VV Cep evolution at low resolution

VV Cep 2017-2019 Campaign
Moderator for this forum: Ernst Pollmann

Re: VV Cep evolution at low resolution

Postby Robin Leadbeater » Sun Jul 30, 2017 12:51 am

Hi Ernst,

Ernst Pollmann wrote:even if the H-alpha emission flux is approximately constant (and it is),


We do not know for certain from your paper that the H alpha line flux is in fact approximately constant. Your conclusion that it is constant depends on the changes in continuum flux near the H alpha wavelength being the same as you saw in the V passband. Your paper does not consider the possibility that this might not be the case. (ie that the continuum shape may change as the V brightness changes, as would be the case for a pulsating star) This is an obvious weakness in the argument which is not considered in your paper but could be verified for example using low resolution spectroscopy or simultaneous V and R photometry.

Note, I am not saying the conclusion in the paper that the H alpha line flux is constant is neccessarily incorrect. It is just that the key fact or at least plausible argument supporting the inference that the V mag and continuum flux near H alpha are correlated is absent from the paper.

Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
 
Posts: 1029
Joined: Mon Sep 26, 2011 4:41 pm

Re: VV Cep evolution at low resolution

Postby Ernst Pollmann » Sun Jul 30, 2017 8:01 am

Hi Robin & Peter,
you can be sure, if we had had a chance in the past, to get reliable R-band data contemporaneous to the EW data, we would have extended our investigation in order to find out something more of the real physical causes of the EW variability. That´s why we are saying in the conclusion: the source of the Halpha variability, originating from an accretion region around the companion, remains unclear.

But with the instrumental possibilities nowadays and in the future, this should be much more easier for people who are interested about that. It is easy to say that there are certain weaknesses related to that point in our investigations, but we are really aware about his. The current interest in VV Cep would be a wonderful opportunity, to begin with a new campaign for the next decades in order to contribute to resolute this mystic secret.

Ernst
Ernst Pollmann
 
Posts: 294
Joined: Mon Sep 26, 2011 7:16 pm

Re: VV Cep evolution at low resolution

Postby HughAllen » Sun Jul 30, 2017 10:12 am

Thanks everyone for some fascinating observations, I have found the conversation very useful.

I am using nu Cep as my calibration star so I think I have the influence of atmospheric extinction under control; nu Cep lies quite close to VV Cep in altitude, and its spectrum is in the Miles database.

Regarding scaling of spectra so that they can most easily be compared I will have a go at Robin's suggestion of presenting the spectra so that they are anchored on the relatively featureless part of the continuum around 6100Å. I am using BASS to process my spectra and I'm still learning about the scaling features - I'll keep you posted how I get on.

I can now better understand some of the uncertainties around measurement of Equivalent Width in my spectra. I'll try to develop a consistent methodology for rectifying the spectra in a narrow region around the H-alpha emission at least to minimise variation due to the way in which I reduce the spectra.

Thanks a lot

Hugh
Hugh Allen
Alpy 600 user
HughAllen
 
Posts: 33
Joined: Mon Mar 23, 2015 7:17 pm
Location: The Mendip Hills, 15 miles south of Bath, UK

Re: VV Cep evolution at low resolution

Postby HughAllen » Wed Aug 02, 2017 7:19 pm

Following advice here I have scaled my spectra so that the average intensity between 6020-6040Å is set to 1 in each case. There is certainly some variation in my response correction, something I'll have to look at. I also developed a reasonably consistent rectification process for estimating the equivalent width and FWHM of the Hα emission. I wouldn't pretend the EW measurements are particularly accurate but there does seem to have been a relative decline since last year. Anyway, monitoring VV Cep is great fun even at low resolution :)
Cheers
Hugh

VV Cep Jul16-Jul17.png
VV Cep Jul16-Jul17.png (81.23 KiB) Viewed 1544 times
Last edited by HughAllen on Thu Aug 03, 2017 6:43 am, edited 1 time in total.
Hugh Allen
Alpy 600 user
HughAllen
 
