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Martian atmospheric O<sub>3</sub> retrieval development for the NOMAD-UVIS spectrometer
The composition of atmospheric trace gases and aerosols is a highly variable and poorly constrained component of the martian atmosphere, and by affecting martian climate and UV surface dose, represents a key parameter in the assessment of suitability for martian habitability. The ExoMars Trace Gas Orbiter (TGO) carries the Open University (OU) designed Ultraviolet and VIsible Spectrometer (UVIS) instrument as part of the Belgian-led Nadir and Occultation for MArs Discovery (NOMAD) spectrometer suite. NOMAD will begin transmitting science observations of martian surface and atmosphere back-scattered UltraViolet (UV) and visible radiation in Spring 2018, which will be processed to derive spatially and temporally averaged atmospheric trace gas and aerosol concentrations, intended to provide a better understanding of martian atmospheric photo-chemistry and dynamics, and will also improve models of martian atmospheric chemistry, climate and habitability. Work presented here illustrates initial development and testing of the OU’s new retrieval algorithm for determining O3 and aerosol concentrations from the UVIS instrument
A Quasiparticle Trap Assisted Transition Edge Sensor for Phonon Mediated Particle Detection
ABSTRACT We have demonstrated the operation of composite superconducting tungsten and aluminum transition‐edge sensors which take advantage of quasiparticle trapping and electrothermal feedback. We call these devices W/Al QETs (quasiparticle‐trap‐assisted electrothermal feedback transition‐edge sensors). The quasiparticle trapping mechanism makes it possible to instrument large surface areas without increasing sensor heat capacity, thus allowing larger absorbers and reducing phonon collection times. The sensor consists of a 30‐nm‐thick superconducting tungsten thin film with Tc∼80 mK deposited on a high‐purity silicon substrate. The W film is patterned into 200 parallel lines segments, each 2 μm wide and 800 μm long. Eight superconducting aluminum thin film pads are electrically connected to each segment, and cover a much larger surface area than the W. When phonons from particle interactions in the silicon crystal impinge on an aluminum pad, Cooper pairs are broken, forming quasiparticles which diffuse to the tungsten lines where they are rapidly thermalized. The W film is voltage biased, and self‐regulates in temperature within its superconducting transition region by electrothermal feedback. Heat deposited in the film causes a current pulse of ∼100 μs duration, which is measured with a series array of dc superconducting quantum interference devices. We have demonstrated an energy resolution of half‐maximum for 6 keV x rays incident on the backside of a 1 cm×1 cm×1 mm (0.25 g) silicon absorber, the highest resolution that has been reported for a fast (duration) calorimetric detector with an absorber mass≳0.1 g. Applications of this technology include dark matter searches and neutrino detection
Doppler Effects from Bending of Light Rays in Curved Space-Times
We study Doppler effects in curved space-time, i.e. the frequency shifts
induced on electromagnetic signals propagating in the gravitational field. In
particular, we focus on the frequency shift due to the bending of light rays in
weak gravitational fields. We consider, using the PPN formalism, the
gravitational field of an axially symmetric distribution of mass. The zeroth
order, i.e. the sphere, is studied then passing to the contribution of the
quadrupole moment, and finally to the case of a rotating source. We give
numerical estimates for situations of physical interest, and by a very
preliminary analysis, we argue that analyzing the Doppler effect could lead, in
principle, in the foreseeable future, to the measurement of the quadrupole
moment of the giant planets of the Solar System.Comment: 16 pages, 2 EPS figures; to appear in the International Journal of
Modern Physics
The Dependence of the Soft X-ray Properties of LMXBs on the Metallicity of Their Environment
We determine the X-ray spectral properties of a sample of low-mass X-ray
binaries (LMXBs) which reside in globular clusters of M31, as well as five
LMXBs in Galactic globular clusters and in the Large Magellanic Cloud using the
ROSAT PSPC. We find a trend in the X-ray properties of the LMXBs as a function
of globular cluster metallicity. The spectra of LMXBs become progressively
softer as the metallicity of its environment increases. The one M31 globular
cluster LMXB in our sample which has a metallicity greater than solar has
spectral properties similar to those of LMXBs in the bulge of M31, but markedly
different from those which reside in low metallicity globular clusters, both in
M31 and the Galaxy. The spectral properties of this high metallicity LMXB is
also similar to those of X-ray faint early-type galaxies. This lends support to
the claim that a majority of the X-ray emission from these X-ray faint
early-type galaxies results from LMXBs and not hot gas, as is the case in their
X-ray bright counterparts.Comment: 5 pages, 2 embedded Postscript figures, uses emulateapj.sty,
Astrophysical Journal Letters, in pres
The discovery of two extremely low luminosity Milky Way globular clusters
We report the discovery of two extremely low luminosity globular clusters in
the Milky Way Halo. These objects were detected in the Sloan Digital Sky Survey
Data Release 5 and confirmed with deeper imaging at the Calar Alto Observatory.
