Oxygen Abundance Distribution in Galactic Disc

arXiv:1408.6103v1 [astro-ph.SR] 26 Aug 2014 o.Nt .Ato.Soc. Astron. R. Not. Mon. ..Korotin S.A. disc. Galactic in distribution abundance Oxygen ..Maciel W.J. cetd eevd;i rgnlform original in ; Received Accepted. aafo ag ooeeu apeo bet ttedif- necessary. the urgently at are objects distances of Galactocentric sample ferent homogeneous abundance large oxygen and a accurate Galaxy from new our data present, in At element pro galaxies. under- this the other of for our distribution responsible affect are and that that duction processes discrepancies those sources of are these standing by there provided that abundance oxygen shows on data the of rmsetocpcsuyo ore uha tr,planetar stars, as H such sources nebulae, of study come oxy- spectroscopic substructures the from Galactic chemical in about the distribution information of abundance General evolution gen as different Galaxy. temporal our used the with astronomi- of of is objects tracer content several it in a in is measured and it content when Earth), ages, metal thus, the the global objects, on constitutes the cal least it of (at element, proxy life abundant a the because most of is third the base This the among Universe. is is the it oxygen in elements view interesting of most point astronomical the From INTRODUCTION 1 ildpnec fteSeI ciiyta,a ebelieve, we as that, ⋆ activity spa- II the SNe reveal to the may of interest disc distribution dependence thin particular a tial Galactic of such in is element of this it Characteristics of II, distribution the SNe study in processes plosive c iaeUiestai,CP 50-0,Sa al,S,Bra SP, São Paulo, 05508-900, Universitária, CEP: Cidade 4 Shevche Branch, Odessa 3 Chile, of Institute 2 Newton Isaac and 1 eateto srnm,Cs etr eev nvriy1 Ciências Geof´ısica Atmosfér University e Astronomia, Reserve de Western Instituto Case Pari Astronomy, Universite of CNRS, Department Paris-Meudon, de Observatoire GEPI, Ode Observatory, Astronomical and Astronomy of Department -al [email protected] E-mail: 04RAS 2014 ic xgnadother and oxygen Since ii ein,aditrtla atr Comparison matter. interstellar and regions, 4 1 ⋆ n ..Kovtyukh V.V. and ..Andrievsky S.M. , 000 α – 21)Pitd1 ac 01(NL (MN 2021 March 15 Printed (2014) 1–7 , eeet r rdcdi ex- in produced are -elements e words: Key discussed. are flattening of mechanisms inmgtbcm atri h ue at fteds.Ti sas su also is H This and disc. nebulae the planetary of clusters, parts open outer hint studied value the a who gradient be in authors (O/H) could flatter radial other there become the that might of found tion slope we abundanc wit Nevertheless, the oxygen observed dex/kpc. find investigate stars We –0.058 to disc. the aim thin for telescope. the Galactic results Hobby-Eberly with fo NLTE the telescope triplet previous Gesellschaft with oxygen our obtained infrared with the spectra combined of cepheid analysis of NLTE ber a performed have We ABSTRACT aay ic–Glx:evolution. Galaxy: – disc Galaxy: tr:audne tr:vrals ehis–Glx:abunda Galaxy: – Cepheids variables: stars: – abundances stars: 1 , 1 2 ..Luck R.E. , csd nvriaed São Paulo, de Universidade da icas zil k ak 51 dsa Ukraine Odessa, 65014 Park, nko y s - . ieo,915Muo ee,France Cedex, Meudon 92125 Diderot, s s ainlUniversity National ssa 90Eci vne lvln,O 44106-7215 OH Cleveland, Avenue, Euclid 0900 o ntne nresye l 20b sn apeof sample other a among using that (2002b) demonstrated stars region. al. even each cepheid et (or Andrievsky in bimodal instance, slopes a For different instead with but structure gradient, multimodal) slope descrip their single for a require, ele not disc that the in evidence distributions growing mental indicates cepheids Galactic with level. on metallicity influence local arms’ the th spiral and in Galactic gas, the distribution the of density efficiency gas the interstellar disc, the by caused is sacag fmtliiylvli iiiyo h Galactic the of vicinity in level interpreted metallicity resonance”. be of corotation can change kpc a 11 as w about associated of is a distance which Galactocentric distribution, fea with metallicity wriggle ”the (2004) the that in al. concluded ture This They et stars. Andrievsky kpc. of sample by 10 larger about confirmed asso- is of was separation distance finding This Galactocentric outer part. with the middle ciated its of from distributions disc abundance Galactic elemental in with aration deal studies al. (both et (2013) Pedicelli 4-7 of al. distribution). about results et iron of the Genovali range by solar and the supported the (2009) also in in is one distribution This steeper flat kpc. much increase the the This changes to increase centre. kpc vicinity steeper Galactic 6.6 the a about toward at show content iron their and of oxygen elements, group h urn ttso h bnac rdetstudies gradient abundance the of status current The ae,Lc ta.(03 eotdaotacersep- clear a about reported (2003) al. et Luck Later, 3 ...Lépine J.R.D. , A T E tl l v2.2) file style X 4 , httedistribu- the that hs aawere data These itiuinin distribution e ii ag num- large a r a Planck Max h ein.Some regions. α ob equal be to pre by pported n iron- and - cs– nces tion ith e - - 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; Lépine 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. trograph even for the faintest cepheids. The most significant In this work we modified our oxygen atomic model first obstacle, which prevents a wide use of this triplet in spec- presented in Mishenina et al. (2000) and then updated in troscopic analyses, is an obvious necessity to apply sophis- Dobrovolskas et al. (2014). Our present model consists of ticated NLTE approximation in order to get correct oxygen 51 O i levels of singlet, triplet and quintet systems and the abundance. This is a well know problem (see e.g. a very ground level of O ii ion. An additional 24 levels of neutral instructive example about extremely strong dependence of oxygen and 15 levels of ions of higher stages were added the derived oxygen abundance on the effective temperature for particle number conservation. The Grotrian diagram of provided by the LTE study of Schuler et al. 2006). our model is shown in Fig. 1. Fine-splitting was taken into The main goal of the present paper was to derive correct account only for the ground level and 3p5P level (the upper NLTE oxygen abundance for a large homogeneous sample level for 777.4 nm triplet). of cepheids using the IR oxygen triplet, and then to con- Two-hundred and forty eight bound-bound transitions struct on this basis the oxygen abundance distribution in were included in the detailed analysis. Photoionization cross- the Galactic thin disc. sections were selected from TOPBASE (Cunto et al. 1993). The paper is organized as follows. Our sample of Accurate quantum-mechanical calculations were employed

