J/MNRAS/507/43 Study of the Spite plateau with GALAH DR3 (Simpson+, 2021)
The GALAH survey: Accreted stars also inhabit the Spite plateau.
Simpson J.D., Martell S.L., Buder S., Bland-Hawthorn J., Casey A.R.,
De Silva G.M., D'Orazi V., Freeman K.C., Hayden M., Kos J., Lewis G.F.,
Lind K., Schlesinger K.J., Sharma S., Stello D., Zucker D.B., Zwitter T.,
Asplund M., Da Costa G., Cotar K., Tepper-Garcia T., Horner J.,
Nordlander T., Ting Y.-S., Wyse R.F.G. (The GALAH Collaboration)
<Mon. Not. R. Astron. Soc. 507, 43-54 (2021)>
=2021MNRAS.507...43S 2021MNRAS.507...43S (SIMBAD/NED BibCode)
ADC_Keywords: Milky Way ; Stars, dwarfs ; Stars, metal-deficient ; Optical ;
Infrared ; Abundances, [Fe/H] ; Effective temperatures
Keywords: stars: abundances - Galaxy: evolution - Galaxy: halo
Abstract:
The European Space Agency (ESA) Gaia mission has enabled the
remarkable discovery that a large fraction of the stars near the solar
neighbourhood are debris from a single in-falling system, the
so-called Gaia-Sausage-Enceladus (GSE). This discovery provides
astronomers for the first time with a large cohort of easily
observable, unevolved stars that formed in a single extragalactic
environment. Here we use these stars to investigate the 'Spite
plateau' - the near-constant lithium abundance observed in unevolved
metal-poor stars across a wide range of metallicities
(-3 < [Fe/H] < -1). Our aim is to test whether individual galaxies
could have different Spite plateaus - e.g. the interstellar medium
could be more depleted in lithium in a lower galactic mass system due
to it having a smaller reservoir of gas. We identified 93 GSE dwarf
stars observed and analysed by the GALactic Archaeology with HERMES
(GALAH) survey as part of its Data Release 3 (DR3). Orbital actions
were used to select samples of GSE stars, and comparison samples of
halo and disc stars. We find that the GSE stars show the same lithium
abundance as other likely accreted stars and in situ Milky Way stars.
Formation environment leaves no imprint on lithium abundances. This
result fits within the growing consensus that the Spite plateau, and
more generally the 'cosmological lithium problem' - the observed
discrepancy between the amount of lithium in warm, metal-poor dwarf
stars in our Galaxy, and the amount of lithium predicted to have been
produced by big bang nucleosynthesis - is the result of lithium
depletion processes within stars.
Description:
In this work, we expand upon previous results, presenting the lithium
abundance of GSE dwarf stars serendipitously observed and analysed. We
explore whether the lithium abundances of GSE stars are consistent
with the abundances of other Galactic populations with likely in situ
and ex situ origins. We test the hypothesis that the Spite plateau is
not the result of galactic chemical evolution.
The spectroscopic data used in this work come from the 588 571 stars
of the Data Release 3 (DR3) of the GALAH survey (Buder et al.
2021MNRAS.506..150B 2021MNRAS.506..150B, Cat. J/MNRAS/506/150) - the combination of
GALAH survey (De Silva et al. 2015MNRAS.449.2604D 2015MNRAS.449.2604D; Martell et al.
2017MNRAS.465.3203M 2017MNRAS.465.3203M, Cat. J/MNRAS/465/3203; Buder et al.
2018MNRAS.478.4513B 2018MNRAS.478.4513B, Cat. J/MNRAS/478/4513), the K2-HERMES survey
(Wittenmyer et al. 2018AJ....155...84W 2018AJ....155...84W; Sharma et al.
2019MNRAS.490.5335S 2019MNRAS.490.5335S), and the TESS-HERMES survey (Sharma et al.
2018MNRAS.473.2004S 2018MNRAS.473.2004S, Cat. J/MNRAS/473/2004). All observations used the
HERMES spectrograph (Sheinis et al. 2015JATIS...1c5002S 2015JATIS...1c5002S) and the
2-degree Field (2dF) fibre positioning system (Lewis et al.
