Metallicity estimations of MW, SMC, and LMC classical Cepheids from the shape of the V- and I-band light curves

Abstract

Context. Estimating metallicity of classical Cepheids is of prime importance for studying metallicity effect on stellar evolution, chemical evolution of galaxies, and ultimately its impact on period-luminosity relation used in the extragalactic distance scale. Aims. We aim first at establishing new empirical relations for estimating the iron content of classical Cepheids for short and longperiods based on Fourier parameters from the V-and I-band light curves. Then we apply these relation to Cepheids from Milky Way (MW), Small and Large Magellanic Clouds (SMC and LMC) available in the literature. Methods. We retrieved metallicity of 586 fundamental mode Cepheids from spectroscopic determinations in the literature and we found well sampled light-curves for 545 of them in different V-band catalogues. We then describe the shape of these light curves by applying Fourier decomposition and we fit empirical relations between the Fourier parameters and the spectroscopic metallicities, separately for short-(2.5 < P < 6.3 days) and for long-period Cepheids (12 < P < 40 days). We verify the accuracy of these relations by applying them to V-band light curves of Cepheids from the Small and Large Magellanic Clouds and comparing the derived metallicities to the literature. We calibrate new interrelations of Fourier parameters to convert these empirical relations into the I-band. We then use these I-band relations to derive the metallicity of fundamental mode Cepheids from OGLE-IV for MW, SMC and LMC (486, 683 and 1678 stars respectively). Last, we map the metallicity distribution in these galaxies for investigating potential application in galactic archeology. Results. For short-period Cepheids, our best fit is given for a relation based on explicit amplitude terms A 1 and A 2 of the first and second harmonic respectively. In the V and I band these empirical relations are found with an intrinsic scatter (rms) of 0.12 dex. This relation performs well for estimating [Fe/H] between about −0.5 and 0.1 dex but remains uncertain outside this range because of the lack of spectroscopic metallicity for the calibration. For the long-period Cepheids we found a metallicity dependence on the Fourier parameters A 1 , φ 21 and R 41 . We found an intrinsic scatter of 0.25 dex when using this relation. These empirical relations in V and I bands are able to derive the mean metallicity of a sample of MW, SMC and LMC Cepheids in agreement with literature values within 1σ. We also show that these relations are precise enough to reconstruct the radial metallicity gradients within the MW from OGLE data. Conclusions. The empirical relations in the V and I bands calibrated in this paper for short and long-period Cepheids provide a new useful tool to estimate the metallicity of Cepheids which are not accessible by spectroscopy. The calibration can be improved with further high-resolution spectroscopic observations of metal-poor Cepheids and homogeneous photometries in V and I bands.