223 research outputs found
Searching for Earth-mass planets around Centauri: precise radial velocities from contaminated spectra
This work is part of an ongoing project which aims to detect terrestrial
planets in our neighbouring star system Centauri using the Doppler
method. Owing to the small angular separation between the two components of the
Cen AB binary system, the observations will to some extent be
contaminated with light coming from the other star. We are accurately
determining the amount of contamination for every observation by measuring the
relative strengths of the H- and NaD lines. Furthermore, we have
developed a modified version of a well established Doppler code that is
modelling the observations using two stellar templates simultaneously. With
this method we can significantly reduce the scatter of the radial velocity
measurements due to spectral cross-contamination and hence increase our chances
of detecting the tiny signature caused by potential Earth-mass planets. After
correcting for the contamination we achieve radial velocity precision of for a given night of observations. We have also
applied this new Doppler code to four southern double-lined spectroscopic
binary systems (HR159, HR913, HR7578, HD181958) and have successfully recovered
radial velocities for both components simultaneously.Comment: accepted for publication in the International Journal of Astrobiology
(published by Cambridge University Press); will appear in a revised form,
subsequent to editorial input by Cambridge University Pres
G-band Spectral Synthesis in Solar Magnetic Concentrations
Narrow band imaging in the G-band is commonly used to trace the small
magnetic field concentrations of the Sun, although the mechanism that makes
them bright has remained unclear. We carry out LTE syntheses of the G-band in
an assorted set of semi-empirical model magnetic concentrations. The syntheses
include all CH lines as well as the main atomic lines within the band-pass. The
model atmospheres produce bright G-band spectra having many properties in
common with the observed G-band bright points. In particular, the contrast
referred to the quiet Sun is about twice the contrast in continuum wavelengths.
The agreement with observations does not depend on the specificities of the
model atmosphere, rather it holds from single fluxtubes to MIcro-Structured
Magnetic Atmospheres. However, the agreement requires that the real G-band
bright points are not spatially resolved, even in the best observations. Since
the predicted G-band intensities exceed by far the observed values, we foresee
a notable increase of contrast of the G-band images upon improvement of the
angular resolution. According to the LTE modeling, the G-band spectrum emerges
from the deep photosphere that produces the continuum. Our syntheses also
predict solar magnetic concentrations showing up in continuum images but not in
the G-band . Finally, we have examined the importance of the CH
photo-dissociation in setting the amount of G-band absorption. It turns out to
play a minor role.Comment: To appear in ApJ, 554 n2 Jun 20, 33 pages and 9 figure
Structure and Evolution of Nearby Stars with Planets. I. Short-Period Systems
Using the Yale stellar evolution code, we have calculated theoretical models
for nearby stars with planetary-mass companions in short-period nearly circular
orbits: 51 Pegasi, Tau Bootis, Upsilon Andromedae, Rho Cancri, and Rho Coronae
Borealis. We present tables listing key stellar parameters such as mass,
radius, age, and size of the convective envelope as a function of the
observable parameters (luminosity, effective temperature, and metallicity), as
well as the unknown helium fraction. For each star we construct best models
based on recently published spectroscopic data and the present understanding of
galactic chemical evolution. We discuss our results in the context of planet
formation theory, and, in particular, tidal dissipation effects and stellar
metallicity enhancements.Comment: 48 pages including 13 tables and 5 figures, to appear in Ap
On Planetary Companions to the MACHO-98-BLG-35 Microlens Star
We present observations of microlensing event MACHO-98-BLG-35 which reached a
peak magnification factor of almost 80. These observations by the Microlensing
Planet Search (MPS) and the MOA Collaborations place strong constraints on the
possible planetary system of the lens star and show intriguing evidence for a
low mass planet with a mass fraction . A giant planet with is excluded from 95%
of the region between 0.4 and 2.5 from the lens star, where is the
Einstein ring radius of the lens. This exclusion region is more extensive than
the generic "lensing zone" which is . For smaller mass planets,
we can exclude 57% of the "lensing zone" for and 14% of
the lensing zone for . The mass fraction corresponds to an Earth mass planet for a lensing star of mass \sim
0.3 \msun. A number of similar events will provide statistically significant
constraints on the prevalence of Earth mass planets. In order to put our limits
in more familiar terms, we have compared our results to those expected for a
Solar System clone averaging over possible lens system distances and
orientations. We find that such a system is ruled out at the 90% confidence
level. A copy of the Solar System with Jupiter replaced by a second Saturn mass
planet can be ruled out at 70% confidence. Our low mass planetary signal (few
Earth masses to Neptune mass) is significant at the confidence
level. If this planetary interpretation is correct, the MACHO-98-BLG-35 lens
system constitutes the first detection of a low mass planet orbiting an
ordinary star without gas giant planets.Comment: ApJ, April 1, 2000; 27 pages including 8 color postscript figure
MOA 2003-BLG-37: A Bulge Jerk-Parallax Microlens Degeneracy
We analyze the Galactic bulge microlensing event MOA-2003-BLG-37. Although
the Einstein timescale is relatively short, t_e=43 days, the lightcurve
displays deviations consistent with parallax effects due to the Earth's
accelerated motion. We show that the chi^2 surface has four distinct local
minima that are induced by the ``jerk-parallax'' degeneracy, with pairs of
solutions having projected Einstein radii, \tilde r_e = 1.76 AU and 1.28 AU,
respectively. This is the second event displaying such a degeneracy and the
first toward the Galactic bulge. For both events, the jerk-parallax formalism
accurately describes the offsets between the different solutions, giving hope
that when extra solutions exist in future events, they can easily be found.
However, the morphologies of the chi^2 surfaces for the two events are quite
different, implying that much remains to be understood about this degeneracy.Comment: 19 pages, 3 figures, 1 table, ApJ, in press, 1 July 200
Improving the Prospects for Detecting Extrasolar Planets in Gravitational Microlensing in 2002
Gravitational microlensing events of high magnification have been shown to be
promising targets for detecting extrasolar planets. However, only a few events
of high magnification have been found using conventional survey techniques.
Here we demonstrate that high magnification events can be readily found in
microlensing surveys using a strategy that combines high frequency sampling of
target fields with online difference imaging analysis. We present 10
microlensing events with peak magnifications greater than 40 that were detected
in real-time towards the Galactic Bulge during 2001 by MOA. We show that Earth
mass planets can be detected in future events such as these through intensive
follow-up observations around the event peaks. We report this result with
urgency as a similar number of such events are expected in 2002.Comment: 11 pages, 3 embedded ps figures including 2 colour, revised version
accepted by MNRA
A Planet Orbiting the Star Rho Coronae Borealis
We report the discovery of near-sinusoidal radial velocity variations of the
G0V star rhoCrB, with period 39.6 days and amplitude 67 m/s. These variations
are consistent with the existence of an orbital companion in a circular orbit.
Adopting a mass of 1.0 M(Sun) for the primary, the companion has minimum mass
about 1.1 Jupiter masses, and orbital radius about 0.23 AU. Such an orbital
radius is too large for tidal circularization of an initially eccentric orbit
during the lifetime of the star, and hence we suggest that the low eccentricity
is primordial, as would be expected for a planet formed in a dissipative
circumstellar disk.Comment: 9 pages, LaTeX, accepted in Astrophys. J. Letter
- âŠ