We present the results of a comprehensive assessment of companions to solar-type stars. A sample of 454 stars, including the Sun, was selected from the Hipparcos catalog with {pi}>40 mas, {sigma}{sub {pi}/{pi}} < 0.05, 0.5 {<=} B - V {<=} 1.0 ({approx}F6-K3), and constrained by absolute magnitude and color to exclude evolved stars. These criteria are equivalent to selecting all dwarf and subdwarf stars within 25 pc with V-band flux between 0.1 and 10 times that of the Sun, giving us a physical basis for the term 'solar-type'. New observational aspects of this work include surveys for (1) very close companions with long-baseline interferometry at the Center for High Angular Resolution Astronomy Array, (2) close companions with speckle interferometry, and (3) wide proper-motion companions identified by blinking multi-epoch archival images. In addition, we include the results from extensive radial-velocity monitoring programs and evaluate companion information from various catalogs covering many different techniques. The results presented here include four new common proper-motion companions discovered by blinking archival images. Additionally, the spectroscopic data searched reveal five new stellar companions. Our synthesis of results from many methods and sources results in a thorough evaluation of stellar and brown dwarf companions to nearby Sun-likemore » stars. The overall observed fractions of single, double, triple, and higher-order systems are 56% {+-} 2%, 33% {+-} 2%, 8% {+-} 1%, and 3% {+-} 1%, respectively, counting all confirmed stellar and brown dwarf companions. If all candidate, i.e., unconfirmed, companions identified are found to be real, the percentages would change to 54% {+-} 2%, 34% {+-} 2%, 9% {+-} 2%, and 3% {+-} 1%, respectively. Our completeness analysis indicates that only a few undiscovered companions remain in this well-studied sample, implying that the majority (54% {+-} 2%) of solar-type stars are single, in contrast to the results of prior multiplicity studies. Our sample is large enough to enable a check of the multiplicity dependence on various physical parameters by analyzing appropriate subsamples. Bluer, more massive stars are seen as more likely to have companions than redder, less massive ones, consistent with the trend seen over the entire spectral range. Systems with larger interaction cross sections, i.e., those with more than two components or long orbital periods, are preferentially younger, suggesting that companions may be stripped over time by dynamical interactions. We confirm the planet-metallicity correlation (i.e., higher metallicity stars are more likely to host planets), but are unable to check it for brown dwarfs due to the paucity of such companions, implying that the brown dwarf desert extends over all separation regimes. We find no correlation between stellar companions and metallicity for B - V < 0.625, but among the redder subset, metal-poor stars ([Fe/H] <-0.3) are more likely to have companions with a 2.4{sigma} significance. The orbital-period distribution of companions is unimodal and roughly log normal with a peak and median of about 300 years. The period-eccentricity relation shows the expected circularization for periods below 12 days, caused by tidal forces over the age of the Galaxy, followed by a roughly flat distribution. The mass-ratio distribution shows a preference for like-mass pairs, which occur more frequently in relatively close pairs. The fraction of planet hosts among single, binary, and multiple systems are statistically indistinguishable, suggesting that planets are as likely to form around single stars as they are around components of binary or multiple systems with sufficiently wide separations. This, along with the preference of long orbital periods among stellar systems, increases the space around stars conducive for planet formation, and perhaps life.« less

/pdf/a-survey-of-stellar-families-multiplicity-of-solar-type-4mf1i754rb.pdf

A survey of stellar families: multiplicity of solar-type stars

We re-examine the stellar kinematics of the solar neighbourhood in terms of the velocity υ� of the Sun with respect to the local standard of rest. We show that the classical determination of its component Vin the direction of Galactic rotation via Str¨ omberg's relation is undermined by the metallicity gradient in the disc, which introduces a correlation between the colour of a group of stars and the radial gradients of its properties. Comparing the local stellar kinematics to a chemodynamical model which accounts for these effects, we obtain (U, V, W)� = (11.1 +0.69 −0.75 , 12.24 +0.47 −0.47 ,7 .25 +0.37 −0.36 )k m s −1 , with additional systematic uncertainties ∼(1, 2, 0.5) km s −1 . In particular, Vis 7 km s −1 larger than previously estimated. The new values of (U, V, W)� are extremely insensitive to the metallicity gradient within the disc.

