The luminosities and distance scales of type II Cepheid and RR Lyrae variables

Abstract

Infrared and optical absolute magnitudes are derived for the type II Cepheids κ Pav and VY Pyx using revised Hipparcos parallaxes and for κ Pav, V553 Cen and SW Tau from pulsational parallaxes. Revised Hipparcos and HST parallaxes for RR Lyrae agree satisfactorily and are combined in deriving absolute magnitudes. Phase‐corrected J, H and Ks mags are given for 142 Hipparcos RR Lyraes based on Two‐Micron All‐Sky Survey observations. Pulsation and trigonometrical parallaxes for classical Cepheids are compared to establish the best value for the projection factor (p) used in pulsational analyses. The MV of RR Lyrae itself is 0.16 ± 0.12 mag brighter than predicted from an MV–[Fe/H] relation based on RR Lyrae stars in the Large Magellanic Cloud (LMC) at a modulus of 18.39 ± 0.05 as found from classical Cepheids. This is consistent with the prediction of Catelan & Cortés that it is overluminous for its metallicity. The results for the metal‐ and carbon‐rich Galactic disc stars, V553 Cen and SW Tau, each with small internal errors (±0.08 mag) have a mean deviation of only 0.02 mag from the period–luminosity (PL) relation established by Matsunaga et al. for type II Cepheids in globular clusters and with a zero‐point based on the same LMC‐scale. Comparing directly the luminosities of these two stars with published data on type II Cepheids in the LMC and in the Galactic bulge leads to an LMC modulus of 18.37 ± 0.09 and a distance to the Galactic Centre of R0= 7.64 ± 0.21 kpc. The data for VY Pyx agree with these results within the uncertainties set by its parallax. Evidence is presented that κ Pav may have a close companion and possible implications of this are discussed. If the pulsational parallax of this star is incorporated in the analyses, the distance scales just discussed will be increased by ∼0.15 ± 0.15 mag. V553 Cen and SW Tau show that at optical wavelengths PL relations are wider for field stars than for those in globular clusters. This is probably due to a narrower range of masses in the latter case.

The VMC Survey – XIII. Type II Cepheids in the Large Magellanic Cloud★

“…As for κ Pav, the pulsational parallax estimated by Feast et al (2008) through BW analysis is about 2σ smaller than the trigonometric parallax measured by HST and adopted here (∆π = 0.67 ± 0.33 mas). Feast et al (2008) investigated the possible causes of the discrepancy with respect to the Hipparcos parallax (van Leeuwen 2007), which was even larger than the HST one, but did not find any definitive explanation. A well known potential problem related with the application of the BW technique is the uncertainty on the projection factor p (see, e.g.…”

“…Indeed, this would allow us to obtain an independent measure of the distance to the LMC and to the GGCs hosting T2CEP variables. (Feast et al 2008). However, VY Pyx turned out to be a peculiar star, unusable as calibrator (see discussion in Benedict et al 2011).…”

“…Table 6. P L(Ks) and P W (Ks) relations for LMC T2CEPs with the ZP calibrated as follows: (2) by imposing a DM LM C,0 =18.46±0.03 mag (from CCs in the LMC Ripepi et al 2012b) in Table 5; (3) by adopting Galactic T2CEP (κ Pav) and RR Lyrae variables with HST parallaxes (Benedict et al 2011) and T2CEPs with BW distance estimates (Feast et al 2008); (4) …”

“…Moreover, according to several authors (see e.g. Feast et al 2008;Feast 2010, and references therein) RV Tauri stars, with periods from about 20 to 150 days and often irregular light curves, are considered as an additional subgroup of the Type II Cepheid class. Their evolutionary phase corresponds to the post Asymptotic Giant Branch phase path towards planetary nebula status.…”

“…Such relations were calibrated by Feast et al (2008) by means of pulsation parallaxes of nearby T2CEPs and used to estimate the distances of the LMC and the Galactic Centre. Subsequent investigations (Matsunaga, Feast, & Menzies 2009;Matsunaga, Feast, & Soszyński 2011) confirmed the existence of such tight P L relations in the J, H, Ks bands for the T2CEPs belonging to the LMC and Small Magellanic Cloud (SMC) found by the OGLE III collaboration (Soszyński et al 2008).…”