Abstract
Abstract We present a new age-dating technique that combines gyrochronology with isochrone fitting to infer ages for FGKM main-sequence and subgiant field stars. Gyrochronology and isochrone fitting are each capable of providing relatively precise ages for field stars in certain areas of the Hertzsprung–Russell diagram (HRD): gyrochronology works optimally for cool main-sequence stars, and isochrone fitting can provide precise ages for stars near the main-sequence turnoff. Combined, these two age-dating techniques can provide precise and accurate ages for a broader range of stellar masses and evolutionary stages than either method used in isolation. We demonstrate that the position of a star on the HRD or color–magnitude diagram can be combined with its rotation period to infer a precise age via both isochrone fitting and gyrochronology simultaneously. We show that incorporating rotation periods with 5% uncertainties into stellar evolution models improves age precision for FGK stars on the main sequence and can, on average, provide age estimates up to three times more precise than isochrone fitting alone. In addition, we provide a new gyrochronology relation, calibrated to the Praesepe cluster and the Sun, that includes a variance model to capture the rotational behavior of stars whose rotation periods do not lengthen with the square root of time and parts of the HRD where gyrochronology has not been calibrated. This publication is accompanied by an open-source Python package (stardate) for inferring the ages of main-sequence and subgiant FGKM stars from rotation periods, spectroscopic parameters, and/or apparent magnitudes and parallaxes.
Highlights
Age is the most difficult stellar property to measure, and the difficulty of age-dating is acute for low mass (GKM) stars on the main sequence (MS)
We present a statistical framework for measuring precise ages of MS stars and subgiants by combining observables that relate, via different evolutionary processes, to stellar age
We tested stardate on simulated data and cluster stars and demonstrated that combining gyrochronology with isochrone fitting improves the precision of age estimates for FGK dwarfs by a factor of 3 over isochrone fitting alone, assuming 5% measurement uncertainties on rotation periods
Summary
Age is the most difficult stellar property to measure, and the difficulty of age-dating is acute for low mass (GKM) stars on the main sequence (MS). GKM dwarfs are difficult to age-date because most of their physical and observable properties do not change rapidly. This is represented in the spacing of isochrones on a Hertzsprung-Russell diagram (HRD) or color-magnitude diagram (CMD). On the MS, isochrones are tightly spaced and, even with very precise measurements of effective temperature and luminosity, the position of a MS star on the HRD may be consistent with range of isochrones spanning several billion years (see Soderblom 2010, for a review of stellar ages). The classical method for measuring stellar ages is isochrone placement, or isochrone fitting, where surface gravity changes resulting from fusion in the core (usually observed via luminosity, L, and effective temperature, Teff, or absolute magnitude and colour) are compared with a set of models that trace stellar evolution across the HRD, or CMD. The method of inferring a star’s age from its rotation period, called ‘gyrochronology’, is much better suited for measuring ages on the MS because MS stars spin down relatively rapidly
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