The Little Satellite that Could...

Derek Buzasi

Univ. of Washington

&

Eureka Scientific

Introduction

“X-ray vision” for stars A brief history of WIRE Some Closeups

Altair Polaris Alpha Cir Procyon Eclipsing Binaries

Future Prospects

How do we know anything about the internal structure of the Sun?

Coherent motions on the solar surface

Leighton (1960) uses Doppler effect to observe oscillatory motion on the Sun

Amplitudes are ~hundreds of meters/sec Periods are ~5 min (~3 mHz)

1975: Things start to get interesting…

What are these things?

They appear to be acoustic waves traveling in a cavity The Sun is ringing like a bell

The study of these waves is called “helioseismology”

Each unique way in which the Sun can oscillate is called a “mode”

Modes

OK – so what good is all this? (Some real physics at last!)

Each mode has a frequency which is characteristic of some “average” sound speed along its travel path

“Low degree” modes penetrate deeply “High degree” modes sample only the surface

By using information from all of the modes, we can model the inside of the Sun!

Good news! There are more than a million modes

Bad news! There are more than a million modes

We can even map the internal rotation

…and details of internal convection

It would be nice to do this for stars, too…

Unfortunately, asteroseismology is much more difficult

Less light (and atmospheric interference) Unresolved sources

But astronomers are trying!

A little about stars…

What we’d like to know mass, age, composition, rotation rate, internal structure, activity, etc.

Ways to find out cluster stars (common origin) binary stars (interact via gravity) field stars (help!)

Wider applications? distance scale, nucleosynthesis, etc.

Hertzsprung-Russell (HR) Diagram

Remember this?

Technical Approaches to Asteroseismology

Two Basic Approaches

1. Look for tiny Doppler shifts (~10 cm/s, or parts per billion) in spectral lines from the ground, where we have big telescopes.

2. Look for tiny variations (parts per million) in the stellar luminosity from space, where the atmosphere isn’t a problem.

Other Uses for ultra-high-precision photometry

Rotational Modulation Granulation (surface signature of

convection) Eclipses & transits

WIRE = Wide-Field Infrared Explorer

1994: Selected by NASA; IR mission designed to study extragalactic star formation for 4 mos1999: Pegasus launch on 4 Mar 19991999: Primary mission failure on 8 Mar 19991999: Conversion to asteroseismology mission begins 30 Apr 1999May 1999 – Sept 2000: Epoch 1Dec 2003 – 23 Oct 2006: Epoch 2Launched 4 Mar 1999; failed 8 Mar 1999

The Star Tracker

• Ball Aerospace CT-601• 52 mm aperture• 512 512 SITe CCD

•7.8° 7.8° field (1 arcmin/pixel)•16-bit ADC•Gain 15e-/ADU

Epoch I

Original mission was 30 April 1999 - 30 September 2000

28 asteroseismology targets; 10 additional targets

Primary targets only Mission termination due to

lack of funding

Epoch II

Mission restart New flight software

included field rotation, making secondary targets usable

Selected Accomplishments

Altair What don’t we know about the brightest star in the northern

sky? Polaris

New insights into an old favorite Alpha Cir

Interaction of rotation and oscillations Procyon

Granulation in a solar-like star Eclipsing Binaries

Old wine in new bottles

Altair

Brightest star in the northern sky Part of the “summer triangle” Sometimes used as a flux standard! WIRE observed for 22 days...

Each “frame” represents a 2-day window

Overall envelope of variability is ~2 ppt

Largest peak is at 15.768 d-

1; amplitude 0.42 mmag

Altair is the brightest δ Scuti star!

Frequency units are “cycles/day” – 15.76 c/d corresponds to roughly 1.5 hours.

9 total modes detected; f1 is easily identified as the fundamental but other IDs are less clear due to the extremely rapid rotation of the star.

HR diagram showing selected models.

The two evolutionary tracks depicted correspond to 1.70 solar masses and v = 150 km/s (solid line), and 1.75 solar masses and v = 200 km/s (dashed line).

The cross indicates observations.

Ages in the shaded area range from 500 to 750 Myr

Polaris

“North Star” Closest and brightest Cepheid

Amplitude has been dropping for decades No longer detectable as variable from the ground

Observed simultaneously using multiple telescopes WIRE SMEI AST

Polaris: Data Quality

Oscillation timing is changing: stellar evolution in real time!

Alpha Circinis

Member of class rapidly oscillating peculiar A-type (roAp) stars First detected in 1970s with ground-based telescopes Typical periods are a few minutes Typical amplitudes are a few parts per thousand or less

Alpha Cir has one well-known mode, with a frequency of 2442 mHz

Combination of ground & space-based observations

Asymmetry in light curve

Amplitude of primary mode is variable: rotation!

Distance Determination via Asteroseismology

Procyon

F5 IV (V ~ 0, so one of the brightest stars in the sky) Historically considered one of the best possible stellar targets for

asteroseismology Higher mass, more evolved star is expected to have larger oscillation

amplitudes based on theory WIRE analysis by Bruntt, Kjeldsen, Buzasi, Bedding

Two time series totalling ~19 days

The light curves of Procyon as seen from WIRE in September 1999 (top) and September 2000 (bottom). Data affected by scattered light have been removed and the correlation with FWHM has been removed. In each panel, only every fifth data point is plotted.

Comparisons: WIRE x 2, MOST, Solar

The plot shows the smoothed power density spectrum for each data set. PDSs permit direct comparison of different time series, since they take into account the different lengths and resolutions of the data sets.

The VIRGO data represent the solar PDS as viewed from space. Note that hydrodynamic models predict Procyon to have somewhat greater granulation “noise” than the Sun does.

The excess power in the MOST data appears to be due to an as yet not understood noise source.

The granulation timescale and granulation PDs are 750 s and 18 ppm2/ µHz, while the amplitude of the p-modes are 5, 10, 15 ppm.

The four panels show the power density spectrum of the WIRE 2000 time series along with different simulations. Each simulation is the mean of five simulations with different seed numbers. The hatched regions show the 1-σ variation for selected simulations.

Simulations for two different white noise levels

Simulations have timescales of the granulation of 250, 750, and 1250 s.

The timescale of the granulation is 750 s but the granulation power densities (PDs) are 10, 18, and 64 ppm2/ µHz.

Best-fit models

WIRE spectra are marked by open box symbols

The 1-σ variation of the simulations is shown by the hatched region.

WIRE 1999 and 2000 results: Procyon

Eclipsing Binaries

A classic astronomer’s tool Orbital timing gives stellar masses Lengths of eclipses give relative stellar radii Depths of eclipses give relative stellar

temperatures Shapes of eclipse light curves give

atmospheric structure

Eclipsing Binaries with WIRE

Good (ground-based TT Aur)…

Better (WIRE, Eri)

Best! (WIRE, Aur)

But wait, there’s more…

Where do we go from here?

The future of asteroseismology lies in space!

END

Fourier Transforms Are Your Friends…(Really!)

Things get worse: here’s a signal-to-noise ratio of 10…

And a signal-to-noise ratio of 0.01…

Real data look like this!

Amazing Appearing Modes!