J. Martin Laming

A MILLION-SECOND CHANDRA VIEW OF CASSIOPEIA A

We introduce a million second observation of the supernova remnant Cassiopeia A with the Chandra X-Ray Observatory. The bipolar structure of the Si-rich ejecta (northeast jet and southwest counterpart) is clearly evident in the new images, and their chemical similarity is confirmed by their spectra. These are most likely due to jets of ejecta as opposed to cavities in the circumstellar medium, since we can reject simple models for the latter. The properties of these jets and the Fe-rich ejecta will provide clues to the explosion of Cas A.

A unified picture of the first ionization potential and inverse first ionization potential effects

We discuss models for coronal abundance anomalies observed in the coronae of the sun and other late-type stars following a scenario first introduced by Schwadron, Fisk & Zurbuchen of the interaction of waves at loop footpoints with the partially neutral gas. Instead of considering wave heating of ions in this location, we explore the effects on the upper chromospheric plasma of the wave ponderomotive forces. These can arise as upward propagating waves from the chromosphere transmit or reflect upon reaching the chromosphere-corona boundary, and are in large part determined by the properties of the coronal loop above. Our scenario has the advantage that for realistic wave energy densities, both positive and negative changes in the abundance of ionized species compared to neutrals can result, allowing both FIP and Inverse FIP effects to come out of the model. We discuss how variations in model parameters can account for essentially all of the abundance anomalies observed in solar spectra. Expected variations with stellar spectral type are also qualitatively consistent with observations of the FIP effect in stellar coronae. Subject headings: Sun: corona – stars: coronaeWe discuss models for coronal abundance anomalies observed in the coronae of the sun and other late-type stars following a scenario first introduced by Schwadron, Fisk, & Zurbuchen of the interaction of waves at loop footpoints with the partially neutral gas. Instead of considering wave heating of ions in this location, we explore the effects on the upper chromospheric plasma of the wave pondermotive forces. These can arise when upward-propagating waves from the chromosphere transmit or reflect upon reaching the chromosphere-corona boundary, and are in large part determined by the properties of the coronal loop above. Our scenario has the advantage that for realistic wave energy densities both positive and negative changes in the abundance of ionized species compared to neutrals can result, allowing both first ionization potential (FIP) and inverse FIP effects to come out of the model. We discuss how variations in model parameters can account for essentially all of the abundance anomalies observed in solar spectra. Expected variations with stellar spectral type are also qualitatively consistent with observations of the FIP effect in stellar coronae.

A Physical Relationship Between Electron-Proton Temperature Equilibration and Mach Number in Fast Collisionless Shocks

The analysis of Balmer-dominated optical spectra from nonradiative (adiabatic) SNRs has shown that the ratio of the electron to proton temperature at the blast wave is close to unity at vS 400 km s-1 but declines sharply down to the minimum value of me/mp dictated by the jump conditions at shock speeds exceeding 2000 km s-1. We propose a physical model for the heating of electrons and ions in non-cosmic-ray-dominated, strong shocks (vS > 400 km s-1) wherein the electrons are heated by lower hybrid waves immediately ahead of the shock front. These waves arise naturally from the cosmic ray pressure gradient upstream from the shock. Our model predicts a nearly constant level of electron heating over a wide range of shock speeds, producing a relationship (Te/Tp)0 v (M-2) that is fully consistent with the observations.

Electron-ion Equilibration in Nonradiative Shocks Associated With SN 1006

The ultraviolet spectrum of SN 1006 observed by the Hopkins Ultraviolet Telescope (HUT) is modeled to infer the degree of electron-ion equilibration at the nonradiative shocks in the remnant. Such an approach is possible since the spectrum has lines (He II λ1640) whose excitation is dominated by electrons, and others (C IV λ1550, N V λ1240, O VI λ1036) in which protons and other ions are more important, and the intensity ratio between these is sensitive to the electron-ion temperature ratio. We find substantially less than full equilibration, marginally consistent with existing plasma simulations but rather more suggestive of even lower equilibration than these studies would predict. The more stringent limits on the electron temperature resulting from this allow a more accurate determination of the distance to SN 1006. Our result is 1.8 ± 0.3 kpc.

