[vsnet-chat 4246] Re: Omega CMa comparison stars

["Sebastian Otero" <varsao@fullzero.com.ar>]

Subject: Re: [vsnet-chat 4213] Re: Omega CMa comparison stars


>>> I suggest this comparison star close to Omega CMa be used in addition to
3 Pup:

   HR 2922 = HD60863    7h35m23s  -28º22'10" (2000)
                        V=4.639  B-V=-0.102  (from Tycho-1)  B8V

** Hi, Thom:
                    One of the main problems in visual estimation is the
choice of stars of very different brightness to make the estimate. In fact,
HD 60863 (p Puppis) is very faint compared with omega CMa. The error we get
by using such a different star is worse than if we use a microvariable.

>>>The other comparison stars that have been suggested are not reliable,
for reasons noted below.
   Delta Col is some distance from Omega CMa and is quite a bit
redder.  The same objection applies to 11 Puppis, which is nearly as
red as Delta Col.

** Yes delta Col is too far away, but 11 Pup is near and it's exactly as
bright as omega CMa at minimum. And this is not variable. If we take the
color in account (lots of messages about it before...) this one is perfect.
And together with k Puppis, they are on a straight line and near the
variable.

>>>What is your reference for k Pup being a Be star, Sebastian?  It's
below our effective -20º Dec limit, so it's not being observed here.
I may know it by its HD number.  Neither "Kappa Pup" nor "k Pup"
is in any of the common catalogues; nor does SIMBAD recognize it.
However, "k Pup" is shown as a V=3.8 star in the AAVSO Atlas and it
appears on other atlases.  There are several stars in Puppis with
lower- and upper-case one character common names.  SIMBAD translates
them to upper-case, which accounts for it not recognizing "k Pup".
Could you provide alternate names or a position for k Pup?  Thanks.

** Of course. k Puppis doesn't have a Flamsteed number as the others (e
Puppis = 11 Pup; l Puppis = 3 Pup)
It is a very nice pair of nearly equal stars. They are HD 61555 and HD
61556.
V= 4.48 and 4.70. Sp types B6V (starB) and B5IV(star A).
It's classified as NSV 3673 but with no range. Hipparcos "confirmed" as a
variable.
PEP(V) from the ground gives it between 3.80 and 3.83 V, that's where the
value 3.82 came from.
Total amplitude measured by Hipparcos is 3.79 - 3.86 V. According to the
published values released.
But I search through the Photometry Epoch data and the "fading" looks very
spurious. It corresponds to a first part of the mission which is full of
doubtful entries for this star: most of the faint measurements are
surrounded by normal ones in question of minutes. Most part of the data show
k Puppis nearly constant between 3.80 and 3.82, as it is sateted everywhere.
So I'd bet that this "variability" is a byproduct of the star being a
double.

These measurements are for the combined brightness so it is impossible to
say which of the stars of the pair would be the "variable".
For individual magnitudes we can find:

HIPPARCOS = A: 4.398 Hp ; B: 4.604 Hp
uvby = A: 4.532 ; B: 4.778
Geneva=  A: 4.468 ; B: 4.683

At this blue color (B-V= -0.16) Geneva and Hipparcos magnitudes are brighter
than V. uvby is more or less the same.
Then:

HIPPARCOS=  A: 4.465 ; B: 4.65 ---> AB= 3.80
uvby = A: 4.532 ; B: 4.778 -----------> AB= 3.895
Geneva= A: 4.483 ; B: 4.697----------> AB= 3.83

From this it is impossible to say which one is "variable" because I think
all measurements are contaminated by the other star's light.
In Hipparcos data BOTH stars are brighter than in Geneva, and in uvby BOTH
stars are systematically fainter. So I wouldn't trust in that 3.895...
For our use (naked eye), the AB combined light is what really matters and
the changes are very little.
All of these stars MUST SURELY BE EXCLUDED from any photometric study of the
star but are usable for visual purposes.

I insist: all we can win using perfectly constant stars, we lose it with
brightness difference or using a star that it is too far away from the
field.

Continuing with k Puppis, the spectral types don't include "e" for emission
lines. Actually I said "Be star" meaning a hot blue star slightly variable
and irregular (who may have or develope emisson lines in the future, as
delta Sco). But it was before checking HIP photometry epoch. Now I don't
think this pair is variable at all.


>>> While an Ae/Be star may have an historical range of only 0.05
magnitude (or may be only "suspected" to be variable), they should not
be used as comparison stars.  Small "outbursts" are always possible
and may not be noticed; e.g., if the comparison brightens by 0.08
magnitude and not noticed as such, the variable will be reported as
having faded by 0.08 mag.  Ae/Be stars should not be used as
comparison stars especially as comparison stars for other Ae/Be
stars.  (Sort of like using one Mira variable as a comparison star
for another Mira.)

** As expressed before it is a great problem for PEP. Visually we can deal
with that.

>>>A supergiant A-star (3 Pup) is likely to be a microvariable,
especially if it shows emission lines in its spectrum.  It could be
used as long as stars for which no variability has ever been found,
such as HR 2922, are included as checks.  Unfortunately, 3 Pup is the
only potential comparison star that is brighter than Omega CMa and
is also similar in color.  One of those unfortunate, but necessary,
compromises when doing photometry of bright stars.


** This is not been confirmed as variable in V.
Lots of measurements seem to indicate it's a 3.96 star (3.95 - 3.97).
Hipparcos gives V= 3.94, but if we take the original Hp magnitude and
convert to V we find it is 3.955 and not 3.94, so again the star it is at
3.96.

It is important to say that one should be careful in using them, but I think
care must be taken in every estimate we make (see vsnet-chat 4199 and 4233).
Every time I estimate a variable, it's like estimating its comparisons since
I take the job of checking if everything is "okay". Using similar comparions
(eg= 3.82, 3.96, 4.20), any difference between them would be perceived and
.... and then I would send an alert !!!!

BUT I know that this may lead to a confussion and that people are not
obligged to check everything every time they observe, so generally speaking
it is important to support the notion of using non-suspected stars for
comparisons.
But there are cases when we lose accuracy when we are actually trying to
achieve it !!

Regards,
Sebastian.





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