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Register a new userModelling the asymmetries of the Suns radial $p$-mode line profiles
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In this paper, we aim to develop a predictive model for solar radial $p$-mode line profiles in the velocity spectrum. Unlike the approach favoured by prior studies, this model is not described by free parameters and we do not use fitting procedures to match the observations. Instead, we use an analytical turbulence model coupled with constraints extracted from a 3D hydrodynamic simulation of the solar atmosphere. We then compare the resulting asymmetries with their observationally derived counterpart. We find that stochastic excitation localised beneath the mode upper turning point generates negative asymmetry for $ u < u_text{max}$ and positive asymmetry for $ u > u_text{max}$. On the other hand, stochastic excitation localised above this limit generates negative asymmetry throughout the $p$-mode spectrum. As a result of the spatial extent of the source of excitation, both cases play a role in the total observed asymmetries. By taking this spatial extent into account and using a realistic description of the spectrum of turbulent kinetic energy, both a qualitative and quantitative agreement can be found with solar observations perfoemed by the GONG network. We also find that the impact of the correlation between acoustic noise and oscillation is negligible for mode asymmetry in the velocity spectrum.
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We report results of a spectropolarimetric and photometric monitoring of the weak-line T Tauri star LkCa4 within the MaTYSSE programme, involving ESPaDOnS at the Canada-France-Hawaii Telescope. Despite an age of only 2Myr and a similarity with prototypical classical T Tauri stars, LkCa4 shows no evidence for accretion and probes an interesting transition stage for star and planet formation. Large profile distortions and Zeeman signatures are detected in the unpolarized and circularly-polarized lines of LkCa4 using Least-Squares Deconvolution (LSD), indicating the presence of brightness inhomogeneities and magnetic fields at the surface of LkCa4. Using tomographic imaging, we reconstruct brightness and magnetic maps of LkCa4 from sets of unpolarized and circularly-polarized LSD profiles. The large-scale field is strong and mainly axisymmetric, featuring a ~2kG poloidal component and a ~1kG toroidal component encircling the star at equatorial latitudes - the latter making LkCa4 markedly different from classical TTauri stars of similar mass and age. The brightness map includes a dark spot overlapping the magnetic pole and a bright region at mid latitudes - providing a good match to the contemporaneous photometry. We also find that differential rotation at the surface of LkCa4 is small, typically ~5.5x weaker than that of the Sun, and compatible with solid-body rotation. Using our tomographic modelling, we are able to filter out the activity jitter in the RV curve of LkCa4 (of full amplitude 4.3km/s) down to a rms precision of 0.055km/s. Looking for hot Jupiters around young Sun-like stars thus appears feasible, even though we find no evidence for such planets around LkCa4.
Solar activity in all its varied manifestations is driven by the magnetic field. Particularly important for many purposes are two global quantities, the Suns total and open magnetic flux, which can be computed from sunspot number records using models. Such sunspot-driven models, however, do not take into account the presence of magnetic flux during grand minima, such as the Maunder minimum. Here we present a major update of a widely used simple model, which now takes into account the observation that the distribution of all magnetic features on the Sun follows a single power law. The exponent of the power law changes over the solar cycle. This allows for the emergence of small-scale magnetic flux even when no sunspots are present for multiple decades and leads to non-zero total and open magnetic flux also in the deepest grand minima, such as the Maunder minimum, thus overcoming a major shortcoming of the earlier models. The results of the updated model compare well with the available observations and reconstructions of the solar total and open magnetic flux. This opens up the possibility of improved reconstructions of sunspot number from time series of cosmogenic isotope production rate.
