Tracing the Milky Way warp and spiral arms with classical Cepheids

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
e-mail: lemasle@uni-heidelberg.de
² Astronomical Observatory, Odessa National University, Shevchenko Park, 65014 Odessa, Ukraine
³ Dipartimento di Fisica, Universitá di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy
⁴ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, Monte Porzio Catone, 00078 Rome, Italy
⁵ Agenzia Spaziale Italiana, Space Science Data Center, Via del Politecnico snc, 00133 Rome, Italy
⁶ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France
⁷ UPJV, Université de Picardie Jules Verne, 33 Rue St. Leu, 80080 Amiens, France
⁸ South African Astronomical Observatory, PO Box 9 7935 Observatory, Cape Town, South Africa
⁹ Southern African Large Telescope Foundation, PO Box 9 7935 Observatory, Cape Town, South Africa
¹⁰ Sternberg Astronomical Institute, Lomonosov Moscow State University, Universitetskij Pr. 13, Moscow 119992, Russia

Received: 5 February 2022
Accepted: 27 August 2022

Abstract

Context. Mapping the Galactic spiral structure is a difficult task since the Sun is located in the Galactic plane and because of dust extinction. For these reasons, molecular masers in radio wavelengths have been used with great success to trace the Milky Way spiral arms. Recently, Gaia parallaxes have helped in investigating the spiral structure in the Solar extended neighborhood.

Aims. In this paper, we propose to determine the location of the spiral arms using Cepheids since they are bright, young supergiants with accurate distances (they are the first ladder of the extragalactic distance scale). They can be observed at very large distances; therefore, we need to take the Galactic warp into account.

Methods. Thanks to updated mid-infrared photometry and to the most complete catalog of Galactic Cepheids, we derived the parameters of the warp using a robust regression method. Using a clustering algorithm, we identified groups of Cepheids after having corrected their Galactocentric distances from the (small) effects of the warp.

Results. We derived new parameters for the Galactic warp, and we show that the warp cannot be responsible for the increased dispersion of abundance gradients in the outer disk reported in previous studies. We show that Cepheids can be used to trace spiral arms, even at large distances from the Sun. The groups we identify are consistent with previous studies explicitly deriving the position of spiral arms using young tracers (masers, OB(A) stars) or mapping overdensities of upper main-sequence stars in the Solar neighborhood thanks to Gaia data.