Purveyors of fine halos III. Chemical abundance analysis of a potential omega Cen associate
Andreas J. Koch-Hansen, Camilla Juul Hansen, Linda Lombardo, Piercarlo Bonifacio, Michael Hanke, Elisabetta Caffau
AAstronomy & Astrophysics manuscript no. ms © ESO 2020November 26, 2020
Purveyors of fine halos
III. Chemical abundance analysis of a potential ω Cen associate (cid:63)
Andreas J. Koch-Hansen , Camilla Juul Hansen , Linda Lombardo , Piercarlo Bonifacio , Michael Hanke , ElisabettaCa ff au Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12, 69120 Heidelberg, Ger-many Max-Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany GEPI, Observatoire de Paris, Université PSL, CNRS, 5 Place Jules Janssen, 92190, Meudon, France
ABSTRACT
Globular clusters (GCs) are important donors to the build-up of the Milky Way (MW) stellar halo, having contributed at the tenpercent level over the Galactic history. Stars that originated from the second generation of dissolved or dissolving clusters can bereadily identified via distinct light-element signatures such as enhanced N and Na and simultaneously depleted C and O abundances.In this paper we present an extensive chemical abundance analysis of the halo star J110842, which was previously kinematicallyassociated with the massive MW GC ω Centauri ( ω Cen), and we discuss viable scenarios from escape to encounter. Based on ahigh-resolution, high signal-to-noise spectrum of this star using the UVES spectrograph, we were able to measure 33 species of 31elements across all nucleosynthetic channels. The star’s low metallicity of [Fe ii / H] = − . ± ± . ω Cen’s metallicity distribution. We find that all of the heavier-element abundances, from α - and Fe-peak elements to neutron-capture elements are closely compatible with ω Cen’s broad abundance distribution. However, given themajor overlap of this object’s abundances with the bulk of all of the MW components, this does not allow for a clear-cut distinctionof the star’s origin. In contrast, our measurements of an enhancement in CN and its position on the Na-strong locus of the Na-Oanticorrelation render it conceivable that it originally formed as a second-generation GC star, lending support to a former associationof this halo star with the massive GC ω Cen.
Key words.
Galaxy: abundances — Galaxy: formation — Galaxy: globular clusters: general — globular clusters: individual: ω Cen — Galaxy: halo — Galaxy: stellar content
1. Introduction
The stellar halo of the Milky Way (MW) galaxy conceivablyformed through a variety of channels. Thus, in situ star forma-tion within the host galaxy is contrasted by an ex situ formation,where the halo stars were born in satellite galaxies and accretedonto the host system only later on. The purported relative impor-tance of either scenario varies in the literature and it is currentlybelieved that our Galaxy experienced a mixture of both, wherethe ex situ component contributed to di ff erent degrees depend-ing on galactocentric radius (Eggen et al. 1962; Searle & Zinn1978; Dekel & Silk 1986; Bullock & Johnston 2005; Zolotovet al. 2009; Cooper et al. 2013; Pillepich et al. 2015; Naidu et al.2020).One important class of donors to the buildup of the MWhalo is the globular clusters (GCs), and there is a wealth of ev-idence for their ongoing tidal disruption and for their accretion,ranging from observations of stellar streams (e.g., Odenkirchenet al. 2001; Lee et al. 2004) and extended envelopes of present-day GCs (e.g., Jordi & Grebel 2010; Kuzma et al. 2018) to thechemodynamical identification of former GC stars in the MW Send o ff print requests to : A.J. Koch-Hansen; e-mail: [email protected] (cid:63) Based on observations obtained at ESO Paranal Observatory, pro-gram 0104.D-0059. halo field (Martell & Grebel 2010; Koch et al. 2019a; Fernández-Trincado et al. 2019; Tang et al. 2019; Hanke et al. 2020b)The key signature to identify a bona fide cluster escapee liesin the chemical anomalies inherent in the multiple populationsof the GSs (Carretta et al. 2009; Milone et al. 2017; Bastian &Lardo 2018; Gratton et al. 2019). As a result of high-temperatureproton-capture reactions in the CNO cycle and its Ne-Na chainin a first generation of massive stars, the second generation thatforms from the ejecta of these polluters is found to be rich in He,N, Na, and Al, while depleted in C, O, and Mg, which leads tothe characteristic anticorrelations (Na-O, Mg-Al) and bimodali-ties (e.g., in CN) observed in any given GC (Cohen 1978; Car-retta et al. 2009; Hanke et al. 2017; Bastian & Lardo 2018). Theremainder of the chemical inventory of the second stellar gen-eration remains largely unaltered by the involved nuclear reac-tions. These chemical patterns are indeed the best tracers of GCescapees, provided they were part of the second generation, asthese characteristic abundances are predominantly found in GCsacross the entire mass range, while absent in young open clus-ters, dwarf galaxies, and in situ halo field stars (Pilachowski et al.1996; Geisler et al. 2007; Bragaglia et al. 2017; Bekki 2019).Based on the these chemical signatures, recent quantita-tive analyses have estimated that about 11% of the stellar MWhalo originated from now defunct GCs (Martell & Grebel 2010;
Article number, page 1 of 9 a r X i v : . [ a s t r o - ph . GA ] N ov & A proofs: manuscript no. ms
