Session organizers: Katja Poppenhaeger and Hans Moritz Günther
Science program
While the majority of the emission from cool stars, stellar systems and the Sun is dominated by thermal processes, recent years saw more and more signatures of non-thermal effects.
A prominent example is the combination of non-thermal coronal heating (tracked by gyrosynchrotron emission at radio wavelengths) and thermal emission (soft X-rays) observed in coronae. In many cases the observational relation between X-ray and radio is described by the Güdel-Benz relation, but neither the theoretical foundation for that is understood, nor does one know what causes many young stars and very late-type stars to differ significantly from this relation. A thorough physical picture of these processes is the keystone to understanding the stellar and solar coronal heating problem.
Other non-thermal pprocesses of interest are for example charge exchange (CX) in solar system objects (and possibly also in diffuse emission of young star forming regions and in exoplanetary atmospheres), and fluorescent emission in active stars.
A key challenge is how to identify and distinguish between different non-thermal mechanisms. The signals are often weak, atomic data for non-thermal processes is scarce and less well tested than collisional ionization calculations. Also, most models concentrate on thermal and time-constant emission. Thus, observers need to fully understand the uncertainty in the atomic models and modelers need to know inherent systematics in the observations.
Our session will provide an introduction to the newest developments in non-thermal astrophysics of cool stars and provide a discussion forum to identify the best strategies to differentiate between non-thermal models and to direct future observations, particularly from wider wavelength ranges.
Talks
Talks should not be longer than 10 minutes to allow ample time for discussions.
| Time | Speaker | Title | slides |
|---|---|---|---|
| 3:00-3:05 | K. Poppenhager | Introduction | |
| 3:05-3:22 | J. A. Carter | Charge-exchange as observed in the vicinity of the Earth and within the Solar System | |
| 3:22-3:40 | H. S. Hudson | The He II 304 Å line wings and charge exchange | |
| 3:40-3:57 | S. J. Wolk | Cool Stars in Really Hot Places | |
| 3:57-4:15 | P. Beiersdorfer | Unusual emission lines of iron between 170 and 200 Å: Markers for non-thermal processes in the corona? | |
| 4:15-4:45 | Coffee break |
||
| 4:45-5:02 | N. S. Brickhouse | How Atomic Rate Uncertainties Affect Fits to X-ray Spectra | |
| 5:02-5:20 | Lalitha Sairam | Relation between chromospheric evaporation and coronal heating - Neupert effect | |
| 5:20-5:37 | A. Kowalski | Non-thermal heating in M dwarf flares: new radiative hydrodynamic models and constraints from observations | |
| 5:37-5:55 | M. Miceli | Hard X-ray emission from the solar corona detected with the SphinX spectrometer | |
| 5:55-6:00 | K. Poppenhaeger | Summary |
Abstracts
Unusual emission lines of iron between 170 and 200 Å: Markers for non-thermal processes in the corona?
P. Beiersdorfer, J. K. Lepson, Elmar TraebertNew iron lines have been observed in the 170 - 200 Å iron M-shell spectral region, which may be markers of non-equilibrium processes. The lines were observed during studies of the emission spectra of iron in the extreme ultraviolet region 150-250 Å recorded at the National Spherical Torus Experiment (NSTX) at Princeton and taken at electron temperatures targeted by the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory. Under certain discharge condition characterized by hot, ionizing plasmas we found significant emission from heretofore unknown M-shell iron lines. In particular, we have observed a strong line of iron, which might be from Fe XI, that lies within the 171 Å band pass channel of the AIA. In addition, we find an unusually strong enhancement of another Fe XI line, which may affect the 193 Å channel of the AIA. Subsequent studies of some of these spectral regions at the electron beam ion trap at Livermore have confirmed the presence of unknown iron lines. The lines may be markers of non-thermal processes in coronal plasma and may compromise the interpretation of the iron M-shell emission in this region under certain conditions. This work was supported by the NASA Solar and Heliospheric program under Contract NNH10AN31I and the DOE OFES General Plasma Science program. Work was performed by Lawrence Livermore National Laboratory and Princeton Plasma Physics Laboratory under the auspices of the U. S. Department of Energy under Contracts DEAC52-07NA27344 and DE-AC02-09CH11466.
How Atomic Rate Uncertainties Affect Fits to X-ray Spectra
Nancy S. BrickhouseX-ray spectra routinely provide diagnostics for the temperature, density, and abundances of thermal plasmas. Non-equilibrium ionization and absorption due to intervening gas or resonance line scattering can also, in principle, be diagnosed from high resolution and/or high signal/noise spectra; however, systematic errors in atomic rates may produce fit residuals that mimic interesting physics. In this talk I will describe error estimates for data in the recent release of ATOMDB (Foster et al. 2012, ApJ, submitted), based on experimental and theoretical considerations. I will also give some examples of how atomic rate errors might affect spectral analyses.
Charge-exchange as observed in the vicinity of the Earth and within the Solar System
J. A. Carter, S. Sembay and A. M. ReadCharge exchange emission is observed locally in the near-Earth environment through X-rays produced via the interactions between highly-charged solar wind ions and neutral hydrogen in the Earth's exosphere. The signal produced may be the strongest diffuse soft X-ray component within an observation, and so an understanding of this component is crucial in determining the background components for astronomers studying diffuse Galactic and extra-galactic X-ray emission. In addition, exospheric charge-exchange provides a diagnostic of the charge-state distribution of the solar wind and the mass transport around the Earth's magnetosheath. We discuss the compositional and temporal information that can be gathered from this emission. We concentrate on the signatures of a passing Coronal Mass Ejection, serendipitously observed by XMM-Newton. We also briefly describe charge exchange processes at other sites in the Solar System, such as around comets or in interplanetary space.
