682 research outputs found
A new probe of the small-scale primordial power spectrum: astrometric microlensing by ultracompact minihalos
The dark matter enclosed in a density perturbation with a large initial
amplitude (delta-rho/rho > 1e-3) collapses shortly after recombination and
forms an ultracompact minihalo (UCMH). Their high central densities make UCMHs
especially suitable for detection via astrometric microlensing: as the UCMH
moves, it changes the apparent position of background stars. A UCMH with a mass
larger than a few solar masses can produce a distinctive astrometric
microlensing signal that is detectable by the space astrometry mission Gaia. If
Gaia does not detect gravitational lensing by any UCMHs, then it establishes an
upper limit on their abundance and constrains the amplitude of the primordial
power spectrum for k~2700 Mpc^{-1}. These constraints complement the upper
bound on the amplitude of the primordial power spectrum derived from limits on
gamma-ray emission from UCMHs because the astrometric microlensing signal
produced by an UCMH is maximized if the dark-matter annihilation rate is too
low to affect the UCMH's density profile. If dark matter annihilation within
UCMHs is not detectable, a search for UCMHs by Gaia could constrain the
amplitude of the primordial power spectrum to be less than 1e-5; this bound is
three orders of magnitude stronger than the bound derived from the absence of
primordial black holes.Comment: 17 pages, 6 figures, references added and minor changes made to match
version published in PR
A High Contrast Imaging Survey of SIM Lite Planet Search Targets
With the development of extreme high contrast ground-based adaptive optics
instruments and space missions aimed at detecting and characterizing Jupiter-
and terrestrial-mass planets, it is critical that each target star be
thoroughly vetted to determine whether it is a viable target given both the
instrumental design and scientific goals of the program. With this in mind, we
have conducted a high contrast imaging survey of mature AFGKM stars with the
PALAO/PHARO instrument on the Palomar 200 inch telescope. The survey reached
sensitivities sufficient to detect brown dwarf companions at separations of >
50 AU. The results of this survey will be utilized both by future direct
imaging projects such as GPI, SPHERE and P1640 and indirect detection missions
such as SIM Lite. Out of 84 targets, all but one have no close-in (0.45-1")
companions and 64 (76%) have no stars at all within the 25" field-of-view. The
sensitivity contrasts in the Ks passband ranged from 4.5 to 10 for this set of
observations. These stars were selected as the best nearby targets for
habitable planet searches owing to their long-lived habitable zones (> 1
billion years). We report two stars, GJ 454 and GJ 1020, with previously
unpublished proper motion companions. In both cases, the companions are stellar
in nature and are most likely M dwarfs based on their absolute magnitudes and
colors. Based on our mass sensitivities and level of completeness, we can place
an upper limit of ~17% on the presence of brown dwarf companions with masses
>40 MJ at separations of 1 arcsecond. We also discuss the importance of
including statistics on those stars with no detected companions in their field
of view for the sake of future companion searches and an overall understanding
of the population of low-mass objects around nearby stars.Comment: Accepted to PASP, Figure 7 available upon reques
Laser-only adaptive optics achieves significant image quality gains compared to seeing-limited observations over the entire sky
Adaptive optics laser guide star systems perform atmospheric correction of
stellar wavefronts in two parts: stellar tip-tilt and high-spatial-order
laser-correction. The requirement of a sufficiently bright guide star in the
field-of-view to correct tip-tilt limits sky coverage. Here we show an
improvement to effective seeing without the need for nearby bright stars,
enabling full sky coverage by performing only laser-assisted wavefront
correction. We used Robo-AO, the first robotic AO system, to comprehensively
demonstrate this laser-only correction. We analyze observations from four years
of efficient robotic operation covering 15,000 targets and 42,000 observations,
each realizing different seeing conditions. Using an autoguider (or a
post-processing software equivalent) and the laser to improve effective seeing
independent of the brightness of a target, Robo-AO observations show a 39+/-19%
improvement to effective FWHM, without any tip-tilt correction. We also
demonstrate that 50% encircled-energy performance without tip-tilt correction
remains comparable to diffraction-limited, standard Robo-AO performance.
Faint-target science programs primarily limited by 50% encircled-energy (e.g.
those employing integral field spectrographs placed behind the AO system) may
see significant benefits to sky coverage from employing laser-only AO.Comment: Accepted for publication in The Astronomical Journal. 7 pages, 6
figure
Multiplicity of the Galactic Senior Citizens: A high-resolution search for cool subdwarf companions
Cool subdwarfs are the oldest members of the low mass stellar population.
