Stringent constraints on single-star evolution theory from masses and radii of well-detached binaries in star clusters (original) (raw)
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Further critical tests of stellar evolution by means of double-lined eclipsing binaries
Monthly Notices of the Royal Astronomical Society, 1997
The most accurately measured stellar masses and radii come from detached, double-lined eclipsing binaries, as compiled by Andersen. We present a detailed quantitative comparison of these fundamental data with evolution models for single stars computed with our evolution code, both with and without the effects of enhanced mixing or overshooting beyond the convective cores. We use the same prescription for overshooting that Schroder, Pols & Eggleton found to reproduce the properties of r Aurigae binaries. For about 80 per cent of the 49 binary systems in the sample, both sets of models provide a good fit to both stars at a single age and metallicity within the observational uncertainties. We discuss possible causes for the discrepancies in the other systems. For only one system, AI Hya, do the enhanced-mixing models provide a significantly better fit to the data. For two others (WX Cep and TZ For) the fit to the enhanced-mixing models is also better. None of the other systems can individually distinguish between the models with and without enhanced mixing. However, the number of systems in a post-main-sequence phase is in much better agreement with the enhanced-mixing models. This test provides supportive evidence for extended mixing in main-sequence stars in the range 2-3M 0 .
Observational constraints from binary stars on stellar evolution models
EAS Publications Series, 2013
Accurate determinations of masses and radii in binary stars, along with estimates of the effective temperatures, metallicities, and other properties, have long been used to test models of stellar evolution. As might be expected, observational constraints are plentiful for mainsequence stars, although some problems with theory remain even in this regime. Models in other areas of the H-R diagram are considerably less well constrained, or not constrained at all. I summarize the status of the field, and provide examples of how accurate measurements can supply stringent tests of stellar theory, including aspects such as the treatment of convection. I call attention to the apparent failure of current models to match the properties of stars with masses of 1.1-1.7 M⊙ that are near the point of central hydrogen exhaustion, possibly connected with the simplified treatment of convective core overshooting.
Disentangling discrepancies between stellar evolution theory and sub-solar mass stars
Astronomy and Astrophysics, 2003
Serious discrepancies have recently been observed between predictions of stellar evolution models in the 0.7-1.1 M⊙ mass range and accurately measured properties of binary stars with components in this mass range. We study one of these objects, the eclipsing binary UV Piscium, which is particularly interesting because Popper (1997) derived age estimates for each component which differed by more than a factor of two. In an attempt to solve this significant discrepancy (a difference in age of 11 Gyr), we compute a large grid of stellar evolution models with the CESAM code for each component. By fixing the masses to their accurately determined values (relative error smaller than 1% for both stars), we consider a wide range of possible metallicities Z (0.01 to 0.05), and Helium content Y (0.25 to 0.34) uncorrelated to Z. In addition, the mixing length parameter αMLT is left as another free parameter. We obtain a best fit in the T eff -radius diagram for a common chemical composition (Z, Y )=(0.012, 0.31), but a different MLT parameter αMLT,A= 0.95±0.12(statistical)+0.30(systematic) and αMLT,B= 0.65±0.07(stat)+0.10(syst). The apparent age discrepancy found by Popper (1997) disappears with this solution, the components being coeval to within 1%. This suggests that fixing αMLT to its solar value (∼1.6), a common hypothesis assumed in most stellar evolutionary models, may not be correct. Secondly, since αMLT is smaller for the less massive component, this suggests that the αMLT parameter may decrease with stellar mass, showing yet another shortcoming of the mixing length theory to explain stellar convection. This trend needs further confirmation with other binary stars with accurate data.
Constraints on stellar evolution theory from precise eclipsing binary data
Symposium - International Astronomical Union, 1984
Previous attempts at a detailed confrontation of eclipsing binary data with theoretical models of main-sequence evolution were faced with the choice between data of inhomogeneous (mostly low) quality for many systems (Kriz, 1969; Lacy, 1979) or accurate values of mass, radius, and temperature (or luminosity) for very few systems only (Popper et al., 1970). In addition, more detailed and homogeneous stellar structure calculations for several compositions were needed. Since 1972, a coordinated photometric and spectroscopic programme at our institute contributes to building a sufficient observational basis for such a test. Among published standard models for the range 1–10 M⊙, Hejlesen's (1980) are the most extensive, agree well with other standard models, and are presented in a format suitable for comparison with binary data. Here we can only outline a few salient new results from this study.
