Norhasliza Yusof

The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150 M⊙ stellar mass limit

Spectroscopic analyses of hydrogen-rich WN5‐6 stars within the young star clusters NGC 3603 and R136 are presented, using archival Hubble Space Telescope and Very Large Telescope spectroscopy, and high spatial resolution near-IR photometry, including MultiConjugate Adaptive Optics Demonstrator (MAD) imaging of R136. We derive high stellar temperatures for the WN stars in NGC 3603 (T∗ ∼ 42±2 kK) and R136 (T∗ ∼ 53± 3 kK) plus clumping-corrected mass-loss rates of 2 ‐ 5 ×10 −5 M⊙ yr −1 which closely agree with theoretical predictions from Vink et al. These stars make a disproportionate contribution to the global ionizing and mechanical wind power budget of their host clusters. Indeed, R136a1 alone supplies ∼7% of the ionizing flux of the entire 30 Doradus region. Compar isons with stellar models calculated for the main-sequence evolution of 85 ‐ 500 M⊙ accounting for rotation suggest ages of ∼1.5 Myr and initial masses in the range 105 ‐ 170 M⊙ for three systems in NGC 3603, plus 165 ‐ 320 M⊙ for four stars in R136. Our high stellar masses are supported by consistent spectroscopic and dynamical mass determinations for the components of NGC 3603 A1. We consider the predicted X-ray luminosity of the R136 stars if they were close, colliding wind binaries. R136c is consistent with a colliding wind binary system. However, short period, colliding wind systems are excluded for R136a WN stars if mass ratios are of order unity. Widely separated systems would have been expected to harden owing to early dynamical encounters with other massive stars within such a high density environment. From simulated star clusters, whose constituents are randomly sampled from the Kroupa initial mass function, both NGC 3603 and R136 are consistent with an tentative upper mass limit of ∼300 M⊙. The Arches cluster is either too old to be used to diagnose the upper mass limit, exhibits a deficiency of very massive stars, or mo re likely stellar masses have been underestimated ‐ initial masses for the most luminous stars in the Arches cluster approach 200 M⊙ according to contemporary stellar and photometric results. The potential for stars greatly exceeding 150 M⊙ within metal-poor galaxies suggests that such pair-instab ility supernovae could occur within the local universe, as has been claimed for SN 2007bi.

Evolution and fate of very massive stars

There is observational evidence that supports the existence of very massive stars (VMS) in the local universe. First, VMS (Mini ≲ 320 M⊙) have been observed in the Large Magellanic Clouds (LMC). Secondly, there are observed supernovae (SNe) that bear the characteristics of pair creation supernovae (PCSNe, also referred to as pair instability SN) which have VMS as progenitors. The most promising candidate to date is SN 2007bi. In order to investigate the evolution and fate of nearby VMS, we calculated a new grid of models for such objects, for solar, LMC and Small Magellanic Clouds (SMC) metallicities, which covers the initial mass range from 120 to 500 M⊙. Both rotating and non-rotating models were calculated using the GENEVA stellar evolution code and evolved until at least the end of helium burning and for most models until oxygen burning. Since VMS have very large convective cores during the main-sequence phase, their evolution is not so much affected by rotational mixing, but more by mass loss through stellar winds. Their evolution is never far from a homogeneous evolution even without rotational mixing. All the VMS, at all the metallicities studied here, end their life as WC(WO)-type Wolf-Rayet stars. Because of very important mass losses through stellar winds, these stars may have luminosities during the advanced phases of their evolution similar to stars with initial masses between 60 and 120 M⊙. A distinctive feature which may be used to disentangle Wolf-Rayet stars originating from VMS from those originating from lower initial masses would be the enhanced abundances of Ne and Mg at the surface of WC stars. This feature is however not always apparent depending on the history of mass loss. At solar metallicity, none of our models is expected to explode as a PCSN. At the metallicity of the LMC, only stars more massive than 300 M⊙ are expected to explode as PCSNe. At the SMC metallicity, the mass range for the PCSN progenitors is much larger and comprises stars with initial masses between about 100 and 290 M⊙. All VMS in the metallicity range studied here produce either a Type Ib SN or a Type Ic SN but not a Type II SN. We estimate that the progenitor of SN 2007bi, assuming a SMC metallicity, had an initial mass between 160 and 175 M⊙. None of models presented in this grid produces gamma-ray bursts or magnetars. They lose too much angular momentum by mass loss or avoid the formation of a black hole by producing a completely disruptive PCSN.

Pair-instability Supernova Simulations: Progenitor Evolution, Explosion, and Light Curves

In recent years, the viability of the pair-instability supernova (PISN) scenario for explaining superluminous supernovae has all but disappeared except for a few slowly-evolving examples. However, PISN are not predicted to be superluminous throughout the bulk of their mass range. In fact, it is more likely that the first PISN we see (if we have not seen one already) will not be superluminous. Here, we present hydrodynamic simulations of PISNe for four stellar models with unique envelope properties spanning the PISN mass range. In addition, we compute synthetic light curves for comparison with current and future observations. We also investigate, in the context of our most massive model, the prospect of mixing in the supernova ejecta alleviating discrepancies between current PISN models and the remaining superluminous candidate events. To this end, we present the first published 3D hydrodynamic simulations of PISNe. After achieving convergence between 1D, 2D, and 3D simulations we examine mixing in the supernova ejecta and its affect on the bolometric light curve. We observe slight deviations from spherical symmetry which increase with the number of dimensions. We find no significant effects on the bolometric light curve, however we conclude that mixing between the silicon and oxygen rich layers caused by the Rayleigh-Taylor instability may affect spectra.

Effective mass of holographic Brownian particle in rotating plasma

The dynamic of string fluctuation under rotating BTZ black hole is studied using the method of \cite{Son:2009vu}. We compare the result with previous computation in \cite{Atmaja:2012jg}, with a different method, for the case of co-rotating string. The result gives definite answer that the end of string identified as an external quark at the boundary behaves as Brownian particle with mass is given an effective mass

Holographic screening length in a hot plasma of two sphere

M_{eff}

MOST EFFECTIVE ENERGY IN THERMONUCLEAR REACTIONS OF SOME LIGHT NUCLEI

equal to the zero temperature mass of external quark

On the nucleosynthesis of phosphorus in massive stars

M_0

The Quantum Hall Effect: Spin‐Charge Locking

times Lorentz factor

ERRATUM: “PAIR-INSTABILITY SUPERNOVAE IN THE LOCAL UNIVERSE” (2014, ApJ, 797, 9)

\gamma

Theoretical study on five dimensional AdS black holes with dilaton backreaction

to the power three. We also extend the computation for general case of rotating black hole where the metric is asymptotically