[vsnet-chat 3354] V382 Vel comparison stars

["Bruce Sumner" <b.sumner@bom.gov.au>]

Mati rightly raises (again) the concern regarding the choice of 
comparison stars. Faced with the lack of real photometry, we often 
have to 'make do' with derived magnitudes extracted from the various 
sky surveys now in existence. Lets look at the four stars Mati 
discusses in the field of V382 Vel. The magnitudes currently used 
originate from Tycho-1, which is known to have large errors at the 
faint end, nominally fainter than 10.0. This essentially covers the 
four stars under discussion. However since Tycho-1 there have been 
several other surveys that include photometry, albeit rather 
inaccurate photometry. Do these calalogues help with the V382 Vel 
sequence? Here is a table:

GSC 0810-   Tycho-1      Tycho-2        USNO-A2.0          UCAC1
	        V    B-V     V     B-V     B     R    'V'     r     'mV'  
01423     9.94  0.51   10.00  0.46   10.4  10.0  10.1    9.92  10.08
00801    10.43  1.27   10.54  1.49   11.7   9.7  10.4   10.05  10.43
00897    10.77  0.38   11.00  0.25         10.8  10.8   10.81  10.98
01225    10.77  0.42   10.82  0.42   11.1  10.9  11.0   10.87  11.13 

The USNO-A2.0 'V' values are from the 'standard' equation 
V=R+0.375(B-V) and the UCAC1 'mV' values are from my own calibration, 
and includes a colour correction for star 00801. All the magnitudes 
listed under V, 'V' or 'mV' are all derived V estimates, and can be 
intercompared between catalogues.

So where does this lead us? Not very far unfortunately. It is fairly 
obvious that Tycho-2 should be used in preference to Tycho-1 
magnitudes, as Tycho-2 is known to be more accurate at the faint end 
than Tycho-1. However even Tycho-2 magnitudes fainter than about 10.5 
need to be used with suspicion (or care, if you rather). The other 
surveys, USNO-A2.0 and UCAC1, provide useful (rough) magnitudes, with 
UCAC1 having the potential to extend Tycho-2 to fainter magnitudes 
with reasonably small errors.

The main problem with this sequence is the red comparison star 00801. 
As Sebastian Otero states, the red colour of this star makes it look 
fainter. In fact using the equation v=V+0.2(B-V) leads to an 
estimated visual magnitude of 10.8 for this star, considerably 
diferent from the 10.4 stated in Tycho-1.

Is there a moral to all this. Yes, there are several things to 
consider if you are trying to create a comparison sequence without 
real photometry:

a) don't use red stars in a sequence. I cannot do better than quote 
from Charles Scovil "In order to keep as close to standard as 
possible we (AAVSO) will use only comp stars with B-V of < 1.0  - 
unless there is absolutely no other star available than one with 
greater than B-V = 1.  Even then we plan not to exceed B-V of 1.5".

b) Use Tycho-2 for stars brighter than 11.0, but beware that the 
absolute errors are larger than the stated errors for stars fainter 
than about 10.5.

c) To extend fainter than Tycho-2, use UCAC1 in the Southern 
Hemisphere, suitable calibrated to V. This will take you to about 
magnitude 16 with errors in the order of 0.1 to 0.2 magnitude for 
'ordinary' i.e. non-red stars.

d) Use USNO A2.0 stars, using the V calibration equation, to confirm 
UCAC1 magnitude estimates in the South, and as a source of faint 
comparison stars in the Northern Hemisphere. However be aware that 
the accuracy of these derived V magnitudes is generally quite poor.

Question - when will some group do a deep whole sky two colour 
photometric (CCD) survey? Wouldn't that be useful? 

Cheers,
Bruce

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