Be Stars: Lessons from LongTerm Multitechnique Observations

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Be StarsLessons from Long-Term Multitechnique
Observations

• Anatoly Miroshnichenko
• University of Toledo

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In Collaboration with
Karen Bjorkman Jon Bjorkman Alex Carciofi
University of Toledo
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Ritter Observatory

• University of Toledo (Ohio state, Midwest USA)
• http//astro1.panet.utoledo.edu/wwwphys/ritter/ri
  tter.html

• Main research areas
• Early-type stars with circumstellar envelopes
• Be, Herbig Ae/Be and Be stars
  - Karen Bjorkman
• Radiative transfer in circumstellar
  envelopes - Jon Bjorkman
• BA-supergiants
  - Nancy Morrison

• Cosmic dust
  - Adolf Witt

• Interstellar medium
  - Steve Federman

• Stellar atmospheres -
  Lawrence Anderson

• Solar system
  - Phil James (Chair)

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Outline

• Brief history of studies of Be stars
• Basic properties of Be stars
• Variations of the spectrum and brightness
• How can we use the variability to get the
  physics?
• Current state of the research

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Classical Be Stars
First discovered group of emission-line stars
Emission lines in the spectrum of ? Cassiopeae
were found in 1867 by visual spectroscopy
200 Be stars are currently known among 1660
B-type stars brighter than V6.5 mag
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Main properties of classical Be stars

• Non-luminous rapidly-rotating objects displaying
  emission-line spectra
• Emission line profile shapes are usually double-
  or single-peaked at a low or moderate spectral
  resolution
• Infrared (IR) radiation excesses
• Polarization of the continuum radiation
• Active emission-line phases may last for decades
  and are followed by no-emission or shell-line
  phases
• Metallic line profiles (e.g., Fe II) suggest that
  the circumstellar gas is involved in a Keplerian
  motion around the star with small radial
  velocities (a few km s-1)

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Basic Stellar Parameters
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Some H? profiles
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IR Excess
1983
1998
? Aqr
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Polarization
? Tau
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Circumstellar Disks
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Origin of the Observed Features
Line emission ionized circumstellar gas
IR excess free-free emission
Polarization Thomson scattering
The polarization spectrum and spectral line
profiles imply a flattened, disk-like, envelope
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Theories of the Be phenomenon

• Elliptical disk model (Struve 1931, ApJ, 73, 94)
  - Keplerian rotation of particles in a
  circumstellar disk.
• No explanation for the disk long-term stability.

• Rotation-pulsation model - changing inflow and
  outflow superposed onto the rotational motion in
  the disk. Variable stellar wind as triggering
  mechanism for the V/R variations (Doazan et al.
  1987, AA, 182, L25).
• No explanation for the disk formation.

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Theories of the Be phenomenon
Wind-compressed disk model (Bjorkman Cassinelli
1993, ApJ, 409, 429) ? a rotating wind produces a
disk. Assumption the outward acceleration is
smaller than the rotation ? the material will
orbit down to the equator before it is
accelerated outwards. But small non-radial
forces act against disk formation.

• A disk can be produced by mass transfer in binary
  systems (Kriz Harmanec 1975, BAICz, 26, 65),
  where the mass gainer spins-up to critical
  rotation.
• Can explain formation of 20?40 of all Be stars
  (Pols et al. 1991, AA, 241, 419) or even less
  (5, Van Bever Vanbeveren (1997, AA, 322,
  116).

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Theories of the Be phenomenon
Non-Radial Pulsations may be a triggering
mechanism for the mass loss from at least
early-type Be stars (Rivinius et al. 2003, AA,
411, 229)
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Problems of the binary model

• Existence of Be/X-ray binaries post
  mass-transfer systems, where mass-transfer cannot
  explain the disk existence

• Lack of detected secondary companions

Small angular separations Lack of monitoring
programs
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Recent lists of Be binaries

