Age-velocity relations with GALEX FUVFUVFUV-determined ages of Sun-like, solar neighborhood stars (original) (raw)
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Astronomy & Astrophysics, 2016
Context. The Mount Wilson Ca ii index log(R HK) is the accepted standard metric of calibration for the chromospheric activity versus age relation for FGK stars. Recent results claim its inability to discern activity levels, and thus ages, for stars older than ∼2 Gyr, which would severely hamper its application to date disk stars older than the Sun. Aims. We present a new activity-age calibration of the Mt. Wilson index that explicitly takes mass and [Fe/H] biases into account; these biases are implicit in samples of stars selected to have precise ages, which have so far not been appreciated. Methods. We show that these selection biases tend to blur the activity-age relation for large age ranges. We calibrate the Mt. Wilson index for a sample of field FGK stars with precise ages, covering a wide range of mass and [Fe/H], augmented with data from the Pleiades, Hyades, M 67 clusters, and the Ursa Major moving group. Results. We further test the calibration with extensive new Gemini/GMOS log(R HK) data of the old, solar [Fe/H] clusters, M 67 and NGC 188. The observed NGC 188 activity level is clearly lower than M 67. We correctly recover the isochronal age of both clusters and establish the viability of deriving usable chromospheric ages for solar-type stars up to at least ∼6 Gyr, where average errors are ∼0.14 dex provided that we explicitly account for the mass and [Fe/H] dimensions. We test our calibration against asteroseismological ages, finding excellent correlation (ρ = +0.89). We show that our calibration improves the chromospheric age determination for a wide range of ages, masses, and metallicities in comparison to previous age-activity relations.
Local Stellar Kinematics from RAVE Data: IV. Solar Neighbourhood Age–Metallicity Relation
Publications of the Astronomical Society of Australia, 2013
We investigated the age–metallicity relation using a sample of 5691 F- and G-type dwarfs from RAdial Velocity Experiment Data Release 3 (RAVE DR3) by applying several constraints. (i) We selected stars with surface gravities log g(cm s−2) ≥ 3.8 and effective temperatures in the 5310leTtextefftext(K)le73005310\le T_{\text{eff}}\text{(K)}\le 73005310leTtextefftext(K)le7300 range and obtained a dwarf sample. (ii) We plotted the dwarfs in metallicity sub-samples in the Ttextefftext−−(J−Ks)0T_{\text{eff}}\text{--}(J-K_s)_0Ttextefftext−−(J−K_s)_0 plane to compare with the corresponding data of González Hernández & Bonifacio (2009) and identified the ones in agreement. (iii) We fitted the reduced dwarf sample obtained from constraints (i) and (ii) to the Padova isochrones and re-identified those which occupy the plane defined by isochrones with ages t ≤ 13 Gyr. (iv) Finally, we omitted dwarfs with total velocity errors larger than 10.63 km s−1. We estimated the ages using the Bayesian procedure of Jørgensen & Lindegren (2005). The largest age–metallicity slope was found for early F...
Kinematics, ages and metallicities for F- and G-type stars in the solar neighbourhood
Monthly Notices of the Royal Astronomical Society, 2005
A new metallicity distribution and an age-metallicity relation are presented for 437 nearby F and G turn-off and sub-giant stars selected from radial velocity data of Nidever et al. Photometric metallicities are derived from uvby − Hβ photometry, and the stellar ages from the isochrones of Bergbusch & VandenBerg as transformed to uvby photometry using the methods of Clem et al. The X (stellar population) criterion of Schuster et al., which combines both kinematic and metallicity information, provides 22 thick-disc stars. σ W = 32 ± 5 km s −1 , V rot = 154 ± 6 km s −1 and [M/H] = −0.55 ± 0.03 dex for these thick-disc stars, which is in agreement with values from previous studies of the thick disc. α-element abundances which are available for some of these thick-disc stars show the typical α-element signatures of the thick disc, supporting the classification procedure based on the X criterion. Both the scatter in metallicity at a given age and the presence of old, metal-rich stars in the age-metallicity relation make it difficult to decide whether or not an age-metallicity relation exists for the older thin-disc stars. For ages greater than 3 Gyr, our results agree with the other recent studies that there is almost no correlation between age and metallicity, ([M/Fe])/ (age) = −0.01 ± 0.005 dex Gyr −1. For the 22 thick-disc stars there is a range in ages of 7-8 Gyr, but again almost no correlation between age and metallicity. For the subset of main-sequence stars with extra-solar planets, the age-metallicity relation is very similar to that of the total sample, very flat, the main difference being that these stars are mostly metal-rich, [M/H] −0.2 dex. However, two of these stars have [M/H] ∼ −0.6 dex and have been classified as thick-disc stars. As for the total sample, the range in ages for these stars with extra-solar planetary systems is considerable with a nearly uniform distribution over 3 age 13 Gyr.
