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Register a new userSelf Regulated Shocks in Massive Star Binary Systems
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In an early-type, massive star binary system, X-ray bright shocks result from the powerful collision of stellar winds driven by radiation pressure on spectral line transitions. We examine the influence of the X-rays from the wind-wind collision shocks on the radiative driving of the stellar winds using steady state models that include a parameterized line force with X-ray ionization dependence. Our primary result is that X-ray radiation from the shocks inhibits wind acceleration and can lead to a lower pre-shock velocity, and a correspondingly lower shocked plasma temperature, yet the intrinsic X-ray luminosity of the shocks, LX remains largely unaltered, with the exception of a modest increase at small binary separations. Due to the feedback loop between the ionizing X-rays from the shocks and the wind-driving, we term this scenario as self regulated shocks. This effect is found to greatly increase the range of binary separations at which a wind-photosphere collision is likely to occur in systems where the momenta of the two winds are significantly different. Furthermore, the excessive levels of X-ray ionization close to the shocks completely suppresses the line force, and we suggest that this may render radiative braking less effective. Comparisons of model results against observations reveals reasonable agreement in terms of log(LX/Lbol). The inclusion of self regulated shocks improves the match for kT values in roughly equal wind momenta systems, but there is a systematic offset for systems with unequal wind momenta (if considered to be a wind-photosphere collision).
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Feedback from massive stars is believed to play a critical role in shaping the galaxy mass function, the structure of the interstellar medium (ISM), and the low efficiency of star formation, but the exact form of the feedback is uncertain. In this paper, the first in a series, we present and test a novel numerical implementation of stellar feedback resulting from momentum imparted to the ISM by radiation, supernovae, and stellar winds. We employ a realistic cooling function, and find that a large fraction of the gas cools to <100K, so that the ISM becomes highly inhomogeneous. Despite this, our simulated galaxies reach an approximate steady state, in which gas gravitationally collapses to form giant molecular clouds (GMCs), dense clumps, and stars; subsequently, stellar feedback disperses the GMCs, repopulating the diffuse ISM. This collapse and dispersal cycle is seen in models of SMC-like dwarfs, the Milky-Way, and z~2 clumpy disk analogues. The simulated global star formation efficiencies are consistent with the observed Kennicutt-Schmidt relation. Moreover, the star formation rates are nearly independent of the numerically imposed high-density star formation efficiency, density threshold, and density scaling. This is a consequence of the fact that, in our simulations, star formation is regulated by stellar feedback limiting the amount of very dense gas available for forming stars. In contrast, in simulations without stellar feedback, i.e. under the action of only gravity and gravitationally-induced turbulence, the ISM experiences runaway collapse to very high densities. In these simulations without feedback, the global star formation rates exceed observed galactic star formation rates by 1-2 orders of magnitude, demonstrating that stellar feedback is crucial to the regulation of star formation in galaxies.
Tidal dissipation due to turbulent viscosity in the convective regions of giant stars plays an important role in shaping the orbits of pre-common envelope systems. Such systems are possible sources of transients and close compact binary systems that will eventually merge and produce detectable gravitational wave signals. Most previous studies of the onset of common envelope episodes have focused on circular orbits and synchronously rotating donor stars under the assumption that tidal dissipation can quickly spin up the primary and circularize the orbit before the binary reaches Roche-lobe overflow (RLO). We test this assumption by coupling numerical models of the post main sequence stellar evolution of massive-stars with the model for tidal dissipation in convective envelopes developed in Vick & Lai (2020) $-$ a tidal model that is accurate even for highly eccentric orbits with small pericentre distances. We find that, in many cases, tidal dissipation does not circularize the orbit before RLO. For a $10~M_odot$ ($15~M_odot$) primary star interacting with a $1.4~M_odot$ companion, systems with pericentre distances within 3 AU (6 AU) when the primary leaves the main sequence will retain the initial orbital eccentricity when the primary grows to the Roche radius. Even in systems that tidally circularize before RLO, the donor star may be rotating subsynchronously at the onset of mass transfer. Our results demonstrate that some possible precursors to double neutron star systems are likely eccentric at the Roche radius. The effects of pre-common envelope eccentricity on the resulting compact binary merit further study.
Most molecular clouds are filamentary or elongated. Among those forming low-mass stars, their long axes tend to be either parallel or perpendicular to the large-scale (10-100 pc) magnetic field (B-field) in the surrounding inter cloud medium. This arises because, along the dynamically dominant B-fields, the competition between self-gravity and turbulent pressure will shape the cloud to be elongated either perpendicular or parallel to the fields. Recent study also suggested that, on the scales of 0.1-0.01 pc, fields are dynamically important within cloud cores forming massive stars. But whether the core field morphologies are inherited from the inter cloud medium or governed by cloud turbulence is under vigorous debate, so is the role played by B-fields in cloud fragmentation at 10 - 0.1 pc scales. Here we report B-field maps covering 100-0.01 pc scales inferred from polarimetric observations of a massive-star forming region, NGC 6334. First, the main filament also lies perpendicular to the ambient field. NGC 6334 hosts young star-forming sites where fields are not severely affected by stellar feedback, and their directions do not change significantly over the entire scale range. This means that the fields are dynamically important. At various scales, we find that the hourglass-shaped field lines are pinched where the gas column density peaks and the field strength is proportional to the 0.4-power of the density. We conclude that B-fields play a crucial role in the fragmentation of NGC 6334.
The potential for hosting photosynthetic life on Earth-like planets within binary/multiple stellar systems was evaluated by modelling the levels of photosynthetically active radiation (PAR) such planets receive. Combinations of M and G stars in: (i) close-binary systems; (ii) wide-binary systems and (iii) three-star systems were investigated and a range of stable radiation environments found to be possible. These environmental conditions allow for the possibility of familiar, but also more exotic forms of photosynthetic life, such as infrared photosynthesisers and organisms specialised for specific spectral niches.
An analytic solution describing an ion-acoustic collisionless shock, self-consistently with the evolution of shock-reflected ions, is obtained. The solution extends the classic soliton solution beyond a critical Mach number, where the soliton ceases to exist because of the upstream ion reflection. The reflection transforms the soliton into a shock with a trailing wave and a foot populated by the reflected ions. The solution relates parameters of the entire shock structure, such as the maximum and minimum of the potential in the trailing wave, the height of the foot, as well as the shock Mach number, to the number of reflected ions. This relation is resolvable for any given distribution of the upstream ions. In this paper, we have resolved it for a simple box distribution. Two separate models of electron interaction with the shock are considered. The first model corresponds to the standard Boltzmannian electron distribution in which case the critical shock Mach number only insignificantly increases from M=1.6 (no ion reflection) to M=1.8 (substantial reflection). The second model corresponds to adiabatically trapped electrons. They produce a stronger increase, from M=3.1 to M=4.5. The shock foot that is supported by the reflected ions also accelerates them somewhat further. A self-similar foot expansion into the upstream medium is also described analytically.
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