c 2014 RAS, MNRAS 000, 1–7 Oxygen abundance distribution in Galactic disc 3

Table 1. Parameters and Abundances for Program cepheids (only header of the table is showed. The full table is available in electronic form)

−1 Object Phase Teﬀ , K log g Vt. km s (Fe/H) (O) RG (kpc)

AA Gem 0.422 5141 1.23 3.92 7.34 8.51 11.55 AA Gem 0.658 5190 1.45 5.85 7.38 8.51 11.55 AA Mon 0.841 5797 2.26 4.90 7.41 8.51 11.36

nm is almost free of the NLTE influence, the lines of triplet O II at 777.4 nm appear to be weaker than the observed ones. 100000 If we decrease by four times the collisional rates calculated by Barklem (2007), as done by Sitnova et al. (2013), we can fit the observed profiles for these two triplets with a single abundance, but the synthetic lines of another IR triplet 80000 at 844.6 nm appear to be significantly increased compared to observations. This is seen even in the Procyon spectrum, although in its cooler atmosphere the collisions with hydrogen atoms begin to be more important. 60000 For our test calculations we used the same stellar parameters and oxygen abundance as Sitnova et al. (2013). Ta-

Energy Energy (cm-1) ble 2 lists the source of observed spectra and stellar param- 40000 eters. We consider that a decrease of all collisional rates calculated by Barklem (2007) is not appropriate, therefore we decided to change the collisional rates only for some transi- 20000 tions. Thus, we decreased by a factor of two the rates that correspond to the IR triplet 777.4 nm, and increased by four times the rates that correspond to 884.6 nm triplet. We also

