111 research outputs found
Selecting Methods to Teach Controversial Topics: A Grounded Theory Study
This grounded theory study examined the perceptions of 14 high school social studies teachers from three school districts in the Central Florida area. They were interviewed to uncover the decision-making process that high school social studies teachers use to choose methodologies when teaching controversial public issues (CPIs). The result was a three-phase model, the CPI Decision-Making Model, in which teachers move through three conceptual phases to decide on a particular methodology. By working through this process, teachers analyze the benefits and drawbacks of different methods for teaching controversial public issues. Significant results from this study included: (a) teachers were choosing to avoid teaching CPIs with standard-level students with student-centered methods, (b) teachers received little to no training in alternative methods and no training in how to deal with controversy in the classroom, (c) teachers possibly overestimated their ability to remain neutral in the classroom, and (d) teachers were learning their methodologies for teaching CPIs through unorthodox means
An ALMA survey of DCN/HCN and DCO/HCO in protoplanetary disks
The deuterium enrichment of molecules is sensitive to their formation
environment. Constraining patterns of deuterium chemistry in protoplanetary
disks is therefore useful for probing how material is inherited or reprocessed
throughout the stages of star and planet formation. We present ALMA
observations at resolution of DCO, HCO, DCN, and
HCN in the full disks around T Tauri stars AS 209 and IM Lup, the
transition disks around T Tauri stars V4046 Sgr and LkCa 15, and the full disks
around Herbig Ae stars MWC 480 and HD 163296. We also present ALMA observations
of HCN in the IM Lup disk. DCN, DCO, and HCO are detected in all
disks, and HCN in all but the IM Lup disk. We find efficient deuterium
fractionation for the sample, with estimates of disk-averaged DCO/HCO
and DCN/HCN abundance ratios ranging from and ,
respectively, which is comparable to values reported for other ISM
environments. The relative distributions of DCN and DCO vary between disks,
suggesting that multiple formation pathways may be needed to explain the
diverse emission morphologies. In addition, gaps and rings observed in both
HCO and DCO emission provide new evidence that DCO bears a
complex relationship with the location of the midplane CO snowline.Comment: 36 pages, 14 figures, updated to match figure order of published
version in Ap
Detecting weak spectral lines in interferometric data through matched filtering
Funding: R.A.L. and J.H. gratefully acknowledge funding from National Science Foundation Graduate Research Fellowships (Grant No. DGE-1144152). R.A.L. also acknowledges funding from the NRAO Student Observing Support Program. K.I.Ö. acknowledges funding from the Alfred P. Sloan Foundation and the David and Lucile Packard Foundation. C.W. acknowledges financial support from the Netherlands Organisation for Scientific Research (NWO, grant 639.041.335) and start-up funds from the University of Leeds, UK.Modern radio interferometers enable observations of spectral lines with unprecedented spatial resolution and sensitivity. In spite of these technical advances, many lines of interest are still at best weakly detected and therefore necessitate detection and analysis techniques specialized for the low signal-to-noise ratio (S/N) regime. Matched filters can leverage knowledge of the source structure and kinematics to increase sensitivity of spectral line observations. Application of the filter in the native Fourier domain improves S/N while simultaneously avoiding the computational cost and ambiguities associated with imaging, making matched filtering a fast and robust method for weak spectral line detection. We demonstrate how an approximate matched filter can be constructed from a previously observed line or from a model of the source, and we show how this filter can be used to robustly infer a detection significance for weak spectral lines. When applied to ALMA Cycle 2 observations of CH3OH in the protoplanetary disk around TW Hya, the technique yields a ≈53% S/N boost over aperture-based spectral extraction methods, and we show that an even higher boost will be achieved for observations at higher spatial resolution. A Python-based open-source implementation of this technique is available under the MIT license at http://github.com/AstroChem/VISIBLE.Publisher PDFPeer reviewe
The coupled physical structure of gas and dust in the IM Lup protoplanetary disk
Funding: IC gratefully acknowledges funding support from the Smithsonian Institution.The spatial distribution of gas and solids in protoplanetary disks determines the composition and formation efficiency of planetary systems. A number of disks show starkly different distributions for the gas and small grains compared to millimeter-centimeter-sized dust. We present new Atacama Large Millimeter/Submillimeter Array observations of the dust continuum, CO, 13CO, and C18O in the IM Lup protoplanetary disk, one of the first systems where this dust-gas dichotomy was clearly seen. The 12CO is detected out to a radius of 970 au, while the millimeter continuum emission is truncated at just 313 au. Based upon these data, we have built a comprehensive physical and chemical model for the disk structure, which takes into account the complex, coupled nature of the gas and dust and the interplay between the local and external environment. We constrain the distributions of gas and dust, the gas temperatures, the CO abundances, the CO optical depths, and the incident external radiation field. We find that the reduction/removal of dust from the outer disk exposes this region to higher stellar and external radiation and decreases the rate of freeze-out, allowing CO to remain in the gas out to large radial distances. We estimate a gas-phase CO abundance of 5% of the interstellar medium value and a low external radiation field (G 0 ≲ 4). The latter is consistent with that expected from the local stellar population. We additionally find tentative evidence for ring-like continuum substructure, suggestions of isotope-selective photodissociation, and a diffuse gas halo.Peer reviewe
An ALMA Survey of H₂CO in Protoplanetary Disks
H₂CO is one of the most abundant organic molecules in protoplanetary disks and can serve as a precursor to more complex organic chemistry. We present an Atacama Large Millimeter/submillimeter Array survey of H₂CO toward 15 disks covering a range of stellar spectral types, stellar ages, and dust continuum morphologies. H₂CO is detected toward 13 disks and tentatively detected toward a fourteenth. We find both centrally peaked and centrally depressed emission morphologies, and half of the disks show ring-like structures at or beyond expected CO snowline locations. Together these morphologies suggest that H₂CO in disks is commonly produced through both gas-phase and CO-ice-regulated grain-surface chemistry. We extract disk-averaged and azimuthally-averaged H₂CO excitation temperatures and column densities for four disks with multiple H₂CO line detections. The temperatures are between 20–50 K, with the exception of colder temperatures in the DM Tau disk. These temperatures suggest that H₂CO emission in disks generally emerges from the warm molecular layer, with some contributions from the colder midplane. Applying the same H₂CO excitation temperatures to all disks in the survey, we find that H₂CO column densities span almost three orders of magnitude (~5 × 10¹¹–5 × 10¹⁴ cm⁻²). The column densities appear uncorrelated with disk size and stellar age, but Herbig Ae disks may have less H₂CO compared to T Tauri disks, possibly because of less CO freeze-out. More H₂CO observations toward Herbig Ae disks are needed to confirm this tentative trend, and to better constrain under which disk conditions H₂CO and other oxygen-bearing organics efficiently form during planet formation
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