VV Cep evolution at low resolution

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VV Cep evolution at low resolution

Postby HughAllen » Fri Jul 07, 2017 10:37 pm

vvCep_Jul16-Jul17composite.png
vvCep_Jul16-Jul17composite.png (79.48 KiB) Viewed 4073 times
Broadly speaking the Hα emission seems to be fading since my first observations last summer 2016, even since this June. Could the processing of the spectra be a factor rather than a real change in the Hα intensity? I attach a composite of the 4 spectra I have obtained since July last year. They are vertically aligned on the Hα emission maximum. My set-up: 8" Meade LX90 at f6.3, Alpy 600 with 23µ slit, Atik 314L camera. For the June and July 2017 spectra I used nu Cep as the calibration star. I used different calibration stars for the 2016 spectra
Cheers
Hugh
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Re: VV Cep evolution at low resolution

Postby Ernst Pollmann » Sat Jul 08, 2017 8:41 am

Hugh,
please have a look to the recent IBVS publication of Ph. Bennet, J. Hopkins and me:
http://ibvs.konkoly.hu/pub/ibvs/6101/6198.pdf

Since the Hα emission originates from a different source (the B-type companion) than the optical continuum of the M supergiant, the EWs calculated relative to the M stars (variable) continuum were corrected for this variability in order to provide a reliable estimate of total Hα emission flux. This correction was done by scaling the previously calculated Hα EWs by a factor of 10^(−0.4∆V) , where ∆V = V − V and V is the mean magnitude of the out-of-eclipse V time series.

We compared in Fig. 3 the intrinsic Halpha line flux to Vmag from July 2004 to October 2016. This plot shows that the Halpha flux is more or less constant (also confirmed in Fig. 4) during this observed time period with a certain amount of stochstic variability (measurement inaccuracy ?).

That means, we can´t expect explicit such a systematicaly decrease of the Halpha emission as it is shown in your diagram. So, I assume that your observed emission decrease has been caused by another effects (reduction?).

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Re: VV Cep evolution at low resolution

Postby HughAllen » Sun Jul 09, 2017 9:39 am

Hi Ernst,
Thank you very much for the link to your paper, I very much enjoyed reading it. I estimated the FWHM and EW values from my 4 spectra:

FWHM, Å EW, Å
2017 05-Jul 11.7 -8.6
2017 03-Jun 11.2 -10.7
2016 22-Aug 11.5 -13.3
2016 03-Jul 11.2 -11.9

I would say the equivalent widths are not rigorously derived (I did not normalise the spectra) and of course unlike in your paper there is no correction for any variation in magnitude of the supergiant. But the general range of equivalent widths across the set of observations looks to be consistent with the range in your paper (figure 3) so maybe my mistake is to draw a trend of weakening Hα emission based on too few data points. I really appreciated your feedback, thanks for taking the time
Cheers
Hugh
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Re: VV Cep evolution at low resolution

Postby Ernst Pollmann » Sun Jul 09, 2017 4:49 pm

Hugh,
the enclosed plot shows you the meanwhile more supplemented time behavior from July 2016 to now. So, here you can immediately estimate the level of coincidence with your EW calculation of your non-normalized spectra.
But in the sense of a better comparability I would recommend to normalize your spectra.
vvcep_Halpha-EW.png
vvcep_Halpha-EW.png (11.96 KiB) Viewed 4041 times

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Re: VV Cep evolution at low resolution

Postby Thilo Bauer » Sat Jul 29, 2017 11:53 am

Hi Hugh,

Just a side note.

Carefully looking to the blue part of your spectra, the orange line is on top, while it is in the middle in the red part, for example. So I suggest there is tilt of the continuum slope.

My first impulse was thinking it should be a problem of color calibration due to different height above horizon and causing atmospheric extinction. This effect is known to tilt the slope of the continuum.

In astronomical photometry, the blue and red part of light, esp. the ratio of the different parts in color spectrum, will vary with height of the target above horizon. Atmospheric (color) extinction is amplified, if the calibration star and the target have been observed through different air mass. The effect is also observed to have seasonal influence (Gutiérrez-Moreno et al., 1982). The effect should also have a significant influence on observation of the eclipse. Therefore, a careful selection of the reference star and time of observation to calibrate the instrumental response is important.

