* Astronomy

Blobrana · L

RE: Globular Clusters

Blobrana · L

Title: The origin of the Milky Way globular clusters
Author: Florent Renaud, Oscar Agertz, Mark Gieles

We present a cosmological zoom-in simulation of a Milky Way-like galaxy used to explore the formation and evolution of star clusters. We investigate in particular the origin of the bimodality observed in the colour and metallicity of globular clusters, and the environmental evolution through cosmic times in the form of tidal tensors. Our results self-consistently confirm previous findings that the blue, metal-poor clusters form in satellite galaxies which are accreted onto the Milky Way, while the red, metal-rich clusters form mostly in situ or, to a lower extent in massive, self-enriched galaxies merging with the Milky Way. By monitoring the tidal fields these populations experience, we find that clusters formed in situ (generally centrally concentrated) feel significantly stronger tides than the accreted ones, both in the present-day, and when averaged over their entire life. Furthermore, we note that the tidal field experienced by Milky Way clusters is significantly weaker in the past than at present-day, confirming that it is unlikely that a power-law cluster initial mass function like that of young massive clusters, is transformed into the observed peaked distribution in the Milky Way with relaxation-driven evaporation in a tidal field.

Read more (3550kb, PDF)

Blobrana · L

The Dark Side of Star Clusters

Observations with ESO's Very Large Telescope in Chile have discovered a new class of "dark" globular star clusters around the giant galaxy Centaurus A. These mysterious objects look similar to normal clusters, but contain much more mass and may either harbour unexpected amounts of dark matter, or contain massive black holes - neither of which was expected nor is understood.
Read more

Blobrana · L

The riddle of the missing stars: Hubble observations cast further doubt on how globular clusters formed

Thanks to the NASA/ESA Hubble Space Telescope, some of the most mysterious cosmic residents have just become even more puzzling. New observations of globular clusters in a small galaxy show they are very similar to those found in the Milky Way, and so must have formed in a similar way. One of the leading theories on how these clusters form predicts that globular clusters should only be found nestled in among large quantities of old stars. But these old stars, though rife in the Milky Way, are not present in this small galaxy, and so, the mystery deepens.
Read more

dave19128 · Member

Globular Cluster Condensation and other Phase-change Condensations in the Universe:

Galaxies may have nucleated following BBN like water droplets condensing on dust and pollen grains in rain clouds by gravitational compression endothermically driving BBN backwards, promoting nearly isothermal collapse, condensing galaxies from the continuum. And this may have occurred in the early universe when the density of the continuum was on the order of galaxy-scale masses fitting within stellar-scale spaces.

Likewise, globular clusters may have condensed within newly-nucleated galaxies during localized endothermic 'helium reionization' (before hydrogen reionization) events, due to gravitational compression.

And 'globules' may have condensed within galaxies (and globular clusters) during endothermic reionization of hydrogen caused by gravitational compression, where invisible primordial globules = cold dark matter. Primordial globules contaminated by stellar metallicity in galactic disks become opaque 'Bok globules', and their outgassing in the form of cometary tails and elephant trunks eroded by OB supergiant stars create (giant) molecular clouds.

And stars condense within metallicity-contaminated Bok globules during endothermic reionization of hydrogen due to gravitational compression, forming their second hydrostatic cores (Larson 1969).

Finally, hot-Jupiter gas-giant planets condense from outer stellar layers of their progenitor stars when their host stars undergo endothermic hydrogen-reionization collapse, forming their second hydrostatic cores.

Ed ~ The latest computer simulations have given very realistic results - and unsurprisingly hot and cold darkmatter (in correct quantities) play a vital role in galaxy and large scale structure formation.

