ([FORMULA] map, 1995) for Hu 2-1, from Kastner et al. (1966) for NGC 2346, from Schwarz et al. (1992) for He 2-36; from Jacoby (1982) - right and from Acker et al. (1992) - left for A 35. North is up, east is left on all maps.

As already noted by Marchenko et al. (1997) for WR stars, the tangential velocity vectors tend to point towards enhanced emission around stars with strong winds, assisted by Roche-lobe overflow in some binaries. A brightening of the surrounding shell in the direction of the stellar motion is clearly apparent for the [WC] star of NGC 40.

For He 2-36 (with a binary nucleus), the bipolar structure shaped by a very strong wind seems to be oriented by the star's motion in the north-western direction and ended by a bright knot. The bipolar structure of NGC 2346 (binary nucleus) appears less confined; the western side of the upper lobe appears slightly deformed by the stellar motion effect. Hu 2-1 is a bipolar PN with a slowly expanding toroid and two wide bipolar lobes; the map of the [NII]/H[FORMULA] line intensity ratio shows a ring-like zone concentric with the toroid and expanding at higher velocity (Miranda, 1995). The shell appears compressed by the motion of the star, as shown in Fig. 3.

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© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998