Barnard's Star and Kapteyn's Star

SolStation.com says Barnard's Star (
http://www.solstation.com/stars/barnards.htm ) is an old disk
star. It's at least 10 billion years old and has a very high
velocity WRT the Sun (about 140 km/s). Its metalicity is low,
less than 32% that of the Sun.

Kapteyn's Star ( http://www.solstation.com/stars/kapteyns.htm )
is described as a halo star. But its characteristics are about
the same: 10 billion years old, high velocity (not given by
SolStation, but it is quite high), and low metalicity (also less
than 32% Sol's). Also it's considered a subdwarf due to that low
metalicity.

So my question is why isn't Barnard's Star also a halo star? How
can they tell the difference? Also, why isn't Barnard's a
subdwarf since it has low metalicity?

--
Dan Tilque

"Dan Tilque" wrote in message
...
SolStation.com says Barnard's Star (
http://www.solstation.com/stars/barnards.htm ) is an old disk
star. It's at least 10 billion years old and has a very high
velocity WRT the Sun (about 140 km/s). Its metalicity is low,
less than 32% that of the Sun.

Kapteyn's Star ( http://www.solstation.com/stars/kapteyns.htm )
is described as a halo star. But its characteristics are about
the same: 10 billion years old, high velocity (not given by
SolStation, but it is quite high), and low metalicity (also less
than 32% Sol's). Also it's considered a subdwarf due to that low
metalicity.

So my question is why isn't Barnard's Star also a halo star? How
can they tell the difference? Also, why isn't Barnard's a
subdwarf since it has low metalicity?

--
Dan Tilque

I think Halo and Disk refer to posistion in the galaxy as opposed to
characteristics of the star.

Out of curiosity does halo refer to the area around the galaxy on it's plane
or above and below the galactic disk, or both.

"F" == Fitzdraco writes:

F "Dan Tilque" wrote in message
F ...
SolStation.com says Barnard's Star (
http://www.solstation.com/stars/barnards.htm ) is an old disk
star. It's at least 10 billion years old and has a very high
velocity WRT the Sun (about 140 km/s). Its metalicity is low, less
than 32% that of the Sun.

Kapteyn's Star ( http://www.solstation.com/stars/kapteyns.htm ) is
described as a halo star. But its characteristics are about the
same: 10 billion years old, high velocity (..), and low metalicity
(also less than 32% Sol's). Also it's considered a subdwarf due to
that low metalicity.

So my question is why isn't Barnard's Star also a halo star? How
can they tell the difference? Also, why isn't Barnard's a subdwarf
since it has low metalicity?

F I think Halo and Disk refer to posistion in the galaxy as opposed
F to characteristics of the star.

In general, yes, but not exclusively.

F Out of curiosity does halo refer to the area around the galaxy on
F it's plane or above and below the galactic disk, or both.

The Galactic halo is the spherical region around the Milky Way, URL:
http://spiff.rit.edu/classes/phys230...mw_struct.html
.

Stars in the Galactic halo tend to have nearly random orbits. In
contrast to the stars in the Galactic disk that all move on roughly
circular orbits in the same direction around the Galactic center,
stars in the halo have orbits that have random inclincations. Some of
them are retrograde. Halo stars do pass through the Galactic disk,
but because they are not travelling in the same direction as the disk
stars, they often have quite high relative velocities.

I'm less sure about how one identifies a star in the Galactic disk as
a halo star, other than by metallicity and velocity.

--
Lt. Lazio, HTML police | e-mail:
No means no, stop rape. | http://patriot.net/%7Ejlazio/
sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html

In article ,
"Dan Tilque" writes:
SolStation.com says Barnard's Star is an old disk
star.
....
Kapteyn's Star is described as a halo star.

Many properties distinguish the disk and halo populations: spatial
distribution, spectral types, velocities, and chemical abundances.
The fact that all of these are correlated is the evidence that these
populations are physically meaningful.

For stars close to the Sun, the main observable difference is
probably the 3-d space motion. I would expect halo stars to have
much larger velocities than disk stars both perpendicular to the
Galactic plane and also in the direction of Galactic rotation. (I
think these are U and W in the usual UVW velocity coordinates, and
I'm using velocities with respect to the Local Standard of Rest, LSR.
If you use a system in which the Galactic center is at rest, halo
stars would have small velocities in U, and the Sun and other disk
stars would have large velocities.)

If you want to pursue this, you can get data from SIMBAD and use
position on the sky, proper motion, parallax, and radial velocity to
convert to velocities with respect to the LSR. (The Sun's speed with
respect to the LSR is only 20 km/s, so you won't be far wrong if you
use heliocentric velocities.) Barnard's star is also known as V2500
Oph, and Kapteyn's star is HD 33793. This might not be a bad
exercise for an advanced undergraduate class. I hope the answer
comes out the way I expect! (Or maybe SolStation is wrong.)


--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)