457 research outputs found
Experimental study on the velocity limits of magnetized rotating plasmas
An experimental study on the physical limits of the rotation velocity of magnetized plasmas is presented. Experiments are performed in the Maryland Centrifugal Experiment (MCX) [R. F. Ellis , Phys. Plasmas 12, 055704 (2005)], a mirror magnetic field plasma rotating azimuthally. The externally applied parameters that control the plasma characteristics-applied voltage, external magnetic field, and fill pressure-are scanned across the entire available range of values. It is found that the plasma rotation velocity does not exceed the Alfven velocity, in agreement with the equilibrium requirements of magnetically confined plasmas. Measured rotation velocities are also lower than the critical ionization velocity in hydrogen, but a strict limit was not observable within MCX parametric capabilities. (c) 2008 American Institute of Physics
Extrasolar Planet Transits Observed at Kitt Peak National Observatory
We obtained J-, H- and JH-band photometry of known extrasolar planet
transiting systems at the 2.1-m Kitt Peak National Observatory Telescope using
the FLAMINGOS infrared camera between October 2008 and October 2011. From the
derived lightcurves we have extracted the mid-transit times, transit depths and
transit durations for these events. The precise mid-transit times obtained help
improve the orbital periods and also constrain transit-time variations of the
systems. For most cases the published system parameters successfully accounted
for our observed lightcurves, but in some instances we derive improved
planetary radii and orbital periods. We complemented our 2.1-m infrared
observations using CCD z'-band and B-band photometry (plus two Hydrogen Alpha
filter observations) obtained with the Kitt Peak Visitor's Center telescope,
and with four H-band transits observed in October 2007 with the NSO's 1.6-m
McMath-Pierce Solar Telescope. The principal highlights of our results are: 1)
our ensemble of J-band planetary radii agree with optical radii, with the
best-fit relation being: (Rp/R*)J = 0.0017 + 0.979 (Rp/R*)optical, 2) We
observe star spot crossings during the transit of WASP-11/HAT-P-10, 3) we
detect star spot crossings by HAT-P-11b (Kepler-3b), thus confirming that the
magnetic evolution of the stellar active regions can be monitored even after
the Kepler mission has ended, and 4) we confirm a grazing transit for
HAT-P-27/WASP-40. In total we present 57 individual transits of 32 known
exoplanet systems.Comment: 33 pages, 6 figures, accepted in Publications of the Astronomical
Society of the Pacifi
Pluto's global surface composition through pixel-by-pixel Hapke modeling of New Horizons Ralph/LEISA data
On July 14th 2015, NASA's New Horizons mission gave us an unprecedented
detailed view of the Pluto system. The complex compositional diversity of
Pluto's encounter hemisphere was revealed by the Ralph/LEISA infrared
spectrometer on board of New Horizons. We present compositional maps of Pluto
defining the spatial distribution of the abundance and textural properties of
the volatiles methane and nitrogen ices and non-volatiles water ice and tholin.
These results are obtained by applying a pixel-by-pixel Hapke radiative
transfer model to the LEISA scans. Our analysis focuses mainly on the large
scale latitudinal variations of methane and nitrogen ices and aims at setting
observational constraints to volatile transport models. Specifically, we find
three latitudinal bands: the first, enriched in methane, extends from the pole
to 55deg N, the second dominated by nitrogen, continues south to 35deg N, and
the third, composed again mainly of methane, reaches 20deg N. We demonstrate
that the distribution of volatiles across these surface units can be explained
by differences in insolation over the past few decades. The latitudinal pattern
is broken by Sputnik Planitia, a large reservoir of volatiles, with nitrogen
playing the most important role. The physical properties of methane and
nitrogen in this region are suggestive of the presence of a cold trap or
possible volatile stratification. Furthermore our modeling results point to a
possible sublimation transport of nitrogen from the northwest edge of Sputnik
Planitia toward the south.Comment: 43 pages, 7 figures; accepted for publication in Icaru
The OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS): Spectral Maps of the Asteroid Bennu
The OSIRIS-REx Visible and Infrared Spectrometer (OVIRS) is a point
spectrometer covering the spectral range of 0.4 to 4.3 microns (25,000-2300
cm-1). Its primary purpose is to map the surface composition of the asteroid
Bennu, the target asteroid of the OSIRIS-REx asteroid sample return mission.
The information it returns will help guide the selection of the sample site. It
will also provide global context for the sample and high spatial resolution
spectra that can be related to spatially unresolved terrestrial observations of
asteroids. It is a compact, low-mass (17.8 kg), power efficient (8.8 W
average), and robust instrument with the sensitivity needed to detect a 5%
spectral absorption feature on a very dark surface (3% reflectance) in the
inner solar system (0.89-1.35 AU). It, in combination with the other
instruments on the OSIRIS-REx Mission, will provide an unprecedented view of an
asteroid's surface.Comment: 14 figures, 3 tables, Space Science Reviews, submitte
The Green Bank Northern Celestial Cap Pulsar Survey - I: Survey Description, Data Analysis, and Initial Results
We describe an ongoing search for pulsars and dispersed pulses of radio
emission, such as those from rotating radio transients (RRATs) and fast radio
bursts (FRBs), at 350 MHz using the Green Bank Telescope. With the Green Bank
Ultimate Pulsar Processing Instrument, we record 100 MHz of bandwidth divided
into 4,096 channels every 81.92 . This survey will cover the entire sky
visible to the Green Bank Telescope (, or 82% of the sky)
and outside of the Galactic Plane will be sensitive enough to detect slow
pulsars and low dispersion measure (30 ) millisecond
pulsars (MSPs) with a 0.08 duty cycle down to 1.1 mJy. For pulsars with a
spectral index of 1.6, we will be 2.5 times more sensitive than previous and
ongoing surveys over much of our survey region. Here we describe the survey,
the data analysis pipeline, initial discovery parameters for 62 pulsars, and
timing solutions for 5 new pulsars. PSR J02145222 is an MSP in a long-period
(512 days) orbit and has an optical counterpart identified in archival data.