2002MNRAS.333..279L 2002MNRAS.333..279L) at the 3.9-m Anglo-Australian Telescope. HERMES
records ∼1000 Å of the optical spectrum at a spectral resolution of
R ≃ 28000 across four non-contiguous sections, which includes the
neutral Li resonance lines at 6708 Å. The spectra were reduced with
a custom iraf pipeline for the GALAH survey (Kos et al.
2017MNRAS.464.1259K 2017MNRAS.464.1259K).
We apply data quality selections to identify a sample of dwarf stars
(defined as surface gravity logg > 3.65 and absolute G magnitude >
1.5) with reliable stellar parameters and abundances. For each star we
require no problems noted in the input data, reduction, analysis, or
iron abundance determination , also five-parameter solution from Gaia
Early Data Release 3 (EDR3; Gaia Collaboration et al.
2021A&A...649A...1G 2021A&A...649A...1G, Cat. I/350) to allow for orbital calculations,
the red camera spectrum (which contains the Li line) signal-to-noise
ratio > 30/pixel and finally no problems noted in the abundance
determination. Giving us a sample of 86 320 dwarf stars with
reliable lithium abundances,
(see section 2 Observation, reduction, and analysis).
As explicited in section 3, for this work it is necessary to identify
stars that are the least affected by Li depletion, and therefore their
current Li abundance is most representative of their birth abundance.
For investigating the Spite plateau, as has been done previously in
the literature (e.g. Melendez et al. 2010A&A...515L...3M 2010A&A...515L...3M), we want to
identify a stellar parameter selection that includes only those stars
hot enough to inhibit significant Li depletion, whilst simultaneously
maximizing our sample size of metal-poor stars. Consequently, we
retain 485 metal-poor dwarf stars with metallicity [Fe/H] < -0.8
that have Teff > 5850 K.
Next, in the section 4, we identify likely accreted GSE stars using
the same method as Feuillet et al. (2020MNRAS.497..109F 2020MNRAS.497..109F), allowing us
to make three additional groups of stars are identified for comparison
to our GSE sample which are retrograde/prograde orbiting stars and
disc stars highly likely to have formed in situ.
Then, as explained in the section 5, to quantify the Spite plateau ALi
abundance, we consider only stars with [Fe/H] < -1.3, because stars
above this metallicity in the prograde halo and disc samples begin to
show a divergence from the Spite plateau. This divergence is likely
the signature of Galactic lithium evolution (Bensby et al.
2018A&A...615A.151B 2018A&A...615A.151B, Cat. J/A+A/615/A151), and not self-depletion or
self-enrichment of lithium in these stars. Our final sample is thus
composed by 251 GALAH metal-poor dwarf stars as shown in the
table1.dat.
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 89 251 *Stellar parameters and lithium abundances for
dwarf stars belonging the Spite plateau
in GALAH DR3
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Note on table1.dat: All stellar parameter and abundance values in this work are
from Buder et al. 2021MNRAS.506..150B 2021MNRAS.506..150B, Cat. J/MNRAS/506/150. The metallicity
range for which GALAH provides reliable metallicities is [Fe/H]≳-3, and
we confirm (as has previously been seen; e.g. Rebolo et al.
1988A&A...192..192R 1988A&A...192..192R; Melendez et al. 2010A&A...515L...3M 2010A&A...515L...3M) that the Spite
plateau is basically flat with metallicity for stars within this metallicity
regime. The four subsamples of stars show essentially identical mean ALi
abundances at ∼ 2.35 with 1σ∼0.12.
