Search

High spectral resolution (R ∼ 40 000) and signal-to-noise optical spectra, obtained at the Very Large Telescope (VLT), are presented for three post-asymptotic giant branch (AGB) candidates selected from the Edinburgh-Cape (EC) Faint Blue Object Survey. The stellar atmospheric parameters and chemical compositions, derived using sophisticated non-local thermodynamic equilibrium calculations, reveal that EC 14102-1337 and EC 20068-7324 are both in an evolved post-horizontal branch (HB) evolutionary state. However, EC 11507-2253 is most likely a post-AGB star.

High spectral resolution (R ∼ 40000) and signal-to-noise ratio observations of five high Galactic latitude early-type stars taken from the Edinburgh-Cape (EC) Faint Blue Object Survey are presented. These were required to complete a magnitude range-limited survey of young B-type objects with 11 < V < 15. Of the five stars, four were rejected on the grounds that they are either subluminous (subdwarf or horizontal branch), were part of a binary system or possessed colours later than the (U - B) = -0.5 cut-off employed. The remaining star in the data set, EC 19596-5356, is found to exhibit normal young B-type stellar properties. A kinematic analysis reveals that an origin in the Galactic disc appears likely for all the stars in the sample. Some statistics are drawn about the number density of young stars in the Galactic halo.

We compare existing high spectral resolution (R=40,000) CaII K observations towards 88 mainly B-type stars, and new observations at R=10,000 towards 3 stars, with 21-cm HI emission-line profiles, in order to search for optical absorption towards known intermediate and high velocity cloud complexes. Given certain assumptions, limits to the gas phase abundance of CaII are estimated for the cloud components. We use the data to derive the following distances from the Galactic plane (z); 1) Tentative lower z-height limits of 2800 pc and 4100 pc towards Complex C using lack of absorption in the spectra of HD 341617 and PG 0855+294. 2) A weak lower z-height of 1400 pc towards Complex WA-WB using lack of absorption in EC 09470-1433 and weak lower limit of 2470 pc with EC 09452-1403. 3) An upper z-height of 2470 pc towards a southern intermediate velocity cloud (IVC) with v_LSR=-55 km/s using PG 2351+198. 4) Detection of a possible IVC in CaK absorption at v_LSR=+52 km/s using EC 20104-2944. No associated HI in emission is detected. At this position, normal Galactic rotation predicts velocities of up to +25 km/s. The detection puts an upper z-height of 1860 pc to the cloud. 5) Tentative HI and CaK detections towards an IVC at +70 km/s in the direction of HVC Complex WE, sightline EC 06387-8045, indicating that the IVC may be at a z-height lower than 1770 pc. 6) Detection of CaK absorption in the spectrum of PG 0855+294 in the direction of IV20, indicating that this IVC has a z-height smaller than 4100 pc. 7) A weak lower z-height of 4300 pc towards a small HVC with v_LSR=+115 km/s at l,b=200,+52, using lack of absorption in the CaK spectrum of PG 0955+291.

High-resolution (R 40 000) echelle spectroscopic observations of 13 high-latitude early-type stars are presented. These stars comprise the final part of a complete magnitude range limited sample based on low-resolution spectroscopy of targets drawn from the Palomar-Green survey. The magnitude range under consideration is 13 ≤ B PG ≤ 14.6, corresponding to an approximate distance limit for main-sequence B-type objects of 5 ≤ d ≤ 40 kpc. Three stars are found to be apparently normal, young stars, based on their positions on the (T eff , log g) diagram, normal abundance patterns and relatively large projected rotational velocities. A further star, PG 1209+263, was found to belong to the chemically peculiar (CP) silicon star class of objects. The remainder are evolved subluminous stars lying on post-horizontal branch (post-HB) tracks, with the exception of PG 2120+062, which appears to be in a post-asymptotic giant branch evolutionary stage. For the young stars in the sample, we have derived distance and age estimates through comparison of the atmospheric parameters with recent theoretical evolutionary models. We discuss formation scenarios by comparing times-of-flight and evolutionary time-scales. It is found that all stars could have formed in the Galactic disc and been ejected from there soon after their birth, with the exception of PG 1209+263. The adopted proper motion is found to be a crucial factor in the kinematical analysis. We also present some number densities for young B-type halo stars, which indicate that they are extremely scarce objects.

