Variability Mechanisms Variability Mechanisms Introductory Course - - PowerPoint PPT Presentation
Variability Mechanisms Variability Mechanisms Introductory Course on Variable Stars Introductory Course on Variable Stars Z. Mikulek 1,2 , G. Szsz 1 1 Astrophysics Division, Department of Theoretical Physics and Astrophysics Faculty of
11/24/08 Introductory Course on Variable Stars: Variability Mechanisms 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
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Types of Variable Stars Types of Variable Stars
Basic Classification of Variability Mechanisms Basic Classification of Variability Mechanisms extrinsic and intrinsic variables extrinsic and intrinsic variables
Extrinsic Variable Stars Extrinsic Variable Stars rotating variables, magnetic variables, stellar activity rotating variables, magnetic variables, stellar activity
Binary Stars Binary Stars eclipsing binaries, interacting binaries eclipsing binaries, interacting binaries
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
Stellar Activity Stellar Activity
activity activity
"Somewhere, something incredible is waiting to be known." (C. Sagan)
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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
Major classification criteria Major classification criteria: :
light curves light curves
spectra spectra
RV curves RV curves
line variations line variations (intensity, EWs) (intensity, EWs)
H-R Diagram H-R Diagram
evolutionary state evolutionary state
"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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A) extrinsic variable stars A) extrinsic variable stars
Extrinsic and Intrinsic Variable Stars
Photo by Jan Kondziolka
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B) intrinsic variable stars B) intrinsic variable stars
Extrinsic and Intrinsic Variable Stars
Photo by Jan Kondziolka
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system geometry system geometry changes due to the fact changes due to the fact that: that:
the star is component the star is component
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
Rotating Variables
Rotating variable stars Rotating variable stars
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axial asymmetry is caused by axial asymmetry is caused by inclined magnetic field inclined magnetic field
the period refers to the the period refers to the rotational period rotational period of the object
P Prot
rot ~ from 10
~ from 10-4
s (pulsars) to several years (CP stars)
Magnetic Variables
Magnetic variable stars Magnetic variable stars
V901 Ori:
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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 regions active regions
photospheric spots photospheric spots – significantly cooler (and darker) – significantly cooler (and darker) than surrounding surface than surrounding surface
RS CVn RS 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 activity Stellar activity
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the period refers to the the period refers to the orbital period
axial anisotropy is caused by axial anisotropy is caused by mutual eclipses mutual eclipses of the
system components system components
the anisotropy has a shape of two opposite coaxial pairs the anisotropy has a shape of two opposite coaxial pairs
Eclipsing Binaries
Eclipsing binaries Eclipsing binaries
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axial anisotropy is caused by axial anisotropy is caused by tidal deformation tidal deformation of the
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 important is also reflection effect reflection effect especially in systems especially in systems with accretion disc (can increase with accretion disc (can increase T Teff
eff by 1000 K)
by 1000 K)
Interacting Binaries
Interacting binaries Interacting binaries
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
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intrinsic variables is caused is caused by variability of their physical properties by variability of their physical properties
in stellar surroundings in 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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Non-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 Surroundings Non-stationary Processes in Stellar Surroundings
expanding envelopes of evolved stars expanding envelopes of evolved stars (novae, SNs) (novae, SNs)
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major sources of non-stationary processes in IBs: major sources of non-stationary processes in IBs: A) accretion disc A) accretion disc – disc-shaped structure formed by – disc-shaped structure formed by accreting gas that carries angular momentum accreting gas that carries angular momentum
reprocesses light of the system companions reprocesses light of the system companions
has it's has it's own energy source
turbulent shearing carries turbulent shearing carries angular momentum away angular momentum away while shifts accreting gas while shifts accreting gas inwards, releasing inwards, releasing gravitational energy gravitational 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 Systems Mass Transfer in Binary Star Systems T Teff
eff ~ 10
~ 103
3 K
K
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major sources of non-stationary processes in IBs: major sources of non-stationary processes in IBs: B) accretion stream B) 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 accretor around accretor
Mass Transfer in Binary Star Systems
