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An ATel from Wolfgang Vollmann and Costantino Sigismondi today (https://www.astronomerstelegram.org/?read=15800) suggests that the red hypergiant RW Cep might undergoing a dimming event similar to that observed in Betelgeuse a few years ago. It normally shows irregular variations between magnitudes 6.0 and 7.3. However, it has recently been fading as is currently mag 7.7 (see VSS light curve).
Worth following to see how faint it gets.Attachments:
Some (PEP) photometry in the IR might be interesting. Betelgeuse hardly faded at all in H and J. There is no history for RW Cep in those bands in the AAVSO database though. Does anyone in the BAA have this capability ?
Here is a spectrum tonight (R=1000 using an ALPY 600 with a narrower than usual 10um slit)
https://britastro.org/specdb/data_graph.php?obs_id=13110
It is very red but clearly heavily reddened by interstellar or circumstellar dustVSX suggests a very wide range of spectrum classifications ranging from G to M2 but Keenan suggests K2i in his 1989 catalogue.
https://articles.adsabs.harvard.edu/pdf/1989ApJS…71..245K
Using the B,V brightness values in SIMBAD, this would imply for a K2i star an E(B-V) = 0.99. Dereddening my spectrum by this amount indeed gives a good match to the Pickles standard for K2i both in terms of the continuum shape and more importantly in the spectral line details (My spectrum is a slightly higher resolution which would explain the higher apparent intensity of the lines in my spectrum)
https://britastro.org/specdb/view_image.php?obs_id=13110Cheers
Robin
EDIT- trying to get the link working.. and failing 🙁 cut and paste it if you need toI suspect the M2 classification may have come from the colour index which would match an M star without reddening but the lack of molecular bands in the spectrum immediately shows we are dealing with a hotter star here (significantly hotter than Betelgeuse)
Cheers
RobinA high resolution spectrum (R=15000) taken tonight shows H alpha to be in emission
https://britastro.org/specdb/data_graph.php?obs_id=13128
There are a number of publicly available archived high resolution spectra taken with the ELODIE spectrograph at Observatoire Haute Provence between 1998 and 2005. None of these show H alpha in emission so this may be unique to this dimming.
Plots of the ELODIE spectra and comparisons with my spectra at both low and high resolution can be seen here on my website
http://www.threehillsobservatory.co.uk/astro/RW_Cep/rwcep_elodie_archive_THO_2022-12-19_lowres.png
http://www.threehillsobservatory.co.uk/astro/RW_Cep/rwcep_elodie_archive_THO_2022-12-19_Halpha.png
Is there any professional interest in these observations ? If so who should I be talking to for guidance on what specifically I should be focussing on (resolution, specific regions/lines)
Cheers
RobinRobin,
contact Prof Constantino Sigismondi at ICRA. His email is in the ATel I linked at the top
JeremyAnother look at this star this evening. I was comparing it to stars E (=7.3) and H (=7.8) on chart 312.02. I definitely think its brightness is between these two stars and a bit closer to E than to H. My estimate was E(1)V(2)H that is magnitude 7.5 (7×50 bins, 18:55 UT). Interestingly the AAVSO plot for this star is showing some quite varied estimates recently even between CCD observations so I am not quite sure what is going on here. What are other people seeing?
Duncan
It seems from this survey RW Cep is dimming by ~ the same amount in Ic and V
http://kws.cetus-net.org/~maehara/VSdata.py
which is significantly different to the Betelgeuse event.
(Thanks to “VY Canis Majoris” on Cloudy Nights for the heads up on this)
https://www.cloudynights.com/topic/854288-rw-cephei-great-dimming/?p=12378723Cheers
RobinThis star has now returned to its normal range of magnitudes. There is a paper that has appeared in the Astronomical Journal (https://iopscience.iop.org/article/10.3847/1538-3881/ace59d) where the authors have imaged the star during the great dimming using interferometry. They say that they saw an asymmetry in the stars photosphere which suggests a mass ejection of material. This would have cooled and formed a dust cloud that dimmed the star. This is similar to what happened to Betelgeuse in 2020.
The H alpha emission I mentioned in the VSSC article on RW Cep (and referenced in the paper) has also reduced in the past 3 months as the brightness increased so it looks like it is associated with the dimming.
Cheers
RobinAttachments:
The dimmed spectrum divided by the current spectrum (green)
Attachments:
An ApJL pre-print “Time-Evolution Images of the Hypergiant RW Cephei During the Re-brightening Phase Following the Great Dimming” appears on ArXiv today: https://arxiv.org/abs/2408.11906
This is a follow-up on the 2022 December Great Dimming episode. The authors present analysis of the re-brightening during 2023. They demonstrate that the west side of RW Cep, initially obscured during the dimming phase, reappeared during the subsequent re-brightening phase and the diameter became larger by 8%. They suggest that the dimming of RW Cep was a result from a recent surface mass ejection event, generating a dust cloud that partially obstructed the photosphere.
It is interesting to see that they present a light curve from 1900 to 2024, and other analyses, which to a great extent relies on amateur observations (from the AAVSO database, which encorporates data from the BAA VSS). This shows the value of long term amateur observations.
RW Cep can readily be monitored with binoculars as it varies between mag ~6.2-7.8 We have obs in the BAA VSS database going back to 1969 made by:
S W Albrighton, C M Allen, M Barrett, B J Beesley, M R Bell, P Bibbings, N M Bone, A Brown, P R Clayton, M Currie, H J Davies, D Dobbs, S J Evans, G Fleming, R B I Fraser, V J Freeman, A Gardner, M Glennon, A Good, T Gough, B H Granslo, D Hale-Sutton, M A Hapgood, W Harris, C Henshaw, T L Heywood, P W Hornby, R K Hunt, G M Hurst, J E Isles, B Jobson, S Johnston, S Koushiappas, M Long, T Lubek, B MacDonald, C Mann, T Markham, L McCalman, I A Middlemist, I Miller, P Mulligan, I P Nartowicz, C Newman, M J Nicholls, W Parkes, J Parkinson, R Pearce, D A Pickup, G Pointer, M Poxon, P Quadt, G Ramsay, D W Robinson, T G Saville, D R B Saw, A Smeaton, D M Swain, M D Taylor, G Thompson, J Toone, C Watkins, J Whinfrey, N White, G Winstanley, J D Wise, W J Worraker, E Yusuf
Since March 2024, RW Cep has started dimming again as shown in the VSS light curve
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