Upgrading the Solar-Stellar Connection:
news about activity in Cool Stars
news about activity in Cool Stars
A splinter session of the 18th Cool Stars Workshop
Organizers: Moritz Günther, Katja Poppenhaeger, Paola Testa
Email: hguenther (at) cfa.harvard.edu, kpoppenhaeger (at) cfa.harvard.edu, ptesta (at) cfa.harvard.edu
The solar-stellar connection
Coronal magnetic activity is what makes Cool Stars special and distinguishes them from hotter stars without magnetic dynamos. Coronal activity also shapes the environment of many extrasolar planets, giving rise to a completely new perspective why we care about magnetic phenomena. While coronal magnetic activity has been studied for a long time, there are new and exciting insights from recent solar and stellar missions, and we would like to provide a forum to bring the stellar and solar astronomers together and discuss synergies from both fields.
In two key areas there is significant observational progress in the last few years:
1) New solar observations, especially IRIS (Interface Region Imaging Spectrograph) and SDO/AIA (Solar Dynamics Observatory, Atmospheric Imaging Assembly) have significantly improved the spatial and temporal resolution of solar observations, as well as their spatial/temporal/temperature coverage.
2) The sample size of stellar activity surveys has exploded, specifically due to the data from Kepler and COROT. We are now at a crucial stage to understand and reap the wealth of such observations, especially in the light of future high-energy missions like Athena+, which is now very likely to been selected by ESA for the L2 launch opportunity.
There is an intimate connection between the activity of our Sun and cool stars. Almost all interpretation of coronal activity starts from the Sun, where observations with a spatial and temporal resolution that is unmatched in stellar astrophysics are possible. On the other hand, active stars provide for more energetic events than commonly observed in our Sun, so that the stellar perspective can help to extend models to a wider range, e.g. to explain the so so-called super-flares.
Program
Talks
Talk should not be longer than 9 min to allow ample time for discussion.
Haiku slides
Attendees of the session are welcome to send a one-slide pdf to the organizers, which they can present at the splinter session in 1 minute. This can be a new result you would like to show, an important figure from your poster, information about a new observing mission, or a conference announcement, for example. Please send an email with the slide of your proposed haiku to hguenther (at) cfa.harvard.edu and kpoppenhaeger (at) cfa.harvard.edu.
Abstracts
A fully parametrized model of the activity pattern of a Solar-like star
Simon BorgnietFuture high-resolution spectrographs are expected to push back the radial velocity detection limits at the level of a few cm/s, giving theoretically access to low planetary masses such as Earth-like planets. However, according to many studies, stellar magnetic activity will induce radial velocity jitter at the level of the m/s, ie far over the expected performances of the future spectrographs, even in the case of a low activity star. It will thus seriously undermine the possibility of detecting an Earth twin in the habitable zone of its host star, unless precise correction can be done. In the context of the modelling of such stellar jitter, we present a fully parametrized model of the activity pattern of a Solar-like star and of its impact on radial velocity jitter. The model includes dark spots, bright faculae and the attenuation of the convective blueshift. It has been compared to the Solar pattern over a full Solar cycle for validation. Being fully parametrized, it is straightforward to transfer to other spectral types and stellar properties (this is a work in progress). It will allow to predict the radial velocity signature for a wide range of stars and activity levels. It also opens new perspectives in terms of correcting the activity radial velocity signature.
Convection and dynamo action in sun-like stars or the new concept of spot-dynamos
Sacha BrunWe will present recent advances made in understanding dynamo action in the Sun and solar-like stars. We will discuss how large scale flows are established, how they vary with rotation rate and how this impact dynamo action in stars. We will also discuss the new concept of spot-dynamo, e.g. non linear dynamo generating self consistently rising omega-loop, that, we believe, will be key to interpret ever growing observations of stellar magnetism.
