1,423 research outputs found
Thermal Structure and Radius Evolution of Irradiated Gas Giant Planets
We consider the thermal structure and radii of strongly irradiated gas giant
planets over a range in mass and irradiating flux. The cooling rate of the
planet is sensitive to the surface boundary condition, which depends on the
detailed manner in which starlight is absorbed and energy redistributed by
fluid motion. We parametrize these effects by imposing an isothermal boundary
condition below the photosphere, and then constrain
from the observed masses and radii. We compute the dependence of
luminosity and core temperature on mass, and core entropy,
finding that simple scalings apply over most of the relevant parameter space.
These scalings yield analytic cooling models which exhibit power-law behavior
in the observable age range , and are confirmed by
time-dependent cooling calculations. We compare our model to the radii of
observed transiting planets, and derive constraints on . Only HD
209458 has a sufficiently accurate radius measurement that is
tightly constrained; the lower error bar on the radii for other planets is
consistent with no irradiation. More accurate radius and age measurements will
allow for a determination of the correlation of with the
equilibrium temperature, informing us about both the greenhouse effect and
day-night asymmetries.Comment: submitted to apj. 14 pages, 20 figure
Frequency Analysis of Reflex Velocities of Stars with Planets
Since it has become possible to discovery planets orbiting nearby solar-type
stars through very precise Doppler-shift measurements, the role of methods used
to analyze such observations has grown significantly. The widely employed
model-dependent approach based on the least-squares fit of the Keplerian motion
to the radial-velocity variations can be, as we show, unsatisfactory. Thus, in
this paper, we propose a new method that may be easily and successfully applied
to the Doppler-shift measurements. This method allows us to analyze the data
without assuming any specific model and yet to extract all significant features
of the observations. This very simple idea, based on the subsequent subtraction
of all harmonic components from the data, can be easily implemented. We show
that our method can be used to analyze real 16 Cygni B Doppler-shift
observations with a surprising but correct result which is substantially
different from that based on the least-squares fit of a Keplerian orbit.
Namely, using frequency analysis we show that with the current accuracy of this
star's observations it is not possible to determine the value of the orbital
eccentricity which is claimed to be as high as 0.6.Comment: AASLaTeX + 5 figures (eps files), 22 pages, two figures delated,
typos corrections; accepted for publication in Ap
A Search for Optical Laser Emission from Alpha Centauri AB
A search for laser light from the directions of Alpha Centauri A and B was
performed by examining 15362 optical, high-resolution spectra obtained between
2004 and 2018. None of the spectra exhibit laser emission lines. The threshold
was 10% of the continuum intensity of the spectra of both stars at all
wavelengths between 3850 and 6900 {\AA}. This search would have revealed
optical laser light from the directions of Alpha Cen B if the laser had a power
at least 1.4 to 5.4 MW (depending on wavelength) and was positioned within the
1 arcsecond field of view (projecting to 1.3 AU), for a benchmark 10-meter
laser launcher. For Alpha Cen A, the laser power must be 3 times greater for
detection. Lasers of smaller aperture would also have been detected but would
require more power. Considering all optical surveys, a growing desert is
emerging in the search for extraterrestrial technology.Comment: Dedicated to the late Frank Drak
Observations of magnetic fields on solar-type stars
Magnetic-field observations were carried out for 29 G and K main-sequence stars. The area covering-factors of magnetic regions tends to be greater in the K dwarfs than in the G dwarfs. However, no spectral-type dependence is found for the field strengths, contrary to predictions that pressure equilibrium with the ambient photospheric gas pressure would determine the surface field strengths. Coronal soft X-ray fluxes from the G and K dwarfs correlate well with the fraction of the stellar surface covered by magnetic regions. The dependence of coronal soft X-ray fluxes on photospheric field strengths is consistent with Stein's predicted generation-rates for Alfven waves. These dependences are inconsistent with the one dynamo model for which a specific prediction is offered. Finally, time variability of magnetic fields is seen on the two active stars that have been extensively monitored. Significant changes in magnetic fields are seen to occur on timescales as short as one day
Causes of sprawl: A portrait from space
We study the extent to which US urban development is sprawling and consider what determines differences in sprawl across space. Using remote-sensing data to track the evolution of land use on a grid of 8.7 billion 30x30 metre cells, we measure sprawl as the amount of undeveloped land surrounding an average urban dwelling. On this measure, while the extent of sprawl remained roughly unchanged between 1976 and 1992, it varied dramatically across metropolitan areas. Ground water availability, temperate climate, rugged terrain, decentralized employment, early public transport infrastructure, uncertainty about metropolitan growth, and unincorporated land in the urban fringe all increase sprawl.urban sprawl; land development; remote sensing
Marcy G. Beyer Interview
Transcript of an oral history interview with Marcy G. Meyer by John Ernst on her experiences during the Vietnam War on May 31, 1997
Planet Formation with Migration
In the core-accretion model, gas-giant planets form solid cores which then
