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Stellar Magnetic Fields



 
 
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  #1  
Old November 19th 10, 10:04 AM posted to sci.astro.research
Robert L. Oldershaw
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Posts: 617
Default Stellar Magnetic Fields

I have noticed something that I think is quite interesting concerning
the Global Surface Dipole B Fields of stars that have measurable B
fields.

White Dwarf: R ~ 10^9 cm; B ~10^6 G

Red Dwarf: R ~ 10^10 cm; B ~ 10^4 G

F-G-K "Dwarfs": R ~ 10^11 cm; B ~ 10^2 G

Red Giants: R ~/ 10^12 cm; B ~/ 1 G

The pattern is like a 1/R^2 progression.

IF there were Kerr-Newman objects at the center of stars, with R ~
10^5.5 cm and B ~ 10^13 G, then the surface dipole B fields for the
stars would just be the 1/R^2 values of the central field.

It has not escaped my attention that neutron star-like objects would
make rather efficient nucleating objects for the formation of stars.

This is an unorthodox and speculative idea, but consider that star
formation and the explanation of stellar surface B fields are two
major unresolved problems in astrophysics.

When a star goes supernova, what's left behind? Right an ultracompact
object with an radius not far from 10^5.5 cm and an average B of about
10^13 G.

Worth considering objectively, I think.

RLO
www.amherst.edu/~rloldershaw
  #2  
Old November 19th 10, 12:59 PM posted to sci.astro.research
Phillip Helbig---undress to reply
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Posts: 629
Default Stellar Magnetic Fields

In article , "Robert L.
Oldershaw" writes:

I have noticed something that I think is quite interesting concerning
the Global Surface Dipole B Fields of stars that have measurable B
fields.


The pattern is like a 1/R^2 progression.


Which is rather generic and probably easy to reproduce by various means.

IF there were Kerr-Newman objects at the center of stars, with R ~
10^5.5 cm and B ~ 10^13 G, then the surface dipole B fields for the
stars would just be the 1/R^2 values of the central field.


This is taking a data set and retroactively fitting some adjustable
parameters to match. Shouldn't a theory PREDICT stuff? What use is a
theory with adjustable parameters so that one can fit various data sets
by adjusting the parameters?

It has not escaped my attention that neutron star-like objects would
make rather efficient nucleating objects for the formation of stars.


How did the first neutron-star like objects form?

This is an unorthodox and speculative idea, but consider that star
formation and the explanation of stellar surface B fields are two
major unresolved problems in astrophysics.


I don't know much about the second but the first is not a major
unresolved problem. Sure, not all the details are known, but the basic
stuff has been known for decades.

When a star goes supernova, what's left behind? Right an ultracompact
object with an radius not far from 10^5.5 cm and an average B of about
10^13 G.


Keep in mind that no-one has actually observed this; it is a conclusion
based on the models of stellar structure and evolution which, above, you
seem rather comfortable about doubting.
  #3  
Old November 23rd 10, 10:21 AM posted to sci.astro.research
Robert L. Oldershaw
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Posts: 617
Default Stellar Magnetic Fields

There is some history to the idea of ultracompact objects at the
centers of stars.

Independently Gamow and Landau proposed the idea in 1937. Fermi
(1950s) discussed the possibility that stars with ultracompact cores
might have the properties of Red Supergiant stars. Then Thorne and
Zytkow worked on the idea starting in 1975.

For a review of early and later efforts see: Eich, C. et al,
Astrophysical Journal, 346, 277-283, 1989.

Previous authors appear to have only considered the applicability of
the idea to Giant and Supergiant stars. The model discussed in this
thread involves Kerr-Newman objects rather than neutron cores, and
proposes that such objects might be found at the centers of all stars,
rather than just the largest stars.

One cannot fail to notice that young stars are often associated with
Herbig-Haro phenomena, which are prodigious, narrow, high-energy jets
emitted from the centers of the young stars. When one can identify the
source of high-energy jets in other contexts, such as in AGN, quasars,
pulsars, black holes, and Gamma-ray bursts, there is usually an
ultracompact object involved. Possibly the H-H phenomena have a
similar origin?

RLO
www.amherst.edu/~rloldershaw
  #4  
Old November 25th 10, 12:30 AM posted to sci.astro.research
Thomas Smid
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Posts: 151
Default Stellar Magnetic Fields

On Nov 19, 9:04*am, "Robert L. Oldershaw"
wrote:
I have noticed something that I think is quite interesting concerning
the Global Surface Dipole B Fields of stars that have measurable B
fields.

White Dwarf: R ~ 10^9 cm; B ~10^6 G

Red Dwarf: R ~ 10^10 cm; B ~ 10^4 G

F-G-K "Dwarfs": R ~ 10^11 cm; B ~ 10^2 G

Red Giants: R ~/ 10^12 cm; B ~/ 1 G

The pattern is like a 1/R^2 progression.


