The Dachs photometric system

Basic Reference

Dachs J., Schmidt-Kaler Th. 1975, A&AS 21, 81


100 bright equatorial and southern stars of spectral type O, B, A, F, to G0 and of luminosity classes I through V have been measured with a photoelectric photometer equipped with two interference filters of 13 Å and 157 Å bandwidth centered on Halpha. The stars are evenly distributed in right ascension and intended to provide a standard system of Halpha line photometry. The observing technique and the method used in establishing the standard system are described. The resulting photometric indices alpha of the Halpha line strength are compiled in table 3. They are compared with the equivalent widths of Halpha known for 25 programme stars. With Crawford's beta index of the strength of the Hbeta line for 94 stars in common with Crawford's lists. With Feinstein's photoelectric Halpha indices measured with a pair of interference filters of 54 Å and 159 Å bandwidth, and with the spectrophotometric Ralpha index of Halpha line strength measured by Andrews for 36 of our programme stars. The advantage of using an interference filter of very small half-bandwidth in photoelectric Halpha line photometry is emphasized.

Principal objectives of the alpha system are: (1) The value of alpha is an excellent luminosity criterion especially for early-type supergiants and heavily reddened OB type stars of spectral type later than about O9. (2) By measuring simultaneously the alpha and beta indices for OB type stars it is possible to discriminate between emission-line stars of low luminosity and non-emission stars of high luminosity of the same spectral type. (3) Stars with variable hydrogen-line emission and peculiarities in their spectra may be identified from their location in the alpha - beta diagram which deviates from the mean relation for normal stars.

Several stars with variable strength of alpha emission are noted. The shell event which occured in dzeta Oph between 1971 and 1973 is also found in our observations.


Access to the data and references

Asiago Database on Photometric System (ADPS)

Indices and Parameters



 Filter   wavelength   Width   Name
              [Å]       [Å]

   Narrow      6560.5        13    Baird Atomic B11(X)
   Wide        6566.        157    Baird Atomic B2


EMI 9558A
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Last update: 21 May 1996