Young Solar-like Stars Research Articles

Delayed Development of Cool Plasmas in X-Ray Flares from the Young Sun-like Star κ 1 Ceti

The Neutron star Interior Composition Explorer (NICER) X-ray observatory observed two powerful X-ray flares equivalent to superflares from the nearby young solar-like star κ 1 Ceti in 2019. NICER follows each flare from the onset through the early decay, collecting over 30 counts s−1 near the peak, enabling a detailed spectral variation study of the flare rise. The flare in September varies quickly in ∼800 s, while the flare in December has a few times longer timescale. In both flares, the hard-band (2–4 keV) light curves show typical stellar X-ray flare variations with a rapid rise and slow decay, while the soft X-ray light curves, especially of the September flare, have prolonged flat peaks. The time-resolved spectra require two temperature plasma components at kT ∼0.3–1 and ∼2–4 keV. Both components vary similarly, but the cool component lags by ∼200 s with a four to six times smaller emission measure (EM) compared to the hot component. A comparison with hydrodynamic flare loop simulations indicates that the cool component originates from X-ray plasma near the magnetic loop footpoints that mainly cools via thermal conduction. The time lag represents the travel time of the evaporated gas through the entire flare loop. The cool component has a several times smaller EM than its simulated counterpart, suggesting a suppression of conductive cooling, possibly by the expansion of the loop cross-sectional area or turbulent fluctuations. The cool component’s time lag and EM ratio provide important constraints on the flare loop geometry.

X-ray activity of the young solar-like star Kepler-63 and the structure of its corona

The X-ray satellite XMM-Newton has so far revealed coronal cycles in seven solar-like stars. Of these, the youngest stars ϵ Eridani (~400 Myr) and ι Horologii (~600 Myr) display the shortest X-ray cycles and the smallest amplitudes, defined as the variation of the X-ray luminosity between the maximum and minimum of the cycle. The X-ray cycle of ϵ Eridani was characterised by applying a novel technique that allowed us to model the corona of a solar-like star in terms of magnetic structures, such as those observed on the Sun (active regions, cores of active regions, and flares), at various filling factors. The high surface coverage of the magnetic structures on ϵ Eridani (65%–95%) that emerged from that study was suggested to be responsible for the low cycle amplitude in the X-ray band. It was also hypothesised that the basal surface coverage with magnetic structures may be higher on the corona of solar-like stars while they are young. To investigate this hypothesis, we started the X-ray monitoring campaign of Kepler-63 in 2019. The star had been observed once before, in 2014, with XMM-Newton. With an age of 210 ± 45 Myr and a photospheric cycle of 1.27 yr, Kepler-63 is the youngest star so far to be observed in X-rays in order to reveal its coronal cycle. Our campaign comprised four X-ray observations of Kepler-63 spanning 10 months (i.e. three-fifths of its photospheric cycle). The long-term X-ray light curve did not reveal a periodic variation of the X-ray luminosity, but a factor two change would be compatible with the considerable uncertainties in the low signal-to-noise data for this relatively distant star. In the case of ϵ Eridani, we describe the coronal emission measure distribution (EMD) of Kepler-63 with magnetic structures such as those observed on the Sun. The best match with the observations is found for an EMD composed of cores and flares of GOES Class C and M following the canonical flare frequency distribution. More energetic flares are occasionally present but they do not contribute significantly to the quasi-stationary high-energy component of the emission measure probed with our modelling. This model yields a coronal filling factor of 100%. This complete coverage of the corona with X-ray-emitting magnetic structures is consistent with the absence of an X-ray cycle, confirming the analogous results derived earlier for ϵ Eridani. Finally, combining our results with the literature on stellar X-ray cycles, we establish an empirical relation between the cycle amplitude LX,max/LX,min and the X-ray surface flux, FX,surf. From the absence of a coronal cycle in Kepler-63, we infer that stars with higher X-ray flux than Kepler-63 must host an EMD that comprises a significant fraction of higher energy flares than those necessary to model the corona of Kepler-63, that is, flares of Class X or higher. Our study opens a new path for studies of the solar-stellar analogy and the joint exploration of resolved and unresolved variability in stellar X-ray light curves.

Extreme energetic particle events by superflare-asssociated CMEs from solar-like stars.

