87 research outputs found
Light Element Abundance Inhomogeneities in Globular Clusters: Probing Star Formation and Evolution in the Early Milky Way
Abundance patterns of the elements C, N, and O are sensitive probes of
stellar nucleosynthesis processes and, in addition, O abundances are an
important input for stellar age determinations. Understanding the nature of the
observed distribution of these elements is key to constraining protogalactic
star formation history. Patterns deduced from low-resolution spectroscopy of
the CN, CH, NH, and CO molecules for low-mass stars in their core-hydrogen or
first shell-hydrogen burning phases in the oldest ensembles known, the Galactic
globular star clusters, are reviewed. New results for faint stars in NGC 104
(47 Tuc, C0021-723) reveal that the bimodal, anticorrelated pattern of CN and
CH strengths found among luminous evolved stars is also present in stars
nearing the end of their main-sequence lifetimes. In the absence of known
mechanisms to mix newly synthesized elements from the interior to the
observable surface layers of such unevolved stars, those particular
inhomogeneities imply that the original material from which the stars formed
some 15 billion years ago was chemically inhomogeneous in the C and N elements.
However, in other clusters, observations of abundance ratios and C isotope
ratios suggest that alterations to surface chemical compositions are produced
as stars evolve from the main sequence through the red giant branch. Thus, the
current observed distributions of C, N, and O among the brightest stars (those
also observed most often) may not reflect the true distribution from which the
protocluster cloud formed. The picture which is emerging of the C, N and O
abundance patterns within globular clusters may be one whichComment: 12 pages in uuencoded compressed postscript (including figures), to
appear in the Canadian Journal of Physics (Special Issue in Honor of G.
Herzberg
Mapping the Galactic Halo I. The `Spaghetti' Survey
We describe a major survey of the Milky Way halo designed to test for
kinematic substructure caused by destruction of accreted satellites. We use the
Washington photometric system to identify halo stars efficiently for
spectroscopic followup. Tracers include halo giants (detectable out to more
than 100 kpc), blue horizontal branch stars, halo stars near the main sequence
turnoff, and the ``blue metal-poor stars'' of Preston et al (1994). We
demonstrate the success of our survey by showing spectra of stars we have
identified in all these categories, including giants as distant as 75 kpc. We
discuss the problem of identifying the most distant halo giants. In particular,
extremely metal-poor halo K dwarfs are present in approximately equal numbers
to the distant giants for V fainter than 18, and we show that our method will
distinguish reliably between these two groups of metal-poor stars. We plan to
survey 100 square degrees at high galactic latitude, and expect to increase the
numbers of known halo giants, BHB stars and turnoff stars by more than an order
of magnitude. In addition to the strong test that this large sample will
provide for the question `was the Milky Way halo accreted from satellite
galaxies?', we will improve the accuracy of mass measurements of the Milky Way
beyond 50 kpc via the kinematics of the many distant giants and BHB stars we
will find. We show that one of our first datasets constrains the halo density
law over galactocentric radii of 5-20 kpc and z heights of 2-15 kpc. The data
support a flattened power-law halo with b/a of 0.6 and exponent -3.0. More
complex models with a varying axial ratio may be needed with a larger dataset.Comment: 55 pages, 22 figures, to appear in the Astronomical Journa
Study of HST counterparts to Chandra X-ray sources in the Globular Cluster M71
We report on archival Hubble Space Telescope (HST) observations of the
globular cluster M71 (NGC 6838). These observations, covering the core of the
globular cluster, were performed by the Advanced Camera for Surveys (ACS) and
the Wide Field Planetary Camera 2 (WFPC2). Inside the half-mass radius (r_h =
1.65') of M71, we find 33 candidate optical counterparts to 25 out of 29
Chandra X-ray sources while outside the half-mass radius, 6 possible optical
counterparts to 4 X-ray sources are found. Based on the X-ray and optical
properties of the identifications, we find 1 certain and 7 candidate
cataclysmic variables (CVs). We also classify 2 and 12 X-ray sources as certain
and potential chromospherically active binaries (ABs), respectively. The only
star in the error circle of the known millisecond pulsar (MSP) is inconsistent
with being the optical counterpart. The number of X-ray faint sources with
L_x>4x10^{30} ergs/s (0.5-6.0 keV) found in M71 is higher than extrapolations
from other clusters on the basis of either collision frequency or mass. Since
the core density of M71 is relatively low, we suggest that those CVs and ABs
are primordial in origin.Comment: 12 pages, 6 figures. Accepted for publication in Astronomy and
Astrophysic
Multiple populations in globular clusters. Lessons learned from the Milky Way globular clusters
Recent progress in studies of globular clusters has shown that they are not
simple stellar populations, being rather made of multiple generations. Evidence
stems both from photometry and spectroscopy. A new paradigm is then arising for
the formation of massive star clusters, which includes several episodes of star
formation. While this provides an explanation for several features of globular
clusters, including the second parameter problem, it also opens new
perspectives about the relation between globular clusters and the halo of our
Galaxy, and by extension of all populations with a high specific frequency of
globular clusters, such as, e.g., giant elliptical galaxies. We review progress
in this area, focusing on the most recent studies. Several points remain to be
properly understood, in particular those concerning the nature of the polluters
producing the abundance pattern in the clusters and the typical timescale, the
range of cluster masses where this phenomenon is active, and the relation
between globular clusters and other satellites of our Galaxy.Comment: In press (The Astronomy and Astrophysics Review
Impact of distance determinations on Galactic structure. II. Old tracers
Here we review the efforts of a number of recent results that use old tracers
to understand the build up of the Galaxy. Details that lead directly to using
these old tracers to measure distances are discussed. We concentrate on the
following: (1) the structure and evolution of the Galactic bulge and inner
Galaxy constrained from the dynamics of individual stars residing therein; (2)
the spatial structure of the old Galactic bulge through photometric
observations of RR Lyrae-type stars; (3) the three\--dimensional structure,
stellar density, mass, chemical composition, and age of the Milky Way bulge as
traced by its old stellar populations; (4) an overview of RR Lyrae stars known
in the ultra-faint dwarfs and their relation to the Galactic halo; and (5)
different approaches for estimating absolute and relative cluster ages.Comment: Review article, 80 pages (25 figures); Space Science Reviews, in
press (chapter of a special collection resulting from the May 2016 ISSI-BJ
workshop on Astronomical Distance Determination in the Space Age
Blue Straggler Stars: Early Observations that Failed to Solve the Problem
In this chapter, I describe early ideas on blue stragglers, and various
observations (some published, some not) that promised but failed to resolve the
question of their origin. I review the data and ideas that were circulating
from Allan Sandage's original discovery in 1953 of "anomalous blue stars" in
the globular cluster M3, up until about 1992, when what seems to have been the
only previous meeting devoted to Blue Straggler Stars (BSSs) was held at the
Space Telescope Science Institute.Comment: Chapter 2, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
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