The clusters, Koposov 1 and Koposov 2, are located at kpc and
appear to have old stellar populations and luminosities of only
mag. Their observed sizes of pc are well within the expected tidal
limit of 10 pc at that distance. Together with Palomar 1, AM 4 and
Whiting 1, these new clusters are the lowest luminosity globulars orbiting the
Milky Way, with Koposov 2 the most extreme. Koposov 1 appears to lie close to
distant branch of the Sagittarius stream. The half-mass relaxation times of
Koposov 1 and 2 are only and Myr respectively (2 orders of
magnitude shorter than the age of the stellar populations), so it would seem
that they have undergone drastic mass segregation. Since they do not appear to
be very concentrated, their evaporation timescales may be as low as . These discoveries show that the structural parameter space of
globular clusters in the Milky Way halo is not yet fully explored. They also
add, through their short remaining survival times, significant direct evidence
for a once much larger population of globular clusters.Comment: accepted for publication in ApJ, minor revision
The Dwarf Galaxy Population of the Dorado group down to Mv=-11
We present V and I CCD photometry of suspected low-surface brightness dwarf
galaxies detected in a survey covering ~2.4 deg^2 around the central region of
the Dorado group of galaxies. The low-surface brightness galaxies were chosen
based on their sizes and magnitudes at the limiting isophote of 26.0V\mu. The
selected galaxies have magnitudes brighter than V=20 (Mv=-11 for an assumed
distance to the group of 17.2 Mpc), with central surface brightnesses \mu0>22.5
V mag/arcsec^2, scale lengths h>2'', and diameters > 14'' at the limiting
isophote. Using these criteria, we identified 69 dwarf galaxy candidates. Four
of them are large very low-surface brightness galaxies that were detected on a
smoothed image, after masking high surface brightness objects. Monte Carlo
simulations performed to estimate completeness, photometric uncertainties and
to evaluate our ability to detect extended low-surface brightness galaxies show
that the completeness fraction is, on average, > 80% for dwarf galaxies with
and 22.5<\mu0<25.5 V mag/arcsec^2, for the range of sizes
considered by us (D>14''). The V-I colors of the dwarf candidates vary from
-0.3 to 2.3 with a peak on V-I=0.98, suggesting a range of different stellar
populations in these galaxies. The projected surface density of the dwarf
galaxies shows a concentration towards the group center similar in extent to
that found around five X-ray groups and the elliptical galaxy NGC1132 studied
by Mulchaey and Zabludoff (1999), suggesting that the dwarf galaxies in Dorado
are probably physically associated with the overall potential well of the
group.Comment: 32 pages, 16 postscript figures and 3 figures in GIF format, aastex
v5.0. To appear in The Astronomical Journal, January 200
Magellanic Cloud Periphery Carbon Stars IV: The SMC
The kinematics of 150 carbon stars observed at moderate dispersion on the
periphery of the Small Magellanic Cloud are compared with the motions of
neutral hydrogen and early type stars in the Inter-Cloud region. The
distribution of radial velocities implies a configuration of these stars as a
sheet inclined at 73+/-4 degrees to the plane of the sky. The near side, to the
South, is dominated by a stellar component; to the North, the far side contains
fewer carbon stars, and is dominated by the neutral gas. The upper velocity
envelope of the stars is closely the same as that of the gas. This
configuration is shown to be consistent with the known extension of the SMC
along the line of sight, and is attributed to a tidally induced disruption of
the SMC that originated in a close encounter with the LMC some 0.3 to 0.4 Gyr
ago. The dearth of gas on the near side of the sheet is attributed to ablation
processes akin to those inferred by Weiner & Williams (1996) to collisional
excitation of the leading edges of Magellanic Stream clouds. Comparison with
pre LMC/SMC encounter kinematic data of Hardy, Suntzeff, & Azzopardi (1989) of
carbon stars, with data of stars formed after the encounter, of Maurice et al.
(1989), and Mathewson et al. (a986, 1988) leaves little doubt that forces other
than gravity play a role in the dynamics of the H I.Comment: 30 pages; 7 figures, latex compiled, 1 table; to appear in AJ (June
2000
The Nature of the Density Clump in the Fornax Dwarf Spheroidal Galaxy
We have imaged the recently discovered stellar overdensity located
approximately one core radius from the center of the Fornax dwarf spheroidal
galaxy using the Magellan Clay 6.5m telescope with the Magellan Instant Camera
(MagIC). Superb seeing conditions allowed us to probe the stellar populations
of this overdensity and of a control field within Fornax to a limiting
magnitude of R=26. The color-magnitude diagram of the overdensity field is
virtually identical to that of the control field with the exception of the
presence of a population arising from a very short (less than 300 Myr in
duration) burst of star formation 1.4 Gyr ago. Coleman et al. have argued that
this overdensity might be related to a shell structure in Fornax that was
created when Fornax captured a smaller galaxy. Our results are consistent with
this model, but we argue that the metallicity of this young component favors a
scenario in which the gas was part of Fornax itself.Comment: 24 pages including 8 figures and 3 tables. Accepted by Astronomical
Journa
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