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See also:
J/MNRAS/506/150 : The GALAH+ Survey DR3 (Buder+, 2021)
J/MNRAS/465/3203 : GALAH observational overview (Martell+, 2017)
J/MNRAS/478/4513 : GALAH Survey DR2 (Buder+, 2018)
J/MNRAS/473/2004 : TESS-HERMES Survey Data Release 1 catalog (Sharma+, 2018)
I/350 : Gaia EDR3 (Gaia Collaboration, 2020)
J/A+A/615/A151 : Lithium abundances in 714 F and G dwarf stars (Bensby+,2018)
II/246 : 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)
J/MNRAS/493/5195 : Milky Way halo stars ages and kinematics (Das+, 2020)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 16 A16 --- 2MASS 2MASS identifier (rstar_id) (1)
18- 32 I15 --- GALAH GALAH identifier (sobject_id) (2)
34- 52 I19 --- GaiaEDR3 Gaia EDR3 source identifier
(dr3sourceid) (3)
54- 57 I4 K Teff Spectroscopic effective temperature (teff)
59- 62 F4.2 [cm/s2] logg Surface gravity (logg)
64- 68 F5.2 [Sun] [Fe/H] Fe atomic abundance from Fe lines
(final, 1D-NLTE) (feh)
70- 73 F4.2 [Sun] ALi Lithium abundance from synthesis of the
6708 Å Li line and includes non-local
thermodynamic equilibrium (LTE) corrections
(a_li) (4)
75- 89 A15 --- Group Stellar group classification (5)
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Note (1): Linked to 2MASS Cutri et al., 2003, Cat. II/246
Note (2): Linked to GALAH DR3, Buder et al. 2021MNRAS.506..150B 2021MNRAS.506..150B,
Cat J/MNRAS/506/150
Note (3): Linked to GaiaEDR3, Gaia Collaboration 2021A&A...649A...1G 2021A&A...649A...1G, Cat. I/350
Note (4): We use the form ALi = log[nLi/nH] + 12, where nLi and nH are the
number densities of lithium and hydrogen, respectively.
Note (5): Consequently, it appears four groups of stars as follows:
GSE = 37 GSE stars (-0.5 < JΨ < 0.5) Mpc.km/s and
sqrt(JR) > 30 (kpc.km/s)0.5,
ALi = 2.37 ± 0.02, σ_ALi = 0.12 ± 0.02
Retrograde halo = 45 Retrograde orbiting halo stars
JΨ < 0 Mpc.km/s and
sqrt(JR) < 25 (kpc.km/s)0.5,
ALi = 2.37 ± 0.02, σ_ALi = 0.15 ± 0.03
Prograde halo = 34 Prograde orbiting stars that will be a mixture
of halo and dynamically thick disc stars
(0 < JΨ < 1) Mpc.km/s and
20 < sqrt(JR) < 25 (kpc.km/s)0.5,
ALi = 2.35 ± 0.02, σ_ALi = 0.10 ± 0.01
Disc stars = 40 Disc stars which are highly likely to have formed
in situ JΨ > 0.9 Mpc.km/s and
sqrt(JR) < 15 (kpc.km/s)0.5,
ALi = 2.33 ± 0.02, σALi = 0.14 ± 0.02
We identify likely accreted GSE stars using the same method as
Feuillet et al. (2020MNRAS.497..109F 2020MNRAS.497..109F), who cleanly selected GSE
members as those with Galactic orbits that had angular momentum
JΨ ∼ 0 and large radial action JR. For our sample of GALAH DR3
dwarfs, JΨ-sqrt(JR) space is divided into pairs of prograde
(JΨ > 0) and retrograde (JΨ < 0) bins for a range of
sqrt(JR).
More, we have not used abundance information as a chemical tag of
GSE stars (e.g. Mg, Mn, and Al as used by Das egt al.
2020MNRAS.493.5195D 2020MNRAS.493.5195D, Cat. J/MNRAS/493/5195), as this would limit us
to only the metal-rich stars, due to the limitations of HERMES
spectra for metal-poor stars.
In the metallicity range -2.5 < [Fe/H] < -2.0, there are 95 dwarf
stars in GALAH DR3 with reliable parameters, of which 59 (62 per cent)
have a reliable ALi, but only 16 (17 per cent) have [α/Fe],
10 (11 per cent) have [Mg/Fe], 10 (11 per cent) have [Mn/Fe], and
none have [Al/Fe]. These 95 dwarf stars are not classified in the
above groups.
(see section 4 GAIA-Sausage-Enceladus member selection and section 5
The lithium plateau and enrichment in GSE).
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History:
From electronic version of the journal
(End) Luc Trabelsi [CDS] 01-Jul-2024