Astron. Astrophys. 347, 69-76 (1999)

4. Discussion

The target stars are plotted in the plane in Fig. 1, and appear to scatter above the zero-age main-sequence (ZAMS), although this diagram is of course insufficient to class the objects as young, high-mass hydrogen burning stars. However, they are unlikely to be evolved, low mass stars on the basis of their generally large projected rotational velocities; in stark contrast to the post-blue horizontal branch (post-BHB) sample presented in Paper I. In addition, the surface gravities appear to be too high for the objects to be post-asymptotic giant branch stars (post-AGB, cf.  McCausland et al. 1992); in any case, none of the compositional peculiarities often observed in post-BHB and post-AGB stars are observed in this sample.

Fig. 1. The diagram for the programme objects (solid squares), where the theoretical positions of the zero-age hydrogen burning main-sequence (ZAMS), blue horizontal branch (BHB) and post-asymptotic giant branch (post-AGB) are given (Bertelli et al. 1994; Sweigart 1987; Schönberner 1993respectively). Also shown are some candidate post-AGB stars (crosses) studied previously by us (Hambly et al. 1996a; Conlon et al. 1994; McCausland et al. 1992and the post-BHB stars (open triangles) from the Hambly et al. (1997) PG sample (Paper I).

The similarity between the spectra of this sample and those of our Galactic disk comparision stars is illustrated in Fig. 2, where we plot sections of spectra for PG 2229+099 plus the comparison HR 6588. Once convolved with the appropriate rotational broadening function, corresponding spectra are effectively identical. The validity for using the bright Galactic disk B-type stars as standards of Population I chemical composition has been verified by the abundance values found in Paper I. From Table 7, there is no strong evidence for any departures from a Population I chemical composition in any of the PG-targets. In brief, all the available evidence points to the objects being distant young high-mass Population-I B-type stars. We now discuss each object separately:

Fig. 2. Comparison between target and standard spectra

PG 0009+036. This object (PHL 2726) has been analysed recently by Schmidt et al. (1996) using intermediate resolution optical and ultraviolet spectra. We find similar atmospheric parameters to that study; in addition, the diffuse helium-line spectrum infers a normal composition. Unfortunately, no other abundance information is available due to the absorption lines being washed out by the large projected rotational velocity ( kms^-1). PG 0009+036 is at a large Galactic latitude (of 60^o) which combined with the derived stellar parameters places it 9 kpc below the Galactic plane. However, this object is a classic `runaway' as it exhibits a large peculiar motion ( kms^-1). Furthermore, the kinematical analysis shows that this star could have formed and have been subsequently ejected from the Galactic disk.

PG 0855+294. This object was originally discovered in the Case low-dispersion Northern sky survey (CBS 29, Sanduleak & Pesch 1984). The moderately large projected rotational velocity precludes any abundance measurements other than that for helium, carbon and magnesium which appear normal. The kinematic analysis demonstrates that this star could quite reasonably have had time to travel to its current position.

PG 0914+001. This object is potentially the most interesting in the sample as it is the faintest and yet is clearly young, due to the large projected rotational velocity ( kms^-1) and the presence of emission in the early Balmer lines (see Fig. 3). The latter introduces a larger uncertainty in our surface gravity estimate (0.3 dex), while the former leads to few absorption features being observed in the spectrum - thus making the model-atmosphere analysis difficult.

Fig. 3. Balmer series for PG 0914+001, showing emission in the earliest lines.

Our choice of atmospheric parameters yield a normal, photospheric helium composition of 11.00.3 dex. Furthermore, these imply an evolutionary age that is less than the time of flight obtained from the kinematical analysis, placing PG 0914+001 some 16.9 kpc above the Galactic plane and at a galactocentric radius () of 33.2 kpc! However, it is important to consider the effect of errors in the derived atmospheric parameters and the radial velocity - as these in turn will affect our estimates of the stellar distance and life-time. We have therefore considered a (, ) pair that is consistent within our observational errors, but which will minimize the ratio of the kinematic time-of-flight to the evolutionary age. For example, the atmospheric parameters (13 000 K, 3.6 dex) lead to an evolutionary age, , that is greater than the time-of-flight, (MIN ), needed for PG 0914+001 to attain its current Galactic position and the results of this error analysis are entered as a second line in Table 8. Furthermore, this choice of (, ) pair lead to more `realistic' values of z-distance and galactocentric radius.

PG 0934+145. The available abundance estimates for this star, although somewhat discordant, are not significantly different from those of the Population I analogue. The large peculiar radial motion implies a large velocity perpendicular to and away from the Galactic plane - and is strong evidence that PG 0934+145 originated in the Galactic disk.

PG 0955+291. This object also presents a Population I chemical composition and a large projected rotational velocity. Although PG 0955+291 is at a moderatively large distance ( kpc) above the Galactic plane, there is no reason to suppose that it can not have travelled to its current position within its estimated lifetime, given its peculiar motion orthogonal to and away from the Galactic plane.

PG 1205+228. A model atmosphere analysis of this star (HO+23 B) has been presented previously (Conlon et al. 1989); in addition, a kinematic analysis appears in Conlon et al. (1990). This star is most certainly a Galactic disk runaway, at Galactic latitude and with a large positive radial velocity of km s^-1.

PG 2219+094. This broad-lined star ( km s^-1) proved difficult to analyse. However, it is clearly young and the abundance estimates for carbon and magnesium are Population I . The kinematical analysis requires flight-times that are greater than the stellar evolutionary age, despite considering the effect of errors in the derived atmospheric parameters and radial velocity measurement. Although apparently situated at a moderately large distance ( kpc) below the Galactic plane, it would be unwise to postulate halo formation as the origin, given the relatively small Galactic latitude of this star and without a much more precise measurement of the star's velocity perpendicular to the plane. Recently, Thejll et al. (1997) presented proper motion measurements for PG 2219+094 and several other PG stars in common with this paper and Paper I. These are discussed further in Sect. 5.

PG 2229+099. This sharp-lined star presents a reasonably rich metal-line spectrum and therefore has well defined abundance estimates, which show no significant deviation from a Population I chemical composition. Indeed, the observed spectrum is very similiar to that of the comparison star HR 6588 (see Fig. 2). PG 2229+099 is observed at a moderately large z-distance below the plane. However, the kinematical error analysis demonstrates that this object could have originated in the Galactic disk.

PG 2345+241. A normal metallicity was derived for this star, the moderately low projected rotational velocity permitting the reliable determination of abundance estimates for many species. PG 2345+241 is a young object, with  Myr. However, it lies a mere 2.9 kpc below the plane and has a large positive radial velocity, making it possible to attain its current Galactic location within its lifetime.

© European Southern Observatory (ESO) 1999

Online publication: June 18, 1999
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