Stellar Activity Level Research Articles

The habitability of Proxima Centauri b

Proxima b is a planet with a minimum mass of 1.3 MEarth orbiting within the habitable zone (HZ) of Proxima Centauri, a very low-mass, active star and the Sun's closest neighbor. Here we investigate a number of factors related to the potential habitability of Proxima b and its ability to maintain liquid water on its surface. We set the stage by estimating the current high-energy irradiance of the planet and show that the planet currently receives 30 times more EUV radiation than Earth and 250 times more X-rays. We compute the time evolution of the star's spectrum, which is essential for modeling the flux received over Proxima b's lifetime. We also show that Proxima b's obliquity is likely null and its spin is either synchronous or in a 3:2 spin-orbit resonance, depending on the planet's eccentricity and level of triaxiality. Next we consider the evolution of Proxima b's water inventory. We use our spectral energy distribution to compute the hydrogen loss from the planet with an improved energy-limited escape formalism. Despite the high level of stellar activity we find that Proxima b is likely to have lost less than an Earth ocean's worth of hydrogen before it reached the HZ 100-200 Myr after its formation. The largest uncertainty in our work is the initial water budget, which is not constrained by planet formation models. We conclude that Proxima b is a viable candidate habitable planet.

RAPIDLY ROTATING, X-RAY BRIGHT STARS IN THEKEPLERFIELD

We present Kepler light curves and optical spectroscopy of twenty X-ray bright stars located in the Kepler field of view. The stars, spectral type FK, show evidence for rapid rotation including chromospheric activity 100 times or more above the Sun at maximum and flaring behavior in their light curves. Eighteen of our objects appear to be (sub)giants and may belong to the class of FK Com variables, that is evolved rapidly spinning single stars with no excretion disk and high levels of chromospheric activity. Such stars are rare and are likely the result of W UMa binary mergers, a process believed to produce the FK Com class of variable and their descendants. The FK Com stage, including the presence of an excretion disk, is short-lived but leads to longer-lived stages consisting of single, rapidly rotating evolved (sub)giants with high levels of stellar activity.

Using planetary transits to estimate magnetic cycles lengths in Kepler stars

AbstractObservations of various solar-type stars along decades showed that they could have magnetic cycles, just like our Sun. These observations yield a relation between the rotation period Prot and the cycle length Pcycle of these stars. Two distinct branches for the cycling stars were identified: active and inactive, classified according to stellar activity level and rotation rate. In this work, we determined the magnetic activity cycle for 6 active stars observed by the Kepler telescope. The method adopted here estimates the activity from the excess in the residuals of the transit light curves. This excess is obtained by subtracting a spotless model transit from the light curve, and then integrating over all the residuals during the transit. The presence of long term periodicity is estimated from the analysis of a Lomb-Scargle periodogram of the complete time series. Finally, we investigate the rotation-cycle period relation for the stars analysed here.

Is the activity level of HD 80606 influenced by its eccentric planet?

Aims: Several studies suggest that the activity level of a planet-host star can be influenced by the presence of a close-by orbiting planet. Moreover, the interaction mechanisms that have been proposed, magnetic interaction and tidal interaction, exhibit a very different dependence on orbital separation between the star and the planet. A detection of activity enhancement and characterization of its dependence on planetary orbital distance can, in principle, allow us to characterize the physical mechanism behind the activity enhancement. Methods: We used the HARPS-N spectrograph to measure the stellar activity level of HD 80606 during the planetary periastron passage and compared the activity measured to that close to apastron. Being characterized by an eccentricity of 0.93 and an orbital period of 111 days, the system's extreme variation in orbital separation makes it a perfect target to test our hypothesis. Results: We find no evidence for a variation in the activity level of the star as a function of planetary orbital distance, as measured by all activity indicators employed log($R'_{HK}$), H$_\alpha$, NaI, and HeI. None of the models employed, whether magnetic interaction or tidal interaction, provides a good description of the data. Conclusions: We find no evidence for star-planet interaction in HD\,80606 at the moment of the periastron passage of its very eccentric planet. The straightforward explanation for the non-detection is the absence of interaction as a result of a low magnetic field strength on either the planet or the star and of the low level of tidal interaction between the two. However, we cannot exclude two scenarios: i) the interaction can be instantaneous and of magnetic origin, being concentrated on the substellar point and its surrounding area, and ii) the interaction can lead to a delayed activity enhancement. (abridged)

