Galaktische Raumkoordinaten und Periodenverteilung naher Mira-Sterne

Abstract

Cartesian galactic coordinates have been calculated for those Mira type variable stars whose visual (mean) maxima magnitudes were available, down to distance moduli 11 ^m_· 0 inclusively. The latter are determined from the period-luminosity relations for long and short period Miras, respectively, as derived by Ferrari (1973, 1976, 1977) from Clayton & Feast’s statistical parallaxes (1969), with the additional assumption that short period Miras are subject to overtone pulsations: formulas (1) and (2).

In the more recent analysis by Foy et alii (1975) we do not find any reason to abandon the outlines of our concept, since his results for the overwhelming majority of Mira-stars sufficiently agree with C l a y t on & Feast’s, last but not least in the confirmation of the uniqueness of the visual luminosity-period relation for M-type Miras regardless of the intensity of OH-features in their spectra. As to the rather poor group of stars with periods from 150 to 200 days, we prefer Clayton & Feast’s mean value, since it is based on a considerably greater sample than Foy’s. But, in our opinion, the natural demarcation line between both branches of the period-luminosity relation must be drawn at about 160 days (instead of 150 days), including the possibility that they overlap in a small realm between 160 and 170 days.

The X, Y, Z coordinates (in pc) of the present catalogue are calculated with the assumption that interstellar absorption depends only on distance and galactic latitude according to formula (3). Under the headings I, II, III logarithmical reductions Δlog₁₀R to three other models of absorption are added. Cases I and II: superimposed density waves of absorbing material parallel to our neighbouring spiral arms are assumed. Case I: maxima of absorption being centered amidst the bright spiral arms with amplitude ± 55 percent of the mean value. Case II: maxima of absorption at the inner edges of the spiral arms with amplitude ± 50 percent of the mean value (Cf.: Ferrari & Jenkner, 1973, but with slightly different coefficients of absorption). Case III: Van Herk’s absorption formula (4). The remarks in the last column of this catalogue are concerned with the occurrence of rare spectral types: C (including the obsolete types R and N), K-M, or S. For the latter ones, distance corrections according to Foy’s result that they are overluminous by 0 ^m_· 3 are given in Table 4. With “E ? ” we have denoted a few stars with periods from 160 to 170 days, tentatively treated according to formula (2), while the same, if treated according to formula (1) in most cases would fall beyond the limiting distance modulus 11 ^m_· 0.

In order to complete the basic material for the statistics of periods in a clearly defined volume of space, we formed an additional list of those photographically observed Miras which have, most probably, distance moduli smaller than 11 ^m_· 0. Since the latter, individually, sometimes might be rather uncertain, instead the apparent photographic magnitudes of maxima and, for statistical purposes only, rough (Z)-coordinates in fractions of kiloparsec from our unpublished working catalogue are given.

Period frequencies have been counted in ten-day intervals, and smoothed out according to formula (5) for Miras of spectral type M,inside and outside the arbitrarily chosen planes of demarcation |Z| = 250 pc (Table 5). The most remarkable feature there is the fact that several more or less pronounced maxima of frequency are located, as far as one can see it, around precisely the same period values in both populations, notwithstanding great differences in the relative heights of corresponding maxima here and there. This observation is opposite to the assumption that the often stressed difference of the mean periods of “disc” and “halo” populations be the result of a general evolutionary trend of periods growing gradually shorter in individual stars when leaving the regions of their origin in the galactic disc.

The problem of the physical meaning of distinct groups of Miras according to period could not be cleared up by the selection of such close pairs of Miras which, according to the rules of probability, might be considered as physically connected. Even in these rare cases, ratios of periods as low as 0.78 are found.