36 research outputs found
Phase Dependent Spectroscopy of Mira Variable Stars
Spectroscopic measurements of Mira variable stars, as a function of phase, probe the stellar atmospheres and underlying pulsation mechanisms. For example, measuring variations in TiO, VO, and ZrO with phase can be used to help determine whether these molecular species are produced in an extended region above the layers where Balmer line emission occurs or below this shocked region. Using the same methods, the Balmer-line increment, where the strongest Balmer line at phase zero is H-delta and not H-alpha can be measured and explanations tested, along with another peculiarity, the absence of the H-epsilon line in the spectra of Miras when other Balmer lines are strong. We present new spectra covering the spectral range from 6200 Angstroms to 9000 Angstroms of 20 Mira variables. A relationship between variations in the CaII IR triplet and H-alpha as a function of phase support the hypothesis that H-epsilon's observational characteristics result from an interaction of H-epsilon photons with the CaII H line. New periods and epochs of variability are also presented for each star
Multiwavelength Study of Pulsation and Dust Production in Mira Variables Using Optical Interferometry for Constraints
Optical interferometry is a technique by which the diameters and indeed the direct pulsations of stars are routinely being measured. As a follow-on to a 7 year interferometric campaign to measure the pulsations of over 100 mira variables, our team has been using the Spitzer Space Telescope to obtain 95 mid-infrared spectra of 25 miras during their pulsations over one year while simultaneously ascertaining their near-infrared diameters using the Palomar Testbed Interferometer. These data will then be combined with modeling from NLTE and radiative transfer codes to place hard constraints on our understanding of these stars and their circumstellar environments. We present some initial results from this work and discuss the next steps toward fully characterizing the atmosphere, molecular photosphere and dust production in mira variables.Some of the work on this project was supported through a NASA grant to the PI and team associated with Spitzer program GO50717
Pulsating Stellar Atmospheres
We review the basic concepts, the present state of theoretical models, and
the future prospects for theory and observations of pulsating stellar
atmospheres. Our emphasis is on radially pulsating cool stars, which dynamic
atmospheres provide a general example for the differences with standard static
model atmospheres.Comment: 9 pages, 2 figs, LaTex, in Proc. of IAU Symp 189, "Fundamental
Stellar Properties...", eds. T. R. Bedding, A. J. Booth and J. Davis, Kluwer,
p.253, 199
Spatially resolving the outer atmosphere of the M giant BK Vir in the CO first overtone lines with VLTI/AMBER
The mass-loss mechanism in normal K--M giant stars with small variability
amplitudes is not yet understood, although they are the majority among red
giant stars. We present high-spatial and high-spectral resolution observations
of the 2.3 micron CO lines in the M7 giant BK Vir with a spatial resolution of
9.8 mas and a spectral resolution of 12000, using AMBER at the Very Large
Telescope Interferometer (VLTI). The angular diameters observed in the CO lines
are 12--31% larger than those measured in the continuum. We also detected
asymmetry in the CO line-forming region. The data taken 1.5 months apart show
possible time variation on a spatial scale of 30 mas (corresponding to 3 x
stellar diameter) at the CO band head. Comparison of the observed data with the
MARCS photospheric model shows that whereas the observed CO line spectrum can
be well reproduced by the model, the angular sizes observed in the CO lines are
much larger than predicted by the model. Our model with two additional CO
layers above the MARCS photosphere reproduces the observed spectrum and
interferometric data in the CO lines simultaneously. This model suggests that
the inner CO layer at ~1.2 stellar radii is very dense and warm with a CO
column density of ~10^{22} cm^{-2} and temperatures of 1900--2100K, while the
outer CO layer at 2.5--3.0 stellar radii is characterized by column densities
of 10^{19}--10^{20} cm^{-2} and temperatures of 1500--2100K. Our AMBER
observations of BK Vir have spatially resolved the extended molecular outer
atmosphere of a normal M giant in the individual CO lines for the first time.
The temperatures derived for the CO layers are higher than or equal to the
uppermost layer of the MARCS photospheric model, implying the operation of some
heating mechanism in the outer atmosphere.Comment: 10 pages, 9 figures, accepted for publication in Astronomy and
Astrophysic
Crop pests and predators exhibit inconsistent responses to surrounding landscape composition
The idea that noncrop habitat enhances pest control and represents a winâwin opportunity to conserve biodiversity and bolster yields has emerged as an agroecological paradigm. However, while noncrop habitat in landscapes surrounding farms sometimes benefits pest predators, natural enemy responses remain heterogeneous across studies and effects on pests are inconclusive. The observed heterogeneity in species responses to noncrop habitat may be biological in origin or could result from variation in how habitat and biocontrol are measured. Here, we use a pest-control database encompassing 132 studies and 6,759 sites worldwide to model natural enemy and pest abundances, predation rates, and crop damage as a function of landscape composition. Our results showed that although landscape composition explained significant variation within studies, pest and enemy abundances, predation rates, crop damage, and yields each exhibited different responses across studies, sometimes increasing and sometimes decreasing in landscapes with more noncrop habitat but overall showing no consistent trend. Thus, models that used landscape-composition variables to predict pest-control dynamics demonstrated little potential to explain variation across studies, though prediction did improve when comparing studies with similar crop and landscape features. Overall, our work shows that surrounding noncrop habitat does not consistently improve pest management, meaning habitat conservation may bolster production in some systems and depress yields in others. Future efforts to develop tools that inform farmers when habitat conservation truly represents a winâwin would benefit from increased understanding of how landscape effects are modulated by local farm management and the biology of pests and their enemies
Crop pests and predators exhibit inconsistent responses to surrounding landscape composition
The idea that noncrop habitat enhances pest control and represents a winâwin opportunity to conserve biodiversity and bolster yields has emerged as an agroecological paradigm. However, while noncrop habitat in landscapes surrounding farms sometimes benefits pest predators, natural enemy responses remain heterogeneous across studies and effects on pests are inconclusive. The observed heterogeneity in species responses to noncrop habitat may be biological in origin or could result from variation in how habitat and biocontrol are measured. Here, we use a pest-control database encompassing 132 studies and 6,759 sites worldwide to model natural enemy and pest abundances, predation rates, and crop damage as a function of landscape composition. Our results showed that although landscape composition explained significant variation within studies, pest and enemy abundances, predation rates, crop damage, and yields each exhibited different responses across studies, sometimes increasing and sometimes decreasing in landscapes with more noncrop habitat but overall showing no consistent trend. Thus, models that used landscape-composition variables to predict pest-control dynamics demonstrated little potential to explain variation across studies, though prediction did improve when comparing studies with similar crop and landscape features. Overall, our work shows that surrounding noncrop habitat does not consistently improve pest management, meaning habitat conservation may bolster production in some systems and depress yields in others. Future efforts to develop tools that inform farmers when habitat conservation truly represents a winâwin would benefit from increased understanding of how landscape effects are modulated by local farm management and the biology of pests and their enemies