1,195 research outputs found
Eigenoscillations of the Differentially Rotating Sun: I. 22-year, 4000-year, and quasi-biennial modes
Retrograde waves with frequencies much lower than the rotation frequency
become trapped in the solar radiative interior. The eigenfunctions of the
compressible, nonadiabatic, Rossby-like modes (-mechanism and
radiative losses taken into account) are obtained by an asymptotic method
assuming a very small latitudinal gradient of rotation, without an arbitrary
choice of other free parameters. An integral dispersion relation for the
complex eigenfrequencies is derived as a solution of the boundary value
problem. The discovered resonant cavity modes (called R-modes) are
fundamentally different from the known r-modes: their frequencies are functions
of the solar interior structure, and the reason for their existence is not
related to geometrical effects. The most unstable R-modes are those with
periods of 1--3 yr, 18--30 yr, and 1500--20000 yrs; these three separate period
ranges are known from solar and geophysical data. The growing times of those
modes which are unstable with respect to the -mechanism are and years, respectively. The amplitudes of the R-modes are
growing towards the center of the Sun. We discuss some prospects to develop the
theory of R-modes as a driver of the dynamics in the convective zone which
could explain, e.g., observed short-term fluctuations of rotation, a control of
the solar magnetic cycle, and abrupt changes of terrestrial climate in the
past.Comment: 17 pages, 6 figures, To appear in Astronomy and Astrophysic
XMM-Newton observations of EF Eridani: the textbook example of low-accretion rate polars
Archival X-ray observations of EF Eridani obtained in a low state revealed
distinct X-ray detections at a luminosity L_X ~ 2 10^{29} erg/s, three orders
of magnitude below its high state value. The plasma temperature was found to be
as low as kT \loa 2 keV, a factor 10 below the high state. The X-ray/UV/IR
spectral energy distribution suggests faint residual accretion rather than
coronal emission as being responsible for the low-state X-ray emission. EF Eri
thus showed a clear transition from being shock-dominated in the high state to
be cyclotron-dominated in the low state. From the optical/UV spectral energy
distribution we re-determine the photospheric temperature of the white dwarf to
\~10000K. Contrary to earlier claims, WD model atmospheres produce sufficient
UV flux to reproduce the published GALEX flux and orbital modulation.Comment: A&A, in pres
V405 Peg (RBS 1955): A Nearby, Low-Luminosity Cataclysmic Binary
(Abridged). The cataclysmic binary V405 Peg, originally discovered as ROSAT
Bright Source (RBS) 1955 (= 1RXS J230949.6+213523), shows a strong contribution
from a late-type secondary star in its optical spectrum, which led Schwope et
al. to suggest it to be among the nearest cataclysmic binaries. We present
extensive optical observations of V405 Peg. Time-series spectroscopy shows the
orbital period, Porb, to be 0.1776469(7) d (= 4.2635 hr), or 5.629 cycle/d. We
classify the secondary as M3 - M4.5. Astrometry with the MDM 2.4m telescope
gives a parallax 7.2 +- 1.1 milli-arcsec, and a relative proper motion of 58
mas/yr. Our best estimate of the distance yields d = 149 (+26, -20) pc. The
secondary stars's radial velocity has K2 = 92 +- 3 km/s, indicating a fairly
low orbital inclination if the masses are typical. Extensive I-band time-series
observations in the show the system varying between a minimum brightness level
of I = 14.14 and states of enhanced activity about 0.2 mag brighter. While the
low-state shows an ellipsoidal modulation, an additional photometric modulation
appears in the high state, with 0.1 mag amplitude and period 220-280 min. The
frequency of this modulation appears to be stable for a month or so, but no
single period was consistently detected from one observing season to the next.
We estimate the system luminosity by combining optical measurements with the
archival X-ray spectrum. The implied mass accretion rate is orders of
magnitudes below the predictions for the standard angular momentum loss above
the period gap. The system may possibly belong to a largely undiscovered
population of hibernating CVs.Comment: 11 figures; 7 of these are .png or .jpg to save space. In press for
Publications of the Astronomical Society of the Pacifi
Performance of the ATLAS Muon Drift-Tube Chambers at High Background Rates and in Magnetic Fields
The ATLAS muon spectrometer uses drift-tube chambers for precision tracking.
The performance of these chambers in the presence of magnetic field and high
radiation fluxes is studied in this article using test-beam data recorded in
the Gamma Irradiation Facility at CERN. The measurements are compared to
detailed predictions provided by the Garfield drift-chamber simulation
programme
The changing accretion states of the intermediate polar MU Camelopardalis
We study the timing and spectral properties of the intermediate polar MU
Camelopardalis (1RXS J062518.2+733433) to determine the accretion modes and
the accretion geometry from multi-wavelength, multi-epoch observational data.
