An investigation of chromospheric activity spanning the Vaughan-Preston gap: impact on stellar ages (original) (raw)
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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.
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.
The X-ray luminosity of solar-mass stars in the intermediate age open cluster NGC 752
Astronomy and Astrophysics, 2008
Aims. While observational evidence shows that most of the decline in a star's X-ray activity occurs between the age of the Hyades (∼ 8 × 10 8 yrs) and that of the Sun, very little is known about the evolution of stellar activity between these ages. To gain information on the typical level of coronal activity at a star's intermediate age, we studied the X-ray emission from stars in the 1.9 Gyr old open cluster NGC 752. Methods. We analysed a ∼ 140 ks Chandra observation of NGC 752 and a ∼ 50 ks XMM-Newton observation of the same cluster. We detected 262 X-ray sources in the Chandra data and 145 sources in the XMM-Newton observation. Around 90% of the catalogued cluster members within Chandra's field-of-view are detected in the X-ray. The X-ray luminosity of all observed cluster members (28 stars) and of 11 cluster member candidates was derived. Results. Our data indicate that, at an age of 1.9 Gyr, the typical X-ray luminosity of the cluster members with M = 0.8 − 1.2 M ⊙ is L X = 1.3 × 10 28 erg s −1 , so approximately a factor of 6 less intense than that observed in the younger Hyades. Given that L X is proportional to the square of a star's rotational rate, the median L X of NGC 752 is consistent, for t > ∼ 1 Gyr, to a decaying rate in rotational velocities v rot ∝ t −α with α ∼ 0.75, steeper than the Skumanich relation (α ≃ 0.5) and significantly steeper than observed between the Pleiades and the Hyades (where α < 0.3), suggesting that a change in the rotational regimes of the stellar interiors is taking place at t ∼ 1 Gyr.
Chromospheric activity as age indicator
Astronomy & Astrophysics, 2013
Context. Chromospheric activity has been calibrated and widely used as an age indicator. However, it has been suggested that the viability of this age indicator is, in the best case, limited to stars younger than about 1.5 Gyr. Aims. I aim to define the age range for which chromospheric activity is a robust astrophysical clock. Methods. I collected literature measurements of the S-index in field stars, which is a measure of the strength of the H and K lines of the Ca II and a proxy for chromospheric activity, and exploited the homogeneous database of temperature and age determinations for field stars provided by the Geneva-Copenhagen survey of the solar neighbourhood. Results. Field data, inclusive data previously used to calibrate chromospheric ages, confirm the result found using open cluster data, i.e. there is no decay of chromospheric activity after about 2 Gyr. Conclusions. The only existing indication supporting the viability of chromospheric ages older than 2 Gyr is the similarity of chromospheric activity levels in the components of 35 dwarf binaries. However, even in the most optimistic scenario, uncertainty in age determination for field stars and lack of sufficient data in open clusters make any attempt of calibrating an age activity relationship for old stars premature. The hypothesis that chromospheric activity follows the Skumanich law, i.e. that it is proportional to t −1/2 , should be relaxed.
A photometric and radial-velocity analysis of the intermediate-age open cluster NGC 752
Publications of the Astronomical Society of the Pacific, 1994
Using all available proper-motion and radial-velocity data, including new radial-velocity observations obtained for this investigation, probable members of the open cluster NGC 752 have been identified. Photoelectric data on six systems have been transformed and collated to form an internally consistent sample on the BVsystem. Binaries have been identified using photometric and radial-velocity data, including a photographic survey for variability and the radial-velocity observations of this study. Analysis of the data leads to the following cluster parameters and their probable errors: E(B-V) =0.035±0.005 mag, [Fe/H]=-0.15±0.05 dex, and (m-Af) = 8.25±0.10 mag. The spread in color among stars in the color-magnitude diagram (cmd) along the main sequence from the turnoff to the unevolved main sequence is the consequence of a rich population of binaries. Due to its age and the comprehensive data available for the cluster, NGC 752 provides an ideal test of a variety of evolutionary phenomena. Comparison with theoretical isochrones normalized in an internally consistent manner leads to the conclusion that the morphology and distribution of stars in the cmd can best be matched using models that include convective overshoot, particularly those of Schaller et al. (1992, A&AS, 96, 269). Despite their differences, the traditional and the overshoot isochrones both imply very similar ages, 1.9 ±0.2 Gyr and 1.7 ±0.1 Gyr, respectively, for the cluster. The Li abundances for the giants confirm that the giant branch is dominated by clump stars and first-ascent giants below the luminosity of the clump. The position and size of the Li dip among the main-sequence stars, compared to the Hyades, is readily explained by stellar evolution with convective overshoot. It is predicted that among turnoff stars in the intermediate-age range Li will cease to be a unique function of age at a given color. Chromospheric flux is shown to be a well-defined function of color for single, unevolved stars, identical to that found for the Hyades, and the relation for NGC 752 falls within the Vaughan-Preston gap. However, the slope of the relation requires that increasing color implies increasing age for the bluer portion of the weak-emission boundary. The combined effect of small samples, random errors, emission limits, a possible selection bias in favor of turnoff stars, and metallicity corrections is to render highly questionable any interpretation of time-variable star formation within the Galaxy based upon chromospheric ages.
