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Both stars and planets can lose mass through an expansive wind outflow, often constrained or channeled by magnetic fields that form a surrounding magnetosphere. The very strong winds of massive stars are understood to be driven by line-scattering of the star’s radiative momentum, while in the Sun and even lower-mass stars a much weaker mass loss arises from the thermal expansion of a mechanically heated corona. In exoplanets around such low-mass stars, the radiative heating and wind interaction can lead to thermal expansion or mechanical ablation of their atmospheres. Stellar magnetospheres result from the internal trapping of the wind outflow, while planetary magnetospheres are typically shaped by the external impact from the star’s wind. But in both cases the stressing can drive magnetic reconnection that results in observable signatures such as X-ray flares and radio outbursts. This review will aim to give an overview of the underlying physics of these processes with emphasis on their similarities and distinctions for stars vs. planets.
In this invited review talk I summarize some of the recent observational advances in understanding mass loss from low-mass stars. This can take the form of a relatively steady wind, or stochastically occurring coronal mass ejections (CMEs). In recent years, there has been an expansion of observational signatures used to probe mass loss in low-mass stars. These observational tools span the electromagnetic spectrum. There has also been a resurgence of interest in this topic because of its potential impact on exoplanet space weather and habitability. The numerous recent observational and theoretical results also point to the complexities involved, rather than using simple scalings from solar understanding. This underscores the need to understand reconnection and eruption processes on magnetically active stars as a tool to putting our Sun in context.
Mass loss plays a key role in the evolution of massive stars and their environment. High mass-loss events are traced by complex circumstellar ejecta and intricate line profiles across the upper Hertzsprung-Russell diagram for massive stars in different evolutionary stages. The basic physics of radiation-driven stellar wind for hot stars is well understood. However, the driving mechanisms and related instabilities for their enhanced mass-loss episodes and the driving mechanisms for the mass loss of cool stars are still debated. In this review, the mass-loss characteristics and the possible mechanisms will be surveyed for an observational set of prominent massive stellar populations that experience outflows, strong stellar winds, and periods of enhanced and eruptive mass loss; massive young stellar objects, OB-type stars, red supergiants, warm hypergiants, luminous blue variables, and Wolf-Rayet stars.
We have recently hit the milestone of 5,000 exoplanets discovered. In stark contrast with the Solar System, most of the exoplanets we know to date orbit extremely close to their host stars, causing them to lose copious amounts of gas through atmospheric escape at some stage in their lives. In some planets, this process can be so dramatic that they shrink in timescales of a few million to billions of years, imprinting features in the demographics of transiting exoplanets. Depending on the transit geometry, ionizing conditions, and atmospheric properties, a planetary outflow can be observed using transmission spectroscopy in the ultraviolet, optical or near-infrared. In this review, we will discuss the main techniques to observe evaporating exoplanets and their results. To date, we have evidence that at least 28 exoplanets are currently losing their atmospheres, and the literature has reported at least 42 non-detections.
Red supergiants (RSGs) are evolved massive stars in a stage preceding core-collapse supernova. Understanding evolved-phases of these cool stars is key to understanding the cosmic matter cycle of our Universe, since they enrich the cosmos with newly formed elements. However, the physical processes that trigger mass loss in their atmospheres are still not fully understood, and remain one of the key questions in stellar astrophysics. We use a new method to study the extended atmospheres of these cold stars, exploring the effect of a stellar wind for both a simple radiative equilibrium model and a semi-empirical model that accounts for a chromospheric temperature structure. We then can compute the intensities, fluxes and visibilities matching the observations for the different instruments at the Very Large Telescope Interferometer (VLTI). Specifically, when comparing with the atmospheric structure of HD 95687 based on published VLTI/AMBER data, we find that our model can accurately match these observations in the K-band, showing the enormous potential of this methodology to reproduce extended atmospheres of RSGs.
