3,206 research outputs found
On the IMF in a Triggered Star Formation Context
The origin of the stellar initial mass function (IMF) is a fundamental issue
in the theory of star formation. It is generally fit with a composite power
law. Some clues on the progenitors can be found in dense starless cores that
have a core mass function (CMF) with a similar shape. In the low-mass end,
these mass functions increase with mass, albeit the sample may be somewhat
incomplete; in the high-mass end, the mass functions decrease with mass. There
is an offset in the turn-over mass between the two mass distributions. The
stellar mass for the IMF peak is lower than the corresponding core mass for the
CMF peak in the Pipe Nebula by about a factor of three. Smaller offsets are
found between the IMF and the CMFs in other nebulae. We suggest that the offset
is likely induced during a starburst episode of global star formation which is
triggered by the formation of a few O/B stars in the multi-phase media, which
naturally emerged through the onset of thermal instability in the cloud-core
formation process. We consider the scenario that the ignition of a few massive
stars photoionizes the warm medium between the cores, increases the external
pressure, reduces their Bonnor?Ebert mass, and triggers the collapse of some
previously stable cores. We quantitatively reproduce the IMF in the low-mass
end with the assumption of additional rotational fragmentation.Comment: 3 figure
The SDSS Damped Lya Survey: Data Release 1
We present the results from an automated search for damped Lya (DLA) systems
in the quasar spectra of Data Release 1 from the Sloan Digital Sky Survey
(SDSS-DR1). At z~2.5, this homogeneous dataset has greater statistical
significance than the previous two decades of research. We derive a statistical
sample of 71 damped Lya systems (>50 previously unpublished) at z>2.1 and
measure HI column densities directly from the SDSS spectra. The number of DLA
systems per unit redshift is consistent with previous measurements and we
expect our survey has >95% completeness. We examine the cosmological baryonic
mass density of neutral gas Omega_g inferred from the damped Lya systems from
the SDSS-DR1 survey and a combined sample drawn from the literature. Contrary
to previous results, the Omega_g values do not require a significant correction
from Lyman limit systems at any redshift. We also find that the Omega_g values
for the SDSS-DR1 sample do not decline at high redshift and the combined sample
shows a (statistically insignificant) decrease only at z>4. Future data
releases from SDSS will provide the definitive survey of DLA systems at z~2.5
and will significantly reduce the uncertainty in Omega_g at higher redshift.Comment: 12 pages, includes color figures. Accepted to PASP, April 20 200
Embryonic Pattern Scaling Achieved by Oppositely Directed Morphogen Gradients
Morphogens are proteins, often produced in a localised region, whose
concentrations spatially demarcate regions of differing gene expression in
developing embryos. The boundaries of expression must be set accurately and in
proportion to the size of the one-dimensional developing field; this cannot be
accomplished by a single gradient. Here, we show how a pair of morphogens
produced at opposite ends of a developing field can solve the pattern-scaling
problem. In the most promising scenario, the morphogens effectively interact
according to the annihilation reaction and the switch occurs
according to the absolute concentration of or . In this case embryonic
markers across the entire developing field scale approximately with system
size; this cannot be achieved with a pair of non-interacting gradients that
combinatorially regulate downstream genes. This scaling occurs in a window of
developing-field sizes centred at a few times the morphogen decay length.Comment: 24 pages; 11 figures; uses iopar
Effects of Differential Jump Training on Balance Performance in Female Volleyball Players
The purpose of this study was to determine whether coordinative jump training that induces neuromuscular stimuli can affect balance performance, associated with injury risk, in elite-level female volleyball players. During the competitive season, the balance performance of 12 elite female players (highest Austrian division) was obtained via a wobble board (WB; 200 Hz) placed on an AMTI force plate (1000 Hz). Three identically repeated measurements defined two intervals (control and intervention phases), both comparable in duration and regular training. The intervention included 6 weeks of differential training (8 sessions of 15–20 min) that delivered variations in dynamics around the ankle joints. Multilevel mixed models were used to assess the effect on postural control. WB performance decreased from 27.0 ± 13.2% to 19.6 ± 11.3% during the control phase and increased to 54.5 ± 16.2% during the intervention (β = 49.1 ± 3.5; p < 0.001). Decreased sway area [cm²] (β = −7.5 ± 1.6; p < 0.001), anterior–posterior (β = −4.1 ± 0.4; p < 0.001) and mediolateral sway [mm] (β = −2.7 ± 0.6; p = 0.12), and mean velocity [mm∙s−1] (β = −9.0 ± 3.6; p < 0.05) were observed during the intervention compared with the control phase. Inter-limb asymmetry was reduced (β = −41.8 ± 14.4; p < 0.05). The applied training concept enhanced balance performance and postural control in elite female volleyball players. Due to the low additional physiological loads of the program and increased injury risk during the competitive season, we recommend this intervention for supporting injury prevention during this period
Evidence for Two Superconducting Gaps in
We have measured the Raman spectra of polycrystalline MgB from 25 {\cm}
to 1200 {\cm}. When the temperature was decreased below the superconducting
transition temperature , we observed a superconductivity-induced
redistribution in the electronic Raman continuum. Two pair-breaking peaks
appear in the spectra, suggesting the presence of two superconducting gaps.
Furthermore, we have analyzed the measured spectra using a quasi
two-dimensional model in which two s-wave superconducting gaps open on two
sheets of Fermi surface. For the gap values we have obtained (2.7 meV) and (6.2 meV). Our results suggest
that a conventional phonon-mediated pairing mechanism occurs in the planar
boron bands and is responsible for the superconductivity of MgB.Comment: 3 figure
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