25 research outputs found
A Search for Star-Disk Interaction Among the Strongest X-ray Flaring Stars in the Orion Nebula Cluster
The Chandra Orion Ultradeep Project observed hundreds of young, low-mass
stars undergoing highly energetic X-ray flare events. The 32 most powerful
cases have been modeled with the result that the magnetic structures
responsible for these flares can be many stellar radii in extent. In this
paper, we model the observed spectral energy distributions of these 32 stars in
order to determine, in detail for each star, whether there is circumstellar
disk material situated in sufficient proximity to the stellar surface for
interaction with the large magnetic loops inferred from the observed X-ray
flares. Our spectral energy distributions span the wavelength range 0.3-8 um
(plus 24 um for some stars), allowing us to constrain the presence of dusty
circumstellar material out to >10 AU from the stellar surface in most cases.
For 24 of the 32 stars in our sample the available data are sufficient to
constrain the location of the inner edge of the dusty disks. Six of these (25%)
have spectral energy distributions consistent with inner disks within reach of
the observed magnetic loops. Another four stars may have gas disks interior to
the dust disk and extending within reach of the magnetic loops, but we cannot
confirm this with the available data. The remaining 14 stars (58%) appear to
have no significant disk material within reach of the large flaring loops.
Thus, up to ~40% of the sample stars exhibit energetic X-ray flares that
possibly arise from a magnetic star-disk interaction, and the remainder are
evidently associated with extremely large, free-standing magnetic loops
anchored only to the stellar surface.Comment: Accepted to the ApJ; 26 pages, 6 tables, 6 figure
Activated Magnetospheres of Magnetars
Like the solar corona, the external magnetic field of magnetars is twisted by
surface motions of the star. The twist energy is dissipated over time. We
discuss the theory of this activity and its observational status. (1) Theory
predicts that the magnetosphere tends to untwist in a peculiar way: a bundle of
electric currents (the "j-bundle") is formed with a sharp boundary, which
shrinks toward the magnetic dipole axis. Recent observations of shrinking hot
spots on magnetars are consistent with this behavior. (2) Continual discharge
fills the j-bundle with electron-positron plasma, maintaining a nonthermal
corona around the neutron star. The corona outside a few stellar radii strongly
interacts with the stellar radiation and forms a "radiatively locked" outflow
with a high e+- multiplicity. The locked plasma annihilates near the apexes of
the closed magnetic field lines. (3) New radiative-transfer simulations suggest
a simple mechanism that shapes the observed X-ray spectrum from 0.1 keV to 1
MeV: part of the thermal X-rays emitted by the neutron star are reflected from
the outer corona and then upscattered by the inner relativistic outflow in the
j-bundle, producing a beam of hard X-rays.Comment: 23 pages, 7 figures; review chapter in the proceedings of ICREA
Workshop on the High-Energy Emission from Pulsars and Their Systems, Sant
Cugat, Spain, April 201
Critical role of gap junction communication, calcium and nitric oxide signaling in bystander responses to focal photodynamic injury
Ionizing and nonionizing radiation affect not only directly targeted cells but also
surrounding “bystander” cells. The underlying mechanisms and therapeutic role of
bystander responses remain incompletely deined. Here we show that photosentizer
activation in a single cell triggers apoptosis in bystander cancer cells, which are
electrically coupled by gap junction channels and support the propagation of a Ca2+
wave initiated in the irradiated cell. The latter also acts as source of nitric oxide
(NO) that diffuses to bystander cells, in which NO levels are further increased by
a mechanism compatible with Ca2+-dependent enzymatic production. We detected
similar signals in tumors grown in dorsal skinfold chambers applied to live mice.
