Detailed Chemical Abundances in the r-Process-Rich Ultra-Faint Dwarf Galaxy Reticulum 2

1.2 MB·

by  Ian U. Roederer

Accepted for publication in the Astronomical Journal
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
DETAILED CHEMICAL ABUNDANCES IN THE R-PROCESS-RICH ULTRA-FAINT DWARF GALAXY
RETICULUM 2†
Ian U. Roederer,1,2 Mario Mateo,1 John I. Bailey III,1 Yingyi Song,1 Eric F. Bell,1 Jeffrey D. Crane,3
Sarah Loebman,1 David L. Nidever,1,4,5 Edward W. Olszewski,5 Stephen A. Shectman,3
Ian B. Thompson,3 Monica Valluri,1 Matthew G. Walker6
Accepted for publication in the Astronomical Journal
ABSTRACT
The ultra-faint dwarf galaxy Reticulum 2 (Ret 2) was recently discovered in images obtained by
6 the Dark Energy Survey. We have observed the four brightest red giants in Ret 2 at high spectral
1
resolutionusingtheMichigan/MagellanFiberSystem. Wepresentdetailedabundancesforasmanyas
0
20elementsperstar, including12elementsheavierthantheFegroup. Weconfirmpreviousdetection
2
of high levels of r-process material in Ret 2 (mean [Eu/Fe] = +1.69 ± 0.05) found in three of these

n stars (mean [Fe/H] = −2.88 ± 0.10). The abundances closely match the r-process pattern found in
a the well-studied metal-poor halo star CS 22892–052. Such r-process-enhanced stars have not been
J found in any other ultra-faint dwarf galaxy, though their existence has been predicted by at least

5 one model. The fourth star in Ret 2 ([Fe/H] = −3.42 ± 0.20) contains only trace amounts of Sr
1 ([Sr/Fe] = −1.73 ± 0.43) and no detectable heavier elements. One r-process enhanced star is also
enhancedinC(natal[C/Fe]≈+1.1). Thisisonlythethirdsuchstarknown, whichsuggeststhatthe

] nucleosynthesis sites leading to C and r-process enhancements are decoupled. The r-process-deficient
R
star is enhanced in Mg ([Mg/Fe] = +0.81 ± 0.14), and the other three stars show normal levels of
S α-enhancement (mean [Mg/Fe] =+0.34 ± 0.03). The abundances of other α and Fe-group elements
. closely resemble those in ultra-faint dwarf galaxies and metal-poor halo stars, suggesting that the
h
p nucleosynthesis that led to the large r-process enhancements either produced no light elements or
- produced light-element abundance signatures indistinguishable from normal supernovae.
o
Subject headings: galaxies: dwarf — galaxies: individual (Reticulum 2) — nuclear reactions, nucle-
r
osynthesis, abundances — stars: abundances
t
s
a
[
1. INTRODUCTION beendetectedinRet2(Westmeieretal.2015). Geringer-
1 Images obtained by the Dark Energy Survey (DES; Sameth et al. (2015) report a possible gamma-ray signal
v Diehl et al. 2014) have recently revealed 17 new faint detectiontowardRet2thatmaybeconsistentwithdark
0 Milky Way satellite candidates toward the general di- matter annihilation.
7 Jietal.(2015b)presentedthefirstabundancesderived
rection of the Magellanic Clouds (Bechtol et al. 2015;
0 fromhigh-resolutionspectroscopyofnineofthebrightest
Drlica-Wagner et al. 2015; Koposov et al. 2015a). One
4 stars in Ret 2. Their startling finding is that seven of
of these, Reticulum 2 (Ret 2), is the nearest (≈ 30 kpc)
0 theseninestarsarehighlyenhancedinmaterialproduced
ultra-faintdwarfgalaxy(UFD;M =−2.7)totheMilky
. V by the rapid neutron-capture process (r process). This
1 Way in the southern hemisphere. Reconnaissance spec-
is remarkable on several levels.
0 troscopy of Ret 2 (Koposov et al. 2015b; Simon et al.
6 2015; Walker et al. 2015) have confirmed Ret 2 has low First, no previous studies of dozens of stars in nine
1 mean metallicity ([Fe/H] =−2.6), a significant metallic- UFDgalaxieshaverevealedlargeenhancementsofheavy
: ity dispersion (σ ≈ 0.5), and a high mass-to-light elementsproducedbyr-processnucleosynthesis(Kochet
v [Fe/H] al. 2008; Feltzing et al. 2009; Frebel et al. 2010, 2014;
i ratio ((cid:38) 500 in solar units). Though located relatively
X Norris et al. 2010a,b; Simon et al. 2010; Gilmore et al.
close to the Large Magellanic Cloud (LMC), it remains
2013;Kochetal.2013;Ishigakietal.2014a;Koch&Rich
r unclear whether Ret 2 was originally a satellite of the
a Magellanic system (Koposov et al. 2015b). No H i has 2014; Roederer & Kirby 2014; Fran¸cois et al. 2015; Ji et
al. 2015a). Typically, elements produced by neutron-
capture (n-capture) reactions are difficult to detect in
†Thispaperincludesdatagatheredwiththe6.5meterMag- the UFD galaxies, and their abundances are among the
ellanTelescopeslocatedatLasCampanasObservatory,Chile.
1Department of Astronomy, University of Michigan, 1085 S. lowest known in any stars.
UniversityAve.,AnnArbor,MI48109,USA;[email protected] Second, stars with such high levels of r-process mate-
2Joint Institute for Nuclear Astrophysics and Center for the rial ([Eu/Fe] >+1 and [Ba/Eu] < 0; hereafter known as
EvolutionoftheElements(JINA-CEE),USA r-IIstars,Christliebetal.2004)arerelativelyrareamong
3Carnegie Observatories, 813 Santa Barbara St., Pasadena,
the metal-poor stars in the solar neighborhood (∼ 3%;
CA91101,USA
4Large Synoptic Survey Telescope, 950 North Cherry Ave., Barklem et al. 2005). About 20 r-II stars are known,
Tucson,AZ85719,USA and all are located in the halo, with the exception of
5Steward Observatory, 933 North Cherry Ave, Tucson, AZ one star in UMi (Shetrone et al. 2001; Sadakane et al.
85719,USA
2004;Aokietal.2007b)andoneintheMilkyWaybulge
6McWilliams Center for Cosmology, Department of Physics,
Carnegie Mellon University, 5000 Forbes Ave., Pittsburgh, PA (Johnson et al. 2013). Until now, none of the stars in
15213,USA the r-II class have been physically associated with each
2 Roederer et al.
other, and finding a galaxy teeming with such stars is 2. OBSERVATIONS
unprecedented.
There are only four stars brighter than the horizontal
Third, there have been few opportunities to test the
branch that are confirmed members of Ret 2 (Simon et
rolethatenvironmentplaysintheformationofr-IIstars
al. 2015; Walker et al. 2015). Table 1 lists the g mag-
and r-process nucleosynthesis more generally. Ji et al.
nitudes and g−r colors, adopted from the photometric
(2015b)assumedthatUFDgalaxieslikeRet2formedin
catalogsthatKoposovetal.(2015a)generatedfrompub-
minihalos with ∼ 106 M(cid:12) of H (e.g., Bromm & Larson lic DES images. The DES names are listed in Table 1.