VARIATION IN THE PRE-TRANSIT BALMER LINE SIGNAL AROUND THE HOT JUPITER HD 189733B

ABSTRACT As followup to our recent detection of a pre-transit signal around HD 189733 b, we obtained full pre-transit phase coverage of a single planetary transit. The pre-transit signal is again detected in the Balmer lines but with variable strength and timing, suggesting that the bow shock geometry reported in our previous work does not describe the signal from the latest transit. We also demonstrate the use of the Ca ii H and K residual core flux as a proxy for the stellar activity level throughout the transit. A moderate trend is found between the pre-transit absorption signal in the 2013 data and the Ca ii H flux. This suggests that some of the 2013 pre-transit hydrogen absorption can be attributed to varying stellar activity levels. A very weak correlation is found between the Ca ii H core flux and the Balmer line absorption in the 2015 transit, hinting at a smaller contribution from stellar activity compared to the 2013 transit. We simulate how varying stellar activity levels can produce changes in the Balmer line transmission spectra. These simulations show that the strength of the 2013 and 2015 pre-transit signals can be reproduced by stellar variability. If the pre-transit signature is attributed to circumplanetary material, its evolution in time can be described by accretion clumps spiraling toward the star, although this interpretation has serious limitations. Further high-cadence monitoring at Hα is necessary to distinguish between true absorption by transiting material and short-term variations in the stellar activity level.

A Pragmatic Bayesian Perspective on Correlation Analysis : The exoplanetary gravity - stellar activity case.

We apply the Bayesian framework to assess the presence of a correlation between two quantities. To do so, we estimate the probability distribution of the parameter of interest, ρ, characterizing the strength of the correlation. We provide an implementation of these ideas and concepts using python programming language and the pyMC module in a very short (∼ 130 lines of code, heavily commented) and user-friendly program. We used this tool to assess the presence and properties of the correlation between planetary surface gravity and stellar activity level as measured by the log([Formula: see text]) indicator. The results of the Bayesian analysis are qualitatively similar to those obtained via p-value analysis, and support the presence of a correlation in the data. The results are more robust in their derivation and more informative, revealing interesting features such as asymmetric posterior distributions or markedly different credible intervals, and allowing for a deeper exploration. We encourage the reader interested in this kind of problem to apply our code to his/her own scientific problems. The full understanding of what the Bayesian framework is can only be gained through the insight that comes by handling priors, assessing the convergence of Monte Carlo runs, and a multitude of other practical problems. We hope to contribute so that Bayesian analysis becomes a tool in the toolkit of researchers, and they understand by experience its advantages and limitations.

The Appearance and Disappearance of Exocomet Gas Absorption

CaII K-line (3933 Å) absorption profiles observed towards 15 A-type and two B-type stars with known (or suspected) debris disks, in addition to archival spectral data for three A-type stars, are presented. Inspection of the extracted CaII K-line absorption lines has determined that one late B-type (HD 58647) and four new A-type (HD 56537, HD 64145, HD 108767, and HD 109573) stellar systems exhibit short-term (night-to-night) absorption variation within these profiles. This variability is due to the liberation of gas from falling evaporating bodies (the so-called FEBs activity) on the grazing approach of exocomets towards their parent star. Also we present archival spectra of the FeIλ3860 Å and CaII K-lines towardsβPictoris (HD 39060) that, for the first time, reveal the absorption signatures of FEB activity at similar velocities inbothline profiles. FEB-hosting stars seem to be of an earlier type (on average spectral type Al.8 as compared with A3.1) than those where no FEB activity has been observed. This could be due to stellar activity levels (such as chromospheric activity or nonradial pulsations) being of a higher level within the atmospheres of these hotter A-type stars.

