95 research outputs found
Investigating APOKASC Red Giant Stars with Abnormal Carbon to Nitrogen Ratios
The success of galactic archaeology and the reconstruction of the formation
history of our galaxy critically relies on precise ages for large populations
of stars. For evolved stars in the red clump and red giant branch, the carbon
to nitrogen ratio ([C/N]) has recently been identified as a powerful diagnostic
of mass and age that can be applied to stellar samples from spectroscopic
surveys such as SDSS/APOGEE. Here, we show that at least 10\% of red clump
stars and % of red giant branch stars deviate from the standard
relationship between [C/N] and mass. {We use the APOGEE-\kepler\ (APOKASC)
overlap sample to show that binary interactions are %the majority contributors
to these responsible for the majority of these outliers and that stars with
%any indicators of current or previous binarity should be excluded from
galactic archaeology analyses that rely on [C/N] abundances to infer stellar
masses. We also show that the %standard DR14 APOGEE analysis overestimates the
surface gravities for even moderately rotating giants (vsini km/s)}Comment: Accepted at the Astrophysical Journal, in process of publicatio
Chemical tagging can work: Identification of stellar phase-space structures purely by chemical-abundance similarity
Chemical tagging promises to use detailed abundance measurements to identify
spatially separated stars that were in fact born together (in the same
molecular cloud), long ago. This idea has not yielded much practical success,
presumably because of the noise and incompleteness in chemical-abundance
measurements. We have succeeded in substantially improving spectroscopic
measurements with The Cannon, which has now delivered 15 individual abundances
for ~100,000 stars observed as part of the APOGEE spectroscopic survey, with
precisions around 0.04 dex. We test the chemical-tagging hypothesis by looking
at clusters in abundance space and confirming that they are clustered in phase
space. We identify (by the k-means algorithm) overdensities of stars in the
15-dimensional chemical-abundance space delivered by The Cannon, and plot the
associated stars in phase space. We use only abundance-space information (no
positional information) to identify stellar groups. We find that clusters in
abundance space are indeed clusters in phase space. We recover some known
phase-space clusters and find other interesting structures. This is the
first-ever project to identify phase-space structures at survey-scale by blind
search purely in abundance space; it verifies the precision of the abundance
measurements delivered by The Cannon; the prospects for future data sets appear
very good.Comment: accepted for publication in the Ap
The Chemodynamics of the Stellar Populations in M31 from APOGEE Integrated Light Spectroscopy
We present analysis of nearly 1,000 near-infrared, integrated light spectra
from APOGEE in the inner 7 kpc of M31. We utilize full spectrum fitting
with A-LIST simple stellar population spectral templates that represent a
population of stars with the same age, [M/H], and [/M]. With this, we
determine the mean kinematics, metallicities, abundances, and ages of
the stellar populations of M31's bar, bulge, and inner disk (4-7 kpc). We
find a non-axisymmetric velocity field in M31 resulting from the presence of a
bar. The bulge of M31 is metal-poor relative to the disk ([M/H] =
dex), features minima in metallicity on either side
of the bar ([M/H] -0.2), and is enhanced in abundance
([/M] = ). The disk of M31 within 7 kpc
is enhanced in both metallicity ([M/H] = ) and
abundance ([/M] = ). Both of these
structural components are uniformly old at 12 Gyr. We find the
metallicity increases with distance from the center of M31, with the steepest
gradient along the disk major axis ( dex/kpc). This gradient is
the result of changing light contributions from the metal-poor bulge and
metal-rich disk. The chemodynamics of stellar populations encodes information
about a galaxy's chemical enrichment, star formation history, and merger
history, allowing us to discuss new constraints on M31's formation. Our results
provide a stepping stone between our understanding of the Milky Way and other
external galaxies
The chemical properties of the Milky Way's on-bar and off-bar regions: evidence for inhomogeneous star formation history in the bulge
Numerous studies of integrated starlight, stellar counts, and kinematics have
confirmed that the Milky Way is a barred galaxy. However, far fewer studies
have investigated the bar's stellar population properties, which carry valuable
independent information regarding the bar's formation history. Here we conduct
a detailed analysis of chemical abundance distributions ([Fe/H] and [Mg/Fe]) in
the on-bar and off-bar regions to study the azimuthal variation of star
