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Astron. Astrophys. 341, 451-457 (1999)

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Interstellar reddening from the Hipparcos and Tycho catalogues

II. Nearby dust features at the NGP associated with approaching HI gas

J. Knude and E. Hog

Niels Bohr Institute for Astronomy, Geophysics and Physics, Juliane Maries Vej 32, DK-2100 Kobenhavn O, Denmark (e-mail: indus@astro.ku.dk)

Received 24 September 1998 / Accepted 7 October 1998


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 [FORMULA] 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 [FORMULA] v. distance features extracted independently from the Hipparcos and Tycho Catalogues. Presently about [FORMULA] 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 21 cm gas with deviating velocities. Some features are revealed immediately in the [FORMULA] v. distance diagram for the north galactic cap. Color excesss - distance data for [FORMULA] reveal two dust features at [FORMULA]50 pc and [FORMULA]300 pc 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 21 cm feature with [FORMULA] in the range [FORMULA] km s-1. The 300 pc feature may be related to three distinct hydrogen features: the edge of a [FORMULA] km s-1 structure, and a fragmented loop with [FORMULA] between -30 and -25 km s-1. 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 [FORMULA] [FORMULA] 5 1020 cm- 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 50 pc where the proposed Perseus Superbubble is expected to be located between 50 and 80 pc.

Key words: ISM: clouds – ISM: dust, extinction – galaxy: solar neighbourhood

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© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998