84 research outputs found
The inner regions of protoplanetary disks
To understand how planetary systems form in the dusty disks around
pre-main-sequence stars a detailed knowledge of the structure and evolution of
these disks is required. While this is reasonably well understood for the
regions of the disk beyond about 1 AU, the structure of these disks inward of 1
AU remains a puzzle. This is partly because it is very difficult to spatially
resolve these regions with current telescopes. But it is also because the
physics of this region, where the disk becomes so hot that the dust starts to
evaporate, is poorly understood. With infrared interferometry it has become
possible in recent years to directly spatially resolve the inner AU of
protoplanetary disks, albeit in a somewhat limited way. These observations have
partly confirmed current models of these regions, but also posed new questions
and puzzles. Moreover, it has turned out that the numerical modeling of these
regions is extremely challenging. In this review we give a rough overview of
the history and recent developments in this exciting field of astrophysics.Comment: 45 pages with 14 Figures. to appear in Annual Review of Astronomy and
Astrophysics (2010, Vol. 48
Current status of NLTE analysis of stellar atmospheres
Various available codes for NLTE modeling and analysis of hot star spectra
are reviewed. Generalizations of standard equations of kinetic equilibrium and
their consequences are discussed.Comment: in Determination of Atmospheric Parameters of B-, A-, F- and G-Type
Stars, E. Niemczura et al. eds., Springer, in pres
A simplistic pedagogical formulation of a thermal speed distribution using a relativistic framework
A novel pedagogical technique is presented that can be used in the
undergraduate (UG) class to formulate a relativistically extended Kinetic
Theory of Gases and thermal speed distribution, while assuming the basic
thermal symmetry arguments of the famous Maxwell-Boltzmann distribution as
presented at the UG level. The adopted framework can be used by students to
understand the physics in a thermally governed system at high temperature and
speeds, without having to indulge in high level tensor based mathematics, as
has been done by the previous works in the subject. Our approach, a logical
extension of that proposed by Maxwell, will first recapitulate what is taught
and known in the UG class and then present a methodology inspired from the
Maxwell-Boltzmann framework that will help students to understand and derive
the physics of relativistic thermal systems. The methodology uses simple tools
well known to undergraduates and involves a component of computational
techniques that can be used to involve students in this exercise. We have tried
to place the current work in a larger perspective in regard to the earlier
works done and emphasize on it's simplicity and accessibility to students.
Towards the end, interesting implications of the relativistically extended
distribution are presented and compared with the Maxwell-Boltzmann results at
various temperatures.Comment: 13 pages, 5 figures, Publication accepted in Pramana - Journal of
Physics (Indian Academy of Sciences). Revised version has an additional
section, discussing previous work on relativistic Kinetic Theory in section
2.1 and comparison with these in section 6. Arguments for formulating a
relativistic thermal speed distributions have been enriched and made more
clear and categorical in section
Central Powering of the Largest Lyman-alpha Nebula is Revealed by Polarized Radiation
High-redshift Lyman-alpha blobs are extended, luminous, but rare structures
that appear to be associated with the highest peaks in the matter density of
the Universe. Their energy output and morphology are similar to powerful radio
galaxies, but the source of the luminosity is unclear. Some blobs are
associated with ultraviolet or infrared bright galaxies, suggesting an extreme
starburst event or accretion onto a central black hole. Another possibility is
gas that is shock excited by supernovae. However some blobs are not associated
with galaxies, and may instead be heated by gas falling into a dark matter
halo. The polarization of the Ly-alpha emission can in principle distinguish
between these options, but a previous attempt to detect this signature returned
a null detection. Here we report on the detection of polarized Ly-alpha from
the blob LAB1. Although the central region shows no measurable polarization,
the polarized fraction (P) increases to ~20 per cent at a radius of 45 kpc,
forming an almost complete polarized ring. The detection of polarized radiation
is inconsistent with the in situ production of Ly-alpha photons, and we
conclude that they must have been produced in the galaxies hosted within the
nebula, and re-scattered by neutral hydrogen.Comment: Published in the August 18 issue of Nature. 1750 words, 3 figures,
and full Supplementary Information. Version has not undergone proofing.
