J/A+A/704/A44 The metal-poor tail of APOGEE I (Montelius+, 2025)
The metal-poor tail of the APOGEE survey.
I. Uncovering [Fe/H]←2.5 stars from the inner Galaxy to the Magellanic Clouds.
Montelius M., Starkenburg E., Woudenberg H., Angrilli Muglia A.,
Ardern-Arentsen A., Viswanathan A., Bystrom A., Helmi A., Martin N.,
Matsuno T., Navarrete C., Navarro J.
<Astron. Astrophys. 704, A44 (2025)>
=2025A&A...704A..44M 2025A&A...704A..44M (SIMBAD/NED BibCode)
ADC_Keywords: Milky Way ; Magellanic Clouds ; Spectra, infrared ;
Stars, metal-deficient
Keywords: Galaxy: abundances - Galaxy: bulge - Galaxy: halo - galaxies: dwarf -
Magellanic Clouds
Abstract:
The most metal-poor stars in our Galaxy contain important clues of its
earliest history, particularly those occupying the inner regions of
the Galaxy. In the search for such metal-poor stars, large
spectroscopic surveys are an invaluable tool. However, the spectra of
metal-poor stars can be difficult to analyse because of the relative
lack of available lines, which can also lead to misclassification.
We aim to identify the stars observed by the APOGEE survey that are
below the metallicity limit of APOGEE's analysis. For the highest
confidence candidates, we will classify the orbital properties of the
stars to investigate whether their orbital distribution matches what
we would expect for stars that are this metal-poor, and to select
especially interesting targets for spectroscopic follow-up purposes.
We examine the properties derived by APOGEE for metal-poor stars from
the literature to find signatures of stars with a metallicity below
the range of the grid used for spectral analysis. Once identified
within APOGEE, we compute the orbits of our metal-poor candidates
using AGAMA.
The calibrated APOGEE stellar parameters provide a clear signature of
the most metal-poor stars in the survey, indicated by null values for
their metallicities while having effective temperatures and surface
gravities determined by the pipeline. From comparison with the
literature, we find that, within a temperature range of 3700 - 6700 K
and above a threshold of SNR>30, the vast majority of APOGEE stars
without calibrated metallicities are very metal-poor. Additional
cleaning by visual inspection of the spectra improves the purity of
the sample further. The radial velocities provided by APOGEE, Gaia DR3
positions and astrometry along with spectrophotometric distances
derived in this work, allow for the computation of their orbits. In
this work, we carefully select 289 very metal-poor red giant stars
(likely below [Fe/H]=-2.5) from the APOGEE results. Our sample
contains 16 very metal-poor member candidates of the Magellanic Clouds
as well as 14 very metal-poor stars with orbits confined to the inner
Galaxy, and 13 inner Galaxy halo interlopers. These samples
significantly add to the very metal-poor stars known in these regions
and allow for a more detailed picture of early chemical evolution
across different environments.
Description:
We present three tables of targets of interest based on the selection
done in this paper. All stars presented are part of our high
confidence selection, likely to have metallicities [Fe/H]←2.5. The
first table contains stars we link to the Magellanic Clouds, while the
second and third tables are of stars in the Inner Galaxy
(RGC<3.5kpc), split into the Inner sample (Apocentre<5kpc) and Outer
sample (Apocentre>5kpc).
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablec1.dat 99 16 Observational data for Magellanic Cloud stars
tabled1.dat 108 14 Observational data for Inner sample stars
tabled2.dat 108 13 Observational data for Outer sample stars
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See also:
III/286 : APOGEE-2 DR17 final allStar catalog (Abdurro'uf+, 2022)
Byte-by-byte Description of file: tablec1.dat
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Bytes Format Units Label Explanations
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1- 19 I19 --- GaiaDR3 Gaia DR3 source_id
21- 28 F8.4 deg RAdeg Gaia DR3 right ascension (J2000)
30- 37 F8.4 deg DEdeg Gaia DR3 declination (J2000)
39- 43 A5 --- Field APOGEE field
45- 48 F4.2 mas/yr pmRA Proper motion in RA
50- 54 F5.3 mas/yr e_pmRA Error on pmRA
56- 60 F5.2 mas/yr pmDE Proper motion in DE
62- 66 F5.3 mas/yr e_pmDE Error on pmDE
68- 72 F5.2 kpc Dist Distance from this work
74- 77 F4.1 kpc e_Dist Error on distance
79- 84 F6.1 K Teff APOGEE effective temperature
86- 89 F4.1 K e_Teff APOGEE error on TefF
91- 93 F3.1 [cm/s2] logg APOGEE log of surface gravity
95- 97 F3.1 [cm/s2] e_logg APOGEE error on logg
99 A1 --- Lit [o] Flag for stars with literature
metallicity
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Byte-by-byte Description of file: tabled1.dat tabled2.dat
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Bytes Format Units Label Explanations
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1- 2 A2 --- Code Designation in paper
4- 22 I19 --- GaiaDR3 Gaia DR3 source_id
24- 31 F8.4 deg RAdeg Gaia DR3 right ascension (J2000)
33- 40 F8.4 deg DEdeg Gaia DR3 declination (J2000)
42- 46 F5.2 kpc Dist Distance from this work
48- 50 F3.1 kpc e_Dist Error on distance
52- 57 F6.1 K Teff APOGEE effective temperature
59- 62 F4.1 K e_Teff APOGEE error on Teff
64- 66 F3.1 [cm/s2] logg APOGEE log of surface gravity
68- 70 F3.1 [cm/s2] e_logg APOGEE error on logg
72- 75 F4.2 kpc RGC Galactocentric radius
77- 81 F5.2 kpc medapo Median apocentre distance
83- 87 F5.2 kpc zmax Maximum distance in z
89- 98 F10.2 kpc.km/s2 E Orbital Energy
100-106 F7.2 kpc.km/s medLz Median z component of angular momentum
108 A1 --- Lit [*p] Flag for stars with literature
metallicity (1)
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Note (1): Flag as follows:
* = stars observed by Ardern-Arentsen et al. (2024MNRAS.530.3391A 2024MNRAS.530.3391A,
Cat. J/MNRAS/530/3391)
p = stars observed by Martin et al. (2024A&A...692A.115M 2024A&A...692A.115M, Cat. J/A+A/692/A115)
or Andrae et al. (2023ApJS..267....8A 2023ApJS..267....8A)
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Acknowledgements:
Martin Montelius, montelius(at)astro.rug.nl
(End) Patricia Vannier [CDS] 01-Oct-2025