We measure the circular velocity curve vc(R) of the Milky Way with the highest precision to date across Galactocentric distances of 5 ≤ R ≤ 25 kpc. Our analysis draws on the six-dimensional ...phase-space coordinates of 23,000 luminous red giant stars, for which we previously determined precise parallaxes using a data-driven model that combines spectral data from APOGEE with photometric information from WISE, 2MASS, and Gaia. We derive the circular velocity curve with the Jeans equation assuming an axisymmetric gravitational potential. At the location of the Sun we determine the circular velocity with its formal uncertainty to be = with systematic uncertainties at the ∼2%-5% level. We find that the velocity curve is gently but significantly declining at (−1.7 0.1) km s−1 kpc−1, with a systematic uncertainty of 0.46 km s−1 kpc−1, beyond the inner 5 kpc. We exclude the inner 5 kpc from our analysis due to the presence of the Galactic bar, which strongly influences the kinematic structure and requires modeling in a nonaxisymmetric potential. Combining our results with external measurements of the mass distribution for the baryonic components of the Milky Way from other studies, we estimate the Galaxy's dark halo mass within the virial radius to be = and a local dark matter density of = .
Abstract
Chemical abundances of Milky Way disk stars are empirical tracers of its enrichment history. However, they capture joint-information that is valuable to disentangle. In this work, we ...quantify how individual abundances evolve across the present-day Galactic radius, at fixed supernovae contribution (Fe/H, Mg/Fe). We use 18,135 Apache Point Observatory Galactic Evolution Experiment Data Release 17 red clump stars and 7943 GALactic Archaeology with HERMES Data Release 3 main-sequence stars to compare the abundance distributions conditioned on (Fe/H, Mg/Fe) across 3–13 kpc and 6.5–9.5 kpc, respectively. We examine 15 elements: C, N, Al, K (light), O, Si, S, Ca, (
α
), Mn, Ni, Cr, Cu, (iron-peak) Ce, Ba (
s
-process) and Eu (
r
-process). We find that the conditional neutron-capture and light elements most significantly trace variations in the disk’s enrichment history, with absolute conditional radial gradients ≤0.03 dex kpc
−1
. The other elements studied have absolute conditional gradients ≲0.01 dex kpc
−1
. We uncover structured conditional abundance variations with Fe/H for the low-
α
, but not the high-
α
,
sequence. The average scatter between the mean conditional abundances at different radii is
σ
intrinsic
≈ 0.02 dex (Ce, Eu, Ba
σ
intrinsic
> 0.05 dex). These results serve as a measure of the magnitude via which different elements trace Galactic radial enrichment history once fiducial supernovae correlations are accounted for. Furthermore, we uncover subtle systematic variations in moments of the conditional abundance distributions and bimodal differences in Al/Fe. These suggest a nonuniform enrichment of each chemical cell, and will presumably constrain chemical evolution models of the Galaxy.
Abstract
Stellar streams from globular clusters (GCs) offer constraints on the nature of dark matter and have been used to explore the dark matter halo structure and substructure of our Galaxy. ...Detection of GC streams in other galaxies would broaden this endeavor to a cosmological context, yet no such streams have been detected to date. To enable such exploration, we develop the
Hough Stream Spotter
(
HSS
), and apply it to the Pan-Andromeda Archaeological Survey (PAndAS) photometric data of resolved stars in M31's stellar halo. We first demonstrate that our code can re-discover known dwarf streams in M31. We then use the
HSS
to blindly identify 27 linear GC stream-like structures in the PAndAS data. For each
HSS
GC stream candidate, we investigate the morphologies of the streams and the colors and magnitudes of all stars in the candidate streams. We find that the five most significant detections show a stronger signal along the red giant branch in color–magnitude diagrams than spurious non-stream detections. Lastly, we demonstrate that the
HSS
will easily detect globular cluster streams in future Nancy Grace Roman Space Telescope data of nearby galaxies. This has the potential to open up a new discovery space for GC stream studies, GC stream gap searches, and for GC stream-based constraints on the nature of dark matter.
