11/24/08 Introductory Course on Variable Stars: Variability Mechanisms 1 Variability Mechanisms Variability Mechanisms 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 Introductory Course on Variable Stars Introductory Course on Variable Stars
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11/24/08 Introductory Course on Variable Stars: Variability Mechanisms 1
Variability MechanismsVariability Mechanisms
Z. Mikulášek 1,2, G. Szász 1
1 Astrophysics Division, Department of Theoretical Physics and AstrophysicsFaculty of Science, Masaryk University, Brno, Czech Republic
2 Observatory and Planetarium of Johann PalisaVŠB – Technical University of Ostrava, Czech Republic
Introductory Course on Variable StarsIntroductory Course on Variable Stars
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OutlineOutline
1.1. Types of Variable StarsTypes of Variable Stars
2.2. Basic Classification of Variability MechanismsBasic Classification of Variability Mechanisms extrinsic and intrinsic variablesextrinsic and intrinsic variables
3.3. Extrinsic Variable StarsExtrinsic Variable Starsrotating variables, magnetic variables, stellar activityrotating variables, magnetic variables, stellar activity
5.5. Intrinsic Variable StarsIntrinsic Variable Starsnon-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 surfaceprocesses on stellar surface
6.6. Stellar ActivityStellar Activityoptical 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 activityactivity
"Somewhere, something incredible is waiting to be known." (C. Sagan)
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1. Types of Variable Stars1. Types of Variable Stars
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 criteriaMajor classification criteria::
light curveslight curves spectraspectra
RV curvesRV curves line variations line variations
"Physics is like sex. Sure, it may give some practical results but it's not why we do it." (R. Feynman)
Photo by Jan Kondziolka
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2. Basic Classification2. Basic Classification
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 starsA) extrinsic variable stars
Extrinsic and Intrinsic Variable Stars
Photo by Jan Kondziolka
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2. Basic Classification2. Basic Classification
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 starsB) intrinsic variable stars
Extrinsic and Intrinsic Variable Stars
Photo by Jan Kondziolka
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3. Extrinsic Variables3. Extrinsic Variables
system geometry system geometry changes due to the fact changes due to the fact that:that:
observed star rotatesobserved star rotates
the star is component the star is component of binary systemof 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-zerosight must be non-zero
radiation field must radiation field must decline in axial symmetrydecline in axial symmetry due to rotation axis due to rotation axis
Rotating Variables
Rotating variable starsRotating variable stars
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3. Extrinsic Variables3. Extrinsic Variables
axial asymmetry is caused by axial asymmetry is caused by inclined magnetic fieldinclined magnetic field
observed light variations are strictly periodicobserved light variations are strictly periodic
the period refers to the the period refers to the rotational periodrotational period of the object of the object
PProtrot
~ from 10 ~ from 10-4-4 s (pulsars) to several years (CP stars) s (pulsars) to several years (CP stars)
Magnetic Variables
Magnetic variable starsMagnetic variable stars
V901 Ori:- mCP star (Ap)- dipolar field- small amp.
- spotty surface
- same Teff
- different SED
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3. Extrinsic Variables3. Extrinsic Variables
axial anisotropy is caused by enormous local magnetic axial anisotropy is caused by enormous local magnetic fields forming so called fields forming so called active regionsactive regions
RS CVnRS CVn – photospheric spots can cover half of the – photospheric spots can cover half of the stellar surface causing light variations in tenths of mag.stellar surface causing light variations in tenths of mag.
