67 research outputs found
Using APOGEE Wide Binaries to Test Chemical Tagging with Dwarf Stars
Stars of a common origin are thought to have similar, if not nearly
identical, chemistry. Chemical tagging seeks to exploit this fact to identify
Milky Way subpopulations through their unique chemical fingerprints. In this
work, we compare the chemical abundances of dwarf stars in wide binaries to
test the abundance consistency of stars of a common origin. Our sample of 31
wide binaries is identified from a catalog produced by cross-matching APOGEE
stars with UCAC5 astrometry, and we confirm the fidelity of this sample with
precision parallaxes from Gaia DR2. For as many as 14 separate elements, we
compare the abundances between components of our wide binaries, finding they
have very similar chemistry (typically within 0.1 dex). This level of
consistency is more similar than can be expected from stars with different
origins (which show typical abundance differences of 0.3-0.4 dex within our
sample). For the best measured elements, Fe, Si, K, Ca, Mn, and Ni, these
differences are reduced to 0.05-0.08 dex when selecting pairs of dwarf stars
with similar temperatures. Our results suggest that APOGEE dwarf stars may
currently be used for chemical tagging at the level of 0.1 dex or at the
level of 0.05 dex when restricting for the best-measured elements in
stars of similar temperatures. Larger wide binary catalogs may provide
calibration sets, in complement to open cluster samples, for on-going
spectroscopic surveys.Comment: 21 pages, 14 figures, accepted for publication in Ap
Constraining the Mass Profiles of Stellar Systems: Schwarzschild Modeling of Discrete Velocity Datasets
(ABRIDGED) We present a new Schwarzschild orbit-superposition code designed
to model discrete datasets composed of velocities of individual kinematic
tracers in a dynamical system. This constitutes an extension of previous
implementations that can only address continuous data in the form of (the
moments of) velocity distributions, thus avoiding potentially important losses
of information due to data binning. Furthermore, the code can handle any
combination of available velocity components, i.e., only line-of-sight
velocities, only proper motions, or a combination of both. It can also handle a
combination of discrete and continuous data. The code finds the distribution
function (DF, a function of the three integrals of motion E, Lz, and I3) that
best reproduces the available kinematic and photometric observations in a given
axisymmetric gravitational potential. The fully numerical approach ensures
considerable freedom on the form of the DF f(E,Lz,I3). This allows a very
general modeling of the orbital structure, thus avoiding restrictive
assumptions about the degree of (an)isotropy of the orbits. We describe the
implementation of the discrete code and present a series of tests of its
performance based on the modeling of simulated datasets generated from a known
DF. We find that the discrete Schwarzschild code recovers the original orbital
structure, M/L ratios, and inclination of the input datasets to satisfactory
accuracy, as quantified by various statistics. The code will be valuable, e.g.,
for modeling stellar motions in Galactic globular clusters, and those of
individual stars, planetary nebulae, or globular clusters in nearby galaxies.
This can shed new light on the total mass distributions of these systems, with
central black holes and dark matter halos being of particular interest.Comment: ApJ, in press; 51 pages, 11 figures; manuscript revised following
comments by refere
Made-to measure galaxy models - I Methodology
We re-derive the made-to-measure method of Syer and Tremaine 1996, for
modelling stellar systems and individual galaxies, and demonstrate how
extensions to the made-to-measure method may be implemented and used. We
illustrate the enhanced made-to-measure method by determining the mass-to-light
ratio of a galaxy modelled as a Plummer sphere. From the standard galactic
observables of surface brightness and line-of-sight velocity dispersion
together with the h_4 Gauss-Hermite coefficient of the line-of-sight velocity
distribution, we successfully recover the true mass-to-light ratio of our toy
galaxy. Using kinematic data from Kleyna et al 2002, we then estimate the
mass-to-light ratio of the dwarf spheroidal galaxy Draco achieving a V-band
value of 539 \pm 136 M_{\odot} / L_{\odot}. We describe the main aspects of
creating a made-to-measure galaxy model and show how the key modelling
parameters may be determined.Comment: 18 pages, 15 figure
Magnetic Mixing in Red Giant and Asymptotic Giant Branch Stars
The available information on isotopic abundances in the atmospheres of
low-mass Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stars
requires that episodes of extensive mixing occur below the convective envelope,
reaching down to layers close to the hydrogen burning shell (Cool Bottom
Processing). Recently \cite{Busso:2007jw} suggested that dynamo-produced
buoyant magnetic flux tubes could provide the necessary physical mechanisms and
also supply sufficient transport rates. Here, we present an
dynamo in the envelope of an RGB/AGB star in which shear and rotation drain via