2004). Ji et al. concluded that the r-process material Throughout this paper, we refer to these stars by the
originated in a single neutron-star merger (e.g., Goriely shorter names listed in Table 1.
et al. 2011; Ramirez-Ruiz et al. 2015) or any other rare We observed these four stars using one arm of the
site with high r-process yields (∼ 10−4.5 M(cid:12) of Eu per Michigan/Magellan Fiber System (M2FS) and MSpec
event). Theamountofr-processmaterialrequiredscales double spectrograph (Mateo et al. 2012; Bailey et al.
withtheassumedHmassoftheminihalo. Thedilutionof 2012) mounted on the Nasmyth platform at the 6.5 m
metalsintoHmaynotbeuniformacrossthegalaxy,and Landon Clay Telescope (Magellan II) at Las Campanas
allr-processmaterialmaynotbeincorporatedintostars, Observatory, Chile. We observed four high-probability
but this sets a lower limit on the amount of r-process members of Ret 2 and one blank sky position simulta-
material necessary. neously on 2015 November 14 and 16, with a total inte-
Previous attempts to identify the astrophysical gration time of 6.67 h. Both observations were taken in
source(s) of the r process have focused on character- dark time.
izing the r-process abundance pattern in detail (e.g., Our observations were made using the HiRes mode
Sneden et al. 1996, 2003; Hill et al. 2002; Christlieb of M2FS with 95 µm entrance slits. This setup de-
et al. 2004; Frebel et al. 2007; Siqueira Mello et al. livered spectral resolving power R ≡ λ/∆λ ∼ 30,000,
2013) and modeling the physical conditions of nucle- as measured from isolated Th or Ar emission lines in
osynthesis to reproduce this pattern (e.g., Wanajo et the comparison lamp spectra. We use a new custom
al. 2003; Kratz et al. 2007; Qian & Wasserburg 2007; order-isolation filter to observe orders 66–86, which cov-
Hayek et al. 2009). Other approaches include estimat- ers roughly 4150 ≤ λ ≤ 5430 ˚A for each target. Fig-
ing the frequency of r-II stars in the field (Barklem et ure 1 illustrates sections of the M2FS spectra of these
al. 2005), estimating the frequency of r-II stars exhibit- four stars.
ingradialvelocityvariationsthatrevealunseencompan- Some data reduction (merging data from different
ions(Hansenetal.2011,2015b),searchingforkinematic CCD chip amplifiers, stacking images, masking cosmic
properties sharedby r-process-enhanced stars (Roederer rays, and subtracting scattered light) was performed us-
2009), searching for subtle abundance patterns among ing custom routines. Standard IRAF routines were used
the lighter elements in r-II stars (Roederer et al. 2014c), to perform all other tasks (flatfielding, extraction, wave-
probing the extent to which r-process material can be length calibration, spectra co-addition, velocity shifting,
detected in trace amounts in metal-poor stars (Roed- and continuum normalization). Sky contamination was
erer 2013; Roederer et al. 2014a), modeling the light found to be negligible, so no sky subtraction was per-
curves associated with r-process events that might be formed. Table1alsoliststhesignal-to-noise(S/N)ratios
found in transient surveys (Metzger et al. 2010; Goriely perpixelfortheco-addedspectraatseveralwavelengths.
et al. 2011; Kasen et al. 2015), and searching for short-
lived radioactive nuclides produced by the r process in 3. ANALYSIS
deep-sea crust or sediments (Wallner et al. 2015; Ho- 3.1. Radial Velocities
tokezaka et al. 2015). Others pursue this question by
WemeasureradialvelocitiesusingtheIRAFfxcor task
modeling the chemical evolution of r-process elements
to cross-correlate the order containing the Mg i b lines
integrated across the Galactic halo (e.g., Mathews et al.
againstatemplate. WeusethehighestS/Nspectrumin
1992; Ishimaru & Wanajo 1999; Argast et al. 2004; Mat-
our sample (Star 4, observation 2) as the template. We
teucci et al. 2014; Cescutti et al. 2015), within dwarf
setthezeropointofthisspectrumbymeasuringtheoffset
spheroidal (dSph) galaxies (Tsujimoto & Shigeyama
between observed and laboratory wavelengths of several
2014),orwithincosmologically-motivatedgalaxymerger
Mg i and Fe i lines. We adopt laboratory wavelengths
models and simulations (Komiya et al. 2014; Ishimaru
fromtheAtomicSpectraDatabase(ASD)oftheNational
et al. 2015; Shen et al. 2015; van de Voort et al. 2015).
Institute of Standards and Technology (NIST; Kramida
Thegrowingpreponderanceofevidenceappearstopoint
etal.2015). Thevelocityzeropointofthetemplateisac-
towards neutron star mergers as a dominant source of
curate to ∼ 0.2 km s−1. We reproduce the radial veloci-
r-process material, but definitive observational evidence
ties of two standard stars (CD−43◦2527 and HD 48381)
is still lacking.
observedoneachnighttowithin0.5km s−1 r.m.s.(Udry
Here, we present a detailed abundance analysis of four
et al. 1999). Roederer et al. (2016) reported radial ve-
stars in Ret 2. We confirm the abundances of Fe, Ba,
and Eu reported by Ji et al. (2015b), and we expand locitymeasurementuncertainties∼0.7–1.0km s−1using
the chemical inventory to include 20 elements from C the same M2FS entrance slits based on repeat observa-
(Z = 6) to Dy (Z = 66). We also present upper limits tions of stars with comparable S/N ratios. We estimate
forsixotherelements. Wepresentthenewobservational that the uncertainties on our radial velocity measure-
material in Section 2, outline our analysis in Section 3, ments do not exceed 1.0 km s−1. The two velocity mea-
describeourresultsinSection4, anddiscusstheirimpli- surements for each star show no evidence of variations.
cations in Section 5. We report mean heliocentric radial velocities, Vr, for
each star in Table 1. We calculate heliocentric correc-
Detailed Abundances in Ret 2 3
TABLE 1
Basic Stellar Data
Namea Nameb Namec g g−r Vr S/N S/N S/N
(kms−1) 4300˚A 4800˚A 5200˚A
Star1 DESJ033447.94−540525.0 Ret2-80 17.50 0.59 +62.2 17 26 33
Star2 DESJ033523.85−540407.5 ··· 16.47 0.80 +65.5 15 26 35
Star3 DESJ033531.14−540148.2 Ret2-115 17.57 0.56 +59.7 25 40 48
Star4 DESJ033607.75−540235.6 Ret2-178 17.39 0.58 +62.0 26 45 59
a Thisstudy
b Simonetal.(2015)
c Walkeretal.(2015)
etal.2011),whichassumeslocalthermodynamicequilib-
TABLE 2
rium(LTE)fortheline-forminglayersoftheatmosphere.
