A cloud of gas falling towards the central black hole in the Milky Way. Jordi Miralda Escudé ICREA, Institut de Ciències del Cosmos University of Barcelona /IEEC. 26-6-2012

The observed properties of the cloud, Gillessen et al. Over the last 10 years, observations have revealed an object approaching Sgr A*, with infrared L-band luminosity 5 L, temperature 550 K, and line emission in Brγ and HeI lines, moving in mid 2011 at 1700 km/s and 1000 AU from the black hole. It is a dusty gas cloud with density ne =105.5 cm-3 and mass 10-5 M. It can be photoionized by the known radiation from O and B stars in the disk of young stars. It is resolved, with a size ~ 100 AU, and a tail following behind. The orbit has been measured: the period is 137 years, highly eccentric (e=0.94), the apocenter was at 8000 AU, the pericenter will be at 250 AU, in summer 2013! Self-gravity is negligible, the cloud must be pressure-confined by the hot external medium and kept at T~104 K by photoionization; ram-pressure and tides are shearing and shocking it. Its evolution will become a probe to the accreting flow medium of Sgr A*.

Ideas for the origin of the cloud Burkert et al. propose the cloud originated from the collision of stellar winds from the known O stars. Problems: no good candidate winds, unclear that a pressure-confined cloud can form and reach the observed position without disrupting, unexplained origin of dust. Murray-Clay & Loeb proposed the cloud accompanies a low-mass star that formed in the star disk in the recent starburst, and has a circumstellar disk that is being photoevaporated. Problems: the star cannot be scattered into the observed highly eccentric orbit without destroying the disk. Alternative idea: a low-mass star flew by a stellar black hole, and a disk was formed from the tidal debris around the star. The disk absorbed angular momentum from the star and expanded, and is now giving rise to a photoevaporation wind which produces the observed cloud. The cloud is being recreated after each peribothron passage.

The collision or flyby hypothesis Many stellar black holes should be present in the nuclear region of the Milky Way, owing to mass segregation from a ~ 10 pc region: ~ 5000 within 0.2 pc, ~ 500 within 0.04 pc or 8000 AU. If a ~ M star flies by a stellar black hole within ~ 2R at ~ 1000 km/s, the tidal effect can pull out the stellar envelope. Some material flies out unbound and some more falls back and makes a small disk around the star. The rate of these collisions can be up to 1 every ~ 106 years. This yields a very small probability for producing the cloud during a flyby on the present orbit, but a reasonable probability if the resulting star with a small disk can produce a cloud over many orbits (e.g. ~ 103 orbits with a period ~ 130 years).

The collision or flyby hypothesis: 4 basic conditions The cloud must be formed from a slow wind: v ≈ (30 AU)/(50 yr) ≈ 4 km/s The tidal encounter must form a disk of 10-3 to 10-2 M and must spin up the star, and then the star must transfer angular momentum to the disk, and the disk must expand, which reduces the wind speed it can generate. The disk is tidally truncated at ≈ 1 AU at every peribothron. The mass loss from photoevaporation needs to occur from a region of size 5 to 10 AU. Perhaps a small fraction of gas can be ejected from the disk at every peribothron, giving rise to gas streams of ≈ 10-5 M that are maintained out of equilibrium in the region at 5 to 10 AU and photoevaporate at every orbit.

What do we expect in this model? In all models, the cloud will be disrupted in mid 2013 and the evolution of the debris will provide a powerful test of the flow of hot gas around the central black hole. The basic difference with the isolated cloud model is that the central star will move on along its exact Keplerian orbit, and that a new cloud with recombination line emission will reemerge after some time. During the peribothron passage, shocks in the outer disk may produce an infrared flare. Schartmann et al. 2012

Summary The cloud of gas moving towards the Galactic Center, reaching peribothron in a year and a half, provides an important opportunity for studying the accretion flow around Sgr A*. Previous models of an isolated diffuse cloud or a star with a circumstellar disk scattered from the young disk have some problems. A different model: an old star was “tidally harrassed “ by a black hole, and a disk was formed which gradually expanded as it absorbed angular momentum from the star. At every peribothron passage the disk ejects gas streams that are photoevaporated and create a cloud like the observed one.