735 research outputs found
Space and Ground Based Pulsation Data of Eta Bootis Explained with Stellar Models Including Turbulence
The space telescope MOST is now providing us with extremely accurate low
frequency p-mode oscillation data for the star Eta Boo. We demonstrate in this
paper that these data, when combined with ground based measurements of the high
frequency p-mode spectrum, can be reproduced with stellar models that include
the effects of turbulence in their outer layers. Without turbulence, the l=0
modes of our models deviate from either the ground based or the space data by
about 1.5-4.0 micro Hz. This discrepancy can be completely removed by including
turbulence in the models and we can exactly match 12 out of 13 MOST frequencies
that we identified as l=0 modes in addition to 13 out of 21 ground based
frequencies within their observational 2 sigma tolerances. The better agreement
between model frequencies and observed ones depends for the most part on the
turbulent kinetic energy which was taken from a 3D convection simulation for
the Sun.Comment: 13 pages, 7 figures, ApJ in pres
On the interpretation of echelle diagrams for solar-like oscillations. Effect of centrifugal distortion
This work aims at determining the impact of slow to moderate rotation on the
regular patterns often present in solar-like oscillation spectra. We focus on
the well-known asteroseismic diagnostic echelle diagrams, examining how
rotation may modify the estimates of the large and small spacings, as well as
the identification of modes. We illustrate the work with a real case: the
solar-like star Bootis. The modeling takes into account rotation effects
on the equilibrium models through an effective gravity and on the oscillation
frequencies through both perturbative and non-perturbative calculations. We
compare the results of both type of calculations in the context of the regular
spacings (like the small spacings and the scaled small spacings) and echelle
diagrams. We show that for echelle diagrams the perturbative approach remains
valid for rotational velocities up to 40-50 km/s. We show that for the
rotational velocities measured in solar-like stars, theoretical oscillation
frequencies must be corrected up to the second-order in terms of rotation rate,
including near degeneracy effects. For rotational velocities of about 16 km/S
and higher, diagnostics on large spacings and on modal identification through
echelle diagrams can be significantly altered by the presence of the
components of the rotationally split modes. We found these effects to be
detectable in the observed frequency range. Analysis of the effects of rotation
on small spacings and scaled small spacings reveals that these can be of the
order of, or even larger than surface effects, typically turbulence,
microscopic diffusion, etc. Furthermore, we show that scaled spacings are
significantly affected by stellar distortion even for small stellar rotational
velocities (from 10-15 km/s) and therefore some care must be taken when using
them as indicators for probing deep stellar interiors.Comment: 10 pages,5 figures, accepted for publication in ApJ;
http://iopscience.iop.org/0004-637X/721/1/537
Possible Signatures of a Cold-Flow Disk from MUSE using a z=1 galaxy--quasar pair towards SDSSJ1422-0001
We use a background quasar to detect the presence of circum-galactic gas
around a low-mass star forming galaxy. Data from the new Multi Unit
Spectroscopic Explorer (MUSE) on the VLT show that the host galaxy has a
dust-corrected star-formation rate (SFR) of 4.70.2 Msun/yr, with no
companion down to 0.22 Msun/yr (5 ) within 240 kpc (30"). Using a
high-resolution spectrum (UVES) of the background quasar, which is fortuitously
aligned with the galaxy major axis (with an azimuth angle of only
), we find, in the gas kinematics traced by low-ionization lines,
distinct signatures consistent with those expected for a "cold flow disk"
extending at least 12 kpc (). We estimate the mass accretion
rate to be at least two to three times larger than the SFR,
using the geometric constraints from the IFU data and the HI column density of
obtained from a {\it HST}/COS NUV spectrum. From
a detailed analysis of the low-ionization lines (e.g. ZnII, CrII, TiII, MnII,
SiII), the accreting material appears to be enriched to about 0.4
(albeit with large uncertainties: ), which is
comparable to the galaxy metallicity (), implying a
large recycling fraction from past outflows. Blue-shifted MgII and FeII
absorptions in the galaxy spectrum from the MUSE data reveal the presence of an
outflow. The MgII and FeII doublet ratios indicate emission infilling due to
scattering processes, but the MUSE data do not show any signs of fluorescent
FeII* emission.Comment: 17 pages, 11 figures, in press (ApJ), minor edits after the proofs.
