New Study Shows Very First Stars Not Monstrous

I have often heard in the last discussion the thesis that the first
stars should have been enormous, what would explain the accumulation of
heavy elements in galaxies very early in the story of the universe
according to the prevailing BB theory.

A recent study just published by JPL proposes the contrary. The first
stars should have been big but not THAT big (around 30-50 Solar masses)

The explanation is as follows:

quote
The team's simulations reveal that matter in the vicinity of the forming
stars heats up to higher temperatures than previously believed, as high
as 50,000 Kelvin (90,000 degrees Fahrenheit), or 8.5 times the surface
temperature of the sun. Gas this hot expands and escapes the gravity of
the developing star, instead of falling back down onto it. This means
the stars stop growing earlier than predicted, reaching smaller final sizes.
end quote

The press release of JPL is he
http://www.jpl.nasa.gov/news/news.cf...y&auid=9845190

More details can be found he
http://www-tap.scphys.kyoto-u.ac.jp/...tarstop_e.html

jacob navia wrote in
:
[Mod. note: quoted text trimmed -- please do this yourself -- mjh]
More details can be found he
http://www-tap.scphys.kyoto-u.ac.jp/...tarstop_e.html


Even more details he

http://arxiv.org/abs/0906.1607

I don't think you'll have an easy time convincing anyone that ~300 solar
masses is 'small'. Additionally, I'll place a dollar on the notion that in
that mass range you are running against the clock with respect to the
star's ability to survive vs the amount of time it needs to accrete more
matter.

Though the article is right, raw luminosity ought to blow gass out. I just
figured the limit would be higher...

On Nov 11, 11:20*pm, jacob navia wrote:

A recent study just published by JPL proposes the contrary. The first
stars should have been big but not THAT big (around 30-50 Solar masses)

The explanation is as follows:

quote
The team's simulations reveal that matter in the vicinity of the forming
stars heats up to higher temperatures than previously believed, as high
as 50,000 Kelvin (90,000 degrees Fahrenheit), or 8.5 times the surface
temperature of the sun. Gas this hot expands and escapes the gravity of
the developing star, instead of falling back down onto it. This means
the stars stop growing earlier than predicted, reaching smaller final sizes.
end quote

The kinetic energy of a particle required to escape from the surface
of star with a mass of 30 solar masses and a radius of (let's say) 10
solar radii would correspond to a temperature of about 7*10^7 K (see
my page http://www.plasmaphysics.org.uk/research/sun.htm ). So a gas
of 50,000 K couldn't even escape the gravitational field of the star
from a distance of less than about 1000 times the stellar radius.

Thomas

Le 14/11/11 22:36, Thomas Smid a écrit :

The kinetic energy of a particle required to escape from the surface
of star with a mass of 30 solar masses and a radius of (let's say) 10
solar radii would correspond to a temperature of about 7*10^7 K (see
my page http://www.plasmaphysics.org.uk/research/sun.htm ). So a gas
of 50,000 K couldn't even escape the gravitational field of the star
from a distance of less than about 1000 times the stellar radius.

Thomas

Mmmmm, I can't see any logical error in your reasoning... I went
to your site and yes, it looks OK.

I think the explanation lies in that it is NOT only kinetic energy
that makes the gas go away. It is also radiation pressure, i.e. the
energy of the photons that impact the atoms of the gas. That makes the
difference.

If I am not completely mistaken of course.

Thomas Smid wrote in
:

On Nov 11, 11:20*pm, jacob navia wrote:

A recent study just published by JPL proposes the contrary. The first
stars should have been big but not THAT big (around 30-50 Solar
masses)

The explanation is as follows:

quote
The team's simulations reveal that matter in the vicinity of the
forming stars heats up to higher temperatures than previously
believed, as high as 50,000 Kelvin (90,000 degrees Fahrenheit), or
8.5 times the surface temperature of the sun. Gas this hot expands
and escapes the gravity of the developing star, instead of falling
back down onto it. This means the stars stop growing earlier than
predicted, reaching smaller final sizes. end quote

The kinetic energy of a particle required to escape from the surface
of star with a mass of 30 solar masses and a radius of (let's say) 10
solar radii would correspond to a temperature of about 7*10^7 K (see
my page http://www.plasmaphysics.org.uk/research/sun.htm ). So a gas
of 50,000 K couldn't even escape the gravitational field of the star
from a distance of less than about 1000 times the stellar radius.

Thomas

The point is not that the gas escapes the star, but that the photon
emissions from the star blow back the gas so it cannot fall into the star.

If you think about it, there's obviously a tipping point where the
presssure of electromagnetic radiation balances out the attractive force of
gravitation.

On Nov 15, 10:49*am, jacob navia wrote:
Le 14/11/11 22:36, Thomas Smid a écrit :



The kinetic energy of a particle required to escape from the surface
of star with a mass of 30 solar masses and a radius of (let's say) 10
solar radii would correspond to a temperature of about 7*10^7 K (see
my pagehttp://www.plasmaphysics.org.uk/research/sun.htm). So a gas
of 50,000 K couldn't even escape the gravitational field of the star
from a distance of less than about 1000 times the stellar radius.

Thomas

Mmmmm, I can't see any logical error in your reasoning... I went
to your site and yes, it looks OK.

I think the explanation lies in that it is NOT only kinetic energy
that makes the gas go away. It is also radiation pressure, i.e. the
energy of the photons that impact the atoms of the gas. That makes the
difference.

If I am not completely mistaken of course.


I was merely commenting on your quote from the press release, which
clearly seems to suggest that heating of the gas is responsible for
the outflow, not radiation pressure. And after having a look now at
the more detailed background information available from the link you
mentioned ( http://www-tap.scphys.kyoto-u.ac.jp/...tarstop_e.html
), this is very much confirmed as the authors explicitly say that
'photoevaporation' following the heating by EUV radiation is the
mechanism at work here. But the results also seems to indicate that
this primarily happens at rather large distances (100 AU), so 50,000
K might be sufficient after all here for the assumed model.

However, the authors do not give any explanation how photoionization
could possibly raise the gas temperature by such an amount. The point
is that the photoelectrons have such a small mass that it takes
thousands of elastic collisions to transfer their energy to ions, but
they will actually recombine again after just a few collisions (as the
recombination cross section and the elastic collision cross section
have the same order of magnitude for electron energies of around
10eV). I have worked on this issue some years back in connection with
ionospheric physics (where by coincidence the gas densities are quite
similar the the protostar environment here) and the result was that
there is no way that the photoelectrons can transfer any significant
amount of energy to the ions or neutrals. They only lose energy either
through electron-impact excitation or ionization of neutrals or by
recombination, and neither of those increase in any way the kinetic
energy of the ions and neutrals in the gas (see my page
http://www.plasmaphysics.org.uk/research/elspec.htm ). The only
mechanisms that could raise the overall gas temperature would involve
molecules (i.e. photodissociation and excitation of vibration or
rotational states (which then could be turned into kinetic energy))..

Thomas