Andrew Yee[_1_]
May 6th 08, 04:42 PM
Observatoire de Paris
Paris, France
Contact:
Jean-Pierre Luminet
Observatoire de Paris, LUTH and CNRS
Tel: 33 1 45 07 74 23 Fax: 33 1 45 07 79 71
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 flambeed 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 'flambeed 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 flambeed 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
IMAGE CAPTIONS:
[Figure 1:
http://www.obspm.fr/actual/nouvelle/may08/crepe-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.
Paris, France
Contact:
Jean-Pierre Luminet
Observatoire de Paris, LUTH and CNRS
Tel: 33 1 45 07 74 23 Fax: 33 1 45 07 79 71
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 flambeed 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 'flambeed 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 flambeed 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
IMAGE CAPTIONS:
[Figure 1:
http://www.obspm.fr/actual/nouvelle/may08/crepe-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.