ABSTRACT The disk mass is among the most important input parameter for every planet formation model to determine the number and masses of the planets that can form. We present an ALMA 887 m survey of ...the disk population around objects from ∼2 to 0.03 M in the nearby ∼2 Myr old Chamaeleon I star-forming region. We detect thermal dust emission from 66 out of 93 disks, spatially resolve 34 of them, and identify two disks with large dust cavities of about 45 au in radius. Assuming isothermal and optically thin emission, we convert the 887 m flux densities into dust disk masses, hereafter Mdust. We find that the relation is steeper than linear and of the form Mdust ∝ (M*)1.3-1.9, where the range in the power-law index reflects two extremes of the possible relation between the average dust temperature and stellar luminosity. By reanalyzing all millimeter data available for nearby regions in a self-consistent way, we show that the 1-3 Myr old regions of Taurus, Lupus, and Chamaeleon I share the same relation, while the 10 Myr old Upper Sco association has a steeper relation. Theoretical models of grain growth, drift, and fragmentation reproduce this trend and suggest that disks are in the fragmentation-limited regime. In this regime millimeter grains will be located closer in around lower-mass stars, a prediction that can be tested with deeper and higher spatial resolution ALMA observations.
Community-based research (CBR) refers to collaborative investigation by academics and non-academic community members that fosters positive change on a local level. Despite recent trends toward ...engaged scholarship, few publications demonstrate how to effectively integrate CBR into academic course work or take advantage of its potential for achieving community change. Community-Based Research: Teaching for Community Impact fills these gaps by providing:
An overview of language and methods used by professionals engaged in CBR
A framework for orienting CBR toward concrete community outcomes
Effective ways to integrate CBR into course content, student-driven projects, and initiatives spanning disciplines, curricula, campuses and countries
Lessons learned in working toward positive outcomes for students and in communities
This text is designed for faculty, graduate students, service-learning and other engaged learning and scholarship practitioners, alliance members, special interest groups, and organizations that desire to strengthen student learning and utilize research for improvement in their communities.
Context.
The discovery of giant planets orbiting very low mass stars (VLMS) and the recent observed substructures in disks around VLMS is challenging planet formation models. Specifically, radial ...drift of dust particles is a catastrophic barrier in these disks, which prevents the formation of planetesimals and therefore planets.
Aims.
We aim to estimate if structures, such as cavities, rings, and gaps, are common in disks around VLMS and to test models of structure formation in these disks. We also aim to compare the radial extent of the gas and dust emission in disks around VLMS, which can give us insight about radial drift.
Methods.
We studied six disks around VLMS in the Taurus star-forming region using ALMA Band 7 (~340 GHz) at a resolution of ~0.1″. The targets were selected because of their high disk dust content in their stellar mass regime.
Results.
Our observations resolve the disk dust continuum in all disks. In addition, we detect the
12
CO (
J
= 3−2) emission line in all targets and
13
CO (
J
= 3−2) in five of the six sources. The angular resolution allows the detection of dust substructures in three out of the six disks, which we studied by using UV-modeling. Central cavities are observed in the disks around stars MHO 6 (M 5.0) and CIDA 1 (M 4.5), while we have a tentative detection of a multi-ringed disk around J0433. We estimate that a planet mass of ~0.1
M
Jup
or ~0.4
M
Saturn
is required for a single planet to create the first gap in J0433. For the cavities of MHO 6 and CIDA 1, a Saturn-mass planet (~0.3
M
Jup
) is required. The other three disks with no observed structures are the most compact and faintest in our sample, with the radius enclosing 90% of the continuum emission varying between ~13 and 21 au. The emission of
12
CO and
13
CO is more extended than the dust continuum emission in all disks of our sample. When using the
12
CO emission to determine the gas disk extension
R
gas
, the ratio of
R
gas
∕
R
dust
in our sample varies from 2.3 to 6.0. One of the disks in our sample, CIDA 7, has the largest
R
gas
∕
R
dust
ratio observed so far, which is consistent with models of radial drift being very efficient around VLMS in the absence of substructures.
Conclusions.
Given our limited angular resolution, substructures were only directly detected in the most extended disks, which represent 50% of our sample, and there are hints of unresolved structured emission in one of the bright smooth sources. Our observations do not exclude giant planet formation on the substructures observed. A comparison of the size and luminosity of VLMS disks with their counterparts around higher mass stars shows that they follow a similar relation.
We present ground-based and Swift photometric and spectroscopic observations of the candidate tidal disruption event (TDE) ASASSN-14li, found at the centre of PGC 043234 (d ... 90 Mpc) by the All-Sky ...Automated Survey for SuperNovae (ASAS-SN). The source had a peak bolometric luminosity of ... and a total integrated energy of ... erg radiated over the ~6 months of observations presented. The UV/optical emission of the source is well fitted by a blackbody with roughly constant temperature of T ... 35,000 K, while the luminosity declines by roughly a factor of 16 over this time. The optical/UV luminosity decline is broadly consistent with an exponential decline, ..., with t0 ... 60 d. ASASSN-14li also exhibits soft X-ray emission comparable in luminosity to the optical and UV emission but declining at a slower rate, and the X-ray emission now dominates. Spectra of the source show broad Balmer and helium lines in emission as well as strong blue continuum emission at all epochs. We use the discoveries of ASASSN-14li and ASASSN-14ae to estimate the TDE rate implied by ASAS-SN, finding an average rate of ... per galaxy with a 90 per cent confidence interval of ... per galaxy. ASAS-SN found roughly 1 TDE for every 70 Type Ia supernovae in 2014, a rate that is much higher than that of other surveys. (ProQuest: ... denotes formulae/symbols omitted.)
