Next Generation Cloud Computing: New Trends and Research Directions

The landscape of cloud computing has significantly changed over the last decade. Not only have more providers and service offerings crowded the space, but also cloud infrastructure that was traditionally limited to single provider data centers is now evolving. In this paper, we firstly discuss the changing cloud infrastructure and consider the use of infrastructure from multiple providers and the benefit of decentralising computing away from data centers. These trends have resulted in the need for a variety of new computing architectures that will be offered by future cloud infrastructure. These architectures are anticipated to impact areas, such as connecting people and devices, data-intensive computing, the service space and self-learning systems. Finally, we lay out a roadmap of challenges that will need to be addressed for realising the potential of next generation cloud systems.Comment: Accepted to Future Generation Computer Systems, 07 September 201

Mantle formation, coagulation and the origin of cloud/core-shine: II. Comparison with observations

Many dense interstellar clouds are observable in emission in the near-IR, commonly referred to as "Cloudshine", and in the mid-IR, the so-called "Coreshine". These C-shine observations have usually been explained with grain growth but no model has yet been able to self-consistently explain the dust spectral energy distribution from the near-IR to the submm. We want to demonstrate the ability of our new core/mantle evolutionary dust model THEMIS (The Heterogeneous dust Evolution Model at the IaS), which has been shown to be valid in the far-IR and submm, to reproduce the C-shine observations. Our starting point is a physically motivated core/mantle dust model. It consists of 3 dust populations: small aromatic-rich carbon grains; bigger core/mantle grains with mantles of aromatic-rich carbon and cores either made of amorphous aliphatic-rich carbon or amorphous silicate. We assume an evolutionary path where these grains, when entering denser regions, may first form a second aliphatic-rich carbon mantle (coagulation of small grains, accretion of carbon from the gas phase), second coagulate together to form large aggregates, and third accrete gas phase molecules coating them with an ice mantle. To compute the corresponding dust emission and scattering, we use a 3D Monte-Carlo radiative transfer code. We show that our global evolutionary dust modelling approach THEMIS allows us to reproduce C-shine observations towards dense starless clouds. Dust scattering and emission is most sensitive to the cloud central density and to the steepness of the cloud density profile. Varying these two parameters leads to changes, which are stronger in the near-IR, in both the C-shine intensity and profile. With a combination of aliphatic-rich mantle formation and low-level coagulation into aggregates, we can self-consistently explain the observed C-shine and far-IR/submm emission towards dense starless clouds.Comment: Paper accepted for publication in A&A with companion paper "Mantle formation, coagulation and the origin of cloud/core-shine: I. Dust scattering and absorption in the IR", A.P Jones, M. Koehler, N. Ysard, E. Dartois, M. Godard, L. Gavila

Towards an open cloud marketplace: vision and first steps

As one of the most promising, emerging concepts in Information Technology (IT), cloud computing is transforming how IT is consumed and managed; yielding improved cost efficiencies, and delivering flexible, on-demand scalability by reducing computing infrastructures, platforms, and services to commodities acquired and paid-for on-demand through a set of cloud providers. Today, the transition of cloud computing from a subject of research and innovation to a critical infrastructure is proceeding at an incredibly fast pace. A potentially dangerous consequence of this speedy transition to practice is the premature adoption, and ossification, of the models, technologies, and standards underlying this critical infrastructure. This state of affairs is exacerbated by the fact that innovative research on production-scale platforms is becoming the purview of a small number of public cloud providers. Specifically, the academic research communities are effectively excluded from the opportunity to contribute meaningfully to the evolution not to mention innovation and healthy mutation of cloud computing technologies. As the dependence on our society and economy on cloud computing increases, so does the realization that the academic research community cannot be shut out from contributing to the design and evolution of this critical infrastructure. In this article we provide an alternative vision that of an Open Cloud eXchange (OCX) a public cloud marketplace, where many stakeholders, rather than just a single cloud provider, participate in implementing and operating the cloud, thus creating an ecosystem that will bring the innovation of a broader community to bear on a much healthier and more efficient cloud marketplace

An Evolutionary Model for Collapsing Molecular Clouds and Their Star Formation Activity

We present an idealized, semi-empirical model for the evolution of gravitationally contracting molecular clouds (MCs) and their star formation rate (SFR) and efficiency (SFE). The model assumes that the instantaneous SFR is given by the mass above a certain density threshold divided by its free-fall time. The instantaneous number of massive stars is computed assuming a Kroupa IMF. These stars feed back on the cloud through ionizing radiation, eroding it. The main controlling parameter of the evolution turns out to be the maximum cloud mass, \Mmax. This allows us to compare various properties of the model clouds against their observational counterparts. A giant molecular cloud (GMC) model (\Mmax \sim 10^5 \Msun) adheres very well to the evolutionary scenario recently inferred by Kawamura et al. (2009) for GMCs in the Large Magellanic Cloud. A model cloud with \Mmax \approx 2000 \Msun evolves in the Kennicutt-Schmidt diagram first passing through the locus of typical low- to-intermediate mass star-forming clouds, and then moving towards the locus of high-mass star-forming ones over the course of $\sim 10$ Myr. Also, the stellar age histograms for this cloud a few Myr before its destruction agree very well with those observed in the $\rho$-Oph stellar association, whose parent cloud has a similar mass, and imply that the SFR of the clouds increases with time. Our model thus agrees well with various observed properties of star-forming MCs, suggesting that the scenario of gravitationally collapsing MCs, with their SFR regulated by stellar feedback, is entirely feasible and in agreement with key observed properties of molecular clouds.Comment: Version accepted for publication in ApJ. At referee's suggestion, includes comparison with numerical models in addition to comparison with observational dat

