The luminosities of type II Cepheids and RR Lyrae variables

TL;DR

This paper reviews the luminosity relations of type II Cepheids and RR Lyrae variables, highlighting their period-luminosity trends, differences across environments, and implications for distance scale calibration.

Contribution

It provides a comprehensive comparison of luminosity relations across different environments and discusses the implications for cosmic distance measurements.

Findings

Type II Cepheids in globular clusters show a narrow period-luminosity relation.

Field Cepheids in the LMC exhibit scatter and deviations at longer periods.

Various zero-points for luminosity relations are consistent within 0.1 mag.

Abstract

Recent work on the luminosities of type II Cepheids (CephIIs) and RR Lyrae variables is reviewed.In the near infrared (JHKs) the CephIIs in globular clusters show a narrow, linear, period-luminosity relation over their whole period range (about 1 to 100 days). The CephIIs in the general field of the LMC follow this relation for periods shorter than about 20 days. At longer period (the region of the RV Tau stars), the LMC field stars have a significant scatter and in the mean are more luminous than the PL relation. The OGLEIII optical data for the LMC field variables show similar trends. Infrared colours of stars in the RV Tau period range show marked mean differences between three groupings; the Galactic field, the LMC field, and globular clusters. In the case of the Galactic field, at least, this may be strongly influenced by selection effects. In the period range about 4 to 20 days (the W Vir range) there are stars lying above the PL relation which may be recognized by their light curves and are all likely to be binaries. The bright Galactic variable, kappa Pav probably belongs to this group. There is evidence that CephIIs in the general field (LMC and Galaxy) have a wider range of masses than those in globular clusters. At present the CephII PL zero-point depends on the pulsation parallaxes of two stars. Zero-points of RR Lyrae M(V)-[Fe/H] and Ks-log P relations can be obtained from trigonometrical, statistical and pulsation parallaxes. These zero-points are compared with those for CephIIs and with the classical Cepheid scale using variables of these three types in the LMC. Within the uncertainties (about 0.1mag) the various scales are in agreement.