965 research outputs found
Lifetime of the embedded phase of low-mass star formation and the envelope depletion rates
Motivated by a considerable scatter in the observationally inferred lifetimes
of the embedded phase of star formation, we study the duration of the Class 0
and Class I phases in upper-mass brown dwarfs and low-mass stars using
numerical hydrodynamics simulations of the gravitational collapse of a large
sample of cloud cores. We resolve the formation of a star/disk/envelope system
and extend our numerical simulations to the late accretion phase when the
envelope is nearly totally depleted of matter. We adopted a classification
scheme of Andre et al. and calculate the lifetimes of the Class 0 and Class I
phases (\tau_C0 and \tau_CI, respectively) based on the mass remaining in the
envelope. When cloud cores with various rotation rates, masses, and sizes (but
identical otherwise) are considered, our modeling reveals a sub-linear
correlation between the Class 0 lifetimes and stellar masses in the Class 0
phase with the least-squares fit exponent m=0.8 \pm 0.05. The corresponding
correlation between the Class I lifetimes and stellar masses in the Class I is
super-linear with m=1.2 \pm 0.05. If a wider sample of cloud cores is
considered, which includes possible variations in the initial gas temperature,
cloud core truncation radii, density enhancement amplitudes, initial gas
density and angular velocity profiles, and magnetic fields, then the
corresponding exponents may decrease by as much as 0.3. The duration of the
Class I phase is found to be longer than that of the Class~0 phase in most
models, with a mean ratio \tau_CI / \tau_C0 \approx 1.5--2. A notable exception
are YSOs that form from cloud cores with large initial density enhancements, in
which case \tau_C0 may be greater than \tau_CI. Moreover, the upper-mass (>=
1.0 Msun) cloud cores with frozen-in magnetic fields and high cloud core
rotation rates may have the \tau_CI / \tau_C0 ratios as large as 3.0--4.0.
(Abdridged).Comment: Accepted for publication by The Astrophysical Journa
Properties of the Youngest Protostars in Perseus, Serpens, and Ophiuchus
We present an unbiased census of deeply embedded protostars in Perseus,
Serpens, and Ophiuchus, assembled by combining large-scale 1.1 mm Bolocam
continuum and Spitzer Legacy surveys. We identify protostellar candidates based
on their mid-infrared properties, correlate their positions with 1.1 mm core
positions, and construct well-sampled SEDs using our extensive wavelength
coverage (lam=1.25-1100 micron). Source classification based on the bolometric
temperature yields a total of 39 Class 0 and 89 Class I sources in the three
cloud sample. We compare to protostellar evolutionary models using the
bolometric temperature-luminosity diagram, finding a population of low
luminosity Class I sources that are inconsistent with constant or monotonically
decreasing mass accretion rates. This result argues strongly for episodic
accretion during the Class I phase, with more than 50% of sources in a
``sub-Shu'' (dM/dt < 1e-6 Msun/yr) accretion state. Average spectra are
compared to protostellar radiative transfer models, which match the observed
spectra fairly well in Stage 0, but predict too much near-IR and too little
mid-IR flux in Stage I. Finally, the relative number of Class 0 and Class I
sources are used to estimate the lifetime of the Class 0 phase; the three cloud
average yields a Class 0 lifetime of 1.7e5 yr, ruling out an extremely rapid
early accretion phase. Correcting photometry for extinction results in a
somewhat shorter lifetime (1.1e5 yr). In Ophiuchus, however, we find very few
Class 0 sources (N(Class0)/N(ClassI)=0.1-0.2), similar to previous studies of
that cloud. The observations suggest a consistent picture of nearly constant
average accretion rate through the entire embedded phase, with accretion
becoming episodic by at least the Class I stage, and possibly earlier.Comment: 31 pages, 19 figures, 8 tables; accepted for publication in Ap
El 'Proyecto educativo de centro' en el ámbito universitario. Reflexiones y propuestas a partir de la experiencia de la Facultad de Derecho de la Universidad de Barcelona (2005-2016)
Este estudio valora la oportunidad y la viabilidad del Proyecto Educativo de "centro", o de Facultad, en un contexto universitario. Para ello se define qué es un proyecto educativo de centro y se identifica el origen del concepto en la literatura y en nuestro sistema educativo así como su aceptación en la universidad española. En segundo lugar se enumeran los elementos que, eventualmente, pueden configurar un proyecto educativo de centro universitario, y se hace a partir de la experiencia académico-docente de la Facultad de Derecho de la Universidad de Barcelona; a partir de este caso, se relacionan los elementos que han integrado el núcleo del proyecto educativo de centro y los elementos complementarios; además se proponen algunas líneas por donde podría discurrir la evolución de este proyecto educativo de centro. La presente aportación, en definitiva, aspira a fomentar un debate, sobre todo entre las facultades de Derecho, y por extensión también entre el resto de Facultades de las universidades públicas españolas, acerca de la conveniencia de disponer y de publicar un Proyecto Educativo de Facultad explícito que marque y oriente nuestra acción docente, que sea un eficaz instrumento de planificación educativa, que fije el marco educativo de los estudiantes, y que se convierta en elemento de identidad institucional