• Pavlovski et al. (1997, AAS, 125, 75) 16
  spectroscopic binaries

• Gies (2000, ASP Conf.Ser, 214, 668) 40
• objects including 13 non-confirmed

Our statistics 12 binaries out of 24 Be stars
brighter than 4m 55 binaries out of 243 Be
stars brighter than 7m
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Bright Be binaries (V lt 4m)
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Recent observational results

• New binaries among bright Be stars have been
  found through high-res. spectroscopic monitoring
  (? Aqr, ? Cas)

• Be binaries usually have H? profiles with either
  1 or more than 2 emission peaks
• Statistics of Be stars with complex profiles
• ESO program 34 of 77
• French program 16 of 137
• Indian program 15 of 44

• Be binaries with large periods (100 ? 200 days)
  have stronger emission lines

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Orbital period vs. EW (H?)

• ? Be/Xray binaries
• ? B1?4 Be binaries
• ? B5?8 Be binaries

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Modeling the Be Star Disks
Input parameters 1. Stellar (Teff , log g ,
L) 2. Circumstellar ?0 ,Te(r) , geometry,
density distribution (??0 r?n)
Observational data 1. Spectral energy
distribution 2. Spectral line profiles 3.
Polarization spectrum
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Typical Modeling Results
Disk opening angle a few degrees Statistical
studies suggest opening angles from 5 to 25?40
degrees
Density at the disk base 10?11?10?12 g cm?3
Density distribution slope 2.5?3.5
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Theoretical Disk Structure
from Carciofi Bjorkman (2004, Polariz. Conf.,
Hawaii)
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Modeling Results for ? Tauri
from Carciofi Bjorkman (2004, Polariz. Conf.,
Hawaii)
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Problems with Snapshot Modeling
Theoretical (simplified assumptions) Smooth
density distribution Uncertain disk
size Parameter space degeneracy (?0 ? disk scale
height)
Observational Non-contemporaneous data Limited
spectral range
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Line Strength Variations
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Complex Profiles
? Oph
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Variations
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? Aquarii H? Variations
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What can We Get from Variability?
Reveal the true basic stellar parameters and
content of the system
Determine the circumstellar contribution to the
continuum
Learn about the mass loss history and mass
distribution in the disk
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Be star spectroscopy at the Ritter Observatory

• 1-meter telescope with a fiberfed echelle
  spectrograph and a 1150x1150-pixel CCD in the
  Coude focus

• 9 non-overlapping orders, 70 Å each, range
  5285-6600 Å. Includes spectral lines of FeII 5317
  6383, HeI 5876, NaI 5889 5895, SiII 6347
  6371, and H?

• Spectral resolving power R (?/??) 26000

• Spectra of stars brighter than 7.5 mag can be
  obtained in 1 hour with a signal-to-noise ratio
  of 100

• 1700 spectra of 45 Be stars obtained in
  1991-2004

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Ritter data statistics (as of 2004/04/29)
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Some results from Ritter program

• Discovery of 2 new Be stars, HD 4881 (V6.2, B9
  V) and HD 5839 (V6.7, B9 V) (Miroshnichenko et
  al. 1999, MNRAS, 302, 612).

• Periodic RV variations (84.3 d) of the emission
  peak and absorption wings of the H? profile in ?
  Aquarii are discovered during the normal B-star
  phase (Bjorkman et al. 2002, ApJ, 573, 812).

• Periodic RV variations (205 d) in the H? line of
  ? Cassiopeae (Miroshnichenko, Bjorkman, Krugov
  2002, PASP, 114, 1226).

• New orbital solution for the ? Scorpii binary and
  monitoring of the initial disk formation stages
  (Miroshnichenko et al. 2001, AA, 377, 485 2003,
  AA, 408, 305).

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? Cassiopeae
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? Aquarii
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? Aquarii
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? Scorpii
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? Scorpii
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? Scorpii
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Observational Constraints
Outer disk radius 20 30 R (Roche lobe size)
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Conclusions

• Long-term multi-wavelength and multi-technique
  observational studies are needed

• Such observations give us information about
• - the disk structure
• - the system content and fundamental parameters
• - the mass loss origin and evolutionary state

• Most of the needed observations can be obtained
  with relatively small telescopes