The first chemical abundance analysis of K giants in the inner Galactic disc
aa r X i v : . [ a s t r o - ph . GA ] J un Astronomy&Astrophysicsmanuscript no. 14809 c (cid:13)
ESO 2018September 3, 2018 L etter to the E ditor The first chemical abundance analysis of K giantsin the inner Galactic disc ⋆ T. Bensby , A. Alves-Brito , M.S. Oey , D. Yong , and J. Mel´endez European Southern Observatory, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago 19, Chile Departamento de Astronom´ıa y Astrof´ısica, Pontificia Universidad Cat´olica de Chile, Santiago, Chile Department of Astronomy, University of Michigan, Ann Arbor, MI 48109-1042, USA Research School of Astronomy and Astrophysics, Australian National University, Weston, ACT 2611, Australia Centro de Astrof´ısica, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, PortugalReceived 16 April 2010 / Accepted 11 June 2010
ABSTRACT
Aims.
The elemental abundance structure of the Galactic disc has been extensively studied in the solar neighbourhood using long-lived stars such as F and G dwarfs or K and M giants. These are stars whose atmospheres preserve the chemical composition oftheir natal gas clouds, and are hence excellent tracers of the chemical evolution of the Galaxy. As far as we are aware, there are nosuch studies of the inner Galactic disc, which hampers our ability to constrain and trace the origin and evolution of the Milky Way.Therefore, we aim in this study to establish the elemental abundance trend(s) of the disc(s) in the inner regions of the Galaxy.
Methods.
Based on equivalent width measurements in high-resolution spectra obtained with the MIKE spectrograph on the MagellanII telescope on Las Campanas in Chile, we determine elemental abundances for 44 K-type red giant stars in the inner Galactic disc,located at Galactocentric distances of 4-7 kpc. The analysis method is identical to the one recently used on red giant stars in theGalactic bulge and in the nearby thin and thick discs, enabling us to perform a truly di ff erential comparison of the di ff erent stellarpopulations. Results.
We present the first detailed elemental abundance study of a significant number of red giant stars in the inner Galactic disc.We find that these inner disc stars show the same type of chemical and kinematical dichotomy as the thin and thick discs show in thesolar neighbourhood. The abundance trends of the inner disc agree very well with those of the nearby thick disc, and also to those ofthe Bulge. The chemical similarities between the Bulge and the Galactic thick disc stellar populations indicate that they have similarchemical histories, and any model trying to understand the formation and evolution of either of the two should preferably incorporateboth of them.
Key words.
Galaxy: disc — Galaxy: bulge — Galaxy: formation — Galaxy: evolution — stars: abundances
1. Introduction
The inner Galactic disc is one of the least studied regions ofthe Milky Way because of high interstellar extinction and con-tamination by background Bulge stars. Apart from a few stud-ies of bright hot OB stars (e.g., Daflon & Cunha 2004) andCepheids (e.g., Luck et al. 2006), which both trace the mostrecent young disc stellar population, almost no information isavailable about the detailed abundance structure of the innerGalactic disc. Open questions are for instance, whether the innerGalactic disc show the same clear kinematic and chemical di-chotomy as the Galactic disc in the solar neighbourhood, wherethe thin and thick discs stand out as two distinct stellar popula-tions?Recent studies have revealed that the Galactic bulge andthe Galactic thick disc have very similar abundance trendswhich reflect similar, and possibly even a shared, chemi-cal histories (Mel´endez et al. 2008; Bensby et al. 2009, 2010;Alves-Brito et al. 2010). A restriction of these studies is thattheir thick disc samples have been observed in the so-lar neighbourhood, and if the Bulge has a secular origin
Send o ff print requests to : T. Bensby, e-mail: [email protected] ⋆ This paper includes data gathered with the 6.5 meter MagellanTelescopes located at the Las Campanas Observatory, Chile (e.g., Kormendy & Kennicutt 2004; Howard et al. 2009), mod-els show that it likely has to be from gas and stars in the innerparts of the Galactic disc (e.g., Rahimi et al. 2010). Both the in-ner and the local disc will help us put constraints on how theseGalactic components formed, if we can verify the existence ofan inner Galactic thick disc and di ff erentially compare it withthe Bulge.Here we will present the first results regarding detailed el-emental abundances in 44 red giant stars that are located at 4-7 kpc from the Galactic centre. They have been analysed usingthe same method as in the recent study of red giants in the Bulgeand nearby thin and thick discs by Alves-Brito et al. (2010). Wewill focus on four α -elements (Mg, Si, Ca and Ti) and omit mostof the analysis details and results for other iron-peak elementsfor a coming publication.
