1,069 research outputs found
An Accurate, Easy to Use Abundace Scale for Globular Clusters Based on 2.2um Spectra of Giant Stars
We present a new method for the determination of [Fe/H] for globular
clusters. This new method is based on moderate resolution (R~1500) near-IR
spectroscopy in the K-band of 6 to 10 of the brightest giants in a cluster. Our
calibration is derived from spectra of 105 stars in 15 globular clusters. From
measurements of the equivalent widths of three features in these spectra, Na,
Ca, and CO, we are able to reproduce the Zinn & West (1984) abundance scale as
updated by Harris (1996) to better than 0.10 dex for clusters with near solar
[Fe/H] down to an [Fe/H] of -1.8. Three advantages of this method are that it
can be used for metal rich, heavily reddened globulars in crowded fields, it
does not require any knowledge of any other cluster or stellar parameters such
as reddening, distance, or luminosity, and it requires only minimal telescope
time. If stellar (J-K)0 and MK values are available as well, the accuracy of
the [Fe/H] estimate is further improved. Observations of as few as three stars
per cluster still gives an [Fe/H] estimate wich is nearly as reliable as that
based on two to three times as many stars.
The accuracy of an [Fe/H] value based on observations of CO absorption alone
is significantly less than that which results from the three spectroscopic
indices. However, we predict that space-based observations of this feature in
the integrated light of stellar systems will prove to be of great value for
abundance determinations at distances as far as the Coma cluster of galaxies.Comment: 53 pages, 16 Postscript figures. Submitted to the A
Low loss Ge-on-Si waveguides operating in the 8â14 ”m atmospheric transmission window
Germanium-on-silicon waveguides were modeled, fabricated and characterized at wavelengths ranging from 7.5 to 11 ”m. Measured waveguide losses are below 5 dB/cm for both TE and TM polarization and reach values of ⌠1 dB/cm for ℠10 ”m wavelengths for the TE polarization. This work demonstrates experimentally for the first time that Ge-on-Si is a viable waveguide platform for sensing in the molecular fingerprint spectral region. Detailed modeling and analysis is presented to identify the various loss contributions, showing that with practical techniques losses below 1 dB/cm could be achieved across the full measurement range
A Near-Infrared Photometric Study of the Low Latitude Globular Clusters Liller 1, Djorgovski 1, HP 1, and NGC 6528
Images recorded through J, H, Ks, 2.2 micron continuum, and 2.3 micron CO
filters are used to investigate the stellar contents of the low Galactic
latitude globular clusters NGC 6528, Liller 1, Djorgovski 1, and HP 1, as well
as surrounding bulge fields. Metallicities are estimated for the latter three
clusters by comparing the colors and CO indices of giant branch stars with
those in other clusters and the bulge, while reddenings are estimated from the
colors of bright bulge stars in the surrounding fields. In some cases the
metallicities and reddenings are significantly different from previous
estimates.Comment: 32 pages, and 28 figure
Kinematics of the Galactic Globular Cluster System: New Radial Velocities for Clusters in the Direction of the Inner Galaxy
HIRES on the Keck I telescope has been used to measure the first radial
velocities for stars belonging to eleven, heavily-reddened globular clusters in
the direction of the inner Galaxy. The question of kinematic substructuring
among the Galactic globular cluster system is investigated using an updated
catalog of globular cluster distances, metallicities and velocities. It is
found that the population of metal-rich globular clusters shows significant
rotation at all Galactocentric radii. For the metal-rich clusters within 4 kpc
of the Galactic center, the measured rotation velocity and line-of-sight
velocity dispersion are similar to those of bulge field stars. We investigate
claims that the metal-rich clusters are associated with the central Galactic
bar by comparing the kinematics of the innermost clusters to that of the atomic
hydrogen in the inner Galaxy. The longitude-velocity diagram of both metal-rich
and metal-poor clusters bears a remarkable similarity to that of the gas,
including the same non-circular motions which have traditionally been
interpreted as evidence for a Galactic bar, or, alternatively, a
non-axisymmetric bulge. However, uncertainties in the existing
three-dimensional Galactocentric positions for most of the clusters do not yet
allow an unambiguous discrimination between the competing scenarios of