turbulent dissipation and Poynting flux. In this context, if the dynamo is to
sustain throughout either phase, convection must resupply shear. Under this
condition, volume-averaged, peak toroidal field strengths of
G (RGB) and G (AGB) are
possible at the base of the convection zone. If the magnetic fields are
concentrated in flux tubes, the corresponding field strengths are comparable to
those required by Cool Bottom Processing.Comment: Replaced to correct small error in published version: In \S 2.1,
paragraphs 2 and 3 incorrectly refer to the poloidal field when qualitatively
discussing magnetic diffusion in the shear zone. The correct physical
interpretation is that the toroidal field diffuses through the shear zone
consistent with the value of $\beta_\phi
Fragile Binary Candidates in the SDSS DR8 spectroscopic archive
We present a catalog of 80 very wide fragile binary candidates (projected
separations > 10000 AU) from the SDSS DR8 spectral archive. The pairs were
selected based on proper motion, radial velocity, metallicity and photometric
parallax criteria. The angular separations of these pairs range from 3" to
250". The peak in the metallicity distribution of these pairs is about -0.5 dex
of solar metallicity. Space motions and reduced proper motion diagrams indicate
all these pairs are members of the disk. The chromospheric activity index SHK
of each component in 38 binary candidates having spectra of high
signal-to-noise ratio and member stars of three open clusters (NGC2420, M67 and
NGC6791) were measured. The SHK vs. color relation for these binary candidates
is consistent with the trend seen in these open clusters. The ages implied by
this relation suggest that fragile wide pairs can survive longer than 8 Gyr.Comment: 17 pages, 12 figures, accepted by A
Unraveling UBC 274: a morphological, kinematical and chemical analysis of a disrupting open cluster
We do a morphological, kinematic and chemical analysis of the disrupting
cluster UBC 274 (2.5 Gyr, pc) to study its global properties. We use
HDBSCAN to obtain a new membership list up to 50 pc from its centre and up to
magnitude using Gaia EDR3 data. We use high resolution and high
signal-to-noise spectra to obtain atmospheric parameters of 6 giants and
subgiants, and individual abundances of 18 chemical species. The cluster has a
highly eccentric (0.93) component, tilted 10 deg with respect to the
plane of the Galaxy, which is morphologically compatible with the result of a
test-particle simulation of a disrupting cluster. Our abundance analysis shows
that the cluster has a subsolar metallicity of [Fe/H]. Its
chemical pattern is compatible with that of Ruprecht 147, of similar age but
located closer to the Sun, with the remarkable exception of neutron-capture
elements, which present an overabundance of . The
cluster's elongated morphology is associated with the internal part of its
tidal tail, following the expected dynamical process of disruption. We find a
significant sign of mass segregation where the most massive stars appear 1.5
times more concentrated than other stars. The cluster's overabundance of
neutron-capture elements can be related to the metallicity dependence of the
neutron-capture yields due to the secondary nature of these elements, predicted
by some models. UBC 274 presents a high chemical homogeneity at the level of
dex in the sampled region of its tidal tails.Comment: Accepted by A&
Hubble Space Telescope Proper Motion (HSTPROMO) Catalogs of Galactic Globular Clusters. I. Sample Selection, Data Reduction and NGC 7078 Results
We present the first study of high-precision internal proper motions (PMs) in
a large sample of globular clusters, based on Hubble Space Telescope (HST) data
obtained over the past decade with the ACS/WFC, ACS/HRC, and WFC3/UVIS
instruments. We determine PMs for over 1.3 million stars in the central regions
of 22 clusters, with a median number of ~60,000 stars per cluster. These PMs
have the potential to significantly advance our understanding of the internal
kinematics of globular clusters by extending past line-of-sight (LOS) velocity
measurements to two- or three-dimensional velocities, lower stellar masses, and
larger sample sizes. We describe the reduction pipeline that we developed to
derive homogeneous PMs from the very heterogeneous archival data. We
demonstrate the quality of the measurements through extensive Monte-Carlo
simulations. We also discuss the PM errors introduced by various systematic
effects, and the techniques that we have developed to correct or remove them to
the extent possible. We provide in electronic form the catalog for NGC 7078 (M
15), which consists of 77,837 stars in the central 2.4 arcmin. We validate the
catalog by comparison with existing PM measurements and LOS velocities, and use
it to study the dependence of the velocity dispersion on radius, stellar
magnitude (or mass) along the main sequence, and direction in the plane of the
sky (radial/tangential). Subsequent papers in this series will explore a range
of applications in globular-cluster science, and will also present the PM
catalogs for the other sample clusters.Comment: 34 pages, 22 figures (3 in low res), 30 tables, accepted for
publication in ApJ on October 20, 201
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