183 research outputs found
High Resolution Spectroscopy of SN1987A's Rings: He I 10830 and H-alpha from the Hotspots
We present the first high-dispersion spectroscopy of He I 10830 from the
hotspots in the ring around SN1987A, obtained at Gemini South, spatially
resolving the near and far sides of the ring. We compare these line profiles to
similar echelle spectra of H and [N II] 6583 obtained at the Magellan
Observatory. We find that the He I profiles are much broader than H-alpha or [N
II], but the He I profiles also have different shapes -- they have enhanced
emission at high speeds, with extra blueshifted emission on the north side of
the ring, and extra redshifted emission on the south side. To explain this, we
invoke a simple geometric picture where the extra He I emission traces hotter
gas from faster shocks that strike the apex of the hotspots directly, while the
H-alpha preferentially traces cooler lower-ionization gas from slower
transverse shocks that penetrate into the sides of the ring.Comment: 3 pages. To appear in proceedings: "Supernova 1987A: 20 Years After:
Supernovae and Gamma-Ray Bursters" AIP, New York, eds. S. Immler, K.W.
Weiler, and R. McCra
UV spectroscopy of the blue supergiant SBW1: the remarkably weak wind of a SN 1987A analog
The Galactic blue supergiant SBW1 with its circumstellar ring nebula
represents the best known analog of the progenitor of SN 1987A. High-resolution
imaging has shown H-alpha and IR structures arising in an ionized flow that
partly fills the ring's interior. To constrain the influence of the stellar
wind on this structure, we obtained an ultraviolet (UV) spectrum of the central
star of SBW1 with the HST Cosmic Origins Spectrograph (COS). The UV spectrum
shows none of the typical wind signatures, indicating a very low mass-loss
rate. Radiative transfer models suggest an extremely low rate below 10
Msun/yr, although we find that cooling timescales probably become comparable to
or longer than the flow time below 10 Msun/yr. We therefore adopt this
latter value as a conservative upper limit. For the central star, the model
yields =21,0001000 K, 510 ,
and roughly Solar composition except for enhanced N abundance. SBW1's very low
mass-loss rate may hinder the wind's ability to shape the surrounding nebula.
The very low mass-loss rate also impairs the wind's ability to shed angular
momentum; the spin-down timescale for magnetic breaking is more than 500 times
longer than the age of the ring. This, combined with the star's slow rotation
rate, constrain merger scenarios to form ring nebulae. The mass-loss rate is at
least 10 times lower than expected from mass-loss recipes, without any account
of clumping. The physical explanation for why SBW1's wind is so weak presents
an interesting mystery.Comment: 12 pages, 6 figs. submitted to MNRAS. comments welom
The Transition Zone in Balmer-Dominated Shocks
We examine the structure of the post-shock region in supernova remnants
(SNRs). The ``shock transition zone'' is set up by charge transfer and
ionization events between atoms and ions, and has a width
cm , where is the total pre-shock density (including
both atoms and ions). For Balmer-dominated SNRs with shock velocity km s, the Rankine-Hugoniot conditions for ion velocity and
temperature are obeyed instantly, leaving the full width at half-maximum (FWHM)
of the broad H line versus relation intact. However, the spatial
variation in the post-shock densities is relevant to the problem of Ly
resonant scattering in young, core-collapse SNRs. Both two- (pre-shock atoms
and ions) and three-component (pre-shock atoms, broad neutrals and ions) models
are considered. We compute the spatial emissivities of the broad () and
narrow () H lines; a calculation of these emissivities in SN
1006 is in general agreement with the computed ones of Raymond et al. (2007).