membership in a rigidly rotating bar, or in a bulge which is an oblate
isotropic rotator. We conclude that the majority of metal-rich clusters within
the central 4 kpc of the Galaxy are probably associated with the bulge/bar, and
not the thick disk. (ABRIDGED)Comment: 18 pages, including 7 of 13 postscript figures. Figures 1-6 available
at http://astro.caltech.edu/~pc. Accepted for publication in the Astronomical
Journa
Mid-infrared intersubband absorption from p-Ge quantum wells grown on Si substrates
Mid-infrared intersubband absorption from p-Ge quantum wells with Si0.5Ge0.5 barriers grown on a Si substrate is demonstrated from 6 to 9âÎŒm wavelength at room temperature and can be tuned by adjusting the quantum well thickness. Fourier transform infra-red transmission and photoluminescence measurements demonstrate clear absorption peaks corresponding to intersubband transitions among confined hole states. The work indicates an approach that will allow quantum well intersubband photodetectors to be realized on Si substrates in the important atmospheric transmission window of 8â13âÎŒm
Mid-Infrared Intersubband Absorption from P-Ge Quantum Wells on Si
Mid-infrared intersubband absorption from p-Ge quantum wells with Si0.5Ge0.5 barriers grown on a Si substrate is demonstrated from 6 to 9 ÎŒm wavelength at room temperature and can be tuned by adjusting the quantum well thickness. Fourier transform infra-red spectroscopy measurements demonstrate clear absorption peaks corresponding to intersubband transitions among confined hole states. The work indicates an approach that will allow quantum well intersubband photodetectors to be realized on Si substrates in the important atmospheric transmission window of 8â13 ÎŒm
Integrated Germanium-on-silicon Waveguides for Mid-infrared Photonic Sensing Chips
Germanium-on-silicon waveguides are designed, fabricated and characterized with a novel near-field infrared spectroscopy technique that allows on-chip investigation of the in-coupling efficiency. On-chip propagation along bends and straight sections up to 0.8 mm is demonstrated around λ = 6 Όm
Chemical Abundance Study of One Red Giant Star in NGC 5694 : A Globular Cluster with Dwarf Spheroidals' Chemical Signature?
We report the abundance analysis of one red giant branch star in the
metal-poor outer halo globular cluster NGC 5694. We obtain [Fe/H] = -1.93,
based on the ionized lines, and our metallicity measurement is in good
agreement with previous estimates. We find that [Ca+Ti/2Fe] and [Cu/Fe] of NGC
5694 are about 0.3 -- 0.4 dex lower than other globular clusters with similar
metallicities, but similar to some LMC clusters and stars in some dwarf
spheroidal galaxies. Differences persist, however, in the abundances of neutron
capture elements. The unique chemical abundance pattern and the large
Galactocentric distance (30 kpc) and radial velocity (-138.6 +/- 1.0 km/sec)
indicate that NGC 5694 had an extragalactic origin.Comment: ApJL accepte
Benchmarking the Use of Heavily-Doped Ge Against Noble Metals for Plasmonics and Sensing in the Mid-Infrared
Despite the recent introduction of heavily-doped semiconductors for mid-infrared plasmonics, it still remains an open point whether such materials can compete with noble metals. We employ a whole set of figures of merit to thoroughly assess the use of heavily-doped Ge on Si as a mid-infrared plasmonic material and benchmark it against standard noble metals such as Au. In doing this, we design and model high-performance, CMOS compatible mid-infrared plasmonic sensors based on experimental material data reaching plasma frequencies up to about 1950 cmâ1. We demonstrate that plasmonic Ge sensors can provide signal enhancements for vibrational spectroscopy above 3 orders of magnitude, thus representing a viable alternative to noble metals
Observation of charge-density-wave excitations in manganites
In the optical conductivity of four different manganites with commensurate
charge order (CO), strong peaks appear in the meV range below the ordering
temperature T_{CO}. They are similar to those reported for one-dimensional
charge density waves (CDW) and are assigned to pinned phasons. The peaks and
their overtones allow one to obtain, for La{1-n/8}Ca{n/8}$MnO{3} with n = 5, 6,
the electron-phonon coupling, the effective mass of the CO system, and its
contribution to the dielectric constant. These results support a description of
the CO in La-Ca manganites in terms of moderately weak-coupling and of the CDW
theory.Comment: To be published on Phys. Rev. Let
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