Enhanced Momentum Feedback from Clustered Supernovae

Young stars typically form in star clusters, so the supernovae (SNe) they produce are clustered in space and time. This clustering of SNe may alter the momentum per SN deposited in the interstellar medium (ISM) by affecting the local ISM density, which in turn affects the cooling rate. We study the effect of multiple SNe using idealized 1D hydrodynamic simulations which explore a large parameter space of the number of SNe, and the background gas density and metallicity. The results are provided as a table and an analytic fitting formula. We find that for clusters with up to ~100 SNe the asymptotic momentum scales super-linearly with the number of SNe, resulting in a momentum per SN that can be an order of magnitude larger than for a single SN, with a maximum efficiency for clusters with 10-100 SNe. We argue that additional physical processes not included in our simulations -- self-gravity, breakout from a galactic disk, and galactic shear -- can slightly reduce the momentum enhancement from clustering, but the average momentum per SN still remains a factor of 4 larger than the isolated SN value when averaged over a realistic cluster mass function for a star-forming galaxy. We conclude with a discussion of the possible role of mixing between hot and cold gas, induced by multi-dimensional instabilities or preexisting density variations, as a limiting factor in the buildup of momentum by clustered SNe, and suggest future numerical experiments to explore these effects.Comment: 19 pages, 26 figures, revised to reflect accepted version. Discussion regarding resolution effects has changed; additional analysis into galactic and gravitational effects has been adde

Forests and Carbon: A Synthesis of Science, Management, and Policy for Carbon Sequestration in Forests

The goal of this volume is to provide guidance for land managers and policymakers seeking to understand the complex science and policy of forest carbon as it relates to tangible problems of forest management and the more abstract problems of addressing drivers of deforestation and negotiating policy frameworks for reducing CO2 emissions from forests. It is the culmination of three graduate seminars at the Yale School of Forestry & Environmental Studies focused on carbon sequestration in forest ecosystems and their role in addressing climate change

The momentum budget of clustered supernova feedback in a 3D, magnetised medium

While the evolution of superbubbles driven by clustered supernovae has been studied by numerous authors, the resulting radial momentum yield is uncertain by as much as an order of magnitude depending on the computational methods and assumed properties of the surrounding interstellar medium (ISM). In this work, we study the origin of these discrepancies, and seek to determine the correct momentum budget for a homogeneous ISM. We carry out 3D hydrodynamic (HD) and magnetohydrodynamic (MHD) simulations of clustered supernova explosions, using a Lagrangian method and checking for convergence with respect to resolution. We find that the terminal momentum of a shell driven by clustered supernovae is dictated primarily by the mixing rate across the contact discontinuity between the hot and cold phases, and that this energy mixing rate is dominated by numerical diffusion even at the highest resolution we can complete, 0.03 $M_\odot$. Magnetic fields also reduce the mixing rate, so that MHD simulations produce higher momentum yields than HD ones at equal resolution. As a result, we obtain only a lower limit on the momentum yield from clustered supernovae. Combining this with our previous 1D results, which provide an upper limit because they allow almost no mixing across the contact discontinuity, we conclude that the momentum yield per supernova from clustered supernovae in a homogeneous ISM is bounded between $2\times 10^5$ and $3\times 10^6$ $M_\odot$ km s$^{-1}$. A converged value for the simple homogeneous ISM remains elusive.Comment: 13 page, 11 figures, revised to reflect the published version. Added a deeper analysis estimating physical and numeric mixing rates (Section 4). Added addition simulations to the resolution stud

Activity-dependent trafficking of lysosomes in dendrites and dendritic spines.

In neurons, lysosomes, which degrade membrane and cytoplasmic components, are thought to primarily reside in somatic and axonal compartments, but there is little understanding of their distribution and function in dendrites. Here, we used conventional and two-photon imaging and electron microscopy to show that lysosomes traffic bidirectionally in dendrites and are present in dendritic spines. We find that lysosome inhibition alters their mobility and also decreases dendritic spine number. Furthermore, perturbing microtubule and actin cytoskeletal dynamics has an inverse relationship on the distribution and motility of lysosomes in dendrites. We also find trafficking of lysosomes is correlated with synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type glutamate receptors. Strikingly, lysosomes traffic to dendritic spines in an activity-dependent manner and can be recruited to individual spines in response to local activation. These data indicate the position of lysosomes is regulated by synaptic activity and thus plays an instructive role in the turnover of synaptic membrane proteins

What is the clinical workup for failure to thrive?