Martell et al. 2011; Koch et al. 2019a; Hanke et al. 2020b) ,an order of magnitude that is bolstered by simulations (e.g.,Reina-Campos et al. 2020). Typically, such studies employ low-resolution spectroscopy, which is suitable to determine CN-bandstrengths, but is not su ffi cient to perform detailed chemical abun-dance analyses that inform us about the chemical properties ofthe progenitor cluster (cf. Ramírez et al. 2012; Lind et al. 2015;Hendricks et al. 2016; Majewski et al. 2017; Fernández-Trincadoet al. 2016, 2017). Taking the chemical identification of poten-tially former GC stars in the halo field one step further, Hankeet al. (2020b) added the kinematic dimensions a ff orded by theastrometry from the second data release (DR2) of the Gaia mis-sion (Lindegren et al. 2018; Gaia Collaboration et al. 2018). Thisallowed us not only to detect extra-tidal stars around known GCs,but also to trace back stars with common phase-space portions topotentially common progenitors (see also Savino & Posti 2019).In addition to disrupting GCs in the present-day MW halo, wealso need to consider the accretion of (dwarf) galaxy satellites.On the one hand, this leads to the donation of their GC systems,thereby increasing the census of the GC population in the Galaxy(Cohen 2004; Forbes & Bridges 2010; Law & Majewski 2010;Carretta et al. 2017; Massari et al. 2019; Myeong et al. 2019;Kruijssen et al. 2019; Koch & Côté 2019; Forbes 2020). On theother hand, the most massive GCs in the MW system are oftenconsidered the former nuclei of disrupted dwarf galaxies, leadingto broad abundance spreads and pronounced multiple popula-tions that are unequalled in the lower-mass star clusters (Bassino& Muzzio 1995; Sarajedini & Layden 1995; Forbes et al. 2004;Hilker et al. 2004; Kayser et al. 2006; Johnson & Pilachowski2010). Of these clusters the most massive GC in the MW sys-tem, ω Centauri ( ≡ NGC 5139; herafter ω Cen), has long beendiscussed as the core of a dwarf galaxy satellite (e.g., Lee et al.1999; Bekki & Freeman 2003; Romano et al. 2007; Valcarce &Catelan 2011); its metallicities show a broad range from − . − . ff & Kraft 1996;Hilker et al. 2004; McWilliam & Smecker-Hane 2005; Johnson& Pilachowski 2010; Villanova et al. 2014; Magurno et al. 2019;Johnson et al. 2020).Based on a large spectroscopic sample from the RAdial Ve-locity Experiment (RAVE; Steinmetz et al. 2006; Kordopatiset al. 2013), Fernández-Trincado et al. (2015) kinematically as-sociated 15 halo stars with ω Cen that were either subject to high-velocity ejections from it some 200 Myr ago or that had had closeencounters with this particular GC at high relative velocities.In this work we present a high-resolution, high signal-to-noise(S / N) chemical abundance analysis of one of these candidates,the metal-poor ([Fe / H] ∼ − − As the relative velocity in the close en-counter with ω Cen, at v rel =
275 km s − , is rather high, Fernández-Trincado et al. (2015) concluded that it is unlikely that this star(and other similar ones) directly escaped from the GC, ratherthat it encountered and interacted with the GC between 45 and290 Myr ago . The full sample of the RAVE- ω Cen associates This value depends, among other factors, on the adopted fraction offirst-generation stars that were lost from the GCs at early times. WhileKoch et al. (2019a) adopt a fraction of 56%, the recent analysis of Hankeet al. (2020b) assigns larger values of 50–80%, which would raise theinferred halo fraction to the 20% level. This star was observed as part of the CERES project (“Chemical Evo-lution of R-process Elements in Stars”) and thus has the alternate IDCES1108 − Depending on the adopted Galactic potential. The orbital analysis ofFernández-Trincado et al. (2015) also relied on proper motions from the shows chemical abundances that are consistent with the stellarpopulations of this object; however, the RAVE spectra only al-lowed the determination of the α -elements Mg, Si, and Ti, and Aland Ni. For J110842 only Al and Ni could be determined. There-fore, a more complete sampling of the abundance space has yetto be conducted, in order to investigate whether the origin of thisstar is indeed similar to that of ω Cen stars and, if so, whether itclassifies as a first- or second-generation star.This paper is organized as follows. In Sect. 2 we describe thedata, followed by details of the abundance analysis in Sect. 3.The resulting chemical abundances are presented in Sect. 4. Wediscuss the results in the context of ω Cen’s chemodynamicalproperties in Sect. 5.
2. Data
Star J110842 was observed on the night of March 03, 2020, withthe Ultraviolet and Visual Echelle Spectrograph (UVES; Dekkeret al. 2000) at the Very Large Telescope (Program 0104.D-0059; P.I. C.J. Hansen). We employed the 390 /
564 setting withdichroic 1, leading to a broad wavelength coverage of 3285–4518, 4623–5600, and 5672–6647 Å, and a high spectral resolv-ing power of R ∼ / N of ∼
50, 120, and 150 per pixel at 4000, 5200, and6200 Å. The data were reduced in the standard manner using theESO UVES reduction workflow recipes (version 6.1.3) that per-form bias correction, order tracing, flat fielding, and wavelengthcalibration using calibration data that were obtained on the sameday as the observations.We measured the star’s radial velocity from a cross-correlation against a template of similar stellar parameters usingthe Image Reduction and Analysis Facility (IRAF) fxcor task.This yielded a heliocentric velocity of v HC = ± − ,which agrees well with the values reported from the lower-resolution RAVE and Gaia spectra to within 0.4 km s − (see Ta-ble 1). Table 1.
Properties of the target star.
Parameter Value Reference α (J2000.0) 11:08:42.12 1 δ (J2000.0) − G G BP G RP L L (cid:12) v HC ± − T e ff ±
50 K 3log g ± ξ ± − / H] − .
10 3 d ( Gaia ) 4.6 + . − . kpc 4 µ α − ± µ δ ± References. (1): Lindegren et al. (2018); (2): Gaia Collaboration et al.(2018); (3): This work; (4): Bailer-Jones et al. (2018).UCAC4 (Zacharias et al. 2013), which, however, are in good agreementwith the latest
Gaia values.Article number, page 2 of 9.J. Koch-Hansen et al.: Abundance analysis of an ω Cen associate
3. Chemical abundance analysis
We performed a standard abundance analysis that employed amixture of equivalent width (EW) measurements, carried out viaGaussian fits with the IRAF splot task, and spectrum synthesis.Here we employed the same line list as in Koch & McWilliam(2014, see Table 2) with further additions in the syntheses fromBiémont et al. (2000), Den Hartog et al. (2003), Den Hartog et al.(2006), Lawler et al. (2007), Lawler et al. (2008), Lawler et al.(2009), Sneden et al. (2009), and Hansen et al. (2013).Hyperfine splitting was included where appropriate. The
Table 2.
