Canada’s Space Seismology Project: The MOST recent news

Jaymie M. Matthews, MOST Mission Scientist
Dept. of Physics & Astronomy
University of British Columbia

The promise of stellar seismology

It’s difficult to imagine that a 15-cm optical telescope carried aboard a microsatellite the size of a small photocopier could provide any fundamental information about the nature of the Universe. But that is exactly what is expected of the MOST (Microvariability and Oscillations of STars) project, the Canadian Space Agency’s first science microsat and the first all-Canadian space astronomy mission.

The MOST instrument is designed to detect brightness oscillations in stars down to a level of a few parts per million - the amplitude of the Sun’s five-minute oscillations seen in integrated light. Those oscillations (caused by turbulent sound waves propagating within the Sun) allow us to probe seismically the otherwise hidden solar interior. Such low-level oscillations in bright nearby stars other than the Sun are currently beyond ground-based detection limits even for networks of large telescopes, but are within reach of a small customised telescope in Earth orbit.

The MOST Science Team (with members at UBC, Toronto, Montreal, St. Mary’s, Harvard and Vienna) will use the oscillation data to probe the structures of neighbouring stars and determine the ages of some of the oldest stars in our Galaxy - making a key contribution to the vigorous ongoing debate over the age of the Universe. David Guenther (St. Mary’s, and MOST Science Team member) reviews some of the exciting potential of stellar seismology in another article in this issue of Cassiopeia.

MOST’s photometric sensitivity will also allow us to detect stochastic variations in hot stellar winds, testing various proposed mechanisms of driving and turbulence, and better estimating the contribution of those winds to the ISM. Analysis of these data will be spearheaded by Tony Moffat (Montreal, MOST Science Team). The tiny mirror of MOST will also be trained on several stars believed to harbour close-in giant planets, so we can measure the light curve due to the planet’s varying illumination with orbital phase, and hence infer its size, albedo and atmospheric composition. The models to support this exciting application of MOST are being developed by Sara Seager (IAS, Princeton; MOST Support Scientist) and Dimitar Sasselov (Harvard; MOST Science Team).

An ultraprecise photometer in space

Every aspect of the design of the MOST mission - from the Instrument optics, structure, detectors and electronics, the bus design, and the intended orbit - all must satisfy what has become the MOST Bible: its photometric error budget. The MOST Instrument Team (led by Jaymie Matthews and MOST Instrument Scientist Rainer Kuschnig) has tried to identify all possible sources of noise and drift (both random and systematic) and assign each a maximum allowable threshold so that MOST can reach its required micromag sensitivity. The list of sources ranges from the obvious (e.g., photon statistics, scattered light, granulation noise in the target star itself) to the subtle (e.g., scintillation flashes in the glass optics caused by cosmic rays in orbit, sub-pixel QE variations in the CCD). The photometric performance model includes over 50 parameters, and new features are being continually added. We know there must be some insidious factors we have overlooked, so each component of the error budget is weighted towards a very pessimistic/conservative value, to allow an additional margin of error for the unknown.

Rainer Kuschnig has incorporated this budget into an IDL-based data simulation package we can use to test different aspects of the MOST design and performance. An example of a simulated light curve is shown in Figure 1, the result of monitoring a V=3 solar-type star for 10 days. Note that the largest variations are due to the intrinsic granulation noise of the stellar photosphere, whose amplitude is thousands of times larger than the oscillation signal which is also imbedded in the simulation. However, the signature of the high-frequency oscillations is easily seen in the resulting Fourier amplitude spectrum (Figure 2), even in the ‘raw’ data with only CCD bias corrections applied.

One of the side-effects of the MOST design, simulation and prototype testing process has been a new respect amongst the MOST Team and our colleagues for the capabilities of a tiny 15-cm optical telescope in space, when equipped with the proper instrument.

Figure 1. A simulation of photometry obtained by MOST of a star oscillating like the Sun, which includes noise/drift sources in the Instrument, the spacecraft, and the target star itself. The signal itself is unrecognisable in this light curve, having an amplitude thousands of times smaller than the full range of variation seen here. Gaps in the data are assumed when the satellite passes through the South Atlantic Anomaly (a region of enhanced radiation flux) during its 800-km-high polar orbit. From that vantage point, MOST will be able to "stare" at targets for as long as 7 weeks and thereby attain impressive frequency-domain resolution.

Figure 2. A Fourier transform of the time series data in Figure 1 (continued for a total of 20 days) reveals the subtle oscillations with frequencies near 3 Mhz (periods near 5 min, like the solar oscillations). The equal spacing of the eigenmodes is the signature of the star’s high-overtone, low-degree p-modes, and the fine structure is sensitive to the star’s core composition and hence, main sequence age. Even the low-frequency ‘noise’ is actually data, since it contains information about the temporal behaviour of granulation in an unresolved star. Note that this is an amplitude spectrum of ‘raw’ unprocessed data (except for CCD bias subtraction). Reduction of the data, including corrections for the known orbital frequency and its harmonics, will lower the r.m.s. noise level at high frequencies to even less than 0.8 micromag. This is also an amplitude spectrum;in a power spectrum (more often shown in astronomical publications), the apparent signal-to-noise would be squared.

The price tag

The MOST mission is a good illustration of how Canadian scientists and industry can team up to achieve lofty goals on a modest budget. Even a small space telescope to do ultraprecise rapid photometry of stars would normally require a large (and expensive) stable platform. Thanks to miniature attitude control technology developed by Dynacon Enterprises Ltd. (Toronto), it is possible to do this with a low-mass microsat, at a cost about 5 - 10 times less than similar missions planned by France and Denmark.

