34 research outputs found
Hydrogen H line polarization in solar flares. Theoretical investigation of atomic polarization by proton beams considering self-consistent NLTE polarized radiative transfer
Context. We present a theoretical review of the effect of impact polarization
of a hydrogen H line due to an expected proton beam bombardment in
solar flares. Aims. Several observations indicate the presence of the linear
polarization of the hydrogen H line observed near the solar limb above
5% and preferentially in the radial direction. We theoretically review the
problem of deceleration of the beam originating in the coronal reconnection
site due to its interaction with the chromospheric plasma, and describe the
formalism of the density matrix used in our description of the atomic processes
and the treatment of collisional rates. Methods. We solve the self-consistent
NLTE radiation transfer problem for the particular semiempirical chromosphere
models for both intensity and linear polarization components of the radiation
field. Results. In contrast to recent calculations, our results show that the
energy distribution of the proton beam at H formation levels and
depolarizing collisions by background electrons and protons cause a significant
reduction of the effect below 0.1%. The radiation transfer solution shows that
tangential resonance-scattering polarization dominates over the impact
polarization effect in all considered models. Conclusions. In the models
studied, proton beams are unlikely to be a satisfying explanation for the
observed linear polarization of the H line.Comment: 11 pages, 11 figures, accepted for publication in A&
THEMIS as particle detector: Spectropolarimetry of solar flares
The progressive phases of three solar flares have beenobserv ed with THEMIS in July 2000, using the multiline spectropolarimetric MTR mode. A preliminary analysis of the characteristics of the polarization of the Hα and Hβ lines
observed at the beginning of the progressive phase of one of these flares is presented
Absence of linear polarization in Halpha emission of solar flares
High sensitivity observations of Halpha polarization of 30 flares of
different sizes and disk positions are reported. Both filter and spectrographic
techniques have been used. The ZIMPOL system eliminates spurious polarizations
due to seeing and flat-field effects. We didn't find any clear linear
polarization signature above our sensitivity level which was usually better
than 0.1%. The observations include an X17.1 flare with gamma-ray lines
reported by the RHESSI satellite. These results cast serious doubts on previous
claims of linear polarization at the one percent level and more, attributed to
impact polarization. The absence of linear polarization limits the anisotropy
of energetic protons in the H emitting region. The likely causes are
isotropization by collisions with neutrals in the chromosphere and defocusing
by the converging magnetic field.Comment: to be published in A&A, 8 pages, 6 figure
Highly Ionized Potassium Lines in Solar X-ray Spectra and the Abundance of Potassium
The abundance of potassium is derived from X-ray lines observed during flares
by the RESIK instrument on the solar mission CORONAS-F between 3.53 A and 3.57
A. The lines include those emitted by He-like K and Li-like K dielectronic
satellites, which have been synthesized using the CHIANTI atomic code and newly
calculated atomic data. There is good agreement of observed and synthesized
spectra, and the theoretical behavior of the spectra with varying temperature
estimated from the ratio of the two GOES channels is correctly predicted. The
observed fluxes of the He-like K resonance line per unit emission measure gives
log A(K) = 5.86 (on a scale log A(H) = 12), with a total range of a factor 2.9.
This is higher than photospheric abundance estimates by a factor 5.5, a
slightly greater enhancement than for other elements with first ionization
potential (FIP) less than about 10 eV. There is, then, the possibility that
enrichment of low-FIP elements in coronal plasmas depends weakly on the value
of the FIP which for K is extremely low (4.34 eV). Our work also suggests that
fractionation of elements to form the FIP effect occurs in the low chromosphere
rather than higher up, as in some models.Comment: 14 pages, 3 figure
Polarization Diagnostics for Cool Core Cluster Emission Lines
The nature of the interaction between low-excitation gas filaments at ~104 K, seen in optical line emission, and diffuse X-ray emitting coronal gas at ~107 K in the centers of galaxy clusters remains a puzzle. The presence of a strong, empirical correlation between the two gas phases is indicative of a fundamental relationship between them, though as yet of undetermined cause. The cooler filaments, originally thought to have condensed from the hot gas, could also arise from a merger or the disturbance of cool circumnuclear gas by nuclear activity. Here, we have searched for intrinsic line emission polarization in cool core galaxy clusters as a diagnostic of fundamental transport processes. Drawing on developments in solar astrophysics, direct energetic particle impact induced polarization holds the promise to definitively determine the role of collisional processes such as thermal conduction in the ISM physics of galaxy clusters, while providing insight into other highly anisotropic excitation mechanisms such as shocks, intense radiation fields, and suprathermal particles. Under certain physical conditions, theoretical calculations predict of the order of 10% polarization. Our observations of the filaments in four nearby cool core clusters place stringent upper limits ( 0.1%) on the presence of emission line polarization, requiring that if thermal conduction is operative, the thermal gradients are not in the saturated regime. This limit is consistent with theoretical models of the thermal structure of filament interfacesPeer reviewe
Imaging Spectroscopy of a White-Light Solar Flare
