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Ca iiK interstellar observations towards early-type disc and halo stars, abundances and distances of intermediate- and high-velocity clouds

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Abstract:

ABSTRACT

We present Ca ii K (air= 3933.661 Å) interstellar observations towards 20 early-type stars, to place lower distance limits to intermediate- and high-velocity clouds (IHVCs) in their lines of sight. The spectra are also employed to estimate the Ca abundance in the low-velocity gas towards these objects, when combined with Leiden–Dwingeloo 21-cm H isurvey data of spatial resolution . Nine of the stars, which lie towards IHVC complexes H, K and gp, were observed with the intermediate dispersion spectrograph on the Isaac Newton Telescope at a resolution R=/Δ of 9000 (∼33 km s−1) and signal-to-noise ratio (S/N) per pixel of 75–140. A further nine objects were observed with the Utrecht Echelle Spectrograph on the William Herschel Telescope at R= 40 000 (∼7.5 km s−1) and S/N per pixel of 10–25. Finally, two objects were observed in both Ca iiK and Na iD lines using the 2D COUDÉ spectrograph on the McDonald 2.7-m telescope at R= 35 000 (∼8.5  km s−1) . The abundance of Ca ii K {log10(A) = log10[N(Ca iiK)] − log10[N(H i)]} plotted against H icolumn density for the objects in the current sample with heights above the Galactic plane (z) exceeding 1000 pc is found to obey the Wakker & Mathis (2000) relation. Also, the reduced column density of Ca iiK as function of z is consistent with the larger sample taken from Smoker et al. (2003). Higher S/N observations than those previously taken towards HVC complex H stars HD 13256 and HILT 190 reinforce the assertion that this lies at a distance exceeding 4000 pc. No obvious absorption is detected in observations of ALS 10407 and HD 357657 towards IVC complex gp. The latter star has a spectroscopically estimated distance of ∼2040 pc, although this was derived assuming the star lies on the main sequence and without any reddening correction being applied. Finally, no Ca iiK absorption is detected towards two stars along the line of sight to complex K, namely PG 1610+529 and PG 1710+490. The latter is at a distance of ∼700 pc, hence placing a lower distance limit to this complex, where previously only an upper distance limit of 6800 pc was available.

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

Document Type: Research Article

DOI: http://dx.doi.org/10.1111/j.1365-2966.2006.10443.x

Affiliations: Astrophysics and Planetary Science Research Division, Department of Physics and Astronomy, The Queen's University of Belfast, University Road, Belfast BT7 1NN

Publication date: July 21, 2006

bsc/mnr/2006/00000370/00000001/art00012
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