Linelist λ E.P. EW[Å] Species [eV] log g f [mÅ]6300.31 O i − i − i − i − i − i − i − i − i − Notes.
Table 2 is available in its entirety in electronic form via the CDS. main abundance analysis was carried out using the ATLAS gridof one-dimensional, 72-layer, plane-parallel, line-blanketed Ku-rucz models without convective overshoot and the α -enhancedopacity distribution functions AODFNEW (Castelli & Kurucz2003). We further assumed that local thermodynamic equilib-rium (LTE) holds for all species. All computations relied on thestellar abundance code MOOG (Sneden 1973, 2014 version) un-less noted otherwise. To derive stellar parameters, we used photometry provided by
Gaia
DR2. We populated the parameter space using computedATLAS9 model atmosphere grids by Castelli & Kurucz (2003).This contains theoretical values of G BP − G RP , A G , E( G BP − G RP ),and bolometric corrections for each set of e ff ective temperatures( T e ff ), surface gravities (log g ), and metallicities ([Fe / H]) in therange of 3500 ≤ T e ff ≤ ≤ log g ≤
4, and − ≤ [Fe / H] ≤ + . G BP − G RP ) was computed using the reddeninglaw of Fitzpatrick et al. (2019). In order to determine the bestsuite of the stellar parameters T e ff and log g for our target starJ110842, the following iterative procedure was used:1. The initial metallicity was estimated via the literature valueof − . T e ff was derived by interpo-lating in G BP − G RP ;2. The bolometric correction was derived by interpolation fromthe new T e ff ;3. log g was derived using the above bolometric correction and T e ff ;4. A G and E( G BP − G BP ) were derived by interpolating in T e ff ;5. G and G BP − G RP were de-reddened using the reddening mapsby Schlafly & Finkbeiner (2011, A V = ff erence in tempera-ture between successive runs was less than 50 K.The microturbulence velocities ( ξ ) in each step were esti-mated using the calibration of Mashonkina et al. (2017). Here we note that the final value of 2.19 km s − provides an excellentbalance in the plot of line-by-line abundances with equivalentwidths of the Fe i lines.The final photometric parameter set of T e ff = g = i / H] = − .
84 and [Fe ii / H] = − .
10 dex, re-spectively. Thus, there is a pronounced ionization imbalanceseen in this star when employing the photometrically derivedsurface gravity. Moreover, no equilibrium of the Fe i abundancewith excitation potential could be reached upon using the pho-tometric temperature. This is a well-known problem for starsmore metal poor than about − / or theone-dimensional treatment of the atmospheres. Therefore, fol-lowing the recommendation of Mucciarelli & Bonifacio (2020),we adopt in the following the stellar parameters derived photo-metrically above and we continue by choosing the Fe abundancefrom the ionized species as the metallicity scale of star J110842. The statistical errors on our abundance ratios were determinedvia the standard deviation and the number of measured linesper element used to derive its abundance. Furthermore, we per-formed a systematic error analysis by varying each stellar pa-rameter about its respective uncertainty: T e ff ±
50 K, log g ± / H] ± ξ ± − . We further ran the iden-tical analyses as above using solar-scaled opacity distributions(ODFNEW) and take one-quarter of the ensuing deviation tomimic an ignorance of the α -enhancement in the star of 0.1 dex.The respective deviations of the abundance ratios from the bonafide results from the unaltered atmospheres are listed in Table 3;a conservative upper limit to the total systematic uncertainty interms of the squared sum of all contributions is given in the lastcolumn, although strong correlations between the impacts fromthe various atmospheric parameters can be expected (see, e.g.,McWilliam et al. 1995; Hanke et al. 2020a).
4. Results
All abundance results and the errors as described above are listedin Table 4. These values adopt the solar abundance scale of As-plund et al. (2009). In the following figures we place our resultsinto context with the MW halo, bulge, and disks, and ω Cen. For ω Cen we used the data of Johnson & Pilachowski (2010) andSimpson et al. (2020), who chemodynamically extracted clustercandidates from the GALAH survey (De Silva et al. 2015). Herewe also show the abundance ratios of those stars. Figures 1, 3,and 4 show the abundance comparison, where we restrict our-selves to those elements in common between our study and thatof ω Cen. The remaining elements, though not explicitly shown,are discussed individually below.
At [Fe ii / H] = − ω Cen’s metallicity distribution (Fig. 1, top). This distribution haslong been known to show a large dispersion and covers a rangeof more than 1.7 dex (Johnson & Pilachowski 2010). As thisobject is commonly considered the nucleus of a formerly moremassive dwarf galaxy, such a large spread and the occurrence of
Article number, page 3 of 9 & A proofs: manuscript no. ms
Table 3.
Systematic error analysis. T e ff log g [M / H] ξ Species ±
50 K ± ± ± − ODF Sys.CH (G-band) ± ∓ ∓ ∓ − i ± ± ± ∓ − i ± ∓ ∓ ∓ i ± ∓ ∓ ∓ i ± ± ∓ ∓ i ± ∓ ∓ ∓ ii ∓ ± ± ∓ − i ± ∓ ∓ ∓ − ii ∓ ± ± ∓ − i ± ∓ ∓ ∓ i ± ∓ ∓ ∓ i ± ∓ ∓ ∓ i ± ∓ ∓ ∓ ii ∓ ± ± ∓ − i ± ∓ ∓ ∓ i ± ∓ ∓ ∓ i ± ∓ ∓ ∓ i ∓ ± ± ∓ − ii ± ± ± ∓ − ii ∓ ± ± ∓ − ii ∓ ± ± ∓ − ii ± ± ± ∓ − ii ± ± ± ∓ − ii ± ± ± ∓ − ii ± ± ± ∓ − ii ± ± ± ∓ − ii ± ± ± ∓ − ii ± ± ± ∓ − ii ± ± ± ∓ − ii ± ± ± ∓ − ii ± ± ± ∓ − ii ± ∓ ∓ ∓ Table 4.
