1,375 research outputs found
The Calibration of Stromgren uvby-Hbeta Photometry for Late-Type Stars -- a Model Atmosphere Approach
We aim to test the power of theoretical calibrations based on a new
generation of MARCS models by comparisons with observational photomteric data.
We calculate synthetic uvby-Hbeta colour indices from synthetic spectra. A
sample of 388 field stars as well as stars in globular clusters is used for a
direct comparison of the synthetic indices versus empirical data and for
scrutinizing the possibilities of theoretical calibrations for temperature,
metallicity and gravity. We show that the temperature sensitivity of the
synthetic (b-y) colour is very close to its empirical counterpart, whereas the
temperature scale based upon Hbeta shows a slight offset. The theoretical
metallicity sensitivity of the m1 index (and for G-type stars its combination
with c1) is somewhat larger than the empirical one, based upon spectroscopic
determinations. The gravity sensitivity of the synthetic c1 index shows a
satisfactory behaviour when compared to obervations of F stars. For stars
cooler than the sun a deviation is significant in the c1-(b-y) diagram. The
theoretical calibrations of (b-y), (v-y) and c1 seem to work well for Pop II
stars and lead to effective temperatures for globular cluster stars supporting
recent claims by Korn et al. (2007) that atomic diffusion occurs in stars near
the turnoff point of NGC 6397. Synthetic colours of stellar atmospheres can
indeed be used, in many cases, to derive reliable fundamental stellar
parameters. The deviations seen when compared to observational data could be
due to incomplete linelists but are possibly also due to effects of assuming
plane-parallell or spherical geometry and LTE
Evolution of O Abundance Relative to Fe
We present a three-component mixing model for the evolution of O abundance
relative to Fe, taking into account the contributions of the first very massive
(> 100 solar masses) stars formed from Big Bang debris. We show that the
observations of O and Fe abundances in metal-poor stars in the Galaxy by
Israelian et al. and Boesgaard et al. can be well represented both
qualitatively and quantitatively by this model. Under the assumption of an
initial Fe ([Fe/H] = -3) and O inventory due to the prompt production by the
first very massive stars, the data at -3 < [Fe/H] < -1 are interpreted to
result from the addition of O and Fe only from type II supernovae (SNII) to the
prompt inventory. At [Fe/H] = -1, SNII still contribute O while both SNII and
type Ia supernovae contribute Fe. During this later stage, (O/Fe) sharply drops
off to an asymptotic value of 0.8(O/Fe)_sun. The value of (O/Fe) for the prompt
inventory at [Fe/H] = -3 is found to be (O/Fe) = 20(O/Fe)_sun. This result
suggests that protogalaxies with low ``metallicities'' should exhibit high
values of (O/Fe). The C/O ratio produced by the first very massive stars is
expected to be much less than 1 so that all the C should be tied up as CO and
that C dust and hydrocarbon compounds should be quite rare at epochs
corresponding to [Fe/H] < -3.Comment: 25 pages, 8 postscript figures, to appear in Ap
The Emergence of the Thick Disk in a CDM Universe II: Colors and Abundance Patterns
The recently emerging conviction that thick disks are prevalent in disk
galaxies, and their seemingly ubiquitous old ages, means that the formation of
the thick disk, perhaps more than any other component, holds the key to
unravelling the evolution of the Milky Way, and indeed all disk galaxies. In
Paper I, we proposed that the thick disk was formed in an epoch of gas rich
mergers, at high redshift. This hypothesis was based on comparing N-body/SPH
simulations to a variety of Galactic and extragalactic observations, including
stellar kinematics, ages and chemical properties.Here examine our thick disk
formation scenario in light of the most recent observations of extragalactic
thick disks. In agreement, our simulted thick disks are old and relatively
metal rich, with V-I colors that do not vary significantly with distance from
the plane. Further, we show that our proposal results in an enhancement of
alpha-elements in thick disk stars as compared with thin disk stars, consistent
with observations of the relevant populations of the Milky Way. We also find
that our scenario naturally leads to the formation of an old metal weak stellar
halo population with high alpha-element abundances.Comment: submitted to Ap
Dynamical Evolution of Galaxies in Clusters
Tidal forces acting on galaxies in clusters lead to a strong dynamical
evolution. In order to quantify the amount of evolution, I run self-consistent
N-body simulations of disk galaxies for a variety of models in the
hierarchically forming clusters. The tidal field along the galactic orbits is
extracted from the simulations of cluster formation in the Omega_0=1;
Omega_0=0.4; and Omega_0=0.4, Omega_Lambda=0.6 cosmological scenarios. For
large spiral galaxies with the rotation speed of 250 km/s, tidal interactions
truncate massive dark matter halos at 30 +- 6 kpc, and thicken stellar disks by
a factor 2 to 3, increasing Toomre's parameter to Q > 2 and halting star
formation. Low density galaxies, such as the dwarf spheroidals with the
circular velocity of 20 km/s and the extended low surface brightness galaxies
with the scale length of 10-15 kpc, are completely disrupted by tidal shocks.
