Variability Mechanisms Variability Mechanisms Introductory Course on Variable Stars Introductory Course on Variable Stars Z. Mikulášek 1,2 , G. Szász 1 1 Astrophysics Division, Department of Theoretical Physics and Astrophysics Faculty of Science, Masaryk University, Brno, Czech Republic 2 Observatory and Planetarium of Johann Palisa VŠB – Technical University of Ostrava, Czech Republic 11/24/08 Introductory Course on Variable Stars: Variability Mechanisms 1
Outline Outline "Somewhere, something incredible is waiting to be known." (C. Sagan) 1. 1. Types of Variable Stars Types of Variable Stars 2. 2. Basic Classification of Variability Mechanisms Basic Classification of Variability Mechanisms extrinsic and intrinsic variables extrinsic and intrinsic variables 3. 3. Extrinsic Variable Stars Extrinsic Variable Stars rotating variables, magnetic variables, stellar activity rotating variables, magnetic variables, stellar activity 4. 4. Binary Stars Binary Stars eclipsing binaries, interacting binaries eclipsing binaries, interacting binaries 5. 5. Intrinsic Variable Stars Intrinsic Variable Stars non-stationary processes in stellar surroundings, non-stationary processes in stellar surroundings, mass transfer in binary star systems, non-stationary mass transfer in binary star systems, non-stationary processes on stellar surface processes on stellar surface 6. 6. Stellar Activity Stellar Activity optical observations, radio and space-based optical observations, radio and space-based observations, causes and models of stellar and solar observations, causes and models of stellar and solar activity activity 11/24/08 Introductory Course on Variable Stars: Variability Mechanisms 2
1. Types of Variable Stars 1. Types of Variable Stars "Physics is like sex. Sure, it may give some practical results but it's not why we do it." (R. Feynman) We have 3 types of women: We have 3 types of women: to marry with, to marry with, to avoid of, to avoid of, to be friends with. to be friends with. (Kama Sutra 200AC) (Kama Sutra 200AC) We have more than 50 types We have more than 50 types of variable stars. of variable stars. Major classification criteria: : Major classification criteria light curves light curves spectra spectra RV curves RV curves line variations line variations (intensity, EWs) (intensity, EWs) Photo by Jan Kondziolka H-R Diagram H-R Diagram evolutionary state evolutionary state 11/24/08 Introductory Course on Variable Stars: Variability Mechanisms 3
2. Basic Classification 2. Basic Classification Extrinsic and Intrinsic Variable Stars We have 2 basic types of variable stars according to We have 2 basic types of variable stars according to their variability mechanism: their variability mechanism: A) extrinsic variable stars A) extrinsic variable stars Photo by Jan Kondziolka 11/24/08 Introductory Course on Variable Stars: Variability Mechanisms 4
2. Basic Classification 2. Basic Classification Extrinsic and Intrinsic Variable Stars We have 2 basic types of variable stars according to We have 2 basic types of variable stars according to their variability mechanism: their variability mechanism: B) intrinsic variable stars B) intrinsic variable stars Photo by Jan Kondziolka 11/24/08 Introductory Course on Variable Stars: Variability Mechanisms 5
3. Extrinsic Variables 3. Extrinsic Variables Rotating Variables Rotating variable stars Rotating variable stars system geometry system geometry changes due to the fact changes due to the fact that: that: observed star rotates observed star rotates the star is component the star is component of binary system of binary system inclination angle of the inclination angle of the rotation axis to the line of rotation axis to the line of sight must be non-zero sight must be non-zero radiation field must radiation field must decline in axial symmetry decline in axial symmetry due to rotation axis due to rotation axis 11/24/08 Introductory Course on Variable Stars: Variability Mechanisms 6
3. Extrinsic Variables 3. Extrinsic Variables Magnetic Variables Magnetic variable stars Magnetic variable stars axial asymmetry is caused by inclined magnetic field inclined magnetic field axial asymmetry is caused by observed light variations are strictly periodic observed light variations are strictly periodic the period refers to the rotational period rotational period of the object of the object the period refers to the -4 s (pulsars) to several years (CP stars) P rot ~ from 10 -4 s (pulsars) to several years (CP stars) P rot ~ from 10 V901 Ori: - mCP star (Ap) - dipolar field - small amp. - spotty surface - same T eff - different SED 11/24/08 Introductory Course on Variable Stars: Variability Mechanisms 7
3. Extrinsic Variables 3. Extrinsic Variables Stellar Activity Stellar activity Stellar activity axial anisotropy is caused by enormous local magnetic axial anisotropy is caused by enormous local magnetic fields forming so called active regions active regions fields forming so called photospheric spots – significantly cooler (and darker) – significantly cooler (and darker) photospheric spots than surrounding surface than surrounding surface RS CVn – photospheric spots can cover half of the – photospheric spots can cover half of the RS CVn stellar surface causing light variations in tenths of mag. stellar surface causing light variations in tenths of mag. 11/24/08 Introductory Course on Variable Stars: Variability Mechanisms 8
4. Binary Stars 4. Binary Stars Eclipsing Binaries Eclipsing binaries Eclipsing binaries the period refers to the orbital period orbital period of the system of the system the period refers to the axial anisotropy is caused by mutual eclipses mutual eclipses of the of the axial anisotropy is caused by system components system components the anisotropy has a shape of two opposite coaxial pairs the anisotropy has a shape of two opposite coaxial pairs of cones (umbra and penumbra) with common apex of cones (umbra and penumbra) with common apex 11/24/08 Introductory Course on Variable Stars: Variability Mechanisms 9
4. Binary Stars 4. Binary Stars Interacting Binaries Interacting binaries Interacting binaries axial anisotropy is caused by tidal deformation tidal deformation of the of the axial anisotropy is caused by components in close binary system components in close binary system gravity changes over the component surface gravity changes over the component surface geometrical projection towards line of sight varies geometrical projection towards line of sight varies important is also reflection effect reflection effect especially in systems especially in systems important is also with accretion disc (can increase T T eff by 1000 K) with accretion disc (can increase eff by 1000 K) active mass transfer active mass transfer forms: forms: accretion streams accretion streams accretion discs accretion discs bright spots bright spots loosing angular loosing angular momentum due to momentum due to the mass loss the mass loss 11/24/08 Introductory Course on Variable Stars: Variability Mechanisms 10
4. Intrinsic Variables 4. Intrinsic Variables "The universe is not required to be in perfect harmony with human ambition." (C. Sagan) observed variability of the intrinsic variables intrinsic variables is caused is caused observed variability of the by variability of their physical properties by variability of their physical properties in stellar surroundings in stellar surroundings in surface layers (stellar activity) (stellar activity) in surface layers in sub-surface layers (pulsations) (pulsations) in sub-surface layers in core (rapid phases of stellar evolution, SNs) (rapid phases of stellar evolution, SNs) in core 11/24/08 Introductory Course on Variable Stars: Variability Mechanisms 11
4. Intrinsic Variables 4. Intrinsic Variables Non-stationary Processes in Stellar Surroundings Non-stationary Processes in Stellar Surroundings Non-stationary Processes in Stellar Surroundings envelopes of young stellar objects (T Tau, FU Ori) (T Tau, FU Ori) envelopes of young stellar objects expanding envelopes of evolved stars (novae, SNs) (novae, SNs) expanding envelopes of evolved stars 11/24/08 Introductory Course on Variable Stars: Variability Mechanisms 12
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