2,411 research outputs found
Early Results from the Wisconsin H-Alpha Mapper Southern Sky Survey
After a successful eleven-year campaign at Kitt Peak, we moved the Wisconsin
H-Alpha Mapper (WHAM) to Cerro Tololo in early 2009. Here we present some of
the early data after a few months under southern skies. These maps begin to
complete the first all-sky, kinematic survey of the diffuse H-alpha emission
from the Milky Way. Much of this emission arises from the Warm Ionized Medium
(WIM), a significant component of the ISM that extends a few kiloparsecs above
the Galactic disk. While this first look at the data focuses on the H-alpha
survey, WHAM is also capable of observing many other optical emission lines,
revealing fascinating trends in the temperature and ionization state of the
WIM. Our ongoing studies of the physical conditions of diffuse ionized gas will
continue from the southern hemisphere following the H-alpha survey. In
addition, future observations will cover the full velocity range of the
Magellanic Stream, Bridge, and Clouds to trace the ionized gas associated with
these neighboring systems.Comment: 4 pages, 2 figures. To appear in "The Dynamic ISM: A celebration of
the Canadian Galactic Plane Survey," ASP Conference Serie
Photoionization of High Altitude Gas in a Supernova-Driven Turbulent Interstellar Medium
We investigate models for the photoionization of the widespread diffuse
ionized gas in galaxies. In particular we address the long standing question of
the penetration of Lyman continuum photons from sources close to the galactic
midplane to large heights in the galactic halo. We find that recent
hydrodynamical simulations of a supernova-driven interstellar medium have low
density paths and voids that allow for ionizing photons from midplane OB stars
to reach and ionize gas many kiloparsecs above the midplane. We find ionizing
fluxes throughout our simulation grids are larger than predicted by one
dimensional slab models, thus allowing for photoionization by O stars of low
altitude neutral clouds in the Galaxy that are also detected in Halpha. In
previous studies of such clouds the photoionization scenario had been rejected
and the Halpha had been attributed to enhanced cosmic ray ionization or
scattered light from midplane H II regions. We do find that the emission
measure distributions in our simulations are wider than those derived from
Halpha observations in the Milky Way. In addition, the horizontally averaged
height dependence of the gas density in the hydrodynamical models is lower than
inferred in the Galaxy. These discrepancies are likely due to the absence of
magnetic fields in the hydrodynamic simulations and we discuss how
magnetohydrodynamic effects may reconcile models and observations.
Nevertheless, we anticipate that the inclusion of magnetic fields in the
dynamical simulations will not alter our primary finding that midplane OB stars
are capable of producing high altitude diffuse ionized gas in a realistic
three-dimensional interstellar medium.Comment: ApJ accepted. 17 pages, 7 figure
Verification of BOUT++ by the method of manufactured solutions
BOUT++ is a software package designed for solving plasma fluid models. It has been used to simulate a wide range of plasma phenomena ranging from linear stability analysis to 3D plasma turbulence and is capable of simulating a wide range of drift-reduced plasma fluid and gyro-fluid models. A verification exercise has been performed as part of a EUROfusion Enabling Research project, to rigorously test the correctness of the algorithms implemented in BOUT++, by testing order-of-accuracy convergence rates using the Method of Manufactured Solutions (MMS). We present tests of individual components including time-integration and advection schemes, non-orthogonal toroidal field-aligned coordinate systems and the shifted metric procedure which is used to handle highly sheared grids. The flux coordinate independent approach to differencing along magnetic field-lines has been implemented in BOUT++ and is here verified using the MMS in a sheared slab configuration. Finally, we show tests of three complete models: 2-field Hasegawa-Wakatani in 2D slab, 3-field reduced magnetohydrodynamics (MHD) in 3D field-aligned toroidal coordinates, and 5-field reduced MHD in slab geometry
Negatively Charged Strangelet Search using the E864 Spectrometer at the AGS
We provide a status report on the progress of searching for negatively
charged strangelets using the E864 spectrometer at the AGS. About 200 million
recorded events representing approximately 14 billion 10% central interactions
of Au + Pt at 11.5 GeV/c taken during the 1996-1997 run of the experiment are
used in the analysis. No strangelet candidates are seen for charges Z=-1 and
Z=-2, corresponding to a 90% confidence level for upper limits of strangelet
production of ~1 x 10^{-8} and ~4 x 10^{-9} per central collision respectively.
