New Period-Luminosity and Period-Color relations of classical Cepheids: I. Cepheids in the Galaxy

Abstract

Abstract. 321 Galactic fundamental-mode Cepheids with good B, V, and (in most cases) I photometry by Berdnikov et al. (2000) and with homogenized color excesses E(B − V) based on Fernie et al. (1995) are used to determine their period-color (P-C) relation in the range 0.4 < log P < ∼ 1.6. The agreement with colors from different model calculations is good to poor. 0 . Also the (B − V) 0 , (V − I) 0 twocolor diagrams differ between Cepheids in the Galaxy and the Clouds, attributed both to the effects of metallicity differences on the spectral energy distributions of the Cepheids and to a shift in the effective temperature of the middle of the instability strip for LMC and SMC relative to the Galaxy by about ∆ log T e ∼ 0.02 at M V = −4. m 0, hotter for both LMC and SMC. Differences in the period-color relations between the Galaxy and the Clouds show that there cannot be a universal P-L relation from galaxy-to-galaxy in any given color. The inferred non-uniqueness of the slope of the P-L relations in the Galaxy, LMC, and SMC is born out by the observations. The Cloud Cepheids follow a shallower overall slope of the P-L relations in B, V, and I than the Galactic ones. LMC and SMC Cepheids are brighter in V than in the Galaxy by up to 0. m 5 at short periods (log P = 0.4) and fainter at long periods (log P > ∼ 1.4). The latter effect is enhanced by a suggestive break of the P-L relation of LMC and SMC at log P = 1.0 towards still shallower values as shown in a forthcoming paper.

A study on the universality and linearity of the Leavitt law in the LMC and SMC galaxies

“…The test of the linearity hypothesis lead us to find that the values of the slopes of the LL behave exponentially as the range of periods increases, giving clear evidence of nonlinearity. In particular, we find that the LMC LL presents a break around 10 days, according to the results reported by Tammann, Sandage & Reindl (2003), , Kanbur et al (2007) and Koen, Kanbur & Ngeow (2007). A clear explanation of the behaviour of the LMC PL relation, in the range of short-periods is unknown until now.…”

“…With respect to non-linearity, the Russian astronomer Boris Kukarkin found in the thirties that the PL relations of external galaxies present a break around 10 days, as it was mentioned by Fernie (1969) and references therein. Tammann, Sandage & Reindl (2003) found a break of the Period-Colour (PC) relation at 10 days in the LMC galaxy. confirmed these break based on the statistical F -test.…”

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

“…In the case of the Galaxy, we considered the calibration of Cepheids in open clusters and associations listed in Table 3 of the paper by Tammann et al (2003). In Fig. 14 we plotted this sample of Cepheids over a set of isochrones with metal content Z = 0.010 and Y = 0.26 (this value of the metallicity corresponds to the lower limit of metallicity for the Galactic Cepheids as given in Romaniello et al 2008).…”

“…(5) and (16), and Fig. 15 of Tammann et al (2003). Because of the small number of objects, it is impossible to draw the lower boundary of the observed strip as done for the LMC and the SMC.…”

The effect of metallicity on the Cepheid period-luminosity relation

“…The SMC is considered 0.44 mag more distant (e.g. Cioni et al 2000) and the distances to Galactic stars were taken from Fouque et al (2003), Barnes et al (2003) and Tammann et al (2003). Photometry and reddening are taken from Laney & Stobie (1994).…”