Simulations of the formation of stellar discs in the Galactic Centre via cloud–cloud collisions

Young massive stars in the central parsec of our Galaxy are best explained by star formation within at least one, and possibly two, massive self-gravitating gaseous discs. With help of numerical simulations, we here consider whether the observed population of young stars could have originated from a large angle collision of two massive gaseous clouds at   R ≃ 1 pc  from Sgr A*. In all the simulations performed, the post-collision gas flow forms an inner, nearly circular gaseous disc and one or two eccentric outer filaments, consistent with the observations. Furthermore, the radial stellar mass distribution is always very steep,  Σ_*∝ R ⁻²  , again consistent with the observations. All of our simulations produce discs that are warped by between 30° and 60°, in accordance with the most recent observations. The three-dimensional velocity structure of the stellar distribution is sensitive to initial conditions (e.g. the impact parameter of the clouds) and gas cooling details. For example, the runs in which the inner disc is fed intermittently with material possessing fluctuating angular momentum result in multiple stellar discs with different orbital orientations, contradicting the observed data. In all the cases the amount of gas accreted by our inner boundary condition is large, enough to allow Sgr A* to radiate near its Eddington limit over ∼10⁵ yr. This suggests that a refined model would have physically larger clouds (or a cloud and a disc such as the circumnuclear disc) colliding at a distance of a few parsecs rather than 1 pc as in our simulations.     

Constraining the number of compact remnants near Sgr A*

Due to dynamical friction stellar mass black holes and neutron stars are expected to form high-density cusps in the inner parsec of our Galaxy. These compact remnants, expected to number around 20 000, may be accreting cold dense gas present there, and give rise to potentially observable X-ray emission. Here we build a simple but detailed time-dependent model of such emission. The possibility that these accretion flows are radiatively inefficient is taken into account and brings in some uncertainty in the conclusions. Despite this uncertainty, we find that at least several X-ray sources of this nature should be detectable with Chandra at any one time. Turning this issue around, we also ask a question of what current observational constraints might be telling us about the total number of compact remnants. In our 'best guess' model, a cusp of ∼40 000 remnants overpredicts the number of discrete sources and the total X-ray luminosity of the inner parsec, and is hence ruled out. In the most radiatively inefficient scenario that we consider, the radiative efficiency is set to be as small as  ɛ= 10⁻⁵  . In this rather unlikely scenario, a cusp of ∼40 000 black holes would be allowed by the data, but several individual sources should still be visible. Future observations of the distribution and orbits of the cold ionized gas in the inner parsec of our Galaxy will put tighter constraints on the cusp of compact remnants.     

Variable accretion and emission from the stellar winds in the Galactic Centre

We present numerical simulations of stellar wind dynamics in the central parsec of the Galactic Centre, studying in particular the accretion of gas on to Sgr A*, the supermassive black hole. Unlike our previous work, here we use state-of-the-art observational data on orbits and wind properties of individual wind-producing stars. Since wind velocities were revised upwards and non-zero eccentricities were considered, our new simulations show fewer clumps of cold gas and no conspicuous disc-like structure. The accretion rate is dominated by a few close 'slow-wind stars' ( v _w≤ 750 km s⁻¹), and is consistent with the Bondi estimate, but variable on time-scales of tens to hundreds of years. This variability is due to the stochastic infall of cold clumps of gas, as in earlier simulations, and to the eccentric orbits of stars. The present models fail to explain the high luminosity of Sgr A* a few hundred years ago implied by Integral observations, but we argue that the accretion of a cold clump with a small impact parameter could have caused it. Finally, we show the possibility of constraining the total mass-loss rate of the 'slow-wind stars' using near infrared observations of gas in the central few arcseconds.     

Possible evidence for the disc origin for the powering of jets in Sgr A* and nearby elliptical galaxies

