Chromospheric Age Research Articles

Age-velocity relations with GALEXFUV-determined ages of Sun-like, solar neighborhood stars

A relationship between chromospheric activity and age is calibrated for FGK dwarf stars using GALEXFUV magnitudes and Gaia(G_{BP} - G) colors. Such a calibration between GALEXFUV magnitudes and stellar age has utility in population studies of dwarfs for further understanding of the chemical evolution of the Milky Way. As an illustration of one such application we have investigated a population of Sun-like, solar neighborhood stars for their metallicities and velocity dispersions; a cross-matched sample of FGK type dwarf stars from Casagrande et al. (2011) with the Gaia and GALEX catalogs. Using calibrated relationships between FUV magnitudes and age, we determined a chromospheric activity indicator, Q, and stellar age, tau , for each dwarf. We constructed age-velocity (AVR) and age-metallicity (AMR) relations with empirically-determined FUV ages. Power law fits to AVR plots are consistent with heating mechanism models within the literature. We further demonstrate that perigalactic distance and eccentricity versus FUV-age plots are consistent with an “inside out” formation history model.

The Solar Twin Planet Search

Context. It is well known that the magnetic activity of solar-type stars decreases with age, but it is widely debated in the literature whether there is a smooth decline or if there is an early sharp drop until 1–2 Gyr that is followed by a relatively inactive constant phase. Aims. We revisited the activity-age relation using time-series observations of a large sample of solar twins whose precise isochronal ages and other important physical parameters have been determined. Methods. We measured the Ca II H and K activity indices using ≈9000 HARPS spectra of 82 solar twins. In addition, the average solar activity was calculated through asteroids and Moon reflection spectra using the same instrumentation. Thus, we transformed our activity indices into the S Mount Wilson scale (SMW), recalibrated the Mount Wilson absolute flux and photospheric correction equations as a function of Teff, and then computed an improved bolometric flux normalized activity index log R′HK (Teff) for the entire sample. Results. New relations between activity and the age of solar twins were derived by assessing the chromospheric age-dating limits using log R′HK (Teff). We measured an average solar activity of SMW = 0.1712 ± 0.0017 during solar magnetic cycles 23–24 covered by HARPS observations, and we also inferred an average of SMW = 0.1694 ± 0.0025 for cycles 10–24, anchored on a sunspot number correlation of S index versus. We also found a simple relation between the average and the dispersion of the activity levels of solar twins. This enabled us to predict the stellar variability effects on the age-activity diagram, and consequently, to estimate the chromospheric age uncertainties that are due to the same phenomena. The age-activity relation is still statistically significant up to ages around 6–7 Gyr, in agreement with previous works using open clusters and field stars with precise ages. Conclusions. Our research confirms that Ca II H & K lines remain a useful chromospheric evolution tracer until stars reach ages of at least 6–7 Gyr. We found evidence that for the most homogenous set of old stars, the chromospheric activity indices seem to continue to decrease after the solar age toward the end of the main sequence. Our results indicate that a significant part of the scatter observed in the age-activity relation of solar twins can be attributed to stellar cycle modulations effects. The Sun seems to have a normal activity level and variability for its age.

The Solar Twin Planet Search

Context . It is well known that the magnetic activity of solar-type stars decreases with age, but it is widely debated in the literature whether there is a smooth decline or if there is an early sharp drop until 1–2 Gyr that is followed by a relatively inactive constant phase. Aims . We revisited the activity-age relation using time-series observations of a large sample of solar twins whose precise isochronal ages and other important physical parameters have been determined. Methods . We measured the Ca II H and K activity indices using ≈9000 HARPS spectra of 82 solar twins. In addition, the average solar activity was calculated through asteroids and Moon reflection spectra using the same instrumentation. Thus, we transformed our activity indices into the S Mount Wilson scale ( S MW ), recalibrated the Mount Wilson absolute flux and photospheric correction equations as a function of T eff , and then computed an improved bolometric flux normalized activity index log R ′ HK ( T eff ) for the entire sample. Results . New relations between activity and the age of solar twins were derived by assessing the chromospheric age-dating limits using log R ′ HK ( T eff ). We measured an average solar activity of S MW = 0.1712 ± 0.0017 during solar magnetic cycles 23–24 covered by HARPS observations, and we also inferred an average of S MW = 0.1694 ± 0.0025 for cycles 10–24, anchored on a sunspot number correlation of S index versus. We also found a simple relation between the average and the dispersion of the activity levels of solar twins. This enabled us to predict the stellar variability effects on the age-activity diagram, and consequently, to estimate the chromospheric age uncertainties that are due to the same phenomena. The age-activity relation is still statistically significant up to ages around 6–7 Gyr, in agreement with previous works using open clusters and field stars with precise ages. Conclusions . Our research confirms that Ca II H & K lines remain a useful chromospheric evolution tracer until stars reach ages of at least 6–7 Gyr. We found evidence that for the most homogenous set of old stars, the chromospheric activity indices seem to continue to decrease after the solar age toward the end of the main sequence. Our results indicate that a significant part of the scatter observed in the age-activity relation of solar twins can be attributed to stellar cycle modulations effects. The Sun seems to have a normal activity level and variability for its age.

