135 research outputs found
Coulomb-Volkov approach of ionization by extreme ultraviolet laser pulses in the subfemtosecond regime
In conditions where the interaction betweeen an atom and a short
high-frequency extreme ultraviolet laser pulse is a perturbation, we show that
a simple theoretical approach, based on Coulomb-Volkov-type states, can make
reliable predictions for ionization. To avoid any additional approximation, we
consider here a standard case : the ionization of hydrogen atoms initially in
their ground state. For any field parameter, we show that the method provides
accurate energy spectra of ejected electrons, including many above threshold
ionization peaks, as long as the two following conditions are simultaneously
fulfilled : (i) the photon energy is greater than or equal to the ionization
potential ; (ii) the ionization process is not saturated. Thus, ionization of
atoms or molecules by the high order harmonic laser pulses which are generated
at present may be addressed through this Coulomb-Volkov treatment.Comment: 19 pages including 5 figures and figure caption
Disk Loss and Disk Renewal Phases in Classical Be Stars I: Analysis of Long-Term Spectropolarimetric Data
(Abridged) Classical Be stars occasionally transition from having a gaseous
circumstellar disk (''Be phase'') to a state in which all observational
evidence for the presence of these disks disappears (''normal B-star phase'').
We present one of the most comprehensive spectropolarimetric views to date of
such a transition for two Be stars, pi Aquarii and 60 Cygni. 60 Cyg's disk loss
episode was characterized by a monotonic decrease in emission strength over a
time-scale of 1000 days, consistent with the viscous time-scale of the disk,
assuming alpha is 0.14. pi Aqr's disk loss was episodic in nature and occurred
over a time-scale of 2440 days. An observed time lag between the behavior of
the polarization and H-alpha in both stars indicates the disk clearing
proceeded in an ''inside-out'' manner. We determine the position angle of the
intrinsic polarization to be 166.7 +/- 0.1 degrees for pi Aqr and 107.7 +/- 0.4
degrees for 60 Cyg, and model the observed polarization during the quiescent
diskless phase of each star to determine the interstellar polarization along
the line of sight. Minor outbursts observed during the quiescent phase of each
star shared similar lifetimes as those previously reported for mu Cen,
suggesting that the outbursts represent the injection and subsequent viscous
dissipation of individual blobs of material into the inner circumstellar
environments of these stars. We also observe deviations from the mean intrinsic
polarization position angle during polarization outbursts in each star,
indicating deviations from axisymmetry. We propose that these deviations might
be indicative of the injection (and subsequent circularization) of new blobs
into the inner disk, either in the plane of the bulk of the disk material or in
a slightly inclined (non-coplanar) orbit.Comment: 30 pages, 14 figures; accepted in Ap
Observing the Earth as an exoplanet with LOUPE, the Lunar Observatory for Unresolved Polarimetry of Earth
The detections of small, rocky exoplanets have surged in recent years and
will likely continue to do so. To know whether a rocky exoplanet is habitable,
we have to characterise its atmosphere and surface. A promising
characterisation method for rocky exoplanets is direct detection using
spectropolarimetry. This method will be based on single pixel signals, because
spatially resolving exoplanets is impossible with current and near-future
instruments. Well-tested retrieval algorithms are essential to interpret these
single pixel signals in terms of atmospheric composition, cloud and surface
coverage. Observations of Earth itself provide the obvious benchmark data for
testing such algorithms. The observations should provide signals that are
integrated over the Earth's disk, that capture day and night variations, and
all phase angles. The Moon is a unique platform from where the Earth can be
observed as an exoplanet, undisturbed, all of the time. Here, we present LOUPE,
the Lunar Observatory for Unresolved Polarimetry of Earth, a small and robust
spectropolarimeter to observe our Earth as an exoplanet.Comment: 14 pages, 3 figures, submitted in special Issue of Planetary and
Space Science on Scientific Preparations for Lunar Exploratio
The Effect of Multiple Scattering on the Polarization from Binary Star Envelopes. I. Self- and Externally Illuminated Disks
We present the results of a Monte Carlo radiative transfer code that
calculates the polarization produced by multiple Thomson scattering and
variable absorptive opacity in a circumstellar disk around one component of a
close detached binary system. We consider in detail the polarization variations
over the binary cycle that result from the disk's illumination by the external
star and by its own volume emission. We identify key features of these
polarization phase curves and investigate their behavior as functions of
optical depth, albedo, and inclination for geometrically thin and thick disks.