Recent VLBA observation indicates the existence of an elongated (jet) structure in the compact radio source Sgr A*. This is hard to explain in the context of advection-dominated accretion flow (ADAF) model for this source. On the other hand, the mass accretion rate favoured by ADAF is 10–20 times smaller than that favoured by the hydrodynamical simulation based on Bondi capture. If the latter were adopted, the predicted radio flux would significantly exceed the observation. A similar situation exists in the case of nearby giant ellipticals, where the canonical ADAF model – the widely assumed standard model for these sources – also significantly overpredicts the radio flux. Based on these facts, in this paper we propose a truncated ADAF model for Sgr A* and three ellipticals M87, NGC 4649 and NGC 4636. We assume that the accretion disc is truncated at a certain radius R _tr within which the jet forms by extracting the energy of the disc. The radio flux is greatly suppressed owing to the radiative truncation of the disc and the fits to the observational data are excellent. For example, for Sgr A*, the model fits the observational spectrum very well from radio including the 'excess' below the break frequency to hard X-ray under a high accretion rate near the simulation value, and the predicted size-frequency relationship is also in excellent agreement with the observation; for M87, the predicted upper limit of the jet location is 24 R _g, in excellent agreement with the observational result that the jet is formed on scales smaller than 30 R _g, and the ≈20 per cent variability at ∼1 keV – which is hard to explain in another model that succeeded in explaining the low radio flux of M87 – is also marginally interpreted. The success of the model supplies possible evidence for the disc rather than the hole origin for the powering of jets.     

X-rays from cusps of compact remnants near galactic centres

Compact remnants – stellar mass black holes and neutron stars formed in the inner few parsec of galactic centres are predicted to sink into the central parsec due to dynamical friction on low-mass stars, forming a high concentration cusp. Same physical region may also contain very high-density molecular clouds and accretion discs that are needed to fuel supermassive black hole (SMBH) activity. Here we estimate gas capture rates on to the cusp of stellar remnants, and the resulting X-ray luminosity, as a function of the accretion disc mass. At low disc masses, most compact objects are too dim to be observable, whereas in the high disc case most of them are accreting at their Eddington rates. We find that for low accretion disc masses, compact remnant cusps may be more luminous than the central SMBHs. This 'diffuse' emission may be of importance for local moderately bright active galactic nuclei (AGNs), especially low-luminosity AGNs. We also briefly discuss how this expected emission can be used to put constraints on the black hole cusp near our Galactic Centre.     

Gravitational instability of discs with dissipative coronae around supermassive black holes

We study the dynamical structure of a self-gravitating disc with coronae around a supermassive black hole. Assuming that the magnetorotational instability responsible for generating the turbulent stresses inside the disc is also the source for a magnetically dominated corona, a fraction of the power released when the disc matter accretes is transported to and dissipated in the corona. This has a major effect on the structure of the disc and its gravitational (in)stability according to our analytical and self-consistent solutions. We determine the radius where the disc crosses the inner radius of gravitational instability and forms the first stars. Not only the location of this radius which may extend to very large distances from the central black hole, but also the mass of the first stars highly depends on the input parameters, notably the viscosity coefficient, the mass of the central object and the accretion rate. For accretion discs around quasi-stellar objects (QSOs) and the Galactic Centre, we determine the self-gravitating radius and the mass of the first clumps. Comparing the cases with a corona and without a corona for typical discs around QSOs or the Galactic Centre, when the viscosity coefficient is around 0.3, we show that the self-gravitating radius decreases by a factor of approximately 2, but the mass of the fragments increases with more or less the same factor. The existence of a corona implies a more gravitationally unstable disc according to our results. The effect of a corona on the instability of the disc is more effective when the viscosity coefficient increases.     

On the effects of the stellar magnetic field on the structure of T Tauri accretion discs

The structure of accretion discs around magnetic T Tauri stars is calculated numerically using a particle hydrodynamical code, in which magnetic interaction is included in the framework of King's diamagnetic blob accretion model. Setting up the calculation so as to simulate the density structure of a quasi-steady disc in the equatorial plane of a T Tauri star, we find that the central star's magnetic field typically produces a central hole in the disc and spreads out the surface density distribution. We argue that this result suggets a promising mechanism for explaining the unusual flatness (IR excess) of T Tauri accretion disc spectra.     

