Norman R. Simon

A Plea For Reexamining Heavy Element Opacities In Stars

It is shown that increasing the opacity due to heavy elements by a factor of 2-3 leads to classical Cepheid models which reproduce observed period ratios at evolutionary masses and luminosities. Thus the mass anomalies are removed in both the double-mode and bump Cepheid regimes. The proposed increases may also serve to energize {3 Cephei variables, thus solving yet another important problem in the theory of pulsating stars. We argue that opacity changes of this order are not implausible and urge further work in this important area.

A MEG study of sleep.

A 64-channel, whole cortex magnetoencephalographic system was employed to obtain sleep data from three healthy subjects. Based upon visual inspection of the signals and the corresponding power spectra, we were able to discern a number of features characterizing the evolution of sleep. These included: (1) the transition from records dominated by the alpha rhythm to records in which alpha is attenuated and slower waves increase; (2) the appearance of sleep spindles, particularly in the parietal channels; and, perhaps most interesting, (3) a slow wave phase whose multichannel spectral signature is a broad rounded maximum in the frequency region around 0.5 Hz. Topographical features of the sleep record were also studied. In two of our subjects, rough lateral symmetry was apparent. As their sleep deepened, the distribution of signal power over the head changed such that the maximum moved in the forward and lateral directions, with parietal and temporal signals strengthening relative to the occipital. The records of the third subject showed a tendency toward right dominance, while topographic changes with sleep depth were minimal. Only one of the subjects was able to sustain the deep, slow-wave stage. Here, characteristic multi-detector outbursts appeared, lasting between 150 and 500 ms. During these intervals, widespread topographic patterns were sustained over the head (often with striking dipolar or quadrupolar forms), while crude source modeling yielded two persisting dipoles, laterally paired. Thus, these outbursts seem to represent large-scale, quasi-static configurations of brain activity perhaps related to the K-complexes, which occur earlier in sleep. Finally, we compare our results with those of previous investigators, including work on human electroencephalographic data and research reported by Steriade et al. from animal studies.

Evidence favoring nonevolutionary cepheid masses

It is suggested that the bumps on the velocity curves of nonlinear Cepheid models may be understood as the consequence of a resonance between the fundamental and the second overtone modes of the pulsating star. Thus the presence of bumps can be inferred from linear calculations. The region of the observational instability strip containing bumps is compared with the location of the calculated resonances. As was the case for the nonlinear calculations, it is found that masses considerably less than evolutionary values are required to bring agreement between the observations and the theory. (AIP)

A provisional RR lyrae distance scale

Hydrodynamic pulsation models are matched with observations of globular cluster RRc stars to derive masses, luminosities and temperatures as functions of two observables: pulsation period, P1, and Fourier phase parameter, phi sub 31. We find that mean RRc masses and luminosities increase, and mean temperatures fall, with decreasing cluster metallicity. The Oosterhoff dichotomy is explained as mainly a temperature effect, while hints of a new dichotomy (in RRc mass and luminosity) are found among certain Oo II clusters. A provisional RR Lyrae distance scale emerges for the RRc stars in the form of a relation between luminosity and the two observables, P1 and phi sub 31. Finally, the scheme we propose passes three independent tests: (1) it reproduces the observed hierarchy of relative luminosity among a large sample of RRc stars in Omega Centauri; (2) it yields mean RRc masses which are fully consistent with the RRd masses derived for the clusters M68 and M15; and (3) it gives an LMC distance modulus in agreement with that obtained by other methods.

An explanation for the blue sequence of variable stars.

Variable stars blue sequence, proposing mu mechanism and beta mechanism as explanation for radial pulsations

ON CEPHEIDS AT MAXIMUM AND MINIMUM LIGHT

It has been known for many decades that the spectral type of Cepheids at light maximum is constant with period. We use hydrodynamic pulsation models to explain this result in terms of the outward reach of the hydrogen ionization front. On the other hand, we show that, at minimum light, the Cepheid photospheric temperature is mainly a function of amplitude. A number of observed Cepheids have published temperatures that seem too hot at both maximum and minimum. We attribute this to an overestimation of the reddening for these stars. A list is given.

