Filamentary Accretion Flows in the Embedded Serpens South Protocluster

One puzzle in understanding how stars form in clusters is the source of mass -- is all of the mass in place before the first stars are born, or is there an extended period when the cluster accretes material which can continuously fuel the star formation process? We use a multi-line spectral survey of the southern filament associated with the Serpens South embedded cluster-forming region in order to determine if mass is accreting from the filament onto the cluster, and whether the accretion rate is significant. Our analysis suggests that material is flowing along the filament's long axis at a rate of ~30Msol/Myr (inferred from the N2H+ velocity gradient along the filament), and radially contracting onto the filament at ~130Msol/Myr (inferred from HNC self-absorption). These accretion rates are sufficient to supply mass to the central cluster at a similar rate to the current star formation rate in the cluster. Filamentary accretion flows may therefore be very important in the ongoing evolution of this cluster.Comment: 19 pages, 8 figures, 2 tables; accepted for publication in Ap

The zero-dimensional O(N) vector model as a benchmark for perturbation theory, the large-N expansion and the functional renormalization group

We consider the zero-dimensional O(N) vector model as a simple example to calculate n-point correlation functions using perturbation theory, the large-N expansion, and the functional renormalization group (FRG). Comparing our findings with exact results, we show that perturbation theory breaks down for moderate interactions for all N, as one should expect. While the interaction-induced shift of the free energy and the self-energy are well described by the large-N expansion even for small N, this is not the case for higher-order correlation functions. However, using the FRG in its one-particle irreducible formalism, we see that very few running couplings suffice to get accurate results for arbitrary N in the strong coupling regime, outperforming the large-N expansion for small N. We further remark on how the derivative expansion, a well-known approximation strategy for the FRG, reduces to an exact method for the zero-dimensional O(N) vector model.Comment: 13 pages, 13 figure

Wheat growth responses to soil mechanical impedance are dependent on phosphorus supply

Increased mechanical impedance induced by soil drying or compaction causes reduction in plant growth and crop yield. However, how mechanical impedance interacts with nutrient stress has been largely unknown. Here, we investigated the effect of mechanical impedance on the growth of wheat seedlings under contrasting phosphorus (P) supply in a sand culture system which allows the mechanical impedance to be independent of water and nutrient availability. Two wheat genotypes containing the Rht-B1a (tall) or Rht-B1c (gibberellin-insensitive dwarf) alleles in the Cadenza background were used and their shoot and root traits were determined. Mechanical impedance caused a significant reduction in plant growth under sufficient P supply, including reduced shoot and root biomass, leaf area and total root length. By contrast, under low P supply, mechanical impedance did not affect biomass, tiller number, leaf length, and nodal root number in both wheat genotypes, indicating that the magnitude of the growth restriction imposed by mechanical impedance was dependent on P supply. The interaction effect between mechanical impedance and P level was significant on most plant traits except for axial and lateral root length, suggesting an evident physical and nutritional interaction. Our findings provide valuable insights into the integrated effects of plants in response to both soil physical and nutritional stresses. Understanding the response patterns is critical for optimizing soil tillage and nutrient management in the field

Resting-state anticorrelations between medial and lateral prefrontal cortex: Association with working memory, aging, and individual differences

We examined how variation in working memory (WM) capacity due to aging or individual differences among young adults is associated with intrinsic or resting-state anticorrelations, particularly between (1) the medial prefrontal cortex (MPFC), a component of the default-mode network (DMN) that typically decreases in activation during external, attention-demanding tasks, and (2) the dorsolateral prefrontal cortex (DLPFC), a component of the fronto-parietal control network that supports executive functions and WM and typically increases in activation during attention-demanding tasks. We compared the magnitudes of MPFC-DLPFC anticorrelations between healthy younger and older participants (Experiment 1) and related the magnitudes of these anticorrelations to individual differences on two behavioral measures of WM capacity in two independent groups of young adults (Experiments 1 and 2). Relative to younger adults, older adults exhibited reductions in WM capacity and in MPFC-DLPFC anticorrelations. Within younger adults, greater MPFC-DLPFC anticorrelation at rest correlated with greater WM capacity. These findings show that variation in MPFC-DLPFC anticorrelations, whether related to aging or to individual differences, may reflect an intrinsic functional brain architecture supportive of WM capacity.National Institutes of Health (U.S.) (National Institute on Aging Grant R21 AG030770)National Institutes of Health (U.S.) (Grant T32 GM007484)Barbara J. Weedon Fund Fellowshi

