Kinsuk Acharyya

The effect of cooling on time dependent behaviour of accretion flows around black holes

We present the results of several numerical simulations of two dimensional axi-symmetric accretion flows around black holes using Smoothed Particle Hydrodynamics (SPH) in the presence of cooling effects. We consider both stellar black holes and super-massive black holes. We observe that due to both radial and vertical oscillation of shock waves in the accretion flow, the luminosity and average thermal energy content of the inner disk exhibit very interesting behaviour. When power density spectra are taken, quasi-periodic variabilities are seen at a few Hz and also occasionally at hundreds of Hz for stellar black holes. For super-massive black holes, the time scale of the oscillations ranges from hours to weeks. The power density spectra have a flat top behavior with average rms amplitude of a few percent and a broken power-law behavior. The break frequency is generally found to be close to the frequency where the shock oscillates.

Deep observations of O2 toward a low-mass protostar with Herschel-HIFI

Context. According to traditional gas-phase chemical models, O-2 should be abundant in molecular clouds, but until recently, attempts to detect interstellar O-2 line emission with ground-and space-based observatories have failed. Aims. Following the multi-line detections of O-2 with low abundances in the Orion and. Oph A molecular clouds with Herschel, it is important to investigate other environments, and we here quantify the O-2 abundance near a solar-mass protostar. Methods. Observations of molecular oxygen, O-2, at 487 GHz toward a deeply embedded low-mass Class 0 protostar, NGC 1333IRAS 4A, are presented, using the Heterodyne Instrument for the Far Infrared (HIFI) on the Herschel Space Observatory. Complementary data of the chemically related NO and CO molecules are obtained as well. The high spectral resolution data are analysed using radiative transfer models to infer column densities and abundances, and are tested directly against full gas-grain chemical models.Results. The deep HIFI spectrum fails to show O-2 at the velocity of the dense protostellar envelope, implying one of the lowest abundance upper limits of O-2/H-2 at = 6x 10-9 (3s). The O-2/CO abundance ratio is less than 0.005. However, a tentative (4.5s) detection of O-2 is seen at the velocity of the surrounding NGC 1333 molecular cloud, shifted by 1 km s-1 relative to the protostar. For the protostellar envelope, pure gas-phase models and gas-grain chemical models require a long pre-collapse phase (similar to 0.7-1 x 106 years), during which atomic and molecular oxygen are frozen out onto dust grains and fully converted to H2O, to avoid overproduction of O2 in the dense envelope. The same model also reproduces the limits on the chemically related NO molecule if hydrogenation of NO on the grains to more complex molecules such as NH2OH, found in recent laboratory experiments, is included. The tentative detection of O-2 in the surrounding cloud is consistent with a low-density PDR model with small changes in reaction rates.

Effective grain surface area in the formation of molecular hydrogen in interstellar clouds

Aims. In the interstellar clouds, molecular hydrogens are formed from atomic hydrogen on grain surfaces. An atomic hydrogen hops around till it finds another one with which it combines. This necessa rily implies that the average recombination time, or equivalently, the effective grain surface area depends on the relative numbers of atomic hydrogen influx rate and the number of sites on the grain. Our a im is to discover this dependency. Methods. We perform a numerical simulation to study the recombination of hydrogen on grain surfaces in a variety of cloud conditions. We use a square lattice (with a periodic boundary condition) of various sizes on two types of grains, namely, amorphous carbon and olivine. Results. We find that the steady state results of our simulation match v ery well with those obtained from a simpler analytical consideration ,

Effects of initial condition and cloud density on the composition of the grain mantle