El programa institucional de mejora de la calidad docente de la Facultat de Derecho de la Universidad de Barcelona (2004-2012)
Esta comunicación muestra las líneas básicas del programa institucional de mejora de la calidad docente de la Facultad de Derecho de la Universidad de Barcelona. Si en CIDUI 2011 analizábamos los factores causales y determinantes del rendimiento académico en la universidad española actual y se proponían, con cierto carácter general, acciones de mejora vinculadas a cada categoría causal, en esta ocasión se describe una experiencia concreta y real: se presentan las 15 principales acciones o engranajes que actúan de forma simultánea y en ámbitos o direcciones diferentes para conseguir el doble objetivo de mejorar la calidad docente e incrementar el rendimiento académico en un marco de exigencia y rigor
Bolocam Survey for 1.1 mm Dust Continuum Emission in the c2d Legacy Clouds. II. Ophiuchus
We present a large-scale millimeter continuum map of the Ophiuchus molecular
cloud. Nearly 11 square degrees, including all of the area in the cloud with
visual extinction more than 3 magnitudes, was mapped at 1.1 mm with Bolocam on
the Caltech Submillimeter Observatory (CSO). By design, the map also covers the
region mapped in the infrared with the Spitzer Space Telescope. We detect 44
definite sources, and a few likely sources are also seen along a filament in
the eastern streamer. The map indicates that dense cores in Ophiuchus are very
clustered and often found in filaments within the cloud. Most sources are
round, as measured at the half power point, but elongated when measured at
lower contour levels, suggesting spherical sources lying within filaments. The
masses, for an assumed dust temperature of 10 K, range from 0.24 to 3.9 solar
masses, with a mean value of 0.96 solar masses. The total mass in distinct
cores is 42 solar masses, 0.5 to 2% of the total cloud mass, and the total mass
above 4 sigma is about 80 solar masses. The mean densities in the cores are
quite high, with an average of 1.6 x 10^6 per cc, suggesting short free-fall
times. The core mass distribution can be fitted with a power law with slope of
2.1 plus or minus 0.3 for M>0.5 solar masses, similar to that found in other
regions, but slightly shallower than that of some determinations of the local
IMF. In agreement with previous studies, our survey shows that dense cores
account for a very small fraction of the cloud volume and total mass. They are
nearly all confined to regions with visual extinction at least 9 mag, a lower
threshold than found previously.Comment: 47 pages, 16 figures, accepted for Ap
Proximate, minerals and antinutrient assessment of the leaves and tenderstem of Leptadenia hastate (pers.) decne
The study evaluated the proximate, minerals and antinutrient contents of the plant Leptadenia hastata (Pers.) decne (Asclepiadaceae). Nutrient and antinutritional factors were determined using standard analytical methods and parameters subjected to statistical analysis. Manganese (Mn), Calcium (Ca) and Cobalt (Co) were found to be significantly (p<0.001) higher in the leaves when compared to the tender stem while Iron (Fe), Zinc (Zn) and Magnesium (Mg) were found to be significantly (p<0.001) higher in the tender stems when compared to the leaves. The percentages of Moisture content, Crude proteins and Total ash were found to be significantly (p<0.05) higher in the leaves when compared to the tender stem while crude fiber and Nitrogen Free Extract (NFE) was significantly (p<0.05) higher in the stem when compared to the leaves. Oxalate and phytic acid were anti nutrient found to be significantly (p<0.001) higher in the leaves when compared to the tender stem. However, Copper (Cu), Lead (Pb), Potassium (K) and Crude fat were not significantly (p>0.05) different when compared in the leaves and tender stem. Correlation analysis indicated that antinutrient contents in the tender stems will not affect the mineral absorption in the leaves while negative correlation between Fe and Cu in the stems could affect the absorption of iron. Therefore, Leptadenia hastata leave and tender stem consumed in soup and local salad in northern Nigeria proved to be rich in minerals and nutrients to support its dietary role as a consumed vegetable.Keywords: Leptadenia hastata, Minerals, Proximate, Antinutrients, Tender stems, Leave
Episodic accretion at early stages of evolution of low mass stars and brown dwarfs: a solution for the observed luminosity spread in HR diagrams?
