Direct Production of 56Fe In Silicon Quasi-Equilibria and the Problem of 58Ni

We consider two questions of importance to the science of nucleosynthesis: (1) Can 56Fe, rather than 56Ni, have been synthesized in its observed ratio to 28Si within a silicon-burning quasi-equilibrium? (2) What accounts for the large abundance of 58Ni? The answer to the first question is in the negative, be- cause a large overabundance of other neutron-rich nuclei, especially 54Fe or 52Cr, appears in the quasi- equilibrium distributions which yield the observed abundance ratio for 56Fe/28Si. The large 58Ni abun- dance seems to imply one of the following: (1) the iron peak is synthesized in an e-process dominated by Ni, in which case the lighter a-particle nuclei are probably synthesized in a cooler mass zone of the same object; (2) the iron peak represents a superposition of at least two considerably different types of equilibrium processes, one accounting for Ni and the other accounting for Fe and 58Ni; or (3) 58Ni is due to the p-process. Determination of the proper means of production of 58Ni is one of the most im- portant problems associated with nucleosynthesis today

Search for Important Weak Interaction Nuclei in Presupernova Evolution

A search is made for the most important electron captures and beta-decays after core silicon buring in massive stars. A nuclear statistical equilibrium code is used to compute isotopic abundances. Electron capture and beta-decay rates are estimated for the 150 most abundant istopes in a simplifiec fashion which generally includes the strongest transitions. These estimates are made for nuclei in the fp-shell and use techniques similar to Fuller, Fowler, & Newman (1982a), and are compared to them. The general behaviour of Y is examined. These methods are then used to follow a typical stellar trajectory, seeking the most important weak interactions during the formation of the iron core. Ranked lists of nuclei are given, to prioritize more detailed studies of individual nuclei. Beta-decays are found to be an important modification to the evolution below Y = 0.4 as the core approaches a state dynamic equilibrium between electron captures and beta-decays

Thermonuclear Origin of Rare Neutron-Rich Isotopes

Many rare neutron-rich isotopes in the range 16\u3c~Z≲34 can be synthesized from seed nuclei exposed to explosive carbon burning. This process, which involves no new astrophysical parameters, can solve most of the outstanding problems in the thermonuclear synthesis of elements in the range Z≲34

Nucelosynthesis of Rare Nuclei from Seed Nuclei in Explosive Carbon Burning

We demonstrate that a Population I concentration of primordial heavy seed nuclei, when present in the carbon- and oxygewrich core of the presupernova star and exposed to the temperature and free nucleon densities of explosive carbon burning, is efficiently transmuted into the rare species 365, 40K, 43Ar, 43Ca, 46Ca, 48Ca, 405c, 45Ti, 0Ti, 50V, 62Ni, 64Ni, 68Zn, 70Zn, and 76Ge in approximately their solar-system abundance ratios. If the temperature is high enough (T0 2.1) that at least 10 percent of the Fe and Ni seed nuclei undergo (p, ) reactions during the explosion, the nuclei 60Cu, 63Zn, 71Ga, 73Ce, and perhaps 75As can also be synthesized. These nuclei comprise virtually all of the relatively rare neutron-rich nuclei in the range 36 \u3c A \u3c 76 except 04Cr and 50Fe. We emphasize the dependence of the results upon the rates of important nuclear reactions, most of which are unknown. The temperature, density, and timescale of the carbon explosion all have a strong influence on the results as well. We therefore regard this paper as an exploratory survey of a very difficult problem that will give increasing information about nucleosynthesis as nuclear facts and models of nuclear explosions become more secure

Models of Type I X-ray Bursts from GS 1826-24: A Probe of rp-Process Hydrogen Burning

The X-ray burster GS 1826-24 shows extremely regular Type I X-ray bursts whose energetics and recurrence times agree well with thermonuclear ignition models. We present calculations of sequences of burst lightcurves using multizone models which follow the rp-process nucleosynthesis with an extensive nuclear reaction network. The theoretical and observed burst lightcurves show remarkable agreement. The models naturally explain the slow ~5s rise and long ~100s tails of these bursts, as well as their dependence on mass accretion rate. This comparison provides further evidence for solar metallicity in the accreted material in this source, and constrains the distance to the source. The main difference is that the observed lightcurves do not show the distinct two-stage rise of the models. This may reflect the time for burning to spread over the stellar surface, or may indicate that our treatment of heat transport or nuclear physics needs to be revised. The trends in burst properties with accretion rate are well-reproduced by our spherically symmetric models which include chemical and thermal inertia from the ashes of previous bursts. Changes in the covering fraction of the accreted fuel are not required.Comment: 5 pages, 4 figures, to appear in ApJ letter