Abundance results. Abundance ratios for ionized species are given relative to Fe ii . For iron itself, [Fe / H] is listed. The line-to-line scatter, σ , and number of measured lines, N , indicate the statistical error. Species [X / Fe] σ N Species [X / Fe] σ N Species [X / Fe] σ N CH (G-band) − S Mn i − H La ii H O i S Fe i − ii S Na i ii − ii S Mg i i H Nd ii S Si i i − ii S Ca i i − ii S Sc ii H Zn i ii S Ti i ii < S Dy ii S Ti ii ii ii − S V i ii ii − S Cr i − ii H Pb i S Notes. “H” indicates that hyperfine structure was accounted for; “S” denotes abundances that were derived from spectrum synthesis. very metal-poor stars down to − . Fernández-Trincado et al. (2015) noted that ω Cen is special inthat it covers a broad range in all of their analyzed abundancepatterns to the point that it overlaps with all MW components.They concluded that any similarity in these properties is “notvery useful” to constrain the origin of the stars. However, theirelement abundances, drawn from the RAVE survey, were only
Article number, page 4 of 9.J. Koch-Hansen et al.: Abundance analysis of an ω Cen associate N o r m a li z ed M D F [ M g / F e ] [ S i / F e ] [Fe/H] [ C a / F e ] Fig. 1.
Chemical abundances of J110842 (red star) in comparison with ω Cen (gray squares: Johnson & Pilachowski 2010; blue squares: Simp-son et al. 2020) and the MW halo (Roederer et al. 2014) and disks(Bensby et al. 2014), shown as black dots. The 329 candidate ω Cenassociates from Fernández-Trincado et al. (2015) are indicated as redpoints. The error bar accounts for statistical and systematic uncertain-ties. The top panel shows the error-weighted metallicity distributionfrom Johnson et al. (2020), on which is highlighted the 1 σ error rangeof the metallicity determined in the present work. limited to Fe, Al, and Ni . Therefore, we highlight here some ofthe elements that provide a greater clue to any potential origin ofJ110842.One of the characteristics of GCs is the presence of multi-ple stellar generations and the ensuing light-element variationsas a result of p -capture reactions in an early generation of stars(Kayser et al. 2008; Carretta et al. 2009; Bastian & Lardo 2018).These variations include bimodalities in CN and associated anti-correlations with CH strength. We measured the strength of thecommonly used CH and CN bands using the index definitions ofNorris et al. (1981), which was then translated into a δ S(3839)index to remove dependencies on evolutionary status (Martell &Grebel 2010; Koch et al. 2019a). The main uncertainty in this Abundances for the other α -elements (Mg, Si, Ti) were reportedfor the remainder of the ω Cen candidates in Fernández-Trincado et al.(2015), but had not been derived for star J110842. quantity is the required absolute magnitude of the star, whichrelies on its distance that still shows large errors (see Table 1).At δ S(3839) = / Fe] we conclude that this instead argues in favor of aC-normal star (e.g., Kirby et al. 2015; Koch et al. 2019b), withvalues that are appropriate for its luminosity of 1224 L (cid:12) (GaiaCollaboration et al. 2018).Similarly, genuine GCs are infamous for having a pro-nounced Na-O anticorrelation, which is also prominently seen in ω Cen (Norris & Da Costa 1995b; Gratton et al. 2011; Simpsonet al. 2020) and particularly extended below − [ N a / F e ] -2 -1.8-1.6-1.4-1.2-1 -0.8-0.6-0.4-0.2 [ F e / H ] Fig. 2.
Sodium-oxygen anticorrelation using data from Marino et al.(2011) and Simpson et al. (2020), color-coded by metallicity. StarJ110842 is indicated by a red star. We also indicate the empirical sep-arations into first, second, and extreme generations (dashed black) andthe simplistic dilution model (red line) by Carretta et al. (2009).
While the value of [O / Fe] for J110842 at 0.65 dex is compat-ible with that of a metal-poor, α -enhanced halo star, the [Na / Fe]value of 0.55 dex is rather high and makes this star fall intothe regime of Na-strong second-generation GC stars. We notethat all our abundances have been derived in an LTE framework.However, interpolating the grid of non-LTE (NLTE) correctionsby Lind et al. (2011) yields a departure from the LTE abun-dance of ∼ − .
06 dex, while the correction for oxygen is nullfor the parameters similar to our star (Sitnova et al. 2013). Thus,even if NLTE corrections are accounted for, we cannot exclude asecond-generation origin for this star based on its Na abundance,and marginally supported by its O abundance and CN strength.Finally, we note that GCs often show strong variations inAl that mildly correlate with Mg, owing to the hot branches ofproton-burning. However, the spectral range of our UVES set-ting did not allow us to determine an Al abundance from the6696 Å line. Conversely, the blue line at 3961 Å lies in the wingof the strong Ca H line, making a meaningful abundance de-termination di ffi cult. Instead, we use the value of Fernández-Trincado et al. (2015) for further discussion. Considering thehigher Mg and O abundances in our star, their adopted [Al / Fe]of 0.38 dex lies at the low branch of Al abundances, which isconsistent with an association with a second stellar populationin a GC (e.g., Carretta et al. 2013).
Article number, page 5 of 9 & A proofs: manuscript no. ms α -elements: Mg, Si, Ca, Ti The α -elements in J110842 present few surprises. At an [ α / Fe]value of 0.42 ± α -plateau delineatedby metal-poor halo stars and the bulk of ω Cen’s broad abundancespace (Figs. 1,3). This indicates enrichment via standard nucle-osynthesis in supernovae of type II (SNe II) and does not allowus to further investigate the question of a peculiar origin of thisstar based on these chemical tracers.
