[vsnet-chat 5480] Re: [vsnet-sequence 176] Re: vsnet-chat 5472 [HR Car]

[Thom Gandet <tgandet@mindspring.com>]

Tycho magnitudes should be used only with great care when setting up
sequences.  I agree with Sebastian that if there are comps with good
ground UBV (especially UBVRI) photometry, then those stars ought to
be used.  At the very least, try to find a check star with good
ground-based UBV photometry.

If ground data isn't available and Tycho magnitudes are used, compare
the mean with Vt the median Hipparcos magnitude. They should be very
close for stars at least as late as A0.  If the s.e. of Vt is larger
than about 0.016, look at the Hipparcos epoch data first.  Arne or
Brian may have more to say about this issue.

A good example - take a look at a the Hipparcos/Tycho light curve
for HD 45151 (HIP 30606 (an 8th magnitude star) at:
http://vizier.u-strasbg.fr/viz-bin/vizExec/Vgraph?I/239/30606&0.

The Hipparcos data, taken roughly contemporaneous with the Tycho
data, has much smaller scatter, whereas the Tycho data is all over
the place.  The star is classed as Hipparcos "C" variability type -
essentially constant.  There is no ground-based evidence for
variability.

Regards,
Thom Gandet


Sebastian Otero wrote:
>
> There is no need to rely on Tycho-2 magnitudes for a bright star when there
> is plenty of V data from the ground.

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