[vsnet-alert 11010] Re: Observe the hot disc of R CrB in the I band

Josch Hambsch hambsch at telenet.be
Sat Jan 31 21:55:37 JST 2009


I could do tonight BVI observations for R CrB for a couple of hours if this 
is of interest?

Josch

http://www.astronomie.be/hambsch
----- Original Message ----- 
From: "Wolfgang Renz" <w_renz at onlinehome.de>
To: "AAVSO-PHOTOMETRY" <aavso-photometry at mira.aavso.org>; "vsnet-alert 
(mailing list)" <vsnet-alert at ooruri.kusastro.kyoto-u.ac.jp>; "vsnet-rcb 
(mailing list)" <vsnet-rcb at ooruri.kusastro.kyoto-u.ac.jp>; "BAV-Forum" 
<forum at bav-astro.de>
Sent: Friday, January 30, 2009 6:17 PM
Subject: [vsnet-alert 11004] Observe the hot disc of R CrB in the I band


Hello

As R CrB is now in a very faint minimum, the following might be
of interest for the ones who can take filtered images.

The fadings itself of R CrB (Simbad: G0Iab:pe, GCVS: C0,0 (F8pep),
2MASS: 5.36 J, 5.09 H, 4.56 Ks) seems to be in general relative
gray (= its color doesn't change much during the fading):
- Near maximum
  (1982PASP...94..172F: obs mag / 2002A&A...394..617Y: <m>):
  U-B = ~ 0.07 / 0.06
  B-V = ~ 0.61 / 0.58
  V-R = ~ 0.49 / 0.42
  V-I = ~ 0.70 / 0.18
- At ~ 7 magV (from Simbad UBV data):
  U-B = ~ 0.29
  B-V = ~ 0.77
  V-I = ~ 0.7 (see below)
- At ~ 8.5 magV (see below)
  B-V = ~ 0.7
  V-R = ~ 0.4
- At ~ 10.7 magV (see below)
  V-R = ~ 0.53

The notes to the BSC say:
 Hydrogen deficient carbon star. At minimum, emission of CaII, NaI,
ScII, TiII, SrII, and FeII. Spectra at 2 and 10 micrometers indicate a
thick dust shell either around R CrB or a companion.

The derived color-brightness relations in 2002A&A...394..617Y:
    U-B = 0.402 * V - 2.31
    B-V = 0.280 * V - 1.08
    V-R = 0.301 * V - 1.36
    V-I = 0.430 * V - 1.94
    V-J = 0.418 * V - 1.62
for the 5.775-6.025 magV range due to its pulsation obviously
doesn't hold for fadings.
 The lightcurves in the V and J band (probably U to J) look pretty
similar. In L and M band its dominated by the warm disc. The K
band shows a transit between the two light curves. Their modeled
dust shell itself seems to be faint, but pretty blue:
    V  ~~ 14.6
    U-B ~~ -0.65
    B-V ~~ -0.28
    V-R ~~ -0.41
    R-I ~~ +1:47
 So this look like as there is Rayleigh scattering at very small
dust grains in the UBVR bands and already the very beginning
of "thermal" emission of the grains in I band !

As a hot body of already a bit below 300°C can be detected with
CCDs that are sensitive to ~ 1100 nm (e.g. a mid-hot soldering
iron can be detected this way long before it starts to glow in the
visual range due to emitting light below ~ 700 nm), the dust must
be pretty warm to give a significant signal in the I band (especially
if the I band filter is just a RG6 long-pass filter and doesn't have a
880 nm short-cut interference layer).

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Therefore it will be most interesting for amateurs and pros to
observe R CrB with the available silicon CCDs in the V and/or
R plus in the I band (at least and especially when it approaches
a very faint minimum (< 14 magV when the dust starts to contri-
bute significantly to the total brightness).
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

The color excess E(B-V) of R CrB is just 0.05 mag (Asplund et al.
1997).

A light curve with all the obs submitted to the AAVSO of R CrB can
be viewed at:
<http://www.aavso.org/cgi-bin/lcg.pl?name=R+CrB&lastdays=800&start=&stop=2454900&obscode=&obstotals=on&grid=on&type=png&pointsize=1&width=800&height=600&mag1=5&mag2=16&mean=7&visual=on&uband=on&bband=on&v=on&r=on&iband=on&j=on&h=on&unvalidated=on>
Unfortunatly there are no I band measurements up to now yet to
verify the above.

It would be great of one of you could take a VRI sequence and check
if the V-I is really >= +1.0 mag or the R-I > +1.4 mag.

Clear skies
 Wolfgang

-- 
Wolfgang Renz, Karlsruhe, Germany
Rz.BAV = WRe.vsnet = RWG.AAVSO


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