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Astronomical Data Analysis Software and Systems VI ASP Conference Series, Vol. 125, 1997 Gareth Hunt and H. E. Payne, eds.

Asteroseismology--Observing for a SONG
Rob Seaman IRAF Group,1 NOAO,2 PO Box 26732, Tucson, AZ 85726 Caty Pilachowski, Sam Barden Kitt Peak National Observatory Abstract. The Stellar Oscillations Network Group (SONG) seeks to study p-mode (acoustic) oscillations in solar typ e stars. These are difficult phenomena to detect due to the limited amplitude of the oscillations in integrated light. Success will require continuous observing sessions over many pulsation cycles, preferably with multiple telescop es staggered in longitude, similar to the GONG pro ject. Such oscillations are b est detected as a b eat frequency relative to some very regular inertial observing cadence. The phase of the cadence must b e maintained, b oth b etween widely separated telescop es and b etween observing sessions that may b e separated by months. We discuss techniques to improve the observing efficiency and the likelihood of detection. Precisely identical data handling and reduction steps for tens or hundreds of thousands of sp ectra are critical to success.

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Introduction

The detection and study of acoustic oscillations in solar-like stars offer a new constraint on stellar models, b eyond the global prop erties of mass, radius, age, and chemical comp osition. For stars with well-measured global prop erties, knowledge of frequencies and frequency splittings of p-mode oscillations will p ermit detailed comparison with stellar models at a level unprecedented outside the solar system. For stars whose global prop erties are less well known, the addition of asteroseismological data may allow the determination of mass and age for individual field stars. The observation of many solar-like stars will also help us to understand how solar p-modes are excited and damp ed. The Stellar Oscillations Network Group (SONG) at NOAO has undertaken to develop methodology for precise equivalent width measurements of Balmer lines in solar typ e stars. Typical oscillation frequencies in solar typ e stars are a few mHz, or a p eriod of a few minutes. The amplitude of sp ectral line vari° ations in integrated sunlight is ab out 5 m° but larger amplitudes (20­30 mA) A, are exp ected in warmer stars. Time series of sp ectra are obtained over many
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Image Reduction and Analysis Facility, distributed by NOAO National Optical Astronomy Observatories, operated by the Asso ciation of Universities for Research in Astronomy, Inc. (AURA) under co operative agreement with the NSF.

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© Copyright 1997 Astronomical Society of the Pacific. All rights reserved.


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nights at precisely timed intervals. Exp osure times are limited to a few minutes or less by the need to preserve time resolution. Very high S/N ratios ( 1000) are needed p er exp osure to detect the weak signal. With telescop es of modest ap erture (4 meters or less), only the brightest stars can b e observed. Such measurements can b e made with commonly available sp ectrographs. With the exp ectation of developing a world-wide network of telescop es to make asteroseismic observations, we are developing techniques of data acquisition, data reduction, and analysis that can b e easily propagated to observatories around the world. Of greatest concern is high observing efficiency so that oscillations can b e detected during observing runs of reasonable length. 2. Observations and Reductions

While the p otential of asteroseismology is great, so are the difficulties in detecting oscillations in solar typ e stars. In integrated light, the amplitudes of oscillations are small: a few meters p er second in radial velocity or a few to a few dozen parts p er million in the strength of sp ectral lines. New techniques in sp ectroscopy, data reduction, and analysis are needed to obtain this precision. 2.1. Exp osure Scheduling

