Transcript The Hydrogen Ionization Front-Photosphere

The Hydrogen Ionization
Front-Photosphere
interaction and the PeriodColor relations of classical
variable stars.
Shashi M. Kanbur,
SUNY Oswego
Collaborators
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Chow Choong Ngeow
D. Leonard, N. Tanvir, M. Hendry
L. Macri, T. Barnes, S. Nikolaev
A. Nanthakumar, C. Koen
G. Feiden, D. Crain, R. Stevens, C.
Phelps, D. Wallace, J. Young, S.
Scott.
The Hydrogen Ionization front
(HIF).
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Region of rapid temperature change near
the surface of a star where the
temperature is changing and hence
hydrogen is ionizing.
Together with this there is a very sharp
rise in opacity.
Stellar photosphere is defined as the
location where optical depth = 2/3.
HIF and photosphere not comoving as star
pulsates.
The HIF-photosphere interaction
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In certain situations, the photosphere can
lie at the base of the HIF.
Further movement in very hard due to
opacity wall.
Then the temperature of the photosphere
is very close to the temperature at which
Hydrogen ionizes.
In this situation, the color of the star is
the temperature at which Hydrogen
ionizes.
The HIF-photosphere interaction
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Saha ionization equation used in stellar
pulsation models.
Temperature at which Hydrogen ionizes is
somewhat independent of density for low
densities.
Thus, when the HIF-photosphere are
engaged, temperature of stellar
photosphere is somewhat independent of
global stellar properties, such as period, at
low densities.
This can lead to changes in the periodcolor relation.
The Period-Color Relation
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Because the photosphere and HIF are
either engaged or not, such changes can
be sudden.
Only occurs when the interaction is at low
densities.
Because the HIF lies further in the mass
distribution as the L/M ratio changes, the
nature and extent of the HIF-photosphere
interaction changes with period and
metallicity and pulsation phase.
Period-Color Relations in Cepheids
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Galactic Cepheids obey a flat PC relation at
maximum light.
LMC/SMC Cepheids obey a flat PC relation at
maximum light for Cepheids with periods greater
than 10 days.
In Galactic Cepheids, HIF and photosphere are
only engaged at maximum light.
In SMC/LMC Cepheids, always engaged, but only
at low densities for Cepheids with periods greater
than 10 days.
LMC Cepheids show a disengagement at all
phases for periods greater than 10 days.
SMC
LMC & GAL
Figure 2: The photospheric density (1/V, where V is the specific volume) at maximum (top) and minimum (bottom) light in the theoretical models. The left panel shows the results from the SMC models with two ML relations. The rights panel show the comparison between the LMC
models (open and solid squares) and the Galactic models (crosses). The right panel is adopted from KN.
The Cepheid PL relation and H0
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The Cepheid PL/PC relations are really
from the PLC relation.
Changes in one are reflected in changes in
the other.
Changes in the PC/PL relation at certain
phases have some effect in the mean light
PC/PL relation.
If the LMC Cepheid PL relation is truly
non-linear and linear relation is used,
then…..
The Cepheid PL relations and H0
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Can affect H0 by as much as 2% (Ngeow
and Kanbur 2006).
If the goal is to reduce errors on the
accuracy of H0 estimates to below 5% via
a method independent of CMB (Spergel et
al 2006), then this is important especially
as..
Other work is attempting to reduce zero
point uncertainties (Macri et al…)
Cepheid Pulsation and Evolution
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Equally important to understand this
effect in terms of pulsation physics.
Relation to Hertzpsrung progression?
Why at 10 days?
Period-Color/Amplitude-Color
relations.
logL(max) – logL(min) =
4(logT(max) – 4logT(min))
RR Lyraes
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PC relation at minimum light is flat.
AC relation at maximum light such
that higher amplitude stars are
driven to bluer colors at maximum
light.
PC relation at minimum light used to
estimate reddening.
Could also use AC relations.