31 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
Distances and ages of globular clusters using Hipparcos parallaxes of local subdwarfs
We discuss the impact of Population II and Globular Cluster (GCs) stars on
the derivation of the age of the Universe, and on the study of the formation
and early evolution of galaxies, our own in particular. The long-standing
problem of the actual distance scale to Population II stars and GCs is
addressed, and a variety of different methods commonly used to derive distances
to Population II stars are briefly reviewed. Emphasis is given to the
discussion of distances and ages for GCs derived using Hipparcos parallaxes of
local subdwarfs. Results obtained by different authors are slightly different,
depending on different assumptions about metallicity scale, reddenings, and
corrections for undetected binaries. These and other uncertainties present in
the method are discussed. Finally, we outline progress expected in the near
future.Comment: Invited review article to appear in: `Post-Hipparcos Cosmic Candles',
A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in press. 22
pages including 3 tables and 2 postscript figures, uses Kluwer's crckapb.sty
LaTeX style file, enclose
The stellar halo of the Galaxy
Stellar halos may hold some of the best preserved fossils of the formation
history of galaxies. They are a natural product of the merging processes that
probably take place during the assembly of a galaxy, and hence may well be the
most ubiquitous component of galaxies, independently of their Hubble type. This
review focuses on our current understanding of the spatial structure, the
kinematics and chemistry of halo stars in the Milky Way. In recent years, we
have experienced a change in paradigm thanks to the discovery of large amounts
of substructure, especially in the outer halo. I discuss the implications of
the currently available observational constraints and fold them into several
possible formation scenarios. Unraveling the formation of the Galactic halo
will be possible in the near future through a combination of large wide field
photometric and spectroscopic surveys, and especially in the era of Gaia.Comment: 46 pages, 16 figures. References updated and some minor changes.
Full-resolution version available at
http://www.astro.rug.nl/~ahelmi/stellar-halo-review.pd
The Distances of the Magellanic Clouds
The present status of our knowledge of the distances to the Magellanic Clouds
is evaluated from a post-Hipparcos perspective. After a brief summary of the
effects of structure, reddening, age and metallicity, the primary distance
indicators for the Large Magellanic Cloud are reviewed: The SN 1987A ring,
Cepheids, RR Lyraes, Mira variables, and Eclipsing Binaries. Distances derived
via these methods are weighted and combined to produce final "best" estimates
for the Magellanic Clouds distance moduli.Comment: Invited review article to appear in ``Post Hipparcos Cosmic
Candles'', F. Caputo & A. Heck (Eds.), Kluwer Academic Publ., Dordrecht, in
pres
Influence of aerobic fitness on gastrointestinal barrier integrity and microbial translocation following a fixed-intensity military exertional heat stress test
Purpose: Exertional-heat stress adversely disrupts gastrointestinal (GI) barrier integrity, whereby subsequent microbial translocation (MT) can result in potentially serious health consequences. To date, the influence of aerobic fitness on GI barrier integrity and MT following exertional-heat stress is poorly characterised. Method: Ten untrained (UT; VO2maxâ=â45â±â3 ml·kgâ1·minâ1) and ten highly trained (HT; VO2maxâ=â64â±â4 ml·kgâ1·minâ1) males completed an ecologically valid (military) 80-min fixed-intensity exertional-heat stress test (EHST). Venous blood was drawn immediately pre- and post-EHST. GI barrier integrity was assessed using the serum dual-sugar absorption test (DSAT) and plasma Intestinal Fatty-Acid Binding Protein (I-FABP). MT was assessed using plasma Bacteroides/total 16S DNA. Results: UT experienced greater thermoregulatory, cardiovascular and perceptual strain (pâ<â0.05) than HT during the EHST. Serum DSAT responses were similar between the two groups (pâ=â0.59), although Î I-FABP was greater (pâ=â0.04) in the UT (1.14â±â1.36 ng·mlâ1) versus HT (0.20â±â0.29 ng·mlâ1) group. Bacteroides/Total 16S DNA ratio was unchanged (Î; -0.04â±â0.18) following the EHST in the HT group, but increased (Î; 0.19â±â0.25) in the UT group (pâ=â0.05). Weekly aerobic training hours had a weak, negative correlation with Î I-FABP and Bacteroides/total 16S DNA responses. Conclusion: When exercising at the same absolute workload, UT individuals are more susceptible to small intestinal epithelial injury and MT than HT individuals. These responses appear partially attributable to greater thermoregulatory, cardiovascular, and perceptual strain
Debris streams in the solar neighbourhood as relicts from the formation of the Milky Way
It is now generally believed that galaxies were built up through gravitational amplification of primordial fluctuations and the subsequent merging of smaller precursor structures. The stars of the structures that assembled to form the Milky Way should now make up much or all of its bulge and halo, in which case one hopes to find âfossil â evidence for those precursor structures in the present distribution of halo stars. Confirmation that this process is continuing came with the discovery of the Sagittarius dwarf galaxy, which is being disrupted by the Milky Way, but direct evidence that this process provided the bulk of the Milky Wayâs population of old stars has so far been lacking. Here we show that about ten per cent of the metalâpoor stars in the halo of the Milky Way, outside the radius of the Sunâs orbit, come from a single coherent structure that was disrupted during or soon after the Galaxyâs formation. This object had a highly inclined orbit about the Milk