269 research outputs found
The spectroscopic Hertzsprung-Russell diagram of Galactic massive stars
The distribution of stars in the Hertzsprung-Russell diagram narrates their
evolutionary history and directly assesses their properties. Placing stars in
this diagram however requires the knowledge of their distances and interstellar
extinctions, which are often poorly known for Galactic stars. The spectroscopic
Hertzsprung-Russell diagram (sHRD) tells similar evolutionary tales, but is
independent of distance and extinction measurements. Based on spectroscopically
derived effective temperatures and gravities of almost 600 stars, we derive for
the first time the observational distribution of Galactic massive stars in the
sHRD. While biases and statistical limitations in the data prevent detailed
quantitative conclusions at this time, we see several clear qualitative trends.
By comparing the observational sHRD with different state-of-the-art stellar
evolutionary predictions, we conclude that convective core overshooting may be
mass-dependent and, at high mass (), stronger than previously
thought. Furthermore, we find evidence for an empirical upper limit in the sHRD
for stars with between 10000 and 32000 K and, a strikingly large
number of objects below this line. This over-density may be due to inflation
expanding envelopes in massive main-sequence stars near the Eddington limit.Comment: 5 pages, 2 figures, 1 table; accepted for publication in A&A Letter
Models of the circumstellar medium of evolving, massive runaway stars moving through the Galactic plane
At least 5 per cent of the massive stars are moving supersonically through
the interstellar medium (ISM) and are expected to produce a stellar wind bow
shock. We explore how the mass loss and space velocity of massive runaway stars
affect the morphology of their bow shocks. We run two-dimensional axisymmetric
hydrodynamical simulations following the evolution of the circumstellar medium
of these stars in the Galactic plane from the main sequence to the red
supergiant phase. We find that thermal conduction is an important process
governing the shape, size and structure of the bow shocks around hot stars, and
that they have an optical luminosity mainly produced by forbidden lines, e.g.
[OIII]. The Ha emission of the bow shocks around hot stars originates from near
their contact discontinuity. The H emission of bow shocks around cool
stars originates from their forward shock, and is too faint to be observed for
the bow shocks that we simulate. The emission of optically-thin radiation
mainly comes from the shocked ISM material. All bow shock models are brighter
in the infrared, i.e. the infrared is the most appropriate waveband to search
for bow shocks. Our study suggests that the infrared emission comes from near
the contact discontinuity for bow shocks of hot stars and from the inner region
of shocked wind for bow shocks around cool stars. We predict that, in the
Galactic plane, the brightest, i.e. the most easily detectable bow shocks are
produced by high-mass stars moving with small space velocities.Comment: 22 pages, 24 figure
BINSTAR: a new binary stellar evolution code
We provide a detailed description of a new stellar evolution code, BINSTAR, which has been developed to study interacting binaries. Based on the stellar evolution code STAREVOL, it is specifically designed to study low- and intermediate-mass binaries. We describe the state-of-the-art input physics, which includes treatments of tidal interactions, mass transfer and angular momentum exchange within the system. A generalised Henyey method is used to solve simultaneously the stellar structure equations of each component as well as the separation and eccentricity of the orbit. Test simulations for cases A and B mass transfer are presented and compared with available models. The results of the evolution of Algol systems are in remarkable agreement with the calculations of the Vrije Universiteit Brussel (VUB) group, thus validating our code. We also computed a large grid of models for various masses (2 ≤ M/M⊙ ≤ 20) and seven metallicities (Z = 0.0001, 0.001, 0.004, 0.008, 0.01, 0.02, 0.03) to provide a useful analytical parameterisation of the tidal torque constant E2, which allows the determination of the circularisation and synchronisation timescales for stars with a radiative envelope and convective core. The evolution of E2 during the main sequence shows noticeable differences compared to available models. In particular, our new calculations indicate that the circularisation timescale is constant during core hydrogen burning. We also show that E2 weakly depends on core overshooting but is substantially increased when the metallicity becomes lower
Observing the disintegration of the A68A iceberg from space
Icebergs impact the physical and biological properties of the ocean where they drift, depending on the degree of melting. We use satellite imagery and altimetry to quantify the area, thickness, and volume change of the massive A68A iceberg from its calving off the Larsen-C Ice Shelf in July 2017 until January 2021, when it disintegrated. A68A thinned from 235 ± 9 to 168 ± 10 m, on average, and lost 802 ± 34 Gt of ice in 3.5 years, 254 ± 17 Gt of which was through basal melting (a lower bound for the immediate fresh water input into the ocean). Basal melting peaked at 7.2 ± 2.3 m/month in the Northern Scotia Sea and an estimated 152 ± 61 Gt of freshwater was released off South Georgia, potentially altering the local ocean properties, plankton occurrence and conditions for predators
Programming Protocol-Independent Packet Processors
P4 is a high-level language for programming protocol-independent packet
processors. P4 works in conjunction with SDN control protocols like OpenFlow.