2000
The age-metallicity relation of the solar neighbourhood is studied using a sample of 552 late-type dwarfs. This sample was built from the intersection of photometric catalogues with chromospheric activity surveys of the Mount Wilson group. For these stars, metallicities were estimated from uvby data, and ages were calculated from their chromospheric emission levels using a new metallicity-dependent chromospheric activity-age relation developed by Rocha-Pinto & Maciel (1998). A careful estimate of the errors in the chromospheric age is made. The errors in the chromospheric indices are shown to include partially the effects of the stellar magnetic cycles, although a detailed treatment of this error is still beyond our knowledge. It is shown that the results are not affected by the presence of unresolved binaries in the sample. We derive an age-metallicity relation which confirms the mean trend found by previous workers. The mean metallicity shows a slow, steady increase with time, amo...
Chemical Evolution in the Milky Way: Rotation-based Ages for APOGEE-Kepler Cool Dwarf Stars
The Astrophysical Journal, 2020
We use models of stellar angular momentum evolution to determine ages for ∼ 500 stars in the APOGEE-Kepler Cool Dwarfs sample. We focus on lower main-sequence stars, where other age-dating tools become ineffective. Our age distributions are compared to those derived from asteroseismic and giant samples and solar analogs. We are able to recover gyrochronological ages for old, lower-mainsequence stars, a remarkable improvement over prior work in hotter stars. Under our model assumptions, our ages have a median relative uncertainty of 14%, comparable to the age precision inferred for more massive stars using traditional methods. We investigate trends of galactic α-enhancement with age, finding evidence of a detection threshold between the age of the oldest α-poor stars and that of the bulk α-rich population. We argue that gyrochronology is an effective tool reaching ages of 10-12 Gyr in K-and early M-dwarfs. Finally, we present the first effort to quantify the impact of detailed abundance patterns on rotational evolution. We estimate a ∼ 15% bias in age for cool, α-enhanced (+ 0.4 dex) stars when standard solar-abundance-pattern rotational models are used for age inference, rather than models that appropriately account for α-enrichment. * Hubble Fellow tain abundances as proxies for age (e.g., Bensby et al. 2014; Martig et al. 2016; Tucci Maia et al. 2016; Feltzing et al. 2017). Other studies have argued against the thin-and thick-disk model of the Milky Way: as our ability to estimate precise stellar ages, composition, and kinematics has improved, the historical two-population hypothesis has evolved into a continuum of galactochemical structure and evolution (e.g., Bovy et al. 2012a,b; Buder et al. 2019). The study of chemical evolution in stellar populations is impossible without precise ages. To date, most investigations have used isochrones to estimate ages (Nordström et al. 2004; Haywood et al. 2013; Buder et al. 2019). While easy to implement, isochrone ages are most useful for cluster stars, which provide an ensemble of stars at a single age, or for field stars that have aged to about one-third of their main-sequence lifetimes (Pont & Eyer 2004; Soderblom 2010) and thus move substantially on the Hertzsprung-Russell (H-R) diagram. In this regime, isochrone ages are, at best, precise to 15%
Dynamical evidence of the age--metallicity relation in the Milky Way disk
Astronomy & Astrophysics, 2006
We studied the relationship between the average stellar abundance of several elements and the orbital evolution of stars in the neighbourhood of the Sun, using both observational data for 325 late-type dwarfs in a volume-complete sample and simulations of the orbital diffusion. Metallicities, ages, and initial position and velocities for the simulated stars are sampled from empirical distributions of these quantities in the Milky Way. We found that that there is a relationship between the average stellar abundance of Fe, Na, Si, Ca, Ni, and Ba and the mean orbital radius of stars currently passing through the solar neighbourhood. The greater the difference between the mean orbital radius and the solar Galactocentric distance, the more deficient the star is, on average, in these chemical species. The stars that take a longer time to come from their birthplaces to arrive in the present solar neighbourhood are more likely to be more metal-poor than those that were born here. This resul...
The age-activity-rotation relationship in solar-type stars
Astronomy & Astrophysics, 2004
We present Ca II K line chromospheric fluxes measured from high-resolution spectra in 35 G dwarf stars of 5 open clusters to determine the age-activity-rotation relationship from the young Hyades and Praesepe (0.6 Gyr) to the old M 67 (∼4.5 Gyr) through the two intermediate age clusters IC 4651 and NGC 3680 (∼1.7 Gyr). The full amplitude of the activity index within a cluster is slightly above 60 % for all clusters but one, NGC 3680, in which only two stars were observed. As a comparison, the same Solar Ca II index varies by ∼40% during a solar cycle. Four of our clusters (Hyades and Praesepe, IC 4651 and NGC 3680) are pairs of twins as far as age is concerned; the Hyades have the same chromospheric-activity level as Praesepe, at odds with early claims based on X-ray observations. Both stars in NGC 3680 are indistinguishable, as far as chromospheric activity is concerned, from those in the coeval IC 4651. This is a validation of the existence of an age-activity relationship. On the other hand, the two intermediate age clusters have the same activity level as the much older M 67 and the Sun. Our data therefore shows that a dramatic decrease in chromospheric activity takes place in solar stars between the Hyades and the IC 4651 age, of about 1 Gyr. Afterwards, activity remains virtually constant for more than 3 Gyr. We have also measured v sin i for all of our stars and the average rotational velocity shows the same trend as the chromospheric-activity index. We briefly investigate the impact of this result on the age determinations of field G dwarfs in the solar neighborhood; the two main conclusions are that a consistent group of "young" stars (i.e. as active as Hyades stars) is present, and that it is virtually impossible to give accurate chromospheric ages for stars older than ∼2 Gyr. The observed abrupt decline in activity explains very well the Vaughan-Preston gap.
