Near-UV observation

VV Cep 2017-2019 Campaign
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Peter Somogyi
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Joined: Sun Jul 13, 2014 8:56 am

Re: Near-UV observation

Post by Peter Somogyi »

Update: looking into the database H-alpha profiles change within 1 year, the Balmer lines jump (3889, 3970) seems to be just following normally the same increase. Seeing not much change in the UV then.
Ernst Pollmann
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Joined: Mon Sep 26, 2011 7:16 pm

Re: Near-UV observation

Post by Ernst Pollmann »

Hi Peter,
that is a very good and interesting result.
First, I marked the identified lines by comparison of the publications of Goedicke (1943) and Hack et al. (A&AS, 95, 1992).
The comparison spectra of Hack et al. shows the variability depending on eclipsing phase. I think particularly with the observation of the CaII lines, you should be able to observe the chromospheric eclipse. That starts about 1.5 years before mid-eclipse, or sometime around Jan 2017.
vvcep_near-UV.png
vvcep_near-UV.png (163.43 KiB) Viewed 4631 times
Remember what Phil Bennet wrote here in that context:

Spectral evidence of chromospheric eclipse should appear in the near UV or blue-violet (around 4000 A) about this time. I really don't know what the best optical lines to observe might be since I've never worked there (the space UV is much cleaner because the cool star doesn't contribute significant flux). In the space UV, spectral line absorption from the supergiant's outer atmosphere continued to show for another year or so outside this time (so you might actually see some absorption *now* from the upcoming 2017-2018 eclipse).

However, Ca II H (3933 A) is a good traditional diagnostic, but there are many low excitation lines of neutral species (particularly Fe I), and of singly-ionized elements (Ti II, V II, Sc II) from slightly more excited energy levels in the near UV (3400-4000 A). Fe II lines are generally seen in emission, but develop strong chromospheric absorption near eclipse. One strong Fe II lines that might be observable at longer wavelengths is Fe II 4233 A, but this will be difficult because the red star's spectrum dominates here and should be subtracted for best results (you can use the mid-eclipse spectrum for this, or in a pinch, alpha Ori or mu Cep).


Ernst Pollmann
Peter Somogyi
Posts: 420
Joined: Sun Jul 13, 2014 8:56 am

Re: Near-UV observation

Post by Peter Somogyi »

Ernst,

Thank you for the analysis, exciting to learn these!
Seems I've just missed Fe I 4067.977 seen on Paolo's spectrum, possibly it wouldn't fit together with both Ti II lines (3900, 3913). But, since I plan to shoot 2 wavelength regions with this grating in the long term, maybe I'll consider that line. Continuing soon!

Peter
Phil Bennett
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Joined: Mon Aug 24, 2015 7:40 am

Re: Near-UV observation

Post by Phil Bennett »

Hi Paolo -- your near-UV observation of VV Cep with mu Cep subtracted is very nice! Just about everything in the subtracted spectrum looks real to me.

The Balmer lines show the same inverse P-Cygni profile seen in our Hubble Space Telescope observations. The term "inverse P Cygni" refers to the characteristic line profile: a strong emission peak on the blue side of the line that quickly gives way to strong, deep absorption near line centre, with broad absorption continuing in the red wing of the line. It's called inverse P-Cygni because the profile are reversed from that of typical lines of the star P Cygni, which shows blue absorption and red emission profiles typical of formation in outflowing stellar winds. Therefore, inverse P Cygni profiles are indicative of a reverse flow: infall, or accretion.

In VV Cep, the emission is coming from the hot wind accretion region immediately around the hot star. Some of the broad absorption comes from just outside the accretion region, but the sharp, deep absorption near line centre comes from the extended atmosphere of the red supergiant. It is this component that will strength (greatly) as eclipse progresses. We are probably already seeing the initial stages of chromospheric ingress into the 2017-2018 eclipse.

I urge you to keep monitoring this region in exactly the way you have here. This will provide an *extremely* useful archival time series of the eclipse progress, especially if you can observe this every week or so. The subtraction of mu Cep spectra is really useful also.

Please keep these observations coming!

Phil
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