Posts: 33
Joined: Mon Mar 23, 2015 7:17 pm
Location: The Mendip Hills, 15 miles south of Bath, UK

Re: VV Cep evolution at low resolution

Postby Robin Leadbeater » Thu Aug 03, 2017 12:24 am

Hi Hugh,

That's pretty good from ~4100-6800. Dont forget also that as the brightness changes due to pulsation of the cool star, the continuum shape will change naturally in any case to some degree due to the change in temperature of the cool star (and the shape change will be even more during eclipse as the hot star component disappears) The instrument response at far ends where the sensitivity drops off are always the most difficult to get right, particularly at the blue end where the response is dropping off fastest and atmospheric extinction and chromatism in the atmosphere and optics are most pronounced. The key thing is to keep everything as identical as possible between measuring the reference and target spectra, including focus and positioning of the star on the slit. Also when you generate the instrument response using the reference star, first check that it works perfectly on the reference star, which it should of course. If there is any discrepancy between your corrected measured spectrum and the library spectrum, you can go back and try to improve your instrument response calculation.

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
 
Posts: 1029
Joined: Mon Sep 26, 2011 4:41 pm

Re: VV Cep evolution at low resolution

Postby Ernst Pollmann » Thu Aug 03, 2017 7:55 am

Hi Hugh,
please be careful with the rating of the EW in your spectra. The measured EW in VV Cep depends mainly on two things:

1) of the continuum brightness Vmag under the emission. If Vmag is growing, the EW decreases and vice versa. This is demonstrated in the attached Fig.
The reason is the process of normalization, the division of the non-normalized spectrum through the (varying) pseudo-continuum.

2) on a intrinsic flux variability within certain limitations. The causes of that variability is still unknown so far (discontinuity in mass flow from the M star to the accretion disk ?).

This means for example: as long as Vmag had been unknown during the exposure of your spectra, as long one can not say the EW has changed.
vvcep_EW versus Vmag.png
vvcep_EW versus Vmag.png (17.65 KiB) Viewed 1529 times

Ernst
Ernst Pollmann
 
Posts: 294
Joined: Mon Sep 26, 2011 7:16 pm

Re: VV Cep evolution at low resolution

Postby Robin Leadbeater » Thu Aug 03, 2017 7:26 pm

Hi Ernst,

The problems is, EW is not a useful measurement for emission lines as the flux in emission lines is generally independent of the continuum, as you pointed out. To measure the strength of emission lines and how they are changing, you need to calibrate the spectrum in absolute flux. This can be done by absolute calibrating low resolution spectra, for example using V mag values, to give a true value of flux at any wavelength without the need to assume a correlation between continuum flux in the V mag band and in the continuum at the emission line being measured (which is approximate at best and may not be true at all). Here is a very good recent example on the group of how this is done.

viewtopic.php?f=5&t=1784&start=70#p8931

Note particularly near the end of the animation the way the continuum at H alpha continues to rise while the continuum at 5000A remains constant. This is an example of why you should not try to use V mag to predict H alpha continuum flux.

Who is doing photometry during this campaign Ernst? Do you have V mag values for dates corresponding to Hugh's spectra which we can use to calibrate the spectra in absolute flux?

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
 
Posts: 1029
Joined: Mon Sep 26, 2011 4:41 pm

Re: VV Cep evolution at low resolution

Postby Ernst Pollmann » Fri Aug 04, 2017 1:42 pm

Hi Robin,
the diagram Halpha EW v. Vmag should demonstrate the quantitative influence of the continuum brightness under the measured emission. Of course it was not the intention, to use this correlation for prediction of the Halpha continuum flux.
In case of VV Cep one have to consider that EW & Vmag must be assigned different physically sources. Vmag represents the physical happenings of and in the M star (among others pulsation), whereas the Halpha emission reflects the recombination of ionized hydrogen gas of the accretion disk.
However, both locations of origin are spatial not resolved of the observer. With other words: Halpha EW & Vmag are physically not correlated to each other. This is clearly to see when Halpha EW is corrected by Vmag (following Fig.).
Image1.png

