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Radial Abundance Gradients in the Outer Galactic Disk As Traced by Main-Sequence OB Stars? G

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Radial Abundance Gradients in the Outer Galactic Disk As Traced by Main-Sequence OB Stars? G A&A 625, A120 (2019) Astronomy https://doi.org/10.1051/0004-6361/201834554 & c ESO 2019 Astrophysics 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., 20921-400 Rio de Janeiro, RJ, Brazil e-mail: [email protected] 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 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 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 −1 −1 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. stars: abundances – stars: early-type 1. Introduction Milky Way on the largest scales. Differences in the history of the halo evolution may affect the chemical distribution in the outer To understand how large spiral galaxies like the Milky Way disk at galactocentric distances greater than 10 kpc, whereas the formed and evolved to their current state is a major goal in Galac- inner gradients remain unchanged (e.g., Chiappini et al. 2001). tic research. Observational results from analyses of the Galac- Massive stars are considered the main drivers of the Galac- tic structure and chemical composition combined with modern ticchemo-dynamicalevolution(Crowther2012;Matteucci2014). models of Galactic chemical evolution are required to constrain They are the first stars to process hydrogen to heavier elements how galaxies form and evolve (Maiolino & Mannucci 2019). and to return these processed elements to the interstellar medium. However, the observational picture of the Milky Way is incom- Their strong winds and supernova explosions are important trig- plete and still lacks some major pieces. In particular the outer gers of star formation (Crowther 2012). Also, due to their short disk is poorly mapped and poorly understood. This is unfor- lifetimes, OB stars are naturally bound to their place of birth in tunate, as Galactic radial gradients of elemental abundances the Galactic plane (Schulz 2012). Thus they provide observational are important observational constraints for models of Galac- constraintsonthepresentstateoftheGalaxythroughdetailedanal- tic chemical evolution (Maiolino & Mannucci 2019; Wang et al. ysesoftheirphotospheres.Theabundanceradialgradientsderived 2019). These gradients are influenced by the evolution of the from these objects can be used to infer 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 published by EDP Sciences A120, page 1 of 13 A&A 625, A120 (2019) was changing with time (Ian Thompson, priv. comm.; the CCD 1.0 was replaced in 2009). In addition, a strong contamination in the line profiles was introduced by the use of a B star as a milky 0.9 flat. As a result, the spectra that were reduced using the standard pipeline displayed a relatively strong feature (about 20−30%) 0.8 in the central regions (∼20 Å) on the orders 1–12 on the blue arm (i.e., 5000−4400 Å). Thus, the spectra had to be re-reduced 0.7 using the data reduction packages written in Python and pro- Normalized flux vided by Carnegie Observatories1. This resulted in better qual- 0.6 ity spectra (corrected for the blaze function and CCD defect) obtained through a careful flat-field selection, using quartz lamp 0.5 spectra, as shown in Fig.1 for the star ALS 14013, in the region 4670 4680 4690 4700 4710 4720 4680−4695 Å. Wavelength (Å) Some intrinsic characteristics of early-type stars such as fast Fig. 1. Example spectra in the region 4670−4720 Å for the star rotation, binarity, and stellar winds, can contribute to make the ALS 14013 reduced with the pipeline (gray line) and re-reduced spectral analysis of massive OB stars challenging. Stellar rota- (blue line). The feature introduced by the CCD defect in the region tion (v sin i) becomes a dominant broadening mechanism and 4680−4695 Å has been adequately removed. affects in particular the weak metal lines present in the spectra of OB stars; these are smeared at v sin i ≈ 100 km s−1 or higher, preventing an accurate abundance analysis. In this study, we samples studied in previous works concentrate in the “local” disk selected only those stars with sharp-lined spectra (with v sin i < and typically include less than about ten stars outside the solar 70 km s−1) in order to avoid any systematics and larger errors that circle. could be introduced in our abundance results due to high stel- In this work we present stellar parameters (effective tem- lar rotation. Recently, Cazorla et al.(2017) analyzed two fast- perature, surface gravity and v sin i, microturbulent and macro- rotating stars (ALS 864 and ALS 18675) from our full sample. turbulent velocities) and oxygen and silicon abundances in A majority of massive stars are found in binary or multi- non-LTE for a sample of 28 main-sequence OB stars located ple star systems (Sana et al. 2012, 2013) and some of them may toward the Galactic anticenter, between galactocentric radii ∼9.5 present double (or even multiple) lines in their spectra (double- and 15.6 kpc, plus three stars in the solar neighborhood. The lined spectroscopic binaries, SB2). In such cases one has to dis- sample analyzed here is part of the larger sample of 136 OB stars entangle the component spectra (e.g. Harmanec et al. 2004) and in the outer disk analyzed by Garmany et al.(2015), where mul- treat the resulting spectra individually. Massive stars with mod- tiplicity, photometric effective temperature, and projected rota- erate to strong winds may also display emission lines in the tional velocity (v sin i) were determined. center of the hydrogen lines. Depending on the intensity of the This paper is structured as follows: Section2 describes the tar- emission lines, the wings of hydrogen lines that are generally get selection and the observations; Sect.3 explains the method- used to determine the surface gravity may be affected. ology used to analyse the stellar spectra and Sect.4 explains how From the original sample of 136 stars described in the distances of the stars were obtained. In Sect.5 we discuss the Garmany et al.(2015), stars with broad line profiles ( v sin i > radial distribution of the silicon and oxygen abundances; our con- 70 km s−1), strong emission profiles in the center of H lines, and clusions are presented in Sect.6. clear double lines were eliminated for this study. One additional star (ALS 16807) was also discarded (see Sect. 3.2). 2. Observations and sample selection Single-epoch spectra like those analyzed in this paper are not adequate to identify and study binary systems. However, in order The target stars were selected from the catalogs by Reed(1998, to discard potential single-lined binaries, we did a search in the 2003), using the reddening-free index Q to select stars with spec- literature for radial velocity measurements or any information on tral types earlier than B2.
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