Comparison with Previous Studies
We derive the effective temperatures (T ), log of the
eff
surface gravity (log g), microturbulence (v ), and model
Quantity Study (cid:104)∆(cid:105)a σ N t
metallicity ([M/H]) using an iterative process. We start
Vr (kms−1) Simonetal.(2015) +1.2±0.8 1.6 4 from an initial guess appropriate for typical metal-poor
Walkeretal.(2015) −0.3±0.4 0.7 3
red giant stars, but this initial set of parameters has no
Koposovetal.(2015b) −0.3 ··· 2
substantial impact on the final set. We estimate T by
Telffog(gK) JJiieettaall..((22001155bb)) +0+.0327±±407.15 09.239 44 requiringnocorrelationbetweentheE.P.ofFeilineesffand
[Fe/H] Simonetal.(2015) −0.20±0.10 0.18 3 the abundances derived from them. We simultaneously
Walkeretal.(2015) −0.22±0.28 0.48 3 requirenocorrelationbetweentheseabundancesandthe
Koposovetal.(2015b) −0.68 ··· 2
line strength, which sets v . Spectroscopic T values
Jietal.(2015b) +0.04±0.05 0.09 4 t eff
[Ba/Fe] Jietal.(2015b) −0.11±0.20 0.35 3 liketheseareknowntobesystematicallycoolerbyafew
[Eu/Fe] Jietal.(2015b) −0.11±0.07 0.11 3 percentthanthosederivedfromcolor-T relations(e.g.,
eff
a Differencesinthesenseofourstudyminuspreviousstudy Johnson2002). Recentmeasuresoftheangulardiameter
ofthenearbymetal-poorgiantHD122563byCreeveyet
tions using the IRAF rvcorrect task. The velocities in al.(2012)favorphotometricT scales,sowecorrectour
eff
Table 1 represent an unweighted mean of our two ob- spectroscopicT valuestoaphotometricscaleusingthe
eff
servations. We find reasonable agreement between our relation given by Frebel et al. (2013). The uncertainty
radial velocities and those measured by Koposov et al. in T is dominated by the scatter in this calibration.
eff
(2015b), Simon et al. (2015), and Walker et al. (2015), Wenextcalculatelog gfromthesetofY2 α-enhanced
as reported in Table 2. isochrones(Demarqueetal.2004)usingouradjustedT
eff
as the input. We assume a uniform old age (13 Gyr; cf.,
3.2. Equivalent Widths and Atomic Data e.g.,Brownetal.2012)forallstars. Wepropagateuncer-
We measure equivalent widths (EWs) from the spec- taintiesfromT intolog gusing104 MonteCarlocalcu-
eff
tra using a semi-automated routine that fits Voigt (or lations. The 13 Gyr [Fe/H] =−2.5 and −3.3 isochrones
Gaussian)lineprofilestocontinuum-normalizedspectra. predictlog gdifferentbyonly0.06dexforredgiants,and
As discussed in Roederer et al. (2014b), the user must anassumedageof10Gyrinsteadof13Gyronlychanges
visually inspect all lines, and poor fits can be modified the predicted log g by 0.03 dex. These are insignificant
byhand. Table3liststheEWsmeasuredfromourdata. compared to the uncertainty introduced by T .
eff
Roederer et al. (2016) showed that the EWs measured Wethenrederiveabundancesusingthenewlog gvalue,
from M2FS HiRes spectra taken with the same 95 µm recalculating the v value and ignoring the slight corre-
t
entrance slits agree to better than 1 m˚A with those de- lation between E.P. and abundance. We also set [M/H]
rived from MIKE spectra, which were shown by Bedell equal to the Fe abundance derived from Fe ii lines. (No
et al. (2014) and Roederer et al. (2014b) to agree with Fe ii lines are measurable in Star 1, so we set [M/H]
high-resolution spectra taken at several observatories. equal to the abundance derived from Fe i lines.) We it-
Table 3 also includes the wavelengths (λ), identifica- erate these steps until the model converges. Throughout
tions, excitation potentials (E.P.), and loggf values for this process we cull lines (∼ 10% of the lines) whose de-
all lines. We use loggf values from recent laboratory rived abundance deviates from the mean by more than
studies whenever possible, since these investigations fre- 2σ. The final model atmosphere parameters and their
quentlydeliver∼5%precision(0.02dex)orbetter(e.g., uncertainties are listed in Table 4. Our Teff and log g
Lawler et al. 2009). valuesareinexcellentagreementwiththevaluesderived
by Ji et al. (2015b), as shown in Table 2.
3.3. Stellar Parameters and Abundance Analysis Wederivemostabundancesusinganadaptationofthe
batch mode capabilities of MOOG. This method gener-
We base our estimates of the stellar parameters on
ates theoretical EWs that are forced to match the mea-
measures that can be extracted from the spectra them-
sured ones by adjusting the abundance of each element.
selves. We interpolate model atmospheres from the one-
Lines that are blended or broadened by hyperfine split-
dimensional plane-parallel ATLAS9 grid of α-enhanced
ting (HFS) structure or isotope shifts (IS) are measured
models (Castelli & Kurucz 2003). We compute Fe abun-
by comparing to a set of synthetic spectra generated by
dances from Fe i and Fe ii lines using a recent version of
MOOG. We adopt r-process isotopic fractions for Ba,
thespectrumanalysiscodeMOOG(Sneden1973;Sobeck
4 Roederer et al.
TABLE 3
Equivalent Widths and Atomic Data
Species λ E.P. loggf Ref. EWStar1 EWStar2 EWStar3 EWStar4
(˚A) (eV) (m˚A) (m˚A) (m˚A) (m˚A)
Mgi 4571.10 0.00 −5.62 1 45.2 90.8 53.0 39.2
Mgi 4702.99 4.33 −0.38 1 63.9 55.7 59.5 71.1
References. — (1) Kramida et al. 2015; (2) Aldenius et al. 2007; (3) Lawler & Dakin 1989, using
HFSfromKurucz&Bell1995;(4)Lawleretal.2013;(5)Woodetal.2013;(6)Lawleretal.2014;(7)
Sobecketal.2007;(8)Nilssonetal.2006;(9)Boothetal.1984;(10)DenHartogetal.2011forboth
loggf valueandHFS;(11)Ruffonietal.2014;(12)Woodetal.2014;(13)Kramidaetal.2015,using
HFSfromKurucz&Bell1995;(14)Roederer&Lawler2012;(15)Bi´emontetal.2011;(16)Ljunget
al. 2006; (17) Palmeri et al. 2005; (18) Wickliffe et al. 1994; (19) Kramida et al. 2015, using HFS/IS
fromMcWilliam1998whenavailable;(20)Lawleretal.2001a,usingHFSfromIvansetal.2006;(21)
Lawler et al. 2009; (22) Li et al. 2007, using HFS from Sneden et al. 2009; (23) Ivarsson et al. 2001,
using HFS from Sneden et al. 2009; (24) Den Hartog et al. 2003, using HFS/IS from Roederer et al.
2008awhenavailable;(25)Lawleretal.2006,usingHFS/ISfromRoedereretal.2008awhenavailable;
(26)Lawleretal.2001b,usingHFS/ISfromIvansetal.2006;(27)DenHartogetal.2006;(28)Lawler
etal.2001c,usingHFSfromLawleretal.2001dwhenavailable;(29)Wickliffeetal.2000
Note. —Theword“synth”intheEWcolumnsindicatesthatabundanceswerederivedbyspectral
synthesismatching. Theword“limit”indicatesthata3σupperlimitwasderivedfromanon-detection.
The complete version of Table 3 is available in the online edition of the Journal. A short version is
shownheretoillustrateitsformandcontent.