The age-mass-metallicity-activity relation for solar-type stars: comparisons with asteroseismology and the NGC 188 open cluster

The Mount Wilson Ca II index log$(R'_{\rm HK})$ is the accepted standard metric of calibration for the chromospheric activity versus age relation of FGK stars. Recent results claim its inability to discern activity levels, and thus ages, for stars older than $\sim$2 Gyr, which would severely hamper its application to date disk stars older than the Sun. We present a new activity-age calibration of the Mt. Wilson index explicitly taking into account mass and $[$Fe/H$]$ biases, implicit in samples of stars selected to have precise ages, that have so far gone unappreciated. 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, M67 clusters, and the Ursa Major moving group. We further test the calibration with extensive new Gemini/GMOS log$(R'_{\rm HK})$ data of the old, solar $[$Fe/H$]$ clusters M67 and NGC 188. The observed NGC 188 activity level is clearly lower than M67. 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 $\sim$6 Gyr, average errors being $\sim$0.14 dex provided that the mass and $[$Fe/H$]$ dimensions are explicitly accounted for. We test our calibration against asteroseismological ages, finding excellent correlation ($\rho$ = +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.

ON THE AGE OF GLIESE 86

Gliese 86 is a nearby planet hosting a visual binary consisting of a K-type primary, Gl 86 A, and a white dwarf secondary, Gl 86 B. In this work, we present a model atmosphere analysis of the M {sub A} = 0.83 M {sub ☉} primary, whose chemistry unambiguously identifies it as a 10 Gyr old disk star. For the secondary, this gives rise to a progenitor mass M {sub prog} = 1.11 ± 0.05 M {sub ☉}, and—by inference with its local sibling o {sup 2} Eri—a white dwarf mass M {sub B} = 0.49 ± 0.02 M {sub ☉}. The discrepancy with the 2-3 Gyr chromospheric age of Gl 86 A, as first noted by Rocha-Pinto, Castilho and Maciel, is thus most plausibly the result of a former accretion of mass and angular momentum from the distant degenerate. In consequence, this also implies that with respect to shorter-period systems, like Sirius or Procyon, one must expect significant wind accretion when they evolve through the planetary nebula phase, such that the system ages cannot be reliably determined from their currently bright primaries.

Chromospheric activity as age indicator

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. I aim to define the age range for which chromospheric activity is a robust astrophysical clock. 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. 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. 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 the inverse square root of the age, should be relaxed.

The star formation rate in the Milky Way: Results from stars and planetary nebulae

AbstractThe star formation rate (SFR) is still a poorly known characteristic of the Milky Way, especially concerning the possibility of an irregular SFR, compared to a constant rate. Some recent results based on the distribution of dwarf stars with chromospheric ages suggest at least two major bursts in the past 10 Gyr, while other investigations are consistent with an approximately constant SFR. The SFR also shows important spatial variations, particularly concerning the radial variations along the galactic disc. In this work, we investigate two different problems relative to the galactic SFR: (i) We estimate the star formation rate in the galactic disc based on the age distribution of the planetary nebula central stars (CSPN), and compare these results with previous investigations based on dwarf stars. The CSPN ages were derived on the basis of five different methods, involving the observed nebular metallicities and kinematical properties; (ii) We derive radial abundance gradients from several elements in planetary nebulae, and compare these results with recent determinations based on younger objects, such as HII regions and cepheid variables. Since the gradients are linked to the formation process of the galactic disc, we can estimate the spatial variation of the SFR. Preliminary results indicate that at least one major star formation burst is obtained, as well as a relatively smooth variation of the SFR along the galactocentric radius.

