INTERSTELLAR K AND H LINES AS DISTANCE INDICATORS

The intensities of two Ca II resonance lines (H and K) in the spectra of O- and B-type stars have been used as distance indicators for many years. The work prior to 1952 has been discussed in in a comprehensive paper by Binnendijk, who also pointed out the intensity-distance relation may be different in different directions. The subject has been reviewed by Munch (1952). A recent study is due to Hobbs (1974). The equivalent width of interstellar K has been measured for a great many stars at low galactic latitude; however, for stars at high galactic latitude, the observational material is sparse and the nature and amount of the material at high galactic latitudes in unclear. The existing principal studies on stars at high galactic latitudes are those of Munch (1952), Munch and Zirin (1961), Greenstein (1968); Rickard (1972), Cohen (1974) and Cohen and Meloy (1975).

In the following we summarize the available information on the interstellar K and H lines in the spectra of quasars.

3C 273. Williams records that Dr. G. Preston, of Lick Observatory, obtained a number of optical spectra of the object at 48 Å/mm in which the interstellar H and K lines of Ca II are visible; a typical spectrum is reproduced in Figure 1 of Williams. However, no equivalent widths or any other details of these observations have yet been published.

Greenstein and Schmidt state: 'Interstellar Ca II absorption lines have been found by Preston (private communication) and subsequently also on a Palomar spectrum with a dispersion of 85 Å/mm.'

Greenstein (1968) has reported the equivalent width of the interstellar K line in 38 objects, including 3C 273B. In his Table 1, the W(K) value for 3C 273B is recorded as '0.50?' (in Å). Also in the text the following statement occurs: 'The quasi-stellar source 3C 273B was available on two plates at 38 Å/mm; unfortunately both are slightly contaminated by moonlight.' No high dispersion study of the spectrum of 3C 48 has ever been published.

The expected equivalent width of the interstellar K line in quasars is expected to be of the same order of magnitude as that in the high galactic latitude stars, i.e. W(3934) = 0.050 to 0.250 Å (Cohen, 1974).

The following quasars are known to show many ( > 10) absorption lines in their spectra:

The spectra of most of these quasars have been obtained at higher dispersion than those of quasars which show only emission lines. Amongst these absorption lines quasars, those with a * symbol show a line close to 3934 Å and it would be reasonable to identify it as Ca II 3934 Å. However, the interpretation of the intensity of this line in such quasars is beset with the difficulty that the observed intensity is the sum of circumstellar and interstellar contributions. Equivalent width data for the absorption lines are available for only a few of the quasars. In 1331+170 (Carswell et al., 1975), there are two lines whose equivalent width is recorded as about 0.2 Å; this does not necessarily imply, however, that all lines stronger than this have been detected. The equivalent widths of the weakest absorption lines recorded in Markarian 132 and 0453-423 are 0.2 Å (Adams et al., 1978) and 0.3 Å, respectively. At present, all that can be said as regards the absence of 3934 Å in the three quasars is that the available data are consistent with our expectations. Morton and Morton (1972) have taken two spectra of Ton 1530 covering the region 4435-4655 Å and they give equivalent widths for five lines. Quasar 2126-158 has not been investigated shortward of 4153 Å (Young et al., 1979). No equivalent width data on any of the remaining quasars are available. It is of interest to note that PKS 0237-23 and PHL 1222 appear in Table 2.

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