202 research outputs found
Investor Sentiment and Pre-Issue Markets
What role do sentiment investors play in the pricing of newly listed stocks? We derive conditions under which we can distinguish between sentiment and rational pricing behavior and test for the rationality of small investors’ demand for new stock issues using data from pre-issue (or ‘grey’) markets in Europe. Under sentiment, the model predicts asymmetric relations between the prices at which small investors trade new stock issues in the grey market and i) the subsequent issue price set by the investment bank, ii) prices in the early after-market, and iii) the degree of stock price reversal in the long run. Our empirical results suggest that sentiment demand is present
and influences the pricing of newly listed firms
Generating all subsets of a finite set with disjoint unions
If X is an n-element set, we call a family G of subsets of X a k-generator
for X if every subset of X can be expressed as a union of at most k disjoint
sets in G. Frein, Leveque and Sebo conjectured that for n > 2k, the smallest
k-generators for X are obtained by taking a partition of X into classes of
sizes as equal as possible, and taking the union of the power-sets of the
classes. We prove this conjecture for all sufficiently large n when k = 2, and
for n a sufficiently large multiple of k when k > 2.Comment: Final version, with some additional explanations added in the proof
Constraints on Deep-seated Zonal Winds Inside Jupiter and Saturn
The atmospheres of Jupiter and Saturn exhibit strong and stable zonal winds.
How deep the winds penetrate unabated into each planet is unknown. Our
investigation favors shallow winds. It consists of two parts.
The first part makes use of an Ohmic constraint; Ohmic dissipation associated
with the planet's magnetic field cannot exceed the planet's net luminosity.
Application to Jupiter (J) and Saturn (S) shows that the observed zonal winds
cannot penetrate below a depth at which the electrical conductivity is about
six orders of magnitude smaller than its value at the molecular-metallic
transition. Measured values of the electrical conductivity of molecular
hydrogen yield radii of maximum penetration of 0.96R_J and 0.86R_S, with
uncertainties of a few percent of R. At these radii, the magnetic Reynolds
number based on the zonal wind velocity and the scale height of the magnetic
diffusivity is of order unity. These limits are insensitive to difficulties in
modeling turbulent convection. They permit complete penetration along cylinders
of the equatorial jets observed in the atmospheres of Jupiter and Saturn.
The second part investigates how deep the observed zonal winds actually do
penetrate. Truncation of the winds in the planet's convective envelope would
involve breaking the Taylor-Proudman constraint on cylindrical flow. This would
require a suitable nonpotential acceleration which none of the obvious
candidates appears able to provide. Accelerations arising from entropy
gradients, magnetic stresses, and Reynolds stresses appear to be much too weak.
These considerations suggest that strong zonal winds are confined to shallow,
stably stratified layers, with equatorial jets being the possible exception
Super-Reflection in Fluid Discs: Corotation Amplifier, Corotation Resonance, Rossby Waves, and Overstable Modes
In differentially rotating discs with no self-gravity, density waves cannot
propagate around the corotation, where the wave pattern rotation speed equals
the fluid rotation rate. Waves incident upon the corotation barrier may be
super-reflected (commonly referred to as corotation amplifier), but the
reflection can be strongly affected by wave absorptions at the corotation
resonance/singularity. The sign of the absorption is related to the Rossby wave
zone very near the corotation radius. We derive the explicit expressions for
the complex reflection and transmission coefficients, taking into account wave
absorption at the corotation resonance. We show that for generic discs, this
absorption plays a much more important role than wave transmission across the
corotation barrier. Depending on the sign of the gradient of the specific
vorticity of the disc the corotation resonance can either enhance or diminish
the super-reflectivity, and this can be understood in terms of the location of
the Rossby wave zone relative to the corotation radius. Our results provide the
explicit conditions (in terms of disc thickness, rotation profile and specific
vorticity gradient) for which super-reflection can be achieved. Global
overstable disc modes may be possible for discs with super-reflection at the
corotation barrier.Comment: 16 pages, 5 figures, MNRAS in pres
Statistical Study of the Early Solar System's Instability with 4, 5 and 6 Giant Planets
Several properties of the Solar System, including the wide radial spacing and
orbital eccentricities of giant planets, can be explained if the early Solar
System evolved through a dynamical instability followed by migration of planets
in the planetesimal disk. Here we report the results of a statistical study, in
which we performed nearly 10^4 numerical simulations of planetary instability
starting from hundreds of different initial conditions. We found that the
dynamical evolution is typically too violent, if Jupiter and Saturn start in
the 3:2 resonance, leading to ejection of at least one ice giant from the Solar
System. Planet ejection can be avoided if the mass of the transplanetary disk
of planetesimals was large (M_disk>50 M_Earth), but we found that a massive
disk would lead to excessive dynamical damping (e.g., final e_55 < 0.01
compared to present e_55=0.044, where e_55 is the amplitude of the fifth
eccentric mode in the Jupiter's orbit), and to smooth migration that violates
constraints from the survival of the terrestrial planets. Better results were
obtained when the Solar System was assumed to have five giant planets initially
and one ice giant, with the mass comparable to that of Uranus and Neptune, was
ejected into interstellar space by Jupiter. The best results were obtained when
the ejected planet was placed into the external 3:2 or 4:3 resonance with
Saturn and M_disk ~ 20 M_Earth. The range of possible outcomes is rather broad
in this case, indicating that the present Solar System is neither a typical nor
expected result for a given initial state, and occurs, in best cases, with only
a ~5% probability (as defined by the success criteria described in the main
text). The case with six giant planets shows interesting dynamics but does