We report the results of our spectropolarimetric monitoring of the weak-line T-Tauri stars (wTTSs) Par 1379 and Par 2244, within the MaTYSSE (Magnetic Topologies of Young Stars and the Survival of close-in giant Exoplanets) programme. Both stars are of a similar mass (1.6 and 1.8 M$_{odot}$) and age (1.8 and 1.1 Myr), with Par 1379 hosting an evolved low-mass dusty circumstellar disc, and with Par 2244 showing evidence of a young debris disc. We detect profile distortions and Zeeman signatures in the unpolarized and circularly polarized lines for each star, and have modelled their rotational modulation using tomographic imaging, yielding brightness and magnetic maps. We find that Par 1379 harbours a weak (250 G), mostly poloidal field tilted $65^{circ}$ from the rotation axis. In contrast, Par 2244 hosts a stronger field (860 G) split 3:2 between poloidal and toroidal components, with most of the energy in higher order modes, and with the poloidal component tilted $45^{circ}$ from the rotation axis. Compared to the lower mass wTTSs, V819 Tau and V830 Tau, Par 2244 has a similar field strength, but is much more complex, whereas the much less complex field of Par 1379 is also much weaker than any other mapped wTTS. We find moderate surface differential rotation of $1.4times$ and $1.8times$ smaller than Solar, for Par 1379 and Par 2244, respectively. Using our tomographic maps to predict the activity related radial velocity (RV) jitter, and filter it from the RV curves, we find RV residuals with dispersions of 0.017 kms$^{-1}$ and 0.086 kms$^{-1}$ for Par 1379 and Par 2244, respectively. We find no evidence for close-in giant planets around either star, with $3sigma$ upper limits of 0.56 and 3.54 M$_{text{jup}}$ (at an orbital distance of 0.1 au).
Non-radial modes are excited in classical pulsators, both in Cepheids and in RR Lyrae stars. Firm evidence come from the first overtone pulsators, in which additional shorter period mode is detected with characteristic period ratio falling in between 0.60 and 0.65. In the case of first overtone Cepheids three separate sequences populated by nearly 200 stars are formed in the Petersen diagram, i.e. the diagram of period ratio versus longer period. In the case of first overtone RR Lyrae stars (RRc stars) situation is less clear. A dozen or so such stars are known which form a clump in the Petersen diagram without any obvious structure. Interestingly, all first overtone RR Lyrae stars for which precise space-borne photometry is available show the additional mode, which suggests that its excitation is common. Motivated by these results we searched for non-radial modes in the OGLE-III photometry of RRc stars from the Galactic bulge. We report the discovery of 147 stars, members of a new group of double-mode, radial-non-radial mode pulsators. They form a clear and tight sequence in the Petersen diagram, with period ratios clustering around 0.613 with a signature of possible second sequence with higher period ratio (0.631). The scatter in period ratios of the already known stars is explained as due to population effects. Judging from the results of space observations this still mysterious form of pulsation must be common among RRc stars and with our analysis of the OGLE data we just touch the tip of the iceberg.
We present the results of complex seismic analysis of the prototype star SX Phoenicis. This analysis consists of a simultaneous fitting of the two radial-mode frequencies, the corresponding values of the bolometric flux amplitude (the parameter $f$) and of the intrinsic mode amplitude $varepsilon$. The effects of various parameters as well as the opacity data are examined. With each opacity table it is possible to find seismic models that reproduce the two observed frequencies with masses allowed by evolutionary models appropriate for the observed values of the effective temperature and luminosity. All seismic models are in the post-main sequence phase. The OPAL and OP seismic models are in hydrogen shell-burning phase and the OPLIB seismic model has just finished an overall contraction and starts to burn hydrogen in a shell. The OP and OPLIB models are less likely due to the requirement of high initial hydrogen abundance ($X_0=0.75)$ and too high metallicity ($Zapprox 0.004$) as for a Population II star. The fitting of the parameter $f$, whose empirical values are derived from multi-colour photometric observations, provides constraints on the efficiency of convective transport in the outer layers of the star and on the microturbulent velocity in the atmosphere. Our complex seismic analysis with each opacity data indicates low to moderately efficient convection in the stars envelope, described by the mixing length parameter of $alpha_{rm MLT}in (0.0,~0.7)$, and the microturbulent velocity in the atmosphere of about $xi_{rm t}in(4,~8)~kms$.
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