Some astrophysical phenomena are periodic, such as the transit of an extrasolar planet in front of its host star, or the oscillation modes with which stars vibrate. To study these variations, astronomers must be able to observe them as they repeat themselves, which requires uninterrupted observations over long periods of time.
Uninterrupted observations for 3 months !
With a few hours of observing time per day at the usual mid-latitude astronomical sites (which may be lost to clouds anyway), the first few measurements have yielded new scientific insights. Dome C, located near the South Pole at a latitude of 75?S, with a 3-month long night every year, is therefore an exceptional site for such observations, second only to space.
? E. Aristidi ? G. Dargaud
At Dome C, the circumpolar stars are turning around the South Celestial Pole during the 3 southern winter?s months, without never going below the horizon; that is to say one hundred days during which the astronomers can observe these stars continuously.
The first winterovers suggest that the site has excellent weather conditions, with nearly 90% clear time, i.e. without even cirrus cloud. Very precise intensity measurements are often limited by scintillation in the atmosphere, and this effect is at least 3.5 times weaker at Dome C than at mid-latitude sites, particularly those in Chile. These conditions are uniquely suitable to long-duration monitoring programmes.
Detecting partial eclipses
The detection of planets around other stars (extrasolar planets or exoplanets) and the analysis of their characteristics (orbital period, mass, chemical composition of their atmosphere...) constitute one of the central problems of contemporary astrophysics. Since the first exoplanet discovery in 1995, more than 280 have been found, mostly indirectly. In a decade or two, instruments may be able to image these planets directly - see To observe small details on astrophysical objects ? but, with a few exceptions, the methods currently used rely on detecting the effect of the planet on the star it orbits. For example, when a planet passes between its star and an observer, the observer first sees a drop in the brightness of the star, which returns to normal when the planet no longer blocks the starlight. This drop in brightness (a transit) is about 1% or less, lasts around 2 hours (the time for the planet in its orbit around the star to pass from one edge of the star to the other), and is repeated very regularly (every time the planet completes an orbit around its star). This transit method allows us to determine the period and size of the planet's orbit (from the interval between transits) and the size of the planet (from the duration of the transit and the change in the star's brightness). By measuring the motion of the star (using other techniques) it is even possible to measure the planet?s mass. The combination of planetary mass and size gives clues to a fundamental characteristic of the planet: its composition. Planets can be made primarily of light gases (hydrogen and helium, like Jupiter and Saturn in our own Solar system), ices (like Uranus and Neptune) or rocks (like the Earth and Mars).
? IMAGE - NASA
Dome C, located on the high Antarctic plateau, preserved from the depression systems which turn around the continent. Moreover, the site is almost in the center of ?the auroral belt?, as one can see on this satellite picture. The astronomical observations are thus not polluted by these magnetic phenomena
The transit method offers a further advantage: During a transit, starlight travels through the planet's atmosphere, allowing astronomers to detect atoms and molecules that make it up. Because the planet is a million to a billion times fainter than the star, direct detection of the atmosphere would require telescopes much larger than we have today. This technique could be used to detect sodium, methane and water vapour in exoplanetary atmospheres, and even signs of planets being evaporated by very nearby stars.
? F. Bouchy et al (2005)
The transit method consists in detecting a drop in brightness when an exoplanet passes between its star and an observer. The brightness?s curve presented here (F. Bouchy et al., 2005) shows the gas planet?s presence of approximately 1.15 Jovian masses, orbiting around its star in only 2.2 days. This type of planet is called ?hot Jupiter?.
The greatest drawback of this detection method is that very few planets pass directly infront of their host star in their orbits: It is estimated that, out of a sample of 1000 Sun-like stars, only one will have a giant planet (like Jupiter) observable in transit, and only planets very close to their star (orbiting in only a few days) will be detected. To find these rare systems, the brightness of a great many stars must be monitored very precisely and continuously, so as not to miss the transit event. This is why only forty transiting planets have been detected to date.
Dome C, located on the high Antarctic plateau, is a very attractive site for the detection of exoplanetary transits. At Dome C, many stars - those which are close enough to the South Celestial Pole ? never go below the horizon during the winter (see picture), and can be observed continuously for 3 months during the southern winter. Very precise monitoring of the brightness of such stars is the purpose of the ASTEP project (Antarctica Search for Transiting Extrasolar Planets, http://pleiades.unice.fr/astep/). As a first step, since April 2008, a 10cm telescope has been permanently pointing at a star field centred on the South Celestial Pole. From the winter of 2010, a 40cm telescope will observe every star in a one square degree field (the size of 4 full moons). Between 3000 and 8000 dwarf stars per field will be monitored, not only to detect Jupiter-like planets around faint stars, but also to characterize the photometric quality of the sky at Dome C, in preparation for launching a more ambitious transit research programme from the ground.
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