The limits are nearly uniform over a wide range of masses and are valid only
for strangelets which are stable or have lifetimes greater than ~50 ns.Comment: 6 pages, 4 figures; Talk at SQM'98, Padova, Italy (July 20-24, 1998
Vertical structure of a supernova-driven turbulent magnetized ISM
Stellar feedback drives the circulation of matter from the disk to the halo
of galaxies. We perform three-dimensional magnetohydrodynamic simulations of a
vertical column of the interstellar medium with initial conditions typical of
the solar circle in which supernovae drive turbulence and determine the
vertical stratification of the medium. The simulations were run using a stable,
positivity-preserving scheme for ideal MHD implemented in the FLASH code. We
find that the majority (\approx 90 %) of the mass is contained in
thermally-stable temperature regimes of cold molecular and atomic gas at T <
200 K or warm atomic and ionized gas at 5000 K < T < 10^{4.2} K, with strong
peaks in probability distribution functions of temperature in both the cold and
warm regimes. The 200 - 10^{4.2} K gas fills 50-60 % of the volume near the
plane, with hotter gas associated with supernova remnants (30-40 %) and cold
clouds (< 10 %) embedded within. At |z| ~ 1-2 kpc, transition-temperature (10^5
K) gas accounts for most of the mass and volume, while hot gas dominates at |z|
> 3 kpc. The magnetic field in our models has no significant impact on the
scale heights of gas in each temperature regime; the magnetic tension force is
approximately equal to and opposite the magnetic pressure, so the addition of
the field does not significantly affect the vertical support of the gas. The
addition of a magnetic field does reduce the fraction of gas in the cold (< 200
K) regime with a corresponding increase in the fraction of warm (~ 10^4 K) gas.
However, our models lack rotational shear and thus have no large-scale dynamo,
which reduces the role of the field in the models compared to reality. The
supernovae drive oscillations in the vertical distribution of halo gas, with
the period of the oscillations ranging from ~ 30 Myr in the T < 200 K gas to ~
100 Myr in the 10^6 K gas, in line with predictions by Walters & Cox.Comment: Accepted for publication in ApJ. Replacement corrects an error in the
observed CNM pressure distribution in Figure 15 and associated discussio
Density probability distribution functions of diffuse gas in the Milky Way
In a search for the signature of turbulence in the diffuse interstellar
medium in gas density distributions, we determined the probability distribution
functions (PDFs) of the average volume densities of the diffuse gas. The
densities were derived from dispersion measures and HI column densities towards
pulsars and stars at known distances. The PDFs of the average densities of the
diffuse ionized gas (DIG) and the diffuse atomic gas are close to lognormal,
especially when lines of sight at |b|=5 degrees are
considered separately. The PDF of at high |b| is twice as wide as that
at low |b|. The width of the PDF of the DIG is about 30 per cent smaller than
that of the warm HI at the same latitudes. The results reported here provide
strong support for the existence of a lognormal density PDF in the diffuse ISM,
consistent with a turbulent origin of density structure in the diffuse gas.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letter
Inflation and quintessence with nonminimal coupling
The nonminimal coupling (NMC) of the scalar field to the Ricci curvature is
unavoidable in many cosmological scenarios. Inflation and quintessence models
based on nonminimally coupled scalar fields are studied, with particular
attention to the balance between the scalar potential and the NMC term in the
action. NMC makes acceleration of the universe harder to achieve for the usual
potentials, but it is beneficial in obtaining cosmic acceleration with unusual
potentials. The slow-roll approximation with NMC, conformal transformation
techniques, and other aspects of the physics of NMC are clarified.Comment: 36 pages, LaTeX. Typos in Eq. (2.5) correcte
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