A5039117039- Astrophysical Phenomena and Observations
- Astrophysics and Star Formation Studies
- Stellar, planetary, and galactic studies
- Galaxies: Formation, Evolution, Phenomena
- Astro and Planetary Science
- Astronomy and Astrophysical Research
- Gamma-ray bursts and supernovae
- High-pressure geophysics and materials
- Astrophysics and Cosmic Phenomena
- Pulsars and Gravitational Waves Research
- Mechanics and Biomechanics Studies
- Heat Transfer Mechanisms
- Solar and Space Plasma Dynamics
- Molecular Spectroscopy and Structure
- Relativity and Gravitational Theory
- Astronomical Observations and Instrumentation
- Geophysics and Gravity Measurements
- Nuclear Physics and Applications
- X-ray Spectroscopy and Fluorescence Analysis
- Cosmology and Gravitation Theories
- Atomic and Molecular Physics
- Fluid Dynamics Simulations and Interactions
- Phase Equilibria and Thermodynamics
- Magnetic confinement fusion research
- Scientific Measurement and Uncertainty Evaluation
University of Leicester
2015-2024
Tel Aviv University
2014
Max Planck Institute for Astrophysics
2001-2011
Max Planck Society
2002-2007
Goddard Space Flight Center
2000-2002
Universities Space Research Association
2001-2002
National Academies of Sciences, Engineering, and Medicine
2001
University of Arizona
1997-2001
National Research Council
2000-2001
Los Alamos National Laboratory
1996
While gas accretion onto some massive black holes (MBHs) at the centers of galaxies actively powers luminous emission, vast majority MBHs are considered dormant. Occasionally, a star passing too near MBH is torn apart by gravitational forces, leading to bright panchromatic tidal disruption flare (TDF). high-energy transient Swift J164449.3+573451 ("Sw 1644+57") initially displayed none theoretically anticipated (nor previously observed) TDF characteristics, we show that observations (Levan...
We report the definite spectroscopic identification of ≃40 OB supergiants, giants, and main-sequence stars in central parsec Galaxy. Detection their absorption lines has become possible with high spatial spectral resolution sensitivity adaptive optics integral field spectrometer SPIFFI/SINFONI on ESO VLT. Several these appear to be helium- nitrogen-rich. Almost all ≃80 massive now known (central arcsecond excluded) reside one two somewhat thick (⟨ /R⟩ ≃ 0.14) rotating disks. These stellar...
We present new observations of the nuclear star cluster in central parsec Galaxy with adaptive optics assisted, integral field spectrograph SINFONI on ESO/VLT. Our work allows spectroscopic detection early- and late-type stars to mK ⩾ 16, more than 2 mag deeper our previous data sets. result a total sample 177 bona fide early-type stars. find that most these Wolf Rayet (WR), O-, B-stars reside two strongly warped disks between 08 12'' from Sgr A*, as well compact concentration (the S-star...
Most supermassive black holes (SMBHs) are accreting at very low levels and difficult to distinguish from the galaxy centers where they reside. Our own Galaxy's SMBH provides a uniquely instructive exception, we present close-up view of its quiescent X-ray emission based on 3 mega-second Chandra observations. Although is elongated aligns well with surrounding disk massive stars, can rule out concentration low-mass coronally active stars as origin lack predicted Fe Kalpha emission. The...
Many models of cosmological gamma-ray bursts involve the sudden release $\sim 10^{51}$ erg which produce shells expand at relativistic speeds (Lorentz $Γ$ factors $10^{2-3}$). We investigate kinematic limits on source size due to observed time structure in three types bursts: short spikes, FREDs (Fast Rise, Exponentail decay), and long complex bursts. The emitting shell keeps up with photons it produces reducing apparent durations by $Γ^2$ so that sizes can be very large ($2cΓ^2 T_{dur})....
