J/A+A/707/A308 The metal-poor tail of the APOGEE survey. II. (Montelius+, 2026)
The metal-poor tail of the APOGEE survey.
II. Spectral analysis of Mg and Si in very metal-poor APOGEE spectra.
Montelius M., Angrilli Muglia A., Starkenburg E., Kobayashi C.,
Ardern-Arentsen A., Gran F., Jablonka P., Martin N., Navarro J., Sestito F.,
Venn K.A., Vitali S.
<Astron. Astrophys. 707, A308 (2026)>
=2026A&A...707A.308M 2026A&A...707A.308M (SIMBAD/NED BibCode)
ADC_Keywords: Milky Way ; Magellanic Clouds ; Spectra, infrared ;
Stars, metal-deficient
Keywords: stars: abundances - stars: chemically peculiar - Galaxy: halo -
galaxies: dwarf - infrared: stars
Abstract:
Very metal-poor stars are important tracers of the early chemical
evolution history of the Milky Way. Infrared H-band spectroscopic
surveys, such as APOGEE, are less affected by extinction in the more
dust-obscured environments of our Galaxy. However, H-band spectra
contain very limited spectral information for stars at the most
metal-poor tail ([Fe/H]←2.5) due to the weakness of available Fe
lines in FGK stars in this wavelength range.
The first paper in this series successfully identified a sample of 327
very metal-poor stars (with [Fe/H]←2) from the APOGEE database, out
of which 289 stars are on the red giant branch. These spectra were not
properly analysed by the APOGEE main pipeline due to their very
metal-poor nature. In this work, we measure metallicities for these
stars, using the abundances of the elements Mg and Si. We demonstrate
that the elements Mg and Si have good quality absorption lines despite
the challenging combination of (low) metallicity, wavelength regime,
spectral resolution, and signal-to-noise ratios available for these
spectra. A specialized pipeline is designed to measure the abundance
of Mg and Si in APOGEE spectra, which yields a robust estimate of the
overall metallicity. In order to provide reliable measurements, we
test 3 different sets of assumptions for Mg and Si enhancement.
We present Mg and Si abundances as well as overall metallicities for
327 stars, all of which had previously gotten null values from the
main APOGEE pipeline for either the calibrated [M/H] or [Fe/H].
Typical uncertainties for our measurements are 0.2dex. We find in our
sample 5 stars with unusually high Si/Mg abundances above 0.5, a
signature we connect to globular cluster stars, and that might be
related to specific supernova events. Our data suggest a concentration
of high Si/Mg stars in the Sextans dwarf galaxy. Other dwarf galaxies
are found to be in good agreement with results found in the
literature.
Our derived metallicities range between -3.1≤[M/H]≤-2.25, thereby
pushing the metal-poor tail of APOGEE results down by 0.6dex.
Description:
We present observational data for our targets along with abundances
from our spectroscopic analysis. The stellar parameters from APOGEE
that were used in the analysis are also included. Flags for the
results of the visual inspection; the division into galactic regions,
dwarf galaxies, and globular clusters; and for stars with known
metallicity, are also provided.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 260 427 Observational data, stellar parameters and
abundances
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See also:
I/355 : Gaia DR3 Part 1. Main source (Gaia Collaboration, 2022)
III/286 : APOGEE-2 DR17 final allStar catalog (Abdurro'uf+, 2022)
Byte-by-byte Description of file: tablea1.dat
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Bytes Format Units Label Explanations
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1- 19 I19 --- GaiaDR3 Gaia DR3 source_id (source_id)
21- 33 F13.9 deg RAdeg APOGEE right ascensions (J2000) (RA)
35- 49 F15.11 deg DEdeg APOGEE declination (J2000) (DEC)
51- 59 F9.4 K Teff APOGEE effective temperature (TEFF)
61- 71 F11.7 K e_Teff APOGEE effective temperature uncertainty
(TEFF_ERR)
73- 82 F10.8 [cm/s2] logg APOGEE surface gravity (LOGG)
84- 94 F11.9 [cm/s2] e_logg APOGEE surface gravity uncertainty (LOGG_ERR)
96 I1 --- Flag [0-3] Visual inspection flag (Flag)
98 I1 --- Sample [0/2] Confidence sample flag (Sample)
100-117 F18.14 --- S/N ? APOGEE signal-to-noise ratio (S/N)
119-137 F19.16 [-] [M/H] ? Weighted metallicity (M_H)
139-156 F18.16 [-] e_[M/H] ? Weighted metallicity uncertainty (MHunc)
158-163 F6.3 [-] [Mg/H] ? Measured magnesium abundance (Mg_H)
165-182 F18.16 [-] e_[Mg/H] ? Lower magnesium uncertainty (MgHlower)
184-201 F18.16 [-] E_[Mg/H] ? Upper magnesium uncertainty (MgHupper)
203-208 F6.3 [-] [Si/H] ? Measured sillicon abundance (Si_H)
210-227 F18.16 [-] e_[Si/H] ? Lower silicon uncertainty (SiHlower)
229-246 F18.16 [-] E_[Si/H] ? Upper silicon uncertainty (SiHupper)
248 A1 --- HasSpec [*] Has literature metallicity flag
(has_spec)
250-258 A9 --- Region Kinematic region, dwarf galaxy, or
globular cluster (region)
260 A1 --- EmpCand [*] Extremely metal-poor star candidate flag
(emp_cand)
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Acknowledgements:
Martin Montelius, montelius(at)astro.rug.nl
(End) Patricia Vannier [CDS] 10-Feb-2026