Stellar Activity
Stellar activityStellar activity
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4. Binary Stars4. Binary Stars
the period refers to the the period refers to the orbital periodorbital period of the system of the system
axial anisotropy is caused by axial anisotropy is caused by mutual eclipsesmutual eclipses of the of the system componentssystem 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 apexof cones (umbra and penumbra) with common apex
Eclipsing Binaries
Eclipsing binariesEclipsing binaries
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4. Binary Stars4. Binary Stars
axial anisotropy is caused by axial anisotropy is caused by tidal deformationtidal deformation of the of the components in close binary systemcomponents in close binary system
gravity changes over the component surfacegravity changes over the component surface
geometrical projection towards line of sight variesgeometrical projection towards line of sight varies
important is also important is also reflection effectreflection effect especially in systems especially in systems with accretion disc (can increase with accretion disc (can increase TT
effeff by 1000 K) by 1000 K)
Interacting Binaries
Interacting binariesInteracting binaries
active mass transferactive mass transfer forms:forms: accretion streamsaccretion streams accretion discsaccretion discs bright spotsbright spots
loosing angular loosing angular momentum due to momentum due to the mass lossthe mass loss
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4. Intrinsic Variables4. Intrinsic Variables
observed variability of the observed variability of the intrinsic variablesintrinsic variables is caused is caused by variability of their physical propertiesby variability of their physical properties
in stellar surroundingsin stellar surroundings
in surface layers in surface layers (stellar activity)(stellar activity)
in sub-surface layers in sub-surface layers (pulsations)(pulsations)
in core in core (rapid phases of stellar evolution, SNs)(rapid phases of stellar evolution, SNs)
"The universe is not required to be in perfect harmony with human ambition." (C. Sagan)
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4. Intrinsic Variables4. Intrinsic VariablesNon-stationary Processes in Stellar Surroundings
envelopes of young stellar objects envelopes of young stellar objects (T Tau, FU Ori)(T Tau, FU Ori)
Non-stationary Processes in Stellar SurroundingsNon-stationary Processes in Stellar Surroundings
expanding envelopes of evolved stars expanding envelopes of evolved stars (novae, SNs)(novae, SNs)
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4. Intrinsic Variables4. Intrinsic Variables
major sources of non-stationary processes in IBs:major sources of non-stationary processes in IBs:
A) accretion discA) accretion disc – disc-shaped structure formed by – disc-shaped structure formed by accreting gas that carries angular momentumaccreting gas that carries angular momentum
reprocesses light of the system companionsreprocesses light of the system companions has it's has it's own energy sourceown energy source
turbulent shearing carries turbulent shearing carries angular momentum awayangular momentum awaywhile shifts accreting gas while shifts accreting gas inwards, releasing inwards, releasing gravitational energygravitational energy
mechanisms responsible mechanisms responsible for the turbulent shearing for the turbulent shearing are mostly not continuous are mostly not continuous and lead to erruptive and lead to erruptive behavior behavior (dwarf novae)(dwarf novae)
Mass Transfer in Binary Star Systems
Mass Transfer in Binary Star SystemsMass Transfer in Binary Star Systems
TTeffeff
~ 10 ~ 1033 K K
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4. Intrinsic Variables4. Intrinsic Variables
major sources of non-stationary processes in IBs:major sources of non-stationary processes in IBs:
B) accretion streamB) accretion stream – a stream of gas passing through – a stream of gas passing through Lagrangian point L1 that mostly forms accretion disc Lagrangian point L1 that mostly forms accretion disc around accretoraround accretor
Mass Transfer in Binary Star Systems
Mass Transfer in Binary Star SystemsMass Transfer in Binary Star Systems
accretion stream accretion stream behavior is generally behavior is generally non-stationarynon-stationary
thethe bright spot bright spot is is formed on its formed on its intersection with intersection with accretion discaccretion disc
variations of the variations of the bright spot are bright spot are source of source of flickeringflickering
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4. Intrinsic Variables4. Intrinsic Variables
the most frequent non-stationary process in stellar the most frequent non-stationary process in stellar photosphere is caused by accretion:photosphere is caused by accretion:
classical novaeclassical novae – close binaries (MS dwarf + WD) – close binaries (MS dwarf + WD)
accreting mass causes slight contraction that accreting mass causes slight contraction that heats up core of the degenerated companionheats up core of the degenerated companion
thin surface layer of hydrogen is heated up thin surface layer of hydrogen is heated up enough to start-up explosive nuclear fusionenough to start-up explosive nuclear fusion
the system brightens ~ 7-19 mag during outburstthe system brightens ~ 7-19 mag during outburst light decay takes light decay takes
several monthsseveral months quiet phase takes 10quiet phase takes 1055 yr yr
Non-stationary Processes on Stellar Surface
