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Mag. and Mag.range

thn's picture
thn
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Joined: 2013-08-17

Hello!

I'm new to observing variable stars. The Nova Delphini brought me here! Now I also got interested in observing carbon stars and downloaded a few lists: One from C. Buil based on Stephenson, 1989, catalogue of Cool Galactic Carbon Stars. The other an observation list from the Astroleague. Furthermore I have the GCVS catalogue integrated in my planetarium-software. Plus the VSX from this site. Sometimes I confer to the Simbad-queries too.

What puzzles me: The information varies a lot from source to source. Example: WZ Cas:  Mag.range 6.9-11.0 (Buil, Astroleague), 9.4-11.0 p (GCVS), 9.4-11.4 p (VSX)

Questions:

1. Where do I get the most reliable information with regard to visual observation?

2. What does the mag. code: p mean, photographic I guess, but how does it translate to visual observations?

3. Is there a general rule what the V mag. means with regards to variable stars? Is it the value at maximum or some sort of average? Information varies a lot too. For WZ Cas I got: 6m9 (Buil, Astroleague), 7m06 (Simbad),  7m04 (HD, "Extended Hipparcos"). But this doen' fit he value 9.4p at maximum from VSX for example.

Thanks in advance for help! Also, if you could direct me to a page where I can read about these topics in detail I would be very grateful. I hope this is the right board for this kind of questions, if not I'd ask the moderator to move it to the correct board.

Clear skies for Germany,

Thomas

Greetings
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thn
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Joined: 2013-08-17

> Clear skies for Germany ROFL. That was wishful thinking. We have bad weather over here. What I meant was Clear skies! from Germany. Thomas

Carbon stars and different passbands
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Sebastian Otero
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Joined: 2010-09-19

Hi, Thomas,
carbon stars are the reddest stars you can observe, the redder they are the more difficult it will be to make an estimate of them!
The differences in the magnitude ranges you mentioned are not actual mistakes but caused by different passbands being used in the observations.

I'll try to answer your questions.

1) Just check that the ranges given are for the V passband. VSX is the best source for that because it is being updated constantly. We are even trying to convert all ranges to visual ranges to help the observers and avoid this source of confusion. I'll update the range of this star ASAP.

2) Yes, p is photographic. Usually p ~ B. So the redder the star the larger the difference between visual observations and photographic magnitudes. Carbon stars have B-V colors between 2 and 5 or even larger! This accounts for the differences you mention. The typical mira (normal red giants) will show a difference of ~1.5 mag. between p and V ranges but the difference will always be larger for carbon stars (The 7th mag. figures are the visual magnitudes in this case)

3) SIMBAD and HIPPARCOS don't give a range. The HIPPARCOS magnitude is just a mean magnitude and the magnitudes presented in other databases might come from a published source that it may be derived from a single observation. So you need to go to VSX to see ranges (taking the passband issue mentioned above into account). You can also check the Light Curve Generator to see what other observers are reporting on your stars. That will give you an idea of the visual range. For carbon stars most observers make estimates several tens of a mag. fainter than V, but that's another story.

I hope this helps.

Cheers,
Sebastian

-----------------------
Sebastian Otero
VSX Team
American Association of Variable Star Observers

Thanks!
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thn
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Joined: 2013-08-17

Thanks Sebastian!

That was very informative and cleared up most of my  questions. (Now I wish it would also clear up the bad weather). This will help me judge the information given by various sources. I will also study the light curves from AAVSO and might even make some contributions.

One last question if I may: Is there a trick, a technique, to estimate very red stars more accurately? They look rather faint to me so I think I'm in danger of underestimating them.

Best greetings,

Thomas

Estimating carbon stars
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Sebastian Otero
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Joined: 2010-09-19

About how to estimate them, there are two things that make it difficult:
*there are no comparison stars of the same color.
*you need to avoid staring at them because Purkinje effect will come into play and when you jump from the comp stars to the variable if you don't make quick glances its brightness will seem to go up.
*Although this will be a problem for bright stars, for faint stars you will find that these very red stars are dificult to detect at the limiting magnitude because the rods sensitive to faint light sources are also color blind. Thus a bluer star of the same magnitude will be detected while a carbon star won't.

Don't use averted vision for these stars, specially if they are faint. I would recommend not observing them as they approach your limiting magnitude because your estimates will have a large error (the amplitude of the star will seem much larger than its actual V amplitude).
Observe them when you can postively detect them with direct vision, but using quick glances.

I think these are the only stars that are difficult to estimate no matter what you do. It is even difficult to get standard V mags for them because of their peculiar colors.

Cheers,
Sebastian

Welcome aboard Thomas
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PKV
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Thomas,  I would also recommend looking at the Visual Observing Forum.  There are many threads on observing practices and techniques.  Kevin Paxson - PKV

Thanks!
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thn
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Thank you Sebastian - I will pay close attention to your advice.

@Kevin: Thanks - you mean _this_ board, right? I will do.


CS, Thomas

I meant to say that you may
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PKV
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I meant to say that you may want check out the Long Period Variable Forum.  You could find yourself migrating to Mira's, SR's and maybe RV Tau's.  Many Mira's are carbon stars.  Kevin Paxson - PKV

AAVSO 49 Bay State Rd. Cambridge, MA 02138 aavso@aavso.org 617-354-0484