TASTE IV: Refining ephemeris and orbital parameters for HAT‐P‐20b and WASP‐1b

AbstractWe present four new light curves of the transiting exoplanets WASP‐1b and HAT‐P‐20b, observed within the TASTE (The Asiago Search for Transit timing variations of Exoplanets) project. We re‐analyzed light curves from the literature in a homogeneous way, calculating a refined ephemeris and orbital‐physical parameters for both objects. WASP‐1b does not show any significant Transit Timing Variation signal at the 120 s level. As for HAT‐P‐20b, we detected a deviation from our re‐estimated linear ephemeris that could be ascribed to the presence of a perturber or, more probably, to a previously unnoticed high level of stellar activity. The rotational period of HAT‐P‐20 A that we obtained from archival data (Prot ≃ 14.5 d), combined with its optical variability and strong emission of Ca ii H & K lines, is consistent with a young stellar age (<1 Gyr) and support the hypothesis that stellar activity may be responsible of the measured deviations of the transit times. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Keplersuper-flare stars: what are they?

The Kepler mission has led to the serendipitous discovery of a significant number of `super flares' - white light flares with energies between 10^33 erg and 10^36 erg - on solar-type stars. It has been speculated that these could be `freak' events that might happen on the Sun, too. We have started a programme to study the nature of the stars on which these super flares have been observed. Here we present high-resolution spectroscopy of 11 of these stars and discuss our results. We find that several of these stars are very young, fast-rotating stars where high levels of stellar activity can be expected, but for some other stars we do not find a straightforward explanation for the occurrence of super flares.

Indications for an influence of hot Jupiters on the rotation and activity of their host stars

The magnetic activity of planet-hosting stars is an important factor to estimate the atmospheric stability of close-in exoplanets and the age of their host stars. It has long been speculated that close-in exoplanets can influence the stellar activity level. However, testing for tidal or magnetic interaction effects in samples of planet-hosting stars is difficult because stellar activity hinders exoplanet detection, so that stellar samples with detected exoplanets show a bias towards low activity for small exoplanets. We aim to test if exoplanets in close orbits influence the stellar rotation and magnetic activity of their host stars, and have developed a novel approach to test for such systematic activity enhancements. We use wide (several 100 AU) binary systems in which one of the stellar components is known to have an exoplanet, while the second stellar component does not have a detected planet and therefore acts as a negative control. We use the stellar coronal X-ray emission as an observational proxy for magnetic activity, and analyze observations performed with Chandra and XMM-Newton. We find that in two systems for which strong tidal interaction can be expected the planet-hosting primary displays a much higher magnetic activity level than the planet-free secondary. In three systems for which weaker tidal interaction can be expected the activity levels of both stellar components are in agreement. Our observations indicate that the presence of Hot Jupiters may inhibit the spin-down of host stars with thick outer convective layers. Possible causes for such an effect include a transfer of angular momentum from the planetary orbit to the stellar rotation through tidal interaction, or differences during the early evolution of the system, where the host star may decouple from the protoplanetary disk early due to a gap opened by the forming Hot Jupiter.

Is it possible to detect planets around young active G and K dwarfs?

Theoretical predictions suggest that the distribution of planets in very young stars could be very different to that typically observed in Gyr old systems that are the current focus of radial velocity surveys. However, the detection of planets around young stars is hampered by the increased stellar activity associated with young stars, the signatures of which can bias the detection of planets. In this paper we place realistic limitations on the possibilities for detecting planets around young active G and K dwarfs. The models of stellar activity based on tomographic imaging of the G dwarf HD 141943 and the K1 dwarf AB Dor and also include contributions from plage and many small random starspots. Our results show that the increased stellar activity levels present on young Solar-type stars strongly impacts the detection of Earth-mass and Jupiter mass planets and that the degree of activity jitter is directly correlated with stellar \vsinis. We also show that for G and K dwarfs, the distribution of activity in individual stars is more important than the differences in induced radial velocities as a function of spectral type. We conclude that Jupiter mass planets can be detected close-in around fast-rotating young active stars, Neptune-mass planets around moderate rotators and that Super-Earths are only detectable around very slowly rotating stars. The effects of an increase in stellar activity jitter by observing younger stars can be compensated for by extending the observational base-line to at least 100 epochs.

