Astronomy and Astrophysics – Astrophysics
Scientific paper
Apr 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...443l..25k&link_type=abstract
Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X), vol. 443, no. 1, p. L25-L28
Astronomy and Astrophysics
Astrophysics
7
Coupled Modes, Stellar Oscillations, Stellar Rotation, Stellar Spectrophotometry, Variable Stars, Variations, Cross Correlation, Data Reduction, Doppler Effect, Line Spectra, Spectrum Analysis
Scientific paper
We obtained spectra of the rapidly rotating Delta Scuti star upsilon UMa with the Advanced Fiber Optic Echelle Spectrometer (AFOE) on five successive nights in 1993 April, at a cadence of one spectrum every 5 minutes over time periods averaging 4 hours on each night. Cross-correlations of the spectra with a template spectrum from a slowly rotating star of similar spectral type yielded the pattern of features propagating across the lines, averaged over all spectral lines recorded. The spacing of the features in wavelength and their speed of motion across the line profile depend on the azimuthal order m and frequency v of propagating oscillation modes. Using a Doppler imaging analysis, we computed nightly absolute value of m-v diagrams; these show several resolved modes with effective azimuthal order m ranging from about 2 up to about 11, and frequencies between 130 and 170 microHz (i.e., periods between 2.1 and 1.6 hours). We identify the observed modes as propagating prograde modes; the corresponding retrograde modes are not observed. Viewed in a frame corotating with the star with rotation speed of 116 km/s as derived from these data, modes with m approximately 7 and with m approximately 11 have approximately the same frequency (70 microHz). However, their relative amplitude changes substantially from night to night, suggesting that (1) the coherence time of the modes is not longer than about 1 day, or (2) a possible coupling between modes of similar intrinsic frequencies causes an alternating pattern of modal amplitude, or (3) beats are being observed between unresolved modes of similar wavelength and frequency.
Brown Tim
Horner Scott
Korzennik Sylvain G.
Nisenson Peter
Noyes Robert W.
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