70 research outputs found
Growth of green wood based on a phase field model
Tree engineering is a young discipline utilizing trees as structural elements, where the determination of limit loads in tree trunks is of great importance. Simple numerical models underestimate the load-bearing capacity of green wood in contrast to experimental bending tests. A well-known reason for this is the residual stress state of the living tree lowering compressive stress towards the trunks surface. This results in an overall stress state, which increases the load capacity, since the tensile strength of wood is commonly higher than its compressive strength. By determining the residual growth stress, a more accurate evaluation of the load-bearing capacity of a living tree is possible. The residual stress state is a non-linear and time dependent function in thickness direction of the trunk. In order to simulate growth and growth stress, a phase field model is employed.
The morphology of a tree is the result of innumerable and often temporary environmental stimuli, which also change and interact with the genetically predisposed growth tropisms. Therefore, we use image processing to capture the individual tree morphology of an existing tree, which is based within the phase field model as predefined growth direction. This is the basis for primary growth in the model. Additionally the model simulates the secondary growth, which corresponds to the thickness of the trunk. Except in tropical areas, this growth is associated with growth rings, which we assign as an attribute to the modelled material. While in the branch structure several tropisms (e.g. gravitropism) are responsible for the off-centre accumulation of woody material, in the stem region we only follow the stress-induced growth. This mechanism can respond to either the principal tensile stress or the principal compressive stress in our model, as this difference is observed in hardwoods and softwoods.
Since the wood matrix represents an anisotropic material with a distinct fiber direction, we approach it in our model by a transversely isotropic constitutive law, whose principal direction coincides with the growth direction
Nothing to hide: An X-ray survey for young stellar objects in the Pipe Nebula
We have previously analyzed sensitive mid-infrared observations to establish
that the Pipe Nebula has a very low star-formation efficiency. That study
focused on YSOs with excess infrared emission (i.e, protostars and pre-main
sequence stars with disks), however, and could have missed a population of more
evolved pre-main sequence stars or Class III objects (i.e., young stars with
dissipated disks that no longer show excess infrared emission). Evolved
pre-main sequence stars are X-ray bright, so we have used ROSAT All-Sky Survey
data to search for diskless pre-main sequence stars throughout the Pipe Nebula.
We have also analyzed archival XMM-Newton observations of three prominent areas
within the Pipe: Barnard 59, containing a known cluster of young stellar
objects; Barnard 68, a dense core that has yet to form stars; and the Pipe
molecular ring, a high-extinction region in the bowl of the Pipe. We
additionally characterize the X-ray properties of YSOs in Barnard 59. The ROSAT
and XMM-Newton data provide no indication of a significant population of more
evolved pre-main sequence stars within the Pipe, reinforcing our previous
measurement of the Pipe's very low star formation efficiency.Comment: Accepted for publication in Ap
A Spitzer Census of Star Formation Activity in the Pipe Nebula
The Pipe Nebula, a large nearby molecular cloud lacks obvious signposts of
star formation in all but one of more than 130 dust extinction cores that have
been identified within it. In order to quantitatively determine the current
level of star formation activity in the Pipe Nebula, we analyzed 13 square
degrees of sensitive mid-infrared maps of the entire cloud, obtained with the
Multiband Imaging Photometer for Spitzer (MIPS) at wavelengths of 24 micron and
70 micron to search for candidate Young Stellar Objects (YSOs) in the
high-extinction regions. We argue that our search is complete for class I and
typical class II YSOs with luminosities of Lbol~0.2 Lo and greater. We find
only 18 candidate YSOs in the high-extinction regions of the entire Pipe cloud.
Twelve of these sources are previously known members of a small cluster
associated with Barnard 59, the largest and most massive dense core in the
cloud. With only six candidate class I and class II YSOs detected towards
extinction cores outside of this cluster, our findings emphatically confirm the
notion of an extremely low level of star formation activity in the Pipe Nebula.
