In biology, "symbiosis" refers to two organisms that live close to
and interact with one another. Astronomers have long studied a class of
stars — called symbiotic stars — that co-exist in a similar way. Using
data from NASA’s Chandra X-ray Observatory and other telescopes, astronomers are gaining a better understanding of how volatile this close stellar relationship can be.
R Aquarii (R Aqr, for short) is one of the best known of the symbiotic stars. Located at a distance of about 710 light years
from Earth, its changes in brightness were first noticed with the naked
eye almost a thousand years ago. Since then, astronomers have studied
this object and determined that R Aqr is not one star, but two: a small,
dense white dwarf and a cool red, giant star.
The red giant star has its own interesting properties. In billions of
years, our Sun will turn into a red giant once it exhausts the hydrogen
nuclear fuel in its core and begins to expand and cool. Most red giants
are placid and calm, but some pulsate with periods between 80 and 1,000
days like the star Mira and undergo large changes in brightness. This subset of red giants is called "Mira variables."
The red giant in R Aqr is a Mira variable and undergoes steady
changes in brightness by a factor of 250 as it pulsates, unlike its
white dwarf companion that does not pulsate. There are other striking
differences between the two stars. The white dwarf is about ten thousand
times brighter than the red giant. The white dwarf has a surface temperature of some 20,000 K
while the Mira variable has a temperature of about 3,000 K. In
addition, the white dwarf is slightly less massive than its companion
but because it is much more compact, its gravitational field is
stronger. The gravitational force of the white dwarf pulls away the
sloughing outer layers of the Mira variable toward the white dwarf and
onto its surface.
Occasionally, enough material will accumulate on the surface of the white dwarf to trigger thermonuclear fusion
of hydrogen. The release of energy from this process can produce a
nova, an asymmetric explosion that blows off the outer layers of the
star at velocities of ten million miles per hour or more, pumping energy
and material into space. An outer ring of material provides clues to
this history of eruptions. Scientists think a nova explosion in the
year 1073 produced this ring.
Evidence for this explosion comes from optical telescope data, from Korean records of a “guest” star at the position of R Aqr in 1073 and information from Antarctic ice cores. An inner ring was generated by an eruption in the early 1770s. Optical data (red) in a new composite image of R Aqr shows the inner ring. The outer ring is about twice as wide as the inner ring, but is too faint to be visible in this image.
Evidence for this explosion comes from optical telescope data, from Korean records of a “guest” star at the position of R Aqr in 1073 and information from Antarctic ice cores. An inner ring was generated by an eruption in the early 1770s. Optical data (red) in a new composite image of R Aqr shows the inner ring. The outer ring is about twice as wide as the inner ring, but is too faint to be visible in this image.
Since shortly after Chandra launched in 1999,
astronomers began using the X-ray telescope to monitor the behavior of R
Aqr, giving them a better understanding of the behavior of R Aqr in
more recent years. Chandra data (blue) in this composite reveal a jet of
X-ray emission that extends to the upper left. The X-rays have likely
been generated by shock waves, similar to sonic booms around supersonic planes, caused by the jet striking surrounding material.
As astronomers have made observations of R Aqr with Chandra over the
years, in 2000, 2003, and 2005, they have seen changes in this jet.
Specifically, blobs of X-ray emission are moving away from the stellar
pair at speeds of about 1.4 million and 1.9 million miles per hour.
Despite travelling at a slower speed than the material ejected by the
nova, the jets encounter little material and do not slow down much. On
the other hand, matter from the nova sweeps up a lot more material and
slows down significantly, explaining why the rings are not much larger
than the jets.
Using the distances of the blobs from the binary, and assuming that
the speeds have remained constant, a team of scientists from the
Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass,
estimated that eruptions in the 1950s and 1980s produced the blobs.
These eruptions were less energetic and not as bright as the nova
explosion in 1073.
In 2007 a team led by Joy Nichols from CfA reported the possible
detection of a new jet in R Aqr using the Chandra data. This implies
that another eruption occurred in the early 2000s. If these less
powerful and poorly understood events repeat about every few decades,
the next one is due within the next 10 years.
Some binary star systems containing white dwarfs have been observed
to produce nova explosions at regular intervals. If R Aqr is one of
these recurrent novas, and the spacing between the 1073 and 1773 events
repeats itself, the next nova explosion should not occur again until the
2470s. During such an event the system may become several hundred times
brighter, making it easily visible to the naked eye, and placing it
among the several dozen brightest stars.
Close monitoring of this stellar couple will be important for trying to understand the nature of their volatile relationship.
Rodolfo ("Rudy") Montez of the Harvard-Smithsonian Center for
Astrophysics (CfA) in Cambridge, Mass, presented these results at the
230th meeting of the American Astronomical Society in Austin, TX. His
co-authors are Margarita Karovska, Joy Nichols, and Vinay Kashyap, all
from CfA.
NASA's Marshall Space Flight Center in Huntsville, Alabama, manages
the Chandra program for NASA’s Science Mission Directorate in
Washington. The Smithsonian Astrophysical Observatory in Cambridge,
Massachusetts, controls Chandra’s science and flight operations.
Fast Facts for R Aquarii:
Scale: Image is 4.2 x 3.4 arcmin (About 0.86 x 0.70 light years).
Category: Normal Stars & Star Clusters, White Dwarfs & Planetary Nebulas
Coordinates (J2000): RA 23h 49m 34.6s | Dec -15° 17' 04"
Constellation: Aquarius
Observation Date: 3 pointings between Sep 2001 and Oct 2005
Observation Time: 34 hours 58 minutes (individual times: 6.3 hours, 18.5 hours, 10.1 hours)
Obs. ID: 651, 4546, 5438
Instrument: ACIS
Color Code: X-ray (Cyan); Optical (Red)
Distance Estimate: About 710 light years
Source: NASA’s Chandra X-ray Observatory
