436 research outputs found
An improved prism energy analyzer for neutrons
The effects of two improvements of an existing neutron energy analyzer consisting of stacked silicon prism rows are presented. First we tested the effect of coating the back of the prism rows with an absorbing layer to suppress neutron scattering by total reflection and by refraction at small angles. Experiments at HZB showed that this works perfectly. Second the prism rows were bent to shift the transmitted wavelength band to larger wavelengths. At HZB we showed that bending increased the transmission of neutrons with a wavelength of 4.9 . Experiments with a white beam at the EROS reflectometer at LLB showed that bending of the energy analyzing device to a radius of 7.9 m allows to shift the transmitted wavelength band from 0 to 9 to 2 to 1
Neutron energy analysis by silicon prisms
Neutron energy analysing allows to measure at di fferent wavelengths at the same time thus avoiding losses due to monochromatization. We built and tested a refractive energy analysing device made from small prisms, where losses only occur due to the attenuation in the material. We measured the refraction and the transmission of MgF2 and Si prisms at the V14 reflectometer in Berlin at 4.9 Angstroem to check their applicability. The experimentally determined linear attenuation coe cients are 0.055 cm 1 for the MgF2 and 0.03 cm 1 for the Si prisms. An energy analyser consisting of silicon prism layers was measured at the EROS reflectometer at the LLB in a white neutron beam. The useful wavelength band was 2.4 to 7.6 Angstroem . At 6.7 Angstroem a wavelength resolution of 5 and a transmission of 53 were achieved. The surface roughness of the prisms could be determined to be 0.011 0.006 de
HST/ACS Images of the GG Tauri Circumbinary Disk
Hubble Space Telescope Advanced Camera for Surveys images of the young binary
GG Tauri and its circumbinary disk in V and I bandpasses were obtained in 2002
and are the most detailed of this system to date. The confirm features
previously seen in the disk including: a "gap" apparently caused by shadowing
from circumstellar material; an asymmetrical distribution of light about the
line of sight on the near edge of the disk; enhanced brightness along the near
edge of the disk due to forward scattering; and a compact reflection nebula
near the secondary star. New features are seen in the ACS images: two short
filaments along the disk; localized but strong variations in disk intensity
("gaplets"); and a "spur" or filament extending from the reflection nebulosity
near the secondary. The back side of the disk is detected in the V band for the
first time. The disk appears redder than the combined light from the stars,
which may be explained by a varied distribution of grain sizes. The brightness
asymmetries along the disk suggest that it is asymmetrically illuminated by the
stars due to extinction by nonuniform circumstellar material or the illuminated
surface of the disk is warped by tidal effects (or perhaps both). Localized,
time-dependent brightness variations in the disk are also seen.Comment: 28 pages, 7 figures, accepted for publication in the Astronomical
Journa
HST Studies of the WLM Galaxy. I. The Age and Metallicity of the Globular Cluster
We have obtained V and I images of the lone globular cluster that belongs to
the dwarf Local Group irregular galaxy known as WLM. The color-magnitude
diagram of the cluster shows that it is a normal old globular cluster with a
well-defined giant branch reaching to M_V=-2.5, a horizontal branch at
M_V=+0.5, and a sub-giant branch extending to our photometry limit of M_V=+2.0.
A best fit to theoretical isochrones indicates that this cluster has a
metallicity of [Fe/H]=-1.52\pm0.08 and an age of 14.8\pm0.6 Gyr, thus
indicating that it is similar to normal old halo globulars in our Galaxy. From
the fit we also find that the distance modulus of the cluster is 24.73\pm0.07
and the extinction is A_V=0.07\pm0.06, both values that agree within the errors
with data obtained for the galaxy itself by others. We conclude that this
normal massive cluster was able to form during the formation of WLM, despite
the parent galaxy's very small intrinsic mass and size.Comment: 14 pages, 5 figures, 1 tabl
HST/NICMOS Observations of Massive Stellar Clusters Near the Galactic Center
We report Hubble Space Telescope (HST) Near-infrared Camera and Multi-object
Spectrometer (NICMOS) observations of the Arches and Quintuplet clusters, two
extraordinary young clusters near the Galactic Center. For the first time, we
have identified main sequence stars in the Galactic Center with initial masses
well below 10 Msun. We present the first determination of the initial mass
function (IMF) for any population in the Galactic Center, finding an IMF slope
which is significantly more positive (Gamma approx -0.65) than the average for
young clusters elsewhere in the Galaxy (Gamma approx -1.4). The apparent
turnoffs in the color-magnitude diagrams suggest cluster ages which are
consistent with the ages implied by the mixture of spectral types in the
clusters; we find tau(age) approx 2+/-1 Myr for the Arches cluster, and
tau(age) approx 4+/-1 Myr for the Quintuplet. We estimate total cluster masses
