AAVSO Alert Notice 694 announces a spectroscopy observing campaign on the very bright Be star gamma Cas. Please see the notice for details and observing instructions.
Many thanks, and Good observing,
Elizabeth O. Waagen, AAVSO HQ
In a note I added to Alert Notice 694 on gamma Cas on May 21, I said that Principal Investigator Ernst Pollmann was particularly requesting higher-resolution spectra of gam Cas. However I said the requested resolution was 1000 or higher, when I should have said 10000 or higher. My apologies for this error.
Campaign Monitoring on gamma Cas
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.
Yes I know that. I was just reminding observers to also add their spectra to BeSS to ensure that all amateur Be star spectra can continue to be found in one place rather than them being split between various databases. (IMO the fragmentation that is already happening between the AAVSO and BAA spectroscopic databases for example is unfortunate and as the "new kids on the block" is something AAVSO should have considered and addressed)
AAVSO Alert Notice 707 contains an update of Ernst Pollmann's spectroscopy campaign on gamma Cas 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 MOVES TO THE THREADS FOR ALERT NOTICE 707 - please post your comments and questions there. (Please do not use this link from from Alert Notice 694 any longer.)
Many thanks, and Good observing,
Elizabeth O. Waagen, AAVSO HQ
fortunately the announcement of the HeI6678 campaign of gamma Cas did lead to the delivery of several useful spectra of different observers.
The attached overview shows beside the profile variability clear recognicable V/R variation from V/R > 1 to V/R <1.
But in order to detect short-term local ejections (or whatever they might be), it will be necessary to take spectra for several hours (see spectra 2020-04-05/08/16) when ever possible.
Thank you to all the observers who contributed with their spectra to this first encouraged results.
beside the monitoring of the mysterious short-term flares, also the V/R behavior of the HeI6678 emission components has to be take into account. Even if there are some observation results received in the past, the quasi-periodic character, which have been found, seems to be still very unsure. See:
Attached you can see a period analysis based on the old and very new (campaign) data performed with the PDM method. As you can recognize, the data set is still rather small for a reliable analysis, but there are signs of a 420 days period.
This result might mean, that there exist a small rotating Helium ring around the central (primary) star with different distribution of matter inside (one-armed density distribution). Thank you so much for your very worthwhile spectra so far, but it would be nice, if you could observe the star further !!
Dear Ernst and observers,
For an experiment (and reproduceability/flat analyis), I have made full night series of 3 nights.
I most likely haven't detected any flare, but seeing absorption velocity changes (emission peaks precisely in the middle), repeating in a given range during days - and similar change in 1 hour on 2020-JUL-04. I admit had to change my flat taking method fixing severe problems at this high SNR (and pixel shifts really causing continuum changes at low gradients), but this kind of change of the absorptions is obvious (R~17000, per-exposure calibration) and symmetric.
Spectra uploaded to avspec (5-6 spectra grouped and summed, 350-450 sec of exposures depending on my saturation estimate).
I am wondering whether if this kind of variablility of the absorptions is known, and need confirmation whether such a full night serie is meaningful and whether worth continue. Shooting the same target during a night suits me well, because being able to sleep (whilst shooting other targets is not an option for me).
a first overview of all spectra suggested me, to build three groups of observations (see attachment).
As you can see, after your 2020-07-05/06 (2459036) observation a short-term flare did appear, which had been recorded by Kevin Gurney (2459037.45).
A second flare have been recorded after your 2020-07-07/08 (2459038) observation, again by K. Gurney at 2020-07-10.
It seems that this kind of profile variations are not easy to observe because of its coincidence character. Inspite of that, your way to record spectra for several hours seems to be the only right way to capture such a kind of events.
Best wishes, Ernst
I would like to inform about the current state of the campaign.
First I want to emphasize that the campaign is accompanied now by the professional researcher Jonathan Labadie-Bartz from the University Sao-Paulo (Brasil). He is very interested in collaboration and in our EW and V/R data.
His first effort has been to find a plausible way to subtract the photospheric rotational broadened absorption profil of the central star, which is laying under the double-peak emission.
The design of a synthetic spectrum, on the base of the well known parameter of the central star, seems to be a reasonable and practical solution.
Our next step will be, spectra of ongoing further observations to correct in that way. The links below are showing the current state of the EW and V/R monitoring.
Thanks a lot to all the campaign member for contributing so far with their spectra.
Interesting campaign results
The straight line slop (see bottom link) of the Hα-EW (top) shows an increase of approx. 1.3 Å/year for the period presented here as a result of the exo-photospheric material input into the Be star disk.In addition, both straight lines show a clearly synchronous-linear time course, with the special feature that the Hα-EW minimum (November 2001) can also be found well in the course of the HeI6678-EW straight line. The obvious correlation between the EW's of Hα and HeI6678, generated by mass ejection from the surface of the primary star into the circumstellar disk, however, is subject to rapid fluctuations in HeI6678, which are not directly recognizable in Hα. This is due to the fact that such a mass input is relatively small compared to the already existing massive disk and therefore may not be immediately visible. The investigations will be continued.
our professional consulter Dr. Myron Smith wrote recently:
Based on your work on correlation of Halpha and HeI6678 EW´s (see the link below) , it is clear that gCas is in a state now of rapidly building up its disk.
This is a condition for which a proposal a few of us wrote recently for XMM (x-ray satellite) time could fulfill.
The star becomes visible to the satellite once again on January 6th.
Do you think a short time before this we could get a very up to date description of what the star is doing (Christmas-New Years Day timeframe)? We would really appreciate it.
Gamma Cas (often categorized as uninteresting and well known star) shows at present a high interesting, not only for professionals, but for amateurs too an unique developement, that could end (very soon?) in a super nova event (or so).