We present high-resolution (R = λ/Δλ ∼ 40 000) Ca II K interstellar observations (λ a i r = 3933.66 A) towards 88 mainly B-type stars, of which 74 are taken from the Edinburgh-Cape or Palomar-Green surveys, and 81 have‖b‖ > 25°. The majority of the data come from previously existing spectroscopy, although also included are 18 new observations of stars with echelle spectra taken with UVES on the Very Large Telescope UT2 (Kueyen). Some 49 of the sample stars have distance estimates above the Galactic plane (‖z‖) ≥ 1 kpc, and are thus good probes of the halo interstellar medium. Of the 362 interstellar Ca K components that we detect, 75 (21 per cent) have absolute values of their LSR velocity values exceeding 40 km s - 1 . In terms of the deviation velocity for the sightlines with distance estimates, 46/273 (17 per cent) of components have velocity values exceeding those predicted by standard Galactic rotation by more than 40 km s - 1 . Combining this data set with previous observations, we find that the median value of the reduced equivalent width (REW) of stars with ‖z‖ ≥ 1 kpc (EW × sin ‖b‖) is ∼115 mA (n = 80), similar to that observed in extragalactic sightlines by Bowen. Using data of all z distances, the REW at infinity is found to be ∼130 mA, with the scaleheight (l) of the Ca II K column density distribution being 800 pc (n = 196) and reduced column density at infinity of log[N(Ca II K) cm - 2 ] ∼12.24. This implies that ∼30 per cent of Can K absorption occurs at distances exceeding ∼1 kpc. For nine sightlines with distance exceeding 1 kpc and with a companion object within 5°, we find that all but two have values of Ca II reduced equivalent width the same to within ∼20 per cent, when the REW of the nearest object is extrapolated to the distance of the further of the pair, and assuming l = 800 pc. For 29 of our sightlines with ‖z‖ ≥ 1 kpc and a H I detection from the Leiden-Dwingeloo survey (beamsize of 0.5°), we find log(N(Ca II K)/N(H i)) ranging from -7.4 to -8.4. Values of the Can K abundance relative to neutral hydrogen (log[N(Ca II K) cm - 2 ] - log[N(H I) cm - 2 ]) are found to be more than ∼0.5 dex higher in stars with distances exceeding 100 pc, when compared with the (log[N(Ca II K) cm - 2 ] - log[N(H t o t ) cm - 2 ]) values found in nearby sightlines such as those in Wakker & Mathis (2000). Finally, stellar Ca II K equivalent widths of the sample are determined for 26 objects.

Absolute and differential abundance analyses have been performed from high-resolution, high signal-to-noise ratio optical (Keck I) spectra for three evolved Galactic halo stars, namely PG 1704 + 222, HD 341617 and LS IV -04 01. Their derived atmospheric parameters indicate that all three objects are undergoing a post-asymptotic giant branch (post-AGB) phase of evolution. A differential abundance analysis reveals HD 341617 as having a mild carbon deficiency of 0.74 dex, possibly due to the star having evolved off the AGB before the onset of the third dredge-up. Although such carbon underabundances are typical of hot post-AGB objects, the same trend is not observed in PG 1704 + 222, where the carbon abundance is found to be consistent with those derived for nitrogen and oxygen. Hence, a dredge-up scenario need not be invoked to explain the chemical composition of PG 1704 + 222. For LS IV -04 01 no iron deficiency is apparent relative to magnesium and silicon, and hence a gas-dust separation event in the AGB progenitor need not be invoked for this star.

A preliminary search for stars that may have formed coevally with the apparently young halo B-type star PHL 346 has been performed with the 2dF multifibre spectrograph on the Anglo-Australian Telescope (AAT). Candidates were selected for spectroscopy from APM scans of B and R Schmidt plates centred on PHL 346. A total of 476 stars of spectral type A or F were found; radial velocity estimates and more accurate spectral type assignments narrowed the number of possible coeval candidates to 6 A-type and 14 F-type stars. A statistical analysis of these results using a comparison with a control field suggests that the number of A-type or F-type candidate stars around PHL 346 is not unexpected, and that they need not be associated with PHL 346. A number of ways to improve the project are suggested.

19 B-type stars, selected from the Palomar-Green Survey, have been observed at infrared wavelengths to search for possible infrared excesses, as part of an ongoing programme to investigate the nature of early-type stars at high Galactic latitudes. The resulting infrared fluxes, along with Stromgren photometry, are compared with theoretical flux profiles to determine whether any of the targets show evidence of circumstellar material, which may be indicative of post-main-sequence evolution. Eighteen of the targets have flux distributions in good agreement with theoretical predictions. However, one star, PG 2120+062, shows a small near-infrared excess, which may be due either to a cool companion of spectral type F5-F7, or to circumstellar material, indicating that it may be an evolved object such as a post-asymptotic giant branch star, in the transition region between the asymptotic giant branch and planetary nebula phase, with the infrared excess due to recent mass loss during giant branch evolution.

