33 research outputs found
The Age of the Milky Way Inner Halo
The Milky Way galaxy is observed to have multiple components with distinct
properties, such as the bulge, disk, and halo. Unraveling the assembly history
of these populations provides a powerful test to the theory of galaxy formation
and evolution, but is often restricted due to difficulties in measuring
accurate stellar ages for low mass, hydrogen-burning stars. Unlike these
progenitors, the "cinders" of stellar evolution, white dwarf stars, are
remarkably simple objects and their fundamental properties can be measured with
little ambiguity from spectroscopy. Here I report observations and analysis of
newly formed white dwarf stars in the halo of the Milky Way, and a comparison
to published analysis of white dwarfs in the well-studied 12.5 billion-year-old
globular cluster Messier 4. From this, I measure the mass distribution of the
remnants and invert the stellar evolution process to develop a new relation
that links this final stellar mass to the mass of their immediate progenitors,
and therefore to the age of the parent population. By applying this technique
to a small sample of four nearby and kinematically-confirmed halo white dwarfs,
I measure the age of local field halo stars to be 11.4 +/- 0.7 billion years.
This age is directly tied to the globular cluster age scale, on which the
oldest clusters formed 13.5 billion years ago. Future (spectroscopic)
observations of newly formed white dwarfs in the Milky Way halo can be used to
reduce the present uncertainty, and to probe relative differences between the
formation time of the last clusters and the inner halo.Comment: Published in Nature, 2012, 486, 90. Second version corrects a missing
reference (#10) in the third paragraph and Figure 1 captio
Ages for exoplanet host stars
Age is an important characteristic of a planetary system, but also one that
is difficult to determine. Assuming that the host star and the planets are
formed at the same time, the challenge is to determine the stellar age.
Asteroseismology provides precise age determination, but in many cases the
required detailed pulsation observations are not available. Here we concentrate
on other techniques, which may have broader applicability but also serious
limitations. Further development of this area requires improvements in our
understanding of the evolution of stars and their age-dependent
characteristics, combined with observations that allow reliable calibration of
the various techniques.Comment: To appear in "Handbook of Exoplanets", eds. Deeg, H.J. & Belmonte,
J.A, Springer (2018
Possible explosion crater origin of small lake basins with raised rims on Titan
The Cassini mission discovered lakes and seas comprising mostly methane in the polar regions of Titan. Lakes of liquid nitrogen may have existed during the epochs of Titan’s past in which methane was photochemically depleted, leaving a nearly pure molecular nitrogen atmosphere and, thus, far colder temperatures. The modern-day small lake basins with sharp edges have been suggested to originate from dissolution processes, due to their morphological similarity to terrestrial karstic lakes. Here we analyse the morphology of the small lake basins that feature raised rims to elucidate their origin, using delay-Doppler processed altimetric and bathymetric data acquired during the last close flyby of Titan by the Cassini spacecraft. We find that the morphology of the raised-rim basins is analogous to that of explosion craters from magma–water interaction on Earth and therefore propose that these basins are from near-surface vapour explosions, rather than karstic. We calculate that the phase transition of liquid nitrogen in the near subsurface during a warming event can generate explosions sufficient to form the basins. Hence, we suggest that raised-rim basins are evidence for one or more warming events terminating a nitrogen-dominated cold episode on Titan
Phenomenology of glider collisions in cellular automaton Rule 54 and associated logical gates
Rule 54, a two state, three neighbor cellular automaton in Wolfram's systems of nomenclature, is less complex that Rule 110, but nevertheless possess a rich and complex dynamics. We provide a systematic and exhaustive analysis of glider behavior and interactions, including a catalog of collisions. Many of them shows promise are computational elements. © 2005 Elsevier Ltd. All rights reserved