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Fig. 1:
Three orbits around the supermassive black hole at the Galactic
Center. The red line shows the orbit of the gas cloud as determined by
the observations, which is interpreted as the inward ejecta of a nova
explosion. The black line shows the orbit of the nova that produced
it, while the blue line shows the orbit of the outward ejecta, which
are interpreted as the 'tail' feature in the observations. The cross
marks the position of the black hole. The line of nodes marks the
intersection of the plane of all three orbits with the plane of sky,
which are highly inclined to each other.
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Fig. 2:
Expanding shell of Nova Cygni 1992, the brightest nova in recent
history. This picture was taken by the Hubble Space Telescope in
1994. It shows a ring-like clumpy structure, which in projection
appears slightly elliptical to the observer.
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It was a big surprise when a group of astronomers at the Max Planck
Institute for Extraterrestrial Physics recently found a gas cloud that
is falling nearly straight towards the central black hole of our
galaxy. Over the next few years, the gas cloud will be completely
disrupted and ultimately parts will be swallowed by the black hole,
resulting in largely increased X-ray emission. While this will tell
the scientists more about the processes in the very close surroundings
of the Galactic Center, the detection also immediately raised the
question of the origin of this strange phenomenon. Right away several
suggestions were made. Now scientists at MPA propose an interesting
new model: The cloud could be the shell of gas ejected at high
velocity by a nova that exploded some ten years ago.
Novae are a well-known phenomenon in close binary systems where a very
compact white dwarf star accretes matter from an ordinary low-mass
companion star. Because of the high gravity at the white dwarf surface
the material collecting there becomes strongly compressed and
extremely hot. Once a critical amount of matter has been accumulated,
a thermonuclear explosion sets in that expels a large part of this
envelope, expanding with high velocity. Nova shells appear in
different shapes, some more spherical, others more ring-like. Fig. 2
shows an example of a ring-like nova, Nova Cygni 1992, in visible
light, two years after its explosion.
The scientists noticed that the properties of the observed gas
cloud, in particular its mass (about 1/100000 solar mass), its speed
(on the order of 1000km/s), and the appearance of dust are in
surprising agreement with those of observed nova shells. The observers
had derived these properties analyzing detailed long-wavelength
infrared images. The observations of the cloud show two bright parts,
a 'head' and a 'tail', with more diffuse brightness in between, where
the part closer to the black hole is already influenced by the
increasing gravity. Now, a ring-like shell of tenuous radiating gas,
seen nearly edge-on, appears as a strongly squeezed ellipse and then
shows two bright outer edges because there our line of sight passes
through the largest column density of gas. This can indeed explain the
two structures, head and tail, observed in the expanding cloud.
The scientists at MPA calculated orbits of ejecta from a nova
explosion to investigate how such a ring structure will appear to the
observer. They took an explosion around the year 2000 and a typical
expansion velocity of 500 km/s, which corresponds to the presently
observed separation of head and tail. As the ejecta have differently
directed velocities the different parts of the ring move on different
orbits around the central black hole. The projection of these parts
on the sky traces the observed motions of the gas cloud. Fig. 1 shows
the orbits of the two brightest visible parts. The cloud (the
prominent head feature) is interpreted as the inward ejecta of the
nova. The outward ejecta of the nova move on an orbit of lower
eccentricity whose projection on the plane of the sky yields the tail
structure.
The peculiar extreme eccentricity of the observed cloud orbit would
seem to require some explanation, but here in the nova model it
naturally results from the addition of two differently directed
velocities, that of the nova system itself and that of the inward
ejecta.
The surprising agreement between several basic parameters of a nova
explosion and the observed cloud appears to strongly hint to a
nova origin. If true, this raises important questions about
stellar evolution close to the Galactic Center, as the occurrence of
such a low-mass binary system bears witness of a stellar population
much older than that of the young luminous stars presently observed
there.
F. Meyer, E. Meyer-Hofmeister
Original publication:
Meyer, F., Meyer-Hofmeister, E.,
"A nova origin of the gas cloud at the Galactic Center?",
A&A 546, L2, 2012
Link:
Galactic Black Hole disrupts Gas Cloud,
MPE Press release December 14, 2011
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