BETA CEPHEI PULSATION ANOMALIES: POTENTIAL NEW WINDOWS into the INSTABILITIES and EVOLUTION of EARLY B STARS

B.A. Goldberg and R.J. Bambery, Jet Propulsion Laboratory/Caltech
R.S. Polidan, NASA Goddard Spaceflight Center
R.A. Crowe and J.T. Gathright, University of Hawaii at Hilo
G.C.L. Aikman and G.J. Odgers, Dominion Astrophysical Observatory

Abstract

We have obtained Voyager Ultraviolet Spectrometer (UVS) measurements of well-known Beta Cephei stars, which now total more than 1500 hours (> 300 pulsation cycles!) and which constitute the most comprehensive coherent data set that can address fundamental pulsation properties of a significant cross-section of the group. The extended measurement sequences for individual stars, which cover many successive pulsation cycles at wavelengths where pulsation amplitudes reach a maximum, can provide far more comprehensive tests of pulsation stability than any ground-based data. The focus of our analysis is on: (1) the determination of the origin and significance of a far-UV (FUV) light-curve ``instability anomaly'' discovered in pre-1990 Voyager UVS data; (2) an extension of the search for secular changes in key pulsation parameters in individual stars; (3) the search for additional evidence of pulsation anomalies in extended measurement sequences; and (4) the discrimination of radial from non-radial pulsation modes using temperature information derived from UV flux measurements. The first of these studies is being supported by a ground-based data set which we acquired during 1990-91 that contains more than 100 hours of simultaneous and near-simultaneous high-resolution spectroscopic observations, as well as UBV photometric observations. Analysis has been initiated at NASA's Goddard Spaceflight Center (NASA/GSFC), the University of Hawaii at Hilo (UH Hilo), and the Dominion Astrophysical Observatory (DAO). This paper constitutes a progress report.

Scientific Objectives

Three classes of problems are to be addressed: (1) those of fundamental importance which have eluded investigators for many decades; (2) those of unknown significance which have recently been identified because of the availability of new types of data; (3) those which require long-term analysis and represent a logical continuation of work already in progress. The scientific objectives which can be addressed directly with the available data are listed below, followed by the more general program goals.

(1) Determine the origin and significance of the ``instability anomaly'' present in the FUV light curves.

(2) Determine cycle-to-cycle pulsational stability from Voyager UVS data and identify any additional ``pulsation anomalies'' that may be present.

(3) Determine the role of atmospheric shock waves. Investigate the possible relationships of shock waves to the pulsation mechanism and to the observed range in behaviour within the group.

(4) Determine whether there exist unique relationships between pulsation amplitude and observable characteristics such as line profile variations, light and velocity curve shapes, etc.

(5) Institute an analysis to discriminate radial from non-radial pulsation modes using temperatures derived from the Voyager UVS data.

(6) Continue the analysis of the long-term (tens of years) pulsational stability of stars such as BW Vulpeculae and assess the evolutionary significance.

• Further define the evolutionary state of Beta Cephei stars.

• Explain the wide range in pulsation amplitudes present within the Beta Cephei group.

• Identify, if possible, the Beta Cephei pulsation mechanism.

• Determine the significance of Beta Cephei instability in the context of the variability of early B stars.

Description of Voyager UVS Data

Voyager uses objective grating spectrometers with wavelength coverage of 500-1700 Å. Spectral resolutions of approximately 18 Å for point sources and 30 Å for diffuse sources are achieved. Instrumental sensitivity is optimized for the 800-1200 Å region. Typical limiting fluxes at 1050 Å in the far-UV are 1.0 x 10E-12 ergs/cm/cm/sec/Å for Voyager 1 (5.0 x 10E-13 for Voyager 2). Most of the sky can be observed. In-flight performance of the UV spectrometers has been reviewed by Broadfoot et al. (Journal of Geophysical Research, 86, 8259, 1981).

Description of Ground-based Data

Measurements at the DAO were made using CCD sensors on both the Cassegrain spectrograph of the 1.8-meter telescope and the coudé spectrograph of the 1.2-meter telescope. Spectral resolutions were approximately 0.3 Å and 0.1 Å respectively; time resolution was typically in the range of 2-4 minutes with a S/N exceeding 50:1. The lines of Si III, He I, Mg II, O II and C III in the wavelength range 4450-4600 Å were among those observed. For observations with the University of Hawaii 2.2-meter telescope on Mauna Kea, a CCD sensor was used on the coudé spectrograph. The spectral resolution was 0.2 Å and the time resolution was 10-15 minutes with a S/N in the range 100-150. In addition, simultaneous UBV photometric observations were obtained on the University of Hawaii 0.6-meter ``Air Force'' telescope. The ground-based program has been an outstanding success, having produced almost 100 hours of simultaneous data. The most extensive simultaneous data sets are for the extreme star BW Vulpeculae. Simultaneous data have also been obtained for Beta Cephei, as well as Nu Eridani, Delta Ceti and 12 Lacertae.

Program Status

• All data for this program have probably been acquired.
• Data analysis is currently in progress at NASA/GSFC, UH Hilo, and DAO.
• Examples of Voyager UVS light curves are given below. Observations have been folded into one cycle. The ``instability anomaly'', shown for Nu Eridani (Fig. 1) and for Beta Cephei (Fig. 2), is manifested in the form of unexplained fluctuations in intensity near the phase of maximum UV flux. In Fig. 3, UVS data for Beta Cephei obtained during May 1991 show that the fluctuations near maximum are still present a decade later. Moreover, the magnitude of the fluctuations during 1991 appears to be comparable to those discovered in the pre-1990 data.

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