RSAA News of the Month: May 2005
Searching for Younger Suns
ANU telescope helps select candidates for possible new solar systems
For the past three years Prof. Mike Bessell has been using the ANU 2.3m telescope at Siding Spring
Observatory to search for young, nearby stars. In particular, he
has been investigating stars which are at age where current theory
suggests that planets may be forming around them. Mike is part of an
international team which also includes astronomers from the Carnegie Institution of
Washington, the Spitzer
Science Centre, UCLA, and the
European Southern Observatory.
The search has been concentrated on identifying members of two groups
of stars, the Beta Pictoris and the TW Hydrae associations. Both
groups of stars are very young. The Beta Pictoris association is 12
million years (12Myr) old and the TW Hydrae group is only 10 Myr. This
is the age range where new stars may be surrounded by a dusty disk of
material, left over from the formation of the star. These
"protoplanetary disks" may be forming planets. Observations
by optical and infrared telescopes can detect such disks, and may see
features which indicate the presence of planets.
Part of the Scorpius-Centaurus association.
This image covers the Milky Way from east of Scorpius to west of the
Southern Cross.
The Beta Pictoris and TW Hydrae associations are moving out of this
region of the sky as the Sco-Cen association breaks up.
Study of groups moving out of Sco-Cen is providing valuable information
on how newborn stars populate the galaxy.
Image: Bessell, Sutherland and Buxton (RSAA)
The main problem with finding these stars is that because they are
so close, only a few hundred light years away, they are scattered
over huge areas of sky. First step in identifying them is done by
looking at the "proper motions", the directions and
speeds that the stars are moving across the line of sight. Nearby
stars have large proper motion, and most have been measured by the
Hipparcos satellite. Stars which have similar proper motions were
probably born in the same region of space, at the same time.
X-ray satellites, like the Chandra observatory,
provide an indication of age. Very young stars emit strong X-ray
flares. First pass in the search program was to find stars with large
proper motion and a history of X-ray flares. This work was done by Dr
Ben Zuckerman and the USA team members, who then sent the shortlist to
the optical observatories for the next stage in the identification
process.
Prof. Bessell has done the vital spectroscopic observations on the
southern candidates. Before a star can be positively identified as a
member of an association, there are two more measurements needed,
radial velocity and distance. Proper motions only measures a star's
movement across the line of sight in two dimensions. Motion along the
line of sight (radial velocity) is one of the parameters that Mike
obtains from his spectroscopy.
Left: The 2.3m telescope at Siding Spring Observatory.
Two instruments are normally mounted at the Nasmyth focii of the
telescope.
In this image a CCD imager is mounted at the left-hand focus and
the Double Beam Spectrograph (DBS) used for this research is
on the right-hand focus.
Right: Close-up of the DBS.
Spectroscopy also tells the temperature of the star and this allows an
estimate of the brightness and distance to be made. A star's
spectrum is an even better indicator of youth than X-ray emission;
Mike searches the spectra Lithium and for strong emission lines of
Hydrogen. From the chemistry and temperature the age can be
determined and an estimate made of the mass of the star.
Combining proper motions, radial velocity and distance gives the
motions of the candidates in three dimensions and ties down their
membership of the moving groups. The Beta Pictoris group is a good
example of the identification process at work. Zuckerman's group
originally considered 22,000 stars whose proper motions made them
possible members. Choosing only those whose space motions were
within a few kilometers per second of each other and of Beta
Pictoris, and which also showed at least one indicator of extreme
youth, the list was reduced to just 18 members.
The 2.3m observations have found many young, nearby, low mass stars
that are now being observed by Zuckerman and collaborators. They
are searching for protoplanetary disks and evidence of planets
using the largest telescopes on Earth; the Keck Telescopes in
Hawaii, the VLT in
Chile, and the Hubble Space
Telescope (HST).
The team has found several protoplanetary disks in the TW Hydrae and
Beta Pictoris associations. Beta Pictoris was in fact one of the
first such discs found, and has been widely studied. For more
information on Beta Pic and its possible planets, click
here.
HST and VLT imaging of one of the TW Hydrae stars identified by the
2.3m, 2M1207334, show that it has a planetary companion, the first
ever to be directly photographed. For more information, click
here.
Last month they announced the discovery of a companion to AB
Pictoris. Spectroscopy shows that it is around 13 Jupiter masses. This
is massive enough that it is, at present, impossible to tell if it is
a very large planet or a very small Brown Dwarf star. For the AB
Pictoris companion discovery paper, click here.
Left: The first image of an extra-Solar planet.
The red object is a planet about 5 times the mass of Jupiter,
orbiting the brown dwarf star 2M1207334.
The planet is nearly twice as far from its star as Neptune is from the Sun.
Image: European Southern Observatory, VLT.
Right: Image of AB Pictoris and its companion planet (or
brown dwarf).
The dark obstruction is an occulting disk used to block the light
from AB Pic and
stop it saturating the image and hiding the companion.
Image: Chauvin, Lagrange, Zuckerman, et.al.
Work is continuing, using the 2.3m and the 4m Anglo Australian
Telescope. to examine fainter candidates for membership of the
associations.
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