Transcript Sco
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 Lʘ Projected rotation velocity (v sin i): up to break-up Vbreak-up = 436 (M/Mʘ)1/2(R/Rʘ)1/2 km/s Basic Stellar Parameters Evolutionary tracks for single stars (numbers are initial masses in Mʘ) Zero-age mainsequence Circumstellar Disks of Be Stars Properties: • • • • • • • • 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 In a binary system, the mass gainer spins-up to critical rotation (Křiž & Harmanec 1976, Bull. Astr. Inst. Czekh., 26, 65) Non-radial pulsations may be a triggering mechanism for the mass loss from at least earlytype Be stars (Rivinius et al. 2003, A&A, 411, 229) Non-Radial Pulsations Line Formation in Disks McDonald Observatory R=60000 Disk Size Effects Continuum Excess Radiation 1998 small or no disk Opt+IR UV Aqr 1983 large disk Scorpii R.A. 16h 01m, Dec. 2238 (J2000) Parameters of Sco Optical brightness without disk, V=2.32 mag Spectral type B0.3 IV Distance, D = 12315 pc Luminosity, log L/Lʘ = 4.40.1 Surface temperature, Teff = 27500500 K Surface gravity, log g = 4.0 (typical for a dwarf) This is a binary system with an angular separation at apoastron of 0.2 arcseconds Orbital period, P = 10.60.1 years (uncertain) Eccentricity, e = 0.940.01 Secondary, 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 Explanations The brightness decrease in 2004/5 can be due to a decreasing mass loss from the primary The disk became a ring Observed 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 Rʘ It 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 lobe Disk may become denser and line emission will rise Single- 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