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21.
Sarbani Basu 《Astrophysics and Space Science》2003,284(1):153-164
Inversions of oscillation frequencies have proved to be extremelypowerful in studying the structure of the Sun. We examine theconditions under which we can invert oscillation frequenciesof other stars to study their structure. We show that despite thevery limited number of modes that are expected to be observed in otherstars, we can perform inversions that will give localized informationof the stellar core. These results can limit the space of admissiblemodels of a given star. 相似文献
22.
M. C. Rabello-Soares Sarbani Basu J. Christensen-Dalsgaard 《Monthly notices of the Royal Astronomical Society》1999,309(1):35-47
The observed solar p-mode frequencies provide a powerful diagnostic of the internal structure of the Sun and permit us to test in considerable detail the physics used in the theory of stellar structure. Among the most commonly used techniques for inverting such helioseismic data are two implementations of the optimally localized averages (OLA) method, namely the subtractive optimally localized averages (SOLA) and multiplicative optimally localized averages (MOLA). Both are controlled by a number of parameters, the proper choice of which is very important for a reliable inference of the solar internal structure. Here we make a detailed analysis of the influence of each parameter on the solution and indicate how to arrive at an optimal set of parameters for a given data set. 相似文献
23.
An attempt is made to infer the structure of the solar convection zone from observedp-mode frequencies of solar oscillations. The differential asymptotic inversion technique is used to find the sound speed in
the solar envelope. It is found that envelope models which use the Canuto-Mazzitelli (CM) formulation for calculating the
convective flux give significantly better agreement with observations than models constructed using the mixing length formalism.
This inference can be drawn from both the scaled frequency differences and the sound speed difference. The sound speed in
the CM envelope model is within 0.2% of that in the Sun except in the region withr > 0.99R
⊙. The envelope models are extended below the convection zone, to find some evidence for the gravitational settling of helium
beneath the base of the convection zone. It turns out that for models with a steep composition gradient below the convection
zone, the convection zone depth has to be increased by about 6 Mm in order to get agreement with helioseismic observations. 相似文献
24.
We show that the explicit assumption of a chemically inhomogeneous interstellar medium allows a better reproduction of the metallicity distribution of G-dwarfs in the solar neighbourhood. The inhomogeneity is considered by assuming that at any time stars are born with a spread in their metallicities, the spread being a Gaussian in the logarithm of the metallicity around the mean metallicity of that epoch. We show that for various simple models of chemical evolution, the fit to the G-dwarf metallicity curve improves considerably once the above assumption is applied. We show that the parameters obtained from the fitting also give acceptable predictions for the age-metallicity relation. We also find that if we use a G-dwarf metallicity function corrected for the scale height inflation of stars, the conventional models of chemical evolution cannot match the shape of the curve, at least under the instantaneous recycling approximation applied to a chemically homogeneous ISM. Under the inhomogeneous ISM approximation, the predicted shapes are found to be better, though not totally satisfactory. 相似文献
25.
A preliminary method is proposed to handle the intrinsic chemical inhomogeneity in models of chemical evolution of the solar neighbourhood. If we assume a Gaussian spread in metallicity (z[Fe/H]) about a mean metallicity in the birth function of stars, the metallicity distribution of stars as a function of time is investigated, assuming that the mean metallicityz evolves linearly with time. Metallicity ([Fe/H]) histograms are plotted for A-, F-, G-, and K-dwarfs of the local disk population based on our compilation of data from several sources, and compared with the existing ones. We have applied the above method on these distributions to check whether distribution of the local A-, F-, and K-dwarfs could be fitted reasonably well with a single set of parameters derived from a fit to the metallicity distribution of the G-dwarfs. 相似文献
26.
Sarbani Basu 《Monthly notices of the Royal Astronomical Society》1998,298(3):719-728
Frequencies of intermediate-degree f modes of the Sun seem to indicate that the solar radius is smaller than what is normally used in constructing solar models. We investigate the possible consequences of an error in radius on results for solar structure obtained using helioseismic inversions. It is shown that solar sound speed will be overestimated if oscillation frequencies are inverted using reference models with a larger radius. Using solar models with a radius of 695.78 Mm and new data sets, the base of the solar convection zone is estimated to be at a radial distance of 0.7135 ± 0.0005 of the solar radius. The helium abundance in the convection zone as determined using models with an OPAL equation of state is 0.248 ± 0.001, where the errors reflect the estimated systematic errors in the calculation, the statistical errors being much smaller. Assuming that the OPAL opacities used in the construction of the solar models are correct, the surface Z / X is estimated to be 0.0245 ± 0.0006. 相似文献
27.
Using data from the Global Oscillation Network Group (GONG) covering the period from 1995 to 1998, we study the change with solar activity in solar f-mode frequencies. The results are compared with similar changes detected from the Michelson Doppler Imager (MDI) data. We find variations in f-mode frequencies which are correlated with solar activity indices. If these changes are due to variation in solar radius then the implications are that the solar radius decreases by about 5 km from minimum to maximum activity. 相似文献
28.
The tachocline at the base of the convection zone is generally believed to be the seat of the solar dynamo. Here we investigate whether the tachocline shows any detectable change using several 72 day time-series of the Michelson Doppler Imager (MDI) Medium-l data. We do not find any clear evidence of change with time. 相似文献
29.
Rabello-Soares M.C. Basu Sarbani Christensen-Dalsgaard J. Di Mauro M.P. 《Solar physics》2000,193(1-2):345-356
It is likely that precise and reliable frequencies of high-degree modes will soon be available from the SOI/MDI experiment. Here we examine the ability of such modes (with l>300) to resolve the solar structure in the near-surface region. In particular, we investigate inversions to determine the adiabatic exponent 1 as a test of the solar equation of state, as well as the potential of such data to constrain the solar envelope helium abundance. 相似文献