Discovery of frequent superflares on active cool stars opened a new avenue in understanding the properties of eruptive events and their impact on exoplanetary environments. Solar data suggest that coronal mass ejections (CMEs) should be associated with superflares on active solar-like planet hosts and produce solar/stellar energetic particle (SEP/StEP) events. Here, we apply the 2D Particle Acceleration and Transport in the Heliosphere model to simulate the SEPs accelerated via CME-driven shocks from the Sun and young solar-like stars. We derive the scaling of SEP fluence and hardness of energy spectra with CME speed and associated flare energy. These results have crucial implications for the prebiotic chemistry and expected atmospheric biosignatures from young rocky exoplanets as well as the chemistry and isotopic composition of circumstellar disks around infant solar-like stars.

Multiscale behaviour of stellar activity and rotation of the planet host Kepler-30

Context. The Kepler-30 system consists of a G dwarf star with a rotation period of ~16 days and three planets orbiting almost coplanar with periods ranging from 29 to 143 days. Kepler-30 is a unique target with which to study stellar activity and rotation in a young solar-like star accompanied by a compact planetary system. Aims. We use about 4 yr of high-precision photometry collected by the Kepler mission to investigate the fluctuations caused by photospheric convection, stellar rotation, and starspot evolution as a function of timescale. Our main goal is to apply methods for the analysis of time-series to find the timescales of the phenomena that affect the light variations. We correlate those timescales with periodicities in the star and the planetary system. Methods. We model the flux rotational modulation induced by active regions using spot modelling and apply the Multifractal Detrending Moving Average algorithm in standard and multiscale versions to analyse the behaviour of variability and light fluctuations that can be associated with stellar convection and the evolution of magnetic fields on timescales ranging from less than 1 day up to about 35 days. The light fluctuations produced by stellar activity can be described by the multifractal Hurst index that provides a measure of their persistence. Results. The spot modelling indicates a lower limit to the relative surface differential rotation of ΔΩ∕Ω ~ 0.02 ± 0.01 and suggests a short-term cyclic variation in the starspot area with a period of ~34 days, which is close to the synodic period of 35.2 days of the planet Kepler-30b. By subtracting the two time-series of the simple aperture photometry and pre-search data conditioning Kepler pipelines, we reduce the rotational modulation and find a 23.1-day period close to the synodic period of Kepler-30c. This period also appears in the multifractal analysis as a crossover of the fluctuation functions associated with the characteristic evolutionary timescales of the active regions in Kepler-30 as confirmed by spot modelling. These procedures and methods may be greatly useful for analysing current TESS and future PLATO data.

The Impact of Eruptions from Young Stars on Environments of Rocky Exoplanets

Kepler and TESS missions have discovered over 4500 extra solar (exoplanets) around F, G, K and M dwarfs. They also revealed frequent superflares on planet hosting stars, providing a mechanism by which host stars may have profound effects on the physical and chemical evolution of exoplanetary atmospheres. While we can only infer the course of the Sun’s early evolution and how it might have affected the early evolution of the Earth, possibly setting the stage for the origin of life, the observation of planets around sun-like stars allows us to directly observe events which likely took place in our own solar system. A major question this leads to is: what effects do extreme energy fluxes from eruptive events during evolution of G-K planet hosts have on prebiotic chemistry and primitive life forms on primitive planets? To address this question, I will describe recent observations of young solar-like stars as inputs for our 3D MHD models of the corona, the wind and transient events (flares, coronal mass ejections and solar energetic particle events) and discuss their impact on atmospheric erosion and chemistry of our planet. I will then use these constrained energy fluxes to describe our recent atmospheric chemistry models impacted by energetic particles from the young Sun and formation and precipitation of biologically relevant molecules. I will then highlight our results of laboratory experiments of proton irradiation of mildly reduced gas mixtures and their implications to the climate, prebiotic chemistry and the rise of habitability on early Earth and young exoplanets.