As is true for the α -elements, the Fe-peak elements also fol-low the trends outlined by metal-poor halo and ω Cen stars (seeFigures 3,4, and also Cohen 1981; Norris & Da Costa 1995a;Smith et al. 1995; Pancino et al. 2011; Magurno et al. 2019 for ω Cen) and that are mainly set by SN Ia nucleosynthesis (e.g.,Kobayashi et al. 2006). [ S c / F e ] [ T i / F e ] [ V / F e ] [ C r / F e ] Fig. 3.
Same as Fig. 1, but for the lighter Fe-peak elements. Disk valuesfor Sc and V are from Battistini & Bensby (2015).
Copper (bottom panel of Fig. 4), in contrast, has been a mat-ter of high interest in this GC, and Cunha et al. (2002) noted thatCu stays approximately constant and remains below the trendseen in halo stars over a broad metallicity range of ∼ − − . ω Cen is rather the nucleus of a former, more massive sys-tem. The target of this study, J110842, overlaps with the metal-poor halo and the metal-poor tail of the ω Cen distribution. Ourown values and the cited literature values were derived under theassumption of LTE. It appears that the Cu abundances are af-fected by NLTE e ff ects (Andrievsky et al. 2018; Shi et al. 2018),although di ff erent model-atoms and NLTE codes provide di ff er-ent results, especially at low metallicity. This notion is reinforcedby the observed deviation between Cu i and Cu ii abundances ob-served by Roederer & Barklem (2018). Bonifacio et al. (2010)found strong granulation e ff ects on the resonance lines (althoughnot used here); however, the combined e ff ect of granulation andNLTE e ff ects still needs to be investigated. It would therefore beinteresting to reinvestigate the Cu abundances in ω Cen, and inJ110842, using a more sophisticated modeling. [ M n / F e ] [ C o / F e ] [ N i / F e ] [ C u / F e ] Fig. 4.
Same as Fig. 1, but for the remaining Fe-peak elements. Abun-dances for Mn are from Sobeck et al. (2006), Co data are from Battistini& Bensby (2015), and Cu for the MW disks and halo are from Mishen-ina et al. (2002, 2011).
We can also use our measurements to address the broadercontext of Galactic chemical evolution. Here Hawkins et al.(2015) posited that the [Mg / Mn] versus [Al / Fe] plane is a pow-
Article number, page 6 of 9.J. Koch-Hansen et al.: Abundance analysis of an ω Cen associate erful indicator for an origin in major, dwarf-galaxy-like accre-tion events versus in situ formed stars that are enhanced inthe α -elements. In J110842 we measured a very high value for[Mg / Mn] of 1.04 ± ff ects, this overabundance would be even larger (Bergemannet al. 2019). In order to qualify as an accreted object, the [Al / Fe]abundance in J110842 would need to be subsolar at its Mg andMn abundances, according to the distinctive line in Horta et al.(2020). However, since Fernández-Trincado et al. (2015) re-ported on a higher [Al / Fe] of 0.38 dex, this instead argues foran in situ formation or that the birth environment has chemicallyevolved. n -capture elements: Zn, Sr, Y, Zr, Ba, La, Ce, Pr, Nd,Sm, Eu, Gd, Dy, Er, Hf, Pb Our abundance results for the selected heavy elements overlap-ping with the literature for ω Cen are shown in Fig. 5. In addition,the values for J110842 for these elements are in close agreementwith metal-poor halo stars and the metal-poor end of the GCabundance distribution, indicating that the same nucleosyntheticprocesses were at play. [ Z n / F e ] [ Y / F e ] [ B a / F e ] -3 -2 -1 000.51 [ La / F e ] [ E u / F e ] Fig. 5.
Same as Fig. 1, but for the neutron-capture elements ( Z ≥
40 50 60 70 80Z00.511.52 l og / B a J110842Solar r -processSolar s -processBest r + s HD20
Fig. 6.
Abundance distribution of the heavy elements, normalized toBa. Also shown are the solar s - and r -process contributions (Burriset al. 2000) and the best-fit linear combination of the contributions. Fi-nally, we overplot the r -process benchmark star HD 20 as black squares(Hanke et al. 2020a). Among the neutron-capture elements we particularly notethe s -process elements Y, Ba, and La (middle three panels ofFig. 5). Dating back to Lloyd Evans (1983), a bimodal behav-ior has now been established; for instance, from their high-resolution analysis of 113 RR Lyr stars in ω Cen Magurno et al.(2019) reported solar [Y / Fe] values for stars more metal poorthan approximately − / Fe] found by Johnson & Pilachowski (2010)also show a clear bimodality. This has been interpreted in termsof markedly di ff erent stellar populations, where the more metal-rich component was (self-)enriched over a long timescale bylow-mass asymptotic giant branch stars (e.g., Norris & Da Costa1995a; Smith et al. 2000; Cunha et al. 2002), while the suddenincrease in the abundance ratios with [Fe / H] is compatible with ω Cen being the remnant of a more massive dwarf galaxy (Ro-mano et al. 2007; Magurno et al. 2019). The solar value of [Y / Fe]in J110842 at its low metallicity and its solar [Ba / Fe] are fullycompatible with the lower-metallicity component.Figure 6 shows the overall abundance distribution of heavy( Z >
30) elements we were able to measure in J110842. Here wenote that Sr in its spectrum shows very strong resonance lines (at4077 and 4215 Å) that are likely saturated. Therefore, we wereonly able to place a limit of ∼ / Fe] value. Asnoted before, J110842 is mainly characterized by standard nucle-osynthesis and the majority of elements lie between the (solar) s - and r -process distributions, as is expected if we consider, atthis metallicity, that AGB stars have already started to contributesome s -process material to the Galactic chemical evolution (e.g.,Simmerer et al. 2004). A χ fit indicates that J110842 has re-ceived an admixture of s -process elements at the 60% level. An-other assessment of the r - and s -process contributions can bemade in comparison to the cleaner r -process tracer HD 20 (blacksymbols in Fig. 6), which at a metallicity that is lower than theSun’s displays a very clean r -process pattern. However, in thisdirect comparison J110842 also comes out as a r – s mixed starwith not just one main polluter governing its formation. A fur-ther characterization of the donors to the chemical enrichmentof this star, such as SNe or AGB stars, cannot be unambiguously Article number, page 7 of 9 & A proofs: manuscript no. ms made in stars like J110842, where the e ff ects of mixing and di-lution need to be properly dealt with (Magg et al. 2020; Hansenet al. 2020).
