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Observational properties of pulsating subdwarf B stars. Mike Reed Missouri State University

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Observational properties of pulsating subdwarf B stars.

Mike Reed

Missouri State University

With help from many, including Andrzej Baran, Staszek Zola, Michal Siwak, Waldek Ogloza.

Views of 3 pulsating sdB stars

Each with different properties.

We wish to understand them and determine how they resemble other pulsating sdB stars.

Connecting to a larger picture:

What can we learn using Asteroseismology?

*Stellar evolutionary timescales *Cosmochronology *Stratifying of stellarinteriors *Stellar crystallization *Nuclear fusioncross sections *Masses, radii, and luminosities of stars (distance scales and population synthesis) *Diffusive processes *Convection *Neutrinos *Elementary particle physics *Helium flash *radiative levitation *binary evolution *Type I supernovae *Mass exchange and loss *Stellar magnetism *Interstellar enrichment *Electroweak theory *Core/Envelope ratios *semiconvection *Stellar equations of state *Stellar winds *Lollypop to Popsicle ratio.

A Radial Pulsator: l=0

The entire surface changes.

A Nonradial Pulsator: l=1

1 line across the surface.

A Nonradial Pulsator: l=2

2 lines across the surface.

But when many are combined....

It is hard to distinguish the mode.

First Goal:

Determine the spherical harmonics of pulsation frequencies to constrain models.

Mode Identification Methods

Traditional: Frequencies and spacings: Feige 48

Binary interactions: PG1336-018

Feige 48

Observed over several years and from multiple campaigns.

Triplet

Our Model Solution:

Total Mass: 0.4725 Msolar

Shell Mass: 0.0025 Msolar

Teff=29635 K (29,500+/-500)

log g = 5.518 (5.50+/-0.05)

Near core He exhaustion (0.74% by mass)

Predicted a rotation period near 0.4 days, which was detected the following year.

PG1336-018: Observed by WET in 1999 and 2001

- 2.4 hour orbital period.
- Tidal forces are comparable to Coriolis force

- Eclipse Mapping
- Tidal Influence on Pulsations

PG1336-018

An ideal case!

~15 minute eclipses covering ~60% of the pulsator.

- All the in-eclipse modes are new! (Except for 2.)
- But not where we expect them to be from splittings seen in the OoE data.
- Most modes are splittings away from OoE modes.

Results:

PG1336 eclipses do not map pulsations as we expect.

A tipped pulsation axis?

- Tidal forces exceed Coriolis force.
- Pulsation axis will point at companion- similar to roAp stars.
- Orbital motion will precess the pulsation axis, completing one revolution every couple hours.

l=2, m=1

- Number and separation of peaks in the combined FT
- Predictable regions of like phase.
- If divided into regions of like phase, a central peak should show up.

And what did we really see?

Nothing new and/or exciting.

Here is one!

What have we learned?

1 good and 1 mediocre l=1, m=1 identifications.

1 reasonable l=2, m=0 identification.

1 reasonable l=2, m=1 identification.

On to the models for PG1336!

PG0048: An unexpected surprise!

Every night, something new!

Detected a total of 29 frequencies.

But only 1 of them is detected in every good-quality run.

Signatures of stochastic oscillations:

*Highly variable amplitudes.

*Sometimes (or often) damped below detectability

*Combinations of data have reduced amplitudes (because of phase differences)

Simulations of stochastic oscillations

Best fit results for PG0048:

A damping timescale if 4 – 6 hours

and a

re-excitation timescale of 13 – 19 hours.

Results:

Feige 48 solved using traditional methods.

PG1336 shows indications of inclined pulsation axis which can constrain models.

PG0048 shows indications of stochastic oscillations.