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Betelgeuse

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Betelgeuse

Following on from Jeremy's news article on Betelgeuse, what is the time frame of the supernova once it goes? How quickly will it appear to brighten, seconds, minutes, days? And once it is at its brightest, will it appear as a disk, or will it remain as a point source of light?

Thanks.

James

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Time to maximum and diameter

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

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Timescale

It depends entirely on how you look at it.

Neutrino telescopes will notice a great increase in brightness on the scale of seconds to a minute or few.

Optical telescopes will take a day or few, if the many thousands of other SNe which have been observed are anything to go by.

Betelgeuse already shows a disk if your telescope is good enough. It will show an ever bigger disk on timescales between days and millennia.  Compare SN1054, the outside of which is now big enough to have been seen by Messier.

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I see Robin and I posted

I see Robin and I posted pretty much simultaneously.

The neutrinos from SN1987a came in a clump a few seconds long.  The telescope wasn't very sensitive and the SN was at quite a distance so it's likely that it saw only the very peak of the neutrino curve.

However, core collapse is a very rapid process, on the timescale of a minute or so (hence my prediction), and there is no obvious intense source of neutrino emission afterwards. Once the neutrinos get outside the core everything else lying in our direction is essentially transparent so they will not be scattered as is the initial burst of photons.

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Brightness

A supernova at that distance is going to be pretty bright. Are any wavelengths reaching us likely to cause problems?

Happy Christmas/Hannukah/Saturnalia/Yule/Solstice

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Problems

Yup.

All the professionals are going to be bugging us amateurs for telescope time because it saturates the detectors on all their equipment.

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I think we can relax...

As regards wavelengths causing problems, I think that's very unlikely.

It'll produce plenty of UV and X-ray photons, but assuming they come out uniformly in all directions, it wouldn't have the power to do anything serious from a distance of 700 lyr. If it beamed them in a jet in our direction, that might be a different story. Luckily there's strong evidence the rotational pole is inclined at an angle of about 20 degrees to our line of sight, and any jets would be directed out of the poles. So I think we can sleep easy.

But as Xilman says, the world's professional astronomers may struggle to find high-resolution spectrographs that can deal with something that bright.

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Defocus

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

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We live in interesting times

Not that often a variable star is featured on the BBC Radio 4 six o'clock news!

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Not that often a variable star is featured on the BBC R4 news

Indeed, David. Variable stars are of course fascinating and I am hoping the Beeb will now make a New Year's Resolution to make this a weekly event....

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In the news

I thought that they covered the item reasonably well too. Hopefully everyone will be gazing south later this evening, peering through the firework smoke to look at Orion's right shoulder...

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spectrum change

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

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Reason?

Any suggestions as to why that should be?

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contraction ?

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. 

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Curious.

If the temperature had dropped I would expect the continuum to have shifted too --- Wien´s law --- though as that goes only as the first power of (1/T) perhaps the effect might be too small to be easily noticeable. I certainly haven´t noticed it from your spectra but there again, I don´t have much experience in these things.

I´m now wondering whether a neutral grey filter has interposed itself between us and the star.  Something akin to the clouds of dust which appear in the atmospheres of RCB variables. The typical particle size would need to be significantly larger than the wavelength of light or we would see severe reddening.

Curious indeed.

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Shrinkage.

If the radius dropped I would expect the temperature to rise, not fall, as the gravitational potential energy is converted to thermal kinetic energy.

Again, curious.

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contraction

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....

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temperature change

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

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Spectroscopy, temperature and stellar structure.

Thanks for your in-depth explanations.  I´m learning!

Although I'm a molecular spectroscopist by background I am emphatically NOT an astrophysicist. You (personally, not the generic "you"') can´t resolve the rotational and any hyperfine substructure of the molecular bands, which is where I cut my teeth.  Not entirely sure that anyone can.  Betelegeuse is bright enough for spectral resolutions of 100K-1M (my doctoral work was at  a resolution of around 300,000) but do the physical conditions in the star's atmosphere allow that kind of line resolution? That was a rhetorical question. I would be delighted to learn that rotational structure is readily observable, not least because the effective temperature of any species in question could then be nailed down to a very few Kelvin.

