scholarly journals Evolution of Earth-like extended exospheres orbiting solar-like stars

2021 ◽  
Author(s):  
Ada Canet ◽  
Ana I Gómez de Castro
Keyword(s):  
Dynamical Evolution ◽  
Young Stars ◽  
Stellar Winds ◽  
Shock Formation ◽  
Mhd Simulations ◽  
Stellar Ages ◽  
Fast Wind ◽  
Ideal Mhd ◽  
Planetary Environment ◽  
Gaseous Envelopes

Abstract Recent observations of the Earth’s exosphere revealed the presence of an extended hydrogenic component that could reach distances beyond 40 planetary radii. Detection of similar extended exospheres around Earth-like exoplanets could reveal crucial facts in terms of habitability. The presence of these rarified hydrogen envelopes is extremely dependent of the planetary environment, dominated by the ionizing radiation and plasma winds coming from the host star. Radiation and fast wind particles ionize the uppermost layers of planetary atmospheres, especially for planets orbiting active, young stars. The survival of the produced ions in the exosphere of such these planets is subject to the action of the magnetized stellar winds, particularly for unmagnetized bodies. In order to address these star-planet interactions, we have carried out numerical 2.5D ideal MHD simulations using the PLUTO code to study the dynamical evolution of tenuous, hydrogen-rich, Earth-like extended exospheres for an unmagnetized planet, at different stellar evolutionary stages: from a very young, solar-like star of 0.1 Gyr to a 5.0 Gyr star. For each star-planet configuration, we show that the morphology of extended Earth-like hydrogen exospheres is strongly dependent of the incident stellar winds and the produced ions present in these gaseous envelopes, showing that the ionized component of Earth-like exospheres is quickly swept by the stellar winds of young stars, leading to large bow shock formation for later stellar ages.

scholarly journals Can Multi-threaded Flux Tubes in Coronal Arcades Support a Magnetohydrodynamic Avalanche?

Solar Physics ◽  
2021 ◽  
Vol 296 (8) ◽  
Author(s):  
J. Threlfall ◽  
J. Reid ◽  
A. W. Hood
Keyword(s):  
Magnetic Fields ◽  
Three Dimensional ◽  
Coronal Loops ◽  
Flux Tubes ◽  
Single Thread ◽  
Mhd Instabilities ◽  
Mhd Instability ◽  
Mhd Simulations ◽  
Ideal Mhd ◽  
Model Geometry

AbstractMagnetohydrodynamic (MHD) instabilities allow energy to be released from stressed magnetic fields, commonly modelled in cylindrical flux tubes linking parallel planes, but, more recently, also in curved arcades containing flux tubes with both footpoints in the same photospheric plane. Uncurved cylindrical flux tubes containing multiple individual threads have been shown to be capable of sustaining an MHD avalanche, whereby a single unstable thread can destabilise many. We examine the properties of multi-threaded coronal loops, wherein each thread is created by photospheric driving in a realistic, curved coronal arcade structure (with both footpoints of each thread in the same plane). We use three-dimensional MHD simulations to study the evolution of single- and multi-threaded coronal loops, which become unstable and reconnect, while varying the driving velocity of individual threads. Experiments containing a single thread destabilise in a manner indicative of an ideal MHD instability and consistent with previous examples in the literature. The introduction of additional threads modifies this picture, with aspects of the model geometry and relative driving speeds of individual threads affecting the ability of any thread to destabilise others. In both single- and multi-threaded cases, continuous driving of the remnants of disrupted threads produces secondary, aperiodic bursts of energetic release.

scholarly journals UV Wind Variability in B Supergiants and its Implications for Wind Structures

1999 ◽  
Vol 169 ◽  
pp. 214-217
Author(s):  
Derek Massa ◽  
Raman K. Prinja
Keyword(s):  
Spontaneous Generation ◽  
Stellar Winds ◽  
Shock Formation ◽  
Theoretical Studies ◽  
Wind Variability ◽  
Dynamic Spectra

AbstractWe discuss why B supergiant winds are particularly well suited for wind studies, and present or refer to dynamic spectra which suggest the presence of disks, bifurcated winds, shock formation, rotationally modulated winds and the spontaneous generation of wind enhancements. They underscore the strength and richness of wind variability in B supergiants and the challenges these phenomena present to theoretical studies of stellar winds.

scholarly journals Physical Processes Responsible for the Removal of Circumstellar Disks

2000 ◽  
Vol 197 ◽  
pp. 403-413
Author(s):  
Doug Johnstone
Keyword(s):  
Ultraviolet Radiation ◽  
Physical State ◽  
Solar Nebula ◽  
Circumstellar Disks ◽  
Young Stars ◽  
Stellar Winds ◽  
Physical Processes ◽  
Central Source

The most likely processes responsible for the removal of circumstellar disks around young stars are reviewed with emphasis on the physical state of the disk during the period of destruction and the timescale for disk removal. Four disk dispersal mechanisms are discussed in detail: 1) viscous accretion of material onto the central source, 2) close stellar encounters, 3) stellar winds, and 4) photoevaporation by ultraviolet radiation. While viscous accretion is shown to be efficient in the inner regions of disks (r < 10 AU), photoevaporation is the principal process of disk dispersal at large radii (r > 10 AU). The commonly held view that stellar winds removed the remnant Solar Nebula is seriously questioned.

