Chemical abundances of field halo stars -- Implications for the building blocks of the Milky Way
aa r X i v : . [ a s t r o - ph . GA ] A ug Star Clusters: From the Milky Way to the Early UniverseProceedings IAU Symposium No. 351, 2019A. Bragaglia, M.B. Davies, A. Sills & E. Vesperini, eds. c (cid:13) Chemical abundances of field halo stars- Implications for the building blocks of theMilky Way -
Miho N. Ishigaki Tohoku University, Astronomical Institute,6-3email: [email protected]
Abstract.
I would like to review recent efforts of detailed chemical abundance measurements for fieldMilky Way halo stars. Thanks to the advent of wide-field spectroscopic surveys up to a sev-eral kpc from the Sun, large samples of field halo stars with detailed chemical measurementsare continuously expanding. Combination of the chemical information and full six dimensionalphase-space information is now recognized as a powerful tool to identify cosmological accretionevents that have built a sizable fraction of the present-day stellar halo. Future observationalprospects with wide-field spectroscopic surveys and theoretical prospects with supernova nucle-osynthetic yields are also discussed.
Keywords.
Galaxy: halo, Galaxy: abundances, stars: Population II
1. Introduction
Chemical abundances in the photosphere of ancient stars provide fossil records to linkfield stars with their original birth places and thus serve as an essential tool to re-constructthe merging history of our Milky Way Galaxy (Freeman & Bland-Hawthorn 2002). Thestellar halo, which predominantly consists of old stellar populations, is a particularlyinteresting target because orbital velocities of the stars are largely preserved over theGalactic history due to the long dynamical time. This makes the Milky Way stellar haloan ideal laboratory to test theories of galaxy formation and evolution in the context ofthe currently standard ΛCDM cosmology (Bullock & Johnston 2005; Robertson et al.2005; Font et al. 2006; Cooper et al. 2010) based on spatial position, kinematics andchemistry of individual stars.Since the stellar halo is an extremely diffuse component where ∼ M ⊙ of stars aredistributed in a large volume over ∼ −
200 kpc from the Galactic center and the localdensity is much less than 1% (Juric et al. 2008), studies of chemical abundances in fieldhalo stars take significant advantages from wide-field spectroscopic surveys. Large sam-ples of candidate halo stars with low metallicity have been built by prism spectroscopicsurveys (the
HK survey or the
Hamburg/ESO survey ). Their follow-up high-resolutionspectroscopy have revealed characteristic chemical abundances among stars with [Fe/H]lower than − Hipparcos in studying the chemodynamical structure and evolution of our Galaxy (seeFeltzing & Chiba 2013 for a review of this subject). Furthermore, the measured chemicalabundances provide further insights into the stellar birth environment making use ofGalactic chemical evolution models (e.g. Kobayashi et al. 2006) and with individualsupernova yield models (e.g., Woosley & Weaver 1995; Heger & Woosley 2010; Nomoto,Kobayashi, & Tominaga 2013).In this article, I would like to focus on how the chemical information help distinguishingthe origin of individual field halo stars in the following three different categories: (1) localhalo stars that characterize the gross chemodynamical structure of the halo (Section 2),(2) halo stars with full space motions measured and identified as candidate members ofkinematically coherent streams (Section 3), and (3) metal-poor stars that show peculiarchemical abundance patterns (Section 4). Finally, future prospects with on-going andplaned large spectroscopic surveys of the Milky Way are discussed (Section 5).
