1,109 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
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
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
Biorthogonal Renormalization
The biorthogonal formalism extends conventional quantum mechanics to the
non-Hermitian realm. It has, however, been pointed out that the biorthogonal
inner product changes with the scaling of the eigenvectors, an ambiguity whose
physical significance is still being debated. Here, we revisit this issue and
argue when this choice of normalization is of physical importance. We
illustrate in which settings quantities such as expectation values and
transition probabilities depend on the scaling of eigenvectors, and in which
settings the biorthogonal formalism remains unambiguous. To resolve the
apparent scaling ambiguity, we introduce an inner product independent of the
gauge choice of basis and show that its corresponding mathematical structure is
consistent with quantum mechanics. Using this formalism, we identify a deeper
problem relating to the physicality of Hilbert space representations, which we
illustrate using the position basis. Apart from increasing the understanding of
the mathematical foundations upon which many physical results rely, our
findings also pave the way towards consistent comparisons between systems
described by non-Hermitian Hamiltonians.Comment: 19 pages, 3 figure
The Relative Age of the Thin and Thick Galactic Disks
We determine the relative ages of the open cluster NGC 188 and selected
Hipparcos field stars by isochrone fitting, and compare them to the age of the
thick disk globular cluster 47 Tuc. The best fit age for NGC 188 was determined
to be Gyr. The solar metallicity Hipparcos field stars yielded a
slightly older thin disk age, Gyr. Two slightly metal-poor (\feh
= -0.22) field stars whose kinematic and orbital parameters indicate that they
are members of the thin disk were found to have an age of Gyr. The
age for 47 Tuc was determined to be Gyr. All errors are internal
errors due to the uncertainty in the values of metallicity and reddening. Thus,
the oldest stars dated in the thin disk are found to be Gyr
younger than 47 Tuc. Furthermore, as discussed by \citet{Chb99} 47 Tuc has a
similar age to three globular clusters located in the inner part of the
Galactic halo, implying that star formation in the thin disk started within
Gyr of star formation in the halo.Comment: 26 pages, 11 figures, to appear in Ap
The Future is Now: the Formation of Single Low Mass White Dwarfs in the Solar Neighborhood
Low mass helium-core white dwarfs (M < 0.45 Msun) can be produced from
interacting binary systems, and traditionally all of them have been attributed
to this channel. However, a low mass white dwarf could also result from a
single star that experiences severe mass loss on the first ascent giant branch.
A large population of low mass He-core white dwarfs has been discovered in the
old metal-rich cluster NGC 6791. There is therefore a mechanism in clusters to
produce low mass white dwarfs without requiring binary star interactions, and
we search for evidence of a similar population in field white dwarfs. We argue
that there is a significant field population (of order half of the detected
systems) that arises from old metal rich stars which truncate their evolution
prior to the helium flash from severe mass loss. There is a consistent absence
of evidence for nearby companions in a large fraction of low mass white dwarfs.
The number of old metal-rich field dwarfs is also comparable with the
apparently single low mass white dwarf population, and our revised estimate for
the space density of low mass white dwarfs produced from binary interactions is
also compatible with theoretical expectations. This indicates that this channel
of stellar evolution, hitherto thought hypothetical only, has been in operation
in our own Galaxy for many billions of years. One strong implication of our
model is that single low mass white dwarfs should be good targets for planet
searches because they are likely to arise from metal-rich progenitors. We also
discuss other observational tests and implications, including the potential
impact on SN Ia rates and the frequency of planetary nebulae.Comment: ApJ published versio
Abundance Analysis of Planetary Host Stars I. Differential Iron Abundances
We present atmospheric parameters and iron abundances derived from
high-resolution spectra for three samples of dwarf stars: stars which are known
to host close-in giant planets (CGP), stars for which radial velocity data
exclude the presence of a close-in giant planetary companion (no-CGP), as well
as a random sample of dwarfs with a spectral type and magnitude distribution
similar to that of the planetary host stars (control). All stars have been
observed with the same instrument and have been analyzed using the same model
atmospheres, atomic data and equivalent width modeling program. Abundances have
been derived differentially to the Sun, using a solar spectrum obtained with
Callisto as the reflector with the same instrumentation. We find that the iron
abundances of CGP dwarfs are on average by 0.22 dex greater than that of no-CGP
dwarfs. The iron abundance distributions of both the CGP and no-CGP dwarfs are
different than that of the control dwarfs, while the combined iron abundances
have a distribution which is very similar to that of the control dwarfs. All
four samples (CGP, no-CGP, combined, control) have different effective
temperature distributions. We show that metal enrichment occurs only for CGP
dwarfs with temperatures just below solar and approximately 300 K higher than
solar, whereas the abundance difference is insignificant at Teff around 6000 K.Comment: 52 pages (aastex 11pt, preprint style), including 17 figures and 13
tables; accepted for publication in AJ (scheduled for the October 2003 issue
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