Pharmacological blockade of connexin channels signiicantly reduced the extent of
apoptosis in bystander cells, consistent with a critical role played by intercellular
communication, Ca2+ and NO in the bystander effects triggered by photodynamic
therapy
Strongly magnetized pulsars: explosive events and evolution
Well before the radio discovery of pulsars offered the first observational
confirmation for their existence (Hewish et al., 1968), it had been suggested
that neutron stars might be endowed with very strong magnetic fields of
-G (Hoyle et al., 1964; Pacini, 1967). It is because of their
magnetic fields that these otherwise small ed inert, cooling dead stars emit
radio pulses and shine in various part of the electromagnetic spectrum. But the
presence of a strong magnetic field has more subtle and sometimes dramatic
consequences: In the last decades of observations indeed, evidence mounted that
it is likely the magnetic field that makes of an isolated neutron star what it
is among the different observational manifestations in which they come. The
contribution of the magnetic field to the energy budget of the neutron star can
be comparable or even exceed the available kinetic energy. The most magnetised
neutron stars in particular, the magnetars, exhibit an amazing assortment of
explosive events, underlining the importance of their magnetic field in their
lives. In this chapter we review the recent observational and theoretical
achievements, which not only confirmed the importance of the magnetic field in
the evolution of neutron stars, but also provide a promising unification scheme
for the different observational manifestations in which they appear. We focus
on the role of their magnetic field as an energy source behind their persistent
emission, but also its critical role in explosive events.Comment: Review commissioned for publication in the White Book of
"NewCompStar" European COST Action MP1304, 43 pages, 8 figure
Theory and Applications of Non-Relativistic and Relativistic Turbulent Reconnection
Realistic astrophysical environments are turbulent due to the extremely high
Reynolds numbers. Therefore, the theories of reconnection intended for
describing astrophysical reconnection should not ignore the effects of
turbulence on magnetic reconnection. Turbulence is known to change the nature
of many physical processes dramatically and in this review we claim that
magnetic reconnection is not an exception. We stress that not only
astrophysical turbulence is ubiquitous, but also magnetic reconnection itself
induces turbulence. Thus turbulence must be accounted for in any realistic
astrophysical reconnection setup. We argue that due to the similarities of MHD
turbulence in relativistic and non-relativistic cases the theory of magnetic
reconnection developed for the non-relativistic case can be extended to the
relativistic case and we provide numerical simulations that support this
conjecture. We also provide quantitative comparisons of the theoretical
predictions and results of numerical experiments, including the situations when
turbulent reconnection is self-driven, i.e. the turbulence in the system is
generated by the reconnection process itself. We show how turbulent
reconnection entails the violation of magnetic flux freezing, the conclusion
that has really far reaching consequences for many realistically turbulent
astrophysical environments. In addition, we consider observational testing of
turbulent reconnection as well as numerous implications of the theory. The
former includes the Sun and solar wind reconnection, while the latter include
the process of reconnection diffusion induced by turbulent reconnection, the
acceleration of energetic particles, bursts of turbulent reconnection related
to black hole sources as well as gamma ray bursts. Finally, we explain why
turbulent reconnection cannot be explained by turbulent resistivity or derived
through the mean field approach.Comment: 66 pages, 24 figures, a chapter of the book "Magnetic Reconnection -
Concepts and Applications", editors W. Gonzalez, E. N. Parke
Laboratory Studies of Astrophysical Jets
Jets and outflows produced during star-formation are observed on many scales:
from the "micro-jets" extending a few hundred Astronomical Units to the
"super-jets" propagating to parsecs distances. Recently, a new "class" of
short-lived (hundreds of nano-seconds) centimetre-long jets has emerged in the
laboratory as a complementary tool to study these complex astrophysical flows.
Here I will discuss and review the recent work done on "simulating"
protostellar jets in the laboratory using z-pinch machines.Comment: 25 Pages, 11 Figures to appear in Lecture Notes in Physics. Series
Title: Jets from young stars IV: From models to observations and experiments
Editors: P. J. V. Garcia and J. M. T. Ferreira. Publisher: Springe
Spectropolarimetry of stars across the H-R diagram
The growing sample of magnetic stars shows a remarkable diversity in the
properties of their magnetic fields. The overall goal of current studies is to
understand the origin, evolution, and structure of stellar magnetic fields in
stars of different mass at different evolutionary stages. In this chapter we
discuss recent measurements together with the underlying assumptions in the
interpretation of data and the requirements, both observational and
theoretical, for obtaining a realistic overview of the role of magnetic fields
in various types of stars.Comment: 23 pages, 3 figures, chapter 7 of "Astronomical Polarisation from the
Infrared to Gamma Rays", published in Astrophysics and Space Science Library
46
The tearing instability of resistive magnetohydrodynamics
In this chapter we explore the linear onset of one of the most important instabilities of resistive magnetohydrodynamics, the tearing instability. In particular, we focus on two important aspects of the onset of tearing: asymptotic (modal) stability and transient (non-modal) stability. We discuss the theory required to understand these two aspects of stability, both of which have undergone significant development in recent years
Effects of plasma turbulence on the nonlinear evolution of magnetic island in tokamak
Magnetic islands (MIs), resulting from a magnetic field reconnection, are ubiquitous structures in magnetized plasmas. In tokamak plasmas, recent researches suggested that the interaction between an MI and ambient turbulence can be important for the nonlinear MI evolution, but a lack of detailed experimental observations and analyses has prevented further understanding. Here, we provide comprehensive observations such as turbulence spreading into an MI and turbulence enhancement at the reconnection site, elucidating intricate effects of plasma turbulence on the nonlinear MI evolution