124 research outputs found
Ultraviolet Imaging Polarimetry of the Large Magellanic Cloud. II. Models
Motivated by new sounding-rocket wide-field polarimetric images of the Large
Magellanic Cloud, we have used a three-dimensional Monte Carlo radiation
transfer code to investigate the escape of near-ultraviolet photons from young
stellar associations embedded within a disk of dusty material (i.e. a galaxy).
As photons propagate through the disk, they may be scattered or absorbed by
dust. Scattered photons are polarized and tracked until they escape to be
observed; absorbed photons heat the dust, which radiates isotropically in the
far-infrared, where the galaxy is optically thin. The code produces four output
images: near- UV and far-IR flux, and near-UV images in the linear Stokes
parameters Q and U. From these images we construct simulated UV polarization
maps of the LMC. We use these maps to place constraints on the star + dust
geometry of the LMC and the optical properties of its dust grains. By tuning
the model input parameters to produce maps that match the observed polarization
maps, we derive information about the inclination of the LMC disk to the plane
of the sky, and about the scattering phase function g. We compute a grid of
models with i = 28 deg., 36 deg., and 45 deg., and g = 0.64, 0.70, 0.77, 0.83,
and 0.90. The model which best reproduces the observed polarization maps has i
= 36 +2/-5 degrees and g ~0.7. Because of the low signal-to-noise in the data,
we cannot place firm constraints on the value of g. The highly inclined models
do not match the observed centro-symmetric polarization patterns around bright
OB associations, or the distribution of polarization values. Our models
approximately reproduce the observed ultraviolet photopolarimetry of the
western side of the LMC; however, the output images depend on many input
parameters and are nonunique.Comment: Accepted to AJ. 20 pages, 7 figure
Optical Follow-up of New SMC Wing Be/X-ray Binaries
We investigate the optical counterparts of recently discovered Be/X-ray
binaries in the Small Magellanic Cloud. In total four sources, SXP101, SXP700,
SXP348 and SXP65.8 were detected during the Chandra Survey of the Wing of the
SMC. SXP700 and SXP65.8 were previously unknown. Many optical ground based
telescopes have been utilised in the optical follow-up, providing coverage in
both the red and blue bands. This has led to the classification of all of the
counterparts as Be stars and confirms that three lie within the Galactic
spectral distribution of known Be/X-ray binaries. SXP101 lies outside this
distribution becoming the latest spectral type known. Monitoring of the Halpha
emission line suggests that all the sources bar SXP700 have highly variable
circumstellar disks, possibly a result of their comparatively short orbital
periods. Phase resolved X-ray spectroscopy has also been performed on SXP65.8,
revealing that the emission is indeed harder during the passage of the X-ray
beam through the line of sight.Comment: 9 pages, 9 figures, 2 tables, accepted for publication in MNRA
Magnetic Field Measurement with Ground State Alignment
Observational studies of magnetic fields are crucial. We introduce a process
"ground state alignment" as a new way to determine the magnetic field direction
in diffuse medium. The alignment is due to anisotropic radiation impinging on
the atom/ion. The consequence of the process is the polarization of spectral
lines resulting from scattering and absorption from aligned atomic/ionic
species with fine or hyperfine structure. The magnetic field induces precession
and realign the atom/ion and therefore the polarization of the emitted or
absorbed radiation reflects the direction of the magnetic field. The atoms get
aligned at their low levels and, as the life-time of the atoms/ions we deal
with is long, the alignment induced by anisotropic radiation is susceptible to
extremely weak magnetic fields (G). In fact,
the effects of atomic/ionic alignment were studied in the laboratory decades
ago, mostly in relation to the maser research. Recently, the atomic effect has
been already detected in observations from circumstellar medium and this is a
harbinger of future extensive magnetic field studies. A unique feature of the
atomic realignment is that they can reveal the 3D orientation of magnetic
field. In this article, we shall review the basic physical processes involved
in atomic realignment. We shall also discuss its applications to
interplanetary, circumstellar and interstellar magnetic fields. In addition,
our research reveals that the polarization of the radiation arising from the
transitions between fine and hyperfine states of the ground level can provide a
unique diagnostics of magnetic fields in the Epoch of Reionization.Comment: 30 pages, 12 figures, chapter in Lecture Notes in Physics "Magnetic
Fields in Diffuse Media". arXiv admin note: substantial text overlap with
arXiv:1203.557
Core excitation in Coulomb breakup reactions
Within the pure Coulomb breakup mechanism, we investigate the one-neutron
removal reaction of the type A(a,b)X with Be and C
projectiles on a heavy target nucleus Pb at the beam energy of 60
MeV/nucleon. Our intention is to examine the prospective of using these
reactions to study the structure of neutron rich nuclei. Integrated partial
cross sections and momentum distributions for the ground as well as excited
bound states of core nuclei are calculated within the finite range distorted
wave Born approximation as well as within the adiabatic model of the Coulomb
breakup. Our results are compared with those obtained in the studies of the
reactions on a light target where the breakup proceeds via the pure nuclear
mechanism. We find that the transitions to excited states of the core are quite
weak in the Coulomb dominated process as compared to the pure nuclear breakup.Comment: Revtex format, five postscript figures included, to appear in Phys.
