Search Results - (Author, Cooperation:J. C. Wheeler)

2014-05-23

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

2011-06-10

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] Supernova models based on either the degenerate cores of intermediate mass stars (4-8M0, see ref. 2) or the iron cores of more massive stars3, suggest a collapsing core of initial mass 1.4M0 and hence a final gravitational mass of 1.2M0. In the more massive stars the collapse of some material of ...

Electronic Resource

1749-6632

Blackwell Publishing Journal Backfiles 1879-2005

Natural Sciences in General

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] Optical spectra of SN1993J taken at 5:30 UT on 29 April and 4:05 on 9 May 1993 are shown in Fig. 1 together with a spectrum taken 20 April 1993 (ref. 1). These correspond to 32, 43 and 23 days, respectively, after the explosion (assuming that shock break-out occurred 28 March2). All spectra ...

Electronic Resource

1617-4623

Key words Tetracycline repressor ; Inducible promoter ; Drosophila ; Aging

Springer Online Journal Archives 1860-2000

Biology

Abstract The “reverse” tetracycline repressor (rtR) binds a specific DNA element, the tetracycline operator (tetO), only in the presence of tetracycline, or derivatives such as doxycycline (dox). Fusion of rtR to the transcriptional activation domain of herpes virus protein VP16 produces a eukaryotic transactivator protein (rtTA). rtTA has previously been shown to allow dox-dependent transcription of transgenes linked to tetO sequences in mammals. To adapt this system to Drosophila, the Actin5C promoter was used to drive constitutive expression of rtTA in transgenic flies. Three reporter constructs, each encoding E. coli β-galactosidase (β-gal), were also introduced into transgenic flies. In one reporter seven tetO sequences were fused to the Adh core promoter. The other two reporter constructs contain seven tetO sequences fused to the hsp70 core promoter. Feeding of transgenic Drosophila containing the rtTA construct and any one of the three reporter constructs with dox caused up to 100-fold induction of β-gal. Dox induced β-gal expression in all tissues, in larvae and in young and senescent adults. Induction of β-gal in adults had no detectable effect on life span. These results suggest the potential usefulness of this system for testing specific genes for effects on Drosophila development and aging.

Electronic Resource

1572-9672

Springer Online Journal Archives 1860-2000

Physics

Abstract The radial pulsations of very luminous, low-mass models (L/M ∼ 104, solar units), which are possible representatives of the R CrB stars, have been examined. These pulsations are extremely nonadiabatic. We find that there are in some cases at least one extra (“strange”) mode which makes interpretation difficult. The blue instability edges are also peculiar, in that there is an abrupt excursion of the blue edge to the blue for L/M sufficiently large. The range of periods of the model encompasses observed periods of the Cepheid-like pulsations of actual R CrB stars.

Electronic Resource

1572-9532

Springer Online Journal Archives 1860-2000

Physics

Abstract A model for QSOs and the nuclei of Seyfert Galaxies is proposed in which mass lost from stars in a galactic nucleus repeatedly builds to a critical density and then collapses to the center where it accretes onto a massive black hole (≈108 M ⊙), emitting great luminosity. This model describes a means of starting with an ordinary nucleus and developing conditions found in QSOs. By invoking intermittent flashes we overcome a difficulty previously encountered in similar models in which plausible sources of mass in reasonable galactic nuclei fail by a factor of ≈10−2 in fueling a black hole at QSO luminosities.

Electronic Resource

1572-9672

Stars ; Supernovae

Springer Online Journal Archives 1860-2000

Physics

Abstract We review the possible evolutionary paths from massive stars to explosive endpoints as various types of supernovae associated with Population I and hence with massive stars: Type II-P, Type II-L, Type Ib, Type Ic, and the hybrid events SN 1987K and SN 1993J. We identify SN 1954A as another hybrid event from the evidence for both H and He in its spectrum with velocities nearly the same as SN 1983J. Evidence for ejected56Ni mass of ∼0.07 M⊙ suggests that SN II-P underwent standard iron core collapse, not collapse of an O−Ne−Mg core nor thermonuclear explosion of a C−O core. Most SN II-P presumably arise in single stars or wide binaries of ∼10–20 M⊙. There may be indirect evidence for duplicity in some cases in the form of strong Ba II lines, such as characterized SN 1987A. SN II-L are recognizably distinct from typical SN II-P and must undergo a significantly different evolution. Despite indications that SN II-L have small envelopes that may be helium enriched, they are also distinct from events like SN 1993J that must have yet again a different evolution. The SN II-L that share a common Luminosity seem to have ejected a small nickel mass and hence may come from stars with O−Ne−Mg cores. The amount of nickel ejected by the exceptionally bright events, SN 1980K and SN 1979C, remains controversial. SN Ib require the complete loss of the H envelope, either to a binary companion or to a wind. The few identified have relatively large ejecta masses. It is not clear what evolutionary processes distinguish SN Ib's evolving in binary systems from hybrid events that retain some H in the envelope. SN Ic events are both H and He deficient. Binary models that can account for transfer of an extended helium envelope from low mass helium cores, ∼2 to 4 M⊙, imply C−O core masses that are roughly consistent with that deduced from the ejecta mass plus a neutron star, ∼2 to 3 M⊙. It is possible that the hybrid events are the result of Roche lobe overflow and that the “pure” events, SN Ib or SN Ic, result from common envelope evolution.

Electronic Resource