Siobahn M. Morgan

[Fe/H] relations for c‐type RR Lyrae variables based upon Fourier coefficients

[Fe /H]-Φ31-P relations are found for c-type RR Lyrae stars in globular clusters. The relations are analogous to that found by Jurcsik & Kovacs for field ab-type RR Lyrae stars, where a longer period correlates with lower metallicity values for similar values of the Fourier coefficient Φ 31 . The relations obtained here are used to determine the metallicity of field c-type RR Lyrae stars, those within co Cen, the Large Magellanic Cloud and toward the galactic bulge. The results are found to compare favourably to metallicity values obtained elsewhere.

Beat cepheid period ratios from opal opacities.

The discovery of a large number of beat Cepheids in the Large Magellanic Cloud in the MACHO survey, provides an opportunity to compare the characteristics of such Cepheids over a range of metallicities. We produced a large grid of linear nonadiabatic pulsation models using the OPAL opacity tables and with compositions corresponding to those of the Milky Way, and the Large and Small Magellanic Clouds. Using the relationship between the period ratio and the main pulsation period, we are able to define a range of models which correspond to the observed beat Cepheids, and thereby constrain the physical characteristics of the LMC beat Cepheids. We are also able to make some predictions about the nature of the yet-to-be-discovered SMC beat Cepheids.

WWW Database of Variable Star Fourier Coefficients

Fourier coefficients are a valuable tool in the study of a wide variety of pulsating stars. In the case of RR Lyrae stars they, can be used to estimate various physical parameters, such as mass, luminosity, metallicity, and effective temperature. They are frequently used to discriminate between different pulsation modes for both RR Lyrae stars and Cepheids. Fourier coefficients have been published for stars based on many different photometric systems as well as for radial velocity measurements. With the release of coefficients from large-scale surveys and the availability of these data on the Internet, the number of Fourier coefficients available for study has significantly increased, and it is difficult to obtain all available data for individual stars or a given subset of stars. To assist researchers in obtaining and making use of Fourier coefficients, an on-line archive of published values of Fourier coefficients has been established. Users can search the database using a variety of tools, and the data sets extend beyond the Milky Way to include extragalactic variables. The archive is located at the Web site http://nitro9.earth.uni.edu/fourier/.

Masses for Galactic Beat Cepheids

Accurate mass determinations for Cepheids may be used to determine the degree of excess mixing in the interiors of their main-sequence progenitors: the larger the excess mixing, the larger the luminosity of the Cepheid of a given mass, or the smaller the mass of a Cepheid with given luminosity. Dynamical masses determined recently for a few Cepheid binaries indicate excess mixing somewhat stronger than that corresponding to the convective overshoot models by Schaller et al. Beat Cepheids can be used similarly to test main-sequence mixing in stellar interiors. The period ratios for beat Cepheids depend on luminosity, Teff, heavy element abundance, and mass. By comparing pulsational models and the observationally derived luminosity, Teff, metallicities, and period ratios it is possible to obtain masses for these stars, the so-called beat masses. With the old opacities masses much smaller than the evolutionary masses were obtained. With the new OPAL opacities a beat mass close to the dynamical mass was obtained for the binary beat Cepheid Y Carinae, showing that it is now possible to obtain reliable beat masses. In this paper, we determine beat masses for seven Galactic beat Cepheids for which photometric and spectroscopic data are available. We find an average mass around 4.2 ± 0.3 M⊙ for these stars, though the actual error limits for each star may be larger mainly because of uncertainties in E(B-V) and the heavy element abundances. (As derived spectroscopically, beat Cepheids are in general metal-poor, with -0.4 [Fe/H] 0.0). The relation between the derived beat masses and the luminosities again indicates excess mixing that is somewhat larger than that corresponding to the models by Schaller et al.

Pulsars as spiral arm tracers

Pulsars are used to trace the spiral arms of the Galaxy. Present pulsar characteristics are used to divide them into two pulsar types as defined by Huang (1987). The ages of Type II pulsars and the galactic rotation curve are used to determine their position at birth. The spiral arm pattern velocity is used to reconstruct the arms from which the individual Type II pulsars originated. It is found that the best fit to the data requires spiral arm segments with a pitch angle of 14.5 deg.

Theoretical Pulsations of Luminous Blue Variables

Both radial and low degree and order g -mode nonradial pulsations are predicted for luminous blue variables that occur in the blue supergiant region of the Hertzsprung-Russell diagram. It is found that the radial strange modes have very large growth rates due to helium ionization in models at surface effective temperatures between 10,000 and 20,000 K.

Effects of SN 1987A on the interstellar medium around it

It is shown that the interstellar spectrum from SN 1987A in the LMC shows a rather strong component in C IV and Si IV, and possibly a weak component in N V, at a velocity of about 300 + or - 10 Km/s. In the spectra of the neighboring stars HD38268 and R123, a component at this velocity is only seen in low-ionization lines, not in the C IV, or N V lines. The possibility is studied that SN 1987A ionized a measurable fraction of this 300 Km/s cloud within less than one day since the Si IV, C IV, and possibly, N V interstellar lines are seen only about one day after the first light from the supernova explosion was detected. This may be the case, but only if the supernova temperature at outburst was larger than 50,000 K, depending on the density assumed for the interstellar cloud. Also, H and He must have been preionized once. 12 references.