Donald Gerard Luttermoser

INFRARED LIGHT CURVES OF MIRA VARIABLE STARS FROM COBE DIRBE DATA

We have used the COBE DIRBE database to derive near- and mid-infrared light curves for a well-defined sample of 38 infrared-bright Mira variable stars and compared with optical data from the AAVSO. In general the 3.5 and 4.9 lm DIRBE bandpasses provide the light curves with the best signal-to-noise ratio (S/N), with S/N decreasing with wavelength at longer wavelengths. At 25 lm good light curves are only available for � 10% of our stars, and at wavelengths � 60 lm extracting high quality light curves is not possible. The amplitude of variability is typically less in the near-infrared than in the optical and less in the mid-infrared than in the near-infrared, with decreasing amplitude with increasing wavelength. On average there are 0.20 � 0.01 mag variation at 1.25 lm and 0.14 � 0.01 mag variation at 4.9 lm for each magnitude variation in V. The observed amplitudes are consistent with results of recent theoretical models of circumstellar dust shells around Mira variables. For a few stars in our sample we find clear evidence of time lags between the optical and near-infrared maxima of phase � 0.05–0.13, with no lags in the minima. For three stars mid-infrared maximum appears to occur slightly before that in the near-infrared, but after optical maximum. We find three examples of secondary maxima in the rising portions of the DIRBE light curves, all of which have optical counterparts in the AAVSO data, supporting the hypothesis that they are due to shocks rather than newly formed dust layers. We find no conclusive evidence for rapid (hours to days) variations in the infrared brightnesses of these stars.

PHASE-DEPENDENT SPECTROSCOPY OF MIRA VARIABLE STARS

Spectroscopic measurements of Mira variable stars as a function of phase probe the stellar atmospheres and underlying pulsation mechanisms. For example, measuring variations in TiO, VO, and ZrO with phase can be used to help determine whether these molecular species are produced in an extended region above the layers where Balmer line emission occurs or below this shocked region. Using the same methods, the Balmer line increment, where the strongest Balmer line at phase zero is Hδ and not Hα, can be measured and explanations tested, along with another peculiarity, the absence of the H line in the spectra of Mira variables when the other Balmer lines are strong. We present new spectra covering the spectral range from 6200 to 9000 A of 20 Mira variables. A relationship between variations in the Ca II IR triplet and Hα as a function of phase support the hypothesis that Hs observational characteristics result from an interaction of H photons with the Ca II H line. New periods and epochs of variability are also presented for each star.

Radio emissions from substellar companions of evolved cool stars

A number of substellar companions to evolved cool stars have now been reported. Cool giants are distinct from their progenitor main-sequence low-mass stars in a number of ways. First, the mass loss rates of cool giant stars are orders of magnitude greater than for the late-type main-sequence stars. Secondly, on the cool side of the Linsky–Haisch ‘dividing line’, K and M giant stars are not X-ray sources, although they do show evidence for chromospheres. As a result, cool star winds are largely neutral for those spectral types, suggesting that planetary or brown dwarf magnetospheres will not be effective in standing off the stellar wind. In this case, one expects the formation of a bow shock morphology at the companion, deep inside its magnetosphere. We explore radio emissions from substellar companions to giant stars using (a) the radiometric Bode’s law and (b) a model for a bow shock morphology. Stars that are X-ray emitters likely have fully ionized winds, and the radio emission can be at the milliJansky level in favourable conditions. Non-coronal giant stars produce only micro-Jansky level emissions when adjusted for low-level ionizations. If the largely neutral flow penetrates the magnetosphere, a bow shock results that can be strong enough to ionize hydrogen. The incoherent cyclotron emission is sub-micro-Jansky. However, the long wavelength radio emission of Solar system objects is dominated by the cyclotron maser instability (CMI) mechanism. Our study leads to the following two observational prospects. First, for coronal giant stars that have ionized winds, application of the radiometic Bode’s law indicates that long wavelength emission from substellar companions to giant stars may be detectable or nearly detectable with existing facilities. Secondly, for the non-coronal giant stars that have neutral winds, the resultant bow shock may act as a ‘feeder’ of electrons that is well embedded in the companion’s magnetosphere. Incoherent cyclotron emissions are far too faint to be detectable, even with next generation facilities; however, much brighter flux densities may be achievable when CMI is considered.

Vanadium Oxide in the Spectra of Mira Variables

As a preliminary step in deducing Teff and log(g) of Mira variables as a function of phase, a comparison is made between spectra synthesized from LTE stellar atmosphere models and observed spectra. The observed spectra show obvious vanadium oxide (VO) absorption bands. However, the molecular line list used to produce the synthetic spectra does not include the bound-bound VO opacities. The wavenumbers, line oscillator strengths, and lowest energy levels are needed to calculate these opacities. The equations, constants, and experimentally determined factors required to calculate the line oscillator strengths and lowest energy levels from experimentally determined wavenumbers are presented. The effect of including the wavenumbers, line oscillator strengths, and lowest energy levels of the VO B-X (0, 0) band are calculated and show the expected absorption features observed in the spectra of Mira variables. In the VO B-X (0, 0) band the line oscillator strengths range from about 0.05 to 3.