The He II 304 Å line wings and charge exchange
Hugh S. HudsonI report on searches for charge-exchange wings of the Ly-alpha line of He II at 304 Å, as observed in solar flares by the EVE instrument on board the Solar Dynamics Observatory. None of the GOES X-class flares observed thus far by this instrument showed indications of the Orrall-Zirker mechanism, whereby low-energy accelerated particles (alphas, in this case) penetrate partially ionized regions of the atmosphere and emit recombination radiation following charge exchange. The upper limits fall well below the prediction of this effect by Peter et al. (1990). We also find only upper limits for the analogous mechanism for alpha particles in solar particle events, but undergoing charge exchange on H-like and He-like ions in the solar corona and solar wind.
Non-thermal heating in M dwarf flares: new radiative hydrodynamic models and constraints from observations
Adam KowalksiA primary mode of radiative energy release in stellar flares is the optical and near-ultraviolet (NUV) continuum. However, radiative-hydrodynamic models of stellar flares using a solar flare paradigm and the sparse observations of solar and stellar flare continua are all seemingly in disagreement over the type(s) of emission that contribute to the optical/NUV continuum during flares. We have completed a long-term flare monitoring campaign using simultaneous low-resolution (3400-9200A) spectroscopic and broadband photometric observations to fully characterize the optical/NUV white light continuum emission on short timescales. To date, our most significant results come from observations during the decay phase of a megaflare on the dM4.5e star YZ CMi, where we have detected multiple continuum components that contribute to the white light near the Balmer jump (3646A). We present a time-resolved spectral analysis of the continuum components and emission lines for twenty flares observed during our spectroscopic monitoring campaign. We have begun the next-generation of 1D radiative-hydrodynamic models with the RADYN code in order to understand the physics that produce the spectral shape, emission strength, and detailed time-evolution of stellar flares. We will show the first results from these models where we have begun to characterize the atmospheric response to large, non-thermal electron beam fluxes (> 10^11 erg/s/cm^2) that include a gradual phase. We will discuss exciting avenues for the future of these models.
Hard X-ray emission from the solar corona detected with the SphinX spectrometer
Marco MiceliWe present the analysis of the X-ray spectra of the solar corona observed by the SphinX spectrometer on board the CORONAS-PHOTON mission in May 2009 (when the active region AR11017 was present). We model the spectrum extracted from the whole Sun over a time window of 17 days in the 1.3-7 keV energy band. The high statistics and the accurate calibration of the spectrometer allowed us to detect a hard X-ray component that dominates the solar X-ray spectrum above 4 keV. Though this component is consistent with being associated with thermal emission from very hot plasma (at ~6.5 million K) it is not possible to exclude a non-thermal origin. We will discuss both the thermal and non-thermal scenarios in the light of a possible link between hard X-ray emission and widespread nanoflare activity.
Closed magnetic flux at coronal holes boundaries
Dmitry ProsovetskyThe boundaries separating coronal holes and the quiet Sun possess particular properties and their features are determinative for many processes in solar atmosphere. According to SDO observations near borders of coronal holes numerous areas of magnetic field reconnection are located. Observations show that the waves propagating along the surface of the Sun can be reflected and/or refracted from coronal hole boundaries. It is impossible to explain these phenomena in the assumption, that properties of coronal holes are defined only by an open weak unipolar magnetic field. We have investigated coronal hole boundaries according to simultaneous EUV observations on board SOHO/EIT, SDO/AIA and STEREO/EUVI/A/B spacecrafts. It has been found, that boundaries of coronal holes considerably vary location depending from angle of view. The reason for this phenomenon can be the covers of hot loops up to 100 thousand kilometers from photosphere along coronal holes boundaries which shield their emission. The microwave observations at 5.7 (SSRT) and 17 GHz (NoRH) show, that in solar polar areas where the coronal holes are present practically constantly the brightness temperature was increased. It has been established, that the radio emission of such areas on 5.7 GHz is polarized, and the polarization degree is up to 3 percent in the assumption optically thin radiation. The magnetic field in these areas has values up to 30 G. Such values of magnetic field strength hardly can correspond to a weak magnetic field of coronal holes. It is possible to assume the source of microwave emission with these characteristics is loops located on borders of coronal holes. The analysis of dynamics of high-speed solar wind streams in the beginning of 2012 shows, that during the separate periods high-speed solar wind streams corresponding to coronal holes were not observed at L1 point. In these cases the considerable area of coronal holes was covered by loops with coronal temperatures plasma inside. Probably, streams of the particles forming a solar wind were shielded by these loops already at level of a transition zone and the bottom corona. In our opinion the derived results should be taken into account at modeling solar atmosphere and interpretation of observations of solar quiet areas and coronal holes.
Relation between chromospheric evaporation and coronal heating - Neptert effect
Lalitha Sairamno abstract
Cool Stars in Really Hot Places
Scott J. WolkDuring the last three decades, evidence has mounted that star and planet formation is not an isolated process, but is influenced by previous generations of stars. More recently we have become aware that young stars effect their neighborhood and the formation of their peers. Although cool stars form in a range of environments, from isolated globules to rich embedded clusters, the influences of other stars on cool star and planet formation may be most significant in embedded clusters, where hundreds to thousands of cool stars form in close proximity to OB stars. Diffuse X-ray plasma with temperatures of over a million K has now been detected in about a dozen OB star forming regions -- including the Carina Nebula, NGC 281, RCW 38 and the ONC. Here I review the basic result from these regions and discuss the diffuse plasma environment enveloping the young cool stars and planetary systems and the possible impact on the natal systems.