7,298 research outputs found
Lifting the Dusty Veil With Near- and Mid-Infrared Photometry: III. Two-Dimensional Extinction Maps of the Galactic Midplane Using the Rayleigh-Jeans Color Excess Method
We provide new, high-resolution A(Ks) extinction maps of the heavily reddened
Galactic midplane based on the Rayleigh-Jeans Color Excess ("RJCE") method.
RJCE determines star-by-star reddening based on a combination of near- and
mid-infrared photometry. The new RJCE-generated maps have 2 x 2 arcmin pixels
and span some of the most severely extinguished regions of the Galaxy -- those
covered with Spitzer+IRAC imaging by the GLIMPSE-I, -II, -3D, and Vela-Carina
surveys, from 256<l<65 deg and, in general, for |b| <= 1-1.5 deg (extending up
to |b|<=4 deg in the bulge). Using RJCE extinction measurements, we generate
dereddened color-magnitude diagrams and, in turn, create maps based on main
sequence, red clump, and red giant star tracers, each probing different
distances and thereby providing coarse three-dimensional information on the
relative placement of dust cloud structures. The maps generated from red giant
stars, which reach to ~18-20 kpc, probe beyond most of the Milky Way extinction
in most directions and provide close to a "total Galactic extinction" map -- at
minimum they provide high angular resolution maps of lower limits on A(Ks).
Because these maps are generated directly from measurements of reddening by the
very dust being mapped, rather than inferred on the basis of some less direct
means, they are likely the most accurate to date for charting in detail the
highly patchy differential extinction in the Galactic midplane. We provide
downloadable FITS files and an IDL tool for retrieving extinction values for
any line of sight within our mapped regions.Comment: 23 pages, 5 figures, accepted for publication in ApJ
The Frequency of Rapid Rotation Among K Giant Stars
We present the results of a search for unusually rapidly rotating giant stars
in a large sample of K giants (~1300 stars) that had been spectroscopically
monitored as potential targets for the Space Interferometry Mission's
Astrometric Grid. The stars in this catalog are much fainter and typically more
metal-poor than those of other catalogs of red giant star rotational
velocities, but the spectra generally only have signal-to-noise (S/N) of
~20-60, making the measurement of the widths of individual lines difficult. To
compensate for this, we have developed a cross-correlation method to derive
rotational velocities in moderate S/N echelle spectra to efficiently probe this
sample for rapid rotator candidates. We have discovered 28 new red giant rapid
rotators as well as one extreme rapid rotator with a vsini of 86.4 km/s. Rapid
rotators comprise 2.2% of our sample, which is consistent with other surveys of
brighter, more metal-rich K giant stars. Although we find that the temperature
distribution of rapid rotators is similar to that of the slow rotators, this
may not be the case with the distributions of surface gravity and metallicity.
The rapid rotators show a slight overabundance of low gravity stars and as a
group are significantly more metal-poor than the slow rotators, which may
indicate that the rotators are tidally-locked binaries.Comment: Accepted for publication in ApJ. 25 pages, 9 figures, 3 tables.
Tables 1 and 2 are provided in their full form as plain text ancillary file
Top-down machine learning of coarse-grained protein force-fields
Developing accurate and efficient coarse-grained representations of proteins
is crucial for understanding their folding, function, and interactions over
extended timescales. Our methodology involves simulating proteins with
molecular dynamics and utilizing the resulting trajectories to train a neural
network potential through differentiable trajectory reweighting. Remarkably,
this method requires only the native conformation of proteins, eliminating the
need for labeled data derived from extensive simulations or memory-intensive
end-to-end differentiable simulations. Once trained, the model can be employed
to run parallel molecular dynamics simulations and sample folding events for
proteins both within and beyond the training distribution, showcasing its
extrapolation capabilities. By applying Markov State Models, native-like
conformations of the simulated proteins can be predicted from the
coarse-grained simulations. Owing to its theoretical transferability and
ability to use solely experimental static structures as training data, we
anticipate that this approach will prove advantageous for developing new
protein force fields and further advancing the study of protein dynamics,
folding, and interactions
A Two Micron All-Sky Survey View of the Sagittarius Dwarf Galaxy: VI. s-Process and Titanium Abundance Variations Along the Sagittarius Stream
We present high-resolution spectroscopic measurements of the abundances of
titanium (Ti), yttrium (Y) and lanthanum (La) for M giant candidates of the
Sagittarius (Sgr) dwarf spheroidal (dSph) + tidal tail system pre-selected on
the basis of position and radial velocity. The majority of these stars show
peculiar abundance patterns compared to those of nominal Milky Way (MW) stars.