The resulting star formation efficiency for the entire cloud mass is only ~0.06
%.Comment: 23 pages, 8 figures, accepted for publication in Ap
Disentangling protostellar evolutionary stages in clustered environments using Spitzer-IRS spectra and comprehensive SED modeling
When studying the evolutionary stages of protostars that form in clusters,
the role of any intracluster medium cannot be neglected. High foreground
extinction can lead to situations where young stellar objects (YSOs) appear to
be in earlier evolutionary stages than they actually are, particularly when
using simple criteria like spectral indices. To address this issue, we have
assembled detailed SED characterizations of a sample of 56 Spitzer-identified
candidate YSOs in the clusters NGC 2264 and IC 348. For these, we use spectra
obtained with the Infrared Spectrograph onboard the Spitzer Space Telescope and
ancillary multi-wavelength photometry. The primary aim is twofold: 1) to
discuss the role of spectral features, particularly those due to ices and
silicates, in determining a YSO's evolutionary stage, and 2) to perform
comprehensive modeling of spectral energy distributions (SEDs) enhanced by the
IRS data. The SEDs consist of ancillary optical-to-submillimeter
multi-wavelength data as well as an accurate description of the 9.7 micron
silicate feature and of the mid-infrared continuum derived from line-free parts
of the IRS spectra. We find that using this approach, we can distinguish
genuine protostars in the cluster from T Tauri stars masquerading as protostars
due to external foreground extinction. Our results underline the importance of
photometric data in the far-infrared/submillimeter wavelength range, at
sufficiently high angular resolution to more accurately classify cluster
members. Such observations are becoming possible now with the advent of the
Herschel Space Observatory.Comment: Accepted for publication in Ap
Story
This book was completed for Jan Baker\u27s artists\u27 book class.https://digitalcommons.risd.edu/specialcollections_bookmark_stories/1019/thumbnail.jp
"Jane sent me this article, so it must be true!" - How tie strength and emotional tone influence information behavior
Fake news are a threat of the information age, yet many factors that determine their spread, such as emotional tone and tie strength, remain under-researched. Responding to calls for research, we developed an experimental study that explains the impact of emotional tone and tie strength in the context of instant messaging. We hypothesize effects on the willingness to fact-check and intention to share, mediated by sender credibility and news believability. Our results will contribute to the academic literature on various levels: we consider the emotional and relational dimensions of fake news sharing. Furthermore, we provide a multidimensional understanding of the emotionality of strong-tie contacts sharing fake news in a seemingly private and safe environment. For policymakers, we provide insights that help detect fake news, and we provide individuals with persuasion knowledge to self-protect against fake news
Simultaneous X-ray and radio observations of Young Stellar Objects in NGC 1333 and IC 348
Young Stellar Objects (YSOs) and in particular protostars are known to show a
variety of high-energy processes. Observations in the X-ray and centimetric
radio wavelength ranges are thought to constrain some of these processes, e.g.,
coronal-type magnetic activity. There is a well-known empirical correlation of
radio and X-ray luminosities in active stars, the so-called Guedel-Benz
relation. Previous evidence whether YSOs are compatible with this relation
remains inconclusive for the earliest evolutionary stages. The main difficulty
is that due to the extreme variability of these sources, simultaneous
observations are essential. Until now, only few YSOs and only a handful of
protostars have been observed simultaneously in the X-ray and radio range. To
expand the sample, we have obtained such observations of two young clusters
rich in protostars, NGC 1333 and IC 348. While the absolute sensitivity is
lower for these regions than for more nearby clusters like CrA, we find that
even in deep continuum observations carried out with the NRAO Very Large Array,
the radio detection fraction for protostars in these clusters is much lower
than the X-ray detection fraction. Very few YSOs are detected in both bands,
and we find the radio and X-ray populations among YSOs to be largely distinct.
We combine these new results with previous simultaneous Chandra and VLA
observations of star-forming regions and find that YSOs with detections in both
bands appear to be offset toward higher radio luminosities for given X-ray
luminosities when compared to the Guedel-Benz relation, although even in this
sensitive dataset most sources are too weak for the radio detections to provide
information on the emission processes. The considerably improved sensitivity of
the Expanded Very Large Array will provide a better census of the YSO radio
population as well as better constraints on the emission mechanisms.Comment: Accepted for publication in Ap
New M dwarf debris disk candidates in NGC 2547
With only six known examples, M-dwarf debris disks are rare, even though M
dwarfs constitute the majority of stars in the Galaxy. After finding a new M
dwarf debris disk in a shallow mid-infrared observation of NGC 2547, we present
a considerably deeper Spitzer-MIPS image of the region, with a maximum exposure
time of 15 minutes per pixel. Among sources selected from a previously
published membership list, we identify nine new M dwarfs with excess emission
at 24 micron tracing warm material close to the snow line of these stars, at
orbital radii of less than 1 AU. We argue that these are likely debris disks,
suggesting that planet formation is under way in these systems. Interestingly,
the estimated excess fraction of M stars appears to be higher than that of G
and K stars in our sample.Comment: 16 pages, 8 figures; accepted for publication in Ap
Development of a high-throughput colorimetric Zika virus infection assay
Zika virus (ZIKV) is an emerging pathogen that causes congenital infections which may result in birth defects, such as microcephaly. Currently, no approved treatment or vaccination is available. ZIKV can be readily detected in cell culture where virally infected cells are normally stained by specific antibodies. As ZIKV regularly causes a cytopathic effect, we were wondering whether this viral property can be used to quantitatively determine viral infectivity. We here describe the use of an 3-[4,5-dimethyl-2-thiazolyl]-2,5-diphenyl-2H-tetrazolium bromide-(MTT)-based cell viability assay that allows to determine ZIKV-induced cell death. We show that this colorimetric assay quantifies ZIKV infection over a broad range of viral dilutions in both monkey and human cells. It allows to determine inhibitory activities of antivirals that block ZIKV or to define the neutralizing antibody titers of ZIKV antisera. This MTT-based ZIKV detection assay can be evaluated by naked eye or computational tools, has a broad linear range, does not require large equipment or costly reagents, and thus represents a promising alternative to antibody-based assays, in particular in resource-poor settings. We propose to use this simple, fast, and cheap method for quantification of ZIKV neutralizing antibodies and testing of antiviral compounds.Peer reviewe
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