5,604 research outputs found
Stellar Winds on the Main-Sequence II: the Evolution of Rotation and Winds
Aims: We study the evolution of stellar rotation and wind properties for
low-mass main-sequence stars. Our aim is to use rotational evolution models to
constrain the mass loss rates in stellar winds and to predict how their
properties evolve with time on the main-sequence.
Methods: We construct a rotational evolution model that is driven by observed
rotational distributions of young stellar clusters. Fitting the free parameters
in our model allows us to predict how wind mass loss rate depends on stellar
mass, radius, and rotation. We couple the results to the wind model developed
in Paper I of this series to predict how wind properties evolve on the
main-sequence.
Results: We estimate that wind mass loss rate scales with stellar parameters
as . We
estimate that at young ages, the solar wind likely had a mass loss rate that is
an order of magnitude higher than that of the current solar wind. This leads to
the wind having a higher density at younger ages; however, the magnitude of
this change depends strongly on how we scale wind temperature. Due to the
spread in rotation rates, young stars show a large range of wind properties at
a given age. This spread in wind properties disappears as the stars age.
Conclusions: There is a large uncertainty in our knowledge of the evolution
of stellar winds on the main-sequence, due both to our lack of knowledge of
stellar winds and the large spread in rotation rates at young ages. Given the
sensitivity of planetary atmospheres to stellar wind and radiation conditions,
these uncertainties can be significant for our understanding of the evolution
of planetary environments.Comment: 26 pages, 14 figures, 2 tables, to be published in A&
Stellar Winds on the Main-Sequence I: Wind Model
Aims: We develop a method for estimating the properties of stellar winds for
low-mass main-sequence stars between masses of 0.4 and 1.1 solar masses at a
range of distances from the star.
Methods: We use 1D thermal pressure driven hydrodynamic wind models run using
the Versatile Advection Code. Using in situ measurements of the solar wind, we
produce models for the slow and fast components of the solar wind. We consider
two radically different methods for scaling the base temperature of the wind to
other stars: in Model A, we assume that wind temperatures are fundamentally
linked to coronal temperatures, and in Model B, we assume that the sound speed
at the base of the wind is a fixed fraction of the escape velocity. In Paper II
of this series, we use observationally constrained rotational evolution models
to derive wind mass loss rates.
Results: Our model for the solar wind provides an excellent description of
the real solar wind far from the solar surface, but is unrealistic within the
solar corona. We run a grid of 1200 wind models to derive relations for the
wind properties as a function of stellar mass, radius, and wind temperature.
Using these results, we explore how wind properties depend on stellar mass and
rotation.
Conclusions: Based on our two assumptions about the scaling of the wind
temperature, we argue that there is still significant uncertainty in how these
properties should be determined. Resolution of this uncertainty will probably
require both the application of solar wind physics to other stars and detailed
observational constraints on the properties of stellar winds. In the final
section of this paper, we give step by step instructions for how to apply our
results to calculate the stellar wind conditions far from the stellar surface.Comment: 24 pages, 13 figures, 2 tables, Accepted for publication in A&
Stellar activity and planetary atmosphere evolution in tight binary star systems
Context. In tight binary star systems, tidal interactions can significantly
influence the rotational and orbital evolution of both stars, and therefore
their activity evolution. This can have strong effects on the atmospheric
evolution of planets that are orbiting the two stars.
Aims. In this paper, we aim to study the evolution of stellar rotation and of
X-ray and ultraviolet (XUV) radiation in tight binary systems consisting of two
solar mass stars and use our results to study planetary atmosphere evolution in
the habitable zones of these systems.
Methods. We have applied a rotation model developed for single stars to
binary systems, taking into account the effects of tidal interactions on the
rotational and orbital evolution of both stars. We used empirical
rotation-activity relations to predict XUV evolution tracks for the stars,
which we used to model hydrodynamic escape of hydrogen dominated atmospheres.
Results. When significant, tidal interactions increase the total amount of
XUV energy emitted, and in the most extreme cases by up to factor of 50.
We find that in the systems that we study, habitable zone planets with masses
of 1~M can lose huge hydrogen atmospheres due to the extended high
levels of XUV emission, and the time that is needed to lose these atmospheres
depends on the binary orbital separation.For some orbital separations, and when
the stars are born as rapid rotators, it is also possible for tidal
interactions to protect atmospheres from erosion by quickly spinning down the
stars. For very small orbital separations, the loss of orbital angular momentum
by stellar winds causes the two stars to merge. We suggest that the merging of
the two stars could cause previously frozen planets to become habitable due to
the habitable zone boundaries moving outwards.Comment: Accepted for publication by A&
The origin of organic emission in NGC 2071
Context: The physical origin behind organic emission in embedded low-mass
star formation has been fiercely debated in the last two decades. A multitude
of scenarios have been proposed, from a hot corino to PDRs on cavity walls to
shock excitation.
