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Doppler Tests on Local Stars



 
 
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  #51  
Old March 14th 07, 02:36 PM posted to sci.astro.research
Kent Paul Dolan
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Posts: 225
Default Doppler Tests on Local Stars

Joseph Lazio wrote:
Kent Paul Dolan writes:


Huh? Many pulsars are still part of the binary
partner stars with which they began their
existence.


Uhh, no. There is a subset, millisecond or
recycled pulsars, that tend to be binary. As a
whole, though, the population consists of isolated
objects.


Read the word "many" again; it doesn't say "most",
and meant to convey "a large number".

Statistical sampling from an effectively infinite
sample space can be done with really modest numbers
of measurements, I seem to recall something in the
middle 500s normally suffices for common statistical
studies.

The pulsars still bound with their binary companions
I'm guessing will more than suffice for the task at
hand, an independent measurement of orbital speed of
the galaxy at various radii, separate from red shift
[though I'm not sure I've convinced myself yet that
certain changes to "how the universe works" wouldn't
change both measurement methods in the same
directions, thus laying waste to "independent"].

There is no reason to believe that such stars
would have motions different in kind from the rest
of stars in the the galaxy with which they rotate.


Yes, there is. The average velocity of pulsars is
something like 450 km/s. (Ref: D. Lorimer has
done a lot of work on this topic.) That means
that most of them have velocities well in excess
of the typical star, and a good fraction of
pulsars are not bound to the Galaxy.


I didn't state what I meant clearly enough, and
as a result you've misinterpreted it. By "such
stars" I was referring back to pulsars still bound
to their binary companions, for which such
difficulties presumably do not exist.

[As a side issue, since I don't have access
to a technical library, whencefrom comes the
aberrant velocity of the rest of the
pulsars? Is "mono-jet propelled" common, so
that the star is currently still
accelerating away? Is blast asymmetry common
in supernova, so that the star gets a one
time huge boost like a squeezed watermelon
seed?]

Having said that, one can still use pulsars to
probe the structure of the Galaxy.


Moreover, pulsars are excellent tools for
exactly the kind of measurements you're
attempting to evaluate: [...]


No disagreement on the rest of the post.


xanthian.
  #52  
Old March 14th 07, 03:56 PM posted to sci.astro.research
Oh No
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Posts: 433
Default Doppler Tests on Local Stars

Thus spake Kent Paul Dolan
I didn't state what I meant clearly enough, and as a result you've
misinterpreted it. By "such stars" I was referring back to pulsars
still bound to their binary companions, for which such difficulties
presumably do not exist.


Other difficulties exist however. For the populations analysed we went
to some trouble to exclude binaries and multiple star systems because
one would need precise knowledge of the masses and orbits of the
component stars before calculating meaningful radial velocity figures.
We also restricted to populations where there are good Hipparcos
parallax distances (errors less than 10% and 20% were tested), and which
are within 100pc and 200pc of the sun. This was done both to keep errors
to a minimum and because the correlations which we were seeking are only
predicted to be linear within a few hundred parsecs of the sun. There
are not that many pulsars that close, and errors in distances to pulsars
are typically about 40% - larger than the error we were looking for,
which is closer to 25-30%. There are pulsars bound in globular clusters,
but, as I have said elsewhere, we have been collecting data on globular
clusters, but are yet to devise a test which we can apply to them.

Regards

--
Charles Francis
moderator sci.physics.foundations.
substitute charles for NotI to email
  #53  
Old March 15th 07, 10:56 AM posted to sci.astro.research
Joseph Lazio
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Posts: 200
Default Doppler Tests on Local Stars

"KPD" == Kent Paul Dolan writes:

KPD Joseph Lazio wrote:
Kent Paul Dolan writes:


Huh? Many pulsars are still part of the binary partner stars with
which they began their existence.


Uhh, no. There is a subset, millisecond or recycled pulsars, that
tend to be binary. As a whole, though, the population consists of
isolated objects.


KPD Read the word "many" again; it doesn't say "most", and meant to
KPD convey "a large number".

Lorimer (URL:
http://relativity.livingreviews.org/...es/lrr-2005-7/ ) lists the
fraction of pulsars in binaries as being 4%, for pulsars in the
Galactic disk. The fraction is higher in globular clusters, because
in them there can be interactions that form binaries. As a whole,
though, the population consists of isolated objects.


There is no reason to believe that such stars would have motions
different in kind from the rest of stars in the the galaxy with
which they rotate.


Yes, there is. The average velocity of pulsars is something like
450 km/s. (Ref: D. Lorimer has done a lot of work on this topic.)
That means that most of them have velocities well in excess of the
typical star, and a good fraction of pulsars are not bound to the
Galaxy.


KPD I didn't state what I meant clearly enough, and as a result
KPD you've misinterpreted it. By "such stars" I was referring back to
KPD pulsars still bound to their binary companions, for which such
KPD difficulties presumably do not exist.

