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Radial Abundance Gradients in the Outer Galactic Disk As Traced Astronomy & Astrophysics manuscript no. ms34554 c ESO 2019 April 10, 2019 Radial abundance gradients in the outer Galactic disk as traced by main-sequence OB stars⋆ G.A. Bragança1, S. Daflon1, T. Lanz2, K. Cunha1, 3, T. Bensby4, P.J. McMillan4, C. D. Garmany5, J.W. Glaspey5, M. Borges Fernandes1, M.S. Oey6, and I. Hubeny3 1 Observatório Nacional-MCTIC, Rua José Cristino, 77. CEP: 20921-400, Rio de Janeiro, RJ, Brazil e-mail: [email protected] 2 Observatoire de la Cote d’Azur, F-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, SE-221 00 Lund, Sweden 5 National Optical Astronomy Observatory, 950 N. Cherry Ave., Tucson, AZ 85719, USA 6 University of Michigan, Department of Astronomy, 311 West Hall, 1085 S. University Ave., Ann Arbor, MI 48109-1107, USA Received XX Xxxxxxx 2018 / Accepted XX Xxxxxxx 2018 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 tele- scope 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 macroturbu- lence) and silicon and oxygen abundances are presented for 28 stars located beyond 9 kpc from the Galactic centre 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 and −0.09 dex/kpc, respectively, in the region 8.4 ≤ RG ≤ 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. Key words. Galaxy: formation — Galaxy: evolution — Galaxy: radial gradients — Stars: abundances — Stars: early-type 1. Introduction halo evolution may affect the chemical distribution in the outer disk at galactocentric distances greater than 10kpc, whereas the To understand how large spiral galaxies like the Milky Way inner gradients remain unchanged (e.g., Chiappini et al. 2001). formedandevolvedto their currentstate is a major goalin Galac- tic research. Observational results from analyses of the Galac- Massive stars are considered the main drivers of the Galactic tic structure and chemical composition combined with modern chemo-dynamical evolution (Crowther 2012; Matteucci 2008). They are the first stars to process hydrogen to heavier ele- arXiv:1904.04340v1 [astro-ph.SR] 8 Apr 2019 models of Galactic chemical evolution are required to constrain how galaxies form and evolve (Maiolino & Mannucci 2018). ments and to return these processed elements to the interstellar However, the observational picture of the Milky Way is in- medium. Their strong winds and supernova explosions are im- complete and still lacks some major pieces. In particular the portant triggers of star formation (Crowther 2012). Also, due outer disk is poorly mapped and poorly understood. This is un- to their short lifetimes, OB stars are naturally bound to their fortunate, as Galactic radial gradients of elemental abundances place of birth in the Galactic plane (Schulz 2012). Thus they pro- are important observational constraints for models of Galac- vide observational constraints on the present state of the Galaxy tic chemical evolution (Maiolino & Mannucci 2018; Wang et al. through detailed analyses of their photospheres. The abundance 2019). These gradients are influenced by the evolution of the radial gradients derived from these objects can be used to in- Milky Way on the largest scales. Differences in the history of the fer the infall rate and mixing of the gas within the thin disk (Chiappini et al. 2001). ⋆ Based on data obtained with the Magellan Clay telescope at the Las Massive stars have been studied for decades by sev- Campanas observatory and the ESO/MPI telescope at La Silla under the eral groups using photometric and spectroscopic tools (e.g., ESO–ON agreement. Rolleston et al. 2000; Daflon & Cunha 2004). However, the Article number, page 1 of 14 A&A proofs: manuscript no. ms34554 N II N O II O II O II O II O II O II O II O II O II O II He I Si III Si HeII 1.0 0.9 ALS 9129 3.5 0.8 0.7 Normalized flux Normalized ALS 404 3.0 0.6 0.5 4670 4680 4690 4700 4710 4720 Wavelength (Å) ALS 45 2.5 Fig. 1. Example spectra in the region 4670 − 4720 Å for the star ALS 14013 reduced with the pipeline (black line) and re-reduced (red line). The feature introduced by the CCD defect in the region 4680 − 4695 Å has been adequately removed. ALS 18714 2.0 Normalized flux Normalized samples studied in previousworks concentrate in the “local” disk and typically include less than about ten stars outside the solar ALS 7 circle. 1.5 In this work we present stellar parameters (effective temper- ature, surface gravity and v · sin i, microturbulent and macro- turbulent velocities) and oxygen and silicon abundances in non- ALS 14013 LTE for a sample of 28 main-sequence OB stars located towards 1.0 the Galactic anti-center, between galactocentric radii ∼ 9.5 and 15.6kpc, plus three stars in the solar neighbourhood.The sample analyzed here is part of the larger sample of 136 OB stars in the outer disk analysed by Garmany et al. (2015), where multiplic- ity, photometric effective temperature, and projected rotational velocity (v · sin i) were determined. 4660 4670 4680 4690 4700 4710 4720 Wavelength (Å) This paper is structured as follows: Section 2 describes the target selection and the observations; Sect. 3 explains the methodology used to analyse the stellar spectra and Sect. 4 ex- Fig. 2. Example spectra for six observed stars in the region 4660 − plains how the distances of the stars were obtained. In Sect. 3.6 4720 Å. The spectra are vertically shifted for better visualization. Some we discuss the determination of stellar parameters and abun- spectral lines are identified on the top. dances, and in Sect. 5 we discuss the radial distribution of the silicon and oxygen abundances; our conclusions are presented in Sect. 6. give spectra with S/N ∼ 100 per pixel for the stars that have V magnitudes in the range 10 − 14mag. At the time of the observing run, the blue CCD of the MIKE ff 2. Observations and sample selection spectrograph was su ering from a serious disfiguring feature af- fecting a number of orders, and the CCD response function was The target stars were selected from the catalogues by Reed changing with time (Ian Thompson, private communication; the (1998, 2003), using the reddening-free index Q to select stars CCD was replaced in 2009). In addition, a strong contamina- with spectral types earlier than B2. Additional selection criteria tion in the line profiles was introduced by the use of a B star include the stellar magnitudes (V > 10) and the location towards as a milky flat. As a result, the spectra that were reduced using the outer edge of the Galactic disk in order to provide OB stars the standard pipeline displayed a relatively strong feature (about that are likely to be located at large galactocentric distances. A 20 − 30%) in the central regions (∼20Å) of the orders 1-12 of detailed description of the selection criteria of the sample stars the blue arm (i.e., 5000 − 4400Å). Thus, the spectra had to be is presented in Garmany et al. (2015). re-reduced using the data reduction packages written in Python High-resolution spectra were obtained over six nights (Dec and provided by Carnegie Observatories1. This resulted in better 29, 2007 to Jan 3, 2008) for 136 stars with the MIKE spectro- quality spectra (corrected for the blaze function and CCD defect) graph (Bernstein et al. 2003) at the Magellan Clay 6.5m tele- obtained through a careful flat-field selection, using quartz lamp scope at Las Campanas Observatory in Chile. MIKE is a dou- spectra, as shown in Fig. 1 for the star ALS 14013, in the region ble echelle spectrograph that simultaneously records red (4900− 4680 − 4695Å. 9500Å) and blue (3350−5000Å) spectra. The observations were done with a 0.7arcsec slit, providing spectra with nominal reso- 1 The Carnegie Observatories CarPy python lution of about R ≈ 53, 000with the blue arm, as measured in the distribution is available for download at Th-Ar calibration images.
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