High-precision orbital parameters for the short-period spectroscopic binary HD 159656 – an eccentric orbit confirmed for an old but active system

HD 159656 is a seventh magnitude short-period (P ∼ 10 d) binary system with a solar-type (G2/3V) primary component. The single-lined spectroscopic orbital solution has been recomputed with high precision. This work confirms the orbit's elliptical nature (e = 0.116 16 ± 0.000 20) which had been brought into doubt based on an earlier less precise solution (Barker, Evans & Laing 1967) and the comprehensive study conducted by Lucy & Sweeney (1971). Short-period binary orbits of this eccentricity are uncommon, particularly when the system has the considerable age that we estimate to be τise = 9.3 ± 0.6 Gyr. Thus HD 159656 may be a system that challenges our present theories of circularization time-scales. Alternatively, HD 159656 could be considered to be suitable for future studies aiming to establish whether or not a third star is interfering with the circularization process. Apsidal motion, undetectable based on this and the previous solution, may also be revealed in the future, this possibility being greatly increased by the high precision of our results. Attempts to detect any companions directly have so far been unsuccessful. The age claimed provides indirect support for the proposition that the system is pseudo-synchronized. This in turn gives us reason to claim an estimate for the inclination, i = 32 +14 -12 degrees with a corresponding mass ratio q ∼ 0.3 that implies that the secondary is an M3 dwarf, or an unusually low-mass white dwarf, since M 2 ∼ 0.3 M ⊙ . HD 159656 is a standard star in many photometric systems. However, a small sample of our spectra indicates that the Ca II H&K line emission, that we demonstrate arises principally from the primary, is variable. Furthermore, ROSAT data of an X-ray source probably coincident with HD 159656 provide evidence of flare activity. Such behaviour is more typical of M-dwarf stars and so may be more properly associated with the secondary, if it is of this spectral type. If the binary is tidally locked, this could explain the activity of this system, which implies a chromospheric age an order of magnitude less than the isochrone age. The spectra reveal a very low lithium abundance for the primary component, together with many other spectral characteristics and photometric colours that are typical of solar analogues. Thus the primary star represents an example of the behaviour of solar analogues in this astrophysical context.

Ages for Illustrative Field Stars Using Gyrochronology: Viability, Limitations, and Errors

We here develop an improved way of using a rotating star as a clock, set it using the Sun, and demonstrate that it keeps time well. This technique, called gyrochronology, permits the derivation of ages for solar- and late-type main sequence stars using only their rotation periods and colors. The technique is clarified and developed here, and used to derive ages for illustrative groups of nearby, late-type field stars with measured rotation periods. We first demonstrate the reality of the interface sequence, the unifying feature of the rotational observations of cluster and field stars that makes the technique possible, and extends it beyond the proposal of Skumanich by specifying the mass dependence of rotation for these stars. We delineate which stars it cannot currently be used on. We then calibrate the age dependence using the Sun. The errors are propagated to understand their dependence on color and period. Representative age errors associated with the technique are estimated at ~15% (plus possible systematic errors) for late-F, G, K, & early-M stars. Ages derived via gyrochronology for the Mt. Wilson stars are shown to be in good agreement with chromospheric ages for all but the bluest stars, and probably superior. Gyro ages are then calculated for each of the active main sequence field stars studied by Strassmeier and collaborators where other ages are not available. These are shown to be mostly younger than 1Gyr, with a median age of 365Myr. The sample of single, late-type main sequence field stars assembled by Pizzolato and collaborators is then assessed, and shown to have gyro ages ranging from under 100Myr to several Gyr, and a median age of 1.2Gyr. Finally, we demonstrate that the individual components of the three wide binaries XiBooAB, 61CygAB, & AlphaCenAB yield substantially the same gyro ages.

A Survey of Chromospheric Activity in the Solar‐Type Stars in the Open Cluster M67

We present the results of a spectroscopic survey of the Ca II H & K core strengths in a sample of 60 solar-type stars that are members of the solar-age and solar-metallicity open cluster M67. We adopt the HK index, defined as the summed H+K core strengths in 0.1 nm bandpasses centered on the H and K lines, respectively, as a measure of the chromospheric activity that is present. We compare the distribution of mean HK index values for the M67 solar-type stars with the variation of this index as measured for the Sun during the contemporary solar cycle. We find that the stellar distribution in our HK index is broader than that for the solar cycle. Approximately 17% of the M67 sun-like stars exhibit average HK indices that are less than solar minimum. About 7%-12% are characterized by relatively high activity in excess of solar maximum values while 72%-80% of the solar analogs exhibit Ca II H+K strengths within the range of the modern solar cycle. The ranges given reflect uncertainties in the most representative value of the maximum in the HK index to adopt for the solar cycle variations observed during the period A.D. 1976--2004. Thus, ~ 20% - 30% of our homogeneous sample of sun-like stars have mean chromospheric H+K strengths that are outside the range of the contemporary solar cycle. Any cycle-like variability that is present in the M67 solar-type stars appears to be characterized by periods greater than ~ 6 years. Finally, we estimate a mean chromospheric age for M67 in the range of 3.8--4.3 Gyr.

The age-activity-rotation relationship in solar-type stars

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 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 sini 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.

Chromospherically young, kinematically old stars

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.

An Intermittent Star Formation History in a "Normal" Disk Galaxy: The Milky Way.

