69 research outputs found
Sensitivity analysis of the solar rotation to helioseismic data from GONG, GOLF and MDI observations
Accurate determination of the rotation rate in the radiative zone of the sun
from helioseismic observations requires rotational frequency splittings of
exceptional quality as well as reliable inversion techniques. We present here
inferences based on mode parameters calculated from 2088-days long MDI, GONG
and GOLF time series that were fitted to estimate very low frequency rotational
splittings (nu < 1.7 mHz). These low frequency modes provide data of
exceptional quality, since the width of the mode peaks is much smaller than the
rotational splitting and hence it is much easier to separate the rotational
splittings from the effects caused by the finite lifetime and the stochastic
excitation of the modes. We also have implemented a new inversion methodology
that allows us to infer the rotation rate of the radiative interior from mode
sets that span l=1 to 25. Our results are compatible with the sun rotating like
a rigid solid in most of the radiative zone and slowing down in the core (R_sun
< 0.2). A resolution analysis of the inversion was carried out for the solar
rotation inverse problem. This analysis effectively establishes a direct
relationship between the mode set included in the inversion and the sensitivity
and information content of the resulting inferences. We show that such an
approach allows us to determine the effect of adding low frequency and low
degree p-modes, high frequency and low degree p-modes, as well as some g-modes
on the derived rotation rate in the solar radiative zone, and in particular the
solar core. We conclude that the level of uncertainties that is needed to infer
the dynamical conditions in the core when only p-modes are included is unlikely
to be reached in the near future, and hence sustained efforts are needed
towards the detection and characterization of g-modes.Comment: Accepted for publication in Astrophysical journal. 15 pages, 19
figure
On The Determination of MDI High-Degree Mode Frequencies
The characteristic of the solar acoustic spectrum is such that mode lifetimes
get shorter and spatial leaks get closer in frequency as the degree of a mode
increases for a given order. A direct consequence of this property is that
individual p-modes are only resolved at low and intermediate degrees, and that
at high degrees, individual modes blend into ridges. Once modes have blended
into ridges, the power distribution of the ridge defines the ridge central
frequency and it will mask the true underlying mode frequency. An accurate
model of the amplitude of the peaks that contribute to the ridge power
distribution is needed to recover the underlying mode frequency from fitting
the ridge.
We present the results of fitting high degree power ridges (up to l = 900)
computed from several two to three-month-long time-series of full-disk
observations taken with the Michelson Doppler Imager (MDI) on-board the Solar
and Heliospheric Observatory between 1996 and 1999.
We also present a detailed discussion of the modeling of the ridge power
distribution, and the contribution of the various observational and
instrumental effects on the spatial leakage, in the context of the MDI
instrument. We have constructed a physically motivated model (rather than some
ad hoc correction scheme) resulting in a methodology that can produce an
unbiased determination of high-degree modes, once the instrumental
characteristics are well understood.
Finally, we present changes in high degree mode parameters with epoch and
thus solar activity level and discuss their significance.Comment: 59 pages, 38 figures -- High-resolution version at
http://www-sgk.harvard.edu:1080/~sylvain/preprints/ -- Manuscript submitted
to Ap
An Upper Limit on the Temporal Variations of the Solar Interior Stratification
We have analyzed changes in the acoustic oscillation eigenfrequencies
measured over the past 7 years by the GONG, MDI and LOWL instruments. The
observations span the period from 1994 to 2001 that corresponds to half a solar
cycle, from minimum to maximum solar activity.
These data were inverted to look for a signature of the activity cycle on the
solar stratification. A one-dimensional structure inversion was carried out to
map the temporal variation of the radial distribution of the sound speed at the
boundary between the radiative and convective zones. Such variation could
indicate the presence of a toroidal magnetic field anchored in this region.
