2,848 research outputs found
An ALMA Constraint on the GSC 6214-210 B Circum-Substellar Accretion Disk Mass
We present Atacama Large Millimeter/submillimeter Array (ALMA) observations
of GSC 6214-210 A and B, a solar-mass member of the 5-10 Myr Upper Scorpius
association with a 15 2 Mjup companion orbiting at 330 AU
(2.2"). Previous photometry and spectroscopy spanning 0.3-5 m revealed
optical and thermal excess as well as strong H and Pa~ emission
originating from a circum-substellar accretion disk around GSC 6214-210 B,
making it the lowest mass companion with unambiguous evidence of a subdisk.
Despite ALMA's unprecedented sensitivity and angular resolution, neither
component was detected in our 880 m (341 GHz) continuum observations down
to a 3- limit of 0.22 mJy/beam. The corresponding constraints on the
dust mass and total mass are <0.15 Mearth and <0.05 Mjup, respectively, or
<0.003% and <0.3% of the mass of GSC 6214-210 B itself assuming a 100:1
gas-to-dust ratio and characteristic dust temperature of 10-20 K. If the host
star possesses a putative circum-stellar disk then at most it is a meager
0.0015% of the primary mass, implying that giant planet formation has certainly
ceased in this system. Considering these limits and its current accretion rate,
GSC 6214-210 B appears to be at the end stages of assembly and is not expected
to gain any appreciable mass over the next few Myr.Comment: Accepted to ApJ
Design of Lightweight Fills for Road Embankments on Boston\u27s Central Artery/Tunnel Project
The use of lightweight-fill materials for highway construction increased significantly worldwide during the 1990s. Predominant with this trend was the increased use of cellular geosynthetics (geofoams and geocombs), especially block-molded expanded polystyrene (EPS) geofoam, on highway and bridge embankments. EPS geofoam is increasingly recognized as an important tool for reducing overall cost of highways through accelerated construction . Thus, it was appropriate that lightweight-fill materials, mostly EPS, were the materials of choice on Boston’s Central Artery/Tunnel (CA/T) Project, commonly known as the Big Dig . EPS highway embankments have been constructed, as part of a cost-and schedule-initiative, replacing the original design concepts for eight transition highway structures on a recent CA/T construction contract. The use of EPS-block geofoam on the CA/T included the first-time implementation of newly developed NCHRP research and AASHTO based design guidelines, material/product specifications as well as formulating innovative solutions to several technical challenges. These challenges centered on relatively tall and slender EPS fills placed over soft soils subjected to periodic flooding and seismic loading within a crowded urban environment. This paper presents a detailed outline of the design process together with the impacts of the buoyancy conditions and seismic loading on the design of EPS highway embankments. Also included is a discussion of other lightweight-fill materials such as geocombs (considered but not used) and expanded-shale aggregate (used in limited quantities)
Diverse capacity for 2-methylhopanoid production correlates with a specific ecological niche
Molecular fossils of 2-methylhopanoids are prominent biomarkers in modern and ancient sediments that have been used as proxies for cyanobacteria and their main metabolism, oxygenic photosynthesis. However, substantial culture and genomic-based evidence now indicates that organisms other than cyanobacteria can make 2-methylhopanoids. Because few data directly address which organisms produce 2-methylhopanoids in the environment, we used metagenomic and clone library methods to determine the environmental diversity of hpnP, the gene encoding the C-2 hopanoid methylase. Here we show that hpnP copies from alphaproteobacteria and as yet uncultured organisms are found in diverse modern environments, including some modern habitats representative of those preserved in the rock record. In contrast, cyanobacterial hpnP genes are rarer and tend to be localized to specific habitats. To move beyond understanding the taxonomic distribution of environmental 2-methylhopanoid producers, we asked whether hpnP presence might track with particular variables. We found hpnP to be significantly correlated with organisms, metabolisms and environments known to support plant–microbe interactions (P-value<10^−6); in addition, we observed diverse hpnP types in closely packed microbial communities from other environments, including stromatolites, hot springs and hypersaline microbial mats. The common features of these niches indicate that 2-methylhopanoids are enriched in sessile microbial communities inhabiting environments low in oxygen and fixed nitrogen with high osmolarity. Our results support the earlier conclusion that 2-methylhopanoids are not reliable biomarkers for cyanobacteria or any other taxonomic group, and raise the new hypothesis that, instead, they are indicators of a specific environmental niche
Resolved young binary systems and their disks
We have conducted a survey of young single and multiple systems in the
Taurus-Auriga star-forming region with the Atacama Large Millimeter Array
(ALMA), substantially improving both the spatial resolution and sensitivity
with which individual protoplanetary disks in these systems have been observed.
