778 research outputs found
The spectroscopic indistinguishability of red giant branch and red clump stars
Stellar spectroscopy provides useful information on the physical properties
of stars such as effective temperature, metallicity and surface gravity (log
g). However, those photospheric characteristics are often hampered by
systematic uncertainties. The joint spectro-seismo project (APOKASC) of field
red giants has revealed a puzzling offset between the log g determined
spectroscopically and those determined using asteroseismology, which is largely
dependent on the stellar evolutionary status. Therefore, in this letter, we aim
to shed light on the spectroscopic source of the offset using the APOKASC
sample. We analyse the log g discrepancy as a function of stellar mass and
evolutionary status and discuss the impact of He and carbon isotopic ratio.
We first show that for stars at the bottom of the red giant branch, the
discrepancy between spectroscopic and asteroseismic log g depends on stellar
mass. This indicates that the discrepancy is related to CN cycling. We
demonstrate that the C isotopic ratio () has the largest
impact on the stellar spectrum. We find that this log g discrepancy shows a
similar trend in mass as the ratios expected by stellar
evolution theory. Although we do not detect a direct signature of ,
the data suggests that the discrepancy is tightly correlated to the production
of . Moreover, by running a data-driven algorithm (the Cannon) on a
synthetic grid trained on the APOGEE data, we quantitatively evaluate the
impact of various ratios. While we have demonstrated that
impacts all parameters, the size of the impact is smaller than the
observed offset in log g. If further tests confirm that is not the
main element responsible of the log g problem, the number of spectroscopic
effects remaining to be investigated is now relatively limited. [Abridged]Comment: 4 Pages, 6 Figures. Accepted for publication in A&
Ages and kinematics of chemically selected, accreted Milky Way halo stars
We exploit the [Mg/Mn]-[Al/Fe] chemical abundance plane to help identify
nearby halo stars in the 14th data release from the APOGEE survey that have
been accreted on to the Milky Way. Applying a Gaussian Mixture Model, we find a
`blob' of 856 likely accreted stars, with a low disc contamination rate of ~7%.
Cross-matching the sample with the second data release from Gaia gives us
access to parallaxes and apparent magnitudes, which place constraints on
distances and intrinsic luminosities. Using a Bayesian isochrone pipeline, this
enables us to estimate new ages for the accreted stars, with typical
uncertainties of ~20%. Our new catalogue is further supplemented with estimates
of orbital parameters.
The blob stars span a metallicities between -0.5 to -2.5, and [Mg/Fe] between
-0.1 to 0.5. They constitute ~30% of the metal-poor ([Fe/H] < -0.8) halo at
metallicities of ~-1.4. Our new ages are mainly range between 8 to 13 Gyr, with
the oldest stars the metal-poorest, and with the highest [Mg/Fe] abundance. If
the blob stars are assumed to belong to a single progenitor, the ages imply
that the system merged with our Milky Way around 8 Gyr ago and that star
formation proceeded for ~5 Gyr. Dynamical arguments suggest that such a single
progenitor would have a total mass of ~1011Msun, similar to that found by other
authors using chemical evolution models and simulations. Comparing the scatter
in the [Mg/Fe]-[Fe/H] plane of the blob stars to that measured for stars
belonging to the Large Magellanic Cloud suggests that the blob does indeed
contain stars from only one progenitor.Comment: 14 pages, 9 figures, 2 tables, submitted to MNRAS. Comments welcome
Chemical Cartography with LAMOST and Gaia Reveal Azimuthal and Spiral Structure in the Galactic Disk
Chemical Cartography, or mapping, of our Galaxy has the potential to fully
transform our view of its structure and formation. In this work, we use
chemical cartography to explore the metallicity distribution of OBAF-type disk
stars from the LAMOST survey and a complementary sample of disk giant stars
from Gaia DR3. We use these samples to constrain the radial and vertical
metallicity gradients across the Galactic disk. We also explore whether there
are detectable azimuthal variations in the metallicity distribution on top of
the radial gradient. For the OBAF-type star sample from LAMOST, we find a
radial metallicity gradient of [Fe/H]/R
dex/kpc in the plane of the disk and a vertical metallicity gradient of
[Fe/H]/Z dex/kpc in the solar
neighborhood. The radial gradient becomes shallower with increasing vertical
height while the vertical gradient becomes shallower with increasing
Galactocentric radius, consistent with other studies. We also find detectable
spatially-dependent azimuthal variations on top of the radial metallicity
gradient at the level of 0.10 dex. Interestingly, the azimuthal
variations appear be close to the Galactic spiral arms in one dataset (Gaia
DR3) but not the other (LAMOST). These results suggest that there is azimuthal
structure in the Galactic metallicity distribution and that in some cases it is
co-located with spiral arms.Comment: 14 pages, 8 Figures, 2 Tables, accepted for publication in MNRA
From the Inner to Outer Milky Way: A Photometric Sample of 2.6 Million Red Clump Stars
Large pristine samples of red clump stars are highly sought after given that
they are standard candles and give precise distances even at large distances.
However, it is difficult to cleanly select red clumps stars because they can
have the same T and log as red giant branch stars.
Recently, it was shown that the asteroseismic parameters, P and
, which are used to accurately select red clump stars, can be
derived from spectra using the change in the surface carbon to nitrogen ratio
([C/N]) caused by mixing during the red giant branch. This change in [C/N] can
also impact the spectral energy distribution. In this study, we predict the
P, , T and log using 2MASS,
AllWISE, \gaia, and Pan-STARRS data in order to select a clean sample of red
clump stars. We achieve a contamination rate of 20\%, equivalent to what
is achieved when selecting from T and log derived from low
resolution spectra. Finally, we present two red clump samples. One sample has a
contamination rate of 20\% and 405,000 red clump stars. The other
has a contamination of 33\% and 2.6 million red clump stars which
includes 75,000 stars at distances 10 kpc. For |b|>30 degrees we
find 15,000 stars with contamination rate of 9\%. The scientific
potential of this catalog for studying the structure and formation history of
the Galaxy is vast given that it includes millions of precise distances to
stars in the inner bulge and distant halo where astrometric distances are
imprecise.Comment: 18 pages, 13 figures, 2 tables, submitted to MNRA
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