An X-ray activity cycle on the young solar-like star ɛ Eridani

Chromospheric Ca II activity cycles are frequently found in late-type stars, but no systematic programs have been created to search for their coronal X-ray counterparts. The typical time scale of Ca II activity cycles ranges from years to decades. Therefore, long-lasting missions are needed to detect the coronal counterparts. The XMM-Newton satellite has so far detected X-ray cycles in five stars. A particularly intriguing question is at what age (and at what activity level) X-ray cycles set in. To this end, in 2015 we started the X-ray monitoring of the young solar-like star ɛ Eridani, previously observed on two occasions: in 2003 and in early 2015, both by XMM-Newton. With an age of 440 Myr, it is one of the youngest solar-like stars with a known chromospheric Ca II cycle. We collected the most recent Mount Wilson S-index data available for ɛ Eridani, starting from 2002, including previously unpublished data. We found that the Ca II cycle lasts 2.92 ± 0.02 yr, in agreement with past results. From the long-term XMM-Newton lightcurve, we find clear and systematic X-ray variability of our target, consistent with the chromospheric Ca II cycle. The average X-ray luminosity is 2 × 1028erg s−1, with an amplitude that is only a factor of 2 throughout the cycle. We apply a new method to describe the evolution of the coronal emission measure distribution of ɛ Eridani in terms of solar magnetic structures: active regions, cores of active regions, and flares covering the stellar surface at varying filling fractions. Combinations of these three types of magnetic structures can only describe the observed X-ray emission measure of ɛ Eridani if the solar flare emission measure distribution is restricted to events in the decay phase. The interpretation is that flares in the corona of ɛ Eridani last longer than their solar counterparts. We ascribe this to the lower metallicity of ɛ Eridani. Our analysis also revealed that the X-ray cycle of ɛ Eridani is strongly dominated by cores of active regions. The coverage fraction of cores throughout the cycle changes by the same factor as the X-ray luminosity. The maxima of the cycle are characterized by a high percentage of covering fraction of the flares, consistent with the fact that flaring events are seen in the corresponding short-term X-ray lightcurves predominately at the cycle maxima. The high X-ray emission throughout the cycle of ɛ Eridani is thus explained by the high percentage of magnetic structures on its surface.

Stellar magnetic activity and the butterfly diagram of Kepler-63

Context. The study of young solar-type stars is fundamental for better understanding the magnetic activity of the Sun. Most commonly, this activity manifests itself in the form of spots and faculae. As a planet in transit crosses in front of its host star, a dark spot on the stellar surface may be occulted, causing a detectable variation in the light curve. Kepler-63 is a young solar-like star with an age of only 210 Myr that exhibits photometric variations compatible with spot signatures. Because the planet that orbits it is in an almost polar orbit, different latitudes of the star can be probed by the method of spot transit mapping. Aims. The goal of this work is to characterise the spots of Kepler-63 and thus decipher the behaviour of the young Sun. Because planetary orbit is highly oblique, the latitudinal distribution and thus the differential rotation of the spots may be determined. Methods. A total of 150 transits of Kepler-63b were observed in the short-cadence light curve, corresponding to a total duration of about four years. Each transit light curve was fit by a model that simulates planetary transits and allows including starspots on the surface of the host star. This enables the physical characterisation of the spot size, intensity, and location. We determined the spot position in a reference frame that rotates with the star, and thus obtained the latitudinal distribution of the spots. Results. We fit a total of 297 spots and determined their sizes, intensities, and positions. The longitude and latitude of the spots were calculated in a reference frame that rotated with the star. The latitude distribution of spots exhibits a bimodality with a lack of spots around 34°. Moreover, the spot sizes tend to be larger close to the equator, but decrease toward the latitude distribution gap, after which they again increase toward the poles. High-latitude spots dominate the magnetic cycle of Kepler-63. For a mean stellar rotation period of 5.400 d, 59 spots were found at approximately the same longitude and latitude on a later transit. Some of these spots were detected eight transits later. This shows that the lifetimes of spots can be at least 75 d. Conclusions. The geometry of the Kepler-63 system, enabled us to build a starspot butterfly diagram, similar to that of sunspots. It was also possible to infer the differential rotation of Kepler-63 from the spots at different latitudes. This star was found to rotate almost rigidly with a period of 5.400 d and a relative shear close to 0.01% for latitudes lower than 34°, whereas the high latitudes do not follow a well-behaved pattern.

