Milky Way metallicity gradient from Gaia DR2 F/1O double-mode Cepheids

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchofstrasse 12-14, 69120 Heidelberg, Germany
e-mail: lemasle@uni-heidelberg.de
² Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
³ Instituto Milenio de Astrofísica, Santiago, Chile
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-Institut für Astronomie, 69117 Heidelberg, Germany
⁷ Dipartimento di Fisica, Università di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy
⁸ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, Rome, Italy
⁹ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France
¹⁰ UPJV, Université de Picardie Jules Verne, 33 rue St. Leu, 80080 Amiens, France
¹¹ South African Astronomical Observatory, PO Box 9, 7935 Observatory, Cape Town, South Africa
¹² Southern African Large Telescope Foundation, PO Box 9, 7935 Observatory, Cape Town, South Africa
¹³ Sternberg Astronomical Institute, Lomonosov Moscow State University, Universitetskij Pr. 13, Moscow 119992, Russia
¹⁴ Special Astrophysical Observatory of RAS, Nizhnij Arkhyz, Karachai-Circassia 369167, Russia
¹⁵ ASI Science Data Center, Via del Politecnico snc, 00133 Rome, Italy
¹⁶ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany

Received: 8 August 2018
Accepted: 13 September 2018

Abstract

Context. The ratio of the first overtone (1O)/fundamental (F) periods of mixed-mode Cepheids that pulsate simultaneously in these two modes (F/1O) is metallicity-dependent. It can therefore be used to characterize the systems that host such variable stars.

Aims. We want to take advantage of the F/1O double-mode Cepheids listed in the Gaia Data Release 2 (DR2) catalog to derive the metallicity gradient in the Milky Way disk.

Methods. The metallicity is derived from the ratio of the first overtone and fundamental periods provided by Gaia DR2 while the Gaia DR2 parallaxes are used to determine the Galactocentric distances of the stars.

Results. From a visual inspection of the light curves, it turns out that a large fraction (77%) of the Galactic F/1O double-mode Cepheids in Gaia DR2 are spurious detections. Gaia DR2 provides three new bona fide F/1O Cepheids. Combining them with the currently known F/1O Cepheids and using the Gaia DR2 parallaxes for the entire sample, we can derive the metallicity gradient in the Milky Way disk. We find a slope of –0.045 ± 0.007 dex kpc⁻¹ using a bootstrap method, and of –0.040 ± 0.002 dex kpc⁻¹ using a total least squares method. These results are in good agreement with previous determinations of the [Fe/H] gradient in the disk based on canonical Cepheids.

Conclusions. The period ratio of F/1O Cepheids allows for a reliable determination of the metallicity gradient in the Milky Way, and in turn, in other systems that would be difficult to reach via classical spectroscopic methods.