5,913 research outputs found
Quasi-periodic pulsations in solar and stellar flares: re-evaluating their nature in the context of power-law flare Fourier spectra
The nature of quasi-periodic pulsations in solar and stellar flares remains
debated. Recent work has shown that power-law-like Fourier power spectra, also
referred to as 'red' noise processes, are an intrinsic property of solar and
stellar flare signals, a property that many previous studies of this phenomenon
have not accounted for. Hence a re-evaluation of the existing interpretations
and assumptions regarding QPP is needed. Here we adopt a Bayesian method for
investigating this phenomenon, fully considering the Fourier power law
properties of flare signals. Using data from the PROBA2/LYRA, Fermi/GBM,
Nobeyama Radioheliograph and Yohkoh/HXT instruments, we study a selection of
flares from the literature identified as QPP events. Additionally we examine
optical data from a recent stellar flare that appears to exhibit oscillatory
properties. We find that, for all but one event tested, an explicit oscillation
is not required in order to explain the observations. Instead, the flare
signals are adequately described as a manifestation of a power law in the
Fourier power spectrum, rather than a direct signature of oscillating
components or structures. However, for the flare of 1998 May 8, strong evidence
for the existence of an explicit oscillation with P ~ 14-16 s is found in the
17 GHz radio data and the 13-23 keV Yohkoh HXT data. We conclude that, most
likely, many previously analysed events in the literature may be similarly
described in terms of power laws in the flare Fourier power spectrum, without
the need to invoke a narrowband, oscillatory component. As a result the
prevalence of oscillatory signatures in solar and stellar flares may be less
than previously believed. The physical mechanism behind the appearance of the
observed power laws is discussed.Comment: 11 pages, 7 figures, 1 table. Accepted for publication in The
Astrophysical Journa
Two Wide Planetary-mass Companions to Solar-type Stars in Upper Scorpius
At wide separations, planetary-mass and brown dwarf companions to solar-type stars occupy a curious region of
parameter space not obviously linked to binary star formation or solar system scale planet formation. These
companions provide insight into the extreme case of companion formation (either binary or planetary), and
due to their relative ease of observation when compared to close companions, they offer a useful template
for our expectations of more typical planets. We present the results from an adaptive optics imaging survey
for wide (~50–500 AU) companions to solar-type stars in Upper Scorpius. We report one new discovery of a
~14 M_J companion around GSC 06214−00210and confirm that the candidate planetary-mass companion 1RXS
J160929.1−210524 detected by Lafrenière et al. is in fact comoving with its primary star. In our survey, these
two detections correspond to ~4% of solar-type stars having companions in the 6–20 M_J mass and ~200–500 AU
separation range. This figure is higher than would be expected if brown dwarfs and planetary-mass companions
were drawn from an extrapolation of the binary mass function. Finally, we discuss implications for the formation
of these objects
The Role of Multiplicity in Disk Evolution and Planet Formation
The past decade has seen a revolution in our understanding of protoplanetary
disk evolution and planet formation in single star systems. However, the
majority of solar-type stars form in binary systems, so the impact of binary
companions on protoplanetary disks is an important element in our understanding
of planet formation. We have compiled a combined multiplicity/disk census of
Taurus-Auriga, plus a restricted sample of close binaries in other regions, in
order to explore the role of multiplicity in disk evolution. Our results imply
that the tidal influence of a close (<40 AU) binary companion significantly
hastens the process of protoplanetary disk dispersal, as ~2/3 of all close
binaries promptly disperse their disks within <1 Myr after formation. However,
prompt disk dispersal only occurs for a small fraction of wide binaries and
single stars, with ~80%-90% retaining their disks for at least ~2--3 Myr (but
rarely for more than ~5 Myr). Our new constraints on the disk clearing
timescale have significant implications for giant planet formation; most single
stars have 3--5 Myr within which to form giant planets, whereas most close
binary systems would have to form giant planets within <1 Myr. If core
accretion is the primary mode for giant planet formation, then gas giants in
close binaries should be rare. Conversely, since almost all single stars have a
similar period of time within which to form gas giants, their relative rarity
in RV surveys indicates either that the giant planet formation timescale is
very well-matched to the disk dispersal timescale or that features beyond the
disk lifetime set the likelihood of giant planet formation.Comment: Accepted to ApJ; 15 pages, 3 figures, 3 tables in emulateapj forma
A low cost scheme for high precision dual-wavelength laser metrology
A novel method capable of delivering relative optical path length metrology
with nanometer precision is demonstrated. Unlike conventional dual-wavelength
metrology which employs heterodyne detection, the method developed in this work
utilizes direct detection of interference fringes of two He-Ne lasers as well
as a less precise stepper motor open-loop position control system to perform
its measurement. Although the method may be applicable to a variety of
circumstances, the specific application where this metrology is essential is in
an astrometric optical long baseline stellar interferometer dedicated to
precise measurement of stellar positions. In our example application of this
metrology to a narrow-angle astrometric interferometer, measurement of
nanometer precision could be achieved without frequency-stabilized lasers
although the use of such lasers would extend the range of optical path length
the metrology can accurately measure. Implementation of the method requires
very little additional optics or electronics, thus minimizing cost and effort
of implementation. Furthermore, the optical path traversed by the metrology
lasers is identical with that of the starlight or science beams, even down to
using the same photodetectors, thereby minimizing the non-common-path between
metrology and science channels.Comment: 17 pages, 4 figures, accepted for publication in Applied Optic
Entanglement of arbitrary superpositions of modes within two-dimensional orbital angular momentum state spaces
We use spatial light modulators (SLMs) to measure correlations between arbitrary superpositions of orbital angular momentum (OAM) states generated by spontaneous parametric down-conversion. Our technique allows us to fully access a two-dimensional OAM subspace described by a Bloch sphere, within the higher-dimensional OAM Hilbert space. We quantify the entanglement through violations of a Bell-type inequality for pairs of modal superpositions that lie on equatorial, polar, and arbitrary great circles of the Bloch sphere. Our work shows that SLMs can be used to measure arbitrary spatial states with a fidelity sufficient for appropriate quantum information processing systems
The radius and mass of the subgiant star bet Hyi from interferometry and asteroseismology
We have used the Sydney University Stellar Interferometer (SUSI) to measure
the angular diameter of beta Hydri. This star is a nearby G2 subgiant whose
mean density was recently measured with high precision using asteroseismology.
We determine the radius and effective temperature of the star to be
1.814+/-0.017 R_sun (0.9%) and 5872+/-44 K (0.7%) respectively. By combining
this value with the mean density, as estimated from asteroseismology, we make a
direct estimate of the stellar mass. We find a value of 1.07+/-0.03 M_sun
(2.8%), which agrees with published estimates based on fitting in the H-R
diagram, but has much higher precision. These results place valuable
constraints on theoretical models of beta Hyi and its oscillation frequencies.Comment: 3 figures, 3 tables, to appear in MNRAS Letter
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