KEPLER-63b: A GIANT PLANET IN A POLAR ORBIT AROUND A YOUNG SUN-LIKE STAR

We present the discovery and characterization of a giant planet orbiting the young Sun-like star Kepler-63 (KOI-63, $m_{\rm Kp} = 11.6$, $T_{\rm eff} = 5576$ K, $M_\star = 0.98\, M_\odot$). The planet transits every 9.43 days, with apparent depth variations and brightening anomalies caused by large starspots. The planet's radius is $6.1 \pm 0.2 R_{\earth}$, based on the transit light curve and the estimated stellar parameters. The planet's mass could not be measured with the existing radial-velocity data, due to the high level of stellar activity, but if we assume a circular orbit we can place a rough upper bound of $120 M_{\earth}$ (3$\sigma$). The host star has a high obliquity ($\psi$ = $104^{\circ}$), based on the Rossiter-McLaughlin effect and an analysis of starspot-crossing events. This result is valuable because almost all previous obliquity measurements are for stars with more massive planets and shorter-period orbits. In addition, the polar orbit of the planet combined with an analysis of spot-crossing events reveals a large and persistent polar starspot. Such spots have previously been inferred using Doppler tomography, and predicted in simulations of magnetic activity of young Sun-like stars.

The place of the Sun among the Sun-like stars

Context. Monitoring of the photometric and chromospheric HK emission data series of stars similar to the Sun in age and average activity level showed that there is an empirical correlation between the average stellar chromospheric activity level and the photometric variability. In general, more active stars show larger photometric variability. Interestingly, the measurements and reconstructions of the solar irradiance show that the Sun is significantly less variable than indicated by the empirical relationship. Aims. We aim to identify possible reasons for the Sun to be currently outside of this relationship. Methods. We employed different scenarios of solar HK emission and irradiance variability and compared them with available time series of Sun-like stars. Results. We show that the position of the Sun on the diagram of photometric variability versus chromospheric activity changes with time. The present solar position is different from its temporal mean position as the satellite era of continuous solar irradiance measurements has accidentally coincided with a period of unusually high and stable solar activity. Our analysis suggests that although present solar variability is significantly smaller than indicated by the stellar data, the temporal mean solar variability might be in agreement with the stellar data. We propose that the continuation of the photometric program and its expansion to a larger stellar sample will ultimately allow us to constrain the historical solar variability.

Stellar activity as a tracer of moving groups

We present the results of a study of the stellar activity in the solar neighborhood using complete kinematics (galactocentric velocities U,V,W) and the chromospheric activity index $\log R'_{\rm{HK}}$. We analyzed the average activity level near the centers of known moving groups using a sample of 2529 stars and found that the stars near these associations tend to be more active than field stars. This supports the hypothesis that these structures, or at least a significant part of them, are composed of kinematically bound, young stars. We confirmed our results by using Galaxy Evolution Explorer (GALEX) UV data and kinematics taken from the Geneva-Copenhagen Survey for the stars in the sample. Finally, we present a compiled catalog with kinematics and activities for 2529 stars and a list of potential moving group members selected based on their stellar activity level.

