Recent MOST space photometry

The Microvariability and Oscillations of STars (MOST) photometric satellite has already undertaken more than 64 primary campaigns which include some clusters and has obtained observations of >850 secondary stars of which ∼180 are variable. More than half of the variables pulsate, with the majority being of B-type. Since 2006 January, MOST has operated with only a single CCD for both guiding and science. The resulting increase in readout cadence has improved precision for the brightest stars. The 2007 light curve for Procyon confirms the lack of predicted p-modes with photometric amplitudes exceeding 8 ppm as we found in 2004 and 2005. p-modes have been detected in other solar-type stars as well as premain sequence objects, roAp and δ Scuti variables. g-modes have been detected in a range of slowly pulsating B stars, Be stars and β Cephei variables. Differential rotation has been defined for several spotted solar-type stars and limits set to the albedo of certain transiting planets and the presence of other perturbing planets. The mission is expected to continue as long as the experiment operates. 1. MOST The MOST photometric satellite, originally proposed by Slavek Rucinski, was launched on 30 June 2003 into a 101.4 min sun-synchronous orbit (820 km) at a total cost of C$10M. The experiment, more fully described by Walker et al. (2003), consists of a 15/17.3 cm RumakMaksutov telescope feeding two CCDs, one for tracking, the other for science, through a single broadband filter (350 – 700 nm). In the Fabry mode, an image of the telescope entrance pupil covering some 1500 pixels is projected onto the science CCD. An area of the science CCD is also devoted to in-focus photometry of fainter stars. The primary goal was detection of solar-type p-modes with periods of minutes at micromagnitude precision. The engineers achieved a dramatic reduction in tracking jitter early in 2004 to ∼1 arcsec (essentially diffraction limited performance) leading to a significant improvement in Direct imaging photometric precision and recognition that precise photometry was also possible from the guide star counts returned by the tracking CCD. This opened up a much larger number, and broader range, of variables for observation. Exposures of up to 1 minute were possible in both the Fabry and Direct fields with ∼1 sec for guide stars. Early in 2006, the tracking CCD system failed, likely from a particle hit. Since then, both science and tracking have been carried out with the Science CCD which limits Fabry and Direct photometric exposures to between 0.1 and 3 sec. On the other hand, there has been a significant 1 Based on data from the MOST satellite, a Canadian Space Agency mission, jointly operated by Dynacon Inc., the University of Toronto Institute of Aerospace Studies and the University of British Columbia with the assistance of the University of Vienna. improvement in precision for the brightest stars because of a higher read-out cadence coupled with an absence of intermittent x-talk from the tracker electronics. The cost of this improvement is the loss of candidates in the 3.5 6 mag range because they are too faint for short exposures in Fabry mode but too bright for the in-focus Direct mode. The presence of parasitic light, mostly Earth shine, at certain orbital phases has posed the greatest challenge in the data analysis. The amount of the background depends on the stellar coordinates, spacecraft roll and season of the year. Neutralising the impact of the periodic parasitic light involved a considerable effort at both the UBC Physics & Astronomy Department and the Institute für Astronomy, Wien for both the Direct and Fabry photometry (see Rowe et al. 2006, and Reegen et al. 2006). But, with this problem finally solved, the pace of paper writing has greatly increased and a list can be found at http://www.astro.ubc.ca/MOST/science.html. I can only touch on a very few highlights in this brief review. 2. some observing statistics As of June 2007, MOST had been in operation for four years, with scientific observations having been made for 3.5 years. There were 64 campaigns which also included some clusters and data were recorded for >850 secondary stars, of which ∼180 have proved to be variable. Figure 1 is a histogram of the % stars as a function of spectral type. 0 5 10 15 20 25 30 35 40