2. Sample selection and observations
One of the caveats in trying to observe the inner Galactic discin the direction of the Galactic centre is that it is very likelythat the sample will be contaminated by background Bulge stars.However, by pointing towards regions on either side of theBulge, contamination is avoided even if the estimated distancesare greatly in error. Therefore, our targets are located at Galacticlongitudes 330 ◦ − ◦ and 20 ◦ − ◦ (see left panel of Fig. 1).
1. Bensby et al.: The first chemical abundance analysis of K giants in the inner Galactic disc
Fig. 1.
The location of the stars in Galactic X , Y , and Z coordi-nates (distances based on spectroscopic parallaxes). Symbols asin Fig. 2.Because dwarf stars at these distances are too faint to beobserved with high-resolution spectrographs we targeted brightred giants. There is a clear separation between dwarfs and gi-ants in the de-reddened ( J − K ) and ( J − H ) colour space(Bessell & Brett 1988), and we utilised the selection criteria ofMajewski et al. (2003), who successfully selected distant K andM giants from the 2MASS catalogue. To make the sample ashomogeneous as possible and use early spectral types (to avoidTiO bands that gets strong for later types), we selected stars with0 . < ( J − K ) < .
88. This is the intrinsic colour for a K4 gi-ant (Bessell & Brett 1988). The corresponding intrinsic ( V − K )colour for a K4 giant is 3.26 (Bessell & Brett 1988), and its ab-solute magnitude is M V ≈ − .
45 (Keenan & Barnbaum 1999),giving M K = − .
71. The 2MASS
K s magnitudes were trans-formed to standard K magnitudes through K = K s + . the distances can be estimated using: K − M K = − + d .We selected 44 K giants from the 2MASS catalogue that hadestimated Galactocentric distances of 3-7 kpc. During two ob-serving runs in 2007 May and July, high-resolution spectra wereobtained for all 44 giants with the MIKE spectrograph at theMagellan II telescope on Las Campanas in Chile, using a 0 . ′′ wide slit. This resulted in spectra with R ≈
55 000, covering theentire optical spectrum from 3500 to 10 000 Å. Typical signal-to-noise ratios are S / N ≈
100 pixel − at 6000 Å.
3. Analysis
Stellar parameters and elemental abundances were determinedusing exactly the same spectroscopic methods as outlined inAlves-Brito et al. (2010). In short, the analysis is based on equiv-alent width measurements and the ATLAS9 model stellar atmo-spheres by Castelli et al. (1997). The e ff ective temperature ( T e ff )is found by requiring an excitation balance of the Fe i line abun-dances; surface gravity (log g ) by requiring ionisation balancebetween abundances from Fe i and Fe ii lines; and the microtur- Extinctions were calculated as ( A K , E ( J − H ), E ( J − K )) = (0 . , . , . E ( B − V ), where E ( B − V ) is from the maps bySchlegel et al. (1998), corrected using Eq. 1 of Bonifacio et al. (2000). Fig. 2.
Toomre diagram for 43 of the 44 stars (one does nothave measured proper motions). Open circles indicate stars thatmove on more or less circular orbits confined to the plane( v tot <
85 km s − ), and filled circles those that move on orbitsthat are highly eccentric and / or reach far from the Galactic plane( v tot >
85 km s − ).bulence ( ξ t ) by requiring that the Fe i line abundances from areindependent of reduced line strength.We find that all stars have e ff ective temperatures in therange 4000 < T e ff < < log g < .
5, i.e. typical of K giant stars. Typical uncertain-ties are 75 K in T e ff , 0.3 dex in log g , and 0.2 km s − in ξ t , and σ [Fe / H] = . σ [Mg / Fe] = . σ [Si / Fe] = . σ [Ti / Fe] = . σ [Ca / Fe] = .