Rev.
The Effect of Multiple Scattering on the Polarization from Binary Star Envelopes. I. Self- and Externally Illuminated Disks
We present the results of a Monte Carlo radiative transfer code that
calculates the polarization produced by multiple Thomson scattering and
variable absorptive opacity in a circumstellar disk around one component of a
close detached binary system. We consider in detail the polarization variations
over the binary cycle that result from the disk's illumination by the external
star and by its own volume emission. We identify key features of these
polarization phase curves and investigate their behavior as functions of
optical depth, albedo, and inclination for geometrically thin and thick disks.
The polarization due to disk self-illumination is sensitive to the internal
optical characteristics of the disk, while the polarization arising from
external illumination is mainly sensitive to the disk's geometrical thickness.
With appropriate flux weighting, these results, combined with those for an
internally illuminated disk, allow simulation of the polarization signature
from an arbitrary binary-disk system.Comment: 34 pages, 17 figures. Submitted to ApJ, revised in response to
referee comments. Color figures available from
http://www.ruf.rice.edu/~jhoffman/monte/colorfigs.htm
Discovery of the first symbiotic star in NGC6822
We report the discovery of the first symbiotic star (V=21.6, K_S=15.8 mag) in
the Local Group dwarf irregular galaxy NGC6822. This star was identified during
a spectral survey of Ha emission-line objects using the Southern African Large
Telescope (SALT) during its performance-verification phase. The observed strong
emission lines of HI and HeII suggest a high electron density and T* < 130 000
K for the hot companion. The infrared colours allow us to classify this object
as an S-type symbiotic star, comprising a red giant losing mass to a compact
companion. The red giant is an AGB carbon star, and a semi-regular variable,
pulsating in the first overtone with a period of 142 days. Its bolometric
magnitude is M_bol=-4.4 mag.
We review what is known about the luminosities of extragalactic symbiotic
stars, showing that most, possibly all, contain AGB stars. We suggest that a
much larger fraction of Galactic symbiotic stars may contain AGB stars than was
previously realised.Comment: 6 pages, 4 figures, accepted to MNRA
Adaptive optics imaging and optical spectroscopy of a multiple merger in a luminous infrared galaxy
(abridged) We present near-infrared (NIR) adaptive optics imaging obtained
with VLT/NACO and optical spectroscopy from the Southern African Large
Telescope (SALT) of a luminous infrared galaxy (LIRG) IRAS 19115-2124. These
data are combined with archival HST imaging and Spitzer imaging and
spectroscopy, allowing us to study this disturbed interacting/merging galaxy,
dubbed the Bird, in extraordinary detail. In particular, the data reveal a
triple system where the LIRG phenomenon is dominated by the smallest of the
components.
One nucleus is a regular barred spiral with significant rotation, while
another is highly disturbed with a surface brightness distribution intermediate
to that of disk and bulge systems, and hints of remaining arm/bar structure. We
derive dynamical masses in the range 3-7x10^10 M_solar for both. The third
component appears to be a 1-2x10^10 M_solar mass irregular galaxy. The total
system exhibits HII galaxy-like optical line ratios and strengths, and no
evidence for AGN activity is found from optical or mid-infrared data. The star
formation rate is estimated to be 190 M_solar/yr. We search for SNe, super star
clusters, and detect 100-300 km/s outflowing gas from the Bird. Overall, the
Bird shows kinematic, dynamical, and emission line properties typical for cool
ultra luminous IR galaxies. However, the interesting features setting it apart
for future studies are its triple merger nature, and the location of its star
formation peak - the strongest star formation does not come from the two major
K-band nuclei, but from the third irregular component. Aided by simulations, we
discuss scenarios where the irregular component is on its first high-speed
encounter with the more massive components.Comment: 24 pages, 16 figures. Accepted MNRAS version, minor corrections only,
references added. Higher resolution version (1.3MB) is available from
http://www.saao.ac.za/~petri/bird/ulirg_bird_highres_vaisanen_v2.pd
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