2. The global chemodynamical structure of the stellar halo
Thanks to the recent wide-field photometric surveys, our understanding of the stellardistribution in the halo has been dramatically changed over the last few decades (Ivezi´cet al. 2012). It was clearly demonstrated that the stellar halo is far from a smooth andstatic distribution of a single stellar population but it exhibits various spatially coherentsubstructures. While the spatial coherence of stars could be washed out in about 10 Gyrs,as a result of dynamical evolution of the Galaxy, kinematics and chemical abundancesare preserved for a longer time scale and thus provide information on the early Galactichistory (Helmi & White 1999).Dating back to the pioneering work by Eggen, Lynden-Bell, & Sandage (1962), thestrength of combining kinematics and chemistry of field halo stars has been widely rec-ognized (e.g. Norris & Ryan 1989, Chiba & Beers 2000). One of the global nature ofthe stellar halo revealed by full space motions and [Fe/H] of nearby field halo stars isthe presence of at least two structural components, the inner and outer halo populations(Carollo et al. 2007).Detailed elemental abundances in nearby halo stars have provided crucial informationon the nucleosynthesis and chemical evolution of the early Universe. At [Fe/H] . − α -element-to-iron ratios ([(Mg, Si, Ca)/Fe]) have found to be enhanced by ∼ . α -elements was thought to continueup to [Fe/H] ∼ −
1, whose behavior was found to be different from the majority of starsin dwarf spheroidal satellite galaxies with measured elemental abundances (e.g., Venn etal. 2004). Exception to the [ α /Fe]-[Fe/H] trends were also reported (e.g., Carney et al. hemistry of field halo stars and streams ∼
100 stars carefully selected to have similar stellar parameters(effective temperature, surface gravity and [Fe/H]) and have full space motions. Theyhave shown that the sample of nearby halo stars can be separated into the two chemicallydifferent populations; namely, high- α and low- α stars. In addition to the alpha elements,the two populations show systematic difference in various elemental abundances includingC, Ni, Zn (Nissen & Schuster 2011) and in the neutron capture elements (Fishlock et al.2017).An intriguing question is whether the inner and the outer halo populations identified bykinematics and [Fe/H] reported by Carollo et al. (2007) exactly correspond to the high- α and low- α components, respectively, reported by Nissen & Schuster (2010). Ishigaki etal. (2012) carried out high-resolution spectroscopic analyses of stars from Chiba & Beers(2000) to study the difference in detailed elemental abundances among those selected tohave characteristic kinematics of the thick disk, inner and outer halo stars reported byCarollo et al. (2007). They have shown that the [Mg/Fe] ratios in stars kinematicallycompatible with the inner and outer halo stars in Carollo et al. (2007) show a decreasingtrend with [Fe/H] at [Fe/H] > − . > − . ∼ α -rich and the α -poor sequences of stars in the [ α /Fe]-[Fe/H]plane at − . < [Fe/H] < − .
55. By analyzing chemical abundance trends with [Fe/H] forfourteen elements, including CNO, α , and Fe-peak elements, they also showed that thetwo sequences of stars are distinguished by the abundance ratios of O, Mg, S, Al, C+N.Furthermore, the two groups of stars show systematically different kinematics identi-fied in the Galactic longitude ( l ) versus Galactic rest-frame radial velocity (GRV) space,suggesting different origins for the two groups. With the updated chemical abundanceestimates for an expanded sample of ∼ ,
000 stars from SDSS Data Release 13, Hayeset al. (2018) confirmed the presence of the high and low-Mg sequences similar to the pre-vious findings. The studies of halo stars either with precision differential analysis (e.g.,Nissen & Schuster 2010) or with a large statistical sample (e.g., Hayes et al. 2018), demon-strate that, with homogeneously measured various elemental abundances, the chemistryalone could be used to separate different stellar populations for moderately metal-poorhalo stars ( − . [Fe/H] . − . ∼
400 stars in the range 5 < r <
30 kpcand demonstrate that the stars at r >
15 kpc show different trends in elemental abun-dance ratios ([X/Fe]) at [M/H] > − .
1. It has become clear that the chemical dichotomyseen in the solar neighborhood is part of a global structure, extending to at least up toseveral kpc from the Sun.In summary, the key improvement in the last 20 years has been the recognition thatthe chemistry of field halo stars is not represented by a homogeneous α /Fe-enhancement Ishigaki, M. N.over a wide [Fe/H] range but exhibits variation depending on local space motions and/orGalactocentric distances. Whether this chemical diversity corresponds to the global ac-cretion events that are now suggested to make up a large fraction of the local halo (Helmiet al. 2018; Belokurov et al. 2018) remains to be investigated in the next generation sur-veys. The origin and the fraction of the high- α halo component remain elusive. It hasbeen found that the high- α halo stars are chemically indistinguishable from the thickdisk stars (Hawkins et al. 2015). Further studies incorporating all the six dimensionalphase space coordinates together with more detailed neutron capture elemental abun-dances are needed to put more constraints on the origin of high- and low- α stars and theirconnection to the thick disk population. Another remaining question is nucleosyntheticorigin of the chemical difference between the high- and low- α populations. As has beenpointed out by previous studies (e.g., Nissen & Schuster 1997; Fishlock et al. 2017), anadditional contribution of elements from Type Ia supernovae to the gas initially enrichedby core-collapse supernovae does not fully explain the observed chemical difference.
