5,231 research outputs found
The Signature of Single-Degenerate Accretion Induced Collapse
The accretion induced collapse (AIC) of a white dwarf to a neutron star has
long been suggested as a natural theoretical outcome in stellar evolution, but
there has never been a direct detection of such an event. This is not
surprising since the small amount of radioactive nickel synthesized
() implies a relatively dim optical transient. Here we
argue that a particularly strong signature of an AIC would occur for an
oxygen-neon-magnesium (ONeMg) white dwarf accreting from a star that is
experiencing Roche-lobe overflow as it becomes a red giant. In such cases, the
explosion from the AIC collides with and shock-heats
the surface of the extended companion, creating an X-ray flash lasting
followed by an optical signature that peaks at an absolute
magnitude of to and lasts for a few days to a week. These
events would be especially striking in old stellar environments where
hydrogen-rich supernova-like, transients would not normally be expected.
Although the rate of such events is not currently known, we describe observing
strategies that could be utilized with high cadence surveys that should either
detect these events or place strong constraints on their rates.Comment: Revised version accepted for publication in The Astrophysical
Journal, 5 pages, 2 figure
Reconciling ^(56)Ni Production in Type Ia Supernovae with Double Degenerate Scenarios
We combine the observed distribution of Type Ia supernova (SN Ia) ^(56)Ni yields with the results of sub-Chandrasekhar detonation and direct collision calculations to estimate what mass white dwarfs (WDs) should be exploding for each scenario. For collisions, the average exploding WD mass must be peaked at ≈0.75M_☉, significantly higher than the average field WD mass
of ≈0.55–0.60M_☉. Thus, if collisions produce most SNe Ia, then a mechanism must exist that favours higher mass WDs. On the other hand, in old stellar populations, collisions would naturally result in low-luminosity SNe Ia, and we suggest these may be related to 1991bg-like events. For sub-Chandrasekhar detonations, the average exploding WD mass must be peaked at ≈1.1M_☉. This is similar to the average total mass in WD–WD binaries, but it is not
clear whether double degenerate mergers would synthesize sufficient ^(56)Ni to match observed yields. If not, then actual ≈1.1M_☉ WDs would be needed for sub-Chandrasekhar detonations. Since such high-mass WDs are produced relatively quickly in comparison to the age of SN Ia
environments, this would require either accretion on to lower mass WDs prior to ignition or a long time-scale between formation of the ≈1.1M_☉ WD and ignition
Variable stars in the field of the old open cluster Melotte 66
We report the results of photometric monitoring of the Melotte 66 field in
BVI filters. Ten variables were identified with nine being new discoveries. The
sample includes eight eclipsing binaries of which four are W UMa type stars,
one star is a candidate blue straggler. All four contact binaries are likely
members of the cluster based on their estimated distances. Ten blue stars with
U-B<-0.3 were detected inside a 14.8 x 22.8 arcmin^2 field centred on the
cluster. Time series photometry for 7 of them showed no evidence for any
variability. The brightest object in the sample of blue stars is a promising
candidate for a hot subdwarf belonging to the cluster. We show that the
anomalously wide main sequence of the cluster, reported in some earlier
studies, results from a combination of two effects: variable reddening occuring
across the cluster field and the presence of a rich population of binary stars
in the cluster itself. The density profile of the cluster field is derived and
the total number of member stars with 16<V<21 or 2.8<M_{V}<7.8 is estimated
conservatively at about 1100.Comment: 8 pages, 12 figures, accepted to MNRAS - 29 June 200
Variations in tephra stratigraphy created by small-scale surface features in sub-polar landscapes
Financial support for this work was provided by NERC Doctoral Training Partnership Ph.D. studentship NE/L002558/1 to Polly I. J. Thompson.We explore the effect small-scale surface features have on influencing the morphology and grain-size distribution (GSD) of tephra layers within the Quaternary stratigraphy of sub-polar landscapes. Icelandic thúfur, small cryogenic earth mounds, are used to assess how and why the morphology and GSD of tephra layers vary over such formations. Through measurement of tephra layer thickness and GSD, Hekla 1947 and GrÃmsvötn 2011 tephra layers are analysed. Results indicate that such microtopographic features do indeed alter the form of tephra deposits and therefore the tephra layer that is preserved in the stratigraphy. Tephra thickness is significantly greater in hollows than on the thúfur crests. There is greater variation in tephra thickness measurements from thúfur in comparison to control measurements from a surface where thúfur are absent. Thúfur crests contain larger grain sizes than hollows, for both H1947 and G2011 tephras; however this was only statistically significant for the G2011 tephra. Such morphological patterns are thought to arise from an interplay of tephra characteristics, altered topography from the thúfur formations and earth surface processes operating at the sites. This study provides insight into the potential of tephra layer morphology and internal structures as indicators of Quaternary landforms and processes. Additionally, it provides important context for the appropriate sampling of tephra layers to infer volcanological processes, as the characteristics of preserved layers do not necessarily reflect those of the original fall-out.Publisher PDFPeer reviewe
On the Spin content of the Nucleon
A QCD sum rule calculation of Balistky and Ji on the spin content of the
nucleon is done with a different approach to the evaluation of the bilocal
contributions and to the extraction of the nucleon pole residues. The result
obtained is much more numerically stable which puts their conclusion that about
half of the nucleon spin is carried by gluons on firmer ground.Comment: 7 pages, two (eps) figure, minor corrections and one figure adde
Anterior Cervical Infection: Presentation and Incidence of an Uncommon Postoperative Complication.
