88 research outputs found
The Secular Evolution of a Uniform Density Star Cluster Immersed in a Compressible Galactic Tidal Field
Abstract
Nuclear stellar clusters are common in the center of galaxies. We consider the possibility that their progenitors assumed to be globular clusters may have formed elsewhere, migrated to, and assembled near their present location. The main challenge for this scenario is whether globular clusters can withstand the tidal field of their host galaxies. Our analysis suggests that provided the mass-density distribution of background potential is relatively shallow, as in some galaxies with relatively flat surface brightness profiles, the tidal field near the center of galaxies may be shown to be able to compress rather than disrupt a globular cluster at a distance from the center much smaller than the conventionally defined âtidal disruption radiusâ r
t
. To do so, we adopt a previously constructed formalism and consider the secular evolution of star clusters with a homogeneous mass-density distribution. We analytically solve the secular equations in the limit that the mass density of stars in the galactic center approaches a uniform distribution. Our model indicates that a star cluster could travel to distances much smaller than r
t
without disruption, thus potentially contributing to the formation of the nuclear cluster. However, appropriate numerical N-body simulations are needed to confirm our analytic findings.</jats:p
The evolution of circumstellar discs in the galactic centre: an application to the G-clouds
The Galactic Centre is known to have undergone a recent star formation episode a few Myr ago, which likely produced many T Tauri stars hosting circumstellar discs. It has been suggested that these discs may be the compact and dusty ionized sources identified as âG-cloudsâ. Given the Galactic Centreâs hostile environment, we study the possible evolutionary pathways these discs experience. We compute new external photoevaporation models applicable to discs in the Galactic Centre that account for the subsonic launching of the wind and absorption of UV photons by dust. Using evolutionary disc calculations, we find that photoevaporationâs rapid truncation of the disc causes them to accrete onto the central star rapidly. Ultimately, an accreting circumstellar disc has a lifetime âČ 1âMyr, which would fail to live long enough to explain the G-clouds. However, we identify a new evolutionary pathway for circumstellar discs in the Galactic Centre. Removal of disc material by photoevaporation prevents the young star from spinning down due to magnetic braking, ultimately causing the rapidly spinning young star to torque the disc into a âdecretion discâ state which prevents accretion. At the same time, any planetary companion in the disc will trap dust outside its orbit, shutting down photoevaporation. The disc can survive for up to âŒ10âMyr in this state. Encounters with other stars are likely to remove the planet on Myr time-scales, causing photoevaporation to restart, giving rise to a G-cloud signature. A giant planet fraction of âŒ10 per cent can explain the number of observed G-clouds
Hot Jupiters from Secular Planet--Planet Interactions
About 25 per cent of `hot Jupiters' (extrasolar Jovian-mass planets with
close-in orbits) are actually orbiting counter to the spin direction of the
star. Perturbations from a distant binary star companion can produce high
inclinations, but cannot explain orbits that are retrograde with respect to the
total angular momentum of the system. Such orbits in a stellar context can be
produced through secular (that is, long term) perturbations in hierarchical
triple-star systems. Here we report a similar analysis of planetary bodies,
including both octupole-order effects and tidal friction, and find that we can
produce hot Jupiters in orbits that are retrograde with respect to the total
angular momentum. With distant stellar mass perturbers, such an outcome is not
possible. With planetary perturbers, the inner orbit's angular momentum
component parallel to the total angular momentum need not be constant. In fact,
as we show here, it can even change sign, leading to a retrograde orbit. A
brief excursion to very high eccentricity during the chaotic evolution of the
inner orbit allows planet-star tidal interactions to rapidly circularize that
orbit, decoupling the planets and forming a retrograde hot Jupiter.Comment: accepted for publication by Nature, 3 figures (version after proof -
some typos corrected
Accretion of Planetary Material onto Host Stars
Accretion of planetary material onto host stars may occur throughout a star's
life. Especially prone to accretion, extrasolar planets in short-period orbits,
