26,070 research outputs found
The Mass of the Convective Zone in FGK Main Sequence Stars and the Effect of Accreted Planetary Material on Apparent Metallicity Determinations
The mass of the outer convective zone in FGK main sequence stars decreases
dramatically with stellar mass. Therefore, any contamination of a star's
atmosphere by accreted planetary material should affect hotter stars much more
than cool stars. If recent suggestions that high metal abundances in stars with
planets are caused by planetesimal accretion are correct, then metallicity
enhancements in earlier-type stars with planets should be very pronounced. No
such trend is seen, however.Comment: Submitted ApJ Letters March 26th; accepted April 30th. 12 pages, 2
figure
vbyCaHbeta CCD Photometry of Clusters. VI. The Metal-Deficient Open Cluster NGC 2420
CCD photometry on the intermediate-band vbyCaHbeta system is presented for
the metal-deficient open cluster, NGC 2420. Restricting the data to probable
single members of the cluster using the CMD and the photometric indices alone
generates a sample of 106 stars at the cluster turnoff. The average E(b-y) =
0.03 +/- 0.003 (s.e.m.) or E(B-V) = 0.050 +/- 0.004 (s.e.m.), where the errors
refer to internal errors alone. With this reddening, [Fe/H] is derived from
both m1 and hk, using b-y and Hbeta as the temperature index. The agreement
among the four approaches is reasonable, leading to a final weighted average of
[Fe/H] = -0.37 +/- 0.05 (s.e.m.) for the cluster, on a scale where the Hyades
has [Fe/H] = +0.12. When combined with the abundances from DDO photometry and
from recalibrated low-resolution spectroscopy, the mean metallicity becomes
[Fe/H] = -0.32 +/- 0.03. It is also demonstrated that the average cluster
abundances based upon either DDO data or low-resolution spectroscopy are
consistently reliable to 0.05 dex or better, contrary to published attempts to
establish an open cluster metallicity scale using simplistic offset corrections
among different surveys.Comment: scheduled for Jan. 2006 AJ; 33 pages, latex, includes 7 figures and 2
table
vbyCaHbeta CCD Photometry of Clusters. VIII. The Super-Metal Rich, Old Open Cluster NGC 6791
CCD photometry on the intermediate-band vbyCaHbeta system is presented for
the metal-rich, old open cluster, NGC 6791. Preliminary analysis led to [Fe/H]
above +0.4 with an anomalously high reddening and an age below 5 Gyr. A revised
calibration between (b-y)_0 and [Fe/H] at a given temperature shows that the
traditional color-metallicity relations underestimate the color of the turnoff
stars at high metallicity. With the revised relation, the metallicity from hk
and the reddening for NGC 6791 become [Fe/H] = +0.45 +/- 0.04 and E(b-y) =
0.113 +/- 0.012 or E(B-V) = 0.155 +/- 0.016. Using the same technique,
reanalysis of the photometry for NGC 6253 produces [Fe/H] = +0.58 +/-0.04 and
E(b-y) = 0.120 +/- 0.018 or E(B-V) = 0.160 +/- 0.025. The errors quoted include
both the internal and external errors. For NGC 6791, the metallicity from m_1
is a factor of two below that from hk, a result that may be coupled to the
consistently low metal abundance from DDO photometry of the cluster and the
C-deficiency found from high dispersion spectroscopy. E(B-V) is the same value
predicted from Galactic reddening maps. With E(B-V) = 0.15 and [Fe/H] = +0.45,
the available isochrones predict an age of 7.0 +/- 1.0 Gyr and an apparent
modulus of (m-M) = 13.60 +/- 0.15, with the dominant source of the uncertainty
arising from inconsistencies among the isochrones. The reanalysis of NGC 6253
with the revised lower reddening confirms that on both the hk and m_1
metallicity scales, NGC 6253, while less than half the age of NGC 6791, remains
at least as metal-rich as NGC 6791, if not richer.Comment: Accepted for Astronomical Journal. 42 p. latex file includes 11
figures and 3 tables, one of which is a short version of a data table to
appear in online AJ in its entiret
Quantum field effects in coupled atomic and molecular Bose-Einstein condensates
This paper examines the parameter regimes in which coupled atomic and
molecular Bose-Einstein condensates do not obey the Gross-Pitaevskii equation.
