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Supernovae

Supernovae Explosions

This is a small research project suitable for undergraduate Astronomy and Physics students. It was put together by Helmut Jerjen from the Research School of Astronomy and Astrophysics (ANU). Please send me an e-mail if you are planning to do this project. I would be delighted to hear more about your own results. Enjoy and have fun!
INTRODUCTION

Supernovae physics (from Wikipedia): A supernova is a stellar explosion that creates an extremely luminous object that is initially made of plasma ionized form of matter. A supernova may briefly out-shine its entire host galaxy before fading from view over several weeks or months. During this brief period of time, the supernova radiates as much energy as the Sun would emit over about 10 billion years. The explosion expels much or all of a star's material at a velocity of up to a tenth the speed of light, driving a shock wave into the surrounding interstellar gas. This shock wave sweeps up an expanding shell of gas and dust called a supernova remnant.

Most important types:
Supernovae Ia (from Wikipedia): A Type Ia supernova is a sub-category of cataclysmic variable stars that results from the violent explosion of a white dwarf star. A white dwarf is the remnant of a star that has completed its normal life cycle and has ceased nuclear fusion.
Supernovae II (from Wikipedia): Type II supernova, or core-collapse supernova, is a sub-category of cataclysmic variable stars that results from the internal collapse and violent explosion of a massive star. Stars must have at least 9 times the mass of the Sun in order to undergo a core-collapse.

METHOD

Your colleague, Dr Patrick Tisserand, from the European Southern Observatory requests your help again. Working on a time series of images in the Large Magellanic Cloud he detected a flaring star, possibly a supernova located in a distant galaxy behind the LMC. He asks you to produce and analyse the light curve of this star to shed light on its nature.

Download the 455 thumbnail CCD images from here .

Check one of the image headers using IRAF to find out what the coordinates of the star are. Then use the NASA Extragalactic Database to find the name of the host galaxy.

The name of each CCD image reflects the Julian date (actually JD-2450000) when it was taken. Record the accurate date/time (in seconds) for each image. Use the IRAF command imexam to measure the flux of the star (I_*) on each CCD image. Important: because you are doing photometry of a star with a unresolved galaxy as background, you need to set the radius of the aperture for the photometry in a way that you can avoid measuring fluxes from the galaxy. Does it matter if you only measure a fraction of the light from the star to avoid neighbouring stars? Explain. Once you decided on a suitable aperture radius, go and edit the "rimexam" parameter file by writing in IRAF "epar rimexam" -> change parameters if necessary -> quit with ":q". Record the flux of the star using the aperture of your choice and compute the instrumental magnitude in all 455 frames [m_* = -2.5 Log(I_*)] and plot the results vs Julian date. How does the light curve look?

Something must be wrong and you have an idea what the problem could be. As the CCD images were obtained under all sorts of atmospheric conditions not all the data is photometric. Some of the measured fluxes must have been affected by cirrus and clouds.

You can solve this problem by comparing the flux from the star with that of another star in the same field. Choose a second (bright) star in the field that is NOT variable. The flux from that star, I_CS, shall serve in the following as zero point for the differential photometry.

Go back and record the flux of the control star using the same aperture and compute the instrumental magnitudes in all frames [m_CS = -2.5 Log(I_CS)].

Plot m_*-m_CS as a function of time. How does the light curve of the SN candiate look now when compared with the control star? Determine the peak magnitude of the light curve and compare the light curve with those of known supernovae. Is the star a supernova and if so what type?

In the meantime, Patrick photometrically calibrated the first CCD image (sn_0_b-317.8389.fits). He informed you that the star at the postion x=112, y=13 has a B-band magnitude of 17.56+-0.03. Use this information to calibrate the time series of the star. Use M_B(max)=-19.36+/-0.18 mag to estimate the distance of the host galaxy. Is this galaxy already known?

Discuss your results within the context of other people's findings reported on the internet and on ADS by the professional astronomers. What are effects not taken into account in our experiment (2nd and 3rd order effects) and how could the experiment be improved? Try to make a gif movie of the SN explosion like the movie shown here .


The data used for this project have been kindly provided by Dr Patrick Tisserand (RSAA).
Images and movies credits: Dr Patrick Tisserand (RSAA); Hubble Space Telescope, NASA; NASA/CXC/Rutgers/J.Warren & J.Hughes et al.
Comments and feedback: jerjen@mso.anu.edu.au
Last update: July, 2007