Recent abrupt changes of CW Leonis may indicate that we are witnessing the moment that the central carbon star is evolving off the Asymptotic Giant Branch (AGB) and entering into the pre-planetary nebula (PPN) phase. The recent appearance of a red compact peak at the predicted stellar position is possibly an unveiling event of the star, and the radial beams emerging from the stellar position resemble the feature of the PPN Egg Nebula. The increase of light curve over two decades is also extraordinary, and it is possibly related to the phase transition. Decadal-period variations are further found in the residuals of light curves, in the relative brightness of radial beams, and in the extended halo brightness distribution. Further monitoring of the recent dramatic and decadal-scale changes of this most well-known carbon star CW Leonis at the tip of AGB is still highly essential, and will help us gain a more concrete understanding on the conditions for transition between the late stellar evolutionary phases.
This contribution is based on the work published by (Pinzón et al. 2021) in which we computed rotation rates for a sample of 79 young stars (∼3 Myr) in a wide range of stellar masses (from T Tauri Stars to Herbig Ae/Be stars) in in the Orion Star Formation Complex (OSFC). We study whether the magnetospheric accretion scenario (MA), valid for young low mass stars, may be applied over a wide range of stellar masses of not. Under the assumption that stellar winds powered by stellar accretion are the main source for the stellar spin down, the hypothesis of an extension of MA toward higher masses seems plausible. A comparison with Ap/Bp stars suggest that HAeBes should suffer a loss of angular momentum by a factor between 12 and 80 during the first 10 Myr in order to match the magnetic Ap/Bp zone in HR diagram.
V530 Per is a solar-like member of the young open cluster α Persei, with an ultra-short rotation period (P∼0.32d). We report on two spectropolarimetric campaigns using ESPaDOnS, aimed at characterizing the short-term variability of its magnetic activity and large-scale magnetic field. We used time-resolved spectropolarimetric observations obtained in 2006 and 2018 and reconstructed the brightness distribution and large-scale magnetic field geometry of V530 Per through Zeeman-Doppler imaging. Using the same data sets, we also mapped the spatial distribution of prominences through tomography of Hα emission. We reconstruct, at both epochs, a large, dark spot occupying the polar region of V530 Per while smaller (dark and bright) spots were reconstructed at lower latitudes. The maximal field strength reached ∼1 kG. The prominence pattern displayed a stable component that was confined close to the corotation radius. In 2018, we also observed rapidly evolving Hα emitting structures, over timescales ranging from minutes to days. The fast Hα evolution was not linked to any detected photospheric changes in the spot or magnetic coverage.
We have investigated the evolution of 12 “water fountain” sources in real time in the accompanying H2O 2o and SiO masers through our FLASHING (Finest Legacy Acquisitions of SiO-/ H2O 2o-maser Ignitions by Nobeyama Generation) project. It has been confirmed that these masers are excellent probes of new jet blob ejections, acceleration of the material supplied from the parental circumstellar envelope and entrained by the stellar jets yielding its deceleration. Possible periodic variations of the maser emission, reflecting properties of the central dying stars or binary systems, will be further investigated.
Radiation-driven mass-loss is an important, but still highly debated, driver for the evolution of massive stars. Current massive star evolution models rely on the theoretical prediction that low luminosity massive stars experience a sudden increase in mass loss below a stellar effective temperature of about 20 000 K. However, novel radiation-driven mass-loss rate predictions show no such bi-stability jump, which effects the post main-sequence evolution of massive stars. The ULLYSES data set provides a unique opportunity to investigate the theoretical bi-stability jump dichotomy and may help to assess the existence of the bi-stability jump in massive star winds. By utilising UV spectra from ULLYSES combined with X-shooter optical data we obtain empirical mass-loss rate constraints, that are no longer degenerate to the effects of wind clumping, and derive novel empirical constraints on the mass-loss behavior across the temperature range of the bi-stability jump. Current preliminary results do not show a clear presence of a bi-stability jump.
