524 research outputs found
The circumbinary disk of HD 98800B: Evidence for disk warping
The quadruple young stellar system HD 98800 consists of two spectroscopic binary pairs with a circumbinary disk around the B component. Recent work by Boden and collaborators using infrared interferometry and radial velocity data resulted in a determination of the physical orbit for HD 98800B. We use the resulting inclination of the binary and the measured extinction toward the B component stars to constrain the distribution of circumbinary material. Although a standard optically and geometrically thick disk model can reproduce the spectral energy distribution, it cannot account for the observed extinction if the binary and the disk are coplanar. We next constructed a dynamical model to investigate the influence of the A component, which is not in the BaâBb orbital plane, on the B disk. We find that these interactions have a substantial impact on the inclination of the B circumbinary disk with respect to the BaâBb orbital plane. The resulting warp would be sufficient to place material into the line of sight and the noncoplanar disk orientation may also cause the upper layers of the disk to intersect the line of sight if the disk is geometrically thick. These simulations also support that the dynamics of the BaâBb orbit clear the inner region to a radius of~3 AU. We then discuss whether the somewhat unusual properties of the HD 98800B disk are consistent with material remnant from the star formation process or with more recent creation by collisions from larger bodies
Spitzer/MIPS Limits on Asteroidal Dust in the Pulsar Planetary System PSR B1257+1
With the MIPS camera on Spitzer, we have searched for far-infrared emission
from dust in the planetary system orbiting pulsar PSR 1257+12. With accuracies
of 0.05 mJy at 24 um and 1.5 mJy at 70 um, photometric measurements find no
evidence for emission at these wavelengths. These observations place new upper
limits on the luminosity of dust with temperatures between 20 and 1000 K. They
are particularly sensitive to dust temperatures of 100-200 K, for which they
limit the dust luminosity to below of the pulsar's spin-down
luminosity, three orders of magnitude better than previous limits. Despite
these improved constraints on dust emission, an asteroid belt similar to the
Solar System's cannot be ruled out
Abundant Circumstellar Silica Dust and SiO Gas Created by a Giant Hypervelocity Collision in the ~12 Myr HD172555 System
The fine dust detected by IR emission around the nearby Beta Pic analogue
star HD172555 is very peculiar. The dust mineralogy is composed primarily of
highly refractory, non-equilibrium materials, with approximately three-quarters
of the Si atoms in silica (SiO2) species. Tektite and obsidian lab thermal
emission spectra (non-equilibrium glassy silicas found in impact and magmatic
systems) are required to fit the data. The best-fit model size distribution for
the observed fine dust is dn/da = a-3.95 +/- 0.10. This steep a size
distribution, with abundant micron-sized particles, argues for a fresh source
of material within the last 0.1 Myr. The location of the dust with respect to
the star is at 5.8 +/- 0.6 AU (equivalent to 1.9 +/- 0.2 AU from the Sun),
within the terrestrial planet formation region but at the outer edge of any
possible terrestrial habitability zone. The mass of fine dust is 4 x 10^19 - 2
x 10^20 kg, equivalent to a 150 - 200 km radius asteroid. Significant emission
features centered at 4 and 8 um due to fluorescing SiO gas are also found.
Roughly 10^22 kg of SiO gas, formed by vaporizing silicate rock, is also
present in the system, and a separate population of very large, cool grains,
massing 10^21 - 10^22 kg and equivalent to the largest sized asteroid currently
found in the Solar System's main asteroid belt, dominates the solid
circumstellar material by mass. The makeup of the observed dust and gas, and
the noted lack of a dense circumstellar gas disk, strong primary x-ray
activity, or an extended disk of Beta-meteroids argues that the source of the
observed circumstellar materials is a giant hypervelocity (> 10 km sec^-1)
impact between large rocky planetesimals, similar to the ones which formed the
Moon and which stripped the surface crustal material off of Mercury's surface.Comment: 48 Pages, 8 Figures, 4 Tables Accepted for Publication in the
Astrophysical Journal 13-Jun-2009 References, Figures Updated 16-Jun-200
Hydrodynamic Simulations of Propagating Warps and Bending Waves In Accretion Discs
We present the results of a study of propagating warp or bending waves in
accretion discs. Three dimensional hydrodynamic simulations were performed
using SPH, and the results of these are compared with calculations based on the
linear theory of warped discs. We consider primarily the physical regime in
which the dimensionless viscosity parameter `alpha' < H/r, the disc aspect
ratio, so that bending waves are expected to propagate. We also present
calculations in which `alpha' > H/r, where the warps are expected to behave
diffusively. Small amplitude perturbations are studied in both Keplerian and
slightly non Keplerian discs, and we find that the SPH results can be
reasonably well fitted by those of the linear theory. The main results of these
calculations are: (1) the warp in Keplerian discs when `alpha' < H/r propagates
with little dispersion and damps at a rate expected from estimates of the code
viscosity, (2) warps evolve diffusively when `alpha' > H/r, (3) the non
Keplerian discs exhibit a substantially more dispersive behaviour of the warps.