Roughly half my DPhil thesis concerned the rotational structure in the spectrum of CeO at ~2300K. Its spectrum is mind-bogglingly complex for such a simple diatomic molecule. There are at least eight low-lying electronic states with populations high enough to exhibit absorption spectra at 2300K. Well over 100K lines in the absorption spectrum between 300nm and 1200nm were measurable with 1980´s technology. CeO is also known to be an atmospheric constituent of a number of cool stars.

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molecular bands

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"

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effect of temperature on M class spectra

Here for example is a comparison of standard spectra for M2iii (blue Teff 3600K) compared with M6iii (pink Teff 3100K) 

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Spectro

Thanks Robin.

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Has luminosity shifted into the IR?

Betelgeuse is a cool red star with the majority of its flux in the red and infrared, while the majority of observations are visual or with a V filter. It occurred to me the widely discussed drop in luminosity is based on the assumption that what is happening in the visual represents the total luminosity of the star, and this might not be the case. I decided to do a little investigation using spectra from the BAA Spectroscopy Database and V filter magnitudes from the AAVSO International Database to do a rough flux calibration of the spectra.

My first plot shows what passes through a V filter using James Foster's spectrum from 9th April 2018. You can see the V filter is most sensitive to wavelengths where Betelgeuse is not at its brightest. The V filter is approximately like human vision, not a perfect match but reasonable. You can also see how the flux continues to rise to shorter wavelengths towards the infrared.

Next I compared James Foster's spectrum from 9th April 2018 to Robin Leadbeater's recent spectrum from 30th December 2019. The approximate V magnitudes on these dates are 0.4 and 1.4, though due to difficulties with measuring the luminosity of Betelgeuse it is best to consider these with uncertainties of +/-0.1 magnitudes. A magnitude of 0.4 is fairly typical of Betelgeuse in recent years, noting it regularly varies by nearly a magnitude. I did a flux calibration using the V filter profile rather than any specific representative wavelength. This was to avoid any effects due to individual lines skewing the results. This shows a very marked drop in luminosity to longer wavelengths up to at least 7400 Angstroms.

Their aren't spectra in the BAA database with longer wavelengths into the infrared due to the difficulty in making these measurements. So instead I then compared Pickles reference spectra for M2 and M4 (both at luminosity class III). These aren't perfect matches to James and Robin's spectra but they are good enough for a first approximation (M2 typical and M4 now). It can be seen that a drop in luminosity at shorter wavelengths in the visual region does not necessary continue to longer wavelengths, with the flux being about the same from about 8000 Angstroms. These spectra only go to 10,000 Angstroms, but give a hint of stronger flux deeper into the infrared for the cooler M4 spectrum. So I wonder whether the total luminosity has not dropped by as much as it appears. Perhaps some or most of the flux has shifted from the visual into the infrared?

Comments and criticism welcome :-)

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Blanketing

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.  

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Betelgeuse in the IR

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

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Betelgeuse in the IR

Thanks for sharing this Robin. When I first looked on the AAVSO database there were no recent IR measurements, it is great these have been added.

I agree this implies a cooling rather than a change in total luminosity (bolometric).

It is possible the star may have expanded a little rather than contracted as I have seen some saying, to explain the cooling. Though that is pure speculation on my part. A more detailed analysis of the existing data and ideally additional spectra in the IR would help to tie down the change in bolometric luminosity and temperature, to then understand if it has changed size.

Cheers,

Andy

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latest Atel

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

http://www.astronomerstelegram.org/?read=13365

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Spectrum evolution

I am now picking up some evolution in my spectra since the start of the year.  The relative flux around 7500A has been increasing compared with that at 5500A.  This is consistent with the brightness in V dropping more than in the IR 

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the trend continues

The plot updated with a spectrum from 2020-02-04 showing continuing evolution in the red

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Latest Atel

Here is the latest Atel on the subject 

http://www.astronomerstelegram.org/?read=13410

"The recent changes defined by our V-band/Wing photometry seem best explained from changes in the envelop-outer convection atmosphere of this pulsating, unstable supergiant. If these recent light changes are due to an extra-large amplitude light pulse on the ~420-day period, then the next mid-light minimum is expected during late January/early February, 2020. If Betelgeuse continues to dim after that time then other possibilities will have to be considered. The unusual behavior of Betelgeuse should be closely watched."