0 strengthened the coupling between 3p3P level (the upper 1S 1D 1D' 3S' 3P 3D' 3F 5S' 3P 3D' 3F 3G' level for the 844.6 nm triplet lines) and levels of the quintet system 4s5S0 and 3d5D0 due to collisions with hydro- i Figure 1. The Grotrian diagramme for O . gen atoms. For the forbidden transitions we used f=1.0 in Drawin’s formula. With these modifications, we were able to adjust theoretical and observational profiles of oxygen lines for the first 19 levels to find collisional rates with elec- of different multiplets in the spectra of the rather hot A-F trons (Barklem 2007). Using spline interpolation the col- stars, the solar-type stars, as well as the cooler stars. For the lision rates can be interpolated against temperature. The cooler stars we used correcting factor SH = 1, in agreement remaining allowed collisional transitions were approximated with Fabbian et al. (2009), and Sitnova et al. (2013). by the van Regemorter (1962) formula, while for the forbid- In Fig. 2 we show a comparison between observed and den transitions we applied the Allen’s (1973) formula with synthetic profiles for the lines of different multiplets for the collision strength set equal to 1.0. Collisions with hydro- Vega and Procyon. Note that a single oxygen abundance gen atoms were described using Drawin’s formula (Drawin value was used to synthesize all the lines for each star. 1969), as modified by Steenbock & Holweger (1984). An os- Good agreement is seen both for 615.7 nm triplet lines that cillator strength of f=0.001 was used for the forbidden tran- are not affected significantly by NLTE effects, and for IR sitions (Fabbian et al. 2009). Generally, our model is quite triplets, which are significantly affected. Another test was similar to that of Sitnova et al. (2013). The MULTI code applied to the solar spectrum. We used solar atmosphere (Carlsson 1986) was used to compute non-LTE level popu- model of Castelli & Kurucz (2003) that was supplemented lations. This code was modified by Korotin et al. (1999). by VAL-C chromosphere model (Vernazza et al. 1981) with As shown by Sitnova et al. (2013), in order to achieve an explicit distribution of the microturbulent velocity. (Note agreement between oxygen abundances in A stars derived that the influence from the chromosphere on the lines of IR from IR triplet lines, and from the lines of the visual part triplets is very small, no more than 1.5% in their equiva- of the spectrum, it is necessary to decrease the collisional lent widths). Observed profiles were taken from Solar flux rates calculated by Barklem (2007) by a factor of four. In atlas (Kurucz et al. 1984). We also made a comparison for order to adjust oxygen abundances from different multiplets the solar disc center using the Solar atlas of Delbouille et al. in solar spectrum, and in the Vega spectrum, they adopted (1973). A comparison of the theoretical profiles for 630 nm a correcting factor in the Drawin’s formula SH = 1. We line, the lines of 777.4 nm and 844.6 nm triplets with ob- performed test calculations for Vega and Sirius using our served profiles in the solar spectrum is shown in Fig. 3. To oxygen atom model and found that it is not possible to de- synthesize all the lines we used a single oxygen abundance scribe observed line profiles for different multiplets with a log ǫ(O) = 8.71 (the same value as proposed by Scott et al. single oxygen abundance value. While the O i triplet at 615.7 (2009)).

c 2014 RAS, MNRAS 000, 1–7 4 S.A. Korotin et al.

Table 2. Stellar parameters

−1 Star Teﬀ , K log g [Fe/H] Vt. km s (O/H) Res. Source

Sun 5777 4.44 0.0 1.0 8.71 350000 Kurucz et al. (1984) Procyon 6590 4.00 0.0 1.8 8.73 80000 Bagnulo et al. (2003) Vega 9550 3.95 -0.5 2.0 8.59 100000 Takeda et al. (2007) Sirius 9850 4.30 0.4 1.8 8.42 80000 Bagnulo et al. (2003)

1.0 1.0 1.0 1.0 0.9 0.9 0.9 0.8 0.8 0.7 0.8 0.7 RELATIVE FLUX 0.6 0.9

Vega RELATIVE FLUX 0.7 0.6 0.5

6156.7 6157.9 6159.2 8445 8447 V1397 Cyg 7772 7773 7774 7775 7776 0.4 λ (A) 7771 7772 7773 7774 7775 7776 λ (A) 1.0 1.0 1.0 1.0 0.9 0.9 0.9 0.8 0.8 0.8 0.7 0.7 0.7 RELATIVE FLUX 0.9 0.6 0.6 Procyon 0.6

