Astronomy and Astrophysics – Astrophysics
Scientific paper
Mar 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005eso..pres....5.&link_type=abstract
ESO Press Release, 03/2005
Astronomy and Astrophysics
Astrophysics
Scientific paper
Very Large Telescope Finds Planet-Sized Transiting Star
Summary
An international team of astronomers have accurately determined the radius and mass of the smallest core-burning star known until now.
The observations were performed in March 2004 with the FLAMES multi-fibre spectrograph on the 8.2-m VLT Kueyen telescope at the ESO Paranal Observatory (Chile). They are part of a large programme aimed at measuring accurate radial velocities for sixty stars for which a temporary brightness "dip" has been detected during the OGLE survey.
The astronomers find that the dip seen in the light curve of the star known as OGLE-TR-122 is caused by a very small stellar companion, eclipsing this solar-like star once every 7.3 days.
This companion is 96 times heavier than planet Jupiter but only 16% larger. It is the first time that direct observations demonstrate that stars less massive than 1/10th of the solar mass are of nearly the same size as giant planets. This fact will obviously have to be taken into account during the current search for transiting exoplanets.
In addition, the observations with the Very Large Telescope have led to the discovery of seven new eclipsing binaries, that harbour stars with masses below one-third the mass of the Sun, a real bonanza for the astronomers.
PR Photo 06a/05: Brightness "Dip" and Velocity Variations of OGLE-TR-122. PR Photo 06b/05: Properties of Low-Mass Stars and Planets. PR Photo 06c/05: Comparison Between OGLE-TR-122b, Jupiter and the Sun.
The OGLE Survey
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 [1].
These "planetary transits" are of great interest as they allow astronomers to measure in a unique way the mass and the radius of exoplanets. Several surveys are therefore underway which attempt to find these faint signatures of other worlds.
One of these programmes is the OGLE survey which was originally devised to detect microlensing events by monitoring the brightness of a very large number of stars over extended time intervals. During the past years, it has also included a search for periodic, very shallow "dips" in the brightness of stars, caused by the regular transit of small orbiting objects (small stars, brown dwarfs [2] or Jupiter-size planets). The OGLE team has since announced 177 "planetary transit candidates" from their survey of several hundred thousand stars in three southern sky fields, one in the direction of the Galactic Centre, another within the Carina constellation and the third within the Centaurus/Musca constellations.
The nature of the transiting object can however only be established by subsequent radial-velocity observations of the parent star. The size of the velocity variations (the amplitude) is directly related to the mass of the companion object and therefore allows discrimination between stars and planets as the cause of the observed brightness "dip".
A Bonanza of Low-Mass Stars
An international team of astronomers [3] has made use of the 8.2-m VLT Kueyen telescope for this work. Profiting from the multiplex capacity of the FLAMES/UVES facility that permits to obtain high-resolution spectra of up to 8 objects simultaneously, they have looked at 60 OGLE transit candidate stars, measuring their radial velocities with an accuracy of about 50 m/s [4].
This ambitious programme has so far resulted in the discovery of five new transiting exoplanets (see, e.g., ESO PR 11/04 for the announcement of two of those).
Most of the other transit candidates identified by OGLE have turned out to be eclipsing binaries, that is, in most cases common, small and low-mass stars passing in front of a solar-like star. This additional wealth of data on small and light stars is a real bonanza for the astronomers.
Constraining the Relation Between Mass and Radius
Low-mass stars are exceptionally interesting objects, also because the physical conditions in their interiors have much in common with those of giant planets, like Jupiter in our solar system. Moreover, a determination of the sizes of the smallest stars provides indirect, crucial information about the behaviour of matter under extreme conditions [5].
Until recently, very few observations had been made and little was known about low-mass stars. At this moment, exact values of the radii are known only for four stars with masses less than one-third of the mass of the Sun (cf. ESO PR 22/02 for measurements made with the Very Large Telescope Interferometer) and none at all for masses below one-eighth of a solar mass.
This situation is now changing dramatically. Indeed, observations with the Very Large Telescope have so far led to the discovery of seven new eclipsing binaries, that harbour stars with masses below one-third the mass of the Sun.
This new set of observations thus almost triples the number of low-mass stars for which precise radii and masses are known. And even better - one of these stars now turns out to be the smallest known!
Planet-Sized Stars
ESO PR Photo 06a/05
ESO PR Photo 06a/05
Brightness "Dip" and Velocity Variations of OGLE-TR-122
[Preview - JPEG: 400 x 474 pix - 33k] [Normal - JPEG: 800 x 948 pix - 176k]
Caption: The top panel of ESO PR Photo 06a/05 shows the brightness dip of OGLE-TR-122 as measured by OGLE. The signal from the star is reduced by 1.5% for a little more than 3 hours. This is the probable indication that an object passed in front of the star. The bottom panel presents the velocity variations of the star. They were determined with the FLAMES instrument on the VLT. The orbital solution fitting the data is also shown as the solid line. These measurements indicate the presence of a low-mass stellar companion to OGLE-TR-122.
ESO PR Photo 06b/05
ESO PR Photo 06b/05
Properties of Low-Mass Stars and Planets
[Preview - JPEG: 400 x 464 pix - 23k] [Normal - JPEG: 800 x 928 pix - 130k]
Caption: ESO PR Photo 06b/05 illustrates the properties of low-mass stars and planets, expressed in solar units. The newly determined, precise values of the mass and radius of OGLE-TR-122b are indicated as the red dot. The blue symbols are values for low-mass stars, while the black symbols on the left represent exoplanets. Note that the "hot Jupiters" - exoplanets orbiting very close to their host star - are larger than OGLE-TR-122b. The various lines represent theoretical models from G. Chabrier, I. Baraffe and colleagues, showing a good agreement between theory and observations.
The newly found stellar gnome is the companion of OGLE-TR-122, a rather remote star in the Milky Way galaxy, seen in the direction of the southern constellation Carina.
The OGLE programme revealed that OGLE-TR-122 experiences a 1.5 per cent brightness dip once every 7 days 6 hours and 27 minutes, each time lasting just over 3 hours (about 188 min). The FLAMES/UVES measurements, made during 6 nights in March 2004, reveal radial velocity variations of this period with an amplitude of about 20 km/s. This is the clear signature of a very low-mass star, close to the Hydrogen-burning limit, orbiting OGLE-TR-122. This companion received the name OGLE-TR-122b.
As François Bouchy of the Observatoire Astronomique Marseille Provence (France) explains: "Combined with the information collected by OGLE, our spectroscopic data now allow us to determine the nature of the more massive star in the system, which appears to be solar-like".
This information can then be used to determine the mass and radius of the much smaller companion OGLE-TR-122b. Indeed, the depth (brightness decrease) of the transit gives a direct estimate of the ratio between the radii of the two stars, and the spectroscopic orbit provides a unique value of the mass of the companion, once the mass of the larger star is known.
The astronomers find that OGLE-TR-122b weighs one-eleventh of the mass of the Sun and has a diameter that is only one-eighth of the solar one. Thus, although the star is still 96 times as massive as Jupiter, it is only 16% larger than this giant planet!
A Dense Star
"Imagine that you add 95 times its own m
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