The formation of young massive clusters by colliding flows

Young massive clusters (YMCs) are the most intense regions of star formation in galaxies. Formulating a model for YMC formation whilst at the same time meeting the constraints from observations is highly challenging however. We show that forming YMCs requires clouds with densities $\gtrsim$ 100 cm$^{-3}$ to collide with high velocities ($\gtrsim$ 20 km s$^{-1}$). We present the first simulations which, starting from moderate cloud densities of $\sim100$ cm$^{-3}$, are able to convert a large amount of mass into stars over a time period of around 1 Myr, to produce dense massive clusters similar to those observed. Such conditions are commonplace in more extreme environments, where YMCs are common, but atypical for our Galaxy, where YMCs are rare.Comment: 6 pages, 5 figures, accepted for publication in MNRAS Letter

UK Housing Market: Time Series Processes with Independent and Identically Distributed Residuals

The paper examines whether a univariate data generating process can be identified which explains the data by having residuals that are independent and identically distributed, as verified by the BDS test. The stationary first differenced natural log quarterly house price index is regressed, initially with a constant variance and then with a conditional variance. The only regression function that produces independent and identically distributed standardised residuals is a mean process based on a pure random walk format with Exponential GARCH in mean for the conditional variance. There is an indication of an asymmetric volatility feedback effect but higher frequency data is required to confirm this. There could be scope for forecasting the index but this is tempered by the reduction in the power of the BDS test if there is a non-linear conditional variance process

Synthesis of Diblocks Copolymers PCL-b-PLLA and Optimization of its Mechanical Characteristics

Biodegradable polymeric materials have a wide application in medicine, ecology and a number of other branches of industry. Overwhelming majority of such polymers is well exposed to the biodegradation, but they have mechanical properties which don’t correspond to the requirements of application areas. As a rule, these materials possess low elasticity. In this work, optimization of mechanical properties of biodegradable polymers and the search of structure of block polymer on the basis of e-caprolactone and L-lactide with the maximal elasticity are considered. The purpose of the work is to receive the diblock polymer with mechanical properties as close as possible to elastomer for medical devices. The task is reached due to application of design of experiment with the subsequent optimization of the received results. Maximization of elongation at maximum load of received polymer is solved by the greatest possible reduction of crystallinity with a variation of molar weight of both part of diblock: polycaprolactone and polylactide. The absolute maximum of elongation at maximum load of synthesized diblock polymers is found. It is established that diblock with the following structure possesses the best mechanical properties in this class of polymers. Molar weight of the polycaprolactone block is 5000 Da while polylactide block is 7000 Da. Elongation at maximum load of this polymer was about 30%. In the second stage of our research, we try to improve the mechanical properties by making one block partly random (PCL-co-PLLA). Second block was made from homopolymer (PLLA). Maximum received elongation at break is 200%. First block of this polymer contains 25% of lactide, has molar weight of 10000 Da. Second block is the pure polylactide, with molar weight 10000 Da. All the experimental results and mathematical modeling pointing direction to maximum elongation could be achieved by multi-block structures

Synthesis of Diblocks Copolymers PCL-b-PLLA and Optimization of its Mechanical Characteristics

Biodegradable polymeric materials have a wide application in medicine, ecology and a number of other branches of industry. Overwhelming majority of such polymers is well exposed to the biodegradation, but they have mechanical properties which don't correspond to the requirements of application areas. As a rule, these materials possess low elasticity. In this work, optimization of mechanical properties of biodegradable polymers and the search of structure of block polymer on the basis of e-caprolactone and L-lactide with the maximal elasticity are considered. The purpose of the work is to receive the diblock polymer with mechanical properties as close as possible to elastomer for medical devices. The task is reached due to application of design of experiment with the subsequent optimization of the received results. Maximization of elongation at maximum load of received polymer is solved by the greatest possible reduction of crystallinity with a variation of molar weight of both part of diblock: polycaprolactone and polylactide. The absolute maximum of elongation at maximum load of synthesized diblock polymers is found. It is established that diblock with the following structure possesses the best mechanical properties in this class of polymers. Molar weight of the polycaprolactone block is 5000 Da while polylactide block is 7000 Da. Elongation at maximum load of this polymer was about 30%. In the second stage of our research, we try to improve the mechanical properties by making one block partly random (PCL-co-PLLA). Second block was made from homopolymer (PLLA). Maximum received elongation at break is 200%. First block of this polymer contains 25% of lactide, has molar weight of 10000 Da. Second block is the pure polylactide, with molar weight 10000 Da. All the experimental results and mathematical modeling pointing direction to maximum elongation could be achieved by multi-block structures

How large should whales be?

The evolution and distribution of species body sizes for terrestrial mammals is well-explained by a macroevolutionary tradeoff between short-term selective advantages and long-term extinction risks from increased species body size, unfolding above the 2g minimum size induced by thermoregulation in air. Here, we consider whether this same tradeoff, formalized as a constrained convection-reaction-diffusion system, can also explain the sizes of fully aquatic mammals, which have not previously been considered. By replacing the terrestrial minimum with a pelagic one, at roughly 7000g, the terrestrial mammal tradeoff model accurately predicts, with no tunable parameters, the observed body masses of all extant cetacean species, including the 175,000,000g Blue Whale. This strong agreement between theory and data suggests that a universal macroevolutionary tradeoff governs body size evolution for all mammals, regardless of their habitat. The dramatic sizes of cetaceans can thus be attributed mainly to the increased convective heat loss is water, which shifts the species size distribution upward and pushes its right tail into ranges inaccessible to terrestrial mammals. Under this macroevolutionary tradeoff, the largest expected species occurs where the rate at which smaller-bodied species move up into large-bodied niches approximately equals the rate at which extinction removes them.Comment: 7 pages, 3 figures, 2 data table