MOST is primarily funded, and solely supervised, by the CSA. Dynacon Enterprises Ltd. is the Prime Contractor, responsible for the MOST bus and system-level management. The University of Toronto Institute of Aerospace Studies (UTIAS) is subcontracted to Dynacon to work on many of the bus subsystems and well as the ground stations, with technical input from the Canadian aerospace company AeroAstro and the not-for-profit AMSAT corporation. The University of British Columbia (UBC) is responsible for the Instrument (with Spectral Applied Research, formerly part of CRESTech, as its subcontractor), as well as overall Science Planning and Operations. There will MOST ground stations in Vancouver and Toronto, and probably a third one in Austria.

Figure 3. Crouching Kitten, Hidden Telescope. This full-scale model of an earlier version of the MOST design does not allow one to see the telescope and camera nestled in the bottom Instrument Bay, just its entrance aperture and re-deployable door at lower right. The oval at the lower left is meant to represent the passive radiator for the cooled CCD camera, which is actually white, not black. Note the coffee mug attached to the upper right corner. Project leader Matthews has not confirmed at the time of writing whether or not this is an intrinsic part of the Instrument or just a blatant ploy to seek corporate sponsorship from Starbucks for future space astronomy missions.

The entire project budget (including cost-in-kind, donations from industry, and launch) is approx. Can$9.5M, of which about Can$6M is being contributed by CSA. The Ontario Research & Development Challenge Fund contributed Can$1.2M towards infrastructure at UTIAS to create a graduate programme in microsat science, for which MOST is the prototype. Minor funding has come from CFI and NSERC, and industrial donations are acknowledged on our Web site at www.astro.ubc.ca/MOST.

Current Status

		MOST passed its Prelimary Design Review in 1999 and its Critical Design Review in early 2000. UBC hosted a successful international science workshop on MOST in May 2000. MOST is on schedule to be launched in mid-2002. A full-scale mockup of an earlier MOST design is shown in Figure 3. The layout is still very similar, but the payload attachment ring is different (and less prominent) than it appears in the mockup and magnetometer booms now extend from the bottom of the satellite.
		The MOST telescope structure and optics are nearly complete. The telescope optics have been fabricated by Ceravolo Optical Systems (Oxford Mills, Ontario) and are being coated at press-time in NRC facilities in Ottawa. Flight-model CCD header electronics are complete and being tested at UBC, while CCD controller electronics are nearing completion, while testing continues on the engineering boards finished last year. The custom-packaged Marconi CCD detectors and our broadband optical filter (built by Custom Scientific - the makers of the HST WF-PC filters) are awaiting installation.
		The MOST’s snug little home in space - the bus (or as we like to call it, the"Vespa") - is being outfitted. Its on-board computer, RAM disk, Attitude Control System (ACS) node and T&C node are complete and being tested, as are the reaction wheels (i.e., gyroscopes), magnetorquers, rate sensors, and other components of the ACS hardware. Solar arrays are proceeding to fabrication. Batteries and other bus electricals are largely complete. The stack of trays which house these components and make up the main body of the bus (‘sorry, I mean Vespa) is also ready.
		The ground station hardware is being assembled at UTIAS, and a prototype system has been tested successfully (i.e., we can talk to satellites).
		In anticipation of the seismic data on metal-poor subdwarf stars to be returnedby MOST, UBC grad student Evgenya Shkolnik has completed a grid of evolutionary and pulsational models across a wide range of metallicity. This is the first time a systematic model grid of pulsational properties has been compiled for theseimportant stellar targets.

Jaymie Matthews is an Associate Professor in the Department of Physics & Astronomy at the University of British Columbia. He has been obsessed with stellar pulsation since his undergrad days at the University of Toronto (and nights atop the Burton Tower with the 40-cm telescope) working for John Percy. To avoid being infected by extragalactic astronomy, he went to the University of Western Ontario for his grad studies, where he and the late William Wehlau started a fruitful collaboration on the seismology of pulsating magnetic stars. Jaymie began his own oscillation, going from UBC to the Universit\’e de Montr\’eal and back to UBC, where the prospect of a permanent position proved to be an effective damping force.

The MOST Project is a natural culmination of Jaymie’s history of Canadian research collaborations to date. The analysis of the data relies on Fourier techniques which were first applied to astronomical light curves by Bill Wehlau and his students in the 1960’s, and were developed further by John Percy for multiperiodic variable stars. The original inspiration for and early development of the Canadian space astronomy microsat project which evolved into MOST were due to Slavek Rucinski (University of Toronto), current Director of the DDO. One of the classes of prime targets for MOST, the winds of Wolf-Rayet stars, is the direct result of Jaymie’s collaboration with Tony Moffat in Montr\’eal. And key aspects of the philosophy of MOST, including its Fabry imaging concept, can be traced to the fertile mind of Gordon Walker at UBC, who is the only person other than the Mission Scientist who sits on both the MOST Science and Instrument Teams.

When Jaymie isn’t directing space projects, he finds time to sit on the Board of Directors of the H.R. MacMillan Space Centre in Vancouver, the Canadian Gemini Science Steering Committee, and fulfill his responsibilities as Grand Poobah of the prestigious Tycho Brahe Society. Despite the size of the MOST Instrument, Jaymie does generally subscribe to the philosophy "Think Big!".

Jaymie (seen at left) tries to convert white dwarf astronomers to the cause of rapidly oscillating Ap stars at a Whole Earth Telescope workshop in Poland.