We show for the first time that a fully cosmological hydrodynamical simulation can reproduce key properties of the innermost region of the Milky Way (MW). Our high-resolution simulation reproduces ...qualitatively the profile and kinematics of the MW's boxy/peanut-shaped bulge, and hence we can use it to reconstruct and understand the bulge assembly. In particular, the age dependence of the X-shape morphology of the simulated bulge parallels the observed metallicity-dependent split in the red clump stars of the inner Galaxy. We use this feature to propose an observational metric that (after calibrated against a larger set of simulations) might allow us to quantify when the bulge formed from the disk. The metric we propose can be employed with upcoming survey data to constrain the age of the MW bar. From the split in stellar counts we estimate the formation of the 4 kpc scale bar in the simulation to have happened Gyr ago, in good agreement with conventional methods to measure bar formation in simulations. We test the prospects for observationally differentiating the stars that belong to the bulge/bar compared to the surrounding disk, and we find that the inner disk and bulge are practically indistinguishable in both chemistry and ages.
Abstract
Planetary engulfment events have long been proposed as a lithium (Li) enrichment mechanism contributing to the population of Li-rich giants (
A
(Li) ≥ 1.5 dex). Using MESA stellar models and
...A
(Li) abundance measurements obtained by the GALAH survey, we calculate the strength and observability of the surface Li enrichment signature produced by the engulfment of a hot Jupiter (HJ). We consider solar-metallicity stars in the mass range of 1–2
M
⊙
and the Li supplied by a HJ of 1.0
M
J
. We explore engulfment events that occur near the main-sequence turn-off (MSTO) and out to orbital separations of
R
⋆
∼ 0.1 au = 22
R
⊙
. We map our results onto the Hertzsprung–Russell Diagram, revealing the statistical significance and survival time of Li enrichment. We identify the parameter space of masses and evolutionary phases where the engulfment of a HJ can lead to Li enrichment signatures at a 5
σ
confidence level and with meteoritic abundance strengths. The most compelling strengths and survival times of engulfment-derived Li enrichment are found among host stars of 1.4
M
⊙
near the MSTO. Our calculations indicate that planetary engulfment is not a viable enrichment pathway for stars that have evolved beyond the subgiant branch. For these sources, observed Li enhancements are likely to be produced by other mechanisms, such as the Cameron–Fowler process or the accretion of material from an asymptotic giant branch companion. Our results do not account for second-order effects, such as extra mixing processes, which can further dilute Li enrichment signatures.
The orbital properties of stars in the Milky Way disk are signatures of their formation, but they are also expected to change over time due to the dynamical evolution of the Galaxy. Stellar orbits ...can be quantified by the three dynamical actions, Jr, Lz, and Jz, which provide measures of the orbital eccentricity, guiding radius, and non-planarity, respectively. Changes in these dynamical actions over time reflect the strength and efficiency of the evolutionary processes that drive stellar redistributions. We examine how dynamical actions of stars are correlated with their age using two samples of stars with well-determined ages: 78 solar twin stars (with ages precise to ∼5%) and 4376 stars from the APOKASC2 sample (∼20%). We compute actions using spectroscopic radial velocities from previous surveys and parallax and proper motion measurements from Gaia DR2. We find weak gradients with significant scatter for all actions as a function of stellar age. These gradients and their associated variances provide strong constraints on the efficiency of the mechanisms that drive the redistribution of stellar orbits over time and demonstrate that actions are informative as to stellar age. However, the shallow action-age gradients combined with the large dispersion in each action at a given age render the prospect of age inference from orbits of individual stars bleak. Using the precision measurements of Fe/H and /Fe we find that, similarly to our stellar age results, the dynamical actions afford little discriminating power between individual low- and high- stars.
Abstract
Identification of chemically similar stars using elemental abundances is core to many pursuits within Galactic archeology. However, measuring the chemical likeness of stars using abundances ...directly is limited by systematic imprints of imperfect synthetic spectra in abundance derivation. We present a novel data-driven model that is capable of identifying chemically similar stars from spectra alone. We call this relevant scaled component analysis (RSCA). RSCA finds a mapping from stellar spectra to a representation that optimizes recovery of known open clusters. By design, RSCA amplifies factors of chemical abundance variation and minimizes those of nonchemical parameters, such as instrument systematics. The resultant representation of stellar spectra can therefore be used for precise measurements of chemical similarity between stars. We validate RSCA using 185 cluster stars in 22 open clusters in the Apache Point Observatory Galactic Evolution Experiment survey. We quantify our performance in measuring chemical similarity using a reference set of 151,145 field stars. We find that our representation identifies known stellar siblings more effectively than stellar-abundance measurements. Using RSCA, 1.8% of pairs of field stars are as similar as birth siblings, compared to 2.3% when using stellar-abundance labels. We find that almost all of the information within spectra leveraged by RSCA fits into a two-dimensional basis, which we link to Fe/H and
α
-element abundances. We conclude that chemical tagging of stars to their birth clusters remains prohibitive. However, using the spectra has noticeable gain, and our approach is poised to benefit from larger data sets and improved algorithm designs.
