1,707 research outputs found
Optical Albedo Theory of Strongly-Irradiated Giant Planets: The Case of HD 209458b
We calculate a new suite of albedo models for close-in extrasolar giant
planets and compare with the recent stringent upper limit for HD 209458b of
Rowe et al. using MOST. We find that all models without scattering clouds are
consistent with this optical limit. We explore the dependence on wavelength and
waveband, metallicity, the degree of heat redistribution, and the possible
presence of thermal inversions and find a rich diversity of behaviors.
Measurements of transiting extrasolar giant planets (EGPs) at short wavelengths
by MOST, Kepler, and CoRoT, as well as by proposed dedicated multi-band
missions, can complement measurements in the near- and mid-IR using {\it
Spitzer} and JWST. Collectively, such measurements can help determine
metallicity, compositions, atmospheric temperatures, and the cause of thermal
inversions (when they arise) for EGPs with a broad range of radii, masses,
degrees of stellar insolation, and ages. With this paper, we reappraise and
highlight the diagnostic potential of albedo measurements of hot EGPs shortward
of 1.3 m.Comment: 6 pages, 1 table, 1 color figure; accepted to the Astrophysical
Journa
IC 4200: a gas-rich early-type galaxy formed via a major merger
We present the result of radio and optical observations of the S0 galaxy IC
4200. We find that the galaxy hosts 8.5 billion solar masses of HI rotating on
a ~90 deg warped disk extended out to 60 kpc from the centre of the galaxy.
Optical spectroscopy reveals a simple-stellar-population-equivalent age of 1.5
Gyr in the centre of the galaxy and V- and R-band images show stellar shells.
Ionised gas is observed within the stellar body and is kinematically decoupled
from the stars and characterised by LINER-like line ratios.We interpret these
observational results as evidence for a major merger origin of IC 4200, and
date the merger back to 1-3 Gyr ago.Comment: Accepted for publication in Astronomy & Astrophysics; 18 pages, 13
figures; the tables of Appendix C can be downloaded at
http://www.astro.rug.nl/~pserra/IC420
Optical Spectral Singularities and Coherent Perfect Absorption in a Two-Layer Spherical Medium
An optical spectral singularity is a zero-width resonance that corresponds to
lasing at threshold gain. Its time-reversal causes coherent perfect absorption
of light and forms the theoretical basis of antilasing. In this article we
explore optical spectral singularities of a two-layer spherical medium. In
particular, we examine the cases that a gain medium is coated by a thin layer
of high-refractive index glass and a spherical glass covered by a layer of gain
material. In the former case, the coating reduces the minimum radius required
for exciting spectral singularities and gives rise to the formation of clusters
of spectral singularities separated by wide spectral gaps. In the latter case,
the coating leads to a doubling of the number of spectral singularities.Comment: 19 pages, 1 table, 10 figures, accepted for publication in Proc. R.
Soc.
Large-scale HI in nearby radio galaxies: segregation in neutral gas content with radio source size
We present results of a study of neutral hydrogen (HI) in a complete sample
of nearby non-cluster radio galaxies. We find that radio galaxies with large
amounts of extended HI (M_HI >= 10^9 M_solar) all have a compact radio source.
The host galaxies of the more extended radio sources, all of Fanaroff & Riley
type-I, do not contain these amounts of HI. We discuss several possible
explanations for this segregation. The large-scale HI is mainly distributed in
disk- and ring-like structures with sizes up to 190 kpc and masses up to 2 x
10^10 M_solar. The formation of these structures could be related to past
merger events, although in some cases it may also be consistent with a
cold-accretion scenario.Comment: 4 pages, 2 figures. Accepted for publication in A&A Letters. A
version with full resolution figures can be found at
http://www.astro.rug.nl/~emonts/emonts_HIletter_jan07.pd
The Kinematic Properties of the Extended Disks of Spiral Galaxies: A Sample of Edge-On Galaxies
We present a kinematic study of the outer regions (R_25<R<2 R_25) of 17
edge-on disk galaxies. Using deep long-slit spectroscopy (flux sensitivity a
few 10^-19 erg s^-1 cm^-2 arcsec^-2), we search for H-alpha emission, which
must be emitted at these flux levels by any accumulation of hydrogen due to the
presence of the extragalactic UV background and any other, local source of UV
flux. We present results from the individual galaxy spectra and a stacked
composite. We detect H-alpha in many cases well beyond R_25 and sometimes as
far as 2 R_25. The combination of sensitivity, spatial resolution, and
kinematic resolution of this technique thus provides a powerful complement to
21-cm observations. Kinematics in the outer disk are generally disk-like (flat
rotation curves, small velocity dispersions) at all radii, and there is no
evidence for a change in the velocity dispersion with radius. We place strong
limits, few percent, on the existence of counter-rotating gas out to 1.5 R_25.
