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The column headers are self-explanatory and the Hipparcos field is indicated in the last row of the header, so that the detail explanation can be found in the Introduction to the Hipparcos and Tycho Catalogues. The column labelled Sce indicates the source of the new information that has been found and which motivated the reprocessing. It has the following meaning:
Each table, or group of two tables for the largest, refers to a single category of re-examined solutions listed in Sect. 3. No star appears in more than one table.
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Specific comments
HIP 1338:
While the Tycho Catalogue was the basic source for the relative astrometry, a
similar
separation was found in the Digitized Sky Survey and in recent ground based
observations.
HIP 18045:
The SS2 has been used to search for a star close to the position given in the
Hipparcos Input
Catalogue. The Hipparcos target was about 20 arcsec from the actual star and
consequently
the signal recorded was weak and highly variable according to the detector
pointing and too far away
from the true value to allow a convergence of the software. However the SS2
position was good
enough as starting values and the iteration converged to an acceptable
solution.
A note in the
Hipparcos Catalogue indicates that TYC 6451-1246-1 (
) could be the corresponding star. In fact this is very likely a
spurious solution
(nothing seen in the SS2 there) while TYC 6451- 122-1 is within 20 mas of our
solution and
with comparable magnitude (allowing for the attenuation effect due to the
pointing offset).
The new solution provides a reliable parallax and proper motion with standard
errors of about
3 mas.
HIP 27464:
This is probably an optical pair (different proper motions in the Tycho
solutions). The
solution agrees with TYC 4098-5-1.
HIP 31132:
The observation target was 7 arcsec from the star and the proper motion in
right ascension
and declination were so far from the true values that no convergence was
possible. A dedicated
setting in the software overcame this problem and yielded a good solution in
terms of standard
errors but with a questionable goodness-of-fit.
HIP 34226:
A binary with components of similar brightness which accounts for the very large
standard errors.
HIP 41884: Solution at 30 mas from TYC 4862-794-1, but large
pointing offset
(15 arcsec) due to a poor position in the Input Catalogue.
HIP 81402: The initial separation and position angle were taken
from the Input
Catalogue and confirmed by Tycho. The Hipparcos solution for the relative
astrometry is
excellent. However the large standard errors in the astrometric parameters are a
consequence of
the very small magnitude difference between the two components.
HIP 81694: Orbital double star not resolved as double by Tycho.
The target
was at 18 arcsec from the actual star position and is responsible for the
instability of the
signal and the large standard errors.
HIP 86405: No new input data were used but the case by case
processing allows a
better control of the outliers and more observation were a priori
rejected.
The improvement in the final astrometric solution shows up clearly in the number of rejected observations, the quality of the goodness-of-fit and/or the standard error. The solutions refer to the primary components of the double systems. In general the astrometric solutions are not very different from that of the Catalogue, primarily because the double stars involved generated a weak deviation from the single star signal and the absolute astrometry was not too sensitive to an error in the separation of the two components. However the multiplicity data given here supersede the Hipparcos values.
Specific comments
HIP 8035: The relative astrometry of the Hipparcos Catalogue is
proved wrong by
subsequent ground based observations. Using this constraint the standard error
of the
separation concluded from the Hipparcos data is greatly improved. The new
astrometric solution
fits the data more closely.
HIP 13725: The FAST solution retained for the final publication
was
subsequently considered as very doubtful. In addition no companion was seen on
SS1 or SS2 with
separation larger than 5 arcsec. While this do not preclude this star to be a
close binary, we
think that in this case the magnitude difference should be larger than 4, with a
negligible
influence on the Hipparcos signal. A single star solution is then preferable and
yields an
excellent fit.
HIP 38479: The recent ground based observation shows clearly
that the
Hipparcos position of the secondary is wrong by exactly one grid-step. The
final fit is
obtained with no observation rejected, although the goodness-of-fit is not as
good.
HIP 44488: The recent ground based separation (12 arcsec) is
definitely
incompatible with the SS1 image taken 40 years ago with
arcsec. The
new Hipparcos solution is
however excellent and leaves no room for a different separation. One may
hypothesize that the
system is in fact an optical binary comprising a distant star and a faint and
nearby
fast moving star. Additional observations, old or new, should help resolve the
problem.
HIP 69736: The Input Catalogue gives this system a separation of
and a magnitude difference
. From the Digitized Sky Survey a much
larger
separation should be adopted, which eventually leads to a better astrometric
solution. The system is actually
triple, and the largest separation corresponds to the two brightest components.
HIP 71867: There is no double possible with that separation in
the image of the
Digitized Sky Survey. The single star solution is excellent and left no room for
improvement with a double star model.
HIP 76435: Triple system from the Tycho Catalogue. The
separation used here is
for the detached components which yields a better fit to the Hipparcos data.
HIP 79902: The Hipparcos relative astrometry with
arcsec is not
seen in the Digitized Sky Survey. On the other hand the solution using the
Input Catalogue
value of
or the new ground based observation leads to a much better
fit.
These facts account for the very small yielding in this group, since only four new solutions are proposed, all based on actual detection of a companion in images of the Digitized Sky Survey. The subsequent Hipparcos solutions for the separations given here have fairly good standard errors and the final astrometric fits are all of good quality. Given the separations and the distance it is unlikely that the acceleration proposed in the Hipparcos Catalogue should be real, unless the primary is also a close astrometric binary.
HIP 23266: The new solution has a much better fit to the data
than the
published solution, but with larger standard errors due to the error propagation
from the
double star model. The difference between the old and new proper motion is much
larger than
the expected statistical error, although the double star signal is very weak
with
. However the acceleration components in the published solution
are large (respectively
40 and -9 mas/yr2) and not fully independent of the first order term.
The new solutions are given in Table 4 (HIP ) and
Table 5 (HIP
). There are no Hipparcos data in the
multiplicity
columns and the second line of each entry gives the relative astrometry derived
from the
Hipparcos observations and using the approximate
and
provided by
the quoted
source. The change in the goodness of fit is the best indication of the
improvement in the
solutions and at the same time confirms to some extent the relative astrometry
and photometry.
When the double star has magnitude difference less than 1.5 the change is
spectacular.
Due to the error propagation in the double star model, the standard error of the astrometric parameters are usually larger than with the single star model although the goodness-of-fit is much improved by allowing for the duplicity.
HIP21000: This is the most striking case of this group with a
distance twenty times larger and a
much smaller proper motion in the new solution. This star appears as a double
system in the Input Catalogue with
. It was processed with the single star model in Hipparcos
because FAST and NDAC could not
agree on the relative astrometry of the system. With the confirmed separation it
is clear that the double star
model is better, even though the standard errors are larger because of the small
magnitude difference. All the 15
observations were used with no rejection.
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