1,666 research outputs found
On-sky wide field adaptive optics correction using multiple laser guide stars at the MMT
We describe results from the first astronomical adaptive optics system to use
multiple laser guide stars, located at the 6.5-m MMT telescope in Arizona. Its
initial operational mode, ground-layer adaptive optics (GLAO), provides uniform
stellar wavefront correction within the 2 arc minute diameter laser beacon
constellation, reducing the stellar image widths by as much as 53%, from 0.70
to 0.33 arc seconds at lambda = 2.14 microns. GLAO is achieved by applying a
correction to the telescope's adaptive secondary mirror that is an average of
wavefront measurements from five laser beacons supplemented with image motion
from a faint stellar source. Optimization of the adaptive optics system in
subsequent commissioning runs will further improve correction performance where
it is predicted to deliver 0.1 to 0.2 arc second resolution in the
near-infrared during a majority of seeing conditions.Comment: 13 pages, 1 table, 7 figures. Accepted for publication in
Astrophysical Journal. Expected March 200
Ground-layer wavefront reconstruction from multiple natural guide stars
Observational tests of ground layer wavefront recovery have been made in open
loop using a constellation of four natural guide stars at the 1.55 m Kuiper
telescope in Arizona. Such tests explore the effectiveness of wide-field seeing
improvement by correction of low-lying atmospheric turbulence with ground-layer
adaptive optics (GLAO). The wavefronts from the four stars were measured
simultaneously on a Shack-Hartmann wavefront sensor (WFS). The WFS placed a 5 x
5 array of square subapertures across the pupil of the telescope, allowing for
wavefront reconstruction up to the fifth radial Zernike order. We find that the
wavefront aberration in each star can be roughly halved by subtracting the
average of the wavefronts from the other three stars. Wavefront correction on
this basis leads to a reduction in width of the seeing-limited stellar image by
up to a factor of 3, with image sharpening effective from the visible to near
infrared wavelengths over a field of at least 2 arc minutes. We conclude that
GLAO correction will be a valuable tool that can increase resolution and
spectrographic throughput across a broad range of seeing-limited observations.Comment: 25 pages, 8 figures, to be published in Astrophys.
Commissioning the MMT ground-layer and laser tomography adaptive optics systems
A multi-laser adaptive optics system, at the 6.5 m MMT telescope, has been undergoing commissioning in preparation for wide-field, partially corrected as well as narrow-field, diffraction limited science observations in the thermal and near infrared. After several delays due to bad weather, we have successfully closed the full high order ground-layer adaptive optics (GLAO) control loop for the first time in February 2008 using five Rayleigh laser guide stars and a single tilt star. Characterization and automated correction of static aberrations such as non-common path errors were addressed in May 2008. Calibration measurements in preparation for laser tomography adaptive optics (LTAO) operation are planned for the fall of 2008 along with the start of shared-risk GLAO science observations.
We present the results of GLAO observations with the PISCES imager, a 1 - 2.5 µm camera with a field of view of 110 arc seconds. The status of the remaining GLAO commissioning work is also reviewed. Finally, we present plans for commissioning work to implement the LTAO operating mode of the system
Commissioning the MMT ground-layer and laser tomography adaptive optics systems
A multi-laser adaptive optics system, at the 6.5 m MMT telescope, has been undergoing commissioning in preparation for wide-field, partially corrected as well as narrow-field, diffraction limited science observations in the thermal and near infrared. After several delays due to bad weather, we have successfully closed the full high order ground-layer adaptive optics (GLAO) control loop for the first time in February 2008 using five Rayleigh laser guide stars and a single tilt star. Characterization and automated correction of static aberrations such as non-common path errors were addressed in May 2008. Calibration measurements in preparation for laser tomography adaptive optics (LTAO) operation are planned for the fall of 2008 along with the start of shared-risk GLAO science observations.
