Two-Stroke Optimization Scheme for Mesoscopic Refrigerators

Refrigerators use a thermodynamic cycle to move thermal energy from a cold reservoir to a hot one. Implementing this operation principle with mesoscopic components has recently emerged as a promising strategy to control heat currents in micro and nano systems for quantum technological applications. Here, we combine concepts from stochastic and quantum thermodynamics with advanced methods of optimal control theory to develop a universal optimization scheme for such small-scale refrigerators. Covering both the classical and the quantum regime, our theoretical framework provides a rigorous procedure to determine the periodic driving protocols that maximize either cooling power or efficiency. As a main technical tool, we decompose the cooling cycle into two strokes, which can be optimized one by one. In the regimes of slow or fast driving, we show how this procedure can be simplified significantly by invoking suitable approximations. To demonstrate the practical viability of our scheme, we determine the exact optimal driving protocols for a quantum microcooler, which can be realized experimentally with current technology. Our work provides a powerful tool to develop optimal design strategies for engineered cooling devices and it creates a versatile framework for theoretical investigations exploring the fundamental performance limits of mesoscopic thermal machines.Comment: 17 pages, 8 figure

Westerlund 1 as a Template for Massive Star Evolution

With a dynamical mass M_dyn ~ 1.3x10e5 M_sun and a lower limit M_cl>5x10e4 M_sun from star counts, Westerlund 1 is the most massive young open cluster known in the Galaxy and thus the perfect laboratory to study massive star evolution. We have developed a comprehensive spectral classification scheme for supergiants based on features in the 6000-9000A range, which allows us to identify >30 very luminous supergiants in Westerlund 1 and ~100 other less evolved massive stars, which join the large population of Wolf-Rayet stars already known. Though detailed studies of these stars are still pending, preliminary rough estimates suggest that the stars we see are evolving to the red part of the HR diagram at approximately constant luminosity.Comment: To be published in Proceedings of IAU Symposium 250: Massive Stars as Cosmic Engines, held in Kaua'i (Hawaii, USA), Dec 2007, edited by F. Bresolin, P.A. Crowther & J. Puls (Cambridge University Press

The influence of nuclei content on cloud cavitation about a hydrofoil

International audienceThe dynamics of cloud cavitation about a 3D hydrofoil are investigated experimentally in a cavitation tunnel with both an abundance and dearth of freestream nuclei. The rectangular-planform, NACA hydrofoil was tested at a Reynolds number of $1.4 × 10^6$, a cavitation number of 0.55 and an incidence of 6°. High-speed photography of cavitation shedding phenomena was acquired simultaneously with unsteady force measurement to enable identification of cavity shedding modes corresponding with force spectral peaks. Two shedding modes are evident for both the nuclei deplete and abundant cases, although each are driven by different flow phenomena. The high-frequency mode for the nuclei deplete case is driven primarily by large-scale re-entrant jet formation during the growth phase, but shockwave propagation for the collapse phase of the cycle. The weaker low-frequency mode occurs because the strength of shedding at the hydrofoil tip varies at half the fundamental frequency. The dominant mode for the abundant case is the low-frequency mode which is some. times slower than the nuclei deplete case. The high-frequency mode for the nuclei abundant case is due to the propagation of two shockwaves; the passage of the first only partially condenses the cavity, while the second condenses a much larger region of the cavity

Cloud cavitation behaviour on a hydrofoil due to fluid-structure interaction

International audienceDespite recent extensive research into fluid-structure interaction (FSI) of cavitating hydrofoils there remains insufficient experimental data to explain many of these observed phenomena. The cloud cavitation behaviour around a hydrofoil due to the effect of FSI is investigated utilizing rigid and compliant 3D hydrofoils held in a cantilevered configuration in a cavitation tunnel. The hydrofoils have identical undeformed geometry of tapered planform with constant NACA section. The rigid model is made of stainless steel and the compliant model of carbon and glass fibre reinforced epoxy resin with the structural fibres aligned along the span-wise direction to avoid material bend-twist coupling. Tests were conducted at an incidence of 6°, a mean chord based Reynolds number of $0.7 × 10^6$, and cavitation number of 0.8. Force measurements were simultaneously acquired with high-speed imaging to enable correlation of forces with tip bending deformations and cavity physics. Hydrofoil compliance was seen to dampen the higher frequency force fluctuations while showing strong correlation between normal force and tip deflection. The 3D nature of the flow field was seen to cause complex cavitation behaviour with two shedding modes observed on both models

Contamination of short GRBs by giant magnetar flares: significance of downwards revision in distance to SGR 1806-20

We highlight how the downward revision in the distance to the star cluster associated with SGR1806-20 by Bibby et al. reconciles the apparent low contamination of BATSE short GRBs by intense flares from extragalactic magnetars without recourse to modifying the frequency of one such flare per 30 years per Milky Way galaxy. We also discuss the variety in progenitor initial masses of magnetars based upon cluster ages, ranging from ~50 Msun for SGR 1806-20 and 1E 1647-455 in Westerlund 1 to ~15 Msun for SGR 1900+14 and presumably 1E 1841-045 if it originated from one of the massive RSG clusters #2 or #3

Survival Outcomes and Prognostic Factors in Glioblastoma.

