Asteroseismology at VUW

Stars exist as a balance between gravity pulling in and the flow of energy created by nuclear fusion in the core pushing out. When a star runs out of nuclear fuel this outward force gradually ceases, and the star will evolve into a small and extremely dense object. Depending on its mass, it will become a white dwarf, neutron star, or black hole. The vast majority of stars (over 97%) are predicted to evolve into white dwarfs.

The forces sustaining a white dwarf are dominated by quantum and relativistic effects. They contain a core about the size of the Earth which can be considered as a classical gas of nuclides (mainly carbon and oxygen) and a quantum gas of electrons. Deep inside the core, many of the electrons will be travelling at speeds approaching that of light. The core is covered with an atmosphere (of similar thickness to our own) containing mainly hydrogen and/or helium.

Most white dwarfs are hot which makes them appear blue (or colloquially, white) compared to other stars, but their small size means that they are intrinsically faint. They slowly cool as they radiate away their internal energy, causing their radiation to shift to longer wavelengths (become redder) over time. Identifying the coolest white dwarfs in our galaxy places important constraints on the maximum age of our galaxy, and therefore the age of the universe.

As they cool, there are certain temperature regimes where the outflowing energy interacts with the hydrogen or helium in the atmosphere in such a way that drives complicated pulsations. The strong gravity at the surface of a white dwarf means that these pulsations are largely non-radial. These pulsations manifest as periodic temperature variations at the surface, made up of many sinusoidal frequencies, which cause the apparant brightness of the star to change on timescales of a few minutes. The spectrum of frequencies within the pulsations are related to the internal structure and composition of the star.

We are able to identify the frequencies that a star pulsates at by measuring its intensity over time and using the Fourier transform to find the equivalent sinusoids that the lightcurve contains. These frequencies can then be used to constrain numerical models of white dwarf interiors to indirectly measure important physical characteristics of the star. Some stars show non-sinusoidal pulsations, which is related to convection within the stellar atmosphere.

The short timescale of the pulsations means that we must continuosly monitor a single star with integrations of tens of seconds. Most white dwarf research is done with ~1m class telescopes, so it is important to have an efficient photometer that makes the most of the limited number of photons received from these faint stars.

We have developed a flexible photometer system for measuring the intensity of variable stars. The photometer integrates an off-the-shelf CCD camera and GPS receiver with custom timing electronics and software for instrument control and data analysis.

Two variants of the instrument exist with different cameras. The original system operated at VUW uses a Princeton Instruments MicroMax CCD, and is used with the 1m telescope at Mt John University Observatory. A second instrument is operated by the White Dwarf group at the University of Texas in Austin. This instrument uses a Princeton Instruments ProEM CCD, and has been used with the 82" (2.1m) and 36" (0.9m) telescopes at McDonald Observatory and the 24" (0.6m) telescope at Meyer Observatory.

The CCDs in both instruments are back-illuminated 1024×1024px frame-transfer CCDs. The back-illumination provides exceptional quantum efficiency in the blue wavelength region emitted by hot white dwarfs, and the frame-transfer operation allows the cameras to be operated without a mechanical shutter, further increasing the photon collecting ability of the system.

Our timing hardware is built around an Atmel AVR microprocessor (similar to those used in the Arduino platform) which interfaces with the external GPS receiver, acquisition PC, and the CCD electronics. Precise exposure timing is available with a 1 millisecond resolution for exposures less than 65 seconds, or 1 second resolution for longer intervals.

The reduction software we have developed, tsreduce, provides real-time analysis of data. It performs differential aperture photometry on a target star plus several comparison stars to compensate for the impact of cloud and other atmospheric effects. The reduction display shows the raw intensities, differential photometry of the target star, an estimate of the atmospheric seeing, and a Fourier Transform of the target pulsations.

We have developed a second AVR-based unit for controlling the intensity of LEDs in a light-tight box, which allows the instrument and reduction software to be tested on the bench. We are currently adapting this work for an undergraduate physics experiment on CCDs and time series photometery.

Our software and harware specifications are open source, hosted on GitHub.
More information on each project is available in the repositories below.

• Puoko-nui (Acquisition software)
• Karaka (Timer unit)
• tsreduce (Reduction software)
• star-simulator (Variable light-source controller)

More information on our research is available from the references below. This section will be expanded as publications in progress are completed.

The Puoko-nui Photometers
Chote, Sullivan, Harrold & Winget 2014 (poster) [PDF]

Puoko-nui: a flexible high-speed photometric system
Chote et al. 2014 (MNRAS) [PDF] [ADS]

Found: the progenitors of AM CVn and supernovae .Ia
Kilic et al. 2014 (MNRAS) [ADS] [arXiv] [Press]

Asteroseismology of Pulsating White Dwarfs
Chote & Sullivan 2013 (poster) [PDF]

Time series photometry of the helium atmosphere pulsating white dwarf EC 04207−4748
Chote et al. 2013 (MNRAS) [ADS]

The Puoko-nui CCD Time-Series Photometer
Chote & Sullivan 2012 (ASPCS) [PDF] [ADS]

HST and Optical Data Reveal White Dwarf Cooling, Spin, and Periodicities in GW Librae 3-4 Years after Outburst
Szkody et al. 2012 (ApJ) [ADS] [arXiv]