163 research outputs found
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
Yield conditions for deformation of amorphous polymer glasses
Shear yielding of glassy polymers is usually described in terms of the
pressure-dependent Tresca or von Mises yield criteria. We test these criteria
against molecular dynamics simulations of deformation in amorphous polymer
glasses under triaxial loading conditions that are difficult to realize in
experiments. Difficulties and ambiguities in extending several standard
definitions of the yield point to triaxial loads are described. Two
definitions, the maximum and offset octahedral stresses, are then used to
evaluate the yield stress for a wide range of model parameters. In all cases,
the onset of shear is consistent with the pressure-modified von Mises
criterion, and the pressure coefficient is nearly independent of many
parameters. Under triaxial tensile loading, the mode of failure changes to
cavitation.Comment: 9 pages, 8 figures, revte
MAMBO Mapping of Spitzer c2d Small Clouds and Cores
AIMS: To study the structure of nearby (< 500 pc) dense starless and
star-forming cores with the particular goal to identify and understand
evolutionary trends in core properties, and to explore the nature of Very Low
Luminosity Objects (< 0.1 L_sun; VeLLOs). METHODS: Using the MAMBO bolometer
array, we create maps unusually sensitive to faint (few mJy per beam) extended
(approx. 5 arcmin) thermal dust continuum emission at 1.2 mm wavelength.
Complementary information on embedded stars is obtained from Spitzer, IRAS, and
2MASS. RESULTS: Our maps are very rich in structure, and we characterize
extended emission features (``subcores'') and compact intensity peaks in our
data separately to pay attention to this complexity. We derive, e.g., sizes,
masses, and aspect ratios for the subcores, as well as column densities and
related properties for the peaks. Combination with archival infrared data then
enables the derivation of bolometric luminosities and temperatures, as well as
envelope masses, for the young embedded stars. CONCLUSIONS: (abridged) Starless
and star-forming cores occupy the same parameter space in many core properties;
a picture of dense core evolution in which any dense core begins to actively
form stars once it exceeds some fixed limit in, e.g., mass, density, or both,
is inconsistent with our data. Comparison of various evolutionary indicators
for young stellar objects in our sample (e.g., bolometric temperatures) reveals
inconsistencies between some of them, possibly suggesting a revision of some of
these indicators.Comment: Accepted to A&A. In total 46 pages, with 20 pages of tables, figures,
and appendices. High-resolution version of this article at
https://www.xythosondemand.com/home/harvard_iic/Users/jkauffma/Public/mambo_spitzer.pd
Metallicity distribution of red giants in the Inner Galaxy from Near Infrared spectra
In this paper, we present low resolution (R=500) near-infrared spectra for
selected and serendipitous sources in six inner in-plane Galactic fields, with
the aim of analysing the stellar content present. From the equivalent widths of
the main features of the K band spectra (the NaI, CaI and CO bandheads) we have
derived the metallicities of the sources by means of the empirical scale
obtained by Ram\'irez et al. (2000) and Frogel et al. (2001) for luminous red
giants.
Our results show how the mean metallicity of the sample varies with Galactic
longitude. We find two groups of stars, one whose [Fe/H] is similar to the
values obtained for the bulge in other studies (Molla et al. 2000; Schultehis
et al 2003), and a second one with a metallicity similar to that of the inner
parts of the disc (Rocha-Pinto et al. 2006). The relative density of both
groups of stars in our sample varies in a continuous way from the bulge to the
disc. This could hint at the existence of a single component apart from the
disc and bulge, running from l=7 to l=27 and able to transport disc stars
inwards and bulge stars outwards, which could be the Galactic bar that has been
detected in previous works as an overdensity of stars located at those galactic
coordinates (Hammersley et al. 1994, 2000; Picaud et al. 2003).Comment: 20 pages, 23 figures. Accepted for publication in A&
Neural Mechanisms of Interference Control in Working Memory: Effects of Interference Expectancy and Fluid Intelligence
A critical aspect of executive control is the ability to limit the adverse effects of interference. Previous studies have shown activation of left ventrolateral prefrontal cortex after the onset of interference, suggesting that interference may be resolved in a reactive manner. However, we suggest that interference control may also operate in a proactive manner to prevent effects of interference. The current study investigated the temporal dynamics of interference control by varying two factors - interference expectancy and fluid intelligence (gF) - that could influence whether interference control operates proactively versus reactively.A modified version of the recent negatives task was utilized. Interference expectancy was manipulated across task blocks by changing the proportion of recent negative (interference) trials versus recent positive (facilitation) trials. Furthermore, we explored whether gF affected the tendency to utilize specific interference control mechanisms. When interference expectancy was low, activity in lateral prefrontal cortex replicated prior results showing a reactive control pattern (i.e., interference-sensitivity during probe period). In contrast, when interference expectancy was high, bilateral prefrontal cortex activation was more indicative of proactive control mechanisms (interference-related effects prior to the probe period). Additional results suggested that the proactive control pattern was more evident in high gF individuals, whereas the reactive control pattern was more evident in low gF individuals.The results suggest the presence of two neural mechanisms of interference control, with the differential expression of these mechanisms modulated by both experimental (e.g., expectancy effects) and individual difference (e.g., gF) factors
A Spitzer Survey of Young Stellar Clusters within One Kiloparsec of the Sun: Cluster Core Extraction and Basic Structural Analysis
We present a uniform mid-infrared imaging and photometric survey of 36 young,
nearby, star-forming clusters and groups using {\it Spitzer} IRAC and MIPS. We
have confidently identified and classified 2548 young stellar objects using
recently established mid-infrared color-based methods. We have devised and
applied a new algorithm for the isolation of local surface density enhancements
from point source distributions, enabling us to extract the overdense cores of
the observed star forming regions for further analysis. We have compiled
several basic structural measurements of these cluster cores from the data,
such as mean surface densities of sources, cluster core radii, and aspect
ratios, in order to characterize the ranges for these quantities. We find that
a typical cluster core is 0.39 pc in radius, has 26 members with infrared
excess in a ratio of Class II to Class I sources of 3.7, is embedded in a
=0.8 mag cloud clump, and has a surface density of 60 pc. We
examine the nearest neighbor distances among the YSOs in several ways,
demonstrating similarity in the spacings between Class II and Class I sources
but large member clusters appear more dense than smaller clusters. We
demonstrate that near-uniform source spacings in cluster cores are common,
suggesting that simple Jeans fragmentation of parsec-scale cloud clumps may be
the dominant process governing star formation in nearby clusters and groups.
