195 research outputs found
Evidence of two viscous relaxation processes in the collective dynamics of liquid lithium
New inelastic X-ray scattering experiments have been performed on liquid
lithium in a wide wavevector range. With respect to the previous measurements,
the instrumental resolution, improved up to 1.5 meV, allows to accurately
investigate the dynamical processes determining the observed shape of the the
dynamic structure factor, . A detailed analysis of the lineshapes
shows the co-existence of relaxation processes with both a slow and a fast
characteristic timescales, and therefore that pictures of the relaxation
mechanisms based on a simple viscoelastic model must be abandoned.Comment: 5 pages, 4 .PS figure
Collective dynamics of liquid aluminum probed by Inelastic X-ray Scattering
An inelastic X-ray scattering experiment has been performed in liquid
aluminum with the purpose of studying the collective excitations at wavevectors
below the first sharp diffraction peak. The high instrumental resolution (up to
1.5 meV) allows an accurate investigation of the dynamical processes in this
liquid metal on the basis of a generalized hydrodynamics framework. The
outcoming results confirm the presence of a viscosity relaxation scenario ruled
by a two timescale mechanism, as recently found in liquid lithium.Comment: 8 pages, 7 figure
Is HR 6819 a triple system containing a black hole? -- An alternative explanation
HR 6819 was recently proposed to be a triple system consisting of an inner
B-type giant + black hole binary with an orbital period of 40d and an outer Be
tertiary. This interpretation is mainly based on two inferences: that the
emission attributed to the outer Be star is stationary, and that the inner
star, which is used as mass calibrator for the black hole, is a B-type giant.
We re-investigate the properties of HR 6819 by spectral disentangling and an
atmosphere analysis of the disentangled spectra to search for a possibly
simpler alternative explanation for HR 6819. Disentangling implies that the Be
component is not a static tertiary, but rather a component of the binary in the
40-d orbit. The inferred radial velocity amplitudes imply an extreme mass ratio
of M_2/M_1 = 15 +/- 3. We infer spectroscopic masses of 0.4
Msun and 6 Msun for the primary and secondary, which agree well
with the dynamical masses for an inclination of i = 32 deg. This indicates that
the primary might be a stripped star rather than a B-type giant. Evolutionary
modelling suggests that a possible progenitor system would be a tight (P_i ~
2d) B+B binary system that experienced conservative mass transfer. While the
observed nitrogen enrichment of the primary conforms with the predictions of
the evolutionary models, we find no indications for the predicted He
enrichment. We suggest that HR 6819 is a binary system consisting of a stripped
B-type primary and a rapidly-rotating Be star that formed from a previous
mass-transfer event. In the framework of this interpretation, HR 6819 does not
contain a black hole. Interferometry can distinguish between these two
scenarios by providing an independent measurement of the separation between the
visible components.Comment: Submitted to A&A, 13 pages (16 figures and 2 tables); 4 pages
supplementary material (4 figures and 4 tables). Comments are welcom
Evidence of short time dynamical correlations in simple liquids
We report a molecular dynamics (MD) study of the collective dynamics of a
simple monatomic liquid -interacting through a two body potential that mimics
that of lithium- across the liquid-glass transition. In the glassy phase we
find evidences of a fast relaxation process similar to that recently found in
Lennard-Jones glasses. The origin of this process is ascribed to the
topological disorder, i.e. to the dephasing of the different momentum
Fourier components of the actual normal modes of vibration of the disordered
structure. More important, we find that the fast relaxation persists in the
liquid phase with almost no temperature dependence of its characteristic
parameters (strength and relaxation time). We conclude, therefore, that in the
liquid phase well above the melting point, at variance with the usual
assumption of {\it un-correlated} binary collisions, the short time particles
motion is strongly {\it correlated} and can be described via a normal mode
expansion of the atomic dynamics.Comment: 7 pages, 7 .eps figs. To appear in Phys. Rev.
Everything that glitters is not gold: V1315 Cas is not a dormant black hole
The quest for quiet or dormant black holes has been ongoing since several
decades. Ellipsoidal variables possibly indicate the existence of a very
high-mass invisible companion and are thought to be one of the best ways to
find such dormant black holes. This, however, is not a panacea as we show here
with one example. We indeed report the discovery of a new semi-detached
interacting binary, V1315 Cas, discovered as an ellipsoidal variable. Using
data from photometric surveys (ASAS-SN, TESS) and high-resolution spectroscopy,
we derived a nearly circular orbit with an orbital period of
=34.54 d. The binary system consists of an evolved F-type star
primary that is likely still filling its Roche lobe and a B-type star
secondary. Using \textsc{phoebe}2, we derived the following masses and radii:
for the primary, and ; for the secondary, and . Modeling the evolution of
the system with MESA, we found an age of 7.7e7 years. The system is at
the end of a period of rapid non-conservative mass transfer that reversed its
mass ratio, while significantly widening its orbit. The primary shows carbon
depletion and nitrogen overabundance, indicative of CNO processed material
being exposed due to mass transfer. An infrared excess as well as stationary
H emission suggest the presence of a circumstellar or circumbinary
disc. V1315 Cas will likely become a detached stripped star binary.Comment: Accepted in MNRA
Inelastic X-ray scattering study of the collective dynamics in liquid sodium
Inelastic X-ray scattering data have been collected for liquid sodium at
T=390 K, i.e. slightly above the melting point. Owing to the very high
instrumental resolution, pushed up to 1.5 meV, it has been possible to
determine accurately the dynamic structure factor, , in a wide
wavevector range, nm, and to investigate on the dynamical
processes underlying the collective dynamics. A detailed analysis of the
lineshape of , similarly to other liquid metals, reveals the
co-existence of two different relaxation processes with slow and fast
characteristic timescales respectively. The present data lead to the conclusion
that: i) the picture of the relaxation mechanism based on a simple viscoelastic
model fails; ii) although the comparison with other liquid metals reveals
similar behavior, the data do not exhibit an exact scaling law as the principle
of corresponding state would predict.Comment: RevTex, 7 pages, 6 eps figures. Accepted by Phys. Rev.