Data available at http://muse-vlt.eu/science/j1422
Asteroseismology across the HR diagram
High precision spectroscopy provides essential information necessary to fully
exploit the opportunity of probing the internal structure of stars using
Asteroseismology. In this work we discuss how Asteroseismology combined with
High Precision Spectroscopy can establish a detailed view on stellar structure
and evolution of stars across the HR diagramme.Comment: 6 pages, 2 figures - to appear in Precision Spectroscopy in
Astrophysics, (Eds) L. Pasquini, M. Romaniello, N.C. Santos, and A. Correia,
ESO Astrophysics Symposia, 200
Current Issues in Asteroseismology
In this contribution we briefly review some of the current issues and
promises for the future by asteroseismology. We are entering a new phase in
this field driven by the wealth of data that has been collected and data that
will soon be available for asteroseismology across the HR Diagram. Major
difficulties in the descriptions of stellar interiors that arose in the second
half of the 20th century may now be in part addressed and solved (this is the
expectation!) by asteroseismology with unprecedented precision. In this
contribution we list some of the key open questions in stellar physics, the
seismic data we expect to collect in the near future, and some techniques that
will provide the tools to connect data and models.Comment: 9 pages, 2 figures - to appear in Helioseismology, Asteroseismology
and MHD Connections, (Eds) L. Gizon et al., Journal of Physics Conference
Series, 2008 [Revision 1 - english; Revision 2 - references
Coexistence of ferro- and antiferromagnetic order in Mn-doped NiMnGa
Ni-Mn-Ga is interesting as a prototype of a magnetic shape-memory alloy
showing large magnetic field induced strains. We present here results for the
magnetic ordering of Mn-rich Ni-Mn-Ga alloys based on both experiments and
theory. Experimental trends for the composition dependence of the magnetization
are measured by a vibrating sample magnetometer (VSM) in magnetic fields of up
to several tesla and at low temperatures. The saturation magnetization has a
maximum near the stoichiometric composition and it decreases with increasing Mn
content. This unexpected behaviour is interpreted via first-principles
calculations within the density-functional theory. We show that extra Mn atoms
are antiferromagnetically aligned to the other moments, which explains the
dependence of the magnetization on composition. In addition, the effect of Mn
doping on the stabilization of the structural phases and on the magnetic
anisotropy energy is demonstrated.Comment: 4 pages, 3 figure
The mysteries of mammatus clouds: Observations and formation mechanisms
Mammatus clouds are an intriguing enigma of atmospheric fluid dynamics and cloud physics. Most commonly observed on the underside of cumulonimbus anvils, mammatus also occur on the underside of cirrus, cirrocumulus, altocumulus, altostratus, and stratocumulus, as well as in contrails from jet aircraft and pyrocumulus ash clouds from volcanic eruptions. Despite their aesthetic appearance, mammatus have been the subject of few quantitative research studies. Observations of mammatus have been obtained largely through serendipitous opportunities with a single observing system (e.g., aircraft penetrations, visual observations, lidar, radar) or tangential observations from field programs with other objectives. Theories describing mammatus remain untested, as adequate measurements for validation do not exist because of the small distance scales and short time scales of mammatus. Modeling studies of mammatus are virtually nonexistent. As a result, relatively little is known about the environment, formation mechanisms, properties, microphysics, and dynamics of mammatus. This paper presents a review of mammatus clouds that addresses these mysteries. Previous observations of mammatus and proposed formation mechanisms are discussed. These hypothesized mechanisms are anvil subsidence, subcloud evaporation/sublimation, melting, hydrometeor fallout, cloud-base detrainment instability, radiative effects, gravity waves, Kelvin-Helmholtz instability, Rayleigh-Taylor instability, and Rayleigh-Bénard-like convection. Other issues addressed in this paper include whether mammatus are composed of ice or liquid water hydrometeors, why mammatus are smooth, what controls the temporal and spatial scales and organization of individual mammatus lobes, and what are the properties of volcanic ash clouds that produce mammatus? The similarities and differences between mammatus, virga, stalactites, and reticular clouds are also discussed. Finally, because much still remains to be learned, research opportunities are described for using mammatus as a window into the microphysical, turbulent, and dynamical processes occurring on the underside of clouds. © 2006 American Meteorological Society
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