Astronomy and Astrophysics – Astrophysics
Scientific paper
May 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004eso..pres...11.&link_type=abstract
ESO Press Release, 05/2004
Astronomy and Astrophysics
Astrophysics
Scientific paper
VLT Measures Properties of New Jupiter-Size Objects in Very Close Orbits
Summary
A European team of astronomers [1] are announcing the discovery and study of two new extra-solar planets (exoplanets). They belong to the OGLE transit candidate objects and could be characterized in detail. This trebles the number of exoplanets discovered by the transit method; three such objects are now known.
The observations were performed in March 2004 with the FLAMES multi-fiber spectrograph on the 8.2-m VLT Kueyen telescope at the ESO Paranal Observatory (Chile). They enabled the astronomers to measure accurate radial velocities for forty-one stars for which a temporary brightness "dip" had been detected by the OGLE survey. This effect might be the signature of the transit in front of the star of an orbiting planet, but may also be caused by a small stellar companion.
For two of the stars (OGLE-TR-113 and OGLE-TR-132), the measured velocity changes revealed the presence of planetary-mass companions in extremely short-period orbits.
This result confirms the existence of a new class of giant planets, designated "very hot Jupiters" because of their size and very high surface temperature. They are extremely close to their host stars, orbiting them in less than 2 (Earth) days.
The transit method for detecting exoplanets will be "demonstrated" for a wide public on June 8, 2004, when planet Venus passes in front of the solar disc, cf. the VT-2004 programme.
PR Photo 14a/04: Sky Field with OGLE-TR-113 PR Photo 14b/04: Sky Field with OGLE-TR-132 PR Photo 14c/04: Brightness "Dips" Caused by Two Transiting Exoplanets PR Photo 14d/04: Velocity Variations Caused by Two Transiting Exoplanets PR Photo 14e/04: Properties of Known Transiting Exoplanets
Discovering other Worlds
During the past decade, astronomers have learned that our Solar System is not unique, as more than 120 giant planets orbiting other stars were discovered by radial-velocity surveys (cf. ESO PR 13/00, ESO PR 07/01, and ESO PR 03/03).
However, the radial-velocity technique is not the only tool for the detection of exoplanets. When a planet happens to pass in front of its parent star (as seen from the Earth), it blocks a small fraction of the star's light from our view. The larger the planet is, relative to the star, the larger is the fraction of the light that is blocked.
It is exactly the same effect when Venus transits the Solar disc on June 8, 2004, cf. ESO PR 03/04 and the VT-2004 programme website. In the past centuries such events were used to estimate the Sun-Earth distance, with extremely useful implications for astrophysics and celestial mechanics.
Nowadays, planetary transits are gaining renewed importance. Several surveys are attempting to find the faint signatures of other worlds, by means of stellar photometric measurements, searching for the periodic dimming of a star as a planet passes in front of its disc.
One of these, the OGLE survey, was originally devised to detect microlensing events by monitoring the brightness of a very large number of stars at regular intervals. For the past four years, it has also included a search for periodical shallow "dips" of the brightness of stars, caused by the regular transit of small orbiting objects (small stars, brown dwarfs or Jupiter-size planets). The OGLE team has since announced 137 "planetary transit candidates" from their survey of about 155,000 stars in two southern sky fields, one in the direction of the Galactic Centre, the other within the Carina constellation.
Resolving the nature of the OGLE transits
ESO PR Photo 14a/04
ESO PR Photo 14a/04
Sky Field with OGLE-TR-113
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ESO PR Photo 14b/04
ESO PR Photo 14b/04
Sky Field with OGLE-TR-132
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Caption: ESO PR Photo 14a/04 and ESO PR Photo 14b/04 show the sky fields in the southern constellation Carina (The Ship Keel) with the images of two stars, designated OGLE-TR-113 and OGLE-TR-132, around which two transiting exoplanets have been identified. The fields measure 10 x 10 arcmin; North is up and East is left. These photos have been reproduced from the Digitized Sky Survey (DSS) [4].
The OGLE transit candidates were detected by the presence of a periodic decrease of a few percent in brightness of the observed stars. The radius of a Jupiter-size planet is about 10 times smaller than that of a solar-type star [2], i.e. it covers about 1/100 of the surface of that star and hence it blocks about 1 % of the stellar light during the transit.
The presence of a transit event alone, however, does not reveal the nature of the transiting body. This is because a low-mass star or a brown dwarf, as well as the variable brightness of a background eclipsing binary system seen in the same direction, may result in brightness variations that simulate the ones produced by an orbiting giant planet.
However, the nature of the transiting object may be established by radial-velocity observations of the parent star. The size of the velocity variations (the amplitude) are directly related to the mass of the companion object and therefore allow to discriminate between stars and planets as the cause of the observed brightness "dip".
In this way, photometric transit searches and radial-velocity measurements combine to become a very powerful technique to detect new exoplanets. Moreover, it is particularly useful for elucidating their characteristics. While the detection of a planet by the radial velocity method only yields a lower estimate of its mass, the measurement of the transit makes it possible to determine the exact mass, radius, and density of the planet.
The follow-up radial-velocity observations of the 137 OGLE transit candidates is not an easy task as the stars are comparatively faint (visual magnitudes around 16). This can only be done by using a telescope in the 8-10m class with a high-resolution spectrograph.
The nature of the two new exoplanets
ESO PR Photo 14c/04
ESO PR Photo 14c/04
Brightness "Dips" Caused by Two Transiting Exoplanets
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ESO PR Photo 14d/04
ESO PR Photo 14d/04
Velocity Variations Caused by Two Transiting Exoplanets
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Caption: ESO PR Photo 14c/04 shows the brightness "dips" of the stars OGLE-TR-113 (upper) and OGLE-TR-132 (lower), as observed during the OGLE survey. It is now known that they are caused by transiting exoplanets of the new "very hot Jupiter" class. The abscissa represents the orbital phase (one revolution = 1) and the ordinate the relative brightness. As can be seen the brightness variations are of the order of 3 % (upper) and 1% (lower), respectively. ESO PR Photo 14d/04 displays the corresponding radial velocity variations over one planetary revolution, as observed with the FLAMES facility at the 8.2-m VLT Kueyen telescope. The sinusoidal shape reflects the gravitational pull of the planet.
A European team of astronomers [1] therefore made use of the 8.2-m VLT Kueyen telescope. In March 2004, they followed 41 OGLE "top transit candidate stars" during 8 half-nights. They profited from the multiplex capacity of the FLAMES/UVES fiber link facility that permits to obtain high-resolution spectra of 8 objects simultaneously and measures stellar velocities with an accuracy of about 50 m/s.
While the vast majority of OGLE transit candidates turned out to be binary stars (mostly small, cool stars transiting in front of solar-type stars), two of the objects, known as OGLE-TR-113 and OGLE-TR-132, were found to exhibit small velocity variations. When all available observations - light variations, the stellar spectrum and radial-velocity changes - were combined, the astronomers were able to determine that for these two stars, the transiting objects have masses compatible with those of a giant planet like Jupiter.
Interestingly, both new planets were detected around rather remote stars in the Milky W
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