The age-activity-rotation relationship in solar-type stars (original) (raw)
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An investigation of chromospheric activity spanning the Vaughan-Preston gap: impact on stellar ages
Astronomy and Astrophysics, 2009
Context. Chromospheric activity is widely used as an age indicator for solar-type stars based on the early evidence that there is a smooth evolution from young and active to old and inactive stars. However, this transition may require modification as chromospheric activity is not a viable age indicator for stars older than 1 Gyr. Aims. We analysed chromospheric activity in five solar-type stars in two open clusters, in order to study how chromospheric activity evolves with time. Methods. We took UVES high-resolution, high S/N ratio spectra of 3 stars in IC 4756 and 2 in NGC 5822, which were combined with a previously studied data-set and reanalysed here. The emission core of the deep, photospheric Ca II K line was used as a probe of the chromospheric activity. Results. All of the 5 stars in the new sample, including those in the 1.2 Gyr-old NGC 5822, have activity levels comparable to those of Hyades and Praesepe. Conclusions. A likely interpretation of our data is that solar-type-star chromospheric activity, from the age of the Hyades until that of the Sun, does not evolve smoothly. Stars change from active to inactive, crossing the activity range corresponding to the Vaughan-Preston gap, on a time-scale that might be as short as 200 Myr. Evolution before and after such a transition is much less significant than cyclical and long-term variations. We show that data presented in the literature to support a correlation between age and activity could be also interpreted differently in the light of our results. Suggestions have been published that relevant stellar structures and/or dynamos are different for active and inactive stars. These provide a natural explanation for the observations presented here. More observations are required in order to strengthen our results, especially a long-term follow up of our two targets in the 1.2-Gyr old cluster NGC 5822.
The Chromospheric Activity and Ages of M Dwarf Stars in Wide Binary Systems
The Astronomical Journal, 2005
We investigate the relationship between age and chromospheric activity for 139 M dwarf stars in wide binary systems with white dwarf companions. The age of each system is determined from the cooling age of its white dwarf component. The current limit for activity-age relations found for M dwarfs in open clusters is 4 Gyr. Our unique approach to finding ages for M stars allows for the exploration of this relationship at ages older than 4 Gyr. The general trend of stars remaining active for a longer time at later spectral type is confirmed. However, our larger sample and greater age range reveals additional complexity in assigning age based on activity alone. We find that M dwarfs in wide binaries older than 4 Gyr depart from the log-linear relation for clusters and are found to have activity at magnitudes, colors and masses which are brighter, bluer and more massive than predicted by the cluster relation. In addition to our activity-age results, we present the measured radial velocities and complete space motions for 161 white dwarf stars in wide binaries.
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.
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.
Fine structure of the chromospheric activity in Solar-type stars – The Hα line
Astronomy & Astrophysics, 2005
A calibration of Hα as both a chromospheric diagnostic and an age indicator is presented, complementing the works previously done on this subject (Herbig 1985; Pasquini & Pallavicini 1991). The chromospheric diagnostic was built with a statistically significant sample, covering nine years of observations, and including 175 solar neighborhood stars. Regarding the age indicator, the presence of stars for which very accurate ages are determined, such as those belonging to clusters and kinematic groups, lends confidence to our analysis. We also investigate the possibility that stars of the same age might have gone through different tracks of chromospheric decay, identifying-within the same age range-effects of metallicity and mass. These parameters, however, as well as age, seem to be significant only for dwarf stars, losing their meaning when we analyze stars in the subgiant branch. This result suggests that, in these evolved stars, the emission mechanism cannot be magnetohydrodynamical in nature, in agreement with recent models (Fawzy et al. 2002c, and references therein). The Sun is found to be a typical star in its Hα chromospheric flux, for its age, mass and metallicity. As a byproduct of this work, we developed an automatic method to determine temperatures from the wings of Hα, which means the suppression of the error inherent to the visual procedure used in the literature.