0.5 RELATIVE FLUX 6156.7 6157.9 6159.2 7772 7773 7774 7775 7776 8445 8447 0.5 λ (A) V438 Cyg 0.4 7771 7772 7773 7774 7775 7776 λ Figure 2. Profile fitting of the O i line in the Vega and Procyon (A) spectrum Figure 4. Profile fitting of the O i line in the cepheids spectrum. Open circles - observed profiles, continuous line - NLTE profiles, dashed line - LTE profiles calculated with derived NLTE oxygen 1.0 1.0 1.0 abundance.

0.9 0.9 corrections are inherent to the stars with higher eﬀective 0.8

RELATIVE FLUX temperatures and lower gravity). 0.9 0.8 0.7 SUN

6300.0 6300.5 7772.0 7773.0 7774.0 7775.0 8446.5 8447.0 λ (A) 4 OXYGEN ABUNDANCE DISTRIBUTION IN THE DISC Figure 3. Profile fitting for the oxigen lines in the solar spectrum In Fig. 5 we show NLTE oxygen distribution vs. Galacto- centric distance for the combined sample of HET and MPG Resulting NLTE oxygen abundances for the HET stars Telescope stars. are gathered in Table 1. We also list parameters of the stars The region 5-13 kpc is quite well sampled by our oxygen (effective temperature, surface gravities, microturbulent ve- data and the distribution is rather tight. At larger distances locities) as they were derived in Luck & Lambert (2011) for the data are more dispersed. Samples observed with different the HET sample. Our NLTE data for the stars of MPG set telescopes produce results that agree well. can be found in Luck et al. (2013), while atmosphere param- The data in Fig. 5 can be interpolated linearly. In eters and LTE abundances for those stars were published in this case we have the slope ∆(O/H)/∆RG = −0.058 with Luck et al. (2011). Galactocentric distances for our program R2 = 0.61. The slope derived by Luck & Lambert (2011) is stars were calculated in the same way as in Andrievsky et al. quite similar: −0.056 with R2 = 0.47. Those authors did not (2002a). For the solar Galactocentric radius we used 7.9 discuss the possible bimodal character of the gradient. kpc (McNamara et al. 2000). If we compare star-by-star our Nevertheless, despite the less populated outer disc re- NLTE values with LTE values from Luck & Lambert (2011) gion, it seems that abundance distribution here could be and Luck et al. (2011), we get the following result: NLTE- more flat. In Fig. 6 we show the same data as in Fig. 5, but LTE = −0.03±0.13 (LTE results are based on 615.6 nm line assuming a bimodal distribution. We formally adopted posi- and the 630.0 nm forbidden line). The best fits of the NLTE tion of the possible break in the distribution at 11 kpc taking synthetic profiles and observed profiles are given for some into account that in many studies, which report about gra- stars in Fig. 4. The NLTE corrections in this case are ex- dient flattening, the distances of 10-12 kpc are mentioned tremely strong, reaching sometimes 0.8-1.0 dex (the larger (see references in Introduction and Discussion).

c 2014 RAS, MNRAS 000, 1–7 Oxygen abundance distribution in Galactic disc 5

9.2

9.0

(O/H) = 9.28 - 0.058 Rg

9.0

8.8

8.6

8.6 (O/H) (O/H)

8.4

8.2

4 6 8 10 12 14 16 18 20

4 6 8 10 12 14 16 18

Rg, kpc

Figure 5. NLTE oxygen abundance vs. Galactocentric distance. Figure 7. Oxygen abundance distribution. Binned data. Open circles - HET data, crosses - MPG Telescope data.

