Context.
A relevant fraction of massive stars are runaway stars. These stars move with a significant peculiar velocity with respect to their environment.
Aims.
We aim to discover and characterize the ...population of massive and early-type runaway stars in the GOSC and BeSS catalogs using
Gaia
DR3 astrometric data.
Methods.
We present a two-dimensional method in the velocity space to discover runaway stars as those that deviate significantly from the velocity distribution of field stars. Field stars are considered to follow the Galactic rotation curve.
Results.
We found 106 O runaway stars, 42 of which were not previously identified as runaways. We found 69 Be runaway stars, 47 of which were not previously identified as runaways. The dispersion of runaway stars is a few times higher in
Z
and
b
than that of field stars. This is explained by the ejections they underwent when they became runaways. The percentage of runaways is 25.4% for O-type stars, and it is 5.2% for Be-type stars. In addition, we conducted simulations in three dimensions for our catalogs. They revealed that these percentages could increase to ∼30% and ∼6.7%, respectively. Our runaway stars include seven X-ray binaries and one gamma-ray binary. Moreover, we obtain velocity dispersions of ∼5 km s
−1
perpendicular to the Galactic plane for O- and Be-type field stars. These values increase in the Galactic plane to ∼7 km s
−1
for O-type stars due to uncertainties and to ∼9 km s
−1
for Be-type stars due to Galactic velocity diffusion.
Conclusions.
The excellent
Gaia
DR3 astrometric data have allowed us to identify a significant number of O-type and Be-type runaways in the GOSC and BeSS catalogs. The higher percentages and higher velocities found for O-type compared to Be-type runaways underline that the dynamical ejection scenario is more likely than the binary supernova scenario. Our results open the door to identifying new high-energy systems among our runaways by conducting detailed studies.
We find that the formation of MWC 656 (the first Be binary containing a black hole) involves a common envelope phase and a supernova explosion. This result supports the idea that a rapidly rotating ...Be star can emerge out of a common envelope phase, which is very intriguing because this evolutionary stage is thought to be too fast to lead to significant accretion and spin up of the B star. We predict ∼10–100 of B-BH binaries to currently reside in the Galactic disc, among which around 1/3 contain a Be star, but there is only a small chance to observe a system with parameters resembling MWC 656. If MWC 656 is representative of intrinsic Galactic Be-BH binary population, it may indicate that standard evolutionary theory needs to be revised. This would pose another evolutionary problem in understanding black hole (BH) binaries, with BH X-ray novae formation issue being the prime example. Future evolution of MWC 656 with an ∼5 M⊙ BH and with an ∼13 M⊙ main-sequence companion on an ∼60 d orbit may lead to the formation of a coalescing BH–NS (neutron star) system. The estimated Advanced LIGO/Virgo detection rate of such systems is up to ∼0.2 yr−1. This empirical estimate is a lower limit as it is obtained with only one particular evolutionary scenario, the MWC 656 binary. This is only a third such estimate available (after Cyg X-1 and Cyg X-3), and it lends additional support to the existence of so far undetected BH–NS binaries.
We have studied in detail the 0.15–15 GHz radio spectrum of the gamma-ray binary LS 5039 to look for a possible turnover and absorption mechanisms at low frequencies, and to constrain the physical ...properties of its emission. We have analysed two archival Very Large Array monitorings, all the available archival Giant Metrewave Radio Telescope (GMRT) data and a coordinated quasi-simultaneous observational campaign conducted in 2013 with Giant Metrewave Radio Telescope and Westerbork Synthesis Radio Telescope. The data show that the radio emission of LS 5039 is persistent on day, week and year time-scales, with a variability ≲ 25 per cent at all frequencies, and no signature of orbital modulation. The obtained spectra reveal a power-law shape with a curvature below 5 GHz and a turnover at ∼0.5GHz, which can be reproduced by a one-zone model with synchrotron self-absorption plus Razin effect. We obtain a coherent picture for the size of the emitting region of ∼0.85 mas, setting a magnetic field of B ∼ 20 mG, an electron density of n
e ∼ 4 × 105 cm−3 and a mass-loss rate of
$\dot{M}\sim 5\times 10^{-8} \,\mathrm{M_{\odot }\, yr^{-1}}$
. These values imply a significant mixing of the stellar wind with the relativistic plasma outflow from the compact companion. At particular epochs the Razin effect is negligible, implying changes in the injection and the electron density or magnetic field. The Razin effect is reported for the first time in a gamma-ray binary, giving further support to the young non-accreting pulsar scenario.
