Context. Planetary formation models are necessary to understand the characteristics of the planets that are the most likely to survive. Their dynamics, their composition and even the probability of ...their survival depend on the environment in which they form. We therefore investigate the most favorable locations for planetary embryos to accumulate in the protoplanetary disk: the planet traps. Aims. We study the formation of the protoplanetary disk by the collapse of a primordial molecular cloud, and how its evolution leads to the selection of specific types of planets. Methods. We use a hydrodynamical code that accounts for the dynamics, thermodynamics, geometry and composition of the disk to numerically model its evolution as it is fed by the infalling cloud material. As the mass accretion rate of the disk onto the star determines its growth, we can calculate the stellar characteristics by interpolating its radius, luminosity and temperature over the stellar mass from pre-calculated stellar evolution models. The density and midplane temperature of the disk then allow us to model the interactions between the disk and potential planets and determine their migration. Results. At the end of the collapse phase, when the disk reaches its maximum mass, it pursues its viscous spreading, similarly to the evolution from a minimum mass solar nebula (MMSN). In addition, we establish a timeline equivalence between the MMSN and a “collapse-formed disk” that would be older by about 2 Myr. Conclusions. We can save various types of planets from a fatal type-I inward migration: in particular, planetary embryos can avoid falling on the star by becoming trapped at the heat transition barriers and at most sublimation lines (except the silicates one). One of the novelties concerns the possible trapping of putative giant planets around a few astronomical units from the star around the end of the infall. Moreover, trapped planets may still follow the traps outward during the collapse phase and inward after it. Finally, this protoplanetary disk formation model shows the early possibilities of trapping planetary embryos at disk stages that are anterior by a few million years to the initial state of the MMSN approximation.
Asteroseismology with the Kepler space telescope is providing not only an improved characterization of exoplanets and their host stars, but also a new window on stellar structure and evolution for ...the large sample of solar-type stars in the field. We perform a uniform analysis of 22 of the brightest asteroseismic targets with the highest signal-to-noise ratio observed for 1 month each during the first year of the mission, and we quantify the precision and relative accuracy of asteroseismic determinations of the stellar radius, mass, and age that are possible using various methods. We present the properties of each star in the sample derived from an automated analysis of the individual oscillation frequencies and other observational constraints using the Asteroseismic Modeling Portal (AMP), and we compare them to the results of model-grid-based methods that fit the global oscillation properties. We find that fitting the individual frequencies typically yields asteroseismic radii and masses to ~1% precision, and ages to ~2.5% precision (respectively, 2, 5, and 8 times better than fitting the global oscillation properties). The absolute level of agreement between the results from different approaches is also encouraging, with model-grid-based methods yielding slightly smaller estimates of the radius and mass and slightly older values for the stellar age relative to AMP, which computes a large number of dedicated models for each star. The sample of targets for which this type of analysis is possible will grow as longer data sets are obtained during the remainder of the mission.
Context. The CoRoT mission is in its third year of observation and the data from the second long run in the galactic centre direction are being analysed. The solar-like oscillating stars that have ...been observed up to now have given some interesting results, specially concerning the amplitudes that are lower than predicted. We present here the results from the analysis of the star HD 170987. Aims. The goal of this research work is to characterise the global parameters of HD 170987. We look for global seismic parameters such as the mean large separation, maximum amplitude of the modes, and surface rotation because the signal-to-noise ratio in the observations does not allow us to measure individual modes. We also aim to retrieve the parameters of the star and its chemical composition. Methods. We studied the chemical composition of the star through ground-based observations performed with the NARVAL spectrograph. We used several methods to calculate the global parameters from the acoustic oscillations based on CoRoT data. The light curve of the star has been interpolated with inpainting algorithms to reduce the effect of data gaps. Results. We found the power excess related to p modes in the range 400–1200 μHz with a mean large separation of 55.2 ± 0.8 μHz with a probability above 95 % that increases to 55.9 ± 0.2 μHz in a higher frequency range 500–1250 μHz and a rejection level of 1%. A hint of the variation of this quantity with frequency was also found. The rotation period of the star is estimated to be around 4.3 days with an inclination axis of i = 50° $^{+20}_{-13}$. We measured a bolometric amplitude per radial mode in a range 2.4–2.9 ppm around 1000 μHz. Finally we estimate the stellar mass with a grid of models, M = 1.43 ± 0.05 $M_\odot$, the radius, R = 1.96 ± 0.046 $R_\odot$, and the age ~2.4 Gyr.
