140 research outputs found
Rings in the Planetesimal Disk of Beta Pic
The nearby main sequence star Beta Pictoris is surrounded by an edge-on disk
of dust produced by the collisional erosion of larger planetesimals. Here we
report the discovery of substructure within the northeast extension of the disk
midplane that may represent an asymmetric ring system around Beta Pic. We
present a dynamical model showing that a close stellar flyby with a quiescient
disk of planetesimals can create such rings, along with previously unexplained
disk asymmetries. Thus we infer that Beta Pic's planetesimal disk was highly
disrupted by a stellar encounter in the last hundred thousand years.Comment: Accepted by ApJ Letters. LaTeX, 13 pages, 4 figures, full PostScript
file available from http://www.maths.qmw.ac.uk/~jdl
Angular Momentum Transfer in Star-Discs Encounters: The Case of Low-Mass Discs
A prerequisite for the formation of stars and planetary systems is that
angular momentum is transported in some way from the inner regions of the
accretion disc. Tidal effects may play an important part in this angular
momentum transport. Here the angular momentum transfer in an star-disc
encounter is investigated numerically for a variety of encounter parameters in
the case of low mass discs. Although good agreement is found with analytical
results for the entire disc, the loss {\it inside} the disc can be up to an
order of magnitude higher than previously assumed. The differences in angular
momentum transport by secondaries on a hyperbolic, parabolic and elliptical
path are shown, and it is found that a succession of distant encounters might
be equally, if not more, successful in removing angular momentum than single
close encounter.Comment: 11pages, 8 figures, 1 tabl
On the tilting of protostellar disks by resonant tidal effects
We consider the dynamics of a protostellar disk surrounding a star in a
circular-orbit binary system. Our aim is to determine whether, if the disk is
initially tilted with respect to the plane of the binary orbit, the inclination
of the system will increase or decrease with time. The problem is formulated in
the binary frame in which the tidal potential of the companion star is static.
We consider a steady, flat disk that is aligned with the binary plane and
investigate its linear stability with respect to tilting or warping
perturbations. The dynamics is controlled by the competing effects of the m=0
and m=2 azimuthal Fourier components of the tidal potential. In the presence of
dissipation, the m=0 component causes alignment of the system, while the m=2
component has the opposite tendency. We find that disks that are sufficiently
large, in particular those that extend to their tidal truncation radii, are
generally stable and will therefore tend to alignment with the binary plane on
a time-scale comparable to that found in previous studies. However, the effect
of the m=2 component is enhanced in the vicinity of resonances where the outer
radius of the disk is such that the natural frequency of a global bending mode
of the disk is equal to twice the binary orbital frequency. Under such
circumstances, the disk can be unstable to tilting and acquire a warped shape,
even in the absence of dissipation. The outer radius corresponding to the
primary resonance is always smaller than the tidal truncation radius. For disks
smaller than the primary resonance, the m=2 component may be able to cause a
very slow growth of inclination through the effect of a near resonance that
occurs close to the disk center. We discuss these results in the light of
recent observations of protostellar disks in binary systems.Comment: 21 pages, 7 figures, to be published in the Astrophysical Journa
The Inner Rings of Beta Pictoris
We present Keck images of the dust disk around Beta Pictoris at 17.9 microns
that reveal new structure in its morphology. Within 1" (19 AU) of the star, the
long axis of the dust emission is rotated by more than 10 degrees with respect
to that of the overall disk. This angular offset is more pronounced than the
warp detected at 3.5" by HST, and in the opposite direction. By contrast, the
long axis of the emission contours at ~ 1.5" from the star is aligned with the
HST warp. Emission peaks between 1.5" and 4" from the star hint at the presence
of rings similar to those observed in the outer disk at ~ 25" with HST/STIS. A
deconvolved image strongly suggests that the newly detected features arise from
a system of four non-coplanar rings. Bayesian estimates based on the primary
image lead to ring radii of 14+/-1 AU, 28+/-3 AU, 52+/-2 AU and 82+/-2 AU, with
orbital inclinations that alternate in orientation relative to the overall disk
and decrease in magnitude with increasing radius. We believe these new results
make a strong case for the existence of a nascent planetary system around Beta
Pic.Comment: 5 pages, 2 figures, PDF format. Published in ApJL, December 20,200
Stellar Encounters with Massive Star-Disk Systems
The dense, clustered environment in which massive stars form can lead to
interactions with neighboring stars. It has been hypothesized that collisions
and mergers may contribute to the growth of the most massive stars. In this
paper we extend the study of star-disk interactions to explore encounters
between a massive protostar and a less massive cluster sibling using the
publicly available SPH code GADGET-2. Collisions do not occur in the parameter
space studied, but the end state of many encounters is an eccentric binary with
a semi-major axis ~ 100 AU. Disk material is sometimes captured by the
impactor. Most encounters result in disruption and destruction of the initial
disk, and periodic torquing of the remnant disk. We consider the effect of the
changing orientation of the disk on an accretion driven jet, and the evolution
of the systems in the presence of on-going accretion from the parent core.Comment: 11 pages, 10 figures, accepted to Ap
Disc-Disc Encounters between Low-Mass Protoplanetary Accretion Discs
Simulations of the collapse and fragmentation of turbulent molecular clouds
and dense young clusters show that encounters between disc-surrounded stars are
relatively common events which should significantly influence the resulting
disc structure. In turn this should alter the accretion rate of disc matter
onto the star and the conditions under which planet formation occurs. Although
the effects of star-disc encounters have been previously investigated, very
little is known about encounters where both stars are surrounded by discs. In
this paper encounters of such disc-disc systems are studied quantitatively. It
is found that for low-mass discs (= 0.01 M_\sun) the results from
star-disc encounters can be straightforwardly generalized to disc-disc
encounters as long as there is no mass transport between the discs. Differences
to star-disc encounters occur naturally where significant amounts of matter are
transported between the discs. In this case it is found that although the mass
distribution does not change significantly, matter caught onto highly eccentric
orbits is transported surprisingly far inside the disc. The captured mass
partly replenishes the disc, but has a much lower angular momentum. This can
lead to a reduction of the angular momentum in the entire disc and thus
considerably increased accretion shortly after the encounter as well as in the
long term.Comment: 24 pages, 10 figures, accepted by Ap
The Birth of High Mass Stars: Accretion and/or Mergers?
