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Suggestion for a long term observation project

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#1 WRAK

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Posted 03 October 2019 - 07:09 AM

LDS 838 is a faint equal bright pair with visual magnitude around 12.5 and a separation of currently ~2 arcseconds. Resolution will need some aperture beginning with 150mm or 6 inch but should else pose no problem if the observation location is southern enough. This pair moves with an impressive proper motion of >3 arcseconds/yr although this is hard to notice as the background star field is even fainter. Even more impressive is the short orbit period of ~26 years so you could follow this pair over a full orbit if you are young enough with noticeable changes in separation and position angle from year to year. LDS 838 is with a distance of less than 9 light years rather close to our Sun system. Radial velocity is currently not known so it is unclear if this pair comes closer or travels away from us


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#2 Cotts

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Posted 03 October 2019 - 08:27 AM

01H 39M 01.54S

-17° 57' 01.8"

 

In Cetus.

 

Rather faint! 

 

I'll be 96 after the orbit completes......

 

Dave 



#3 ssmith

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Posted 03 October 2019 - 11:18 AM

I'll be 93.

 

Visibility of this pair will be similar to Kruger 60 (DO Ceph) which is much higher in the sky  (9.93, 11.4  1.7" period 44 yrs) so this would be a good test pair. 

 

I haven't split this pair (LDS 838 - BL & UV Ceti) but now that it is becoming visible again in the evening I will give it a go.

 

Here is a photo of the unresolved pair that I took as part of my "closest stars" survey.

 

BL Ceti C9 11-14-18 8fr.jpg

 

 

                       


Edited by ssmith, 04 October 2019 - 08:22 AM.

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#4 WRAK

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Posted 03 October 2019 - 04:48 PM

Interesting image. Good luck for your next attempt.

BTW - mass of the primary is rather 0.15.



#5 fred1871

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Posted 03 October 2019 - 08:10 PM

I'm far enough South, but the magnitudes don't suggest, based on my past experience with faint fairly close pairs, that 6-inch/150mm will be enough. Around mag 12.5 is getting quite dim for pairs around 2 arcseconds - photographically, not difficult, visually it becomes limited by decreasing resolution capability of the eye with dim stars.

The WDS lists the magnitudes at 12.7 and 13.2. Based on spectral types and distance from us, that sounds about right. Gaia has remarkably bright Gmags of 10.5 and 10.8 (rounded); seems very unlikely to match the visual magnitudes.

I'll look up the pair but think I'll use my C9.25 for a bit less eye-strain. It gives near a full magnitude benefit over a 6-inch. That can make the difference between "did I see it?" and a definite sighting. However, I'd be more inclined for a long-term project to image the pair periodically, as that shows both the orbital motion and the rapid proper motion against background stars. It would make a nice series over a decade or two.



#6 ssmith

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Posted 04 October 2019 - 08:34 AM

Hi Wilfried -  yes the mass of the primary that I have listed is clearly in error and i have corrected the photo however the values I have found list the masses as being nearly equal (1.02 & 0.999).  Where did you find the 0.15 value? 
 
As to the visibility of the components I always take the visual magnitudes listed for stars trending toward the red end of the spectrum with a grain of salt - they always seem harder to see than the numbers would suggest.

Edited by ssmith, 04 October 2019 - 12:10 PM.


#7 WRAK

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Posted 04 October 2019 - 10:43 AM

The GAIA DR2 StarHorse catalog (Uni Leipzig) offers now masses for about 80% of the GAIA objects brighter than mag 18. For GJ65 the median masses are ~0.15/0.14 with some statistical spread.
Two other sources would be Benedict et al. 2016 and Kervella et al. 2018 giving slightly smaller masses of ~0.125/0.119.
The mentioned ~12.5mag are own V-filter measurements

#8 ssmith

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Posted 04 October 2019 - 01:14 PM

Thanks Wilfried -

The Kervella paper is very good. Is your 12.5 mag measure the combined magnitude for the pair?

#9 WRAK

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Posted 04 October 2019 - 02:56 PM

Measurements are actually 12.2/12.3 Vmag but with heavily overlapping star disks so I suspect some bias here towards too bright values. Calculation from GAIA G/B/R-mags would give 12.7/13.1 Vmag estimations.

My 6 images so far were done with 10s exposure time - obviously far too long, will try again with 3s exposure time to get less overlap



#10 WRAK

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Posted 08 October 2019 - 01:22 AM

Shorter exposure time did not work out, star disks are still overlapping

#11 fred1871

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Posted 08 October 2019 - 03:11 AM

Measurements are actually 12.2/12.3 Vmag but with heavily overlapping star disks so I suspect some bias here towards too bright values. Calculation from GAIA G/B/R-mags would give 12.7/13.1 Vmag estimations.

My 6 images so far were done with 10s exposure time - obviously far too long, will try again with 3s exposure time to get less overlap

So the likely mags - if derived from Gaia photometry - are close to the WDS listed numbers....

Will be interesting to see what you get if you can get separated star disks. What telescope/camera setup are you using? One of the iTelescopes again?



#12 WRAK

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Posted 14 October 2019 - 03:00 PM

Yes, iT32 with a resolution of 0.63 arcsec/pixel - I hoped this should be good enough for a 2" separation with faint stars but this did not work out so far




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