Outer Convective Layers Research Articles

Scaling Relations for Solar-Like Oscillations: A Review

The scaling relations for solar-like oscillations provide a translation of the features of the stochastic low-degree modes of oscillation in the Sun to predict the features of solar-like oscillations in other stars with convective outer layers. This prediction is based on their stellar mass, radius and effective temperature. Over time, the original scaling relations have been reversed in their use from predicting features of solar-like oscillations to deriving stellar parameters. Updates to the scaling relations as well as their reference values have been proposed to accommodate for the different requirements set by the change in their use. In this review the suggestions for improving the accuracy of the estimates of stellar parameters through the scaling relations for solar-like oscillations are presented together with a discussion of pros and cons of different approaches.

Coupling 1D stellar evolution with 3D-hydrodynamical simulations on-the-fly II: stellar evolution and asteroseismic applications

ABSTRACT Models of stellar structure and evolution are an indispensable tool in astrophysics, yet they are known to incorrectly reproduce the outer convective layers of stars. In the first paper of this series, we presented a novel procedure to include the mean structure of 3D hydrodynamical simulations on-the-fly in stellar models, and found it to significantly improve the outer stratification and oscillation frequencies of a standard solar model. In this work, we extend the analysis of the method; specifically how the transition point between envelope and interior affects the models. We confirm the versatility of our method by successfully repeating the entire procedure for a different grid of 3D hydrosimulations. Furthermore, the applicability of the procedure was investigated across the HR diagram and an accuracy comparable to the solar case was found. Moreover, we explored the implications on stellar evolution and find that the red-giant branch is shifted about $40\, \mathrm{K}$ to higher effective temperatures. Finally, we present for the first time an asteroseismic analysis based on stellar models fully utilizing the stratification of 3D simulations on-the-fly. These new models significantly reduce the asteroseismic surface term for the two selected stars in the Kepler field. We extend the analysis to red giants and characterize the shape of the surface effect in this regime. Lastly, we stress that the interpolation required by our method would benefit from new 3D simulations, resulting in a finer sampling of the grid.

Exterior Companions to Hot Jupiters Orbiting Cool Stars Are Coplanar

Abstract The existence of hot Jupiters has challenged theories of planetary formation since the first extrasolar planets were detected. Giant planets are generally believed to form far from their host stars, where volatile materials like water exist in their solid phase, making it easier for giant planet cores to accumulate. Several mechanisms have been proposed to explain how giant planets can migrate inward from their birth sites to short-period orbits. One such mechanism, called Kozai–Lidov migration, requires the presence of distant companions in orbits inclined by more than ∼40° with respect to the plane of the hot Jupiter’s orbit. The high occurrence rate of wide companions in hot-Jupiter systems lends support to this theory for migration. However, the exact orbital inclinations of these detected planetary and stellar companions is not known, so it is not clear whether the mutual inclination of these companions is large enough for the Kozai–Lidov process to operate. This paper shows that in systems orbiting cool stars with convective outer layers, the orbits of most wide planetary companions to hot Jupiters must be well aligned with the orbits of the hot Jupiters and the spins of the host stars. For a variety of possible distributions for the inclination of the companion, the width of the distribution must be less than ∼20° to recreate the observations with good fidelity. As a result, the companion orbits are likely well aligned with those of the hot Jupiters, and the Kozai–Lidov mechanism does not enforce migration in these systems.

ORBITAL CIRCULARIZATION OF HOT AND COOL KEPLER ECLIPSING BINARIES

ABSTRACT The rate of tidal circularization is predicted to be faster for relatively cool stars with convective outer layers, compared to hotter stars with radiative outer layers. Observing this effect is challenging because it requires large and well-characterized samples that include both hot and cool stars. Here we seek evidence of the predicted dependence of circularization upon stellar type, using a sample of 945 eclipsing binaries observed by Kepler. This sample complements earlier studies of this effect, which employed smaller samples of better-characterized stars. For each Kepler binary we measure e cos ω based on the relative timing of the primary and secondary eclipses. We examine the distribution of e cos ω as a function of period for binaries composed of hot stars, cool stars, and mixtures of the two types. At the shortest periods, hot–hot binaries are most likely to be eccentric; for periods shorter than four days, significant eccentricities occur frequently for hot–hot binaries, but not for hot–cool or cool–cool binaries. This is in qualitative agreement with theoretical expectations based on the slower dissipation rates of hot stars. However, the interpretation of our results is complicated by the largely unknown ages and evolutionary states of the stars in our sample.

