Function Of Stellar Temperature Research Articles

The California Legacy Survey. V. Chromospheric Activity Cycles in Main-sequence Stars

We present optical spectroscopy of 710 solar neighborhood stars collected over 20 years to catalog chromospheric activity and search for stellar activity cycles. The California Legacy Survey stars are amenable to exoplanet detection using precise radial velocities, and we present their Ca ii H and K time series as a proxy for stellar and chromospheric activity. Using the High Resolution Echelle Spectrometer at Keck Observatory, we measured stellar flux in the cores of the Ca ii H and K lines to determine S-values on the Mount Wilson scale and the log(RHK′) metric, which is comparable across a wide range of spectral types. From the 710 stars, with 52,372 observations, 285 stars were sufficiently sampled to search for stellar activity cycles with periods of 2–25 yr, and 138 stars showed stellar cycles of varying length and amplitude. S-values can be used to mitigate stellar activity in the detection and characterization of exoplanets. We used them to probe stellar dynamos and to place the Sun's magnetic activity into context among solar neighborhood stars. Using precise stellar parameters and time-averaged activity measurements, we found tightly constrained cycle periods as a function of stellar temperature between log(RHK′) of −4.7 and −4.9, a range of activity in which nearly every star has a periodic cycle. These observations present the largest sample of spectroscopically determined stellar activity cycles to date.

Planetary entropy production as a thermodynamic constraint for exoplanet habitability

ABSTRACT Any biosphere emerges, lives, and grows producing entropy. Entropy production is a thermodynamic function crucial in the framework of non-equilibrium thermodynamics as it is directly related to the dynamical behaviour of far-from equilibrium systems. The extent of entropy production is proportional to the ability of such systems to dissipate free energy and thus to ‘live’, to evolve, to grow in complexity. Generally, a certain threshold of entropy production must be exceeded for the emergence of complex self-organizing structures. Thus, the entropy production can be considered as the thermodynamic thrust that drives life emergence and evolution. In this perspective, we propose that the value of the planetary entropy production (PEP) can provide a first order estimate of the thermodynamic potential of planetary environment to sustain a complex biosphere. Here we use a simplified approach to evaluate the upper limit to the PEP and to the corresponding free energy as function of stellar temperature and orbital parameters of the planet. We found that only Earth-like planets in the circumstellar habitable zone (CHZ) of G and F stars can have a PEP value higher than the Earth value. Further significant thermodynamic differences exist between the inner and outer edge of the CHZ, with the inner edge being thermodynamically more advantageous for the development of complex biospheres. Interestingly, among the recently proposed habitable exoplanets, the ones belonging to the Hycean planets appear the thermodynamically best candidates.

Neutron capture cross sections ofOs186,Os187, andOs189for the Re-Os chronology

Discrete as well as continuum $\ensuremath{\gamma}$-ray energy spectra from the neutron capture by $^{186}\mathrm{Os}$, $^{187}\mathrm{Os}$, and $^{189}\mathrm{Os}$ have been taken for the first time at $5\ensuremath{\leqslant}{E}_{n}\ensuremath{\leqslant}90$ keV by an anti-Compton NaI(Tl) spectrometer. The detection of a weak discrete \ensuremath{\gamma}-ray, about 0.5% of total \ensuremath{\gamma}-ray strength, demonstrates the high sensitivity of the present measurement. The energy spectra enabled us to accurately determine the reaction cross sections with a small systematic uncertainty. Based on the new cross sections, we reestimate on the basis of a careful reaction cross section calculation the correction factor ${F}_{\ensuremath{\sigma}}$ for the neutron capture on the 9.75-keV first excited state in $^{187}\mathrm{Os}$ as a function of stellar temperature, as required to derive the age of the galaxy within the Re-Os chronology.

Hγ and Hδ Absorption Features in Stars and Stellar Populations

The Hγ and Hδ absorption features are measured in a sample of 455 (out of an original 460) Lick/IDS stars with pseudo-equivalent width indices. For each Balmer feature, two definitions, involving a narrow (~20 A) and a wide (~40 A) central bandpass, are measured. These four new Balmer indices augment 21 indices previously determined by Worthey et al., and polynomial fitting functions that give index strengths as a function of stellar temperature, gravity, and [Fe/H] are provided. The new indices are folded into models for the integrated light of stellar populations, and predictions are given for single-burst stellar populations of a variety of ages and metallicities. Contrary to our initial hopes, the indices cannot break a degeneracy between burst age and burst strength in post-starburst objects, but they are successful mean-age indicators when used with sensitive metallicity indicators. An appendix gives data, advice, and examples of how to transform new spectra to the 25-index Lick/IDS system.

Terminating the Proton-Proton Chain at High Densities.

view Abstract Citations (14) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Terminating the Proton-Proton Chain at High Densities. Bahcall, John N. ; Wolf, Richard A. Abstract The rate of the He3(e , ) reaction is calculated as a function of stellar temperature and den. using the modern theory of nuclear beta-decay. Three tritium-burning sequences of nuclear reactions also studied. Some possible applications are discussed briefly. Publication: The Astrophysical Journal Pub Date: February 1964 DOI: 10.1086/147790 Bibcode: 1964ApJ...139..622B full text sources ADS |