5. Discussion
In our endeavor to find signs of a chemical association of thehalo star J110842 with the massive GC ω Cen, as has been pre-viously suggested from their relative dynamics, we performedan extensive chemical abundance study. We showed that the ma-jority of the heavier elements ( Z ≥
12) is fully compatible withthose seen in ω Cen. This is, however, not surprising given thelarge overlap of the GC abundance space with that of metal-poorhalo field stars at this metallicity ( − . ω Cen, but just had chance encounters when their orbits co-incided. The central escape velocity of the GC is on the orderof 60–100 km s − (Gnedin et al. 2002; Gao et al. 2015), whichcompares to the computed encounter velocity with J110842 inexcess of 200 km s − . On the other hand, extreme horizontalbranch stars at high tangential motions of up to 90–310 km s − have been found in ω Cen, and they are most likely to escapethe GC within the next Myr or so (Gao et al. 2015), in whichcase a former origin of our target star within the GC remainsplausible. Here we note that Lind et al. (2015) associate a halostar with typical GC signatures such as enhanced Al and low Mgabundances with having escaped from ω Cen at high speed. Suchhigh-speed ejections ( >
100 km s − ) are suggested if interactionswith binaries or black holes are evoked (Gvaramadze et al. 2009;Lützgendorf et al. 2012).As to the origin of ω Cen itself, Myeong et al. (2019) haveassociated it with the recently discovered massive Sequoia ac-cretion event, while Ibata et al. (2019) paired it with the Fim-bulthul stream. The latter poses an intriguing parallel to our staras its abundance distribution is very similar to the most metal-poor stream candidate analyzed by Simpson et al. (2020). Thisemphasizes the power of chemical tagging in meaningfully in-vestigating the manifold of eclectic GC-stream-halo-field inter-faces.
Acknowledgements.
The authors thank the anonymous referee for a swift andconstructive report. AJKH gratefully acknowledges funding by the DeutscheForschungsgemeinschaft (DFG, German Research Foundation) – Project-ID138713538 – SFB 881 (“The Milky Way System”), subprojects A03, A05, A11.CJH acknowledges support from the Max Planck Society and from the ChETECCOST Action (CA16117), supported by COST (European Cooperation in Sci-ence and Technology). LL, PB and EC gratefully acknowledge support from theFrench National Research Agency (ANR) funded project “Pristine” (ANR-18-CE31-0017). This work has made use of data from the European Space Agency(ESA) mission
Gaia ( ), processed bythe Gaia
Data Processing and Analysis Consortium (DPAC, ). Funding for the DPAC hasbeen provided by national institutions, in particular the institutions participatingin the
Gaia
Multilateral Agreement. This research made use of atomic data fromthe INSPECT database, version 1.0 ( ). References
Andrievsky, S., Bonifacio, P., Ca ff au, E., et al. 2018, MNRAS, 473, 3377 Asplund, M., Grevesse, N., Sauval, A. J., & Scott, P. 2009, ARA&A, 47, 481Bailer-Jones, C. A. L., Rybizki, J., Fouesneau, M., Mantelet, G., & Andrae, R.2018, AJ, 156, 58Barbuy, B., Friaça, A. C. S., da Silveira, C. R., et al. 2015, A&A, 580, A40Bassino, L. P. & Muzzio, J. C. 1995, The Observatory, 115, 256Bastian, N. & Lardo, C. 2018, ARA&A, 56, 83Battistini, C. & Bensby, T. 2015, A&A, 577, A9Bekki, K. & Freeman, K. C. 2003, MNRAS, 346, L11Bekki, K. 2019, MNRAS, 490, 4007Bensby, T., Feltzing, S., & Oey, M. S. 2014, A&A, 562, A71Bergemann, M., Gallagher, A. J., Eitner, P., et al. 2019, A&A, 631, A80Biémont, E., Garnir, H. P., Palmeri, P., Li, Z. S., & Svanberg, S. 2000, MNRAS,312, 116Bonifacio, P., Ca ff au, E., & Ludwig, H. G. 2010, A&A, 524, A96Bragaglia, A., Carretta, E., D’Orazi, V., et al. 2017, A&A, 607, A44Bullock, J. S. & Johnston, K. V. 2005, ApJ, 635, 931Burris, D. L., Pilachowski, C. A., Armand ro ff , T. E., et al. 2000, ApJ, 544, 302Carretta, E., Bragaglia, A., Gratton, R., & Lucatello, S. 2009, A&A, 505, 139Carretta, E., Gratton, R. G., Bragaglia, A., D’Orazi, V., & Lucatello, S. 2013,A&A, 550, A34Carretta, E., Bragaglia, A., Lucatello, S., et al. 2017, A&A, 600, A118Castelli, F. & Kurucz, R. L. 2003, in IAU Symposium, Vol. 