307 research outputs found
Surface figure measurements of radio telescopes with a shearing interferometer
A new technique for determining the surface figure of large submillimeter wavelength telescopes is presented, which is based on measuring the telescopeâs focal plane diffraction pattern with a shearing interferometer. In addition to the instrumental theory, results obtained using such an interferometer on the 10.4-m diam telescope of the Caltech Submillimeter Observatory are discussed. Using wavelengths near 1 mm, a measurement accuracy of 9 ”m, or λ/115, has been achieved, and the rms surface accuracy has been determined to be just under 30 ”m. The distortions of the primary reflector with changing elevation angle have also been measured and agree well with theoretical predictions of the dish deformation
Infalling Gas Towards the Galactic Center
VLA maps of ammonia emission were made for the Galactic Center region. The
NH3(1,1) and NH3(2,2) transitions were observed in three 2' x 2' fields
covering Sgr A* and the region 3' immediately south of it. In the central 3
parsecs surrounding Sgr A* we find emission which appears to be associated with
the circumnuclear disk (CND), both morphologically and kinematically. This
central emission is connected to a long, narrow 2 pc x 10 pc streamer of clumpy
molecular gas located towards the south, which appears to be carrying gas from
the nearby 20 km/s giant molecular cloud (GMC) to the circumnuclear region. We
find a velocity gradient along the streamer, with progressively higher
velocities as the gas approaches Sgr A*. The streamer stops at the location of
the CND, where the line width of the NH3 emission increases dramatically. This
may be the kinematic signature of accretion onto the CND. The ratio of the
NH3(2,2)/NH3(1,1) emission indicates that the gas is heated at the northern tip
of the streamer, located inside the eastern edge of the CND. The morphology,
kinematics and temperature gradients of the gas all indicate that the southern
streamer is located at the Galactic Center and is interacting with the
circumnuclear region.Comment: 11 pages, 10 figures, accepted by The Astrophysical Journal (figure 1
contours have been corrected
High Resolution Infrared Imaging and Spectroscopy of the Pistol Nebula: Evidence for Ejection
We present new NICMOS/HST infrared images and CGS4/UKIRT Br-alpha (4.05 um)
spectroscopy of the Pistol Star and its associated nebula, finding strong
evidence to support the hypothesis that the Pistol Nebula was ejected from the
Pistol Star. The Pa-alpha NICMOS image shows that the nebula completely
surrounds the Pistol Star, although the line intensity is much stronger on its
northern and western edges. The Br-alpha spectra show the classical ring-like
signature of quasi-spherical expansion, with weak blueshifted emission (V_max
approx -60 km/s) and strong redshifted emission (V_max approx +10 km/s), where
the velocities are with respect to the velocity of the Pistol Star; further,
the redshifted emission appears to be "flattened" in the position-velocity
diagram. These data suggest that the nebula was ejected from the star several
thousand years ago, with a velocity between the current terminal velocity of
the stellar wind (95 km/s) and the present expansion velocity of gas in the
outer shell of the nebula (60 km/s). The Pa-alpha image reveals several
emission-line stars in the region, including two newly-identified emission-line
stars north of the Pistol Star with spectral types earlier than WC8 (T_eff >
50,000 K). The presence of these stars, the morphology of the Pa-alpha
emission, and the velocity field in the gas suggest that the side of the nebula
furthest from us is approaching, and being ionized by, the hot stars of the
Quintuplet, and that the highest velocity redshifted gas has been decelerated
by winds from the Quintuplet stars. We also discuss the possibility that the
nebular gas might be magnetically confined by the ambient magnetic field
delineated by the nearby nonthermal filaments.Comment: Figure 1 is included as a JPG file. Figure 1 and 2 also available at
ftp://quintup.astro.ucla.edu/pistol2
High-Resolution Near-Infrared Polarimetry of a Circumstellar Disk around UX Tau A
We present H-band polarimetric imagery of UX Tau A taken with HiCIAO/AO188 on the Subaru Telescope. UX Tau A has been classified as a pre-transitional disk object, with a gap structure separating its inner and outer disks. Our imagery taken with the 0.15" (21 AU) radius coronagraphic mask has revealed a strongly polarized circumstellar disk surrounding UX Tau A which extends to 120 AU, at a spatial resolution of 0.1" (14 AU). It is inclined by 46 degrees plus or minus 2 degrees as the west side is nearest. Although SED modeling and sub-millimeter imagery suggested the presence of a gap in the disk, with the inner edge of the outer disk estimated to be located at 25 - 30 AU, we detect no evidence of a gap at the limit of our inner working angle (23AU) at the near-infrared wavelength. We attribute the observed strong polarization (up to 66 %) to light scattering by dust grains in the disk. However, neither polarization models of the circumstellar disk based on Rayleigh scattering nor Mie scattering approximations were consistent with the observed azimuthal profile of the polarization degrees of the disk. Instead, a geometric optics model of the disk with nonspherical grains with the radii of 30 micrometers is consistent with the observed profile. We suggest that the dust grains have experienced frequent collisional coagulations and have grown in the circumstellar disk of UX Tau A
Lyot-based Low Order Wavefront Sensor: Implementation on the Subaru Coronagraphic Extreme Adaptive Optics System and its Laboratory Performance
High throughput, low inner working angle (IWA) phase masks coronagraphs are
essential to directly image and characterize (via spectroscopy) earth-like
planets. However, the performance of low-IWA coronagraphs is limited by
