British Astronomical Association
Supporting amateur astronomers since 1890

Secondary menu

Main menu

Home Forums Variable Stars
Terms of use

VV Cep eclipse paper in JBAA

9 posts / 0 new
Last post
Robin Leadbeater's picture
Offline
Last seen: 8 hours 44 min ago
Joined: 05/03/2014 - 00:50
VV Cep eclipse paper in JBAA

Hugh Allen's paper on spectroscopy during the eclipse of VV Cep in the latest edition of the Journal is an impressive example of squeezing the maximum information out of a set of low resolution spectra. The period returning to its original value after an apparent increase last time round is puzzling and unless there is enough wriggle room in the uncertainties, could be tricky to explain astrophysically. Are there any theories while we wait for it to come round again ?

Cheers

Robin

Andy Wilson's picture
Offline
Last seen: 5 hours 39 sec ago
Joined: 29/03/2014 - 16:05
VV Cep eclipse paper in JBAA

Indeed an excellent paper by Hugh. Great to see spectra from multiple observers contributing to Hugh's research on VV Cep.

The shift in central eclipse timing without an apparent long term change in period is a puzzle. It would be interesting to know how accurate the various central timings were over the past hundred years. I wondered whether there was any possibility that different spectral features were used to estimate the central eclipse in early work compared to modern work of the 1990's and onwards, resulting in a systematic shift in observed in the central time.

Cheers,

Andy

Robin Leadbeater's picture
Offline
Last seen: 8 hours 44 min ago
Joined: 05/03/2014 - 00:50
mid eclipse estimates

Hi Andy,

The measurements pre 1997 were all based on the light curves which are indeed tricky to estimate the mid eclipse point from due to the continuing  brightness variability of the red giant throughout the eclipse seen in the AAVSO data in fig 5 of Hugh's paper.  The photometry and spectroscopy were in good agreement within 7 days in 1998 though. 

I have extracted  B-V  from the AAVSO data which is less influenced by brightness variations than using using V I think. I have marked Hugh's spectroscopic mid eclipse and  the expected values based both on the earlier ephemeris and assuming a 60 day increase in period post 1978.

Hugh's value agrees well with the B-V data. The prediction based on a permanent increase in period is clearly not compatible  with the current data but perhaps there is enough wriggle room to say that the original ephemeris is ok and the 1997-1998 eclipse data (both spectroscopic and photometric) was anomalous for some reason? 

Cheers

Robin

Attachments: 
Robin Leadbeater's picture
Offline
Last seen: 8 hours 44 min ago
Joined: 05/03/2014 - 00:50
change in period evidence

The argument for something to have happened though between 1978 and 1998 is convincing from fig 4 of the 1999 Graczyk et al paper if the error bars are to be believed

allenhugh10's picture
Offline
Last seen: 2 weeks 5 days ago
Joined: 02/08/2016 - 22:43
Mid-eclipse estimates

Hi Robin,

That's a very convincing B-V curve - so the photometric and spectroscopic eclipse mid-points are in good agreement, like they were also in the two studies I cited in my paper from the late 1997-98 eclipse. Could some variability in the red supergiant be a factor in the changes in the eclipse measurements? It's hard to understand and makes the next eclipse in 2038 an event to be anticipated!

Cheers

Hugh

Andy Wilson's picture
Offline
Last seen: 5 hours 39 sec ago
Joined: 29/03/2014 - 16:05
Duration of totality

Robin, that is an interesting analysis of the AAVSO data.

I've taken a brief look at a couple of the papers and there is mention of different durations of totality. For the 1997-98 Graczyk et al (1999) found totality lasted 450d in U-B and 477d in B-V. While for the 1976-78 Saito et al (1980) found totality lasted just 300d, and Saito note this is "shorter than those estimated during earlier eclipses".

I've not read them in detail, though I wonder if this is a clue to what is happening..

The duration Hugh found is closer to 300d than 450d, at 332d using H-beta. This appears similar to the AAVSO data analysed by Robin, where by eye it looks to be just over 300d.

Cheers,

Andy

Robin Leadbeater's picture
Offline
Last seen: 8 hours 44 min ago
Joined: 05/03/2014 - 00:50
Duration of totality

Yes I agree. If the continuous brightness variations are  due to radial pulsations of the M star, these would produce changes in the effective radius (and opacity?)  of the M star. These could could asymmetrically shift the mid eclipse point dependent on their timing. (is a 50% increase in effective radius due to pulsations feasible?)

Cheers

Robin

Robin Leadbeater's picture
Offline
Last seen: 8 hours 44 min ago
Joined: 05/03/2014 - 00:50
length of chord

" (is a 50% increase in effective radius due to pulsations feasible?)"

Though of course a smaller change in radius would be sufficient depending on the path of the hot star behind the M star 

Offline
Last seen: 4 days 2 hours ago
Joined: 22/11/2018 - 23:57
VV Cep paper

This is a fascinating thread - thanks for starting it, Robin!

Just thought I’d drop a reminder that if anyone would like to put their thoughts on record, letters to the JBAA are always welcome (and can be sent to me at pjennings@britastro.org).

Roll on 2038!

Philip