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1\13 ESO - Nearest Brown Dwarf- AVO Press Conference- Teachers' Summer School
Part 2 of 3

At this point, the object was thought to be an isolated traveller. 
However, a search through available online catalogues quickly revealed 
that just 7 arcminutes away was a well-known star, Epsilon Indi. The 
two share exactly the same very large proper motion, and thus it was 
immediately clear the two must be related, forming a wide binary 
system separated by more than 1500 times the distance between the Sun 
and the Earth.

Epsilon Indi is one of the 20 nearest stars to the Sun at just 11.8 
light years [5]. It is a dwarf star (of spectral type K5) and with a 
surface temperature of about 4000 degC, somewhat cooler than the Sun. 
As such, it often appears in science fiction as the home of a 
habitable planetary system [6]. That all remains firmly in the realm 
of speculation, but nevertheless, we now know that it most certainly 
has a very interesting companion.

This is a remarkable discovery: Epsilon Indi B is the nearest 
star-like source to the Sun found in 15 years, the highest proper 
motion source found in over 70 years, and with a total luminosity just 
0.002% that of the Sun, one of the intrinsically faintest sources ever 
seen outside the Solar System!

After Proxima and Alpha Centauri, the Epsilon Indi system is also just 
the second known wide binary system within 15 light years. However, 
unlike Proxima Centauri, Epsilon Indi B is no ordinary star.

Brown dwarfs: cooling, cooling, cooling ...
-------------------------------------------

ESO PR Photo 03b/03             Caption: PR Photo 03b/03 shows the
                                near-IR (0.9-2.5 micron) spectrum
                                of Epsilon Indi B, obtained on
                                November 16-17, 2002, with the SOFI
[Preview - JPEG: 480 x 400 pix  multi-mode instrument on the ESO
 - 41k]                         3.5-m New Technology Telescope (NTT)
[Normal - JPEG: 960 x 800 pix   at the La Silla Observatory (Chile)    
- 120k]                         The total integration time is 360      
[Full-Res - JPEG: 2200 x 1834   sec. Regions of strong absorption in   
pix - 304k]                     the Earth's atmosphere have been       
                                removed for clarity. The locations
                                of prominent molecular absorption
                                bands from water (H2O), methane
                                (CH4) and carbon monoxide (CO) in
                                the atmosphere of Epsilon Indi B are
                                indicated. Also labelled are some
                                spectral lines from potassium (KI,
                                at 1.25 and 1.52 micron) and sodium
                                (NaI, at 2.33 micron) atoms. From these
                                data, the spectral type of Epsilon
                                Indi B is determined as T2.5V,
                                corresponding to an effective
                                temperature of 'just' 1000 +- 60 degC.

Within days of its discovery in the database, the astronomers managed 
to secure an infrared spectrum of Epsilon Indi B using the SOFI 
instrument on the ESO 3.5-m New Technology Telescope (NTT) at the La 
Silla Observatory (Chile). The spectrum showed the broad absorption 
features due to methane and water steam in its upper atmosphere, 
indicating a temperature of 'only' 1000 degC. Ordinary stars are never 
this cool - Epsilon Indi B was confirmed as a brown dwarf.

Brown dwarfs are thought to form in much the same way as stars, by the
gravitational collapse of clumps of cold gas and dust in dense 
molecular clouds. However, for reasons not yet entirely clear, some 
clumps end up with masses less than about 7.5% of that of our Sun, or 
75 times the mass of planet Jupiter. Below that boundary, there is not 
enough pressure in the core to initiate nuclear hydrogen fusion, the 
long-lasting and stable source of power for ordinary stars like the 
Sun. Except for a brief early phase where some deuterium is burned, 
these low-mass objects simply continue to cool and fade slowly away 
while releasing the heat left-over from their birth.

Theoretical discussions of such objects began some 40 years ago. They 
were first named 'black dwarfs' and later 'brown dwarfs', in 
recognition of their predicted very cool temperatures. However, they 
were also predicted to be very faint and very red, and it was only in 
1995 that such objects began to be detected.

The first were seen as faint companions to nearby stars, and then 
later, some were found floating freely in the Solar neighbourhood. 
Most brown dwarfs belong to the recently classified spectral types L 
and T, below the long-known cool dwarfs of type M. These are very red 
to human eyes, but L and T dwarfs are cooler still, so much so that 
they are almost invisible at optical wavelengths, with most of their 
emission coming out in the infrared.  [7].

How massive is Epsilon Indi B?
------------------------------
The age of most brown dwarfs detected to date is unknown and thus it 
is hard to estimate their masses. However, it may be assumed that the 
age of Epsilon Indi B is the same as that of Epsilon Indi A, whose age 
is estimated to be 1.3 billion years based on its rotational speed. 
Combining this information with the measured temperature, brightness, 
and distance, it is then possible to determine the mass of Epsilon 
Indi B using theoretical models of brown dwarfs.

Two independent sets of models yield the same result: Epsilon Indi B 
must have a mass somewhere between 4-6% of that of the Sun, or 40-60 
Jupiter masses. The most likely value is around 45 Jupiter masses, 
i.e. well below the hydrogen fusion limit, and definitively confirming 
this new discovery as a bona-fide brown dwarf.

The importance of Epsilon Indi B
--------------------------------
ESO PR Photo 03c/03               Caption: PR Photo 03c/03 displays a
                                  3D map of all known stellar systems
                                  in the solar neighbourhood within a
                                  radius of 12.5 light-years. The Sun
[Preview - JPEG: 469 x 400 pix -  is at the centre and the Epsilon
77k]                              Indi binary system with the newly   
[Normal - JPEG: 937 x 800 pix -   found brown dwarf Epsilon Indi B        
328k]                             lies near the bottom. The colour is     
[Full-Res - JPEG: 2718 x 2321 pix indicative of the temperature and       
- 3.1M]                           the spectral class - white stars are    
[Java Applet]                     (main-sequence) A and F dwarfs;         
                                  yellow stars like the Sun are G
                                  dwarfs; orange stars are K dwarfs;
                                  and red stars are M dwarfs, by far
                                  the most common type of star in the
                                  solar neighbourhood. The blue axes
                                  are oriented along the galactic
                                  coordinate system, and the radii of
                                  the rings are 5, 10, and 15
                                  light-years, respectively. The Java
                                  Applet conveniently provides
                                  detailed information about the stars
                                  in the figure - just move the cursor
                                  over the field. The figure is
                                  adapted from a diagram by Richard
                                  Powell.

PR Photo 03c/03 shows the current census of the stars in the solar
neighbourhood. All these stars have been known for many years, 
including GJ1061, which, however, only had its distance firmly 
established in 1997.  The discovery of Epsilon Indi B, however, is an 
extreme case, never before catalogued, and the first brown dwarf to be 
found within the 12.5 light year horizon.

If current predictions are correct, there should be twice as many 
brown dwarfs as main sequence stars. Consequently, Epsilon Indi B may 
be the first of perhaps 100 brown dwarfs within this distance, still 
waiting to be discovered!

(continued)

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