2,299 research outputs found
Metallicities of M Dwarf Planet Hosts from Spectral Synthesis
We present the first spectroscopic metallicities of three M dwarfs with known
or candidate planetary mass companions. We have analyzed high resolution, high
signal-to-noise spectra of these stars which we obtained at McDonald
Observatory. Our analysis technique is based on spectral synthesis of atomic
and molecular features using recently revised cool-star model atmospheres and
spectrum synthesis code. The technique has been shown to yield results
consistent with the analyses of solar-type stars and allows measurements of M
dwarf [M/H] values to 0.12 dex precision. From our analysis, we find [M/H] =
-0.12, -0.32, and -0.33 for GJ 876, GJ 436, and GJ 581 respectively. These
three M dwarf planet hosts have sub-solar metallicities, a surprising departure
from the trend observed in FGK-type stars. This study is the first part of our
ongoing work to determine the metallicities of the M dwarfs included in the
McDonald Observatory planet search program.Comment: 13 pages, 2 figures, accepted for publication in ApJ
Damping of giant resonances in asymmetric nuclear matter
The giant collective modes in asymmetric nuclear matter are investigated
within a dynamic relaxation time approximation. We derive a coupled dispersion
relation and show that two sources of coupling appear: (i) a coupling of
isoscalar and isovector modes due to different mean-fields acting and (ii) an
explicit new coupling in asymmetric matter due to collisional interaction. We
show that the latter one is responsible for a new mode arising besides
isovector and isoscalar modes.Comment: Varenna conference proceeding
Fictive Impurity Approach to Dynamical Mean Field Theory: a Strong-Coupling Investigation
Quantum Monte Carlo and semiclassical methods are used to solve two and four
site cluster dynamical mean field approximations to the square lattice Hubbard
model at half filling and strong coupling. The energy, spin correlation
function, phase boundary and electron spectral function are computed and
compared to available exact results. The comparision permits a quantitative
assessment of the ability of the different methods to capture the effects of
intersite spin correlations. Two real space methods and one momentum space
representation are investigated. One of the two real space methods is found to
be significantly worse: in it, convergence to the correct results is found to
be slow and, for the spectral function, nonuniform in frequency, with
unphysical midgap states appearing. Analytical arguments are presented showing
that the discrepancy arises because the method does not respect the pole
structure of the self energy of the insulator. Of the other two methods, the
momentum space representation is found to provide the better approximation to
the intersite terms in the energy but neither approximation is particularly
acccurate and the convergence of the momentum space method is not uniform. A
few remarks on numerical methods are made.Comment: Errors in previous versions corrected; CDMFT results adde
A Consistency Test of Spectroscopic Gravities for Late-Type Stars
Chemical analyses of late-type stars are usually carried out following the
classical recipe: LTE line formation and homogeneous, plane-parallel,
flux-constant, and LTE model atmospheres. We review different results in the
literature that have suggested significant inconsistencies in the spectroscopic
analyses, pointing out the difficulties in deriving independent estimates of
the stellar fundamental parameters and hence,detecting systematic errors.
The trigonometric parallaxes measured by the HIPPARCOS mission provide
accurate appraisals of the stellar surface gravity for nearby stars, which are
used here to check the gravities obtained from the photospheric iron ionization
balance. We find an approximate agreement for stars in the metallicity range -1
<= [Fe/H] <= 0, but the comparison shows that the differences between the
spectroscopic and trigonometric gravities decrease towards lower metallicities
for more metal-deficient dwarfs (-2.5 <= [Fe/H] <= -1.0), which casts a shadow
upon the abundance analyses for extreme metal-poor stars that make use of the
ionization equilibrium to constrain the gravity. The comparison with the
strong-line gravities derived by Edvardsson (1988) and Fuhrmann (1998a)
confirms that this method provides systematically larger gravities than the
ionization balance. The strong-line gravities get closer to the physical ones
for the stars analyzed by Fuhrmann, but they are even further away than the
iron ionization gravities for the stars of lower gravities in Edvardsson's
sample. The confrontation of the deviations of the iron ionization gravities in
metal-poor stars reported here with departures from the excitation balance
found in the literature, show that they are likely to be induced by the same
physical mechanism(s).Comment: AAS LaTeX v4.0, 35 pages, 10 PostScript files; to appear in The
Astrophysical Journa
A Unified Theory for the Atmospheres of the Hot and Very Hot Jupiters: Two Classes of Irradiated Atmospheres
We highlight the importance of gaseous TiO and VO opacity on the highly
irradiated close-in giant planets. The atmospheres of these planets naturally
fall into two classes that are somewhat analogous to the M- and L-type dwarfs.
Those that are warm enough to have appreciable opacity due to TiO and VO gases
we term the ``pM Class'' planets, and those that are cooler we term ``pL
Class'' planets. We calculate model atmospheres for these planets, including
pressure-temperature profiles, spectra, and characteristic radiative time
constants. We show that pM Class planets have hot stratospheres 2000 K
and appear ``anomalously'' bright in the mid infrared secondary eclipse, as was
recently found for planets HD 149026b and HD 209458b. This class of planets
absorbs incident flux and emits thermal flux from high in their atmospheres.
Consequently, they will have large day/night temperature contrasts and
negligible phase shifts between orbital phase and thermal emission light
curves, because radiative timescales are much shorter than possible dynamical
timescales. The pL Class planets absorb incident flux deeper in the atmosphere
where atmospheric dynamics will more readily redistribute absorbed energy. This
will lead to cooler day sides, warmer night sides, and larger phase shifts in
thermal emission light curves. Around a Sun-like primary this boundary occurs
at 0.04-0.05 AU. The eccentric transiting planets HD 147506b and HD
17156b alternate between the classes. Thermal emission in the optical from pM
Class planets is significant red-ward of 400 nm, making these planets
attractive targets for optical detection. The difference in the observed
day/night contrast between ups Andromeda b (pM Class) and HD 189733b (pL Class)
is naturally explained in this scenario. (Abridged.)Comment: Accepted to the Astrophysical Journa
Momentum conservation and local field corrections for the response of interacting Fermi gases
We reanalyze the recently derived response function for interacting systems
in relaxation time approximation respecting density, momentum and energy
conservation. We find that momentum conservation leads exactly to the local
field corrections for both cases respecting only density conservation and
respecting density and energy conservation. This rewriting simplifies the
former formulae dramatically. We discuss the small wave vector expansion and
find that the response function shows a high frequency dependence of
which allows to fulfill higher order sum rules. The momentum
conservation also resolves a puzzle about the conductivity which should only be
finite in multicomponent systems
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