Mostly present in the galactic halo, subdwarfs are characterized by their low
metallicity. Measuring their binary fraction and comparing it to solar
metallicity stars could give key insights into the star formation process early
in the history of the Milky Way. However, because of their low luminosity and
relative rarity in the solar neighborhood, binarity surveys of cool subdwarfs
have suffered from small sample sizes and incompleteness. Previous surveys have
suggested that the binary fraction of red subdwarfs is much lower than for
their main sequence cousins. Using the highly efficient RoboAO system, we
present the largest yet high-resolution survey of subdwarfs, sensitive to
angular separations, down to 0.15 arcsec, and contrast ratios, up to 6
magnitude difference, invisible in past surveys. Of 344 target cool subdwarfs,
40 are in multiple systems, 16 newly discovered, for a binary fraction of 11.6
percent and 1.8 percent error. We also discovered 6 triple star systems for a
triplet fraction of 1.7 percent and 0.7 percent error. Comparisons to similar
surveys of solar metallicity dwarf stars gives a 3 sigma disparity in
luminosity between companion stars, with subdwarfs displaying a shortage of low
contrast companions.Comment: 13 pages, 10 figures, submitted to Ap
High-speed imaging and wavefront sensing with an infrared avalanche photodiode array
Infrared avalanche photodiode arrays represent a panacea for many branches of
astronomy by enabling extremely low-noise, high-speed and even photon-counting
measurements at near-infrared wavelengths. We recently demonstrated the use of
an early engineering-grade infrared avalanche photodiode array that achieves a
correlated double sampling read noise of 0.73 e- in the lab, and a total noise
of 2.52 e- on sky, and supports simultaneous high-speed imaging and tip-tilt
wavefront sensing with the Robo-AO visible-light laser adaptive optics system
at the Palomar Observatory 1.5-m telescope. We report here on the improved
image quality achieved simultaneously at visible and infrared wavelengths by
using the array as part of an image stabilization control-loop with
adaptive-optics sharpened guide stars. We also discuss a newly enabled survey
of nearby late M-dwarf multiplicity as well as future uses of this technology
in other adaptive optics and high-contrast imaging applications.Comment: Accepted to Astrophysical Journal. 8 pages, 3 figures and 1 tabl
Twelve-thousand laser-AO observations: first results from the Robo-AO large surveys
Robo-AO is the first AO system which can feasibly perform surveys of thousands of targets. The system has been operating in a fully robotic mode on the Palomar 1.5m telescope for almost two years. Robo-AO has completed nearly 12,000 high-angular-resolution observations in almost 20 separate science programs including exoplanet characterization, field star binarity, young star binarity and solar system observations. We summarize the Robo-AO surveys and the observations completed to date. We also describe the data-reduction pipeline we developed for Robo-AO—the first fully-automated AO data-reduction, point-spread-function subtraction and companion-search pipeline
Robo-AO Discovery and Basic Characterization of Wide Multiple Star Systems in the Pleiades, Praesepe, and NGC 2264 Clusters
We identify and roughly characterize 66 candidate binary star systems in the
Pleiades, Praesepe, and NGC 2264 star clusters based on robotic adaptive optics
imaging data obtained using Robo-AO at the Palomar 60" telescope. Only
10% of our imaged pairs were previously known. We detect companions at
red optical wavelengths having physical separations ranging from a few tens to
a few thousand AU. A 3-sigma contrast curve generated for each final image
provides upper limits to the brightness ratios for any undetected putative
companions. The observations are sensitive to companions with maximum contrast
6 at larger separations. At smaller separations, the mean (best) raw
contrast at 2 arcsec is 3.8 (6), at 1 arcsec is 3.0 (4.5), and
at 0.5 arcsec is 1.9 (3). PSF subtraction can recover close to the full
contrast in to the closer separations. For detected candidate binary pairs, we
report separations, position angles, and relative magnitudes. Theoretical
isochrones appropriate to the Pleiades and Praesepe clusters are then used to
determine the corresponding binary mass ratios, which range from 0.2-0.9 in
. For our sample of roughly solar-mass (FGK type) stars in NGC 2264
and sub-solar-mass (K and early M-type) primaries in the Pleiades and Praesepe,
the overall binary frequency is measured at 15.5% 2%. However, this
value should be considered a lower limit to the true binary fraction within the
specified separation and mass ratio ranges in these clusters, given that
complex and uncertain corrections for sensitivity and completeness have not
been applied.Comment: Accepted to A
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