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[vsnet-be 66] Periods for Be stars
Dear friends:
Since the delta Scorpii event I have been observing and
comparing data for several Be stars. I have found very common the ocurrence
of periods in the order of tens of days.
What mechanism drives that variation? Is it a pulsation or
is it related to binarity ?
There are three different types of variability working in
most of them:
1- variations in the order of a day or so (NRP?).
2- variations in the order of several tenths of days.
3- outbursts lasting from days to years (delta Sco, gamma Cas) and during
which the amplitude of the shorter cycle is increased.
I have compared my visual data with Hipparcos' and observations from
different papers and this are some interesting things on southern bright Be
stars:
- delta Sco: My visual observations clearly show a cycle of +-76 days in
agreement with Gandet's and others spectroscopic studies. The outbursts or
GCAS activity seem to be related with the orbital period: 10.583 years.
Amplitude and quiescent data is of course unknown since the activity is new.
Full range: 1.70 - 2.33 V (up to now!!)
- omega CMa: My visual observations clearly confirm Harmanec's period of
34.675 days. Amplitude is in the order of 0.15 magnitudes. There are 3300
days between the outburst observed by the Hipparcos satellite and the
current one. The light curves are similar. Quiescent state: 3.95 - 4.17 V.
Full range: 3.60 - 4.18 V
- kappa CMa: The 1.40 days period from Balona (1990) is confirmed by
Hipparcos data. My visual observations show there is a cycle similar to the
other stars present: +- 66.8 days. Amplitude is in the order of 0.10 - 0.15
magnitudes.
The star was at its brightest state ever when observed by Hipparcos,
reaching V= 3.43 around JD 2448300.
Current quiescent state: 3.58 - 3.73 V. This star was around 3.9 in past
decades. Full range: 3.43 - 3.97 V.
- mu Cen: Another case: my visual observations show a typical cycle of
57.9 days. Petr Harmanec ran a period analysis on Hipparcos data and found a
similar 55.303 day-period. Amplitude is in the order of 0.10 - 0.15
magnitudes. Quiescent state:
3.35 - 3.54 V. Full range: 2.92 - 3.54 V.
- chi Oph: This is a very strange case: the star is at maximum brightness
for years. I have found a short 0.52 days variation from Hipparcos data (in
the order of 0.05 magnitudes between 4.18 - 4.24 V). However , Balona (1990)
suggested 13.77 days and commented that it lied outside of the normal
distribution of periods. My observations didn't detect long term activity.
There is a 0.15 dimming cught by Hipparcos around JD 2448680, when the star
faded to 4.34 V. Quiescent state: 4.17 - 4.24 V. Full range: GCVS lists a
5.0 V minimum. Could someone provide any information about it??
- alpha Ara: Balona (1990) gives 0.981 day period. From Hipparcos data,
0.498 days seems to fit better. Amplitude is in the order of 0.04
magnitudes. Mean brightness of alpha Arae has increased over the last
decades. It used to lie between 2.9 and 3.0 but curently its quiescent state
is at 2.80 - 2.85 V. Hipparcos detected variations between 2.78 and 2.89 V.
The star wa brighter at the beginning of the mission. Full range: 2.78 -
3.0? V.
- lambda Pav: Another star that shows little activity. BUT again , there
seems to be some cycle this time from Hipparcos data: 22.9 days. Amplitude
is less than 0.1 magnitudes so visual confirmation is very difficult.
Quiescent state: 4.18 - 4.25 V.
Full range: 4.00 - 4.26 V
- eta Cen: Hipparcos has classified it as an EB star and a nice double
waved lightcurve is shown. Cuypers et al (1989) mention this star is a
convincing case for a triple-wave lightcurve. The period in Balona (1990) is
1.927 days but Hipparcos gives 1.28494 days. Amplitude is 0.08 magnitudes
bor both sources.
My visual observations are still few and poor due to lack of constant
comparison stars, but a 18 day-cycle seems to be present with a less than
0.1 mag. amplitude. This star hasn't shown variability of more than 0.2
magnitudes. Is it worth of the GCAS classification it attains in the GCVS??
Full range: 2.30 - 2.49 V.
In short:
STAR -----Var1-----Var2-----Var3
del Sco --?-- +-76 days -- 10.583 years
ome CMa --1.37 d. -- 34.7 d.-- ?
kap CMa --1.40 d.-- +-66.8 d. --?
mu Cen --2.10 d.-- +- 56 d. --?
chi Oph -- +-0.52 d.--? -- ?
Alp Ara -- +-0.49 d.-- ? --?
lam Pav --? -- +-22.9 d. --?
eta Cen --1.285 d.-- +-18 d. --?
Be stars are very interesting!!
Regards,
Sebastian.
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