Modal selection in pulsating stars

Initial-value and periodic nonlinear pulsation integrations are carried out for a series of double-mode Cepheid models (P/sub 1//P/sub 0/approx.0.7) in the vicinity of the resonance ..omega../sub 1/+..omega../sub 0/=..omega../sub 3/. None of the models tested shows persistent double-mode behavior. A new, semiquantitative description of modal selection, based upon the iterative theory of Simon, is introduced to analyze the nonlinear results. In the simplest version of this description, modal section categories emerge which are identical to those of Stellingwerf. Our hydrodynamic results also agree at least partially with Stellingwerfs in that we find a region in the red where the models approach (though never reach) simultaneous instability of both limit cycles. Analysis of resonant effects on modal selection in our calculations leads to the conclusion that models lying between the resonances ..omega../sub 1/+..omega../sub 0/=..omega../sub 3/ and P/sub 2//P/sub 0/=0.5 may yet be viable candidates for double-mode pulsation.

THE RR LYRAE VARIABLES IN THE GLOBULAR CLUSTER M68

New observations, made with the Helen Sawyer Hogg telescope at Las Campanas, have been analyzed in a search for double-mode pulsators (RRd stars) in the metal-poor globular cluster, Messier 68. Of the 30 stars studied, nine have been identified as RRd stars; V33, which exhibited the characteristics of an RRd star in 1950, now appears to be an RRc star. Reliable periods and period ratios have been determined for six of the RRd stars. Masses for these RRd stars, calculated from fitting formulas given by Kovacs et al. (1991), range from 0.75 to 0.90 solar mass, depending on the assumed luminosity and metal abundance. These masses are in the same range as those for the RRd stars in M 15, whose RRd sample resembles that of M68 very closely. Fourier parameters determined for the light curves of the M68 variables show that the RRc stars in the two clusters are also very similar. In particular, on the plot of phase parameter phi sub 31 with period, the M15 and M68 RRc samples are virtually indistinguishable. A comparison of the new M68 observations with observations made 40 yr ago shows that the periods of some of the stars have changed, but the 40 yr interval is too short for detecting period changes caused by evolutionary effects.

CLASSICAL CEPHEID LIGHT CURVES REVISITED.

Fourier decompositions are made of the B, V, R, and I classical Cepheid observations of Moffett and Barnes. The Fourier diagrams for V agree very well with the corresponding plots of Simon and Lee. The B, R, and I diagrams resemble the plots for V, but with small, systematic shifts in the phases 21 and 31. The 2.5-day star, DT Cyg, is confirmed as a likely overtone pulsator by its anomalous position in the Fourier plots. Finally, the authors study the Fourier phase quantity 41, and introduce phase-phase diagrams to look at the Hertzsprung progression. Discontinuities in these diagrams support the idea that the long-period and short-period Cepheids may reach their limit cycles in different ways.

Comparative pulsation calculations with OP and OPAL opacities

Comparative linear nonadiabatic pulsation calculations are presented using the OPAL and Opacity Project opacities. The two sets of opacities include effects due to intermediate coupling and fine structure as well as new abundances. We used two mass luminosity (M-L) relations, one standard (BIT), and one employing substantial convective core overshoot (COV). The two sets of opacities cannot be differentiated on the basis of the stellar pulsation calculations presented here. The BIT relation can model the beat and bump Cepheids with masses between 4 and 7 solar mass, while if the overshoot relation is used, masses between 2 and 6 solar mass are required. In the RR Lyrae regime, we find the inferred masses of globular cluster RRd stars to be little influenced by the choice of OPAL or OP. Finally, the limited modeling we have done is not able to constrain the Cepheid M-L relation based upon period ratios observed in the beat and bump stars.