Using classical Cepheids to study the far side of the Milky Way disk^⋆

I. Spectroscopic classification and the metallicity gradient

¹ Pontificia Universidad Católica de Chile, Instituto de Astrofísica, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
e-mail: jhminniti@uc.cl
² European Southern Observatory, Alonso de Córdova 3107, Santiago, Chile
³ Millennium Institute of Astrophysics, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
⁴ Departamento de Física, Facultad de Ciencias Exactas, Universidad Andrés Bello, Fernández Concha 700, Las Condes, Santiago, Chile
⁵ Vatican Observatory, 00120 Vatican City State, Italy
⁶ INAF-Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Naples, Italy
⁷ Universidad de Concepción, Departamento de Astronomía, Casilla 160-C, Concepción, Chile
⁸ Departamento de Física, Universidade Estadual de Feira de Santana, Av. Transnordestina, s/n, 44036-900 Feira de Santana, BA, Brazil
⁹ Centro de Nanotecnología Aplicada, Facultad de Ciencias, Universidad Mayor, Santiago, Chile
¹⁰ Centro para el Desarrollo de la Nanociencia y la Nanotecnología (CEDENNA), Santiago, Chile

Received: 24 January 2020
Accepted: 15 May 2020

Abstract

Context. Much of what we know about the Milky Way disk is based on studies of the solar vicinity. The structure, kinematics, and chemical composition of the far side of the Galactic disk, beyond the bulge, are still to be revealed.

Aims. Classical Cepheids (CCs) are young and luminous standard candles. We aim to use a well-characterized sample of these variable stars to study the present-time properties of the far side of the Galactic disk.

Methods. A sample of 45 Cepheid variable star candidates were selected from near-infrared time series photometry obtained by the VVV survey. We characterized this sample using high quality near-infrared spectra obtained with VLT/X-shooter. The spectroscopic data was used to derive radial velocities and iron abundances for all the sample Cepheids. This allowed us to separate the CCs, which are metal rich and with kinematics consistent with the disk rotation, from type II Cepheids (T2Cs), which are more metal poor and with different kinematics.

Results. We estimated individual distances and extinctions using VVV photometry and period-luminosity relations, reporting the characterization of 30 CCs located on the far side of the Galactic disk, plus 8 T2Cs mainly located in the bulge region, of which 10 CCs and 4 T2Cs are new discoveries. The remaining seven stars are probably misclassified foreground ellipsoidal binaries. This is the first sizeable sample of CCs in this distant region of our Galaxy that has been spectroscopically confirmed. We use their positions, kinematics, and metallicities to confirm that the general properties of the far disk are similar to those of the well-studied disk on the solar side of the Galaxy. In addition, we derive for the first time the radial metallicity gradient on the disk’s far side. Considering all the CCs with R_GC < 17 kpc, we measure a gradient with a slope of −0.062 dex kpc⁻¹ and an intercept of +0.59 dex, which is in agreement with previous determinations based on CCs on the near side of the disk.