Laser-induced ultrafast electron emission from a field emission tip

We show that a field emission tip electron source that is triggered with a femtosecond laser pulse can generate electron pulses shorter than the laser pulse duration (~100 fs). The emission process is sensitive to a power law of the laser intensity, which supports an emission mechanism based on multiphoton absorption followed by over-the-barrier emission. Observed continuous transitions between power laws of different orders are indicative of field emission processes. We show that the source can also be operated so that thermionic emission processes become significant. Understanding these different emission processes is relevant for the production of sub-cycle electron pulses

The earliest phases of high-mass star formation: a 3 square degree millimeter continuum mapping of Cygnus X

We have made an extensive 1.2mm continuum mosaicing study of the Cygnus X molecular cloud complex using the MAMBO cameras at the IRAM 30 m telescope. We then compared our mm maps with mid-IR images, and have made SiO(2-1) follow-up observations of the best candidate progenitors of high-mass stars. Our complete study of Cygnus X provides, for the first time, an unbiased census of massive young stellar objects. We discover 129 massive dense cores, among which 42 are probable precursors of high-mass stars. Our study qualifies 17 cores as good candidates for hosting massive IR-quiet protostars, while up to 25 cores potentially host high-luminosity IR protostars. We fail to discover the high-mass analogs of pre-stellar dense cores in CygnusX, but find several massive starless clumps that might be gravitationally bound. Since our sample is derived from a single molecular complex and covers every embedded phase of high-mass star formation, it gives the first statistical estimates of their lifetime. In contrast to what is found for low-mass class 0 and class I phases, the IR-quiet protostellar phase of high-mass stars may last as long as their better-known high-luminosity IR phase. The statistical lifetimes of high-mass protostars and pre-stellar cores (~ 3 x 10^4 yr and < 10^3 yr) in Cygnus X are one and two order(s) of magnitude smaller, respectively, than what is found in nearby, low-mass star-forming regions. We therefore propose that high-mass pre-stellar and protostellar cores are in a highly dynamic state, as expected in a molecular cloud where turbulent processes dominate.Comment: 32 pages, 62 figures to be published in Astronomy & Astrophysics journa

Survival Strategies: Doctoral Students’ Perceptions of Challenges and Coping Methods

The purpose of this collective case study was to use a critical dialectical pluralistic (CDP) philosophical lens to investigate select doctoral students’ perceptions about the challenges that they encountered while in a doctorate program and the coping strategies that they found effective in mitigating these challenges. A major goal of CDP is to empower research participants maximally by giving them the role of participant-researchers. Participants were 10 doctoral students enrolled at a Tier-II university in the United States, who were selected via convenience sampling. Each student participated in a face-to-face interview with a member of the research team—consistent with a CDP approach. A qualitative-dominant crossover mixed analysis was used wherein both quantitative and qualitative analyses were used to analyze the qualitative data, with the qualitative analysis phase being dominant. The qualitative analyses (e.g., constant comparison analysis, classical content analysis) revealed the following five themes: compartmentalization of life, outside support systems, justification for participation in program, emotional status, and structure of program. These themes indicated that although challenges are plentiful, particularly in terms of balancing one’s academic life with other obligations, participants found support and encouragement from family, friends, and other doctoral students to be the most beneficial coping strategy. These findings have important implications for the structuring of doctoral programs

Beliefs in national continuity are related to essentialist thinking and to perceptions of the nation as a family

National narratives serve to foster a sense of collective continuity-the perception that the nation has preserved its traits, values and goals across many generations. The present study explores some of the correlates of such perceptions of collective continuity (PCC). We predicted that people who see their nation as more continuous would tend to think about social groups in more strongly essentialist terms and to feel personally attached to other group members (a phenomenon known as identity fusion). An international sample of 307 respondents (predominantly from the United States and India) completed measures of PCC, social essentialism, identity fusion and national identification. Both hypotheses were supported at the level of the level of the full sample, suggesting that perceived national continuity is related to a general cognitive predisposition for essen-tialist thinking and also to one's sense of personal attachment to the nation. However, exploratory analyses by nationality revealed that the results could not be replicated with the Indian participants, potentially as a result of cultural factors. Identity fusion was also more strongly correlated to cultural/essentialist continuity than to historical continuity. Interpretations and directions for future research are discussed

Fragmentation and mass segregation in the massive dense cores of Cygnus X

We present Plateau de Bure interferometer observations obtained in continuum at 1.3 and 3.5 mm towards the six most massive and young (IR-quiet) dense cores in Cygnus X. Located at only 1.7 kpc, the Cygnus X region offers the opportunity of reaching small enough scales (of the order of 1700 AU at 1.3 mm) to separate individual collapsing objects. The cores are sub-fragmented with a total of 23 fragments inside 5 cores. Only the most compact core, CygX-N63, could actually be a single massive protostar with an envelope mass as large as 60 Msun. The fragments in the other cores have sizes and separations similar to low-mass pre-stellar and proto-stellar condensations in nearby protoclusters, and are probably of the same nature. A total of 9 out of these 23 protostellar objects are found to be probable precursors of OB stars with envelope masses ranging from 6 to 23 Msun. The level of fragmentation is globally higher than in the turbulence regulated, monolithic collapse scenario, but is not as high as expected in a pure gravo-turbulent scenario where the distribution of mass is dominated by low-mass protostars/stars. Here, the fractions of the total core masses in the high-mass fragments are reaching values as high as 28, 44, and 100 % in CygX-N12, CygX-N53, and CygX-N63, respectively, much higher than what an IMF-like mass distribution would predict. The increase of the fragmentation efficiency as a function of density in the cores is proposed to be due to the increasing importance of self-gravity leading to gravitational collapse at the scale of the dense cores. At the same time, the cores tend to fragment into a few massive protostars within their central regions. We are therefore probably witnessing here the primordial mass segregation of clusters in formation.Comment: 14 pages, 16 figures, submitted for publication in A&