210, Modelling ofStellar Atmospheres, ed. N. Piskunov, W. W. Weiss, & D. F. Gray, A20Cohen, J. G. 1978, ApJ, 223, 487Cohen, J. G. 1981, ApJ, 247, 869Cohen, J. G. 2004, AJ, 127, 1545Cooper, A. P., D’Souza, R., Kau ff mann, G., et al. 2013, MNRAS, 434, 3348Cunha, K., Smith, V. V., Suntze ff , N. B., et al. 2002, AJ, 124, 379De Silva, G. M., Freeman, K. C., Bland-Hawthorn, J., et al. 2015, MNRAS, 449,2604Dekel, A. & Silk, J. 1986, ApJ, 303, 39Dekker, H., D’Odorico, S., Kaufer, A., Delabre, B., & Kotzlowski, H. 2000, inSociety of Photo-Optical Instrumentation Engineers (SPIE) Conference Se-ries, Vol. 4008, Optical and IR Telescope Instrumentation and Detectors, ed.M. Iye & A. F. Moorwood, 534–545Den Hartog, E. A., Lawler, J. E., Sneden, C., & Cowan, J. J. 2003, ApJS, 148,543Den Hartog, E. A., Lawler, J. E., Sneden, C., & Cowan, J. J. 2006, ApJS, 167,292Eggen, O. J., Lynden-Bell, D., & Sandage, A. R. 1962, ApJ, 136, 748Fernández-Trincado, J. G., Robin, A. C., Vieira, K., et al. 2015, A&A, 583, A76Fernández-Trincado, J. G., Robin, A. C., Moreno, E., et al. 2016, ApJ, 833, 132Fernández-Trincado, J. G., Zamora, O., García-Hernández, D. A., et al. 2017,ApJ, 846, L2Fernández-Trincado, J. G., Beers, T. C., Tang, B., et al. 2019, MNRAS, 488,2864Fitzpatrick, E. L., Massa, D., Gordon, K. D., Bohlin, R., & Clayton, G. C. 2019,ApJ, 886, 108Forbes, D. A., Strader, J., & Brodie, J. P. 2004, AJ, 127, 3394Forbes, D. A. & Bridges, T. 2010, MNRAS, 404, 1203Forbes, D. A. 2020, MNRAS, 493, 847Freeman, K. C. & Rodgers, A. W. 1975, ApJ, 201, L71Gaia Collaboration, Brown, A. G. A., Vallenari, A., et al. 2018, A&A, 616, A1Gao, X.-H., Xu, S.-K., & Chen, L. 2015, Research in Astronomy and Astro-physics, 15, 1639Geisler, D., Wallerstein, G., Smith, V. V., & Casetti-Dinescu, D. I. 2007, PASP,119, 939Gnedin, O. Y., Zhao, H., Pringle, J. E., et al. 2002, ApJ, 568, L23Gratton, R. G., Johnson, C. I., Lucatello, S., D’Orazi, V., & Pilachowski, C. 2011,A&A, 534, A72Gratton, R., Bragaglia, A., Carretta, E., et al. 2019, A&A Rev., 27, 8Gvaramadze, V. V., Gualandris, A., & Portegies Zwart, S. 2009, MNRAS, 396,570Hanke, M., Koch, A., Hansen, C. J., & McWilliam, A. 2017, A&A, 599, A97Hanke, M., Hansen, C. J., Ludwig, H.-G., et al. 2020a, A&A, 635, A104Hanke, M., Koch, A., Prudil, Z., Grebel, E. K., & Bastian, U. 2020b, A&A, 637,A98Hansen, C. J., Bergemann, M., Cescutti, G., et al. 2013, A&A, 551, A57Hansen, C. J., Koch, A., Mashonkina, L., et al. 2020, A&A, 643, 49Hawkins, K., Kordopatis, G., Gilmore, G., et al. 2015, MNRAS, 447, 2046Hendricks, B., Boeche, C., Johnson, C. I., et al. 2016, A&A, 585, A86Hilker, M., Kayser, A., Richtler, T., & Willemsen, P. 2004, A&A, 422, L9Horta, D., Schiavon, R. P., Mackereth, J. T., et al. 2020, arXiv e-prints,arXiv:2007.10374Ibata, R. A., Bellazzini, M., Malhan, K., Martin, N., & Bianchini, P. 2019, NatureAstronomy, 3, 667Johnson, C. I. & Pilachowski, C. A. 2010, ApJ, 722, 1373Johnson, C. I., Dupree, A. K., Mateo, M., et al. 2020, AJ, 159, 254Jordi, K. & Grebel, E. K. 2010, A&A, 522, A71Kayser, A., Hilker, M., Richtler, T., & Willemsen, P. G. 2006, A&A, 458, 777 Article number, page 8 of 9.J. Koch-Hansen et al.: Abundance analysis of an ω Cen associate
Kayser, A., Hilker, M., Grebel, E. K., & Willemsen, P. G. 2008, A&A, 486, 437Kirby, E. N., Guo, M., Zhang, A. J., et al. 2015, ApJ, 801, 125Kobayashi, C., Umeda, H., Nomoto, K., Tominaga, N., & Ohkubo, T. 2006, ApJ,653, 1145Koch, A. & Edvardsson, B. 2002, A&A, 381, 500Koch, A. 2009, Astronomische Nachrichten, 330, 675Koch, A. & McWilliam, A. 2014, A&A, 565, A23Koch, A. & Côté, P. 2019, A&A, 632, A55Koch, A., Grebel, E. K., & Martell, S. L. 2019a, A&A, 625, A75Koch, A., Xi, S., & Rich, R. 2019b, A&A, 627, A70Kordopatis, G., Gilmore, G., Steinmetz, M., et al. 2013, AJ, 146, 134Kruijssen, J. M. D., Pfe ff er, J. L., Reina-Campos, M., Crain, R. A., & Bastian,N. 2019, MNRAS, 486, 3180Kunder, A., Kordopatis, G., Steinmetz, M., et al. 2017, AJ, 153, 75Kuzma, P. B., Da Costa, G. S., & Mackey, A. D. 2018, MNRAS, 473, 2881Law, D. R. & Majewski, S. R. 2010, ApJ, 718, 1128Lawler, J. E., den Hartog, E. A., Labby, Z. E., et al. 2007, ApJS, 169, 120Lawler, J. E., Sneden, C., Cowan, J. J., Ivans, I. I., & Den