The Sun Through Time Güdel, Manuel
Space science reviews,
12/2020, Letnik:
216, Številka:
8
Journal Article
Recenzirano
Odprti dostop
Magnetic activity of stars like the Sun evolves in time because of spin-down owing to angular momentum removal by a magnetized stellar wind. These magnetic fields are generated by an internal dynamo ...driven by convection and differential rotation. Spin-down therefore converges at an age of about 700 Myr for solar-mass stars to values uniquely determined by the stellar mass and age. Before that time, however, rotation periods and their evolution depend on the initial rotation period of a star after it has lost its protostellar/protoplanetary disk. This non-unique rotational evolution implies similar non-unique evolutions for stellar winds and for the stellar high-energy output. I present a summary of evolutionary trends for stellar rotation, stellar wind mass loss and stellar high-energy output based on observations and models.
Aims. We aim to describe the pre-main-sequence and main-sequence evolution of X-ray and extreme-ultaviolet radiation of a solar-mass star based on its rotational evolution starting with a realistic ...range of initial rotation rates. Methods. We derive evolutionary tracks of X-ray radiation based on a rotational evolution model for solar-mass stars and the rotation-activity relation. We compare these tracks to X-ray luminosity distributions of stars in clusters with different ages. Results. We find agreement between the evolutionary tracks derived from rotation and the X-ray luminosity distributions from observations. Depending on the initial rotation rate, a star might remain at the X-ray saturation level for very different time periods, from ≈10 Myr to ≈300 Myr for slow and fast rotators, respectively. Conclusions. Rotational evolution with a spread of initial conditions leads to a particularly wide distribution of possible X-ray luminosities in the age range of 20–500 Myr, before rotational convergence and therefore X-ray luminosity convergence sets in. This age range is crucial for the evolution of young planetary atmospheres and may thus lead to very different planetary evolution histories.
Abstract We present new Chandra X-ray observations of TAP 26, a ≈17 Myr old magnetically active weak-lined T Tauri star that has been reported to host a massive planet in a ≈10.8 day orbit. At a ...separation of a = 0.097 au the planet will be exposed to intense X-ray and UV radiation from the star. The first observation caught the star in a state of elevated X-ray emission with variability on a timescale of a few hours and an X-ray temperature kT x ≈ 2–4 keV. Two subsequent observations 5–10 days later showed slow variability and a lower X-ray flux and temperature ( kT x ≈ 1 keV). We characterize the X-ray emission and estimate the X-ray ionization and heating rates that will need to be incorporated into realistic models of the planet’s atmosphere.
Abstract
Power-law size distributions are the hallmarks of nonlinear energy dissipation processes governed by self-organized criticality (SOC). Here we analyze 75 data sets of stellar flare size ...distributions, mostly obtained from the Extreme-Ultraviolet Explorer and the Kepler mission. We aim to answer the following questions for size distributions of stellar flares. (i) What are the values and uncertainties of power-law slopes? (ii) Do power-law slopes vary with time? (iii) Do power-law slopes depend on the stellar spectral type? (iv) Are they compatible with solar flares? (v) Are they consistent with SOC models? We find that the observed size distributions of stellar flare fluences (or energies) exhibit power-law slopes of
α
E
= 2.09 ± 0.24 for optical data sets observed with Kepler. The observed power-law slopes do not show much time variability and do not depend on the stellar spectral type (M, K, G, F, A, giants). In solar flares, we find that background subtraction lowers the uncorrected value of
α
E
= 2.20 ± 0.22 to
α
E
= 1.57 ± 0.19. Furthermore, most of the stellar flares are temporally not resolved in low-cadence (30 minutes) Kepler data, which causes an additional bias. Taking these two biases into account, the stellar flare data sets are consistent with the theoretical prediction
of SOC models, i.e.,
α
E
= 1.5. Thus, accurate power-law fits require automated detection of the inertial range and background subtraction, which can be modeled with the generalized Pareto distribution, finite-system size effects, and extreme event outliers.
•Earth Archean upper atmosphere studied using state-of-the-art models.•High atmospheric CO2 levels and low solar activity needed to prevent rapid escape to space.•At least 40% of atmospheric gas was ...CO2 at 3.8 billion year ago.•Greenhouse effect from CO2 was strong enough to solve faint young Sun problem.•The Sun must have been born as a slow rotator with low activity.
