28 Tau ARAS monitoring

Re: 28 Tau ARAS monitoring

Postby Ernst Pollmann » Wed Jan 27, 2021 4:51 pm

Dear Jon,
thanks for information of White´s photometric 2017 short-term observations (probably caused by pulsations).
Indeed it is great that TESS observations are planed in fall this year while the periastron passage at September 7th, whereby I hope that all the colleagues in the past will be again contribute with spectra to this campaign.

The accordance of your TESS Halpha observations 2018 - 2020 with our monitoring shown here (above) don´t surprises me. This typical behavior of the Halpha parameter EW, CA, V/R and RV have been observable during each periastron passage we monitored in the past
(see: IBVS Journal Vol.62, No.6239:
https://konkoly.hu/pub/ibvs/6201/6239.pdf
and been also confirmed by J. Katahira.

The exact coincidence of the CA maxima with the minima of V/R and RV and EW as a result of disk precession has never before been observed during the maximum shell phase in the years around 1980, or during the initial shell phase around August/October 1974. It is known that the precession of the disk depends on its size (radius) and its mass due to gravitational effects (Katz et al. 1982, Larwood et al. 1996, Lubow & Ogilvie 2001).

Independend of the disk precession, the individual changes of all this parameters results also from the truncated disk size during the periastron passages. In addition I think, particularly the rapid increase and decrease of the EW, is essential determined by the inclination angle in the observers view and hence of the central dip in Halpha during a periastron passage.

Ernst
Ernst Pollmann
 
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Re: 28 Tau ARAS monitoring

Postby Ernst Pollmann » Mon Feb 01, 2021 3:16 pm

Dear colleagues,
today the periastron passage took place.
The Halpha central abbsorption (CA) seems to have achieved a maximum value, but further observations could show more about the behavior during the next days.
CA-timeseries.png
CA-timeseries.png (90.48 KiB) Viewed 1514 times

Please (if it possible) observe the star to get more data information.
For observers who wants to know more about shell lines in Be stars, see:
"Shell lines in disks around Be stars"; Hanuschik, A&A 295, 423-434, 1995.
Ernst Pollmann
Ernst Pollmann
 
Posts: 415
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Re: 28 Tau ARAS monitoring

Postby Ernst Pollmann » Sun Feb 21, 2021 11:04 am

Dear colleagues!
As is known, the variable angles of inclination of Be star disks manifest themselves in the observer's line of sight as variable depths of the central absorption depression (CA) of the Halpha emission (Hanuschik, A&A 295, 1995;
http://articles.adsabs.harvard.edu/pdf/ ... .295..423H

A variable depth of the central depression in the Halpha emission thus also leads to a reciprocal variability of the equivalent width EW.
In simple words:
at a small angle of inclination (pole on) maximum emission can be observed (central absorption small, CA value F/Fc = large);
at a large angle of inclination (edge ​​on), lower emissions can be observed (central absorption deep, CA value F/Fc = small).

The change in the inclination angle of the disk is mainly caused in the periastron by the (unobservable) secondary star [for its possible nature see: RevMexAA (Serie de Conferencias), 5, 114-116, 1996].

This geometrical regularity can be observed very well in the phase of the periastron passages of the companion star in the Be binary star 28 Tau (Pollmann, IBVS Vol. 62, No. 6239; https://konkoly.hu/pub/ibvs/6201/6239.pdf)

The last period determinations based on radial velocity measurements (P = 218.058 d) date back to 2010 (Nemravova et al.
https://www.aanda.org/articles/aa/pdf/2 ... 885-09.pdf)

The very sharp peaks of the EW and CA changes at 10 periastron passages meanwhile suggest that these should also be used (perhaps preferably) for the future monitoring of the orbital period of the companion star.

The orbital periods presented here:
EW (periastron) = 218.119d (± 0.049)
CA (periastron) = 217.883d (± 0.084)
Periodenanalyse CA und EW_20.2.21.png
Periodenanalyse CA und EW_20.2.21.png (473.37 KiB) Viewed 1438 times

Any comments?
Ernst Pollmann
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Posts: 415
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