STELLAR SURFACE MAGNETO-CONVECTION AS A SOURCE OF ASTROPHYSICAL NOISE. I. MULTI-COMPONENT PARAMETERIZATION OF ABSORPTION LINE PROFILES

We outline our techniques to characterise photospheric granulation as an astrophysical noise source. A four component parameterisation of granulation is developed that can be used to reconstruct stellar line asymmetries and radial velocity shifts due to photospheric convective motions. The four components are made up of absorption line profiles calculated for granules, magnetic intergranular lanes, non-magnetic intergranular lanes, and magnetic bright points at disc centre. These components are constructed by averaging Fe I $6302 \mathrm{\AA}$ magnetically sensitive absorption line profiles output from detailed radiative transport calculations of the solar photosphere. Each of the four categories adopted are based on magnetic field and continuum intensity limits determined from examining three-dimensional magnetohydrodynamic simulations with an average magnetic flux of $200 \mathrm{G}$. Using these four component line profiles we accurately reconstruct granulation profiles, produced from modelling 12 x 12 Mm$^2$ areas on the solar surface, to within $\sim \pm$ 20 cm s$^{-1}$ on a $\sim$ 100 m s$^{-1}$ granulation signal. We have also successfully reconstructed granulation profiles from a $50 \mathrm{G}$ simulation using the parameterised line profiles from the $200 \mathrm{G}$ average magnetic field simulation. This test demonstrates applicability of the characterisation to a range of magnetic stellar activity levels.

KEPLERSTUDIES OF LOW-MASS ECLIPSING BINARIES. I. PARAMETERS OF THE LONG-PERIOD BINARY KIC 6131659

KIC 6131659 is a long-period (17.5 days) eclipsing binary discovered by the Kepler mission. We analyzed six quarters of Kepler data along with supporting ground-based photometric and spectroscopic data to obtain accurate values for the mass and radius of both stars, namely, M1 = 0.922 ± 0.007 M☉, R1 = 0.8800 ± 0.0028 R☉, and M2 = 0.685 ± 0.005 M☉, R2 = 0.6395 ± 0.0061 R☉. There is a well-known issue with low-mass (M ≲ 0.8 M☉) stars (in cases where the mass and radius measurement uncertainties are smaller than 2% or 3%) where the measured radii are almost always 5% to 15% larger than expected from evolutionary models, i.e., the measured radii are all above the model isochrones in a mass–radius plane. In contrast, the two stars in KIC 6131659 were found to sit on the same theoretical isochrone in the mass–radius plane. Until recently, all of the well-studied eclipsing binaries with low-mass stars had periods of less than about three days. The stars in such systems may have been inflated by high levels of stellar activity induced by tidal effects in these close binaries. KIC 6131659 shows essentially no evidence of enhanced stellar activity, and our measurements support the hypothesis that the unusual mass–radius relationship observed in most low-mass stars is influenced by strong magnetic activity created by the rapid rotation of the stars in tidally locked, short-period systems. Finally, using short cadence data, we show that KIC 6131657 has one of the smallest measured non-zero eccentricities of a binary with two main-sequence stars, where ecos ω = (4.57 ± 0.02) × 10−5.

The independency of stellar mass-loss rates on stellar X-ray luminosity and activity level based on solar X-ray flux and solar wind observations

Stellar mass loss rates are an important input ingredient for stellar evolution models since they determine stellar evolution parameters such as stellar spin-down and increase in stellar luminosity through the lifetime of a star. Due to the lack of direct observations of stellar winds from Sun-like stars stellar X-ray luminosity and stellar level of activity are commonly used as a proxy for estimating stellar mass loss rates. However, such an intuitive activity --- mass loss rate relation is not well defined. In this paper, I study the mass loss rate of the Sun as a function of its activity level. I compare in-situ solar wind measurements with the solar activity level represented by the solar X-ray flux. I find no clear dependency of the solar mass flux on solar X-ray flux. Instead, the solar mass loss rate is scattered around an average value of $2\cdot10^{-14}\;M_\odot\;yr^{-1}$. This independency of the mass loss rate on level of activity can be explained by the fact that the activity level is governed by the large modulations in the solar close magnetic flux, while the mass loss rate is governed by the rather constant open magnetic flux. I derive a simple expression for stellar mass loss rates as a function of the stellar ambient weak magnetic field, the stellar radius, the stellar escape velocity, and the average height of the Alfv\'en surface. This expression predicts stellar mass loss rates of $10^{-15}-10^{-12}\;M_\odot\;yr^{-1}$ for Sun-like stars.

