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Campaign Monitoring of the Helium double peak emission at 6678 Å

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Campaign Monitoring of the Helium double peak emission at 6678 Å

Campaign Monitoring of the Helium double peak emission at 6678 Å

I am Ernst Pollmann and my investigation of the variability of the HeI6678 emission of the star is continuing, and I am asking for your assistance in obtaining spectra.

The HeI 6678 Å line in gamma Cas exhibits double-peaked emissions with the well-known V/R variations (changes in the relative strength of the shorter wavelength, “violet”, to the longer wavelength, “red”, peak) on the timescale of several years as well as within a few hours.

Usually HeI lines like 6678 Å are formed in hot circumstellar regions close to the star, and therefore often show more localized activity and their V & R emissions can be unequal.

The V and R emissions are emitted along lines of sight ’to the side’ of the Be star, where the doppler orbital motions cause them to be displaced away from line center. In many cases, like gas the disk extends ‘upward’ (and ‘downward’) from the equator, so that part of the HeI gas is in the foreground in front of the star. This region is cooler than the star and therefore absorbs some of the disk photospheric contribution behind it.

That means an observer is measuring a collection of straight ‘lines of sight’, each of which goes through a point on the CS disk to an end point on the stellar surface. An observer looking at the flux of any line of sight will see the absorption due to the stellar disk from the background point on the star’s surface. But the line of sight is infinitesimally narrow (by definition) so the stellar line profile at that point will be un-broadened, not broadened.

The HeI 6678 double-peaked emission is furthermore filled by a large part of the rotationally broadened photospheric absorption core. The whole line is very weak and can only be measured reliably on the spectra with high S/N. The emission peaks rise only a few percent above the continuum level.

The HeI 6678 emission reflects localized difference in CS temperature and especially density. It is hard to say whether these variations represent only real long-term changes or whether they are partly caused by line blending.

Since HeI 6678 is formed over a smaller volume, any departures from isotropy (i.e. localized concentrations) are much more likely to occur than for Halpha. Therefore, local ejections (or whatever they might be) are more likely to show up, and for sure over rapid timescales.

The goal of the HeI6678 campaign monitoring is to build up short term emission events (transit flares) and their durations. Emission events, especially small ones that have gone largely unnoticed, address the issue of how Be stars lose their mass and build their disks. 

The question is, does it only happen infrequently in large events (which do occur) or importantly in many small  but frequent events. The HeI 6678 emission wings are so well defined that it would be good to measure their RVs. They could describe the motion of the secondary.

Observers, it is for this reason that weekly spectra (especially taken at resolution > 10000) of gamma Cas are so important. If you have the capability of observing at this higher resolution, please do so because it will be extremely helpful. If you cannot observe at higher resolution, please observe at resolution > 1000.

The attached spectra sequence shows a short overview of spectra from February to April 2020, taken with the spectrograph LHIRES III (R ~ 17000) and different telescopes.

Ernst Pollmann

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Campaign Monitoring of the Helium double peak emission at 6678

AAVSO Alert Notice 707 contains an update of Ernst Pollmann's spectroscopy campaign on gamma Cas (originally announced in Alert Notice 694) and gives the science behind his request and also shows some spectra of the line he is studying. Please see the notice for details and observing instructions.

DISCUSSION OF THIS CAMPAIGN IN THIS FORUM MOVES TO THIS THREAD - please post your comments and questions below.            (Do not use the old threads from Alert Notice 694.)

Many thanks, and Good observing,

Elizabeth O. Waagen, AAVSO HQ


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