Observing the gas component of circumplanetary disks around wide-orbit planet-mass companions in the (sub)mm regime

Context. Several detections of wide-orbit planet-mass/substellar companions around young solar-like stars were reported in the last decade. The origin of those possible planets is still unclear, but accretion tracers and VLT/SPHERE observations indicate that they are surrounded by circumplanetary material or even a circumplanetary disk (CPD). Aims. We want to investigate if the gas component of disks around wide-orbit companions is detectable with current (ALMA) and future (ngVLA; sub)mm telescopes and what constraints such gas observations can provide on the nature of the circumplanetary material and the mass of the companion. Methods. We applied the radiation thermochemical disk code PRODIMO to model the dust and gas component of passive CPDs and produced realistic synthetic observables. We considered different companion properties (mass, luminosity), disk parameters (mass, size, and dust properties) and radiative environments (background fields) and compared the resulting synthetic observables to telescope sensitivities and existing dust observations. Results. The main criterion for a successful detection is the size of the CPD. At a distance of about 150 pc, a CPD with an outer radius of about 10 au is detectable with ALMA in about six hours in optically thick CO lines. Other aspects, such as the luminosity, disk inclination, and background radiation fields of the companion, are also relevant and should be considered to optimize the observing strategy for detection experiments. Conclusions. For most of the known wide-orbit planet-mass companions, their maximum theoretical disk size of one-third of the Hill radius would be sufficient to allow detection of CO lines. It is therefore feasible to detect their gas disks and constrain the mass of the companion through the kinematic signature. Even in the case of non-detections such observations provide stringent constraints on disk size and gas mass, and this information is crucial for formation theories.

Constraining the stellar energetic particle flux in young solar-like stars

AbstractAnomalies in the abundance measurements of short lived radionuclides in meteorites indicate that the protosolar nebulae was irradiated by a large number of energetic particles (E≳ 10 MeV), often called solar cosmic rays. The particle flux of the contemporary Sun cannot explain these anomalies, but, similar to T Tauri stars, the young Sun was more active and probably produced enough high energy particles. However, the stellar particle (SP) flux of young stars is essentially unknown. We model the impact of high-energy ionization sources on the chemistry of the circumstellar environment (disks and envelopes). The model includes X-ray radiative transfer and makes use of particle transport models to calculate the individual molecular hydrogen ionization rates. We study the impact on the chemistry via the ionization tracers HCO+ and N2H+. We argue that spatially resolved observations of those molecules combined with detailed models allow for disentangling the contribution of the individual high-energy ionization sources and to put constraints on the SP flux in young stars.

A RESOLVED NEAR-INFRARED IMAGE OF THE INNER CAVITY IN THE GM Aur TRANSITIONAL DISK∗

ABSTRACT We present high-contrast H-band polarized intensity (PI) images of the transitional disk around the young solar-like star GM Aur. The near-infrared direct imaging of the disk was derived by polarimetric differential imaging using the Subaru 8.2 m Telescope and HiCIAO. An angular resolution and an inner working angle of 0.″07 and r ∼ 0.″05, respectively, were obtained. We clearly resolved a large inner cavity, with a measured radius of 18 ± 2 au, which is smaller than that of a submillimeter interferometric image (28 au). This discrepancy in the cavity radii at near-infrared and submillimeter wavelengths may be caused by a 3–4 M Jup planet about 20 au away from the star, near the edge of the cavity. The presence of a near-infrared inner cavity is a strong constraint on hypotheses for inner cavity formation in a transitional disk. A dust filtration mechanism has been proposed to explain the large cavity in the submillimeter image, but our results suggest that this mechanism must be combined with an additional process. We found that the PI slope of the outer disk is significantly different from the intensity slope obtained from HST/NICMOS, and this difference may indicate the grain growth process in the disk.

The Environment of the Young Earth in the Perspective of An Young Sun

AbstractOur Sun, a magnetically mild star, exhibits space weather in the form of magnetically driven solar explosive events (SEE) including solar flares, coronal mass ejections and energetic particle events. We use Kepler data and reconstruction of X-ray and UV emission from young solar-like stars to recover the frequency and energy fluxes from extreme events from active stars including the young Sun. Extreme SEEs from a magnetically active young Sun could significantly perturb the young Earth's magnetosphere, cause strong geomagnetic storms, initiate escape and introduce chemical changes in its lower atmosphere. I present our recent simulations results based on multi-dimensional multi-fluid hydrodynamic and magnetohydrodynamic models of interactions of extreme CME and SEP events with magnetospheres and lower atmospheres of early Earth and exoplanets around active stars. We also discuss the implications of the impact of these effects on evolving habitability conditions of the early Earth and prebiotic chemistry introduced by space weather events at the early phase of evolution of our Sun.

