Evolutionary Signatures In The Formation Of Low-Mass Protostars. II. Toward Reconciling Models And Observations

A long-standing problem in low-mass star formation is the "luminosity problem," whereby protostars are underluminous compared to the accretion luminosity expected both from theoretical collapse calculations and arguments based on the minimum accretion rate necessary to form a star within the embedded phase duration. Motivated by this luminosity problem, we present a set of evolutionary models describing the collapse of low-mass, dense cores into protostars. We use as our starting point the evolutionary model following the inside-out collapse of a singular isothermal sphere as presented by Young & Evans. We calculate the radiative transfer of the collapsing core throughout the full duration of the collapse in two dimensions. From the resulting spectral energy distributions, we calculate standard observational signatures (L(bol), T(bol), L(bol)/L(smm)) to directly compare to observations. We incorporate several modifications and additions to the original Young & Evans model in an effort to better match observations with model predictions; we include (1) the opacity from scattering in the radiative transfer, (2) a circumstellar disk directly in the two-dimensional radiative transfer, (3) a two-dimensional envelope structure, taking into account the effects of rotation, (4) mass-loss and the opening of outflow cavities, and (5) a simple treatment of episodic mass accretion. We find that scattering, two-dimensional geometry, mass-loss, and outflow cavities all affect the model predictions, as expected, but none resolve the luminosity problem. On the other hand, we find that a cycle of episodic mass accretion similar to that predicted by recent theoretical work can resolve this problem and bring the model predictions into better agreement with observations. Standard assumptions about the interplay between mass accretion and mass loss in our model give star formation efficiencies consistent with recent observations that compare the core mass function and stellar initial mass function. Finally, the combination of outflow cavities and episodic mass accretion reduces the connection between observational class and physical stage to the point where neither of the two commonly used observational signatures (T(bol) and L(bol)/L(smm)) can be considered reliable indicators of physical stage.NASA 1224608, 1288664, 1288658, RSA 1377304, NNX 07-AJ72GNSF AST0607793UT Austin University Continuing FellowshipAstronom

crisscrossing Science Episode 012: It\u27s Good to Be the Queen

In this episode, Chris Gaiser (professor of biology at Linfield College) and Mike Crosser (professor of physics at Linfield College) invite Chad Tillberg (associate professor of biology at Linfield College) into the studio to discuss eusocial animals: bees, ants, termites, and, of course, naked mole-rats. Each of these animal groups lives in a nest and has distinct classes of individuals, from workers to queens. Note: This episode discusses the mating rituals of various animals

A Wide-Field CCD Survey for Centaurs and Kuiper Belt Objects

A modified Baker-Nunn camera was used to conduct a wide-field survey of 1428 square degrees of sky near the ecliptic in search of bright Kuiper Belt objects and Centaurs. This area is an order of magnitude larger than any previously published CCD survey for Centaurs and Kuiper Belt Objects. No new objects brighter than red magnitude m=18.8 and moving at a rate 1"/hr to 20"/hr were discovered, although one previously discovered Centaur 1997 CU26 Chariklo was serendipitously detected. The parameters of the survey were characterized using both visual and automated techniques. From this survey the empirical projected surface density of Centaurs was found to be SigmaCentaur(m<18.8)=7.8(+16.0 -6.6)x10^-4 per square degree and we found a projected surface density 3sigma upper confidence limit for Kuiper Belt objects of SigmaKBO(m< 18.8)<4.1x10^-3 per square degree. We discuss the current state of the cumulative luminosity functions of both Centaurs and Kuiper Belt objects. Through a Monte Carlo simulation we show that the size distribution of Centaurs is consistent with a q=4 differential power law, similar to the size distribution of the parent Kuiper Belt Objects. The Centaur population is of order 10^7 (radius > 1 km) assuming a geometric albedo of 0.04. About 100 Centaurs are larger than 50 km in radius, of which only 4 are presently known. The current total mass of the Centaurs is 10^-4 Earth Masses. No dust clouds were detected resulting from Kuiper Belt object collisions, placing a 3sigma upper limit <600 collisionally produced clouds of m<18.8 per year.Comment: 13 pages, 5 figures, Accepted for Publication in A

Development and testing of a risk indexing framework to determine field-scale critical source areas of faecal bacteria on grassland.

This paper draws on lessons from a UK case study in the management of diffuse microbial pollution from grassland farm systems in the Taw catchment, south west England. We report on the development and preliminary testing of a field-scale faecal indicator organism risk indexing tool (FIORIT). This tool aims to prioritise those fields most vulnerable in terms of their risk of contributing FIOs to water. FIORIT risk indices were related to recorded microbial water quality parameters (faecal coliforms [FC] and intestinal enterococci [IE]) to provide a concurrent on-farm evaluation of the tool. There was a significant upward trend in Log[FC] and Log[IE] values with FIORIT risk score classification (r2 =0.87 and 0.70, respectively and P<0.01 for both FIOs). The FIORIT was then applied to 162 representative grassland fields through different seasons for ten farms in the case study catchment to determine the distribution of on-farm spatial and temporal risk. The high risk fields made up only a small proportion (1%, 2%, 2% and 3% for winter, spring, summer and autumn, respectively) of the total number of fields assessed (and less than 10% of the total area), but the likelihood of the hydrological connection of high FIO source areas to receiving watercourses makes them a priority for mitigation efforts. The FIORIT provides a preliminary and evolving mechanism through which we can combine risk assessment with risk communication to end-users and provides a framework for prioritising future empirical research. Continued testing of FIORIT across different geographical areas under both low and high flow conditions is now needed to initiate its long term development into a robust indexing tool

Optic radiation structure and anatomy in the normally developing brain determined using diffusion MRI and tractography