Understanding Astrophysical Noise from Stellar Surface Magneto-convection
Heather CeglaCool, low mass stars with a convective envelope have bubbles of hot, bright plasma rising to the surface where they eventually cool, darken and sink. The motions of these plasma bubbles induce stellar line asymmetries since the radial velocity (RV) shift induced from the uprising granules does not completely cancel the shift from the sinking intergranular lanes. Furthermore, these line asymmetries are constantly changing as the ratio of granular to intergranular lane material continues to change due to magnetic field interplay. The net result for Sun-like stars is shifts in the line profiles on the order of several tens of cm/s. Hence an understanding of magneto-convection and its effects is paramount in any high precision RV study. One particular area impacted is the RV confirmation of Earth-analogs; the astrophysical noise from the host star stellar surface magneto-convection completely swamps the 10 cm/s signal induced from the planet. We aim to understand the physical processes involved here so that we may disentangle the effects of magneto-convection from observed stellar lines. To do so, we start with a state-of-the-art 3D magnetohydrodynamic simulation of the solar surface. Motivated by computational constraints and a desire to breakdown the physics, we parameterize the granulation signal from these simulations. This parameterization is then used to construct model Sun-as-a-star observations with a RV precision far beyond current instrumentation. This parameterization across the stellar disc, for a variety of magnetic field strengths, is presented here, alongside the current results from the model star observations. We find several line characteristics to be correlated with the induced RV shifts. Particularly high correlations were found for the velocity asymmetry (comparing the spectral information content of the blue wing to the red wing) and brightness measurements (approximated by integrating under the model observation profiles), allowing significant granulation noise reduction. The results of this campaign can feed directly into future high precision RV studies, such as the search for habitable, rocky worlds, with the forthcoming ESPRESSO and HIRES spectrographs.
Magnetic Modulation of Stellar Angular Momentum Loss
Cecilia GarraffoAngular Momentum Loss (AML) is important for understanding astrophysical phenomena such as stellar rotation and magnetic activity, close binaries, and cataclysmic variables. Magnetic breaking is the dominant mechanism in the spin down of young late-type stars. We have studied AML as a function of stellar magnetic activity. We argue that the complexity of the field and its latitudinal distribution are crucial for the AML rates. In this talk I will discuss how AML is modulated by magnetic cycles and stellar spin down is not just a simple function of large scale magnetic field strength.
A Deep Rapid Archival Flare Transient Search in the Galactic Bulge
Adam KowalskiDue to their high flare rates and energies combined with a large contrast against the background quiescent emission, the low-mass M dwarfs are the primary target for studying flare rates in the Galaxy. However, high-precision monitoring from Kepler and the Hubble Space Telescope have recently revealed important information on the flare rates of earlier-type, more massive stars. In this talk, I will focus on the properties of flares and flare stars in the optical as revealed by a Hubble Space Telescope/ACS planet search of the Galactic Bulge. We discovered ~100 flare stars, which are likely old (10 Gyr) binary systems with sustained flare activity from tidal spin-up. We will discuss the implications for flare rates in future time-domain surveys and compare to rates and energetics of flares on much younger M dwarf flare stars.
The variability of Sun-like stars: reproducing observed photometric trends
A. I. ShapiroThe Sun and stars with low magnetic activity levels become photometrically brighter when their activity increases. Magnetically more active stars display the opposite behavior and get fainter when their activity increases. We reproduce the observed photometric trends in stellar variations with a model that attributes the variability of the stellar radiative energy flux to the imbalance between the contributions from dark starspots and bright faculae. Our approach allows us to model the stellar photometric variability vs. activity dependence and reproduce the transition from faculae-dominated variability and direct activity–brightness correlation to spot- dominated variability and inverse activity–brightness correlation with increasing chromospheric activity level. The general success of the model in reproducing the behavior of Sun-like stars is a clear indication that the photometric variability of more active stars has the same basic causes as the Sun’s.
