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Astronomy & Astrophysics Seminar
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ASTR 400/500
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+ Graduate and Undergraduate Research Seminar
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Fall 2008
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Titles and Abstracts
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THE SUPERMASSIVE BLACK HOLE AT THE GALACTIC CENTER
Siming Liu, Los Alamos National Laboratory
The supermassive black hole at the Galactic center has been observed
extensively during the past decade. Observations in near-infared
bands with VLT and KECK show stars at a distance of 7.6(+-0.2) kpc
from us orbiting a mass concentration of 3.6(+-0.2) million solar
masses within 45 AU. This mass is centered at the location of the
compact radio source, Sgr A*, whose intrinsic size has been measured
to be 14 r_S in radius at 3.5 millimeter, where r_S =1.1e12
centimeter is the Schwarszchild radius of the black hole. Flares are
routinely observed in the millimeter, NIR and X-ray bands from the
direction of Sgr A*. Their variability, polarization, and correlation
suggest that they are triggered near the event horizon and propagate
outward, and the synchrotron and self-Comptonization dominate the
emission processes. The longer wavelength emissions may also be
associated with outflows from the black hole. Chandra observations
also reveal a concentration of hot plasmas with a temperature of 1.9
keV in the direction of Sgr A*. These observations suggest that Sgr
A* is powered by accretion of the supermassive black hole in stellar
winds and warrantee a detailed study of the accretion process. I will
give a review of these observations and related theoretical work and
discuss ongoing observational and theoretical investigations to
further our understanding of this source.
THE HUNT FOR PARTICLE DARK MATTER
Uwe Oberlack, Rice University
Redundant astrophysical observations imply that baryonic matter makes up less than
5% of the universe. The dominant components, "particle-like" Dark Matter (~25%)
and "like-nothing-else" Dark Energy (~70%), are unexplained in the framework
of standard physics. A natural explanation for the bulk of matter in the universe
is provided by non-relativistic, massive, relic particles. These Weakly
Interacting Massive Particles (WIMPs) are predicted in particular by theories
invoking supersymmetry, and should at some small rate produce detectable signals
from nuclear recoils in sensitive low background experiments. I will provide a
brief overview of direct Dark Matter searches, and elaborate on the XENON suite of
experiments. The XENON Dark Matter program reached a major milestone with the
operation of its first Dark Matter detector, XENON10, at the Gran Sasso
underground laboratory in Italy, providing world-best limits on WIMP-nucleon
cross-sections last year. Meanwhile we are building the next generation detector
XENON100 at the same location, with ten-fold greater fiducial mass and 100 times
reduced background. I will report on the status of XENON100 and its projected
sensitivity, and conclude with an outlook on future prospects.
SIMULATIONS OF DIFFUSIVE SHOCK ACCELERATION AT OBLIQUE RELATIVISTIC SHOCKS
Errol Summerlin, Rice University
Diffusive shock acceleration (DSA) at relativistic shocks is expected to be an important accel-
eration mechanism in a variety of astrophysical objects including extragalactic jets in active
galactic nuclei and gamma ray bursts. In this paper, we present results from a Monte Carlo sim-
ulation of such diffusive acceleration in test-particle, relativistic, oblique, MHD shocks. Simula-
tion output is presented for both large angle and small angle scattering scenarios, and a variety
of shock obliquities including superluminal regimes when the de Hoffman-Teller frame does not
exist. The distribution functions and power-law indices compare favorably with semi-analytic
results in the non-relativistic and ultra-relativistic regimes, as well as published DSA simulation
results in the trans-relativistic regime. Angular distributions are also discussed; these provide
powerful checks on the accuracy of the simulation. New insights into the character of oblique
shocks are addressed, specifically the downstream evolution of the test particle energy spectra
and angular distributions that mark the interplay between diffusive and gyrational properties
in the shock layer.
MONTE CARLO CODE AND BLAZER VARIABILITY
Xuhui Chen, Rice University
I am working on adopting an existing simulation code to the problem of Blazar
Variability. This program uses the Monte Carlo method and the
Fokker Planck equation to deal with time dependent Compton Scattering problems.
In this talk, I will introduce Blazar variability, and then discuss the fundamentals
of the simulation code, what we can do using this code, and what we still need
to do in the future.
PROPERTIES OF BLAZAR POPULATIONS FROM MODELING AND PHENOMENOLOGICAL CENSUS
Eileen Meyer, Rice University
I will discuss the progress of my current project, which is the development of a
versatile, multi-layered program which simulates underlying blazar populations for
the purposes of statistical analysis, including monte carlo. I will put the work
in context, citing recent success with similar methods in diffuse x-ray background
and stellar population synthesis.
