Big Black Holes Cook flambéed Stellar Pancakes

Observatoire de Paris
Paris, France

Matthieu Brassart
Observatoire de Paris, LUTH and CNRS
Max-Planck-Institut fur Astrophysik, Garching
Tel: 33 1 45 07 78 91 Fax: 33 1 45 07 79 71

1 May 2008

*Big Black Holes Cook Flambéed Stellar Pancakes*

According to two astrophysicists from Paris Observatory, the fate of
stars that venture too close to massive black holes could be even more
violent than previously believed. Not only are they crushed by the black
hole's huge gravity, but the process can also trigger a nuclear
explosion that tears the star apart from within. In addition, shock
waves in the pancake star carry a brief and very high peak of
temperature outwards, that could give rise to a new type of X-ray or
gamma-ray bursts.

Scientists have long understood that massive black holes lurking in
galactic nuclei and weighing millions of Suns can disrupt stars that
come too close. Due to intense tidal forces, the black hole's gravity
pulls harder on the nearest part of the star, an imbalance that pulls
the star apart over a period of hours, once it gets inside the so-called
"tidal radius".

Now, Matthieu Brassart and Jean-Pierre Luminet of the Observatoire de
Paris (section of Meudon), France, say the strain of these tidal forces
can also trigger a nuclear explosion powerful enough to destroy the star
from within. They carried out computer simulations of the final moments
of such an unfortunate star's life, as it penetrates deeply into the
tidal field of a massive black hole.

When the star gets close enough the black hole (without falling into),
the tidal forces flatten it into a pancake shape. Previous studies
already performed by Luminet and collaborators twenty years ago had
suggested this flattening would increase the density and temperature
inside the star enough to trigger intense nuclear reactions that would
tear it apart. But other studies had suggested that the picture would be
complicated by shock waves generated during the flattening process, and
that no nuclear explosion should occur.

The new simulations investigate the effects of shock waves in detail,
and find that even when their effects are included, the conditions
favour a nuclear explosion which will completely destroy the star, and
which will be powerful enough to hurl much of the star's matter out of
the black hole's reach.

*Stellar Fireworks*

The tidal disruption of stars by black holes may already have been
observed by X-ray telescopes such as GALEX, XMM and Chandra, although at
a much later stage: several months after the event that rips the star
apart, its matter starts swirling into the hole, heats up and releases
ultraviolet light and X-rays. However, if pancake stars really do
explode, then they could in principle allow these events to be detected
at a much earlier stage. Future observatories, such as the Large
Synoptic Survey Telescope (LSST), which will detect large numbers of
supernovae, could turn up some explosions of this type.

But this might be not the only hazard facing the doomed star. Brassart
and Luminet calculated that the shock waves inside the stellar pancake
carry a brief ( 0.1 s) but very high (above 10**9 K) peak of
temperature outwards from the centre to the surface of the star. This
last result is very promising since it could give rise to a new type of
X-ray or gamma-ray burst, making it possible to see the disruption of
the star immediately if it gets hot enough.

The rate of such 'flambéed pancake stars' is estimated to about 10**-5
event per galaxy. Since almost every galaxy -- including our own Milky
Way -- harbors a massive black hole in its centre, and since the
universe is transparent to hard X and gamma radiation, several events of
this kind per year should be detectable within the full observable universe.

*Conclusion*

The planned high-energy, all-sky surveys are the best suited to detect
more flares of this type because of their large sky coverage. By
providing a quick localization of flambéed stellar pancakes, followed by
the detection of the corresponding afterglows in the optical, infrared,
and radio bands, these missions could bring as much to the understanding
of stellar disruptions by black holes as the Beppo-Sax and Swift
telescopes did for the comprehension of gamma-ray bursts.

*References*

[1] Shock Waves in Tidally Compressed Stars by Massive Black Holes, M.
Brassart & J.-P. Luminet, Astron. Astrophys. 481 (2008) 259-277

[2] For a popular account of tidal disruption and massive black holes, see
also J.-P. Luminet, Le destin de l'univers: trous noirs et energie sombre,
Fayard (Paris, 2006), chap. 21

*Images*

[Figure 1: http://www.obspm.fr/actual/nouvelle/...repe-f1_en.gif (99KB)]

The disruption of a star by the tidal forces of a massive black hole.
The diagram illustrates the progressive deformation of the star when it
plunges deep inside the so-called 'tidal radius' (the size of the star
has been considerably enlarged for clarity).

The upper view shows the deformation of the star in its orbital plane
(seen from above), the middle view shows the deformation in the
perpendicular plane (seen from the side), and the lower view depicts the
magnitude of flattening. From (a) to (d) the tidal forces are weak and
the star remains practically spherical. At (e) the star penetrates the
tidal radius and is doomed to be destroyed. First it become
cigar-shaped, then from (e) to (g) the squeezing of the tidal forces
flattens the star in its orbital plane to the shape of a pancake. Next
the star rebounds, and as it leaves the tidal radius in (h), it starts
to expand. A little further on its orbit the star finally breaks up into
gas fragments. Detailed hydrodynamical simulations taking account of
shock waves have been performed during the crushing phase (e) to (g).
J.-P. Luminet

[Figure 2: http://www.obspm.fr/actual/nouvelle/may08/crepe-f2.gif (18KB)]

Increase in central temperature (in units of initial temperature T* =
10**7 K) for pancake stars penetrating within the tidal radius by
factors respectively 7, 10, 12 and 15. Time is in seconds, t = 0
corresponds to the passage of the star at the closest distance from the
black hole. The maximum central temperature increases as the square of
the penetration factor.

[Figure 3: http://www.obspm.fr/actual/nouvelle/may08/crepe-f3.gif (11KB)]

Evolution of the stellar temperature (in eV ~ 10**4 K) as a function of
time (in seconds) in case of a deep plunging. The solid red line
corresponds to the temperature at the centre of the star. The dashed
line corresponds to the increase in temperature (up to 10**9 K) produced
by the shock wave as it propagates outwards. The duration at half
maximum of the peak of temperature at the shock front is only 0.05 sec.

*Contact*

Jean-Pierre Luminet
Observatoire de Paris, LUTH and CNRS
Tel: 33 1 45 07 74 23 Fax: 33 1 45 07 79 71

On May 6, 9:29*pm, Keith L wrote:
Observatoire de Paris
Paris, France

Matthieu Brassart
Observatoire de Paris, LUTH and CNRS
Max-Planck-Institut fur Astrophysik, Garching
Tel: 33 1 45 07 78 91 Fax: 33 1 45 07 79 71

1 May 2008

*Big Black Holes Cook Flambéed Stellar Pancakes*

According to two astrophysicists from Paris Observatory, the fate of
stars that venture too close to massive black holes could be even more
violent than previously believed. Not only are they crushed by the black
hole's huge gravity, but the process can also trigger a nuclear
explosion that tears the star apart from within. In addition, shock
waves in the pancake star carry a brief and very high peak of
temperature outwards, that could give rise to a new type of X-ray or
gamma-ray bursts.

Such an event may have already been observed, in which the star did
not quite "escape" the tidal zone. See -

http://news.yahoo.com/s/space/200805...tscreamingstar