Interstellar CaiiLine Intensities and the Distances of the OB stars

Abstract

We show that the equivalent widths of the well-known interstellar Ca ii H and K lines can be used to determine the distances to OB stars in our Galaxy. The equivalent widths, measured in the spectra of 147 early-type stars, are strongly related to the Hipparcos parallaxes of those objects. The lines fitted to the parallax-equivalent width data are given by the formulaewhere is in arcseconds and EW is in milliangstroms. The form of the formulae, yielding a finite parallax even for zero absorption, shows that space within %100 pc of the Sun contains very little Ca ii, which is in agreement with the known dimensions of the Local Bubble. Using Ca ii lines for distance determination does not require the knowledge of the absolute magnitude of the object; it is thus well suited for targets for which the absolute calibration is either not precise (OB supergiants) or not available at all (peculiar objects). We also demonstrate that neither the reddening E (B À V ) nor the equivalent widths of interstellar K i and CH lines are suitable candidates for distance estimation, their relation with parallaxes being far less tight than for Ca ii.

The interstellar Ca II distance scale

“…Ca ii lines; since they get stronger with distance, the relations by Megier et al (2005) and the one given in this paper lead to discrepancies at large distances. The formula (1) should give reasonable distance estimates up to 3 kpc in the Galactic plane.…”

“…The form of the function to fit was similar to that in Megier et al (2005): Because of the nonlinear dependence of the column density on the equivalent widths of the doublet lines, the resulting relative errors in N Ca ii may be significantly larger than those in the equivalent widths themselves. While relative errors in parallax are still generally larger, they are not so dominant as in the case of EW(Ca ii H) vs parallax or EW(Ca ii K) vs. parallax relations studied in Megier et al (2005). We thus decided to fit the data using a maximum likelihood algorithm, taking into account errors in both coordinates.…”

“…While the nonuniform, structure-rich distribution of the interstellar matter is well known, some components are markedly more clumpy than Article published by EDP Sciences others, resulting in the lower correlation of the associated spectral features with distance. For example, Galazutdinov (2005) and Megier et al (2005) show that the Ca ii is a better distance indicator than K i, Na i or E B−V .…”

“…In Megier et al (2005), the relation between the equivalent widths of the interstellar Ca ii lines and the distances of Galactic OB stars was recalibrated with the use of the Hipparcos data. This resulted in formulae for estimating distances, applicable in the range from a few hundred parsecs to slightly over a kiloparsec.…”

A method to determine distances to molecular clouds using near-IR photometry

“…The traditional ways of determining distances to isolated clouds utilize the star count method (Bok & Bok 1941) or Wolf diagrams (Wolf 1923), which is limited due to its dependence on the questionable extrapolation of luminosity functions. Additional methods of assigning distances to small dark clouds have involved bracketing the cloud distance by using spectroscopic distances to stars (Hobbs et al 1986) and using the equivalent widths of interstellar Ca II H and K lines of stars close in front of and behind the cloud (Megier et al 2005). However, spectroscopy of a large number of stars projected on a cloud is extremely demanding in terms of telescope time.…”