Mass Transfer in Binary Star Systems Mass Transfer in Binary Star Systems
accretion stream accretion stream behavior is generally behavior is generally non-stationary non-stationary
the the bright spot bright spot is is formed on its formed on its intersection with intersection with accretion disc accretion disc
variations of the variations of the bright spot are bright spot are source of source of flickering flickering
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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 novae classical 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 companion heats 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 fusion enough to start-up explosive nuclear fusion
the system brightens ~ 7-19 mag during outburst the system brightens ~ 7-19 mag during outburst
light decay takes light decay takes several months several months
quiet phase takes 10 quiet phase takes 105
5 yr
yr
Non-stationary Processes on Stellar Surface
Non-stationary Processes on Stellar Surface Non-stationary Processes on Stellar Surface
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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 chromospheres extensive chromospheres
these lines display 2 kinds of variations: these lines display 2 kinds of variations:
short-term short-term (P ~ days) – movement of active regions (P ~ days) – movement of active regions
long-term long-term (P ~ 8-12 yr) – equivalent of solar cycle (P ~ 8-12 yr) – equivalent of solar cycle
Optical Observations
Optical Observations Optical Observations
solar-type stars solar-type stars without any without any
are possibly in some are possibly in some equivalent of equivalent of Maunder Minimum Maunder Minimum
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cool 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 chromospheres extensive 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
Optical Observations
pronounced pronounced white flares white 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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T Tauri stars T Tauri stars also show signs of extreme stellar activity also show signs of extreme stellar activity
Optical Observations
`
variable emission in H variable emission in H and K lines and K lines (evidence for (evidence for presence of extensive presence of extensive chromospheres and chromospheres and chromospheric activity) chromospheric activity)
frequent flares frequent flares
extreme stellar winds extreme 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 chromospheres chromospheres aned extremely aned extremely strong stellar winds strong 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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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 spots extensive photospheric spots (that can cover 50% of stellar (that can cover 50% of stellar surface) surface)
chromospheric activity chromospheric activity
extreme flares extreme flares
all all late-type stars late-type stars generally have chromospheres generally have chromospheres
activity of the most of them is much higher than solar activity 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
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stellar chromospheres ( stellar chromospheres (T T ~ 200,000 K) are visible in ~ 200,000 K) are visible in UV UV
stellar coronae ( stellar coronae (T T ~ 10 ~ 106
6 – 10
– 108
8 K) are visible in
K) are visible in X-ray X-ray
almost all of the almost all of the F-M type stars F-M type stars have both have both chromospheres and hot coronae chromospheres and hot coronae
A-F type stars A-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 loss have coronae due to strong mass loss
giants and supergiants giants and supergiants
Radio and Space-Based Observations
Radio and Space-Based Observations Radio and Space-Based Observations
earlier types than K2 earlier types than K2 – have – have chromospheres and coronae chromospheres and coronae
later types than K2 later types than K2 – have – have
T Tauri stars T Tauri stars have only have only chromospheres chromospheres
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important role of local magnetic fields important role of local magnetic fields
dynamo effect dynamo effect – amplifying random magnetic fields – amplifying random magnetic fields
convective currents and rotation convective 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 corona heating up plasma in stellar chromosphere end corona
stellar activity strongly depends on rotation speed stellar activity strongly depends on rotation speed (dynamo effect strength ~ (dynamo effect strength ~ v vrot
rot 2 2)
)
fast rotating stars fast rotating stars
young stars young stars
close binary companions close binary companions with bound rotation with bound rotation
Causes and Models of Stellar and Solar Activity
Causes and Models of Stellar and Solar Activity Causes and Models of Stellar and Solar Activity
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the model is not applicable to hot stars the model is not applicable to hot stars
A7-F0 type stars A7-F0 type stars – mostly chemically peculiar stars – mostly chemically peculiar stars
A stars A stars – non-active stars – non-active stars
slow rotating A stars slow rotating A stars – magnetic CP stars – magnetic CP stars
O-B type stars O-B type stars – stellar activity is given by – stellar activity is given by intensity of intensity of stellar wind stellar wind ~ ~ T Teff
eff Causes and Models of Stellar and Solar Activity