1,380 research outputs found
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
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
Asteroseismic surface gravity for evolved stars
Context: Asteroseismic surface gravity values can be of importance in
determining spectroscopic stellar parameters. The independent log(g) value from
asteroseismology can be used as a fixed value in the spectroscopic analysis to
reduce uncertainties due to the fact that log(g) and effective temperature can
not be determined independently from spectra. Since 2012, a combined analysis
of seismically and spectroscopically derived stellar properties is ongoing for
a large survey with SDSS/APOGEE and Kepler. Therefore, knowledge of any
potential biases and uncertainties in asteroseismic log(g) values is now
becoming important. Aims: The seismic parameter needed to derive log(g) is the
frequency of maximum oscillation power (nu_max). Here, we investigate the
influence of nu_max derived with different methods on the derived log(g)
values. The large frequency separation between modes of the same degree and
consecutive radial orders (Dnu) is often used as an additional constraint for
the determination of log(g). Additionally, we checked the influence of small
corrections applied to Dnu on the derived values of log(g). Methods We use
methods extensively described in the literature to determine nu_max and Dnu
together with seismic scaling relations and grid-based modeling to derive
log(g). Results: We find that different approaches to derive oscillation
parameters give results for log(g) with small, but different, biases for
red-clump and red-giant-branch stars. These biases are well within the quoted
uncertainties of ~0.01 dex (cgs). Corrections suggested in the literature to
the Dnu scaling relation have no significant effect on log(g). However somewhat
unexpectedly, method specific solar reference values induce biases of the order
of the uncertainties, which is not the case when canonical solar reference
values are used.Comment: 8 pages, 5 figures, accepted for publication by A&
A thorough analysis of the short- and mid-term activity-related variations in the solar acoustic frequencies
The frequencies of the solar acoustic oscillations vary over the activity
cycle. The variations in other activity proxies are found to be well correlated
with the variations in the acoustic frequencies. However, each proxy has a
slightly different time behaviour. Our goal is to characterize the differences
between the time behaviour of the frequency shifts and of two other activity
proxies, namely, the area covered by sunspots and the 10.7cm flux. We define a
new observable that is particularly sensitive to the short-term frequency
variations. We then compare the observable when computed from model frequency
shifts and from observed frequency shifts obtained with the Global Oscillation
Network Group (GONG) for cycle 23. Our analysis shows that on the shortest
time-scales the variations in the frequency shifts seen in the GONG
observations are strongly correlated with the variations in the area covered by
sunspots. However, a significant loss of correlation is still found. We verify
that the times when the frequency shifts and the sunspot area do not vary in a
similar way tend to coincide with the times of the maxima of the quasi-biennial
variations seen in the solar seismic data. A similar analysis of the relation
between the 10.7cm flux and the frequency shifts reveals that the short-time
variations in the frequency shifts follow even more closely those of the 10.7cm
flux than those of the sunspot area. However, a loss of correlation between
frequency shifts and 10.7cm flux variations is still found around the same
times.Comment: 7 pages, 6 figures, accepted for publication in MNRA
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
On the relation between activity-related frequency shifts and the sunspot distribution over the solar cycle 23
The activity-related variations in the solar acoustic frequencies have been
known for 30 years. However, the importance of the different contributions is
still not well established. With this in mind, we developed an empirical model
to estimate the spot-induced frequency shifts, which takes into account the
sunspot properties, such as area and latitude. The comparison between the model
frequency shifts obtained from the daily sunspot records and those observed
suggests that the contribution from a stochastic component to the total
frequency shifts is about 30%. The remaining 70% is related to a global,
long-term variation. We also propose a new observable to investigate the short-
and mid-term variations of the frequency shifts, which is insensitive to the
long-term variations contained in the data. On the shortest time scales the
variations in the frequency shifts are strongly correlated with the variations
in the total area covered by sunspots. However, a significant loss of
correlation is still found, which cannot be fully explained by ignoring the
invisible side of the Sun when accounting for the total sunspot area. We also
verify that the times when the frequency shifts and the sunspot areas do not
vary in a similar way tend to coincide with the times of the maximum amplitude
of the quasi-biennial variations found in the seismic data.Comment: 4 pages, 2 figures, proceedings of the Joint TASC2 - KASC9 Workshop -
SPACEINN - HELAS8 Conference "Seismology of the Sun and the Distant Stars
2016: Using Today's Successes to Prepare the Future". To be published by the
EPJ Web of Conference
Super-Nyquist asteroseismology of solar-like oscillators with Kepler and K2 - expanding the asteroseismic cohort at the base of the red-giant branch
We consider the prospects for detecting solar-like oscillations in the
"super-Nyquist" regime of long-cadence (LC) Kepler photometry, i.e., above the
associated Nyquist frequency of approximately 283 {\mu}Hz. Targets of interest
are cool, evolved subgiants and stars lying at the base of the red-giant
branch. These stars would ordinarily be studied using the short-cadence (SC)
data, since the associated SC Nyquist frequency lies well above the frequencies
of the detectable oscillations. However, the number of available SC target
slots is quite limited. This imposes a severe restriction on the size of the
ensemble available for SC asteroseismic study.We find that archival Kepler LC
data from the nominal Mission may be utilized for asteroseismic studies of
targets whose dominant oscillation frequencies lie as high as approximately 500
{\mu}Hz, i.e., about 1.75- times the LC Nyquist frequency. The frequency
detection threshold for the shorter-duration science campaigns of the
re-purposed Kepler Mission, K2, is lower. The maximum threshold will probably
lie somewhere between approximately 400 and 450 {\mu}Hz. The potential to
exploit the archival Kepler and K2 LC data in this manner opens the door to
increasing significantly the number of subgiant and low-luminosity red-giant
targets amenable to asteroseismic analysis, overcoming target limitations
imposed by the small number of SC slots.We estimate that around 400 such
targets are now available for study in the Kepler LC archive. That number could
potentially be a lot higher for K2, since there will be a new target list for
each of its campaigns.Comment: Accepted for publication in MNRAS; 11 pages, 7 figures; reference
list update
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
- …