64 research outputs found
Multiple Stellar Populations in the Globular Cluster omega Centauri as Tracers of a Merger Event
The discovery of the Sagittarius dwarf galaxy, which is being tidally
disrupted by and merging with the Milky Way, supports the view that the halo of
the Galaxy has been built up at least partially by the accretion of similar
dwarf systems. The Sagittarius dwarf contains several distinct populations of
stars, and includes M54 as its nucleus, which is the second most massive
globular cluster associated with the Milky Way. The most massive globular
cluster is omega Centauri, and here we report that omega Centauri also has
several distinct stellar populations, as traced by red-giant-branch stars. The
most metal-rich red-giant-branch stars are about 2 Gyr younger than the
dominant metal-poor component, indicating that omega Centauri was enriched over
this timescale. The presence of more than one epoch of star formation in a
globular cluster is quite surprising, and suggests that omega Centauri was once
part of a more massive system that merged with the Milky Way, as the
Sagittarius dwarf galaxy is in the process of doing now. Mergers probably were
much more frequent in the early history of the Galaxy and omega Centauri
appears to be a relict of this era.Comment: 7 pages, 3 figures, Latex+nature.sty (included), To appear in
November 4th issue of Natur
The RR Lyrae Distance Scale
We review seven methods of measuring the absolute magnitude M_V of RR Lyrae
stars in light of the Hipparcos mission and other recent developments. We focus
on identifying possible systematic errors and rank the methods by relative
immunity to such errors. For the three most robust methods, statistical
parallax, trigonometric parallax, and cluster kinematics, we find M_V (at
[Fe/H] = -1.6) of 0.77 +/- 0.13, 0.71 +/- 0.15, 0.67 +/- 0.10. These methods
cluster consistently around 0.71 +/- 0.07. We find that Baade-Wesselink and
theoretical models both yield a broad range of possible values (0.45-0.70 and
0.45-0.65) due to systematic uncertainties in the temperature scale and input
physics. Main-sequence fitting gives a much brighter M_V = 0.45 +/- 0.04 but
this may be due to a difference in the metallicity scales of the cluster giants
and the calibrating subdwarfs. White-dwarf cooling-sequence fitting gives 0.67
+/- 0.13 and is potentially very robust, but at present is too new to be fully
tested for systematics. If the three most robust methods are combined with
Walker's mean measurement for 6 LMC clusters, V_{0,LMC} = 18.98 +/- 0.03 at
[Fe/H] = -1.9, then mu_{LMC} = 18.33 +/- 0.08.Comment: Invited review article to appear in: `Post-Hipparcos Cosmic Candles',
A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in press. 21
pages including 1 table; uses Kluwer's crckapb.sty LaTeX style file, enclose
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Global lake thermal regions shift under climate change
Water temperature is critical for the ecology of lakes. However, the ability to predict its spatial and seasonal variation is constrained by the lack of a thermal classification system. Here we define lake thermal regions using objective analysis of seasonal surface temperature dynamics from satellite observations. Nine lake thermal regions are identified that mapped largely contiguously globally, and robustly even for small lakes. The regions differed from other global patterns, and so provide unique information. Using a lake model forced by 21st century climate projections we found that 12, 27 and 66% of lakes will change to a lower latitude thermal region by 2080-2099 for low, medium and high greenhouse gas concentration trajectories (Representative Concentration Pathways 2.6, 6.0 and 8.5) respectively. Under the worst-case scenario, a 79% reduction in the number of lakes in the northernmost thermal region is projected. This thermal region framework will facilitate the global scaling of lake-research
Genetic variability of hepatitis C virus before and after combined therapy of interferon plus ribavirin
We present an analysis of the selective forces acting on two hepatitis C virus genome regions previously postulated to be involved in the viral response to combined antiviral therapy. One includes the three hypervariable regions in the envelope E2 glycoprotein, and the other encompasses the PKR binding domain and the V3 domain in the NS5A region. We used a cohort of 22 non-responder patients to combined therapy (interferon alpha-2a plus ribavirin) for which samples were obtained before initiation of therapy and after 6 or/and 12 months of treatment. A range of 25-100 clones per patient, genome region and time sample were sequenced. These were used to detect general patterns of adaptation, to identify particular adaptation mechanisms and to analyze the patterns of evolutionary change in both genome regions. These analyses failed to detect a common adaptive mechanism for the lack of response to antiviral treatment in these patients. On the contrary, a wide range of situations were observed, from patients showing no positively selected sites to others with many, and with completely different topologies in the reconstructed phylogenetic trees. Altogether, these results suggest that viral strategies to evade selection pressure from the immune system and antiviral therapies do not result from a single mechanism and they are likely based on a range of different alternatives, in which several different changes, or their combination, along the HCV genome confer viruses the ability to overcome strong selective [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]
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