9.2 More massive stars (like cepheid progenitors) have in- suﬃcient time to migrate far from their birth places, there- (O/H) = 9.27 - 0.056 Rg fore this mechanism is not eﬃcient in modifying the metal-

9.0 licity distribution obtained from these objects. Their position should be approximately associated with their parent gas clouds. Since the gas component ﬂows in opposite direc-

(O/H) = 8.96 - 0.033 Rg 8.8 tion from the corotation circle (with respect to stars) due to the inﬂuence of angular momentum loss and gain in vicinity

(O/H) of the corotation circle (Barros et al. 2013) and references 8.6 therein, one can expect some increase in the metallicity out- wards of the corotation circle, which reﬂects a local increase in the gas density, the star formation rate, and therefore the 8.4 metallicity production. An element such as oxygen, being produced by massive short-lived stars, may show an abun-

8.2 dance distribution that reflects the instantaneous mass den- 6 8 10 12 14 16 18 sity distribution in the Galactic disc. In our Fig. 6 a peculiar Rg, kpc region could be associated with RG ≈ 11 kpc. Barros et al. (2013) find an evidence of the mass density gap at Galacto- Figure 6. Bimodal distribution of oxygen abundance. The break centric distance of about 9 kpc. This is somewhat less that position is close to 11 kpc. 11 kpc, but qualitatively conclusion remains the same. If we turn to iron abundance distribution using the most More robust confirmation of this conclusion follows complete data for cepheids provided by Luck & Lambert from Fig. 7. In this figure all cepheids were binned with (2011), their Fig. 1, then we have to state that it is diffi- a step of 0.5 kpc. For each bin we show error of the mean. cult to detect any peculiar point in this distribution. Iron If only one star falls in a bin, then error of the mean was is efficiently produced by SNe Ia stars, whose progenitors adopted to be equal 0.2. A clear change of the general trend have a typical life-time of about 1 Gyr, or slightly more at about 12 kpc and a local increase of the oxygen abun- (Iben & Tutukov 1984), which is comparable to the charac- dance in vicinity of 15 kpc is seen. teristic crossing time of particular region in vicinity of the corotation circle. The movement of the slightly larger number of the SNe Ia progenitors from the inner part of corota- 5 DISCUSSION tion circle towards the outer part, and opposite movement of the smaller number of such stars from the outer part could As shown by Barros et al. (2013), the specific action of the increase the gas pollution with iron at the corotation circle corotation resonance results in orbital trajectory migration itself and in vicinity of it, and therefore veil any ”instanta- of stars and also in the formation of a gas density minimum neous” distribution picture of this element created by SNe in the corotation. For instance, their Fig. 7 demonstrates of type II. how a star situated initially inside the corotation circle af- It should be noted that Daflon & Cunha (2004), based ter some time appears outside it. The star from outside the on observations of OB stars, found a different behaviour of corotation circle, on contrary, migrates inside it. The typi- the oxygen gradient compared to the present work, with a cal time of this change is about 2 Gyr. The star migration strong step near the solar radius (taken as 8.5 kpc). The process itself may disturb the observed radial metallicity OB stars are much younger (a few million years) than the distribution at present time. cepheids, so that in principle, their abundance is really that

