1,104 research outputs found
Molecular astronomy of cool stars and sub-stellar objects
The optical and infrared spectra of a wide variety of `cool' astronomical
objects including the Sun, sunspots, K-, M- and S-type stars, carbon stars,
brown dwarfs and extrasolar planets are reviewed. The review provides the
necessary astronomical background for chemical physicists to understand and
appreciate the unique molecular environments found in astronomy. The
calculation of molecular opacities needed to simulate the observed spectral
energy distributions is discussed
Water vapor on supergiants. The 12 micron TEXES spectra of mu Cephei
Several recent papers have argued for warm, semi-detached, molecular layers
surrounding red giant and supergiant stars, a concept known as a MOLsphere.
Spectroscopic and interferometric analyses have often corroborated this general
picture. Here, we present high-resolution spectroscopic data of pure rotational
lines of water vapor at 12 microns for the supergiant mu Cephei. This star has
often been used to test the concept of molecular layers around supergiants.
Given the prediction of an isothermal, optically thick water-vapor layer in
Local Thermodynamic Equilibrium around the star (MOLsphere), we expected the 12
micron lines to be in emission or at least in absorption but filled in by
emission from the molecular layer around the star. Our data, however, show the
contrary; we find definite absorption. Thus, our data do not easily fit into
the suggested isothermal MOLsphere scenario. The 12 micron lines, therefore,
put new, strong constraints on the MOLsphere concept and on the nature of water
seen in signatures across the spectra of early M supergiants. We also find that
the absorption is even stronger than that calculated from a standard,
spherically symmetric model photosphere without any surrounding layers. A cool
model photosphere, representing cool outer layers is, however, able to
reproduce the lines, but this model does not account for water vapor emission
at 6 microns. Thus, a unified model for water vapor on mu Cephei appears to be
lacking. It does seem necessary to model the underlying photospheres of these
supergiants in their whole complexity. The strong water vapor lines clearly
reveal inadequacies of classical model atmospheres.Comment: Accepted for publication in the Astrophysical Journa
Identifying Young Brown Dwarfs Using Gravity-Sensitive Spectral Features
We report the initial results of the Brown Dwarf Spectroscopic Survey Gravity
Project, to study gravity sensitive features as indicators of youth in brown
dwarfs. Low-resolution (R~2000) J-band and optical (R~1000) observations using
NIRSPEC and LRIS at the W.M. Keck Observatory reveal transitions of TiO, VO, K
I, Na I, Cs I, Rb I, CaH, and FeH. By comparing these features in late-type
giants and in old field dwarfs we show that they are sensitive to the gravity
(g = GM/R^2) of the object. Using low-gravity spectral signatures as age
indicators, we observed and analyzed J-band and optical spectra of two young
brown dwarfs, G 196-3B (20-300 Myr) and KPNO Tau-4 (1-2 Myr), and two possible
low mass brown dwarfs in the sigma Orionis cluster (3-7 Myr). We report the
identification of the phi bands of TiO near 1.24 microns and the A-X band of VO
near 1.18 microns together with extremely weak J-band lines of K I in
KPNO-Tau4. This is the first detection of TiO and VO in the J-band in a
sub-stellar mass object. The optical spectrum of KPNO-Tau4 exhibits weak K I
and Na I lines, weak absorption by CaH, and strong VO bands, also signatures of
a lower gravity atmosphere. G 196-3B shows absorption features in both
wavelength regions like those of KPNO-Tau4 suggesting that its age and mass are
at the lower end of published estimates. Whereas sigma Ori 51 appears to be
consistent with a young sub-stellar object, sigma Ori 47 shows signatures of
high gravity most closely resembling an old L1.5/L0, and can not be a member of
the sigma Orionis cluster.Comment: 14 pages, 4 figures. To appear in the January 10, 2004 issue of the
Astrophysical Journa
The Observed Trend of Boron and Oxygen in Field Stars of the Disk
Oxygen abundances are derived in a sample of 13 field F and G dwarfs and
subgiants with metallicities in the range of -0.75 < [Fe/H] < +0.15. This is
the same sample of stars for which boron abundances have been derived earlier
from archived spectra obtained with the Hubble Space Telescope. In a log-log
comparison of the B versus the O abundances, a slope of m(BO)=1.39 is found,
indicating that in the disk, the abundance of B relative to O is intermediate
between primary and secondary production (hybrid behavior). This relation of B
versus O for disk stars is compared to the same relation for halo stars.Comment: 14 pages, 3 figures, 1 table. In press to The Astronomical Journal
(July 2001
An abundance analysis of the symbiotic star CH Cyg
The photospheric abundances for the cool component of the symbiotic star CH
Cyg were calculated for the first time using high-resolution near-infrared
spectra and the method of of standard LTE analysis and atmospheric models. The
iron abundance for CH Cyg was found to be solar, [Fe/H] = 0.0+/-0.19. The
atmospheric parameters and metallicity for CH Cyg are found to be approximately
equal to those for nearby field M7 giants. The calculated [C/H] = -0.15, [N/H]
= +0.16, [O/H] = -0.07, and the isotopic ratios of 12C/13C and 16O/17O are
close to the mean values for single M giants that have experienced the first
dredge-up. A reasonable explanation for the absence of barium star-like
chemical peculiarities seems to be the high metallicity of CH Cyg. The emission
line technique was explored for estimating CNO ratios in the wind of the giant.Comment: 9 pages, 4 figures, 4 tables; accepted for publication in A&
Frequency dispersion reduction and bond conversion on n-type GaAs by in situ surface oxide removal and passivation
The method of surface preparation on n-type GaAs, even with the presence of an amorphous-Si interfacial passivation layer, is shown to be a critical step in the removal of accumulation capacitance frequency dispersion. In situ deposition and analysis techniques were used to study different surface preparations, including NH4OH, Si-flux, and atomic hydrogen exposures, as well as Si passivation depositions prior to in situ atomic layer deposition of Al2O3. As–O bonding was removed and a bond conversion process with Si deposition is observed. The accumulation capacitance frequency dispersion was removed only when a Si interlayer and a specific surface clean were combined
GaAs interfacial self-cleaning by atomic layer deposition
The reduction and removal of surface oxides from GaAs substrates by atomic layer deposition (ALD) of Al2O3 and HfO2 are studied using in situ monochromatic x-ray photoelectron spectroscopy. Using the combination of in situ deposition and analysis techniques, the interfacial "self-cleaning" is shown to be oxidation state dependent as well as metal organic precursor dependent. Thermodynamics, charge balance, and oxygen coordination drive the removal of certain species of surface oxides while allowing others to remain. These factors suggest proper selection of surface treatments and ALD precursors can result in selective interfacial bonding arrangements
Chemical and electrical characterization of the HfO2/InAlAs interface
InAlAs has the potential to be used as a barrier layer in buried channel quantum well field effect transistor devices due to favorable lattice-matching and carrier confinement properties with InGaAs. Field effect device structures of this nature may also require a high-k oxide deposited on the InAlAs surface to reduce leakage current. This study investigates the impact of surface preparations and atomic layer deposition of HfO2 on these surfaces using x-ray photoelectron spectroscopy to analyse the chemical interactions taking place, as well as the electrical performance of associated capacitor devices. A large concentration of As related surface features is observed at the InAlAs surface, and is attributed to a large D-it response in electrical measurements. (C) 2013 AIP Publishing LLC
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