14 in the abundance ratios.With spectroscopic stellar parameters at hand, “spectro-scopic” parallaxes were re-calculated throughlog π = . g ] − [ M ] − T ]) − . K + BC K − A K + . . (1)Here the notation [ X ] ≡ log( X / X ⊙ ), and the bolometric correc-tion is given by BC K = − .
75 log( T e ff / M = ⊙ .Assuming that the uncertainties in the stellar parameters andthe reddening correction are uncorrelated, the uncertainties inthe distances are calculated to be 30 %. Then we calculatedthe space velocities ( U LSR , V LSR , and W LSR ) using our spec-troscopic parallaxes, the proper motions from the UCAC3 cat-alogue (Zacharias et al. 2010), and radial velocities as measuredfrom the spectra. Uncertainties in the space velocities were cal-culated based on the assumption that the uncertainties in the dis-tances and the proper motions are uncorrelated. Finally, Galacticorbits were calculated with the grinton integrator (Carraro et al.2002; Bedin et al. 2006), which gives the minimum and max-imum distances from the Galactic centre ( R min and R max ), themaximum distance from the Galactic plane ( Z max ), and the ec-centricity of the orbit ( e ).
4. Results and discussion
In the Toomre diagram in Fig. 2 the stars have been codedaccording to the simple assumption that those with v tot >
85 km s − are thick disc stars, and those with lower velocitiesare thin disc stars (e.g., Fuhrmann 2004). Because we do notknow the properties of the inner thick disc, the coding should not
2. Bensby et al.: The first chemical abundance analysis of K giants in the inner Galactic disc be taken literally. It is also obvious that the errors in the calcu-lated space velocities make this classification uncertain. Hence,we just coded those stars that move on more circular orbits andthose that have more kinematically hot orbits. Below we willcall them kinematically hot stars (black circles) and kinemati-cally cold stars (empty circles).In Fig. 3a we see that stars with distances greater than2.5 kpc from the Sun have consistently high [ α/ Fe] values ( α ≡ (Mg + Si + Ti) / α/ Fe] values,and that most of these are kinematically hot stars. We also see afew kinematically hot stars that are located close to the plane andalso have low [ α/ Fe] values. However, Fig. 3c shows that thesestars have kinematic properties that allow them to reach as far as ∼ α/ Fe] values, and are kinematically cold,stay within 1 kpc from the plane. We also note that we have afew stars with high [ α/ Fe] values, which are kinematically hot,but which remain close to the the plane. These are stars that havehighly eccentric orbits. Figure 3d then shows that stars that moveon highly eccentric orbits all have high [ α/ Fe] values, and are allclassified as kinematically hot. For stars with less eccentric or-bits there is a gradual decrease in [ α/ Fe] as the orbits becomemore circular. With a few exceptions, the stars with the least ec-centric orbits have the lowest [ α/ Fe] values. Figure 3e showsthat stars with low [Fe / H] have high [ α/ Fe], with a flat trendthat eventually starts to decrease for metallicities higher than[Fe / H] ≈ − .
3. Also, the stars with cold kinematics generallyhave higher [Fe / H] and lower [ α/ Fe].These connections and correlations between kinematics andchemistry that we see for the inner disc sample is what wesee for disc stars in the solar neighbourhood. Stars with orbitsthat are highly eccentric and / or reach far from the plane gener-ally have high [ α/ Fe] values, and those on more circular orbits,which stay closer to the plane, have low [ α/ Fe] values. Starswith these properties are generally classified as thick disc andthin disc stars, respectively (e.g., Bensby et al. 2005; Fuhrmann2004). That we see the same correlations in the inner Galacticdisc strongly suggests that we have two distinct disc populationsalso in the inner disc, an inner thin disc and an inner thick disc,similar to those in the solar neighbourhood.
