Transcript Lecture 17: Black Holes

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; rampressure 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
1.
2.
3.
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.