This is particularly true in the present orbital phase shortly after the apastron, in which the M star and the B star are about 10-15 AU apart.
Therefore it would be wrong, to determine the absolute continuum flux (with Vmag as parameter) under the emission, in order to find out something of the intrinsic Halpha EW variability.
On the other hand, if one wants to see the eclipse of the B star through the M star in the continuum, then it make sense to calibrate the spectra in absolute flux. I have gathered the photometric Vmag data of the BAV & AAVSO. If you are interested I can make them available.
Image2.png

Ernst
Ernst Pollmann
 
Posts: 294
Joined: Mon Sep 26, 2011 7:16 pm

Re: VV Cep evolution at low resolution

Postby Thilo Bauer » Thu Aug 10, 2017 6:14 pm

Hi Hugh,

not to be misunderstood: I really want to encourage you, to have a closer look onto your data and existing calibration! Probably, you should present an amplified plot of what is going on with your spectra in details in the blue part of your spectra. Just to gain impression what might be worth for all of us to observe and think about the difficulties and pitfalls. I think it will be worth for us all to learn during this critical path of the ongoing occultation to adjust our ongoing measurements.


Hi Robin,

Robin Leadbeater wrote:I am not yet convinced from these plots that there is necessarily an extinction (or other instrument response) problem here as the effect should be seen more severely at the blue end with little difference at the red end, though I agree it is difficult to see from these differently scaled plots.


Yes, I agree. That's why I said it was my "first impulse". At least, the observations, Hugh presented to all of us, are not too long distance over months and I assume observations were taken at similar air mass (I saw, the observations have been taken not too far away from each other in terms of season). So it could be real. For clarification of these effects, we should consider to carefully calibrate flux. All we know, the binary companion shall be present in the blue end. Therefore, low dispersion profiles, S/N in the blue, a very well known (I don't said good, but well-known) flux reference in the blue might be important.

I'm trying to do this in a way to try to observe alpha Lyr and VV Cep at similar air masses. This is not possible in the final end, but alpha Lyr is one of our best known flux standards. The idea behind this is to take spectra of alpha Lyr, then wait and observe VV Cep later. In the meanwhile taking spectra of "something". Just to get an impression, of what my feeling is.

Just my few cents and not really knowing what will be the output of my spectra of VV Cep.

And, Ernst, I'm speaking about low dispersion behaviour in the blue end, not taking into account H-alpha. Both effects could be very anti-correlated, while the star, accretion disk (if any) and other facts must not be correlated in any way. But, this is also fascinating. This will tell more about the nature of the binary, accretion disk, transfer of material. More, than just observe H-alpha alone.

Thilo
Thilo Bauer
 
Posts: 37
Joined: Sun May 31, 2015 4:45 pm

Re: VV Cep evolution at low resolution

Postby Ernst Pollmann » Thu Aug 10, 2017 7:21 pm

Dear Thilo,
I love it to discuss (speculate), particularly about the content of your last paragraph. Let me tell some thoughts to that:
The growing data from observers of the Aras group enables the continously monitoring of the Halpha EW time behavior. This is shown in the following diagram.
vvcep_EW.png

Obviously is the dominant long-term variation with a cycle of approx. 4000 days and two maxima of emission activity. Also the Vmag corrected Halpha flux (red triangles) follow this behavior. In that diagram I have marked the time of the last peri-astron and last ap-astron.

Striking here is a higher emission activity clearly far before the peri-astron. If both stars approx. 10-15 AU are away from each other, then a higher emission activity (caused by mass flow/wind of the M star) particularly around the ap-astron seems to be unlogical from my point of view.

On the other hand we have demonstrated in our IBVS paper No. 6156 that the corrected Halpha flux seems to be more or less constant in time. This leads me to the consideration of the causes (at least) of the emission activity around the ap-astron.

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
Ernst Pollmann
 
Posts: 294
Joined: Mon Sep 26, 2011 7:16 pm

PreviousNext

Return to VV Cep

Who is online

Users browsing this forum: No registered users and 1 guest

x