Hubble Space Telescope transmission spectroscopy of the exoplanet HD 189733b: high-altitude atmospheric haze in the optical and near-ultraviolet with STIS

We present Hubble Space Telescope optical and near-ultraviolet transmission spectra of the transiting hot-Jupiter HD189733b, taken with the repaired Space Telescope Imaging Spectrograph (STIS) instrument. The resulting spectra cover the range 2900-5700 Ang and reach per-exposure signal-to-noise levels greater than 11,000 within a 500 Ang bandwidth. We used time series spectra obtained during two transit events to determine the wavelength dependance of the planetary radius and measure the exoplanet's atmospheric transmission spectrum for the first time over this wavelength range. Our measurements, in conjunction with existing HST spectra, now provide a broadband transmission spectrum covering the full optical regime. The STIS data also shows unambiguous evidence of a large occulted stellar spot during one of our transit events, which we use to place constraints on the characteristics of the K dwarf's stellar spots, estimating spot temperatures around Teff~4250 K. With contemporaneous ground-based photometric monitoring of the stellar variability, we also measure the correlation between the stellar activity level and transit-measured planet-to-star radius contrast, which is in good agreement with predictions. We find a planetary transmission spectrum in good agreement with that of Rayleigh scattering from a high-altitude atmospheric haze as previously found from HST ACS camera. The high-altitude haze is now found to cover the entire optical regime and is well characterised by Rayleigh scattering. These findings suggest that haze may be a globally dominant atmospheric feature of the planet which would result in a high optical albedo at shorter optical wavelengths.

A search for star-planet interactions in theυAndromedae system at X-ray and optical wavelengths

Close-in, giant planets are expected to influence their host stars via tidal or magnetic interaction. But are these effects strong enough in suitable targets known so far to be observed with today's instrumentation? The upsilon And system, an F8V star with a Hot Jupiter, was claimed to undergo cyclic changes in chromospheric activity indicators with its innermost planet's period. We want to investigate the stellar chromospheric and coronal activity over several months. We therefore monitored the star in X-rays as well as at optical wavelengths to test coronal and chromospheric activity indicators for planet-induced variability, making use of the Chandra X-ray Observatory as well as the echelle spectrographs FOCES and HRS at Calar Alto (Spain) and the Hobby-Eberly Telescope (Texas, US). The stellar activity level is low, as seen both in X-rays as in Ca II line fluxes; the chromospheric data show variability with the stellar rotation period. We do not find activity variations in X-rays or in the optical which can be traced back to the planet. Gaining observational evidence for star-planet interactions in X-rays remains challenging.

Using the Sun to estimate Earth-like planets detection capabilities

Stellar activity is a potential important limitation to the detection of low mass extrasolar planets with indirect methods (RV, photometry, astrometry). In previous papers, using the Sun as a proxy, we investigated the impact of stellar activity (spots, plages, convection) on the detectability of an Earth-mass planet in the habitable zone (HZ) of solar-type stars with RV techniques. We extend here the detectability study to the case of astrometry. We used the sunspot and plages properties recorded over one solar cycle to infer the astrometric variations that a Sun-like star seen edge-on, 10 pc away, would exhibit, if covered by such spots/bright structures. We compare the signal to the one expected from the astrometric wobble (0.3 {\mu}as) of such a star surrounded by a one Earth-mass planet in the HZ. We also briefly investigate higher levels of activity. The activity-induced astrometric signal along the equatorial plane has an amplitude of typ. less than 0.2 {\mu}as (rms=0.07 {\mu}as), smaller than the one expected from an Earth-mass planet at 1 AU. Hence, for this level of activity, the detectability is governed by the instrumental precision rather than the activity. We show that for instance a one Earth-mass planet at 1 AU would be detected with a monthly visit during less than 5 years and an instrumental precision of 0.8 {\mu}as. A level of activity 5 times higher would still allow such a detection with a precision of 0.35 {\mu}as. We conclude that astrometry is an attractive approach to search for such planets around solar type stars with most levels of stellar activity.