335 research outputs found
The Extrasolar Planet epsilon Eridani b - Orbit and Mass
Hubble Space Telescope observations of the nearby (3.22 pc), K2 V star
epsilon Eridani have been combined with ground-based astrometric and radial
velocity data to determine the mass of its known companion. We model the
astrometric and radial velocity measurements simultaneously to obtain the
parallax, proper motion, perturbation period, perturbation inclination, and
perturbation size. Because of the long period of the companion, \eps b, we
extend our astrometric coverage to a total of 14.94 years (including the three
year span of the \HST data) by including lower-precision ground-based
astrometry from the Allegheny Multichannel Astrometric Photometer. Radial
velocities now span 1980.8 -- 2006.3. We obtain a perturbation period, P = 6.85
+/- 0.03 yr, semi-major axis, alpha =1.88 +/- 0.20 mas, and inclination i =
30.1 +/- 3.8 degrees. This inclination is consistent with a previously measured
dust disk inclination, suggesting coplanarity. Assuming a primary mass M_* =
0.83 M_{\sun}, we obtain a companion mass M = 1.55 +/- 0.24 M_{Jup}. Given the
relatively young age of epsilon Eri (~800 Myr), this accurate exoplanet mass
and orbit can usefully inform future direct imaging attempts. We predict the
next periastron at 2007.3 with a total separation, rho = 0.3 arcsec at position
angle, p.a. = -27 degrees. Orbit orientation and geometry dictate that epsilon
Eri b will appear brightest in reflected light very nearly at periastron.
Radial velocities spanning over 25 years indicate an acceleration consistent
with a Jupiter-mass object with a period in excess of 50 years, possibly
responsible for one feature of the dust morphology, the inner cavity
Possible Observational Criteria for Distinguishing Brown Dwarfs from Planets
The difference in formation process between binary stars and planetary
systems is reflected in their composition as well as their orbital
architecture, particularly orbital eccentricity as a function of orbital
period. It is suggested here that this difference can be used as an
observational criterion to distinguish between brown dwarfs and planets.
Application of the orbital criterion suggests that with three possible
exceptions, all of the recently-discovered substellar companions discovered to
date may be brown dwarfs and not planets. These criterion may be used as a
guide for interpretation of the nature of sub-stellar mass companions to stars
in the future.Comment: LaTeX, 11 pages including 2 figures, accepted for publication in the
Astrophysical Journal Letter
Astrometric Methods and Instrumentation to Identify and Characterize Extrasolar Planets: A Review
I present a review of astrometric techniques and instrumentation utilized to
search for, detect, and characterize extra-solar planets. First, I briefly
summarize the properties of the present-day sample of extrasolar planets, in
connection with predictions from theoretical models of planet formation and
evolution. Next, the generic approach to planet detection with astrometry is
described, with significant discussion of a variety of technical, statistical,
and astrophysical issues to be faced by future ground-based as well as
space-borne efforts in order to achieve the required degree of measurement
precision. After a brief summary of past and present efforts to detect planets
via milli-arcsecond astrometry, I then discuss the planet-finding capabilities
of future astrometric observatories aiming at micro-arcsecond precision.
Lastly, I outline a number experiments that can be conducted by means of
high-precision astrometry during the next decade, to illustrate its potential
for important contributions to planetary science, in comparison with other
indirect and direct methods for the detection and characterization of planetary
systems.Comment: 61 pages, 8 figures, PASP, accepted (October 2005 issue
Theoretical Spectra and Atmospheres of Extrasolar Giant Planets
We present a comprehensive theory of the spectra and atmospheres of
irradiated extrasolar giant planets. We explore the dependences on stellar
type, orbital distance, cloud characteristics, planet mass, and surface
gravity. Phase-averaged spectra for specific known extrasolar giant planets
that span a wide range of the relevant parameters are calculated, plotted, and
discussed. The connection between atmospheric composition and emergent spectrum
is explored in detail. Furthermore, we calculate the effect of stellar
insolation on brown dwarfs. We review a variety of representative observational
techniques and programs for their potential for direct detection, in light of
our theoretical expectations, and we calculate planet-to-star flux ratios as a
function of wavelength. Our results suggest which spectral features are most
diagnostic of giant planet atmospheres and reveal the best bands in which to
image planets of whatever physical or orbital characteristics.Comment: 47 pages, plus 36 postscript figures; with minor revisions, accepted
to the Astrophysical Journal, May 10, 2003 issu
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
MOST discovers a multimode delta Scuti star in a triple system: HD 61199
A field star, HD 61199 (V ~ 8), simultaneously observed with Procyon by the
MOST (Microvariability & Oscillations of STars) satellite in continuous runs of
34, 17, and 34 days in 2004, 2005, and 2007, was found to pulsate in 11
frequencies in the delta Scuti range with amplitudes from 1.7 down to 0.09
mmag. The photometry also showed variations with a period of about four days.
