160 research outputs found
Properties of quasi-periodic pulsations in solar flares from a single active region
We investigate the properties of a set of solar flares originating from a
single active region (AR) that exhibit QPPs, and look for signs of the QPP
periods relating to AR properties. The AR studied, best known as NOAA 12192,
was unusually long-lived and produced 181 flares. Data from the GOES, EVE,
Fermi, Vernov and NoRH observatories were used to determine if QPPs were
present in the flares. For the soft X-ray GOES and EVE data, the time
derivative of the signal was used. Power spectra of the time series data
(without any form of detrending) were inspected, and flares with a peak above
the 95% confidence level in the spectrum were labelled as having candidate
QPPs. The confidence levels were determined taking account of uncertainties and
the possible presence of red noise. AR properties were determined using HMI
line of sight magnetograms. A total of 37 flares (20% of the sample) show good
evidence of having QPPs, and some of the pulsations can be seen in data from
multiple instruments and in different wavebands. The QPP periods show a weak
correlation with the flare amplitude and duration, but this may be due to an
observational bias. A stronger correlation was found between the QPP period and
duration of the QPP signal, which can be partially but not entirely explained
by observational constraints. No correlations were found with the AR area,
bipole separation, or average magnetic field strength. The fact that a
substantial fraction of the flare sample showed evidence of QPPs using a strict
detection method with minimal processing of the data demonstrates that these
QPPs are a real phenomenon, which cannot be explained by the presence of red
noise or the superposition of multiple unrelated flares. The lack of
correlation between the QPP periods and AR properties implies that the
small-scale structure of the AR is important, and/or that different QPP
mechanisms act in different cases.Comment: 23 pages, 57 figures. Accepted for publication by Astronomy &
Astrophysic
Oscillations in stellar superflares
Two different mechanisms may act to induce quasi-periodic pulsations (QPP) in
whole-disk observations of stellar flares. One mechanism may be
magneto-hydromagnetic (MHD) forces and other processes acting on flare loops as
seen in the Sun. The other mechanism may be forced local acoustic oscillations
due to the high-energy particle impulse generated by the flare (known as
`sunquakes' in the Sun). We analyze short-cadence Kepler data of 257 flares in
75 stars to search for QPP in the flare decay branch or post-flare oscillations
which may be attributed to either of these two mechanisms. About 18 percent of
stellar flares show a distinct bump in the flare decay branch of unknown
origin. The bump does not seem to be a highly-damped global oscillation because
the periods of the bumps derived from wavelet analysis do not correlate with
any stellar parameter. We detected damped oscillations covering several cycles
(QPP), in seven flares on five stars. The periods of these oscillations also do
not correlate with any stellar parameter, suggesting that these may be a due to
flare loop oscillations. We searched for forced global oscillations which might
result after a strong flare. To this end, we investigated the behaviour of the
amplitudes of solar-like oscillations in eight stars before and after a flare.
However, no clear amplitude change could be detected. We also analyzed the
amplitudes of the self-excited pulsations in two delta Scuti stars and one
gamma Doradus star before and after a flare. Again, no clear amplitude changes
were found. Our conclusions are that a new process needs to be found to explain
the high incidence of bumps in stellar flare light curves, that flare loop
oscillations may have been detected in a few stars and that no conclusive
evidence exists as yet for flare induced global acoustic oscillations
(starquakes).Comment: 13 pages, 14 figures, 3 table
Planetary detection limits taking into account stellar noise. II. Effect of stellar spot groups on radial-velocities
The detection of small mass planets with the radial-velocity technique is now
confronted with the interference of stellar noise. HARPS can now reach a
precision below the meter-per-second, which corresponds to the amplitudes of
different stellar perturbations, such as oscillation, granulation, and
activity. Solar spot groups induced by activity produce a radial-velocity noise
of a few meter-per-second. The aim of this paper is to simulate this activity
and calculate detection limits according to different observational strategies.
Based on Sun observations, we reproduce the evolution of spot groups on the
surface of a rotating star. We then calculate the radial-velocity effect
induced by these spot groups as a function of time. Taking into account
oscillation, granulation, activity, and a HARPS instrumental error of 80 cm/s,
we simulate the effect of different observational strategies in order to
efficiently reduce all sources of noise. Applying three measurements per night
of 10 minutes every three days, 10 nights a month seems the best tested
strategy. Depending on the level of activity considered, from log(R'_HK)= -5 to
-4.75, this strategy would allow us to find planets of 2.5 to 3.5 Earth masses
in the habitable zone of a K1V dwarf. Using Bern's model of planetary
formation, we estimate that for the same range of activity level, 15 to 35 % of
the planets between 1 and 5 Earth masses and with a period between 100 and 200
days should be found with HARPS. A comparison between the performance of HARPS
and ESPRESSO is also emphasized by our simulations. Using the same optimized
strategy, ESPRESSO could find 1.3 Earth mass planets in the habitable zone of
early-K dwarfs. In addition, 80 % of planets with mass between 1 and 5 Earth
masses and with a period between 100 and 200 days could be detected.Comment: 11 pages, 11 figures, accepted for publication in A&
Planetary detection limits taking into account stellar noise. I. Observational strategies to reduce stellar oscillation and granulation effects
The radial velocity signature of stellar noise is small, around the
meter-per-second, but already too much for the detection of Earth mass planets
in habitable zones. In this paper, we address the important role played by
observational strategies in averaging out the radial velocity signature of
stellar noise. We also derive the planetary mass detection limits expected in
presence of stellar noise. We start with HARPS asteroseismology measurements
for 4 stars (beta Hyi, alpha Cen A, mu Ara and tau Ceti) available in the ESO
