Mon. Not. R. Astron. Soc. 000, 1–7 (2014) Printed 15 March 2021 (MN LATEX style file v2.2) Oxygen abundance distribution in Galactic disc. S.A. Korotin1⋆, S.M. Andrievsky1,2, R.E. Luck3, J.R.D. L´epine4, W.J. Maciel4 and V.V. Kovtyukh1 1Department of Astronomy and Astronomical Observatory, Odessa National University and Isaac Newton Institute of Chile, Odessa Branch, Shevchenko Park, 65014 Odessa, Ukraine 2GEPI, Observatoire de Paris-Meudon, CNRS, Universite Paris Diderot, 92125 Meudon Cedex, France 3Department of Astronomy, Case Western Reserve University 10900 Euclid Avenue, Cleveland, OH 44106-7215 4Instituto de Astronomia, Geof´ısica e Ciˆencias Atmosf´ericas da Universidade de S˜ao Paulo, Cidade Universit´aria, CEP: 05508-900, S˜ao Paulo, SP, Brazil Accepted. Received ; in original form ABSTRACT We have performed a NLTE analysis of the infrared oxygen triplet for a large num- ber of cepheid spectra obtained with the Hobby-Eberly telescope. These data were combined with our previous NLTE results for the stars observed with Max Planck Gesellschaft telescope with the aim to investigate oxygen abundance distribution in Galactic thin disc. We find the slope of the radial (O/H) gradient value to be equal –0.058 dex/kpc. Nevertheless, we found that there could be a hint that the distribu- tion might become flatter in the outer parts of the disc. This is also supported by other authors who studied open clusters, planetary nebulae and H ii regions. Some mechanisms of flattening are discussed. Key words: stars: abundances – stars: variables: Cepheids – Galaxy: abundances – Galaxy: disc – Galaxy: evolution. 1 INTRODUCTION is caused by the interstellar gas density distribution in the disc, the efficiency of the Galactic spiral arms’ influence on From the astronomical point of view oxygen is among the the gas, and the local metallicity level. most interesting elements in the Universe. This is because The current status of the abundance gradient studies it is the third most abundant element, it constitutes the with Galactic cepheids indicates growing evidence that ele- base of the life (at least on the Earth), and it is used as mental distributions in the disc require, for their description a proxy of the global metal content in several astronomi- arXiv:1408.6103v1 [astro-ph.SR] 26 Aug 2014 not a single slope gradient, but instead a bimodal (or even cal objects, thus, when measured in objects with different multimodal) structure with different slopes in each region. ages, it is a tracer of the temporal evolution of the chemical For instance, Andrievsky et al. (2002b) using a sample of content of our Galaxy. General information about the oxy- cepheid stars demonstrated that among other α- and iron- gen abundance distribution in Galactic substructures comes group elements, oxygen and iron show a steeper increase from spectroscopic study of sources such as stars, planetary of their content toward the Galactic centre. This increase nebulae, H ii regions, and interstellar matter. Comparison at about 6.6 kpc changes the flat distribution in the solar of the data on oxygen abundance provided by these sources vicinity to the much steeper one in the range of about 4-7 shows that there are discrepancies that affect our under- kpc. This is also supported by the results of Pedicelli et al. standing of those processes that are responsible for the pro- (2009) and Genovali et al. (2013) (both studies deal with duction and distribution of this element in our Galaxy and iron distribution). other galaxies. At present, new accurate oxygen abundance Later, Luck et al. (2003) reported about a clear sep- data from a large homogeneous sample of objects at the dif- aration in elemental abundance distributions of the outer ferent Galactocentric distances are urgently necessary. Galactic disc from its middle part. This separation is asso- Since oxygen and other α-elements are produced in ex- ciated with Galactocentric distance of about 10 kpc. This plosive processes in SNe II, it is of particular interest to finding was confirmed by Andrievsky et al. (2004) with a study the distribution of this element in Galactic thin disc. larger sample of stars. They concluded that ”the wriggle fea- Characteristics of such a distribution may reveal the spa- ture in the metallicity distribution, which is associated with tial dependence of the SNe II activity that, as we believe, Galactocentric distance of about 11 kpc can be interpreted as a change of metallicity level in vicinity of the Galactic ⋆ E-mail: [email protected] corotation resonance”. c 2014 RAS 2 S.A. Korotin et al. In contrast, Kovtyukh, Wallerstein & Andrievsky cepheids is presented in Sect. 2. The NLTE approximation (2005), and Luck et al. (2006) did not find any strong that was used to derive oxygen abundance from the