[vsnet-alert 11022] Re: BAV-Forum: Re: Re: Observe the hot disc of R CrB in the I band

[Josch Hambsch]

Hi Arne,

I got 4 images in BVI of R CrB over a period of about 1h (individual 
exposures of 5 min each). What Check and Comp stars should I use.
Presently I have used:

GSC 2039 52 as comp star
GSC 2039 1551 as check star

This gives relative magnitudes differenbces to the comp star of (average of 
3 values)

B 0.848 +/- .008 mag
V 0.822 +/- .007 mag
I  0.678 +/- .011 mag

I could get more measurements in the next days if of interest. I could also 
make a series during a longer period and maybe shorter exposures (2-3 
minutes) if this is of concern.

Regards,

Josch

http://www.astronomie.be/hambsch
----- Original Message ----- 
From: "arne" <arne at aavso.org>
To: "Josch Hambsch" <hambsch at telenet.be>
Cc: "Wolfgang Renz" <w_renz at onlinehome.de>; "AAVSO-PHOTOMETRY" 
<aavso-photometry at mira.aavso.org>; "vsnet-alert (mailing list)" 
<vsnet-alert at ooruri.kusastro.kyoto-u.ac.jp>; "BAV-Forum" 
<forum at bav-astro.de>
Sent: Saturday, January 31, 2009 4:27 PM
Subject: BAV-Forum: Re: [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&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
>>
>
>