1 Introduction

The Deep Survey instrument of the Extreme Ultraviolet Explorer (EUVE) detected three significant absortion features in the Lex/B count rates which were identified with 100 $\mu$ emission features, Bowyer et al. (1995), Berghöfer et al. (1997). The calculation of the thermal pressure in these shadows from the emission measure requires information on the extent of the emitting volume. This paper presents the optical data used to assess this volume. The softness of the Lex/B band implies that only a minor column density $N\rm _H$ is needed to reach unit optical depth. The situation is thus so simple that all the emission must originate in front of the absorbing clouds and the cloud absorbs all emission produced at larger distances. Therefore the cloud distance provides a limit of the emitting volume.

A description of the EUVE mission and details of the instrumentation are given in Bowyer & Malina (1991). The EUVE scans the complete sky but the Deep Survey is about a factor ten more sensitive than the general survey.
Local diffuse interstellar clouds may in principle have their distance determined in two ways. One is the combination of spectral classification and measurement of optical absorption lines in background stars and another is distance and continuum absorption of background stars derived from a precision photometric system. This way an upper distance limit is provided. Lower distance limits are provided from unobscured foreground stars. The approach by means of photometry has the advantage to be relatively fast and to work for a wide spectral and luminosity range. The success rate of confining an absorption feature only a few square degrees and within $\sim$50 pc is possibly largest by the photometric approach. The Tables 1-3 present the $uvby\beta$ data for the three shadow directions lb27-31, lb165-32 and lb329+46. The $uvby\beta$ systems are able to provide photometric parallaxes of an accuracy 15$\%$ and color excess with an uncertainty 0.008 - 0.010 in E_b-y (Knude 1978). The former of these numbers is confirmed by the stars of this program within $d_{\rm phot}$ < 150 pc common to the Hipparcos Catalog. The probability to detect local diffuse clouds with magnitude limited complete samples are discussed in Knude (1981).

  

Table 1: lb165-32 shadow direction. Columns 1, 2, 3 are PPM, HD/DM and Tycho numbers. Columns 4 and 5 Right ascension and declination (2000.0). Columns 6, 7, 8, 9 and 11 V, b-y, m₁, c₁ and $\beta$ in magnitudes. Columns 10 and 12 number of uvby and $\beta$ observations. Last column comments. D: unresolved duplicity in Tycho data, G: listed in GCVS, R: weak indication of duplicity combined with indication of variability in Tycho data, S: suspected duplicity from Tycho data, U: apparent variability, W: suspected variability. Stars without PPM, HD/DM or Tycho identification are listed by a working number in the last column

  

Table 2: lb27-31 shadow direction. Columns 1, 2, 3 are PPM, HD/DM and Tycho numbers. Columns 4 and 5 Right ascension and declination (2000.0). Columns 6, 7, 8, 9 and 11 V, b-y, m₁, c₁ and $\beta$ in magnitudes. Columns 10 and 12 number of uvby and $\beta$ observations. Last column comments. D: unresolved duplicity in Tycho data, G: lised in GCVS, R: weak indication of duplicity combined with indication of variability in Tycho data, S: suspected duplicity from Tycho data, U: apparent variability, W: suspected variability. Stars without PPM, HD/DM or Tycho identification are listed by a working number in the last column

 

Table 2: continued

  

Table 3: lb329+46 shadow direction. Columns 1, 2, 3 are PPM, HD/DM and Tycho numbers. Columns 4 and 5 Right ascension and declination (2000.0). Columns 6, 7, 8, 9 and 11 V, b-y, m₁, c₁ and $\beta$ in magnitudes. Columns 10 and 12 number of uvby and $\beta$ observations. Last column comments. D: unresolved duplicity in Tycho data, G: listed in GCVS, R: weak indication of duplicity combined with indication of variability in Tycho data, S: suspected duplicity from Tycho data, U: apparent variability, W: suspected variability. Stars without PPM, HD/DM or Tycho identification are listed by a working number in the last column