Despite their importance for determining the evolution of the Earth's atmosphere and surface conditions, the evolutionary histories of the Earth's atmospheric CO2 abundance during the Archean eon and the Sun's activity are poorly constrained. In this study, we apply a state-of-the-art physical model for the upper atmosphere of the Archean Earth to study the effects of different atmospheric CO2/N2 mixing ratios and solar activity levels on the escape of the atmosphere to space. We find that unless CO2 was a major constituent of the atmosphere during the Archean eon, enhanced heating of the thermosphere by the Sun's strong X-ray and ultraviolet radiation would have caused rapid escape to space. We derive lower limits on the atmospheric CO2 abundance of approximately 40% at 3.8 billion years ago, which is likely enough to counteract the faint young Sun and keep the Earth from being completely frozen. Furthermore, our results indicate that the Sun was most likely born as a slow to moderate rotating young G-star to prevent rapid escape, putting essential constraints on the Sun's activity evolution throughout the solar system's history. In case that there were yet unknown cooling mechanisms present in the Archean atmosphere, this could reduce our CO2 stability limits, and it would allow a more active Sun.
The first flare on the Sun was observed exactly 150 years ago. During most of the long history, only secondary effects have been noticed, so flares remained a riddle. Now the primary flare products, ...high-energy electrons and ions, can be spatially resolved in hard X-rays (HXRs) and gamma rays on the Sun. Soft X-rays (SXRs) are observed from most stars, including young stellar objects. Structure and bulk motions of the corona are imaged on the Sun in high temperature lines and are inferred from line shifts in stellar coronae. Magnetic reconnection is the trigger for reorganization of the magnetic field into a lower energy configuration. A large fraction of the energy is converted into nonthermal particles that transport the energy to higher density gas, heating it to SXR-emitting temperatures. Flares on young stars are several orders of magnitude more luminous and more frequent; they significantly ionize protoplanetary disks and planetary ionospheres. PUBLICATION ABSTRACT
We review the origin and evolution of the atmospheres of Earth, Venus and Mars from the time when their accreting bodies were released from the protoplanetary disk a few million years after the ...origin of the Sun. If the accreting planetary cores reached masses
≥
0.5
M
Earth
before the gas in the disk disappeared, primordial atmospheres consisting mainly of H
2
form around the young planetary body, contrary to late-stage planet formation, where terrestrial planets accrete material after the nebula phase of the disk. The differences between these two scenarios are explored by investigating non-radiogenic atmospheric noble gas isotope anomalies observed on the three terrestrial planets. The role of the young Sun’s more efficient EUV radiation and of the plasma environment into the escape of early atmospheres is also addressed. We discuss the catastrophic outgassing of volatiles and the formation and cooling of steam atmospheres after the solidification of magma oceans and we describe the geochemical evidence for additional delivery of volatile-rich chondritic materials during the main stages of terrestrial planet formation. The evolution scenario of early Earth is then compared with the atmospheric evolution of planets where no active plate tectonics emerged like on Venus and Mars. We look at the diversity between early Earth, Venus and Mars, which is found to be related to their differing geochemical, geodynamical and geophysical conditions, including plate tectonics, crust and mantle oxidation processes and their involvement in degassing processes of secondary
N
2
atmospheres. The buildup of atmospheric
N
2
,
O
2
, and the role of greenhouse gases such as
CO
2
and
CH
4
to counter the Faint Young Sun Paradox (FYSP), when the earliest life forms on Earth originated until the Great Oxidation Event
≈
2.3 Gyr ago, are addressed. This review concludes with a discussion on the implications of understanding Earth’s geophysical and related atmospheric evolution in relation to the discovery of potential habitable terrestrial exoplanets.