Finally, we compare our results to other similar surveys in the literature and
discuss potential biases in the data to guide further interpretation.Comment: 207 pages, 54 figures, 44 tables. Accepted to ApJ Supplements. Full
resolution version at
http://www.cfa.harvard.edu/~rgutermuth/preprints/gutermuth_clusters_survey.pd
<i>Herschel</i>-HOBYS study of the earliest phases of high-mass star formation in NGC 6357
Aims: To constrain models of high-mass star formation it is important to identify the massive dense cores (MDCs) that are able to form high-mass star(s). This is one of the purposes of the Herschel/HOBYS key programme. Here, we carry out the census and characterise of the properties of the MDCs population of the NGC 6357 H II region.
Methods: Our study is based on the Herschel/PACS and SPIRE 70−500 μm images of NGC 6357 complemented with (sub-)millimetre and mid-infrared data. We followed the procedure established by the Herschel/HOBYS consortium to extract ~0.1 pc massive dense cores using the getsources software. We estimated their physical parameters (temperatures, masses, luminosities) from spectral energy distribution (SED) fitting.
Results: We obtain a complete census of 23 massive dense cores, amongst which one is found to be IR-quiet and twelve are starless, representing very early stages of the star-formation process. Focussing on the starless MDCs, we have considered their evolutionary status, and suggest that only five of them are likely to form a high-mass star.
Conclusions: We find that, contrarily to the case in NGC 6334, the NGC 6357 region does not exhibit any ridge or hub features that are believed to be crucial to the massive star formation process. This study adds support for an empirical model in which massive dense cores and protostars simultaneously accrete mass from the surrounding filaments. In addition, the massive star formation in NGC 6357 seems to have stopped and the hottest stars in Pismis 24 have disrupted the filaments
The G305 star-forming complex: Embedded Massive Star Formation Discovered by Herschel Hi-GAL
We present a Herschel far-infrared study towards the rich massive star-
forming complex G305, utilising PACS 70, 160 {\mu}m and SPIRE 250, 350, and 500
{\mu}m observations from the Hi-GAL survey of the Galactic plane. The focus of
this study is to identify the embedded massive star-forming population within
G305, by combining far-infrared data with radio continuum, H2O maser, methanol
maser, MIPS, and Red MSX Source survey data available from previous studies. By
applying a frequentist technique we are able to identify a sample of the most
likely associations within our multi-wavelength dataset, that can then be
identified from the derived properties obtained from fitted spectral energy
distributions (SEDs). By SED modelling using both a simple modified blackbody
and fitting to a comprehensive grid of model SEDs, some 16 candidate
associations are identified as embedded massive star-forming regions. We derive
a two-selection colour criterion from this sample of log(F70/F500)\geq 1 and
log(F160/F350)\geq 1.6 to identify an additional 31 embedded massive star
candidates with no associated star-formation tracers. Using this result we can
build a picture of the present day star-formation of the complex, and by
extrapolating an initial mass function, suggest a current population of \approx
2 \times 10^4 young stellar objects (YSOs) present, corresponding to a star
formation rate (SFR) of 0.01-0.02 M\odot yr^-1. Comparing this resolved star
formation rate, to extragalactic star formation rate tracers (based on the
Kennicutt-Schmidt relation), we find the star formation activity is
underestimated by a factor of \geq 2 in comparison to the SFR derived from the
YSO population.Comment: Accepted by MNRAS, 16 pages, 8 figures, 3 table
Rapid Evolution of Enormous, Multichromosomal Genomes in Flowering Plant Mitochondria with Exceptionally High Mutation Rates
A pair of species within the genus Silene have evolved the largest known mitochondrial genomes, coinciding with extreme changes in mutation rate, recombination activity, and genome structure
Insulin-regulated Release from the Endosomal Recycling Compartment Is Regulated by Budding of Specialized Vesicles
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