residual pointing errors and other low-order modes. The extent to which
wavefront aberrations upstream of the coronagraph are corrected and calibrated
drives coronagraphic performance. Addressing this issue is essential for
preventing coronagraphic leaks, thus we have developed a Lyot-based low order
wave front sensor (LLOWFS) to control the wavefront aberrations in a
coronagraph. The LLOWFS monitors the starlight rejected by the coronagraphic
mask using a reflective Lyot stop in the downstream pupil plane. The early
implementation of LLOWFS at LESIA, Observatoire de Paris demonstrated an open
loop measurement accuracy of 0.01 lambda/D for tip-tilt at 638 nm when used in
conjunction with a four quadrant phase mask (FQPM) in the laboratory. To
further demonstrate our concept, we have installed the reflective Lyot stops on
the Subaru Coronagraphic Extreme AO (SCExAO) system at the Subaru Telescope and
modified the system to support small IWA phase mask coronagraphs (< 1 lambda/D)
on-sky such as FQPM, eight octant phase mask, vector vortex coronagraph and the
phase induced amplitude apodization complex phase mask coronagraph with a goal
of obtaining milli arc-second pointing accuracy. Laboratory results have shown
the measurement of tip, tilt, focus, oblique and right astigmatism at 1.55 um
for the vector vortex coronagraph. Our initial on-sky result demonstrate the
closed loop accuracy of < 7 x 10-3 lambda/D at 1.6 um for tip, tilt and focus
aberrations with the vector vortex coronagraph.Comment: 9 pages, 9 Figures, Proc. of SPIE Astronomical Telescopes +
Instrumentation 201
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
Coronagraphic Low Order Wave Front Sensor : post-processing sensitivity enhancer for high performance coronagraphs
Detection and characterization of exoplanets by direct imaging requires a
coronagraph designed to deliver high contrast at small angular separation. To
achieve this, an accurate control of low order aberrations, such as pointing
and focus errors, is essential to optimize coronagraphic rejection and avoid
the possible confusion between exoplanet light and coronagraphic leaks in the
science image. Simulations and laboratory prototyping have shown that a
Coronagraphic Low Order Wave-Front Sensor (CLOWFS), using a single defocused
image of a reflective focal plane ring, can be used to control tip-tilt to an
accuracy of 10^{-3} lambda/D. This paper demonstrates that the data acquired by
CLOWFS can also be used in post-processing to calibrate residual coronagraphic
leaks from the science image. Using both the CLOWFS camera and the science
camera in the system, we quantify the accuracy of the method and its ability to
successfully remove light due to low order errors from the science image. We
also report the implementation and performance of the CLOWFS on the Subaru
Coronagraphic Extreme AO (SCExAO) system and its expected on-sky performance.
In the laboratory, with a level of disturbance similar to what is encountered
in a post Adaptive Optics beam, CLOWFS post-processing has achieved speckle
calibration to 1/300 of the raw speckle level. This is about 40 times better
than could be done with an idealized PSF subtraction that does not rely on
CLOWFS.Comment: 10 pages, 7 figures, accepted for publication in PAS
Development of a broadband submillimeter grating spectrometer
One of the central issues in astronomy is the formation and evolution of galaxies at large redshifts. Submillimeter observations are essential to understanding these processes. One of the best prospects for high redshift submillimeter observations is the study of the C_(II) 158 micrometer fine- structure line, which carries about 0.2% of the total luminosity of nearby starburst galaxies. However, current heterodyne receivers at submillimeter observatories have insufficient bandwidth to detect the full extent of highly broadened emission lines. We are developing a broadband grating spectrometer for the Caltech Submillimeter Observatory with a total bandwidth of ~3400 km/s and a velocity resolution of 200 km/s. The detectors will be a linear array of 32 close-packed monolithic silicon bolometers developed at NASA's Goddard Space Flight Center. In order to achieve background-limited sensitivity, the bolometers will be cooled to 100 mK by an adiabatic demagnetization refrigerator. The spectrometer optics will consist of a tunable cryogenic immersion grating using broadband filters as order sorters. The spectrometer will be optimized to operate in the 350 ”m and 450 ”m atmospheric windows. Calculations of the sensitivity of the spectrometer reveal that an ultraluminous infrared galaxy of 10^(12) L_â at a redshift of z = 1 would be detectable at the 3Ï level in the C_(II) line with 20 minutes of integration time
Detection of interstellar CH_3
Observations with the Short Wavelength Spectrometer (SWS) onboard the {\it
Infrared Space Observatory} (ISO) have led to the first detection of the methyl
radical in the interstellar medium. The branch at 16.5
m and the (0) line at 16.0 m have been unambiguously detected
toward the Galactic center SgrA. The analysis of the measured bands gives a
column density of (8.02.4) cm and an excitation
temperature of K. Gaseous at a similarly low excitation
temperature and are detected for the same line of sight. Using
constraints on the column density obtained from and
visual extinction, the inferred abundance is
. The chemically related
molecule is not detected, but the pure rotational lines of are seen
with the Long Wavelength Spectrometer (LWS). The absolute abundances and the
and ratios are inconsistent with published
pure gas-phase models of dense clouds. The data require a mix of diffuse and
translucent clouds with different densities and extinctions, and/or the
development of translucent models in which gas-grain chemistry, freeze-out and
reactions of with polycyclic aromatic hydrocarbons and solid
aliphatic material are included.Comment: 2 figures. ApJL, Accepte
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