Complications are encountered when attempting to implement even the simplest requirements for conducting a valid sequence of asteroseismological observations. SONG hop es to detect stellar oscillations with p eriods of a few minutes or tens of minutes. Naively, the two or three orders of magnitude improvement over this provided by the normal 1 second time resolution of the KPNO CCD cameras should b e quite sufficient for such a detection. For this to b e true, however, the scheduling of the individual CCD exp osures also has to match at least this level of accuracy. Our first proof-of-concept observing night at the telescop e relied on a long free-running sequence of several hundred exp osures to provide this scheduling. Figure 1a is a plot of the running average of the cycle time (from shutter-op en to shutter-op en) versus sequence numb er through the night. The rep eatability of the cycle time is resp ectable-- the mean deviation is well within the 1 second precision of the camera system. The only remarkable thing ab out Figure 1a is a slight increasing trend of the average cycle time from ab out 68.3 s at the b eginning of the observing session to ab out 68.7 s at the end of the session several hours later. We attribute this 0.6% effect to increasing hard disk seek times as the data partition filled up. This small slop e has a disprop ortionately dramatic effect, however, on the quality of the science that can b e p erformed. Figure 1b is a plot of the residuals of the start time of each exp osure compared to an evenly partitioned grid of precisely identical cadence from the start to the end of the observing sequence. These residuals sweep out more than 120 of phase in the nominally regular observing cadence. Without a regular cadence, the straightforward (in principle) detection of stellar oscillations as simple b eat frequencies b ecomes more difficult. The much more complicated mathematical treatment required by unevenly gridded observations jeopardizes the reliability of any detection whatsoever, and at b est comes at the exp ense of observing efficiency. Clearly, some way is required to schedule the observations on regular clock ticks. We refer to this as "cadencing" the observations. A short delay is added to


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Cadenced data--shutter timed to b etter than 0.01 s. the precise clock tick desired. after cadencing. regular observing cadence for arises. Our telescop es do not

each exp osure such that the shutter is op ened on Figures 2a and 2b show the improvement realized Having addressed the question of enforcing a a single observing sequence, the next complication


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provide an inertial observing platform. Unless we account for the 1000 s light travel time amplitude of the Earth's motion around the Sun, we cannot hop e to easily combine data from observing runs separated by any significant fraction of a year. The problem is even more fundamental than this, since, except when a particular target is very near opp osition to the Sun, the heliocentric correction can vary by several seconds over the course of a single day. Note that the amplitude of the barycentric correction for the Solar system is ab out 10 light seconds--ab out 1% of the heliocentric correction. Jupiter contributes ab out half of the barycentric leverage and Saturn most of the rest. At any given ep och the absolute value of the barycentric effect may b e significantly less than the full 10 s, dep ending on Saturn's p osition relative to Jupiter. It is the non-zero second derivative of these motions that will smear out the phase of the observations, since otherwise they could b e treated like the typical target star's radial velocity term of a few km s-1 , which can just b e comp ensated for as a small Doppler correction. For instance, Procyon's radial velocity is -3km s-1 ; this will just require a 1 part in 105 Doppler correction. 2.2. CCD Readout Mo de Variations

As another way to increase efficiency, we investigated the use of a continuous readout mode (CRM) for sp ectra of bright stars to eliminate the time sp ent with the shutter closed. A ma jor concern is the variation of the light across the sp ectrum during the observations. We simulated CRM observations using the Artdata package in IRAF. We created a CRM sp ectrum using the known pixelto-pixel sensitivity variations of the CCD, and allowed the artificial sp ectrum to vary in intensity, p osition on the CCD, and width due to seeing during "readout." The simulated sp ectra contain errors of up to 4% in intensity. The S/N in the final sp ectrum is 500, no b etter than a normal observation in the same time. A second approach for improving the observing efficiency is a stepp ed CCD readout mode producing "miniframe" images. Using a narrow region of interest covering the sp ectrum, multiple integrations can b e obtained within a single image frame. The sp ectrograph shutter is closed b etween integrations, and the region of interest is read out into a buffer; rep eated integrations are app ended to the same buffer. With this stepp ed readout mode, a total integration time of 25­30 seconds can b e obtained each minute without saturating the CCD. This compares to a total integration time of only 15­20 seconds using a conventional readout, nearly doubling our observing efficiency. References Stellar Oscillations Network Group3 Massey, P., et al. 1996, An Observer's Guide to Taking CCD Data with ICE
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http://www.noao.edu/noao/song/ http://www.noao.edu/kpno/manuals/ice/ice.html