We report observations of a white-light solar flare (SOL2010-06-12T00:57,
M2.0) observed by the Helioseismic Magnetic Imager (HMI) on the Solar Dynamics
Observatory (SDO) and the Reuven Ramaty High-Energy Solar Spectroscopic Imager
(RHESSI). The HMI data give us the first space-based high-resolution imaging
spectroscopy of a white-light flare, including continuum, Doppler, and magnetic
signatures for the photospheric FeI line at 6173.34{\AA} and its neighboring
continuum. In the impulsive phase of the flare, a bright white-light kernel
appears in each of the two magnetic footpoints. When the flare occurred, the
spectral coverage of the HMI filtergrams (six equidistant samples spanning
\pm172m{\AA} around nominal line center) encompassed the line core and the blue
continuum sufficiently far from the core to eliminate significant Doppler
crosstalk in the latter, which is otherwise a possibility for the extreme
conditions in a white-light flare. RHESSI obtained complete hard X-ray and
\Upsilon-ray spectra (this was the first \Upsilon-ray flare of Cycle 24). The
FeI line appears to be shifted to the blue during the flare but does not go
into emission; the contrast is nearly constant across the line profile. We did
not detect a seismic wave from this event. The HMI data suggest stepwise
changes of the line-of-sight magnetic field in the white-light footpoints.Comment: 14 pages, 7 figures, Accepted by Solar Physic
X-Ray Spectroscopy of Stars
(abridged) Non-degenerate stars of essentially all spectral classes are soft
X-ray sources. Low-mass stars on the cooler part of the main sequence and their
pre-main sequence predecessors define the dominant stellar population in the
galaxy by number. Their X-ray spectra are reminiscent, in the broadest sense,
of X-ray spectra from the solar corona. X-ray emission from cool stars is
indeed ascribed to magnetically trapped hot gas analogous to the solar coronal
plasma. Coronal structure, its thermal stratification and geometric extent can
be interpreted based on various spectral diagnostics. New features have been
identified in pre-main sequence stars; some of these may be related to
accretion shocks on the stellar surface, fluorescence on circumstellar disks
due to X-ray irradiation, or shock heating in stellar outflows. Massive, hot
stars clearly dominate the interaction with the galactic interstellar medium:
they are the main sources of ionizing radiation, mechanical energy and chemical
enrichment in galaxies. High-energy emission permits to probe some of the most
important processes at work in these stars, and put constraints on their most
peculiar feature: the stellar wind. Here, we review recent advances in our
understanding of cool and hot stars through the study of X-ray spectra, in
particular high-resolution spectra now available from XMM-Newton and Chandra.
We address issues related to coronal structure, flares, the composition of
coronal plasma, X-ray production in accretion streams and outflows, X-rays from
single OB-type stars, massive binaries, magnetic hot objects and evolved WR
stars.Comment: accepted for Astron. Astrophys. Rev., 98 journal pages, 30 figures
(partly multiple); some corrections made after proof stag
The weak-line T Tauri star V410Tau I. A multi-wavelength study of variability
We present the results of an intensive coordinated monitoring campaign in the
optical and X-ray wavelength ranges of the low-mass, pre-main sequence star
V410Tau carried out with the aim to study the relation between various
indicators for magnetic activity that probe emission from different atmospheric
layers: optical photometric star spot (rotation) cycle, chromospheric Halpha
emission, and coronal X-rays. Two X-ray pointings were carried out with the
Chandra satellite simultaneously with the optical observations, and centered
near the maximum and minimum levels of the optical lightcurve. A relation of
their different count levels to the rotation period of the dominating spot is
not confirmed by a third Chandra observation carried out some months later,
during another minimum of the 1.87d cycle. Similarly we find no indications for
a correlation of the Halpha emission with the spots' rotational phase. The
extraordinary stability of the largest spot is confirmed by long-term
photometric and radial velocity measurements. Joining our optical photometry
with previous data we provide a new estimate for the dominant periodicity of
V410Tau. This updated value removes systematic offsets of the time of minimum
observed in data taken over the last decade. Furthermore, the combination of
the new data with published measurements taken during the last decade allows us
to examine long-term changes in the mean light level of the photometry of
V410Tau. A variation on the timescale of 5.4yr is suggested. Assuming that this
behavior is truely cyclic V410Tau is the first pre-main sequence star on which
an activity cycle is detected.Comment: 16 pages, accepted for publication in A&
MOLÉCULES DANS LES ATMOSPHÈRES STELLAIRES
Les molécules sont abondantes dans les étoiles froides. Elles influent sur la stratification de l'atmosphère et le mode de transfert de l'énergie et on doit donc en tenir compte lors de la recherche des conditions physiques dans les atmosphères stellaires froides. Par ailleurs la spectroscopie moléculaire permet la mesure des abondances des éléments légers et fournit donc des données aux théories de nucléosynthèse et d'évolution stellaire.Molecules are abundant in cold stellar atmospheres. They have an effect upon the thermal stratification of the atmosphere and the energy transfert ; so one must include them in any research of the physical conditions in cold stellar atmospheres. From the study of molecular spectrum it is possible to know the abundances of light elements which are data for the nucleo-synthesis and stellar evolution theories
Flare line impact polarization. Na D2 589 nm line polarization in the 2001 June 15 flare
International audienc