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Register a new userThe Effect of Metallicity on the Cepheid Period-Luminosity Relation
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We present preliminary results of an observational campaign devoted at establishing the influence of chemical composition on the Cepheid Period-Luminosity relation. The data are in good agreement with theoretical predictions based on non-linear convective models, suggesting a fairly strong dependence of the Period-Luminosity relation on metallicity in the sense of more metal rich stars being intrinsically fainter than otherwise expected. Our data indicate that the error on the inferred distance can be as large as 10% if the role of metallicity is neglected.
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We have assessed the influence of the stellar iron content on the Cepheid Period-Luminosity (PL) relation by relating the V band residuals from the Freedman et al (2001) PL relation to [Fe/H] for 68 Galactic and Magellanic Cloud Cepheids. The iron abundances were measured from FEROS and UVES high-resolution and high signal-to-noise optical spectra. Our data indicate that the stars become fainter as metallicity increases, until a plateau or turnover point is reached at about solar metallicity. This behavior appears at odds both with the PL relation being independent from iron abundance and with Cepheids becoming monotonically brighter as metallicity increases (e.g. Kennicutt et al 1998, Sakai et al 2004).
We use parallax data from the Gaia second data release (GDR2), combined with parallax data based on Hipparcos and HST data, to derive the period-luminosity-metallicity (PLZ) relation for Galactic classical cepheids (CCs) in the V,K, and Wesenheit WVK bands. An initial sample of 452 CCs are extracted from the literature with spectroscopically derived iron abundances. Reddening values, pulsation periods, and mean magnitudes are taken from the literature. Based on nine CCs with a goodness-of-fit (GOF) statistic <8 and with an accurate non-Gaia parallax, a parallax zero-point offset of -0.049 +- 0.018 mas is derived. Selecting a GOF statistic <8 removes about 40% of the sample most likely related due to binarity. Excluding first overtone and multi-mode cepheids and applying some other criteria reduces the sample to about 200 stars. The derived PL(Z) relations depend strongly on the parallax zero-point offset. The slope of the PL relation is found to be different from the relations in the LMC at the 3 sigma level. Fixing the slope to the value found in the LMC leads to a distance modulus (DM) to the LMC of order 18.7 mag, larger than the canonical distance. The canonical DM of around 18.5 mag would require a parallax zero-point offset of order $-0.1$ mas. Given the strong correlation between zero point, period and metallicity dependence of the PL relation, and the parallax zero-point offset there is no evidence for a metallicity term in the PLZ relation. The GDR2 release does not allow us to improve on the current distance scale based on CCs. The value of and the uncertainty on the parallax zero-point offset leads to uncertainties of order 0.15 mag on the distance scale. The parallax zero-point offset will need to be known at a level of 3 microas or better to have a 0.01 mag or smaller effect on the zero point of the PL relation and the DM to the LMC.
We present the results of an observational campaign undertaken to assess the influence of the iron content on the Cepheid Period-Luminosity relation. Our data indicate that this dependence is not well represented by a simple linear relation. Rather, the behaviour is markedly non monotonic, with the correction peaking at about solar metallicity and declining for higher and lower values of [Fe/H].
The dependency of the Cepheid Period-Luminosity Relation on chemical composition at different wavelengths is assessed via direct detailed abundance analysis of Galactic and Magellanic Cepheids, as derived from high resolution, high signal-to-noise spectra. Our measurements span one order of magnitude in iron content and allow to rule out at the ~ 9 sigma level the universality of the Period-Luminosity Relation in the V band, with metal rich stars being fainter than metal poor ones by ~0.3 mag. The dependency is less pronounced in the K band. Its magnitude and statistical significance decisively depend on detailed distance measurements to individual stars, as inferred via the Infrared Surface Brightness Method.
Classical Cepheids (DCEPs) are the most important primary indicators for the extragalactic distance scale. Establishing the dependence on metallicity of their period--luminosity and period--Wesenheit (PL/PW) relations has deep consequences on the estimate of the Hubble constant (H$_0$). We aim at investigating the dependence on metal abundance ([Fe/H]) of the PL/PW relations for Galactic DCEPs. We combined proprietary and literature photometric and spectroscopic data, gathering a total sample of 413 Galactic DCEPs (372 fundamental mode -- DCEP_F and 41 first overtone -- DCEP_1O) and constructed new metallicity-dependent PL/PW relations in the near infra-red (NIR) adopting the Astrometric Based Luminosity. We find indications that the slopes of the PL$(K_S)$ and PW$(J,K_S)$ relations for Galactic DCEPs might depend on metallicity when compared to the Large Magellanic Cloud relationships. Therefore, we have used a generalized form of the PL/PW relations to simultaneously take into account the metallicity dependence of the slope and intercept of these relations. We calculated PL/PW relations which, for the first time, explicitly include a metallicity dependence of both the slope and intercept terms. Although the insufficient quality of the available data makes our results not yet conclusive, they are relevant from a methodological point of view. The new relations are linked to the geometric measurement of the distance to the Large Magellanic Cloud and allowed us to estimate a {it Gaia} DR2 parallax zero point offset $Delta varpi$=0.0615$pm$0.004 mas from the dataset of DCEPs used in this work.
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