We present the results of GLAO observations with the PISCES imager, a 1 - 2.5 µm camera with a field of view of 110 arc seconds. The status of the remaining GLAO commissioning work is also reviewed. Finally, we present plans for commissioning work to implement the LTAO operating mode of the system
Adaptive Optics Nulling Interferometric Constraints on the Mid-Infrared Exozodiacal Dust Emission around Vega
We present the results of mid-infrared nulling interferometric observations
of the main-sequence star alpha Lyr (Vega) using the 6.5 m MMT with its
adaptive secondary mirror. From the observations at 10.6 microns, we find that
there is no resolved emission from the circumstellar environment (at
separations greater than 0.8 AU) above 2.1% (3 sigma limit) of the level of the
stellar photospheric emission. Thus, we are able to place an upper limit on the
density of dust in the inner system of 650 times that of our own solar system's
zodiacal cloud. This limit is roughly 2.8 times better than those determined
with photometric excess observations such as those by IRAS. Comparison with
far-infrared observations by IRAS shows that the density of warm dust in the
inner system (< 30 AU) is significantly lower than cold dust at larger
separations. We consider two scenarios for grain removal, the sublimation of
ice grains and the presence of a planetary mass "sweeper." We find that if
sublimation of ice grains is the only removal process, a large fraction (> 80%)
of the material in the outer system is ice.Comment: 11 pages, 1 figure, Accepted to The Astrophysical Journal Letter
Mid-Infrared Imaging of the Post-AGB Star AC Herculis with the MMT Adaptive Optics System
We utilized the MMT's unique deformable secondary adaptive optics system to
produce high-resolution (FWHM=0.3"), very high Strehl mid-infrared (9.8, 11.7 &
18 micron) images of the post-AGB star AC Her. The very high (98+/-2%) Strehls
achieved with Mid-IR AO led naturally to an ultra-stable PSF independent of
airmass, seeing, or location on the sky. We find no significant difference
between AC Her's morphology and our unresolved PSF calibration stars (mu UMa &
alpha Her) at 9.8, 11.7, & 18 microns. Our current observations do not confirm
any extended Mid-IR structure around AC Her. These observations are in conflict
with previously reported Keck (seeing-limited) 11.7 and 18 micron images which
suggested the presence of a resolved ~0.6" edge-on circumbinary disk. We
conclude that AC Her has no extended Mid-IR structure on scales greater than
0.2" (R<75 AU). These first results of Mid-IR AO science are very encouraging
for future high accuracy Mid-IR imaging with this technique.Comment: To appear in the November 20, 2003 issue of the Astrophysical Journal
Letters. The preprint has 7 pages and 3 figures (one in color; but prints OK
in B&W
Here Comes Grosz
Program for the eighth annual RISD Cabaret held in Cellar at the top of the Waterman Building. Publicity, posters, cartoons and program designed by Yoon Cho, Yu-Kyung Chung,Arther Jones, Scott King, Richard Lloyd and Polly Spencer.https://digitalcommons.risd.edu/liberalarts_cabaret_programs/1007/thumbnail.jp
High Resolution Images of Orbital Motion in the Trapezium Cluster: First Scientific Results from the MMT Deformable Secondary Mirror Adaptive Optics System
We present the first scientific images obtained with a deformable secondary
mirror adaptive optics system. We utilized the 6.5m MMT AO system to produce
high-resolution (FWHM=0.07'') near infrared (1.6 um) images of the young (~1
Myr) Orion Trapezium theta 1 Ori cluster members. A combination of high spatial
resolution and high signal to noise allowed the positions of these stars to be
measured to within ~0.003'' accuracies. Including previous speckle data
(Weigelt et al. 1999), we analyze a six year baseline of high-resolution
observations of this cluster. Over this baseline we are sensitive to relative
proper motions of only ~0.002''/yr (4.2 km/s at 450 pc). At such sensitivities
we detect orbital motion in the very tight theta 1 Ori B2B3 (52 AU separation)
and theta 1 Ori A1A2 (94 AU separation) systems. Such motions are consistent
with those independently observed by Schertl et al. (2003) with speckle
interferometry, giving us confidence that these very small (~0.002''/yr)
orbital motions are real. All five members of the theta 1 Ori B system appear
likely gravitationally bound. The very lowest mass member of the theta 1 Ori B
system (B4) has K' ~11.66 and an estimated mass of ~0.2 Msun. There was very
little motion (4+/-15 km/s) detected of B4 w.r.t B1 or B2, hence B4 is possibly
part of the theta 1 Ori B group. We suspect that if this very low mass member
is physically associated it most likely is in an unstable (non-hierarchical)
orbital position and will soon be ejected from the group. The theta 1 Ori B
system appears to be a good example of a star formation ``mini-cluster'' which
may eject the lowest mass members of the cluster in the near future. This
``ejection'' process could play a major role in the formation of low mass stars
and brown dwarfs.Comment: To appear in the December 10, 2003 issue of the Astrophysical Journal
21 pages, 14 figures (some in color, but print OK in B&W
Monte Carlo Simulations of Metasomatic Enrichment in the Lithosphere and Implications for the Source of Alkaline Basalts
