4,687 research outputs found
Strong late-time circumstellar interaction in the peculiar supernova iPTF14hls
We present a moderate-resolution spectrum of the peculiar Type II supernova
iPTF14hls taken on day 1153 after discovery. This spectrum reveals the clear
signature of shock interaction with dense circumstellar material (CSM). We
suggest that this CSM interaction may be an important clue for understanding
the extremely unusual photometric and spectroscopic evolution seen over the
first 600 days of iPTF14hls. The late-time spectrum shows a double-peaked
intermediate-width H-alpha line indicative of expansion speeds around 1000
km/s, with the double-peaked shape hinting at a disc-like geometry in the CSM.
If the CSM was highly asymmetric, perhaps in a disc or torus that was ejected
from the star 3-6 years prior to explosion, then the CSM interaction could have
been overrun and hidden below the SN ejecta photosphere from a wide range of
viewing angles. In that case, CSM interaction luminosity would have been
thermalized well below the photosphere, possibly sustaining the high luminosity
without exhibiting the traditional observational signatures of strong CSM
interaction (narrow H-alpha emission and X-rays). Variations in density
structure of the CSM could account for the multiple rebrightenings of the
lightcurve. We propose that enveloped CSM interaction as seen in some recent
SNe, rather than an entirely new explosion mechanism, may be adequate to
explain the peculiar evolution of iPTF14hls.Comment: 6 pages, 5 figures, submitted to MNRAS with referee respons
Extreme isolation of WN3/O3 stars and implications for their evolutionary origin as the elusive stripped binaries
Recent surveys of the Magellanic Clouds have revealed a subtype of Wolf-Rayet
(WR) star with peculiar properties. WN3/O3 spectra exhibit both WR-like
emission and O3 V-like absorption - but at lower luminosity than O3 V or WN
stars. We examine the projected spatial distribution of WN3/O3 stars in the LMC
as compared to O-type stars. Surprisingly, WN3/O3 stars are among the most
isolated of all classes of massive stars; they have a distribution similar to
red supergiants dominated by initial masses of 10-15 , and are far
more dispersed than classical WR stars or luminous blue variables (LBVs). Their
lack of association with clusters of O-type stars suggests strongly that WN3/O3
stars are not the descendants of single massive stars (30 or
above). Instead, they are likely products of interacting binaries at lower
initial mass (10-18 ). Comparison with binary models suggests a
probable origin with primaries in this mass range that were stripped of their H
envelopes through non-conservative mass transfer by a low-mass secondary. We
show that model spectra and positions on the Hertzsprung-Russell diagram for
binary stripped stars are consistent with WN3/O3 stars. Monitoring radial
velocities with high-resolution spectra can test for low-mass companions or
runaway velocities. With lower initial mass and environments that avoid very
massive stars, the WN3/O3 stars fit expectations for progenitors of Type Ib and
possibly Type Ibn supernovae.Comment: Accepted for publication in MNRA
Massive stars dying alone: The extremely remote environment of SN 2009ip
We present late-time HST images of the site of supernova (SN) 2009ip taken
almost 3 yr after its bright 2012 luminosity peak. SN 2009ip is now slightly
fainter in broad filters than the progenitor candidate detected by HST in 1999.
The current source continues to be dominated by ongoing late-time CSM
interaction that produces strong H-alpha emission and a weak pseudo-continuum,
as found previously for 1-2 yr after explosion. The intent of these
observations was to search for evidence of recent star formation in the local
(1kpc; 10 arcsec) environment around SN 2009ip, in the remote outskirts of its
host spiral galaxy NGC 7259. We can rule out the presence of any massive
star-forming complexes like 30 Dor or the Carina Nebula at the SN site or
within a few kpc. If the progenitor of SN 2009ip was really a 50-80 Msun star
as archival HST images suggested, then it is strange that there is no sign of
this type of massive star formation anywhere in the vicinity. A possible
explanation is that the progenitor was the product of a merger or binary mass
transfer, rejuvenated after a lifetime that was much longer than 4-5 Myr,
allowing its natal H II region to have faded. A smaller region like the Orion
Nebula would be an unresolved but easily detected point source. This is ruled
out within 1.5 kpc around SN 2009ip, but a small H II region could be hiding in
the glare of SN 2009ip itself. Later images after a few more years have passed
are needed to confirm that the progenitor candidate is truly gone and to test
for the presence of a small H II region or cluster at the SN position.Comment: 8 pages, 5 figs. submitted to MNRA
The CRaTER Special Issue of Space Weather: Building the observational foundation to deduce biological effects of space radiation
