Supernova Patrol

Arkansas Academy of Sciences Meeting, 1999 April 2-3, Arkansas Tech University

A SUPERNOVA PATROL PROGRAM

R.Tut Campbell and Jeff Robertson, Physical Science Department, Arkansas Tech University, Russellville AR, 72801-2222, Jeff.Robertson@mail.atu.edu.

ABSTRACT

A Meade LX200 12-inch telescope equipped with an SBIG ST-6 CCD camera is being used to systematically monitor some 200+ spiral galaxies that lie less than 300 million light years from our Milky Way, in hopes of capturing the rise in light from a supernova outburst. Supernovae (of type-Ia) are important astrophysical objects
because of their use as absolute distance indicators.

INTRODUCTION

Type Ia supernovae are thought to be interacting binary star systems in which a white dwarf star is gaining mass via an accretion disk from a companion. As the white dwarf accretes more and more matter and goes over the Chandrasaekar limit (1.4 solar masses) for a degenerate electron gas, a thermonuclear explosion results. Since this
critical limit of 1.4 solar masses consistently sets the amount of material in the blast, these types of supernovae are more useful as standard candles than other types. Early detection of new supernovae is important because of the need to get vital spectroscopic information for type classification while the system is still bright and spectroscopy still possible.

Equally as important is the observation of the outburst maximum light, which gives us information about its distance given the assumption of a standard candle (1.4 solar mass explosion). Assuming the luminosity can be established for the type Ia supernovae, its absolute magnitude is then known. Observation of its apparent magnitude yields the distance to it and the galaxy in which it resides by the distance modulus, m - M = 5 log d - 5, where m and M are the apparent and absolute magnitudes, and d is the distance in parsecs to the supernova and host galaxy.
Supernovae are bright enough to be observed at cosmologically important distances, thereby increasing their significance in the search for those parameters governing the expansion rate of the universe (i.e. Hubble constant).

Earliest possible observations are desired as the supernova fades from maximum light during the weeks after the outburst. Early detection of supernovae candidates requires vigilance, patience, considerable skill and stamina. The appearance of a supernova in any one galaxy is an extremely rare occurrence, but by monitoring many galaxies, the likelihood of discovery increases. Also, observing efficiency at the telescope is paramount because of this reason. Observation of a hundred or so galaxies every clear night is demanding, unglamorous work and has been all but
been abandoned by professional astronomers hoping that someone else will rise to the occasion. Most new supernovae discoveries are actually made by amateurs who have a significant amount of skill and are armed with new sophisticated but inexpensive CCD imaging cameras.

PROGRAM

We are carrying out a successful supernovae patrol program here at Arkansas Tech University. We initiated the monitoring program specifically to identify new extragalactic supernovae hopefully pre-maximum light. A Meade 12" LX-200 telescope and a SBIG ST6CCD imaging camera is used by student R. Tut Campbell and mentor Dr. Jeff Robertson. The use of this automated telescope is important as it increases the number of targets that can be imaged in a single night. The use of the CCD camera is equally as important so as to increase the number of faint targets that are usually left untouched by visual observers.

The first phase involved imaging of a few hundred galaxies in order to use these star fields as templates. The images of the galaxy and surrounding star field must be transformed into transparent overlays to be used on site at the telescope. Later, an image of a candidate galaxy is taken and upon CCD readout, the overlay is placed on top of the computer screen to check for any "new"stars in or near the galaxy that are not in the template. In this way, rapid identificationcan take place without the need for exhaustive photometry and data analysis, or relying on memorization of many star fields. An announcement can be initiated over the internet to the Super-Nova-Web, Variable Star Network, or the American Association of Variable Star Observers for confirmation. Successful confirmation is usually followed up quite rapidly with spectroscopic identification from professionals at major observatories and some minimal fan-fair for the discoverer(s).

There are more than enough galaxies accessible by the ATU telescope to ensure some degree of success. During several weeks of testing the telescope we imaged some prospective galaxy candidates for the preparation of the galaxy star field templates and to evaluate the characteristics of the CCD imaging camera. Two of these galaxies were later reported to have newly identified supernovae. Upon retro-inspection, our images clearly show the candidate supernovae! These most certainly would have been identified far in advance if our patrol program had been in operation with the template overlays in place. Needless to say we are quite optimistic about the probable success of our continued supernovae monitoring program. An example of a host galaxy and a new supernova with its corresponding overlay image template for easy discrimination is shown in Figure 1 below.

FIGURE 1:

We would like to acknowledge Arkansas Tech University, the Arkansas Academy of Sciences and the Arkansas Space Grant Consortium.

HOW SUPERNOVAS FORM

Although many stars like our sun can remain stable for billions of years, more massive stars can race through their entire life cycles in a relatively short 10 million years or so, ending in a cataclysmic explosion called a supernova that literally tears the aging star apart. A supernova remnant is the expanding gaseous nebula created by these titanic explosions. Supernova remnants are of interest to many areas of astrophysics.

Young supernova remnants (less than a few thousand years after the SN explosion) still show large enrichments from the heavy elements that were created deep inside the star before it exploded. Observations of the light from these objects provides our only direct test of the process of nucleosynthesis, whereby lighter elements are fused into heavier elements in the centers of stars.

Some supernova remnants remain visible for tens of thousands of years before finally blending back into the general interstellar medium, or tenuous gas in the regions between the stars.

As a supernova shock wave travels outward from the point of the explosion, it sweeps up the gas of the interstellar medium into a roughly spherical, expanding shell. This gas "glows" because of heating from particle collisions. The light given off from these objects spans the entire electromagnetic spectrum from X-rays and ultraviolet light through the optical, infrared and radio. Analysis of this light provides information on the chemical composition and physical conditions (temperature, density, and variations thereof) in these normally invisible regions of interstellar space. Hence, supernova remnants can be used as probes of the structure of the interstellar medium, as well as providing information on the physics of shock waves in conditions much different from those that can be created in a laboratory on earth.

Shock waves from supernovas play an important role in energizing the interstellar medium and by compressing clouds of gas and dust in the interstellar medium may be responsible for starting new cycles of star formation.

Article by William P. Blair, taken from his webpage at
http://fuse.pha.jhu.edu/~wpb/