One hypothesis for the origin of alkaline lavas erupted on oceanic islands and in intracontinental settings is that they represent the melts of amphibole-rich veins in the lithosphere (or melts of their dehydrated equivalents if metasomatized lithosphere is recycled into the convecting mantle). Amphibole-rich veins are interpreted as cumulates produced by crystallization of low-degree melts of the underlying asthenosphere as they ascend through the lithosphere. We present the results of trace-element modelling of the formation and melting of veins formed in this way with the goal of testing this hypothesis and for predicting how variability in the formation and subsequent melting of such cumulates (and adjacent cryptically and modally metasomatized lithospheric peridotite) would be manifested in magmas generated by such a process. Because the high-pressure phase equilibria of hydrous near-solidus melts of garnet lherzolite are poorly constrained and given the likely high variability of the hypothesized accumulation and remelting processes, we used Monte Carlo techniques to estimate how uncertainties in the model parameters (e.g. the compositions of the asthenospheric sources, their trace-element contents, and their degree of melting; the modal proportions of crystallizing phases, including accessory phases, as the asthenospheric partial melts ascend and crystallize in the lithosphere; the amount of metasomatism of the peridotitic country rock; the degree of melting of the cumulates and the amount of melt derived from the metasomatized country rock) propagate through the process and manifest themselves as variability in the trace-element contents and radiogenic isotopic ratios of model vein compositions and erupted alkaline magma compositions. We then compare the results of the models with amphibole observed in lithospheric veins and with oceanic and continental alkaline magmas. While the trace-element patterns of the near-solidus peridotite melts, the initial anhydrous cumulate assemblage (clinopyroxene ± garnet ± olivine ± orthopyroxene), and the modelled coexisting liquids do not match the patterns observed in alkaline lavas, our calculations show that with further crystallization and the appearance of amphibole (and accessory minerals such as rutile, ilmenite, apatite, etc.) the calculated cumulate assemblages have trace-element patterns that closely match those observed in the veins and lavas. These calculated hydrous cumulate assemblages are highly enriched in incompatible trace elements and share many similarities with the trace-element patterns of alkaline basalts observed in oceanic or continental setting such as positive Nb/La, negative Ce/Pb, and similiar slopes of the rare earth elements. By varying the proportions of trapped liquid and thus simulating the cryptic and modal metasomatism observed in peridotite that surrounds these veins, we can model the variations in Ba/Nb, Ce/Pb, and Nb/U ratios that are observed in alkaline basalts. If the isotopic compositions of the initial low-degree peridotite melts are similar to the range observed in mid-ocean ridge basalt, our model calculations produce cumulates that would have isotopic compositions similar to those observed in most alkaline ocean island basalt (OIB) and continental magmas after ~0·15 Gyr. However, to produce alkaline basalts with HIMU isotopic compositions requires much longer residence times (i.e. 1–2 Gyr), consistent with subduction and recycling of metasomatized lithosphere through the mantle. EM magmas cannot readily be explained without appealing to other factors such as a heterogeneous asthenosphere. These modelling results support the interpretation proposed by various researchers that amphibole-bearing veins represent cumulates formed during the differentiation of a volatile-bearing low-degree peridotite melt and that these cumulates are significant components of the sources of alkaline OIB and continental magmas. The results of the forward models provide the potential for detailed tests of this class of hypotheses for the origin of alkaline magmas worldwide and for interpreting major and minor aspects of the geochemical variability of these magmas
The impact of ENSO on Southern African rainfall in CMIP5 ocean atmosphere coupled climate models
We study the ability of 24 ocean atmosphere global coupled models from the Coupled Model Intercomparison Project 5 (CMIP5) to reproduce the teleconnections between El Niño Southern Oscillation (ENSO) and Southern African rainfall in austral summer using historical forced simulations, with a focus on the atmospheric dynamic associated with El Niño. Overestimations of summer rainfall occur over Southern Africa in all CMIP5 models. Abnormal westward extensions of ENSO patterns are a common feature of all CMIP5 models, while the warming of the Indian Ocean that happens during El Niño is not correctly reproduced. This could impact the teleconnection between ENSO and Southern African rainfall which is represented with mixed success in CMIP5 models. Large-scale anomalies of suppressed deep-convection over the tropical maritime continent and enhanced convection from the central to eastern Pacific are correctly simulated. However, regional biases occur above Africa and the Indian Ocean, particularly in the position of the deep convection anomalies associated with El Niño, which can lead to the wrong sign in rainfall anomalies in the northwest part of South Africa. From the near-surface to mid-troposphere, CMIP5 models underestimate the observed anomalous pattern of pressure occurring over Southern Africa that leads to dry conditions during El Niño years
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