Re: Doubles in Tycho-2 and Tycho-1

ross_shuart Apr 1, 2002

I thought I'd add an explanation for those wondering about why the
newer, larger catalogue seems to have less complete coverage.

The Tycho catalogue (Tycho-1 from here on) as published in 1997 was
based solely on observations from the Hipparcos spacecraft. If it
resolved a double star, there were two entries in the catalogue, each
with their own position and proper motion. Guide 6 used Tycho-1.

The ACT catalogue was published shortly after and provided better
proper motions than Tycho-1, using the roughly 85-year time base from
the epoch of the Astrographic Catalogue. About 93% of the Tycho-1
stars are in the ACT. Guide 7 uses the ACT data, if available. If a
star is not in the ACT, the original Tycho-1 data are available.

The result of this is that for Guide 6 and 7, if Tycho-1 resolved the
pair, Guide displays two stars.

Tycho-2, published in 2000, was based not only on a new analysis of
the spacecraft data, but also on about 140 ground-based catalogues
published during the last century. This provided better proper motions
than using just the AC and Tycho-2 positions.

Typically, the ground-based catalogues only had resolutions of about 2
or 3 arcseconds. A pair closer than this was measured as a blended
image. For consistency in proper motion determination, the Tycho-2
positions were averaged to form a photocenter. This was then used in
the data reduction. The result is a photocentric solution for the
pair, yielding a common proper motion and a single position. Plotting
stars from this database yields perfectly superimposed symbols.

The Tycho Double Star Catalogue (TDSC) provides accurate positions and
proper motions for the pairs in Tycho-2. When this is combined with
the Tycho-2 data, the pairs with photocenter determinations will go
back to being pairs.

Special note: this actually won't work completely for epsilon Lyrae
because the TDSC (# 48456) has no proper motion for the B component,
so for epochs away from 1991, it will be misplaced by 6 arcseconds per
century (a lot for a 2" pair). I don't know why they didn't just copy
the Hipparcos value.

The solution of using binary orbit data to compute positions is even
better. I'm looking forward to watching binary pairs orbit their
center of gravity while moving across the sky. That brings up the
issue of mass ratios - Bill, I will e-mail you about that when I can
get to my notes.

Ross Shuart