We address the evolution of lithium in Population II dwarf stars under the joint effects of microscopic diffusion and tachocline mixing. This process relies on analytical developments and is also ...constrained by helioseismology observations. It was successfully applied to solar analogs but never investigated in halo stars. It is induced in the upper radiation zone by rotation and a slight differential rotation in latitude. Consequently we modeled different possible rotation histories of halo stars, showing that the initial rotation rate had no impact on lithium in the framework of tachocline mixing. We find a negligible impact of pre-main-sequence evolution on super( 7)Li independent of metallicity provided that image. On the contrary, microscopic diffusion and tachocline turbulence act on the long term of main sequence and shape the current image pattern from the turnoff down to 5000 K. The tachocline mixing models fit the super(7)Li-image relation better than the pure microscopic diffusion models. We address the issue of warm super(7)Li-poor stars and conclude that a moderate mass transfer from a companion could explain their composition. Finally, we discuss the lithium lighter isotope. The pre-main-sequence and main-sequence super( 6)Li depletion we compute seems difficult to reconcile with the current observations.
Phenomenological models of convection adopt characteristic length scales that are chosen to fit solar or stellar observations. We investigate whether changes in these length scales are required ...between the Sun and low mass stars on the red giant branch (RGB). The question is addressed jointly in the frameworks of the mixing length theory and the full spectrum of turbulence model. For both models, the convective length scale is assumed to be a fixed fraction of the local pressure scale height. We use constraints provided by the observed effective temperatures and linear radii independently. We consider a sample of 38 nearby giants and subgiants for which surface temperatures and luminosities are known accurately and the radii are determined by interferometry to better than 10%. We computed dedicated models for the few cases where the stellar masses were determined by asteroseismological measurements. First we calibrated the solar models. With the same physics, we then computed RGB models for masses between 0.9 M⊙ and 2.5 M⊙ and metallicities ranging from Fe/H = −0.34 to solar. The evolution is followed up to 103 L⊙. Special attention is given to the opacities and the non-grey atmosphere models used as boundary conditions for which the model of convection is the same as in the interior. For both the mixing length theory and the full spectrum of turbulence models, the characteristic solar length scale for convection has to be slightly reduced to fit the lower edge of the observed RGB. The corresponding models also agree more closely with the expected mass distribution on the RGB and the seismic constraints. These results are robust regardless of effective temperatures determined spectroscopically or radii determined interferometrically are used.
We study the effects of different descriptions of the solar surface convection on the eigenfrequencies of p modes. 1D evolution calculations of the whole Sun and 3D hydrodynamic and ...magnetohydrodynamic simulations of the current surface are performed. These calculations rely on realistic physics. Averaged stratifications of the 3D simulations are introduced in the 1D solar evolution or in the structure models. The eigenfrequencies obtained are compared to those of 1D models relying on the usual phenomenologies of convection and to observations of the Michelson Doppler Imager instrument aboard the Solar Heliospheric Observatory (SoHO). We also investigate how the magnetic activity could change the eigenfrequencies and the solar radius, assuming that, 3 Mm below the surface, the upgoing plasma advects a 1.2 kG horizontal field. All models and observed eigenfrequencies are fairly close below 3 mHz. Above 3 mHz the eigenfrequencies of the phenomenological convection models are above the observed eigenfrequencies. The frequencies of the models based on the 3D simulations are slightly below the observed frequencies. Their maximum deviation is 3 μHz at 3 mHz but drops below 1 μHz at 4 mHz. Replacing the hydrodynamic by the magnetohydrodynamic simulation increases the eigenfrequencies. The shift is negligible below 2.2 mHz and then increases linearly with frequency to reach 1.7 μHz at 4 mHz. The impact of the simulated activity is a 14 mas shrinking of the solar layers near the optical depth unity.