An Optically Dark GRB Observed by HETE-2: GRB 051022

GRB 051022 was detected at 13:07:58 on 22 October 2005 by HETE-2. The location of GRB 051022 was determined immediately by the flight localization system. This burst contains multiple pulses and has a rather long duration of about 190 seconds. The detections of candidate X-ray and radio afterglows were reported, whereas no optical afterglow was found. The optical spectroscopic observations of the host galaxy revealed the redshift z = 0.8. Using the data derived by HETE-2 observation of the prompt emission, we found the absorption N_H = 8.8 -2.9/+3.1 x 10^22 cm^-2 and the visual extinction A_V = 49 -16/+17 mag in the host galaxy. If this is the case, no detection of any optical transient would be quite reasonable. The absorption derived by the Swift XRT observations of the afterglow is fully consistent with those obtained from the early HETE-2 observation of the prompt emission. Our analysis implies an interpretation that the absorbing medium could be outside external shock at R ~ 10^16 cm, which may be a dusty molecular cloud.Comment: 6 pages, 2 figures, accepted for publication in PASJ lette

Fundamental Strings, Holography, and Nonlinear Superconformal Algebras

We discuss aspects of holography in the AdS_3 \times S^p near string geometry of a collection of straight fundamental heterotic strings. We use anomalies and symmetries to determine general features of the dual CFT. The symmetries suggest the appearance of nonlinear superconformal algebras, and we show how these arise in the framework of holographic renormalization methods. The nonlinear algebras imply intricate formulas for the central charge, and we show that in the bulk these correspond to an infinite series of quantum gravity corrections. We also makes some comments on the worldsheet sigma-model for strings on AdS_3\times S^2, which is the holographic dual geometry of parallel heterotic strings in five dimensions.Comment: 25 page

HETE-2 Observations of the X-Ray Flash XRF 040916

A long X-ray flash was detected and localized by the instruments aboard the High Energy Transient Explorer II (HETE-2) at 00:03:30 UT on 2004 September 16. The position was reported to the GRB Coordinates Network (GCN) approximately 2 hours after the burst. This burst consists of two peaks separated by 200 s, with durations of 110 s and 60 s. We have analyzed the energy spectra of the 1st and 2nd peaks observed with the Wide Field X-Ray Monitor (WXM) and the French Gamma Telescope (FREGATE). We discuss the origin of the 2nd peak in terms of flux variabilities and timescales. We find that it is most likely part of the prompt emission, and is explained by the long-acting engine model. This feature is similar to some bright X-ray flares detected in the early afterglow phase of bursts observed by the Swift satellite.Comment: 9 pages, 4 figures, Accepted for publication in PAS

Evidence for Type Ia Supernova Diversity from Ultraviolet Observations with the Hubble Space Telescope

We present ultraviolet (UV) spectroscopy and photometry of four Type Ia supernovae (SNe 2004dt, 2004ef, 2005M, and 2005cf) obtained with the UV prism of the Advanced Camera for Surveys on the Hubble Space Telescope. This dataset provides unique spectral time series down to 2000 Angstrom. Significant diversity is seen in the near maximum-light spectra (~ 2000--3500 Angstrom) for this small sample. The corresponding photometric data, together with archival data from Swift Ultraviolet/Optical Telescope observations, provide further evidence of increased dispersion in the UV emission with respect to the optical. The peak luminosities measured in uvw1/F250W are found to correlate with the B-band light-curve shape parameter dm15(B), but with much larger scatter relative to the correlation in the broad-band B band (e.g., ~0.4 mag versus ~0.2 mag for those with 0.8 < dm15 < 1.7 mag). SN 2004dt is found as an outlier of this correlation (at > 3 sigma), being brighter than normal SNe Ia such as SN 2005cf by ~0.9 mag and ~2.0 mag in the uvw1/F250W and uvm2/F220W filters, respectively. We show that different progenitor metallicity or line-expansion velocities alone cannot explain such a large discrepancy. Viewing-angle effects, such as due to an asymmetric explosion, may have a significant influence on the flux emitted in the UV region. Detailed modeling is needed to disentangle and quantify the above effects.Comment: 17 pages, 13 figures, accepted by Ap