while relatively rare, constitute a significant fraction of the known
population, and these planets are subject to dynamical and atmospheric
influences that can drive significant mass loss. Theoretical models frame
expectations regarding the rates and extent of this planetary accretion. For
instance, tidal interactions between planets and stars may drive complete
orbital decay during the main sequence. Many planets that survive their stars'
main sequence lifetime will still be engulfed when the host stars become red
giant stars. There is some observational evidence supporting these predictions,
such as a dearth of close-in planets around fast stellar rotators, which is
consistent with tidal spin-up and planet accretion. There remains no clear
chemical evidence for pollution of the atmospheres of main sequence or red
giant stars by planetary materials, but a wealth of evidence points to active
accretion by white dwarfs. In this article, we review the current understanding
of accretion of planetary material, from the pre- to the post-main sequence and
beyond. The review begins with the astrophysical framework for that process and
then considers accretion during various phases of a host star's life, during
which the details of accretion vary, and the observational evidence for
accretion during these phases.Comment: 18 pages, 5 figures (with some redacted), invited revie
Rings in the Solar System: a short review
Rings are ubiquitous around giant planets in our Solar System. They evolve
jointly with the nearby satellite system. They could form either during the
giant planet formation process or much later, as a result of large scale
dynamical instabilities either in the local satellite system, or at the
planetary scale. We review here the main characteristics of rings in our solar
system, and discuss their main evolution processes and possible origin. We also
discuss the recent discovery of rings around small bodies.Comment: Accepted for the Handbook of Exoplanet
A new concept for the combination of optical interferometers and high-resolution spectrographs
The combination of high spatial and spectral resolution in optical astronomy
enables new observational approaches to many open problems in stellar and
circumstellar astrophysics. However, constructing a high-resolution
spectrograph for an interferometer is a costly and time-intensive undertaking.
Our aim is to show that, by coupling existing high-resolution spectrographs to
existing interferometers, one could observe in the domain of high spectral and
spatial resolution, and avoid the construction of a new complex and expensive
instrument. We investigate in this article the different challenges which arise
from combining an interferometer with a high-resolution spectrograph. The
requirements for the different sub-systems are determined, with special
attention given to the problems of fringe tracking and dispersion. A concept
study for the combination of the VLTI (Very Large Telescope Interferometer)
with UVES (UV-Visual Echelle Spectrograph) is carried out, and several other
specific instrument pairings are discussed. We show that the proposed
combination of an interferometer with a high-resolution spectrograph is indeed
feasible with current technology, for a fraction of the cost of building a
whole new spectrograph. The impact on the existing instruments and their
ongoing programs would be minimal.Comment: 27 pages, 9 figures, Experimental Astronomy; v2: accepted versio
Transiting extrasolar planetary candidates in the Galactic bulge
More than 200 extrasolar planets have been discovered around relatively
nearby stars, primarily through the Doppler line shifts owing to the reflex
motions of their host stars, and more recently through transits of some planets
across the face of the host stars. The detection of planets with the shortest
known periods, 1.2 to 2.5 days, has mainly resulted from transit surveys which
have generally targeted stars more massive than 0.75 M_sun. Here we report the
results from a planetary transit search performed in a rich stellar field
towards the Galactic bulge. We discovered 16 candidates with orbital periods
between 0.4 and 4.2 days, five of which orbit stars of 0.44 to 0.75 M_sun. In
two cases, radial-velocity measurements support the planetary nature of the
companions. Five candidates have orbital periods below 1.0 day, constituting a
new class of ultra-short-period planets (USPPs), which occur only around stars
of less than 0.88 M_sun. This indicates that those orbiting very close to more
luminous stars might be evaporatively destroyed, or that jovian planets around
lower-mass stars might migrate to smaller radii.Comment: To appear in October 5, 2006 issue of Natur
Turbulence and galactic structure
