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Crash Course in Stellar Pulsation. Ryan Maderak A540 April 27, 2005. Mechanisms. k mechanism Compression of partial ionization zones -> ionization -> small change in T k ~ r / T 3.5 , increase r -> increase k g mechanism - PowerPoint PPT Presentation
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Crash Course in Stellar Pulsation
Ryan MaderakA540
April 27, 2005
Mechanisms mechanism
Compression of partial ionization zones -> ionization -> small change in T
T3.5, increase -> increase mechanism
Heat flow into partial ionization zone from higher temperature layers
So, compression -> higher -> energy buildup -> energy release -> expansion
Mechanisms mechanism
Compression -> higher T -> higher energy production rate -> expansion
stochastic excitation convective turbulence -> acoustic noise ->
solar-type oscillations oscillatory convection
convective + g-mode in rotating stars -> oscillatory modes
tidal interaction periodic fluid motion -> non-radial modes
HR Diagram
Gautschy & Saio, 1995
Main Sequence Solar-type stars
solar-type oscillations expected more precise photometry needed
~mag greatest amp. at ~1.5 MSun
Main Sequence roAp = rapidly oscillating Ap stars
P = 5-15 min, multi-periodic, ~50 mmag ~2 MSun
magnetically modulated rotational splitting overlap with Scuti instability strip, but
excitation mechanism uncertain in He II zone suppressed by diffusion of He convection + B ? in Si IV zone?
Main Sequence
Gautschy & Saio, 1996
Main Sequence Scuti
P = 0.01-0.2 days, 0.003 to 0.9 mag, multi-periodic (up to 12 modes observed)
1.5 – 2.5 Msun, A0 – F5 IV - V, disk population non-radial p-modes, driven by in He II zone amp. limited by coupling between p and g
modes “stable” stars observed within Scuti
instability strip suspected to be very low amplitude variables more precise photometry needed
Main Sequence Scuti
http://users.skynet.be/bho/deltascutis.htm
Main Sequence Slowly Pulsating B Stars (SPB)
P = 1 – 3 days, low amp., multi-periodic
2.5 – 5 Msun, B3 – B8 IV driven g-modes can be thought of as an extension of
the Cephei instability to longer periods
Main Sequence Cephei
P = 0.1 – 0.6 days, 0.01 – 0.3 mag majority multi-periodic, a few non-radial
7 – 8 Msun, O8 – O6 p-modes, driven by in the “z-bump” metalicity dependent pulsational stability
Cep strip extends farther blue-ward for higher metalicity stars
Cep-type variability appears in at least a few cases to be transient
Spica exhibited Cep variability from ~1890 to 1972
Main Sequence Cephei
http://www.aavso.org/vstar/vsots/winter05.shtml
Main Sequence Be stars
exhibit photometric and line profile variability with periods of <1 day
found within the Cep/SPB instability region -> “z-bump” driving
MS 60 – 120 Msun
models suggest driving from CNO burning driving may be one of the factors which
determines the high mass cutoff of the MS
Horizontal Branch RR Lyrae
P = 0.3 – 1.2 days, 0.2 – 2 mag < 0.75 Msun, A – F, prominent in globular clusters driven, but convective flux is thought to be
important important standard candles for clusters, but the
P-L relationship is metalicity dependent the period decreases as cluster metalicity increases
(for fixed Teff) careful calibration and stellar evolution models needed
Horizontal Branch RR Lyrae
http://www.dur.ac.uk/john.lucey/astrolab/pulsating.html
Horizontal Branch RR Lyrae
RRab: asymmetric light curves, longer periods, higher amp.
RRc: nearly sinusoidal light curves, shorter periods, lower amp.
RRd: bi-periodic RRab’s exhibit a periodic change in
light curve shape and amp. -> “Blazhko” effect
coupling between B and rotation?
Horizontal Branch P-L Relation
http://zebu.uoregon.edu/~soper/MilkyWay/cepheid.html
Horizontal Branch “Classical” Cepheids
P = 1 – 135 days, ~0.01 – 2 mag > 4 – 5 MSun, F at maximum light, G -
K at minimum light stars above 4 – 5 MSun pass through
the instability strip during each of one or more blue loops
for ~4 MSun -> bi-periodic cepheid
Horizontal Branch Classical Cepheid
http://www.astronomynotes.com/ismnotes/s5.htm
Horizontal Branch “Classical” Cepheids
masses from evolution versus pulsation theories did not agree historically, but improved opacities solved the problem
but pulsational models using the improved values give periods that are metalicity dependent
careful abundance measurements are needed to use the P-L relationship accurately
AGB W Virginis (Population II Cepheids)
P = 0.8 – 35 days, 0.3 – 1.2 mag M ~ 0.5 MSun
cross instability strip in late HB or early AGB evolution
fundamental or 1st harmonic, driven by He II and H/He I zones
instability strip is wider for metal poor stars
AGB W Virginis
http://www.astronomynotes.com/ismnotes/s5.htm
AGB RV Tau
P = 30 – 150 days, 1.5 – 2 mag M = 0.5 – 0.7 MSun, F – G at maximum light,
K – M at minimum light driven by H and He I zones characteristic “double peak” pattern
resonances between fundamental and 1st harmonic
chaotic motion of multiple atmospheric layers low-dimensional chaotic attractors
AGB RV Tauri
AGB RV Tau
various irregularities change in depth of primary and secondary minima changes in period
relatively few known ~130 (GCVS) duration of phase only ~500yr believed to be post-AGB/proto-planetary
have experienced significant mass loss RVb: long term (600 – 1500 day) variation in
mean brightness eclipsing binary? episodic mass loss? dust shell
eclipse?
AGB Mira
P = 80 – 1000 days, 2.5 – 11 mag low-mass, Me – Se First variable discovered: 1595 fundamental, driven by H and He I
zones coupling between pulsation and
convection
AGB Mira
AGB Semi-Regular
P = 20 – 2000+, ~0.01 – 2 mag, multi-periodic
occupy same part of HR diagram as Mira’s – physically similar
distinguished by amplitude difference due to mass, composition, age
SRb: power spectra exhibit broadened mode-envelopes
stochastic excitation?
AGB Semi-Regular
Planetary Nebula PG1159 (variable planetary nebula
nuclei = PNNV) P = 7 – 30 min g-modes, driven by C and/or O K-shell
ionization Teff = 70000 – 170000, strong C, He,
and O features
Cooling Track DB-type variable WD (DBV)
P = 140 – 1000 seconds, non-radial M ~ 0.6 MSun, Teff = 21500 – 24000 g-modes, driven by He II zone complicated power spectra
need high time resolution and long data sets to resolve peaks -> WET
Cooling Track ZZ Ceti (DA-type variable WD)
Similar to DBV g-modes may be driven by ionization
of a surface H layer lower Teff -> blue edge of instability
~13000K H rich, with almost no He or metals
Future Work Larger samples of Cepheids and RR Lyrae’s
---> more accurate determination of metalicity dependence of P-L
Continued high time resolution, long duration astroseismology -> better understanding of interior structure and excitation mechanisms
Better theory of convection -> better understanding of coupling between convection and pulsation
References Carrol, B.W., & Ostlie, D.A. 1996, “An
Introduction to Modern Astrophysics,” Addison-Wesley, Reading, MA.
Gautschy, A., & Saio, H. 1995, ARA&A, 34, 551.
Gautschy, A., & Saio, H. 1996, ARA&A, 33, 75.
“GCVS Variability Types.” http://www.sai.msu.su/groups/cluster/gcvs/gcvs/iii/vartype.txt