31 research outputs found
On the quasi-isometric rigidity of graphs of surface groups
Let be a word hyperbolic group with a cyclic JSJ decomposition that
has only rigid vertex groups, which are all fundamental groups of closed
surface groups. We show that any group quasi-isometric to is
abstractly commensurable with .Comment: 54 pages, 10 figures, comments welcom
Equations over hyperbolic groups
We show that the Diophantine problem, for quadratic equations over a non-elementary torsion-free hyperbolic group, is NP-complete. Furthermore, given a finite system of equations over a torsion-free hyperbolic group , there is an algorithm which constructs a canonical -diagram and a complete set of induced -NTQ systems, for . Finally, the class of -limit groups is the same as that of iterated generalized doubles over
One-zone models for spheroidal galaxies with a central supermassive black-hole. Self-regulated Bondi accretion
By means of a one-zone evolutionary model, we study the co-evolution of
supermassive black holes and their host galaxies, as a function of the
accretion radiative efficiency, dark matter content, and cosmological infall of
gas. In particular, the radiation feedback is computed by using the
self-regulated Bondi accretion. The models are characterized by strong
oscillations when the galaxy is in the AGN state with a high accretion
luminosity. We found that these one-zone models are able to reproduce two
important phases of galaxy evolution, namely an obscured-cold phase when the
bulk of star formation and black hole accretion occur, and the following
quiescent hot phase in which accretion remains highly sub-Eddington. A
Compton-thick phase is also found in almost all models, associated with the
cold phase. An exploration of the parameter space reveals that the closest
agreement with the present-day Magorrian relation is obtained, independently of
the dark matter halo mass, for galaxies with a low-mass seed black hole, and
the accretion radiative efficiency ~0.1.Comment: Accepted for publication in A&A, 12 pages, 5 figure
The giant planet orbiting the cataclysmic binary DP Leonis
Planets orbiting post-common envelope binaries provide fundamental
information on planet formation and evolution, especially for the yet nearly
unexplored class of circumbinary planets. We searched for such planets in \odp,
an eclipsing short-period binary, which shows long-term eclipse-time
variations. Using published, reanalysed, and new mid-eclipse times of the white
dwarf in DP\,Leo, obtained between 1979 and 2010, we find agreement with the
light-travel-time effect produced by a third body in an elliptical orbit. In
particular, the measured binary period in 2009/2010 and the implied radial
velocity coincide with the values predicted for the motion of the binary and
the third body around the common center of mass. The orbital period, semi-major
axis, and eccentricity of the third body are P_c = 28.0 +/- 2.0 yrs, a_c = 8.2
+/- 0.4 AU, and e_c = 0.39 +/- 0.13. Its mass of M_c sin(i_c) = 6.1 +/- 0.5 M_J
qualifies it as a giant planet. It formed either as a first generation object
in a protoplanetary disk around the original binary or as a second generation
object in a disk formed in the common envelope shed by the progenitor of the
white dwarf. Even a third generation origin in matter lost from the present
accreting binary can not be entirely excluded. We searched for, but found no
evidence for a fourth body.Comment: Accepted by A&
He star evolutionary channel to intermediate-mass binary pulsar PSR J1802-2124
The intermediate-mass binary pulsars (IMBPs) are characterized by relatively
long spin periods (10 - 200 ms) and massive (\ga 0.4 M_{\odot}) white dwarf
(WD) companions. Recently, precise mass measurements have been performed for
the pulsar and the WD in the IMBP PSR J1802-2124. Some observed properties,
such as the low mass of the pulsar, the high mass of the WD, the moderately
long spin period, and the tight orbit, imply that this system has undergone a
peculiar formation mechanism. In this work, we attempt to simulate the detailed
evolutionary history of PSR J1802-2124. We propose that a binary system
consisting of a neutron star (NS, of mass ) and an He star (of
mass ), and with an initial orbital period of 0.5 d, may have
been the progenitor of PSR J1802-2124. Once the He star overflows its Roche
lobe, He-rich material is transferred onto the NS at a relatively high rate of
, which is significantly higher
than the Eddington accretion rate. A large amount of the transferred material
is ejected from the vicinity of the NS by radiation pressure and results in the
birth of a mildly recycled pulsar. Our simulated results are consistent with
the observed parameters of PSR J1802-2124. Therefore, we argue that the NS + He
star evolutionary channel may be responsible for the formation of most IMBPs
with orbital periods \la 3 \rm d.Comment: 4 pages, 3 figures, Astronomy and Astrophysics in pres
Low-Mass Binary Induced Outflows from Asymptotic Giant Branch Stars
A significant fraction of planetary nebulae (PNe) and proto-planetary nebulae
(PPNe) exhibit aspherical, axisymmetric structures, many of which are highly
collimated. The origin of these structures is not entirely understood, however