Near-infrared reddening of extra-galactic GMCs in a face-on geometry

[Abridged] We describe the near-infrared reddening signature of giant molecular clouds (GMCs) in external galaxies. In particular, we examine the E(J-H) and E(H-K) color-excesses, and the effective extinction law observed in discrete GMC regions. We also study the effect of the relative scale height of the GMC distribution to the color-excesses, and to the observed mass function of GMCs. We perform Monte Carlo radiative transfer simulations with 3D models of stellar radiation and clumpy dust distributions, resembling a face-on geometry. The scattered light is included in the models, and near-infrared color maps are calculated from the simulated data. The effective near-infrared reddening law, i.e. the ratio E(J-H)/E(H-K), has a value close to unity in GMC regions. The ratio depends on the relative scale height of GMCs, xi, and for xi values 0.1...0.75 we find the typical ratios of 0.6...1.1. The effective extinction law turns out to be very flat in GMC regions. We find the ratios of apparent extinctions of A(H)/A(K)=1.35...1.55 and A(J)/A(H)=1.15. The effect of the scattered flux on the effective reddening law, as well as on the effective extinction law, is significant. Regarding the GMC mass function, we find no correlation between the input and observed slopes of the mass functions. Rather, the observed slope reflects the parameter $\xi$ and the dynamical range of the mass function. We estimate that only a fraction of 10...20 % of the total mass of GMCs is recovered, if the observed color-excess values are transformed to masses using the Galactic reddening law. In the case of individual clouds the fraction can vary between ~0...50 %.Comment: 8 pages, 10 figures, accepted for publication in A&A. Added missing histograms in Fig.

Physical State of Molecular Gas in High Galactic Latitude Translucent Clouds

The rotational transitions of carbon monoxide (CO) are the primary means of investigating the density and velocity structure of the molecular interstellar medium. Here we study the lowest four rotational transitions of CO towards high-latitude translucent molecular clouds (HLCs). We report new observations of the J = (4-3), (2-1), and (1-0) transitions of CO towards eight high-latitude clouds. The new observations are combined with data from the literature to show that the emission from all observed CO transitions is linearly correlated. This implies that the excitation conditions which lead to emission in these transitions are uniform throughout the clouds. Observed 13CO/12CO (1-0) integrated intensity ratios are generally much greater than the expected abundance ratio of the two species, indicating that the regions which emit 12CO (1-0) radiation are optically thick. We develop a statistical method to compare the observed line ratios with models of CO excitation and radiative transfer. This enables us to determine the most likely portion of the physical parameter space which is compatible with the observations. The model enables us to rule out CO gas temperatures greater than 30K since the most likely high-temperature configurations are 1 pc-sized structures aligned along the line of sight. The most probable solution is a high density and low temperature (HDLT) solution. The CO cell size is approximately 0.01 pc (2000 AU). These cells are thus tiny fragments within the 100 times larger CO-emitting extent of a typical high-latitude cloud. We discuss the physical implications of HDLT cells, and we suggest ways to test for their existence.Comment: 19 pages, 13 figures, 2 tables, emulateapj To be published in The Astrophysical Journa

Photoionization of Galactic Halo Gas by Old Supernova Remnants

We present new calculations on the contribution from cooling hot gas to the photoionization of warm ionized gas in the Galaxy. We show that hot gas in cooling supernova remnants (SNRs) is an important source of photoionization, particularly for gas in the halo. We find that in many regions at high latitude this source is adequate to account for the observed ionization so there is no need to find ways to transport stellar photons from the disk. The flux from cooling SNRs sets a floor on the ionization along any line of sight. Our model flux is also shown to be consistent with the diffuse soft X-ray background and with soft X-ray observations of external galaxies. We consider the ionization of the clouds observed towards the halo star HD 93521, for which there are no O stars close to the line of sight. We show that the observed ionization can be explained successfully by our model EUV/soft X-ray flux from cooling hot gas. In particular, we can match the H alpha intensity, the S++/S+ ratio, and the C+* column. From observations of the ratios of columns of C+* and either S+ or H0, we are able to estimate the thermal pressure in the clouds. The slow clouds require high (~10^4 cm^-3 K) thermal pressures to match the N(C+*)/N(S+) ratio. Additional heating sources are required for the slow clouds to maintain their ~7000 K temperatures at these pressures, as found by Reynolds, Hausen & Tufte (1999).Comment: AASTeX 5.01; 34 pages, 2 figures; submitted to Astrophysical Journa

On the chemistry of hydrides of N atoms and O+^+ ions

Previous work by various authors has suggested that the detection by Herschel/HIFI of nitrogen hydrides along the low density lines of sight towards G10.6-0.4 (W31C) cannot be accounted for by gas-phase chemical models. In this paper we investigate the role of surface reactions on dust grains in diffuse regions, and we find that formation of the hydrides by surface reactions on dust grains with efficiency comparable to that for H$_2$ formation reconciles models with observations of nitrogen hydrides. However, similar surface reactions do not contribute significantly to the hydrides of O$^+$ ions detected by Herschel/HIFI present along many sight lines in the Galaxy. The O$^+$ hydrides can be accounted for by conventional gas-phase chemistry either in diffuse clouds of very low density with normal cosmic ray fluxes or in somewhat denser diffuse clouds with high cosmic ray fluxes. Hydride chemistry in dense dark clouds appears to be dominated by gas-phase ion-molecule reactions.Comment: 19 pages, 4 figures, 4 tables Accepted for publication in Ap