723 research outputs found
Phase Errors in Diffraction-Limited Imaging: Contrast Limits for Sparse Aperture Masking
Bispectrum phase, closure phase and their generalisation to kernel-phase are
all independent of pupil-plane phase errors to first-order. This property, when
used with Sparse Aperture Masking (SAM) behind adaptive optics, has been used
recently in high-contrast observations at or inside the formal diffraction
limit of large telescopes. Finding the limitations to these techniques requires
an understanding of spatial and temporal third-order phase effects, as well as
effects such as time-variable dispersion when coupled with the non-zero
bandwidths in real observations. In this paper, formulae describing many of
these errors are developed, so that a comparison can be made to fundamental
noise processes of photon- and background-noise. I show that the current
generation of aperture-masking observations of young solar-type stars, taken
carefully in excellent observing conditions, are consistent with being limited
by temporal phase noise and photon noise. This has relevance for plans to
combine pupil-remapping with spatial filtering. Finally, I describe calibration
strategies for kernel-phase, including the optimised calibrator weighting as
used for LkCa 15, and the restricted kernel-phase POISE technique that avoids
explicit dependence on calibrators.Comment: 11 pages, 5 figures, resubmitted to MNRAS after responding to
referee's comment
Effects of moderate abundance changes on the atmospheric structure and colours of Mira variables (Research Note)
Aims. We study the effects of moderate deviations from solar abundances upon
the atmospheric structure and colours of typical Mira variables. Methods. We
present two model series of dynamical opacity-sampling models of Mira variables
which have (1) 1 solar metallicity 3 and (2) "mild" S-type C/O abundance ratio
([C/O]=0.9) with typical Zr enhancement (solar +1.0). These series are compared
to a previously studied solar-abundance series which has similar fundamental
parameters (mass, luminosity, period, radius) that are close to those of o Cet.
Results. Both series show noticeable effects of abundance upon stratifications
and infrared colours but cycle-to-cycle differences mask these effects at most
pulsation phases, with the exception of a narrow-water-filter colour near
minimum phase.Comment: 4 pages, 3 figures, accepted for A&
Integrated photonic building blocks for next-generation astronomical instrumentation II: the multimode to single mode transition
There are numerous advantages to exploiting diffraction-limited
instrumentation at astronomical observatories, which include smaller
footprints, less mechanical and thermal instabilities and high levels of
performance. To realize such instrumentation it is imperative to convert the
atmospheric seeing-limited signal that is captured by the telescope into a
diffraction-limited signal. This process can be achieved photonically by using
a mode reformatting device known as a photonic lantern that performs a
multimode to single-mode transition. With the aim of developing an optimized
integrated photonic lantern, we undertook a systematic parameter scan of
devices fabricated by the femtosecond laser direct-write technique. The devices
were designed for operation around 1.55 {\mu}m. The devices showed (coupling
and transition) losses of less than 5% for F/# 12 injection and the
total device throughput (including substrate absorption) as high as 75-80%.
Such devices show great promise for future use in astronomy.Comment: 12 pages, 9 figure
Multiband processing of multimode light: combining 3D photonic lanterns with waveguide Bragg gratings
The first demonstration of narrowband spectral filtering of multimode light
on a 3D integrated photonic chip using photonic lanterns and waveguide Bragg
gratings is reported. The photonic lanterns with multi-notch waveguide Bragg
gratings were fabricated using the femtosecond direct-write technique in
boro-aluminosilicate glass (Corning, Eagle 2000). Transmission dips of up to 5
dB were measured in both photonic lanterns and reference single-mode waveguides
with 10.4-mm-long gratings. The result demonstrates efficient and symmetrical
performance of each of the gratings in the photonic lantern. Such devices will
be beneficial to space-division multiplexed communication systems as well as
for units for astronomical instrumentation for suppression of the atmospheric
telluric emission from OH lines.Comment: 5 pages, 4 figures, accepted to Laser & Photonics Review
Simulating a dual beam combiner at SUSI for narrow-angle astrometry
The Sydney University Stellar Interferometer (SUSI) has two beam combiners,
i.e. the Precision Astronomical Visible Observations (PAVO) and the
Microarcsecond University of Sydney Companion Astrometry (MUSCA). The primary
beam combiner, PAVO, can be operated independently and is typically used to
measure properties of binary stars of less than 50 milliarc- sec (mas)
separation and the angular diameters of single stars. On the other hand, MUSCA
was recently installed and must be used in tandem with the for- mer. It is
dedicated for microarcsecond precision narrow-angle astrometry of close binary
stars. The performance evaluation and development of the data reduction
pipeline for the new setup was assisted by an in-house computer simulation tool
developed for this and related purposes. This paper describes the framework of
the simulation tool, simulations carried out to evaluate the performance of
each beam combiner and the expected astrometric precision of the dual beam
combiner setup, both at SUSI and possible future sites.Comment: 28 pages, 23 figures, accepted for publication in Experimental
Astronomy. The final publication is available at http://link.springer.co
Mapping the Shores of the Brown Dwarf Desert. I. Upper Scorpius
We present the results of a survey for stellar and substellar companions to 82 young stars in the nearby OB association Upper Scorpius. This survey used nonredundant aperture mask interferometry to achieve typical contrast
limits of ΔK ~5-6 at the diffraction limit, revealing 12 new binary companions that lay below the detection limits
of traditional high-resolution imaging; we also summarize a complementary snapshot imaging survey that discovered
seven directly resolved companions. The overall frequency of binary companions (~35 +5 -4% at separations of
6-435 AU) appears to be equivalent to field stars of similar mass, but companions could be more common among
lower mass stars than for the field. The companion mass function has statistically significant differences compared to several suggested mass functions for the field, and we suggest an alternate lognormal parameterization of the mass function. Our survey limits encompass the entire brown dwarf mass range, but we only detected a single companion that might be a brown dwarf; this deficit resembles the so-called brown dwarf desert that has been observed by radial velocity planet searches. Finally, our survey’s deep detection limits extend into the top of the planetary mass function, reaching 8-12 MJup for half of our sample. We have not identified any planetary companions at high confidence (≳99.5%), but we have identified four candidate companions at lower confidence (≳97.5%) that merit additional follow-up to confirm or disprove their existence
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