7,419 research outputs found
The potential for super-Nyquist asteroseismology with TESS
The perfect 30-min cadence of the full-frame images from the Transiting
Exoplanet Survey Satellite (TESS) will impose a hard Nyquist limit of 24
d ( Hz). This will be problematic for asteroseismology
of stars with oscillation frequencies at or around that Nyquist limit, which
will have insurmountable Nyquist ambiguities. TESS does offer some observing
slots at shorter cadences, but these will be limited in number and competitive,
while the full frame images will be the main data product for many types of
variable stars. We show that the Nyquist ambiguities can be alleviated if, when
TESS resumes observations after a downlink, integrations are not resumed at
perfect cadence with those before the downlink. The time spent idling before
integrations are resumed need only be around five minutes for satisfactory
results, and observing time can be recouped from the downlink event if the
telescope does not wait for a return to perfect cadence before resuming
integrations. The importance of imperfect cadence after downlink is discussed
in light of phase coverage of transit events.Comment: 7 pages; 9 figures, all of which use colour but have been checked for
readability under grey-scale printing; 2 small tables; accepted for
publication in MNRA
Super-Nyquist Asteroseismology with Future Space Missions
We propose a photometric technique for future space missions that overcomes
the problem of Nyquist aliases. These aliases result from typically long
cadences of observation imposed by telemetry constraints. The proposed method
is to introduce a periodic modulation to the sampling rate. Suitable
combinations of the frequency and the amplitude of this modulation allow the
true peaks to be distinguished from the aliases. We provide an analytical proof
of the validity of this method and some demonstrations with simulated data. We
also propose to divide a long cadence into two unequal parts, aiming at
reproducing the intrinsic amplitude spectrum of stars without a severe smearing
effect due to long exposures. The two exposures can be summed to recover the
photon statistics if the user is interested in doing so. Based on these
proposals, a specific recommendation for the PLATO mission is made to maximise
its capability of photometry for asteroseismology, without serious interference
with its other scientific missions.Comment: 13 pages, fig11a was replaced with the correct one. Proceedings of
the PHOST "Physics of Oscillating Stars" conference (2-7 Sept. 2018,
Banyuls-sur-mer, France), Edited by J. Ballot, S. Vauclair, & G. Vauclai
The modeling of diffuse boundaries in the 2-D digital waveguide mesh
The digital waveguide mesh can be used to simulate the propagation of sound waves in an acoustic system. The accurate simulation of the acoustic characteristics of boundaries within such a system is an important part of the model. One significant property of an acoustic boundary is its diffusivity. Previous approaches to simulating diffuse boundaries in a digital waveguide mesh are effective but exhibit limitations and have not been analyzed in detail. An improved technique is presented here that simulates diffusion at boundaries and offers a high degree of control and consistency. This technique works by rotating wavefronts as they pass through a special diffusing layer adjacent to the boundary. The waves are rotated randomly according to a chosen probability function and the model is lossless. This diffusion model is analyzed in detail, and its diffusivity is quantified in the form of frequency dependent diffusion coefficients. The approach used to measuring boundary diffusion is described here in detail for the 2-D digital waveguide mesh and can readily be extended for the 3-D case
Deriving the orbital properties of pulsators in binary systems through their light arrival time delays
We present the latest developments to the phase modulation method for finding
binaries among pulsating stars. We demonstrate how the orbital elements of a
pulsating binary star can be obtained analytically, that is, without converting
time delays to radial velocities by numerical differentiation. Using the time
delays directly offers greater precision, and allows the parameters of much
smaller orbits to be derived. The method is applied to KIC9651065, KIC10990452,
and KIC8264492, and a set of the orbital parameters is obtained for each
system. Radial velocity curves for these stars are deduced from the orbital
elements thus obtained.Comment: 12 pages, 26 figures, 8 tables, accepted for publication in MNRA
New low-mass members of the Octans stellar association and an updated 30-40 Myr lithium age
The Octans association is one of several young stellar moving groups recently
discovered in the Solar neighbourhood, and hence a valuable laboratory for
studies of stellar, circumstellar disc and planetary evolution. However, a lack
of low-mass members or any members with trigonometric parallaxes means the age,
distance and space motion of the group are poorly constrained. To better
determine its membership and age, we present the first spectroscopic survey for
new K and M-type Octans members, resulting in the discovery of 29 UV-bright
K5-M4 stars with kinematics, photometry and distances consistent with existing
members. Nine new members possess strong Li I absorption, which allow us to
estimate a lithium age of 30-40 Myr, similar to that of the Tucana-Horologium
