68 research outputs found
Coronal heating through braiding of magnetic field lines
Cool stars like our Sun are surrounded by a million degree hot outer
atmosphere, the corona. Since more than 60 years the physical nature of the
processes heating the corona to temperatures well in excess of those on the
stellar surface remain puzzling. Recent progress in observational techniques
and numerical modeling now opens a new window to approach this problem. We
present the first coronal emission line spectra synthesized from
three-dimensional numerical models describing the evolution of the dynamics and
energetics as well as of the magnetic field in the corona. In these models the
corona is heated through motions on the stellar surface that lead to a braiding
of magnetic field lines inducing currents which are finally dissipated. These
forward models enable us to synthesize observed properties like (average)
emission line Doppler shifts or emission measures in the outer atmosphere,
which until now have not been understood theoretically, even though many
suggestions have been made in the past. As our model passes these observational
tests, we conclude that the flux braiding mechanism is a prime candidate for
being the dominant heating process of the magnetically closed corona of the Sun
and solar-like stars.Comment: 4 pages, 3 figures, submitted to Ap
Limits on the Boron Isotopic Ratio in HD 76932
Data in the 2090 A B region of HD 76932 have been obtained at high S/N using
the HST GHRS echelle at a resolution of 90,000. This wavelength region has been
previously identified as a likely candidate for observing the B11/B10 isotopic
splitting.
The observations do not match a calculated line profile extremely well at any
abundance for any isotopic ratio. If the B abundance previously determined from
observations at 2500 A is assumed, the calculated line profile is too weak,
indicating a possible blending line. Assuming that the absorption at 2090 A is
entirely due to boron, the best-fit total B abundance is higher than but
consistent with that obtained at 2500 A, and the best-fit isotopic ratio
(B11/B10) is in the range ~10:1 to ~4:1. If the absorption is not entirely due
to B and there is an unknown blend, the best-fit isotopic ratio may be closer
to 1:1. Future observations of a similar metal-poor star known to have
unusually low B should allow us to distinguish between these two possibilities.
The constraints that can be placed on the isotopic ratio based on comparisons
with similar observations of HD 102870 and HD 61421 (Procyon) are also
discussed.Comment: Accepted for Nov 1998 Ap
HST Observations of Heavy Elements in Metal-Poor Galactic Halo Stars
We present new abundance determinations of neutron-capture elements Ge, Zr,
Os, Ir, and Pt in a sample of 11 metal-poor (-3.1 <= [Fe/H] <= -1.6) Galactic
halo giant stars, based on Hubble Space Telescope UV and Keck I optical
high-resolution spectroscopy. The stellar sample is dominated by r-process-rich
stars such as the well-studied CS 22892-052 and bd+173248, but also includes
the r-process-poor, bright giant HD 122563. Our results demonstrate that
abundances of the 3rd r-process peak elements Os, Ir and Pt in these metal-poor
halo stars are very well-correlated among themselves, and with the abundances
of the canonical r-process element Eu (determined in other studies), thus
arguing for a common origin or site for r-process nucleosynthesis of heavier
(Z>56) elements. However, the large (and correlated) scatters of
[Eu,Os,Ir,Pt/Fe] suggests that the heaviest neutron-capture r-process elements
are not formed in all supernovae. In contrast, the Ge abundances of all program
stars track their Fe abundances, very well. An explosive process on iron-peak
nuclei (e.g., the alpha-rich freeze-out in supernovae), rather than neutron
capture, appears to have been the dominant synthesis mechanism for this element
at low metallicities -- Ge abundances seem completely uncorrelated with Eu.Comment: 35 pages, 5 tables, 7 figures; To appear in the Astrophysical Journa
A Laser System for the Spectroscopy of Highly-Charged Bismuth Ions
We present and characterize a laser system for the spectroscopy on
highly-charged ^209Bi^82+ ions at a wavelength of 243.87 nm. For absolute
frequency stabilization, the laser system is locked to a near-infra-red laser
stabilized to a rubidium transition line using a transfer cavity based locking
scheme. Tuning of the output frequency with high precision is achieved via a
tunable rf offset lock. A sample-and-hold technique gives an extended tuning
range of several THz in the UV. This scheme is universally applicable to the
stabilization of laser systems at wavelengths not directly accessible to atomic
or molecular resonances. We determine the frequency accuracy of the laser
system using Doppler-free absorption spectroscopy of Te_2 vapour at 488 nm.
