The photometric variability of massive stars due to gravity waves excited by core convection

Massive stars die in catastrophic explosions that seed the interstellar medium with heavy elements and produce neutron stars and black holes. Predictions of the explosion's character and the remnant mass depend on models of the star's evolutionary history. Models of massive star interiors can be empirically constrained by asteroseismic observations of gravity wave oscillations. Recent photometric observations reveal a ubiquitous red noise signal on massive main sequence stars; a hypothesized source of this noise is gravity waves driven by core convection. We present three-dimensional simulations of massive star convection extending from the star's centre to near its surface, with realistic stellar luminosities. Using these simulations, we predict the photometric variability due to convectively driven gravity waves at the surfaces of massive stars, and find that gravity waves produce photometric variability of a lower amplitude and lower characteristic frequency than the observed red noise. We infer that the photometric signal of gravity waves excited by core convection is below the noise limit of current observations, and thus the red noise must be generated by an alternative process.

Generalized quasi-linear approximation and non-normality in Taylor-Couette spiral turbulence

Taylor-Couette flow is well known to admit a spiral turbulence state in which laminar and turbulent patches coexist around the cylinder. This flow state is quite complex, with delicate internal structure, and it can be traced into certain regimes of linear stability. This behaviour is believed to be connected to the non-normality of the linear operator, which is itself a function of the control parameters. Using spiral turbulence in both linearly stable and unstable regimes, we investigate the effectiveness of the generalized quasi-linear approximation (GQL), an extension of quasi-linear theory designed to capture the essential aspects of turbulent flows. We find that GQL performs much better in the supercritical regime than the subcritical. By including only a small number of modes in the nonlinear interactions, GQL simulations maintain a turbulent-like state when in the supercritical regime. However, a much larger number is required to avoid returning to the laminar state when in the subcritical regime. This article is part of the theme issue 'Taylor-Couette and related flows on the centennial of Taylor's seminal Philosophical Transactions paper (part 1)'.

The magnetorotational instability prefers three dimensions

The magnetorotational instability (MRI) occurs when a weak magnetic field destabilizes a rotating, electrically conducting fluid with inwardly increasing angular velocity. The MRI is essential to astrophysical disc theory where the shear is typically Keplerian. Internal shear layers in stars may also be MRI-unstable, and they take a wide range of profiles, including near-critical. We show that the fastest growing modes of an ideal magnetofluid are three-dimensional provided the shear rate, S, is near the two-dimensional onset value, S _c . For a Keplerian shear, three-dimensional modes are unstable above S ≈ 0.10S _c , and dominate the two-dimensional modes until S ≈ 2.05S _c . These three-dimensional modes dominate for shear profiles relevant to stars and at magnetic Prandtl numbers relevant to liquid-metal laboratory experiments. Significant numbers of rapidly growing three-dimensional modes remainy well past 2.05S _c . These finding are significant in three ways. First, weakly nonlinear theory suggests that the MRI saturates by pushing the shear rate to its critical value. This can happen for systems, such as stars and laboratory experiments, that can rearrange their angular velocity profiles. Second, the non-normal character and large transient growth of MRI modes should be important whenever three-dimensionality exists. Finally, three-dimensional growth suggests direct dynamo action driven from the linear instability.

Flow-Induced Symmetry Breaking in Growing Bacterial Biofilms

Bacterial biofilms represent a major form of microbial life on Earth and serve as a model active nematic system, in which activity results from growth of the rod-shaped bacterial cells. In their natural environments, ranging from human organs to industrial pipelines, biofilms have evolved to grow robustly under significant fluid shear. Despite intense practical and theoretical interest, it is unclear how strong fluid flow alters the local and global architectures of biofilms. Here, we combine highly time-resolved single-cell live imaging with 3D multiscale modeling to investigate the mechanisms by which flow affects the dynamics of all individual cells in growing biofilms. Our experiments and cell-based simulations reveal three quantitatively different growth phases in strong external flow and the transitions between them. In the initial stages of biofilm development, flow induces a downstream gradient in cell orientation, causing asymmetrical dropletlike biofilm shapes. In the later developmental stages, when the majority of cells are sheltered from the flow by the surrounding extracellular matrix, buckling-induced cell verticalization in the biofilm core restores radially symmetric biofilm growth, in agreement with predictions of a 3D continuum model.

Numerical simulations of internal wave generation by convection in water

Water's density maximum at 4°C makes it well suited to study internal gravity wave excitation by convection: an increasing temperature profile is unstable to convection below 4°C, but stably stratified above 4°C. We present numerical simulations of a waterlike fluid near its density maximum in a two-dimensional domain. We successfully model the damping of waves in the simulations using linear theory, provided we do not take the weak damping limit typically used in the literature. To isolate the physical mechanism exciting internal waves, we use the spectral code dedalus to run several simplified model simulations of our more detailed simulation. We use data from the full simulation as source terms in two simplified models of internal-wave excitation by convection: bulk excitation by convective Reynolds stresses, and interface forcing via the mechanical oscillator effect. We find excellent agreement between the waves generated in the full simulation and the simplified simulation implementing the bulk excitation mechanism. The interface forcing simulations overexcite high-frequency waves because they assume the excitation is by the "impulsive" penetration of plumes, which spreads energy to high frequencies. However, we find that the real excitation is instead by the "sweeping" motion of plumes parallel to the interface. Our results imply that the bulk excitation mechanism is a very accurate heuristic for internal-wave generation by convection.