To investigate the nature of the longer period, 45 days of time-resolved
spectroscopy was obtained at the Thueringer Landessternwarte Tautenburg in
2004. The radial velocity measurements indicate that HD 61199 is a triple
system. A delta Scuti pulsator with a rich eigenspectrum in a multiple system
is promising for asteroseismology. Our objectives were to identify which of the
stars in the system is the delta Scuti variable and to obtain the orbital
elements of the system and the fundamental parameters of the individual
components, which are constrained by the pulsation frequencies of the delta
Scuti star. Classical Fourier techniques and least-squares multi-sinusoidal
fits were applied to the MOST photometry to identify the pulsation frequencies.
The groundbased spectroscopy was analysed with least-squares-deconvolution
(LSD) techniques, and the orbital elements derived with the KOREL and ORBITX
routines. Asteroseismic models were also generated. The photometric and
spectroscopic data are compatible with a triple system consisting of a close
binary with an orbital period of 3.57 days and a delta Scuti companion (HD
61199,A) as the most luminous component. The delta Scuti star is a rapid
rotator with about vsin i = 130 km/s and an upper mass limit of about 2.1 Msun.
For the close binary components, we find they are of nearly equal mass, with
lower mass limits of about 0.7 Msun.Comment: 11 pages, 14 figures, accepted by A&
The Discovery of a Planetary Companion to 16 Cygni B
High precision radial velocity observations of the solar-type star 16 Cygni B
taken at McDonald Observatory and at Lick Observatory, have each independently
discovered periodic radial-velocity variations indicating the presence of a
Jovian-mass companion to this star. The orbital fit to the combined data gives
a period of 800.8 days, a velocity amplitude of 43.9 m/s, and an eccentricity
of 0.63. This is the largest eccentricity of any planetary system discovered so
far. Assuming that 16 Cygni B has a mass of 1.0 Msun, this implies a mass for
the companion of 1.5 sin i Jupiter masses. While the mass of this object is
well within the range expected for planets, the large orbital eccentricity
cannot be explained simply by the standard model of growth of planets in a
protostellar disk. It is possible that this object was formed in the normal
manner with a low eccentricity orbit, and has undergone post-formational
orbital evolution, either through the same process which formed the ``massive
eccentric'' planets around 70 Virginis and HD114762, or by gravitational
interactions with the companion star 16 Cygni A. It is also possible that the
object is an extremely low mass brown dwarf, formed through fragmentation of
the collapsing protostar. We explore a possible connection between stellar
photospheric Li depletion, pre-main sequence stellar rotation, the presence of
a massive proto-planetary disk, and the formation of a planetary companion.Comment: 18 pages, 3 PostScript figures, Latex, uses aaspp4 macros, submitted
to Astrophysical Journa
The production of trust during organizational change
This paper investigates the relationships between organizational change and trust in management. It is argued that organizational change represents a critical episode for the production and destruction of trust in management. Although trust in management is seen as a semi stable psychological state, changes in organizations make trust issues salient and organizational members attend to and process trust relevant information resulting in a reassessment of their trust in management. The direction and magnitude of change in trust is dependent on a set of change dimensions that reflect trust relevant experiences and information. We distinguish between dimensions related to trust relevant consequences of the change and trust relevant aspects of how the change process is performed. Empirical results indicate that increases in post change emotional stress and the use of referential accounts for justifying change are both negatively related to post change trust in management. The use of ideological accounts and participation were found to be positively related to post change trust in management, so was perceived decision quality. Findings also indicate that the effects of change on trust are negatively moderated by tenure
1955: Abilene Christian College Bible Lectures - Full Text
Delivered in the Auditorium of Abilene Christian College, February, 1955
ABILENE, TEXAS
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A mathematical model of brain glucose homeostasis
This is an Open Access article distributed under the terms of the Creative Commons Attribution Licens
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