A GRANULATION “FLICKER”-BASED MEASURE OF STELLAR SURFACE GRAVITY

ABSTRACT In our previous work we found that high-quality light curves, such as those obtained by Kepler, may be used to measure stellar surface gravity via granulation-driven light curve “flicker” (F 8). Here, we update and extend the relation originally presented by Bastien et al. in 2013 after calibrating F 8 against a more robust set of asteroseismically derived surface gravities. We describe in detail how we extract the F 8 signal from the light curves, including how we treat phenomena, such as exoplanet transits and shot noise, that adversely affect the measurement of F 8. We examine the limitations of the technique, and, as a result, we now provide an updated treatment of the F 8-based log g ?> error. We briefly highlight further applications of the technique, such as astrodensity profiling or its use in other types of stars with convective outer layers. We discuss potential uses in current and upcoming space-based photometric missions. Finally, we supply F 8-based log g ?> values, and their uncertainties, for 27,628 Kepler stars not identified as hosts of transiting planets, with 4500 K < T eff < 7150 K, 2.5 < log g ?> < 4.6, K p ≤ 13.5, and overall photometric amplitudes <10 parts per thousand.

Sun-like Stars: magnetic fields, cycles and exoplanets

AbstractIn Sun-like stars, magnetic fields are generated in the outer convective layers. They shape the stellar environment, from the photosphere to planetary orbits. Studying the large-scale magnetic field of those stars enlightens our understanding of the field properties and gives us observational constraints for field generation dynamo models. It also sheds light on how “normal” the Sun is among Sun-like stars. In this contribution, I will review the field properties of Sun-like stars, focusing on solar twins and planet hosting stars. I will discuss the observed large-scale magnetic cycles, compare them to stellar activity cycles, and link that to what we know about the Sun. I will also discuss the effect of large-scale stellar fields on exoplanets, exoplanetary emissions (e.g. radio), and habitability.

Indications for an influence of hot Jupiters on the rotation and activity of their host stars

The magnetic activity of planet-hosting stars is an important factor to estimate the atmospheric stability of close-in exoplanets and the age of their host stars. It has long been speculated that close-in exoplanets can influence the stellar activity level. However, testing for tidal or magnetic interaction effects in samples of planet-hosting stars is difficult because stellar activity hinders exoplanet detection, so that stellar samples with detected exoplanets show a bias towards low activity for small exoplanets. We aim to test if exoplanets in close orbits influence the stellar rotation and magnetic activity of their host stars, and have developed a novel approach to test for such systematic activity enhancements. We use wide (several 100 AU) binary systems in which one of the stellar components is known to have an exoplanet, while the second stellar component does not have a detected planet and therefore acts as a negative control. We use the stellar coronal X-ray emission as an observational proxy for magnetic activity, and analyze observations performed with Chandra and XMM-Newton. We find that in two systems for which strong tidal interaction can be expected the planet-hosting primary displays a much higher magnetic activity level than the planet-free secondary. In three systems for which weaker tidal interaction can be expected the activity levels of both stellar components are in agreement. Our observations indicate that the presence of Hot Jupiters may inhibit the spin-down of host stars with thick outer convective layers. Possible causes for such an effect include a transfer of angular momentum from the planetary orbit to the stellar rotation through tidal interaction, or differences during the early evolution of the system, where the host star may decouple from the protoplanetary disk early due to a gap opened by the forming Hot Jupiter.

Disparity between Hα and Hβ in SN 2008 in: Inhomogeneous external layers of type IIP supernovae?

We study disparity between H\alpha\ and H\beta\ in early spectra of the type IIP supernova SN 2008in. The point is that these lines cannot be described simultaneously in a spherically-symmetric model with the smooth density distribution. It is shown that an assumption of a clumpy structure of external layers of the envelope resolves the problem. We obtain estimates of the velocity at the inner border of the inhomogeneous zone (\approx6100 km s^{-1}), the filing factor of inhomogeneities (\leq0.5), and the mass of the inhomogeneous layers (~ 0.03 M_{\odot}). The amplitude of flux fluctuations in the early spectrum of H\alpha\ (\Delta F/F ~ 10^{-2}) imposes a constraint on the size of inhomogeneities (\leq 200 km s^{-1}). A detection of fluctuations in the early H\alpha\ of type IIP supernovae might become an observational test of the inhomogeneous structure of their envelopes. We propose also the indirect test of the clumpy structure of external layers: the study of properties of the initial radiation outburst due to the shock breakout. The inhomogeneous structure of external layers of type IIP supernovae could be an outcome of density perturbations and density inversion in outer convective layers of presupernova red supergiant.

The Venus–Earth–Jupiter spin–orbit coupling model

A Venus-Earth-Jupiter spin-orbit coupling model is constructed from a combination of the Venus- Earth-Jupiter tidal-torquing model and the gear e ect. The new model produces net tangential torques that act upon the outer convective layers of the Sun with periodicities that match many of the long-term cycles that are found in the 10 Be and 14 C proxy records of solar activity.

