An attempt is made to formulate the relationship between age and chromospheric emission (CE) in late-type dwarf stars. Evidence is reviewed that a deterministic relationship of this type actually ...exists, and that for stars of known age, either a power-law relation or a curve corresponding to a constant star formation rate fits equally well. Further observations should be able to demonstrate either that there is a real excess of young stars near the sun or that the evolution of CE for a low-mass star goes through a slow initial decline, a rapid decline at intermediate ages, and finally a slow decline for old stars like the sun. 100 refs.
IAU Commission 29 - Stellar Spectra has been one of the IAU commissions from the onset, until its dissolution at the most recent IAU General Assembly in Honolulu in 2015. This commission belonged to ...IAU Division G (“Stars and Stellar Physics”), the latter committed with fostering research in stellar astrophysics. Within the general field of stellar astrophysics, stellar spectroscopy plays a key role, as stellar spectra are a powerful tool providing a view into the detailed physical properties of stars and the physical processes occuring within them.
High-resolution echelle spectra of more than 100 F, G, and K dwarfs in the Pleiades are reported. Chromospheric activity in these stars is measured via comparisons of the profiles of H-alpha and the ...Ca II IR triplet to chromospherically inactive field stars. Consistent dereddened colors are determined from the available photometry and temperatures are derived. Most G and K dwarfs in the Pleiades rotate slowly, but about 20 percent of the stars are ultrafast rotators (UFRs). That fraction of UFRs is independent of color, and the highest rotation rates are found among the K dwarfs. The Pleiades exhibit a broad range in the strength of chromospheric emission at any one color. Most G and K dwarfs in the Pleiades show H-alpha and the IR triple in absorption, with filling in of the line cores.
We use high signal-to-noise ratio (~150-450), high-resolution (R ~ 45,000) Keck HIRES spectroscopy of 13 candidate post-T Tauri stars (PTTs) to derive basic physical parameters, lithium abundances, ...and radial velocities. We place our stars in the Mv-Teff plane for use in determining approximate ages from pre-main-sequence isochrones, and we confirm these using three relative age indicators in our analysis: Li abundances, chromospheric emission, and the kinematic U-V plane. Using the three age criteria we identify five stars (HIP 54529, HIP 62758, HIP 63322, HIP 74045, and HIP 104864) as probable PTTs with ages between 10 and 100 Myr. We confirm HIP 54529 as an SB2 star and HIP 63322 as an SB1 star. We also examine irregular photometric variability of PTTs using the Hipparcos photometry annex. Two of our PTTs exhibit near-IR excesses compared to Kurucz model flux; while recent work suggests classical T Tauri stars evince similar JHK excesses presumably indicative of nonphotospheric (disk) emission, our results may be illusory artifacts of the chosen I-band normalization. The near-IR excesses we see in a literature-based sample of PTTs appear to be artifacts of previous spectral type-based Teff values. Indeed, comparison of the homology of their observed and model photospheric spectral energy distributions suggests that photometric temperatures are more reliable than temperatures based on spectral standards for the cooler temperature ranges of the stars in this sample. We conclude that our age-oriented analysis is a robust means to select samples of nearby, young, isolated PTTs that otherwise masquerade as normal field stars.
We consider a survey of rotation among F, G, and K dwarfs of the Pleiades in the context of other young clusters (Alpha Persei and the Hyades) and pre-main-sequence (PMS) stars (in Taurus-Auriga and ...Orion) in order to examine how the angular momentum of a star like the sun evolves during its early life on the main sequence. The rotation of PMS stars can be evolved into distributions like those seen in the young clusters if there is only modest, rotation-independent angular momentum loss prior to the ZAMS. Even then, the ultrafast rotators (UFRs, or ZAMS G and K dwarfs with v sin i equal to or greater than 30 km/s) must owe their extra angular momentum to their conditions of formation and to different angular momentum loss rates above a threshold velocity, for it is unlikely that these stars had angular momentum added as they neared the ZAMS, nor can a spread in ages within a cluster account for the range of rotation seen. Only a fraction of solar-type stars are thus capable of becoming UFRs, and it is not a phase that all stars experience. Simple scaling relations (like the Skumanich relation) applied to the observed surface rotation rates of young solar-type stars cannot reproduce the way in which the Pleiades evolve into the Hyades. We argue that invoking internal differential rotation in these ZAMS stars can explain several aspects of the observations and thus can provide a consistent picture of ZAMS angular momentum evolution.
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