Many mathematical models have investigated the impact of expanding access to antiretroviral therapy (ART) on new HIV infections. Comparing results and conclusions across models is challenging because ...models have addressed slightly different questions and have reported different outcome metrics. This study compares the predictions of several mathematical models simulating the same ART intervention programmes to determine the extent to which models agree about the epidemiological impact of expanded ART.
Twelve independent mathematical models evaluated a set of standardised ART intervention scenarios in South Africa and reported a common set of outputs. Intervention scenarios systematically varied the CD4 count threshold for treatment eligibility, access to treatment, and programme retention. For a scenario in which 80% of HIV-infected individuals start treatment on average 1 y after their CD4 count drops below 350 cells/µl and 85% remain on treatment after 3 y, the models projected that HIV incidence would be 35% to 54% lower 8 y after the introduction of ART, compared to a counterfactual scenario in which there is no ART. More variation existed in the estimated long-term (38 y) reductions in incidence. The impact of optimistic interventions including immediate ART initiation varied widely across models, maintaining substantial uncertainty about the theoretical prospect for elimination of HIV from the population using ART alone over the next four decades. The number of person-years of ART per infection averted over 8 y ranged between 5.8 and 18.7. Considering the actual scale-up of ART in South Africa, seven models estimated that current HIV incidence is 17% to 32% lower than it would have been in the absence of ART. Differences between model assumptions about CD4 decline and HIV transmissibility over the course of infection explained only a modest amount of the variation in model results.
Mathematical models evaluating the impact of ART vary substantially in structure, complexity, and parameter choices, but all suggest that ART, at high levels of access and with high adherence, has the potential to substantially reduce new HIV infections. There was broad agreement regarding the short-term epidemiologic impact of ambitious treatment scale-up, but more variation in longer term projections and in the efficiency with which treatment can reduce new infections. Differences between model predictions could not be explained by differences in model structure or parameterization that were hypothesized to affect intervention impact.
Cyanobacteria are oxygenic phototrophic prokaryotes that have evolved to produce ultraviolet-screening mycosporine-like amino acids (MAAs) to lessen harmful effects from obligatory exposure to solar ...UV radiation. The cyanobacterial MAA biosynthetic cluster is formed by a gene encoding 2-
-5-
-valiolone synthase (EVS) located immediately upstream from an
methyltransferase (OMT) encoding gene, which together biosynthesize the expected MAA precursor 4-deoxygadusol. Accordingly, these genes are typically absent in non-producers. In this study, the relationship between gene cluster architecture and constitutive production of MAAs was evaluated in cyanobacteria isolated from various Brazilian biomes. Constitutive production of MAAs was only detected in strains where genes formed a co-linear cluster. Expectedly, this production was enhanced upon exposure of the strains to UV irradiance and by using distinct culture media. Constitutive production of MAAs was not detected in all other strains and, unexpectedly, production could not be induced by exposure to UV irradiation or changing growth media. Other photoprotection strategies which might be employed by these MAA non-producing strains are discussed. The evolutionary and ecological significance of gene order conservation warrants closer experimentation, which may provide a first insight into regulatory interactions of genes encoding enzymes for MAA biosynthesis.
The impact of stellar multiplicity on the evolution of planet-forming disks is still the subject of debate. Here we present and analyze disk structures around ten multiple stellar systems that were ...included in an unbiased, high spatial resolution survey performed with ALMA of 32 protoplanetary disks in the Taurus star-forming region. At the unprecedented spatial resolution of ~0.12′′ we detect and spatially resolve the disks around all primary stars, and those around eight secondary and one tertiary star. The dust radii of disks around multiple stellar systems are smaller than those around single stars in the same stellar mass range and in the same region. The disks in multiple stellar systems also show a steeper decay of the millimeter continuum emission at the outer radius than disks around single stars, suggestive of the impact of tidal truncation on the shape of the disks in multiple systems. However, the observed ratio between the dust disk radii and the observed separation of the stars in the multiple systems is consistent with analytic predictions of the effect of tidal truncation only if the eccentricities of the binaries are rather high (typically >0.5) or if the observed dust radii are a factor of two smaller than the gas radii, as is typical for isolated systems. Similar high-resolution studies targeting the gaseous emission from disks in multiple stellar systems are required to resolve this question.
Field edge habitat in homogeneous agricultural landscapes can serve multiple purposes including enhanced biodiversity, water quality protection, and habitat for beneficial insects, such as native ...bees and natural enemies. Despite this ecosystem service value, adoption of field border plantings, such as hedgerows, on large-scale mono-cropped farms is minimal. With profits primarily driving agricultural production, a major challenge affecting hedgerow plantings is linked to establishment costs and the lack of clear economic benefits on the restoration investment. Our study documented that hedgerows are economically viable to growers by enhancing beneficial insects and natural pest control and pollination on farms. With pest control alone, our model shows that it would take 16 yr to break even from insecticide savings on the US$4,000 cost of a typical 300-m hedgerow field edge planting. By adding in pollination benefits by native bees, where honey bees (Apis mellifera L.) may be limiting, the return time is reduced to 7 yr. USDA cost share programs allow for a quicker return on a hedgerow investment. Our study shows that over time, small-scale restoration can be profitable, helping to overcome the barrier of cost associated with field edge habitat restoration on farms.
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