TABLE 4 TABLE 5
Stellar Parameters Abundances Derived from Individual
Lines
Name Teff logg vt [M/H]
(K) (kms−1) Star Species λ(˚A) log(cid:15) σlog(cid:15)
Star1 5020(140) 2.09(0.38) 2.00(0.3) −2.7(0.2) Star1 Mgi 4571.10 +5.28 0.20
Star2 4710(140) 1.22(0.40) 2.85(0.3) −2.9(0.2) Star1 Mgi 4702.99 +5.14 0.26
Star3 4855(140) 1.63(0.39) 2.40(0.2) −3.3(0.2)
Note. —The completeversion ofTable5 is
Star4 4810(140) 1.50(0.39) 2.15(0.2) −2.8(0.2)
availableintheonlineeditionoftheJournal. A
shortversionisshownheretoillustrateitsform
Nd,Sm,andEu(Snedenetal.2008). WederiveCabun-
andcontent.
dancesbymatchingsyntheticspectratotheobservedCH
A2∆−X2Π G band from 4290–4330 ˚A, using a line list derived from lines of the neutral species. Similarly, we
addσ forelementXinquadraturewithσ forelement
provided by B. Plez (2007, private communication). We II stat
Y when Y is derived from lines of the ionized species.
derive3σ upperlimitsbasedonnon-detectionsusingthe
Our derived metallicities are in reasonable agreement
method described in Roederer et al. (2014b).
with those derived by Ji et al. (2015b), Simon et al.
4. RESULTS (2015), and Walker et al. (2015). The mean metallic-
ity difference calculated from two stars in common with
Table 5 lists the abundances derived from each line
Koposov et al. (2015b) is −0.68 dex. Table 2 also lists
in each star. Tables 6 and 7 list the mean abundances
these comparisons. We are encouraged by the general
and their uncertainties. For element X, the logarithmic
agreementinlightofthedifferentmethods,spectralcov-
abundance is defined as the number of atoms of element
erage, and S/N ratios that are used to derive [Fe/H].
X per 1012 H atoms, log(cid:15)(X) ≡ log (N /N )+12.0.
10 X H Table 2 also compares our derived [Ba/Fe] and [Eu/Fe]
For elements X and Y, the logarithmic abundance ra-
ratios with those of Ji et al. These values are in good
tio relative to the solar ratio is defined as [X/Y] ≡
agreement.
log (N /N )−log (N /N ) . These ratios are con-
10 X Y 10 X Y (cid:12)
structed by referencing abundances derived from species
4.1. Neutron-Capture Elements
inthesameionizationstate(i.e.,neutralstoneutralsand
ions to ions). We adopt the solar reference abundances Figure 2 compares the [Sr/Fe], [Ba/Fe], and [Eu/Fe]
given in Asplund et al. (2009). ratiosintheRet2starswiththoseinotherUFDgalaxies
Four sets of uncertainties are listed in Tables 6 and and halo stars in the solar neighborhood. The Sr ii line
7. These are calculated as described in Roederer et al. at 4215 ˚A is detectable (Figure 3) in the most metal-
(2014b), based on the methodology of McWilliam et al. poor star in Ret 2, Star 3, but no other transitions of
(1995). The statistical uncertainty, σ , is given by any n-capture element are detectable in our spectrum
stat
equation A17 of McWilliam et al. and includes uncer- of this star. The upper limit derived from the Ba ii
tainties in the EW, line profile fitting, and loggf values. line at 4554 ˚A is among the lowest for any stars with
The total uncertainty, σ , is given by equation A16 of [Fe/H] <−3 in the UFD galaxies.
tot
McWilliametal.andincludesthestatisticaluncertainty The n-capture abundancesobservedin the otherthree
and uncertainties in the model atmosphere parameters. stars in Ret 2 are unprecedented among the UFD galax-
We use the other two uncertainties, σ and σ , when ies. Allthreeshowextremelyenhancedratiosof[Ba/Fe],
I II
constructingabundanceratiosamongdifferentelements. [Eu/Fe], and other heavy elements. Figure 3 com-
We add σ for element X in quadrature with σ for pares a piece of the spectrum of Star 4 (T = 4810 K,
I stat eff
element Y when computing the ratio [X/Y] when Y is [Fe/H]=−2.93,[Eu/Fe]=+1.64)withtwoothermetal-
Detailed Abundances in Ret 2 5
TABLE 6
Mean Abundances I
Star1 Star2
Species N log(cid:15) [X/Fe]a σstat σtot σI σII N log(cid:15) [X/Fe]a σstat σtot σI σII
Fei 21 +4.74 −2.76 0.09 0.19 0.00 0.00 30 +4.55 −2.95 0.11 0.21 0.00 0.00
Feii 0 ··· ··· ··· ··· ··· ··· 4 +4.55 −2.95 0.14 0.20 0.00 0.00
C(CH) 1 +5.92 +0.25 0.25 0.31 0.31 0.31 1 +6.08 +0.60 0.25 0.32 0.32 0.32
Mgi 3 +5.17 +0.33 0.08 0.20 0.12 0.12 4 +5.03 +0.38 0.09 0.24 0.15 0.24
Cai 0 ··· ··· ··· ··· ··· ··· 0 ··· ··· ··· ··· ··· ···
Scii 0 ··· ··· ··· ··· ··· ··· 1 <+0.55 <+0.35 0.21 0.26 0.31 0.26
Tii 4 +2.85 +0.66 0.08 0.19 0.12 0.12 4 +2.57 +0.56 0.11 0.21 0.15 0.25
Tiii 3 +2.31 +0.12 0.11 0.19 0.22 0.12 3 +2.38 +0.39 0.10 0.17 0.23 0.17
Vi 0 ··· ··· ··· ··· ··· ··· 0 ··· ··· ··· ··· ··· ···
Cri 2 +2.64 −0.23 0.10 0.19 0.13 0.13 3 <+2.96 <+0.27 ··· ··· ··· ···
Crii 0 ··· ··· ··· ··· ··· ··· 0 ··· ··· ··· ··· ··· ···