The B-type Binaries Characterisation Programme II. VFTS 291: A stripped star from a recent mass transfer phase
Recent studies of massive binaries with putative black hole companions have
uncovered a phase of binary evolution that has not been observed before,
featuring a bloated stripped star that very recently ceased transferring mass
to a main-sequence companion. In this study, we focus on the candidate system
VFTS 291, a binary with an orbital period of 108 d and a high semi-amplitude
velocity ( km s). Through our analysis of the
disentangled spectra of the two components, together with dynamical and
evolutionary arguments, we identify a narrow-lined star of ~1.5-2.5
dominating the spectrum, and an early B-type main-sequence companion of
. The low mass of the narrow-lined star, and the high
mass ratio, suggest that VFTS 291 is a post-mass-transfer system, with the
narrow-lined star being bloated and stripped of its hydrogen-rich envelope,
sharing many similarities with other recently discovered stripped stars. Our
finding is supported by our detailed binary evolution models, which indicate
that the system can be well explained by an initial configuration consisting of
an 8.1 primary with an 8 companion in a 7 d orbital period.
While some open questions remain, particularly concerning the surface helium
enrichment of the stripped star and the rotational velocity of the companion,
we expect that high-resolution spectroscopy may help reconcile our estimates
with theory. Our study highlights the importance of multi-epoch spectroscopic
surveys to identify and characterize binary interaction products, and provides
important insights into the evolution of massive binary stars.Comment: 25 pages, 17 figures. Published by MNRAS, Advance Acces
Density fluctuations and single-particle dynamics in liquid lithium
The single-particle and collective dynamical properties of liquid lithium
have been evaluated at several thermodynamic states near the triple point. This
is performed within the framework of mode-coupling theory, using a
self-consistent scheme which, starting from the known static structure of the
liquid, allows the theoretical calculation of several dynamical properties.
Special attention is devoted to several aspects of the single-particle
dynamics, which are discussed as a function of the thermodynamic state. The
results are compared with those of Molecular Dynamics simulations and other
theoretical approaches.Comment: 31 pages (in preprint format), 14 figures. Submitted to Phys. Rev.
The young massive SMC cluster NGC 330 seen by MUSE III. Stellar parameters and rotational velocities
The origin of initial rotation rates of stars, and how a star's surface
rotational velocity changes during the evolution, either by internal angular
momentum transport or due to interactions with a binary companion, remain open
questions in stellar astrophysics. Here, we aim to derive the physical
parameters and study the distribution of (projected) rotational velocities of
B-type stars in the 35 Myr-old, massive cluster NGC 330 in the Small Magellanic
Cloud. NGC 330 is in an age range where the number of post-interaction binaries
is predicted to be high near the cluster turnoff (TO). We develop a
simultaneous photometric and spectroscopic grid-fitting method adjusting
atmosphere models on multi-band Hubble Space Telescope photometry and Multi
Unit Spectroscopic Explorer spectroscopy. This allows us to homogeneously
constrain the physical parameters of over 250 B and Be stars, brighter than
mF814W = 18.8 mag. The rotational velocities of Be stars in NGC 330 are
significantly higher than the ones of B stars. The rotational velocities vary
as a function of the star's position in the color-magnitude diagram,
qualitatively following predictions of binary population synthesis. A
comparison to younger clusters shows that stars in NGC 330 rotate more rapidly
on average. The rotational velocities of the 35 Myr old population in NGC 330
quantitatively agree with predictions for a stellar population that underwent
significant binary interactions: the bulk of the B stars could be single stars
or primaries in pre-interaction binaries. The rapidly spinning Be stars could
be mass and angular momentum gainers in previous interactions, while those Be
stars close to the TO may be spun-up single stars. The slowly rotating,
apparently single stars above the TO could be merger products. The different
vsini-characteristics of NGC 330 compared to younger populations can be
understood in this framework.Comment: 18 pages (incl. appendix), 15 figures, 3 tables, accepted for
publication in A&
A massive helium star with a sufficiently strong magnetic field to form a magnetar
Magnetars are highly magnetized neutron stars; their formation mechanism is
unknown. Hot helium-rich stars with spectra dominated by emission lines are
known as Wolf-Rayet stars. We observe the binary system HD 45166 using
spectropolarimetry, finding that it contains a Wolf-Rayet star with a mass of 2
solar masses and a magnetic field of 43 kilogauss. Stellar evolution
calculations indicate that this component will explode as a type Ib or IIb
supernova, and the strong magnetic field favors a magnetar remnant. We propose
that the magnatized Wolf-Rayet star formed by the merger of two lower mass
helium stars.Comment: Published in Science on the 18 August 2023. Radial velocities,
spectra, and software available in: https://zenodo.org/record/8042656 ESO
press release: www.eso.org/public/news/eso231
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