Magnetic fields of Sun-like stars

AbstractMagnetic fields play an important role at all stages of stellar evolution. In Sun-like stars, they are generated in the outer convective layers. Studying the large-scale magnetic fields of these stars enlightens our understanding of the field properties and gives us observational constraints for the field generation models. In this review, I summarise the current observational picture of the large-scale magnetic fields of Sun-like stars, in particular solar-twins and planet-host stars. I will discuss the observations of large-scale magnetic cycles, and compare these to the solar cycle.

Stochastically excited oscillations in the upper main sequence

AbstractConvective envelopes in stars on the main sequence are usually connected only with stars of spectral types F5 or later. However, observations as well as theory indicate that the convective outer layers in hotter stars, despite being shallow, are still effective and turbulent enough to stochastically excite oscillations. Because of the low amplitudes, exploring stochastically excited pulsations became possible only with space missions such as Kepler and CoRoT. Here I review the recent results and discuss among others, pulsators such as δ Scuti, γ Doradus, roAp, β Cephei, Slowly Pulsating B and Be stars, all in the context of solar-like oscillations.

Effect of convective outer layers modeling on non-adiabatic seismic observables of δ Scuti stars

Context. The identification of pulsation modes in δ Scuti stars is mandatory for constraining the theoretical stellar models. The nonadiabatic observables used in the photometric identification methods depend, however, on convection modeling in the external layers. Aims. We determine how the treatment of convection in the atmospheric and sub-atmospheric layers affects the mode identification and what information about the thermal structure of the external layers can be obtained from amplitude ratios and phase lags in Stromgren photometric bands. Methods. We derive non-adiabatic parameters for δ Scuti stars by using, for the first time, stellar models with the same treatment of convection in the interior and in the atmosphere. We compute classical non-gray mixing length models, and as well non-gray “full spectrum of turbulence” (FST) models. Furthermore, we compute the photometric amplitudes and phases of pulsation by using the colors and the limb-darkening coefficients as derived from the same atmosphere models as used in the stellar modeling. Results. We show that the non-adiabatic phase-lag is mainly sensitive to the thermal gradients in the external layers, hence to the treatment of convection, and that this sensitivity is also clearly reflected in the multi-color photometric phase differences.

A possible activity cycle in Proxima Centauri

Several late-type stars present activity cycles resembling the Solar one. This fact has been observed mostly in stars ranging from F to K, i.e., in stars with a radiative core and an outer convective layer. This work aims at studying whether an activity cycle can be detected in the dM5.5e star Proxima Centauri, which is supposed to be completely convective. We present periodical medium-resolution echelle observations covering the complete visual range, which were taken at the CASLEO Argentinean Observatory. These observations are distributed over 7 years. We discarded the spectra that present flare activity, and analyze the remaining activity levels using four different statistical techniques to look for a period of activity. We find strong evidence of a cyclic activity, with a period of around 442 days. We also estimate that the Ca II S index varies around 130% due to activity variations outside of flares.

An analysis of the effect of turbulent stress on the evolution of an 8 M⊙ start

On the basis of the mixing-length theory, macroscopic turbulent stress is incorporated into the hydrostatic equilibrium equation in our model of stellar structure and evolution. For an 8 M ⊙ star from the main sequence stage to the early AGB stage, the effect of turbulence was followed through calculation of the ratio of turbulence stress gradient to gravity. The results are: during the stages of nuclear burning the ratio is almost negligibly small; however, during the RGB and early AGB stages, there exists a thin layer in the outer convective layer where the ratio amounts to several units, while outside the thin layer the ratio may attain 65%. These facts have obvious effects on the central temperature of the AGB star and on the star's structure and evolutionary features at the onset of thermal pulsation.

Orbital period study of the Algol-type eclipsing binary system TW Draconis

The century-long times of light minimum of the Algol-type eclipsing binary star, TW Dra (BD +64°1077, Sp A5V+K2III), are investigated by considering a new pattern of period change. Two sudden period increases and two successive period decreases are discovered to superimpose on a rapid secular increase (d P/d t=+4.43×10 −6 days/year). The secular increase may be caused by a dynamical mass transfer from the secondary to the primary component (d m/d t=6.81×10 −7 M ⊙/year) that is in agreement with the semi-detached configuration of the system and with the existence of a hot spot and a gaseous stream in the binary system. The irregular period jumps superimposed on the secular increase can be explained by the structure variation of the K2-type giant via instabilities of the outer convective layer or via magnetic activity cycles.

