Tue 7 June | 14:30 - 15:45 | Session 1.1: Solar/stellar variability: observational properties and theory
14:30 M. Giampapa (invited): Photometric Variability in the Sun and Sun-like Stars
The rich array of solar magnetic field-related phenomena we see occurs not only on stellar counterparts of our Sun but in stars that represent significant departures in their fundamental parameters from those of the Sun. Though these phenomena appear energetically negligible when compared to the total luminosity of stars, they nevertheless govern the angular momentum evolution and modulate the radiative and particle output of the Sun and late-type stars. The term “The Solar-Stellar Connection” has been coined to describe the solar-stellar synergisms in the investigation of the generation, emergence and coupling of magnetic fields with the outer solar-stellar atmosphere to produce what we broadly refer to as magnetic activity. With the discovery of literally thousands of planets beyond our solar system, the Solar-Stellar-Planet Connection is quickly emerging as a new area of investigation of the impacts of magnetic activity on exoplanet atmospheres. In parallel with this rapid evolution in our perspectives is the advent of transformative facilities for the study of the Sun and the dynamic Universe. The primary focus of this invited talk will be on photometric variations in solar-type stars and the Sun. These brightness variations are associated with thermal homogeneities typically defined by magnetic structures that are also spatially coincident with key radiative proxies. Photometric variability in solar-type stars and the Sun includes transient brightening, rotational modulation by cool spots and cycle-related variability, each with a characteristic signature in time and wavelength. The emphasis of this presentation will be on the relationship between broadband photometric variations and magnetic field-related activity in solar-type stars and the Sun. Facets of this topic will be discussed both retrospectively and prospectively as we enter a revolutionary, new era for astronomy.
14:50 R. A. García: Probing Stellar Magnetism with Space Photometry of Solar Analogs
The surface magnetic field has substantial influence on various stellar properties that can be probed through various techniques. With the advent of new space-borne facilities such as CoRoT and Kepler, uninterrupted long high-precision photometry is available for hundred of thousand of stars. This number will substantially grow through the forthcoming TESS and PLATO missions. The unique Kepler observations –covering up to 4 years with a 30-min cadence– allows studying stellar variability with different origins such as pulsations, convection, surface rotation, or magnetism at several time scales from hours to years. We study the pho- tospheric magnetic activity of solar-like stars by means of the variability induced in the observed signal by starspots crossing the visible disk. We constructed a solar photometric magnetic activity proxy, Sph from SPM/VIRGO/SoHO, as if the Sun was a distant star and we compare it with several solar well-known magnetic proxies. The results validate this approach. Thus, we compute the Sph proxy for a set of CoRoT and Kepler solar-like stars for which pulsations were already detected. After characterizing the rotation and the magnetic properties of ∼300 solar-like stars, we use their seismic properties to characterize 18 solar analogs for which we study their magnetism. This allows us to put the Sun into context of its siblings.
15:02 C. Marvin: Measurements of Absolute Calcium II H & K in FGKM Stars
M dwarfs are the most numerous stars in the universe, yet they still lack absolute chromospheric Ca II H and K (R'HK ) calibrations to effectively compare their activity with FGK stars. We scale high-S/N, high-resolution template spectra, obtained by co-adding multiple HARPS spectra of the same star, to PHOENIX stellar atmosphere models, and obtain chromospheric line measurements of Ca II H & K in physical units of 106 M dwarfs. We also derive new Mt. Wilson S-index to R'HK conversions appropriate for cooler stars, ranging from 0.82 ≤ (B − V ) ≤ 2.00. We establish a chromospheric activity database by combining archival data of FGK stars and using our technique to extend absolute chromospheric measurements to M dwarfs. Our results show that using model atmospheres provide a reliable way to scale uncalibrated spectra and also estimate photospheric flux for M dwarfs, but note that accurate stellar parameter determination is essential to compare chromospheric emission of different spectral types.
15:14   R. Haywood: Radial-Velocity Variability of the Sun as a Star with HARPS and HARPS-N
Since we can resolve the surface of the Sun directly, we can explore the origin of radial-velocity variations induced by individual solar surface features such as faculae/plage, sunspots and granulation. I will present my recent investigation of the radial-velocity variations of the Sun as a star, based on high-resolution HARPS spectra of reflected sunlight and simultaneous images from the Solar Dynamics Observatory. We found that faculae are the dominant source of activity-induced radial-velocity variations, via suppression of convective blueshift. We investigated possible proxies for activity-induced radial-velocity variations and found that optical lightcurves can only provide a partial representation of these signals; the full-disc magnetic flux, however, is an excellent tracer. In addition to this dataset, the HARPS-N spectrograph has been operating with a new solar telescope feed since 2015 July. I will present results from the first year observations, which show radial-velocity variations of up to 7-8 m/s. Identifying proxies for solar radial-velocity variations is key to understanding the radial-velocity variability of other Sun-like stars, and is also essential for other investigations such as exoplanet detection surveys.
15:26 J. R. Barnes: Photospheric Acne at the Bottom of the Main Sequence
Starspots are an important manifestation of stellar activity and yet their distribution patterns on the lowest mass stars is not well known. Time series spectra of fully convective M dwarfs taken in the red-optical with UVES reveal numerous line profile distortions which are interpreted as starspots. We derive Doppler images for four M4.5V - M9V stars and find that contrast ratios corresponding to photosphere-spot temperature differences of only 200-300 K are sufficient to model the timeseries spectra. Although more starspot structure is found at high latitudes, spots are reconstructed at a range of phases and latitudes with mean spot filling factors of only a few per cent. The occurrence of low-contrast spots at predominantly high latitudes is in general likely to be responsible for the low amplitude photometric variability seen in late-M dwarfs. The recovered starspot patterns are used to assess their effect on precision radial velocity surveys aimed at detecting planets around this population of stars.
