1,555 research outputs found
Segue 1: An Unevolved Fossil Galaxy from the Early Universe
We present Magellan/MIKE and Keck/HIRES high-resolution spectra of six red
giant stars in the dwarf galaxy Segue 1. Including one additional Segue 1 star
observed by Norris et al. (2010), high-resolution spectra have now been
obtained for every red giant in Segue 1. Remarkably, three of these seven stars
have metallicities below [Fe/H] = -3.5, suggesting that Segue 1 is the least
chemically evolved galaxy known. We confirm previous medium-resolution analyses
demonstrating that Segue 1 stars span a metallicity range of more than 2 dex,
from [Fe/H] = -1.4 to [Fe/H] = -3.8. All of the Segue 1 stars are
alpha-enhanced, with [alpha/Fe] ~ 0.5. High alpha-element abundances are
typical for metal-poor stars, but in every previously studied galaxy [alpha/Fe]
declines for more metal-rich stars, which is typically interpreted as iron
enrichment from supernova Ia. The absence of this signature in Segue 1
indicates that it was enriched exclusively by massive stars. Other light
element abundance ratios in Segue 1, including carbon-enhancement in the three
most metal-poor stars, closely resemble those of metal-poor halo stars.
Finally, we classify the most metal-rich star as a CH star given its large
overabundances of carbon and s-process elements. The other six stars show
remarkably low neutron-capture element abundances of [Sr/H] < -4.9 and [Ba/H] <
-4.2, which are comparable to the lowest levels ever detected in halo stars.
This suggests minimal neutron-capture enrichment, perhaps limited to a single
r-process or weak s-process synthesizing event. Altogether, the chemical
abundances of Segue 1 indicate no substantial chemical evolution, supporting
the idea that it may be a surviving first galaxy that experienced only one
burst of star formation.Comment: ApJ, accepted, 20 pages (emulateapj), 9 figure
Development of a Multi-Probe Kelvin Scanner Device for Industrially-Relevant Characterization of Surface-Activated Carbon Fiber Reinforced Thermoplastic Composites
Carbon fiber reinforced thermoplastic (CFRTP) composites are becoming increasingly attractive materials in manufacturing due to their lightweight nature, mechanical strength, and corrosion resistance. Surface activation of these materials is usually required during processing to increase the bond strength of assemblies (aerospace and automotive industries) or improve adhesion with implants (biomedical industry). Industrially-relevant, nondestructive quality control methods for assessing the activation state of these materials do not currently exist, however. Applying principles discovered through the use of scanning probe microscopy, a multiple-probe Kelvin scanning (MPKS) device has been developed that can assess the uniformity of the activation state of plasma-treated CFRTP surfaces. The device can distinguish between control and plasma-treated samples and its measurements have been correlated with shear bond strength of epoxy-bonded assemblies. With the multiple probes increasing measurement speed, the automated device can be scaled for use in manufacturing-relevant environments and improve upon current quality control practices
Spectroscopic Confirmation of the Dwarf Galaxies Hydra II and Pisces II and the Globular Cluster Laevens 1
We present Keck/DEIMOS spectroscopy of stars in the recently discovered Milky
Way satellites Hydra II, Pisces II, and Laevens 1. We measured a velocity
dispersion of 5.4 (+3.6 -2.4) km/s for Pisces II, but we did not resolve the
velocity dispersions of Hydra II or Laevens 1. We marginally resolved the
metallicity dispersions of Hydra II and Pisces II but not Laevens 1.
Furthermore, Hydra II and Pisces II obey the luminosity-metallicity relation
for Milky Way dwarf galaxies ( = -2.02 +/- 0.08 and -2.45 +/- 0.07,
respectively), whereas Laevens 1 does not ( = -1.68 +/- 0.05). The
kinematic and chemical properties suggest that Hydra II and Pisces II are dwarf
galaxies, and Laevens 1 is a globular cluster. We determined that two of the
previously observed blue stars near the center of Laevens 1 are not members of
the cluster. A third blue star has ambiguous membership. Hydra II has a radial
velocity = 303.1 +/- 1.4 km/s, similar to the leading arm of the
Magellanic stream. The mass-to-light ratio for Pisces II is 370 (+310 -240)
M_sun/L_sun. It is not among the most dark matter-dominated dwarf galaxies, but
it is still worthy of inclusion in the search for gamma rays from dark matter
self-annihilation.Comment: Accepted to ApJ. v2 has been revised in response to the referee's
repor
Are the Ultra-Faint Dwarf Galaxies Just Cusps?
We develop a technique to investigate the possibility that some of the
recently discovered ultra-faint dwarf satellites of the Milky Way might be cusp
caustics rather than gravitationally self-bound systems. Such cusps can form
when a stream of stars folds, creating a region where the projected 2-D surface
density is enhanced. In this work, we construct a Poisson maximum likelihood
test to compare the cusp and exponential models of any substructure on an equal
footing. We apply the test to the Hercules dwarf (d ~ 113 kpc, M_V ~ -6.2, e ~
0.67). The flattened exponential model is strongly favored over the cusp model
in the case of Hercules, ruling out at high confidence that Hercules is a cusp
catastrophe. This test can be applied to any of the Milky Way dwarfs, and more
generally to the entire stellar halo population, to search for the cusp
catastrophes that might be expected in an accreted stellar halo.Comment: Accepted for publication in ApJ Letters. Minor revisions from version
- …