We present evolutionary models for young low mass stars and brown dwarfs
taking into account episodic phases of accretion at early stages of the
evolution, a scenario supported by recent large surveys of embedded protostars.
An evolution including short episodes of vigorous accretion (\mdot \ge 10^{-4}
\msolyr) followed by longer quiescent phases (\mdot < 10^{-6} \msolyr) can
explain the observed luminosity spread in HR diagrams of star forming regions
at ages of a few Myr, for objects ranging from a few Jupiter masses to a few
tenths of a solar mass. The gravitational contraction of these accreting
objects strongly departs from the standard Hayashi track at constant \te. The
best agreement with the observed luminosity scatter is obtained if most of the
accretion shock energy is radiated away. The obtained luminosity spread at 1
Myr in the HR diagram is equivalent to what can be misinterpreted as a
10 Myr age spread for non-accreting objects. We also predict a significant
spread in radius at a given \te, as suggested by recent observations. These
calculations bear important consequences on our understanding of star formation
and early stages of evolution and on the determination of the IMF for young
( a few Myr) clusters. Our results also show that the concept of a stellar
birthline for low-mass objects has no valid support.Comment: 8 pages, 5 figures, accepted for publication in APJ Lette
Comparing Star Formation on Large Scales in the c2d Legacy Clouds: Bolocam 1.1 mm Dust Continuum Surveys of Serpens, Perseus, and Ophiuchus
We have undertaken an unprecedentedly large 1.1 millimeter continuum survey
of three nearby star forming clouds using Bolocam at the Caltech Submillimeter
Observatory. We mapped the largest areas in each cloud at millimeter or
submillimeter wavelengths to date: 7.5 sq. deg in Perseus (Paper I), 10.8 sq.
deg in Ophiuchus (Paper II), and 1.5 sq. deg in Serpens with a resolution of
31", detecting 122, 44, and 35 cores, respectively. Here we report on results
of the Serpens survey and compare the three clouds. Average measured angular
core sizes and their dependence on resolution suggest that many of the observed
sources are consistent with power-law density profiles. Tests of the effects of
cloud distance reveal that linear resolution strongly affects measured source
sizes and densities, but not the shape of the mass distribution. Core mass
distribution slopes in Perseus and Ophiuchus (alpha=2.1+/-0.1 and
alpha=2.1+/-0.3) are consistent with recent measurements of the stellar IMF,
whereas the Serpens distribution is flatter (alpha=1.6+/-0.2). We also compare
the relative mass distribution shapes to predictions from turbulent
fragmentation simulations. Dense cores constitute less than 10% of the total
cloud mass in all three clouds, consistent with other measurements of low
star-formation efficiencies. Furthermore, most cores are found at high column
densities; more than 75% of 1.1 mm cores are associated with Av>8 mag in
Perseus, 15 mag in Serpens, and 20-23 mag in Ophiuchus.Comment: 32 pages, including 18 figures, accepted for publication in Ap
A complete 1.1mm survey of Perseus with Bolocam
We have completed a 7.5 square degree λ=1.1mm map of Perseus
using Bolocam. Our map is the largest unbiased survey of Perseus at millimeter wavelengths to date, and covers the same area as the c2d Spitzer IRAC and MIPS maps of Perseus. We find that that the mass function shape is similar to that seen in other clouds and to the local IMF. Despite the large area surveyed, few new sources are found outside the known cluster regions
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