Initially imposed higher amplitude nonlinear warping disturbances were studied
in Keplerian discs. The results indicate that nonlinear warps can lead to the
formation of shocks, and that the evolution of the warp becomes less wave-like
and more diffusive in character. This work is relevant to the study of the
warped accretion discs that may occur around Kerr black holes or in misaligned
binary systems. The results indicate that SPH can accurately model the
hydrodynamics of warped discs, even when using rather modest numbers of
particles.Comment: 14 pages, 9 figures, to appear in MNRA
Debris disks around Sun-like stars
We have observed nearly 200 FGK stars at 24 and 70 microns with the Spitzer
Space Telescope. We identify excess infrared emission, including a number of
cases where the observed flux is more than 10 times brighter than the predicted
photospheric flux, and interpret these signatures as evidence of debris disks
in those systems. We combine this sample of FGK stars with similar published
results to produce a sample of more than 350 main sequence AFGKM stars. The
incidence of debris disks is 4.2% (+2.0/-1.1) at 24 microns for a sample of 213
Sun-like (FG) stars and 16.4% (+2.8/-2.9) at 70 microns for 225 Sun-like (FG)
stars. We find that the excess rates for A, F, G, and K stars are statistically
indistinguishable, but with a suggestion of decreasing excess rate toward the
later spectral types; this may be an age effect. The lack of strong trend among
FGK stars of comparable ages is surprising, given the factor of 50 change in
stellar luminosity across this spectral range. We also find that the incidence
of debris disks declines very slowly beyond ages of 1 billion years.Comment: ApJ, in pres
Tourism income and economic growth in Greece: Empirical evidence from their cyclical components
This paper examines the relationship between the cyclical
components of Greek GDP and international tourism income for
Greece for the period 1976â2004. Using spectral analysis the authors
find that cyclical fluctuations of GDP have a length of about nine
years and that international tourism income has a cycle of about
seven years. The volatility of tourism income is more than eight
times the volatility of the Greek GDP cycle. VAR analysis shows that
the cyclical component of tourism income is significantly influencing
the cyclical component of GDP in Greece. The findings support the
tourism-led economic growth hypothesis and are of particular
interest and importance to policy makers, financial analysts and
investors dealing with the Greek tourism industry
Three-dimensional Calculations of High and Low-mass Planets Embedded in Protoplanetary Discs
We analyse the non-linear, three-dimensional response of a gaseous, viscous
protoplanetary disc to the presence of a planet of mass ranging from one Earth
mass (1 M) to one Jupiter mass (1 M) by using the ZEUS hydrodynamics
code. We determine the gas flow pattern, and the accretion and migration rates
of the planet. The planet is assumed to be in a fixed circular orbit about the
central star. It is also assumed to be able to accrete gas without expansion on
the scale of its Roche radius. Only planets with masses M \gsim 0.1 M
produce significant perturbations in the disc's surface density. The flow
within the Roche lobe of the planet is fully three-dimensional. Gas streams
generally enter the Roche lobe close to the disc midplane, but produce much
weaker shocks than the streams in two-dimensional models. The streams supply
material to a circumplanetary disc that rotates in the same sense as the
planet's orbit. Much of the mass supply to the circumplanetary disc comes from
non-coplanar flow. The accretion rate peaks with a planet mass of approximately
0.1 M and is highly efficient, occurring at the local viscous rate. The
migration timescales for planets of mass less than 0.1 M, based on torques
from disc material outside the planets' Roche lobes, are in excellent agreement
with the linear theory of Type I (non-gap) migration for three-dimensional
discs. The transition from Type I to Type II (gap) migration is smooth, with
changes in migration times of about a factor of 2. Starting with a core which
can undergo runaway growth, a planet can gain up to a few M with little
migration. Planets with final masses of order 10 M would undergo large
migration, which makes formation and survival difficult.Comment: Accepted by MNRAS, 18 pages, 13 figures (6 degraded resolution).