We conducted CO J=1→0 emission line observations for nearby AGB stars using the Nobeyama 45 m telescope. Comparing our results with those from CO J=3→2 observations with JCMT, the circumstellar envelopes observed in CO J=1→0 look more extended than J=3→2. Thus, we could trace the outer, cold parts of the envelopes. We also found four stars in which the CO/13CO ratio changes dramatically outward, but the change implies the effect of selective photodissociation by interstellar ultraviolet radiation, not the third dredge up in the stellar interior. We moreover found two unique stars with aspherical envelope morphology.
Atmospheric escape is a fundamental phenomenon shaping the structure and evolution of planetary atmospheres. Physics of planetary winds range from global processes such as tidal interactions with the host star, through large-scale hydrodynamic outflow, to essentially microphysical kinetic effects, including Jeans-like escape and the interaction of planetary atmospheres with stellar winds and the own magnetic fields of planets. Each of these processes is expected to be most relevant for planets of different properties and at different stages in planetary and stellar evolution. Thus, it is expected that the hydrodynamic outflow guides the evolution of hydrogen-dominated atmospheres of planets having low masses (below that of Neptune) and/or close-in orbits, while the kinetic effects are most important for the long-term evolution of planets with secondary atmospheres, similar to the inner planets in the Solar System. Finally, each of these processes is affected by the interaction with stellar winds.
The stellar wind from low-mass stars affects the evolution of the whole stellar system in various ways. To better describe its quantitative contributions, we need to understand the theoretical aspects of stellar wind formation. Here, we present an overview of the theoretical models of stellar wind. The classical thermally-driven wind model fails in reproducing the anti-correlation between the coronal temperature and wind speed observed in the solar wind, thus needs modification with magnetic-energy injection. Specifically, energy input by Alfvén wave is likely to be important. Indeed, a number of solar-wind observations are well reproduced by the Alfvén-wave models, although it could be risky to directly apply the Alfvén-wave models to general low-mass stars. For a better description of stellar wind from low-mass stars with a variety of activity levels, the hybrid model would be better, in which we consider the effect of flux emergence as well as Alfvén wave.
In the regime of hot stars, winds were not seen as a common thing until the era of UV astronomy. Since we have access to the UV wavelength range, it has become clear that winds are not an exotic phenomenon limited to some special objects, but actually ubiquitous among hot and massive stars. The opacities due to spectral lines are the decisive ingredient that allows hot, massive stars to launch powerful winds. While the fundamental principles of these so-called line-driven winds have been realized decades ago, their proper quantitative prediction is still a major challenge today. Established theoretical and empirical descriptions have allowed us to make major progress on all astrophysical scales. However, we are now reaching their limitations as we still lack various fundamental insights on the nature of hot star winds, thereby hampering us from drawing deeper conclusions, not least when dealing with stellar or sub-stellar companions. This has spawned a new generation of researchers searching for answers with a yet unprecedented level of detail in observational and new theoretical approaches.
In these proceedings, the fundamental principles of driving hot star winds will be briefly reviewed. Starting from the classical CAK theory and its extensions, over Monte Carlo and recent comoving-frame-based simulations, the different methods to describe and model the acceleration of hot star winds will be introduced. The review continues with briefly discussing instabilities as well as qualitative and quantitative insights for OB- and Wolf-Rayet-star winds. Moreover, the challenges of companions and their impact on radiation-driven winds are outlined.
Stars lose mass and angular momentum during their lifetimes. Observations of H-alpha absorption of a number of low mass stars, show prominences transiting the stellar disc and being ejected into the extended stellar wind. Analytic modelling have shown these M-dwarf coronal structures growing to be orders of magnitude larger than their solar counterparts. This makes prominences responsible for mass and angular momentum loss comparable to that due to the stellar wind. We present results from a numerical study which used magnetohydrodynamic simulations to model the balance between gravity, magnetic confinement, and rotational acceleration. This allows us to study the time dependent nature of prominence formation. We demonstrate that a prominence, formed beyond the co-rotation radius, is ejected into the extended stellar wind in the slingshot prominence paradigm. Mass, angular momentum flux and ejection frequency have been calculated for a representative cool star, in the so-called Thermal Non-Equilibrium (TNE) regime.