[1] The United States is preparing for exploration beyond low-Earth Orbit (LEO). However, the space radiation environment poses significant risks. The radiation hazard is potentially severe but not sufficiently well characterized to determine if long missions outside LEO can be accomplished with acceptable risk [Cucinotta et al., 2001; Schwadron et al., 2010; Cucinotta et al., 2010]. Radiation hazards may be over- or under-stated through incomplete characterization in terms of net quantities such as accumulated dose. Time-dependent characterization often changes acute risk estimates [NCRP, 1989; Cucinotta, 1999; Cucinotta et al., 2000; George et al., 2002]. For example, events with high accumulated doses but sufficiently low dose rates (/h) pose significantly reduced risks. Protons, heavy ions, and neutrons all contribute significantly to the radiation hazard. However, each form of radiation presents different biological effectiveness. As a result, quality factors and radiation-specific weighting factors are needed to assess biological effectiveness of different forms of radiation [e.g., NCRP 116, 1993] (Figure 1). More complete characterization must account for time-dependent radiation effects according to organ type, primary and secondary radiation composition, and acute effects (vomiting, sickness, and, at high exposures, death) versus chronic effects (such as cancer)
Optimal Collision Avoidance Trajectories for Unmanned/Remotely Piloted Aircraft
The post-911 environment has punctuated the force-multiplying capabilities that Remotely Piloted Aircraft (RPA) provides combatant commanders at all echelons on the battlefield. Not only have unmanned aircraft systems made near-revolutionary impacts on the battlefield, their utility and proliferation in law enforcement, homeland security, humanitarian operations, and commercial applications have likewise increased at a rapid rate. As such, under the Federal Aviation Administration (FAA) Modernization and Reform Act of 2012, the United States Congress tasked the FAA to provide for the safe integration of civil unmanned aircraft systems into the national airspace system (NAS) as soon as practicable, but not later than September 30, 2015. However, a necessary entrance criterion to operate RPAs in the NAS is the ability to Sense and Avoid (SAA) both cooperative and noncooperative air traffic to attain a target level of safety as a traditional manned aircraft platform. The goal of this research effort is twofold: First, develop techniques for calculating optimal avoidance trajectories, and second, develop techniques for estimating an intruder aircraft\u27s trajectory in a stochastic environment. This dissertation describes the optimal control problem associated with SAA and uses a direct orthogonal collocation method to solve this problem and then analyzes these results for different collision avoidance scenarios
Vibration of hollow cylindrical shells with partial constrained layer damping
A theoretical analysis is presented for determining the natural frequencies and damping factors for isotropic circular cylindrical shell type structures. Equations of motion for a cylindrical shell and a three-layer cylindrical shell fully and partially treated, with a viscoelastic core, are derived from equilibrium. The assumed mode method or Galerkin method are used to find the equivalent mass and stiffness matrices from which the natural frequencies can then be obtained. The effects on the natural frequency and damping factor due to various viscoelastic core thicknesses, viscoelastic shear moduli, constraining layer thickness, constraining layer Young\u27s moduli, and viscoelastic coverage length are discussed. The results reveal for the cylinder studied, that an optimal coverage length exists for achieving maximum damping, along with an optimal viscoelastic shear modulus
Government Payments: Economic Impact on Southeastern Peanut Farms
Southeastern peanut farms with diversified field crops utilize government payments to supplement market receipts. Production in 2002 represented growing conditions under adverse weather, while 2003 represented optimal conditions. Representative farm analysis provides insight into allocation of market receipts and government payments for meeting variable costs and fixed costs.Crop Production/Industries,
Isotropic-medium three-dimensional cloaks for acoustic and electromagnetic waves
We propose a generalization of the two-dimensional eikonal-limit cloak
derived from a conformal transformation to three dimensions. The proposed cloak
is a spherical shell composed of only isotropic media; it operates in the
transmission mode and requires no mirror or ground plane. Unlike the well-known
omnidirectional spherical cloaks, it may reduce visibility of an arbitrary
object only for a very limited range of observation angles. In the
short-wavelength limit, this cloaking structure restores not only the
trajectories of incident rays, but also their phase, which is a necessary
ingredient to complete invisibility. Both scalar-wave (acoustic) and transverse
vector-wave (electromagnetic) versions are presented.Comment: 17 pages, 12 figure
IV. Fossil Fishes From The Miocene Ellensburg Formation, South Central Washington
The Ellensburg Formation was named for sediments deposited in the Kittitas Valley along the Yakima River near Ellensburg, Washington (Russell, 1893, 1900). Similar beds are present to the south along the leeward front of the emerging central Cascade Mountains; including the Nile, Selah, Yakima, and Toppenish basins. Further south along the Columbia River, portions of the Dalles Group, Rhododendron Formation, and Sandy River Mudstone are likely temporal equivalents; the latter two of which are found on the windward side of the uplifting Cascade Range (Farooqui, et al., 1981; Evarts et al., 2009).Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/146545/1/MP 204vol4.pdfDescription of MP 204vol4.pdf : Main Articl
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