association and bracketed by the firm lithium depletion boundary ages of the
Beta Pictoris (20 Myr) and Argus/IC 2391 (50 Myr) associations. Several stars
also show hints in our medium-resolution spectra of fast rotation or
spectroscopic binarity. More so than other nearby associations, Octans is much
larger than its age and internal velocity dispersion imply. It may be the
dispersing remnant of a sparse, extended structure which includes some younger
members of the foreground Octans-Near association recently proposed by
Zuckerman and collaborators.Comment: Accepted for publication in MNRAS (16 pages, 5 tables
Spectrophotometric Libraries, Revised Photonic Passbands and Zero-points for UBVRI, Hipparcos and Tycho Photometry
We have calculated improved photonic passbands for the UBV RI, Hipparcos and
Tycho Hp,BT,VT standard systems using the extensive spectrophotometric
libraries of NGSL and MILES. Using the Hp passband, we adjusted the absolute
flux levels of stars in the spectrophotometric libraries so their synthetic Hp
magnitudes matched the precise Hipparcos catalog value. Synthetic photometry
based on the renormalized fluxes were compared to the standard UBVRI and BT, VT
magnitudes and revised synthetic zero-points were determined. The Hipparcos and
Tycho photometry system zero-points were also compared to the V magnitude
zero-points of the SAAO UBVRI system, the homogenized UBV system and the
Walraven V B system. The confusion in the literature concerning broadband
magnitudes, fluxes, passbands and the choice of appropriate mean wavelengths is
detailed and discussed in an appendix.Comment: 44 pages, including 16 figures and a 12 page appendi
New members of the TW Hydrae Association and two accreting M-dwarfs in Scorpius-Centaurus
We report the serendipitous discovery of several young mid-M stars found
during a search for new members of the 30-40 Myr-old Octans Association. Only
one of the stars may be considered a possible Octans(-Near) member. However,
two stars have proper motions, kinematic distances, radial velocities,
photometry and Li I 6708AA measurements consistent with membership in the 8-10
Myr-old TW Hydrae Association. Another may be an outlying member of TW Hydrae
but has a velocity similar to that predicted by membership in Octans. We also
identify two new lithium-rich members of the neighbouring Scorpius-Centaurus OB
Association (Sco-Cen). Both exhibit large 12 and 22 micron excesses and strong,
variable H-alpha emission which we attribute to accretion from circumstellar
discs. Such stars are thought to be incredibly rare at the ~16 Myr median age
of Sco-Cen and they join only one other confirmed M-type and three higher-mass
accretors outside of Upper Scorpius. The serendipitous discovery of two
accreting stars hosting large quantities of circumstellar material may be
indicative of a sizeable age spread in Sco-Cen, or further evidence that disc
dispersal and planet formation time-scales are longer around lower-mass stars.
To aid future studies of Sco-Cen we also provide a newly-compiled catalogue of
305 early-type Hipparcos members with spectroscopic radial velocities sourced
from the literature.Comment: 12 pages. Accepted for publication in MNRA
FM stars II: a Fourier view of pulsating binary stars - determining binary orbital parameters photometrically for highly eccentric cases
Continuous and precise space-based photometry has made it possible to measure the orbital frequency modulation of pulsating stars in binary systems with extremely high precision over long time spans. Frequency modulation caused by binary orbital motion manifests itself as a multiplet with equal spacing of the orbital frequency in the Fourier transform. The amplitudes and phases of the peaks in these multiplets reflect the orbital properties, hence the orbital parameters can be extracted by analysing such precise photometric data alone. We derive analytically the theoretical relations between the multiplet properties and the orbital parameters, and present a method for determining these parameters, including the eccentricity and the argument of periapsis, from a quintuplet or a higher order multiplet. This is achievable with the photometry alone, without spectroscopic radial velocity measurements. We apply this method to Kepler mission data of KIC 8264492, KIC 9651065, and KIC 10990452, each of which is shown to have an eccentricity exceeding 0.5. Radial velocity curves are also derived from the Kepler photometric data. We demonstrate that the results are in good agreement with those obtained by another technique based on the analysis of the pulsation phases
Acoustic modeling using the digital waveguide mesh
The digital waveguide mesh has been an active area of music acoustics research for over ten years. Although founded in 1-D digital waveguide modeling, the principles on which it is based are not new to researchers grounded in numerical simulation, FDTD methods, electromagnetic simulation, etc. This article has attempted to provide a considerable review of how the DWM has been applied to acoustic modeling and sound synthesis problems, including new 2-D object synthesis and an overview of recent research activities in articulatory vocal tract modeling, RIR synthesis, and reverberation simulation. The extensive, although not by any means exhaustive, list of references indicates that though the DWM may have parallels in other disciplines, it still offers something new in the field of acoustic simulation and sound synth
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