Conclusion
Instead of using the H-alpha intensity for comparing spectra, try to normalize spectra based on continuum intensity to study possible reasons of the effect. If you want to study H-alpha intensity only, normalize spectra based on the base level of the continuum around H-alpha.

Literature
Gutiérrez-Moreno, A., G. Cortes, and H. Moreno. A study of atmospheric extinction at Cerro Tololo Inter-American observatory. Publications of the Astronomical Society of the Pacific 94.560 (1982): 722.
http://adsabs.harvard.edu/abs/1982PASP...94..722G

Good luck,

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Re: VV Cep evolution at low resolution

Postby Robin Leadbeater » Sat Jul 29, 2017 4:00 pm

Hi Thilo,

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.

Hugh,

Since relative flux units are arbitrary and have no physical meaning, It is good practise to always scale plots in relative flux to 1 relative to some specified point in the continuum, typically 5500A is used for example. (confusingly this is often called normalising but strictly it is just rescaling. Strictly normalising (rectification) is where the continuum is divided out so the continuum is 1 at all wavelengths) With a cool stars it can be difficult to pick a suitable continuum region. I would suggest in this case normalising relative to the average value from 6050 to 6150A say for example and comparing the spectra again. (ISIS has a function to do this, also it is easy to replot and compare spectra even if they are not scaled the same using Tim Lester's PlotSpectra program)

Cheers
Robin
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Re: VV Cep evolution at low resolution

Postby Robin Leadbeater » Sat Jul 29, 2017 4:10 pm

Hugh,

Your EW measurements should be valid independent of any flux scaling factor as the calculation of EW is done relative to the local continuum. The result can be very sensitive to how this continuum is determined (either manually or by the software)however and is particularly difficult at low resolution in cool stars like this where the region of interest contains so much structure and blending of lines. I would say this is a case where the uncertainty in your EW measurement is likely to be large compared with measurements made at higher resolution.

Robin
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Re: VV Cep evolution at low resolution

Postby Robin Leadbeater » Sat Jul 29, 2017 5:01 pm

Where low resolution spectra could be useful however is to test Ernst's assumption in his paper that changes in the flux in the V band can be used to correct EW for changes in the continuum flux at H alpha. This is not clear from the paper currently, since if the variations in brightness are pulsation (and therefore temperature) driven, one would expect the flux changes to be less in the continuum around H alpha compared with in the V band.

Robin
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Re: VV Cep evolution at low resolution

Postby Ernst Pollmann » Sat Jul 29, 2017 7:06 pm

Dear colleagues,
EW is a useful measure for spectra in which lines are formed in the same source and similar region as the continuum, but here they are not. The H-alpha emission is formed around the hot star in a region completely separate from the M star -- they simply appear superimposed because the sources are spatially unresolved. What happens is that the H-alpha emission sits on top of the M star continuum, but these features are from unrelated sources.

When the EW is computed, the H-alpha flux is divided by the M star's continuum flux and that varies. So, even if the H-alpha emission flux is approximately constant (and it is), you see an apparent anti-correlation because when the M star is faint, the H-alpha flux is divided by a smaller number (resulting in a larger EW) than when the M star is bright (with smaller EW).

There is NO physical correlation of V and H-alpha emission (coefficient of "correlation" R = 0.18 in Fig. 4 of the IBVS paper). There still is some apparently random variation in H-alpha emission on short time scales, but it going to take more work to get a result out of this.

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Re: VV Cep evolution at low resolution

Postby Peter Somogyi » Sat Jul 29, 2017 9:01 pm

Dear Ernst and Robin,

"...correlation R = 0.18..."

In case professional need emerges to produce absolute H-alpha fluxes, I already have a photometric slit for the LHires - but need mentoring (with detailed howto with VV Cep as example) and 2-3 more observers for confirming the results and make the picture complete. Otherwise I skip this human intensive effort.

- Peter
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