[vsnet-alert 11037] Re: BAV-Forum: Re: Re: Observe the hot disc of R CrB in the I band

Tue Feb 3 09:06:39 JST 2009

Well, Arne Henden's just made some photometry available in the AAVSO Variable Star Database (or whatever VSD stands for) for around this star, and it includes the below noted GSC comp star

<http://mira.aavso.org/cgi-bin/vsp.pl?action=render&name=r+crb&ra=&dec=&charttitle=&chartcomment=&aavsoscale=Choose&fov=&resolution=100&maglimit=&ccdtable=on&othervars=&chartid=&Submit=Plot+Chart>

Now, using that info I've just done a quick butcher's hook, some simple arithmetic, and the B-V from the below report of JH's looks to be pretty much the same as at maximum as per the below info of Wolfgang's, and V-I seems to be less than 0.1 redder than at maximum, possibly even the same, because I can't tell whether the pasp paper Wolfgang quotes is using Johnson or Cousins I, offhand.

That is, on quicklook, seems coloured no different to the range it has at max, to speak of.

Other's arithmetic may differ or I'm missing the point here.

That's the trouble with stars, they don't read the journals.

Cheers

John

[NB no superhumps were harmed in the writing of this email]

===================

Josch Hambsch reported :-

"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
>>
>
>