Observation of Large-Delta-Magnitude Close Binaries with Shaped Aperture Masks

In this paper, the motivation, history, mechanism of operation, and practical usage of shaped aperture masks are discussed in the context of binary star observation and discovery. Introduction Binary star systems are important subjects for astronomical study. Records of star separations and position angles, taken over a number of years, allow us to establish orbits and calculate the dynamical mass of binary systems via Kepler’s Third Law. The dynamical mass, when coupled with other information, yields the mass of each constituent star, the single most sensitive parameter affecting stellar evolution. In close pairs with a large brightness difference, the light from the brighter star often overwhelms the light from the dimmer star. The faint stars are thus difficult to isolate, and so they are relatively under-observed. Many of these overlooked stars are late-M stars, which are cool, faint, and red (Figure 1). The chargecoupled device (CCD) cameras used to observe these stars has low sensitivities at red wavelengths where late-M stars radiate the most energy, further exacerbating the problem (Figure 2). Recent adaptive optics (AO) observations of late-M stars showed that some mass estimates were off by a whopping factor of two or more (Dupuy et al. 2010). With accurate speckle observations and orbital analyses of a larger number of short-period binaries with late-M secondaries, we can refine our stellar evolutionary models and strengthen the conclusions we draw from them. First, however, we must discover these stars! Figure 1. The Hertzsprung–Russell diagram (Encyclopædia Britanica). We are most interested in the late-M secondary stars that appear in the lower-right of the diagram. Vol. 11 No. 1S Journal of Double Star Observations | September 2015 Foley et al. 344 Figure 2. A typical spectrum of an M0 star (left) compared to a typical CCD sensitivity profile (right) (“Andor Luca-S 658M Specifications”; Richmond 2011). This star has significant energy output at wavelengths longer than 800 nm. The camera’s sensitivity is low in this range, though the camera can still detect infrared light. Shaped aperture masks and shaped pupil masks are innovative solutions to the double-star, largedelta-magnitude problem, and have been used successfully in practice (Debes et al. 2002; Debes & Ge 2004, Daley 2014). The masks’ strategically-shaped contours diffract light in such a way that the light destructively interferes with itself in prescribed image regions. These regions, known as discovery zones, provide exceptional contrast. Astronomers can use this contrast to their advantage: if a telescope is centered on the primary star of a double-star system, the light from the bright primary will disperse about the image, leaving the discovery zones largely unaffected and facilitating the detection of any faint companion stars therein. Modern pupil masks are proposed for use in exoplanet discovery and observation because of the extremely high contrast requirements of this area of research. Host stars can outshine hot Jupiters by factors exceeding 10! and Neptunes by factors around 10! (Kasdin 2014). Consequently, aperture masks must be designed with great care and machined precisely to allow such observations to take place. The inherently high contrast ratio requirements surrounding exoplanet study are an expensive barrier to entry. Luckily for smaller observatories, there is a considerable amount of science that can be performed on binary stars at more modest contrast ratios closer to 10! —ratios that may be possible to attain without specialty equipment. Further, small, ground-based telescopes have easily accessible apertures near which a mask can be placed, as opposed to larger telescopes with available pupils further down the optical path. The terms shaped aperture mask and shaped pupil mask imply different placements, but the masks’ functions are identical. Speckle interferometry can be used with shaped masks to resolve regions within the atmospheric seeing limit; thus, these masks are theoretically capable of distinguishing stars that simultaneously have large delta magnitudes and small angular separations. There is much to learn from binary stars with magnitude differences between 3.0 and 8.0 and angular separations near the seeing limit (less than a few arc seconds). Shaped masks may enable small observatories to study stars meeting these criteria. For the same distance from Earth, more closely spaced binaries have shorter-period orbits, meaning star motion can be tracked over a shorter timeframe to arrive at meaningful orbits and mass estimates. Adaptive optics systems can be used in large observatories to replace the speckle process, but the system’s higher cost is not a realistic investment for some astronomers. The shaped masks that can be readily manufactured have azimuthally-limited discovery zones. This is not an issue during observation where the approximate position of the secondary star is known, but when it is not—as in the case of binary star discovery—one must repeatedly rotate the mask to survey the primary star’s surroundings. For example, if a symmetric mask has 20 degrees of visibility on each side, Vol. 11 No. 1S Journal of Double Star Observations | September 2015 Foley et al. 345 it has a 40-degree discovery window and must be positioned in at least nine different orientations to completely scan the 360-degree neighborhood of a primary star. In practice, each new discovery zone should slightly overlap the previous to avoid problems when the secondary happens to fall on a discovery zone border. Manual realignments would be slow and tedious, making the rotation of shaped masks an ideal target for automation.