Tuesday, May 16, 2017

Enigma din R Crateris / The R Crateris Riddle

The telescope I'm  using most often is pretty  old at 20 years  but not a ''classic'' because it is home made.
It is a Dobsonian with a 125mm F/7 very good primary mirror made by late Gavril Beches.
Currently the mounting is under refurbishment ,some parts gave up after two decades of stargazing.

The book where I got the target for the observation below  is a classic , ''Pleasures of the Telescope'' by Garrett Putnam Serviss.

For a long time my pet  targets were double stars.
But since more than one year I'm addicted to a Deep Sky craze.
The  second week of May being around Full Moon , there were not   chances to see many galaxies in my small telescope...
On 10 and 12 of May I was looking for another type of  targets.
Like trying  to see Messier 68 , a globular cluster but without success.

Serviss suggest , while being in that part of the firmamant , to have a look to Delta Corvi which is a double star and to R Crateris ,a variable red star.
I started my star hopping with Algorab or Delta Corvi , an old friend because is the starting point when searching for Messier 104 ''Sombrero Galaxy''.
This time ,thanks to Serviss , I looked a bit more careful and ... yes , Delta Corvi is a not so  easy double star of very unequal stars ,separated by 24''  , split at 35x , if you look carefuly.
The dim companion is better seen at 69x /University 12.5mm ortho.

From Delta Corvi , I navigated toward West to Gienah Corvi /Gamma Corvi.
And here my science ended , I was not able to manage alone on the bright Moon lit sky ,among those dim and rare stars.
The Toshimi Taki atlas helped me to notice a triangular asterism located to the West of Gienah , composed of 31 Crateris -Eta Crt-Zeta Crt  .
The lower  side of this triangle ,namely 31 Crt- Zeta Crt  is pointing toward Gamma Crt.
From Gamma Crt the next step is bringing us to Alpha Crateris  or Alkes.
R Crateris is in the same field at 69x with Alpha Crt.
Here is a finder chart made using Stellarium and Paint.

R Crateris is the middle star of a boomerang shaped asterism of three stars.
The  color of the star was  ruddy and R Crt was the brighter  in the asterism.
Serviss is not mentioning that R Crt is also a double star.
Stellarium is stating R Crt have a separation of 63'' and PA= 268 degree.
I haven't searched in big data bases but I believe the companion is the small star of 9.9 mv we see in this print screen from Aladin Lite.

Checking with Stellarium,the position angle of the small star fit very well : R Crt is to the East of Alpha Crt .

The 9.9 mv star is located  between R Crateris and Alkes which lie to the West of R Crt .
This  mean a   PA of  about 270 deg against 268 degrees listed by Stellarium.
The difference in RA between R Crateris and the supposed companion is of about four seconds of time which mean 60'' against the separation of 63'' listed by Stellarium.

Again ,data agree well.

According to Simbad , R Crateris is a ''Helium burning asymptotic giant branch star'' :

However ,there is something  that I don't understand.
Stellarium is stating R Crt to be of 8.95 mv.
According to  Vizier the maximum brightness of R Crt is of 9.8 mv.

Stellarium is also showing the  9.8 mv brightness  in the row about range of variability.
But visualy ,R Crt was the brightest in the trio of stars shown in the screen-print.
Much brighter I dare to say...

So ,the ''riddle'' I found in R Crateris is this : how can a star of 8.95mv have a range of variability from 9.8 to 11.2 ?
Is this due to the different photometric systems used ?

Well ,it was fun to take a look at this giant red star.
But in the stellar world the red giant stars  are pretty cool .
So ,should I say it was ...cool ?

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