Background

HR 1217 is the non-generate star whose amplitude spectrum most resembles that of the sun. It has a series of alternating even and odd degree p modes, all of which are amplitude modulated with the 12.45-day rotation period of the star. It is clear that this magnetic Ap star is an oblique rotator and oblique pulsator. All of the modes are non-normal: i.e. they are distorted by the magnetic field.

Xcov 20 will cover 1.7 rotation periods and allow us to resolve fully the rotational sidelobes of each mode which, in turn, will give us the information we need to model the "shape" of the mode - determine its degree and how it is perturbed from a normal mode. This is not possible with any other type of star, and, so far, for no other Ap star can it be done for both even and odd degree modes.

Xcov 20 will provide data for us to examine:

1. how p modes of different degrees interact with the global magnetic field;
2. how the magnetic field perturbs the pulsations frequencies, possibly constraining the internal magnetic field strength (which has never been done for any star);
3. how the surface inhomogeneities perturb the pulsation frequencies (again, never done for any star);
4. how non-adiabaticity and pulsation near to the critical frequency affect the pulsation, putting new constraints on the atmospheric structure of a very peculiar star;
5. how the magnetic field perturbs and interacts with the eigenmodes - a difficult and little-studied subject.

For more information, consult the full HR 1217 Scientific Justification.

Observing Instructions

Two sample light curves, from the times of pulsation maximum and minimum, are shown in the at the bottom of this page, along with their amplitude spectra. These were obtained with the SAAO 1-m telescope and are the kind of data you should obtain under photometric conditions with the same sized telescope. Since the noise is scintillation limited, larger aperture telescopes will give lower noise levels, smaller aperture will give higher noise levels.

The rotational ephemeris of HR 1217 is currently problematical Xcov 20 will solve this. Kurtz et al. (1989) give an ephemeris of HJD(pulsation maximum) = 2446744.15 + 12.4572E (1) Using the rotational period of Kurtz & Marang (1987). Mathys & Hubrig (1997) found the rotation period of Kurtz & Marang did not fit their magnetic data. They found an ephemeris for magnetic maximum (= pulsation maximum) of HJD(magnetic maximum) = 2440577.230 + 12.4610E (2) Leone et al. (2000) made new magnetic field measurements and found that the period of Kurtz & Marang fits their data, and that of Mathys and Hubrig does not. They give: HJD(magnetic maximum) = 2448513.1 + 12.4572E (3) although they comment that the period is probably slightly longer than the one they used.

All of this leaves us with some uncertainty about when to expect pulsation maximum. Ephemeris (1) leads us to expect maxima at JD2451864.1 (15 Nov) and JD2451876.5 (28 Nov); Ephemeris (2) leads us to expect maxima at JD2451866.9 (18 Nov) and JD2451879.4 (31 Nov); Ephemeris (3) leads us to expect maxima at JD2451864.1 (15 Nov) and JD2451876.5 (28 Nov) - the same as the first one. I (DWK) did our original ephemeris predictions using ephemeris (2) of Mathys & Hubrig as it was the most recent. All this leaves us with some uncertainty about when maximum will occur. However, Mike Reed is observing at CTIO with the 1.5-m telescope 3-16 November, so we will know the answer by the time the rest of you start your runs. All of this is only important before the run to the extent that your expectations about what amplitude you should be seeing at the telescope may be off by a day or two until we get the ephemeris updated.

The coordinates are:

Control Center: The Xcov20 control center is at Iowa State University's International Institute of Theoretical and Applied Physics (IITAP) from 14-25 November, and at the Observatoire Midi-Pyrénées Laboratoire d'Astrophysique from 25 November to 5 December.

Contacting the Control Center: Consult the Phone Contact List for information on phone, fax, and email for the control center. See the Xcov20 People List for contact information for the other Xcov20 active sites.

Time Check: Please do a clock time check each night before taking data and verify your time with HQ at the beginning of the run, and as often during the run as possible. HQ will have an accurate clock available for phone clock checks. We had many problems with time during Xcov17, so please, check the time before each run!
There is also a link to a Java Clock on the Xcov 20 homepage that works reasonable well for those of you with internet access at your telescope. A time check will still be required on the first night of observations for verification.

Data Transfer: There are two ways to send your data and logs at the end of each night.

1. By anonymous ftp to: wet.physics.iastate.edu
   Note that this is different than the wet.physics.iastate.edu address you are probably used to!
   If you use the ftp option, load your data into the "wet/incoming/xcov20/" directory.
   
