Observations

November 1995

Work at NOAO began with an observing run of 5 nights in November, 1995, on the Coude Feed Telescope. Spectra from 390-520 nm, including the H-alpha, H-beta, and H-gamma absorption lines, were obtained for the magnitude 3.7, F6IV star Alpha Triangulum at a dispersion of 0.45A/pixel. The spectra also covered the Mgb lines at 518 nm and the Ca II K line. While the nights were spectroscopic rather than photometric, we were able to obtain several hours of continuous observations each night. The Coude Feed Telescope has an aperture of only 0.9 meters, too small to obtain the necessary signal to assure the detection of acoustic oscillations. The data are useful, however, for developing data reduction and analysis procedures. The observations were obtained at a cadence of about 68 seconds, with an integration time of 45 seconds.

Analysis of the timing data suggested that improvements were needed to assure that the cadence remained constant, and to maximize the observing efficiency. We estimate that the maximum per-pixel signal-to-noise ratio we can expect in a 45 second exposure at this magnitude is about 600/1.

Following this run, new ICE ccd control code was prepared by Rob Seaman of the IRAF group to allow us to obtain spectra in uniform time steps based on a heliocentric clock to eliminate variations in the timing due to the motion of the Earth in its orbit.

Our data from the November run show a peak in the power spectrum at a frequency of about 660 micro-Hertz (corresponding to periods near 25 minutes). This is the expected frequency for Alpha Triangulum, but the signal is very weak and may be due simply to noise.

February 1996

In February, 1996, we had observing runs on the 2.1-m with the Goldcam CCD spectrograph (Feb. 2-8 UT) and on the Coude Feed Telescope (Feb. 2-6 UT), again with the coude spectrograph. We observed the F5IV-V star Procyon with both telescopes. The same cadence was used (20s) but the Coude Feed spectra were offset by 10s from the spectra obtained with the 2.1-m. The first two nights of the runs were cloudy, but we obtained approximately 4.5 nights worth of data with the 2.1m telescope and 2.5 nights with the Coude Feed. Both data sets have similar spectral resolution to the Alpha Triangulum data, about half an Angstrom per pixel. Reductions of the spectra are underway.

March 1996

In March, 1996, we returned to the Coude Feed Telescope to attempt to confirm the detection of oscillations in the G0IV star Eta Boo by Kjeldsen et al. Approximately 3 of the 5 nights (March 8-12 UT) were clear, and data reductions will be complete soon.

During the first part of the night on this run, we also attempted to obtain spectra of Procyon and Sirius using a "ramp" readout mode of the CCD. This mode allows us to improve the observing efficiency with more or less continuous readout of the spectrum. These observations are being reduced to determine if the signal-to-noise ration of the spectra is adequate to permit precise measurements of the variation of the hydrogen lines.

Sam Barden has also run Monte Carlo simulations (click here for the figure) which show how many nights of data will be needed to achieve a 90% probability of detection oscillations in Procyon at the expected amplitude and frequency. With a 20 second cadence, approximately 20 clear nights (160 hours) of observations are needed. Reducing the cadence to 5 seconds would allow a 90% confidence detection in only 4 nights.

May 1996

An important test of the equivalent width method consists in applying it to integrated solar light. This has been done by Kjeldsen et al. (1995), but they did not detect an obvious excess in the power spectrum at 3 mHz at their noise level of 5.1 ppm. We have recently repeated this experiment with a fast CCD camera that provides spectra at very high signal-to-noise levels. On May 25, 1996, we obtained a four-hour sequence of spectra at a sampling rate of 15 seconds with the stellar spectrograph at the McMath-Pierce telescope on Kitt Peak using the integrated light feed and one camera of the Zurich Imaging Stokes Polarimeter (ZIMPOL) I. 128 spectra were averaged in real-time at a rate of 10 spectra per second for every sampling period. After averaging along the spectral lines, the signal-to-noise ratio per 0.027 nm wavelength bin was about 20,000. The power spectrum showed a peak in the equivalent width of Hbeta at the expected frequency of 3.1 mHz. However, this result could not be confirmed in later observations.

December 1996

6 continuous days of solar integrated light were observed with ZIMPOL in down-town Tucson. There is a slight excess of power around 3 mHz and some of the peaks correspond to the frequencies associated with low-order p mode oscillations. However, this result is tentative and needs to be confirmed with a more rigorous data analysis.
Last change: September 29, 1997