offer significant advantages relative to the five planet case.Comment: To appear in The Astronomical Journa
Corotational Damping of Diskoseismic C-modes in Black Hole Accretion Discs
Diskoseismic c-modes in accretion discs have been invoked to explain
low-frequency variabilities observed in black-hole X-ray binaries. These modes
are trapped in the inner-most region of the disc and have frequencies much
lower than the rotation frequency at the disc inner radius. We show that
because the trapped waves can tunnel through the evanescent barrier to the
corotational wave zone, the c-modes are damped due to wave absorption at the
corotation resonance. We calculate the corotational damping rates of various
c-modes using the WKB approximation. The damping rate varies widely depending
on the mode frequency, the black hole spin parameter and the disc sound speed,
and is generally much less than 10% of the mode frequency. A sufficiently
strong excitation mechanism is needed to overcome this corotational damping and
make the mode observable.Comment: 10 pages, 5 figures, MNRAS in pres
Perturbed disks get shocked. Binary black hole merger effects on accretion disks
The merger process of a binary black hole system can have a strong impact on
a circumbinary disk. In the present work we study the effect of both central
mass reduction (due to the energy loss through gravitational waves) and a
possible black hole recoil (due to asymmetric emission of gravitational
radiation). For the mass reduction case and recoil directed along the disk's
angular momentum, oscillations are induced in the disk which then modulate the
internal energy and bremsstrahlung luminosities. On the other hand, when the
recoil direction has a component orthogonal to the disk's angular momentum, the
disk's dynamics are strongly impacted, giving rise to relativistic shocks. The
shock heating leaves its signature in our proxies for radiation, the total
internal energy and bremsstrahlung luminosity. Interestingly, for cases where
the kick velocity is below the smallest orbital velocity in the disk (a likely
scenario in real AGN), we observe a common, characteristic pattern in the
internal energy of the disk. Variations in kick velocity simply provide a phase
offset in the characteristic pattern implying that observations of such a
signature could yield a measure of the kick velocity through electromagnetic
signals alone.Comment: 10 pages, 13 figures. v2: Minor changes, version to be published in
PR
Dynamical evolution of the young stars in the Galactic center: N-body simulations of the S-stars
We use N-body simulations to study the evolution of the orbital
eccentricities of stars deposited near (<0.05 pc) the Milky Way massive black
hole (MBH), starting from initial conditions motivated by two competing models
for their origin: formation in a disk followed by inward migration; and
exchange interactions involving a binary star. The first model predicts modest
eccentricities, lower than those observed in the S-star cluster, while the
second model predicts higher eccentricities than observed. The N-body
simulations include a dense cluster of 10 M_sun stellar black holes (SBHs),
expected to accumulate near the MBH by mass segregation. Perturbations from the
SBHs tend to randomize the stellar orbits, partially erasing the dynamical
signatures of their origin. The eccentricities of the initially highly
eccentric stars evolve, in 20 Myr (the S-star lifespan), to a distribution that
is consistent at the ~95 % level with the observed eccentricity distribution.
In contrast, the eccentricities of the initially more circular orbits fail to
evolve to the observed values in 20 Myr, arguing against the disk migration
scenario. We find that 20 % - 30 % of the S-stars are tidally disrupted by the
MBH over their lifetimes, and that the S-stars are not likely to be ejected as
hypervelocity stars outside the central 0.05 pc by close encounters with
stellar black holes.Comment: 6 pages, 2 figures. Minor corrections, Sumitted to Ap
Near-infrared observations of water-ice in OH/IR stars
A search for the near-infrared water-ice absorption band was made in a number
of very red OH/IR stars which are known to exhibit the 10um silicate
absorption. As a by-product, accurate positions of these highly reddened
objects are obtained. We derived a dust mass loss rate for each object by
modelling the spectral energy distribution and the gas mass loss rate by
solving the equation of motion for the dust drag wind. The derived mass loss
rates show a strong correlation with the silicate optical depth as well as that
of the water-ice. The stars have a high mass loss rate (> 1.0E-4 Msun/yr) with
an average gas-to-dust mass ratio of 110. In objects which show the 3.1um
water-ice absorption, the near-IR slope is much steeper than those with no
water-ice. Comparison between our calculated mass loss rates and those derived
from OH and CO observations indicates that these stars have recently increased
their mass loss rates.Comment: 10 pages, 6 figures : accepted for publication in A&
Tidal Heating Models for the Radii of the Inflated Transiting Giant Planets WASP-4b, WASP-6b, WASP-12b, and TrES-4
In order to explain the inflated radii of some transiting extrasolar giant
planets, we investigate a tidal heating scenario for the inflated planets
WASP-4b, WASP-6b, WASP-12b, WASP-15b, and TrES-4. To do so, we assume that they
retain a nonzero eccentricity, possibly by dint of continuing interaction with
a third body. We calculate the amount of extra heating in the envelope that is
then required to fit the radius of each planet, and we explore how this
additional power depends on the planetary atmospheric opacity and on the mass
of a heavy-element central core. There is a degeneracy between the core mass
and the heating . Therefore, in the case
of tidal heating, there is for each planet a range of the couple that can lead to the same radius, where is the tidal
dissipation factor and is the eccentricity. With this in mind, we also
investigate the case of the non-inflated planet HAT-P-12b, which can admit
solutions combining a heavy-element core and tidal heating. A substantial
improvement of the measured eccentricities of such planetary systems could
simplify this degeneracy by linking the two unknown parameters . Further independent constraints on either of these parameters
would, through our calculations, constrain the other.Comment: Accepted in ApJ; 17 pages, 3 figures, 6 tables (emulateapj format);
expanded explanatory tex
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