SN1957D, located in one of the spiral arms of M83, is one of the small number of extragalactic supernovae (SNe) that has remained detectable at radio and optical wavelengths during the decades after ...its explosion. Here, we report the first detection of SN1957D in X-rays, as part of a 729 ks observation of M83 with Chandra. The X-ray luminosity (0.3-8 keV) is (ProQuest: Formulae and/or non-USASCII text omitted) x 10 super(37) erg s super(-1). The spectrum is hard and highly self-absorbed compared to most sources in M83 and to other young supernova remnants (SNRs), suggesting that the system is dominated at X-ray wavelengths by an energetic pulsar and its pulsar wind nebula. The high column density may be due to absorption within the SN ejecta. Hubble Space Telescope WFC3 images resolve the SNR from the surrounding emission and the local star field. Photometry of stars around SN1957D, using WFC3 images, indicates an age of less than 10 super(7) years and a main-sequence turnoff mass more than 17 M sub(middot in circle). New spectra obtained with Gemini-South show that the optical spectrum continues to be dominated by broad OIII emission lines, the signature of fast-moving SN ejecta. The width of the broad lines has remained about 2700 km s super(-1) (FWHM). The OIII flux dropped precipitously between 1989 and 1991, but continued monitoring shows the flux has been almost constant since. In contrast, radio observations over the period 1990-2011 show a decline rate Sv ~ t super(-4.0), far steeper than the rate observed earlier, suggesting that the primary shock has overrun the edge of a pre-SN wind.
Low success rates in developmental mathematics courses have caused a growing concern for many institutions including four-year universities. As a result, institutions have adopted various course ...redesign models intended to improve developmental mathematics course completion rates. The co-requisite model of instruction is one alternative pathway for students with developmental needs. This model allows students to enroll in the required general education gateway mathematics course concurrent with a developmental mathematics lab, offering student-centered instruction and just-in-time support for student learning. This study contributes to the current body of knowledge by investigating relationships between possible placement variables and student performance in the co-requisite model.
We present the results of detailed monitoring of the radio emission from the Type Ic supernova SN 1994I from three days after optical discovery on 1994 March 31 until eight years later at age 2927 ...days on 2002 April 5. The data were mainly obtained using the Very Large Array at the five wavelengths of Delta *l Delta *l1.3, 2.0, 3.6, 6.2, and 21 cm and from the Cambridge 5 km Ryle Telescope at Delta *l2.0 cm. Two additional measurements were obtained at millimeter wavelengths. This data set represents the most complete, multifrequency radio observations ever obtained for a Type Ib/c supernova. The radio emission evolves regularly in both time and frequency and is well described by established supernova emission/absorption models. It is the first radio supernova with sufficient data to show that it is clearly dominated by the effects of synchrotron self-absorption at early times.
We describe the radio evolution of supernova (SN) 2001ig in NGC 7424, from 700 d of multifrequency monitoring with the Australia Telescope Compact Array (ATCA) and the Very Large Array (VLA). We find ...that deviations of the radio light curves at each frequency from the standard ‘minishell’ model are consistent with density modulations in the circumstellar medium (CSM), which seem to recur with a period near 150 d. One possibility is that these are due to enhanced mass loss from thermal pulses in an asymptotic giant branch star progenitor. A more likely scenario, however, is that the progenitor was a Wolf–Rayet (WR) star, whose stellar wind collided with that from a massive hot companion on an eccentric 100-d orbit, leading to a regular build-up of CSM material on the required time and spatial scales. Recent observations of ‘dusty pinwheels’ in WR binary systems lend credibility to this model. Since such binary systems are also thought to provide the necessary conditions for envelope stripping which would cause the WR star to appear as a Type Ib/c SN event rather than a Type II, these radio observations of SN 2001ig may provide the key to linking Type Ib/c SNe to Type IIb events, and even to some types of gamma-ray bursts.
Living retention trees are being used in managed forests to promote a variety of values, including the maintenance of biological diversity. Federal forest plans for the northwestern USA include ...guidelines that require the retention of a minimum of 15% basal area in harvest units, with the goal of facilitating the development of late-seral stand structure, which is an important habitat element for old-growth forest-dependent species. However, effective levels and patterns of green-tree retention are unknown. We present results of a treatment consisting of 15% basal area, evenly dispersed retention (15%D). We quantified changes in the ectomycorrhiza (EM) community after the 15%D treatment, both near and away from retention trees. Pretreatment samples were obtained between 1 and 24 months before tree harvest. Post-treatment samples were collected within 14-25 months of harvest. In areas 8-25 m from retention trees, there was a 50% decline in the number of EM types per soil core from before to after treatment. Soil cores taken >5 m from retention trees exhibited a shift in EM community structure. EM-type richness was positively correlated with fine-root-tip density. We demonstrate the potential for retention trees to act as refugia for recolonization of newly established seedlings by ectomycorrhizal fungi.
We present radio observations of the optically bright Type IIn supernova SN 1995N. We observed the SN at radio wavelengths with the Very Large Array for 11 years. We also observed it at low radio ...frequencies with the Giant Metrewave Radio Telescope at various epochs within 6.5 - 10 years since explosion. Although there are indications of an early optically thick phase, most of the data are in the optically thin regime so it is difficult to distinguish between synchrotron self absorption and free-free absorption (FFA) mechanisms. However, the information from other wavelengths indicates that FFA is the dominant absorption process. Model fits of radio emission with FFA give reasonable physical parameters. Making use of X-ray and optical observations, we derive the physical conditions of the shocked ejecta and the shocked circumstellar matter.
Student veterans face some barriers to fully engaging in the postsecondary education experience, in addition to sometimes struggling to complete their degrees. This article highlights a university ...program that annually engages groups of student veterans in meaningful service-learning experiences through a partnership with the Appalachian Trail Conservancy. The theoretical foundation, program structure, and lessons learned are shared so that other institutions or organizations may similarly engage their veteran population.
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