J/A+A/699/A251ivo://CDS.VizieR/J/A+A/699/A251doi:10.26093/cds/vizier/bc-nv/lcfbCDSYang H.Liu J.Soria R.Spada F.Wang S.Fang X.Li X.2026-08-03T06:55:40Z2025-07-10T07:36:24ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frstellar-activitystellar-agesvisible-astronomyGyrochronology and the rotation-activity relationship are standard techniques to determine the evolution phase and dynamo process of low-mass stars, based on their slowing down. Gyrochronology identifies two tracks in color-period diagrams: the convective (younger, faster rotating) and the interface (older, slower rotating) phases, separated by a transition ('gap') without a precise estimate of its duration. Instead, the rotation-activity relation identifies two stages: the saturated (faster, with higher activity) and unsaturated (slower, with declining activity) regimes. The mismatch in the definition of the evolutionary phases has so far raised many issues in physics and mathematics and hampered the understanding of how the internal dynamo processes affect the observable properties. To address this problem, we seek a unified scheme that shows a one-to-one mapping from gyrochronology to the rotation-activity relationship. We combine LAMOST spectra, the Kepler mission and two open clusters to obtain the chromospheric activity R'HK of 6846 stars and their rotation periods. We use R'HK and rotation period to investigate the rotation-activity relationship. Instead of the traditional two-interval model, we apply a three-interval model to fit the rotation-activity relationship in the range of the Rossby number Ro<0.7. We also use the X-ray data to verify our new model. We find that the rotation-activity relationship is best fitted by three intervals in the rang of Ro<0.7. We associate those intervals to the convective, gap and interface phases of gyrochronology. The mean Ro of the C-to-g and g-to-I transition is ~0.022 and ~0.15, respectively. The g-to-I transition is on the edge of the intermediate-period gap, indicating that the transition of surface brightness from spot-dominated to the facula-dominated can be associated with the transition from gap to I sequence. Furthermore, based on previous studies, we suggest an additional epoch at late times of the I phase (Ro>0.7; weakened magnetic breaking phase) from the perspective of activity. We further use the three-interval models to fit the period-activity relationship in temperature bins and determine the duration of the transition phase as a function of effective temperature. By comparing the critical temperature and period of the g-to-I transition with the slowly rotating sequence of 10 young open clusters whose ages range from 1Myr to 2.5Gyr, we conclude that our new model finds the pure I sequence without fast rotating outliers, which defines the zero-age I sequence (ZAIS). We propose that there is an ambiguous consensus on when the I sequence starts to work. This ambiguity is from the visually convergent sequence of the color-period diagrams in open clusters. This visually convergent sequence is younger than the ZAIS and is actually the pre-I sequence that can be associated with the stall of the spin-down. Our results unify the rotation-activity relationship and gyrochronology for the stellar evolution of low-mass stars, for which we coined the "CgIW" scenario.2025A&A...699A.251Yhttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/699/A251CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicerelated-toV/133 : Kepler Input Catalog (Kepler Mission Team, 2009)V/156 : LAMOST DR7 catalogs (Luo+, 2019)I/355 : Gaia DR3 Part 1. 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Should Not be used for identification.meta.recordintGaiaDR3Gaia DR3 IDmeta.id;meta.mainKIC? Kepler input catalog IDmeta.idintnullableRAJ2000Right ascension (J2000)degpos.eq.ra;meta.mainDEJ2000Declination (J2000)degpos.eq.dec;meta.mainSpTypeSpectrum type given by the LAMOST pipelinemeta.note;src.morph.typecharTeffEffective temperatureKphys.temperature.effectiveintloggSurface gravitylog(cm.s**-2)phys.gravityfloat[Fe/H]Metallicityphys.abund.ZfloatSindexChromospheric activity in terms of the Mount Wilson Observatory scalespect.indexfloate_Sindexrsm uncertainty of S-indexstat.errorfloatlog(R'HK)Normalized chromospheric activityphot.flux;arith.ratiofloate_log(R'HK)rms uncertainty of log(R'HK)stat.errorfloatProtRotation perioddtime.periodfloattaugGlobal convective turnover timedtime.epochfloate_taugrms uncertainty of the global convective turnover timedstat.errorfloatB-VColor index B-Vmagphot.color;em.opt.B;em.opt.VfloatLbol? Bolometric luminositysolLumphys.luminositydoublenullablePhaseEvolutionary phase (MS or post-MS)time.agecharClusterField star (fs) or open clustermeta.codecharSimbadSimbad column added by the CDSmeta.ref.urlchar