Monthly Notices of the Royal Astronomical Society, 2011
A new method is presented to describe the evolution of the orbital-parameter distributions for an initially universal binary population in star clusters by means of the currently largest existing library of N -body models. It is demonstrated that a stellar-dynamical operator, Ω M ecl ,r h dyn (t), exists, which uniquely transforms an initial (t = 0) orbital parameter distribution function for binaries, D in , into a new distribution, D M ecl ,r h (t), depending on the initial cluster mass, M ecl , and half-mass radius, r h , after some time t of dynamical evolution. For D in the distribution functions derived by Kroupa (1995a,b) are used, which are consistent with constraints for pre-main sequence and Class I binary populations. Binaries with a lower energy and a higher reduced-mass are dissolved preferentially. The Ω-operator can be used to efficiently calculate and predict binary properties in clusters and whole galaxies without the need for further N -body computations. For the present set of N -body models it is found that the binary populations change their properties on a crossing time-scale such that Ω M ecl ,r h dyn (t) can be well parametrized as a function of the cluster density, ρ ecl . Furthermore it is shown that the binary-fraction in clusters with similar initial velocity dispersions follows the same evolutionary tracks as a function of the passed number of relaxation-times. Present-day observed binary populations in star clusters put constraints on their initial stellar densities, ρ ecl , which are found to be in the range 10 2 ρ ecl ( r h )/M ⊙ pc −3 2 × 10 5 for open clusters and a few×10 3 ρ ecl ( r h )/M ⊙ pc −3 10 8 for globular clusters, respectively.
Star cluster evolution, dynamical age estimation and the kinematical signature of star formation
Monthly Notices of the Royal Astronomical Society, 1995
We distribute 400 stars in N bin = 200 binary systems in clusters with initial half mass radii 0.077 ≤ R 0.5 ≤ 2.53 pc and follow the subsequent evolution of the stellar systems by direct N-body integration. The stellar masses are initially paired at random from the KTG(1.3) initial stellar mass function. The initial period distribution is flat ranging from 10 3 to 10 7.5 days, but we also perform simulations with a realistic distribution of periods which rises with increasing P > 3 days and which is consistent with pre-main sequence observational constraints. For comparison we simulate the evolution of single star clusters. After an initial relaxation phase, all clusters evolve according to the same n(t) ∝ exp(−t/τ e ) curve, where n(t) is the number density of stars in the central 2 pc sphere at time t and τ e ≈ 230 Myrs. All clusters have the same lifetime τ . n(t) and τ are thus independent of (i) the inital proportion of binaries and (ii) the initial R 0.5 . Mass segregation measures the dynamical age of the cluster: the mean stellar mass inside the central region increases approximately linearly with age. The proportion of binaries in the central cluster region is a sensitive indicator of the initial cluster concentration: it declines within approximately the first 10-20 initial relaxation times and rises only slowly with age, but for initial R 0.5 < 0.8 pc, it is always significantly larger than the binary proportion outside the central region. If most stars form in binaries in embedded clusters that are dynamically equivalent to a cluster specified initially by (N bin , R 0.5 ) = (200, 0.85 pc), which is located at the edge of a 1.5 × 10 5 M ⊙ molecular cloud with a diameter of 40 pc, then we estimate that at most about 10 per cent of all pre-main sequence stars achieve near escape velocities from the molecular cloud. The large ejection velocities resulting from close encounters between binary systems imply a distribution of young stars over large areas surrounding star forming sites. This 'halo' population of a molecular cloud complex is expected to have a significantly reduced binary proportion (about 15 