1999A&A...341..451K

The Hipparcos and Tycho Catalogues offer an interesting possibility to study the local distribution of the interstellar reddening from the combination of data contained in the catalogues: Hipparcos parallaxes, Tycho B-V and spectral and luminosity classification compiled from the literature, mainly the Michigan Catalogs. In some cases the distances to intermediate velocity (IV) gas features may be estimated from positional coincidence between HI gas in narrow velocity intervals and E_B–V v. distance features extracted independently from the Hipparcos and Tycho Catalogues. Presently about 33000 sight lines are available for interstellar studies. Apart from the IV Arch the other possible dust counterparts are identified without a priori knowledge of the positions of the 21cm gas with deviating velocities. Some features are revealed immediately in the E_B–V v. distance diagram for the north galactic cap. Color excesss-distance data for b>60° reveal two dust features at ≃50pc and ≃300pc with color excesses much larger than the average NGP reddening. We present circumstantial evidence that the distances of these features may pertain mainly to gas with intermediate negative velocity. The nearest NGP dust spike is sharply defined and may be related to a 21cm feature with V_lsr in the range [-37|-27]km/s. The 300pc feature may be related to three distinct hydrogen features: the edge of a [-60|-57.5]km/s structure, and a fragmented loop with V_lsr between -30 and -25km/s. These results indicate that high velocity gas may be local, a suggestion gaining ground since the discovery of low velocity bridges. Another result is that gas to dust ratio along the sight lines at the positional associations are found to N_H/E_B–V≲5x10²⁰cm^–2.mag^–1, an order of magnitude smaller than the canonical value. If this implies depletion it is no wonder that interstellar absorption lines with velocities matching the IV gas have not been observed. Since dust is predominantly destroyed by shocks in the warm neutral/ionized part of the interstellar medium and we discuss features with larger than normal dust/gas ratio the IV gas could be cold. There is just an indication of a shell like structure between 30 and 50pc where the proposed Perseus Superbubble is expected to be located between 50 and 80pc.