2,461 research outputs found
On deriving p-mode parameters for inclined solar-like stars
Thanks to their high quality, new and upcoming asteroseismic observations -
with CoRoT, Kepler, and from the ground... - can benefit from the experience
gained with helioseismology. We focus in this paper on solar-like oscillations,
for which the inclination of the rotation axis is unknown. We present a
theoretical study of the errors of p-mode parameters determined by means of a
maximum-likelihood estimator, and we also analyze correlations and biases. We
have used different, complementary approaches: we have performed either
semi-analytical computation of the Hessian matrix, fitting of single mean
profiles, or Monte Carlo simulations. We give first analytical approximations
for the errors of frequency, inclination and rotational splitting. The
determination of the inclination is very challenging for the common case of
slow rotators (like the Sun), making difficult the determination of a reliable
rotational splitting. Moreover, due to the numerous correlations, biases - more
or less significant - can appear in the determination of various parameters in
the case of bad inclination fittings, especially when a locking at 90 degrees
occurs. This issue concerning inclination locking is also discussed.
Nevertheless, the central frequency and some derived parameters such as the
total power of the mode are free of such biases.Comment: 9 pages, 6 figures, to appear in A&
Spectroscopic measurements of temperature and plasma impurity concentration during magnetic reconnection at the Swarthmore Spheromak Experiment
Electron temperature measurements during counterhelicity spheromak merging studies at the Swarthmore Spheromak Experiment (SSX) [M. R. Brown, Phys. Plasmas 6, 1717 (1999)] are presented. VUV monochromator measurements of impurity emission lines are compared with model spectra produced by the non-LTE excitation kinematics code PRISMSPECT [J. J. MacFarlane et al., in Proceedings of the Third Conference on Inertial Fusion Science and Applications (2004)] to yield the electron temperature in the plasma with 1 µs time resolution. Average T_e is seen to increase from 12 to 19 eV during spheromak merging. Average C III ion temperature, measured with a new ion Doppler spectrometer (IDS) [C. D. Cothran et al., Rev. Sci. Instrum. 77, 063504 (2006)], likewise rises during spheromak merging, peaking at ~22 eV, but a similar increase in T_i is seen during single spheromak discharges with no merging. The VUV emission line measurements are also used to constrain the concentrations of various impurities in the SSX plasma, which are dominated by carbon, but include some oxygen and nitrogen. A burst of soft x-ray emission is seen during reconnection with a new four-channel detector (SXR). There is evidence for spectral changes in the soft x-ray emission as reconnection progresses, although our single-temperature equilibrium spectral models are not able to provide adequate fits to all the SXR data
The Octave (Birmingham - Sheffield Hallam) automated pipeline for extracting oscillation parameters of solar-like main-sequence stars
The number of main-sequence stars for which we can observe solar-like
oscillations is expected to increase considerably with the short-cadence
high-precision photometric observations from the NASA Kepler satellite. Because
of this increase in number of stars, automated tools are needed to analyse
these data in a reasonable amount of time. In the framework of the asteroFLAG
consortium, we present an automated pipeline which extracts frequencies and
other parameters of solar-like oscillations in main-sequence and subgiant
stars. The pipeline uses only the timeseries data as input and does not require
any other input information. Tests on 353 artificial stars reveal that we can
obtain accurate frequencies and oscillation parameters for about three quarters
of the stars. We conclude that our methods are well suited for the analysis of
main-sequence stars, which show mainly p-mode oscillations.Comment: accepted by MNRA
The relation between and for solar-like oscillations
Establishing relations between global stellar parameters and asteroseismic
quantities can help improve our understanding of stellar astrophysics and
facilitate the interpretation of observations. We present an observed relation
between the large frequency separation, , and the frequency of
maximum power, . We find that is proportional to
, allowing prediction of to about 15 per cent
given . Our result is further supported by established scaling
relations for and and by extended stellar model
calculations, which confirm that can be estimated using this
relation for basically any star showing solar-like oscillations in the
investigated range (0.5<M/Msol<4.0).Comment: 5 pages, 8 figures, Letter accepted by MNRA
Sounding stellar cycles with Kepler - I. Strategy for selecting targets
The long-term monitoring and high photometric precision of the Kepler
satellite will provide a unique opportunity to sound the stellar cycles of many
solar-type stars using asteroseismology. This can be achieved by studying
periodic changes in the amplitudes and frequencies of the oscillation modes
observed in these stars. By comparing these measurements with conventional
ground-based chromospheric activity indices, we can improve our understanding
of the relationship between chromospheric changes and those taking place deep
in the interior throughout the stellar activity cycle. In addition,
asteroseismic measurements of the convection zone depth and differential
rotation may help us determine whether stellar cycles are driven at the top or
at the base of the convection zone. In this paper, we analyze the precision
that will be possible using Kepler to measure stellar cycles, convection zone
depths, and differential rotation. Based on this analysis, we describe a
strategy for selecting specific targets to be observed by the Kepler
Asteroseismic Investigation for the full length of the mission, to optimize
their suitability for probing stellar cycles in a wide variety of solar-type
stars.Comment: accepted for publication in MNRA
A new efficient method for determining weighted power spectra: detection of low-frequency solar p-modes by analysis of BiSON data
We present a new and highly efficient algorithm for computing a power
spectrum made from evenly spaced data which combines the noise-reducing
advantages of the weighted fit with the computational advantages of the Fast
Fourier Transform (FFT). We apply this method to a 10-year data set of the
solar p-mode oscillations obtained by the Birmingham Solar Oscillations Network
(BiSON) and thereby uncover three new low-frequency modes. These are the l=2,
n=5 and n=7 modes and the l=3, n=7 mode. In the case of the l=2, n=5 modes,
this is believed to be the first such identification of this mode in the
literature. The statistical weights needed for the method are derived from a
combination of the real data and a sophisticated simulation of the instrument
performance. Variations in the weights are due mainly to the differences in the
noise characteristics of the various BiSON instruments, the change in those
characteristics over time and the changing line-of-sight velocity between the
stations and the Sun. It should be noted that a weighted data set will have a
more time-dependent signal than an unweighted set and that, consequently, its
frequency spectrum will be more susceptible to aliasing.Comment: 11 pages, 7 Figures, accepted for publication in MNRAS, Figure 6 had
to be reduced in size to upload and so may be difficult to view on screen in
.ps versio
Why should we correct reported pulsation frequencies for stellar line-of-sight Doppler velocity shifts?
In the age of Kepler and Corot, extended observations have provided estimates
of stellar pulsation frequencies that have achieved new levels of precision,
regularly exceeding fractional levels of a few parts in . These high
levels of precision now in principle exceed the point where one can ignore the
Doppler shift of pulsation frequencies caused by the motion of a star relative
to the observer. We present a correction for these Doppler shifts and use
previously published pulsation frequencies to demonstrate the significance of
the effect. We suggest that reported pulsation frequencies should be routinely
corrected for stellar line-of-sight velocity Doppler shifts, or if a
line-of-sight velocity estimate is not available, the frame of reference in
which the frequencies are reported should be clearly stated.Comment: 5 pages, 1 figure, accepted for publication in MNRAS Letter
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