3. Chemistry of kinematically interesting stars
Along with the global structure, substructures in kinematic spaces have been iden-tified in the solar neighborhood (Helmi et al. 1999; Chiba & Beers 2000; Arifyanto &Fuchs 2006; Dettbarn et al. 2007; Kepley et al. 2007; Klement et al. 2008; Klement etal. 2009; Smith et al. 2009; Smith 2016 and Liang et al. 2018 for recent reviews of thissubject). These kinematically interesting halo stars, that are often referred to as ”kine-matic streams” are considered to have originated from accretion of dwarf galaxies orglobular clusters to the Milky Way halo. Chemistry has provided the most stringent testto distinguish the origin of these streams.One of the best known kinematic streams is the H99 stream, which was identified bykinematics mostly provided by the Hipparcos satellite (Helmi et al. 1999,Chiba & Beers2000). Roederer et al. (2010) analyzed high-resolution spectra of 13 candidate memberstars selected to have kinematics consistent with the H99 stream. It was shown that thecandidate member stars have a wide range of [Fe/H], which rules out the possibility thatthe H99 is originated from a dissolved star cluster. Instead, the [X/Fe] ratios are nearlyhomogeneous and their scatter is found to be small. While the Galactic dwarf satellitegalaxies that have metallicity similar to the H99 stream show evolution in [X/Fe] with[Fe/H], the abundance ratios of the H99 stars are nearly constant with [Fe/H] except forneutron capture elements. No signature of chemical enrichment by Type Ia supernovaeor AGB stars are found. The observed abundance pattern do not stand out compared tothe bulk of field halo stars that have similar [Fe/H].Another well studied kinematic stream is KFR08, which was originally discovered byKlement et al. (2008) based on velocities from the RAVE survey. Follow up studies haveconfirmed the presence of this stream (Klement et al. 2009; Bobylev et al. 2010) basedon independent data sets. The nature of the stream, however, remains elusive due tothe uncertainties in distances and kinematics as well as a small number of candidatemember stars (Klement et al. 2011). Liu et al. (2015) analyzed high-resolution spectrafor 16 candidate member stars of the KFR08 stream and estimated detailed elementalabundances of 14 elements. They have found that the 16 stars have a scatter in [Fe/H] aslarge as 0.29 dex and therefore, it is unlikely they originated from the same star cluster. Byquantifying similarity in chemical abundances among these stars by the method proposedby Mitschang et al. (2013), three of the 16 stars are found to show [X/Fe] ratios close eachother. On the other hand, their vertical velocities ( W ) exhibit a large dispersion, whichdoes not support the hypothesis that the three chemically similar stars were formed in the hemistry of field halo stars and streams α /Fe], [Na/Fe] and [Ni/Fe] ratios and higher [Ba/Y] ratiocompared to the bulk of the field halo stars that share a similar [Fe/H] (Suda et al. 2008).The amount of the offset from the trend of field stars is similar to that found for low- α stars as reported by Nissen & Schuster (2010). The direction of the offsets in [X/Fe] fromthe field stars is similar to those reported for dwarf spheroidal galaxies, which mightsuggest that they were originally born in a dwarf galaxy that was accreted to the MilkyWay in the past.The origin of nearby field halo stars on highly retro-grade orbits has been debated for awhile. It has been proposed that the stars are tidal debris of a galaxy which once hostedthe ω -Centauri ( ω Cen) globular cluster (e.g., Mizutani et al. 2003). Majewski et al.(2012) analyzed high-resolution ( R ∼ , ∼ ω Cen stars. This findingsuggests that these retro-grade stars are likely tidal debris of ω Cen itself. With a largecompilation of ∼
800 literature abundance data in the SAGA database (Suda et al. 2008)cross matched with Gaia DR2, Matsuno et al. (2019) show that the highly retro-gradestars show a different trend in the [ α -Fe]-[Fe/H] space from that seen among high-energyorbit stars. These retrograde stars show a [ α /Fe]-[Fe/H] down turn, which is often called”knee” at [Fe/H] ∼ .
