351 research outputs found
The brown dwarf desert as a consequence of orbital migration
We show that the dearth of brown dwarfs in short-period orbits around
Solar-mass stars - the brown dwarf desert - can be understood as a consequence
of inward migration within an evolving protoplanetary disc. Brown dwarf
secondaries forming at the same time as the primary star have masses which are
comparable to the initial mass of the protoplanetary disc. Subsequent disc
evolution leads to inward migration, and destruction of the brown dwarf, via
merger with the star. This is in contrast with massive planets, which avoid
this fate by forming at a later epoch when the disc is close to being
dispersed. Within this model, a brown dwarf desert arises because the mass at
the hydrogen burning limit is coincidentally comparable to the initial disc
mass for a Solar mass star. Brown dwarfs should be found in close binaries
around very low mass stars, around other brown dwarfs, and around Solar-type
stars during the earliest phases of star formation.Comment: MNRAS (Letters), in pres
Imaging Cool Giant Planets in Reflected Light: Science Investigations and Synergy with Habitable Planets
Planned astronomical observatories of the 2020s will be capable of obtaining
reflected light photometry and spectroscopy of cool extrasolar giant planets.
Here we explain that such data are valuable both for understanding the origin
and evolution of giant planets as a whole and for preparing for the
interpretation of similar datasets from potentially habitable extrasolar
terrestrial planets in the decades to follow.Comment: Science white paper submitted to the Astro 2020 Decadal Survey on
Astronomy and Astrophysics. Replace version to fix typo in co-signer name and
add figure credit
Binary Black Hole Mergers from Planet-like Migrations
If supermassive black holes (BHs) are generically present in galaxy centers,
and if galaxies are built up through hierarchical merging, BH binaries are at
least temporary features of most galactic bulges. Observations suggest,
however, that binary BHs are rare, pointing towards a binary lifetime far
shorter than the Hubble time. We show that, regardless of the detailed
mechanism, all stellar-dynamical processes are insufficient to reduce
significantly the orbital separation once orbital velocities in the binary
exceed the virial velocity of the system. We propose that a massive gas disk
surrounding a BH binary can effect its merger rapidly, in a scenario analogous
to the orbital decay of super-jovian planets due to a proto-planetary disk. As
in the case of planets, gas accretion onto the secondary (here a supermassive
BH) is integrally connected with its inward migration. Such accretion would
give rise to quasar activity. BH binary mergers could therefore be responsible
for many or most quasars.Comment: 8 pages, submitted to ApJ Letter
On the determination of Jupiter's satellite-dependent Love numbers from Juno gravity data
The Juno gravity experiment, among the nine instruments onboard the spacecraft, is aimed at studying the interior structure of Jupiter to gain insight into its formation. Doppler data collected during the first two gravity-dedicated orbits completed by Juno around the gas giant have already provided a measurement of Jupiter's gravity field with outstanding accuracy, answering crucial questions about its interior composition. The large dataset that will be collected throughout the remaining phases of the mission until the end in July 2021 might allow to determine Jupiter's response to the satellite-dependent tidal perturbation raised by its moons, and even to separate the static and dynamic effects. We report on numerical simulations performed over the full science mission to assess the sensitivity of Juno gravity measurements to satellite-dependent tides on Jupiter. We assumed a realistic simulation scenario that is coherent with the result of data analysis from the first gravity passes. Furthermore, we implemented a satellite-dependent tidal model within the dynamical model used to fit the simulated Doppler data. The formal uncertainties resulting from the covariance analysis show that Juno is indeed sensitive to satellite-dependent tides on Jupiter raised by the inner Galilean satellites (the static Love numbers of degree and order 2 of Io, Europa and Ganymede can be determined respectively to 0.28%, 4.6% and 5.3% at 1 sigma). This unprecedented determination, that will be carried out towards the end of the mission, could further constrain the interior structure of the planet, allowing to discern among interior models and improving existing theories of planetary tidal response
Magnetically modulated accretion in T Tauri stars
We examine how accretion on to T Tauri stars may be modulated by a
time-dependent `magnetic gate' where the inner edge of the accretion disc is
disrupted by a varying stellar field. We show that magnetic field variations on
time-scales shorter than 10^5 yr can modulate the accretion flow, thus
providing a possible mechanism both for the marked photometric variability of T
Tauri stars and for the possible conversion of T Tauri stars between classical
and weak line status. We thus suggest that archival data relating to the
spectrophotometric variability of T Tauri stars may provide an indirect record
of magnetic activity cycles in low-mass pre-main-sequence stars.Comment: LaTeX file (requires mn.sty), 4 pages, no figures or tables. To
appear in MNRAS
Global Star Formation Rates in Disk Galaxies and Circumnuclear Starbursts from Cloud Collisions
We invoke star formation triggered by cloud-cloud collisions to explain
global star formation rates of disk galaxies and circumnuclear starbursts.
Previous theories based on the growth rate of gravitational perturbations
ignore the dynamically important presence of magnetic fields. Theories based on
triggering by spiral density waves fail to explain star formation in systems
without such waves. Furthermore, observations suggest gas and stellar disk
instabilities are decoupled. Following Gammie, Ostriker & Jog (1991), the cloud
collision rate is set by the shear velocity of encounters with initial impact
parameters of a few tidal radii, due to differential rotation in the disk.
This, together with the effective confinement of cloud orbits to a two
dimensional plane, enhances the collision rate above that for particles in a
three dimensional box. We predict Sigma_{SFR}(R) proportional to Sigma_{gas}
Omega (1-0.7 beta). For constant circular velocity (beta = 0), this is in
agreement with recent observations (Kennicutt 1998). We predict a B-band
Tully-Fisher relation: L_{B} proportional to v_{circ}^{7/3}, also consistent
with observations. As additional tests, we predict enhanced star formation in
regions with relatively high shear rates, and lower star formation efficiencies
in clouds of higher mass.Comment: 27 pages including 3 figures and 2 tables. Accepted to ApJ. Expanded
statistical analysis of cloud SF efficiency test. Stylistic changes. Data for
figures available electronically at
http://astro.berkeley.edu/~jt/disksfr.htm
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