HAZMAT I: The Evolution of Ultraviolet Emission from Early M Stars
Evgenya ShkolnikWith the recent discoveries of several super-earths orbiting M dwarfs well within their habitable zones (0.1 to 0.4 AU), and with many more such planets to come, it is critical to assess the evolution of the high-energy radiation environment of these systems. We have begun the HAZMAT (HAbitable Zones and M dwarf Activity across Time) program by first measuring the drop in near-UV and far-UV flux in early M stars from 10 Myr to several Gyr using photometry from NASA's Galaxy Evolution Explorer (GALEX). We focus this study on the confirmed low-mass members of nearby young moving groups, the Hyades cluster, and old field stars. We show a relatively slow decline in UV flux up until at least 650 Myr with a sharper drop in the old M dwarfs. Yet without confirmed M dwarfs in nearby star clusters with ages of 1-2 Gyr, mapping the precise evolution at these older ages is not currently possible. The UV data also provide much-needed constraints to M dwarf upper-atmosphere models, which are currently insufficient for predicting UV emission from M dwarfs. Our analysis will aid empirically motivated upper-atmospheric modeling for the young and old M stars, which can then be used to predict the extreme-UV fluxes most critical to the evolution of a planetary atmosphere. The HAZMAT program is the first comprehensive study of the UV history of M stars.
Radius variability induced by dynamo magnetic fields: the Sun vs. low-mass stars
Frederico SpadaWe investigate the impact of the magnetic fields associated with the dynamo on the internal structure and the global parameters (i.e., radius, luminosity, effective temperature) of the Sun and of solar-like stars. Although magnetic fields are usually not taken into account in standard stellar evolutionary codes, they have both direct effects (by contributing to the total pressure) and indirect effects (by inhibiting or suppressing convection), which can alter the internal equilibrium structure. Many theoretical studies have shown that magnetic fields are a promising mechanism to explain the so-called radius discrepancy in low-mass stars - i.e., the discrepancies between observed and modelled stellar radii and effective temperatures for young, active stars, usually reported at the ~10% and ~5% levels, respectively. More recently, radius variations have been reported for the Sun, based on the measurements by the Solar Disk Sextant (SDS) experiment, with an amplitude of up to 0.02% over the whole solar cycle. The possibility to reconcile both the solar and the stellar observations within the framework of a unique theoretical picture will be discussed.
Bright Hot Impacts by Erupted Fragments Falling Back on the Sun: A Template for Stellar Accretion
Paola TestaImpacts of falling fragments observed after the eruption of a filament in a solar flare on 7 June 2011 are similar to those inferred for accretion flows on young stellar objects. As imaged in the ultraviolet (UV)-extreme UV range by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory, many impacts of dark, dense matter display uncommonly intense, compact brightenings. High-resolution hydrodynamic simulations show that such bright spots, with plasma temperatures increasing from ~104 to ~106 kelvin, occur when high-density plasma (>>1010 particles per cubic centimeter) hits the solar surface at several hundred kilometers per second, producing high-energy emission as in stellar accretion. The high-energy emission comes from the original fragment material and is heavily absorbed by optically thick plasma, possibly explaining the lower mass accretion rates inferred from x-rays relative to UV-optical-near infrared observations of young stars.
Magnetism in cool stars: empirical trends with age and rotation
A. A. VidottoWe investigate how the large-scale surface magnetic fields of cool dwarf stars, reconstructed using the Zeeman-Doppler Imaging technique, vary with age, rotation period, Rossby number and X-ray emission. Our sample consists of 104 magnetic maps of 76 stars, from accreting pre-main sequence to main-sequence objects, spanning ages from ~1 Myr to ~10 Gyr. For non-accreting dwarfs we empirically find that the unsigned average large-scale surface magnetic field relates to age as age-0.655± 0.045. This relation has a similar power dependency to that identified in the seminal work of Skumanich (1972). We also find in our data evidence for a linear-type dynamo, in which the surface field is linearly dependent on the rotation rate. The trends we find for large-scale stellar magnetism from ZDI studies are consistent with the trends found from Zeeman broadening measurements, which are sensitive to the unsigned large- and small-scale magnetic field. These similarities indicate that the fields recovered from both techniques are coupled to each other, suggesting that small- and large-scale fields could share the same dynamo field generation processes. Our results are relevant for investigations of rotational evolution of low-mass stars and give important observational constraints for stellar dynamo studies.