The Sun’s magnetic activity has steadily declined during its main-sequence life. While the solar photospheric luminosity was about 30% lower 4.6 Gyr ago when the Sun arrived on the main sequence ...compared to present-day levels, its faster rotation generated enhanced magnetic activity; magnetic heating processes in the chromosphere, the transition region, and the corona induced ultraviolet, extreme-ultraviolet, and X-ray emission about 10, 100, and 1000 times, respectively, the present-day levels, as inferred from young solar-analog stars. Also, the production rate of accelerated, high-energy particles was orders of magnitude higher than in present-day solar flares, and a much stronger wind escaped from the Sun, permeating the entire solar system. The consequences of the enhanced radiation and particle fluxes from the young Sun were potentially severe for the evolution of solar-system planets and moons. Interactions of high-energy radiation and the solar wind with upper planetary atmospheres may have led to the escape of important amounts of atmospheric constituents. The present dry atmosphere of Venus and the thin atmosphere of Mars may be a product of early irradiation and heating by solar high-energy radiation. High levels of magnetic activity are also inferred for the pre-main sequence Sun. At those stages, interactions of high-energy radiation and particles with the circumsolar disk in which planets eventually formed were important. Traces left in meteorites by energetic particles and anomalous isotopic abundance ratios in meteoritic inclusions may provide evidence for a highly active pre-main sequence Sun. The present article reviews these various issues related to the magnetic activity of the young Sun and the consequent interactions with its environment. The emphasis is on the phenomenology related to the production of high-energy photons and particles. Apart from the activity on the young Sun, systematic trends applicable to the entire main-sequence life of a solar analog are discussed.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Million-Degree Plasma Pervading the Extended Orion Nebula Güdel, Manuel; Briggs, Kevin R.; Montmerle, Thierry ...
Science (American Association for the Advancement of Science),
01/2008, Letnik:
319, Številka:
5861
Journal Article
Recenzirano
Odprti dostop
Most stars form as members of large associations within dense, very cold (10 to 100 kelvin) molecular clouds. The nearby giant molecular cloud in Orion hosts several thousand stars of ages less than ...a few million years, many of which are located in or around the famous Orion Nebula, a prominent gas structure illuminated and ionized by a small group of massive stars (the Trapezium). We present x-ray observations obtained with the X-ray Multi-Mirror satellite XMM-Newton, revealing that a hot plasma with a temperature of 1.7 to 2.1 million kelvin pervades the southwest extension of the nebula. The plasma flows into the adjacent interstellar medium. This x-ray outflow phenomenon must be widespread throughout our Galaxy.
Context. The feedback of star formation to the parent cloud is conventionally examined through the study of molecular outflows. Little is known, however, about the effect that atomic ejecta tracing ...fast shocks can have on small scales or on global cloud properties. Aims. Our immediate objective is to study the morphology of protostellar ejecta through far-infrared atomic lines, compare them to other outflow tracers, and associate them with their driving sources. The main goal is to study the feedback from atomic jet emission that is excited by fast shocks on the parent cloud material, and examine the relative importance of atomic jets as regulators of the star formation process. Methods. We employed O i and C ii line maps of the NGC 1333 star-forming region observed with Herschel/PACS. We studied the detailed morphology and velocity distribution of the O i line using channel and line-centroid maps. We derived the momentum, energy, and mass flux for all the bipolar outflows traced by O i line emission. We compared the O i morphology to CO and H2 emission, and its dynamical and kinematic properties to the emission corresponding to CO outflows. Results. We find that the line-centroid maps can trace velocity structures down to 5 km s-1 which is a factor of ~20 beyond the nominal velocity resolution reached by Herschel/PACS. These maps reveal an unprecedented degree of details that significantly assist in the association and characterization of outflows. We associate most of the O i emission with ejecta from embedded protostars. The spatial distribution of the O i emission closely follows the CO emission pattern and strongly correlates to the spatial distribution of the H2 emission, with the latter indicating excitation in shocks. The O i momentum accounts for only ~1% of the momentum carried by the large-scale CO outflows. The energy released in shocks, however, corresponds to 50–100% of the energy carried away by outflows. Mass-flux estimates of the atomic gas range between 10-6 and 10-7M⊙ km s-1, which is in line with previous estimations. Conclusions. The detailed study of line-centroid shifts in maps introduced for the first time in this study can provide an invaluable tool for the study of outflows. Assuming that the O i corresponds to an atomic jet that also drives the CO outflow, we find that the mass traced by O i corresponds to a small fraction compared to the mass corresponding to the CO outflows. This may indicate that only a small fraction of the mass of atomic jets is excited in shocks, which is consistent with the sizes of the shock-cooling zones compared to the total jet length. The estimated ratios of the jet to the outflow momenta and energies are also consistent with the results of two-component, nested jet and outflow simulations, where jets are associated with episodic accretion events. The mass flux from the O i calculated here and in other studies shows no clear evolutionary pattern with the age of the parent source, which is in support of the scenario that jets are associated with episodic accretion events. The energy contribution of the jet-induced shocks to the cloud, is important and can equal the energy deposited by outflows. We therefore conclude that the atomic component, either ejected or formed in shocks plays an important role in generating and maintaining turbulence in clouds but also in dissipating the cloud gas.
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