341 research outputs found
The X-ray Remnant of SN1987A
We present high resolution Chandra observations of the remnant of SN1987A in
the Large Magellanic Cloud. The high angular resolution of the Chandra X-ray
Observatory (CXO) permits us to resolve the X-ray remnant. We find that the
remnant is shell-like in morphology, with X-ray peaks associated with some of
the optical hot spots seen in HST images. The X-ray light curve has departed
from the linear flux increase observed by ROSAT, with a 0.5-2.0 keV luminosity
of 1.5 x 10^35 erg/s in January 2000. We set an upper limit of 2.3 x 10^34
ergs/s on the luminosity of any embedded central source (0.5 - 2 keV). We also
present a high resolution spectrum, showing that the X-ray emission is thermal
in origin and is dominated by highly ionized species of O, Ne, Mg, and Si.Comment: 16 pages, 3 figures, Accepted for publication in ApJ Letter
The Effect of Sodium Hydroxide on Niobium Carbide Precipitates in Thermally Sensitised 20Cr-25Ni-Nb Austenitic Stainless Steel
Niobium-stabilised austenitic stainless steel (20Cr-25Ni-Nb) has been immersed in sodium hydroxide, which is used as a corrosion inhibitor. The work shows how NbC precipitates may be degraded by use of pH 13 NaOH. Initial electrochemical measurements indicate that there is no benefit to this pretreatment as regards long-term corrosion inhibition, and post corrosion imaging shows the initiation of pitting corrosion at Nb-rich precipitates still present in the microstructure
Telescope to Observe Planetary Systems (TOPS): a high throughput 1.2-m visible telescope with a small inner working angle
The Telescope to Observe Planetary Systems (TOPS) is a proposed space mission
to image in the visible (0.4-0.9 micron) planetary systems of nearby stars
simultaneously in 16 spectral bands (resolution R~20). For the ~10 most
favorable stars, it will have the sensitivity to discover 2 R_E rocky planets
within habitable zones and characterize their surfaces or atmospheres through
spectrophotometry. Many more massive planets and debris discs will be imaged
and characterized for the first time. With a 1.2m visible telescope, the
proposed mission achieves its power by exploiting the most efficient and robust
coronagraphic and wavefront control techniques. The Phase-Induced Amplitude
Apodization (PIAA) coronagraph used by TOPS allows planet detection at 2
lambda/d with nearly 100% throughput and preserves the telescope angular
resolution. An efficient focal plane wavefront sensing scheme accurately
measures wavefront aberrations which are fed back to the telescope active
primary mirror. Fine wavefront control is also performed independently in each
of 4 spectral channels, resulting in a system that is robust to wavefront
chromaticity.Comment: 12 pages, SPIE conference proceeding, May 2006, Orlando, Florid
CHARIS Science: Performance Simulations for the Subaru Telescope's Third-Generation of Exoplanet Imaging Instrumentation
We describe the expected scientific capabilities of CHARIS, a high-contrast
integral-field spectrograph (IFS) currently under construction for the Subaru
telescope. CHARIS is part of a new generation of instruments, enabled by
extreme adaptive optics (AO) systems (including SCExAO at Subaru), that promise
greatly improved contrasts at small angular separation thanks to their ability
to use spectral information to distinguish planets from quasistatic speckles in
the stellar point-spread function (PSF). CHARIS is similar in concept to GPI
and SPHERE, on Gemini South and the Very Large Telescope, respectively, but
will be unique in its ability to simultaneously cover the entire near-infrared
, , and bands with a low-resolution mode. This extraordinarily broad
wavelength coverage will enable spectral differential imaging down to angular
separations of a few , corresponding to 0.\!\!''1. SCExAO
will also offer contrast approaching at similar separations,
0.\!\!''1--0.\!\!''2. The discovery yield of a CHARIS survey will
depend on the exoplanet distribution function at around 10 AU. If the
distribution of planets discovered by radial velocity surveys extends unchanged
to 20 AU, observations of 200 mostly young, nearby stars targeted
by existing high-contrast instruments might find 1--3 planets. Carefully
optimizing the target sample could improve this yield by a factor of a few,
while an upturn in frequency at a few AU could also increase the number of
detections. CHARIS, with a higher spectral resolution mode of , will
also be among the best instruments to characterize planets and brown dwarfs
like HR 8799 cde and And b.Comment: 13 pages, 7 figures, proceedings from SPIE Montrea
Early Ultraviolet, Optical and X-Ray Observations of the Type IIP SN 2005cs in M51 with Swift
We report early photospheric-phase observations of the Type IIP Supernova
(SN) 2005cs obtained by Swift's Ultraviolet-Optical and X-Ray Telescopes.
Observations started within two days of discovery and continued regularly for
three weeks. During this time the V-band magnitude remained essentially
constant, while the UV was initially bright but steadily faded until below the
brightness of an underlying UV-bright HII region. This UV decay is similar to
SNe II observed by the International Ultraviolet Explorer. UV grism spectra
show the P-Cygni absorption of MgII 2798A, indicating a photospheric origin of
the UV flux. Based on non-LTE model atmosphere calculations with the CMFGEN
code, we associate the rapid evolution of the UV flux with the cooling of the
ejecta, the peak of the spectral energy distribution (SED) shifting from ~700A
on June 30th to ~1200A on July 5th. Furthermore, the corresponding
recombination of the ejecta, e.g., the transition from FeIII to FeII, induces
considerable strengthening of metal line-blanketing at and above the
photosphere, blocking more effectively this fading UV flux. SN2005cs was not
detected in X-rays, and the upper limit to the X-ray luminosity yields a limit
to the mass loss rate of the progenitor of about 10^-5 solar masses per year.
