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Hi Andy,
Good point. The V (and R) band does seem to sit in a region which is particularly sensitive to the degree of molecular band absorption which in turn is sensitive to temperature. That additional absorbed flux has to end up elsewhere in the spectrum, most likely in the IR so changes in V mag probably do overestimate the change in total flux as you say.
Here for example is a comparison of standard spectra for M2iii (blue Teff 3600K) compared with M6iii (pink Teff 3100K)
No the individual lines cannot be resolved (Even if the spectrometer had enough resolution they would be broadened in the stellar spectrum to the point where they merged). The depth of the merged bands however gives a measure of the spectral class and therefore Teff even in low resolution spectra. The referenced Atel for example just used photometry in a particular targeted 705nm waveband to estimate the temperature change.
“Measures of Wing TiO-band (705 nm) and near-IR colors indicate that currently Betelgeuse has relatively strong TiO-bands and has a corresponding lower photospheric temperature of T~3580 K (relative to T~ 3660 K near maximum brightness”
Wein’s law is not too much help here as we are seeing just the tail of the black body distribution and stars like these are not really black bodies in any case due to the contribution of the deep absorption lines which varies with temperature
Looking at the Pickles standard spectra (see attached) The change in Betelgeuse looks to correspond to a change in spectral class of around 1-1.5 points eg M2 to M3.5 That is equivalent to a drop in Teff of around 200-300K
The implication is that the contraction in this case could be the result of a reduction in energy production rather than the normal radial pulsation cycle. I am not convinced yet that we are in that situation though rather than the coincidence of the combined effect of the normal pulsations with different periods. If it keeps dropping though….
The short answer is I don’t know. Perhaps the star’s photosphere has reduced in radius by ~30% (and hence halved in surface area) at approximately constant effective temperature ? The original Atel
http://www.astronomerstelegram.org/?read=13341
notes a small drop in Teff, indicated by an increase in the TiO band depths, which I am also seeing but the temperature drop quoted there does not seem to be sufficient on its own to explain the drop in luminosity. Since luminosity goes as T^4, the temperature would need to drop by ~%16 to halve the luminosity.
Interestingly the changes in the spectrum are rather subtle considering the luminosity has roughly halved. Here is a current low resolution spectrum compared with one from the MILES library taken 2000/2001 when the V mag brightness was typically ~0.5
Apparently there’s a lot of money to be made from reducing latency
A point source that bright would be a problem for an amateur setup too but perhaps not if you defocus
http://www.threehillsobservatory.co.uk/astro/SA200_SEPSA_Vega.png
Type II supernovae are a pretty heterogeneous bunch but a typical time to maximum at visible wavelengths is around 10 days. (Earlier at Gamma, a few hours when the shock wave breaks out? and Neutrinos even earlier) Assuming a velocity of explosion of say ~10000 km/s that gives a diameter of around 0.6 arcsec after 10 days if my back of envelope calculation is correct
If the redshift based distance (~80Mpc) is correct, mag 14.4 works out at an impressive absolute magnitude of -20.2 , thought to be boosted by interaction with circumstellar material in the case of type IIn
Robin
I have just posted the image again here
https://britastro.org/comment/7489#comment-7489
and strangely the original image now appears everywhere it should ! Work that one out
It appears on my pc (win 7 /chrome) but not in another chromium based browser or on my phone (android/chrome)
Here it is again in case it was a one off glitch
I see there is a box in the user account settings that has to be ticked to allow these messages to be received. Mine was ticked so I assume it is set like that by default. (The system seems to be working for me)
Many forums have a personal messaging system that allows members to communicate privately within the forum eg cloudy nights , stargazers lounge etc . Is that a possibility?
This is how it looked in the guider 2019-12-01T17:18:12 3 hours after the discovery was announced (South is up)
Interesting. Your values match nicely the levels when the disc was present. It could be another way of monitoring disc loss. I will point Paul to this and see if he has any comments. Meanwhile the V mag appears to have now returned to the same level it was a year ago. The H alpha EW is still continuing its steady fall though.
I am tempted to upload a classification spectrum (A G2v star with superimposed methane absorption bands)
A few years back it seemed every sixth former was launching cameras into the stratosphere. I know our local school did and they came along to our society to talk about it. I remember Julian Wessel did a nice night flight a couple of years ago, though there doesn’t seem to be any evidence of the further promised flights around
He did have the advantage of the whole of continental Europe to land in and decent roads to make a fast rendezvous !
Robin
Here is some feedback from Paul Roche concerning the drop in V mag and evolution of the H alpha emission line. Please keep an eye on the spectra in the BAA database and if there has not been an observation for a week or two consider taking one.
Thanks!
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Feedback from Paul Roche 20-11-2019
It certainly looks like behaviour similar to that prior to previous disc-loss events (weakening H alpha, fading V mag). It would be interesting to see if system is becoming bluer as well, as tends to happen when the red circumstellar disc dissipates. I’d be particularly interested in any infrared observations (I band, or even JHK bands – although that’s getting very tricky!), as that’s another good probe of the extent of the circumstellar disc.
There’s nothing much happening in X-rays at the moment as far as I can see, but I’d like to keep an eye on as many wavelengths as possible as we typically see an interaction between the neutron star and the disc material if the disc expands/moves outward and reaches the (wide) NS orbit eventually (assuming the disc material is being expelled outwards from the star… if it “collapses inwards”, we won’t see any interaction and the NS will continue it’s gradual spin-down).
It’s hard to predict when the H alpha emission will disappear (if it does…) and the normal B-star absorption becomes visible again, but weekly observations are fine for now. If it appears that the decrease in H alpha EW is accelerating, it would be worth trying to increase cadence to every couple of days to capture those final stages.
