View Full Version : Star parallax, solar deflection and HIPPARCOS.

2008-Dec-15, 07:07 PM
This is a path I meandered down a few months
ago and is, I think, worth setting down here.
I was thinking of the distance to a White Dwarf
star, WD2154-512 which was listed as 47.59LYs
(lightyears) in a catalogue. Looking it up I
found straightaway it was part of a system
GJ841a&b. The a component was magnitude 10.4
and there was a HIPPARCOS distance of 16.2
parsecs which was 52.92LYs.

For various reasons I wondered if the real
distance might be nearer 60LYs and I looked
in more detail. The actual parallax from the
satellite was 61.63mas+&-2.67mas(milli aecsec).
Well the smaller the parallax the larger the
distance so taking the lower estimate gave
55.32LYs. Getting nearer my 60LYs.

Then I started thing about the deflection of
light by the Sun. It is 0.8secs about, either
side of the Suns limb so could it be a milli
arc second or two at the Earths distance? A
search found several recent articles
discussing this and a figure up to 4mas
was mentioned. This effect would give larger
parallaxes as the sightline to star climbs
away at 90 degrees to the ecliptic with lesser
errors at different angles. But it seems
inconceivable that the maths used by the
HIPPARCOS team did not take this effect
into consideration. And I would not cast
dispersions on one of the greatest science
results from this spacecraft.

At this point I picked up my latest copy of
Astronomy Now for Aug. There was an interesting
article about mapping the galaxy. And star
distances were being revised slightly to
greater distances because the HIPPARCOS
results had been revised due to a better
model of the spacecraft performance.


How absolutely spiffing!

So I do not yet know the new distance to
WD2154-512 but it seems a good bet the new
error bars will include 59LYs. Search and ye
shall find:)

2008-Dec-15, 10:37 PM
For various reasons I wondered if the real
distance might be nearer 60LYs and I looked
in more detail.

Would you mind saying what these various reasons are?

2008-Dec-16, 06:34 PM
In my ATM idea I listed a group of white dwarf
stars that may give a burst of radiation these
last few years when irradiated by neutrinos
from SN1987a. Naturally I look at the GCN
notices to see if any bursts have been
localised at positions very near the
coordinates of these stars. A burst at the
beginning of August was about 2 degrees from
one. Only I had expected a burst 4 years ago
according to the original distance I had.
Thus the above research.

Not much chance it was my star as a redshift
of z=2.2 was measured from the afterglow.
Just curious that I was able to find a larger
distance for the star given this pointer.

2008-Dec-17, 07:12 PM
Small correction, I meant the September issue
of Astronomy Now. I was reading it in August.