Scaled to the target wavelength of 244 nm, we achieve a frequency uncertainty
of \sigma_{244nm} = 6.14 MHz (one standard deviation) over six days of
operation.Comment: Contribution to the special issue on "Trapped Ions" in "Applied
Physics B
A Far Ultraviolet Spectroscopic Explorer Survey of Coronal Forbidden Lines in Late-Type Stars
We present a survey of coronal forbidden lines detected in Far Ultraviolet
Spectroscopic Explorer (FUSE) spectra of nearby stars. Two strong coronal
features, Fe XVIII 974 A and Fe XIX 1118 A, are observed in 10 of the 26 stars
in our sample. Various other coronal forbidden lines, observed in solar flares,
also were sought but not detected. The Fe XVIII feature, formed at log T (K) =
6.8, appears to be free of blends, whereas the Fe XIX line can be corrupted by
a C I multiplet. FUSE observations of these forbidden iron lines at spectral
resolution R ~ 15,000 provides the opportunity to study dynamics of hot coronal
plasmas. We find that the velocity centroid of the Fe XVIII feature deviates
little from the stellar rest frame, confirming that the hot coronal plasma is
confined. The observed line widths generally are consistent with thermal
broadening at the high temperatures of formation and show little indication of
additional turbulent broadening. The fastest rotating stars, 31 Com, alpha Aur
Ab, and AB Dor, show evidence for excess broadening beyond the thermal
component and the photospheric v sin i. The anomalously large widths in these
fast rotating targets may be evidence for enhanced rotational broadening
consistent with emission from coronal regions extending an additional delta R ~
0.4-1.3 R_star above the stellar photosphere or represent the turbulent
broadening caused by flows along magnetic loop structures. For the stars in
which Fe XVIII is detected, there is an excellent correlation between the
observed Rontgensatellit} (ROSAT) 0.2-2.0 keV soft X-ray flux and the coronal
forbidden line flux. As a result, Fe XVIII is a powerful new diagnostic of
coronal thermal conditions and dynamics that can be utilized to study high
temperature plasma processes in late-type stars.Comment: 29 pages, 7 figures, AASTEX v5.0, accepted by Ap
Oscillator Strengths and Damping Constants for Atomic Lines in the J and H Bands
We have built a line list in the near-infrared J and H bands (1.00-1.34,
1.49-1.80 um) by gathering a series of laboratory and computed line lists.
Oscillator strengths and damping constants were computed or obtained by fitting
the solar spectrum.
The line list presented in this paper is, to our knowledge, the most complete
one now available, and supersedes previous lists.Comment: Accepted, Astrophysical Journal Supplement, tentatively scheduled for
the Sep. 1999 Vol. 124 #1 issue. Text and tables also available at
http://www.iagusp.usp.br/~jorge
Improved Laboratory Transition Probabilities for Er II and Applications to the Erbium Abundances of the Sun and Five r-Process Rich, Metal-Poor Stars
Recent radiative lifetime measurements accurate to +/- 5% (Stockett et al.