Aims: The aim of this paper is to determine the location and the
corresponding physical conditions of the gas responsible for organics emission
lines. The outflows around the small protocluster NGC 2071 are an ideal testbed
to differentiate between various scenarios.
Methods: Using Herschel-HIFI and the SMA, observations of CH3OH, H2CO and
CH3CN emission lines over a wide range of excitation energies were obtained.
Comparisons to a grid of radiative transfer models provide constraints on the
physical conditions. Comparison to H2O line shape is able to trace gas-phase
synthesis versus a sputtered origin.
Results: Emission of organics originates in three spots: the continuum
sources IRS 1 ('B') and IRS 3 ('A') as well as a outflow position ('F').
Densities are above 10 cm and temperatures between 100 to 200 K.
CH3OH emission observed with HIFI originates in all three regions and cannot be
associated with a single region. Very little organic emission originates
outside of these regions.
Conclusions: Although the three regions are small (<1,500 AU), gas-phase
organics likely originate from sputtering of ices due to outflow activity. The
derived high densities (>10 cm) are likely a requirement for organic
molecules to survive from being destroyed by shock products. The lack of
spatially extended emission confirms that organic molecules cannot (re)form
through gas-phase synthesis, as opposed to H2O, which shows strong line wing
emission. The lack of CH3CN emission at 'F' is evidence for a different history
of ice processing due to the absence of a protostar at that location and recent
ice mantle evaporation.Comment: 10 Pages, 8 figures, Accepted for Astronomy and Astrophysic
A Critique of Current Magnetic-Accretion Models for Classical T-Tauri Stars
Current magnetic-accretion models for classical T-Tauri stars rely on a
strong, dipolar magnetic field of stellar origin to funnel the disk material
onto the star, and assume a steady-state. In this paper, I critically examine
the physical basis of these models in light of the observational evidence and
our knowledge of magnetic fields in low-mass stars, and find it lacking.
I also argue that magnetic accretion onto these stars is inherently a
time-dependent problem, and that a steady-state is not warranted.
Finally, directions for future work towards fully-consistent models are
pointed out.Comment: 2 figure
Stellar wind interaction and pick-up ion escape of the Kepler-11 "super-Earths"
We study the interactions between stellar wind and the extended
hydrogen-dominated upper atmospheres of planets and the resulting escape of
planetary pick-up ions from the 5 "super-Earths" in the compact Kepler-11
system and compare the escape rates with the efficiency of the thermal escape
of neutral hydrogen atoms. Assuming the stellar wind of Kepler-11 is similar to
the solar wind, we use a polytropic 1D hydrodynamic wind model to estimate the
wind properties at the planetary orbits. We apply a Direct Simulation Monte
Carlo Model to model the hydrogen coronae and the stellar wind plasma
interaction around Kepler-11b-f within a realistic expected heating efficiency
range of 15-40%. The same model is used to estimate the ion pick-up escape from
the XUV heated and hydrodynamically extended upper atmospheres of Kepler-11b-f.
From the interaction model we study the influence of possible magnetic moments,
calculate the charge exchange and photoionization production rates of planetary
ions and estimate the loss rates of pick-up H+ ions for all five planets. We
compare the results between the five "super-Earths" and in a more general sense
also with the thermal escape rates of the neutral planetary hydrogen atoms. Our
results show that for all Kepler-11b-f exoplanets, a huge neutral hydrogen
corona is formed around the planet. The non-symmetric form of the corona
changes from planet to planet and is defined mostly by radiation pressure and
gravitational effects. Non-thermal escape rates of pick-up ionized hydrogen
atoms for Kepler-11 "super-Earths" vary between approximately 6.4e30 1/s and
4.1e31 1/s depending on the planet's orbital location and assumed heating
efficiency. These values correspond to non-thermal mass loss rates of
approximately 1.07e7 g/s and 6.8e7 g/s respectively, which is a few percent of
the thermal escape rates.Comment: 8 pages, 3 figures, accepted to A&
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Results of an aqueous source term model for a radiological risk assessment of the Drigg LLW Site, U.K.
A radionuclide source term model has been developed which simulates the biogeochemical evolution of the Drigg low level waste (LLW) disposal site. The DRINK (DRIgg Near field Kinetic) model provides data regarding radionuclide concentrations in groundwater over a period of 100,000 years, which are used as input to assessment calculations for a groundwater pathway. The DRINK model also provides input to human intrusion and gaseous assessment calculations through simulation of the solid radionuclide inventory. These calculations are being used to support the Drigg post closure safety case. The DRINK model considers the coupled interaction of the effects of fluid flow, microbiology, corrosion, chemical reaction, sorption and radioactive decay. It represents the first direct use of a mechanistic reaction-transport model in risk assessment calculations
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