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DEM L71: Supernova Origin Revealed
NASA's Chandra X-ray Observatory image of the supernova
remnant DEM L71 reveals a hot inner cloud (aqua) of glowing iron
and silicon surrounded by an outer blast wave. Data from the
Chandra observation show that the central ten-million-degree
Celsius cloud is the remains of a supernova explosion that
destroyed a white dwarf star.
DEM L71 presents a textbook example of the double-shock structure
expected to develop when a star explodes and ejects matter at
high speeds into the surrounding interstellar gas. The expanding
ejecta drive an outward-moving shock wave that races ahead of the
ejecta into the interstellar gas (bright outer rim). The pressure
behind this shock wave drives an inward-moving shock wave that
heats the ejecta, seen as the aqua cloud.
The clear separation of the shocked matter and the heated ejecta
in the Chandra image allowed astronomers to determine the mass
and composition of the ejecta. The computed ejected mass was
found to be comparable to the mass of the Sun. This and the X-ray
spectrum, which exhibits a high concentration of iron atoms
relative to oxygen and silicon, convincingly show that the ejecta
are the remains of an exploded white
dwarf star. The size and temperature of the remnant indicate
that it is several thousand years old.
Astronomers have identified two major types of supernovas: Type
II, in which a massive star explodes; and Type Ia, in which a
white dwarf star explodes because it has pulled too much material
from a nearby companion star onto itself (see illustration
below). If the mass of the white dwarf becomes greater than about
1.4 times the mass of the Sun, it becomes unstable and is blown
apart in a thermonuclear explosion. This was the case in DEM L71.
One of the major goals of the study of supernova remnants is to
determine the type of supernova explosion. The identification of
DEM L71 as the remnant of an exploded white dwarf, or Type Ia
supernova, represents a major step forward in understanding more
about the ways in which stars explode.
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