The overall [Ti/Fe], [Y/Fe], [La/Fe] and [La/Y] patterns with [Fe/H] of the Sgr
stream plus Sgr core do resemble those seen in the Large Magellanic Cloud (LMC)
and other dSphs, only shifted [Fe/H] by ~+0.4 from the LMC and by ~+1 dex from
the other dSphs; these relative shifts reflect the faster and/or more efficient
chemical evolution of Sgr compared to the other satellites, and show that Sgr
has had an enrichment history more like the LMC than the other dSphs. By
tracking the evolution of the abundance patterns along the Sgr stream we can
follow the time variation of the chemical make-up of dSph stars donated to the
MW halo by Sgr. This evolution demonstrates that while the bulk of the stars
currently in the Sgr dSph are quite unlike those of the MW halo, an increasing
number of stars farther along the Sgr stream have abundances like MW halo
stars, a trend that shows clearly how the MW halo could have been contributed
by present day satellite galaxies even if the present chemistry of those
satellites is now different from typical halo field stars. Finally, we analyze
the chemical abundances of a moving group of M giants among the Sgr leading arm
stars at the North Galactic Cap, but having radial velocities unlike the
infalling Sgr leading arm debris there. Through use of "chemical
fingerprinting", we conclude that these northern hemisphere M giants also are
Sgr stars, likely trailing arm debris overlapping the leading arm in the north.Comment: Accepted for publication in Ap
Probing the Galactic Potential with Next-Generation Observations of Disk Stars
Near-future surveys promise a dramatic improvement in the number and
precision of astrometric, photometric and spectroscopic measurements of stars
in the Milky Way's disk. We examine the impact of such surveys on our
understanding of the Galaxy by "observing" particle realizations of
non-axisymmetric disk distributions orbiting in an axisymmetric halo with
appropriate errors and then attempting to recover the underlying potential
using a Markov Chain Monte Carlo (MCMC) approach. We demonstrate that the
azimuthally averaged gravitational force field in the Galactic plane--and
hence, to a lesser extent, the Galactic mass distribution--can be tightly
constrained over a large range of radii using a variety of types of surveys so
long as the error distribution of the measurements of the parallax, proper
motion and radial velocity are well-understood and the disk is surveyed
globally. One advantage of our method is that the target stars can be selected
non-randomly in real or apparent-magnitude space to ensure just such a global
sample without biasing the results. Assuming we can always measure the
line-of-sight velocity of a star with at least 1 km/s precision, we demonstrate
that the force field can be determined to better than ~1% for Galactocentric
radii in the range R=4-20 kpc We conclude that near-future surveys, like SIM
Lite, Gaia, and VERA, will provide the first precise mapping of the
gravitational force field in the region of the Galactic disk.Comment: 41 pages and 10 figures, accepted for publication in Ap
CADIS has seen the Virgo overdensity and parts of the Monoceros and `Orphan' streams in retrospect
We reanalyze deep star counts in five CADIS fields. The data are presented as
vertical density distributions of stars perpendicular to the Galactic plane. In
three fields the profiles are consistent with each other, while in two fields
significant overdensities of stars are found. The overdensity in one field can
be associated with the Virgo overdensity which can be traced right into the
disk of the Milky Way. Using this detection we estimate the mass of the Virgo
overdensity and show that this is equivalent to the stellar content of a Local
Group dwarf spheroidal galaxy. The overdensity in the second field is more
difficult to associate with a previously known overdensity. We suggest that it
is related both to the Monoceros stream and the recently discovered Orphan
stream.Comment: 4 pages, 3 figures, accepted as Research Note by Astron. Astrophy
Comparing the Dark Matter Halos of Spiral, Low Surface Brightness and Dwarf Spheroidal Galaxies
We consider dark masses measured from kinematic tracers at discrete radii in
galaxies for which baryonic contributions to overall potentials are either
subtracted or negligible. Recent work indicates that rotation curves due to
dark matter (DM) halos at intermediate radii in spiral galaxies are remarkably
similar, with a mean rotation curve given by
. Independent
studies show that while estimates of the dark mass of a given dwarf spheroidal
(dSph) galaxy are robust only near the half-light radius, data from the Milky
Way's (MW's) dSph satellites are consistent with a narrow range of mass
profiles. Here we combine published constraints on the dark halo masses of
spirals and dSphs and include available measurements of low surface brightness
galaxies for additional comparison. We find that most measured MW dSphs lie on
the extrapolation of the mean rotation curve due to DM in spirals. The union of
MW-dSph and spiral data appears to follow a mass-radius relation of the form
, or
equivalently a constant acceleration , spanning 0.02\la r \la 75 kpc. Evaluation at
specific radii immediately generates two results from the recent literature: a
common mass for MW dSphs at fixed radius and a constant DM central surface
density for galaxies ranging from MW dSphs to spirals. However, recent
kinematic measurements indicate that M31's dSph satellites are systematically
less massive than MW dSphs of similar size. Such deviations from what is
otherwise a surprisingly uniform halo relation presumably hold clues to
individual formation and evolutionary histories.Comment: ApJL in press (minor edits to text in order to match version in
press
Mapping the substructure in the Galactic halo with the next generation of astrometric satellites
We run numerical simulations of the disruption of satellite galaxies in a
Galactic potential to build up the entire stellar halo, in order to investigate
what the next generation of astrometric satellites will reveal by observing the
halo of the Milky Way. We generate artificial DIVA, FAME and GAIA halo
catalogues, in which we look for the signatures left by the accreted
satellites. We develop a method based on the standard Friends-of-Friends
algorithm applied to the space of integrals of motion. We find this simple
method can recover about 50% of the different accretion events, when the
observational uncertainties expected for GAIA are taken into account, even when
the exact form of the Galactic potential is unknown. The recovery rate for DIVA
and FAME is much smaller, but these missions, like GAIA, should be able to test
the hierarchical formation paradigm on our Galaxy by measuring the amount of
halo substructure in the form of nearby kinematically cold streams with for
example, a two-point correlation function in velocity space.Comment: 10 pages, 9 figures, submitted to MNRAS. High resolution color
figures available from http://www.strw.leidenuniv.nl/~ahelmi/astrom.htm
Voltage controlled spin injection in a (Ga,Mn)As/(Al,Ga)As Zener diode
The spin polarization of the electron current in a
p-(Ga,Mn)As-n-(Al,Ga)As-Zener tunnel diode, which is embedded in a
light-emitting diode, has been studied theoretically. A series of
self-consistent simulations determines the charge distribution, the band
bending, and the current-voltage characteristics for the entire structure. An
empirical tight-binding model, together with the Landauer- Buttiker theory of
coherent transport has been developed to study the current spin polarization.
This dual approach allows to explain the experimentally observed high magnitude
and strong bias dependence of the current spin polarization.Comment: Submitted to Phys. Rev. B Rapid Communication
- …