The clinical evaluation of failure to thrive (FTT) includes a thorough history and physical examination; observation of parent-child interactions; observation and documentation of the child's feeding patterns; and a home visit by an appropriately trained health care professional (Strength of Recommendation [SOR]: C). Further diagnostic testing should be performed as indicated by positive findings from the history and physical exam or if the child's weight has not improved at follow-up (SOR: C)

How Do Lessons Learned on the International Space Station (ISS) Help Plan Life Support for Mars?

How can our experience in developing and operating the International Space Station (ISS) guide the design, development, and operation of life support for the journey to Mars? The Mars deep space Environmental Control and Life Support System (ECLSS) must incorporate the knowledge and experience gained in developing ECLSS for low Earth orbit, but it must also meet the challenging new requirements of operation in deep space where there is no possibility of emergency resupply or quick crew return. The understanding gained by developing ISS flight hardware and successfully supporting a crew in orbit for many years is uniquely instructive. Different requirements for Mars life support suggest that different decisions may be made in design, testing, and operations planning, but the lessons learned developing the ECLSS for ISS provide valuable guidance

Momentum injection by clustered supernovae: testing subgrid feedback prescriptions

Using a 1D Lagrangian code specifically designed to assess the impact of multiple, time-resolved supernovae (SNe) from a single-star cluster on the surrounding medium, we test three commonly used feedback recipes: delayed cooling (e.g. used in the GASOLINE-2 code), momentum-energy injection (a resolution-dependent transition between momentum-dominated feedback and energy-dominated feedback used, e.g. in the FIRE-2 code), and simultaneous energy injection (e.g. used in the EAGLE simulations). Our work provides an intermediary test for these recipes: we analyse a setting that is more complex than the simplified scenarios for which many were designed, but one more controlled than a full galactic simulation. In particular, we test how well these models reproduce the enhanced momentum efficiency seen for an 11 SN cluster simulated at high resolution (0.6 pc; a factor of 12 enhancement relative to the isolated SN case) when these subgrid recipes are implemented in low resolution (20 pc) runs. We find that: (1) the delayed cooling model performs well – resulting in 9 times the momentum efficiency of the fiducial isolated SN value – when SNe are clustered and 1051 erg are injected per SN, while clearly overpredicting the momentum efficiency in the single SN test case; (2) the momentum-energy model always achieves good results, with a factor of 5 boost in momentum efficiency; and (3) injecting the energy from all SNe simultaneously does little to prevent overcooling and greatly underproduces the momentum deposited by clustered SNe, resulting in a factor of 3 decrease in momentum efficiency on the average.We thank the anonymous reviewer for their useful comments and suggestions. This work was supported by the National Science Foundation (NSF) through grants AST-1405962 (ESG and MRK), AST-1229745 (PM) andDGE 1339067 (ESG), by the Australian Research Council through grant FT180100375 (MRK) and by National Aeronautics and Space Administration (NASA) through a contract to the WFIRST-EXPO Science Investigation Team (15-WFIRST15-0004), administered by the Goddard Space Flight Center (PM). This work made use of resources and services from the National Computational Infrastructure (NCI), which is supported by the Australian Government. MRK thanks the Simons Foundation, which contributed to this work through its support for the Simons Symposium ‘Galactic Superwinds: Beyond Phenomenology’

Fully Secure Attribute Based Encryption from Multilinear Maps

We construct the first fully secure attribute based encryption (ABE) scheme that can handle access control policies expressible as polynomial-size circuits. Previous ABE schemes for general circuits were proved secure only in an unrealistic selective security model, where the adversary is forced to specify its target before seeing the public parameters, and full security could be obtained only by complexity leveraging, where the reduction succeeds only if correctly guesses the adversary’s target string x*, incurring a 2^{|x^*|} loss factor in the tightness of the reduction. At a very high level, our basic ABE scheme is reminiscent of Yao’s garbled circuits, with 4 gadgets per gate of the circuit, but where the decrypter in our scheme puts together the appropriate subset of gate gadgets like puzzle pieces by using a cryptographic multilinear map to multiply the pieces together. We use a novel twist of Waters’ dual encryption methodology to prove the full security of our scheme. Most importantly, we show how to preserve the delicate information-theoretic argument at the heart of Waters’ dual system by enfolding it in an information-theoretic argument similar to that used in Yao’s garbled circuits