Forum Replies Created
-
Posts
-
It does indeed seem the fading of Betelgeuse has indeed been greatly exaggerated
I see there is now a recent H,J point in the AAVSO database (The first for over a year)
It shows that Betelgeuse is not fading in the IR where most of the flux is so it does suggest there has not been any significant drop in bolometric luminosity, just a small drop in temperature which has produced an exaggerated effect at V mag, a region sensitive to changes in the depth of the molecular absorption bands.
Cheers
Robin
The latest Atel from the team who originally announced the dimming has some figures on photosphere temperature changes based on IR Wing band filters. They seem to tally reasonably with our estimates from the spectrum change
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.