Abstract
The elemental abundances of planet host stars can shed light on the conditions of planet forming environments. We test if individual abundances of 130 known/candidate planet hosts in APOGEE ...are statistically different from those of a reference doppelgänger sample. The reference set comprises objects selected with the same
T
eff
,
log
g
, Fe/H, and Mg/H as each Kepler Object of Interest (KOI). We predict twelve individual abundances (X = C, N, O, Na, Al, Si, Ca, Ti, V, Cr, Mn, Ni) for the KOIs and their doppelgängers using a local linear model of these four parameters, training on ASPCAP abundance measurements for a sample of field stars with high-fidelity (signal-to-noise ratio > 200) APOGEE observations. We compare element prediction residuals (model–measurement) for the two samples and find them to be indistinguishable, given a high-quality sample selection. We report median intrinsic dispersions of ∼0.038 dex and ∼0.041 dex, for the KOI and doppelgänger samples, respectively, for these elements. We conclude that the individual abundances at fixed
T
eff
,
log
g
, Fe/H, and Mg/H are unremarkable for known planet hosts. Our results establish an upper limit on the abundance precision required to uncover any chemical signatures of planet formation in planet host stars.
ABSTRACT
Individual abundances in the Milky Way disc record stellar birth properties e.g. age, birth radius (Rbirth), and capture the diversity of the star-forming environments over time. Assuming an ...analytical relationship between (Fe/H, α/Fe), and Rbirth, we examine the distributions of individual abundances X/Fe of elements C, O, Mg, Si, Ca (α), Al (odd-z), Mn (iron-peak), Y, and Ba (neutron-capture) for stars in the Milky Way. We want to understand how these elements might differentiate environments across the disc. We assign tracks of Rbirth in the α/Fe versus Fe/H plane as informed by expectations from simulations for ∼59 000 GALAH stars in the solar neighborhood (R ∼ 7−9 kpc) which also have inferred ages. Our formalism for Rbirth shows that older stars (∼10 Gyrs) have an Rbirth distribution with smaller mean values (i.e. $\bar{R}_{\mbox{birth}} \sim 5\pm 0.8$ kpc) compared to younger stars (∼6 Gyrs; $\bar{R}_{\mbox{birth}} \sim 10\pm 1.5$ kpc), for a given Fe/H, consistent with inside–out growth. The α-, odd-z, and iron-peak element abundances decrease as a function of Rbirth, whereas the neutron-capture abundances increase. The Rbirth–Fe/H gradient we measure is steeper compared to the present-day gradient (−0.066 dex kpc−1 versus −0.058 dex kpc−1), which we also find true for Rbirth–X/Fe gradients. These results (i) showcase the feasibility of relating the birth radius of stars to their element abundances, (ii) demonstrate that the Milky Way abundance gradients across Rbirth have evolved to be shallower over time, and (iii) offer an observational comparison to element abundance distributions in hydrodynamical simulations.
ABSTRACT
The present-day age–metallicity relation (AMR) is a record of the star formation history of galaxies, as this traces the chemical enrichment of the gas over time. We use a zoomed-in ...cosmological simulation that reproduces key signatures of the Milky Way (MW), g2.79e12 from the NIHAO-UHD project, to examine how stellar migration and satellite infall shape the AMR across the disc. We find in the simulation, similar to the MW, the AMR in small spatial regions (R, z) shows turning points that connect changes in the direction of the relations. The turning points in the AMR in the simulation are a signature of late satellite infall. This satellite infall has a mass radio similar as that of the Sagittarius dwarf to the MW (∼0.001). Stars in the apex of the turning points are young and have nearly not migrated. The late satellite infall creates the turning points via depositing metal-poor gas in the disc, triggering star formation of stars in a narrow metallicity range compared to the overall AMR. The main effect of radial migration on the AMR turning points is to widen the metallicity range of the apex. This can happen when radial migration brings stars born from the infallen gas in other spatial bins, with slightly different metallicities, into the spatial bin of interest. These results indicate that it is possible that the passage of the Sagittarius dwarf galaxy played a role in creating the turning points that we see in the AMR in the Milky Way.
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