44 research outputs found
Linking dwarf galaxies to halo building blocks with the most metal-poor star in Sculptor
Current cosmological models indicate that the Milky Way's stellar halo was
assembled from many smaller systems. Based on the apparent absence of the most
metal-poor stars in present-day dwarf galaxies, recent studies claimed that the
true Galactic building blocks must have been vastly different from the
surviving dwarfs. The discovery of an extremely iron-poor star (S1020549) in
the Sculptor dwarf galaxy based on a medium-resolution spectrum cast some doubt
on this conclusion. However, verification of the iron-deficiency and
measurements of additional elements, such as the alpha-element Mg, are
mandatory for demonstrating that the same type of stars produced the metals
found in dwarf galaxies and the Galactic halo. Only then can dwarf galaxy stars
be conclusively linked to early stellar halo assembly. Here we report
high-resolution spectroscopic abundances for 11 elements in S1020549,
confirming the iron abundance of less than 1/4000th that of the Sun, and
showing that the overall abundance pattern mirrors that seen in low-metallicity
halo stars, including the alpha-elements. Such chemical similarity indicates
that the systems destroyed to form the halo billions of years ago were not
fundamentally different from the progenitors of present-day dwarfs, and
suggests that the early chemical enrichment of all galaxies may be nearly
identical.Comment: 16 pages, including 2 figures. Accepted for publication in Nature. It
is embargoed for discussion in the press until formal publication in Natur
Transiting extrasolar planetary candidates in the Galactic bulge
More than 200 extrasolar planets have been discovered around relatively
nearby stars, primarily through the Doppler line shifts owing to the reflex
motions of their host stars, and more recently through transits of some planets
across the face of the host stars. The detection of planets with the shortest
known periods, 1.2 to 2.5 days, has mainly resulted from transit surveys which
have generally targeted stars more massive than 0.75 M_sun. Here we report the
results from a planetary transit search performed in a rich stellar field
towards the Galactic bulge. We discovered 16 candidates with orbital periods
between 0.4 and 4.2 days, five of which orbit stars of 0.44 to 0.75 M_sun. In
two cases, radial-velocity measurements support the planetary nature of the
companions. Five candidates have orbital periods below 1.0 day, constituting a
new class of ultra-short-period planets (USPPs), which occur only around stars
of less than 0.88 M_sun. This indicates that those orbiting very close to more
luminous stars might be evaporatively destroyed, or that jovian planets around
lower-mass stars might migrate to smaller radii.Comment: To appear in October 5, 2006 issue of Natur
A search for new members of the β Pictoris, Tucana–Horologium and ε Cha moving groups in the RAVE data base
We report on the discovery of new members of nearby young moving groups, exploiting the full power of combining the Radial Velocity Experiment (RAVE) survey with several stellar age diagnostic methods and follow-up high-resolution optical spectroscopy. The results include the identification of one new and five likely members of the β Pictoris moving group, ranging from spectral types F9 to M4 with the majority being M dwarfs, one K7 likely member of the ε Cha group and two stars in the Tucana–Horologium association. Based on the positive identifications, we foreshadow a great potential of the RAVE data base in progressing towards a full census of young moving groups in the solar neighbourhood
Exploring the Universe with Metal-Poor Stars
The early chemical evolution of the Galaxy and the Universe is vital to our
understanding of a host of astrophysical phenomena. Since the most metal-poor
Galactic stars (with metallicities down to [Fe/H]\sim-5.5) are relics from the
high-redshift Universe, they probe the chemical and dynamical conditions of the
Milky Way and the origin and evolution of the elements through nucleosynthesis.