Their debris contribute to the diffuse intracluster light. The tidal effects
are significant not only in the core but throughout the cluster and can be
parametrized by the critical tidal density. The tidally-induced evolution
results in the transformation of the infalling spirals into S0 galaxies and in
the depletion of the LSB population. In the low Omega_0 cosmological models,
clusters form earlier and produce stronger evolution of galaxies.Comment: accepted to Ap
Disentangling the Hercules stream
Using high-resolution spectra of nearby F and G dwarf stars, we have
investigated the detailed abundance and age structure of the Hercules stream.
We find that the stars in the stream have a wide range of stellar ages,
metallicities, and element abundances. By comparing to existing samples of
stars in the solar neighbourhood with kinematics typical of the Galactic thin
and thick disks we find that the properties of the Hercules stream distinctly
separate into the abundance and age trends of the two disks. Hence, we find it
unlikely that the Hercules stream is a unique Galactic stellar population, but
rather a mixture of thin and thick disk stars. This points toward a dynamical
origin for the Hercules stream, probably caused by the Galactic bar.Comment: Accepted for publication in ApJ Letter
Enrichment of the r-process Element Europium in the Galactic Halo
We investigate the enrichment of europium, as a representative of r-process
elements, in the Galactic halo. In present chemical evolution models, stars are
assumed to be formed through shock processes by supernovae (SNe). The
enrichment of the interstellar medium is calculated by a one-zone approach. The
observed large dispersions in [Eu/Fe] for halo stars, converging with
increasing metallicity, can be explained with our models. In addition, the mass
range of SNe for the {\it r}-process site is constrained to be either stars of
or .Comment: 5 pages (including 4 figures), LaTeX, uses aas2pp4.sty, accepted to
ApJ
Loschmidt echo and fidelity decay near an exceptional point
Non-Hermitian classical and open quantum systems near an exceptional point
(EP) are known to undergo strong deviations in their dynamical behavior under
small perturbations or slow cycling of parameters as compared to Hermitian
systems. Such a strong sensitivity is at the heart of many interesting
phenomena and applications, such as the asymmetric breakdown of the adiabatic
theorem, enhanced sensing, non-Hermitian dynamical quantum phase transitions
and photonic catastrophe. Like for Hermitian systems, the sensitivity to
perturbations on the dynamical evolution can be captured by Loschmidt echo and
fidelity after imperfect time reversal or quench dynamics. Here we disclose a
rather counterintuitive phenomenon in certain non-Hermitian systems near an EP,
namely the deceleration (rather than acceleration) of the fidelity decay and
improved Loschmidt echo as compared to their Hermitian counterparts, despite
large (non-perturbative) deformation of the energy spectrum introduced by the
perturbations. This behavior is illustrated by considering the fidelity decay
and Loschmidt echo for the single-particle hopping dynamics on a tight-binding
lattice under an imaginary gauge field.Comment: 11 pages, 6 figures, to appear in Annalen der Physi
WIYN/Hydra Detection of Lithium Depletion in F Stars of the Young Open Cluster M35 and Implications for the Development of the Lithium Gap
We report discovery of significant depletion of Li on the surfaces of F dwarf
stars in the 150-Myr-old open cluster M35, analagous to a feature in the
700-Myr-old Hyades cluster that has been referred to as the ``Li gap.'' We have
caught the gap in the act of forming: using high resolution, high S/N,
WIYN/Hydra observations, we detect Li in all but a few M35 F stars; the maximum
depletion lies at least 0.6-0.8 dex below minimally depleted (or undepleted)
stars. The M35 Li depletion region, a) is quite wide, with clear depletion seen
from 6000K to 6700K or hotter; b) shows a significant dispersion in Li
abundance at all T_eff, even with stars of the same T_eff; and c) contains
undepleted stars (as well as depleted ones) in the (narrow) classical Hyades
gap region, which itself shows no undepleted stars. All of these M35 Li
depletion properties support rotationally-induced slow mixing as the primary
physical mechanism that forms the gap, and argues against other proposed
mechanisms, particularly diffusion and steady main sequence mass loss. When
viewed in the context of the M35 Li depletion properties, the Hyades Li gap may
well be wider than is usually recognized.Comment: 14 Pages, 3 figures. Accepted to ApJ Letter
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