These results suggest that thin disks extend well beyond R_25; however, we also
find a few puzzling anomalies. In ESO 323-G033 we find two emission regions
that have velocities close to the systemic velocity rather than the expected
rotation velocity. These low relative velocities are unlikely to be simply due
to projection effects and so suggest that these regions are not on disk-plane,
circular orbits. In MCG-01-31-002 we find emission from gas with a large
velocity dispersion that is co-rotating with the inner disk.Comment: 18 pages, 14 figures, accepted for publication in Ap
Infrared dust emission in the outer disk of M51
We examine faint infrared emission features detected in Spitzer Space
Telescope images of M51, which are associated with atomic hydrogen in the outer
disk and tidal tail at R greater than R_25 (4.9', ~14 kpc at d=9.6 Mpc). The
infrared colors of these features are consistent with the colors of dust
associated with star formation in the bright disk. However, the star formation
efficiency (as a ratio of star formation rate to neutral gas mass) implied in
the outer disk is lower than that in the bright disk of M51 by an order of
magnitude, assuming a similar relationship between infrared emission and star
formation rate in the inner and outer disks.Comment: 13 pages in manuscript form, 2 figures; download PDF of manuscript
with original-resolution Figure 1 at
http://www.eg.bucknell.edu/physics/thornley/thornleym51.pd
Vacuum friction in rotating particles
We study the frictional torque acting on particles rotating in empty space.
At zero temperature, vacuum friction transforms mechanical energy into light
emission and produces particle heating. However, particle cooling relative to
the environment occurs at finite temperatures and low rotation velocities.
Radiation emission is boosted and its spectrum significantly departed from a
hot-body emission profile as the velocity increases. Stopping times ranging
from hours to billions of years are predicted for materials, particle sizes,
and temperatures accessible to experiment. Implications for the behavior of
cosmic dust are discussed.Comment: 4 figures, 10 pages, includes paper and supplementary information in
the appendi
From gas to galaxies
The unsurpassed sensitivity and resolution of the Square Kilometer Array
(SKA) will make it possible for the first time to probe the continuum emission
of normal star forming galaxies out to the edges of the universe. This opens
the possibility for routinely using the radio continuum emission from galaxies
for cosmological research as it offers an independent probe of the evolution of
the star formation density in the universe. In addition it offers the
possibility to detect the first star forming objects and massive black holes.
In deep surveys SKA will be able to detect HI in emission out to redshifts of
and hence be able to trace the conversion of gas into stars
over an era where considerable evolution is taking place. Such surveys will be
able to uniquely determine the respective importance of merging and accreting
gas flows for galaxy formation over this redshift range (i.e. out to when the
universe was only one third its present age). It is obvious that only SKA will
able to see literally where and how gas is turned into stars.
These and other aspects of SKA imaging of galaxies will be discussed.Comment: To be published in New Astronomy Reviews, Elsevier, Amsterdam as part
of "Science with the Square Kilometre Array", eds. C. Carilli and S.
Rawlings. 18 pages + 13 figures; high resolution version and other chapters
of "Science with the Square Kilometre Array" available at
http://www.skatelescope.org/pages/science_gen.ht
Modelling of laboratory data of bi-directional reflectance of regolith surface containing Alumina
Bidirectional reflectance of a surface is defined as the ratio of the
scattered radiation at the detector to the incident irradiance as a function of
geometry. The accurate knowledge of the bidirectional reflection function (BRF)
of layers composed of discrete, randomly positioned scattering particles is
very essential for many remote sensing, engineering, biophysical applications
and in different areas of Astrophysics. The computations of BRF's for plane
parallel particulate layers are usually reduced to solve the radiative transfer
equation (RTE) by the existing techniques. In this work we present our
laboratory data on bidirectional reflectance versus phase angle for two sample
sizes of 0.3 and 1 of Alumina for the He-Ne laser at 632.8 nm (red) and
543.5nm(green) wavelength. The nature of the phase curves of the asteroids
depends on the parameters like- particle size, composition, porosity, roughness
etc. In our present work we analyse the data which are being generated using
single scattering phase function i.e. Mie theory considering particles to be
compact sphere. The well known Hapke formula will be considered along with
different particle phase function such as Mie and Henyey Greenstein etc to
model the laboratory data obtained at the asteroid laboratory of Assam
University.Comment: 5 pages, 5 figures [accepted for publication in Publications of the
Astronomical Society of Australia (PASA) on 8 June, 2011
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