Overall, Swift presents a unique opportunity to capture the early and fast
evolution of Type II SNe in the UV, providing additional constraints on the
reddening, the SED shortward of 4000A, and the ionization state and temperature
of the photon-decoupling regions.Comment: 15 pages, 6 figures. Accepted for publication by Astrophysical
Journa
HST Time-Series Photometry of the Transiting Planet of HD 209458
We have observed 4 transits of the planet of HD 209458 using the STIS
spectrograph on HST. Summing the recorded counts over wavelength between 582 nm
and 638 nm yields a photometric time series with 80 s time sampling and
relative precision of about 1.1E-4 per sample. The folded light curve can be
fit within observational errors using a model consisting of an opaque circular
planet transiting a limb-darkened stellar disk. In this way we estimate the
planetary radius R_p = 1.347 +/- 0.060 R_Jup, the orbital inclination i = 86.68
+/- 0.14 degrees, the stellar radius R_* = 1.146 +/- 0.050 R_solar, and one
parameter describing the stellar limb darkening. Our estimated radius is
smaller than those from earlier studies, but is consistent within measurement
errors, and is also consistent with theoretical estimates of the radii of
irradiated Jupiter-like planets. Satellites or rings orbiting the planet would,
if large enough, be apparent from distortions of the light curve or from
irregularities in the transit timings. We find no evidence for either
satellites or rings, with upper limits on satellite radius and mass of 1.2
Earth radii and 3 Earth masses, respectively. Opaque rings, if present, must be
smaller than 1.8 planetary radii in radial extent. The high level of
photometric precision attained in this experiment confirms the feasibility of
photometric detection of Earth-sized planets circling Sun-like stars.Comment: 28 pages, 9 figures. To be published in the Astrophysical Journa
Recommended from our members
Global Estimates of CO Sources with High Resolution by Adjoint Inversion of Multiple Satellite Datasets (MOPITT, AIRS, SCIAMACHY, TES)
We combine CO column measurements from the MOPITT, AIRS, SCIAMACHY, and TES satellite instruments in a full-year (May 2004âApril 2005) global inversion of CO sources at 4°Ă5° spatial resolution and monthly temporal resolution. The inversion uses the GEOS-Chem chemical transport model (CTM) and its adjoint applied to MOPITT, AIRS, and SCIAMACHY. Observations from TES, surface sites (NOAA/GMD), and aircraft (MOZAIC) are used for evaluation of the a posteriori solution. Using GEOS-Chem as a common intercomparison platform shows global consistency between the different satellite datasets and with the in situ data. Differences can be largely explained by different averaging kernels and a priori information. The global CO emission from combustion as constrained in the inversion is . This is much higher than current bottom-up emission inventories. A large fraction of the correction results from a seasonal underestimate of CO sources at northern mid-latitudes in winter and suggests a larger-than-expected CO source from vehicle cold starts and residential heating. Implementing this seasonal variation of emissions solves the long-standing problem of models underestimating CO in the northern extratropics in winter-spring. A posteriori emissions also indicate a general underestimation of biomass burning in the GFED2 inventory. However, the tropical biomass burning constraints are not quantitatively consistent across the different datasets.Engineering and Applied Science
TOPS: a small space telescope using phase induced-amplitude apodization (PIAA) to image rocky and giant exo-planets
The Telescope to Observe Planetary Systems (TOPS) is a proposed space mission to image planetary systems of nearby stars simultaneously in a few wide spectral bands covering the visible light (0.4-0.9 ÎŒm). It achieves its power by combining a high accuracy wavefront control system with a highly efficient Phase-Induced Amplitude Apodization (PIAA) coronagraph which provides strong suppression very close to the star (within 2 λ/D). The PIAA coronagraphic technique opens the possibility of imaging Earthlike planets in visible light with a smaller telescope than previously supposed. If sized at 1.2-m, TOPS would image and characterize many Jupiter-sized planets, and discover 2 RE rocky planets within habitable zones of the â10 most favorable stars. With a larger 2-m aperture, TOPS would have the sensitivity to reveal Earth-like planets in the habitable zone around â20 stars, and to characterize any found with low resolution spectroscopy. Unless the occurrence of Earth-like planets is very low (ηâ <~ 0.2), a useful fraction of the TPF-C scientific program would be possible with aperture much smaller than the baselined 8 by 3.5m for TPF, with its more conventional coronagraph. An ongoing laboratory experiment has successfully demonstrated high contrast coronagraphic imaging within 2 λ/d with the PIAA coronagraph / focal plane wavefront sensing scheme envisioned for TOPS
The NASA X-Ray Mission Concepts Study
The 2010 Astrophysics Decadal Survey recommended a significant technology development program towards realizing the scientific goals of the International X-ray Observatory (IXO). NASA has undertaken an X-ray mission concepts study to determine alternative approaches to accomplishing IXO's high ranking scientific objectives over the next decade given the budget realities, which make a flagship mission challenging to implement. The goal of the study is to determine the degree to which missions in various cost ranges from 2B could fulfill these objectives. The study process involved several steps. NASA released a Request for Information in October 2011, seeking mission concepts and enabling technology ideas from the community. The responses included a total of 14 mission concepts and 13 enabling technologies. NASA also solicited membership for and selected a Community Science Team (CST) to guide the process. A workshop was held in December 2011 in which the mission concepts and technology were presented and discussed. Based on the RFI responses and the workshop, the CST then chose a small group of notional mission concepts, representing a range of cost points, for further study. These notional missions concepts were developed through mission design laboratory activities in early 2012. The results of all these activities were captured in the final X-ray mission concepts study report, submitted to NASA in July 2012. In this presentation, we summarize the outcome of the study. We discuss background, methodology, the notional missions, and the conclusions of the study report
- âŠ