summary of experiences from observations of the be binary sco anatoly miroshnichenko university of...

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