Journal Article

Ca <span class="smallCaps">ii</span> K interstellar observations towards early-type disc and halo stars

J. V. Smoker, W. R. J. Rolleston, H. R. M. Kay, D. Kilkenny, R. Morras, M. Arnal, F. P. Keenan, C. J. Mooney, P. L. Dufton, R. S. I. Ryans, N. C. Hambly, D. O'Donoghue and H. McGillivray

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 346, issue 1, pages 119-134
Published in print November 2003 | ISSN: 0035-8711
Published online November 2003 | e-ISSN: 1365-2966 | DOI:
Ca ii K interstellar observations towards early-type disc and halo stars

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We present high-resolution (R=λ/Δλ∼ 40 000) Ca ii K interstellar observations (λair= 3933.66 Å) 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 m Å (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 m Å, 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 Ca ii 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 Ca ii 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(Htot) 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.

Keywords: stars: early-type; ISM: clouds; ISM: general; ISM: structure

Journal Article.  8728 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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