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1\22 ESO - Distant World in Peril Discovered from La Silla- ALMA- AVO
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        Information from the European Southern Observatory
 ESO Press Release 03/03
 22 January 2003                                           [ESO Logo]
 For immediate release
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Distant World in Peril Discovered from La Silla

Giant Exoplanet Orbits Giant Star

Summary

When, in a distant future, the Sun begins to expand and evolves into a
"giant" star, the surface temperature on the Earth will rise 
dramatically and our home planet will eventually be incinerated by 
that central body.

Fortunately for us, this dramatic event is several billion years away.
However, that sad fate will befall another planet, just discovered in
orbit about the giant star HD 47536, already within a few tens of 
millions of years. At a distance of nearly 400 light-years from us, it 
is the second-remotest planetary system discovered to date [1].

This is an interesting side-result of a major research project, now
carried out by a European-Brazilian team of astronomers [2]. In the 
course of a three-year spectroscopic survey, they have observed about 
80 giant stars in the southern sky with the advanced FEROS 
spectrograph on the 1.52-m telescope installed at the ESO La Silla 
Observatory (Chile). It is one of these stars that has just been found 
to host a giant planet. This is only the fourth such case known and 
with a diameter of about 33 million km (or 23.5 times that of our 
Sun), HD 47536 is by far the largest of those giant stars [1].

The distance of the planet from the star is still of the order of 300
million km (or twice the distance of the Earth from the Sun), a safe
margin now, but this will not always be so. The orbital period is 712
days, i.e., somewhat less than two Earth years, and the planet's mass 
is 5 - 10 times that of Jupiter.

The presence of exoplanets in orbit around giant stars, some of which 
will eventually perish into their central star (be "cannibalized"), 
provides a possible explanation of the anomalous abundance of certain 
chemical elements that is observed in the atmospheres of some stars, 
cf. ESO PR 10/01.

This interesting discovery bodes well for coming observations of
exoplanetary systems with new, more powerful instruments, like HARPS 
to be installed next year at the ESO 3.6-m telescope on La Silla and 
also the Very Large Telescope Interferometer (VLTI) at Paranal.

PR Photo 05a/02: Giant stars observed in this programme (HR-diagram)
PR Photo 05b/02: Giant star HD 47536.  PR Photo 05c/02: "Velocity 
curve" of HD 47536.  PR Photo 05d/02: Distance distribution of known 
exoplanets.
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Stellar evolution

The structure and evolution of stars like our Sun are quite well 
understood.  They are born by contraction in immense clouds of dust 
and gas and when the central density and temperature become high 
enough, nuclear fusion ignites in their interiors. Then follows a long 
period of relative calm - the Sun is now in this phase - that ends 
when the nuclear fuel runs out. A direct result is that the star 
begins to expand and soon becomes a "giant". During this phase, the 
surface temperature drops somewhat (but is still several thousand 
degrees) and the colour of the star changes from yellow to red.

In the case of the Sun, this will happen some billion years from now. 
At some moment, our star will become larger and the surface of our 
home planet will become exceedingly hot, incinerating whatever 
remaining lifeforms that cannot protect themselves.

Later, the Sun will shred its outer layers into space and a small, hot 
core will become visible. This final stage of stellar evolution can be 
observed as beautiful "Planetary Nebulae", e.g. the Dumbbell Nebula of 
which an impressive VLT photo is available (ESO PR Photos 38a-b/98).

A spectroscopic survey of giant stars
-------------------------------------
ESO PR Photo 05a/03            Caption: PR Photo 05a/03 shows part
                               of the Hertzsprung-Russell (HR)
                               diagram [3] - a very useful way to
                               illustrate stellar evolution.
[Preview - JPEG: 400 x 467 pix Plotting the temperature of
128k                           solar-type stars (abscissa; in
[Normal - JPEG: 800 x 933 pix -degrees Kelvin or as a "colour
288k]                          index") against their intrinsic
                               brightness (ordinate; in solar
                               units) reveals a typical
                               distribution (hotter stars are less
                               bright than cooler stars) that
                               reflect their different evolutionary
                               stages. With time, the position of
                               the Sun in this diagram (now at the
                               lower left) will migrate towards the
                               upper right as it expands and
                               becomes brighter. This direction
                               corresponds to increasing radius.
                               The approximately 80 stars plotted
                               here are those that are being
                               spectroscopically observed within
                               the present programme; cf. the text.
                               The positions and names of four
                               giant stars that are known to host
                               planets are marked [1]. The largest
                               and brightest of them is HD 47536,
                               as indicated by its upper-right
                               position, relative to the three
                               others.

Since 1999, a European-Brazilian team of astronomers [2] has been 
studying a selection of comparatively bright giant stars with the goal 
to learn more about their physical properties. In particular, detailed 
spectra have been obtained by means of the advanced FEROS spectrograph 
on the 1.52-m telescope that is installed at the ESO La Silla 
Observatory in Chile, cf. ESO PR 03/99.

About 80 stars have been regularly observed with this instrument, in 
order to search for possible velocity variations [4]. In PR Photo 
05a/03, their temperature and intrinsic brightness are plotted in the 
so-called Hertzsprung-Russell diagram [3], a very useful way of 
illustrating stellar evolution.

The background for this ambitious research project is that recent
observations indicate that some giant stars may undergo small velocity
variations with periods from days to years. While short-term 
variations are likely to be caused by oscillations in their extended 
and tenous atmospheres, there are at least three possible causes for 
long-term variations: 1) the gravitational pull of one or more 
orbiting planets, 2) radial pulsations of the entire star, or 3) 
variable surface patterns due to stellar activity.

Which of these possibilities are behind the observed velocity 
variations?  How many of those stars pulsate? Do some of them possess 
planets and if so, are planetary systems around giant stars common or 
not?

"These are very fundamental questions" says team leader Johny Setiawan 
of the Kiepenheuer-Institut in Freiburg (Germany), "and the present 
discovery was somehow unexpected. Many of our giant stars show similar 
long-period velocity variations which we suspect are due to stellar 
activity".

(continued)

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