4. Using chemistry to directly identify accreted stars
Stars that exhibit chemistry similar to those found in dwarf spheroidal galaxies cur-rently orbiting the Milky Way have been known for a while, although they are rel-atively rare. The most remarkable classical example is the three stars, BD+80 ◦ − . − . − . α (Mg, Si, and Ca) -to-iron and [(Sr, Ba)/Fe] ratios with large variations in Fe-peakelements for the three stars.Thanks to the large spectroscopic surveys, candidate stars with extreme chemical pat-terns are more efficiently found. Xing et al. (2019) have analyzed one of candidate ofstars that have very low [ α /Fe] ratios identified by the LAMOST survey. A follow-upspectroscopy with Subaru/HDS has confirmed that this star shows the [Mg/Fe] ratio of − . − .
2. Such a low [Mg/Fe] ratio is unusual for the halo star with compa-rable metallicity, while it is similar to stars in classical dwarf spheroidal galaxies such asUrsa Minor. On the other hand, the star shows a remarkable enhancement in r-processelements, with the abundance pattern comparable to the solar-system r-process pattern.Sakari et al. (2019) reported a low- α and mildly r-process enhanced star, RAVEJ093730.5-062655, originally identified in the RAVE survey. Sakari et al. (2019) madedetailed comparison of the observed abundances with yield models of Type Ia super-novae to investigate whether the abundances are explained by contribution of Fe fromType Ia supernovae. Although the existing yield models of Type Ia supernovae do notexactly reproduce all the observed elemental abundances, this is more likely formed outof gas enriched with Fe from Type Ia supernovae. Its retro-grade orbit clearly suggeststhat this star has come from an accreted dwarf galaxy.Both of the low- α stars of Sakari et al. (2019) and Xing et al. (2019) exhibit en-hancement of r-process elements that are more frequently seen among much lower [Fe/H]stars. In fact there is a growing evidence that r-process enhanced stars are originatedfrom dwarf galaxies (Roederer et al. 2018).For the case of stars with metallicity lower than [Fe/H] ∼ −
3, that are collectively calledextremely metal-poor (EMP) stars, the observed abundances are generally believed tobe the result of only one or a few supernovae of the very first stars in the Universe (e.g.,Audouze & Silk 1995).A sign of stochastic chemical enrichment has been seen among EMP stars as a largescatter in observed elemental abundance ratios. The most remarkable feature is the pres-ence of carbon enhanced stars that do not show enhancement in s-process elements(CEMP-no; Yong et al. 2013, Placco et al. 2014 but see Norris & Yong 2019 for the effectof 3D/NLTE effects on the Fe and C abundance measurements for EMP stars). Sincethe fraction of binary stars among the CEMP-no is not particularly high compared tonormal EMP stars, it is unlikely their atmospheric composition was modified by a binarymass transfer and thus are thought to reflect the abundance of gas from which thesestars formed. The origin of the CEMP-no stars has been debated for a while. The pro-posed scenarios include rotating massive first stars (Maeder et al. 2015), faint supernovae(Umeda & Nomoto 2003, Iwamoto et al. 2005), inhomogeneous metal-mixing (Hartwig& Yoshida 2019) or the result of the properties of dusts that were responsible for coolingthe gas from which these stars have formed (Chiaki et al. 2017). As the detailed ele-mental abundances become available for EMP stars, it becomes clear that some fractionof CEMP-no stars also show enhancement of intermediate-mass elements, including Na,Mg, Al, or Si (Bonifacio et al. 2018, Aoki et al. 2018). The diversity in other elementalabundance seen in CEMP-no stars suggests that multiple mechanisms are required tofully explain the carbon enhancement (Yoon et al. 2016).Recent large statistical sampling of EMP stars have identified stars that show sig-nificantly lower [ α /Fe] ratios than the majority of halo stars with similar metallicities(Cohen et al. 2013; Caffau et al. 2013; Bonifacio et al. 2018). Unlike the low- α stars with[Fe/H] ∼ − α /Fe] ratios remain largely unknown. Kobayashi hemistry of field halo stars and streams α stars in the sample of Bonifacio et al. (2018) are more likely to have been enriched bymore than one supernova of the first stars.These studies imply that the chemically peculiar EMP stars have formed in the en-vironment dominated by stochastic chemical enrichment. Such characteristic patternsare frequently reported in ultra-faint dwarf galaxies currently orbiting around the MilkyWay (e.g., Koch et al. 2008; Tolstoy et al. 2009; Salvadori et al. 2015) and some of theclassical dwarf galaxies (e.g.Venn et al. 2012).