Mni 0 ··· ··· ··· ··· ··· ··· 0 ··· ··· ··· ··· ··· ···
Nii 0 ··· ··· ··· ··· ··· ··· 1 <+4.57 <+1.30 ··· ··· ··· ···
Zni 0 ··· ··· ··· ··· ··· ··· 0 ··· ··· ··· ··· ··· ···
Srii 1 +0.70 +0.59 0.48 0.51 0.51 0.47 1 +0.46 +0.54 0.23 0.26 0.27 0.30
Y ii 3 −0.13 +0.42 0.16 0.21 0.26 0.17 5 −0.34 +0.41 0.19 0.24 0.29 0.23
Zrii 1 +0.69 +0.87 0.37 0.40 0.42 0.38 2 <+0.65 <+1.02 ··· ··· ··· ···
Tci 0 ··· ··· ··· ··· ··· ··· 0 ··· ··· ··· ··· ··· ···
Rui 1 <+2.35 <+3.36 ··· ··· ··· ··· 1 <+1.93 <+3.13 ··· ··· ··· ···
Baii 1 +0.22 +0.80 0.17 0.24 0.25 0.17 1 +0.32 +1.09 0.21 0.24 0.25 0.28
Laii 1 −0.24 +1.42 0.17 0.22 0.26 0.18 2 −0.49 +1.36 0.20 0.25 0.30 0.25
Ceii 1 +0.09 +1.27 0.12 0.19 0.24 0.14 5 −0.11 +1.26 0.19 0.24 0.29 0.23
Prii 1 −0.27 +1.77 0.41 0.43 0.46 0.41 2 −0.66 +1.57 0.26 0.30 0.35 0.30
Ndii 6 +0.35 +1.69 0.14 0.19 0.24 0.15 7 −0.07 +1.46 0.17 0.23 0.28 0.22
Smii 2 +0.11 +1.91 0.15 0.20 0.25 0.16 3 −0.17 +1.82 0.18 0.24 0.28 0.23
Euii 2 −0.49 +1.75 0.27 0.32 0.33 0.27 2 −0.75 +1.68 0.14 0.18 0.23 0.20
Gdii 2 <+0.64 <+2.33 ··· ··· ··· ··· 2 <+0.37 <+2.25 ··· ··· ··· ···
Tbii 0 ··· ··· ··· ··· ··· ··· 1 <−0.78 <+1.87 ··· ··· ··· ···
Dyii 1 <+0.52 <+2.18 ··· ··· ··· ··· 1 +0.09 +1.94 0.32 0.40 0.40 0.39
a [Fe/H]givenforFeiandFeii
TABLE 7
Mean Abundances II
Star3 Star4
Species N log(cid:15) [X/Fe]a σstat σtot σI σII N log(cid:15) [X/Fe]a σstat σtot σI σII
Fei 28 +4.08 −3.42 0.11 0.20 0.00 0.00 51 +4.57 −2.93 0.06 0.18 0.00 0.00
Feii 2 +4.24 −3.26 0.10 0.17 0.00 0.00 6 +4.71 −2.79 0.07 0.15 0.00 0.00
C(CH) 1 +5.14 +0.13 0.15 0.25 0.19 0.19 1 +5.76 +0.26 0.15 0.25 0.19 0.19
Mgi 4 +4.99 +0.81 0.08 0.21 0.14 0.23 4 +5.01 +0.34 0.05 0.19 0.09 0.20
Cai 1 +3.08 +0.16 0.28 0.32 0.30 0.34 2 +3.76 +0.36 0.13 0.21 0.14 0.23
Scii 1 <+0.79 <+0.90 ··· ··· ··· ··· 1 <+0.70 <+0.34 ··· ··· ··· ···
Tii 3 +1.88 +0.35 0.13 0.21 0.17 0.24 12 +2.51 +0.50 0.05 0.17 0.08 0.20
Tiii 4 +1.94 +0.25 0.14 0.19 0.24 0.18 9 +2.55 +0.39 0.06 0.15 0.20 0.09
Vi 6 <+1.21 <+0.70 ··· ··· ··· ··· 6 <+1.24 <+0.24 ··· ··· ··· ···
Cri 1 <+2.23 <+0.01 ··· ··· ··· ··· 5 +2.49 −0.22 0.06 0.18 0.08 0.20
Crii 1 <+3.35 <+0.97 ··· ··· ··· ··· 3 +3.50 +0.65 0.06 0.15 0.20 0.09
Mni 0 ··· ··· ··· ··· ··· ··· 2 <+2.38 <−0.12 ··· ··· ··· ···
Nii 1 <+3.95 <+1.15 ··· ··· ··· ··· 1 +3.50 +0.21 0.09 0.19 0.11 0.21
Zni 3 <+2.14 <+1.00 ··· ··· ··· ··· 1 +1.96 +0.33 0.13 0.21 0.14 0.23
Srii 1 −2.12 −1.73 0.42 0.44 0.47 0.43 1 +0.24 +0.16 0.25 0.29 0.29 0.30
Y ii 9 <−1.00 <+0.05 ··· ··· ··· ··· 7 −0.29 +0.29 0.08 0.16 0.21 0.11
Zrii 2 <+1.22 <+1.90 ··· ··· ··· ··· 3 +0.36 +0.57 0.19 0.23 0.26 0.21
Tci 1 <+0.56 <+3.98 ··· ··· ··· ··· 1 <+0.73 <+3.66 ··· ··· ··· ···
Rui 1 <+1.81 <+3.48 ··· ··· ··· ··· 1 <+1.77 <+2.95 ··· ··· ··· ···
Baii 1 <−2.59 <−1.51 ··· ··· ··· ··· 1 +0.06 +0.67 0.09 0.16 0.17 0.18
Laii 8 <−1.08 <+1.08 ··· ··· ··· ··· 5 −0.44 +1.26 0.14 0.20 0.24 0.16
Ceii 6 <−0.21 <+1.47 ··· ··· ··· ··· 5 −0.13 +1.08 0.09 0.17 0.21 0.11
Prii 4 <−0.97 <+1.57 ··· ··· ··· ··· 3 −0.71 +1.36 0.16 0.21 0.25 0.17
Ndii 16 <−0.56 <+1.28 ··· ··· ··· ··· 18 −0.07 +1.30 0.09 0.16 0.21 0.11
Smii 10 <−1.04 <+1.26 ··· ··· ··· ··· 14 −0.27 +1.56 0.09 0.17 0.21 0.11
Euii 1 <−1.66 <+1.08 ··· ··· ··· ··· 2 −0.63 +1.64 0.06 0.13 0.14 0.14
Gdii 3 <−0.56 <+1.63 ··· ··· ··· ··· 1 −0.26 +1.46 0.18 0.23 0.27 0.20
Tbii 1 <−0.52 <+2.44 ··· ··· ··· ··· 0 <−0.39 <+2.10 ··· ··· ··· ···
Dyii 1 <−0.24 <+1.92 ··· ··· ··· ··· 1 +0.19 +1.88 0.12 0.20 0.22 0.18
a [Fe/H]givenforFeiandFeii
6 Roederer et al.
CS 22892−052
TABLE 8
5
Corrected [C/Fe] ratios
Star 4
Star logg [C/Fe] [C/Fe]
4
(observed) (corrected)
Star1 2.09 +0.25 +0.27
ux Star 3 Star2 1.22 +0.60 +1.15
e fl 3 Star3 1.63 +0.13 +0.38
ativ Star4 1.50 +0.26 +0.68
Rel Star 2 poor red giants. One star is in the UFD galaxy Seg 2
2
(T =4566K,[Fe/H]=−2.96,[Eu/Fe]<−0.30;Roed-
eff
erer&Kirby2014). Thatstudydetectedatotalofseven
Star 1
lines of n-capture elements in the star in Seg 2: 2 lines
1 of Sr ii and 5 lines of Ba ii. In contrast, numerous
lines of n-capture elements are detectable in Star 4 in
CH CH Ret2. Thespectrumofthisstarcloselyresemblesthatof
0
themetal-poorr-process-enhancedfieldgiantCS22892–
4290 4300 4310 4320
° 052 (T = 4800 K, [Fe/H] = −3.10, [Eu/Fe] = +1.64;
Wavelength (A) eff
Sneden et al. 2003). The only substantial differences
between the spectra of Star 4 and CS 22892–052 are
CS 22892−052 Ba II the CH features, since CS 22892–052 is C-enhanced
5
([C/Fe] = +0.88, Sneden et al.). These three stars in
Ret 2 have a mean metallicity ([Fe/H]= −2.88 ± 0.10)
Star 4
in the same range as most r-II stars found in the halo,
4 roughly−3.2(cid:46)[Fe/H](cid:46)−2.6(e.g.,Barklemetal.2005;
Roederer et al. 2014c).