POPULATION STATISTICS STUDY OF RADIO AND GAMMA-RAY PULSARS FROM THE GALACTIC DISK
Peter L. Gonthier, Hope College
Results of our pulsar population synthesis of radio and gamma-ray pulsars from the
Galactic disk will be presented. Over the past several years, a program has been
developed to simulate pulsar birth, evolution, and emission using Monte Carlo
techniques. Our goal is to understand the beam geometry and luminosity models of
both radio and gamma-ray emission regions. From our studies of radio pulsars that
have clearly identifiable core and cone components, in which we fit the
polarization sweep as well as the pulse profiles to constrain the viewing
geometry, we develop a model describing the ratio of radio core-to-cone
luminosities. In this model, short period pulsars are more cone-dominated than in
our previous studies. Using a recently improved radio beam geometry, we present
the preliminary predictions for observing these pulsars with FERMI and AGILE. We
suggest that the correlations of the radio pulse profile with the gamma-ray
profile will constrain high-energy emission regions distinguishing between slot
gap and outer gap models.
DARK STARS: A NEW PHASE OF STELLAR EVOLUTION SUPPORTED BY DARK MATTER ANNIHILATION
Paolo Gondolo, Univ. of Utah
The first phase of stellar evolution in the history of the universe may be
Dark Stars, powered by dark matter heating rather than fusion. Weakly
interacting massive particles can annihilate and provide an important heat
source for the first stars in the universe. This talk presents the story of
these Dark Stars: how they form, how long they might live, and what they
might become at the end of their life.
EMISSION MAPS FROM THE (M)HD CODE AstroBEAR
Jacob Palmer, Rice
An important step in testing numerical simulations is to compare them to observations. In order to do this, the simulations must be able to produce emission maps. I am in the process of adding a post-processing routine to the (M)HD code AstroBEAR that will calculate H-alpha emission, as well as various emission lines of OI, OII, NI, NII, and SII. This talk will cover the methods used to obtain these emission maps from AstroBEAR output and some of the problems encountered. I will also discuss briefly the goal of adding a new cooling routine to AstroBEAR that will take into account more of the microphysics involved in cooling.
NEW RESULTS FROM THE PIERRE AUGER OBSERVATORY
Stefan Westerhoff, Univ. of Wisconsin, Madison
The Pierre Auger Observatory in Malargue, Argentina, is
the world's largest detector for the study of the origin
of ultrahigh energy cosmic rays. The experiment stretches
over 3000 km^2 and measures cosmic rays with energies
above 10^18 eV using two complementary detector types: an
array of 1600 particle detectors on the ground, and four
fluorescence detectors overlooking the ground array from
the periphery. The Observatory was recently completed,
but data taking started already at the beginning of 2004.
Results from the first years of operation include a
measurement of the energy spectrum and the chemical
composition of ultrahigh energy cosmic rays, as well
as first indications that the arrival direction
distribution of the highest energy cosmic rays is not
isotropic. In this talk, I will review the most recent
results, with a special emphasis on the arrival directions
and possible correlation with known astrophysical sources.
NEON AND SULFUR IN THE UNIVERSE: OBSERVATIONS WITH SPITZER
Reginald J. Dufour, Rice
Neon and Sulfur are two significant elements produced by alpha-capture onto carbon
and oxygen in massive stars and ejected into the ISM as supernovae. Until
recently, determination of the abundances of these elements in the ISM of galaxies
from optical studies of H II region spectra have been limited by observing only
the lines of minority ionization states (Ne++ and S+). With the Spitzer Space
Telescope IRS we can observe emission lines of [Ne II], [Ne III], [S III] and
[S IV], which are the dominant ionization states in photoionized nebulae. I will
report on such observations and results for two spiral galaxies, M83 and M33, and
motivation for observations of a third galaxy, NGC 6822.
INVESTIGATING THE SPECTRAL, PHYSICAL, AND KINETIC PROPERTIES OF SAGITTARIUS A* THROUGH GRMHD AND MONTE-CARLO/FOKKER-PLANCK SIMULATIONS
Guy Hilburn, Rice
I will present research which involves simulating the physical and kinetic properties of the accretion disk of Sagittarius A* by use of HARM, a GRMHD code. Electron and photon distributions are then calculated through use of a Monte-Carlo/Fokker-Planck code, allowing for study of the time-dependent emission through synchrotron, bremsstrahlung, and Compton scattering processes, and the evolution of the electron distribution. Conclusions on the nature of electron injection and heating in the region can be drawn from these tools.
SOFT X-RAY EMISSION OF MAGNETARS: RESONANT CYCLOTRON SCATTERING
Zorawar Wadiasingh, Rice
Soft X-ray spectra of soft gamma repeaters and anomalous X-ray pulsars have been customarily fit with an empirical blackbody plus power-law model. Resonant cyclotron scattering (RCS) has recently emerged as a physical model for soft X-ray emission in magnetar candidates. A non-thermal spectrum emerges when blackbody emission from a magnetar's surface is distorted and upscattered through efficient multiple resonant cyclotron scattering with warm non-relativistic plasma in the magnetosphere. I shall discuss the RCS model and its application to soft X-ray spectra of magnetar candidates.