LS I +61 303 is a gamma-ray binary that exhibits an outburst at GHz frequencies each orbital cycle of ≈26.5 d and a superorbital modulation with a period of ≈4.6 yr. We have performed a detailed ...study of the low-frequency radio emission of LS I +61 303 by analysing all the archival Giant Metrewave Radio Telescope data at 150, 235 and 610 MHz, and conducting regular LOw Frequency ARray observations within the Radio Sky Monitor (RSM) at 150 MHz. We have detected the source for the first time at 150 MHz, which is also the first detection of a gamma-ray binary at such a low frequency. We have obtained the light curves of the source at 150, 235 and 610 MHz, all of them showing orbital modulation. The light curves at 235 and 610 MHz also show the existence of superorbital variability. A comparison with contemporaneous 15-GHz data shows remarkable differences with these light curves. At 15 GHz we see clear outbursts, whereas at low frequencies we see variability with wide maxima. The light curve at 235 MHz seems to be anticorrelated with the one at 610 MHz, implying a shift of ∼0.5 orbital phases in the maxima. We model the shifts between the maxima at different frequencies as due to the expansion of a one-zone emitting region assuming either free–free absorption or synchrotron self-absorption with two different magnetic field dependences. We always obtain a subrelativistic expansion velocity, in some cases being close to the stellar wind one.
We present a statistical analysis of the properties of the obscuring material around active galactic nuclei (AGN). This study represents the first of its kind for an ultra-hard X-ray (14–195 keV;
...Swift
/BAT), volume-limited (
D
L
< 40 Mpc) sample of 24 Seyfert (Sy) galaxies (BCS
40
sample) using high angular resolution infrared data and various torus models: smooth, clumpy, and two-phase torus models and clumpy disc+wind models. We find that torus models (i.e. without including the polar dusty wind component) and disc+wind models provide the best fits for a comparable number of galaxies, 8 out of 24 (33.3%) and 9 out of 24 (37.5%), respectively. We find that the best-fit models depend on the hydrogen column density (
N
H
X−ray
), which is related to the X-ray (unobscured or obscured) and optical (Sy1/Sy2) classification. In particular, smooth, clumpy, and two-phase torus models are best at reproducing the infrared (IR) emission of AGN with relatively high hydrogen column density (median value of log (
N
H
X−ray
cm
−2
) = 23.5 ± 0.8; i.e. Sy2). However, clumpy disc+wind models provide the best fits to the nuclear IR spectral energy distributions (SEDs) of Sy1/1.8/1.9 (median value of log (
N
H
X−ray
cm
−2
) = 21.0 ± 1.0) – specifically, in the near-infrared (NIR) range. The success of the disc+wind models in fitting the NIR emission of Sy1 galaxies is due to the combination of adding large graphite grains to the dust composition and self-obscuration effects caused by the wind at intermediate inclinations. In general, we find that the Seyfert galaxies having unfavourable (favourable) conditions, namely, nuclear hydrogen column density and Eddington ratio, for launching IR dusty polar outflows are best-fitted with smooth, clumpy, and two-phase torus (disc+wind) models, confirming the predictions from simulations. Therefore, our results indicate that the nature of the inner dusty structure in AGN depends on the intrinsic AGN properties.