ux Star 3 Figure4illustratesthedetailedabundancepatternsof
e fl 3 each of the four stars observed in Ret 2. The abundance
v
ati pattern found in star CS 22892–052 is shown for com-
Rel Star 2 parison. Theonlyadjustmentmadetothispatternisits
2
overallnormalization. Thispatternisasuperbrepresen-
tation for the elements detected in Star 1, Star 2, and
Star 1
Star 4 in Ret 2. The ratio between the lighter n-capture
1
elements (Sr, Y, and Zr; 38 ≤ Z ≤ 40) and the heav-
ier ones (Ba and beyond; Z ≥ 56) is also well-matched,
which is not always the case in metal-poor stars with
0
r-process material (e.g., McWilliam 1998; Johnson &
4530 4540 4550 4560
° Bolte 2002; Aoki et al. 2005; Roederer et al. 2010). This
Wavelength (A)
comparison also suggests that the radioactive actinides
232Th and 238U may be present and detectable in Ret 2,
CS 22892−052Mg I Mg I Mg I which could offer an independent check of the age of the
5
r-process material in this system (cf., e.g., Cowan et al.
[Fe/H] = −3.10
1997; Cayrel et al. 2001), but no lines of these elements
Star 4 are covered in our spectra. We conclude that r-process
4
nucleosynthesis is responsible for the production of the
[Fe/H] = −2.93
heavy elements observed in Ret 2.
x
u Star 3
e fl 3 4.2. Carbon
v [Fe/H] = −3.42
Relati 2 Star 2 areThsuep[eCr-/sFoela]rr,aatsiosshionwanllinfoFuirgusrtear2s.oHbosewrevveedr,inCRisedte2-
[Fe/H] = −2.95 pletedatthesurfaceasstarsascendtheredgiantbranch.
We use the corrections given by Placco et al. (2014) to
Star 1
1 estimate the natal [C/Fe] ratios for these stars. These
[Fe/H] = −2.76 are listed in Table 8. The magnitude of the corrections
increases with decreasing log g, as expected. Star 2 is
0 classifiedasaC-enhancedmetal-poor(CEMP)starusing
5160 5170 5180 5190 a common definition of C-enhancement, [C/Fe] > +0.7
Wavelength (A°) (Aoki et al. 2007a). The enhanced C in Star 2 is appar-
ent from the strong absorption near 4310 and 4323 ˚A in
Fig. 1.— SelectionsofthenormalizedM2FSspectraoffourstars the top panel of Figure 1.
inRet2andCS22892–052aroundtheCHGband(top),Baiiline
The fraction of CEMP stars in the field increases with
at4554˚A(middle),andMgibtriplet(bottom). Thespectrahave
beenoffsetverticallyfordisplaypurposes. decreasingmetallicity(e.g., Frebeletal.2006;Carolloet
al. 2012; Lee et al. 2013b; Placco et al. 2014). CEMP
Detailed Abundances in Ret 2 7
Fig. 2.— Comparison of abundance ratios in Ret2 (colored points) with stars in other UFD galaxies (black crosses) and halo giants
(graydots). TheUFDsampleincludesdatafromBooI(Feltzingetal.2009;Norrisetal.2010a;Gilmoreetal.2013;Ishigakietal.2014a),
BooII(Koch&Rich2014;Jietal.2015a),CVnII(Fran¸coisetal.2015),Com(Frebeletal.2010),Her(Kochetal.2008,2013;Fran¸cois
etal.2015),LeoIV(Simonetal.2010),Seg1(Norrisetal.2010b;Frebeletal.2014),Seg2(Roederer&Kirby2014),andUMaII(Frebel
etal.2010). ThehalosampleincludesonlythegiantsfromRoedereretal.(2014b). Upperlimitsareomittedfromthecomparisonsamples
forclarity. ThecolorsoftheRet2datapointsindicatethedominantnucleosyntheticoriginsofeachelement:yellow,He-burning;green,C
andOburningandα-captureinhydrostaticorexplosivenucleosynthesis;blue,Fe-groupelementsformedinnuclearstatisticalequilibrium
(NSE) during explosive nucleosynthesis; red, r-process nucleosynthesis. The dotted lines mark the solar ratios. Note the expanded scale
ontheverticalaxesforthe[Sr/Fe],[Ba/Fe],and[Eu/Fe]panels.
stars with no enhancement of slow n-capture process ment of both C and r-process material in these stars.
(s-process) material, like those in Ret 2, likely reflect Not all r-process stars in Ret 2 are C-enhanced. We in-
their natal composition (e.g., Ryan et al. 2005; Norris et ferfromthisthattheCandr-processenhancementsmay
al. 2013; Starkenburg et al. 2014; Hansen et al. 2015a,c). not be co-produced by the same nucleosynthesis site or
TheseCEMPstarsmaycontainmetalsfromoneorafew mechanism. Byextension,perhapstheC-andr-process-
zero-metallicity Pop III stars (cf., e.g., Bromm & Loeb enhancements in other CEMP-r stars are also not di-
2003; Umeda & Nomoto 2005; Cooke & Madau 2014; rectly related, as previous studies have concluded.
Ishigaki et al. 2014b), although they are not the only
classofstarsproposedtohaveformedfromtheremnants 4.3. Magnesium through Zinc
of Pop III stars (e.g., Aoki et al. 2014). A few CEMP Figure2alsocomparestheabundanceratiosamongα-
starshavebeenidentifiedintheUFDgalaxies(Frebelet andFe-groupelementsinRet2withthoseinotherUFD
al. 2010; Norris et al. 2010b; Lai et al. 2011; Gilmore et galaxies and halo giants. In all cases, the Ret 2 abun-
al. 2013), and we add one star in Ret 2 to this small but dance ratios fall well within the range found for other
growing inventory. UFD galaxies and halo stars.
TheCEMPstarinRet2isalsosubstantiallyenhanced The [Mg/Fe] ratio is higher in Star 3, the most metal-
inr-processmaterial(CEMP-r,accordingtothenomen- poor star in Ret 2 observed by us, by ≈ 0.45 dex com-
clatureofBeers&Christlieb2005). Onlytwootherstars, pared to the other three stars. This difference is about
CS 22892–052 (Sneden et al. 1994, 1996) and CS 22945– three times larger than the measurement uncertainties.
017 (Roederer et al. 2014c) are members of the CEMP-r Figure 5 illustrates portions of the spectra around sev-
class. No compelling explanation exists for the enhance- eral Mg i lines in Star 3 and Star 4, which have different
8 Roederer et al.
Star 4 in Ret 2 UFD Star in Seg 2 UFD CS 22892−052 (r−process−rich halo giant)
CH CH CH CH CH CH CH CH
x
u
fl 1
e
v
ti
a
Rel Zr I Sm II Nd II Gd II
Ce II
Ce II Zr II
Zr II
Y II
Eu II Sr II
0
4200 4205 4210 4215 4220
°
Wavelength (A)
Fig. 3.— Spectra of Star4 in Ret2 (black line), star SDSSJ021933.13+200830.2 in Seg2 (bold blue line), and the field giant
CS22892–052 (thin red line). The SDSSJ021933.13+200830.2 and CS22892–052 spectra were taken using the R ∼ 41,000 setting of
theMagellanInamoriKyoceraEchelle(MIKE)spectrograph(seeRoederer&Kirby2014andRoedereretal.2014bfordetails). Allstars
havesimilarparametersandmetallicities,buttheirCandn-captureabundancesdifferdramatically. Linesofn-capturespeciesaremarked
belowthespectra,linesofCHaremarkedabove,andunmarkedlinesarisefromα-orFe-groupelements.
