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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.
I just made a few checks indoors using a red LED with my 350D DSLR, 18mm lens with a Star Analyser 200 on the front. There was a slight reduction in dispersion close in but once beyond 5m the change was very small
Relative to the dispersion at 10m, the dispersion reduced by
5m =0.9% 1m = 3.5% 0.5m = 5.7%
Cheers
Robin
That does look rather like pincushion distortion
https://en.wikipedia.org/wiki/Distortion_(optics)#Radial_distortion
(In stellar spectra, atmospheric refraction can also bend the spectrum but this cannot be the cause here)
Lens distortions will also distort the spectrum along the dispersion direction as well as bending the spectrum. The DSLR lens is probably better in this respect than the CCTV lens. What does the image look like without the grating eg eg does the roof line look curved as you move from top to bottom of the frame ?
If the lens is distorting the image significantly that will make precise calibration difficult. Once defined you could correct the distortion first in software first which should straighten everything out and linearise the dispersion. I don’t have any direct experience of handling this with meteors and wide angle lenses though. Perhaps Bill Ward has come across it and has some ideas on how to deal with it ?
Cheers
Robin
Hi Derek,
>I have a suspicion that I’m going to get slightly different dispersion distances for n = +1 and n = -1, possibly because of the grating, being plastic, it’s not completely flat but is curving. Depending on where the dot occurs, if it’s on a part that’s more curved, relative to the opposite side of the zero order image, we might get discrepancies there.
That would be really strange, I think. The light from the laser dot on the distant object illuminates the full grating (just like a star illuminates the full aperture if the telescope.) The grating then produces a number of beams at different angles according to the spectrum order, which are then brought to focus by the lens. Distortions in the grating could blur the spectrum because the effective line spacing varies across the grating but I don’t understand how it could affect the +- orders differently. If you are seeing different dispersions in +- orders I suspect It would be more likely to be due to aberrations in the lens (eg barrel/pincushion type distortions for example.
Cheers
Robin
Hi Derek
>But I think once done and I have a value of the number of pixels the green LASER is dispersed in the first order diffraction, as long as the equipment stays the same, lens, focal length, grating, do you think the calibration will no longer be necessary ? Oh, I think I need to follow up with Robin’s comments on distance. I will need to do some exposures with various objects at different distances painted with the LASER. Robin, do you think it’s important to know the actual distances? A problem I have is, I don’t know the distances – they are just roof tops, aerials etc. Is there a way to work out their distances?
I can’t add much I am afraid as expected the dispersion to be a constant of the setup so the effect of the distance to the source came as a surprise to me ! Perhaps it only becomes significant as the focal length of the lens becomes a significant fraction of the distance. I think I would start with something close, a couple of metres away say compared with a few tens of metres to get a feel for the magnitude of the effect.
Cheers
Robin
