60 research outputs found
Mode visibilities in radial velocity and intensity Sun-as-a-star helioseismic measurements
We analyze more than 5000 days of Sun-as-a-star radial velocity GOLF and
intensity VIRGO observations to measure the visibilities of the l=0, 1, 2, and
3 modes and the m-amplitude ratios of the l=2 and 3 modes in the solar acoustic
spectrum. We provide observational values that we compare to theoretical
predictions.Comment: SOHO 24 / GONG 2010 conference, to be published in JPC
Misleading variations in estimated rotational frequency splittings of solar p modes: Consequences for helio- and asteroseismology
The aim of this paper is to investigate whether there are any 11-yr or
quasi-biennial solar cycle-related variations in solar rotational splitting
frequencies of low-degree solar p modes. Although no 11-yr signals were
observed, variations on a shorter timescale (~2yrs) were apparent. We show that
the variations arose from complications/artifacts associated with the
realization noise in the data and the process by which the data were analyzed.
More specifically, the realization noise was observed to have a larger effect
on the rotational splittings than accounted for by the formal uncertainties.
When used to infer the rotation profile of the Sun these variations are not
important. The outer regions of the solar interior can be constrained using
higher-degree modes. While the variations in the low-l splittings do make large
differences to the inferred rotation rate of the core, the core rotation rate
is so poorly constrained, even by low-l modes, that the different inferred
rotation profiles still agree within their respective 1sigma uncertainties. By
contrast, in asteroseismology, only low-l modes are visible and so higher-l
modes cannot be used to constrain the rotation profile of stars. Furthermore,
we usually only have one data set from which to measure the observed low-l
splitting. In such circumstances the inferred internal rotation rate of a main
sequence star could differ significantly from estimates of the surface rotation
rate, hence leading to spurious conclusions. Therefore, extreme care must be
taken when using only the splittings of low-l modes to draw conclusions about
the average internal rotation rate of a star.Comment: 10 pages, 7 figures, accepted for publication in MNRA
CoRoT reveals a magnetic activity cycle in a Sun-like star
The 11-year activity cycle of the Sun is a consequence of a dynamo process
occurring beneath its surface. We analyzed photometric data obtained by the
CoRoT space mission, showing solar-like oscillations in the star HD49933, for
signatures of stellar magnetic activity. Asteroseismic measurements of global
changes in the oscillation frequencies and mode amplitudes reveal a modulation
of at least 120 days, with the minimum frequency shift corresponding to maximum
amplitude as in the Sun. These observations are evidence of a stellar magnetic
activity cycle taking place beneath the surface of HD49933 and provide
constraints for stellar dynamo models under conditions different from those of
the Sun.Comment: Brevia text and supporting online material, published in Scienc
Probing the Deep End of the Milky Way with New Oscillating Kepler Giants
The Kepler mission has been a success in both exoplanet search and stellar
physics studies. Red giants have actually been quite a highlight in the Kepler
scene. The Kepler long and almost continuous four-year observations allowed us
to detect oscillations in more than 15,000 red giants targeted by the mission.
However by looking at the power spectra of 45,000 stars classified as dwarfs
according to the Q1-16 Kepler star properties catalog, we detected red-giant
like oscillations in 850 stars. Even though this is a small addition to the
known red-giant sample, these misclassified stars represent a goldmine for
galactic archeology studies. Indeed they happen to be fainter (down to Kp~16)
and more distant (d>10kpc) than the known red giants, opening the possibility
to probe unknown regions of our Galaxy. The faintness of these red giants with
detected oscillations is very promising for detecting acoustic modes in red
giants observed with K2 and TESS. In this talk, I will present this new sample
of red giants with their revised stellar parameters derived from
asteroseismology. Then I will discuss about the distribution of their masses,
distances, and evolutionary states compared to the previously known sample of
red giants.Comment: 5 pages, 4 figures, Proceedings of the KASC9-TASC2 meetin
Seismic inference of 57 stars using full-length Kepler data sets
We present stellar properties (mass, age, radius, distances) of 57 stars from
a seismic inference using full-length data sets from Kepler. These stars
comprise active stars, planet-hosts, solar-analogs, and binary systems. We
validate the distances derived from the astrometric Gaia-Tycho solution.
