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Crimson
2004-Nov-15, 10:44 PM
Beta Centauri Weighs In (http://KenCroswell.com/BetaCentauri.html):

"[T]he eleventh brightest star in the night" has a distance of "about 330 light-years, nearly 200 light-years less than the Hipparcos satellite determined."

pghnative
2004-Nov-15, 10:50 PM
The article states that orbital motion resulted in an error in the measured parallax. Is this possible? I would think that they would have taken repeated measurements over at least one year to be able to subtract out the effects of proper motion.

Evan
2004-Nov-16, 12:01 AM
The Hipparcos data is known to be inaccurate in the case of binary stars with a period close to one year.

<added later>

pghnative,

The problem is that the only parallax data that matters is that taken six months apart from opposite sides of the Earths orbit. If the orbital period of a binary is close to a year then the stars may be placed in the maximum different locations when the two different measurements are taken, giving a false parallax reading. This also applies to binaries with other orbital periods, especially those that are multiples of a year.

pghnative
2004-Nov-16, 01:52 PM
pghnative,

The problem is that the only parallax data that matters is that taken six months apart from opposite sides of the Earths orbit.Understood


If the orbital period of a binary is close to a year then the stars may be placed in the maximum different locations when the two different measurements are taken, giving a false parallax reading. This also applies to binaries with other orbital periods, especially those that are multiples of a year.Ah, now I get it -- so that even if you took measurements in January, July and then again in January (to subtract out proper motion error in the July reading), if the period is about a year, then there would be very little net motion from January to January.

Crimson
2004-Nov-16, 07:15 PM
From Section 6.1 of the paper linked to by the article:

"The Hipparcos catalogue recognises Beta Cen as a binary system but has no mention of the dual nature of the primary component of Beta Cen and lists only the faint third-magnitude companion. We conjecture that the Hipparcos value for the parallax is seriously affected by the binary nature of the primary of Beta Cen. In fact, the Hipparcos observations of spectroscopic binaries were not fitted in general with an orbital model to derive the value for the parallax. While reprocessing of the Hipparcos astrometric data has been performed for many single-lined spectroscopic binaries (see for example Pourbaix and Boffin 2003 and references therein), it has not been done for Beta Cen."

Evan
2004-Nov-16, 07:28 PM
Here is a quote from the FAQ at the Hipparcos web site:


Q. Concerning binaries and the effect on parallaxes, L. Szabados has reported (Hipparcos Venice '97, ESA SP-402, page 657, available at this site) that undetected orbital motion in Cepheid binaries "falsifies the trigonometric parallax determined from the Hipparcos measurements". What does the project have to say about that? Are all the Hipparcos parallaxes suspect? [Jan 98]

A. It may be worth you taking a look at Volume 3 of the published catalogue, ESA SP-1200, describing the catalogue construction, and especially Chapter 20 "Verification of Parallaxes" and Chapter 22 "Analysis of Double Star Results". Basically, we do not expect the parallaxes to be significantly perturbed by (detected or undetected) binarity, unless the orbital period is close to one year, in which case the photocentric motion may confuse the shift in the system's apparent position due to parallax. ASCII Discs 5 and 6 (in Volume 17) provide the intermediate astrometric data necessary to carry out a rigorous re-analysis of the astrometric data, taking into account subsequent ground-based information on orbital parameters of specific objects. To our knowledge this has not been carried out for these objects. So some suspicion is always appropriate, but we are not aware of specific errors confirmed by a thorough analysis.

[my emphasis]

http://astro.estec.esa.nl/Hipparcos/www-faq.html

eburacum45
2004-Nov-16, 07:38 PM
The Celestia space sim uses Hipparchos data to fix the positions of 10,000 stars; it is not terribly accurate for stars more than 100 ly away, and binary stars completely throw the results out.
I wanted to put a fictional planet around Psi Serpentis (http://www.orionsarm.com/worlds/Daffy.html), but the star was in completely the wrong position, so I had to add my own; since then the mainbrains at Celestia have amended that star.
Given good information they will change the Celestia database; they are in the process of adding hundreds of accurate binaries for the next version, or so I understand.
I'd better make sure they know about this.

Evan
2004-Nov-16, 08:12 PM
eburacum45,

Thanks. Just what I need. Something else to fritter away my time. By that I mean Celestia (http://www.orionsarm.com/reviews/Celestia.html)

:D