strong reason to not use a simple linear gradient to describe the IR triplet is detaily described in Sect. 3. The oxygen abun- spatial abundance distributions obtained with even more dance distribution in the disc is the subject of Sect. 4. The expanded sample of the cepheid stars. The same conclusion Discussion and Conclusion summarize our results from the was made by Luck et al. (2011) and Luck & Lambert (2011) point of view of the recent observational and theoretical (but they noted an increased scatter of the abundance data works on abundance gradients in the Galactic disc. outward the distance of 10 kpc). In part, this conclusion may be affected by including the new cepheids covering more distant parts of the disc in Galactocentric longitude, 2 OUR SAMPLE OF STARS that could clean up any specific features in the radial abundance distribution. Continuing our program of the Galactic abundance gradi- Genovali et al. (2014) have also used a single gradient ent investigation (Andrievsky et al. 2002a,b,c; Luck et al. value to describe the iron abundance distribution over a 2003; Andrievsky et al. 2004; Luck et al. 2006; L´epine et al. broad range of Galactocentric distances from 5 to 19 kpc. 2011), we study the oxygen distribution in the thin disc with Lemasle et al. (2013) also do not support the flattening of a large sample of Galactic cepheids. Our sample consists of the abundance distribution for some elements in the outer two subsamples. One contains the Hobby-Eberly Telescope Galactic disc, but at the same time they do not provide the (HET) data described in detail by Luck & Lambert (2011). corresponding data for oxygen and iron. The second is composed by the MPG Telescope data, which As we can see, the situation with characteristics of the are described in Luck et al. (2013). The observational data abundance distribution in Galactic disc of such astrophys- for both samples were collected by one of the authors (REL). ically important elements as oxygen and iron is far from The list of the studied stars (only HET data) and their spec- being clear. All the data on oxygen distribution from the tra can be found in Table 1 (electronically available in its giant and supergiant stars F-G-K stars come from analysis full form). of either forbidden oxygen lines at 630.03 and 636.38 nm, or 615.6-615.8 triplet, but there are known problems of the re- liable oxygen abundance determination from this restricted 3 METHOD OF ANALYSIS sample of the lines. For example, two forbidden lines can hardly be detected in the supergiant spectra if the effective The NLTE approximation was used to derive oxygen abun- temperature of the star is higher than 5500 K, but this is the dance in our stars. For this we used the O i triplet : 777.4 case for a large amount of cepheids. Moreover, these lines are nm. Study of the NLTE deviations in the IR oxygen triplet often polluted with terrestrial absorptions. Since this region was initiated by many authors in the past (e.g. Kiselman is dominated by terrestrial bands, the additional measures 1991; Takeda 1992; Carlsson & Judge 1993; Paunzen et al. of the spectra cleaning are needed. Among the triplet lines, 1999; Reetz 1999; Mishenina et al. 2000; Przybilla et al. only the line 615.8 nm is more or less detectable in the super- 2000; Fabbian et al. 2009; Sitnova et al. 2013). As shown by giant spectra. It cannot be detected in the medium-to-high those authors NLTE effects significantly strengthen these resolution spectra of the stars with effective temperature lines. At the same time available O i atomic models were lower than 5500-6000 K. Thus, in some cases, in order to not always able to reproduce observed triplet profiles. For in- derive oxygen abundance for a large sample of the cepheid stance, Przybilla et al. (2000) found different oxygen abun- stars one needs to use either forbidden lines (for cooler stars, dance in A and F stars derived from IR lines and lines in or for phases of minimum with lower temperature), or triplet visual part of the spectrum. lines (for the hotter stars). The most complete atomic models published by A good solution would be to use of the 777.1-777.4 nm Sitnova et al. (2013) and Fabbian et al. (2009) take into ac- triplet. It affords the most suitable opportunity to derive count the most recent collisional rate values between atoms oxygen abundance in F-G-K supergiant stars because: 1) and electrons (Barklem 2007). Nevertheless, the lack of de- its lines are quite strong and very well shaped over a wide tailed calculations describing collisions with hydrogen atoms range of the effective temperature, 2) these lines are prac- force the use of Drawin’s formula (Drawin 1969) with a very tically not blended, 3) they are reachable with many spec- uncertain correcting factor varying from 0 to 1.