Tag Archives: MASCARA

Gravity darkening of planet-host MASCARA-4

MASCARA is one of WASP’s competitor transit-search projects, so let’s celebrate a neat result from TESS data of transits of MASCARA-4b. The host star, MASCARA-4, is a hot, fast-rotating A-type star. As a result of its fast rotation, the equatorial regions are being flung outwards by centrifugal forces, such that the star has a flattened, oblate shape. As a result, the force of gravity will be less at the equator than at the poles of the star, and that means that the equatorial regions will be slightly cooler and so a bit dimmer (in outline, that’s because gravity inward pull is balanced by gas pressure, and so lower gravity means lower pressure, and the temperature of a gas is related to its temperature through the perfect gas law). This effect is called “gravity darkening”.

The star spins around its axis (thick line) while the planet orbits at an oblique angle.

In a new paper, John Ahlers et al have detected the effect of gravity darkening on a transit lightcurve of the hot Jupiter MASCARA-4b. The planet has a misaligned orbit, first coming onto the stellar face near the equator, and then moving towards a pole. That means it moves from slightly cooler regions to slightly hotter regions, and that changes the amount of light occulted by the planet.

If gravity darkening is not taken into account then the model fit is a bit too deep at the start and a bit too shallow at the end of the transit. One of the benefits of detecting this effect of gravity darkening is that it then tells us the true angle between the star’s spin axis and the planet’s orbit (whereas other methods, such as Doppler tomography, only tell us the projection of that angle onto the sky).

A first planet for the Next Generation Transit Survey

The latest transit survey to announce their first planet is the Next Generation Transit Survey. While the planet NGTS-1b has a fairly normal mass and radius for a hot Jupiter, it is unusual in being found transiting an M0-type dwarf, a star of only 0.6 solar radii. Thus the planet is nearly a quarter as big as the star, in terms of radius, the highest planet-to-star ratio yet found.

NGTS is an array of twelve 20-cm telescopes sited at Cerro Paranal in Chile, and has been accumulating survey data since 2016.

Next-Generation Transit Survey

It is important to realise that the newer survey NGTS does not supersede WASP, but instead complements it, being designed to do a different task. WASP, and similar surveys such as HATnet and KELT, use camera lenses (typically 200-mm f/1.8 or 85-mm f/1.2) to survey large swathes of sky. The data is good enough to detect transits of Jupiter- and Saturn-sized planets, but not smaller ones.

NGTS was designed to find smaller planets, down to Neptune and possibly super-Earth size. To do that it uses bigger optics, being telescopes rather than camera lenses, with a much better plate scale (more CCD pixels per chunk of sky). This gives much better photometry, but at the price of a much smaller field of view. A smaller field of view means covering many fewer bright stars.

Indeed, NGTS has a field of view comparable to the Kepler field (1% of the sky), though since it will raster several fields it will add up to sky coverage comparable to that of the Kepler K2 mission phase.

Thus WASP, running with 200-mm lenses surveying much of the sky, finds Jupiters and Saturns transiting stars of typically V = 9 to 13. NGTS can find smaller planets, and is aimed at finding Neptunes, but will likely find them transiting fainter stars of typically V = 13 to 14 (and perhaps, as with K2, an occasional brighter one).

Meanwhile, WASP-South has recently been running with wider, 85-mm lenses, which cover the whole Southern sky and target stars of V = 6.5 to 11.5. Hence the two surveys are entirely complementary: WASP aiming for large, Jupiter-sized planets around very bright stars, while NGTS aims for Neptune-sized planets around much fainter stars.

The main competition for WASP is now KELT and MASCARA, whereas the main competition for NGTS is the ongoing K2 mission. Of course NASA’s forthcoming TESS mission, set for launch in 2018, should out-compete all of the ground-based surveys.