Age-velocity relations with GALEX FUVFUVFUV-determined ages of Sun-like, solar neighborhood stars
Astrophysics and Space Science
A relationship between chromospheric activity and age is calibrated for FGK dwarf stars using GALEX$FUV$ F U V magnitudes and Gaia$(G_{BP} - G)$ ( G B P − G ) colors. Such a calibration between GALEX$FUV$ F U V 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 FUVFUVFUV F U V magnitudes and age, we determined a chromospheric activity indicator, QQQ Q , and stellar age, tau\tau tau τ , for each dwarf. We constructed age-velocity (AVR) and age-metallicity (AMR) relations with empirically-determined FUVFUVFUV F U V ages. Power law fits to AVR plots are consistent with heating mechanism models within...
Fine structure of the chromospheric activity in Solar-type stars ? The H? line
Astronomy and Astrophysics, 2005
A calibration of Hα as both a chromospheric diagnostic and an age indicator is presented, complementing the works previously done on this subject . The chromospheric diagnostic was built with a statistically significant sample, covering nine years of observations, and including 175 solar neighborhood stars. Regarding the age indicator, the presence of stars for which very accurate ages are determined, such as those belonging to clusters and kinematic groups, lends confidence to our analysis. We also investigate the possibility that stars of the same age might have gone through different tracks of chromospheric decay, identifying -within the same age range -effects of metallicity and mass. These parameters, however, as well as age, seem to be significant only for dwarf stars, losing their meaning when we analyze stars in the subgiant branch. This result suggests that, in these evolved stars, the emission mechanism cannot be magnetohydrodynamical in nature, in agreement with recent models (Fawzy et al. 2002c, and references therein). The Sun is found to be a typical star in its Hα chromospheric flux, for its age, mass and metallicity. As a byproduct of this work, we developed an automatic method to determine temperatures from the wings of Hα, which means the suppression of the error inherent to the visual procedure used in the literature.
Rotation and Activity of Pre–Main‐Sequence Stars
The Astrophysical Journal, 2007
Rotation and activity are key parameters in stellar evolution and can be used to probe basic stellar physics. Here we present a study of rotation (measured as projected rotational velocity v sin i) and chromospheric activity (measured as Hα equivalent width) based on an extensive set of high-resolution optical spectra obtained with the MIKE instrument on the 6.5 m Magellan Clay telescope. Our targets are 74 F-M dwarfs in the young stellar associations η Chamaeleontis, TW Hydrae, β Pictoris, and Tucana-Horologium, spanning ages from 6 to 30 Myr. While the Hα equivalent widths for most F and G stars are consistent with pure photospheric absorption, most K and M stars show measurable chromospheric emission. By comparing Hα equivalent width in our sample to results in the literature, we see a clear evolutionary sequence: Chromospheric activity declines steadily from the T Tauri phase to the main sequence. Using activity as an age indicator, we find a plausible age range for the Tuc-Hor association of 10-40 Myr. Between 5 and 30 Myr, we do not see evidence for rotational braking in the total sample, thus angular momentum is conserved, in contrast to younger stars. This difference indicates a change in the rotational regulation at ∼5-10 Myr, possibly because disk braking cannot operate longer than typical disk lifetimes, allowing -2the objects to spin up. On timescales of ∼ 100 Myr there is some evidence for weak rotational braking, possibly due to stellar winds. The rotation-activity relation is flat in our sample; in contrast to main-sequence stars, there is no linear correlation for slow rotators. We argue that this is because young stars generate their magnetic fields in a fundamentally different way from main-sequence stars, and not just the result of a saturated solar-type dynamo. By comparing our rotational velocities with published rotation periods for a subset of stars, we determine ages of 13 +7 −6 Myr and 9 +8 −2 Myr for the η Cha and TWA associations, respectively, consistent with previous estimates. Thus we conclude that stellar radii from evolutionary models by are in agreement with the observed radii within ±15%.
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.