VV Cep evolution at low resolution

VV Cep 2017-2019 Campaign
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Re: VV Cep evolution at low resolution

Postby Thilo Bauer » Thu Aug 10, 2017 8:24 pm

Hi Ernst,

Before we start discussion about what might be the case taken out of observations, I would like to step back and ask, how to observe. Everything counts.

My personal opinion and feeling is, that we may learn a lot, especially with this occultation. My concerns are about how to find a good calibration. This is not meant in a negative way, because we all may learn during the continuous observation cycle of the occultation which currently takes place. And the combination of all measures is important to gain a complete picture.

I tried first observations during OHP 2015 and also OHP 2017, where I tried to obtain a first spectrum with a high S/N taking a one hour exposure (co-added).

So far I need "more input" to compare the different observations over years and find hints to quantitatively explore the results. It may happen we do mistakes during observations. This is sometimes the case, if I'm getting tired during the night, finding defocussed stars doubled in my spectra. This was the case last weekend, when I tried to get a new spectrum at the beginning of the occultation, with the risk of my camera destroyed by rain. I hope not to find this every day.

That's why I wanted to continue with discussion especially focussed on how to calibrate my observations. This is not very easy. On one side there are not many well-known calibration standards. During my work at the Hoher List observatory (in the early 1990ies), we had discussions of how to use calibration stars of the Landolt sequence from CCD observations. I found this not very convenient, but painful in the final end. Air masses may not be very well corrected and even the distances of certain nearby galaxies observed have been measured faulty by factors of ten and needed to be corrected over time in literature. One particular reason is, these stars are distributed across equatorial longitude, but not equally distributed over latitude to correct for air mass for different observatories. The few known flux standards are also not very well distributed over the Northern hemisphere. A few of them, proposed by ESO later, are white dwarfs. More or less constant in flux, but less than the number of fingers of a hand, not equally distributed. Then, I was asking myself how to calibrate colors (or spectral bands) found in the open stellar cluster database. A few star clusters well measured, many of them providing large uncertainties in magnitudes and colors, however. Later I read a paper about the magnitude of alpha Lyr, which proposed a correction to 0.03 mag instead of the zero-point. I mean, what is a zero-point worth, if it shifts? Finally, I assume, nobody of us has ever taken a picture of an open stellar cluster to calibrate the local influence of atmosphere from air mass 1 to horizon. I did such photometric observations with my DSLR once with one single cluster, but hesitate to use it a a reference for my spectra, because the observation has been taken years ago.

On the other hand we observe a suspected faint blue star being companion of a red super-giant. Therfore, I would assume, we need a calibration especially in the blue. May Steve Shore do some comments on my findings, as I'm trying to learn every day...

So my conclusion is, there is not much to consider as "comparable" reference. I found alpha Lyra to be one of the most prominent stars with large S/N and known flux. This could be especially worthy with observations in the blue spectral range to study the companion.

Any more considerations or hints how to observe and calibrate?

Best regards,

Thilo
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Re: VV Cep evolution at low resolution

Postby Thilo Bauer » Fri Aug 11, 2017 3:21 pm

Hi Ernst,

Ernst Pollmann wrote:What, if we would assume that the B star does have its own disk, beside the accretion disk? We would have in that case a superposition of two objects with emission activity. An accretion emission activity (fed by mass flow/wind from the M star) and additionally an emission activity coming from the (fictive) B star disk. With this scenario and with the individual dynamic of this second disk we could explain:

1) the long-term cycle as one-armed density enhancement in the B star disk,
2) the flux variability with shorter periods as it is described in our IBVS paper,
3) we would have an explanation for the independence of the Halpha flux variation from Vmag.

What´s your opinion?

Ernst


This is a good idea.
Should we better refer to "low dispersion", not "low resolution"?

While cross-reading a paper, about a WR star binary system (where Paul Luckas participated) I had the idea to think about what lines in our low resolution spectra are assigned to which component of the system and worth being discussed separately. In the paper I read the authors took several lines out of the spectrum each of which is typical for the isolated component discussed. So the RVs could be plotted individually for each of the binary components of a WR system.

Let's do some brain storming what is in our low dispersion spectra from 370 nm to 800 nm.

H-alpha and other emission lines correspond to the "disk" and mass flow.
While another post might be quicker than me, I will deep dive into the spectra of the super giant and B star so see what might be separated from each other.

Another question was discussed by Robin: What about the blue end? Is this critical and sensitive to calibration? In how far?

Thilo
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Re: VV Cep evolution at low resolution

Postby Ernst Pollmann » Fri Aug 11, 2017 4:18 pm

Hi Thilo,
I sent my considerations (from Thu Aug 10, 2017 7:21 pm) also to Phil Bennet. But I assume he is in vacation at present.

Ernst
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Re: VV Cep evolution at low resolution

Postby HughAllen » Sat Sep 02, 2017 4:23 pm

vvcep_27thAug2017_Hbetazoom.png
vvcep_27thAug2017_Hbetazoom.png (101.89 KiB) Viewed 213 times
I made my last observation on 27th August and chose to look at the evolution since last year of the Hβ emission in the attached graphic. I have scaled the curves to normalise on a region around 4890Å. The eclipse seems to be well underway....
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Re: VV Cep evolution at low resolution

Postby HughAllen » Sat Sep 09, 2017 9:54 am

vvcep_080917v270817.png
vvcep_080917v270817.png (83.24 KiB) Viewed 165 times
For the first time in my own series of observations, last night's VV Cep spectrum showed no further decline in the Balmer emission intensity. Maybe even a slight brightening. This is very much in agreement with what Jim Ferreira observed during a series of Alpy observations in August, and with Ernst Pollmann's fascinating high resolution spectra. I guess only a long time series is going to untangle the eclipse from other factors specific to the Be star. Or could there be variations in the opacity of the atmosphere of the red supergiant?
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Re: VV Cep evolution at low resolution

Postby Ernst Pollmann » Sat Sep 09, 2017 12:44 pm

Hi Hugh,
Phil Bennett wrote here:
Since we are now in atmospheric eclipse, any variation in the M star size due to pulsation would be expected to modulate the transmitted H-alpha flux (from the vicinity of the B star) correspondingly.

The attached plot gives an impression of the long-term process of the eclipse. The duration from the first to the second contact took approx. 165 days (~ 5.5 month) in 1997. That means a decrease of ~ 2 Å/month for Hα.
As you can see in that plot, this time the definition of the first contact is very difficult. If we would estimate this at ~ 2457900 (end of May), then the second contact would be to expect ~ at the end of October.
vvcep.png

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Re: VV Cep evolution at low resolution

Postby HughAllen » Sun Sep 17, 2017 10:14 am

VVCep_Halphazoom_270817-140917.png
VVCep_Halphazoom_270817-140917.png (42.79 KiB) Viewed 81 times
Thanks Ernst for your interesting feedback. I captured another spectrum on Thursday night with again little apparent change in the Hα intensity
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