Local kinematics and the local standard of rest

We present and discuss new determinations of metallicity, rotation, age, kinematics, and Galactic orbits for a com- plete, magnitude-limited, and kinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our ∼63 000 new, accurate radial-velocity observations for nearly 13 500 stars allow identification of most of the binary stars in the sample and, together with published uvbyβ photometry, Hipparcos parallaxes, Tycho-2 proper motions, and a few earlier radial velocities, complete the kinematic information for 14 139 stars. These high-quality velocity data are supplemented by effective temperatures and metallicities newly derived from recent and/or revised calibrations. The remaining stars either lack Hipparcos data or have fast rotation. Am ajor effort has been devoted to the determination of new isochrone ages for all stars for which this is possible. Particular attention has been given to a realistic treatment of statistical biases and error estimates, as standard techniques tend to under- estimate these effects and introduce spurious features in the age distributions. Our ages agree well with those by Edvardsson et al. (1993), despite several astrophysical and computational improvements since then. We demonstrate, however, how strong observational and theoretical biases cause the distribution of the observed ages to be very different from that of the true age distribution of the sample. Among the many basic relations of the Galactic disk that can be reinvestigated from the data presented here, we revisit the metallicity distribution of the G dwarfs and the age-metallicity, age-velocity, and metallicity-velocity relations of the Solar neighbourhood. Our first results confirm the lack of metal-poor G dwarfs relative to closed-box model predictions (the "G dwarf problem"), the existence of radial metallicity gradients in the disk, the small change in mean metallicity of the thin disk since its formation and the substantial scatter in metallicity at all ages, and the continuing kinematic heating of the thin disk with an efficiency consistent with that expected for a combination of spiral arms and giant molecular clouds. Distinct features in the distribution of the V component of the space motion are extended in age and metallicity, corresponding to the effects of stochas- tic spiral waves rather than classical moving groups, and may complicate the identification of thick-disk stars from kinematic criteria. More advanced analyses of this rich material will require careful simulations of the selection criteria for the sample and the distribution of observational errors.

/pdf/the-geneva-copenhagen-survey-of-the-solar-neighbourhood-ages-xdt2chbt8c.pdf

The Geneva-Copenhagen survey of the Solar neighbourhood - Ages, metallicities, and kinematic properties of ~14 000 F and G dwarfs

This article presents and discusses a critical compilation of accurate, fundamental determinations of stellar masses and radii. We have identified 95 detached binary systems containing 190 stars (94 eclipsing systems, and α Centauri) that satisfy our criterion that the mass and radius of both stars be known within errors of ±3% accuracy or better. All of them are non-interacting systems, and so the stars should have evolved as if they were single. This sample more than doubles that of the earlier similar review by Andersen (Astron Astrophys Rev 3:91–126, 1991), extends the mass range at both ends and, for the first time, includes an extragalactic binary. In every case, we have examined the original data and recomputed the stellar parameters with a consistent set of assumptions and physical constants. To these we add interstellar reddening, effective temperature, metal abundance, rotational velocity and apsidal motion determinations when available, and we compute a number of other physical parameters, notably luminosity and distance. These accurate physical parameters reveal the effects of stellar evolution with unprecedented clarity, and we discuss the use of the data in observational tests of stellar evolution models in some detail. Earlier findings of significant structural differences between moderately fast-rotating, mildly active stars and single stars, ascribed to the presence of strong magnetic and spot activity, are confirmed beyond doubt. We also show how the best data can be used to test prescriptions for the subtle interplay between convection, diffusion, and other non-classical effects in stellar models. The amount and quality of the data also allow us to analyse the tidal evolution of the systems in considerable depth, testing prescriptions of rotational synchronisation and orbital circularisation in greater detail than possible before. We show that the formulae for pseudo-synchronisation of stars in eccentric orbits predict the observed rotations quite well, except for very young and/or widely separated stars. Deviations do occur, however, especially for stars with convective envelopes. The superior data set finally demonstrates that apsidal motion rates as predicted from General Relativity plus tidal theory are in good agreement with the best observational data. No reliable binary data exist, which challenge General Relativity to any significant extent. The new data also enable us to derive empirical calibrations of M and R for single (post-) main-sequence stars above $${0.6\,M_{\odot}}$$
 . Simple, polynomial functions of T
 eff, log g and [Fe/H] yield M and R within errors of 6 and 3%, respectively. Excellent agreement is found with independent determinations for host stars of transiting extrasolar planets, and good agreement with determinations of M and R from stellar models as constrained by trigonometric parallaxes and spectroscopic values of T
 eff and [Fe/H]. Finally, we list a set of 23 interferometric binaries with masses known to be better than 3%, but without fundamental radius determinations (except α Aur). We discuss the prospects for improving these and other stellar parameters in the near future.

