Betelgeuse

[Francois Teyssier]

ATel: changes in Betelgeuse




Author: E, F. Guinan, R. J. Wasatonic (Villanova Univ.) and T. J.
Calderwood (AAVSO)
Queries: edward.guinan@villanova.edu
Posted: 8 Dec 2019; 03:57 UT
Subjects:Infra-Red, Optical, Ultra-Violet, A Comment, Star, Variables

V-band and Wing TiO-band and Near-IR photometry of the semi-regular variable red supergiant, Betelgeuse (alpha Ori; M1.5 - M2.5 Iab) has been carried out over last 25+ years. This photometry was joined by complementary B & V photometry from T. Calderwood (AAVSO). Betelgeuse and Antares are the two nearest red supergiant core-collapse Type-II supernova (SN II) progenitors.
Photometry from this season shows the star has been declining in brightness since October 2019, now reaching a modern all-time low of V = +1.12 mag on 07 December 2019 UT. Betelgeuse undergoes complicated quasi-periodic brightness variations with a dominant period of ~420 +/-15 days. But also Betelgeuse has longer-term (5 - 6 years) and shorter term (100 - 180 days) smaller brightness changes. Currently this is the faintest the star has been during our 25+ years of continuous monitoring and 50 years of photoelectric V-band observations. The light variations are complicated and arise from pulsations as well from the waxing and waning of large super-granules on the star's convective surface.

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- typically V ~ 0.2-0.3 mag). This is an opportune time to secure photometry, spectroscopy, spectropolarimetry and if possible interferometry
and Adaptive Optics(AO) imaging.

Betelgeuse is being frequently monitored during 2019/20 with HST by Andrea Dupree (CfA) as the leading part of the 2019/20 CfA MOB program (the.mob@cfa.harvard).

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[James Foster]

I'll be shooting a Ori more in the CaK band (shooting now) and also try H-alpha.....any other non IR bands (<7200A) to look at?

James

[Robin Leadbeater]

While waiting for the skies to clear properly to continue with the TESS Be star and X Per campaigns , I took a quick medium resolution (R~5500 spectrum of Betelgeuse covering ~6500-6850A. To put the region into context here is the spectrum overlaid on the MILES library spectrum of Betelgeuse

betelgeuse_20191228_890_Leadbeater_MILES.png (34.74 KiB) Viewed 30652 times

and here is the spectrum overlaid on spectra from UVES and ELODIE taken at times when the brightness was more typical, around 0.6 Visual

betelgeuse_20191228_890_Leadbeater.png (66.64 KiB) Viewed 30652 times

There are no dramatic differences but the underlying "pseudo continuum" dips more in my spectrum indicating perhaps an ~10% increase in depth of the dominating TiO band in this region. A carefully flux calibrated lower resolution full range spectrum would likely show this more clearly however.

Cheers
Robin

[Peter Somogyi]

Fresh result from yesterday, scanned near-UV and NIR ranges, allowing comparative exploration:
1. 3870-4010A (CaII region):
R~5300A, LHires III 2400/mm, 35 micron slit 1 x 20 min (more than enough signal), bin2x2, luckily had an earlier own spectrum in 2017:

alfori_3870A_4010A_2017_vs_20191230.png (58.26 KiB) Viewed 30599 times

The 2 CaII components (3933A, 3967A) have higher EWs.
Continuous U-mag drop (see aavso) may mean that only the continuum has dropped, not the core CaII emission.

Note: James's spectrum of 2019-11-18 show similar state of the core CaII emission as mine in 2017, however can't compare the one of his 2019-12-11 due to resolution difference.

2. 8250-8950A, Ca II triplet region, R~5000 (rare setup: 15 micron slit + IR filter + 600/mm grating + bin 1x1)
Comparison against INDO-US (year 2004, https://www.noao.edu/cflib/):

alfori_8250A_8950A_lhires_vs_INDO_US2004.png (60.23 KiB) Viewed 30599 times

Whilst absorption is dominant, ratio show outstanding difference at CaII triplet.

3. 7100A-7800A (checking the often mentioned K I 7699), same setup:

alfori_7100A_7800A_lhires_vs_INDO_US2004.png (56.16 KiB) Viewed 30599 times

There is really some change at 7699, however not so outstanding (same INDO-US spectrum used).
For visual purpose, here I made a telluric removal using other hot stars (multipe ones to have a check, this reduction is not perfect in the 7605-7680 range, similarily problemmtic as more INDO-US spectra).
But I agree with Robin, most prominent change here is the higher depth of TiO band, best to observe that at low resolution + full range.

Submitting spectra to aavso is in progress. Hope to see new comparative results from others.
Peter

[Olivier GARDE]

Here's my contribution with an echelle spectrum taken last night (R=11000)



And to compare the evolution before/after the falling brightness, I recovered a spectrum taken in February 2005 by Elodie spectrograph at OHP :



and the superposition of the 2 spectra :



I looked in detail the 2 spectra over all wavelengths and I did not see any significant evolution before and after the decrease in brightness ....

[Robin Leadbeater]

Carefully flux calibrated ALPY 600 spectrum (using repeat spectra and 2 reference stars) confirms small increase in depth of molecular bands compared with MILES spectrum from 2000-2001. Spectrum added to BAA database

Cheers
Robin

betelgeuse_telrem_20191230_949_Leadbeater.png (70.16 KiB) Viewed 30535 times

[Simon de Visscher]

Hi,

Here are some first results I had with the ALPY600+ST10xme mounted on a AP130EDT (processing with IRIS): I managed to get two spectra on Dec 25th and 29th. Just be careful with my results, these are the first "serious" ones I manage to get with the Alpy, so it might very well be that some things are still not under control (most probably on the processing side).

As we can see these are not identical. Unless someone has a spectrum taken at the same time I can only speculate about the most probable systematic uncertainties:

a) The ref star used on Dec 25th is Mu Ori, which is...a variable star (good shot...)
b) the weather conditions was varying quite fast on Dec 29th (fast varying fog/humidity), so it might be that the the atmospheric transparency varied between the ref and betelgeuse measurements.

betelgeuse.png (65.5 KiB) Viewed 30475 times

Olivier and I did some comparisons with his own measurement (see before), and the agreement is generally ok-ish although not perfect. Now no clue if that may come a change of the continuum global shape in 1-2 days time.

Cheers,

Simon

[Joan Guarro Flo]

Hello,

Whee have I to send this spectra, please ?

Joan.

[James Foster]

Here's my low resolution (R=517) of a Ori:

I took the best 9 of 17 to lower errors due to scintillation. Except for the OII tellurics, Balmer, & CaII lines, all other lines shown are speculative.
I'll try to hit a Ori with the LISA IR (6300-9600A), but my Atik460 has been acting up lately and may have to be shipped to the UK for service; hopefully
I'll make it before Brexit!

James

[Francois Teyssier]

Hello Joan,
You can send me the spectrum
I will open a page in the database

Simon:
Trés correct.
Toujours utiliser la même étoile de référence, comme tu le notes dans ton message.
Et des durées d'exposition sufisamment longues pour éviter les effets de bords sur la fente (minimum 30 secondes). Pas facile avec un objet brillant ... Raison pour laquelle Véga est une trés mauvaise étoile de référence avec un Alpy ou un LISA.

An excellent target for UVEX as shown by the very interesting evolution shown by Peter
and also for high resolution spectroscopy ( R = 50000 )

François