2007, J. Phys. B 40, 4529) using laser-induced fluorescence (LIF) on 8
even-parity and 62 odd-parity levels of Er II have been combined with new
branching fractions measured using a Fourier transform spectrometer (FTS) to
determine transition probabilities for 418 lines of Er II. This work moves Er
II onto the growing list of rare earth spectra with extensive and accurate
modern transition probability measurements using LIF plus FTS data. This
improved laboratory data set has been used to determine a new solar
photospheric Er abundance, log epsilon = 0.96 +/- 0.03 (sigma = 0.06 from 8
lines), a value in excellent agreement with the recommended meteoric abundance,
log epsilon = 0.95 +/- 0.03. Revised Er abundances have also been derived for
the r-process-rich metal-poor giant stars CS 22892-052, BD+17 3248, HD 221170,
HD 115444, and CS 31082-001. For these five stars the average Er/Eu abundance
ratio, = 0.42, is in very good agreement with the
solar-system r-process ratio. This study has further strengthened the finding
that r-process nucleosynthesis in the early Galaxy which enriched these
metal-poor stars yielded a very similar pattern to the r-process which enriched
later stars including the Sun.Comment: 20 pages, 4 tables, 10 figures; To be published in the Astrophysical
Journal Supplemen
The coronal line regions of planetary nebulae NGC6302 and NGC6537: 3-13um grating and echelle spectroscopy
We report on advances in the study of the cores of NGC6302 and NGC6537 using
infrared grating and echelle spectroscopy. In NGC6302, emission lines from
species spanning a large range of ionization potential, and in particular
[SiIX]3.934um, are interpreted using photoionization models (including CLOUDY),
which allow us to reestimate the central star's temperature to be about
250000K. All of the detected lines are consistent with this value, except for
[AlV] and [AlVI]. Aluminium is found to be depleted to one hundredth of the
solar abundance, which provides further evidence for some dust being mixed with
the highly ionized gas (with photons harder than 154eV). A similar depletion
pattern is observed in NGC6537. Echelle spectroscopy of IR coronal ions in
NGC6302 reveals a stratified structure in ionization potential, which confirms
photoionization to be the dominant ionization mechanism. The lines are narrow
(< 22km/s FWHM), with no evidence of the broad wings found in optical lines
from species with similar ionization potentials, such as [NeV]3426A. We note
the absence of a hot bubble, or a wind blown bipolar cavity filled with a hot
plasma, at least on 1'' and 10km/s scales. We also provide accurate new
wavelengths for several of the infrared coronal lines observed with the
echelle.Comment: Accepted for publication in MNRA
Further evidence for a variable fine-structure constant from Keck/HIRES QSO absorption spectra
[Abridged] We previously presented evidence for a varying fine-structure
constant, alpha, in two independent samples of Keck/HIRES QSO spectra. Here we
present a detailed many-multiplet analysis of a third Keck/HIRES sample
containing 78 absorption systems. We also re-analyse the previous samples,
providing a total of 128 absorption systems over the redshift range
0.2<z_abs<3.7. All three samples separately yield consistent, significant
values of da/a. The analyses of low- and high-z systems rely on different
ions/transitions with very different dependencies on alpha, yet they also give
consistent results. We identify additional random errors in 22 high-z systems
characterized by transitions with a large dynamic range in apparent optical
depth. Increasing the statistical errors on da/a for these systems gives our
fiducial result, a weighted mean da/a=(-0.543+/-0.116)x10^-5, representing
4.7-sigma evidence for a smaller weighted mean alpha in the absorption clouds.
Assuming that da/a=0 at z_abs=0, the data marginally prefer a linear increase
in alpha with time: dota/a=(6.40+/-1.35)x10^-16 yr^-1. The two-point
correlation function for alpha is consistent with zero over 0.2-13 Gpc comoving
scales and the angular distribution of da/a shows no significant dipolar
anisotropy. We therefore have no evidence for spatial variations in da/a. We
extend our previous searches for possible systematic errors, identifying
atmospheric dispersion and isotopic structure effects as potentially the most
significant. However, overall, known systematic errors do not explain the
results. Future many-multiplet analyses of QSO spectra from different
telescopes and spectrographs will provide a now crucial check on our Keck/HIRES
results.Comment: 31 pages, 25 figures (29 EPS files), 8 tables. Accepted by MNRAS.
Colour versions of Figs. 6, 8 & 10 and text version of Table 3 available at
http://www.ast.cam.ac.uk/~mim/pub.htm
On Solving the Coronal Heating Problem
This article assesses the current state of understanding of coronal heating,
outlines the key elements of a comprehensive strategy for solving the problem,
and warns of obstacles that must be overcome along the way.Comment: Accepted by Solar Physics; Published by Solar Physic
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