/pdf/accurate-masses-and-radii-of-normal-stars-modern-results-and-30f6pk9dvf.pdf

Accurate masses and radii of normal stars: Modern results and applications

While the strong anticorrelation between chromospheric activity and age has led to the common use of the Ca II H
and K emission index (R'_(HK) = L_(HK)/L_(bol)) as an empirical age estimator for solar-type dwarfs, existing activity-age relations
produce implausible ages at both high and low activity levels.We have compiled R'_(HK)
HK data from the literature
for young stellar clusters, richly populating for the first time the young end of the activity-age relation. Combining the
cluster activity data with modern cluster age estimates and analyzing the color dependence of the chromospheric activity
age index,we derive an improved activity-age calibration for F7-K2 dwarfs (0:5 mag < B - V < 0.9 mag). We
also present a more fundamentally motivated activity-age calibration that relies on conversion of R'_(HK) values through
the Rossby number to rotation periods and then makes use of improved gyrochronology relations. We demonstrate that
our new activity-age calibration has typical age precision of ~0.2 dex for normal solar-type dwarfs aged between the
Hyades and the Sun (~0.6-4.5 Gyr). Inferring ages through activity-rotation-age relations accounts for some color-dependent
effects and systematically improves the age estimates (albeit only slightly). We demonstrate that coronal
activity as measured through the fractional X-ray luminosity (R_X = L_X/L_(bol)) has nearly the same age- and rotation inferring
capability as chromospheric activity measured through R'_(HK). As a first application of our calibrations, we
provide new activity-derived age estimates for a volume-limited sample of the 108 solar-type field dwarfs within
16 pc.

/pdf/improved-age-estimation-for-solar-type-dwarfs-using-activity-3a3h4zjus7.pdf

Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics

We present a new method, using Bayesian estimation, to determine stellar ages and their uncertainties from observational data and theoretical isochrones. The result for an individual star is obtained as the relative posterior probability density as function of the age ("G function"). From this can be derived the most probable age and confidence intervals. The convoluted morphology of isochrones and strong non-linearities make the age determination by any method difficult and susceptible to statistical biases, and as a result age uncertainties havee often been underestimated in the literature. From simulations we find that the G functions provide a general, robust and reliable way to quantify age information. Resulting age estimates are at least as accurate as those obtained with conventional isochrone fitting methods, and in some cases much better, especially when the observational uncertainties are large. We also find that undetected binaries, on the whole, have a surprisingly small effect on the age determinations. For a stellar sample, the individual G functions can be combined to derive the star formation history of the population; this will be developed in a forthcoming paper. For a coeval population the combination simplifies to computing the product of the individual G functions, and we apply that method to estimate the ages of the two open clusters IC 4651 and M67, using Padova isochrones and photometric data from the literature. For IC 4651 we find an estimated age of 1.56 +/- 0.03 Gyr, assuming a true distance modulus of 9.80. For M67 we find 4.05 +/- 0.05 Gyr for true distance modulus 9.48. The small formal errors of these age estimates do not include the ( much larger) uncertainties from calibration and model errors, but illustrate the statistical power of combining G functions. Our statistical approach to the age determination problem is well suited for the mass treatment of data resulting from large-scale surveys such as the Gaia mission. (Less)

/pdf/determination-of-stellar-ages-from-isochrones-bayesian-4xmpos2v2a.pdf

Determination of stellar ages from isochrones: Bayesian estimation versus isochrone fitting