Near-infrared imaging polarimetry of LkCa 15: A possible warped inner disk.

We present high-contrast H-band polarized intensity images of the transitional disk around the young solar-like star LkCa 15. By utilizing Subaru/HiCIAO for polarimetric differential imaging, the angular resolution and the inner working angle reach 0.07 and r = 0″.1, respectively. We obtained a clearly resolved gap (width ≲ 27 au) at ~48 au from the central star. This gap is consistent with images reported in previous studies. We also confirmed the existence of a bright inner disk with a misaligned position angle of 13° ±4° with respect to that of the outer disk, i.e., the inner disk is possibly warped. The large gap and the warped inner disk both point to the existence of a multiple planetary system with a mass of ≲ 1 M Jup.

CONNECTING FLARES AND TRANSIENT MASS-LOSS EVENTS IN MAGNETICALLY ACTIVE STARS

We explore the ramification of associating the energetics of extreme mag- netic reconnection events with transient mass loss in a stellar analogy with solar eruptive events. We establish energy partitions relative to the total bolometric radiated flare energy for different observed components of stellar flares, and show that there is rough agreement for these values with solar flares. We apply an equipartition between the bolometric radiated flare energy and kinetic energy in an accompanying mass ejection, seen in solar eruptive events and expected from reconnection. This allows an integrated flare rate in a particular waveband to be used to estimate the amount of associated transient mass loss. This approach is supported by a good correspondence between observational flare signatures on high flaring rate stars and the Sun, which suggests a common physical origin. If the frequent and extreme flares that young solar-like stars and low-mass stars experience are accompanied by transient mass loss in the form of coronal mass ejections, then the cumulative effect of this mass loss could be large. We find that for young solar-like stars and active M dwarfs, the total mass lost due to transient magnetic eruptions could have significant impacts on disk evolution, and thus planet formation, and also exoplanet habitability.

Stellar activity as noise in exoplanet detection – I. Methods and application to solar-like stars and activity cycles

The detection of exoplanets using any method is prone to confusion due to the intrinsic variability of the host star. We investigate the effect of cool starspots on the detectability of the exoplanets around solar-like stars using the radial velocity method. For investigating this activity-caused "jitter" we calculate synthetic spectra using radiative transfer, known stellar atomic and molecular lines, different surface spot configurations, and an added planetary signal. Here, the methods are described in detail, tested and compared to previously published studies. The methods are also applied to investigate the activity jitter in old and young solar-like stars, and over a solar-like activity cycles. We find that the mean full jitter amplitude obtained from the spot surfaces mimicking the solar activity varies during the cycle approximately between 1 m/s and 9 m/s. With a realistic observing frequency a Neptune mass planet on a one year orbit can be reliably recovered. On the other hand, the recovery of an Earth mass planet on a similar orbit is not feasible with high significance. The methods developed in this study have a great potential for doing statistical studies of planet detectability, and also for investigating the effect of stellar activity on recovered planetary parameters.

FLECHAS – A new échelle spectrograph at the University Observatory Jena

AbstractThe new échelle spectrograph FLECHAS (Fibre Linked ECHelle Astronomical Spectrograph) is in operation at the Nasmyth‐focus of the 0.9 m telescope of the University Observatory Jena. FLECHAS is equipped with a sensitive back‐illuminated and midband coated CCD‐detector, as well as with a calibration unit for flatfield and wavelength‐calibration. The spectrograph covers the spectral range between about 3900 and 8100 Å and exhibits a resolving power of R ∼ 9300. In this article all technical characteristics of FLECHAS are described and examples of the first astronomical observations obtained with the new instrument in July 2013 at the University Observatory Jena are presented, among them the first light spectra taken with FLECHAS, simultaneous imaging and spectroscopic observations, the determination of the detection limit of the instrument, the spectroscopy of stars of different spectral types and of faint extended objects, as well as the Li‐line detection in the spectra of young solar‐like stars. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

