Ana M. Larson

A CA II LAMBDA-8662 INDEX OF CHROMOSPHERIC ACTIVITY: THE CASE OF 61 CYGNI A

The stellar spectra obtained in our precise velocity program include the Ca II infrared triplet line at lambda-8662. Changes in the core flux of this line, quantified with our Delta-EW8662 index, reflect changes in stellar chromospheric activity. In our 12 years of Ca II observations for 61 Cygni A, we detect a solar-type cycle of 7.22 years and a rotational period of 36.21 days which confirm earlier results based on the H and K lines. The stability of the derived rotation period over 12 years suggests that the regions of chromospheric activity are either long-lived or formed within a limited-longitudal range.

A LOW-AMPLITUDE PERIODICITY IN THE RADIAL VELOCITY AND CHROMOSPHERIC EMISSION OF BETA GEMINORUM

Using high-quality spectra obtained with the hydrogren fluoride technique at the Canada-France-Hawaii 3.6-meter telescope and Dominion Astrophysical Observatory 1.22-meter telescope, we have detected a low-amplitude periodicity in both the radial velocity and chromospheric emission of Beta Geminorum. In addition, we have detected a long-term change (time scale > 12 years) in thi stars chromospheric emission which is reminiscent of solar-type magnetic cycles believed present in giants of similar spectral type. The radial-velocity variations can be fitted by a sinusoid with amplitude and period of K = 46.23 ± 3.9 m s-1, P=584.65 ± 3.3 days. A similar, statistically significant (false-alarm probability of 1%) period is found in the residuals from the long-term trend in the chromospheric emission: P = 587.7 ± 12 days. However, because of the weakness of the signal, K = 0.583 ± 19 mA, this detection needs confirmation. Although these independently determined periods are in agreement with each other, they are not consistent with the maximum rotation period of 178 days derived from this stars v sin i = 2.5 km s-1 (Gray 1982). Either the observed v sin i value of Beta Geminorum needs to be revised below 0.76 km s-1 or some alternative to rotational modulation of surface phenomena must be found to explain this stars periodicity.

The Luminosity Function of M30: Evidence for Rapidly Rotating Cores in the Cluster Giants?

The observed luminosity function (LF) of M30 shows an excess in the number of red giants relative to the number of turnoff stars as compared with the predictions of canonical models. This well-known problem is reexamined in the light of new stellar evolutionary models that have been constructed using an improved equation of state formulation as well as recent opacities and nuclear reaction rates. Neither the improvements to the basic physics of stars nor the adoption of different choices, within reason, for the cluster distance (and hence age), chemical composition, or initial mass function have any appreciable effect on the LF discrepancy. However, it seems to be possible to reconcile theory with observation if the cluster stars retain significant amounts of angular momentum throughout their evolution. Stellar models, which treat rotation in the simple spherically symmetric approximation and which conserve angular momentum—leading to structures on the giant branch with rapidly rotating cores and slowly rotating envelopes—predict LFs in good agreement with that observed for M30. The amount of angular momentum that is required to achieve this consistency appears to be insufficient to have detectable consequences for the tightness of the observed color-magnitude diagram (though this needs to be checked using a two-dimensional code) or to affect predicted age versus turnoff-luminosity relations at more than the few percent level.

The Puzzling K and Early-M Giants: A Summary of Precise Radial Velocity Results for 15 Stars

Nearly a decade ago, “yellow giants” were introduced as a new class of low-amplitude radial-velocity variable stars. In this report we discuss new results for 12 spectral type K and early-M giants based on long-term monitoring using both the hydrogen-fluoride and iodine-cell techniques. We compare these results with those of published data for 3 additional stars ( ϒ Cephei, β Geminorum, and β Ophiuchi), and discuss possible implications for the underlying physical mechanism(s).