The Astronomical Journal, 2008
We present a spectroscopic analysis of over 38,000 low-mass stars from the Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5). Analysis of this unprecedentedly large sample confirms the previously detected decrease in the fraction of magnetically active stars (as traced by Hα emission) as a function of the vertical distance from the Galactic plane. The magnitude and slope of this effect vary as a function of spectral type. Using simple 1D dynamical models, we demonstrate that the drop in activity fraction can be explained by thin-disk dynamical heating and a rapid decrease in magnetic activity. The timescale for this rapid activity decrease changes according to the spectral type. By comparing our data to the simulations, we calibrate the age-activity relation at each M dwarf spectral type. We also present evidence for a possible decrease in the metallicity as a function of height above the Galactic plane. In addition to our activity analysis, we provide line measurements, molecular band indices, colors, radial velocities, 3D space motions, and mean properties as a function of spectral type for the SDSS DR5 low-mass star sample. 785 786 WEST ET AL.
A detailed spectroscopic analysis of the open cluster NGC 5460★
Monthly Notices of the Royal Astronomical Society, 2011
Within the context of a large project aimed at studying early F-, A-and late B-type stars we present the abundance analysis of the photospheres of 21 members of the open cluster NGC 5460, an intermediate age cluster (log t∼ 8.2) previously unstudied with spectroscopy. Our study is based on medium and high resolution spectra obtained with the FLAMES instrument of the ESO/VLT. We show that cluster members have a nearly solar metallicity, and that there is evidence that the abundances of magnesium and iron are correlated with the effective temperature, exhibiting a maximum around T eff =10500 K. No correlations are found between abundances and projected equatorial velocity, except for marginal evidence of barium being more abundant in slower than in faster rotating stars. We discovered two He-weak stars, and a binary system where the hotter component is a HgMn star. We provide new estimates for the cluster distance (720 ± 50 pc), age (log t=8.2 ± 0.1), and mean radial velocity (−17.9 ± 5.2 km s −1 ).
Chromospherically young, kinematically old stars
Astronomy & Astrophysics, 2002
We have investigated a group of stars known to have low chromospheric ages, but high kinematical ages. Isochrone, chemical and lithium ages are estimated for them. The majority of stars in this group show lithium abundances much smaller than expected for their chromospheric ages, which is interpreted as an indication of their old age. Radial velocity measurements in the literature also show that they are not close binaries. The results suggest that they can be formed from the coalescence of short-period binaries. Coalescence rates, calculated taking into account several observational data and a maximum theoretical time scale for contact, in a short-period pair, predict a number of coalesced stars similar to what we have found in the solar neighbourhood.
The Ages of the Hyades, Praesepe, and Coma Star Clusters
Publications of the Astronomical Society of the Pacific, 1969
If one uses the revised T e , (B-V) relations and bolometric corrections for main-sequence stars derived by Morton and Adams (1968) and Iben's evolutionary tracks for Population I stars (X = 0.71, Z = 0.02), it is found that the Hyades and Praesepe clusters are about 9 X 10 8 years old, while-the Coma cluster has an age of about 6.5 X 10 8 years. From a comparison of color magnitude diagrams with evolutionary tracks computed by Auman (1965) and Iben (1965a,b), the Hyades and Praesepe clusters were thought to have an age of between 4 X 10 8 years and 5 X 10 8 years (Barbaro, Dellaporta, and Nobili 1966; Kraft 1967) while the Coma cluster was expected to be somewhat younger since it contains slightly bluer stars. The revised relations between effective temperature (T e), bolometric correction (BC), and color indices of main-sequence stars as derived from interferometric measurements of stellar diameters together with accurate model atmospheres (Morton and Adams 1968), indicate much lower effective temperatures than previously estimated for spectral types between B8 and A5 (c£, Harris 1963); for instance for (B-V) = 0*00, T e has been revised from 10,700° K to 9600° K. This implies considerably larger ages for star clusters in which the turn-off is in this spectral range, since the turn-off is moved farther down the main sequence in the M v , log T e diagram and hence the stars at the turn-off must be less massive than previously estimated. In order to evaluate the magnitude of this effect, timelines were computed for an artificial star cluster that consists of 45 stars with masses between 1.5 39Ω and 2.25 2W©. The 45 mass values were chosen at random from a collection of 180 values that ° Contributions from the Lick Observatory, No. 304.