archive plus very precise measurements of alpha Cen B. This sample covers
different spectral types, from G2 to K1 and different evolutionary stage, from
subgiant to dwarf stars. Since the span of our data ranges between 5 to 8 days,
we will have access to oscillation modes and granulation phenomena, without
important contribution of activity noise which is present at larger time
scales. For those 5 stars, we generate synthetic radial velocity measurements
after fitting corresponding models of stellar noise in Fourier space. These
measurements allows us to study the radial velocity variation due to stellar
noise for different observational strategies as well as the corresponding
planetary mass detection limits. Applying 3 measurements per night of 10
minutes exposure each, 2 hours apart, seems to average out most efficiently the
stellar noise considered. For quiet K1V stars as alpha Cen B, such a strategy
allows us to detect planets of ~3 times the mass of Earth with an orbital
period of 200 days, corresponding to the habitable zone of the star. Since
activity is not yet included in our simulation, these detection limits
correspond to a case, which exist, where the host star has few magnetic
features. In this case stellar noise is dominated by oscillation modes and
granulation phenomena.Comment: 12 pages, 6 figures, Accepted for publication in A&
The radius and mass of the close solar twin 18 Sco derived from asteroseismology and interferometry
The growing interest in solar twins is motivated by the possibility of
comparing them directly to the Sun. To carry on this kind of analysis, we need
to know their physical characteristics with precision. Our first objective is
to use asteroseismology and interferometry on the brightest of them: 18 Sco. We
observed the star during 12 nights with HARPS for seismology and used the PAVO
beam-combiner at CHARA for interferometry. An average large frequency
separation Hz and angular and linear radiuses of mas and R were estimated. We used these
values to derive the mass of the star, M.Comment: 5 pages, 5 figure
Uncertainties in models of stellar structure and evolution
Numerous physical aspects of stellar physics have been presented in Ses- sion
2 and the underlying uncertainties have been tentatively assessed. We try here
to highlight some specific points raised after the talks and during the general
discus- sion at the end of the session and eventually at the end of the
workshop. A table of model uncertainties is then drawn with the help of the
participants in order to give the state of the art in stellar modeling
uncertainties as of July 2013.Comment: Proc. of the workshop "Asteroseismology of stellar populations in the
Milky Way" (Sesto, 22-26 July 2013), Astrophysics and Space Science
Proceedings, (eds. A. Miglio, L. Girardi, P. Eggenberger, J. Montalban
About the p-mode frequency shifts in HD 49933
We study the frequency dependence of the frequency shifts of the low-degree p
modes measured in the F5V star HD 49933, by analyzing the second run of
observations collected by the CoRoT satellite. The 137-day light curve is
divided into two subseries corresponding to periods of low and high stellar
activity. The activity-frequency relationship is obtained independently from
the analysis of the mode frequencies extracted by both a local and a global
peak-fitting analyses, and from a cross-correlation technique in the frequency
range between 1450 microHz and 2500 microHz. The three methods return
consistent results. We show that the frequency shifts measured in HD 49933
present a frequency dependence with a clear increase with frequency, reaching a
maximal shift of about 2 microHz around 2100 microHz. Similar variations are
obtained between the l=0 and l=1 modes. At higher frequencies, the frequency
shifts show indications of a downturn followed by an upturn, consistent between
the l=0 and 1 modes. We show that the frequency variation of the p-mode
frequency shifts of the solar-like oscillating star HD 49933 has a comparable
shape to the one observed in the Sun, which is understood to arise from changes
in the outer layers due to its magnetic activity.Comment: 5 pages, 3 figures, 1 table, Accepted for publication in A\&
Cycle dependence of a quasi-biennial variability in the solar interior
We investigated the solar cycle dependence on the presence and periodicity of the Quasi-Biennial Oscillation (QBO). Using helioseismic techniques, we used solar oscillation frequencies from the Global Oscillations Network Group (GONG), Michelson Doppler Imager (MDI), and Helioseismic and Magnetic Imager (HMI) in the intermediate-degree range to investigate the frequency shifts over Cycles 23 and 24. We also examined two solar activity proxies, the F10.7 index and the Mg ii index, for the last four solar cycles to study the associated QBO. The analyses were performed using Empirical Mode Decomposition (EMD) and the Fast Fourier Transform (FFT). We found that the EMD analysis method is susceptible to detecting statistically significant Intrinsic Mode Functions (IMFs) with periodicities that are overtones of the length of the data set under examination. Statistically significant periodicities, which were not due to overtones, were detected in the QBO range. We see a reduced presence of the QBO in Cycle 24 compared to Cycle 23. The presence of the QBO was not sensitive to the depth to which the p-mode travelled, nor the average frequency of the p-mode. The analysis further suggested that the magnetic field responsible for producing the QBO in frequency shifts of p-modes is anchored above approximately 0.95 R⊙
Estimating the p-mode frequencies of the solar twin 18 Sco
Solar twins have been a focus of attention for more than a decade, because
their structure is extremely close to that of the Sun. Today, thanks to
high-precision spectrometers, it is possible to use asteroseismology to probe
their interiors. Our goal is to use time series obtained from the HARPS
spectrometer to extract the oscillation frequencies of 18 Sco, the brightest
solar twin. We used the tools of spectral analysis to estimate these
quantities. We estimate 52 frequencies using an MCMC algorithm. After
examination of their probability densities and comparison with results from
direct MAP optimization, we obtain a minimal set of 21 reliable modes. The
identification of each pulsation mode is straightforwardly accomplished by
comparing to the well-established solar pulsation modes. We also derived some
basic seismic indicators using these values. These results offer a good basis
to start a detailed seismic analysis of 18 Sco using stellar models.Comment: 12 pages, 6 figures, to be published in A&
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