per cent or less) and a significantly increased proportion of stars with depleted or completely removed circumstellar disks. In this scenario, the distributed population is expected to have a similar proportion of binaries as the Galactic field (about 50 per cent). If a distributed population shows orbital parameter distributions not affected by stimulated evolution (e.g. as in Taurus-Auriga) then it probably originated in a star-formation mode in which the binaries form in relative isolation rather than in embedded clusters. The Hyades Cluster luminosity function suggests an advanced dynamical age. The Pleiades luminosity function data suggest a distance modulus m − M = 6, rather than 5.5. The total proportion of binaries in the central region of the Hyades and Pleiades Clusters are probably 0.6-0.7. Any observational luminosity function of a Galactic cluster must be corrected for unresolved binaries when studying the stellar mass function. Applying our parametrisation for open cluster evolution we estimate the birth masses of both clusters. We find no evidence for different dynamical properties of stellar systems at birth in the Hyades, Pleiades and Galactic field stellar samples. Parametrising the depletion of low mass stars in the central cluster region by the ratio, ζ(t), of the stellar luminosity function at the 'H 2 -convection peak' (M V ≈ 12) and 'H − plateau' (M V ≈ 7), we find good agreement with the Pleiades and Hyades ζ(t) values. The observed proportion of binary stars in the very young Trapezium Cluster is consistent with the early dynamical evolution of a cluster with a very high initial stellar number density.
Empirical tests of pre-main-sequence stellar evolution models with eclipsing binaries
New Astronomy Reviews, 2014
We examine the performance of standard pre-main-sequence (PMS) stellar evolution models against the accurately measured properties of a benchmark sample of 26 PMS stars in 13 eclipsing binary (EB) systems having masses 0.04-4.0 M ⊙ and nominal ages ≈1-20 Myr. We provide a definitive compilation of all fundamental properties for the EBs, with a careful and consistent reassessment of observational uncertainties. We also provide a definitive compilation of the various PMS model sets, including physical ingredients and limits of applicability. No set of model isochrones is able to successfully reproduce all of the measured properties of all of the EBs. In the H-R diagram, the masses inferred for the individual stars by the models are accurate to better than 10% at 1 M ⊙ , but below 1 M ⊙ they are discrepant by 50-100%. Adjusting the observed radii and temperatures using empirical relations for the effects of magnetic activity helps to resolve the discrepancies in a few cases, but fails as a general solution. We find evidence that the failure of the models to match the data is linked to the triples in the EB sample; at least half of the EBs possess tertiary companions. Excluding the triples, the models reproduce the stellar masses to better than ∼10% in the H-R diagram, down to 0.5 M ⊙ , below which the current sample is fully contaminated by tertiaries. We consider several mechanisms by which a tertiary might cause changes in the EB properties and thus corrupt the agreement with stellar model predictions. We show that the energies of the tertiary orbits are comparable to that needed to potentially explain the scatter in the EB properties through injection of heat, perhaps involving tidal interaction. It seems from the evidence at hand that this mechanism, however it operates in detail, has more influence on the surface properties of the stars than on their internal structure, as the lithium abundances are broadly in good agreement with model predictions. The EBs that are members of young clusters appear individually coeval to within 20%, but collectively show an apparent age spread of ∼50%, suggesting true age spreads in young clusters. However, this apparent spread in the EB ages may also be the result of scatter in the EB properties induced by tertiaries.