The observational consequences of the merger scenario for massive star
formation are explored and contrasted with the gradual accumulation of mass by
accretion. Protostellar mergers may produce high luminosity infrared flares
lasting years to centuries followed by a luminosity decline on the
Kelvin-Helmholtz time-scale of the merger product. Mergers may be surrounded by
thick tori of expanding debris, impulsive wide-angle outflows, and shock
induced maser and radio continuum emission. Collision products are expected to
have fast stellar rotation and a large multiplicity fraction. Close encounters
or mergers will produce circumstellar debris disks with an orientation that
differs form that of a pre-existing disk. The extremely rare merger of two
stars close to the upper-mass end of the IMF may be a possible pathway to
hypernova generated gamma-ray bursters. While accretional growth can lead to
the formation of massive stars in isolation or in loose clusters, mergers can
only occur in high-density cluster environments. It is proposed that the
outflow emerging from the OMC1 core in the Orion molecular cloud was produced
by a protostellar merger that released between to ergs less
than a thousand years ago
Stellar Encounters with the Beta Pictoris Planetesimal System
We use data from the Hipparcos Catalog and the Barbier-Brossat & Figon (2000)
catalog of stellar radial velocities to test the hypothesis that the Beta Pic
planetesimal disk was disrupted by a close stellar encounter. We trace the
space motions of 21,497 stars and discover 18 that have passed within 5 pc of
Beta Pic in the past 1 Myr. Beta Pic's closest encounter is with the K2III star
HIP 27628 (0.6 pc), but dynamically the most important encounter is with the
F7V star HIP 23693 (0.9 pc). We calculate the velocity and eccentricity changes
induced by the 18 perturbations and conclude that they are dynamically
significant if planetesimals exist in a Beta Pic Oort cloud. We provide a
first-order estimate for the evolutionary state of a Beta Pic Oort cloud and
conclude that the primary role of these stellar perturbations would be to help
build a comet cloud rather than destroy a pre-existing structure. The stellar
sample is 20% complete and motivates future work to identify less common close
interactions that would significantly modify the observed circumstellar disk.
For future radial velocity study we identify six stars in the Hipparcos Catalog
that may have approached Beta Pic to within 0.1 pc and therefore remain as
candidate disk perturbers.Comment: 23 pages, 5 figures, Accepted for publication in Ap
An alpha theory of time-dependent warped accretion discs
The non-linear fluid dynamics of a warped accretion disc was investigated in
an earlier paper by developing a theory of fully non-linear bending waves in a
thin, viscous disc. That analysis is here extended to take proper account of
thermal and radiative effects by solving an energy equation that includes
viscous dissipation and radiative transport. The problem is reduced to simple
one-dimensional evolutionary equations for mass and angular momentum, expressed
in physical units and suitable for direct application. This result constitutes
a logical generalization of the alpha theory of Shakura & Sunyaev to the case
of a time-dependent warped accretion disc. The local thermal-viscous stability
of such a disc is also investigated.Comment: 16 pages, 3 figures, to be published in MNRA
Capture Formed Binaries via Encounters with Massive Protostars
Most massive stars are found in the center of dense clusters, and have a
companion fraction much higher than their lower mass siblings; the massive
stars of the Trapezium core in Orion have ~ 1.5 companions each. This high
multiplicity could be a consequence of formation via a capture scenario, or it
could be due to fragmentation of the cores that form the massive stars. During
stellar formation circumstellar disks appear to be nearly ubiquitous. Their
large radii compared to stellar sizes increase the interaction radius
significantly, suggesting that disk interactions with neighboring stars could
assist in capturing binary companions. This mechanism has been studied for
stars of approximately solar mass and found to be inefficient. In this paper we
present simulations of interactions between a 22 Msun star-disk system and less
massive impactors, to study the disk-assisted capture formation of binaries in
a regime suited to massive stars. The formation of binaries by capture is found
to be much more efficient for massive capturers. We discuss the effects of a
mass dependent velocity dispersion and mass segregation on the capture rates,
and consider the long term survival of the resultant binaries in a dense
cluster.Comment: 16 pages, 10 figures, accepted to Ap
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