We found no systematic variation with time of the stratification at the base
of the convection zone. However we can set an upper limit to any fractional
change of the sound speed at the level of .Comment: 11 pages, 5 figures, to appear in Ap
An Upper Limit on the Reflected Light from the Planet Orbiting the Star tau Bootis
The planet orbiting tau Boo at a separation of 0.046 AU could produce a
reflected light flux as bright as 1e-4 relative to that of the star. A spectrum
of the system will contain a reflected light component which varies in
amplitude and Doppler-shift as the planet orbits the star. Assuming the
secondary spectrum is primarily the reflected stellar spectrum, we can limit
the relative reflected light flux to be less than 5e-5. This implies an upper
limit of 0.3 for the planetary geometric albedo near 480 nm, assuming a
planetary radius of 1.2 R_Jup. This albedo is significantly less than that of
any of the giant planets of the solar system, and is not consistent with
certain published theoretical predictions.Comment: 5 pages, 1 figure, accepted by ApJ Letter
A High-Eccentricity Low-Mass Companion to HD 89744
HD 89744 is an F7 V star with mass 1.4 M, effective temperature 6166 K, age
2.0 Gy and metallicity [Fe/H]= 0.18. The radial velocity of the star has been
monitored with the AFOE spectrograph at the Whipple Observatory since 1996, and
evidence has been found for a low mass companion. The data were complemented by
additional data from the Hamilton spectrograph at Lick Observatory during the
companion's periastron passage in fall 1999. As a result, we have determined
the star's orbital wobble to have period P = 256 d, orbital amplitude K = 257
m/s, and eccentricity e = 0.7. From the stellar mass we infer that the
companion has minimum mass m2 sin i = 7.2 MJup in an orbit with semi-major axis
a2 = 0.88 AU. The eccentricity of the orbit, among the highest known for
extra-solar planets, continues the trend that extra-solar planets with
semi-major axes greater than about 0.15 AU tend to have much higher
eccentricities than are found in our solar system. The high metallicity of the
parent star reinforces the trend that parent stars of extra-solar planets tend
to have high metallicityComment: AASTEX-LateX v5.0, 7 pages w/ 3 figures, to be published in ApJ
How much do helioseismological inferences depend upon the assumed reference model?
We investigate systematic uncertainties in determining the profiles of the
solar sound speed, density, and adiabatic index by helioseismological
techniques. We find that rms uncertainties-averaged over the sun of ~ 0.2%-0.4%
are contributed to the sound speed profile by each of three sources: 1)the
choice of assumed reference model, 2) the width of the inversion kernel, and 3)
the measurements errors. The density profile is about an order of magnitude
less well determined by the helioseismological measurements. The profile of the
adiabatic index is determined to an accuracy of about 0.2% . We find that even
relatively crude reference models yield reasonably accurate solar parameters.Comment: Accepted for publication in ApJ . Related material at
http://www.sns.ias.edu/~jn
A New Spectroscopic and Photometric Analysis of the Transiting Planet Systems TrES-3 and TrES-4
We report new spectroscopic and photometric observations of the parent stars
of the recently discovered transiting planets TrES-3 and TrES-4. A detailed
abundance analysis based on high-resolution spectra yields [Fe/H] , K, and for TrES-3,
and [Fe/H] , K, and for TrES-4. The accuracy of the effective temperatures is supported
by a number of independent consistency checks. The spectroscopic orbital
solution for TrES-3 is improved with our new radial-velocity measurements of
that system, as are the light-curve parameters for both systems based on newly
acquired photometry for TrES-3 and a reanalysis of existing photometry for
TrES-4. We have redetermined the stellar parameters taking advantage of the
strong constraint provided by the light curves in the form of the normalized
separation (related to the stellar density) in conjunction with our
new temperatures and metallicities. The masses and radii we derive are
M_\star=0.928_{-0.048}^{+0.028} M_{\sun},R_\star = 0.829_{-0.022}^{+0.015}
R_{\sun}, and M_\star = 1.404_{-0.134}^{+0.066} M_{\sun},
R_\star=1.846_{-0.087}^{+0.096} R_{\sun} for TrES-3 and TrES-4, respectively.
With these revised stellar parameters we obtain improved values for the
planetary masses and radii. We find , for TrES-3, and
, for TrES-4. We confirm TrES-4 as the planet with the largest
radius among the currently known transiting hot Jupiters.Comment: 42 pages, 10 tables, 8 figures. Accepted for publication in the
Astrophysical Journa
The SOI-MDI high-latitude jet: the evidence for and against
The apparent detection of a prograde jet at latitude 75° and at a radius of about 0.95R⊙ in some inversions of rotation data from SOI--MDI (Schou et al., 1998) has excited considerable interest, but whether the jet really exists in the solar interior is certainly not yet firmly established. The detection of the feature is sensitive both to the inversion techniques used and to the methods of mode parameter estimation used to generate the input data. In particular, the feature is much more apparent in Regularized Least-Squares inversions than in inversions using an Optimally Localized Average approach, and is not detected at all in the present GONG data when analysed with the GONG peakfinding algorithm, or indeed in SOI data when analysed with the GONG algorithm. Therefore in this poster we examine critically the current evidence for the source and existence of this jet in the light of forward and inverse analyses
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