PLC Relations Research Articles

The Cepheid Luminosity Scale

The distances of clusters containing classical Cepheids are central to the calibration of the period-luminosity relation. As a step in improving the reliability of the calibration, uvbys photometry was obtained for B stars in eight of these clusters. These data were then used to determine the distances of the clusters and, thus, the absolute magnitudes of the Cepheids and the zero point of the PLC relation (See Schmidt 1984 for details).

On Possible Causes of Scatter in the Period‐Luminosity‐Colour Relation of Cepheids

AbstractSome possible causes of scatter in the PLC relation of cepheids are investigated and estimates arising from the presence of rotation, magnetic field, and companion star are presented.

On the period-luminosity-colour relation for galactic Cepheids

A period-luminosity-colour relation is derived from the data of 29 galactic Cepheids with known distances collected from the literature, and is then compared with the simpler period-luminosity relation. The insignificant reduction in the dispersion seems to argue against the introduction of the colour term. It is, however, demonstrated that this evidence may be fictitious and due to the use of Cepheids, whose membership in a star aggregate is still doubtful. The exclusion of these stars leads to a PLC relation which shows a significantly smaller dispersion than that given by the corresponding PL relation.

Cepheid space distribution and the structure of the galaxy

On the basis of the PLC relation (1) or the PL relation by Van den Bergh (2) and the PC relation by Deanet al. (1978), the distances of 284 galactic cepheids with photoelectric observations have been derived. The space distribution of these cepheids with 111 additional ones without photoelectric observations, is studied. In spite of the strong influence of the absorption matter, which makes a great number of distant cepheids unknown (Figure 4), a conclusion is drawn that the cepheids do not trace spiral arms with only one possible exception: the Carina arm. The cepheidz-coordinate distribution confirms the finding of Fernie (1968) that the cepheid layer is inclined towards the formal galactic plane. On the basis of cepheid space density, a number of vast star complexes (Table I) are identified in which other young objects, together with cepheids fall. The existence of these complexes is explained by star formation in giant molecular clouds. The cepheid mean period increase towards the galactic centre is most probably connected with the existence of a ring between the Sun and the centre of the Galaxy, with the highest density of hydrogen and the highest rate of star formation.