37 research outputs found
The role of turbulent pressure as a coherent pulsational driving mechanism: the case of the delta Scuti star HD 187547
HD 187547 was the first candidate that led to the suggestion that solar-like
oscillations are present in delta Scuti stars. Longer observations, however,
show that the modes interpreted as solar-like oscillations have either very
long mode lifetimes, longer than 960 days, or are coherent. These results are
incompatible with the nature of `pure' stochastic excitation as observed in
solar-like stars. Nonetheless, one point is certain: the opacity mechanism
alone cannot explain the oscillation spectrum of HD 187547. Here we present new
theoretical investigations showing that convection dynamics can intrinsically
excite coherent pulsations in the chemically peculiar delta Scuti star HD
187547. More precisely, it is the perturbations of the mean Reynold stresses
(turbulent pressure) that drives the pulsations and the excitation takes place
predominantly in the hydrogen ionization zone.Comment: 8 pages, 4 figures, accepted to Ap
Effective induction heating around strongly magnetized stars
Planets that are embedded in the changing magnetic fields of their host stars
can experience significant induction heating in their interiors caused by the
planet's orbital motion. For induction heating to be substantial, the planetary
orbit has to be inclined with respect to the stellar rotation and dipole axes.
Using WX~UMa, for which the rotation and magnetic axes are aligned, as an
example, we show that for close-in planets on inclined orbits, induction
heating can be stronger than the tidal heating occurring inside Jupiter's
satellite Io; namely, it can generate a surface heat flux exceeding
2\,W\,m. An internal heating source of such magnitude can lead to
extreme volcanic activity on the planet's surface, possibly also to internal
local magma oceans, and to the formation of a plasma torus around the star
aligned with the planetary orbit. A strongly volcanically active planet would
eject into space mostly SO, which would then dissociate into oxygen and
sulphur atoms. Young planets would also eject CO. Oxygen would therefore be
the major component of the torus. If the O{\sc i} column density of the torus
exceeds 10\,cm, the torus could be revealed by detecting
absorption signatures at the position of the strong far-ultraviolet O{\sc i}
triplet at about 1304\,\AA. We estimate that this condition is satisfied if the
O{\sc i} atoms in the torus escape the system at a velocity smaller than
1--10\,km\,s. These estimates are valid also for a tidally heated
planet.Comment: 8 pages, 6 figures, accepted for publication in Ap
Model atmospheres of chemically peculiar stars: Self-consistent empirical stratified model of HD24712
High-resolution spectra of some chemically peculiar stars clearly demonstrate
the presence of strong abundance gradients in their atmospheres. However, these
inhomogeneities are usually ignored in the standard scheme of model atmosphere
calculations, braking the consistency between model structure and
spectroscopically derived abundance pattern. In this paper we present first
empirical self-consistent stellar atmosphere model of roAp star HD24712, with
stratification of chemical elements included, and which is derived directly
from the observed profiles of spectral lines without time-consuming simulations
of physical mechanisms responsible for these anomalies. We used the LLmodels
stellar model atmosphere code and DDAFIT minimization tool for analysis of
chemical elements stratification and construction of self-consistent
atmospheric model. Empirical determination of Pr and Nd stratification in the
atmosphere of HD24712 is based on NLTE line formation for Prii/iii and Ndii/iii
with the use of the DETAIL code. Based on iterative procedure of stratification
analysis and subsequent re-calculation of model atmosphere structure we
constructed a self-consistent model of HD24712, i.e. the model which
temperature-pressure structure is consistent with results of stratification
analysis. It is shown that stratification of chemical elements leads to the
considerable changes in model structure as to compare with non-stratified
homogeneous case. We find that accumulation of REE elements allows for the
inverse temperature gradient to be present in upper atmosphere of the star with
the maximum temperature increase of about 600K.Comment: Comments: Accepted by A&A, 16 pages, 10 figures, 3 table
Direct evidence of a full dipole flip during the magnetic cycle of a sun-like star
Context. The behaviour of the large-scale dipolar field, during a starâs magnetic cycle, can provide valuable insight into the stellar dynamo and associated magnetic field manifestations such as stellar winds.
Aims. We investigate the temporal evolution of the dipolar field of the K dwarf 61 Cyg A using spectropolarimetric observations covering nearly one magnetic cycle equivalent to two chromospheric activity cycles.
Methods. The large-scale magnetic field geometry is reconstructed using Zeeman Doppler imaging, a tomographic inversion technique. Additionally, the chromospheric activity is also monitored.
Results. The observations provide an unprecedented sampling of the large-scale field over a single magnetic cycle of a star other than the Sun. Our results show that 61 Cyg A has a dominant dipolar geometry except at chromospheric activity maximum. The dipole axis migrates from the southern to the northern hemisphere during the magnetic cycle. It is located at higher latitudes at chromospheric
activity cycle minimum and at middle latitudes during cycle maximum. The dipole is strongest at activity cycle minimum and much weaker at activity cycle maximum.
