|
|
Thread Tools | Display Modes |
#51
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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. -- 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 |
#54
|
|||
|
|||
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
|
|||
|
|||
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. -- 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 |
#56
|
|||
|
|||
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. -- 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 |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Gravitational Doppler | [email protected] | Astronomy Misc | 138 | March 28th 07 07:44 PM |
Gravitational Doppler | [email protected] | Astronomy Misc | 0 | July 31st 06 08:44 AM |
Gravitational Doppler | [email protected] | Astronomy Misc | 12 | July 28th 06 08:41 AM |
redshift Vs doppler shift | Maximus | Misc | 0 | July 1st 05 10:19 AM |