344 research outputs found
Electron gas polarization effect induced by heavy H-like ions of moderate velocities channeled in a silicon crystal
We report on the observation of a strong perturbation of the electron gas
induced by 20 MeV/u U ions and 13 MeV/u Pb ions channeled in
silicon crystals. This collective response (wake effect) in-duces a shift of
the continuum energy level by more than 100 eV, which is observed by means of
Radiative Electron Capture into the K and L-shells of the projectiles. We also
observe an increase of the REC probability by 20-50% relative to the
probability in a non-perturbed electron gas. The energy shift is in agreement
with calculations using the linear response theory, whereas the local electron
density enhancement is much smaller than predicted by the same model. This
shows that, for the small values of the adiabaticity parameter achieved in our
experiments, the density fluctuations are not strongly localized at the
vicinity of the heavy ions
Single Transfer-Excitation Resonance Observed Via the Two-Photon Decay in He-like GeÂłâ°âș
We measured the 2E1 decay of the 1s2s 1S0 â1s2 1S0 transition in He-like germanium for 12- to 19-MeV/u Ge31+ +H2 collisions. The resonant population of the 2s2p 1P1 state by transfer excitation was isolated due to its cascading to the 1s2s 1S0 state. The experimental cross sections compare well with calculations using dielectronic recombination rates. The method gives the unique possibility to populate selectively the 1S0 state in heavy He-like ions
A new strongly X-ray flaring M9 dwarf in the solar neighborhood
We report on the discovery of a very low mass (VLM) star in the solar
neighborhood, originally identified as an optical counterpart of a flaring
X-ray source detected in the ROSAT All-Sky survey. Optical spectroscopy and
infrared photometry consistently reveal a spectral type of M9 \pm 0.5 and a
distance of ~11 pm 2pc. The optical counterpart of 1RXS J115928.5-524717 shows
a large proper motion of 1.08\pm0.06 "/year. 1RXS~J115928.5-524717 is the
fourth object among the VLM stars displaying a huge X-ray flare, reaching the
unprecedent value of L_X/L_{bol}~0.1.Comment: 8 pages, 5 figures. A&A, accepte
Isotope shift in the dielectronic recombination of three-electron ^{A}Nd^{57+}
Isotope shifts in dielectronic recombination spectra were studied for Li-like
^{A}Nd^{57+} ions with A=142 and A=150. From the displacement of resonance
positions energy shifts \delta E^{142,150}(2s-2p_1/2)= 40.2(3)(6) meV
(stat)(sys)) and \delta E^{142,150}(2s-2p_3/2) = 42.3(12)(20) meV of 2s-2p_j
transitions were deduced. An evaluation of these values within a full QED
treatment yields a change in the mean-square charge radius of ^{142,150}\delta
= -1.36(1)(3) fm^2. The approach is conceptually new and combines the
advantage of a simple atomic structure with high sensitivity to nuclear size.Comment: 10 pages, 3 figures, accepted for publication in Physical Review
Letter
The enigmatic young brown dwarf binary FU Tau: accretion and activity
FU Tau belongs to a rare class of young, wide brown dwarf binaries. We have
resolved the system in a Chandra X-ray observation and detected only the
primary, FU Tau A. Hard X-ray emission, presumably from a corona, is present
but, unexpectedly, we detect also a strong and unusually soft component from FU
Tau A. Its X-ray properties, so far unique among brown dwarfs, are very similar
to those of the T Tauri star TW Hya. The analogy with TW Hya suggests that the
dominating soft X-ray component can be explained by emission from accretion
shocks. However, the typical free-fall velocities of a brown dwarf are too low
for an interpretation of the observed X-ray temperature as post-shock region.
On the other hand, velocities in excess of the free-fall speed are derived from
archival optical spectroscopy, and independent pieces of evidence for strong
accretion in FU Tau A are found in optical photometry. The high X-ray
luminosity of FU Tau A coincides with a high bolometric luminosity confirming
an unexplained trend among young brown dwarfs. In fact, FU Tau A is
overluminous with respect to evolutionary models while FU Tau B is on the 1 Myr
isochrone suggesting non-contemporaneous formation of the two components in the
binary. The extreme youth of FU Tau A could be responsible for its peculiar
X-ray properties, in terms of atypical magnetic activity or accretion.
