341 research outputs found
DIRECT Distances to Local Group Galaxies
The two nearby galaxies, M31 and M33, are stepping stones for most of our
current efforts to understand the evolving universe at large scales. We are
undertaking a long term project, called DIRECT, to improve the direct distance
estimate to M31 and M33. The massive photometry we have obtained as part of our
project over the past 3 years provides us with very good light curves for known
and new Cepheid variables, a large number of eclipsing binaries and other
variable stars. Some recent results on the 470+ new Cepheids are discussed, as
well as our correction for effects of blending using archival HST images.Comment: 5 pages, 4 figures, invited contribution at IAU Symp. 176, Budapest
1999, "Impact of Large-Scale Surveys on Pulsating Star Research", ASP Conf.
Se
The effect of temperature evolution on the interior structure of HO-rich planets
For most planets in the range of radii from 1 to 4 R, water is a
major component of the interior composition. At high pressure HO can be
solid, but for larger planets, like Neptune, the temperature can be too high
for this. Mass and age play a role in determining the transition between solid
and fluid (and mixed) water-rich super-Earth. We use the latest high-pressure
and ultra-high-pressure phase diagrams of HO, and by comparing them
with the interior adiabats of various planet models, the temperature evolution
of the planet interior is shown, especially for the state of HO. It
turns out that the bulk of HO in a planet's interior may exist in
various states such as plasma, superionic, ionic, Ice VII, Ice X, etc.,
depending on the size, age and cooling rate of the planet. Different regions of
the mass-radius phase space are also identified to correspond to different
planet structures. In general, super-Earth-size planets (isolated or without
significant parent star irradiation effects) older than about 3 Gyr would be
mostly solid.Comment: Accepted by ApJ, in print for March 2014 (14 pages, 3 colored
figures, 1 table
The persistence of oceans on Earth-like planets: insights from the deep-water cycle
In this paper we present a series of models for the deep water cycle on
super-Earths experiencing plate tectonics. The deep water cycle can be modeled
through parameterized convection models coupled with a volatile recycling
model. The convection of the silicate mantle is linked to the volatile cycle
through the water-dependent viscosity. Important differences in surface water
content are found for different parameterizations of convection. Surface oceans
are smaller and more persistent for single layer convection, rather than
convection by boundary layer instability. Smaller planets have initially larger
oceans but also return that water to the mantle more rapidly than larger
planets. Super-Earths may therefore be less habitable in their early years than
smaller planets, but their habitability (assuming stable surface conditions),
will persist much longer.Comment: 14 pages; accepted for publication at the Astrophysical Journa
REMOVING DEGENERACY OF MICROLENSING LIGHT CURVES THROUGH NARROW-BAND PHOTOMETRY OF GIANTS
The standard light curve of a microlensing event provides only two
constraints on the six unknown parameters of the lens. We show that narrow-band
photometry during a microlensing event of a giant star can in addition
determine the angular radius of the Einstein ring and the proper motion of the
lens. This possibility results from the fact that the extended atmospheres of
giants emit the cores of resonant lines primarily from a narrow ring (limb
brightening). The radius of the emission ring can be determined to a precision
of 20% by detailed spectroscopic observations of the source after the lensing
event has ended. A considerable fraction of the clump giant stars in the bulge
have a ring radius >10^{12} cm, within the range of Einstein radii for
sub-solar mass lenses. The extended thin ring also provides a sensitive probe
of possible planetary companions as it sweeps across the lens plane. The ring
signature can be detected photometrically, using a narrow-band filter centered
on the CaII K line at 3933 A.Comment: 13 pages, uuencoded file, 4 figures include
A Detailed Model Grid for Solid Planets from 0.1 through 100 Earth Masses
This paper describes a new grid for the mass-radius relation of 3-layer
exoplanets within the mass range of 0.1 through 100 Earth Masses. The 3 layers
are: Fe (epsilon iron), MgSiO3 (including both the perovskite phase,
post-perovskite phase, and its dissociation at ultra-high pressures), and H2O
(including Ices Ih, III, V, VI, VII, X, and the superionic phase along the
melting curve). We discuss the current state of knowledge about the equations