Rapid planetesimal formation in turbulent circumstellar disks

During the initial stages of planet formation in circumstellar gas disks, dust grains collide and build up larger and larger bodies. How this process continues from metre-sized boulders to kilometre-scale planetesimals is a major unsolved problem: boulders are expected to stick together poorly, and to spiral into the protostar in a few hundred orbits owing to a 'headwind' from the slower rotating gas. Gravitational collapse of the solid component has been suggested to overcome this barrier. But even low levels of turbulence will inhibit sedimentation of solids to a sufficiently dense midplane layer, and turbulence must be present to explain observed gas accretion in protostellar disks. Here we report that boulders can undergo efficient gravitational collapse in locally overdense regions in the midplane of the disk. The boulders concentrate initially in transient high pressure regions in the turbulent gas, and these concentrations are augmented a further order of magnitude by a streaming instability driven by the relative flow of gas and solids. We find that gravitationally bound clusters form with masses comparable to dwarf planets and containing a distribution of boulder sizes. Gravitational collapse happens much faster than radial drift, offering a possible path to planetesimal formation in accreting circumstellar disks.

Weight loss prior to clinical AIDS as a predictor of survival. Multicenter AIDS Cohort Study Investigators

In this analysis the aim was to determine the independent effect of moderate to severe weight loss prior to an AIDS diagnosis on survival after AIDS. The study was conducted as part of the Multicenter AIDS Cohort Study (MACS), a longitudinal study of HIV-1-seropositive gay or bisexual men. Measured weight and self-reported weight loss data were collected semiannually from 1984 through 1993. The study population included 962 HIV-1-seropositive men who developed clinical AIDS during the follow-up period. Median survival after AIDS was significantly lower for men with measured weight loss of > or = 4.5 kg 3-9 months and 3-15 months prior to AIDS, or who had lost > 10% of their baseline body weight compared with men with less weight loss or weight gain. Men with self-reported unintentional weight loss of > or = 4.5 kg 3-9 months prior to AIDS had significantly poorer survival (median = 1.05 years vs. 1.48 years; p = 0.0001) compared with men not reporting weight loss. After adjusting for potential confounding factors, men in the high measured weight loss group 3-9 months prior to AIDS still had significantly poorer survival [relative hazard (RH) = 1.36; p = 0.02]. Similar trends were seen for the two longer intervals prior to AIDS (RH = 1.38, p = 0.01; and RH = 1.50, p = 0.02, respectively). Men who self-reported weight loss > or = 4.5 kg 3-9 months prior to AIDS also had significantly poorer survival after AIDS (RH = 1.43; p = 0.002) in multivariate analysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Lymphocytic subpopulation profiles in bronchoalveolar lavage fluid and peripheral blood from tobacco and marijuana smokers

The effect of heavy, habitual marijuana use compared with tobacco smoking on the composition of bronchoalveolar and peripheral blood lymphocytic phenotypes was examined. Bronchoalveolar lavage (BAL) and peripheral blood (PB) samples were taken from 14 nonsmokers (NS), 14 tobacco smokers (TS), 19 heavy, habitual marijuana smokers (MS), and 9 marijuana and tobacco smokers (MTS). In BAL fluid, marijuana use was associated with significantly higher alveolar macrophage concentrations, whereas tobacco smoking was associated with significantly higher alveolar macrophage, as well as higher bronchoalveolar lymphocyte and neutrophil concentrations. The bronchoalveolar T-lymphocytic phenotypic profiles of marijuana users differed from those of tobacco smokers. Tobacco, not marijuana, was found to have a significant effect toward lower percentages of bronchoalveolar CD4 cells, toward higher concentrations of bronchoalveolar CD8 cells, and toward lower bronchoalveolar CD4:CD8 ratios. Marijuana use had a significant effect toward lower percentages of bronchoalveolar CD8 cells. In peripheral blood, marijuana, but not tobacco, use was associated with significantly higher percentages of CD4 cells, lower percentages of CD8 cells, and higher CD4:CD8 ratios. These findings suggest that tobacco and marijuana have effects on bronchoalveolar and peripheral blood immunoregulatory T-lymphocytic subpopulations that differ in type or magnitude.