HARP/ACSIS: A submillimetre spectral imaging system on the James Clerk Maxwell Telescope

This paper describes a new Heterodyne Array Receiver Programme (HARP) and Auto-Correlation Spectral Imaging System (ACSIS) that have recently been installed and commissioned on the James Clerk Maxwell Telescope (JCMT). The 16-element focal-plane array receiver, operating in the submillimetre from 325 to 375 GHz, offers high (three-dimensional) mapping speeds, along with significant improvements over single-detector counterparts in calibration and image quality. Receiver temperatures are $\sim$120 K across the whole band and system temperatures of $\sim$300K are reached routinely under good weather conditions. The system includes a single-sideband filter so these are SSB figures. Used in conjunction with ACSIS, the system can produce large-scale maps rapidly, in one or more frequency settings, at high spatial and spectral resolution. Fully-sampled maps of size 1 square degree can be observed in under 1 hour. The scientific need for array receivers arises from the requirement for programmes to study samples of objects of statistically significant size, in large-scale unbiased surveys of galactic and extra-galactic regions. Along with morphological information, the new spectral imaging system can be used to study the physical and chemical properties of regions of interest. Its three-dimensional imaging capabilities are critical for research into turbulence and dynamics. In addition, HARP/ACSIS will provide highly complementary science programmes to wide-field continuum studies, and produce the essential preparatory work for submillimetre interferometers such as the SMA and ALMA.Comment: MNRAS Accepted 2009 July 2. 18 pages, 25 figures and 6 table

Enhanced efficacy and increased long-term toxicity of CNS-directed, AAV-based combination therapy for Krabbe disease

Infantile globoid cell leukodystrophy (GLD, Krabbe disease) is a demyelinating disease caused by the deficiency of the lysosomal enzyme galactosylceramidase (GALC) and the progressive accumulation of the toxic metabolite psychosine. We showed previously that central nervous system (CNS)-directed, adeno-associated virus (AAV)2/5-mediated gene therapy synergized with bone marrow transplantation and substrate reduction therapy (SRT) to greatly increase therapeutic efficacy in the murine model of Krabbe disease (Twitcher). However, motor deficits remained largely refractory to treatment. In the current study, we replaced AAV2/5 with an AAV2/9 vector. This single change significantly improved several endpoints primarily associated with motor function. However, nearly all (14/16) of the combination-treated Twitcher mice and all (19/19) of the combination-treated wild-type mice developed hepatocellular carcinoma (HCC). 10 out of 10 tumors analyzed had AAV integrations within the Rian locus. Several animals had additional integrations within or near genes that regulate cell growth or death, are known or potential tumor suppressors, or are associated with poor prognosis in human HCC. Finally, the substrate reduction drug L-cycloserine significantly decreased the level of the pro-apoptotic ceramide 18:0. These data demonstrate the value of AAV-based combination therapy for Krabbe disease. However, they also suggest that other therapies or co-morbidities must be taken into account before AAV-mediated gene therapy is considered for human therapeutic trials

The electric double layer has a life of its own

Using molecular dynamics simulations with recently developed importance sampling methods, we show that the differential capacitance of a model ionic liquid based double-layer capacitor exhibits an anomalous dependence on the applied electrical potential. Such behavior is qualitatively incompatible with standard mean-field theories of the electrical double layer, but is consistent with observations made in experiment. The anomalous response results from structural changes induced in the interfacial region of the ionic liquid as it develops a charge density to screen the charge induced on the electrode surface. These structural changes are strongly influenced by the out-of-plane layering of the electrolyte and are multifaceted, including an abrupt local ordering of the ions adsorbed in the plane of the electrode surface, reorientation of molecular ions, and the spontaneous exchange of ions between different layers of the electrolyte close to the electrode surface. The local ordering exhibits signatures of a first-order phase transition, which would indicate a singular charge-density transition in a macroscopic limit

The Economic Resource Receipt of New Mothers

U.S. federal policies do not provide a universal social safety net of economic support for women during pregnancy or the immediate postpartum period but assume that employment and/or marriage will protect families from poverty. Yet even mothers with considerable human and marital capital may experience disruptions in employment, earnings, and family socioeconomic status postbirth. We use the National Survey of Families and Households to examine the economic resources that mothers with children ages 2 and younger receive postbirth, including employment, spouses, extended family and social network support, and public assistance. Results show that many new mothers receive resources postbirth. Marriage or postbirth employment does not protect new mothers and their families from poverty, but education, race, and the receipt of economic supports from social networks do

Herzfeld instability versus Mott transition in metal-ammonia solutions

Although most metal-insulator transitions in doped insulators are generally viewed as Mott transitions, some systems seem to deviate from this scenario. Alkali metal-ammonia solutions are a brilliant example of that. They reveal a phase separation in the range of metal concentrations where a metal-insulator transition occurs. Using a mean spherical approximation for quantum polarizable fluids, we argue that the origin of the metal-insulator transition in such a system is likely similar to that proposed by Herzfeld a long time ago, namely, due to fluctuations of solvated electrons. We also show how the phase separation may appear: the Herzfeld instability of the insulator occurs at a concentration for which the metallic phase is also unstable. As a consequence, the Mott transition cannot occur at low temperatures. The proposed scenario may provide a new insight into the metal-insulator transition in condensed-matter physics.Comment: 9 pages, 4 figure