19 research outputs found
Nonparametric Econometrics: The np Package
We describe the R np package via a series of applications that may be of interest to applied econometricians. The np package implements a variety of nonparametric and semiparametric kernel-based estimators that are popular among econometricians. There are also procedures for nonparametric tests of significance and consistent model specification tests for parametric mean regression models and parametric quantile regression models, among others. The np package focuses on kernel methods appropriate for the mix of continuous, discrete, and categorical data often found in applied settings. Data-driven methods of bandwidth selection are emphasized throughout, though we caution the user that data-driven bandwidth selection methods can be computationally demanding.
Perturbations induced by a molecular cloud on the young stellar disc in the Galactic Centre
The Galactic centre (GC) is a crowded environment: observations have revealed
the presence of (molecular, atomic and ionized) gas, of a cusp of late-type
stars, and of ~100 early-type stars, about half of which lying in one or
possibly two discs. In this paper, we study the perturbations exerted on a thin
stellar disc (with outer radius ~0.4 pc) by a molecular cloud that falls
towards the GC and is disrupted by the supermassive black hole (SMBH). The
initial conditions for the stellar disc were drawn from the results of previous
simulations of molecular cloud infall and disruption in the SMBH potential. We
find that most of the gas from the disrupted molecular cloud settles into a
dense and irregular disc surrounding the SMBH. If the gas disc and the stellar
disc are slightly misaligned (~5-20 deg), the precession of the stellar orbits
induced by the gas disc significantly increases the inclinations of the stellar
orbits (by a factor of ~3-5 in 1.5 Myr) with respect to the normal vector to
the disc. Furthermore, the distribution of orbit inclinations becomes
significantly broader. These results might be the clue to explain the broad
distribution of observed inclinations of the early-type stars with respect to
the normal vector of the main disc. We discuss the implications for the
possibility that fresh gas was accreted by the GC after the formation of the
disc(s) of early-type stars.Comment: 12 pages, 12 figures, 2 tables, accepted for publication in MNRA
Clumps in the Outer Disk by Disk Instability: Why They are Initially Gas Giants and the Legacy of Disruption
We explore the initial conditions for fragments in the extended regions
( AU) of gravitationally unstable disks. We combine analytic
estimates for the fragmentation of spiral arms with 3D SPH simulations to show
that initial fragment masses are in the gas giant regime. These initial
fragments will have substantial angular momentum, and should form disks with
radii of a few AU. We show that clumps will survive for multiple orbits before
they undergo a second, rapid collapse due to H dissociation and that it is
possible to destroy bound clumps by transporting them into the inner disk. The
consequences of disrupted clumps for planet formation, dust processing, and
disk evolution are discussed. We argue that it is possible to produce
Earth-mass cores in the outer disk during the earliest phases of disk
evolution.Comment: Accepted for publication in Icarus. The arguments have been greatly
expanded (from v1) to address comments by the referee
Nonparametric Econometrics: The np Package
We describe the R np package via a series of applications that may be of interest to applied econometricians. The np package implements a variety of nonparametric and semiparametric kernel-based estimators that are popular among econometricians. There are also procedures for nonparametric tests of significance and consistent model specification tests for parametric mean regression models and parametric quantile regression models, among others. The np package focuses on kernel methods appropriate for the mix of continuous, discrete, and categorical data often found in applied settings. Data-driven methods of bandwidth selection are emphasized throughout, though we caution the user that data-driven bandwidth selection methods can be computationally demanding.ISSN:1548-766
2007) The np package
Abstract We describe the R np package via a series of applications that may be of interest to applied econometricians. This vignette is based o