accrete gaseous envelopes. Tidal interactions with disk gas cause a core to
undergo inward type-I migration in 10^4 to 10^5 years. Cores must form faster
than this to survive. Giant planets clear a gap in the disk and undergo inward
type-II migration in <10^6 years if observed disk accretion rates apply to the
disk as a whole. Type-II migration times exceed typical disk lifetimes if
viscous accretion occurs mainly in the surface layers of disks. Low turbulent
viscosities near the midplane may allow planetesimals to form by coagulation of
dust grains. The radius r of such planetesimals is unknown. If r<0.5 km, the
core formation time is shorter than the type-I migration timescale and cores
will survive. Migration is substantial in most cases, leading to a wide range
of planetary orbits, consistent with the observed variety of extrasolar
systems. When r is of order 100m and midplane alpha is of order 3 times 10^-5,
giant planets similar to those in the Solar System can form.Comment: 12 pages including 4 figure
Precise radial velocities of giant stars. IV. A correlation between surface gravity and radial velocity variation and a statistical investigation of companion properties
Since 1999, we have been conducting a radial velocity survey of 179 K giants
using the CAT at UCO/Lick observatory. At present ~20-100 measurements have
been collected per star with a precision of 5 to 8 m/s. Of the stars monitored,
145 (80%) show radial velocity (RV) variations at a level >20 m/s, of which 43
exhibit significant periodicities. Our aim is to investigate possible
mechanism(s) that cause these observed RV variations. We intend to test whether
these variations are intrinsic in nature, or possibly induced by companions, or
both. In addition, we aim to characterise the parameters of these companions. A
relation between log g and the amplitude of the RV variations is investigated
for all stars in the sample. Furthermore, the hypothesis that all periodic RV
variations are caused by companions is investigated by comparing their inferred
orbital statistics with the statistics of companions around main sequence
stars. A strong relation is found between the amplitude of the RV variations
and log g in K giant stars, as suggested earlier by Hatzes & Cochran (1998).
However, most of the stars exhibiting periodic variations are located above
this relation. These RV variations can be split in a periodic component which
is not correlated with log g and a random residual part which does correlate
with log g. Compared to main-sequence stars, K giants frequently exhibit
periodic RV variations. Interpreting these RV variations as being caused by
companions, the orbital param eters are different from the companions orbiting
dwarfs. Intrinsic mechanisms play an important role in producing RV variations
in K giants stars, as suggested by their dependence on log g. However, it
appears that periodic RV variations are additional to these intrinsic
variations, consistent with them being caused by companions.Comment: 10 pages, accepted by A&
The metal-rich nature of stars with planets
With the goal of confirming the metallicity "excess" present in stars with
planetary-mass companions, we present in this paper a high-precision
spectroscopic study of a sample of dwarfs included in the CORALIE extrasolar
planet survey. The targets were chosen according to the basic criteria that 1)
they formed part of a limited volume and 2) they did not present the signature
of a planetary host companion. A few stars with planets were also observed and
analysed; namely, HD 6434, HD 13445, HD 16141, HD 17051, HD 19994, HD 22049, HD
28185, HD 38529, HD 52265, HD 190228, HD 210277 and HD 217107. For some of
these objects there had been no previous spectroscopic studies. The
spectroscopic analysis was done using the same technique as in previous work on
the metallicity of stars with planets, thereby permitting a direct comparison
of the results. The work described in this paper thus represents the first
uniform and unbiased comparison between stars with and without planetary-mass
companions in a volume-limited sample. The results show that 1) stars with
planets are significantly metal-rich, and 2) that the source of the metallicity
is most probably "primordial". The results presented here may impose serious
constraints on planetary system formation and evolution models.Comment: 14 pages, 7 figures, A&A in pres
Space-borne global astrometric surveys: the hunt for extra-solar planets
The proposed global astrometry mission {\it GAIA}, recently recommended
within the context of ESA's Horizon 2000 Plus long-term scientific program,
appears capable of surveying the solar neighborhood within 200 pc for
the astrometric signatures of planets around stars down to the magnitude limit
of =17 mag, which includes late M dwarfs at 100 pc. Realistic end-to-end
simulations of the GAIA global astrometric measurements have yielded first
quantitative estimates of the sensitivity to planetary perturbations and of the
ability to measure their orbital parameters. Single Jupiter-mass planets around
normal solar-type stars appear detectable up to 150 pc (12 mag) with
probabilities 50 per cent for orbital periods between 2.5 and
8 years, and their orbital parameters measured with better than 30 per
cent accuracy to about 100 pc. Jupiter-like objects (same mass and period as
our giant planet) are found with similar probabilities up to 100 pc.These first
experiments indicate that the {\it GAIA} results would constitute an important
addition to those which will come from the other ongoing and planned
planet-search programs. These data combined would provide a formidable testing
ground on which to confront theories of planetary formation and evolution.Comment: 13 pages, 10 figures, uses mn.sty, accepted by MNRA
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