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MOST found evidence for solar-type oscillations in the K2 giant star HD 20884
We found evidence for radial p-modes and nonradial mixed modes in theoscillation spectrum of the K giant HD 20884 based on 20.6 days ofnearly continuous high-precision photometry obtained by the Canadianmicrosatellite MOST Oscillation frequencies range from 5-31 ?Hz(periods of about 2.3 d - 9 hr) with luminosity amplitudes between about300 and 950 ppm and mode lifetimes exceeding 10 days are indicated. Themode identifications are based on searches of a large grid of models fora best fit to the frequencies and temperature of HD 20884. The latteris better constrained now by spectroscopy obtained at the David DunlapObservatory as part of this work.Based on data from the MOST (Microvariability & Oscillation ofSTars) satellite, a Canadian Space Agency mission jointly operated byDynacon, Inc., the University of Toronto Institute of Aerospace Studies,and the University of British Columbia, with assistance from theUniversity of Vienna, Austria.
| Nonradial p-modes in the G9.5 giant ? Ophiuchi? Pulsation model fits to MOST photometry
The G9.5 giant ? Oph shows evidence of radial p-mode pulsationsin both radial velocity and luminosity. We re-examine the observedfrequencies in the photometry and radial velocities and find a bestmodel fit to 18 of the 21 most significant photometric frequencies. Theobserved frequencies are matched to both radial and nonradial modes inthe best model fit. The small scatter of the frequencies about the modelpredicted frequencies indicate that the average lifetimes of the modescould be as long as 10-20 d. The best fit model itself, constrained onlyby the observed frequencies, lies within ±1? of ?Oph's position in the HR-diagram and the interferometrically determinedradius.Based on data from the MOST satellite, a Canadian Space Agency missionjointly operated by Dynacon, Inc., the University of Toronto Instituteof Aerospace Studies, and the University of British Columbia, withassistance from the University of Vienna, Austria.
| Detection of solar-like oscillations in the red giant star ? Ophiuchi by MOST spacebased photometry
Context: Solar-like oscillations have been discovered in a few redgiants, including ? Oph, through spectroscopy. Acoustic modesaround 60 ?Hz were clearly seen in this star, but daily aliasing ofthe groundbased data made it impossible to unambiguously isolate thep-mode frequencies in the eigenspectrum, and hence the correct value ofthe large spacing, to asteroseismically constrain the mass of thispulsating star. Aims: We obtained about 28 days of contiguoushigh-precision photometry of ? Oph in May-June 2005 with theMOST (Microvariability & Oscillations of STars) satellite. Thethorough time sampling removes the ambiguity of the frequencyidentifications based on the groundbased discovery data. Methods:We identify equidistant peaks in the Fourier spectrum of the MOSTphotometry in the range where the p-modes were seen spectroscopically.Those peaks are searched by autocorrelation of the power spectrum toestimate the value of the large separation in the p-mode eigenspectrum.Having isolated the oscillation modes, we determine their modeparameters (frequency, amplitude and line width) by fitting thedistribution of peaks to Lorentzian profiles. Results: The clearseries of equidistant peaks in the power spectrum, with amplitudes fromabout 30 to 130 ppm, are consistent with radial modes spaced by a meanvalue of ( 5.3 ± 0.1) ?Hz. This large separation matches oneof the two possibilities allowed by the groundbased observations thusconstraining the stellar models to a much greater extent than previouslypossible. The line widths and Lorentzian fits indicate a rather shortaverage mode lifetime: (2.7+0.6-0.8}) days.Based on data from the MOST satellite, a Canadian Space Agency mission,jointly operated by Dynacon Inc., the University of Toronto Institutefor Aerospace Studies, and the University of British Columbia, with theassistance of the University of Vienna.
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