Non-stationary Processes on Stellar SurfaceNon-stationary Processes on Stellar Surface
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6. Stellar Activity6. Stellar Activity
most solar-type stars show prominent emission cores in most solar-type stars show prominent emission cores in H and K lines, the evidence of H and K lines, the evidence of extensive chromospheresextensive chromospheres
these lines display 2 kinds of variations:these lines display 2 kinds of variations:
short-termshort-term (P ~ days) – movement of active regions (P ~ days) – movement of active regions
long-termlong-term (P ~ 8-12 yr) – equivalent of solar cycle (P ~ 8-12 yr) – equivalent of solar cycle
Optical Observations
Optical ObservationsOptical Observations
solar-type stars solar-type stars without any without any observable activity observable activity are possibly in some are possibly in some equivalent of equivalent of Maunder MinimumMaunder Minimum
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6. Stellar Activity6. Stellar Activity
optical observations of optical observations of cool MS starscool MS stars (especially red (especially red dwarfs) also show prominent emission in H and K lines dwarfs) also show prominent emission in H and K lines that reveals presence of that reveals presence of extensive chromospheresextensive chromospheres
these stars are mostly also intrinsic variables called these stars are mostly also intrinsic variables called flare flare stars stars that show frequent optical flares, lasting several that show frequent optical flares, lasting several minutes and increasing luminosity of the star in 2 orders minutes and increasing luminosity of the star in 2 orders of magnitude (in extreme cases)of magnitude (in extreme cases)
Optical Observations
pronounced pronounced white flareswhite flares (one order of (one order of magnitude magnitude stronger than stronger than solar and much solar and much more frequent)more frequent)
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6. Stellar Activity6. Stellar Activity
T Tauri stars T Tauri stars also show signs of extreme stellar activityalso show signs of extreme stellar activity
Optical Observations
`
variable emission in H variable emission in H and K linesand K lines (evidence for (evidence for presence of extensive presence of extensive chromospheres and chromospheres and chromospheric activity)chromospheric activity)
frequent flaresfrequent flares
extreme stellar windsextreme stellar winds (several orders of (several orders of magnitude stronger than magnitude stronger than solar)solar)
giants and supergiants giants and supergiants evidently have evidently have extensive extensive chromosphereschromospheres aned extremely aned extremely strong stellar windsstrong stellar winds (those causing serious mass loss that has significant (those causing serious mass loss that has significant impact on stellar evolution)impact on stellar evolution)
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6. Stellar Activity6. Stellar Activity
RS CVn stars RS CVn stars are a very special case showing 3 kinds of are a very special case showing 3 kinds of stellar activity:stellar activity:
Optical Observations
extensive photospheric spotsextensive photospheric spots (that can cover 50% of stellar (that can cover 50% of stellar surface)surface)
chromospheric activitychromospheric activity
extreme flaresextreme flares
all all late-type stars late-type stars generally have chromospheresgenerally have chromospheres
activity of the most of them is much higher than solaractivity of the most of them is much higher than solar
this conclusions are also supported by non-optical this conclusions are also supported by non-optical observationsobservations
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6. Stellar Activity6. Stellar Activity
stellar chromospheres (stellar chromospheres (TT ~ 200,000 K) are visible in ~ 200,000 K) are visible in UVUV
stellar coronae (stellar coronae (TT ~ 10 ~ 1066 – 10 – 1088 K) are visible in K) are visible in X-ray X-ray
almost all of thealmost all of the F-M type stars F-M type stars have both have both chromospheres and hot coronaechromospheres and hot coronae
A-F type starsA-F type stars do not have hot coronae do not have hot coronae
O-B type stars O-B type stars have coronae due to strong mass losshave coronae due to strong mass loss
giants and supergiantsgiants and supergiants
Radio and Space-Based Observations
Radio and Space-Based ObservationsRadio and Space-Based Observations
earlier types than K2earlier types than K2 – have – have chromospheres and coronaechromospheres and coronae
later types than K2later types than K2 – have – have only chromospheresonly chromospheres
T Tauri stars T Tauri stars have only have only chromosphereschromospheres
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6. Stellar Activity6. Stellar Activity
important role of local magnetic fieldsimportant role of local magnetic fields
dynamo effectdynamo effect – amplifying random magnetic fields – amplifying random magnetic fields
convective currents and rotationconvective currents and rotation – move frozen – move frozen magnetic field from stellar interior towards surface magnetic field from stellar interior towards surface where it dissipates creating MHD waves and thus where it dissipates creating MHD waves and thus transferring the energy over photosphere and thus transferring the energy over photosphere and thus heating up plasma in stellar chromosphere end coronaheating up plasma in stellar chromosphere end corona