c 2014 RAS, MNRAS 000, 1–7 6 S.A. Korotin et al. of the surrounding gas. Costa et al. (2004) derived some flat- variables, which is supported by recent additional observa- tening at Galactocenttric distances larger than 10 kpc on tional data on these objects (Fu et al. 2009; Pedicelli et al. the basis of a set of Galactic planetary nebulae, and a pre- 2009). Dynamical calculations valid for the Milky Way disc vious work in this series (Andrievsky et al. 2004, 2013) led over a time span of about 6 Gyr seem to confirm these results to the same conclusion based on cepheid variables. More (Curir et al. 2014). recent results by Lemasle et al. (2013) suggest a constant Additionally, some recent work on the time variation of slope along the Galactic radius, although their own data are the abundance gradients as a function of the redshift suggest not inconsistent with some flattening at large Galactocen- that the variations are indeed very small for z < 0.5 (cf. tric distances. More recent results of the same group based Gibson et al. 2013; Pilkington et al. 2012), which comprise on accurate iron abundances of a large sample of Galac- the age bracket of most of the objects mentioned above. tic cepheids (Genovali et al. 2014) are consistent with a lin- Alternatively, the data at galactocentric distances larger ear gradient over a broad range of Galactocentric distances than 14 kpc in principle may become more scattered. In such given by -0.06 dex/kpc, very similar to the results shown in a case, the outer parts of the Galaxy may show abundances Figure 5. that reflect local events that influence the abundances more Open cluster data seem to indicate the same tendency than coherent evolution as implied by a gradient. Unfor- presented in this paper, as recently discussed by Yong et al. tunately, this type of analysis is not really possible with (2012) based on α- and iron-group element abundances cepheids (the natural lack of the cepheid stars at large dis- of new sample of objects in the outer Galactic disc. For tances). Perhaps in the future giants might be used to con- instance, the plot for representative α-element magnesiun tinue the project. shows a break of the distribution at 13 kpc with some flattening of the distribution at larger radii (see their Fig. 31a). In principle, similar behaviour is inherent to the cepheid 6 CONCLUSION (Mg/H) distribution (Fig. 31d). The same conclusion was also made earlier by Summarizing our present study, one can note that there is Magrini et al. (2009), who claimed that there is a flatten- a growing observational evidence about the flattening of α- ing of the iron and α-element distribution (or even plateau) element distributions (including oxygen) in the discs of dif- at the radii larger than 12 kpc. Cescutti et al. (2007) suc- ferent galaxies including our Galaxy. These data come from ceeded to model the flattening of the gradients produced by spectroscopic analyses of the open clusters, planetary neb- open clusters, as well as by cepheids, hot stars and red giant ulae, H II regions, hot stars, red giant stars, and now, from stars. cepheids. Lépine et al. (2013) performing an average of abun- Since oxygen is produced by massive short-lived stars, dances of several alpha elements in order to decrease the its abundance distribution should follow an instantaneous effect of individual errors on measurements, suggest (their gas distribution in the Galactic disc. As recently concluded Figure 6) that there are two distinct levels of alpha abun- by Barros et al. (2013) based on several previous theoretical dances, with some overlap in Galactic radius. studies, the radial gas density profile has a gap at the corota- A recent study based on Galactic H ii regions and fea- tion circle, and an increased density on both sides of it. Our turing the outer disc object NGC 2579 (Esteban et al. 2013) observational data comprisong a large sample of cepheids, is also consistent with a flattened gradient at large distances analysed taking into account NLTE effects on oxygen abun- from the Galactic center. dance determinations, seem to support this assumption, and The flattening of the gradients at large Galactocentric show that the corotation resonance in the Galactic disc can distance seems to be an universal property of disc galax- have really significant impact on the abundance gradients ies. For example Stasinska et al. (2013) considered PNe and in its vicinity. In particular, our oxygen abundance distri- H ii regions in the spiral galaxy NGC 300 and found that bution reveals a clear change of the general trend at about oxygen and other element abundance gradients from PNe 12 kpc with the subsequent flattening in the range from 12 are significantly shallower than those from H ii regions, and to 15 kpc, or even small increase of the oxygen abundance this may indicate a steepening of the metallicity gradient in toward 15 kpc. NGC 300. Very clear flattening of the oxygen abundance distribution in the outer parts of the spiral galaxy M83 was reported by Bresolin et al. (2009) (see their Fig. 9). Recently, ACKNOWLEDGMENTS Sanchez et al. (2014) presented the largest and most homogeneous catalog of H ii regions in several hundreds of galaxies SAK and SMA acknowledges SCOPES grant No. IZ73Z0- from CALIFA survey. Many of these galaxies show a flatten- 152485 for financial support. Authors thank the anonymous ing in the oxygen abundance. refree for her/his valuable comments that significantly im- These results may in principle be affected by the time proved the paper. variation of the radial abundance gradients in the Galactic disc, but some recent work based on four different sample of Galactic planetary nebulae suggest that this variation was REFERENCES probably very small during the past 4 to 5 Gyr, so that the gradients indicated by these different objects are essen- Allen, C.W., 1973, Astrophysical Quantities (Athlone tially indistinguishable (Maciel & Costa 2013). Therefore, Press) the planetary nebula gradient is not expected to be very dif- Andrievsky S.M., Kovtyukh V.V., Luck R.E., et al., 2002a, ferent from the gradient observed in H ii regions and cepheid A&A, 381, 32

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