C.J. Mooney

An analysis of the optical spectra of the post-asymptotic giant branch stars LSIV -12 111 and HD 341617

High spectral resolution and signal-to-noise observations of the absorption and emission line spectra in two post- asymptotic-giant-branch (PAGB) stellar candidates, LSIV 12 111 and HD 314617 are discussed. The absorption line spectra have been analysed using non-LTE model atmosphere techniques to determine stellar atmospheric parameters and chemical compositions, both in absolute terms and relative to a standard star, HD 13841. The atmospheric parameters dier from previous estimates based on LTE model atmospheres, probably due to non-LTE eects. In turn these imply stellar masses that are generally larger than have been previously estimated. Both PAGB candidates have relative uniform underabundances of metals with mean values of 0.35 dex for LSIV 12 111 and 0.50 dex for HD 314617. Furthermore, their abundance patterns are remarkably similar to that observed for optically bright, F-type PAGBs. From the emission spectra, the plasma parameters and angular size of the circumstellar material are constrained, and these are consistent with previous estimates and with a PAGB evolutionary stage.

High-resolution spectroscopy of globular cluster post-Asymptotic Giant Branch stars

We present model atmosphere analyses of high resolution Keck and VLT optical spectra for three evolved stars in globular clusters, viz. ZNG-1 in M 10, ZNG-1 in M 15 and ZNG-1 in NGC 6712. The derived atmospheric parameters and chemical compositions confirm the programme stars to be in the post-Asymptotic Giant Branch (post-AGB) evolutionary phase. Differential abundance analyses reveal CNO abundance patterns in M 10 ZNG-1, and possibly M 15 ZNG-1, which suggest that both objects may have evolved off the AGB before the third dredge-up occurred. The abundance pattern of these stars is similar to the third class of optically, bright post-AGB objects discussed by van Winckel (1997). Furthermore, M 10 ZNG-1 exhibits a large C underabundance (with � (C/O) ∼− 1.6 dex), typical of other hot post-AGB objects. Differential � (α/Fe) abundance ratios in both M 10 ZNG-1 and NGC 6712 ZNG-1 are found to be approximately 0.0 dex, with the Fe abundance of the former being in disagreement with the cluster metallicity of M 10. Given that the Fe absorption features in both M 10 ZNG-1 and NGC 6712 ZNG-1 are well observed and reliably modelled, we believe that these differential Fe abundance estimates to be secure. However, our Fe abundance is difficult to explain in terms of previous evolutionary processes that occur on both the Horizontal Branch and the AGB.

Ca II K interstellar observations towards early-type disc and halo stars – distances to intermediate- and high-velocity clouds

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.