by adding the masses of observed stars down to the 50% completeness limit, and
then extrapolating down to a lower mass cutoff of 1 Msun. Using this method, we
find > 10^4 Msun for the total mass of the Arches cluster. Such a determination
for the Quintuplet cluster is complicated by the double-valued mass-magnitude
relationship for clusters with ages > 3 Myr. We find a lower limit of 6300 Msun
for the total cluster mass, and suggest a best estimate of twice this value
which accounts for the outlying members of the cluster. Both clusters have
masses which place them as the two most massive clusters in the Galaxy.Comment: accepted by ApJ higher resolution versions of figures 1 and 2 can be
found at: ftp://quintup.astro.ucla.edu/nicmos1
High resolution imaging of the GG Tau system at 267 GHz
Studying circumbinary disks is critical to understanding the formation
mechanisms of binary stars. While optical or mid-infrared images reveal the
scattered mission, millimeter observations provide direct measurements of the
dust thermal emission. We study the properties of the circumbinary disk around
the well-known, multiple young stellar object GG Tau with the highest possible
sensitivity and spatial resolution. We mapped the continuum emission of GG Tau
at 267 GHz using the IRAM Plateau de Bure interferometer equipped with upgraded
receivers and LO systems. An angular resolution of 0.45"x0.25" was achieved,
corresponding to a linear resolution of 65x35 AU. The GG Tau A circumbinary
disk is observed as an extremely clearly defined narrow ring. The width of the
ring is not resolved. Emission from the central binary is detected and clearly
separated from the ring: it coincides with the GG Tau Aa position and may
therefore trace a circumstellar disk around this star. The mass ratio of the
circumbinary to circumprimary material is ~80.Comment: 5 pages, 2 figures, accepted for publication in A&
NICMOS Imaging of a Damped Lyman-alpha Absorber at z=1.89 toward LBQS 1210+1731 : Constraints on Size and Star Formation Rate
We report results of a high-resolution imaging search (in rest frame
H- and optical continuum) for the galaxy associated with the damped
Lyman- (DLA) absorber at toward the quasar
LBQS 1210+1731, using HST/NICMOS. After PSF subtraction, a feature is seen in
both the broad-band and narrow-band images, at a projected separation of
0.25\arcsec from the quasar. If associated with the DLA, the object would be
kpc in size with a flux of Jy in
the F160W filter, implying a luminosity at {\AA} in
the rest frame of L at ,
for . However, no significant H- emission is seen,
suggesting a low star formation rate (SFR) (3 upper limit of 4.0
M yr), or very high dust obscuration.
Alternatively, the object may be associated with the host galaxy of the quasar.
H-band images obtained with the NICMOS camera 2 coronagraph show a much fainter
structure kpc in size and containing four knots of
continuum emission, located 0.7\arcsec away from the quasar. We have probed
regions far closer to the quasar sight-line than in most previous studies of
high-redshift intervening DLAs. The two objects we report mark the closest
detected high-redshift DLA candidates yet to any quasar sight line. If the
features in our images are associated with the DLA, they suggest faint,
compact, somewhat clumpy objects rather than large, well-formed proto-galactic
disks or spheroids.Comment: 52 pages of text, 19 figures, To be published in Astrophysical
Journal (accepted Dec. 8, 1999
The Luminosity Function of M3
We present a high precision, large sample luminosity function (LF) for the
Galactic globular cluster M3. With a combination of ground based and Hubble
Space Telescope data we cover the entire radial extent of the cluster. The
observed LF is well fit by canonical standard stellar models from the red giant
branch (RGB) tip to below the main sequence turnoff point. Specifically,
neither the RGB LF-bump nor subgiant branch LF indicate any breakdown in the
standard models. On the main sequence we find evidence for a flat initial mass
function and for mass segregation due to the dynamical evolution of the
cluster.Comment: 18 pages, 13 figures. ApJ, in pres
Limits on the Optical Brightness of the Epsilon Eridani Dust Ring
The STIS/CCD camera on the {\em Hubble Space Telescope (HST)} was used to
take deep optical images near the K2V main-sequence star Eridani in
an attempt to find an optical counterpart of the dust ring previously imaged by
sub-mm observations. Upper limits for the optical brightness of the dust ring
are determined and discussed in the context of the scattered starlight expected
from plausible dust models. We find that, even if the dust is smoothly
distributed in symmetrical rings, the optical surface brightness of the dust,
as measured with the {\em HST}/STIS CCD clear aperture at 55 AU from the star,
cannot be brighter than about 25 STMAG/". This upper limit excludes some
solid grain models for the dust ring that can fit the IR and sub-mm data.
Magnitudes and positions for 59 discrete objects between 12.5" to 58"
from Eri are reported. Most if not all of these objects are likely
to be background stars and galaxies.Comment: Revision corrects author lis
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