Reduced and processed data products are available here:
http://obswww.unige.ch/people/matthew.hayes/LymanAlpha/LabPol
Modeling Molecular-Line Emission from Circumstellar Disks
Molecular lines hold valuable information on the physical and chemical
composition of disks around young stars, the likely progenitors of planetary
systems. This invited contribution discusses techniques to calculate the
molecular emission (and absorption) line spectrum based on models for the
physical and chemical structure of protoplanetary disks. Four examples of
recent research illutrate these techniques in practice: matching resolved
molecular-line emission from the disk around LkCa15 with theoertical models for
the chemistry; evaluating the two-dimensional transfer of ultraviolet radiation
into the disk, and the effect on the HCN/CN ratio; far-infrared CO line
emission from a superheated disk surface layer; and inward motions in the disk
around L1489 IRS.Comment: 6 pages, no figures. To appear in "The Dense Interstellar Medium in
Galaxies", Procs. Fourth Cologne-Bonn-Zermatt-Symposiu
High energy emission from microquasars
The microquasar phenomenon is associated with the production of jets by X-ray
binaries and, as such, may be associated with the majority of such systems. In
this chapter we briefly outline the associations, definite, probable, possible,
and speculative, between such jets and X-ray, gamma-ray and particle emission.Comment: Contributing chapter to the book Cosmic Gamma-Ray Sources, K.S. Cheng
and G.E. Romero (eds.), to be published by Kluwer Academic Publishers,
Dordrecht, 2004. (19 pages
Non-thermal emission processes in massive binaries
In this paper, I present a general discussion of several astrophysical
processes likely to play a role in the production of non-thermal emission in
massive stars, with emphasis on massive binaries. Even though the discussion
will start in the radio domain where the non-thermal emission was first
detected, the census of physical processes involved in the non-thermal emission
from massive stars shows that many spectral domains are concerned, from the
radio to the very high energies.
First, the theoretical aspects of the non-thermal emission from early-type
stars will be addressed. The main topics that will be discussed are
respectively the physics of individual stellar winds and their interaction in
binary systems, the acceleration of relativistic electrons, the magnetic field
of massive stars, and finally the non-thermal emission processes relevant to
the case of massive stars. Second, this general qualitative discussion will be
followed by a more quantitative one, devoted to the most probable scenario
where non-thermal radio emitters are massive binaries. I will show how several
stellar, wind and orbital parameters can be combined in order to make some
semi-quantitative predictions on the high-energy counterpart to the non-thermal
emission detected in the radio domain.
These theoretical considerations will be followed by a census of results
obtained so far, and related to this topic... (see paper for full abstract)Comment: 47 pages, 5 postscript figures, accepted for publication in Astronomy
and Astrophysics Review. Astronomy and Astrophysics Review, in pres
Intrinsic Shapes of Elliptical Galaxies
Tests for the intrinsic shape of the luminosity distribution in elliptical
galaxies are discussed, with an emphasis on the uncertainties. Recent
determinations of the ellipticity frequency function imply a paucity of nearly
spherical galaxies, and may be inconsistent with the oblate hypothesis.
Statistical tests based on the correlation of surface brightness, isophotal
twisting, and minor axis rotation with ellipticity have so far not provided
strong evidence in favor of the nearly oblate or nearly prolate hypothesis, but
are at least qualitatively consistent with triaxiality. The possibility that
the observed deviations of elliptical galaxy isophotes form ellipses are due to
projection effects is evaluated. Dynamical instabilities may explain the
absence of elliptical galaxies flatter than about E6, and my also play a role
in the lack of nearly-spherical galaxies
Discovery of Radio Emission from the Brown Dwarf LP944-20
Brown dwarfs are classified as objects which are not massive enough to
sustain nuclear fusion of hydrogen, and are distinguished from planets by their
ability to burn deuterium. Old (>10 Myr) brown dwarfs are expected to possess
short-lived magnetic fields and, since they no longer generate energy from
collapse and accretion, weak radio and X-ray emitting coronae. Several efforts
have been undertaken in the past to detect chromospheric activity from the
brown dwarf LP944-20 at X-ray and optical wavelengths, but only recently an
X-ray flare from this object was detected. Here we report on the discovery of
quiescent and flaring radio emission from this source, which represents the
first detection of persistent radio emission from a brown dwarf, with
luminosities that are several orders of magnitude larger than predicted from an
empirical relation between the X-ray and radio luminosities of many stellar
types. We show in the context of synchrotron emission, that LP944-20 possesses
an unusually weak magnetic field in comparison to active dwarf M stars, which
might explain the null results from previous optical and X-ray observations of
this source, and the deviation from the empirical relations.Comment: Accepted to Natur
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