[Aavso-photometry] Resend V5558 Sgr, change in R-I but not B-V

[arne]

Peter Nelson wrote:
> Further to last nights observations, note a significant change to R-I whilst
> B-V has remained essentially constant, since the 18th.
> 
>  
> 
> V5558 SGR 2454368.96596 SEP 25.4660 6.13 I O NLX 8.7 070716 0.03 Y Comp
> I=7.592
> 
> V5558 SGR 2454368.96565 SEP 25.4657 6.51 R O NLX 8.7 070716 0.03 Y Comp
> R=8.098 
> 
> V5558 SGR 2454368.96503 SEP 25.4650 8.20 B O NLX 8.7 070716 0.03 Y Comp
> B=9.907 
> 
> V5558 SGR 2454368.96438 SEP 25.4644 7.33 V O NLX 8.7 070716 0.01 Y Comp
> V=8.726
> 
> B-V = 0.87, R-I = 0.38
> 
>  
> 
> V5558 SGR 2454361.9972 SEP 18.4972 6.95 I O NLX 8.7 070615 0.1 Y Comp
> I=7.592 
> 
> V5558 SGR 2454361.99666 SEP 18.4967 6.98 R O NLX 8.7 070615 0.03 Y Comp
> R=8.098 
> 
> V5558 SGR 2454361.99581 SEP 18.4958 9.16 B O NLX 8.7 070615 0.05 Y Comp
> B=9.907 
> 
> V5558 SGR 2454361.99425 SEP 18.4942 8.29 V O NLX 8.7 070615 0.13 Y Comp
> V=8.726
> 
> B-V = 0.87, R-I = 0.03
> 
novae typically have strong Halpha emission, and later stages have
basically a nebular spectrum (emission lines and little continuum).
That makes interpretation of broad-band colors tricky at times.  Let's
look at the equivalent colors for Peter:

   Date    V    B-V   V-Rc  Rc-Ic
2454361  8.29  0.87  1.31  0.03
2454368  7.33  0.87  0.82  0.38

and for comparison, here are two Sonoita datapoints close to these
dates:

   Date    V    B-V   V-Rc  Rc-Ic
2454358  8.15  0.91  1.33  0.13
2454368  7.36  0.88  0.80  0.48

so, while the numbers differ slightly, the overall wavelength behavior
and temporal trends are the same.  The important thing to note is that
the Rc filter contains Halpha, and so all colors that include this
filter are highly dependent on the strength of that line.  That is why
(V-Rc) is so large - if this were a normal star, (V-Rc) ~ 0.5*(B-V)
or about 0.5; values larger than this indicate the star is brighter
than normal at Rc and we are probably seeing Halpha emission.  Likewise,
(Rc-Ic) should also be about 0.5*(B-V); values less than this indicate
that Rc is too bright, again confirming Halpha emission.

For the latest outburst, you see (V-Rc) and (Rc-Ic) getting closer to
normal, meaning the continuum has increased and the relative strength
of Halpha compared to the continuum has decreased.  This is normal
for an outburst.  As it fades, you should see Halpha reappearing and
the colors again becoming discrepant.  (B-V), on the other hand, doesn't
contain strong emission lines at this point in the outburst, and so
you just see a reddened continuum which doesn't change much between
the outburst and "quiescent" stages.  If this nova suddenly fades,
which may be indicative of dust formation, then you should see it
redden and (B-V) get larger.  That is why observing in multiple,
standardized filters can give you a lot of physical insight into
what is causing the light changes.

This nova is starting to set in the southwest for northern hemisphere
observers - you folks in the south need to be vigilent and keep monitoring
this peculiar object as long as you can!
Arne