PSR J06365129 is an MSP in a very short-period (96 minutes) orbit with a
very low mass companion (8 ). PSR J06455158 is an isolated MSP
with a timing residual RMS of 500 ns and has been added to pulsar timing array
experiments. PSR J14347257 is an isolated, intermediate-period pulsar that
has been partially recycled. PSR J18164510 is an eclipsing MSP in a
short-period orbit (8.7 hours) and may have recently completed its spin-up
phase.Comment: 18 pages, 10 figures, 5 tables, accepted by Ap
The Infrared Sensor Suite for SnowEx 2017
SnowEx is a winter airborne and field campaign designed to measure snow-water equivalent in forested landscapes. A major focus of Year 1 (2016-17) of NASA's SnowEx campaign will be an extensive field program involving dozens of participants from U.S. government agencies and from many universities and institutions, both domestic and foreign. Along with other instruments, two infrared (IR) sensors will be flown on a Naval Research Laboratory P-3 aircraft. Surface temperature is a critical input to hydrologic models and will be measured during the SnowEx mission. A Quantum Well Infrared Photodetector (QWIP) IR imaging camera system will be flown along with a KT-15 remote thermometer to aid in the calibration of the IR image data. Together, these instruments will measure surface temperature of snow and ice targets to an expected accuracy of less than 1C
The Green Bank Northern Celestial Cap Pulsar Survey II: The Discovery and Timing of Ten Pulsars
We present timing solutions for ten pulsars discovered in 350 MHz searches
with the Green Bank Telescope. Nine of these were discovered in the Green Bank
Northern Celestial Cap survey and one was discovered by students in the Pulsar
Search Collaboratory program in analysis of drift-scan data. Following
discovery and confirmation with the Green Bank Telescope, timing has yielded
phase-connected solutions with high precision measurements of rotational and
astrometric parameters. Eight of the pulsars are slow and isolated, including
PSR J09302301, a pulsar with nulling fraction lower limit of 30\% and
nulling timescale of seconds to minutes. This pulsar also shows evidence of
mode changing. The remaining two pulsars have undergone recycling, accreting
material from binary companions, resulting in higher spin frequencies. PSR
J05572948 is an isolated, 44 \rm{ms} pulsar that has been partially recycled
and is likely a former member of a binary system which was disrupted by a
second supernova. The paucity of such so-called `disrupted binary pulsars'
(DRPs) compared to double neutron star (DNS) binaries can be used to test
current evolutionary scenarios, especially the kicks imparted on the neutron
stars in the second supernova. There is some evidence that DRPs have larger
space velocities, which could explain their small numbers. PSR J1806+2819 is a
15 \rm{ms} pulsar in a 44 day orbit with a low mass white dwarf companion. We
did not detect the companion in archival optical data, indicating that it must
be older than 1200 Myr.Comment: 9 pages, 5 figure
Elusive Ethylene Detected in Saturns Northern Storm Region
The massive eruption at 40 deg. N (planetographic latitude) on Saturn in 2010 December has produced significant and lasting effects in the northern hemisphere on temperature and species abundances. The northern storm region was observed on many occasions in 2011 by Cassini's Composite Infrared Spectrometer (CIRS). In 2011 May, temperatures in the stratosphere greater than 200 K were derived from CIRS spectra in the regions referred to as "beacons" (warm regions in the stratosphere). Ethylene has been detected in the beacon region in Saturn's northern storm region using CIRS. Ground-based observations using the high-resolution spectrometer Celeste on the McMath-Pierce Telescope on 2011 May 15 were used to confirm the detection and improve the altitude resolution in the retrieved profile. The derived ethylene profile from the CIRS data gives a C2H4 mole fraction of 5.9 +/- 4.5 x 10(exp -7) at 0.5 mbar, and from Celeste data it gives 2.7 +/- 0.45 x 10(exp -6) at 0.1 mbar. This is two orders of magnitude higher than the amount measured in the ultraviolet at other latitudes prior to the storm. It is also much higher than predicted by photochemical models, indicating that perhaps another production mechanism is required or a loss mechanism is being inhibited
The Green Bank North Celestial Cap Pulsar Survey. IV: Four New Timing Solutions
We present timing solutions for four pulsars discovered in the Green Bank
Northern Celestial Cap (GBNCC) survey. All four pulsars are isolated with spin
periods between 0.26s and 1.84s. PSR J00382501 has a 0.26s
period and a period derivative of ,
which is unusually low for isolated pulsars with similar periods. This low
period derivative may be simply an extreme value for an isolated pulsar or it
could indicate an unusual evolution path for PSR J00382501, such as a
disrupted recycled pulsar (DRP) from a binary system or an orphaned central
compact object (CCO). Correcting the observed spin-down rate for the Shklovskii
effect suggests that this pulsar may have an unusually low space velocity,
which is consistent with expectations for DRPs. There is no X-ray emission
detected from PSR J00382501 in an archival swift observation, which suggests
that it is not a young orphaned CCO. The high dispersion measure of PSR
J1949+3426 suggests a distance of 12.3kpc. This distance indicates that PSR
J1949+3426 is among the most distant 7% of Galactic field pulsars, and is one
of the most luminous pulsars.Comment: 7 pages, 5 figure
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