Ensemble analysis of the stellar properties reveals a trend of mixing-length
parameter with the surface gravity and effective temperature. We derive a
linear relationship with the seismic quantity to
estimate the stellar age. Finally, we define the stellar regimes where the
Kjeldsen et al (2008) empirical surface correction for 1D model frequencies is
valid.Comment: 4-page proceedings from Seismology of the Sun and the Distant Stars
2016, TASC/KASC, Azores, Portugal, corrected references in v
Are short-term variations in solar oscillation frequencies the signature of a second solar dynamo?
In addition to the well-known 11-year solar cycle, the Sun's magnetic
activity also shows significant variation on shorter time scales, e.g. between
one and two years. We observe a quasi-biennial (2-year) signal in the solar
p-mode oscillation frequencies, which are sensitive probes of the solar
interior. The signal is visible in Sun-as-a-star data observed by different
instruments and here we describe the results obtained using BiSON, GOLF, and
VIRGO data. Our results imply that the 2-year signal is susceptible to the
influence of the main 11-year solar cycle. However, the source of the signal
appears to be separate from that of the 11-year cycle. We speculate as to
whether it might be the signature of a second dynamo, located in the region of
near-surface rotational shear.Comment: 6 pages, 2 figures, proceedings for SOHO-24/GONG 2010 conference, to
be published in JPC
Seismic constraints on rotation of Sun-like star and mass of exoplanet
Rotation is thought to drive cyclic magnetic activity in the Sun and Sun-like
stars. Stellar dynamos, however, are poorly understood owing to the scarcity of
observations of rotation and magnetic fields in stars. Here, inferences are
drawn on the internal rotation of a distant Sun-like star by studying its
global modes of oscillation. We report asteroseismic constraints imposed on the
rotation rate and the inclination of the spin axis of the Sun-like star HD
52265, a principal target observed by the CoRoT satellite that is known to host
a planetary companion. These seismic inferences are remarkably consistent with
an independent spectroscopic observation (rotational line broadening) and with
the observed rotation period of star spots. Furthermore, asteroseismology
constrains the mass of exoplanet HD 52265b. Under the standard assumption that
the stellar spin axis and the axis of the planetary orbit coincide, the minimum
spectroscopic mass of the planet can be converted into a true mass of 1.85
(+0.52,-0.42) M_Jupiter, which implies that it is a planet, not a brown dwarf.Comment: Published in Proceedings of the National Academy of Sciences (5
pages, 5 figures, 3 tables). Available at
http://www.pnas.org/cgi/doi/10.1073/pnas.130329111
The influence of metallicity on stellar differential rotation and magnetic activity
Observations of Sun-like stars over the last half-century have improved our
understanding of how magnetic dynamos, like that responsible for the 11-year
solar cycle, change with rotation, mass and age. Here we show for the first
time how metallicity can affect a stellar dynamo. Using the most complete set
of observations of a stellar cycle ever obtained for a Sun-like star, we show
how the solar analog HD 173701 exhibits solar-like differential rotation and a
7.4-year activity cycle. While the duration of the cycle is comparable to that
generated by the solar dynamo, the amplitude of the brightness variability is
substantially stronger. The only significant difference between HD 173701 and
the Sun is its metallicity, which is twice the solar value. Therefore, this
provides a unique opportunity to study the effect of the higher metallicity on
the dynamo acting in this star and to obtain a comprehensive understanding of
the physical mechanisms responsible for the observed photometric variability.
The observations can be explained by the higher metallicity of the star, which
is predicted to foster a deeper outer convection zone and a higher facular
contrast, resulting in stronger variability.Comment: Submitted to Ap
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