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