Concave accretion discs and X-ray reprocessing

Spectra of Seyfert 1s are commonly modelled as emission from an X-ray-illuminated flat accretion disc orbiting a central black hole. This provides both reprocessed and direct components of the X-ray emission, as required by observations of individual objects, and possibly a fraction of the cosmological X-ray background. There is some observational motivation for us to at least consider the role that an effectively concave disc surface might play: (1) a reprocessed fraction ≳1/2 in some Seyferts and possibly in the X-ray background, and (2) the commonality of a sharp iron line peak for Seyferts at 6.4 keV despite a dependence of peak location on inclination angle for flat disc models. Here it is shown that a concave disc may not only provide a larger total fraction of reprocessed photons, but can also reprocess a much larger fraction of photons in its outer regions compared with a flat disc. This reduces the sensitivity of the 6.4-keV peak location to the inner disc inclination angle because the outer regions are less affected by Doppler and gravitational effects. If the X-ray source is isotropic, the reprocessed fraction is directly determined by the concavity. If the X-ray source is anisotropic, the location of iron line peak can still be determined by concavity but the total reflected fraction need not be as large as for the isotropic emitter case. The geometric calculations herein are applicable to general accretion disc systems illuminated from the centre.     

Self-gravitating warped discs around supermassive black holes

We consider warped equilibrium configurations for stellar and gaseous discs in the Keplerian force field of a supermassive black hole, assuming that the self-gravity of the disc provides the only acting torques. Modelling the disc as a collection of concentric circular rings and computing the torques in the non-linear regime, we show that stable, strongly warped precessing equilibria are possible. These solutions exist for a wide range of disc-to-black-hole mass ratios   M _d/ M _bh  , can span large warp angles of up to  ±∼120°  , have inner and outer boundaries, and extend over a radial range of a factor of typically two to four. These equilibrium configurations obey a scaling relation such that in good approximation     where     is the (retrograde) precession frequency and Ω is a characteristic orbital frequency in the disc. Stability was determined using linear perturbation theory and, in a few cases, confirmed by numerical integration of the equations of motion. Most of the precessing equilibria are found to be stable, but some are unstable. The main result of this study is that highly warped discs near black holes can persist for long times without any persistent forcing other than by their self-gravity. The possible relevance of this to galactic nuclei is briefly discussed.     

Variability associated with α accretion disc theory for standard and advection-dominated discs

The α turbulent viscosity formalism for accretion discs must be interpreted as a mean field theory, modelling a steady state only on spatial or time-scales greater than those of the turbulence. The extent of the scale separation determines the relative precision error (RPE) of the predicted luminosity L _ν. Turbulence and the use of α implies that (1) field line stretching gives a magnetic pressure  α²/6 of the total pressure generally, and a one-to-one relation between α and the pressure ratio for thin discs, and (2) large turbulent scales in advection-dominated accretion flows (ADAFs) predict a lower L _ν precision than thin discs for a given observation duration and central mass. The allowed variability (or RPE) at frequency ν increases with the size of the contributing region. For X-ray binary ADAFs, the RPE ∼ 5 per cent at R  ≤ 1000 Schwarzchild radii ( R _s) for averages over  1000 s. However, current data for galaxies like NGC 4258 and M87 give RPEs in L _ν of 50–100 per cent even at R  ≤ 100  R _S. More data are required, but systematic deviations from ADAF predictions are more significant than random deviations, and may constrain properties of the turbulence, the accretion mode, the assumption of a steady state or the accretion rate.     

Do accretion discs regulate the rotation of young stars?

We present a photometric study of I -band variability in the young cluster IC 348. The main purpose of the study was to identify periodic stars. In all, we find 50 periodic stars, of which 32 were previously unknown. For the first time in IC 348, we discover periods in significant numbers of lower-mass stars  ( M < 0.25 M_⊙)  and classical T Tauri stars. This increased sensitivity to periodicities is a result of the enhanced depth and temporal density of our observations, compared with previous studies. The period distribution is at first glance similar to that seen in the Orion nebula cluster (ONC), with the higher-mass stars  ( M > 0.25 M_⊙)  showing a bi-modal period distribution concentrated around periods of 2 and 8 d, and the lower-mass stars showing a uni-modal distribution, heavily biased towards fast rotators. Closer inspection of the period distribution shows that the higher-mass stars show a significant dearth of fast rotators, compared to the ONC, whilst the low-mass stars are rotating significantly faster than those in Orion. We find no correlation between rotation period and K – L colour or Hα equivalent width.
We also present a discussion of our own IC 348 data in the context of previously published period distributions for the ONC, the Orion flanking fields and NGC 2264. We find that the previously claimed correlation between infrared excess and rotation period in the ONC might, in fact, result from a correlation between infrared excess and mass. We also find a marked difference in period distributions between NGC 2264 and IC 348, which presents a serious challenge to the disc-locking paradigm, given the similarity in ages and disc fractions between the two clusters.