In its current form, OpenFlow explicitly specifies protocol headers on which it
operates. This set has grown from 12 to 41 fields in a few years, increasing
the complexity of the specification while still not providing the flexibility
to add new headers. In this paper we propose P4 as a strawman proposal for how
OpenFlow should evolve in the future. We have three goals: (1)
Reconfigurability in the field: Programmers should be able to change the way
switches process packets once they are deployed. (2) Protocol independence:
Switches should not be tied to any specific network protocols. (3) Target
independence: Programmers should be able to describe packet-processing
functionality independently of the specifics of the underlying hardware. As an
example, we describe how to use P4 to configure a switch to add a new
hierarchical label
Pulsational pair-instability supernovae in gravitational-wave and electromagnetic transients
Current observations of binary black-hole ({BBH}) merger events show support
for a feature in the primary BH-mass distribution at
, previously interpreted as a signature of
pulsational pair-instability (PPISN) supernovae. Such supernovae are expected
to map a wide range of pre-supernova carbon-oxygen (CO) core masses to a narrow
range of BH masses, producing a peak in the BH mass distribution. However,
recent numerical simulations place the mass location of this peak above
. Motivated by uncertainties in the progenitor's
evolution and explosion mechanism, we explore how modifying the distribution of
BH masses resulting from PPISN affects the populations of gravitational-wave
(GW) and electromagnetic (EM) transients. To this end, we simulate populations
of isolated {BBH} systems and combine them with cosmic star-formation rates.
Our results are the first cosmological BBH-merger predictions made using the
\textsc{binary\_c} rapid population synthesis framework. We find that our
fiducial model does not match the observed GW peak. We can only explain the
peak with PPISNe by shifting the expected CO core-mass
range for PPISN downwards by . Apart from being
in tension with state-of-the art stellar models, we also find that this is
likely in tension with the observed rate of hydrogen-less super-luminous
supernovae. Conversely, shifting the mass range upward, based on recent stellar
models, leads to a predicted third peak in the BH mass function at
. Thus we conclude that the
feature is unlikely to be related to PPISNe.Comment: Accepted for publication in MNRAS. 19 pages, 8 figures includings
appendice
Self-Enrichment in Globular Clusters: Is There a Role for the Super-Asymptotic Giant Branch Stars?
In four globular clusters (GCs) a non negligible fraction of stars can be
interpreted only as a very helium rich population. The evidence comes from the
presence of a "blue" main sequence in Cen and NGC 2808, and from the
the very peculiar horizontal branch morphology in NGC 6441 and NGC 6388.
Although a general consensus is emerging on the fact that self--enrichment is a
common feature among GCs, the helium content required for these stars is
Y\simgt0.35, and it is difficult to understand how it can be produced without
any --or, for Cen, without a considerable--associated metal
enhancement. We examine the possible role of super--AGB stars, and show that
they may provide the required high helium. However, the ejecta of the most
massive super--AGBs show a global CNO enrichment by a factor of 4, due
to the dredge--out process occurring at the second dredge up stage. If these
clusters show no evidence for this CNO enrichment, we can rule out that at
least the most massive super--AGBs evolve into O--Ne white dwarfs and take part
in the formation of the second generation stars. This latter hypothesis may
help to explain the high number of neutron stars present in GCs. The most
massive super--AGBs would in fact evolve into electron--capture supernovae.
