4,485 research outputs found
Mode identification in rapidly rotating stars
Context: Recent calculations of pulsation modes in rapidly rotating polytropic models and models based on the Self-Consistent Field method have shown that the frequency spectrum of low degree pulsation modes can be described by an empirical formula similar to Tassoul's asymptotic formula, provided that the underlying rotation profile is not too differential.
Aims: Given the simplicity of this asymptotic formula, we investigate whether it can provide a means by which to identify pulsation modes in rapidly rotating stars.
Methods: We develop a new mode identification scheme which consists in scanning a multidimensional parameter space for the formula coefficients which yield the best-fitting asymptotic spectra. This mode identification scheme is then tested on artificial spectra based on the asymptotic formula, on random frequencies and on spectra based on full numerical eigenmode calculations for which the mode identification is known beforehand. We also investigate the effects of adding random frequencies to mimic the effects of chaotic modes which are also expected to show up in such stars.
Results: In the absence of chaotic modes, it is possible to accurately find a correct mode identification for most of the observed frequencies provided these frequencies are sufficiently close to their asymptotic values. The addition of random frequencies can very quickly become problematic and hinder correct mode identification. Modifying the mode identification scheme to reject the worst fitting modes can bring some improvement but the results still remain poorer than in the case without chaotic modes
Determining the metallicity of the solar envelope using seismic inversion techniques
The solar metallicity issue is a long-lasting problem of astrophysics,
impacting multi- ple fields and still subject to debate and uncertainties.
While spectroscopy has mostly been used to determine the solar heavy elements
abundance, helioseismologists at- tempted providing a seismic determination of
the metallicity in the solar convective enveloppe. However, the puzzle remains
since two independent groups prodived two radically different values for this
crucial astrophysical parameter. We aim at provid- ing an independent seismic
measurement of the solar metallicity in the convective enveloppe. Our main goal
is to help provide new information to break the current stalemate amongst
seismic determinations of the solar heavy element abundance. We start by
presenting the kernels, the inversion technique and the target function of the
inversion we have developed. We then test our approach in multiple
hare-and-hounds exercises to assess its reliability and accuracy. We then apply
our technique to solar data using calibrated solar models and determine an
interval of seismic measurements for the solar metallicity. We show that our
inversion can indeed be used to estimate the solar metallicity thanks to our
hare-and-hounds exercises. However, we also show that further dependencies in
the physical ingredients of solar models lead to a low accuracy. Nevertheless,
using various physical ingredients for our solar models, we determine
metallicity values between 0.008 and 0.014.Comment: Accepted for publication in MNRA
RUBIS: a simple tool for calculating the centrifugal deformation of stars and planets
In this article we present RUBIS (Rotation code Using Barotropy conservation
over Isopotential Surfaces), a fully Python-based centrifugal deformation
program available at https://github.com/pierrehoudayer/RUBIS. The code has been
designed to calculate the centrifugal deformation of a star or planet resulting
from a given cylindrical rotation profile, starting from a spherically
symmetric non-rotating model. Furthermore, it can handle models with
discontinuities in the density profile.
The underlying assumption in RUBIS is that the relationship between density
and pressure is preserved during the deformation process. This leads to many
procedural simplifications. For instance, RUBIS only needs to solve Poisson'
equation, either in spheroidal or spherical coordinates depending on whether
the 1D model has discontinuities or not.
In this paper, we present the benefits of using RUBIS to deform polytropic
models and more complex barotropic structures, thus providing, to a certain
extent, insights into baroclinic models. The resulting structures can be used
for a wide range of applications, including the seismic study of models.
Finally, we illustrate how RUBIS is beneficial specifically in the analysis of
Jupiter's gravitational moments, thanks to its ability to handle discontinuous
models while retaining a high accuracy compared to current methods
Are the stars of a new class of variability detected in NGC~3766 fast rotating SPB stars?