ιHorologi, the first coronal activity cycle in a young solar-like star

Context: The shortest chromospheric (Ca II H&K) activity cycle (1.6 yr) has been recently discovered in the young (~600 Myr) solar-like star iota Hor. Coronal X-ray activity cycles have only been discovered in a few stars other than the Sun, all of them with an older age and a lower activity level than iota Hor. Aims: We intended to find the X-ray coronal counterpart of the chromospheric cycle for i Hor. This represents the first X-ray cycle observed in an active star, as well as the paradigm of the first coronal cycles in the life of a solar-like star. Methods: We monitored i Hor with XMM-Newton observations spanning almost two years. The spectra of each observation are fit with two-temperature coronal models to study the long-term variability of the star. Results: We find a cyclic behavior in X-rays very similar to the contemporaneous chromospheric cycle. The continuous chromospheric monitoring for more than three cycle lengths shows a trend toward decreasing amplitude, apparently modulated by a longer term trend. The second cycle is disrupted prior to reaching its maximum, followed by a brief episode of chaotic variability before the cyclic behavior resumes, only to be disrupted again after slightly more than one cycle. Conclusions: We confirm the presence of an activity cycle of ~1.6 yr in i Hor both in X-rays and Ca II H&K. It is likely subject to the modulation of a longer, not yet constrained second cycle. The 1.6 yr cycle is the shortest coronal one observed to date, and i Hor represents the most active star for which a coronal activity cycle has been found. This cycle is probably representative of the first coronal cycles in the life of a solar-like star, at the age when life started on Earth.

Probing the size of a magnetosphere of a young solar-like star

AbstractWe have obtained high resolution spectra of the pre-main sequence binary system KH 15D (V582 Mon) while the star is fully visible, fully occulted, and during several ingress and egress events over the course of five contiguous observing seasons. The Hα line profile is a standard probe of the magnetospheric accretion flows on young stars such as KH 15D. We use these time series data to map out the size of the magnetosphere and find that it changes size from one observing season to the next.

Simulations of Turbulent Convection in Rotating Young Solarlike Stars: Differential Rotation and Meridional Circulation

We present the results of three-dimensional simulations of the deep convective envelope of a young (10 Myr) 1 M? star, obtained with the anelastic spherical harmonic code. Since young stars are known to be faster rotators than their main-sequence counterparts, we have systematically studied the impact of the stellar rotation speed, by considering stars spinning up to 5 times as fast as the Sun. The aim of these nonlinear models is to understand the complex interactions between convection and rotation. We discuss the influence of the turbulence level and of the rotation rate on the intensity and the topology of the mean flows. For all of the computed models, we find a solar-type superficial differential rotation, with an equatorial acceleration, and meridional circulation that exhibits a multicellular structure. Even if the differential rotation contrast ?? decreases only marginally for high rotation rates, the meridional circulation intensity clearly weakens according to our simulations. We have also shown that, for Taylor numbers above a certain threshold (Ta 109), the convection can develop a vacillating behavior. Since simulations with high turbulence levels and rotation rates exhibit strongly cylindrical internal rotation profiles, we have considered the influence of baroclinic effects at the base of the convective envelope of these young Suns to see whether such effects can modify the otherwise near-cylindrical profiles to produce more conical, solarlike profiles.

Fast transition between classical and weak lined T Tauri stars due to external UV dissipation

The discovery of optical jets immersed in the strong UV radiation field of the Rosette Nebula sheds new light on, but meanwhile poses challenges to, the study of externally irradiated jets. The jet systems in the Rosette are found to have a high state of ionization and show unique features. In this paper, we investigate the evolutionary status of the jet-driving sources for young solar-like stars. To our surprise, these jet sources indicate unexpected near infrared properties with no excess emission. They are bathed in harsh external UV radiation such that evaporation leads to a fast dissipation of their circumstellar material. This could represent a transient phase of evolution of young solar-like stars between classical and weak lined T Tauri stars. Naked T Tauri stars formed in this way have indistinguishable evolutionary ages from those of classical T Tauri stars resulting from the same episode of star formation. However, it would be hard for such sources to be identified if they are not driving an irradiated jet in a photoionized medium.

The Solar Wind and the Sun in the Past

Exposure to the solar wind can have significant long term consequences for planetary atmospheres, especially for planets such as Mars that are not protected by global magnetospheres. Estimating the effects of solar wind exposure requires knowledge of the history of the solar wind. Much of what we know about the Sun’s past behavior is based on inferences from observations of young solar-like stars. Stellar analogs of the weak solar wind cannot be detected directly, but the interaction regions between these winds and the interstellar medium have been detected and used to estimate wind properties. I here review these observations, with emphasis on what they suggest about the history of the solar wind.