5941 research outputs found
Sort by
Evolution of multiple supernova remnants
No full textOpen AccessHeating of the interstellar medium (ISM) by multiple supernova (SN) explosions is at the heart of producing galaxy-scale outflows in starburst galaxies. Standard models of outflows assume a high efficiency of SNe in heating the gas to X-ray emitting temperatures and filling the central region of starburst with hot gas, in order to launch vigorous outflows. We use hydrodynamical simulations to study the efficiency of multiple SNe in heating the ISM and filling the volume with gas of high temperatures. We argue that it is important for SN remnants to have a large filling factor and a large heating efficiency. For this, they have to be clustered in space and time, and keep exploding until the hot gas percolates through the whole region, in order to compensate for the radiative loss. In the case of a limited number of SNe, we find that although the filling factor can be large, the heating efficiency declines after reaching a large value. In the case of a continuous series of SNe, the hot gas (T ≥ 3 × 106 K) can percolate through the whole region after the total volume filling factor reaches a threshold of ∼0.3. The efficiency of heating the gas to X-ray temperatures can be ≥0.1 after this percolation epoch, which occurs after a period of ≈10 Myr for a typical starburst SN rate density of νSN ≈ 10−9 pc−3 yr−1 and gas density of n ≈ 10 cm−3 in starburst nuclei regions. This matches the recent observations of a time delay of similar order between the onset of star formation and galactic outflows. The efficiency to heat gas up to X-ray temperatures (≥106.5 K) roughly scales as ν0.2SNn−0.6. For a typical SN rate density and gas density in starburst nuclei, the heating efficiency is ∼0.15, also consistent with previous interpretations from X-ray observations. We discuss the implications of our results with regard to observational diagnostics of ionic ratios and emission measures in starburst nuclei regions
Spot-free catalysis using gold carbon nanotube & gold graphene composites for hydrogen evolution reaction
No full textRestricted Access.Hydrogen has been proposed as the green fuel of the future in the wake of depleting fossil fuels. Recently, carbon paste electrodes (CPE) modified with nanomaterials as electrocatalysts have drawn wide attention for hydrogen evolution reaction (HER) in acid medium. The CPEs are advantageous owing to their chemical stability and ease of fabrication. Their applications for HER without any modification, however, are hampered on account of large hydrogen overpotential associated with carbon surface. In the present study, CPE has been modified with novel gold composites as electro-catalysts for HER in acid medium. The nanocomposites have shown ∼100 fold increased current density than unmodified CPE at −0.3 V. Most strikingly for the first time, this study has quantitatively brought out the difference in catalysis between surfactant capped and pristine gold nanoparticles in terms of their application as spot-free catalysts towards hydrogen gas production by electrochemical route
Crystal structure of 4-methoxyphenyl 2-oxo-2H-chromene-3-carboxylate
No full textOpen AccessIn the title compound, C16H9BrO4, the coumarin ring system is approximately planar, with an r.m.s deviation of the ten fitted non-H atoms of 0.031 Å, and forms a dihedral angle of 25.85 (10)° with the bromobenzene ring. The carbonyl atoms are syn. In the crystal, molecules are connected along [001] via C—H⋯O interactions, forming C(6) chains. Neighbouring C(6) chains are connected via several π–π interactions [range of centroid–centroid distances = 3.7254 (15)–3.7716 (16) Å], leading to sheets propagating in the bc plane
Crystal structure of 4-bromophenyl-2-oxo-2H-chromene-3-carboxylate
No full textRestricted Access.In the title compound, C16H9BrO4, the coumarin ring system is approximately planar, with an r.m.s deviation of the ten fitted non-H atoms of 0.031 Å, and forms a dihedral angle of 25.85 (10)° with the bromobenzene ring. The carbonyl atoms are syn. In the crystal, molecules are connected along [001] via C—H⋯O interactions, forming C(6) chains. Neighbouring C(6) chains are connected via several π–π interactions [range of centroid–centroid distances = 3.7254 (15)–3.7716 (16) Å], leading to sheets propagating in the bc plan