New optical spectroscopy of the high-mass X-ray binary microquasar LS I +61 303 is presented. Eccentric orbital fits to our radial velocity measurements yield updated orbital parameters in good ...agreement with previous work. Our orbital solution indicates that the periastron passage occurs at radio phase 0.23 and the X-ray/radio outbursts are triggered 2.5–4 d after the compact star passage. The spectrum of the optical star is consistent with a B0 V spectral type and contributes ~65 per cent of the total light, the remainder being the result of emission by a circumstellar disc. We also measure the projected rotational velocity to be v sin i≃ 113 km s−1.
Star clusters form in dense, hierarchically collapsing gas clouds. Bulk kinetic energy is transformed to turbulence with stars forming from cores fed by filaments. In the most compact regions, ...stellar feedback is least effective in removing the gas and stars may form very efficiently. These are also the regions where, in high-mass clusters, ejecta from some kind of high-mass stars are effectively captured during the formation phase of some of the low mass stars and channeled into the latter to form multiple populations. Star formation epochs in star clusters are generally set by gas flows that determine the abundance of gas in the cluster. We argue that there is likely only one star formation epoch after which clusters remain essentially clear of gas by cluster winds. Collisional dynamics is important in this phase leading to core collapse, expansion and eventual dispersion of every cluster. We review recent developments in the field with a focus on theoretical work.
Observational and theoretical studies point to microquasars (MQs) as possible counterparts of a significant fraction of the unidentifiedgamma-ray sources detected so far. At present, a proper ...scenario to explain the emission beyond soft X-rays from these objects is not known, nor what the precise connection is between the radio and the high-energy radiation. We develop a new model where the MQ jet is dynamically dominated by cold protons and radiatively dominated by relativistic leptons. The matter content and power of the jet are both related with the accretion process. The magnetic field is assumed to be close to equipartition, although it is attached to and dominated by the jet matter. For the relativistic particles in the jet, their maximum energy depends on both the acceleration efficiency and the energy losses. The model takes into account the interaction of the relativistic jet particles with the magnetic field and all the photon and matter fields. Such interaction produces significant amounts of radiation from radio to very high energies through synchrotron, relativistic Bremsstrahlung, and inverse Compton (IC) processes. Variability of the emission produced by changes in the accretion process (e.g. via orbital eccentricity) is also expected. The effects of the gamma-ray absorption by the external photon fields on the gamma-ray spectrum have been taken into account, revealing clear spectral features that might be observed. This model is consistent to the accretion scenario, energy conservation laws, and current observational knowledge, and can provide deeper physical information of the source when tested against multiwavelength data.
Context. Gamma-ray binaries allow us to study physical processes such as particle acceleration up to TeV energies as well as very high energy gamma-ray emission and absorption with changing ...geometrical configurations on a periodic basis. These sources produce outflows of radio-emitting particles whose structure can be imaged with very long baseline interferometry (VLBI). LS 5039 is a gamma-ray binary that has shown variable VLBI structures in the past. Aims. We aim to characterise the radio morphological changes of LS 5039 and determine if they are either repeatable or erratic. Methods. We observed LS 5039 with the VLBA at 5 GHz during five consecutive days to cover the 3.9-day orbit and an extra day to distinguish between orbital or secular variability. We also compiled the available high-resolution radio observations of the source to study its morphological variability at different orbital phases. We used a simple model to interpret the obtained images. Results. The new observations show that the morphology of LS 5039 up to projected distances of 10 milliarcsec changes within 24 h. The observed radio morphological changes display a periodic orbital modulation. Multifrequency and multiepoch VLBI observations confirm that the morphological periodicity is stable on timescales of years. Using a simple model we show that the observed behaviour is compatible with the presence of a young non-accreting pulsar with an outflow behind it. The morphology is reproduced for inclinations of the orbit of 60–75°. For masses of the companion star in the range 20–50 M⊙, this range of inclinations implies a mass of the compact object of 1.3–2.7 M⊙. Conclusions. The periodic orbital modulation of the radio morphology of LS 5039, stable over several years, suggests that all gamma-ray binaries are expected to show a similar behaviour. The changes in the radio structure of LS 5039 are compatible with the presence of a young non-accreting neutron star, which suggests that the known gamma-ray binaries contain young pulsars.
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