Light curves in different observed energy ranges (optical, UV, X-ray) are
extracted. The timescales of variability in these light curves are determined
using Analysis of Variance. Phase-resolved X-ray spectra are created with
respect to the most prominent detected periodicities and each fitted with an
identical model, to quantify the differences in the fitted components. The
published tentative value for the spin period is unambiguously identified with
the rotation period of the white dwarf. We detect a distinct soft X-ray
component that can be reproduced well by a black body. The analysis of data
obtained at different epochs demonstrates that the system is changing its
accretion geometry from disk-dominated to a combination of disk- plus
stream-dominated, accompanied with a significant change in brightness at
optical wavelengths.Comment: Accepted for publication in Astron. Astrophys., 13 pages, 10 figure
Performance of the ATLAS Precision Muon Chambers under LHC Operating Conditions
For the muon spectrometer of the ATLAS detector at the large hadron collider
(LHC), large drift chambers consisting of 6 to 8 layers of pressurized drift
tubes are used for precision tracking covering an active area of 5000 m2 in the
toroidal field of superconducting air core magnets. The chambers have to
provide a spatial resolution of 41 microns with Ar:CO2 (93:7) gas mixture at an
absolute pressure of 3 bar and gas gain of 2?104. The environment in which the
chambers will be operated is characterized by high neutron and background with
counting rates of up to 100 per square cm and second. The resolution and
efficiency of a chamber from the serial production for ATLAS has been
investigated in a 100 GeV muon beam at photon irradiation rates as expected
during LHC operation. A silicon strip detector telescope was used as external
reference in the beam. The spatial resolution of a chamber is degraded by 4 ?m
at the highest background rate. The detection efficiency of the drift tubes is
unchanged under irradiation. A tracking efficiency of 98% at the highest rates
has been demonstrated
Resolution and Efficiency of the ATLAS Muon Drift-Tube Chambers at High Background Rates
The resolution and efficiency of a precision drift-tube chamber for the ATLAS
muon spectrometer with final read-out electronics was tested at the Gamma
Irradiation Facility at CERN in a 100 GeV muon beam and at photon irradiation
rates of up to 990 Hz/square cm which corresponds to twice the highest
background rate expected in ATLAS. A silicon strip detector telescope was used
as external reference in the beam. The pulse-height measurement of the read-out
electronics was used to perform time-slewing corrections which lead to an
improvement of the average drift-tube resolution from 104 microns to 82 microns
without irradiation and from 128 microns to 108 microns at the maximum expected
rate. The measured drift-tube efficiency agrees with the expectation from the
dead time of the read-out electronics up to the maximum expected rate
Calculation of Spectral Darkening and Visibility Functions for Solar Oscillations
Calculations of spectral darkening and visibility functions for the
brightness oscillations of the Sun resulting from global solar oscillations are
presented. This has been done for a broad range of the visible and infrared
continuum spectrum. The procedure for the calculations of these functions
includes the numerical computation of depth-dependent derivatives of the
opacity caused by p modes in the photosphere. A radiative-transport code was
used for this purpose to get the disturbances of the opacities from temperature
and density fluctuations. The visibility and darkening functions are obtained
for adiabatic oscillations under the assumption that the temperature
disturbances are proportional to the undisturbed temperature of the
photosphere. The latter assumption is the only way to explore any opacity
effects since the eigenfunctions of p-mode oscillations have not been obtained
so far. This investigation reveals that opacity effects have to be taken into
account because they dominate the violet and infrared part of the spectrum.
Because of this dominance, the visibility functions are negative for those
parts of the spectrum. Furthermore, the darkening functions show a
wavelength-dependent change of sign for some wavelengths owing to these opacity
effects. However, the visibility and darkening functions under the assumptions
used contradict the observations of global p-mode oscillations, but it is
beyond doubt that the opacity effects influence the brightness fluctuations of
the Sun resulting from global oscillations
Evolution of dust and ice features around FU Orionis objects
(abridged) We present spectroscopy data for a sample of 14 FUors and 2 TTauri
stars observed with the Spitzer Space Telescope or with the Infrared Space
Observatory (ISO). Based on the appearance of the 10 micron silicate feature we
define 2 categories of FUors. Objects showing the silicate feature in
absorption (Category 1) are still embedded in a dusty and icy envelope. The
shape of the 10 micron silicate absorption bands is compared to typical dust
compositions of the interstellar medium and found to be in general agreement.
Only one object (RNO 1B) appears to be too rich in amorphous pyroxene dust, but
a superposed emission feature can explain the observed shape. We derive optical
depths and extinction values from the silicate band and additional ice bands at
6.0, 6.8 and 15.2 micron. In particular the analysis of the CO_2 ice band at
15.2 micron allows us to search for evidence for ice processing and constrains
whether the absorbing material is physically linked to the central object or in
the foreground. For objects showing the silicate feature in emission (Category
2), we argue that the emission comes from the surface layer of accretion disks.
Analyzing the dust composition reveals that significant grain growth has
already taken place within the accretion disks, but no clear indications for
crystallization are present. We discuss how these observational results can be
explained in the picture of a young, and highly active accretion disk. Finally,
a framework is proposed as to how the two categories of FUors can be understood
in a general paradigm of the evolution of young, low-mass stars. Only one
object (Parsamian 21) shows PAH emission features. Their shapes, however, are
often seen toward evolved stars and we question the object's status as a FUor
and discuss other possible classifications.Comment: accepted for publication in ApJ; 63 pages preprint style including 8
tables and 24 figure
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