1,238 research outputs found
The Quiet-Sun Photosphere and Chromosphere
The overall structure and the fine structure of the solar photosphere outside
active regions are largely understood, except possibly important roles of a
turbulent near-surface dynamo at its bottom, internal gravity waves at its top,
and small-scale vorticity. Classical 1D static radiation-escape modelling has
been replaced by 3D time-dependent MHD simulations that come closer to reality.
The solar chromosphere, in contrast, remains ill-understood although its
pivotal role in coronal mass and energy loading makes it a principal research
area. Its fine structure defines its overall structure, so that hard-to-observe
and hard-to-model small-scale dynamical processes are the key to understanding.
However, both chromospheric observation and chromospheric simulation presently
mature towards the required sophistication. The open-field features seem of
greater interest than the easier-to-see closed-field features.Comment: Accepted for special issue "Astrophysical Processes on the Sun" of
Phil. Trans. Royal Soc. A, ed. C. Parnell. Note: clicking on the year in a
citation opens the corresponding ADS abstract page in the browse
Addendum: "The Dynamics of M15: Observations of the Velocity Dispersion Profile and Fokker-Planck Models" (ApJ, 481, 267 [1997])
It has recently come to our attention that there are axis scale errors in
three of the figures of Dull et al. (1997, hereafter D97). D97 presented
Fokker-Planck models for the collapsed-core globular cluster M15 that include a
dense, centrally concentrated population of neutron stars and massive white
dwarfs, but do not include a central black hole. In this Addendum, we present
corrected versions of Figures 9, 10, and 12, and an expanded version of Figure
6. This latter figure, which shows the full run of the velocity dispersion
profile, indicates that the D97 model predictions are in good agreement with
the moderately rising HST-STIS velocity dispersion profile for M15 reported by
Gerssen et al. (2002, astro-ph/0209315). Thus, a central black hole is not
required to fit the new STIS velocity measurements, provided that there is a
sufficient population of neutron stars and massive white dwarfs. This
conclusion is consistent with the findings of Gerssen et al. (2002,
astro-ph/0210158), based on a reapplication of their Jeans equation analysis
using the corrected mass-to-light profile (Figure 12) for the D97 models.Comment: 4 pages, 4 figures, submitted to Ap
Aperture Increase Options for the Dutch Open Telescope
This paper is an invitation to the international community to participate in
the usage and a substantial upgrade of the Dutch Open Telescope on La Palma
(DOT, \url{http://dot.astro.uu.nl}).
We first give a brief overview of the approach, design, and current science
capabilities of the DOT.
The DOT database (\url{http://dotdb.phys.uu.nl/DOT}) now contains many
tomographic image sequences with 0.2-0.3 arcsec resolution and up to multi-hour
duration. You are welcome to pull them over for analysis.
The main part of this contribution outlines DOT upgrade designs implementing
larger aperture. The motivation for aperture increase is the recognition that
optical solar physics needs the substantially larger telescope apertures that
became useful with the advent of adaptive optics and viable through the DOT's
open principle, both for photospheric polarimetry at high resolution and high
sensitivity and for chromospheric fine-structure diagnosis at high cadence and
full spectral sampling.
Realization of an upgrade requires external partnership(s). This report about
DOT upgrade options therefore serves also as initial documentation for
potential partners.Comment: in press,"Physics of Chromospheric Plasmas" (Coimbra), ASP 368, 573
(2007
Convective Dynamos and the Minimum X-ray Flux in Main Sequence Stars
The objective of this paper is to investigate whether a convective dynamo can
account quantitatively for the observed lower limit of X-ray surface flux in
solar-type main sequence stars. Our approach is to use 3D numerical simulations
of a turbulent dynamo driven by convection to characterize the dynamic
behavior, magnetic field strengths, and filling factors in a non-rotating
stratified medium, and to predict these magnetic properties at the surface of
cool stars. We use simple applications of stellar structure theory for the
convective envelopes of main-sequence stars to scale our simulations to the
outer layers of stars in the F0--M0 spectral range, which allows us to estimate
the unsigned magnetic flux on the surface of non-rotating reference stars. With
these estimates we use the recent results of \citet{Pevtsov03} to predict the
level of X-ray emission from such a turbulent dynamo, and find that our results
compare well with observed lower limits of surface X-ray flux. If we scale our
predicted X-ray fluxes to \ion{Mg}{2} fluxes we also find good agreement with
the observed lower limit of chromospheric emission in K dwarfs. This suggests
that dynamo action from a convecting, non-rotating plasma is a viable
alternative to acoustic heating models as an explanation for the basal emission
level seen in chromospheric, transition region, and coronal diagnostics from
late-type stars.Comment: ApJ, accepted, 30 pages with 7 figure
On a mechanism for enhancing magnetic activity in tidally interacting binaries
We suggest a mechanism for enhancing magnetic activity in tidally interacting
binaries. We suppose that the deviation of the primary star from spherical
symmetry due to the tidal influence of the companion leads to stellar pulsation
in its fundamental mode. It is shown that stellar radial pulsation amplifies
torsional Alfv{\'e}n waves in a dipole-like magnetic field, buried in the
interior, according to the recently proposed swing wave-wave interaction
(Zaqarashvili 2001). Then amplified Alfv{\'e}n waves lead to the onset of
large-scale torsional oscillations, and magnetic flux tubes arising towards the
surface owing to magnetic buoyancy diffuse into the atmosphere producing
enhanced chromospheric and coronal emission.Comment: Accepted in Ap
A Systematic Investigation on the Hemalog
Peer Reviewe
Stellar Iron Abundances: non-LTE Effects
We report new statistical equilibrium calculations for Fe I and Fe II in the
atmosphere of Late-Type stars. We used atomic models for Fe I and Fe II having
respectively 256 and 190 levels, as well as 2117 and 3443 radiative
transitions. Photoionization cross-sections are from the Iron Project. These
atomic models were used to investigate non-LTE effects in iron abundances of
Late-Type stars with different atmospheric parameters.