Stellar Winds from Young Stars

1985 ◽  
Vol T11 ◽  
pp. 71-75 ◽  
Author(s):  
Nino Panagia
Keyword(s):  
Young Stars ◽  
Stellar Winds

scholarly journals Radial-velocity jitter of stars as a function of observational timescale and stellar age

2019 ◽  
Vol 632 ◽  
pp. A37 ◽  
Author(s):  
Stefan S. Brems ◽  
Martin Kürster ◽  
Trifon Trifonov ◽  
Sabine Reffert ◽  
Andreas Quirrenbach
Keyword(s):  
Radial Velocity ◽  
Young Stars ◽  
Data Sets ◽  
Activity Levels ◽  
Sampled Data ◽  
Planet Detection ◽  
Dwarf Stars ◽  
Stellar Ages ◽  
Irregularly Sampled Data ◽  
Instrumental Precision

Context. Stars show various amounts of radial-velocity (RV) jitter due to varying stellar activity levels. The typical amount of RV jitter as a function of stellar age and observational timescale has not yet been systematically quantified, although it is often larger than the instrumental precision of modern high-resolution spectrographs used for Doppler planet detection and characterization. Aims. We aim to empirically determine the intrinsic stellar RV variation for mostly G and K dwarf stars on different timescales and for different stellar ages independently of stellar models. We also focus on young stars (≲30 Myr), where the RV variation is known to be large. Methods. We use archival FEROS and HARPS RV data of stars which were observed at least 30 times spread over at least two years. We then apply the pooled variance (PV) technique to these data sets to identify the periods and amplitudes of underlying, quasiperiodic signals. We show that the PV is a powerful tool to identify quasiperiodic signals in highly irregularly sampled data sets. Results. We derive activity-lag functions for 20 putative single stars, where lag is the timescale on which the stellar jitter is measured. Since the ages of all stars are known, we also use this to formulate an activity–age–lag relation which can be used to predict the expected RV jitter of a star given its age and the timescale to be probed. The maximum RV jitter on timescales of decades decreases from over 500 m s−1 for 5 Myr-old stars to 2.3 m s−1 for stars with ages of around 5 Gyr. The decrease in RV jitter when considering a timescale of only 1 d instead of 1 yr is smaller by roughly a factor of 4 for stars with an age of about 5 Myr, and a factor of 1.5 for stars with an age of 5 Gyr. The rate at which the RV jitter increases with lag strongly depends on stellar age and reaches 99% of the maximum RV jitter over a timescale of a few days for stars that are a few million years old, up to presumably decades or longer for stars with an age of a few gigayears.

scholarly journals Protostellar Outflows at the EarliesT Stages (POETS)

2018 ◽  
Vol 619 ◽  
pp. A107 ◽  
Author(s):  
A. Sanna ◽  
L. Moscadelli ◽  
C. Goddi ◽  
V. Krishnan ◽  
F. Massi
Keyword(s):  
Spectral Index ◽  
Young Stars ◽  
Radio Continuum ◽  
Continuum Emission ◽  
Stellar Winds ◽  
Distance Measurements ◽  
Very Large Array ◽  
Star Forming ◽  
Continuum Source ◽  
Star Forming Regions

Context. Weak and compact radio continuum and H2O masers are preferred tracers of the outflow activity nearby very young stars. Aims. We want to image the centimeter free–free continuum emission in the range 1–7 cm (26–4 GHz), which arises in the inner few 1000 au from those young stars also associated with bright H2O masers. We seek to study the radio continuum properties in combination with the H2O maser kinematics to quantify the outflow energetics powered by single young stars. Methods. We made use of the Karl G. Jansky Very Large Array (VLA) in the B configuration at K band and the A configuration at both Ku and C bands in order to image the radio continuum emission toward 25 H2O maser sites with an angular resolution and thermal rms on the order of 0.′′1 and 10 μJy beam−1, respectively. These targets add to our pilot study of 11 maser sites previously presented. The sample of H2O maser sites was selected among those regions that have accurate distance measurements, obtained through maser trigonometric parallaxes, and H2O maser luminosities in excess of 10−6 L⊙. Results. We present high-resolution radio continuum images of 33 sources belonging to 25 star-forming regions. In each region, we detect radio continuum emission within a few 1000 au of the H2O masers’ position; 50% of the radio continuum sources are associated with bolometric luminosities exceeding 5 × 103 L⊙, including W33A and G240.32 + 0.07. We provide a detailed spectral index analysis for each radio continuum source, based on the integrated fluxes at each frequency, and produce spectral index maps with the multifrequency synthesis deconvolution algorithm of CASA. The radio continuum emission traces thermal bremsstrahlung in (proto)stellar winds and jets that have flux densities at 22 GHz below 3 mJy and spectral index values between − 0.1 and 1.3. We prove a strong correlation (r > 0.8) between the radio continuum luminosity (Lrad) and the H2O maser luminosity (LH2O) of (L8 GHz∕mJy kpc2) = 103.8 × (LH2O L⊙)0.74. This power-law relation is similar to that between the radio continuum and bolometric luminosities, which confirms earlier studies. Since H2O masers are excited through shocks driven by (proto)stellar winds and jets, these results provide support to the idea that the radio continuum emission around young stars is dominated by shock ionization, and this holds over several orders of magnitude of stellar luminosites (1–105 L⊙).