Stochastic field equations for coupled atomic and molecular condensates are
derived using the functional positive-P representation. These equations
describe the full quantum state of the coupled condensates and include the
commonly used Gross-Pitaevskii equation as the noiseless limit. The model
includes all interactions between the particles, background gas losses,
two-body losses and the numerical simulations are performed in three
dimensions. It is found that it is possible to differentiate the quantum and
semiclassical behaviour when the particle density is sufficiently low and the
coupling is sufficiently strong.Comment: 4 postscript figure
Caby Photometry of the Hyades: Comparisons to the Field Stars
Intermediate-band photometry of the Hyades cluster on the Caby system is
presented for dwarf stars ranging from spectral type A through late K. A mean
hk, b-y relation is constructed using only single stars without anomalous
atmospheres and compared to the field stars of the solar neighborhood. For the
F dwarfs, the Hyades relation defines an approximate LOWER bound in the
two-color diagram, consistent with an [Fe/H] between +0.10 and +0.15. These
index-color diagrams follow the common convention of presenting stars with
highest abundance at the bottom of the plot although the index values for the
metal-rich stars are numerically larger. For field F dwarfs in the range [Fe/H]
between +0.4 and -1.0, [Fe/H] = -5.6 delta-hk + 0.125, with no evidence for a
color dependence in the slope. For the G and K dwarfs, the Hyades mean relation
crosses the field star distribution in the two-color diagram, defining an
approximate UPPER bound for the local disk stars. Stars found above the Hyades
stars fall in at least one of three categories: [Fe/H] below -0.7, [Fe/H] above
that of the Hyades, or chromospherically active. It is concluded that, contrary
to the predictions of model atmospheres, the hk index for cool dwarfs at a
given color hits a maximum value for stars below solar composition and, with
increasing [Fe/H] above some critical value, declines. This trend is
consistent, however, with the predictions from synthetic indices based upon
much narrower Ca filters where the crossover is caused by the metallicity
sensitivity of b-y.Comment: 13 pages, 9 eps figures, 1 tex table, 1 ascii tabl
Coulomb corrections to bremsstrahlung in electric field of heavy atom at high energies
The differential and partially integrated cross sections are considered for
bremsstrahlung from high-energy electrons in atomic field with the exact
account of this field. The consideration exploits the quasiclassical electron
Green's function and wave functions in an external electric field. It is shown
that the Coulomb corrections to the differential cross section are very
susceptible to screening. Nevertheless, the Coulomb corrections to the cross
section summed up over the final-electron states are independent of screening
in the leading approximation over a small parameter ( is
a screening radius, is the electron mass, ). Bremsstrahlung from
an electron beam of the finite size on heavy nucleus is considered as well.
Again, the Coulomb corrections to the differential probability are very
susceptible to the beam shape, while those to the probability integrated over
momentum transfer are independent of it, apart from the trivial factor, which
is the electron-beam density at zero impact parameter. For the Coulomb
corrections to the bremsstrahlung spectrum, the next-to-leading terms with
respect to the parameters ( is the electron energy) and
are obtained.Comment: 13 pages, 4 figure
Monopole Vector Spherical Harmonics
Eigenfunctions of total angular momentum for a charged vector field
interacting with a magnetic monopole are constructed and their properties
studied. In general, these eigenfunctions can be obtained by applying vector
operators to the monopole spherical harmonics in a manner similar to that often
used for the construction of the ordinary vector spherical harmonics. This
construction fails for the harmonics with the minimum allowed angular momentum.
These latter form a set of vector fields with vanishing covariant curl and
covariant divergence, whose number can be determined by an index theorem.Comment: 21 pages, CU-TP-60
Gamma-Ray Burst Sequences in Hardness Ratio-Peak Energy Plane
The narrowness of the distribution of the peak energy of
spectrum of gamma-ray bursts (GRBs) and the unification of GRB population are
great puzzles yet to be solved. We investigate the two puzzles based on the
global spectral behaviors of different GRB population in the
plane (HR the spectral hardness ratio) with BATSE and HETE-2 observations. It
is found that long GRBs and XRFs observed by HETE-2 seem to follow the same
sequence in the plane, with the XRFs at the low end of this
sequence. The long and short GRBs observed by BATSE follow significantly
different sequences in the plane, with most of the short GRBs
having a larger hardness ratio than the long GRBs at a given .
These results indicate that the global spectral behaviors of the long GRB
sample and the XRF sample are similar, while that of short GRBs is different.
The short GRBs seem to be a unique subclass of GRBs, and they are not the
higher energy extension of the long GRBs (abridged).Comment: 9 pages, 3 figure
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