Paper with high-resolution figures available at
http://www.astro.ex.ac.uk/people/mbate
New Debris Disks Around Nearby Main Sequence Stars: Impact on The Direct Detection of Planets
Using the MIPS instrument on the Spitzer telescope, we have searched for
infrared excesses around a sample of 82 stars, mostly F, G, and K main-sequence
field stars, along with a small number of nearby M stars. These stars were
selected for their suitability for future observations by a variety of
planet-finding techniques. These observations provide information on the
asteroidal and cometary material orbiting these stars - data that can be
correlated with any planets that may eventually be found. We have found
significant excess 70um emission toward 12 stars. Combined with an earlier
study, we find an overall 70um excess detection rate of % for mature
cool stars. Unlike the trend for planets to be found preferentially toward
stars with high metallicity, the incidence of debris disks is uncorrelated with
metallicity. By newly identifying 4 of these stars as having weak 24um excesses
(fluxes 10% above the stellar photosphere), we confirm a trend found in
earlier studies wherein a weak 24um excess is associated with a strong 70um
excess. Interestingly, we find no evidence for debris disks around 23 stars
cooler than K1, a result that is bolstered by a lack of excess around any of
the 38 K1-M6 stars in 2 companion surveys. One motivation for this study is the
fact that strong zodiacal emission can make it hard or impossible to detect
planets directly with future observatories like the {\it Terrestrial Planet
Finder (TPF)}. The observations reported here exclude a few stars with very
high levels of emission, 1,000 times the emission of our zodiacal cloud,
from direct planet searches. For the remainder of the sample, we set relatively
high limits on dust emission from asteroid belt counterparts
Spitzer IRAC Photometry for Time Series in Crowded Fields
We develop a new photometry algorithm that is optimized for time
series in crowded fields and that is particularly adapted to faint and/or
heavily blended targets. We apply this to the 170 targets from the 2015
microlensing campaign and present the results of three variants of
this algorithm in an online catalog. We present detailed accounts of the
application of this algorithm to two difficult cases, one very faint and the
other very crowded. Several of 's instrumental characteristics that
drive the specific features of this algorithm are shared by and
, implying that these features may prove to be a useful starting point
for algorithms designed for microlensing campaigns by these other missions.Comment: accepted for publication in The Astrophysical Journal Supplement,
online catalog available at http://www.astronomy.ohio-state.edu/Spitzer2015
Magnetospheric Emission from Extrasolar Planets
The magnetospheric emissions from extrasolar planets represent a science
frontier for the next decade. All of the solar system giant planets and the
Earth produce radio emissions as a result of interactions between their
magnetic fields and the solar wind. In the case of the Earth, its magnetic
field may contribute to its habitability by protecting its atmosphere from
solar wind erosion and by preventing energetic particles from reaching its
surface. Indirect evidence for at least some extrasolar giant planets also
having magnetic fields includes the modulation of emission lines of their host
stars phased with the planetary orbits, likely due to interactions between the
stellar and planetary magnetic fields. If magnetic fields are a generic
property of giant planets, then extrasolar giant planets should emit at radio
wavelengths allowing for their direct detection. Existing observations place
limits comparable to the flux densities expected from the strongest emissions.
Additional sensitivity at low radio frequencies coupled with algorithmic
improvements likely will enable a new means of detection and characterization
of extrasolar planets within the next decade.Comment: Science white paper for Astro2010; submitted to PSF pane
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