Star 1 Star 2
1.5 CS 22892−052 1.5 CS 22892−052
(r−process) (r−process)
1.0 1.0
0.5 0.5
(cid:161) (cid:161)
g g
o o
l 0.0 l 0.0
−0.5 −0.5
−1.0 SrZr BaCeNdSmGdDy −1.0 SrZr BaCeNdSmGdDy
Y LaPr EuTb Y LaPr EuTb
4400 5500 6600 7700 8800 9900 4400 5500 6600 7700 8800 9900
AtomicNumber AtomicNumber
Star 3 Star 4
−0.5 CS 22892−052 1.5 CS 22892−052
(r−process) (r−process)
−1.0 1.0
−1.5 0.5
(cid:161) (cid:161)
g g
o o
l −2.0 l 0.0
−2.5 −0.5
−3.0 −1.0 SrZr BaCeNdSmGdDy
Y LaPr EuTb
4400 5500 6600 7700 8800 9900 4400 5500 6600 7700 8800 9900
AtomicNumber AtomicNumber
Fig. 4.— The n-capture abundance patterns compared with the r-process-enhanced standard star CS22892–052 (Sneden et al. 2003,
2009;Roedereretal.2009). ThepatternsarenormalizedtoEu(Z=63)inStar1,Star2,andStar4andtoSr(Z=38)inStar3. Note
thedifferentscaleontheverticalaxisforStar3(lowerleftpanel).
[Fe/H] but similar [Mg/H] ratios. This comparison sup- asevidenceforenrichmentdominatedbyhigh-masscore-
ports our assertion that the [Mg/Fe] ratio is enhanced collapse supernovae.
in Star 3. Enhanced [Mg/Fe] appears to be a genuine [Ti i/Fe] is high in three of the four stars. The Ti i
characteristic of some stars in the UFD galaxies. Two of abundance has been derived from 4–12 lines in these
these Mg-enhanced stars (in Boo I and Seg 1) are also three stars. No line appears to yield systematically high
C-enhanced(Norrisetal.2010b;Laietal.2011;Gilmore abundancesinRet2,andRoedereretal.(2014b)didnot
et al. 2013), but the one in Ret 2 and several others identifyanyoftheselinesasyieldingsystematically-high
in Com, Her, and UMa II are not (Koch et al. 2008; abundances in metal-poor halo giants. This indicates
Frebel et al. 2010). The enhanced [Mg/Fe] ratios in the that the high [Ti i/Fe] ratios cannot be attributed to a
C-normal stars have been interpreted (e.g., Koch et al.) handful of blended or poorly-measured Ti i lines. We
Detailed Abundances in Ret 2 9
ders of magnitude lower. Higher quality spectra of this
Star 3 [Mg/H] = −2.61 [Fe/H] = −3.26
star could settle the issue. Regardless, it seems likely
Star 4 [Mg/H] = −2.59 [Fe/H] = −2.79
that at least two n-capture enrichment events occurred
x1
u inRet2,whichdistinguishesitfromotherUFDgalaxies.
e fl None of the other [X/Fe] ratios in Figure 6 differ by
v Fe I
Relati Fe I Fe I CTi+a I II Fe I msaomreplteh.aWn2eσcbonetcwluedeentthhaetRtehte2αstaanrsdaFned-gthroeucpomelpeamreisnotns
Mg I studiedinRet2wereproducedinsimilarproportionsby
Mg I
Mg I the progenitors responsible for enriching Ret 2 and the
0 stars in the halo comparison sample.
5170 5180 5190
° Roederer et al. (2014b) performed this comparison for
Wavelength (A)
stars in the r-II class. That study concluded that the
abundances of elements from Mg to Zn in the r-II stars
Fig. 5.— Spectra of Stars 3 and 4 around the Mg i lines at areindistinguishablefromthoseinstarswithnormallev-
5167.32,5172.68,and5183.60˚A.BothstarshavesimilarTeff and els of r-process material at the limit of the data, about
logg. TheFeilinesaresignificantlyweakerinStar3,yettheMgi
3.5% (0.015 dex). This result was interpreted to mean
lineshavenearlyequalstrengths.
that the nucleosynthetic sites responsible for the large
notethat[Tiii/Fe]isnothigh, however, suggestingthat
r-process enhancements in the r-II stars did not pro-
there is probably not a genuine excess of Ti in the Ret 2
duce any α- or Fe-group element abundance signatures
stars.
distinct from normal core-collapse supernovae. Alterna-
The[X/Fe]ratiosshowminimalornodispersionwithin
tively, the r-process nucleosynthetic site(s) produced no
Ret 2, except for C, Mg, and the n-capture elements
α orFe-groupelementsatall. Weproposethatthiscon-
discussed previously. Furthermore, none of the [Mg/Fe],
clusion also applies to the stars examined in Ret 2.
[Ca/Fe], or [Ti/Fe] ratios lie significantly below the α-
enhanced plateau ([α/Fe] ∼ +0.3) in any of the Ret 2
5. DISCUSSIONANDCONCLUSIONS
stars observed.
Thepresent-daystellarmassofRet2(∼2.6×103 M ;
(cid:12)
4.4. Outliers in the [X/Fe] Ratios Bechtol et al. 2015) is roughly two orders of magnitude
less than the minimum mass necessary to fully sample
In Figure 6, we perform a detailed quantitative com-
a standard IMF. Assuming that Ret 2 still retains a
parisonbetweentheabundanceratiosineachofthestars
substantial fraction of its stars, we would expect to ob-
inRet2andhalogiantswithsimilarT and[Fe/H]that
eff servetheeffectsofstochasticallysamplingtheIMF. This
have been analyzed using similar methods. This com-
evidence may be found in the enhanced [C/Fe] ratio in
parison allows us to identify any element ratios that are
Star 2andenhanced[Mg/Fe]ratioinStar 3. Thenormal
outliers relative to the majority of halo stars. The sys-
α-andFe-groupabundancesfoundinmoststarsofferno
tematicabundanceuncertaintieslargelycanceloutwhen
compelling evidence of substantial variations at the high
using this differential approach. Elements detected in
end of the IMF in Ret 2.
multiple ionization states are compared and illustrated
The UFD enrichment models of Lee et al. (2013) rely
separately in Figure 6. We exclude from the comparison
on stochastic sampling of the mass function of super-
sample anystars thatexhibit highlevels of s-processen-
novae whose n-capture yields are more strongly mass-
richment,sincethischaracteristicfrequentlysignalsthat
dependent than yields of lighter elements like Ti. These
the present-day surface composition of the star has been
models predict that some n-capture-rich stars should be
polluted by a companion and probably does not reflect
found in the UFDs, and the r-process-enhanced stars
the natal composition.