We study the illumination of accretion disks in vicinity compact objects by an overlying X-ray source. Our approach differs from previous works subject that we relax simplifying assumption constant gas density used these studies; instead determine hydrostatic balance which is solved simultaneously with ionization and radiative transfer a plane-parallel geometry. calculate temperature profile illuminated layer reprocessed spectra for range physical conditions, values photon index Γ...
The central parsec around the supermassive black hole in Galactic center (GC) hosts more than 100 young and massive stars. Outside cusp (R ∼ 1'') majority of these O Wolf–Rayet (W–R) stars reside a main clockwise system, plus second, less prominent disk or streamer system at large angles with respect to system. Here we present results from new observations GC AO-assisted near-infrared imager NACO integral field spectrograph SINFONI on ESO/VLT. These include detection 27 reliably measured...
We compute the effect of an orbiting gas disc in promoting coalescence a central supermassive black hole binary. Unlike earlier studies, we consider finite mass with explicit time dependence: do not assume that necessarily adopts steady state or spatially constant accretion rate, i.e. merging was somehow inserted into pre--existing disc. tidal torque binary on disc, and binary's gravitational radiation. study effects star formation simple energy feedback framework. The spectrum differs...
We present the first systematic analysis of X-ray variability Sgr A* during Chandra Observatory's 2012 Visionary Project. With 38 High Energy Transmission Grating Spectrometer observations spaced an average 7 days apart, this unprecedented campaign enables detailed study emission from supermassive black hole at high spatial, spectral and timing resolution. In 3 Ms observations, we detect 39 flares A*, lasting a few hundred seconds to approximately 8 ks, ranging in 2–10 keV luminosity ∼1034...
We hypothesise that planets are made by tidal downsizing of migrating giant planet embryos. The proposed scheme for formation consists these steps: (i) a massive young protoplanetary disc fragments at R ~ several tens to hundreds AU on gaseous clumps with masses few Jupiter masses; (ii) the cool and contract, simultaneously migrate closer in parent star; (iii) as earlier suggested Boss (1998), dust sediments inside gas form terrestrial mass solid cores; (iv) if core becomes more than 10...
Recent well-resolved numerical simulations of active galactic nuclei (AGN) feedback have shown that its effects on the host galaxy may be not only negative but also positive. In late gas-poor phase, AGN blows gas away and terminates star formation. However, in gas-rich phase(s), outflows trigger formation by over-compressing cold dense thus provide positive their hosts. this paper we study AGN-triggered starburst effect. We show rate burst increases until counteracts locally outflow...
In the context of massive fragmenting protoplanetary discs, Boss (1998) suggested that grains can grow and sediment inside giant planet embryos formed at R ~ 5 AU away from star. Several authors since then criticised suggestion. Convection may prevent grain sedimentation, cannot even form so close to parent star as cooling is too inefficient these distances. Here we reconsider sedimentation process by but an embryo formed, expected in light constraints, 100 AU. Such are much less dense also...
Tidal Downsizing is the modern version of Kuiper (1951) scenario planet formation. Detailed simulations self-gravitating discs, gas fragments, dust grain dynamics, and evolutionary calculations are summarised here used to build a predictive formation model population synthesis. A new interpretation exoplanetary debris disc data, Solar System's origins, links between planets brown dwarfs offered. This contrasted with current observations predictions Core Accretion theory. Observations that...
We numerically model fragmentation of a gravitationally unstable gaseous disc under conditions that may be appropriate for the formation young massive stars observed in central parsec our Galaxy. In this study, we adopt simple prescription with locally constant cooling time. find that, times just short enough to induce fragmentation, form top-heavy Initial Mass Function (IMF), as Galactic Centre (GC). For shorter times, fragments much more vigorously, leading lower average stellar masses....
Few dozens of young high mass stars orbit Sgr A* at distances as short 0.1 parsec, where star formation should be quenched by the strong tidal shear from A*. The puzzling stellar population is believed to come into existence in one two ways: (i) "normal" several tens parsec a very massive cluster that then spiraled in, or (ii) situ self-gravitating disk. We propose constrain these scenarios via expected X-ray emission low have formed alongside stars. To this end we compare recent Chandra...