Since the pioneering observations of Spite & Spite in 1982, the constant lithium abundance of metal-poor (Fe/H < -1.3) halo stars near the turnoff has been attributed to a cosmological origin. Closer ...analysis, however, revealed that the observed abundance lies at super(7)Li 6 0.4 dex below the predictions of big bang nucleosynthesis (BBN). The measurements of deuterium abundances along the lines of sight toward quasars, and the recent data from the Wilkinson Microwave Anisotropy Probe (WMAP), have independently confirmed this gap. We suggest here that part of the discrepancy (from 0.2 to 0.3 dex) is explained by a first generation of stars that efficiently depleted lithium. Assuming that the models for lithium evolution in halo turnoff stars, as well as the super(7)Li, estimates are correct, we infer that between one-third and one-half of the baryonic matter of the early halo (i.e., 610 super(9) M sub( )) was processed through Population III stars. This new paradigm proposes a very economical solution to the lingering difficulty of understanding the properties of the Spite plateau and its lack of star-to-star scatter down to Fe/H = -2.5. It is moreover in agreement both with the absence of lithium in the most iron-poor turnoff star currently known (HE 1327-2326) and also with new trends of the plateau suggesting its low-metallicity edge may be reached around Fe/H = -2.5. We discuss the role of turbulent mixing associated with enhanced supernovae explosions in the early interstellar medium in this picture. We suggest how it may explain the small scatter and also other recent observational features of the lithium plateau. Finally, we show that other chemical properties of the extremely metal-poor stars (such as carbon enrichment) are also in agreement with significant Population III processing in the halo, provided these models include mass loss and rotationally induced mixing.
The lifetime of solar-like stars, the envelope structure of more massive stars, and stellar acoustic frequencies largely depend on the radiative properties of the stellar plasma. Up to now, these ...complex quantities have been estimated only theoretically. The development of the powerful tools of helio- and astero- seismology has made it possible to gain insights on the interiors of stars. Consequently, increased emphasis is now placed on knowledge of the monochromatic opacity coefficients. Here we review how these radiative properties play a role, and where they are most important. We then concentrate specifically on the envelopes of
β
Cephei variable stars. We discuss the dispersion of eight different theoretical estimates of the monochromatic opacity spectrum and the challenges we need to face to check these calculations experimentally.
Seismic constraints on open clusters Piau, L.; Ballot, J.; Turck-Chièze, S.
Astronomy & astrophysics,
02/2005, Letnik:
430, Številka:
2
Journal Article
Recenzirano
Odprti dostop
The aim of this theoretical and modelling paper is to derive knowledge on the global and structural parameters of low-mass stars using asteroseismology and taking advantage of the stellar collective ...behavior within open clusters. We build stellar models and compute the seismic signal expected from main sequence objects in the $0.8{-}1.6\, M_{\odot}$ range. We first evaluate apparent magnitudes and oscillations-induced luminosity fluctuations expected in the Hyades, the Pleiades and the α Persei clusters. The closest cluster presents a feasible challenge to observational asteroseismology in the present and near future. The remainder of the work therefore focuses on the Hyades. We combine seismological and classical computations to address three questions: what can be inferred about 1) mass; 2) composition; and 3) extension of outer convection zones of solar analogs in the Hyades. The first issue relies on the strong sensitivity of the large separation to mass. We show that seismic constraints provide masses to a precision level ($0.05 \,M_{\odot}$) that is competitive with the actual mass estimations from binary systems. Then large separations ($\Delta \nu$) and second differences ($\delta_2 \nu$) are used to respectively constrain metal and helium fractions in the Hyades. When plotted for several masses, the relation of effective temperature ($T_\mathrm{eff}$) vs. large separation ($\Delta \nu$) is found to be strongly dependent on the metal content. Besides this the second difference main modulation is related to the second ionization of helium. An accuracy in the helium mass fraction of 0.02 to 0.01 can be achieved provided mass and age are accurately known, which is the case for a few Hyades binary systems. The second difference modulations are also partly due to the discontinuity in stellar stratification at the convective envelope/radiative core transition. They permit direct insight in the stellar structure. We compute acoustic radii of the convective bases for different values of the mixing length theory parameter $\alpha_\mathrm{MLT}$ in convection modelling, i.e. different convective efficiency in the superadiabatic layers. For a given effective temperature we show that the acoustic radius changes with convection efficiency. This suggests that seismology can provide constraints on the extension of outer convection and also more generally on the direct approaches of convection and dynamical phenomena being currently developed.
PDF
