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Register a new userFundamental Parameters of He-Weak and He-Strong Stars
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We carried out low resolution spectroscopic observations in the wavelength range 3400-4700 A of 20 He-weak and 8 He-strong stars to determine their fundamental parameters by means of the Divan-Chalonge-Barbier (BCD) spectrophotometric system. For a few He-weak stars we also estimate the effective temperatures and the angular diameters by integrating absolute fluxes observed over a wide spectral range. Non-LTE model calculations are carried out to study the influence of the He/H abundance ratio on the emergent radiation of He-strong stars and on their Teff determination. We find that the effective temperatures, surface gravities and bolometric absolute magnitudes of He-weak stars estimated with the BCD system and the integrated flux method are in good agreement between each other, and they also agree with previous determinations based on several different methods. The mean discrepancy between the visual absolute magnitudes derived using the Hipparcos parallaxes and the BCD values is on average 0.3 mag for He-weak stars, while it is 0.5 mag for He-strong stars. For He-strong stars, we note that the BCD calibration, based on stars in the solar environment, leads to overestimated values of Teff. By means of model atmosphere calculations with enhanced He/H abundance ratios we show that larger He/H ratios produce smaller BD which naturally explains the Teff overestimation. We take advantage of these calculations to introduce a method to estimate the He/H abundance ratio in He-strong stars. The BD of HD 37479 suggests that the Teff of this star remains fairly constant as the star spectrum undergoes changes in the intensity of H and He absorption lines. Data for the He-strong star HD 66765 are reported for the first time.
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To enlarge our database of Chemically Peculiar (CP) stars, we compiled published data concerning the He-weak and He-rich stars observed by high-resolution spectroscopy techniques during last decades. Twenty He-weak and 28 He-rich stars have been added to the database. We have also distinguished roAp stars from stars previously identified as Ap stars. To deepen our knowledge on statistical overview of the abundance anomalies versus the physical parameters of stars, we compared our data with previous compilations. We applied statistical tests on our data and found interesting correlations for effective temperature and surface gravity for all type of stars and a few correlations for projected rotation velocity only for He-rich stars. Because of the lack of the data we couldnt check whether being a member of binary system is affecting on chemical peculiarities of those stars.
Spectral analyses of H-deficient post-AGB stars have shown that a small group of four extremely hot objects exists which have almost pure He absorption-line spectra in the optical. These are classified as O(He) stars. For their evolution there are two scenarios: They could be the long-sought hot successors of RCrB stars, which have not been identified up to now. If this turns out to be true, then a third post-AGB evolutionary sequence is revealed, which is probably the result of a double-degenerate merging process. An alternative explanation might be that O(He) stars are post early-AGB stars. These depart from the AGB just before they experience their first thermal pulse (TP) which will then occur as a late thermal pulse (LTP). This would be a link to the low-mass He-enriched sdO stars and low-mass, particularly He-rich PG1159 stars.
To comprehend the recent Brookhaven National Laboratory experiment E788 on $^4_Lambda$He, we have outlined a simple theoretical framework, based on the independent-particle shell model, for the one-nucleon-induced nonmesonic weak decay spectra. Basically, the shapes of all the spectra are tailored by the kinematics of the corresponding phase space, depending very weakly on the dynamics, which is gauged here by the one-meson-exchange-potential. In spite of the straightforwardness of the approach a good agreement with data is acheived. This might be an indication that the final-state-interactions and the two-nucleon induced processes are not very important in the decay of this hypernucleus. We have also found that the $pi+K$ exchange potential with soft vertex-form-factor cutoffs $(Lambda_pi approx 0.7$ GeV, $Lambda_K approx 0.9$ GeV), is able to account simultaneously for the available experimental data related to $Gamma_p$ and $Gamma_n$ for $^4_Lambda$H, $^4_Lambda$He, and $^5_Lambda$He.
We use the next-to-leading-order (NLO) amplitude in an effective field theory (EFT) for ${}^3$He + ${}^4$He $rightarrow {}^7$Be + $gamma$ to perform the extrapolation of higher-energy data to solar energies. At this order the EFT describes the capture process using an s-wave scattering length and effective range, the asymptotic behavior of $^7$Be and its excited state, and short-distance contributions to the E1 capture amplitude. We use a Bayesian analysis to infer the multi-dimensional posterior of these parameters from capture data below 2 MeV. The total $S$-factor $S(0)= 0.578^{+0.015}_{-0.016}$ keV b at 68% degree of belief. We also find significant constraints on $^3$He-$^4$He scattering parameters.
We report on a combined theoretical and numerical study of counterflow turbulence in superfluid $^{4}$He in a wide range of parameters. The energy spectra of the velocity fluctuations of both the normal-fluid and superfluid components are strongly anisotropic. The angular dependence of the correlation between velocity fluctuations of the two components plays the key role. A selective energy dissipation intensifies as scales decrease, with the streamwise velocity fluctuations becoming dominant. Most of the flow energy is concentrated in a wavevector plane which is orthogonal to the direction of the counterflow. The phenomenon becomes more prominent at higher temperatures as the coupling between the components depends on the temperature and the direction with respect to the counterflow velocity.
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