Radial abundance gradients in the outer Galactic disk as traced by main-sequence OB stars^⋆

¹ Observatório Nacional-MCTIC, Rua José Cristino, 77., 20921-400 Rio de Janeiro, RJ, Brazil
e-mail: daflon@on.br
² Observatoire de la Côte d’Azur, 06304 Nice, France
³ Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA
⁴ Lund Observatory, Department of Astronomy and Theoretical physics, Box 43, 221 00 Lund, Sweden
⁵ National Optical Astronomy Observatory, 950 N. Cherry Ave., Tucson, AZ 85719, USA
⁶ Department of Astronomy, University of Michigan, 311 West Hall, 1085 S. University Ave., Ann Arbor, MI 48109-1107, USA

Received: 1 November 2018
Accepted: 2 April 2019

Abstract

Context. Elemental abundance gradients in galactic disks are important constraints for models of how spiral galaxies form and evolve. However, the abundance structure of the outer disk region of the Milky Way is poorly known, which hampers our understanding of the spiral galaxy that is closest to us and that can be studied in greatest detail. Young OB stars are good tracers of the present-day chemical abundance distribution of a stellar population and because of their high luminosities they can easily be observed at large distances, making them suitable to explore and map the abundance structure and gradients in the outer regions of the Galactic disk.

Aims. Using a sample of 31 main-sequence OB stars located between galactocentric distances 8.4−15.6 kpc, we aim to probe the present-day radial abundance gradients of the Galactic disk.

Methods. The analysis is based on high-resolution spectra obtained with the MIKE spectrograph on the Magellan Clay 6.5-m telescope on Las Campanas. We used a non-NLTE analysis in a self-consistent semi-automatic routine based on TLUSTY and SYNSPEC to determine atmospheric parameters and chemical abundances.

Results. Stellar parameters (effective temperature, surface gravity, projected rotational velocity, microturbulence, and macroturbulence) and silicon and oxygen abundances are presented for 28 stars located beyond 9 kpc from the Galactic center plus three stars in the solar neighborhood. The stars of our sample are mostly on the main-sequence, with effective temperatures between 20 800−31 300 K, and surface gravities between 3.23−4.45 dex. The radial oxygen and silicon abundance gradients are negative and have slopes of −0.07 dex kpc⁻¹ and −0.09 dex kpc⁻¹, respectively, in the region 8.4 ≤ R_G ≤ 15.6 kpc.

Conclusions. The obtained gradients are compatible with the present-day oxygen and silicon abundances measured in the solar neighborhood and are consistent with radial metallicity gradients predicted by chemodynamical models of Galaxy Evolution for a subsample of young stars located close to the Galactic plane.