5. Key questions for the future
The observations of chemistry of field halo stars have yielded various intriguing ques-tions to be addressed in the next generation observing facilities. One of such questionswould be how to quantify the relative contribution of substructures to the smooth halocomponent. Quantification of halo populations that have different birth places (e.g., in-situ, kicked-out, accreted) is the central issue to test the Galaxy formation model as hasbeen addressed by Unavane et al. (1996). Detailed chemical information is essential tomake further progress in this issue since phase-space coordinates can be largely washedout as the result of the dynamical evolution of the Galaxy. A drawback of the chemicalanalysis is that observations tend to be incomplete compared to the photometric sampleand thus frequently suffer from selection bias. In this case it would be difficult to obtaina quantitative conclusion about the fraction of stars with given chemistry in the wholestellar halo population.Cosmological simulations incorporating the chemical evolution in the building blocksof the Galaxy provide a powerful tool to quantify and interpret the emerging chemicalobservations (e.g., Font et al. 2006; Zolotov et al. 2010; Tissera et al. 2013). Techniquesto compare observations with these theoretical predictions have been investigated by e.g.,Schlaufman et al. (2012) and Lee et al. (2015). These studies provide a step forward tomake full use of spectroscopic data from large surveys on-going and planed in the nearfuture such as WEAVE (Dalton et al. 2014), 4MOST (de Jong et al. 2014),
Milky WayMapper survey planned as part of SDSS-V (Kollmeier et al. 2017), and the PFS (Takadaet al. 2014).For the theoretical side, some of the chemical signatures seen in field halo stars are likelyconnected to specific nucleosynthesis mechanisms in supernovae of the earliest generationof stars (e.g., Ezzeddine et al. 2019). Further investigations of theoretical yield models arecrucial to better understand the stellar birth environment. In fact, it has been pointed outthat the elemental abundances of the Sun are not fully explained by neither traditionalnor modern core-collapse and Type Ia supernova yield models (Simionescu et al. 2019).With increasingly large sample of high-resolution spectroscopic samples, it would beinteresting to compare the elemental abundance patterns to grids of supernova yieldmodels to obtain their statistical properties (Tominaga et al. 2014, Placco et al. 2015,Ishigaki et al. 2018). These studies have been used to investigate the possible origin ofextremely metal-poor stars in terms of the physical properties of the very first generationof stars. It is still difficult to reproduce observed abundances of all the key elements by anygiven supernova yield models. This is partly due to the still unknown physical mechanismof stellar evolution and supernova nucleosynthesis. Ishigaki, M. N.
6. Summary
Important observational results on the chemistry of field halo stars described in thisarticle can be summarized as follows: • The chemistry of nearby field halo stars with [Fe/H] & − . α -elements aswell as several other elements. This is likely connected to the global structural compo-nents such as the dual halo structure and hints at the formation of the Milky Way withaccretions of dwarf galaxies. • Some of the kinematic streams show characteristic abundance patterns that havehelped distinguishing their birth places (dwarf galaxies, star clusters or the Galacticdisk). • Chemically interesting field halo stars at [Fe/H] & − α -enhanced stellar population downto [Fe/H] ∼ −
1, that is distinct from currently surviving dwarf satellite galaxies, to anew picture of highly complex stellar populations in both kinematics and chemistry.At the same time, these findings provide intriguing questions to be answered in futureobservational and theoretical efforts.
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