The Interstellar Ti II Distance Scale

Abstract

We measured the equivalent widths ($$EW$$s) of interstellar Ti II absorption lines at $$\lambda$$ 3383.759 for about 250 reddened objects and found a good correlation of the $$EW$$s with distances to the background stars, estimated using Gaia DR3 parallaxes. Hipparcos trigonometric parallaxes were used for very bright objects which were not observed by Gaia. The Ti II based distance estimation procedure is similar to the well known Ca II-method (Megier et al., 2005, 2009). However, there are at least 3 advantages of the Ti II method: in contrast to interstellar Ca II H and K lines, blending of Ti II with stellar lines is not an issue even for late B stars; Ti II is to a much less extent influenced by the saturation effect; only a single line must be measured, i.e. there is only a single measurement error to be taken into account. The relation of between Ti II $$EW$$ and distance is given by simple equation $$d\approx 30EW$$, where $$d$$ is the distance in pc and $$EW$$ is the equivalent width of the Ti II $$\lambda$$ 3383.759 line in mÅ.