Their envelopes would be easily ejected out of the cluster, but the remnant
neutron stars remain into the clusters, thanks to their small supernova natal
kicks.Comment: version accepted for publication in The Astrophysical Journal Letter
Core-collapse supernova progenitor constraints using the spatial distributions of massive stars in local galaxies
We study the spatial correlations between the H emission and
different types of massive stars in two local galaxies, the Large Magellanic
Cloud (LMC) and Messier 33. We compare these to correlations derived for
core-collapse supernovae (CCSNe) in the literature to connect CCSNe of
different types with the initial masses of their progenitors and to test the
validity of progenitor mass estimates which use the pixel statistics method. We
obtain samples of evolved massive stars in both galaxies from catalogues with
good spatial coverage and/or completeness, and combine them with coordinates of
main-sequence stars in the LMC from the SIMBAD database. We calculate the
spatial correlation of stars of different classes and spectral types with
H emission. We also investigate the effects of distance, noise and
positional errors on the pixel statistics method. A higher correlation with
H emission is found to correspond to a shorter stellar lifespan, and we
conclude that the method can be used as an indicator of the ages, and therefore
initial masses, of SN progenitors. We find that the spatial distributions of
type II-P SNe and red supergiants of appropriate initial mass (9
) are consistent with each other. We also find the distributions of
type Ic SNe and WN stars with initial masses 20
consistent, while supergiants with initial masses around 15 are a
better match for type IIb and II-L SNe. The type Ib distribution corresponds to
the same stellar types as type II-P, which suggests an origin in interacting
binaries. On the other hand, we find that luminous blue variable stars show a
much stronger correlation with H emission than do type IIn SNe.ERC, STF
The lack of carbon stars in the Galactic bulge
In order to explain the lack of carbon stars in the Galactic bulge, we have
made a detailed study of thermal pulse - asymptotic giant branch stars by using
a population synthesis code. The effects of the oxygen overabundance and the
mass loss rate on the ratio of the number of carbon stars to that of oxygen
stars in the Galactic bulge are discussed. We find that the oxygen
overabundance which is about twice as large as that in the solar neighbourhood
(close to the present observations) is insufficient to explain the rareness of
carbon stars in the bulge. We suggest that the large mass loss rate may serve
as a controlling factor in the ratio of the number of carbon stars to that of
oxygen stars.Comment: 16 pages, 5 figure
Population III X-ray Binaries and their Impact on the Early Universe
The first population of X-ray binaries (XRBs) is expected to affect the
thermal and ionization states of the gas in the early Universe. Although these
X-ray sources are predicted to have important implications for high-redshift
observable signals, such as the hydrogen 21-cm signal from cosmic dawn and the
cosmic X-ray background, their properties are poorly explored, leaving
theoretical models largely uninformed. In this paper we model a population of
X-ray binaries arising from zero metallicity stars. We explore how their
properties depend on the adopted initial mass function (IMF) of primordial
stars, finding a strong effect on their number and X-ray production efficiency.
We also present scaling relations between XRBs and their X-ray emission with
the local star formation rate, which can be used in sub-grid models in
numerical simulations to improve the X-ray feedback prescriptions.
Specifically, we find that the uniformity and strength of the X-ray feedback in
the intergalactic medium is strongly dependant on the IMF. Bottom-heavy IMFs
result in a smoother distribution of XRBs, but have a luminosity orders of
magnitude lower than more top-heavy IMFs. Top-heavy IMFs lead to more spatially
uneven, albeit strong, X-ray emission. An intermediate IMF has a strong X-ray
feedback while sustaining an even emission across the intergalactic medium.
These differences in X-ray feedback could be probed in the future with
measurements of the cosmic dawn 21-cm line of neutral hydrogen, which offers us
a new way of constraining population III IMF.Comment: Accepted for publication in MNRAS, 17 pages, 9 figure
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