Basic physical parameters of a selected sample of evolved stars
Astronomy & Astrophysics, 2006
We present the detailed spectroscopic analysis of 72 evolved stars, which were previously studied for accurate radial velocity variations. Using one Hyades giant and another well studied star as the reference abundance, we determine the [Fe/H] for the whole sample. These metallicities, together with the T eff values and the absolute V-band magnitude derived from Hipparcos parallaxes, are used to estimate basic stellar parameters (ages, masses, radii, (B−V) 0 and log g) using theoretical isochrones and a Bayesian estimation method. The (B−V) 0 values so estimated turn out to be in excellent agreement (to within ∼ 0.05 mag) with the observed (B−V), confirming the reliability of the T eff -(B−V) 0 relation used in the isochrones. On the other hand, the estimated log g values are typically 0.2 dex lower than those derived from spectroscopy; this effect has a negligible impact on [Fe/H] determinations. The estimated diameters θ have been compared with limb darkening-corrected ones measured with independent methods, finding an agreement better than 0.3 mas within the 1 < θ < 10 mas interval (or, alternatively, finding mean differences of just 6 %). We derive the age-metallicity relation for the solar neighborhood; for the first time to our knowledge, such a relation has been derived from observations of field giants rather than from open clusters and field dwarfs and subdwarfs. The age-metallicity relation is characterized by close-to-solar metallicities for stars younger than ∼ 4 Gyr, and by a large [Fe/H] spread with a trend towards lower metallicities for higher ages. In disagreement with other studies, we find that the [Fe/H] dispersion of young stars (less than 1 Gyr) is comparable to the observational errors, indicating that stars in the solar neighbourhood are formed from interstellar matter of quite homogeneous chemical composition. The three giants of our sample which have been proposed to host planets are not metal rich; this result is at odds with those for main sequence stars. However, two of these stars have masses much larger than a solar mass so we may be sampling a different stellar population from most radial velocity searches for extrasolar planets. We also confirm the previous indication that the radial velocity variability tends to increase along the RGB, and in particular with the stellar radius. 2 L. da Silva et al.: Basic physical parameters of Giant stars
The Binarity of Milky Way F,G,K Stars as a Function of Effective Temperature and Metallicity
The Astrophysical Journal, 2014
We estimate the fraction of F,G,K stars with close binary companions by analysing multi-epoch stellar spectra from SDSS and LAMOST for radial velocity (RV) variations. We employ a Bayesian method to infer the maximum likelihood of the fraction of binary stars with orbital periods of 1000 days or shorter, assuming a simple model distribution for a binary population with circular orbits. The overall inferred fraction of stars with such a close binary companion is 43.0% ± 2.0% for a sample of F, G, K stars from SDSS SEGUE, and 30% ± 8.0% in a similar sample from LAMOST. The apparent close binary fraction decreases with the stellar effective temperature. We divide the SEGUE and LEGUE data into three subsamples with different metallicity ([Fe/H] < −1.1; −1.1 < [Fe/H] < −0.6; −0.6 < [Fe/H]), for which the inferred close binary fractions are 56 ± 5.0%, 56.0 ± 3%, and 30 ± 5.7%. The metal-rich stars from our sample are therefore substantially less likely to possess a close binary companion than otherwise similar stars drawn from metal-poor populations. The different ages and formation environments of the Milky Way's thin disk, thick disk and halo may contribute to explaining these observations. Alternatively metallicity may have a significant effect on the formation and/or evolution of binary stars.
Monthly Notices of the Royal Astronomical Society
We investigate the vertical metallicity gradients of five mono-age stellar populations between 0 and 11 Gyr for a sample of 18 435 dwarf stars selected from the cross-matched Tycho-Gaia Astrometric Solution and Radial Velocity Experiment (RAVE) Data Release 5. We find a correlation between the vertical metallicity gradients and age, with no vertical metallicity gradient in the youngest population and an increasingly steeper negative vertical metallicity gradient for the older stellar populations. The metallicity at disc plane remains almost constant between 2 and 8 Gyr, and it becomes significantly lower for the 8 < τ ≤ 11 Gyr population. The current analysis also reveals that the intrinsic dispersion in metallicity increases steadily with age. We discuss that our results are consistent with a scenario that (thin) disc stars formed from a flaring (thin) star-forming disc.