of state (EOS) that influence these calculations and the improvements used in
the new grid. For the 2-layer model, we demonstrate the utility of contours on
the mass-radius diagrams. Given the mass and radius input, these contours can
be used to quickly determine the important physical properties of a planet
including its p0 (central pressure), p1/p0 (core-mantle boundary pressure over
central pressure), CMF (core mass fraction) or CRF (core radius fraction). For
the 3-layer model, a curve segment on the ternary diagram represents all
possible relative mass proportions of the 3 layers for a given mass-radius
input. These ternary diagrams are tabulated into Table3 with the intent to make
comparison to observations easier. How the presence of Fe in the mantle affects
the mass-radius relations is also discussed in a separate section. A dynamic
and interactive tool to characterize and illustrate the interior structure of
exoplanets built upon models in this paper is available on the website:
http://www.cfa.harvard.edu/~lzengComment: submitted to PASP, including 29 pages, 5 figures, 3 tables. For
associated files and interactive tool, see http://www.cfa.harvard.edu/~lzeng
New Bounds on Omega Baryons from Observational Big Bang Nucleosynthesis
We re-examine the systematic errors in the determination of the primordial
helium abundance, . We find that the systematics are significantly
larger than the statistical errors. The uncertainty in (the determination of)
, is thus, larger than is currently claimed. Furthermore, most of
the systematics lead to underestimate of . The new upper bound
allows cosmological models with no non-baryonic dark matter in which
.Comment: 11 pages of uuencoded compressed PostScript with 2 figure
Formation of Carbon Dwarfs
We consider the formation of dwarf carbon stars via accretion from a carbon
AGB companion in light of the new 107 object sample of Downes et al. (2004).
This sample is now large enough to allow good mass determination via comparison
of a composite spectrum to theoretical atmospheric models. Carbon dwarfs of
spectral type M are indeed main sequence M dwarfs with enhanced metallicity and
carbon abundance. We also calculate the predicted abundance of both M and of
F/G carbon dwarfs, and show that the latter should be falsifiable in the near
future.Comment: submitte
Mass-Radius Relation for Rocky Planets based on PREM
Several small dense exoplanets are now known, inviting comparisons to Earth
and Venus. Such comparisons require translating their masses and sizes to
composition models of evolved multi-layer-interior planets. Such theoretical
models rely on our understanding of the Earth's interior, as well as
independently derived equations of state (EOS), but have so far not involved
direct extrapolations from Earth's seismic model -PREM. In order to facilitate
more detailed compositional comparisons between small exoplanets and the Earth,
we derive here a semi-empirical mass-radius relation for two-layer rocky
planets based on PREM: , where CMF stands for Core Mass Fraction. It is
applicable to 18 M and CMF of 0.00.4. Applying this
formula to Earth and Venus and several known small exoplanets with radii and
masses measured to better than 30\% precision gives a CMF fit of
.Comment: 3 figures, 2 tables, ApJ, accepte
The Chromaticity of Microlensing (Surface Imaging of Stars)
Gravitational microlensing is not achromatic when the star being microlensed
is resolved by the lens. We show how narrow-band photometry and moderate- to
high-resolution spectroscopy can be used to reconstruct the stellar surface
intensity distribution of a microlensed star. Such microlens imaging can
provide a unique opportunity to study the surfaces of normal red giants where
Doppler imaging is not applicable due to slow rotation.}Comment: 5 pages, 2 figures, LaTex, invited talk 12th IAP Colloquium on "The
Astrophysical Returns of Microlensing Surveys", eds. R.Ferlet and
J.P.Maillar
First Results From New 3D Spectral Simulations Of Anelastic Turbulent Convection
We have adapted the anelastic spectral code of Barranco & Marcus (2006) to
simulate a turbulent convective layer with the intention of studying the
effectiveness of turbulent eddies in dissipating external shear (e.g. tides).
We derive the anelastic equations, show the time integration scheme we use to
evolve these equations and present the tests we ran to confirm that our code
does what we expect. Further we apply a perturbative approach to find an
approximate scaling of the effective eddy viscosity with frequency, and find
that it is in general agreement with an estimate obtained by applying the same
procedure to a realistic simulation of the upper layers of the solar convective
zone.Comment: 32 pages, 12 figure
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