We found that most Fe I lines in metal-poor stars are formed in conditions
far from LTE. We derived metallicity corrections of about 0.3 dex with respect
to LTE values, for the case of stars with [Fe/H] ~ -3.0. Fe II is found not to
be affected by significant non-LTE effects. The main non-LTE effect invoked in
the case of Fe I is overionization by ultraviolet radiation, thus classical
ionization equilibrium is far to be satisfied. An important consequence is that
surface gravities derived by LTE analysis are in error and should be corrected
before final abundances corrections.
This apparently solves the observed discrepancy between spectroscopic surface
gravities derived by LTE analyses and those derived from Hipparcos parallaxes.
A table of non-LTE [Fe/H] and log g values for a sample of metal-poor late-type
stars is given.Comment: 22 pages, 9 figures, 1 table, ApJ style, accepte
The system parameters of DW Ursae Majoris
We present new constraints on the system parameters of the SW Sextantis star
DW Ursae Majoris, based on ultraviolet (UV) eclipse observations with the
Hubble Space Telescope. Our data were obtained during a low state of the
system, in which the UV light was dominated by the hot white dwarf (WD)
primary. Eclipse analysis, using the full Roche lobe geometry, allows us to set
firm limits on the masses and radii of the system components and the distance
between them: 0.67 \leq M_1/M_sun \leq 1.06, 0.008 \leq R_1/R_sun \leq 0.014,
M_2/M_sun > 0.16, R_2/R_sun > 0.28 and a/R_sun > 1.05. For q = M_2/M_1 < 1.5
the inclination must satisfy i > 71 degrees. Using Smith & Dhillon's
mass-period relation for CV secondaries, our estimates for the system
parameters become M_1/M_sun = 0.77 \pm 0.07, R_1/R_sun = 0.012 \pm 0.001,
M_2/M_sun = 0.30 \pm 0.10, R_2/R_sun = 0.34 \pm 0.04, q =0.39 \pm 0.12, i = 82
\pm 4 degrees and a/R_sun = 1.14 \pm 0.06. We have also estimated the spectral
type of the secondary, M3.5 \pm 1.0, and distance to the system, d =930 \pm 160
pc, from time-resolved I- and K-band photometry. Finally, we have repeated
Knigge et al.'s WD model atmosphere fit to the low-state UV spectrum of DW UMa
in order to account for the higher surface gravity indicated by our eclipse
analysis. In this way we obtained a second estimate for the distance, d = 590
\pm 100 pc, which allows us to obtain a second estimate for the spectral type
of the secondary, M7 \pm 2.0. We conclude that the true value for the distance
and spectral type will probably be in between the values obtained by the two
methods.Comment: 23 pages including 5 figures and 3 tables. Accepted for publication
in Ap
HST Observations of Chromospheres in Metal Deficient Field Giants
HST high resolution spectra of metal-deficient field giants more than double
the stars in previous studies, span about 3 magnitudes on the red giant branch,
and sample an abundance range [Fe/H]= -1 to -3. These stars, in spite of their
age and low metallicity, possess chromospheric fluxes of Mg II (2800 Angstrom)
that are within a factor of 4 of Population I stars, and give signs of a
dependence on the metal abundance at the lowest metallicities. The Mg II k-line
widths depend on luminosity and correlate with metallicity. Line profile
asymmetries reveal outflows that occur at lower luminosities (M_V = -0.8) than
detected in Ca K and H-alpha lines in metal-poor giants, suggesting mass
outflow occurs over a larger span of the red giant branch than previously
thought, and confirming that the Mg II lines are good wind diagnostics. These
results do not support a magnetically dominated chromosphere, but appear more
consistent with some sort of hydrodynamic, or acoustic heating of the outer
atmospheres.Comment: 36 pages, 12 figures, 7 tables, and accepted for publication in The
Astronomical Journa
Irradiation of the secondary star in X-ray Nova Scorpii 1994 (=GRO J1655--40)
We have obtained intermediate resolution optical spectra of the black-hole
candidate Nova Sco 1994 in June 1996, when the source was in an X-ray/optical
active state (R~15.05). We measure the radial velocity curve of the secondary
star and obtain a semi-amplitude of 279+/-10 km/s; a value which is 30 per cent
larger than the value obtained when the source is in quiescence. Our large
value for K_2 is consistent with 60 +9,-7 per cent of the secondary star's
surface being heated; compared to 35 per cent, which is what one would expect
if only the inner face of the secondary star were irradiated. Effects such as
irradiation-induced flows on the secondary star may be important in explaining
the observed large value for K_2.Comment: 5 pages, 2 figures, accepted by MNRA
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