Publications

We present complete spectropolarimetric coverage of the Type II supernova (SN) 2023ixf ranging from 1 to 120 days after explosion. Polarimetry was obtained with the Kast double spectrograph on the Shane 3m telescope at Lick Observatory. As the ejecta interact with circumstellar material (CSM) during the first week, the intrinsic polarization of SN 2023ixf is initially high at $\lesssim$1%, dropping steeply within days down to $\sim$ 0.4% when the ejecta sweep up the optically-thick CSM. The continuum polarization stays low at $\sim$ 0.2% thereafter, until it rises again to $\sim$ 0.6% as the ejecta transition to the nebular phase. We model this evolution using a combination of archival and newly-computed 2D polarized radiative-transfer models. In this context, we interpret the early-time polarization as arising from an aspherical CSM with a pole-to-equator density contrast $\gtrsim$ 3. We propose that the surge in polarization at late times originates from an asymmetric distribution of $^{56}$Ni deep in the ejecta. The distinct sources of asymmetries at early and late times are consistent with the temporal evolution of the observed polarization and the polarization angle in SN 2023ixf.

A combined computational/experimental study on LiNi1/3Co1/3Mn1/3O2 is presented. Both density functional theory and experiments are used to probe the active redox pairs and changes in electronic structure of LiNi1/3Co1/3Mn1/3O2 during intercalation or deintercalation of Li. The phase stability and voltage curve of this material are also shown in this paper. Both the experimental and computational data show that LiNi1/3Co1/3Mn1/3O2 material is a high-capacity stable electrode for advanced rechargeable lithium ion batteries.

The low-coordinate monovalent cobalt complex (IPr)Co[N(SiMe3)DIPP] [2, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; DIPP = 2,6-diisopropylphenyl], supported by bulky amide and N-heterocyclic carbene (NHC) ligands and its 9-diazofluorene (FluN2) adduct (IPr)Co[N(SiMe3)DIPP](FluN2) (3) are described. Complex 3 was characterized as possessing a high-spin divalent cobalt center antiferromagnetically coupled to a ligand-based radical, resulting in an overall triplet spin ground state (S = 1). Both 2 and 3 are catalyst precursors for the homocoupling of FluN2 in benzene under ambient conditions to produce 1,2-di(9H-fluoren-9-ylidene)hydrazine (8) and 9,9′-bifluorenylidene (9) in a ratio of 1:8.1. A switch in product selectivity was observed for the reaction in the polar solvent tetrahydrofuran (THF), or in the presence of exogenous good L-type ligands such as tert-butylnitrile, to generate the corresponding hydrazine 8 as the major product. A mechanistic study was carried out to rationalize the observed product distributions. The reaction exhibits first-order rate dependence on both the FluN2 and cobalt catalyst (2) concentrations (monitored by 1H NMR spectroscopy), and 3 was identified as the catalytic resting state. Theoretical calculations were carried out to simulate the production of hydrazine 8 and olefin 9. The result predicted turnover frequencies (TOFs) of 4.6 × 10–7 and 2.3 × 10–6 s–1 for the generation of 8 and 9 in benzene, respectively, in good agreement with the experimentally observed product ratio. Modeling the reaction in media with higher polarity such as THF resulted in a more favorable kinetic barrier toward the formation of hydrazine 8 due to the stabilization of the more polar C–N bond-forming transition state (8, TOF = 2.6 × 10–5 s–1 vs 9, TOF = 6.4 × 10–6 s–1, in THF). Moreover, simulation of the potential energy surface with a coordinated L-type donor, such as acetonitrile, suggests that the selectivity switch could also result from a modified ligand field, rendering diazoalkane adduct 3 more nucleophilic and lowering the barrier of rate-limiting C–N bond formation to give hydrazine 8.

Due to its relatively slow clinical progression, B cell chronic lymphocytic leukemia (B-CLL) is classically described as a disease of accumulation rather than proliferation. However, evidence for various forms of clonal evolution suggests that B-CLL clones may be more dynamic than previously assumed. We used a nonradioactive, stable isotopic labeling method to measure B-CLL cell kinetics in vivo. Nineteen patients drank an aliquot of deuterated water (2H2O) daily for 84 days, and 2H incorporation into the deoxyribose moiety of DNA of newly divided B-CLL cells was measured by gas chromatography/mass spectrometry, during and after the labeling period. Birth rates were calculated from the kinetic profiles. Death rates were defined as the difference between calculated birth and growth rates. These analyses demonstrated that the leukemic cells of each patient had definable and often substantial birth rates, varying from 0.1% to greater than 1.0% of the entire clone per day. Those patients with birth rates greater than 0.35% per day were much more likely to exhibit active or to develop progressive disease than those with lower birth rates Thus, B-CLL is not a static disease that results simply from accumulation of long-lived lymphocytes. Rather, it is a dynamic process composed also of cells that proliferate and die, often at appreciable levels. The extent to which this turnover occurs has not been previously appreciated. A correlation between birth rates and disease activity and progression appears to exist, which may help identify patients at risk for worsening disease in advance of clinical deterioration.

Abstract not Available.

The AGN STORM 2 collaboration targeted the Seyfert 1 galaxy Mrk 817 for a year-long multiwavelength, coordinated reverberation mapping campaign including HST, Swift, XMM-Newton, NICER, and ground-based observatories. Early observations with NICER and XMM revealed an X-ray state ten times fainter than historical observations, consistent with the presence of a new dust-free, ionized obscurer. The following analysis of NICER spectra attributes variability in the observed X-ray flux to changes in both the column density of the obscurer by at least one order of magnitude ($N_\mathrm{H}$ ranges from $2.85\substack{+0.48\\ -0.33} \times 10^{22}\text{ cm}^{-2}$ to $25.6\substack{+3.0\\ -3.5} \times 10^{22} \text{ cm}^{-2}$) and the intrinsic continuum brightness (the unobscured flux ranges from $10^{-11.8}$ to $10^{-10.5}$ erg s$^{-1}$ cm$^{-2}$ ). While the X-ray flux generally remains in a faint state, there is one large flare during which Mrk 817 returns to its historical mean flux. The obscuring gas is still present at lower column density during the flare but it also becomes highly ionized, increasing its transparency. Correlation between the column density of the X-ray obscurer and the strength of UV broad absorption lines suggests that the X-ray and UV continua are both affected by the same obscuration, consistent with a clumpy disk wind launched from the inner broad line region.