BACKGROUND: IDH-wildtype glioblastoma is the most common malignant primary brain tumour in adults. As there is limited information on prognostic factors outside of clinical trials; thus, we conducted a retrospective study to characterise the glioblastoma population at our centre. METHODS: Demographic, tumour molecular profiles, treatment, and survival data were collated for patients diagnosed with glioblastoma at our centre between July 2011 and December 2015. We used multivariate proportional hazard model associations with survival. RESULTS: 490 patients were included; 60% had debulking surgery and 40% biopsy only. Subsequently, 56% had standard chemoradiotherapy, 25% had non-standard chemo/radio-therapy, and 19% had no further treatment. Overall survival was 9.2 months. In the multivariate analysis, longer survival was associated with debulking surgery vs. biopsy alone (14.9 vs. 8 months) (HR 0.54 [95% CI 0.41-0.70]), subsequent treatment after diagnosis (HR 0.12 [0.08-0.16]) (standard chemoradiotherapy [16.9 months] vs. non-standard regimens [9.2 months] vs. none [2.0 months]), tumour MGMT promotor methylation (HR 0.71 [0.58-0.87]), and younger age (hazard ratio vs. age < 50: 1.70 [1.26-2.30] for ages 50-59; 3.53 [2.65-4.70] for ages 60-69; 4.82 [3.54-6.56] for ages 70+). CONCLUSIONS: The median survival for patients with glioblastoma is less than a year. Younger age, debulking surgery, treatment with chemoradiotherapy, and MGMT promotor methylation are independently associated with longer survival

The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150 Msun stellar mass limit

Spectroscopic analyses of H-rich WN5-6 stars within the young star clusters NGC 3603 and R136 are presented, using archival HST & VLT spectroscopy, & high spatial resolution near-IR photometry. We derive high T* for the WN stars in NGC 3603 (T*~42+/-2 kK) & R136 (T*~53+/-3 kK) plus clumping-corrected dM/dt ~ 2-5x10^-5 Msun/yr which closely agree with theoretical predictions. These stars make a disproportionate contribution to the global budget of their host clusters. R136a1 alone supplies ~7% of N(LyC) of the entire 30 Dor region. Comparisons with stellar models calculated for the main-sequence evolution of 85-500 Msun suggest ages of ~1.5 Myr & M_init in the range 105 - 170 Msun for 3 systems in NGC 3603, plus 165-320 Msun for 4 stars in R136. Our high stellar masses are supported by dynamical mass determinations for the components of NGC 3603 A1. We consider the predicted L_X of the R136 stars if they were close, colliding wind binaries. R136c is consistent with a colliding wind binary system. However, short period, colliding wind systems are excluded for R136a WN stars if mass ratios are of order unity. Widely separated systems would have been expected to harden owing to early dynamical encounters with other massive stars in such a dense environment. From simulated star clusters, whose constituents are randomly sampled from the Kroupa IMF, both clusters are consistent with a tentative upper mass limit of ~300 Msun. The Arches cluster is either too old, exhibits a deficiency of very massive stars, or more likely stellar masses have been underestimated - M_init for the most luminous stars in the Arches cluster approach 200 Msun according to contemporary stellar & photometric results. The potential for stars greatly exceeding 150 Msun within metal-poor galaxies suggests that such pair-instability SNe could occur within the local universe, as has been claimed for SN 2007bi (abridged).Comment: 20 pages, 14 figures, accepted for MNRAS. Version with higher resolution figures is available from http://pacrowther.staff.shef.ac.uk/R136.pdf See also http://www.eso.org/public/news/eso1030/ from Wed 21 from noon (CEST

METIS - the Mid-infrared E-ELT Imager and Spectrograph

METIS, the Mid-infrared ELT Imager and Spectrograph (formerly called MIDIR), is a proposed instrument for the European Extremely Large Telescope (E-ELT), currently undergoing a phase-A study. The study is carried out within the framework of the ESO-sponsored E-ELT instrumentation studies. METIS will be designed to cover the E-ELT science needs at wavelengths longward of 3um, where the thermal background requires different operating schemes. In this paper we discuss the main science drivers from which the instrument baseline has been derived. Specific emphasis has been given to observations that require very high spatial and spectral resolution, which can only be achieved with a ground-based ELT. We also discuss the challenging aspects of background suppression techniques, adaptive optics in the mid-IR, and telescope site considerations. The METIS instrument baseline includes imaging and spectroscopy at the atmospheric L, M, and N bands with a possible extension to Q band imaging. Both coronagraphy and polarimetry are also being considered. However, we note that the concept is still not yet fully consolidated. The METIS studies are being performed by an international consortium with institutes from the Netherlands, Germany, France, United Kingdom, and Belgium.Comment: 15 pages, to be published in Proc SPIE 7014: Ground-based & Airborne Instrumentation for Astronomy I