These ALMA observations can resolve binary separations as small as 25--30 AU
and have an average 3 detection level of 0.35 mJy, equivalent to a disk
mass of M for an M3 star. Our sample was
constructed from stars that have an infrared excess and/or signs of accretion
and have been classified as Class II. For the binary and higher order multiple
systems observed, we detect mm continuum emission from one or
more stars in all of our target systems. Combined with previous surveys of
Taurus, our 21 new detections increase the fraction of millimeter-detected
disks to over 75% in all categories of stars (singles, primaries, and
companions) earlier than spectral type M6 in the Class II sample. Given the
wealth of other information available for these stars, this has allowed us to
study the impact of multiplicity with a much larger sample. While millimeter
flux and disk mass are related to stellar mass as seen in previous studies, we
find that both primary and secondary stars in binary systems with separations
of 30 to 4200 AU have lower values of millimeter flux as a function of stellar
mass than single stars. We also find that for these systems, the circumstellar
disk around the primary star does not dominate the total disk mass in the
system and contains on average 62% of the total mass.Comment: 30 pages, 12 figures, to be published in the Astrophysical Journa
The Immediate and Short-Term Effects of Transcutaneous Spinal Cord Stimulation and Peripheral Nerve Stimulation on Corticospinal Excitability
Rehabilitative interventions involving electrical stimulation show promise for neuroplastic recovery in people living with Spinal Cord Injury (SCI). However, the understanding of how stimulation interacts with descending and spinal excitability remain unclear. In this study we compared the immediate and short-term (within a few minutes) effects of pairing Transcranial Magnetic Stimulation (TMS) with transcutaneous Spinal Cord stimulation (tSCS) and Peripheral Nerve Stimulation (PNS) on Corticospinal excitability in healthy subjects. Three separate experimental conditions were assessed. In Experiment I, paired associative stimulation (PAS) was applied, involving repeated pairing of single pulses of TMS and tSCS, either arriving simultaneously at the spinal motoneurones (PAS0ms) or slightly delayed (PAS5ms). Corticospinal and spinal excitability, and motor performance, were assessed before and after the PAS interventions in 24 subjects. Experiment II compared the immediate effects of tSCS and PNS on corticospinal excitability in 20 subjects. Experiment III compared the immediate effects of tSCS with tSCS delivered at the same stimulation amplitude but modulated with a carrier frequency (in the kHz range) on corticospinal excitability in 10 subjects. Electromyography (EMG) electrodes were placed over the Tibialis Anterior (TA) soleus (SOL) and vastus medialis (VM) muscles and stimulation electrodes (cathodes) were placed on the lumbar spine (tSCS) and lateral to the popliteal fossa (PNS). TMS over the primary motor cortex (M1) was paired with tSCS or PNS to produce Motor Evoked Potentials (MEPs) in the TA and SOL muscles. Simultaneous delivery of repetitive PAS (PAS0ms) increased corticospinal excitability and H-reflex amplitude at least 5 min after the intervention, and dorsiflexion force was increased in a force-matching task. When comparing effects on descending excitability between tSCS and PNS, a subsequent facilitation in MEPs was observed following tSCS at 30-50 ms which was not present following PNS. To a lesser extent this facilitatory effect was also observed with HF- tSCS at subthreshold currents. Here we have shown that repeated pairing of TMS and tSCS can increase corticospinal excitability when timed to arrive simultaneously at the alpha-motoneurone and can influence functional motor output. These results may be useful in optimizing stimulation parameters for neuroplasticity in people living with SCI
CO and dust properties in the TW Hya disk from high-resolution ALMA observations
We analyze high angular resolution ALMA observations of the TW Hya disk to
place constraints on the CO and dust properties. We present new, sensitive
observations of the CO line at a spatial resolution of 8 AU
(0\farcs14). The CO emission exhibits a bright inner core, a shoulder at
AU, and a prominent break in slope at AU. Radiative
transfer modeling is used to demonstrate that the emission morphology can be
reasonably reproduced with a CO column density profile featuring a steep
decrease at AU and a secondary bump peaking at AU.
Similar features have been identified in observations of rarer CO
isotopologues, which trace heights closer to the midplane. Substructure in the
underlying gas distribution or radially varying CO depletion that affects much
of the disk's vertical extent may explain the shared emission features of the
main CO isotopologues. We also combine archival 1.3 mm and 870 m continuum
observations to produce a spectral index map at a spatial resolution of 2 AU.
The spectral index rises sharply at the continuum emission gaps at radii of 25,
41, and 47 AU. This behavior suggests that the grains within the gaps are no
larger than a few millimeters. Outside the continuum gaps, the low spectral
index values of indicate either that grains up to centimeter
size are present, or that the bright continuum rings are marginally optically
thick at millimeter wavelengths.Comment: 27 pages, 11 figures, accepted by ApJ; FITS image files available at
https://dataverse.harvard.edu/dataset.xhtml?persistentId=doi:10.7910/DVN/PXDKB
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