328 research outputs found
The Nature of SN 1961V
The nature of SN 1961V has been uncertain. Its peculiar optical light curve
and slow expansion velocity are similar to those of super-outbursts of luminous
blue variables (LBVs), but its nonthermal radio spectral index and declining
radio luminosity are consistent with decades-old supernovae (SNe). We have
obtained Hubble Space Telescope STIS images and spectra of the stars in the
vicinity of SN 1961V, and find Object 7 identified by Filippenko et al. to be
the closest to the optical and radio positions of SN 1961V. Object 7 is the
only point source detected in our STIS spectra and only its H-alpha emission is
detected; it cannot be the SN or its remnant because of the absence of
forbidden lines. While the H-alpha line profile of Object 7 is remarkably
similar to that of eta Car, the blue color (similar to an A2Ib supergiant) and
lack of appreciable variability are unlike known post-outburst LBVs. We have
also obtained Very Long Baseline Array (VLBA) observations of SN 1961V at 18
cm. The non-detection of SN 1961V places a lower limit on the size of the
radio-emitting region, 7.6 mas or 0.34 pc, which implies an average expansion
velocity in excess of 4,400 km/s, much higher than the optical expansion
velocity measured in 1961. We conclude the following: (1) A SN occurred in the
vicinity of SN 1961V a few decades ago. (2) If the SN 1961V light maximum
originates from a giant eruption of a massive star, Object 7 is the most
probable candidate for the survivor, but its blue color and lack of significant
variability are different from a post-outburst eta Car. (3) The radio SN and
Object 7 could be physically associated with each other through a binary
system. (4) Object 7 needs to be monitored to determine its nature and
relationship to SN 1961V.Comment: 16 pages, 3 figures, accepted by the Astronomical Journal for the
2004 May issu
Narrow Lines in Type II Supernovae -- Probing the Circumstellar Nebulae of the Progenitors
We have carried out a high-dispersion (R~30,000) echelle spectroscopic survey
of 16 Type II supernovae (SNe) to search for narrow emission lines from
circumstellar nebulae ejected by their massive progenitors. Circumstellar
nebulae, if detected, provide invaluable opportunities to probe SN progenitors.
Of the 16 SNe observed, SN ejecta are clearly detected in 4 SNe and possibly in
another 2 SNe, interstellar gas is detected in 12 SNe, and circumstellar
material is detected only in SN 1978K and SN 1998S. In the case of SN 1978K we
are able to place an upper limit of ~2.2 pc for the size of the circumstellar
ejecta nebula and note that this is more consistent with the typical sizes
observed for ejecta nebulae around luminous blue variables rather than
Wolf-Rayet stars. In the case of SN 1998S, our observations of the narrow lines
\~1 year after the SN explosion show variations compared to early epochs. The
nebular lines we observe from SN 1998S originate from either the low--density,
outer region of a circumstellar nebula or have become dominated by an
interstellar component.Comment: (20 pages, 7 figures, revised from previous version, resubmitted to
AJ, 28 Jan 2002
A Novel Approach to Adolescent Obesity in Rural Appalachia of West Virginia: Educating Adolescents as Family Health Coaches and Research Investigators
A Study of Carbon Features in Type Ia Supernova Spectra
One of the major differences between various explosion scenarios of Type Ia
supernovae (SNe Ia) is the remaining amount of unburned (C+O) material and its
velocity distribution within the expanding ejecta. While oxygen absorption
features are not uncommon in the spectra of SNe Ia before maximum light, the
presence of strong carbon absorption has been reported only in a minority of
objects, typically during the pre-maximum phase. The reported low frequency of
carbon detections may be due to low signal-to-noise data, low abundance of
unburned material, line blending between C II 6580 and Si II 6355, ejecta
temperature differences, asymmetrical distribution effects, or a combination of
these. However, a survey of published pre-maximum spectra reveals that more SNe
Ia than previously thought may exhibit C II 6580 absorption features and relics
of line blending near 6300 Angstroms. Here we present new SN Ia observations
where spectroscopic signatures of C II 6580 are detected, and investigate the
presence of C II 6580 in the optical spectra of 19 SNe Ia using the
parameterized spectrum synthesis code, SYNOW. Most of the objects in our sample
that exhibit C II 6580 absorption features are of the low-velocity gradient
subtype. Our study indicates that the morphology of carbon-rich regions is
consistent with either a spherical distribution or a hemispheric asymmetry,
supporting the recent idea that SN Ia diversity may be a result of off-center
ignition coupled with observer line-of-sight effects.Comment: 10 papges, 9 figures, 3 table
Late-Time HST Photometry of SN 1994I: Hints of Positron Annihilation Energy Deposition
We present multicolor Hubble Space Telescope (HST) WFPC2 broadband
observations of the Type Ic SN 1994I obtained approximately 280 days after
maximum light. We measure the brightness of the SN and, relying on the detailed
spectroscopic database of SN 1994I, we transform the ground-based photometry
obtained at early times to the HST photometric system, deriving light curves
for the WFPC2 F439W, F555W, F675W, and F814W passbands that extend from 7 days
before to 280 days after maximum. We use the multicolor photometry to build a
quasi-bolometric light curve of SN 1994I, and compare it with similarly
constructed light curves of other supernovae. In doing so, we propose and test
a scaling in energy and time that allows for a more meaningful comparison of
the exponential tails of different events.
Through comparison with models, we find that the late-time light curve of SN
1994I is consistent with that of spherically symmetric ejecta in homologous
expansion, for which the ability to trap the Gamma-rays produced by the
radioactive decay of 56Co diminishes roughly as the inverse of time squared. We
also find that by the time of the HST photometry, the light curve was
significantly energized by the annihilation of positrons.Comment: To appear in PAS
Mastering the Hard Stuff: The History of College Concrete-Canoe Races and the Growth of Engineering Competition Culture
This article details the history of college engineering competitions, originating with student concrete-canoe racing in the 1970s, through todayâs multi-million-dollar international multiplicity of challenges. Despite initial differences between engineering educators and industry supporters over the ultimate purpose of undergraduate competitions, these events thrived because they evolved to suit many needs of students, professors, schools, corporations, professional associations, and the engineering profession itself. The twenty-first-century proliferation of university-level competitions in turn encouraged a trickling-down of technical contests to elementary-age children and high schools, fostering the institutionalization of what might be called a competition culture in engineering
Recapitulation of Human Retinal Development from Human Pluripotent Stem Cells Generates Transplantable Populations of Cone Photoreceptors
Transplantation of rod photoreceptors, derived either from neonatal retinae or pluripotent stem cells (PSCs), can restore rod-mediated visual function in murine models of inherited blindness. However, humans depend more upon cone photoreceptors that are required for daylight, color, and high-acuity vision. Indeed, macular retinopathies involving loss of cones are leading causes of blindness. An essential step for developing stem cell-based therapies for maculopathies is the ability to generate transplantable human cones from renewable sources. Here, we report a modified 2D/3D protocol for generating hPSC-derived neural retinal vesicles with well-formed ONL-like structures containing cones and rods bearing inner segments and connecting cilia, nascent outer segments, and presynaptic structures. This differentiation system recapitulates human photoreceptor development, allowing the isolation and transplantation of a pure population of stage-matched cones. Purified human long/medium cones survive and become incorporated within the adult mouse retina, supporting the potential of photoreceptor transplantation for treating retinal degeneration
Relation between Plasmodium falciparum asymptomatic infection and malaria attacks in a cohort of Senegalese children
© 2008 Le Port et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licens
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