Alternatively, rotation and magnetic field effects may reduce the efficiency of
convection which in turn affects the effective temperature and radius of FU Tau
A shifting its position in the HR diagram. Although there is no direct prove of
this latter scenario so far we present arguments for its plausibility.Comment: Accepted for publication in MNRAS; 9 pages, 5 figure
Radiative recombination of bare Bi83+: Experiment versus theory
Electron-ion recombination of completely stripped Bi83+ was investigated at
the Experimental Storage Ring (ESR) of the GSI in Darmstadt. It was the first
experiment of this kind with a bare ion heavier than argon. Absolute
recombination rate coefficients have been measured for relative energies
between ions and electrons from 0 up to about 125 eV. In the energy range from
15 meV to 125 eV a very good agreement is found between the experimental result
and theory for radiative recombination (RR). However, below 15 meV the
experimental rate increasingly exceeds the RR calculation and at Erel = 0 eV it
is a factor of 5.2 above the expected value. For further investigation of this
enhancement phenomenon the electron density in the interaction region was set
to 1.6E6/cm3, 3.2E6/cm3 and 4.7E6/cm3. This variation had no significant
influence on the recombination rate. An additional variation of the magnetic
guiding field of the electrons from 70 mT to 150 mT in steps of 1 mT resulted
in periodic oscillations of the rate which are accompanied by considerable
changes of the transverse electron temperature.Comment: 12 pages, 14 figures, to be published in Phys. Rev. A, see also
http://www.gsi.de/ap/ and http://www.strz.uni-giessen.de/~k
A representative particle approach to coagulation and fragmentation of dust aggregates and fluid droplets
Context: There is increasing need for good algorithms for modeling the
aggregation and fragmentation of solid particles (dust grains, dust aggregates,
boulders) in various astrophysical settings, including protoplanetary disks,
planetary- and sub-stellar atmospheres and dense molecular cloud cores. Here we
describe a new algorithm that combines advantages of various standard methods
into one.
Aims: The aim is to develop a method that 1) can solve for aggregation and
fragmentation, 2) can easily include the effect and evolution of grain
properties such as compactness, composition, etc., and 3) can be built as a
coagulation/fragmentation module into a hydrodynamics simulations.
Methods: We develop a Monte-Carlo method in which we follow the 'life' of a
limited number of representative particles. Each of these particles is
associated with a certain fraction of the total dust mass and thereby
represents a large number of true particles which all are assumed to have the
same properties as their representative particle. Under the assumption that the
total number of true particles vastly exceeds the number of representative
particles, the chance of a representative particle colliding with another
representative particle is negligibly small, and we therefore ignore this
possibility. This now makes it possible to employ a statistical approach to the
evolution of the representative particles.
Results: The method reproduces the known analytic solutions of simplified
coagulation kernels, and compares well to numerical results for Brownian motion
using other methods. For reasonably well-behaved kernels it produces good
results even for moderate number of swarms.Comment: accepted for publication in A&
Rotation and variability of very low mass stars and brown dwarfs near Epsilon Ori
We explore the rotation and activity of very low mass (VLM) objects by means
of a photometric variability study. Our targets in the vicinity of Epsilon Ori
belong to the OriOB1b population in the Orion star-forming complex. In this
region we selected 143 VLM stars and brown dwarfs (BDs), whose photometry in
RIJHK is consistent with membership of the young population. The variability of
these objects was investigated using a densely sampled I-band time series
covering four consecutive nights with altogether 129 data points per object.
Our targets show three types of variability: Thirty objects, including nine
BDs, show significant photometric periods, ranging from 4h up to 100h, which we
interpret as the rotation periods. Five objects, including two BDs, exhibit
variability with high amplitudes up to 1 mag which is at least partly
irregular. This behaviour is most likely caused by ongoing accretion and
confirms that VLM objects undergo a T Tauri phase similar to solar-mass stars.
Finally, one VLM star shows a strong flare event of 0.3 mag amplitude. The
rotation periods show dependence on mass, i.e. the average period decreases
with decreasing object mass, consistent with previously found mass-period
relationships in younger and older clusters. The period distribution of BDs
extends down to the breakup period, where centrifugal and gravitational forces
are balanced. Combining our BD periods with literature data, we found that the
lower period limit for substellar objects lies between 2h and 4h, more or less
independent of age. Contrary to stars, these fast rotating BDs seem to evolve
at constant rotation period from ages of 3 Myr to 1 Gyr, in spite of the
contraction process. Thus, they should experience strong rotational braking.Comment: 17 pages, 13 figures, A&A, in pres
Rotation and accretion of very low mass objects in the SigmaOri cluster
We report on two photometric monitoring campaigns of Very Low Mass (VLM)
objects in the young open cluster around SigmaOrionis. Our targets were
pre-selected with multi-filter photometry in a field of 0.36 sqdeg. For 23 of
these objects, spanning a mass range from 0.03 to 0.7 MS, we detect periodic
variability. Of these, 16 exhibit low-level variability, with amplitudes of
less than 0.2 mag in the I-band, which is mostly well-approximated by a sine
wave. These periodicities are probably caused by photospheric spots co-rotating
with the objects. In contrast, the remaining variable targets show high-level
variability with amplitudes ranging from 0.25 to 1.1 mag, consisting of a
periodic light variation onto which short-term fluctuations are superimposed.
This variability pattern is very similar to the photometric behaviour of
solar-mass, classical T Tauri stars. Low-resolution spectra of a few of these
objects reveal strong Halpha and Ca-triplet emission, indicative of ongoing
accretion processes. This suggests that 5-7% of our targets still possess a
circumstellar disk. In combination with previous results for younger objects,
this translates into a disk lifetime of 3-4 Myr, significantly shorter than for
solar mass stars. The highly variable objects rotate on average slower than the
low-amplitude variables, which is expected in terms of a disk-locking scenario.
There is a trend towards faster rotation with decreasing mass, which might be
caused by shortening of the disk lifetimes or attenuation of magnetic fields.Comment: 19 pages, 14 figures, A&A, in pres
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