New period-luminosity and period-color relations of classical Cepheids

Abstract

Abstract. Photometric data for 593 Cepheids in the LMC, measured by Udalski et al. in the OGLE survey, augmented by 97 longer period Cepheids from other sources, are analyzed for the period-color (P-C) and period-luminosity (P-L) relations, and for the variations of amplitude, light curve shape, and period across the instability strip at constant absolute magnitude. Both the P-C and P-L relations have different slopes for periods smaller and larger than 10 days. The break at 10 days is also seen in the period-amplitude relations, and the compound Fourier combinations of R 21 and Φ 21 introduced by Simon and Lee. The LMC Cepheids are bluer than Galactic Cepheids in the B, V, and I color bands, part of which is due to differential Fraunhofer line blanketing and part to real differences in the temperature boundaries of the instability strip. The LMC strip is hotter by between 80 K and 350 K depending on the period. Hence, both the slopes and (necessarily) the zero points of the P-L relations in B, V, and I must differ between LMC and the revised relations (also given here) for the Galaxy, and in fact they do. The LMC Cepheids are brighter by up to 0.5 mag at log P = 0.4 (2 days) and fainter by 0.2 mag at log P = 1.5 (32 days). These facts complicate the use of Cepheid as precision distance indicators until the reason is found (metallicity differences or other unknown differences) for the non-universality of the P-L and P-C relations. The very large data base permits mapping of various Cepheid properties at different positions within the instability strip, both at constant period and at constant absolute magnitude over the range of 2 < P < 40 days and −2 > M V > −5. Amplitude of the light curves are largest near the blue edge of the strip for periods between 2 and 7 days and longer than 15 days. The sense is reversed for periods between 7 and 15 days. The shape of the light curves varies systematically across the strip. Highly peaked curves (of large amplitude) that necessarily have large values of R 21 of about 0.5, occur near the blue edge of the strip. More symmetrical (small amplitude) light curves that have, thereby, small values of R 21 , generally occur near the red edge of the strip. Consequently, there is a strong correlation of R 21 with color within the strip at a given absolute magnitude. Strong correlations also exist between color and period, and color and amplitude at given absolute magnitudes, for the same reason that has long been known for RR Lyrae stars, based on the sloping lines of constant period in the CMD combined with the variation of amplitude, and now R 21 , with color. The highly peaked light-curve shapes and large amplitudes (indicating a non-linear regime that is overdriven out of the linear regime) near the blue edge of the strip, show that the energy driver for the pulsation (i.e. the negative dissipation) is strongest at the blue edge.

Scaled solar tracks and isochrones in a large region of theZ–Yplane

“…Considering the observations of Cepheids in the Galaxy and the Magellanic Clouds (Tamman et al 2003;Sandage et al 2004Sandage et al , 2009, we show that the observed instability strip is narrower than the theoretical one, and that the slope of the lower boundary of observed stars coincides with the slope of the faint envelope connecting each of the loops of the theoretical tracks for the chemical composition of the MCs. Because of the small number of Cepheids in open clusters and associations of the Milky Way, we cannot draw the lower boundary of the observed strip as we did for the MCs, but in this case, the loops of the theoretical models can also indicate the location of MW Cepheids in the HR-diagram.…”

“…The blue and red dotted lines correspond to the blue and red boundaries of the instability strip, such that the strip contains ∼90% of the Cepheids (Fig. 8 of Sandage et al 2004). The blue and red long-dashed lines are the boundaries of the theoretical strip as represented by the formulae of Caputo et al (2004) for the appropriate chemical composition.…”

“…The bottom of the instability strip was taken from Fig. 8 of the paper on the LMC Cepheids by Sandage et al (2004). The blue and red dotted lines correspond to the blue and red boundaries of the instability strip, such that the strip contains ∼90% of the Cepheids (Fig.…”

The Galactic Cepheid period–luminosity relation revisited using bona fide cluster Cepheids

“…The optimal set exhibits less scatter (rms = 0.21 mag) around the relation M V = (−3.08 ± 0.50) log P + (−0.94 ± 0.42) mag, with considerable uncertainty in both the zero point and the slope. We note that our result is in near exact agreement with the PLR in Sandage et al (2004), although nearly all of the PLRs shown in Fig. 1 agree with our results to within their uncertainties.…”

“…We illustrate this fact in Fig. 1, which shows PLRs based on different methods, including absolute and relative trigonometric parallaxes, BaadeBecker-Wesselink-type methods, as well as the calibration by Sandage et al (2004) which is partially based on Cepheids in Galactic open clusters. In light of such discrepancies, we deem that a self-consistent analysis of membership for CCs is warranted, and here we present a calibration obtained from the preliminary results of our mostly literaturebased, eight-dimensional, self-consistent all-sky census of classical Cepheids in Galactic open clusters.…”