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

Sun Feb 1 18:31:36 EST 2009

Following observations have been obtained during May 2008.
The observations are transformed to the standard system.
The values given are averages from 3 images in each filter, the standard
deviation is given in parentheses (least significant digit).

The comparison stars used are labeled 122, 126, 129 and 142 on AAVSO
chart 1072knh.
For the observations on JD 2454594.42 comparison star 122 has not been
used.
The values for the comp stars are taken from the photometry table for
this AAVSO chart.
Please note that the photometry for these comp stars comes from various
sources (ASAS, TASS, SDSS, CMC14)!
The V-Rc data is based on comp star 142 only. For the other comp stars
no Rc data is available currently.
The observations will be revised and submitted to the AAVSO database as
soon as the field is calibrated.

JD          V         V-Rc      V-Ic 
2454594.42  14.30(2)  +0.37(6)  +1.04(3)
2454595.41  14.32(2)  +0.41(3)  +1.05(4)
2454599.41  14.25(1)  +0.36(3)  +1.00(3)
2454607.41  14.29(4)  +0.35(4)  +1.05(1)

Arne, is this fading not monitored at SRO as intensively as Z UMi's
fading?

Best Regards,
Eric

> -----Original Message-----
> From: aavso-photometry-bounces at aavso.org 
> [mailto:aavso-photometry-bounces at aavso.org] On Behalf Of arne
> Sent: zaterdag 31 januari 2009 16:28
> To: Josch Hambsch
> Cc: AAVSO-PHOTOMETRY; BAV-Forum; vsnet-alert (mailing list); 
> Wolfgang Renz
> Subject: Re: [Aavso-photometry] [vsnet-alert 11010] Re: 
> Observe the hot disc of R CrB in the I band
> 
> 
> 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&s
tart=&stop=2454900&obscode=&obstotals=on&grid=on&type=png&pointsize=1&wi
dth=800&height=600&mag1=5&mag2=16&mean=7&visual=on&uband=on&bband=on&v=o
n&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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