The polarization due to disk self-illumination is sensitive to the internal
optical characteristics of the disk, while the polarization arising from
external illumination is mainly sensitive to the disk's geometrical thickness.
With appropriate flux weighting, these results, combined with those for an
internally illuminated disk, allow simulation of the polarization signature
from an arbitrary binary-disk system.Comment: 34 pages, 17 figures. Submitted to ApJ, revised in response to
referee comments. Color figures available from
http://www.ruf.rice.edu/~jhoffman/monte/colorfigs.htm
Core excitation in Coulomb breakup reactions
Within the pure Coulomb breakup mechanism, we investigate the one-neutron
removal reaction of the type A(a,b)X with Be and C
projectiles on a heavy target nucleus Pb at the beam energy of 60
MeV/nucleon. Our intention is to examine the prospective of using these
reactions to study the structure of neutron rich nuclei. Integrated partial
cross sections and momentum distributions for the ground as well as excited
bound states of core nuclei are calculated within the finite range distorted
wave Born approximation as well as within the adiabatic model of the Coulomb
breakup. Our results are compared with those obtained in the studies of the
reactions on a light target where the breakup proceeds via the pure nuclear
mechanism. We find that the transitions to excited states of the core are quite
weak in the Coulomb dominated process as compared to the pure nuclear breakup.Comment: Revtex format, five postscript figures included, to appear in Phys.
Rev.
UV and EUV Instruments
We describe telescopes and instruments that were developed and used for
astronomical research in the ultraviolet (UV) and extreme ultraviolet (EUV)
regions of the electromagnetic spectrum. The wavelength ranges covered by these
bands are not uniquely defined. We use the following convention here: The EUV
and UV span the regions ~100-912 and 912-3000 Angstroem respectively. The
limitation between both ranges is a natural choice, because the hydrogen Lyman
absorption edge is located at 912 Angstroem. At smaller wavelengths,
astronomical sources are strongly absorbed by the interstellar medium. It also
marks a technical limit, because telescopes and instruments are of different
design. In the EUV range, the technology is strongly related to that utilized
in X-ray astronomy, while in the UV range the instruments in many cases have
their roots in optical astronomy. We will, therefore, describe the UV and EUV
instruments in appropriate conciseness and refer to the respective chapters of
this volume for more technical details.Comment: To appear in: Landolt-Boernstein, New Series VI/4A, Astronomy,
Astrophysics, and Cosmology; Instruments and Methods, ed. J.E. Truemper,
Springer-Verlag, Berlin, 201
Eta Carinae and the Luminous Blue Variables
We evaluate the place of Eta Carinae amongst the class of luminous blue
variables (LBVs) and show that the LBV phenomenon is not restricted to
extremely luminous objects like Eta Car, but extends luminosities as low as
log(L/Lsun) = 5.4 - corresponding to initial masses ~25 Msun, and final masses
as low as ~10-15 Msun. We present a census of S Doradus variability, and
discuss basic LBV properties, their mass-loss behaviour, and whether at maximum
light they form pseudo-photospheres. We argue that those objects that exhibit
giant Eta Car-type eruptions are most likely related to the more common type of
S Doradus variability. Alternative atmospheric models as well as
sub-photospheric models for the instability are presented, but the true nature
of the LBV phenomenon remains as yet elusive. We end with a discussion on the
evolutionary status of LBVs - highlighting recent indications that some LBVs
may be in a direct pre-supernova state, in contradiction to the standard
paradigm for massive star evolution.Comment: 27 pages, 6 figures, Review Chapter in "Eta Carinae and the supernova
imposters" (eds R. Humphreys and K. Davidson) new version submitted to
Springe
QCD Corrections to Spin Dependent Drell-Yan and a Global Subtraction Scheme
We present QCD corrections to the Drell-Yan process in the transversely
polarized, longitudinally polarized and unpolarized cases. The analytical
results are presented in a form valid for all -dimensional regularization
schemes. A universal mass factorization scheme is presented in which the
results reduce to those of dimensional reduction. The connection between the
parton distributions and fragmentation functions of dimensional reduction and
those of dimensional regularization is elucidated in a simple manner. Numerical
results are presented for proton-proton collisions at energies relevant to RHIC
(Relativistic Heavy Ion Collider). The perturbative stability of the transverse
and longitudinal asymmetries is investigated.Comment: 18 pages, 8 figures in 1 postscript fil
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
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