The (dimensionless) spatial shift, , between the centroids
of and is unique for a given shock velocity and ,
the pre-shock ion fraction. Measurements of can be used
to constrain .Comment: 25 pages, 8 figures. Accepted by Astrophysical Journa
Evaluating leadership development in a changing world?:Alternative models and approaches for healthcare organisations
Internationally, healthcare is undergoing a major reconfiguration in a post-pandemic world. To make sense of this change and deliver an integrated provision of care, which improve both patient outcomes and satisfaction for key stakeholders, healthcare leaders must develop an insight into the context in which healthcare is delivered, and leadership is enacted. Formal leadership development programmes (LDPs) are widely used for developing leaders and leadership in healthcare organizations. However, there is a paucity of rigorous evaluations of LDPs. Existing evaluations often focus on individual-level outcomes, with limited attention to long-term outcomes that might emerge across team and organizational levels. Specifically, evaluation models that have been closely associated with or rely heavily on qualitative methods are seldom used in LDP evaluations, despite their relevance for capturing unanticipated outcomes, investigating learning impact over time, and studying collective outcomes at multiple levels. The purpose of this paper is to review the potential of qualitative models and approaches in healthcare leadership development evaluation. This scoping review identifies seventeen evaluation models and approaches. Findings indicate that the incorporation of qualitative and participatory elements in evaluation designs could offer a richer demonstration and context-specific explanations of programme impact in healthcare contexts
Ultraviolet Opacity and Fluorescence in Supernova Envelopes
By the time the expanding envelope of a Type 2 supernova becomes transparent in the optical continuum, most of the gamma-ray luminosity produced by radioactive Fe/Co/Ni clumps propagates into the hydrogen/helium envelope and is deposited there, if at all. The resulting fast electrons excite He 1 and H 1, the two- photon continua of which are the dominant internal sources of ultraviolet radiation. The UV radiation is blocked by scattering in thousands of resonance lines of metals and converted by fluorescence into optical and infrared emission lines that escape freely. We describe results of Monte Carlo calculations that simulate non-LTE scattering and fluorescence in more than five million allowed lines of Ca, Sc, Ti, V, Cr, Mn, Fe, Co, and Ni. For a model approximating conditions in the envelope of SN 1987A, the calculated emergent spectrum resembles the observed one. For the first 2 yr after explosion, the ultraviolet radiation (lambda less than or approximately equals 3000) is largely blocked and converted into a quasi continuum of many thousands of weak optical and infrared emission lines and some prominent emission features, such as the Ca 2 lambdalambda8600 triplet. Later, as the envelope cools and expands, it becomes more transparent, and an increasing fraction of the luminosity emerges in the UV band
Hydrogen Molecules in SN 1987A
The observations of CO and SiO in the infrared spectrum of SN 1987A clearly indicate that molecules can form in the debris of a supernova explosion. Since H2 is not easily observable we compute its abundance theoretically. For conditions typical of the inner (v less than 2500 km/s) envelope of SN 1987A, the fraction of H that is in molecular form rises to approx. 1% by t approx. 800 days. For t less than 500 days the formation is dominated by the gas-phase reactions H + H(+) yields H2(+) + hv; H2(+) + H yields H2 + H(+). Thereafter, the formation is dominated by the reactions H + e yields H(-) + hv; H(-) + H yields H2 + e. At early times the H(-) may absorb approx. 10%-30% of visible photons, contributing to the apparent paucity of H alpha emission. For t greater than 1000 days the abundance of H2 'freezes out' due to the slowing of all reactions. The opacity of the supernova envelope in the range 912 less than lambda less than or approx. equal to 1400 A (the upper limit depending on temperature) is dominated by resonance scattering in the Lyman and Werner bands of H2. The resulting fluorescence emission bands of H2 in the range 1150 less than lambda less than 1650 A may be observable in the UV spectra of supernovae at late times
Supernova Remnant 1987A: Opening the Future by Reaching the Past
We report an up-turn in the soft X-ray light curve of supernova remnant (SNR)
1987A in late 2003 (~6200 days after the explosion), as observed with the
Chandra X-ray Observatory. Since early 2004, the rapid increase of the 0.5-2
keV band X-ray light curve can no longer be described by the exponential
density distribution model with which we successfully fitted the data between
1990 and 2003. Around day ~6200, we also find that the fractional contribution
to the observed soft X-ray flux from the decelerated shock begins to exceed
that of the fast shock and that the X-ray brightening becomes "global" rather
than "spotty". We interpret these results as evidence that the blast wave has
reached the main body of the dense circumstellar material all around the inner
ring. This interpretation is supported by other recent observations, including
a deceleration of the radial expansion of the X-ray remnant, a significant
up-turn in the mid-IR intensities, and the prevalence of the optical hot spots
around the entire inner ring, all of which occur at around day 6000. In
contrast to the soft X-ray light curve, the hard band (3-10 keV) X-ray light
curve increases at a much lower rate which is rather similar to the radio light
curve. The hard X-ray emission may thus originate from the reverse shock where
the radio emission is likely produced. Alternatively, the low increase rate of
the hard X-rays may simply be a result of the continuous softening of the
overall X-ray spectrum.Comment: AASTex preprint style 12 pages including 1 table and 4 figures,
Accepted by ApJ
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