Blobrana · L

Title: Fluorine abundances and the puzzle of globular cluster chemical history
Authors: P. de Laverny, A. Recio-Blanco

The abundance of fluorine in a few Galactic globular clusters is known to strongly vary from star-to-star. These unexpected chemical properties are an additional confirmation of the chemical inhomogeneities already found in several GC, and probably caused by the first generations of stars formed in these systems. The aim of this article is to complement our understanding of the F-behaviour in GC stars and to look for new constraints on the formation histories of their multiple stellar populations. We have collected near-IR spectra of 15 RGB stars belonging to GC spanning a wide range of metallicity: 47 Tuc, M4, NGC6397 and M30. F, Na and Fe abundances have been estimated by spectral synthesis. No anticorrelation between F and Na abundances are found for the most metal-rich cluster of the sample (47 Tuc). In this GC, RGB stars indeed exhibit rather small differences in [F/Fe] unlike the larger ones found for the [Na/Fe] ratios. This reveals a rather inhomogeneous stellar system and a complex chemical evolution history for 47 Tuc . In M4, one star of our study confirms the previous Na-F distribution reported by another group in 2005. For the two very metal-poor GC (NGC6397 and M30), only upper limits of F abundances have been derived. We show that F abundances could be estimated in such metal-poor GC with current telescopes and spectrographs only if unexpected F-rich giants are found and/or exceptional observational conditions are met. The distribution of the F and Na abundances in GC reveal that their RGB members seem to belong to two well-separated regions. All the RGB stars analysed so far in the different GC are indeed found to be either F-rich Na-poor or F-poor Na-rich. Such well-separated bimodal regimes are consistent with the separate formation episodes suspected in most galactic GC.

Read more (88kb, PDF)

Blobrana · L

Stars Reveal the Secrets of Looking Young

Some people are in great shape at the age of 90, while others are decrepit before they're 50. We know that how fast people age is only loosely linked to how old they actually are - and may have more to do with their lifestyle. A new study using both the MPG/ESO 2.2-metre telescope at ESO's La Silla Observatory and the NASA/ESA Hubble Space Telescope reveals that the same is true of star clusters.
Globular clusters are spherical collections of stars, tightly bound to each other by their mutual gravity. Relics of the early years of the Universe, with ages of typically 12-13 billion years (the Big Bang took place 13.7 billion years ago), there are roughly 150 globular clusters in the Milky Way and they contain many of our galaxy's oldest stars.
Read more

Blobrana · L

Title: How did globular clusters lose their gas?
Authors: C.Charbonnel, M.Krause, T.Decressin, G.Meynet, N.Prantzos, R.Diehl

We summarise the results presented in Krause et al. (2012) on the impact of supernova-driven shells and dark-remnant accretion on gas expulsion in globular cluster infancy.

Read more (156kb, PDF)

Blobrana · L

Title: Is there a Size Difference between Red and Blue Globular Clusters?
Authors: J. M. B. Downing

Blue (metal-poor) globular clusters are observed to have half-light radii that are ~20% larger than their red (metal-rich) counterparts. The origin of this enhancement is not clear and differences in either the luminosity function or in the actual size of the clusters have been proposed. I analyse a set of dynamically self-consistent Monte Carlo globular cluster simulations to determine the origin of this enhancement. I find that my simulated blue clusters have larger half-light radii due to differences in the luminosity functions of metal-poor and metal-rich stars. I find that the blue clusters can also be physically larger, but only if they have a substantial number of black holes heating their central regions. In this case the difference between half-light radii is significantly larger than observed. I conclude that the observed difference in half-light radii between red and blue globular clusters is due to differences in their luminosity functions and that half-light radius is not a reliable proxy for cluster size.

Read more (347kb, PDF)

Blobrana · L

Why ancient star clusters are all the same size

Why do the universe's oldest star clusters tend to be roughly the same size? New simulations suggest it's because galaxy mergers destroyed the smaller ones.
Globular star clusters are ancient, spherical blobs of stars. Most are a few hundred thousand times the mass of our sun. The scarcity of much bigger clusters is no surprise, as they would form more rarely - but why are there so few small ones?
Read more