There's still some discussion over the form of the velocity
distribution. It is clear that binary systems tend to have lower
velocities than isolated pulsars, but even "lower" here could mean 100
km/s, much larger than typical main-sequence star velocities.

KPD [As a side issue, since I don't have access to a
KPD technical library, whencefrom comes the aberrant velocity of the
KPD rest of the pulsars? Is "mono-jet propelled" common, so that the
KPD star is currently still accelerating away? Is blast asymmetry
KPD common in supernova, so that the star gets a one time huge boost
KPD like a squeezed watermelon seed?]

Good question. Certainly one possibility is that the supernova that
creates the pulsar is not perfectly symmetric. Even small asymmetries
could lead to a large "kick" to the nascent neutron star. Other more
exotic possibilities have been invoked, also, including such things as
neutrino interactions during the supernova. In addition, while
pulsars themselves tend to be isolated, many main-sequence stars are
in binaries. Thus, a pulsar could get a "kick" during formation from
the disruption of its binary. In some cases, there might be multiple
effects.

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  #54  
Old March 17th 07, 11:15 PM posted to sci.astro.research
Kent Paul Dolan
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Posts: 225
Default Doppler Tests on Local Stars

Joseph Lazio wrote:

There's still some discussion over the form of the
velocity distribution. It is clear that binary
systems tend to have lower velocities than
isolated pulsars, but even "lower" here could mean
100 km/s, much larger than typical main-sequence
star velocities.


How? If the pulsar had that kind of a velocity,
it would surely be far above escape velocity
for its companion, or am I misled by how far
Earth is from Sol, and the low solar system
escape velocity from here?

xanthian.
  #55  
Old March 18th 07, 11:13 PM posted to sci.astro.research
Joseph Lazio
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Posts: 200
Default Distance to SgrA*

"ON" == Oh No writes:

ON The 1993 paper by Reid is interesting. He outline the gamut of
ON methods of determining distance of Sgr A*. Armando Caussade gives
ON a more recent web based report citing this paper at
ON http://www.armandocaussade.com/astro...ic_center.html
[...]
ON Trigonometric parallax. Although Caussade says that Reid has
ON measured this, no parallax error is stated and Reid makes no
ON reference to it in his paper. Nor can I find any other paper
ON referring to such a measurement.

ON Keplerian orbits: Unfortunately to determine the Keplerian orbit
ON we need to know both the radial and the transverse velocities. The
ON method would have to be completely reworked if there is a
ON systematic error in radial velocity, and will give a different
ON result.

I'd check the proceedings of the GC'02 and GC'06 conferences. IIRC, a
combination of the radio astrometry of Sgr A* combined with the IR
measurements of the 'S' stars is providing a complementary and
cooperative approach toward the distance to the Galactic center.
These days, people seem to be using 8 kpc more and more, but 7.5 kpc
is well within the uncertainties, and potentially even a better value.

--
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  #56  
Old March 18th 07, 11:14 PM posted to sci.astro.research
Joseph Lazio
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Posts: 200
Default Doppler Tests on Local Stars

"KPD" == Kent Paul Dolan writes:

KPD Joseph Lazio wrote:
There's still some discussion over the form of the velocity
distribution. It is clear that binary systems tend to have lower
velocities than isolated pulsars, but even "lower" here could mean
100 km/s, much larger than typical main-sequence star velocities.


KPD How?

Not sure I follow. "How" what? How can there be discussion or how
can binary systems have lower velocities than the isolated pulsars?

For the former, remember that there are all kinds of systematic
effects that go into finding pulsars. If one searches the Galactic
plane (as the recent Parkes multi-beam survey did), one would tend to
miss the pulsars escaping the disk. The frequency and the sample time
at which one searches affects how deeply into the Galaxy one can
search, in turn affecting what fraction of the population one might
detect. One's algorithms for processing the data, particularly how
one handles acceleration terms in the search (which are clearly needed
for binary systems), affects how many binaries one finds. Etc.

KPD If the pulsar had that kind of a velocity, it would surely be far
KPD above escape velocity for its companion, or am I misled by how
KPD far Earth is from Sol, and the low solar system escape velocity
KPD from here?

The 100 km/s velocity that I mentioned above is the systemic velocity,
that is, the velocity of the barycenter of the pulsar-companion
system. The velocity of the pulsar and companion about each other
will of course depend how massive they are.

These two may be coupled or at least mildly related, though. As you
note, if the pulsar has too large of a velocity, the system becomes
unbound. So consider a binary system in which one member is a star
just about to go supernova. As it goes supernova, how much mass it
loses relative to the total system mass and the direction of the
"kick" that the pulsar receives during the supernova can both affect
whether the system remains bound. In general, we might expect that
most, but perhaps not all, binary systems become unbound when one
member undergoes a supernova. The observational evidence seems to
bear that out, as most, but not all, pulsars are isolated.

--
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