HeI 6678 V/R periodicity in gamma Cas

American Association of Variable Star Observers (AAVSO)
Wed, 01/29/2020 - 17:45

Dear colleagues!
I am presently reactivating the investigation of HeI 6678 V/R periodicity in gamma Cas.

Stee et al. (1998) proposed that the He excitation and ionization region, responsible for the emission in the HeI 6678 line, extends to 2.3 stellar radii. Thus, the HeI 6678 line has an important diagnostic value of activity close to the stellar surface.

The time-dependent mass loss from the primary component of the gamma Cas binary system assumes that both photospheric and disk density variations lead to the double peak profile variations of HeI 6678.

The attached overview shows the V/R periodicity of the Hel 6678 double-peak emission for the period August 2009 to September 2019. The period analysis of the V/R data of that time section cannot confirm the period, published at IBVS 6103 (2014), probably because of the few data available until 2014.

The 448 day period found today, is based on a much longer monitoring time and a three times larger data set, from the data bases BAA, BeSS and my own spectra. It would be great if interested observers would be willing to take part in this area of Be star research. The spectral resolution should be better than 10000. Please wright me an email in case of any questions.

Ernst Pollmann

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American Association of Variable Star Observers (AAVSO)

Dear colleagues,
here some additional explanations concerning the meaning and purpose of gam Cas He6678 observations:

Monitoring the HeI6678 Profile
Monitoring of HeI6678 reveals the build up of short term emission events 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, more importantly, in many small but frequent events?
Usually HeI lines like 6678A 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. In other words, Hα is generally formed in a symmetric ring around the star, whereas overall HeI emission maybe a little less symmetric, reflecting localized differences in circumstellar temperature, and especially density.

The V (violet) and R (red) emissions are emitted alongside, or ’to the side’, of the Be star, where the Doppler orbital motions cause them to be displaced away from the line center. In many cases involving 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's photospheric contribution behind it.

Since the HeI 6678 emission is formed over a smaller volume, any departures from isotropy (i.e. localized concentrations) are much more likely to occur than for Hα. Therefore, local ejections (or whatever they might be) are more likely to show up, and also occur over rapid timescales. The upper Fig. shows the average profile of HeI6678 profile, taken 1993 Nov. 6, by Prof. Dr. Myron Smith (spectral resolution 45000, S/N ~ 300). The lower Fig. shows the HeI6678 profile taken by me on 2020 February 7. (spectral resolution 17000, S/N ~ 1000). Despite the lower spectral resolution, amateur spectra reveals obviously the same profile characteristic than the professional spectrum. 

Ernst Pollmann