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

Abstract

We compare existing high spectral resolution (R = λ/Δλ ∼40 000) Ca II K observations (λ air = 3933.66 A) towards 88 mainly B-type stars, and new observations taken using the Intermediate dispersion Spectrograph and Imaging System (ISIS) on the William Herschel Telescope at R ∼ 10 000 towards three stars taken from the Palomar-Green Survey, with 21-cm H I 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 Ca II are estimated for the cloud components. We use the data to derive the following distances from the Galactic plane (z). (i) Tentative lower z-height limits of 2800 and 4100 pc towards complex C using lack of absorption in the spectra of HD 341617 and PG 0855+294, respectively. (ii) A weak lower z-height of 1400 pc towards complex WA-WB using lack of absorption in EC 09470-1433 and a weak lower limit of 2470 pc using lack of absorption in EC 09452-1403. (iii) An upper z-height of 2470 pc towards a southern intermediate-velocity cloud (IVC) with υ LSR = -55 km s -1 using PG 2351+198. (iv) Detection of a possible IVC in Ca II absorption at υ LSR = +52 km s -1 using EC 20104-2944. No associated H I in emission is detected. At this position, normal Galactic rotation predicts velocities of up to ∼+25 km s -1 . The detection puts an upper z-height of 1860 pc to the cloud. (v) Tentative H I and Ca II K detections towards an IVC at ∼+70 km s -1 in the direction of high-velocity cloud (HVC) complex WE, sightline EC 06387-8045, indicating that the IVC may be at a z-height lower than 1770 pc. (vi) Detection of Ca II K 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. (vii) A weak lower z-height of 4300 pc towards a small HVC with υ LSR = +115 km s -1 at l, b = 200°, + 52°, using lack of absorption in the Ca II K spectrum of PG 0955+291.