On the morphology and internal kinematics of PNe

1997 ◽  
Vol 180 ◽  
pp. 184-189
Author(s):  
A. Manchado
Keyword(s):  
Stellar Evolution ◽  
Great Degree ◽  
Planetary Nebula ◽  
High Density ◽  
Stellar Winds ◽  
Low Density ◽  
Fast Wind ◽  
Slow Wind

The study of the morphology of planetary nebula (PN) is fundamental for addressing several questions in the context of stellar evolution. An AGB star can loose most of its mass due to strong stellar winds. Kwok et al. (1978) proposed that the interaction of a low-density fast wind with a high-density slow wind, will form the PN. This model can account for the round observed PNe with a great degree of symmetry. However as we will see later, round PNe are not the most common ones. Therefore a mechanism for causing asymmetry has to be invoked. Several processes have been proposed by different authors.

scholarly journals The surprisingly small impact of magnetic fields on the inner accretion flow of Sagittarius A* fueled by stellar winds

2019 ◽  
Vol 492 (3) ◽  
pp. 3272-3293 ◽  
Author(s):  
S M Ressler ◽  
E Quataert ◽  
J M Stone
Keyword(s):  
Angular Momentum ◽  
Magnetic Fields ◽  
Event Horizon ◽  
Initial Conditions ◽  
Rotational Instability ◽  
Stellar Winds ◽  
Accretion Flow ◽  
Sagittarius A ◽  
Mhd Simulations ◽  
Sgr A

ABSTRACT We study the flow structure in 3D magnetohydrodynamic (MHD) simulations of accretion on to Sagittarius A* via the magnetized winds of the orbiting Wolf–Rayet stars. These simulations cover over 3 orders of magnitude in radius to reach ≈300 gravitational radii, with only one poorly constrained parameter (the magnetic field in the stellar winds). Even for winds with relatively weak magnetic fields (e.g. plasma β ∼ 106), flux freezing/compression in the inflowing gas amplifies the field to β ∼ few well before it reaches the event horizon. Overall, the dynamics, accretion rate, and spherically averaged flow profiles (e.g. density, velocity) in our MHD simulations are remarkably similar to analogous hydrodynamic simulations. We attribute this to the broad distribution of angular momentum provided by the stellar winds, which sources accretion even absent much angular momentum transport. We find that the magneto-rotational instability is not important because of (i) strong magnetic fields that are amplified by flux freezing/compression, and (ii) the rapid inflow/outflow times of the gas and inefficient radiative cooling preclude circularization. The primary effect of magnetic fields is that they drive a polar outflow that is absent in hydrodynamics. The dynamical state of the accretion flow found in our simulations is unlike the rotationally supported tori used as initial conditions in horizon scale simulations, which could have implications for models being used to interpret Event Horizon Telescope and GRAVITY observations of Sgr A*.

scholarly journals Long-term stability of the dead-zone in proto-planetary disks

2010 ◽  
Vol 6 (S276) ◽  
pp. 418-419
Author(s):  
Mario Flock ◽  
Neal Turner ◽  
Natalia Dzyurkevich ◽  
Hubert Klahr
Keyword(s):  
Dead Zone ◽  
Cosmic Ray ◽  
X Ray ◽  
Long Term Stability ◽  
Mhd Simulations ◽  
Magnetic Resistivity ◽  
Ideal Mhd ◽  
Self Consistent ◽  
Ionization Fraction

AbstractWe present 3D global non-ideal MHD simulations with a self consistent dynamic evolution of ionization fraction of the gas as result of reduced chemical network. We include X-ray ionization from the star as well as cosmic ray ionization. Based on local gas density and temperature in our chemical network, we determine the magnetic resistivity, which is fed back in MHD simulations. Parameters for dust size and abundance are chosen to have accreting layers and a laminar “dead” mid-plane.

scholarly journals SPIN EVOLUTION OF ACCRETING YOUNG STARS. II. EFFECT OF ACCRETION-POWERED STELLAR WINDS

2012 ◽  
Vol 745 (1) ◽  
pp. 101 ◽  
Author(s):  
Sean P. Matt ◽  
Giovanni Pinzón ◽  
Thomas P. Greene ◽  
Ralph E. Pudritz
Keyword(s):  
Young Stars ◽  
Stellar Winds ◽  
Spin Evolution
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