DETECTION OF ZEEMAN SIGNATURE USING THE LSD (LEAST SQUARE DECONVOLUTION) METHOD
Wei Chen, Rice
With the assumption that most spectral lines exhibit Zeeman signatures with almost
the same shape and that line intensities add up linearly, I wrote a code, using
the least square deconvolution method, to detect Zeeman signatures from the
optimal extracted polarization echelle spectra. As a result, we can see clearly
the signatures from the resulting profile. In fact, LSD of Stokes I spectra also
performs quite well.
Journal Club: DETECTION OF PULSED GAMMA-RAYS ABOVE 25 GEV FROM THE CRAB PULSAR
(MAGIC Collaboration)
Sarah Story, Rice
Since the discovery of gamma-ray emission from pulsars, competing models have been
put forth to explain the way particles are accelerated in pulsar magnetospheres
and the sites of the gamma-ray emission. The most powerful discriminator between
these models is the maximum energy and shape of the high-energy spectral cutoff.
This cutoff has been predicted to occur between a few GeV and a few tens of GeV.
Thus far, observations in this regime have been limited. EGRET, aboard the CGRO,
could not observe above about 5 GeV, and atmospheric Cherenkov telescopes are not
sensitive below about 60 GeV. With recent modifications, however, the MAGIC
telescope has been able to observe at energies down to 25 GeV, and has detected
pulsed emission from the Crab pulsar at that energy with 6.4-sigma confidence.
This severely restricts the possible sites of emission in the Crab. More detailed
information will be available when new data from Fermi is published later this
year.
THE NATURE OF DOUBLE-PEAKED EMISSION LINES IN MWC349A
Lauren Cleeves, Rice
MWC349, a well known emission-line star and the only known natural
laser has been a subject of extensive study at the Maria Mitchell
Association. During my REU at MMA this summer I became involved with
the peculiar object through analysis of the double peaked nature of
its Halpha line. Most mm hydrogen recombination lines and
quasi-thermal near-IR lines observed in the source appear
double-peaked, and this behavior has been ascribed to their formation
in an edge-on circumstellar disk. However, the strongly double-peaked
character of the Halpha emission line has instead been previously
attributed by Hartmann et al. to be self-absorption in the line. We
obtained an Halpha spectrum with the 0.9m coude feed telescope at Kitt
Peak from which I attempted to reconcile the two different
explanations for similar line morphology by showing that the line can
indeed be formed in a circumstellar disk, though only if it forms in a
very narrow ring. Higher-resolution spectroscopy of other optical and
near-IR lines will further clarify the situation and will also provide
a probe into disk structure of interesting object.
LABORATORY ASTROPHYSICS AND REAL EQUATIONS OF STATE: THE NEXT CHALLENGES FOR
ASTROPHYSICAL MHD SIMULATIONS
Robert Carver, Rice
Magneto-hydrodynamics (MHD) simulation codes are an essential tool
in astrophysics research, allowing us to better understand phenomena such as
stellar outflows, supernovae, and accretion disks. These MHD codes have
until recently focused on simulating ideal gases. As more complicated flow
structures and shocks have gained increased interest, these codes have had
to evolve. Recently a new field of study, laboratory astrophysics, has
become increasingly important in the study and understanding of strophysical
MHD flows. To adapt to this new field of study, we have added a real gas
equation of state (EOS) capability to an astrophysics MHD code. This talk
will focus on the required changes to that code and the results from our
first simulation of a laboratory astrophysics experiment.
OBSERVATIONAL AND THEORETICAL INTERPRETATION OF ENERGETIC PARTICLE TRANSPORT
IN SOLAR FLARES
Antoun Daou, Rice
The combination of excellent space-based remote sensing, and image
reconstruction techniques, as well as improvements in numerical modeling,
help enhance our understanding of particle transport in solar flares. A
rigorous analysis of flare hard X-ray emission uses the unprecedented
spectral and spatial resolution of the RHESSI telescope data in order to
better understand the spectral properties of the emitting electron
population in solar flares. A forward-fit to the data, using a Fokker-Planck
kinetic code, completes this study by numerically modeling the particle
transport in phase-space in realistic magnetic geometries and for different
particle injection profiles. I will also expose other projects such as
filament eruption analysis using STEREO/MLSO, the flare-CME-chromospheric
wave relationship, the relationship between microflares and jets,
mechatronix engineering, and science outreach efforts.
EXOPLANETS FROM A TO M
John Johnson, IfA, Hawaii
I will present the results of our search for planets around stars at either
end of the stellar mass scale, from M dwarfs to "retired" A stars. Our results
reveal a strong correlation between stellar mass and planet occurrence,
which indicates that planets form roughly 4 times more efficiently around A
stars like Vega than around M dwarfs like Proxima Centauri. We also find
that planets around A stars all orbit beyond 0.8 AU, which has important
implications for our understanding of planet migration, and for the planning
of future astrometry and direct imaging missions.