We present a detailed discussion of our new 3D numerical models for the accretion stellar winds on to Sgr A*. In most sophisticated models, we put wind sources realistic orbits around A*, include recently discovered 'slow' (vw∼ 300 km s−1), and account optically thin radiative cooling. test approach by first modelling only one-phase 'fast' 1000 s−1). For fixed in space, rate is order , fluctuates ≲10 per cent, good agreement with previous models. contrast, decreases an magnitude following...
Sgr A* is extra-ordinarily dim in all wavelengths requiring a very low accretion rate at the present time. However, radial distance of fraction parsec from A*, two rings populated by young massive stars suggest recent burst star formation rather hostile environment. Here we explore ways creating such stellar with gaseous disk: self-gravity disk, and capturing "old" mass growing them via gas disk. The minimum disk above for first mechanism few tens times larger second one. observed relatively...
We present numerical simulations of stellar wind dynamics in the central parsec Galactic centre, studying particular accretion gas on to Sgr A*, super-massive black hole. Unlike our previous work, here we use state-of-the-art observational data orbits and properties individual wind-producing stars. Since velocities were revised upwards non-zero eccentricities considered, new show fewer clumps cold no conspicuous disc-like structure. The rate is dominated by a few close `slow stars' v_w <...
Young massive stars in the central parsec of our Galaxy are best explained by star formation within at least one, and possibly two, self-gravitating gaseous discs. With help numerical simulations, we here consider whether observed population young could have originated from a large angle collision two clouds R approx. 1 Sgr A*. In all simulations performed, post-collision gas flow forms an inner, nearly circular disc one or eccentric outer filaments, consistent with observations....
We use smoothed particle hydrodynamics simulations with an approximate radiative cooling prescription to model the evolution of a massive (∼100 au) very young protoplanetary disc. also dust growth and gas-grain dynamics second fluid approach. It is found that disc fragments into large number ∼10MJ clumps cool contract slowly. Some evolve on eccentric orbits, delivering them inner tens au, where they are disrupted by tidal forces from star. Dust grows sediments inside clumps, displaying...
{\it Fermi}-LAT has recently detected two gamma ray bubbles disposed symmetrically with respect to the Galactic plane. The have been suggested be in a quasi-steady state, inflated by ongoing star formation over age of Galaxy. Here we propose an alternative picture where are remnants large-scale wide-angle outflow from \sgra, SMBH our Such would natural consequence short but bright accretion event on \sgra\ if it happened concurrently well known inner 0.5 pc Milky Way $\sim 6$ Myr ago. We...
Deposition of a massive ($10^4$ to $10^5 \msun$) giant molecular cloud (GMC) into the inner parsec Galaxy is widely believed explain origin over hundred unusually young stars born there $\sim 6$ Myr ago. An unknown fraction that gas could have been accreted by Sgr A*, supermassive black hole (SMBH) Milky Way. It has recently suggested two observed $\gamma$-ray-emitting bubbles emanating from very center our were inflated this putative activity A*. We run suite numerical simulations test...
It is usually assumed that outflows from luminous AGN are either in the energy-conserving (non-radiative) or momentum-conserving (radiative) regime. We show a non-spherical geometry effects of both regimes may manifest at same time, and it momentum outflow sets $M_{\rm BH}-\sigma$ relation. Considering an initially elliptical distribution gas host galaxy, we non-radiative opens up wide ``escape route'' over paths least resistance. Most energy escapes direction. At directions higher...
It has long been recognized that the main obstacle to accretion of gas on supermassive black holes (SMBHs) is a large specific angular momentum. feared settles in large-scale disc, and would then proceed too inefficiently explain masses observed SMBHs. Here we point out that, while mean momentum bulge very likely be large, deviations from can also significant. Indeed, cosmological simulations show velocity fields flows galaxies are complex. Furthermore, inside bulges distribution further...