The star formation rate history of the Milky Way is derived using the chromospheric age distribution for 552 stars in the solar neighborhood. The stars' sample birth sites are distributed over a very large range of distances because of orbital diffusion and so give an estimate of the global star formation rate history. The derivation incorporates the metallicity dependence of chromospheric emission at a given age and corrections to account for incompleteness, scale height-age correlations, and stellar evolutionary effects. We find fluctuations in the global star formation rate with amplitudes greater than a factor of 2-3 on timescales less than 0.2-1 Gyr. The actual history is likely to be more bursty than found here because of the smearing effect of age uncertainties. There is some evidence for a slow secular increase in the star formation rate, perhaps a record of the accumulation history of our Galaxy. A smooth, nearly constant star formation rate history is strongly ruled out, confirming the result first discovered by Barry using a smaller sample and a different age calibration. This result suggests that galaxies can fluctuate coherently on large scales.

Metallicity effects on the chromospheric activity-age relation for late-type dwarfs

We show that there is a relationship between the age excess, defined as the difference between the stellar isochrone and chromospheric ages, and the metallicity as measured by the index [Fe/H] for late-type dwarfs. The chromospheric age tends to be lower than the isochrone age for metal-poor stars, and the opposite occurs for metal-rich objects. We suggest that this could be an effect of neglecting the metallicity dependence of the calibrated chromospheric emission-age relation. We propose a correction to account for this dependence. We also investigate the metallicity distributions of these stars, and show that there are distinct trends according to the chromospheric activity level. Inactive stars have a metallicity distribution which resembles the metallicity distribution of solar neighbourhood stars, while active stars appear to be concentrated in an activity strip on the log R′HK × [Fe/H] diagram. We provide some explanations for these trends, and show that the chromospheric emission-age relation probably has different slopes on the two sides of the Vaughan-Preston gap.

Chaotic Behaviour of the Temporal Evolution of Star Formation Rate in the Solar Neighborhood

We use statistical data giving the number of stars formed as a function of the age, assigned using the observed deterministic relation between chromospheric activity and age, to show that the temporal behaviour of star formation in the solar neighbourhood must have been chaotic during time intervals of less than 1 Gyr. We also offer evidence that systems of gas and stars typified by that in the solar vicinity show behaviour analogous to that of the logistic map.

The chromospheric age dependence of the birthrate, composition, motions, and rotation of late F and G dwarfs within 25 parsecs of the sun

view Abstract Citations (140) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Chromospheric Age Dependence of the Birthrate, Composition, Motions, and Rotation of Late F and G Dwarfs within 25 Parsecs of the Sun Barry, Don C. Abstract Chromospheric ages are calculated for 115 stars. Reported uncertainties of about 10% in the Vaughan-Preston S values lead to accidental errors in the calculated ages ranging from about 50% at 3×108yr to about 15% at 1010yr. Rotational velocities are plotted against chromospheric age to reveal the rotational spin-down trajectories of solar-type stars of various masses. Also, the stellar birthrate, the [Fe/H] ratios, and the peculiar velocities of the sample stars are presented as a function of chromospheric age. The results confirm that a star's rotation depends only on its mass and age. The smooth, continuous decline of both a star's rotation and its activity across the Vaughan-Preston gap is demonstrated. The simples and only remaining tenable interpretation of the gap is that it is caused by time variations in the stellar birthrate. Publication: The Astrophysical Journal Pub Date: November 1988 DOI: 10.1086/166848 Bibcode: 1988ApJ...334..436B Keywords: Chromosphere; Dwarf Stars; Late Stars; Star Formation; Stellar Composition; Stellar Rotation; F Stars; G Stars; Stellar Motions; Astrophysics; STARS: ABUNDANCES; STARS: CHROMOSPHERES; STARS: EVOLUTION; STARS: FORMATION; STARS: ROTATION; STARS: STELLAR STATISTICS full text sources ADS | data products SIMBAD (104)

Chromospheric activity and ages of solar-type stars

Observations of 15 solar-type stars in the intermediate-age open cluster NGC 752 are reported. A lower resolution analog of the Mount Wilson S index is shown to yield absolute chromospheric surface flux values for these stars with about 60 percent of the sensitivity of the Mount Wilson system. Absolute chromospheric surface fluxes of solar-type stars in eight clusters ranging from 10 million yrs to six billion or more years in age are presented. Two heuristic forms are shown to fit the data about equally well, with no indication of a discontinuity at intermediate ages. These relations can yield chromospheric ages for any G-type dwarf or subgiant with a Mount Wilson S index. The usefulness of this lower resolution approach for studies of chemical and dynamical evolution of the Galaxy as well as of the stellar birth rate is pointed out. 24 references.