pulsating variables. cepheid variables discovered by j. goodricke (1784): prototype: cephei light...
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Cepheid Variables:The Period-Luminosity Relation
The variability period of a Cepheid variable is positively correlated with its luminosity:
MV = -2.80 log10d – 1.43
Cepheid Variablesas Distance Indicators
Measuring a Cepheid’s period
determine its absolute magnitude
Distance indicator!
Cepheids are Ib supergiants, L ~ 103 - 4 L0
=> Identifiable out to several Mpc!
The Instability StripClassical Cepheids
W Virginis Stars: metal-deficient
(Pop. II), Cepheid-like
RR Lyrae Stars: Pop. II; horizontal-branch;
nearly standard-candle luminosity!
Scuti Stars: Evolved F stars
near MS
Increasing Period
Stellar Pulsations
Estimate from sound travel time through the star:
-1/2
Cepheids all have approx. the same surface
temperature.
=> Higher L => Larger R => Smaller => Larger
The Valve MechanismNodal zone is opaque and absorbs more radiative flux than necessary to balance
the weight from higher layers. => Expansion
Upon expansion, nodal zone becomes more transparent, absorbs less radiative flux => weight from higher layers pushes it back inward. => Contraction.
Upon compession, nodal zone becomes more opaque again, absorbs more radiative flux than needed for equilibrium => Expansion
Temperature [K]
log
R [
cm-1])
R ~ T-7/2
Kramer’s Opacity Law
104 105 106 107
Gas gradually becoming
ionized
Gas fully ionized; opacity dominated
by free-free absorption
For the valve mechanism to work:
needs to increase with increasing and T
→ Partial Ionization Zones!
Location of Partial Ionization Zones
~ 104 K
~ 4x104 K
Instability strip:
Walve mechanism driven by He partial ionization zones
LPVs:
Valve mechanism driven by H partial ionization zones
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