A new and homogeneous metallicity scale for Galactic classical Cepheids

I. Physical parameters^{⋆,⋆⋆,⋆⋆⋆}

¹ Dipartimento di Fisica, Università degli Studi di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy
e-mail giuseppe.bono@roma2.infn.it
² INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, Rome, Italy
³ Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Rome, Italy
e-mail ronaldo.dasilva@ssdc.asi.it
⁴ Astronomical Observatory, Odessa National University, Shevchenko Park, 65014 Odessa, Ukraine
⁵ Isaac Newton Institute of Chile, Odessa Branch, Shevchenko Park, 65014 Odessa, Ukraine
⁶ Max-Planck Institute for Astronomy, 69117 Heidelberg, Germany
⁷ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
⁸ Institute for Astro- and Particle Physics, University of Innsbruck, Technikerstr. 25/8, 6020 Innsbruck, Austria
⁹ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
¹⁰ INAF – Osservatorio Astronomico di Teramo, Via Mentore Maggini snc, 64100 Teramo, Italy
¹¹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹² Monash Centre for Astrophysics, School of Physics and Astronomy, Monash University, Melbourne VIC 3800, Australia
¹³ INAF – Osservatorio Astronomico di Bologna, Via Ranzani 1, 40127 Bologna, Italy
¹⁴ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France
¹⁵ UPJV, Université de Picardie Jules Verne, 33 Rue St. Leu, 80080 Amiens, France
¹⁶ Department of Physics and Astronomy, N283 ESC, Brigham Young University, Provo, UT 84601, USA
¹⁷ South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa
¹⁸ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
¹⁹ Max-Planck-Institute for Astrophysics, Karl-Schwarzschild-Str.1, 85741 Garching, Germany
²⁰ University Observatory Munich, Scheinerstr. 1, 81679 M unich, Germany
²¹ Department of Astronomy, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
²² INAF – Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, 34143 Trieste, Italy
²³ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
²⁴ Laboratoire Lagrange, CNRS/UMR 7293, Observatoire de la Côte d’Azur, Bd de l’Observatoire, CS 34229, 06304 Nice, France

Received: 23 March 2018
Accepted: 30 April 2018

Abstract

We gathered more than 1130 high-resolution optical spectra for more than 250 Galactic classical Cepheids. The spectra were collected with the optical spectrographs UVES at VLT, HARPS at 3.6 m, FEROS at 2.2 m MPG/ESO, and STELLA. To improve the effective temperature estimates, we present more than 150 new line depth ratio (LDR) calibrations that together with similar calibrations already available in the literature allowed us to cover a broad range in wavelength (5348 ≤ λ ≤ 8427 Å) and in effective temperature (3500 ≤ T_eff ≤ 7700 K). This gives us the unique opportunity to cover both the hottest and coolest phases along the Cepheid pulsation cycle and to limit the intrinsic error on individual measurements at the level of ~100 K. As a consequence of the high signal-to-noise ratio of individual spectra, we identified and measured hundreds of neutral and ionized lines of heavy elements, and in turn, have the opportunity to trace the variation of both surface gravity and microturbulent velocity along the pulsation cycle. The accuracy of the physical parameters and the number of Fe I (more than one hundred) and Fe II (more than ten) lines measured allowed us to estimate mean iron abundances with a precision better than 0.1 dex. We focus on 14 calibrating Cepheids for which the current spectra cover either the entire or a significant portion of the pulsation cycle. The current estimates of the variation of the physical parameters along the pulsation cycle and of the iron abundances agree very well with similar estimates available in the literature. Independent homogeneous estimates of both physical parameters and metal abundances based on different approaches that can constrain possible systematics are highly encouraged.