They also provide constraints on the nature of the first stars, their
associated supernovae and initial mass function, and early star and galaxy
formation. The Milky Way's dwarf satellites contain a large fraction (~30%) of
the known most metal-poor stars that have chemical abundances that closely
resemble those of equivalent halo stars. This suggests that chemical evolution
may be universal, at least at early times, and that it is driven by massive,
energetic SNe. Some of these surviving, ultra-faint systems may show the
signature of just one such PopIII star; they may even be surviving first
galaxies. Early analogs of the surviving dwarfs may thus have played an
important role in the assembly of the old Galactic halo whose formation can now
be studied with stellar chemistry. Following the cosmic evolution of small
halos in simulations of structure formation enables tracing the cosmological
origin of the most metal-poor stars in the halo and dwarf galaxies. Together
with future observations and additional modeling, many of these issues,
including the reionization history of the Milky Way, may be constrained this
way. The chapter concludes with an outlook about upcoming observational
challenges and ways forward is to use metal-poor stars to constrain theoretical
studies.Comment: 34 pages, 11 figures. Book chapter to appear in "The First Galaxies -
Theoretical Predictions and Observational Clues", 2012 by Springer, eds. V.
Bromm, B. Mobasher, T. Wiklin
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 American Astronomical Society, find out more The Institute of Physics, find out more The Sixth Data Release of the Radial Velocity Experiment (Rave). II. Stellar Atmospheric Parameters, Chemical Abundances, and Distances
We present part 2 of the 6th and final Data Release (DR6 or FDR) of the Radial Velocity Experiment (RAVE), a magnitude-limited (9<I<12) spectroscopic survey of Galactic stars randomly selected in the southern hemisphere. The RAVE medium-resolution spectra (R~7500) cover the Ca-triplet region (8410-8795A) and span the complete time frame from the start of RAVE observations on 12 April 2003 to their completion on 4 April 2013. In the second of two publications, we present the data products derived from 518387 observations of 451783 unique stars using a suite of advanced reduction pipelines focussing on stellar atmospheric parameters, in particular purely spectroscopically derived stellar atmospheric parameters (Teff, log(g), and the overall metallicity), enhanced stellar atmospheric parameters inferred via a Bayesian pipeline using Gaia DR2 astrometric priors, and asteroseismically calibrated stellar atmospheric parameters for giant stars based on asteroseismic observations for 699 K2 stars. In addition, we provide abundances of the elements Fe, Al, and Ni, as well as an overall [alpha/Fe] ratio obtained using a new pipeline based on the GAUGUIN optimization method that is able to deal with variable signal-to-noise ratios. The RAVE DR6 catalogs are cross matched with relevant astrometric and photometric catalogs, and are complemented by orbital parameters and effective temperatures based on the infrared flux method. The data can be accessed via the RAVE Web site (http://rave-survey.org) or the Vizier database
Metal-Poor Stars and the Chemical Enrichment of the Universe
Metal-poor stars hold the key to our understanding of the origin of the
elements and the chemical evolution of the Universe. This chapter describes the
process of discovery of these rare stars, the manner in which their surface
abundances (produced in supernovae and other evolved stars) are determined from
the analysis of their spectra, and the interpretation of their abundance
patterns to elucidate questions of origin and evolution. More generally,
studies of these stars contribute to other fundamental areas that include
nuclear astrophysics, conditions at the earliest times, the nature of the first
stars, and the formation and evolution of galaxies -- including our own Milky
Way. We illustrate this with results from studies of lithium formed during the
Big Bang; of stars dated to within ~1 Gyr of that event; of the most metal-poor
stars, with abundance signatures very different from all other stars; and of
the build-up of the elements over the first several Gyr. The combination of
abundance and kinematic signatures constrains how the Milky Way formed, while
recent discoveries of extremely metal-poor stars in the Milky Way's dwarf
galaxy satellites constrain the hierarchical build-up of its stellar halo from
small dark-matter dominated systems. [abridged]Comment: Book chapter, emulated version, 34 pages; number of references are
limited by publisher; to appear in Vol. 5 of textbook "Planets, Stars and
Stellar Systems", by Springer, in 201
Testing evaluations to the limits: the case of English Health Action Zones
Since the Blair government was elected in Britian in May 1997, there has been an explosion of area-based initiatives aimed at regenerating deprived communities and reducing poverty and social exclusion. All are partnerships that place a high value on collaboration, in contrast to the previous government's faith in competition and quasi-markets. At the same time, these new experiments in 'joined-up government' are expected to put the interests of consumers and communities before the innate conservatism of providers of services. In addition, many of the new area-based initiatives are charged with the responsibility for 'constantly reviewing, evaluating and, where necessary, adapting policies to ensure they achieve the maximum effect