A recent photometric survey in the NGC~3766 cluster led to the detection of
stars presenting an unexpected variability. They lie in a region of the
Hertzsprung-Russell (HR) diagram where no pulsation are theoretically expected,
in between the Scuti and slowly pulsating B (SPB) star instability
domains. Their variability periods, between 0.1--0.7~d, are outside the
expected domains of these well-known pulsators. The NCG~3766 cluster is known
to host fast rotating stars. Rotation can significantly affect the pulsation
properties of stars and alter their apparent luminosity through gravity
darkening. Therefore we inspect if the new variable stars could correspond to
fast rotating SPB stars. We carry out instability and visibility analysis of
SPB pulsation modes within the frame of the traditional approximation. The
effects of gravity darkening on typical SPB models are next studied. We find
that at the red border of the SPB instability strip, prograde sectoral (PS)
modes are preferentially excited, with periods shifted in the 0.2--0.5~d range
due to the Coriolis effect. These modes are best seen when the star is seen
equator-on. For such inclinations, low-mass SPB models can appear fainter due
to gravity darkening and as if they were located between the ~Scuti and
SPB instability strips.Comment: 6 pages, 2 figures, to appear in the proceedings of the IAU Symposium
307, New windows on massive stars: asteroseismology, interferometry, and
spectropolarimetr
Fusarium Oxysporum Infection of Cultivated Coneflowers (Echninacea Angustifolia
The narrow-leaf purple coneflower Echinacea angustifolia D.C. is a perennial composite native to the Central Great Plains. Because of its potential ornamental and pharmacological value, Echinacea is currently being brought into commercial production in South Dakota. A blight or wilt was observed in a field research plot on 1-to 3-year-old plants in the summers of 1993 and 1994. Up to 5% of plants growing in the wetter portion of the field showed symptoms. Affected plants became necrotic along leaf margins, followed by wilting and eventual death. Diseased plants were harvested and brought to the lab for analysis. Examination of roots and stems revealed darkening of the vascular and ground tissues. The pathogen was consistently isolated by plating 0.5 cm surface sterilized stem segments on lactic acid PDA for 3 to 5 days at room temperature. To complete Koch’s postulates, a single hyphal tip of the pathogen was transferred to carnation leaf agar. Micro and macro conidia formed abundantly within 10 days and matched the description of Fusarium oxysporum Schlect. Two to three month old coneflower seedlings were inoculated with the isolated pathogen by placing six mycelial plugs directly on the roots of each plant, just below the soil surface. Infected seedlings showed typical diagnostic symptoms (30% of inoculated seedlings) within three weeks. Non-inoculated plants remained symptomless. Fusarium oxysporum was reisolated from the seedlings as above. This is the first report of F. oxysporum as a pathogen on E. angustifolia
Pulsation modes in rapidly rotating stellar models based on the Self-Consistent Field method
Context: New observational means such as the space missions CoRoT and Kepler
and ground-based networks are and will be collecting stellar pulsation data
with unprecedented accuracy. A significant fraction of the stars in which
pulsations are observed are rotating rapidly.
Aims: Our aim is to characterise pulsation modes in rapidly rotating stellar
models so as to be able to interpret asteroseismic data from such stars.
Methods: The pulsation code developed in Ligni\`eres et al. (2006) and Reese
et al. (2006) is applied to stellar models based on the self-consistent field
(SCF) method (Jackson et al. 2004, 2005, MacGregor et al. 2007).
Results: Pulsation modes in SCF models follow a similar behaviour to those in
uniformly rotating polytropic models, provided that the rotation profile is not
too differential. Pulsation modes fall into different categories, the three
main ones being island, chaotic, and whispering gallery modes, which are
rotating counterparts to modes with low, medium, and high l-|m| values,
respectively. The frequencies of the island modes follow an asymptotic pattern
quite similar to what was found for polytropic models. Extending this
asymptotic formula to higher azimuthal orders reveals more subtle behaviour as
a function of m and provides a first estimate of the average advection of
pulsation modes by rotation. Further calculations based on a variational
principle confirm this estimate and provide rotation kernels that could be used
in inversion methods. When the rotation profile becomes highly differential, it
becomes more and more difficult to find island and whispering gallery modes at
low azimuthal orders. At high azimuthal orders, whispering gallery modes, and
in some cases island modes, reappear.Comment: 16 pages, 11 figures, accepted for publication in A&
Observations on the vibration of axially-tensioned elastomeric pipes conveying fluids
A study of the effect of axial tension on the vibration of a single-span elastomeric pipe clamped at both ends conveying fluid has been carried out both experimentally and theoretically. A new mathematical model using a penalty function technique and the method of kinematic correction and fictitious loads has been developed. The influence of flowing fluid and axial tension on natural frequencies and mode shapes of the system has been described using this model and compared with experimental observations. Linear and non-linear dynamic response of the harmonically excited pipe has also been investigated for varying flow velocities and initial axial tensions
BVRI Surface Photometry of Isolated Spiral Galaxies
A release of multicolor broad band (BVRI) photometry for a subsample of 44
isolated spirals drawn from the Catalogue of Isolated Galaxies (CIG) is
presented. Total magnitudes and colors at various circular apertures, as well
as some global structural/morphological parameters are estimated. Morphology is
reevaluated through optical and sharp/filtered R band images, (B-I) color index
maps, and archive near-IR JHK images from the Two-Micron Survey. The CAS
structural parameters (Concentration, Asymmetry, and Clumpiness) were
calculated from the images in each one of the bands. The fraction of galaxies
with well identified optical/near-IR bars (SB) is 63%, while a 17% more shows
evidence of weak or suspected bars (SAB). The sample average value of the
maximum bar ellipticity is 0.4. Half of the galaxies in the sample shows rings.
We identify two candidates for isolated galaxies with disturbed morphology. The
structural CAS parameters change with the observed band, and the tendencies
they follow with the morphological type and global color are more evident in
the redder bands. In any band, the major difference between our isolated
spirals and a sample of interacting spirals is revealed in the A-S plane. A
deep and uniformly observed sample of isolated galaxies is intended for various
purposes including (i) comparative studies of environmental effects, (ii)
confronting model predictions of galaxy evolution and (iii) evaluating the
change of galaxy properties with redshift.Comment: 44 pages, 9 figures and 7 tables included. To appear in The
Astronomical Journal. For the 43 appendix figures 4.1-4.43 see
http://www.astroscu.unam.mx/~avila/Figs4.1_4.43.tar.gz (7.2 Mb tar.gz file
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