15:45 - 16:15 Coffee Break
- 14:30 M. Giampapa (invited): Photometric Variability in the Sun and Sun-like Stars
Tue 7 June | 16:15 - 17:30 | Session 1.2: Stellar magnetic fields and their impact on the surrounding environment
16:15 S. Marsden (invited): Magnetic Fields on Solar-Type Stars: The Solar-Stellar Connection
Our understanding of solar magnetic fields is significantly more detailed than we can achieve for other stars. However, over the last few decades, techniques such as Zeeman Doppler Imaging (ZDI), have delivered major advances in the study of stellar magnetic fields, to the point where we can now map the global surface magnetic topology of other stars and use these to model the coronal magnetic fields and even the stellar wind produced by the star. These advances now allow us to compare the Sun with other solar-type stars and make increasingly detailed comparisons of the behaviour and generation of their dynamo magnetic fields. In this review I will give an overview of what ZDI has taught us about the magnetic fields of other stars and how our study of the Sun can inform our understanding of the dynamos operating across a range of solar-type stars.
16:35 S. Aigrain (invited): The Effects of Stellar Activity on Detecting and Characterizing Planets
Intrinsic stellar variability associated with magnetic activity, rotation and convection, affects the detection of exoplanets via the transit and radial velocity methods, and the characterisation of their atmospheres. I will review the increasingly sophisticated methods developed in the last few years to mitigate this problem, and outline how stellar variability is likely to impact the field of exoplanets in the future. Planetary transits last a few hours, much shorter than the rotational modulation of star spots (day to weeks), but smaller-scale variability is nonetheless an important limiting factor in our ability to detect transits of Earth analogs in Kepler and Plato data. In radial velocity, the problem is even more severe, as the planet’s signal occurs on the orbital timescale, which can coincide with the range expected for stellar rotation periods or activity cycles - but the spectra used to extract radial velocities contain a wealth of information about stellar activity that can be used to disentangle the two types of signals. Finally, when using transits or phase curves to probe the composition and dynamics of planetary atmospheres, star spots must be accounted for very carefully, as they can mimic or mask planetary atmosphere signals. On the positive side, the sensitivity of planet search and characterisation experiments to stellar activity means that they are a treasure trove of information about stellar activity. The continued success of exoplanet surveys depends on our making the best possible use of this information.
16:55 J. Llama: The Impact of Stellar Activity on High Energy Exoplanet Transits
High energy (X-ray / UV) observations of transiting exoplanets have revealed the presence of extended atmospheres around a number of systems. At these energies, stellar radiation is absorbed in the upper atmosphere of the planet, making X-ray / UV transits an exciting tool for investigating the composition of exoplanetary atmospheres. However, the effects of stellar activity on transits at these wavelengths is far from understood. In X-rays the stellar disk appears limb-brightened, and active regions appear as extended bright features that evolve on a much shorter timescale than in the optical. This makes measuring the true planet-to-star radius ratio challenging. The Sun offers a unique opportunity to study the impact of stellar activity on high energy transits. Using disk resolved soft X-ray and UV images from NASA’s Solar Dynamics Observatory taken over the last solar cycle I will show how both occulted and unocculted active regions can mimic an inflated planetary atmosphere by changing the depth and shape of a transit profile. I will also show how the disk integrated Lyman-α Solar irradiance varies on both short and long timescales and how this variability can also impact our ability to recover the true radius ratio of a transiting exoplanet. Finally, I will present techniques to overcome these challenges in high-energy transits.
17:07 J. D. Alvarado-Gómez: Simulating the Environment Around Planet-Hosting Stars
Recent developments in instrumentation and observational techniques have opened a new window for stellar magnetic field studies. In particular Zeeman Doppler Imaging (ZDI) is now routinely used to recover the large-scale magnetic field topologies of stars different from the Sun, including several planet-hosting stars. These stellar magnetic fields intimately affect the environment around late-type stars by driving the coronal high-energy radiation (EUV/X-rays), transient events (e.g. flares and coronal mass ejections), and the development of stellar winds and astrospheres. These elements can have a strong impact in the evolution of planetary systems via star-planet interactions and erosion of exoplanetary atmospheres. In this context, the results from ZDI data-driven, detailed 3D MHD modeling of the coronal conditions and circumstellar environment around three planet hosting stars are presented. For one of the considered systems (HD 1237), we investigate the interactions of the magnetized stellar wind with the exoplanet, assuming a Jupiter-like magnetosphere around it.
17:19 M. Jardine: Predicting the Wind Speeds of Solar-Like Stars
Some aspects of stellar magnetic activity such as X-ray emission are relatively easy to observe, while others, such as the geometry of the magnetic field are rather more difficult. Indeed, typically only the large-scale (or low-order) components of the field can be mapped. Most elusive of all is the hot, tenuous stellar wind, yet the wind speed is a crucial quantity governing the angular momentum loss of the star. We demonstrate, however, that wind speeds can be predicted reliably even from fairly low-resolution magnetograms. We use an empirically-derived model of the solar wind that predicts the distribution of wind speeds based on surface magnetograms. Using solar magnetic field measurements spanning several magnetic cycles, we demonstrate how changes in the surface magnetic field on various lengthscales affect the coronal structure and hence the wind speed. We find that only low-order field components are needed to characterise the wind speed. We compare the variations in wind speed with variations in the X-ray emission over several cycles and show that while the small-scale field has a significant effect Lx, it has little effect on the wind speed. This suggests that the large number of stellar magnetograms that are becoming available can be used to predict the stellar wind speeds.
- 17:30 End of Session 1
- 16:15 S. Marsden (invited): Magnetic Fields on Solar-Type Stars: The Solar-Stellar Connection