The Spectrum of the Brown Dwarf Gliese 229B

We present a spectrum of the cool K) brown dwarf Gliese 229B. This spectrum, with a (T eff 900 relatively high signal-to-noise ratio per spectral resolution element spans the wavelength range (Z30), from 0.837 k mt o 5.0 km. We identify a total of four di†erent major methane absorption features, includ- ing the fundamental band at 3.3 km, at least four steam bands, and two neutral cesium features. We con-rm the recent detection of carbon monoxide (CO) in excess of what is predicted by thermochemical equilibrium calculations. Carbon is primarily involved in a chemical balance between methane and CO at the temperatures and pressures present in the outer parts of a brown dwarf. At lower temperatures, the balance favors methane, while in the deeper, hotter regions, the reaction reverses to convert methane into CO. The presence of CO in the observable part of the atmosphere is therefore a sensitive indicator of vertical Nows. The high signal-to-noise ratio in the 1 k mt o 2.5 km region permits us to place con- straints on the quantity of dust in the atmosphere of the brown dwarf. We are unable to reconcile the observed spectrum with synthetic spectra that include the presences of dust. The presence of CO but lack of dust may be a clue to the location of the boundaries of the outer convective region of the atmo- sphere: The lack of dust may mean that it is not being conveyed into the photosphere by convection, or that it exists in patchy clouds. If the dust is not in clouds, but rather sits below the outer convective region, we estimate that the boundary between outer convective and inner radiative layers is between 1250 K and 1600 K, in agreement with recent models. Subject headings: circumstellar matter E line: identi-cation E stars: individual (Gliese 229B) E stars: low-mass, brown dwarfs

Inevitability of a magnetic field in the Sun's radiative interior

The gas in the convective outer layers of the Sun rotates faster at the equator than in the polar regions, yet deeper inside (in the radiative zone) the gas rotates almost uniformly1,2,3. There is a thin transition layer between these zones, called the tachocline4. This structure has been measured seismologically1,2,3, but no purely fluid-dynamical mechanism can explain its existence. Here we argue that a self-consistent model requires a large-scale magnetic field in the Sun's interior, as well as consideration of the Coriolis effects in the convection zone and in the tachocline. Turbulent stresses in the convection zone induce (through Coriolis effects) a meridional circulation, causing the gas from the convection zone to burrow downwards, thereby generating the horizontal and vertical shear that characterizes the tachocline. The interior magnetic field stops the burrowing, and confines the shear, as demanded by the observed structure of the tachocline. We outline a dynamical theory of the flow, from which we estimate a field strength of about 10−4 tesla just beneath the tachocline. An important test of this picture, after numerical refinement, will be quantitative consistency between the predicted and observed interior angular velocities.

Synchronization Timescales for Three Solar-Type Stars That Have Jupiter-Mass Companions in Short-Period Orbits

We examine the physical properties of the three currently known star-plus-planet systems that have orbital periods of ~3–5 days (51 Peg, υ And, and τ Boo). The observed properties are, in general, more consistent with the Zahn viscous dissipation mechanism than with the Tassoul hydrodynamic spin-down mechanism (at least in the latter's present formulation). However, a comparison of the data for τ Boo with the Zahn theory suggests that the predicted synchronization timescale tZ might be regarded as an upper limit to the actual time needed to synchronize the outer convective layers, and that the latter may actually be shorter than tZ by a factor of ≥3.

Evidence of magnetic activity in short-period Algol binaries

view Abstract Citations (53) References (61) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Evidence of Magnetic Activity in Short-Period Algol Binaries Richards, Mercedes T. ; Albright, Geary E. Abstract The cool G- to K-type secondaries in short-period Algol-type binaries are expected to be magnetically active like the RS CVn binaries because of their rapid rotation and outer convective layers. There is now growing evidence of such activity, primarily from radio and X-ray observations, but there is still little or no information about most of the other indicators of magnetic activity. In this work, we have collected the evidence for magnetic activity in a selected group of 15 short-period Algols and have examined the possibility that part of the H-alpha emission detected in these systems may indeed arise from the chromosphere of the cool star and not exclusively from circumstellar material. Publication: The Astrophysical Journal Supplement Series Pub Date: September 1993 DOI: 10.1086/191821 Bibcode: 1993ApJS...88..199R Keywords: Cool Stars; Dwarf Stars; Eclipsing Binary Stars; H Alpha Line; Magnetoactivity; Stellar Magnetic Fields; Balmer Series; G Stars; K Stars; Starspots; Stellar Atmospheres; Stellar Flares; Astrophysics; STARS: BINARIES: ECLIPSING; STARS: CIRCUMSTELLAR MATTER; STARS: MAGNETIC FIELDS full text sources ADS | data products SIMBAD (18)

Strong spherical magnetogasdynamic shock waves in rotating stellar atmosphere

An analytic expression for the velocity of magnetogasdynamic shock wave, propagating in rotating inter stellar atmosphere has been obtained by using the method of characteristics and considering the effect of coriolis force. It has been shown that in the outer convective layer of the star Coriolis force and magnetic field both have significant effect on the shock velocity.