We report on a new survey of metallicities, ages, and Galactic orbitsfor a complete, magnitude-limited, and kinematically unbiased all-skysample of 16682 nearby F- and G-dwarfs. Our~63000 new, accurate radialvelocities for nearly 13500 of the stars, combined with Hipparcosparallaxes and Tycho-2 proper motions, complete the kinematic data for14139 stars and allow us to identify most of the binary stars in thesample. Isochrone ages have been determined whenever reliable resultsare possible, with particular attention to realistic error estimates.Among the basic properties of the Galactic disk that can bereinvestigated from our data are the metallicity distribution ofG-dwarfs and the age-metallicity and age-velocity relations of the solarneighbourhood. We confirm the lack of metal-poor G-dwarfs relative toclassical model predictions (the `G-dwarf problem'), the near-constancyof the mean metallicity since the formation of the thin disk, and theappearance of the kinematic signature of the thick disk ~ 10Gyr ago. (Less)

/pdf/the-geneva-copenhagen-survey-of-the-solar-neighbourhood-g0gm6op299.pdf

The Geneva-Copenhagen Survey of the Solar Neighbourhood

We present and discuss new determinations of metallicity, rotation, age, kinematics, and Galactic orbits for a complete, magnitude-limited, and kinematically unbiased sample of 16,682 nearby F and G dwarf stars. Our 63,000 new, accurate radial-velocity observations for nearly 13,500 stars allow identification of most of the binary stars in the sample and, together with published data complete the kinematic information for 14,139 stars. A major effort has been devoted to the determination of new isochrone ages for all stars for which this is possible. Particular attention has been given to a realistic treatment of statistical biases and error estimates, as standard techniques tend to underestimate these effects and introduce spurious features in the age distributions. We demonstrate, however, how strong observational and theoretical biases cause the distribution of the observed ages to be very different from that of the true age distribution of the sample. Our first results confirm the lack of metal-poor G dwarfs relative to closed-box model predictions (the ``G dwarf problem''), the existence of radial metallicity gradients in the disk, the small change in mean metallicity of the thin disk since its formation and the substantial scatter in metallicity at all ages, and the continuing kinematic heating of the thin disk with an efficiency consistent with that expected for a combination of spiral arms and giant molecular clouds. Distinct features in the distribution of the V component of the space motion are extended in age and metallicity, corresponding to the effects of stochastic spiral waves rather than classical moving groups, and may complicate the identification of thick-disk stars from kinematic criteria. (abridged)

/pdf/the-geneva-copenhagen-survey-of-the-solar-neighbourhood-ages-165nm1b6m9.pdf

The Geneva-Copenhagen survey of the Solar neighbourhood: Ages, metallicities, and kinematic properties of 14,000 F and G dwarfs

The G dwarf metallicity distribution of a mass limited sample of stars within 40 pc and south of δ = −26◦ and for which Str̈omgrenuvby andHβ photometry is available is studied. Special care is taken to ensure that the sample consists of stars with a main sequence lifetime longer than the age of the Galactic disk. Furthermore, binary stars, subgiants, active stars, etc. are removed to ensure that the sample is as clean as possible. The sample was limited to the mass interval 0.7 − 1.0M using masses determined by interpolation between theoretical evolutionary tracks. A mass interval was used as opposed to a color interval, as it is more directly comparable with theoretical models. The final sample contains 253 stars. Even though the sample shows a remarkably small low metallicity tail a new scale height weighting method was developed, and used. The resulting G dwarf distribution cannot be reconciled with the simple model for chemical evolution, in fact this dataset shows that the G dwarf problem is even larger than earlier results indicate.

Bjarne Rosenkilde Jørgensen

Papers

The Geneva-Copenhagen survey of the Solar neighbourhood - Ages, metallicities, and kinematic properties of ~14 000 F and G dwarfs

Determination of stellar ages from isochrones: Bayesian estimation versus isochrone fitting

The Geneva-Copenhagen Survey of the Solar Neighbourhood

The Geneva-Copenhagen survey of the Solar neighbourhood: Ages, metallicities, and kinematic properties of 14,000 F and G dwarfs

The G dwarf problem. Analysis of a new data set