Time and M-theory

We review our recent proposal for a background independent formulation of a holographic theory of quantum gravity. The present review incorporates the necessary background material on geometry of canonical quantum theory, holography and spacetime thermodynamics, Matrix theory, as well as our specific proposal for a dynamical theory of geometric quantum mechanics, as applied to Matrix theory. At the heart of this review is a new analysis of the conceptual problem of time and the closely related and phenomenologically relevant problem of vacuum energy in quantum gravity. We also present a discussion of some observational implications of this new viewpoint on the problem of vacuum energy.Comment: 86 pages, 5 figures, LaTeX, typos fixed, references added, and Sec. 6.2 revised; invited review for Int. J. Mod. Phys.

Molecular gas freeze-out in the pre-stellar core L1689B

C17O (J=2-1) observations have been carried out towards the pre-stellar core L1689B. By comparing the relative strengths of the hyperfine components of this line, the emission is shown to be optically thin. This allows accurate CO column densities to be determined and, for reference, this calculation is described in detail. The hydrogen column densities that these measurements imply are substantially smaller than those calculated from SCUBA dust emission data. Furthermore, the C17O column densities are approximately constant across L1689B whereas the SCUBA column densities are peaked towards the centre. The most likely explanation is that CO is depleted from the central regions of L1689B. Simple models of pre-stellar cores with an inner depleted region are compared with the results. This enables the magnitude of the CO depletion to be quantified and also allows the spatial extent of the freeze-out to be firmly established. We estimate that within about 5000 AU of the centre of L1689B, over 90% of the CO has frozen onto grains. This level of depletion can only be achieved after a duration that is at least comparable to the free-fall timescale.Comment: MNRAS letters. 5 pages, 5 figure

Rotation of the pre-stellar core L1689B

The search for the onset of star formation in pre-stellar cores has focussed on the identification of an infall signature in the molecular line profiles of tracer species. The classic infall signature is a double peaked line profile with an asymmetry in the strength of the peaks such that the blue peak is stronger. L1689B is a pre-stellar core and infall candidate but new JCMT HCO+ line profile data, presented here, confirms that both blue and red asymmetric line profiles are present in this source. Moreover, a dividing line can be drawn between the locations where each type of profile is found. It is argued that it is unlikely that the line profiles can be interpreted with simple models of infall or outflow and that rotation of the inner regions is the most likely explanation. A rotational model is developed in detail with a new 3D molecular line transport code and it is found that the best type of model is one in which the rotational velocity profile is in between solid body and Keplerian. It is firstly shown that red and blue asymmetric line profiles can be generated with a rotation model entirely in the absence of any infall motion. The model is then quantitively compared with the JCMT data and an iteration over a range of parameters is performed to minmize the difference between the data and model. The results indicate that rotation can dominate the line profile shape even before the onset of infall.Comment: Accepted by MNRAS, 7 pages, 4 figure

Molecules, ices and astronomy

Molecules in interstellar gas and in interstellar ices play a fundamental role in astronomy. However, the formation of the simplest molecule, molecular hydrogen, is still not fully understood. Similarly, although interstellar ice analogues have received much attention in the laboratory, the evolution of ices in the interstellar medium still requires further study. At UCL we have developed two separate experiments to address these issues and explore the following questions: How is H formed on dust-grain surfaces? What is the budget between internal, kinetic and surface energies in the formation process? What are the astronomical consequences of these results? For ices, we ask: How do molecules desorb from pure and from mixed ices in regions warmed by newly formed stars? What can molecules released from ices tell us about the star-formation process? We put our results in the context of other laboratory work and we describe their application to current problems in astronomy

Desorption From Interstellar Ices

The desorption of molecular species from ice mantles back into the gas phase in molecular clouds results from a variety of very poorly understood processes. We have investigated three mechanisms; desorption resulting from H_2 formation on grains, direct cosmic ray heating and cosmic ray induced photodesorption. Whilst qualitative differences exist between these processes (essentially deriving from the assumptions concerning the species-selectivity of the desorption and the assumed threshold adsorption energies, E_t) all three processes are found to be potentially very significant in dark cloud conditions. It is therefore important that all three mechanisms should be considered in studies of molecular clouds in which freeze-out and desorption are believed to be important. Employing a chemical model of a typical static molecular core and using likely estimates for the quantum yields of the three processes we find that desorption by H_2 formation probably dominates over the other two mechanisms. However, the physics of the desorption processes and the nature of the dust grains and ice mantles are very poorly constrained. We therefore conclude that the best approach is to set empirical constraints on the desorption, based on observed molecular depletions - rather than try to establish the desorption efficiencies from purely theoretical considerations. Applying this method to one such object (L1689B) yields upper limits to the desorption efficiencies that are consistent with our understanding of these mechanisms.Comment: 11 pages, 5 figures, accepted by MNRAS subject to minor revision which has been carried ou

Molecular Clouds as Ensembles of Transient Cores

We construct models of molecular clouds that are considered as ensembles of transient cores. Each core is assumed to develop in the background gas of the cloud, grow to high density and decay into the background. The chemistry in each core responds to the dynamical state of the gas and to the gas-dust interaction. Ices are deposited on the dust grains in the core's dense phase, and this material is returned to the gas as the core expands to low density. The cores of the ensemble number typically one thousand and are placed randomly in position within the cloud, and are assigned a random evolutionary phase. The models are used to generate molecular line contour maps of a typical dark cloud. These maps are found to represent extremely well the characteristic features of observed maps of the dark cloud L673, which has been observed at both low and high resolutions. The computed maps are found to exhibit the general morphology of the observed maps, and to generate similar sizes of emitting regions, molecular column densities, and the separations between peaks of emissions of various molecular species. The models give insight into the nature of molecular clouds and the dynamical processes occurring within them, and significantly constrain dynamical and chemical processes in the interstellar medium.Comment: 29 pages, 8 figures. Accepted for publication in Ap

On Star Formation and the Non-Existence of Dark Galaxies

We investigate whether a baryonic dark galaxy or `galaxy without stars' could persist indefinitely in the local universe, while remaining stable against star formation. To this end, a simple model has been constructed to determine the equilibrium distribution and composition of a gaseous protogalactic disk. Specifically, we determine the amount of gas that will transit to a Toomre unstable cold phase via the H2 cooling channel in the presence of a UV--X-ray cosmic background radiation field. All but one of the models are predicted to become unstable to star formation. Moreover, we find that all our model objects would be detectable via HI line emission, even in the case that star formation is potentially avoided. These results are consistent with the non-detection of isolated extragalactic HI clouds with no optical counterpart (galaxies without stars) by HIPASS. Additionally, where star formation is predicted to occur, we determine the minimum interstellar radiation field required to restore gravothermal stability, which we then relate to a minimum global star formation rate. This leads to the prediction of a previously undocumented relation between HI mass and star formation rate that is observed for a wide variety of dwarf galaxies in the HI mass range 10^8--10^10 M_sun. The existence of such a relation strongly supports the notion that the well observed population of dwarf galaxies represent the minimum rates of self-regulating star formation in the universe. (Barely abridged)Comment: 19 pages, 8 figures, TeX using emulateapj.cls, v2 accepted for publication in ApJ (16/8/5) with one figure deleted and a number of minor clarifying revision