in Ret 2 confirm this prediction. Neutron-star mergers
Theheaviestelementsare,ofcourse,significantlyover-
(e.g., Goriely et al. 2011; Ramirez-Ruiz et al. 2015) or
abundant in Star 1, Star 2, and Star 4 relative to the
rare r-process events associated with core-collapse su-
comparisonsample. InStar 3,theSrandBaabundances
pernovae, like magnetically-induced jets from the proto
arenotonlydeficientrelativetotheotherstarsinRet2,
neutron star (e.g., Fujimoto et al. 2008; Nishimura et al.
buttheyarealsodeficientrelativetothecomparisonhalo
2015), can produce a few 10−2 M of r-process material
sample. This deficiency is, however, quite normal for (cid:12)
and a few 10−5 M of Eu. These are compatible with
starsintheUFDgalaxies. Frebeletal.(2014)suggested (cid:12)
the observations of Ret 2, and the sites associated with
that this may be a result of a single n-capture enrich-
supernovaefulfilltherequirementoftheLeeetal.model.
menteventwithineachgalaxy. WecandetectonlySrin
We can use Ret 2 and other UFDs to estimate the
Star 3, limiting our ability to reliably identify the nucle-
amount of material produced by the main component of
osynthetic process that produced the heaviest elements
the r-process per unit of gas turned into stars. The 10
in Star 3. Candidates include the weak component of
UFD galaxies whose stars have been observed at high
ther process(e.g.,Truranetal.2002;Arcones&Thiele-
spectralresolution(seereferencesincaptiontoFigure2)
mann2013)andchargedparticlereactions(e.g.,Woosley
& Hoffman 1992) operating in core-collapse supernovae have a combined stellar mass of ∼ 1.1×105 M(cid:12) (Mc-
or the weak component of the s process (e.g., Raiteri Connachie 2012; Bechtol et al. 2015). Ji et al. (2015b)
et al. 1991; Pignatari et al. 2008) operating in massive, estimated that ∼ 3×10−5 M(cid:12) of Eu (∼ 3×10−2 M(cid:12)
rapidly-rotating stars. The upper limit on Ba in Star 3 of r-process material) are required to match the ob-
(Figure 4) is nearly able to exclude the possibility that servations of Ret 2. Thus ∼ 3×10−10 M of Eu
(cid:12)
the r-process pattern found in the r-process-enhanced (∼ 3×10−7 M of r-process material) is produced per
(cid:12)
stars is also present in Star 3 at an abundance three or- M of gas turned into stars in the 10 UFD galaxies.
(cid:12)
10 Roederer et al.
Star 1 Star 2
2 2
1 1
e] e]
F 0 F 0
X/ X/
[ [
−1 −1
C O MgSi Ca Ti Cr NiZn Sr Zr Ba C O MgSi Ca Ti Cr NiZn Sr Zr Ba
−2 N NaAl K Sc VMnCoCu Y Eu −2 N NaAl K Sc VMnCoCu Y Eu

Element Element
Star 3 Star 4
2
1
1
0
e] e]
F F 0
X/ X/
[ −1 [
−1
−2
C O MgSi Ca Ti Cr NiZn Sr Zr Ba C O MgSi Ca Ti Cr NiZn Sr Zr Ba
N NaAl K Sc VMnCoCu Y Eu −2 N NaAl K Sc VMnCoCu Y Eu

Element Element
Fig. 6.— Comparison of the abundances in the Ret2 stars with field red giants with similar metallicities. The comparison samples,
drawn from Roederer et al. (2014b), are selected to have T within 250 K and [Fe/H] within 0.3 dex of each Ret2 star. The number of
eff
starsinthecomparisonsamplesrangefrom22to34. Thiscomparisonsample,shownbytheshadedgrayboxes,representsthemean±1σ
standard deviations. Smaller symbols are shown for Ti, V, Cr, and Mn to accommodate ratios from both the neutral and ionized states,
whichmaydiffer. Thedottedlinemarksthesolarratios.
This rate may offer a link between the UFDs and Frebel & Bromm (2012) identified a set of chemical
higher-mass dSph galaxies. Tsujimoto et al. (2015) characteristics that could distinguish a bona fide “first
identified a plateau at [Eu/H] = −1.3 in stars with galaxy,” a system where only one long-lived stellar gen-
[Fe/H] (cid:38) −2 in Dra and several other dSph galaxies. eration formed from the yields of the first Pop III stars.
Drahasatotalstellarmassof∼2.5×105 M initsstars Ret 2 fulfills some of the observational criteria identified
(cid:12)
with[Fe/H]>−2(Tsujimotoetal.),whichtranslatesto byFrebel&Bromm,sincethereisnoevidenceofenrich-
∼ 5×10−5 M of Eu. Using the rate of Eu production ment by Type Ia supernovae or asymptotic giant branch
(cid:12)
we estimatebasedon theUFDgalaxies, thesame stellar stars. However, Ret 2 appears not to be a good candi-
massinDrawouldbeexpectedtoproduce∼7×10−5M date “first galaxy” based on the large internal spread in
(cid:12)
of Eu. The similarity between these two values could re- r-processmaterial. ThelargersampleofJietal.(2015b)
flectanaturalconnectionbetweenther-processmaterial hints that chemical evolution may have occurred within
observedinsurvivingUFDsanddSphgalaxies. Material Ret2,sincethemostmetal-poorstarslackther-process
produced by the main component of the r-process has enhancements observed in the more metal-rich ones.
alsobeenfoundinCarandUMi,twootherdSphgalaxies TheescapevelocitiesoftheUFDsystemslikeRet2are
with stellar masses similar to Dra (Shetrone et al. 2001, ∼ 25–50 km s−1, assuming a NFW halo profile (Navarro
2003;Sadakaneetal.2004;Cohen&Huang2010;Vennet et al. 1996) with virial mass ∼ 108–109 M(cid:12) and a con-
al. 2012). The extreme r-process enhancement observed centration, c, of 10.7–12.5 (Prada et al. 2012). Most
inRet2maybetheresultofalow-probabilitynucleosyn- ((cid:29)99%)ofthemetalsproducedwithinRet2wereprob-
thesis event that becomes inevitable (and less conspicu- ably lost from the system, as has been inferred for other
ous) in more luminous dSph galaxies with stellar masses dwarfgalaxies(e.g.,Kirbyetal.2011;Frebeletal.2014).
(cid:38) 105 M . Tafelmeyer et al. (2010) and Jablonka et Alternatively,simulationsbySmithetal.(2015)indicate
(cid:12)
al. (2015) called for examination of the neutron-capture thatsomelow-massminihaloscouldhavebeenexternally
abundancepatternsinmetal-poorstarsindSphgalaxies enriched by metals from neighboring minihalos prior to
with a range of masses to better understand the galactic formation of their own Pop III star. The star-to-star
metal retention and (Pop II or Pop III) sites that pro- abundancespreadsobservedinsomeelementratios(e.g.,
duce heavy elements. We propose that a focused effort [Fe/H], [C/Fe], [Mg/Fe], [Eu/Fe]) indicate that the ISM
toquantifythefractionofmetal-poorstarswithmaterial of Ret 2 was not fully mixed at the time the low-mass
produced by the main component of the r-process as a stars formed. This indicates that ejecta from at least
function of galaxy mass may also prove enlightening. two supernovae—perhaps even from multiple neighbor-
ingminihalos—formedstarsintheminihalothatevolved