The Evolution of Binary Populations in Young Star Clusters:From the ONC to OB associations
2012
Observations of the binary populations in young, sparse clusters have shown, that almost all stars are part of a binary system at the end of the star formation process. By contrast, the binary frequency of field stars only ∼ 50%. Most stars, and therefore most binaries, are formed in dense star clusters. This rises the question if the natal environments lead to the observed reduction of the binary frequency. In this thesis this question is addressed using numerical Nbody simulations of binary populations in different star cluster environments. First the evolution of binaries in ONC-like star clusters has been investigated. It was found that there the evolution of the normalised number of binaries is independent from the initial binary frequency. This allows to predict the evolution of binary populations in ONC-like clusters without the need of further numerical simulations. In addition it was found, that dynamical interactions preferentially destroy low-mass binaries resulting in a higher binary frequency for high-mass stars in the simulated clusters, in accordance with observation in the ONC. The combination of dynamical evolution with gas-induced orbital decay of embedded binaries is capable to reshape a log-uniform period distribution, as observed in young star clusters, to a log-normal period distribution as observed in the field today. The modelling has been generalised to clusters with arbitrary densities. Performing simulations for clusters with up to eight times higher densities than before with two different initial binary frequencies, it was shown that the evolution of the normalised number of binaries remains independent of the initial binary frequency. The higher the density in a cluster the more binaries are destroyed as to be expected. However, this effect levels out for clusters with central densities exceeding ≈ 3 × 10 4 pc −3. This means that there is a limit beyond which increasing the binary frequency in the clusters does not lead to significantly more binaries being destroyed. Finally it was investigated how the binary population evolves in star clusters that have undergone instantaneous gas expulsion with a resultant fast decrease in stellar density. It was found that the lower the star formation efficiency of a cluster and therefore the faster the decrease in stellar density, the less binaries are destroyed during the evolution while at the same time the more very wide binaries are formed. Comparison of the evolution of the simulated clusters and the observed leaky cluster sequence shows that clusters including a binary population and a ε = 0.3 match the observations of leaky-star clusters remarkably well.
A Test of Stellar Evolution Theory by Visual Binaries
Observational Tests of the Stellar Evolution Theory, 1984
When close visual binaries have good and homogeneous data, they can prove to be precision probes of stellar evolution theory. This has been demonstrated by using an MK spectral classification survey of 170 visual binaries, south of-25° declination and with separations mostly between one and five arcseconds. The binaries have area scanner photometry in UBV colours (Hurly and Warner 1983; Rakos et al. 1982), which have been tested for accuracy. The binaries with the most secure colour-magnitude data were used to test new Yale isochrones (Green et al. 1982), which include a mixing length parameter of 1.5 from the outset of the model calculations. An isochrone composition of Y=0.25, Z=0.04 proved best. Small decreases in the mixing length parameter were suggested for the coolest stars, but the good fit of the binaries to the isochrones (mean age difference between the components =0.0 ±0.1 Gyr) showed that stellar evolution had in general been well modelled by Green et al. A full account of this test has been submitted to The Astrophysical Journal.
The Pisa Stellar Evolution Data Base for low-mass stars
Astronomy & Astrophysics, 2012
Context. The last decade showed an impressive observational effort from the photometric and spectroscopic point of view for ancient stellar clusters in our Galaxy and beyond, leading to important and sometimes surprising results. Aims. The theoretical interpretation of these new observational results requires updated evolutionary models and isochrones spanning a wide range of chemical composition so that the possibility of multipopulations inside a stellar cluster is also taken also into account. Methods. With this aim we built the new "Pisa Stellar Evolution Database" of stellar models and isochrones by adopting a welltested evolutionary code (FRANEC) implemented with updated physical and chemical inputs. In particular, our code adopts realistic atmosphere models and an updated equation of state, nuclear reaction rates and opacities calculated with recent solar elements mixture. Results. A total of 32 646 models have been computed in the range of initial masses 0.30 ÷ 1.10 M for a grid of 216 chemical compositions with the fractional metal abundance in mass, Z, ranging from 0.0001 to 0.01, and the original helium content, Y, from 0.25 to 0.42. Models were computed for both solar-scaled and α-enhanced abundances with different external convection efficiencies. Correspondingly, 9720 isochrones were computed in the age range 8÷15 Gyr, in time steps of 0.5 Gyr. The whole database is available to the scientific community on the web. Models and isochrones were compared with recent calculations available in the literature and with the color-magnitude diagram of selected Galactic globular clusters. The dependence of relevant evolutionary quantities, namely turn-off and horizontal branch luminosities, on the chemical composition and convection efficiency were analyzed in a quantitative statistical way and analytical formulations were made available for reader's convenience. These relations can be useful in several fields of stellar evolution, e.g. evolutionary properties of binary systems, synthetic models for simple stellar populations and for star counts in galaxies, and chemical evolution models of galaxies.