STUDY DESIGN: Retrospective multi-institutional case series.
OBJECTIVE: The anterior cervical discectomy and fusion (ACDF) affords the surgeon the flexibility to treat a variety of cervical pathologies, with the majority being for degenerative and traumatic indications. Limited data in the literature describe the presentation and true incidence of postoperative surgical site infections.
METHODS: A retrospective multicenter case series study was conducted involving 21 high-volume surgical centers from the AOSpine North America Clinical Research Network, selected for their excellence in spine care and clinical research infrastructure and experience. Medical records for 17 625 patients who received cervical spine surgery (levels from C2 to C7) between January 1, 2005, and December 31, 2011, inclusive, were reviewed to identify the occurrence of 21 predefined treatment complications. Patients who underwent an ACDF were identified in the database and reviewed for the occurrence of postoperative anterior cervical infections.
RESULTS: A total of 8887 patients were identified from a retrospective database analysis of 21 centers providing data for postoperative anterior cervical infections (17/21, 81% response rate). A total of 6 postoperative infections after ACDF were identified for a mean rate of 0.07% (range 0% to 0.39%). The mean age of patients identified was 57.5 (SD = 11.6, 66.7% female). The mean body mass index was 22.02. Of the total infections, half were smokers (n = 3). Two patients presented with myelopathy, and 3 patients presented with radiculopathic-type complaints. The mean length of stay was 4.7 days. All patients were treated aggressively with surgery for management of this complication, with improvement in all patients. There were no mortalities.
CONCLUSION: The incidence of postoperative infection in ACDF is exceedingly low. The management has historically been urgent irrigation and debridement of the surgical site. However, due to the rarity of this occurrence, guidance for management is limited to retrospective series
ASASSN-15pz: Revealing Significant Photometric Diversity among 2009dc-like, Peculiar SNe Ia
We report comprehensive multi-wavelength observations of a peculiar Type
Ia-like supernova ("SN Ia-pec") ASASSN-15pz. ASASSN-15pz is a spectroscopic
"twin" of SN 2009dc, a so-called "Super-Chandrasekhar-mass" SN, throughout its
evolution, but it has a peak luminosity M_B,peak = -19.69 +/- 0.12 mag that is
\approx 0.6 mag dimmer and comparable to the SN 1991T sub-class of SNe Ia at
the luminous end of the normal width-luminosity relation. The synthesized Ni56
mass of M_Ni56 = 1.13 +/- 0.14 M_sun is also substantially less than that found
for several 2009dc-like SNe. Previous well-studied 2009dc-like SNe have
generally suffered from large and uncertain amounts of host-galaxy extinction,
which is negligible for ASASSN-15pz. Based on the color of ASASSN-15pz, we
estimate a host extinction for SN 2009dc of E(B-V)_host=0.12 mag and confirm
its high luminosity (M_B, peak[2009dc] \approx -20.3 mag). The 2009dc-like SN
population, which represents ~1% of SNe Ia, exhibits a range of peak
luminosities, and do not fit onto the tight width-luminosity relation. Their
optical light curves also show significant diversity of late-time (>~ 50 days)
decline rates. The nebular-phase spectra provide powerful diagnostics to
identify the 2009dc-like events as a distinct class of SNe Ia. We suggest
referring to these sources using the phenomenology-based "2009dc-like SN
Ia-pec" instead of "Super-Chandrasekhar SN Ia," which is based on an uncertain
theoretical interpretation.Comment: 21 pages, 16 figures, accepted for publication in Ap
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