Extreme Value Theory versus traditional GARCH approaches applied to financial data: a comparative evaluation

Although stock prices fluctuate, the variations are relatively small and are frequently assumed to be normally distributed on a large time scale. But sometimes these fluctuations can become determinant, especially when unforeseen large drops in asset prices are observed that could result in huge losses or even in market crashes. The evidence shows that these events happen far more often than would be expected under the generalised assumption of normally distributed financial returns. Thus it is crucial to model distribution tails properly so as to be able to predict the frequency and magnitude of extreme stock price returns. In this paper we follow the approach suggested by McNeil and Frey in 2000 and combine GARCH-type models with the extreme value theory to estimate the tails of three financial index returns ¿ S&P 500, FTSE 100 and NIKKEI 225 ¿ representing three important financial areas in the world. Our results indicate that EVT-based conditional quantile estimates are more accurate than those from conventional GARCH models assuming normal or Student¿s t distribution innovations when doing not only in-sample but also out-of-sample estimation. Moreover, these results are robust to alternative GARCH model specifications. The findings of this paper should be useful to investors in general, since their goal is to be able to forecast unforeseen price movements and take advantage of them by positioning themselves in the market according to these predictions

Monetary policy uncertainty spillovers in time and frequency domains

We use the recently created monthly Interest Rate Uncertainty measure, to investigate monetary policy uncertainty across the US, Germany, France, Italy, Spain, UK, Japan, Canada, and Sweden in both the time and frequency domains. We find that the largest spillover indices are from innovations in the country itself; however, there are some instances where spillover indices between countries are large. These relationships change over time and we observe large variances in pairwise spillovers during the global financial crisis. We find that most of the volatility is confined to the crisis period. Policy makers should consider accounting for the spillovers from the US, Germany, France and Spain, as we found that they are the most consistent net transmitters of monetary policy uncertainty

A Multivariate Analysis of United States and Global Real Estate Investment Trusts

Using daily data for the period February 2006 to July 2013 we examine the return and volatility linkages between the two main United States REIT sub-sectors and global linkages between the Americas, Europe and the Asia Pacific regions using the BEKK-GARCH and the DCC-GARCH models. We find that there is no evidence of any volatility spillovers between the US sub-sectors. By contrast, we find evidence of volatility spillovers between the Asia Pacific and the Americas, the Asia Pacific and Europe but no spillovers between the United States and Europe. Our results suggest that the REIT market is becoming increasingly globalized and that investors need to consider time varying volatility and correlations across different regions of the world when forming their optimal portfolio-allocations

Laser-Assisted In Vitro Fertilization Facilitates Fertilization of Vitrified-Warmed C57BL/6 Mouse Oocytes with Fresh and Frozen-Thawed Spermatozoa, Producing Live Pups

The utility of cryopreserved mouse gametes for reproduction of transgenic mice depends on development of assisted reproductive technologies, including vitrification of unfertilized mouse oocytes. Due to hardening of the zona pellucida, spermatozoa are often unable to penetrate vitrified-warmed (V-W) oocytes. Laser-assisted in vitro fertilization (LAIVF) facilitates fertilization by allowing easier penetration of spermatozoa through a perforation in the zona. We investigated the efficiency of V-W C57BL/6NTac oocytes drilled by the XYClone laser, compared to fresh oocytes. By using DAP213 for cryoprotection, 83% (1,470/1,762) of vitrified oocytes were recovered after warming and 78% were viable. Four groups were evaluated for two-cell embryo and live offspring efficiency: 1) LAIVF using V-W oocytes, 2) LAIVF using fresh oocytes, 3) conventional IVF using V-W oocytes and 4) conventional IVF using fresh oocytes. First, the groups were tested using fresh C57BL/6NTac spermatozoa (74% motile, 15 million/ml). LAIVF markedly improved the two-cell embryo efficiency using both V-W (76%, 229/298) and fresh oocytes (69%, 135/197), compared to conventional IVF (7%, 12/182; 6%, 14/235, respectively). Then, frozen-thawed C57BL/6NTac spermatozoa (35% motile, 15 million/ml) were used and LAIVF was again found to enhance fertilization efficiency, with two-cell embryo rates of 87% (298/343) using V-W oocytes (P<0.05, compared to fresh spermatozoa), and 73% (195/266) using fresh oocytes. Conventional IVF with frozen-thawed spermatozoa using V-W (6%, 10/168) and fresh (5%, 15/323) oocytes produced few two-cell embryos. Although live offspring efficiency following embryo transfer was greater with conventional IVF (35%, 18/51; LAIVF: 6%, 50/784), advantage was seen with LAIVF in live offspring obtained from total oocytes (5%, 50/1,010; conventional IVF: 2%, 18/908). Our results demonstrated that zona-drilled V-W mouse oocytes can be used for IVF procedures using both fresh and frozen-thawed spermatozoa, producing live pups. The ability to cryopreserve mouse gametes for LAIVF may facilitate management of large-scale transgenic mouse production facilities.National Institutes of Health (U.S.) (NIH National Research Service Award (T32-RR070036))National Institutes of Health (U.S.) (NIH National Cancer Institute Program Project (P01CA10451)