In Fig. 4 we show the detailed abundance trends for four α -elements, comparing the 44 inner disc K giants to the Bulge gi-ants and nearby thin and thick disc giants from Alves-Brito et al.(2010). We emphasise that all stars in these plots have been anal-ysed with the exact same methods, allowing truly di ff erentialcomparisons between the di ff erent populations.We note that especially the Mg abundance trend shows verylittle scatter, and that the inner disc giants have high [Mg / Fe]ratios for [Fe / H] < − . / H]. This is a signature of enrichment by massive stars at lowmetallicities, and a delayed contribution from low mass stars athigher metallicities, consistent with the same signature seen inthe nearby thick disc (e.g., Feltzing et al. 2003). This points tothe existence of an inner thick disc and moreover that this thickdisc does not di ff er much in terms of abundance trends, from thethick disc we see in the solar neighbourhhod. The same trendthat is seen for Mg can also be seen in the Si and Ti plots, butwith larger scatters. No clear trend can be seen in the Ca plot. Fig. 3. [ α / Fe] versus X , | Z | , Z max , e , and [Fe / H]. Symbols as inFig. 2.Furthermore, the abundance trends of the inner disc appearto be very similar to those of the Bulge. This inevitably pointsto a possible connection between the thick disc and the Bulge,implying they both might have formed at the same time (e.g.,Genzel et al. 2008), sharing a similar star-formation rate and ini-tial mass function. A possible scenario could be that the sub-solar part of the Bulge has a secular origin, and has formed frominner disc material (e.g., Shen et al. 2010).The agreement between the Bulge and the thick disc has re-cently also been seen in studies that compare Bulge stars withnearby thick disc stars. For instance, Bensby et al. (2010) pre-sented a detailed abundance analysis of 15 microlensed dwarfstars in the Galactic bulge. These stars were found to share thesame abundance trends as were traced by kinematically selectedthick disc dwarf stars in the solar neighbourhood (Bensby et al.2003, 2005, and 2010, in prep.). Similarly, Mel´endez et al.(2008) and Alves-Brito et al. (2010) found very good agree-ment between the abundance trends of red giants in the Bulgeand thick disc red giants in the solar neighbourhood (see alsoRyde et al. 2010). Similar to this study, the analysis methodsof these studies are internally fully consistent (same methods,model stellar atmospheres, atomic data, etc.). They compare
3. Bensby et al.: The first chemical abundance analysis of K giants in the inner Galactic disc
Fig. 4.
Abundance trends for our inner disc giants (red filled circles) together with Bulge giants (asterisks), nearby thin disc giants(green empty triangles); and nearby thick disc giants (blue empty squares), all from Alves-Brito et al. (2010). Typical error bars areshown in each plot.dwarfs with dwarfs, and giants with giants. Other studies ofred giants in the Bulge (e.g. Fulbright et al. 2007; Zoccali et al.2006; Lecureur et al. 2007) have found that the Bulge is sig-nificantly more α -enhanced at higher metallicities than thinand thick disc stars. As discussed in Bensby et al. (2010) andAlves-Brito et al. (2010), it is likely that those studies su ff erfrom problems with the analysis (especially line blending). Theyalso compare their Bulge giant samples with disc dwarf samples.The combined e ff ect is that their Bulge stars seem spuriouslymore enhanced in the α -elements than the thick disc stars.We further note that none of the inner disc giants are asmetal-rich as some of the most metal-rich Bulge giants. As themetallicity distribution of the thick disc peaks at [Fe / H] ≈ − .
5. Summary
We have presented the first detailed elemental abundance studyof K giants in the inner Galactic disc. Our sample consists of 44stars positioned 4-7 kpc from the Galactic centre, and up to 3 kpcfrom the Galactic plane. The three main results are: – based on elemental abundances and kinematics, we find itlikely that the inner Galactic disc has two distinct stellar pop-ulations: a thin disc and a thick disc; – the abundance trends of the inner Galactic thick disc are sim-ilar to those of the thick disc in the solar neighbourhood; – we confirm, now using inner disc giants, the chemical sim-ilarity between the Galactic thick disc and the metal-poorBulge.Finally, our results do not preclude the possibility that the localthick disc could be in part produced by radial mixing of innerdisc stars (Sch¨onrich & Binney 2009).In a forthcoming paper we will present the analysis of thecurrent sample in detail and also add abundance results for moreelements. That study will also include another similar sample ofgiant stars, but located in the outer Galactic disc. Acknowledgements.
TB and MSO acknowledge support by the NationalScience Foundation, grant AST-0448900. AAB acknowledges grants fromFONDECYT (process 3100013). JM is supported by a Ciˆencia 2007 contract(FCT / MCTES / Portugal and POPH / FSE / EC) and acknowledges support fromPTDC / CTE-AST / / / Portugal).
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4. Bensby et al.: The first chemical abundance analysis of K giants in the inner Galactic disc