Interstellar turbulence is driven over a wide range of scales by processes
including spiral arm instabilities and supernovae, and it affects the rate and
morphology of star formation, energy dissipation, and angular momentum transfer
in galaxy disks. Star formation is initiated on large scales by gravitational
instabilities which control the overall rate through the long dynamical time
corresponding to the average ISM density. Stars form at much higher densities
than average, however, and at much faster rates locally, so the slow average
rate arises because the fraction of the gas mass that forms stars at any one
time is low, ~10^{-4}. This low fraction is determined by turbulence
compression, and is apparently independent of specific cloud formation
processes which all operate at lower densities. Turbulence compression also
accounts for the formation of most stars in clusters, along with the cluster
mass spectrum, and it gives a hierarchical distribution to the positions of
these clusters and to star-forming regions in general. Turbulent motions appear
to be very fast in irregular galaxies at high redshift, possibly having speeds
equal to several tenths of the rotation speed in view of the morphology of
chain galaxies and their face-on counterparts. The origin of this turbulence is
not evident, but some of it could come from accretion onto the disk. Such high
turbulence could help drive an early epoch of gas inflow through viscous
torques in galaxies where spiral arms and bars are weak. Such evolution may
lead to bulge or bar formation, or to bar re-formation if a previous bar
dissolved. We show evidence that the bar fraction is about constant with
redshift out to z~1, and model the formation and destruction rates of bars
required to achieve this constancy.Comment: in: Penetrating Bars through Masks of Cosmic Dust: The Hubble Tuning
Fork strikes a New Note, Eds., K. Freeman, D. Block, I. Puerari, R. Groess,
Dordrecht: Kluwer, in press (presented at a conference in South Africa, June
7-12, 2004). 19 pgs, 5 figure
Early Gas Stripping as the Origin of the Darkest Galaxies in the Universe
The known galaxies most dominated by dark matter (Draco, Ursa Minor and
Andromeda IX) are satellites of the Milky Way and the Andromeda galaxies. They
are members of a class of faint galaxies, devoid of gas, known as dwarf
spheroidals, and have by far the highest ratio of dark to luminous matter. None
of the models proposed to unravel their origin can simultaneously explain their
exceptional dark matter content and their proximity to a much larger galaxy.
Here we report simulations showing that the progenitors of these galaxies were
probably gas-dominated dwarf galaxies that became satellites of a larger galaxy
earlier than the other dwarf spheroidals. We find that a combination of tidal
shocks and ram pressure swept away the entire gas content of such progenitors
about ten billion years ago because heating by the cosmic ultraviolet
background kept the gas loosely bound: a tiny stellar component embedded in a
relatively massive dark halo survived until today. All luminous galaxies should
be surrounded by a few extremely dark-matter-dominated dwarf spheroidal
satellites, and these should have the shortest orbital periods among dwarf
spheroidals because they were accreted early.Comment: Published in Nature (15 February 2007), 28 pages, 8 figures,
Supplementary Information include
Planet Populations as a Function of Stellar Properties
Exoplanets around different types of stars provide a window into the diverse
environments in which planets form. This chapter describes the observed
relations between exoplanet populations and stellar properties and how they
connect to planet formation in protoplanetary disks. Giant planets occur more
frequently around more metal-rich and more massive stars. These findings
support the core accretion theory of planet formation, in which the cores of
giant planets form more rapidly in more metal-rich and more massive
protoplanetary disks. Smaller planets, those with sizes roughly between Earth
and Neptune, exhibit different scaling relations with stellar properties. These
planets are found around stars with a wide range of metallicities and occur
more frequently around lower mass stars. This indicates that planet formation
takes place in a wide range of environments, yet it is not clear why planets
form more efficiently around low mass stars. Going forward, exoplanet surveys
targeting M dwarfs will characterize the exoplanet population around the lowest
mass stars. In combination with ongoing stellar characterization, this will
help us understand the formation of planets in a large range of environments.Comment: Accepted for Publication in the Handbook of Exoplanet
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