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Register a new userThe Effect of Tides on the Population of PN from Interacting Binaries
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We have used the tidal equations of Zahn to determine the maximum orbital distance at which companions are brought into Roche lobe contact with their giant primary, when the primary expands during the giant phases. This is a key step when determining the rates of interaction between giants and their companions. Our stellar structure calculations are presented as maximum radii reached during the red and asymptotic giant branch (RGB and AGB, respectively) stages of evolution for masses between 0.8 and 4.0 Mo (Z=0.001 - 0.04) and compared with other models to gauge the uncertainty on radii deriving from details of these calculations. We find overall tidal capture distances that are typically 1-4 times the maximum radial extent of the giant star, where companions are in the mass range from 1 Jupiter mass to a mass slightly smaller than the mass of the primary. We find that only companions at initial orbital separations between ~320 and ~630 Ro will be typically captured into a Roche lobe-filling interaction or a common envelope on the AGB. Comparing these limits with the period distribution for binaries that will make PN, we deduce that in the standard scenario where all ~1-8 Mo stars make a PN, at most 2.5 per cent of all PN should have a post-common envelope central star binary, at odds with the observational lower limit of 15-20 per cent. The observed over-abundance of post-interaction central stars of PN cannot be easily explained considering the uncertainties. We examine a range of explanations for this discrepancy.
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Context. An important ingredient in binary evolution is the common-envelope (CE) phase. Although this phase is believed to be responsible for the formation of many close binaries, the process is not well understood. Aims. We investigate the characteristics of the population of post-common-envelope binaries (PCEB). As the evolution of these binaries and their stellar components are relatively simple, this population can be directly used to constraint CE evolution. Methods. We use the binary population synthesis code SeBa to simulate the current-day population of PCEBs in the Galaxy. We incorporate the selection effects in our model that are inherent to the general PCEB population and that are specific to the SDSS survey, which enables a direct comparison for the first time between the synthetic and observed population of visible PCEBs. Results. We find that selection effects do not play a significant role on the period distribution of visible PCEBs. To explain the observed dearth of long-period systems, the {alpha}-CE efficiency of the main evolutionary channel must be low. In the main channel, the CE is initiated by a red giant as it fills its Roche lobe in a dynamically unstable way. Other evolutionary paths cannot be constrained more. Additionally our model reproduces well the observed space density, the fraction of visible PCEBs amongst white dwarf (WD)- main sequence (MS) binaries, and the WD mass versus MS mass distribution, but overestimates the fraction of PCEBs with helium WD companions.
We systematically searched for gravity- and Rossby-mode period spacing patterns in Kepler eclipsing binaries with $gamma$ Doradus pulsators. These stars provide an excellent opportunity to test the theory of tidal synchronisation and angular momentum transport in F- and A-type stars. We discovered 35 systems that show clear patterns, including the spectroscopic binary KIC 10080943. Combined with 45 non-eclipsing binaries with $gamma$ Dor components that have been found using pulsation timing, we measured their near-core rotation rates and asymptotic period spacings. We find that many stars are tidally locked if the orbital periods are shorter than 10 days, in which the near-core rotation periods given by the traditional approximation of rotation (TAR) are consistent with the orbital period. Compared to the single stars, $gamma$ Dor stars in binaries tend to have slower near-core rotation rates, likely a consequence of tidal spin-down. We also find three stars that have extremely slow near-core rotation rates. To explain these, we hypothesise that unstable tidally excited oscillations can transfer angular momentum from the star to the orbit, and slow the star below synchronism, a process we refer to as `inverse tides.
Metallicity is known to significantly affect the radial expansion of a massive star: the lower the metallicity, the more compact the star, especially during its post-MS evolution. We study this effect in the context of binary evolution. Using the stellar-evolution code MESA, we computed evolutionary tracks of stars at different metallicities, exploring variations of factors known to affect the radial expansion (e.g. semiconvection, overshooting, rotation). We find observational support for an evolution in which already at metallicity $0.2Z_{odot}$ massive stars remain relatively compact during the Hertzprung-Gap (HG) phase and most of their expansion occurs during core-helium burning (CHeB). Consequently, we show that metallicity has a strong influence on the type of mass transfer evolution in binary systems. At solar metallicity, a case-B mass transfer is initiated shortly after the end of MS, and a giant donor is almost always a rapidly expanding HG star. At lower metallicity, the parameter space for mass transfer from a more evolved CHeB star increases dramatically. This means that envelope stripping and formation of helium stars in low-metallicity environments occurs later in the evolution of the donor, implying a much shorter duration of the Wolf-Rayet phase (even by an order of magnitude) and higher final core masses. This metallicity effect is independent of the impact of metallicity-dependent stellar winds. At very low metallicities, a significant fraction of massive stars in binaries engages in the first episode of mass transfer very late into their evolution, when they already have a well-developed CO core. The remaining lifetime ($< 10^4$ yr) is unlikely to be enough to strip the entire H-rich envelope. We also briefly discuss the extremely small parameter space for mass transfer from massive convective-envelope donors in the context of binary black hole merger formation.
Tidal interactions can play an important role as compact white dwarf (WD) binaries are driven together by gravitational waves (GWs). This will modify the strain evolution measured by future space-based GW detectors and impact the potential outcome of the mergers. Surveys now and in the near future will generate an unprecedented population of detached WD binaries to constrain tidal interactions. Motivated by this, I summarize the deviations between a binary evolving under the influence of only GW emission and a binary that is also experiencing some degree of tidal locking. I present analytic relations for the first and second derivative of the orbital period and braking index. Measurements of these quantities will allow the inference of tidal interactions, even when the masses of the component WDs are not well constrained. Finally, I discuss tidal heating and how it can provide complimentary information.
Binary stars are places of complex stellar interactions. While all binaries are in principle converging towards a state of circularization, many eccentric systems are found even in advanced stellar phases. In this work we discuss the sample of binaries with a red-giant component, discovered from observations of the NASA Kepler space mission. We first discuss which effects and features of tidal interactions are detectable in photometry, spectroscopy and the seismic analysis. In a second step, the sample of binary systems observed with Kepler, is compared to the well studied sample of Verbunt & Phinney (1995, hereafter VP95). We find that this study of circularization of systems hosting evolving red-giant stars with deep convective envelopes is also well applicable to the red-giant binaries in the sample of Kepler stars.
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