Where did you get these R/B figures from? If you look for instance at
http://en.wikipedia.org/wiki/White_dwarf , then you can find there
that the magnetic field of white dwarfs can differ by many orders of
magnitude. So it seems you have been rather selective here in order to
establish the 1/R^2 relationship.

The point is that, independently of the radius, the magnetic field
should definitely depend on the rotational frequency of the star as
well (what is needed for a magnetic field in general is a current, and
you should always get a current if you have a rotating plasma, as the
electrons and protons have different masses and thus will respond
differently to the centrifugal force, and there will thus be a very
slight differential rotation of the positive and negative charges,
i.e. a current and thus a magnetic field).

Following this idea, I looked a while ago I looked into the magnetic
field of the planets, and found that the difference between the
Earth's and Jupiter's magnetic momenta seems indeed to be explained if
one assumes it is associated with the centrifugal force acting on the
matter in its (fluid or gaseous) interior, i.e. if one takes it as
proportional to the centrifugal force of the planet or star

F= M* w^2 *R ,

where M is the total mass, w the angular rotational frequency and R
the radius (give or take a constant factor).

As mentioned, F would determine the magnetic moment. The surface
magnetic field would then decrease with a further factor 1/R^3 (dipole
field), so B would go like

B ~ M * w^2 *1/R^2 .

So the surface magnetic field should not just be given by the 1/R^2
behaviour but also by the angular rotation frequency w and the mass M.

Thomas
  #5  
Old November 25th 10, 01:28 PM posted to sci.astro.research
Jonathan Thornburg [remove -animal to reply][_3_]
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Posts: 137
Default Stellar Magnetic Fields

Thomas Smid wrote:
Where did you get these R/B figures from? If you look for instance at
http://en.wikipedia.org/wiki/White_dwarf , then you can find there
that the magnetic field of white dwarfs can differ by many orders of
magnitude. So it seems you have been rather selective here in order to
establish the 1/R^2 relationship.


Moreover, for the one case for which we have excellent data (the Sun),
the magnetic field on a stellar surface is highly variable in both
space and time, in ways which don't accord well with the magnetic
field of a Kerr-Newman black hole. Magnetic fields of other stars
are also known to vary a lot over space and time, although our data
for other stars doesn't have the spatial resolution that we have for
the Sun.

--
-- "Jonathan Thornburg [remove -animal to reply]"
Dept of Astronomy, Indiana University, Bloomington, Indiana, USA
"Washing one's hands of the conflict between the powerful and the
powerless means to side with the powerful, not to be neutral."
-- quote by Freire / poster by Oxfam
  #6  
Old November 29th 10, 07:40 PM posted to sci.astro.research
Jonathan Thornburg [remove -animal to reply][_3_]
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Posts: 137
Default microquasars (was: Stellar Magnetic Fields)

[[This discussion isn't really about stellar magnetic fields any more,
so I've changed the subject line.]]

Robert L. Oldershaw wrote:
(2) Also see the recent singular example of discrete self-similarity
between microquasars and AGN/radio quasars, which is in press at MNRAS
and is discussed he

http://www.sciencedaily.com/releases...1124114711.htm


Thanks for posting this link, and thanks to the writers/editors of
that story for (for once!) including a link to the actual research!
They're talking about
http://arxiv.org/abs/1008.0394

This is a nice piece of observational work, but it's not clear to me how
it's meaningful to call the their data "a singular example of discrete
self-similarity".

[The Earth is, to a reasonable approximation, spherical in shape.
Saturn is, to a reasonable approximation, spherical in shape.
Does this identity of shapes
[explicable in terms of the Newtonian mechanics
of hydrostatic equilibrium]
constitute an example of discrete self-similarity?]

ciao,

--
-- "Jonathan Thornburg [remove -animal to reply]"
Dept of Astronomy, Indiana University, Bloomington, Indiana, USA
"Washing one's hands of the conflict between the powerful and the
powerless means to side with the powerful, not to be neutral."
-- quote by Freire / poster by Oxfam
  #7  
Old December 1st 10, 09:33 AM posted to sci.astro.research
Robert L. Oldershaw
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Posts: 617
Default Protostar Jets/AGN Jets Self-Similarity!

There has been an new development in the recent discovery of a
magnetic jet in a protostar, which was one of the subjects of my 11/29
post.

The authors of the paper: http://arxiv.org/PS_cache/arxiv/pdf/...011.6254v1.pdf
make the following timely and poignant observation.

"The polarization properties and the magnetic field configuration in
the HH 80-81 [protostar] jet are very similar to those observed in AGN
jets."

The concept that protostar jets might be generated by a central Kerr-
Newman ultra-compact object, or some form of neutron star-like object,
is supported by this new observation of self-similarity between
stellar and galactic phenomena.