Bronchoalveolar lavage cell and lymphocyte phenotype profiles in healthy asbestos-exposed shipyard workers

The cellular and lymphocyte phenotypic composition of bronchoalveolar lavage (BAL) fluid and peripheral blood (PB) from 15 healthy, nonsmoking, asbestos-exposed shipyard workers (AEW) and 10 nonsmoking, age-matched unexposed workers (UEW) were compared. None of the AEW had clinical, radiographic, or physiologic evidence of asbestosis, but six had radiographic evidence of pleural plaques and/or thickening. The mean duration of asbestos exposure was 16.3 +/- 2.3 yr, and the mean period since exposure was 10.8 +/- 0.5 yr. All but three of the AEW and none of the UEW had asbestos bodies detected in the first 20 ml of BAL fluid recovered (0.1 to 35 asbestos bodies/ml). The AEW had a significantly higher mean percentage (19.1 +/- 2.8% versus 9.7 +/- 1.6%) and concentration (31.6 +/- 5.2 x 10(3) cells/ml versus 14.7 +/- 2.5 x 10(3) cells/ml) of BAL lymphocytes compared with that in the UEW, with an increased mean concentration of each phenotype measured. In PB, the mean lymphocyte concentration was also higher in the AEW than in the UEW (2.0 +/- 0.3 x 10(3) cells/ml versus 1.5 +/- 0.3 x 10(3) cells/ml), but the difference was not statistically significant, and there was no increase of any phenotype measured. BAL lymphocytosis did not correlate with exposure history or BAL asbestos body count, but was greater in AEW with pleural abnormality (30.1 +/- 2.9% versus 11.8 +/- 1.6%). BAL concentrations of CD-20, CD-3, and CD-4, but not of CD-8 cells were significantly increased compared with those in the AEW without pleural abnormality. Further longitudinal studies are needed to determine the prognostic significance of these findings.

Cellular and T-lymphocyte subpopulation profiles in bronchoalveolar lavage fluid from patients with acquired immunodeficiency syndrome and pneumonitis

The cellular composition and T-lymphocyte subpopulations of bronchoalveolar lavage (BAL) fluid from 12 patients with acquired immunodeficiency syndrome (AIDS) and active pneumonitis were examined. Differential cell counts were performed on BAL specimens from each patient and from 25 normal subjects. The BAL and peripheral blood (PB) lymphocytes were isolated from 8 patients and 11 subjects. Leu 4 (mature T), Leu 2 (T suppressor), and Leu 3 (T helper) markers were identified by fluorescein-labeled monoclonal antibody agents and counted in an automated flow cytometer. Infectious pneumonitis caused by Pneumocystis carinii and/or cytomegalovirus and/or Mycobacterium avium-intracellulare was diagnosed in all but 1 patient. All but 2 patients demonstrated lymphocytosis in the BAL fluid; only 3 had greater than 1% neutrophils. The BAL cell differentials were not predictive of the type of pneumonitis. The Leu 3/Leu 2 ratios were (mean +/- SEM): 0.08 +/- 0.03, patients' BAL fluid; 1.55 +/- 0.21, subjects' BAL fluid; 0.18 +/- 0.06, patients' PB; 1.42 +/- 0.12, subjects' PB. The marked decrease in Leu 3/Leu 2 ratios primarily reflected severely diminished proportions of Leu 3 positive cells (3 +/- 1.3% compared with a control value of 35 +/- 4.0%), although the proportion of Leu 2 positive cells tended to be elevated as well (46 +/- 7.9% compared with a control value of 22 +/- 2.2%). Bronchoalveolar lavage specimens from patients with AIDS and these types of pneumonitis may contain increased proportions of lymphocytes. The accumulation of lymphocytes, however, does not reflect homing of helper T-lymphocytes to the site of pulmonary infection.

Plasma disappearance, urine excretion, and tissue distribution of ribavirin in rats and rhesus monkeys

Ribavirin has been shown to have broad-spectrum antiviral. To study its tissue distribution and disappearance rate, a single dose of 10 mg/kg which contained 10 microCi of [14C]ribavirin was injected intravenously into rhesus monkeys and intramuscularly into monkeys and rats. Except for peak plasma concentrations and the initial phases of the plasma disappearance and urine excretion curves, no significant difference was observed between plasma, tissue, or urine values for intramuscularly or intravenously injected monkeys. Plasma disappearance curves were triphasic; plasma concentrations of ribavirin were similar for both monkeys and rats. Rats excreted ribavirin in the urine more rapidly and to a greater extent (82% excreted in 24 h) than did monkeys (60% excreted in 72 h). In the rat, only 3% of the injected [14C]ribavirin was detected in expired CO2. Therefore, for both species, urine was the major route for the elimination of labeled ribavirin and its metabolites from the body. In monkeys, the amount of parent drug in blood cells increased through 48 h and remained stable for 72 h, whereas in rats, ribavirin decreased at a rate similar to the plasma disappearance curve. Concentrations of ribavirin at 8 h were consistently higher in monkeys than in rats for all tissues except the brain. Thus, these differences in blood cellular components and organ content and in urine excretion suggested that there was greater tissue retention of ribavirin in monkeys than in rats.