163 research outputs found
Investigating the lack of main-sequence companions to massive Be stars
About 20% of all B-type stars are classical Be stars. The Be phenomenon is
strongly correlated with rapid rotation, the origin of which remains unclear.
It may be rooted in single- or binary-star evolution. In the framework of the
binary channel, the initially more massive star transfers mass and angular
momentum to the original secondary, which becomes a Be star. The system then
evolves into a Be binary with a post-main-sequence companion, which may later
be disrupted in a supernova event. Hence, if the binary channel dominates the
formation of Be stars, one may expect a strong lack of close Be binaries with
main sequence (MS) companions. Through an extensive, star-by-star review of the
literature of a magnitude-limited sample of Galactic early-type Be stars, we
investigate whether Be binaries with MS companions are known to exist. Our
sample is constructed from the BeSS database and cross-matched with all
available literature on the individual stars. Out of an initial list of 505 Be
stars, we compile a final sample of 287 Galactic Be stars earlier than B1.5
with V<=12 mag. Out of those, 13 objects were reported as Be binaries with
known post-MS companions and 11 as binaries with unknown, uncertain or debated
companions. We find no confirmed reports of Be binaries with MS companions. For
the remaining 263 targets, no significant reports of multiplicity exist in the
literature, implying that they are either Be binaries with faint companions, or
truly single. The clear lack of reported MS companions to Be stars, which
stands in contrast to the high number of detected B+B MS binaries, strongly
supports the hypothesis that early-type Be stars are binary interaction
products that spun up after mass and angular momentum transfer from a companion
star. Taken at face value, our results may suggest that a large majority of the
early-type Be stars have formed through binary mass-transfer.Comment: 15 pages (incl. appendix), 6 figures, 3 tables, accepted for
publication in A&
MWC 656 is unlikely to contain a black hole
Context. MWC 656 was reported as the first known Be star with a black-hole
(BH) companion in a 60 d period. The mass of the proposed BH companion is
estimated to be between 4 - 7 MSun. This estimate is based on radial velocity
(RV) measurements derived from the Fe ii 4583 emission line of the Be star disc
and from the He ii 4686 emission line, assumed to be formed in a disc around
the putative BH. Aims. Using new high-resolution spectroscopic data, we
investigate whether MWC 656 truly contains a BH. Methods. We used the
cross-correlation method to calculate the RVs of both the Be star and the He ii
4686 emission line and we derive a new orbital solution. We also performed
disentangling to look for the spectral signature of a companion. Results. We
derive an orbital period of 59.028 +- 0.011 d and a mass ratio q = M_Heii/M_Be
= 0.12 +- 0.03, much lower than the previously reported q = 0.41 +- 0.07.
Adopting a mass of the Be star of M_Be = 7.8 +- 2.0MSun, the companion has a
mass of 0.94 +- 0.34MSun. For the upper limit of M_Be = 16MSun and q = 0.15,
the companion has a mass 2.4MSun. Performing disentangling on mock spectra
shows that the spectral signature of a non-degenerate stellar companion with
such a low mass cannot be retrieved using our data. Conclusions. Our
measurements do not support the presence of a BH companion in MWC 656. The
derived upper limit on the mass of the companion rather indicates that it is a
neutron star, a white dwarf, or a hot helium star. Far-UV data will help to
reject or confirm a hot helium-star companion.Comment: 6 pages, 2 + 6 figures, 3 + 1 tables, accepted for publication in A&
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
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.
Binary-object spectral-synthesis in 3D (BOSS-3D) -- Modelling H-alpha emission in the enigmatic multiple system LB-1
Context: To quantitatively decode the information stored within an observed
spectrum, detailed modelling of the physical state and accurate radiative
transfer solution schemes are required. In the analysis of stellar spectra, the
numerical model often needs to account for binary companions and 3D structures
in the stellar envelopes. The enigmatic binary (or multiple) system LB-1
constitutes a perfect example of such a complex multi-D problem. Aims: To
improve our understanding of the LB-1 system, we directly modelled the
phase-dependent H-alpha line profiles of this system. To this end, we developed
a multi-purpose binary-object spectral-synthesis code in 3D (BOSS-3D). Methods:
BOSS-3D calculates synthetic line profiles for a given state of the
circumstellar material. The standard pz-geometry commonly used for single stars
is extended by defining individual coordinate systems for each involved object
and by accounting for the appropriate coordinate transformations. The code is
then applied to the LB-1 system, considering two main hypotheses, a binary
containing a stripped star and Be star, or a B star and a black hole with a
disc. Results: Comparing these two scenarios, neither model can reproduce the
detailed phase-dependent shape of the H-alpha line profiles. A satisfactory
match with the observations, however, is obtained by invoking a disc around the
primary object in addition to the Be-star disc or the black-hole accretion
disc. Conclusions: The developed code can be used to model synthetic line
profiles for a wide variety of binary systems, ranging from transit spectra of
planetary atmospheres, to post-asymptotic giant branch binaries including
circumstellar and circumbinary discs and massive-star binaries with stellar
winds and disc systems. For the LB-1 system, our modelling provides strong
evidence that each object in the system contains a disc-like structure
Searching for compact objects in the single-lined spectroscopic binaries of the young Galactic cluster NGC 6231
Recent evolutionary computations predict that a few percent of massive OB
stars in binary systems should have a dormant BH companion. Despite several
reported X-ray quiet OB+BH systems over the last couple of years, finding them
with certainty remains challenging. These have great importance as they can be
gravitational wave (GW) source progenitors, and are landmark systems in
constraining supernova kick physics. This work aims to characterise the hidden
companions to the single-lined spectroscopic binaries (SB1s) in the B star
population of the young open Galactic cluster NGC 6231 to find candidate
systems for harbouring compact object companions. With the orbital solutions
for each SB1 previously constrained, we applied Fourier spectral disentangling
to multi-epoch optical VLT/FLAMES spectra of each target to extract a potential
signature of a faint companion, and to identify newly disentangled double-lined
spectroscopic binaries (SB2s). For targets where the disentangling does not
reveal any signature of a stellar companion, we performed atmospheric and
evolutionary modelling on the primary to obtain constraints on the unseen
companion. Seven newly classified SB2 systems with mass ratios down to near 0.1
were identified. From the remaining targets, for which no faint companion could
be extracted from the spectra, four are found to have companion masses in the
predicted mass ranges of neutron stars (NSes) and BHes. Two of these have
companion masses between 1 and 3.5 , making them potential hosts of
NSes (or lower mass main sequence stars). The other two are between 2.5 to 8
and 1.6 and 26 , respectively, and so are identified as
candidates for harbouring BH companions. However, unambiguous identification of
these systems as X-ray quiet compact object harbouring binaries requires follow
up observations.Comment: Submitted to A&
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
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 phonon theory of liquid thermodynamics
Heat capacity of matter is considered to be its most important property
because it holds information about system's degrees of freedom as well as the
regime in which the system operates, classical or quantum. Heat capacity is
well understood in gases and solids but not in the third state of matter,
liquids, and is not discussed in physics textbooks as a result. The perceived
difficulty is that interactions in a liquid are both strong and
system-specific, implying that the energy strongly depends on the liquid type
and that, therefore, liquid energy can not be calculated in general form. Here,
we develop a phonon theory of liquids where this problem is avoided. The theory
covers both classical and quantum regimes. We demonstrate good agreement of
calculated and experimental heat capacity of 21 liquids, including noble,
metallic, molecular and hydrogen-bonded network liquids in a wide range of
temperature and pressure.Comment: 7 pages, 4 figure
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