Conclusions. The behaviour of the large-scale dipolar field during the magnetic cycle resembles the solar magnetic cycle. Our results are further confirmation that 61 Cyg A indeed has a large-scale magnetic geometry that is comparable to the Sunâs, despite being a slightly older and cooler K dwarf
Magnetic fields and chemical peculiarities of the very young intermediate-mass binary system HD 72106
The recently discovered magnetic Herbig Ae and Be stars may provide
qualitatively new information about the formation and evolution of magnetic Ap
and Bp stars. We have performed a detailed investigation of one particularly
interesting binary system with a Herbig Ae secondary and a late B-type primary
possessing a strong, globally ordered magnetic field. Twenty high-resolution
Stokes V spectra of the system were obtained with the ESPaDOnS instrument
mounted on the CFHT. In these observations we see clear evidence for a magnetic
field in the primary, but no evidence for a magnetic field in the secondary. A
detailed abundance analysis was performed for both stars, revealing strong
chemical peculiarities in the primary and normal chemical abundances in the
secondary. The primary is strongly overabundant in Si, Cr, and other iron-peak
elements, as well as Nd, and underabundant in He. The primary therefore appears
to be a very young Bp star. In this context, line profile variations of the
primary suggest non-uniform lateral distributions of surface abundances.
Interpreting the 0.63995 +/- 0.00009 day variation period of the Stokes I and V
profiles as the rotational period of the star, we have modeled the magnetic
field geometry and the surface abundance distributions of Si, Ti, Cr and Fe
using Magnetic Doppler Imaging. We derive a dipolar geometry of the surface
magnetic field, with a polar strength of 1230 G and an obliquity of 57 degrees.
The distributions Ti, Cr and Fe are all qualitatively similar, with an
elongated patch of enhanced abundance situated near the positive magnetic pole.
The Si distribution is somewhat different, and its relationship to the magnetic
field geometry less clear.Comment: Accepted by Monthly Notices of the Royal Astronomical Society,
September 2008. 15 pages, 10 figure
Life Beyond the Solar System: Space Weather and Its Impact on Habitable Worlds
The search of life in the Universe is a fundamental problem of astrobiology
and a major priority for NASA. A key area of major progress since the NASA
Astrobiology Strategy 2015 (NAS15) has been a shift from the exoplanet
discovery phase to a phase of characterization and modeling of the physics and
chemistry of exoplanetary atmospheres, and the development of observational
strategies for the search for life in the Universe by combining expertise from
four NASA science disciplines including heliophysics, astrophysics, planetary
science and Earth science. The NASA Nexus for Exoplanetary System Science
(NExSS) has provided an efficient environment for such interdisciplinary
studies. Solar flares, coronal mass ejections and solar energetic particles
produce disturbances in interplanetary space collectively referred to as space
weather, which interacts with the Earth upper atmosphere and causes dramatic
impact on space and ground-based technological systems. Exoplanets within close
in habitable zones around M dwarfs and other active stars are exposed to
extreme ionizing radiation fluxes, thus making exoplanetary space weather (ESW)
effects a crucial factor of habitability. In this paper, we describe the recent
developments and provide recommendations in this interdisciplinary effort with
the focus on the impacts of ESW on habitability, and the prospects for future
progress in searching for signs of life in the Universe as the outcome of the
NExSS workshop held in Nov 29 - Dec 2, 2016, New Orleans, LA. This is one of
five Life Beyond the Solar System white papers submitted by NExSS to the
National Academy of Sciences in support of the Astrobiology Science Strategy
for the Search for Life in the Universe.Comment: 5 pages, the white paper was submitted to the National Academy of
Sciences in support of the Astrobiology Science Strategy for the Search for
Life in the Univers
The magnetic Bp star 36 Lyncis, I. Magnetic and photospheric properties
This paper reports the photospheric, magnetic and circumstellar gas
characteristics of the magnetic B8p star 36 Lyncis (HD 79158). Using archival
data and new polarised and unpolarised high-resolution spectra, we redetermine
the basic physical properties, the rotational period and the geometry of the
magnetic field, and the photospheric abundances of various elements.}{Based on
magnetic and spectroscopic measurements, we infer an improved rotational period
of d. We determine a current epoch of the longitudinal
magnetic field positive extremum (HJD 2452246.033), and provide constraints on
the geometry of the dipole magnetic field (i\geq 56\degr, G, unconstrained). We redetermine the effective
temperature and surface gravity using the optical and UV energy distributions,
optical photometry and Balmer line profiles ( K,
), and based on the Hipparcos parallax we redetermine the
luminosity, mass, radius and true rotational speed ( \kms). We
measure photospheric abundances for 21 elements using optical and UV spectra,
and constrain the presence of vertical stratification of these elements. We
perform preliminary Doppler Imaging of the surface distribution of Fe, finding
that Fe is distributed in a patchy belt near the rotational equator. Most
remarkably, we confirm strong variations of the H line core which we
interpret as due to occultations of the star by magnetically-confined
circumstellar gas.Comment: Accepted by Astronomy and Astrophysic