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Chart for Alpha Ori

A new BAA VSS visual chart for Betelgeuse has been prepared by John Toone and is available in the original article on the fading of this star. Unusually, this is an 80 degree chart, which includes many of the bright stars in the winter sky. You can also download the chart directly from the VSS website.

The latest observation in the BAA VSS database has the star a mag 1.6 (Gary Poyner, Jan 28, visual).

Further observations are encouraged to see whether the fade is actually bottoming out.

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Minimum time for Betelgeuse?

According to a very recent paper by Constantino Sigismondi, his analyses of its light curve variously predict minimum sometime between Jan 22 and Feb 1. Time will tell whether reality bares this out.

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Video of Betelgeuse workshop

Constantino Sigismondi has sent me the link to the international workshop on Betelgeuse.

This is a "virtual" workshop in which Most of the speakers are beamed in via telecon, so the quality is a bit variable....

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Video of Betelgeuse workshop

Constantino Sigismondi has sent me the link to the international workshop on Betelgeuse.

This is a "virtual" workshop in which Most of the speakers are beamed in via telecon, so the quality is a bit variable....

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Minimum time for Betelgeuse?

American media inform us that Betelgeuse is 'fainting', so perhaps we need some interstellar smelling salts to help it recover from its current swoon...

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Minimum time for Betelgeuse?

Hi Alex

I've been occasionally looking at Orion clips on my meteor video camera. I started to see some dimming of Betelgeuse last year and since then, it was evident just comparing with Rigel.  But last night and Wed morning, I thought?? I noticed Betelgeuse looked as if it was getting brighter again.  Anyone notice something like this in the last 24 hours?  

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International workshop on Betelgeuse

A workshop was organised by Prof. Costantino Sigismondi in Italy a couple of weeks ago to discuss Betelgeuse's dimming. I'm told that a video of the event will be posted on its website, but it's not there yet. Worth keeping an eye out for. In the meantime I have asked Constantino for more info...

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spectrum update
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Still fading

This is a 10d mean visual and V-band light curve for Betelgeuse since the start of the year - taken from the AAVSO light curve generator.  The trend is still slightly downwards, but perhaps levelling out.

Gary

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Betelguese

Due to dimming, is it changing its spectral type ?

Regards,

Jack

Essex  

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spectral type

Yes slightly as seen in the increased depth of the molecular absorption bands (perhaps 1 subdivision eg M2i to M3i?)  

https://britastro.org/comment/7649#comment-7649

https://britastro.org/comment/7861#comment-7861

Interestingly there has also been a reduction in  H alpha absorption since the start of the year

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Images

High resolution images of Betelgeuse are available on the ESO website showing before and after images of the surface taken with the Sphere instrument .Could it be dust that is causing the dimming. hears the link https://www.eso.org/public/news/eso2003/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+EsoTopNews+%28ESO+Top+News%29

Peter

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dusty Betelgeuse
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partial obscuration

Christian Buil has just posted some high (R ~ 38,000) resolution spectra which confirm the reduction and narrowing of the H alpha line seen in my medium resolution spectra. He  also detects a small (4.5km/s) shift in the line.

http://www.spectro-aras.com/forum/viewtopic.php?f=38&t=2433&p=13796#p13792

These observations may be consistent with the partial obscuration of the star as suggested from the VLT images. The speed of rotation of the star is of the right order as can be seen in this reference and the axis of rotation is given there

https://www.obspm.fr/the-slow-rotation-of-the-red.html?lang=eng

As far as I can see, the orientation of the VLT image is not given though unfortunately. 

Jeremy, are you still in contact with Prof. Constantino Sigismondi? I wonder if these observations might be of interest

Cheers

Robin