A CO (J=3-2) Outflow Survey of the Elias 29 Region

We present a 5'x5' integrated intensity map of 12CO (J=3-2) emission from the rho-Ophiuchi cloud core that traces low-luminosity outflow emission from two protostars: Elias 29 and, most likely, LFAM 26. The morphology of the outflow from Elias 29 is bipolar and has a curved axis that traces the S-shaped symmetry seen in H_2 emission. The outflow from LFAM 26 is a new detection and oriented in the east/west direction near the plane of the sky with most of the blue-shifted emission being absorbed by intervening clouds. The outflow axis of this object also appears to intersect a knot of H_2 emission previously attributed to Elias 29. LFAM 26 is a low luminosity source (L_bol = 0.06 L_sun) which, in combination with the observed outflow, makes it a candidate Very Low Luminosity Object (VeLLO). We derive lower limits to the gas column densities and energetics for both outflows. The mechanical luminosities for Elias 29 and LFAM 26 are 6.4 and 10.3 x 10^{-3} L_sun, respectively.Comment: 9 pages, 2 figures, accepted to the ApJ

Functional renormalization group approach to zero-dimensional interacting systems

We apply the functional renormalization group method to the calculation of dynamical properties of zero-dimensional interacting quantum systems. As case studies we discuss the anharmonic oscillator and the single impurity Anderson model. We truncate the hierarchy of flow equations such that the results are at least correct up to second order perturbation theory in the coupling. For the anharmonic oscillator energies and spectra obtained within two different functional renormalization group schemes are compared to numerically exact results, perturbation theory, and the mean field approximation. Even at large coupling the results obtained using the functional renormalization group agree quite well with the numerical exact solution. The better of the two schemes is used to calculate spectra of the single impurity Anderson model, which then are compared to the results of perturbation theory and the numerical renormalization group. For small to intermediate couplings the functional renormalization group gives results which are close to the ones obtained using the very accurate numerical renormalization group method. In particulare the low-energy scale (Kondo temperature) extracted from the functional renormalization group results shows the expected behavior.Comment: 22 pages, 8 figures include

Integrated heterodyne array receivers for submillimeter astronomy

The advent of large format (~100 pixel) spectroscopic imaging cameras at submillimeter wavelengths would fundamentally change the way in which astronomy is performed in this important wavelength regime. While the possibility of such instruments has been discussed for more than two decades, only recently have advances in mixer technology, device fabrication, micromachining, digital signal processing, and telescope design made the construction of such an instrument possible and economical. In our paper, we will present the design concept for a 10×10 heterodyne camera

Arecibo HI Absorption Measurements of Pulsars and the Electron Density at Intermediate Longitudes in the First Galactic Quadrant

We have used the Arecibo telescope to measure the HI absorption spectra of eight pulsars. We show how kinematic distance measurements depend upon the values of the galactic constants R_o and Theta_o, and we select our preferred current values from the literature. We then derive kinematic distances for the low-latitude pulsars in our sample and electron densities along their lines of sight. We combine these measurements with all others in the inner galactic plane visible from Arecibo to study the electron density in this region. The electron density in the interarm range 48 degrees < l < 70 degrees is [0.017 (-0.007,+0.012) (68% c.l.)] cm^(-3). This is 0.75 (-0.22,+0.49) (68% c.l.) of the value calculated by the Cordes & Lazio (2002) galactic electron density model. The model agrees more closely with electron density measurements toward Arecibo pulsars lying closer to the galactic center, at 30 degrees<l<48 degrees. Our analysis leads to the best current estimate of the distance of the relativistic binary pulsar B1913+16: d=(9.0 +/- 3) kpc. We use the high-latitude pulsars to search for small-scale structure in the interstellar hydrogen observed in absorption over multiple epochs. PSR B0301+19 exhibited significant changes in its absorption spectrum over 22 yr, indicating HI structure on a ~500 AU scale.Comment: Accepted by Astrophysical Journal September 200

A gentle introduction to the functional renormalization group: the Kondo effect in quantum dots

The functional renormalization group provides an efficient description of the interplay and competition of correlations on different energy scales in interacting Fermi systems. An exact hierarchy of flow equations yields the gradual evolution from a microscopic model Hamiltonian to the effective action as a function of a continuously decreasing energy cutoff. Practical implementations rely on suitable truncations of the hierarchy, which capture nonuniversal properties at higher energy scales in addition to the universal low-energy asymptotics. As a specific example we study transport properties through a single-level quantum dot coupled to Fermi liquid leads. In particular, we focus on the temperature T=0 gate voltage dependence of the linear conductance. A comparison with exact results shows that the functional renormalization group approach captures the broad resonance plateau as well as the emergence of the Kondo scale. It can be easily extended to more complex setups of quantum dots.Comment: contribution to Les Houches proceedings 2006, Springer styl

Deep roots and soil structure

In this opinion article we examine the relationship between penetrometer resistance and soil depth in the field. Assuming that root growth is inhibited at penetrometer resistances > 2.5 MPa, we conclude that in most circumstances the increases in penetrometer resistance with depth are sufficiently great to confine most deep roots to elongating in existing structural pores. We suggest that deep rooting is more likely related to the interaction between root architecture and soil structure than it is to the ability of a root to deform strong soil. Although the ability of roots to deform strong soil is an important trait, we propose it is more closely related to root exploration of surface layers than deep rooting