Spectral observations in the Ly-α line have shown that atmospheric escape is variable and for the exoplanet HD189733b, the atmospheric evaporation goes from undetected to enhanced evaporation in a 1.5 years interval. To understand the temporal variation in the atmospheric escape, we investigate the effect of flares, winds, and CMEs on the atmosphere of hot Jupiter HD189733b using 3D self-consistent radiation hydrodynamic simulations. We consider four cases: first, the quiescent phase including stellar wind; secondly, a flare; thirdly, a CME; and fourthly, a flare followed by a CME. We find that the flare alone increases the atmospheric escape rate by only 25%, while the CME leads to a factor of 4 increments, in comparison to the quiescent case. We also find that the flare alone cannot explain the observed high blue-shifted velocities seen in the Ly-α. The CME, however, leads to an increase in the velocity of escaping atmospheres, enhancing the blue-shifted transit depth.
The intense extreme ultraviolet radiation heats the upper atmosphere of close-in exoplanets and drives the atmospheric escape. The escaping process determines the planetary evolution of close-in planets. The mass loss rate depends on the UV flux at the planet. We introduce the relevant physical quantities which describe the dominant physics in the atmosphere. We find that the equilibrium temperature and the characteristic temperature determine whether the system becomes energy-limited or recombination-limited. We classify the observed close-in planets using the physical conditions. We also find that many of the Lyman-α absorptions detected planets receive intenser flux than the critical flux which can be determined from physical conditions. Our classification method can quantitatively reveal whether the EUV is not strong enough to drive the outflow or the Lyman- α absorption is not detected for some reason (e.g. stellar wind confinement). We also discuss the thermo-chemical structure of hydrodynamic simulations with the relevant physics.
The winds of hot, massive stars are variable from processes happening on both large and small spatial scales. A particular case of such wind variability is ‘discrete-absorption components’ (DACs) that manifest themselves as outward moving density features in UV resonance line spectra. Such DACs are believed to be caused by large-scale spiral-shaped density structures in the stellar wind. We consider novel 3-D radiation-hydrodynamic models of rotating hot star winds and study the emergence of co-rotating spiral structures due to a local (pseudo-)magnetic spot on the stellar surface. Subsequently, the hydrodynamic models are used to retrieve DAC spectral signatures in synthetic UV spectra created from a 3-D short-characteristics radiative transfer code.
. In this work, we implemented a hydrodynamical solution for fast rotating stars, which leaves high values of mass-loss rates and low terminal velocities of the wind. This 1D density distribution adopts a viscosity mimicking parameter which simulates a quasi-Keplerian motion. Then, it is converted to a volumetric density considering vertical hydrostatic equilibrium using a power-law scale height, as usual in viscous decretion disk models. We calculate the theoretical hydrogen emission lines and the spectral energy distribution utilizing the radiative transfer code HDUST. Our disk-wind structures are in agreement with viscous decretions disk models.
We study the role the the p-mode-like vertical oscillation on the photosphere in driving solar winds in the framework of Alfvén-wave-driven winds. By performing one-dimensional magnetohydrodynamical numerical simulations from the photosphere to the interplanetary space, we discover that the mass-loss rate is raised up to ≈ 4 times as the amplitude of longitudinal perturbations at the photosphere increases. When the longitudinal fluctuation is added, transverse waves are generated by the mode conversion from longitudinal waves in the chromosphere, which increases Alfvénic Poynting flux in the corona. As a result, the coronal heating is enhanced to yield higher coronal density by the chromospheric evaporation, leading to the increase of the mass-loss rate. Our findings clearly show the importance of the p-mode oscillation in the photosphere and the mode conversion in the chromosphere in determining the basic properties of the wind from the sun and solar-type stars.