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Ilya
2007-Feb-03, 12:35 AM
If a nearby main sequence star, say Delta Pavonis, had a quiet, non-pulsing neutron star companion at the distance of 70 AU, would we notice it?

A neutron star at such distance would cause Delta P to move at 5-6 km/sec -Ė easily detectable from Earth. However, with a 400+ year orbit, it will not be at all obvious that the motion is circular -- to a casual observer, this motion will be subsumed into Delta Pís overall velocity vector. Decades of observation will be needed in order to notice Delta P is orbiting something invisible. These observations need not be continuous, of course: if someone measures Delta Pís proper motion, then compares it with observations a century earlier, the orbit will jump out. But if no one ever bothered to do that, neutron star could well remain undetected.

At least thatís what I think. Question for professional astronomers -- if Delta Pavonis had such companion, and we had no particular reason to look for it, when would it be found?

I already posted a similar question in "Small Media at Large", but it seems not to attract attention. Moderators are welcome to remove the other thread.

Spaceman Spiff
2007-Feb-03, 02:32 PM
If a nearby main sequence star, say Delta Pavonis, had a quiet, non-pulsing neutron star companion at the distance of 70 AU, would we notice it?

A neutron star at such distance would cause Delta P to move at 5-6 km/sec -Ė easily detectable from Earth. However, with a 400+ year orbit, it will not be at all obvious that the motion is circular -- to a casual observer, this motion will be subsumed into Delta Pís overall velocity vector. Decades of observation will be needed in order to notice Delta P is orbiting something invisible. These observations need not be continuous, of course: if someone measures Delta Pís proper motion, then compares it with observations a century earlier, the orbit will jump out. But if no one ever bothered to do that, neutron star could well remain undetected.

At least thatís what I think. Question for professional astronomers -- if Delta Pavonis had such companion, and we had no particular reason to look for it, when would it be found?

I already posted a similar question in "Small Media at Large", but it seems not to attract attention. Moderators are welcome to remove the other thread.

If you knew where to look - you would image that area of the sky with Chandra. The neutron star, with a surface temperature of a few million K, would likely be far brighter at X-ray wavelengths than the star it shares an orbit around the CM with. In the case of Delta Pavonis (a 0.98 solar mass main sequence star) that would definitely be true. Whatever the parent star, the neutron star would have a far larger hard/soft X-ray flux ratio. But now that you see it, doesn't mean that you would know that it shares an orbit with Delta Pavonis, although you might suspect it.

Another idea - even at 70 AU, the X-ray flux from the neutron star might be significant enough to cause changes in the illuminated face of Delta Pavonis, altering its spectrum mainly on that side of the star. I haven't worked that through to know how much. Even then, we'd need to be able to spatially resolve Delta Pavonis to see the changes in surface temperature, which we can do with optical interferometry for nearby stars.

Anybody else with ideas?

Nereid
2007-Feb-03, 03:39 PM
HIPPARCOS would probably not have picked up the 'binary' nature of any proper motion it detected, in such a nearby main sequence (MS) star. However, assuming it was observed with the LSST, changes in the proper motion - between HIPPARCOS and LSST, and in the 10-year life of the main LSST project - might; perhaps some OOM calculations would be a good idea?

I'll go along with Spaceman Spiff re x-ray (and, possibly, gamma ray) emission. Again, some OOM calculations would clarify things. Of course, if by 'quiescent' you also mean 'cold' (below, say, 20,000 K), then there'd be no x-ray emission.

Continuing with 'influences on its environment': what effect would such a neutron star have on the MS star's solar wind? I expect it would be pretty dramatic, possibly even dramatic enough to be easily observable - as something like a planetary nebula perhaps? If the pair are in, or near, a giant molecular cloud, the effects would likely be pretty obvious.

Spaceman Spiff
2007-Feb-03, 10:25 PM
by quiescent, I thought Ilya meant not a pulsar (not rapidly rotating).
Off hand, not sure how the spin down time compares to the cooling time.

Ilya
2007-Feb-05, 05:54 PM
If you knew where to look - you would image that area of the sky with Chandra. The neutron star, with a surface temperature of a few million K, would likely be far brighter at X-ray wavelengths than the star it shares an orbit around the CM with. In the case of Delta Pavonis (a 0.98 solar mass main sequence star) that would definitely be true. Whatever the parent star, the neutron star would have a far larger hard/soft X-ray flux ratio. But now that you see it, doesn't mean that you would know that it shares an orbit with Delta Pavonis, although you might suspect it.

Another idea - even at 70 AU, the X-ray flux from the neutron star might be significant enough to cause changes in the illuminated face of Delta Pavonis, altering its spectrum mainly on that side of the star. I haven't worked that through to know how much. Even then, we'd need to be able to spatially resolve Delta Pavonis to see the changes in surface temperature, which we can do with optical interferometry for nearby stars.

Either observation would only occur if someone already suspected something unusual is going on. IOW, such widely separated main sequence/neutron star binary, even a nearby one, probably would not have been noticed as of yet. I did a google search on "neutron star", "binary" and "greatest known separation", and several variations thereof, and found nothing.


by quiescent, I thought Ilya meant not a pulsar (not rapidly rotating).

Yes, that's what I meant.

Spaceman Spiff
2007-Feb-05, 08:13 PM
Either observation would only occur if someone already suspected something unusual is going on. IOW, such widely separated main sequence/neutron star binary, even a nearby one, probably would not have been noticed as of yet. I did a google search on "neutron star", "binary" and "greatest known separation", and several variations thereof, and found nothing.


Not necessarily true. A lot of otherwise "normal" stars are observed by Chandra (although most often via spectroscopy) to understand their hot outer atmospheres. This is outside my field, so I cannot offer many particulars, especially regarding imaging. But in general, it could be that such systems containing a neutron star may be in greater abundance than otherwise suspected.