Hartog, E. A. 2009,ApJS, 182, 51Lawler, J. E., Sneden, C., Cowan, J. J., et al. 2008, ApJS, 178, 71Lee, Y. W., Joo, J. M., Sohn, Y. J., et al. 1999, Nature, 402, 55Lee, K. H., Lee, H. M., Fahlman, G. G., & Sung, H. 2004, AJ, 128, 2838Lind, K., Asplund, M., Barklem, P. S., & Belyaev, A. K. 2011, A&A, 528, A103Lind, K., Koposov, S. E., Battistini, C., et al. 2015, A&A, 575, L12Lindegren, L., Hernández, J., Bombrun, A., et al. 2018, A&A, 616, A2Lloyd Evans, T. 1983, MNRAS, 204, 975Lützgendorf, N., Gualandris, A., Kissler-Patig, M., et al. 2012, A&A, 543, A82Magg, M., Nordlander, T., Glover, S. C. O., et al. 2020, MNRAS, 498, 3703Magurno, D., Sneden, C., Bono, G., et al. 2019, ApJ, 881, 104Majewski, S. R., Schiavon, R. P., Frinchaboy, P. M., et al. 2017, AJ, 154, 94Marino, A. F., Milone, A. P., Piotto, G., et al. 2011, ApJ, 731, 64Martell, S. L. & Grebel, E. K. 2010, A&A, 519, A14Martell, S. L., Smolinski, J. P., Beers, T. C., & Grebel, E. K. 2011, A&A, 534,A136Mashonkina, L., Jablonka, P., Pakhomov, Y., Sitnova, T., & North, P. 2017, A&A,604, A129Massari, D., Koppelman, H. H., & Helmi, A. 2019, A&A, 630, L4McWilliam, A., Preston, G. W., Sneden, C., & Searle, L. 1995, AJ, 109, 2757McWilliam, A. & Smecker-Hane, T. A. 2005, ApJ, 622, L29Milone, A. P., Piotto, G., Renzini, A., et al. 2017, MNRAS, 464, 3636Mishenina, T. V., Kovtyukh, V. V., Soubiran, C., Travaglio, C., & Busso, M.2002, A&A, 396, 189Mishenina, T. V., Gorbaneva, T. I., Basak, N. Y., Soubiran, C., & Kovtyukh, V. V.2011, Astronomy Reports, 55, 689Mucciarelli, A. & Bonifacio, P. 2020, A&A, 640, A87Myeong, G. C., Vasiliev, E., Iorio, G., Evans, N. W., & Belokurov, V. 2019,MNRAS, 1731Naidu, R. P., Conroy, C., Bonaca, A., et al. 2020, ApJ, 901, 48Norris, J., Cottrell, P. L., Freeman, K. C., & Da Costa, G. S. 1981, ApJ, 244, 205Norris, J. E. & Da Costa, G. S. 1995a, ApJ, 447, 680Norris, J. E. & Da Costa, G. S. 1995b, ApJ, 441, L81Odenkirchen, M., Grebel, E. K., Rockosi, C. M., et al. 2001, ApJ, 548, L165Pancino, E., Mucciarelli, A., Sbordone, L., et al. 2011, A&A, 527, A18Pilachowski, C. A., Sneden, C., & Kraft, R. P. 1996, AJ, 111, 1689Pillepich, A., Madau, P., & Mayer, L. 2015, ApJ, 799, 184Ramírez, I., Meléndez, J., & Chanamé, J. 2012, ApJ, 757, 164Reina-Campos, M., Hughes, M. E., Kruijssen, J. M. D., et al. 2020, MNRAS,493, 3422Roederer, I. U. & Barklem, P. S. 2018, ApJ, 857, 2Roederer, I. U., Preston, G. W., Thompson, I. B., et al. 2014, AJ, 147, 136Romano, D., Matteucci, F., Tosi, M., et al. 2007, MNRAS, 376, 405Sarajedini, A. & Layden, A. C. 1995, AJ, 109, 1086Savino, A. & Posti, L. 2019, A&A, 624, L9Schlafly, E. F. & Finkbeiner, D. P. 2011, ApJ, 737, 103Searle, L. & Zinn, R. 1978, ApJ, 225, 357Shi, J. R., Yan, H. L., Zhou, Z. M., & Zhao, G. 2018, ApJ, 862, 71Simmerer, J., Sneden, C., Cowan, J. J., et al. 2004, ApJ, 617, 1091Simpson, J. D., Martell, S. L., Da Costa, G., et al. 2020, MNRAS, 491, 3374Sitnova, T. M., Mashonkina, L. I., & Ryabchikova, T. A. 2013, Astronomy Let-ters, 39, 126Smith, V. V., Cunha, K., & Lambert, D. L. 1995, AJ, 110, 2827Smith, V. V., Suntze ff , N. B., Cunha, K., et al. 2000, AJ, 119, 1239Sneden, C. A. 1973, PhD thesis, The University of Texas at Austin.Sneden, C., Lawler, J. E., Cowan, J. J., Ivans, I. I., & Den Hartog, E. A. 2009,ApJS, 182, 80Sobeck, J. S., Ivans, I. I., Simmerer, J. A., et al. 2006, AJ, 131, 2949Steinmetz, M., Zwitter, T., Siebert, A., et al. 2006, AJ, 132, 1645Suntze ff , N. B. & Kraft, R. P. 1996, AJ, 111, 1913Tang, B., Liu, C., Fernández-Trincado, J. G., et al. 2019, ApJ, 871, 58Valcarce, A. A. R. & Catelan, M. 2011, A&A, 533, A120Villanova, S., Geisler, D., Gratton, R. G., & Cassisi, S. 2014, ApJ, 791, 107Zacharias, N., Finch, C. T., Girard, T. M., et al. 2013, AJ, 145, 44Zolotov, A., Willman, B., Brooks, A. M., et al. 2009, ApJ, 702, 1058, N. B. & Kraft, R. P. 1996, AJ, 111, 1913Tang, B., Liu, C., Fernández-Trincado, J. G., et al. 2019, ApJ, 871, 58Valcarce, A. A. R. & Catelan, M. 2011, A&A, 533, A120Villanova, S., Geisler, D., Gratton, R. G., & Cassisi, S. 2014, ApJ, 791, 107Zacharias, N., Finch, C. T., Girard, T. M., et al. 2013, AJ, 145, 44Zolotov, A., Willman, B., Brooks, A. M., et al. 2009, ApJ, 702, 1058