4 Discussion

The main goal by the present undertaking is to locate the interstellar material responsible for the extreme ultraviolet absorption in space. Meaning that we need color excesses and stellar distances. A priori we expect distances with an accuracy of 15$\%$ depending on the validity of the absolute calibration of the $uvby\beta$ system of course. The color excesses may be checked with the polarimetry in progression, polarimetry is an order of magnitude more accurate than photometry. Preliminary results from the 2.5 m Nordic Optical Telescope for the lb329+46 shadow indicate that the ratio of polarization to reddening is large, Knude and Bowyer (in preparation). The distances uncertainty may be confirmed by the recently published Hippacos Catalog, ESA (1997). The most essential is of course to confirm the small distances found for the shadowing clouds, if possible. The Hipparcos Catalog is complete to $V\approx$ 7.5, depending on color. We have compared the list of stars in Tables 1-3 to the Hipparcos Catalog. Within $D_{uvby \beta}$ < 150 pc, without the application of any lower limit to $\delta$c₁. The search was performed according to the positions from the observing lists. 24 stars were found in Hipparcos. Figure 5 shows the photometric distance versus $\frac{1}{\pi}$. The solid curve is the one - one

  

Figure 5: Photometric distance versus 1./$\pi$ for stars in the shadow directions within a photometric distance of 150 pc. The three deviant points are more or less understandable. A 15$\%$ accuracy in the photometric distances are confirmed for this small sample

relation and the dashed curves indicates the $\pm$15$\%$ accuracy expected in the photometric distances. No Hipparcos error applied. There are three large deviations. All with an excuse though. One is an CN star whose luminosity is not calibrated and thus may have a deviant $\delta c_1$. Another example is the super metal rich A star candidate mentioned before and the third is probably a misidentification by SAT. The difference between the PPM and Hipparcos position is 10 arcsec. The SAT centers the stars automatically and if they are not located at once it scans a small box and is satisfied by any star so the photometry of the star identified is correct but it is probably not the star with the PPM number in the list. With a standard deviation of 15$\%$ 14 of the 21 remaining stars are expected within the dashed lines. 13 - 14 are found. The remaining stars but one are within $\pm$2$\sigma$ and only one just outside the $\pm$2$\sigma$ limits - just as expected with $\sigma$ = 15$\%$.

In Fig. 6 we show our V versus the observed Tycho magnitude $V\rm _T$, ESA (1997).

  

Figure 6: $V_{by} - V\rm _T$. The abscissa is compatible to $V_{\rm Johnson}$ and the ordinate is a V magnitude measured by the Tycho experiment and is n ot transformed to $V\rm _{Johnson}$

The increasing scatter is mainly due to the Tycho photometry. The limiting magnitude noticed in Tables 1-3 is determined by the automatic centering and is about $V\approx$ 11.5. Six stars are not plotted in Fig. 6: 1237 1095 1, 5560 1460 1, 6339 558 1 because there are stars nearby influencing the observations by stray light; 1240 710 1 and 1237 128 1 deviate for no obvious reason and finally 6339 384 1 has suspected duplicity.

The resulting distance - E_b-y diagrams and distribution of color excess across the shadow regions together with the thermal pressure of the hot ISM phase in the

three directions may be consulted in Berghöfer et al. (1997). A first estimate of the hot phase pressure in the (l,b) = (165,-32) shadow, $P/k \approx$19000 cm^-3 K was given in Bowyer et al. (1995).

Acknowledgements

ESO (December 1993) and The Danish Board for Astronomy (November 1994, March 1995) is thanked for granting observing time and travel support for this investigation. Dr. Claus Fabricius is thanked for saving me the trouble identifying the stars common to the Hipparcos and Tycho Catalogs and Dr. Thomas Berghöfer for comments to the manuscript.