[ngcles]

Hi Ken & All,

Yes it has been interesting to follow -- when the sky is clear enough. For the last six weeks here it has either been smoke or dust. Often sufficient to obscure all stars down to second magnitude.

I did get a chance to finally get a look with a relatively clear sky on 22nd December at about 2.30am. My estimate was magnitude +1.3 -- the faintest certainly I've ever seen it. It was notably fainter than Aldebaran, (+0.9) (but variable) and a bit fainter that Pollux (+1.14) and a bit brighter than Bellatrix (+1.62).

Tonight the comparison stars were Adhara (Epsilon Canis Majoris +1.53) and Bellatrix (+1.62) and it was brighter than both these and I arrived at a similar magnitude for Betelgeuse (+1.3). The sky is still somewhat smoke affected but these three stars are all about the same height above the horizon and so, similarly affected by haze.

Recent (about a week old) photometric measures from different observatories have yielded: V = +1.273, +1.294 and +1.286. This is the faintest it has ever been photometrically measured.

The current thinking is this dip in brightness is a superimposition of two different minima within two of Betelgeuse's roughly cyclic pulsations -- 495 days and 2100 days. Call it a resonant super-dip if you like.

The latest update is in ATel #13365 that reads:

As reported earlier in ATel #13341, the luminous red supergiant (a core-collapse SN II progenitor) Betelgeuse (alpha Ori) has become unusually faint with a V ~+1.125 mag (measured on 2019 December 07.25 UT). Since that report Betelgeuse has continued to decrease further in brightness. The most recent measurements made on 2019 December 19.3 UT, 20.2 UT and 22.25 UT are V = +1.273, +1.294 and +1.286 mag, respectively. This appears to be the faintest the star has been measured since photoelectric observations have been carried out of the star. However, photoelectric photometry carried out during late-1926 / early-1927 by Joel Stebbins (1931: Pub. Washburn Obs., 15, 177) indicates that Betelgeuse declined to V’ ~+1.25 mag. At its average maximum brightness light (V ~ 0.3 - 0.4 mag), Betelgeuse is the 6 - 7th brightest star. But by 2019 mid-December the star has slipped to the ~21st brightest star. The red supergiant is now closer in brightness to Bellatrix (V =+1.64 mag) than to Rigel (V =+0.13 mag). Wing three-band Near-IR and TiO photometry carried out at Wasatonic Observatory shows that Betelgeuse is also cooler with an inferred spectral-type near ~M3.5 Iab (Teff ~ 3,545 K from TiO-photometry). This is about 150 K cooler than measured near maximum light. Analysis of the last 25-yrs of V-band and Wing TiO and Near-IR photometry shows a dominant ~425+/-10 day period as well as a long-term ~5.9+/-0.5 year period. The current faintness of Betelgeuse appears to arise from the coincidence of the star being near the minimum light of the ~5.9-yr light-cycle as well as near, the deeper than usual, minimum of the ~425-d period. We plan to continue to monitor the star. If the star continues to follow above periods, light minimum should occur soon. But this needs to be checked. This continues to be an opportune time to carry out complementary measures of Betelgeuse while it is in its current low state and is unusually cool and faint.



For those of you hoping for an imminent core-collapse supernova, you're probably going to be disappointed -- a mundane explanation is more likely. One to keep an eye on certainly so we can gain a better understanding of the late evolution of high-mass stars.

Best,

L.