The absorption-line spectra of early B-type supergiants show significant broadening that implies that an additional broadening mechanism (characterized here as 'macroturbulence') is present in addition to rotational broadening. Using high-resolution spectra with signal-to-noise ratios of typically 500, we have attempted to quantify the relative contributions of rotation and macroturbulence, but even with data of this quality significant problems were encountered. However, for all our targets, a model where macroturbulence dominates and rotation is negligible is acceptable; the reverse scenario leads to poor agreement between theory and observation. Additionally, there is marginal evidence for the degree of broadening increasing with line strength, possibly a result of the stronger lines being formed higher in the atmosphere. Acceptable values of the projected rotational velocity are normally less than or equal to 50 km s - 1 , which may also be a typical upper limit for the rotational velocity. Our best estimates for the projected rotational velocity are typically 10-20 km s - 1 and hence compatible with this limit. These values are compared with those predicted by single star evolutionary models, which are initially rapidly rotating. It is concluded that either these models underestimate the rate of rotational breaking or some of the targets may be evolving through a blue loop or are binaries.

Local thermodynamic equilibrium (LTE) absolute and differential abundances are presented for a peculiar metal-rich B-type star, HD 135485. These suggest that HD 135485 has a general enrichment of ∼0.5 dex in all the metals observed (C, N, O, Ne, Mg, Al, Si, P, S, Cl, Ar, Sc, Ti, Cr, Mn, Fe and Sr), except for nickel. The helium enhancement and hence hydrogen deficiency can account for ≤ 0.2 dex of this enhancement of metals, with the additional enhancement probably being representative of the progenitor gas. However, some of the metals appear to have greater enhancements, which may have occurred during the star's evolution. The significantly larger nitrogen abundance coupled with a modest helium enhancement observed in HD 135485 indicates that carbon–nitrogen (CN) processed material has possibly contaminated the stellar surface. Neon and carbon enhancements may indicate that helium core flashes have also occurred in HD 135485. Some of the iron-group elements (viz. Mn and Ni) appear to have similar abundance patterns to that of silicon Ap stars, but it is uncertain how these abundance patterns formed if they were not present in the progenitor gas. From a kinematical investigation it is unclear whether this star formed in a metal-rich region as implied by its chemical composition. From its position in the Hertzsprung–Russell diagram, HD 135485 would appear to be an evolved star lying close to or on the horizontal branch.

We present high-resolution spectroscopic observations of 21 B-type stars, selected from the Edinburgh–Cape Blue Object Survey. Model atmosphere analyses confirm that 14 of these stars are young, main-sequence B-type objects with Population I chemical compositions. The remaining seven are found to be evolved objects, including subdwarfs, horizontal branch and post-AGB objects. A kinematical analysis shows that all 14 young main-sequence stars could have formed in the disc and subsequently been ejected into the halo. These results are combined with the analysis of a previous subsample of stars taken from the Survey. Of the complete sample, 31 have been found to be young, main-sequence objects, with formation in the disc, and subsequent ejection into the halo, again being found to be a plausible scenario.

We present a parallax measurement for the very cool degenerate WD 0346+246, the serendipitous discovery of which was reported by Hambly et al. We find an absolute parallax of 36±5 mas, yielding a distance estimate of 28±4 pc. The resulting absolute visual magnitude of the object is MV=16.8±0.3, making it the second-lowest luminosity white dwarf currently known. We use the distance estimate and measured proper motion to show that the object has kinematics consistent with membership of the Galactic halo. WD 0346+246 is therefore by far the coolest and least luminous of only a handful of plausible halo white dwarf candidates. As such, the object has relevance to the ongoing debate concerning the results of microlensing experiments and the nature of any baryonic dark matter component to the Galactic halo residing in stellar remnants.

We present new, high-resolution, high signal-to-noise ratio, optical and 21-cm HI observations of the faint blue halo star HD 100340. A differential abundance analysis with respect to the Galactic disc star HR 2387 shows no significant peculiarities. We find that HD 100340 lies at a distance of 3 kpc, and a kinematical analysis strongly supports a `runaway star' origin. We also present improved cloud models for three intermediate velocity clouds seen in Ca II K or H I towards HD 100340.

We have used previously published observations of main-sequence B-type stars lying in 11 young stellar clusters and associations to estimate non-LTE abundances of oxygen and nitrogen over galactocentric distances 6

The paper "Metallicity and photospheric abundances in field K and M dwarfs" was published in Mon. Not. R. Astron. Soc. 299, 753 – 758 (1998). The third author, W.R.J. Rolleston, was accidentally omitted from the final version of the manuscript submitted to the RAS, and so did not appear in the author list of the published paper. His affiliation during this work was Armagh Observatory, College Hill, Armagh BT61 9DG. His present address is Queen's University of Belfast, Belfast BT7 1NN. The authors apologize for this oversight.