RLO
www.amherst.edu/~rloldershaw
  #8  
Old December 18th 10, 09:31 PM posted to sci.astro.research
Robert L. Oldershaw
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Posts: 617
Default Eureka! The Proverbial Smoking Gun

A key hypothesis and organizing concept in this thread is that all
stars have ultracompact nuclear objects at their centers, just as
atoms do.

I have often wished that we could "X-ray a star" and see if a neutron-
star-like object actually could be seen at the center of the star,
well before it goes supernova.

It has been done! See: http://arxiv.org/PS_cache/arxiv/pdf/...007.3981v1.pdf

The object BP Piscium was originally thought to be a protostar because
it has huge, long, twin, pencil-beam jets emitted by its central
object.

Now the situation has changed dramatically. Researchers have realized
that BP Piscium appears to be a run-of-the-mill Red Giant star. They
have also identified an X-ray point source at the center of the star
as the source of the twin jets. The thinking is that the star accreted
extra mass/energy from an external source and is now blowing off
excess mass/energy and angular momentum as the central nucleus adjusts
to the interaction and returns to stability.

This is only one star, but it may be the Rosetta Stone of stellar
structure. More and more pieces of the puzzle are rapidly fitting
together. Very exciting!

RLO
www.amherst.edu/~rloldershaw

[Mod. note: this paper suggests that the X-ray emission comes from the
corona of the star or from star-disc interactions; they certainly do
not say it is a 'point source at the center of the star' -- which is
of course impossible -- or that it is associated with the jets. -- mjh]
  #9  
Old December 19th 10, 10:55 AM posted to sci.astro.research
Robert L. Oldershaw
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Posts: 617
Default Eureka! The Proverbial Smoking Gun


[Mod. note: this paper suggests that the X-ray emission comes from the
corona of the star or from star-disc interactions; they certainly do
not say it is a 'point source at the center of the star' -- which is
of course impossible -- or that it is associated with the jets. -- mjh]


It is true that I "jumped the shark" a bit on my characterization of
the X-ray data. The Chandra results indicate that PB Piscium is a
point source, but the actual size and shape of the X-ray emitting
source are unknown at this point. Assuming the stellar corona is the X-
ray source is arguably the most conservative, and least radical,
possibility.

It turns out that the most important piece of information gained from
the existing X-ray data is that the X-ray flux is 2 or 3 orders of
magnitude below what would be expected for a protostar/T Tauri star.

The identification of BP Pis as a red giant is supported by: (1) low X-
ray flux, (2) isolation of BP Pis far from any gas/dust clouds or
young stars, (3) a surface gravity that is too weak for a protostar,
and (4) a lithium abundance that fits with a RG but not a protostar.

The main piece of possible evidence for a central nuclear ultracompact
object is the humongous pair of parsec-scale jets associated with the
Red Giant star. The definitive prediction of the central K-N
utracompact object hypothesis is that the jets will be observed right
into the center of the star. The more conservative accretion disk
model would definitively predict that the jets are generated external
to the star. So we have a definitive up or down, right or wrong, test
of the type that has been all-too-uncommon in postmodern theoretical
physics. The prospect of such a definitive test is very exciting.

This unusual system may be a "normal" star undergoing a very rare
accretion event that can tell us new things about the internal
structure of "normal" stars. In this case it could be a Rosetta Stone
for interpreting stellar structure. Stay tuned.

RLO
www.amherst.edu/~rloldershaw
  #10  
Old December 19th 10, 02:15 PM posted to sci.astro.research
Phillip Helbig---undress to reply
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Posts: 629
Default Eureka! The Proverbial Smoking Gun

In article , "Robert L.
Oldershaw" writes:

[Mod. note: this paper suggests that the X-ray emission comes from the
corona of the star or from star-disc interactions; they certainly do
not say it is a 'point source at the center of the star' -- which is
of course impossible -- or that it is associated with the jets. -- mjh]


Indeed. Light takes a long time to get from deep within the star, where
it is generated, to the surface---thousands or millions of years, IIRC.
This is due to repeated scattering. It would thus seem strange if a
nice jet were able to come out of the core and retain its shape outside
the star.

It has been known for a long for a long time that the outer layers of
giant stars are very tenuous (in some cases, better than the best vacuum
on Earth) and the core region is very dense. To first order, one can
ignore the outer region and just look at the dense core. But it is a
dense core of a normal star, generating energy by fusion. Keep in mind
that solar models are extremely detailed, making very detailed
predictions involving helioseismology, which have now been observed in
detail. We know much more about the interior of the Sun, and other
stars, than about the interior of the Earth. Of course this is an
"ultra-compact nuclear object" (what else would it be?), but not in the
sense the OP is suggesting.

Solar models are so detailed and so well checked that, after the
experiments were deemed good enough, people decided on "new physics"
(hence confirmed: neutrino oscillations, implying that neutrinos have
mass) rather than doubt the models.
 




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