Directed search for gravitational waves from Scorpius X-1 with initial LIGO data
No full textOpen AccessWe present results of a search for continuously emitted gravitational radiation, directed at the brightest low-mass x-ray binary, Scorpius X-1. Our semicoherent analysis covers 10 days of LIGO S5 data ranging from 50–550 Hz, and performs an incoherent sum of coherent F-statistic power distributed amongst frequency-modulated orbital sidebands. All candidates not removed at the veto stage were found to be consistent with noise at a 1% false alarm rate. We present Bayesian 95% confidence upper limits on gravitational-wave strain amplitude using two different prior distributions: a standard one, with no a priori assumptions about the orientation of Scorpius X-1; and an angle-restricted one, using a prior derived from electromagnetic observations. Median strain upper limits of 1.3×10−24 and 8×10−25 are reported at 150 Hz for the standard and angle-restricted searches respectively. This proof-of-principle analysis was limited to a short observation time by unknown effects of accretion on the intrinsic spin frequency of the neutron star, but improves upon previous upper limits by factors of ∼1.4 for the standard, and 2.3 for the angle-restricted search at the sensitive region of the detector
The role of three-body H2 formation in the fragmentation of primordial gas
No full textOpen AccessIt has been shown that the behaviour of primordial gas collapsing in a dark matter minihalo can depend on the adopted choice of three-body H2 formation rate. The uncertainties in this rate span two orders of magnitude in the current literature, and so it remains a source of uncertainty in our knowledge of Population III star formation. Here, we investigate how the amount of fragmentation in primordial gas depends on the adopted three-body rate. We present the results of calculations that follow the chemical and thermal evolution of primordial gas as it collapses in two dark matter minihaloes. Our results on the effect of three-body rate on the evolution until the first protostar forms agree well with previous studies. However, our modified version of gadget-2 smoothed particle hydrodynamics also includes sink particles, which allows us to follow the initial evolution of the accretion disc that builds up on the centre of each halo, and capture the fragmentation in gas as well as its dependence on the adopted three-body H2 formation rate. We find that the fragmentation behaviour of the gas is only marginally affected by the choice of three-body rate co-efficient, and that halo-to-halo differences are of equal importance in affecting the final mass distribution of stars
CMB distortion from circumgalactic gas
No full textOpen AccessWe study the Sunyaev–Zel'dovich (SZ) distortion of the cosmic microwave background radiation from extensive circumgalactic gas (CGM) in massive galactic haloes. Recent observations have shown that galactic haloes contain a large amount of X-ray emitting gas at the virial temperature, as well as a significant amount of warm O vi absorbing gas. We consider the SZ distortion from the hot gas in those galactic haloes in which the gas cooling time is longer than the halo destruction time-scale. We show that the SZ distortion signal from the hot gas in these galactic haloes at redshifts z ≈ 1–8 can be significant at small angular scales (ℓ ∼ 104), and dominate over the signal from galaxy clusters. The estimated SZ signal for most massive galaxies (halo mass ≥1012.5 M⊙) is consistent with the marginal detection by Planck at these mass scales. We also consider the SZ effect from warm circumgalactic gas. The integrated Compton distortion from the warm O vi absorbing gas is estimated to be y ∼ 10−8, which could potentially be detected by experiments planned for the near future. Finally, we study the detectability of the SZ signal from circumgalactic gas in two types of surveys, a simple extension of the South Pole Telescope survey and a more futuristic cosmic-variance-limited survey. We find that these surveys can easily detect the kinetic Sunyaev–Zel'dovich signal from CGM. With the help of a Fisher matrix analysis, we find that it will be possible for these surveys to constrain the gas fraction in CGM, after marginalizing over cosmological parameters, to ≤33 per cent, in case of no redshift evolution of the gas fraction