The evolution of grain mantles in various interstellar environments is studied. We concentrate mainly on water, methanol and carbon dioxide, which constitute nearly 90 per cent of the grain mantle. We investigate how the production rates of these molecules depend on the relative gas-phase abundances of oxygen and carbon monoxide and constrain the relevant parameter space that reproduces these molecules close to the observed abundances. Allowing the accretion of only H, O and CO on the grains and using the Monte Carlo method, we follow the chemical processes for a few million years. We allow the formation of multilayers on the grains and incorporate the freeze-out effects of accreting O and CO. We find that the formation of these molecules depends on the initial conditions as well as on the average cloud density. Specifically, when the number density of accreting O is less than three times that of CO, methanol is always overproduced. Using the available reaction pathways it appears to be difficult to match the exact observed abundances of all three molecules simultaneously. Only in a narrow region of parameter space are all three molecules produced within the observed limits. Furthermore, we found that the incorporation of the freeze-outs of O and CO leads to an almost steady state on the grain surface. The mantle thickness grows anywhere between 60 and 500 layers in a period of two million years. In addition, we consider a case in which the gas number density changes with time owing to the gradual collapse of the molecular cloud and present the evolution of the composition of different species as a function of the radius of the collapsing cloud.

STICKING OF MOLECULES ON NONPOROUS AMORPHOUS WATER ICE

Accurate modeling of physical and chemical processes in the interstellar medium (ISM) requires detailed knowledge of how atoms and molecules adsorb on dust grains. However, the sticking coefficient, a number between 0 and 1 that measures the first step in the interaction of a particle with a surface, is usually assumed in simulations of ISM environments to be either 0.5 or 1. Here we report on the determination of the sticking coefficient of H2, D2, N2, O2, CO, CH4, and CO2 on nonporous amorphous solid water. The sticking coefficient was measured over a wide range of surface temperatures using a highly collimated molecular beam. We showed that the standard way of measuring the sticking coefficient—the King–Wells method—leads to the underestimation of trapping events in which there is incomplete energy accommodation of the molecule on the surface. Surface scattering experiments with the use of a pulsed molecular beam are used instead to measure the sticking coefficient. Based on the values of the measured sticking coefficient, we suggest a useful general formula of the sticking coefficient as a function of grain temperature and molecule-surface binding energy. We use this formula in a simulation of ISM gas–grain chemistry to find the effect of sticking on the abundance of key molecules both on grains and in the gas phase.

The importance of OH radical–neutral low temperature tunnelling reactions in interstellar clouds using a new model

Recent laboratory experiments using a pulsed Laval nozzle apparatus have shown that reactions between a neutral molecule and the radical OH can occur efficiently at low temperatures despite activation energy barriers if there is a hydrogen-bonded complex in the entrance channel which allows the system to tunnel efficiently under the barrier. Since OH is a major radical in the interstellar medium, this class of reactions may well be important in the chemistry that occurs in the gas phase of interstellar clouds. Using a new gas-grain chemical network with both gas-phase reactions and reactions on the surfaces of dust particles, we studied the role of OH–neutral reactions in dense interstellar clouds at 10, 50, and 100 K. We determined that at least one of these reactions can be significant, especially at the lowest temperatures studied, where the rate constants are large. It was found in particular that the reaction between CH3OH and OH provides an effective and unambiguous gas-phase route to the production of the gaseous methoxy radical (CH3O), which has been recently detected in cold, dense interstsellar clouds. The role of other reactions in this class is explored.

Molecular hydrogen formation during dense interstellar cloud collapse

We study the evolution of molecular hydrogen on the grain surfaces and in the gas phase using both the rate equation (which tracks the average number of various species) and the master equation (which tracks the expectation values of various species). We show that above a certain critical accretion rate of H on the grains, the results from these two methods become identical. We used this result to follow the collapse of a dense interstellar cloud and studied the formation of molecular hydrogen for two different temperatures (T = 10 and 12 K) and two different masses (1 and 10 M ○. ) of the cloud when olivine grains were used. Because at higher temperatures, the recombination is very small for these grains, we also studied similar hydrodynamic processes at higher temperatures (T = 20 and 25 K) when amorphous carbon grains were used. We find that generally, for olivine grains, more than 90 per cent H is converted to H 2 within ∼10 5-7 yr whereas for amorphous grains it takes ∼ 10 6-7 yr. H 2 formed in this manner can be adequate to produce the observed complex molecules.

Binding energy of molecules on water ice: Laboratory measurements and modeling

We measured the binding energy of N

SIMULATIONS OF THE CHEMISTRY IN THE SMALL MAGELLANIC CLOUD

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High Spectral Resolution SOFIA/EXES Observations of C2H2 toward Orion IRc2

, CO, O