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Summary of Experiences from Observations of the Bebinary
Sco
Anatoly Miroshnichenko University of North Carolina at Greensboro
USA
•Properties of Be Stars•Basic Parameters of the Sco System•Observed Behaviour of the System•Conclusions That Can Be Made Now
Properties of Be Stars
Definition: Be stars are rapidly rotating non-supergiant objects of spectral type B that sometimes show hydrogen emission lines in their spectra
Spectral type: O9 – A1 (Teff : ~30000 9000 K)Luminosity: 100 – 3 104 LʘProjected rotation velocity (v sin i): up to break-up
Vbreak-up = 436 (M/Mʘ)1/2(R/Rʘ)1/2 km/s
Basic Stellar ParametersEvolutionary tracks for single stars (numbers are initial masses in Mʘ)
Zero-age main-sequence
Circumstellar Disks of Be StarsProperties:
• Flat near the star
• Disk thickness increases outward from the star
• Density drops with distance from the star as rn
• Density exponent n ~ 2.54.0 (simplified models)
• Disks are temporary (can be present for decades)
• Disks can suddenly appear or disappear
• Disks can change into rings
• Disks can add to the continuum brightness (up to 70% in the optical region)
Theoretical Disk Structure
Carciofi & Bjorkman (2004, Polarization Conf., Hawaii)
Reasons for the Be Phenomenon
Rapid rotation can be intrinsic (from birth) or induced (mass-transfer in a binary system)
Mass loss (disk formation) can be triggered by pulsations or by close passages in binaries
Disk material orbits the star moving outward through viscosity
If mass loss from the star exceeds mass loss from the disk, the material is accumulated near the star
Hypotheses about the Nature 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, A&A, 411, 229)
In a binary system, the mass gainer spins-up to critical rotation (Křiž & Harmanec 1976, Bull. Astr. Inst. Czekh., 26, 65)
Non-Radial Pulsations
Line Formation in Disks
McDonald Observatory R=60000
Disk Size Effects
Continuum Excess Radiation
Aqr
1998 small or no disk
1983 large disk
UV
Opt+IR
ScorpiiR.A. 16h 01m, Dec. 2238 (J2000)
Parameters of ScoOptical brightness without disk, V=2.32 magSpectral type B0.3 IVDistance, D = 12315 pcLuminosity, log L/Lʘ = 4.40.1Surface temperature, Teff = 27500500 KSurface gravity, log g = 4.0 (typical for a dwarf)
This is a binary system with an angular separation at apoastron of 0.2 arcsecondsOrbital period, P = 10.60.1 years (uncertain)Eccentricity, e = 0.940.01Secondary, V ~1.5 mag, Sp.T. ~B3 (uncertain)
Sco without Disk
Orbit of Sco
Average radial velocities of the H emission line
Orbit of Sco
Brightness Variations of Sco
Brightness Variations of Sco
= 0.5 m
= 2.2 m
Recent Brightness Variations
H line in 20002003
Disk in 2001
From Carciofi et al. (2006, ApJ, 652, 1617)
Disk in 2001
H line in 20042007
H EW Evolution
Brightness Spectrum
H Line Width Drop
Seen since March 2005
CII, CIV Line emission?
Possible ExplanationsThe brightness decrease in 2004/5 can be due to a decreasing mass loss from the primary
The disk became a ringObserved consequences:The H line width decreased (no contribution from rapidly rotating part of the disk)The line equivalent width (EW) decreased
What to Expect at Periastron
Current disk size is ~ 20 R1 or ~ 150 RʘIt may grow larger as time goes
Distance between the stars at periastron is d = 24 R1
Primary’s Roche lobe size ~ 0.6 d or ~ 15 R1
Consequences:Some disk material may flow into the secondary’s Roche lobeDisk may become denser and line emission will riseSingle- or triple-peak profiles may be observed
Roche Lobes
Conclusions and Suggestions
• The binary is coming to its next periastron in February May 2011 (observable from the Northern hemisphere)
• Weekly observations are important before periastron and more frequent around it
• Photometry needs to accompany spectroscopy• Radial velocity data will constrain the orbital
period• Impact of the Roche lobe compression on the disk
may be revealed