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Disks of Be Stars &
Their Pulsations &
Qingkang Li
Department of Astronomy
Beijing Normal University
The Third Workshop of SONG, April, 2010
Collaborators:
• J. C. Brown, Dept. of Physics and
Astronomy, University of Glasgow, UK
• J. P. Cassinelli, Dept of Astronomy,
University of Wisconsin-Madison, USA
• R. Ignace, Dept. of Physics and
Astronomy, University of East
Tennessee, USA
• And many others.
Contents
1. Introduction
2. The mystery of the Be stars
3. Be star disk models
4. Discussion
1. Introduction
• Over the course of their lifetimes, hot,
luminous, massive (OB-type) stars lose large
amount of mass in nearly continuous outflow
called stellar winds.
• These winds are driven by scattering of the
star’s continuum radiation in a large
ensemble of spectral lines (Castor, Abbott &
Klein 1975).
• There is extensive evidence for variability
and structure on both small and large scales.
Solar Activities: Coronal Expansion
SOHO Extreme ultraviolet (171 Angstrom)
Wind-Blown Bubbles in
ISM
WR wind bubble
NGC 2359
Superbubble in the
Large Magellanic Cloud
Eta Carinae
OB stars in the HR
diagram
O, B spectral type stars
Teff > 10,000 K
L* > 100 Lsun
M* > 3 Msun
Hot massive pulsators
Pulsating stars in
the HR diagram
βCep variables:
pulsation periods
P~2-10 hrs, loworder p and g
modes, B0-B2
Slowly pulsating B
(SPB) stars: P∽1050 hrs, high-order g
modes, B3-B9
Pulsating Be stars:
P~similar to SPB up
to 100-200 hrs
2. The Mystery of the Be Stars
Be stars are non-supergiant B-type stars whose spectra have,
or had at some time, one or more Balmer lines in emission.
The mystery of the "Be phenomenon" is that the emission,
which is well understood to originate from a flattened
circumstellar disk (e.g. Struve, 1931), can come and go
episodically on time scales of days to decades.
Be stars
• Hot, bright, & rapidly rotating stars.
• The “e” stands for emission lines in the star’s spectrum
Hb
Ha
• Detailed spectra show
emission intensity is split
into peaks to blue and red
of line-center.
• This is from Doppler shift
of gas moving toward and
away from the observer.
• Indicates a disk of gas orbits the star.
Intensity
Hydrogen
spectrum
lo
Wavelength
Cartoon of a Be star and its emission lines
From http://www.astrosurf.org/buil/us/bestar.html
Variability of line profile
From http://www.astrosurf.org/buil/us/bestar.html
Key Properties
• Strong, radiatively driven stellar winds
---Mdot ~ 10-10 to 10-6 MO/yr; v > 1000 km/s
– Driven by line-scattering of the star’s radiation
• Magnetic Activity
– Some have observed dipole field ~103-104 G
– stable; from convective dynamo or fossil?
• Stellar Rotation
– Be stars are generally rapid rotators
– Vrot ~ 200-400 km/s ~ Vcrit/2
– Prot ~ few days
– oblateness and gravity darkening
• Stellar Pulsation
– Many Be stars show Non-Radial Pulsations
(NRP) with m < l = 1 – 4
– Give rise to the line variability.
The Key Puzzle of Be Disks
• Be stars are too old to still
have protostellar disk.
• And most Be stars are not
in close binary systems.
• They thus lack outside
mass source to fall into
disk.
• So disk matter must be
launched from the star.
How do Be stars
do this?
The key theoretical concerns on the Be star
disks are as follows:
1. What source of energy elevates matter to
orbits well above the stellar surface?
2. How does the matter obtain the angular
momentum and orbit around the star with
Keplerian speed with no observed outflow?
3. How are the very high observed disk
density attained?
3. Be star disk models
(1) Wind compressed disk model
(Bjorkman, Cassinelli, 1993, ApJ, 409, 429)
(2) Episodic mass ejection model
(Ando, 1986, A&A, 163, 97; Owocki & Cranmer, 2002, ASP 259,
512, 951; Cranmer, 2009, ApJ, 701, 396)
Observations confirm that Be star lineprofile variability is due to the non-radial
pulsations (NRP). Some mixed modes of
NRP (retrograde) are associated with the
transport of angular momentum up to the
surface and beyond, saying, pulsational
energy may leak out of a hot star into
circumstellar medium.
• low-frequency evanescent pulsations;
• high-frequency resonant wave excitation at the acoustic cutoff
• shock steepening and dissipation;
• wave pressure & angular momentum transfer.
(3) Magnetically torqued disk (MTD) model
(Cassinelli, et al., 2002, ApJ, 578, 951)
MTD+GD
(Brown, et al., 2004, MNRAS, 352, 1061)
X-ray emission from MTD+GD
Li, et al., 2008, ApJ, 672,1174
X-ray emission from MTD+GD
Li, et al., 2008, ApJ, 672,1174
(4) Combination of all stuff
Wind
?
+ B-field
+ Rotation
+ Pulsation
Painted by John C. Brown
4. Discussion
• The disk formation mechanism of Be stars is
one of the key issues to hot star research
both observational and theoretical.
• We need new observational facilities, such
as SONG.
• We require the development of simulations.
• We require the advancement of model
atmospheres and radiative transfer techniques
• We need correct stellar parameters (mass
loss rate, rotation, magnetic fields, and
pulsations).
Thank you very much
for your time!