formation history (SFH) in the inner Galaxy. We find that the on-bar and
off-bar stars at Galactocentric radii 3 5 kpc have remarkably
consistent [Fe/H] and [Mg/Fe] distribution functions and [Mg/Fe]--[Fe/H]
relation, suggesting a common SFH shared by the long bar and the disc. In
contrast, the bar and disc at smaller radii (2 3 kpc) show
noticeable differences, with relatively more very metal-rich ([Fe/H]~0.4) stars
but fewer solar abundance stars in the bar. Given the three-phase star
formation history proposed for the inner Galaxy in Lian et al. (2020b), these
differences could be explained by the off-bar disc having experienced either a
faster early quenching process or recent metal-poor gas accretion. Vertical
variations of the abundance distributions at small suggest a wider
vertical distribution of low- stars in the bar, which may serve as
chemical evidence for vertical heating through the bar buckling process. The
lack of such vertical variations outside the bulge may then suggest a lack of
vertical heating in the long bar.Comment: 10 pages, 5 figures. MNRAS in pres
The Milky Way's bulge star formation history as constrained from its bimodal chemical abundance distribution
We conduct a quantitative analysis of the star formation history (SFH) of the
Milky Way's bulge by exploiting the constraining power of its stellar [Fe/H]
and [Mg/Fe] distribution functions. Using APOGEE data, we confirm the
previously-established bimodal [Mg/Fe]--[Fe/H] distribution within 3 kpc of the
inner Galaxy. Compared to that in the solar vicinity, the high-
population in the bulge peaks at a lower [Fe/H]. To fit these observations, we
use a simple but flexible star formation framework, which assumes two distinct
stages of gas accretion and star formation, and systematically evaluate a wide
multi-dimensional parameter space. We find that the data favor a three-phase
SFH that consists of an initial starburst, followed by a rapid star formation
quenching episode and a lengthy, quiescent secular evolution phase. The
metal-poor, high- bulge stars ([Fe/H]0.15) are formed
rapidly (<2 Gyr) during the early starburst. The density gap between the high-
and low- sequences is due to the quenching process. The metal-rich,
low- population ([Fe/H]>0.0 and [Mg/Fe]<0.15) then accumulates
gradually through inefficient star formation during the secular phase. This is
qualitatively consistent with the early SFH of the inner disk. Given this
scenario, a notable fraction of young stars (age<5 Gyr) is expected to persist
in the bulge. Combined with extragalactic observations, these results suggest
that a rapid star formation quenching process is responsible for bimodal
distributions in both the Milky Way's stellar populations and in the general
galaxy population and thus plays a critical role in galaxy evolution.Comment: 16 pages, 12 figures. MNRAS in pres
High-resolution, H band Spectroscopy of Be Stars with SDSS-III/APOGEE: I. New Be Stars, Line Identifications, and Line Profiles
APOGEE has amassed the largest ever collection of multi-epoch,
high-resolution (R~22,500), H-band spectra for B-type emission line (Be) stars.
The 128/238 APOGEE Be stars for which emission had never previously been
reported serve to increase the total number of known Be stars by ~6%. We focus
on identification of the H-band lines and analysis of the emission peak
velocity separations (v_p) and emission peak intensity ratios (V/R) of the
usually double-peaked H I and non-hydrogen emission lines. H I Br11 emission is
found to preferentially form in the circumstellar disks at an average distance
of ~2.2 stellar radii. Increasing v_p toward the weaker Br12--Br20 lines
suggests these lines are formed interior to Br11. By contrast, the observed IR
Fe II emission lines present evidence of having significantly larger formation
radii; distinctive phase lags between IR Fe II and H I Brackett emission lines
further supports that these species arise from different radii in Be disks.
Several emission lines have been identified for the first time including
~16895, a prominent feature in the spectra for almost a fifth of the sample
and, as inferred from relatively large v_p compared to the Br11-Br20, a tracer
of the inner regions of Be disks. Unlike the typical metallic lines observed
for Be stars in the optical, the H-band metallic lines, such as Fe II 16878,
never exhibit any evidence of shell absorption, even when the H I lines are
clearly shell-dominated. The first known example of a quasi-triple-peaked Br11
line profile is reported for HD 253659, one of several stars exhibiting intra-
and/or extra-species V/R and radial velocity variation within individual
spectra. Br11 profiles are presented for all discussed stars, as are full
APOGEE spectra for a portion of the sample.Comment: accepted in A
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