J/A+A/707/A111 Oldest Milky Way stars (Tomasetti+, 2026)
The oldest Milky Way stars:
New constraints on the age of the Universe and the Hubble constant.
Tomasetti E., Chiappini C., Nepal S., Moresco M., Lardo C., Cimatti A.,
Anders F., Queiroz A.B.A., Limberg G.
<Astron. Astrophys. 707, A111 (2026)>
=2026A&A...707A.111T 2026A&A...707A.111T (SIMBAD/NED BibCode)
ADC_Keywords: Milky Way ; Stars, ages ; Stars, masses ; Optical
Keywords: stars: fundamental parameters - cosmological parameters -
cosmology: observations
Abstract:
In this work, we exploit the most robust, old, and
cosmology-independent age estimates of individual stars from Gaia DR3
to place a lower bound on the age of the Universe, t_U. These
constraints can be used as an anchor point for any cosmological model,
providing an upper limit to the Hubble constant H0.
We consider the stellar age catalog of Nepal et al.
(2024A&A...688A.167N 2024A&A...688A.167N), selecting 3000 of the oldest and most robustly
measured main sequence turn-off (MSTO) and subgiant branch (SGB)
stars, with ages older than 12.5Gyr and associated uncertainty below
1Gyr. Stellar ages are derived via isochrone fitting using the
Bayesian code StarHorse, spanning the uniform range 0-20Gyr, not
considering any cosmological prior knowledge on tU. Applying a
conservative cut in the Kiel diagram, strict quality cuts on both
stellar parameters and posterior probability distribution shapes, and
filtering out potential contaminants, we isolate a final sample of 160
bona-fide stars, representing the most numerous sample of precise and
reliable MSTO and SGB stars ages available to date.
The age distribution of the final sample peaks at
13.6±1.0(stat)±1.4(syst)Gyr. Assuming a maximum formation redshift
for these stars of zf=20, corresponding to a formation delay of
∼0.2Gyr, we obtain a lower bound on tU of tU≥13.8±1.0 (stat)
± 1.4(syst)Gyr. Considering the 10th percentile of the posterior
probability distributions of the individual stars, we find that, at
90% CL, 70 stars favour tU>13Gyr, while none exceeds 14.1Gyr. For
this upper envelope to fall below 13Gyr, a shift of nearly the full
systematic error budget would be required, indicating that such low
values are only attainable under very peculiar assumptions. This work
presents the first statistically significant use of individual stellar
ages as cosmic clocks, opening a new, independent approach for
cosmological studies. While this analysis represents already a
significant step forward, future Gaia data releases will enable even
larger and more precise stellar samples, further strengthening these
constraints.
Description:
Source ID from Gaia DR3, age, age error, mass, mass error.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
tabled1.dat 44 185 Star ID, age and mass
--------------------------------------------------------------------------------
See also:
I/355 : Gaia DR3 Part 1. Main source (Gaia Collaboration, 2022)
I/354 : StarHorse2, Gaia EDR3 photo-astrometric distances (Anders+, 2022)
Byte-by-byte Description of file: tabled1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 19 I19 --- GaiaDR3 Gaia DR3 source id
21- 26 F6.3 Gyr Age [12.53/17.38] 50th percentile of marginalized
StarHorse age PDF
28- 32 F5.3 Gyr e_Age [0.46/0.99] 0.5*(84th-16th) percentile of
marginalized StarHorse age PDF
34- 38 F5.3 Msun Mass [0.76/0.98] 50th percentile of marginalized
StarHorse mass PDF
40- 44 F5.3 Msun e_Mass [0.01/0.03] 0.5*(84th-16th) percentile of
marginalized StarHorse mass PDF
--------------------------------------------------------------------------------
Acknowledgements:
Elena Tomasetti, elena.tomasetti2(at)unibo.it
(End) Patricia Vannier [CDS] 16-Jan-2026