   
2. Via e-mail to: wet.physics.iastate.edu
   
   We will be reducing the data as fast as they come in, and we'll contact you promptly if we have any questions about, or problems with, the data you send.
   
   In case the IITAP mail/ftp servers go down, there is an emergency email address for use in contacting the control center: wholeearthtelescope@yahoo.com.

Observing Logs: We ask that you send a copy of your observation log along with the data, at the same time if you can.

Run start: Please start your run on an integral multiple of ten seconds (times ending in "0"). Check the computer clock just before the start of a run, synchronizing with your master clock to within 0.1 sec if possible.

Start and finish times for HR 1217: Please begin and end runs on HR 1217 at airmass = 2. For northern sites secondary targets may be observed before and after that. At the beginning of the night, please assure that set-up time is allowed before HR 1217 reaches airmass = 2, so that you start on time. Southern sites should observe HR 1217 the whole night.

Integration time: Use 10 seconds integration time for PMT-based systems. Those with CCDs will have to use their judgement (and/or confer with HQ) to obtain the appropriate balance between signal to noise, cycle time, and duty time.

Filters and count rates: Use a Johnson B filter plus whatever ND we have sent you, or you have to keep the count rates manageable. Higher count rates are preferable. To the extent that you have any choice left by the time you get to the telescope, try to get between 500,000 cps and 1,000,000 cps. Please be sure your filters are CLEAN.

Dead Time: We need to know the dead time for your photometer/electronics. At the time of your first data submission, please remind us of your latest and best measurement of the deadtime of your system. During your run please check your deadtime again.

Guide star: We have not selected any standard comparison stars for HR 1217. Please pick your own isolated star and send us your ch2 offsets and telescope plate scale so we can verify exactly which star was observed. The ch2 star is primarily for precision guiding, but there is a chance of serendipitous discovery of a new variable.

Sky calibration: Observers with 3-channel photometers should record sky in all three channels near the beginning and possibly the end of each run to permit accurate cross calibration. Observers with 2 channel photometers should take sky by moving the telescope. We have sometimes in the past used the channel 2 verniers to measure channel 2 sky (only) but we find this is not really reliable. Take sky as often as needed.

Xcov 20 is different from previous WET campaigns. Although HR 1217 is very bright, we are observing through bright moon at many sites, and in some cases with the moon close to the star in the sky. Since we are working at less than 100 micromag precision, sky does matter. Always observe sky at the start and end. Three 10-s integrations are sufficient. More just gives unwanted gaps in the data.

This is how to judge how often sky should be observed:

We do not want the sky counts to vary by more than 0.3% of the star counts, so that we can model the interpolated sky counts precisely. At the beginning of the night measure sky at the end of twilight and 15 minutes later. If the change in the sky counts is less than 0.3% (.003 mag) of the star+sky count rate, then you may lengthen the time until the next sky to 30 minutes. If after 30 minutes the change is still less than 0.3%, you may lengthen the interval further. You must test and judge this for yourselves. When you have a bright moon high in the sky and the star in near the meridian, you may be able to reduce you sky measurements to once per hour. Please do not go longer than that. When the moon is rising or setting, or if you extend your observations into twilight (recommended - we will delete unusable data) then you must do sky more often. In twilight you will have to do them very often. Use the given criterion.

Apertures: NB!!!!!!!!!!! This is different from ALL previous WET campaigns and may be foreign to your experience. Use a 25-30 arcsec aperture. Do NOT use an aperture smaller then 25 arcsec, and if you have any guiding or tracking problems, use 30 arcsec or even 35 arcsec. We do not have a sky background problem with such a bright star. Star images are not just seeing disks: they have diffraction rings and spikes on them; they jitter; they wander; the wind blows the telescope; your guiding is not perfect. A loss of 0.5% of the starlight would not be noticed with a faint star with 2% errors from photon statistics. For HR 1217 it would be a large enough effect to render the data as trash.

Light Shock: Your phototube may suffer from light shock. This manifests itself as a change in sensitivity (either way) when bright starlight is first seen at the beginning of the night. Please expose your phototube to the star in twilight before observing, preferably for 10-15 minutes. This will give it a chance to settle, if it has such a problem.

Good luck to each of you!!

Sample HR 1217 Lightcurves and amplitude spectra.
Click on the plots to download gzipped postscript versions.

Sample light curve at amplitude maximum

Sample light curve at amplitude minimum

Sample raw amplitude spectrum at maximum

Sample amplitude spectrum at maximum, filtered for low frequency noise

Sample amplitude spectrum at minimum