9 research outputs found
Constraining the cometary flux through the asteroid belt during the late heavy bombardment
In the Nice model, the late heavy bombardment (LHB) is related to an orbital
instability of giant planets which causes a fast dynamical dispersion of a
transneptunian cometary disk. We study effects produced by these hypothetical
cometary projectiles on main-belt asteroids. In particular, we want to check
whether the observed collisional families provide a lower or an upper limit for
the cometary flux during the LHB.
We present an updated list of observed asteroid families as identified in the
space of synthetic proper elements by the hierarchical clustering method,
colour data, albedo data and dynamical considerations and we estimate their
physical parameters. We selected 12 families which may be related to the LHB
according to their dynamical ages. We then used collisional models and N-body
orbital simulations to gain insight into the long-term dynamical evolution of
synthetic LHB families over 4 Gyr. We account for the mutual collisions, the
physical disruptions of comets, the Yarkovsky/YORP drift, chaotic diffusion, or
possible perturbations by the giant-planet migration.
Assuming a "standard" size-frequency distribution of primordial comets, we
predict the number of families with parent-body sizes D_PB >= 200 km which
seems consistent with observations. However, more than 100 asteroid families
with D_PB >= 100 km should be created at the same time which are not observed.
This discrepancy can be nevertheless explained by the following processes: i)
asteroid families are efficiently destroyed by comminution (via collisional
cascade), ii) disruptions of comets below some critical perihelion distance (q
<~ 1.5 AU) are common.
Given the freedom in the cometary-disruption law, we cannot provide stringent
limits on the cometary flux, but we can conclude that the observed distribution
of asteroid families does not contradict with a cometary LHB.Comment: accepted in Astronomy and Astrophysic
Conservation of HIV-1 T cell epitopes across time and clades:validation of immunogenic HLA-A2 epitopes selected for the GAIA HIV vaccine
HIV genomic sequence variability has complicated efforts to generate an effective globally relevant vaccine. Regions of the viral genome conserved in sequence and across time may represent the “Achilles’ heel” of HIV. In this study, highly conserved T-cell epitopes were selected using immunoinformatics tools combining HLA-A2 supertype binding predictions with relative global conservation. Analysis performed in 2002 on 10,803 HIV-1 sequences, and again in 2009, on 43,822 sequences, yielded 38 HLA-A2 epitopes. These epitopes were experimentally validated for HLA binding and immunogenicity with PBMCs from HIV-infected patients in Providence, Rhode Island, and/or Bamako, Mali. Thirty-five (92%) stimulated an IFNγ response in PBMCs from at least one subject. Eleven of fourteen peptides (79%) were confirmed as HLA-A2 epitopes in both locations. Validation of these HLA-A2 epitopes conserved across time, clades, and geography supports the hypothesis that such epitopes could provide effective coverage of virus diversity and would be appropriate for inclusion in a globally relevant HIV vaccine