The optic radiation (OR) is a component of the visual system known to be myelin mature very early in life. Diffusion tensor imaging (DTI) and its unique ability to reconstruct the OR in vivo were used to study structural maturation through analysis of DTI metrics in a cohort of 90 children aged 5–18 years. As the OR is at risk of damage during epilepsy surgery, we measured its position relative to characteristic anatomical landmarks. Anatomical distances, DTI metrics and volume of the OR were investigated for age, gender and hemisphere effects. We observed changes in DTI metrics with age comparable to known trajectories in other white matter tracts. Left lateralization of DTI metrics was observed that showed a gender effect in lateralization. Sexual dimorphism of DTI metrics in the right hemisphere was also found. With respect to OR dimensions, volume was shown to be right lateralised and sexual dimorphism demonstrated for the extent of the left OR. The anatomical results presented for the OR have potentially important applications for neurosurgical planning

Effects of radiotransmitters on fecal glucocorticoid metabolite levels of three-toed box turtles in captivity

The increased use of radio-telemetry for studying movement, resource selection, and population demographics in reptiles necessitates closer examination of the assumption that radiotransmitter attachment does not bias study results. We determined the effects of radiotransmitter attachment on fecal glucocorticoid metabolite levels of wild three-toed box turtles (Terrapene carolina triunguis) in captivity. During May 2002 we captured 11 adult three-toed box turtles in central Missouri. We housed turtles in individual pens in a semi-natural outdoor setting. We radio-tagged 6 turtles, and the remaining 5 turtles served as controls. We captured and handled all turtles similarly during treatments. We collected feces daily prior to attachment (14 June–05 July 2002), while transmitters were attached (06 July–02 August 2002), and after transmitters were removed (03 August–24 August 2002). We conducted a standard assay validation and found that the assay accurately and precisely quantified fecal glucocorticoid metabolites of box turtles. We did not find a significant effect of radiotransmitter attachment on fecal glucocorticoid metabolite levels of three-toed box turtles (F1, 9 =0.404, P=0.541). Fecal glucocorticoid metabolite levels of control and treatment turtles increased significantly during the study (F2,166=7.874, P= 0.001), but there was no treatment:period interaction (F2,166 = 0.856, P = 0.427). Additionally, we did not find a significant relationship between glucocorticoid metabolite levels and time in captivity (r2=0.01, F1,179=2.89, P=0.091) or maximum daily temperature (r2\u3c0.01, F1,179=0.301, P=0.584). Our results suggested that radiotransmitter attachment did not significantly increase fecal glucocorticoid metabolite levels in adult three-toed box turtles; however, we conducted our study in captivity and sample sizes were small. Thus, more research is needed to assess potential effects of radiotransmitters on turtles in the wild. We believe this study is the first to validate the use of fecal glucocorticoid metabolite measures for reptiles, which might prove useful in other research studies

Comparison of geomechanical deformation induced by megatonne-scale CO2 storage at Sleipner, Weyburn, and In Salah

Geological storage of CO2 that has been captured at large, point source emitters represents a key potential method for reduction of anthropogenic greenhouse gas emissions. However, this technology will only be viable if it can be guaranteed that injected CO2 will remain trapped in the subsurface for thousands of years or more. A significant issue for storage security is the geomechanical response of the reservoir. Concerns have been raised that geomechanical deformation induced by CO2 injection will create or reactivate fracture networks in the sealing caprocks, providing a pathway for CO2 leakage. In this paper, we examine three large-scale sites where CO2 is injected at rates of ∼1 megatonne/y or more: Sleipner, Weyburn, and In Salah. We compare and contrast the observed geomechanical behavior of each site, with particular focus on the risks to storage security posed by geomechanical deformation. At Sleipner, the large, high-permeability storage aquifer has experienced little pore pressure increase over 15 y of injection, implying little possibility of geomechanical deformation. At Weyburn, 45 y of oil production has depleted pore pressures before increases associated with CO2 injection. The long history of the field has led to complicated, sometimes nonintuitive geomechanical deformation. At In Salah, injection into the water leg of a gas reservoir has increased pore pressures, leading to uplift and substantial microseismic activity. The differences in the geomechanical responses of these sites emphasize the need for systematic geomechanical appraisal before injection in any potential storage sit

Modeling deltaic lobe‐building cycles and channel avulsions for the Yellow River delta, China

River deltas grow by repeating cycles of lobe development punctuated by channel avulsions, so that over time, lobes amalgamate to produce a composite landform. Existing models have shown that backwater hydrodynamics are important in avulsion dynamics, but the effect of lobe progradation on avulsion frequency and location has yet to be explored. Herein, a quasi‐2‐D numerical model incorporating channel avulsion and lobe development cycles is developed. The model is validated by the well‐constrained case of a prograding lobe on the Yellow River delta, China. It is determined that with lobe progradation, avulsion frequency decreases, and avulsion length increases, relative to conditions where a delta lobe does not prograde. Lobe progradation lowers the channel bed gradient, which results in channel aggradation over the delta topset that is focused farther upstream, shifting the avulsion location upstream. Furthermore, the frequency and location of channel avulsions are sensitive to the threshold in channel bed superelevation that triggers an avulsion. For example, avulsions occur less frequently with a larger superelevation threshold, resulting in greater lobe progradation and avulsions that occur farther upstream. When the delta lobe length prior to avulsion is a moderate fraction of the backwater length (0.3–0.5L_b), the interplay between variable water discharge and lobe progradation together set the avulsion location, and a model capturing both processes is necessary to predict avulsion timing and location. While this study is validated by data from the Yellow River delta, the numerical framework is rooted in physical relationships and can therefore be extended to other deltaic systems