[vsnet-alert 11013] Re: Observe the hot disc of R CrB in the I band

arne arne at aavso.org
Sun Feb 1 00:27:41 JST 2009


Josch Hambsch wrote:
> I could do tonight BVI observations for R CrB for a couple of hours if 
> this is of interest?
> 

Hallo Josch,
Any filtered photometry at this very faint minimum would be welcomed
by the professionals.  Usually they are more interested in a single
nightly observation since the minimum lasts so long, but a time series
or two to see what kind of sub-day variation is present can't hurt.

 From last night, approximate photometry (+/- 0.03):
V = 14.89
(B-V) = +0.43
(V-Rc) = +0.26
(Rc-Ic) = +0.52
(V-Ic) = +0.80

We're currently calibrating the field, so that the fainter sequence
stars have real BVRI photometry instead of transformed values from
other sources.  Initial results indicate that they are close, but
will shift by ~0.05mag or so.
Arne

> ----- 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
> 


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