recent evidence suggests that many observed PNe harbor binary systems, which
may play a role in their shaping. In an effort to understand how binaries may
produce such asymmetries, we study the effect of low-mass (< 0.3 M_sun)
companions (planets, brown dwarfs and low-mass main sequence stars) embedded
into the envelope of a 3.0 M_sun star during three epochs of its evolution (Red
Giant Branch, Asymptotic Giant Branch (AGB), interpulse AGB). We find that
common envelope evolution can lead to three qualitatively different
consequences: (i) direct ejection of envelope material resulting in a
predominately equatorial outflow, (ii) spin-up of the envelope resulting in the
possibility of powering an explosive dynamo driven jet and (iii) tidal
shredding of the companion into a disc which facilitates a disc driven jet. We
study how these features depend on the secondary's mass and discuss
observational consequences.Comment: 24 pages, 6 figures, submitted to MNRA
On the formation and evolution of black-hole binaries
We present the results of a systematic study of the formation and evolution
of binaries containing black holes and normal-star companions with a wide range
of masses. We first reexamine the standard formation scenario for close
black-hole binaries, where the spiral-in of the companion in the envelope of a
massive star causes the ejection of the envelope. We estimate the formation
rates for different companion masses and different assumptions about the
common-envelope structure and other model parameters. We find that black-hole
binaries with intermediate- and high-mass secondaries can form for a wide range
of assumptions, while black-hole binaries with low-mass secondaries can only
form with apparently unrealistic assumptions (in agreement with previous
studies). We then present detailed binary evolution sequences for black-hole
binaries with secondaries of 2 to 17 Msun and demonstrate that in these systems
the black hole can accrete appreciably even if accretion is Eddington limited
(up to 7 Msun for an initial black-hole mass of 10 Msun) and that the black
holes can be spun up significantly in the process. We discuss the implications
of these calculations for well-studied black-hole binaries (in particular GRS
1915+105), ultra-luminous X-ray sources and Cygnus X-1. Finally, we discuss how
some of the assumptions in the standard model could be relaxed to allow the
formation of low-mass, short-period black-hole binaries which appear to be very
abundant in Nature. (Abstract abridged)Comment: 21 pages, 9 figures, accepted by MNRAS, Figs. 2a/2b and 5 in very
reduced forma
The progenitor of binary millisecond radio pulsar PSR J1713+0747 (Research Note)
PSR J1713+0747 is a binary system comprising millisecond radio pulsar with a
spin period of 4.57 ms, and a low-mass white dwarf (WD) companion orbiting the
pulsar with a period of 67.8 days. Using the general relativistic Shapiro
delay, the masses of the WD and pulsar components were previously found to be
and (68% confidence),
respectively. Standard binary evolution theory suggests that PSR J1713+0747
evolved from a low-mass X-ray binary (LMXB). Here, we test this hypothesis. We
used a binary evolution code and a WD evolution code to calculate evolutionary
sequences of LMXBs that could result in binary millisecond radio pulsars such
as PSR J1713+0747. During the mass exchange, the mass transfer is
nonconservative. Because of the thermal and viscous instabilities developing in
the accretion disk, the neutron star accretes only a small part of the incoming
material. We find that the progenitor of PSR J1713+0747 can be modelled as an
LMXB including a donor star with mass and an initial
orbital period ranging from 2.40 to 4.15 days. If the cooling timescale of the
WD is 8 Gyr, its present effective temperature is between 3870 and 4120 K,
slightly higher than the observed value. We estimate a surface gravity of .Comment: 6 pages, 4 figures, accepted for publication in Astronomy and
Astrophysic
The Origin of Subdwarf B Star (I): the Formation Channels
Subdwarf B (sdB) stars (and related sdO/sdOB stars) are believed to be helium
core-burning objects with very thin hydrogen-rich envelopes. In recent years it
has become increasingly clear from observational surveys that a large fraction
of these objects are members of binary systems. To better understand their
formation, we here present the results of a detailed investigation of the three
main binary evolution channels that can lead to the formation of sdB stars: the
common envelope (CE) ejection channel, the stable Roche lobe overflow (RLOF)
channel and the double helium white dwarfs (WDs) merger channel. We obtained
the conditions for the formation of sdB stars from each of these channels using
detailed stellar and binary evolution calculations where we modelled the
detailed evolution of sdB stars and carried out simplified binary population
synthesis simulations. The observed period distribution of sdB stars in compact
binaries strongly constrains the CE ejection parameters. We also present the
distribution of sdB stars in the - diagram, the
Hertzsprung-Russell diagram and the distribution of mass functions.Comment: 20 pages, 23 figures, accepted for publication in MNRA