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 H2{}_{2}O-rich planets

For most planets in the range of radii from 1 to 4 R$_{\oplus}$, water is a major component of the interior composition. At high pressure H${}_{2}$O 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 H${}_{2}$O, 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 H${}_{2}$O. It turns out that the bulk of H${}_{2}$O 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, $Y_{\rm P}$. We find that the systematics are significantly larger than the statistical errors. The uncertainty in (the determination of) $Y_{\rm P}$, is thus, larger than is currently claimed. Furthermore, most of the systematics lead to underestimate of $Y_{\rm P}$. The new upper bound allows cosmological models with no non-baryonic dark matter in which $\Omega_{baryons} = \Omega_{BBN} = \Omega_{dyn}$.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: ${\frac{R}{R_\oplus}} = (1.07-0.21\cdot \text{CMF})\cdot (\frac{M}{M_\oplus})^{1/3.7}$, where CMF stands for Core Mass Fraction. It is applicable to 1$\sim$8 M$_{\oplus}$ and CMF of 0.0$\sim$0.4. Applying this formula to Earth and Venus and several known small exoplanets with radii and masses measured to better than $\sim$30\% precision gives a CMF fit of $0.26\pm0.07$.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