THE SLOW TEMPERATURE EQUILIBRATION BEHIND THE SHOCK FRONT OF SN 1006

We report on the observation of O VII Doppler line broadening in a compact knot at the edge of SN 1006 detected with the reflective grating spectrometer on board XMM-Newton. The observed line width of σ = 3.4 ± 0.5 eV at a line energy of 574 eV indicates an oxygen temperature of kT = 528 ± 150 keV. Combined with the observed electron temperature of ~1.5 keV, the observed broadening is direct evidence for temperature nonequilibration in high Mach number shocks and slow subsequent equilibration. The O VII line emission allows an accurate determination of the ionization state of the plasma, which is characterized by a relatively high forbidden line contribution, indicating log net 9.2.

Where Was the Iron Synthesized in Cassiopeia A

We investigate the properties of Fe-rich knots on the east limb of the Cassiopeia A supernova remnant using observations with Chandra/ACIS and analysis methods developed in a companion paper. We use the fitted ionization age and electron temperature of the knots to constrain the ejecta density profile and the Lagrangian mass coordinates of the knots. Fe-rich knots that also have strong emission from Si, S, Ar, and Ca are clustered around mass coordinates q 0.35-0.4 in the shocked ejecta; for ejecta mass 2 M☉, this places the knots 0.7-0.8 M☉ out from the center (or 2-2.1 M☉, allowing for a 1.3 M☉ compact object). We also find an Fe clump that is evidently devoid of line emission from lower mass elements, as would be expected if it were the product of α-rich freezeout; the mass coordinate of this clump is similar to those of the other Fe knots.

Detection of the 67.9 keV and 78.4 ke.V lines associated with the radio-active decay of 44 Ti in Cassiopeia A

We report the detection of the 44Sc nuclear decay lines at 67.9 and 78.4 keV associated with the nuclear decay of 44Ti in Cassiopeia A. The line emission was observed by the Phoswich Detection System instrument on board BeppoSAX, which recently observed the supernova remnant for over 500 ks. The detection of the line emission with a flux of (2.1 ± 0.7) × 10-5 photons cm-2 s-1 in each line (90% confidence) is at the 5 σ significance level, if we can assume that the 12-300 keV continuum is adequately represented by a single power law. However, as the nature of the continuum is not clear, we investigate various other possibilities. A more conservative estimate of the line flux is made by assuming that a power-law continuum is at least a good approximation to the continuum emission for a narrower 30-100 keV energy range. With this limitation, the measured line flux is (1.9 ± 0.9) × 10-5 photons cm-2 s-1, with the detection still at the 3.4 σ significance level. We suggest that together with the Compton Gamma Ray Observatory/COMPTEL measurement of the 44Ca line at 1157 keV of (3.3 ± 0.6) × 10-5 photons cm-2 s-1, a flux for all three lines of (2.5 ± 1.0) × 10-5 photons cm-2 s-1 for Cas A can be adopted. This implies an initial 44Ti mass of (0.8-2.5) × 10-4 M☉.

Enhanced Noble Gases in the Coronae of Active Stars

We have analyzed Chandra High-Energy Transmission Grating spectra of the active RS CVn‐type binary V711 Tauri (HR 1099; HD 22468) in order to examine the chemical composition of its coronae. Observed fluxes and flux upper limits for spectral lines from a range of charge states of iron, covering species Fe xvi‐Fe xxv, have been used to determine the emission measure distribution as a function of temperature, while the observed Fe line-to-continuum ratio has been used to examine the absolute iron abundance, Fe/H. Abundances of elements O, Ne, Mg, Si, S, and Ar relative to both Fe and H have been estimated by comparison of observed line fluxes with predictions based on the emission measure distribution. We confirm results of earlier studies finding the coronae of V711 Tau to be metal-poor and derive an iron abundance of . We find the noble Fe/H p 7.0 5 0.1 gas elements Ne and Ar to be enhanced relative to the local cosmic value and enhanced by an order of magnitude relative to Fe. Very mild enhancements of O and Mg relative to Fe are also discerned. By examination of coronal abundances of Ne relative to Fe culled from the literature, in addition to Ne lines seen in hitherto unpublished Chandra spectra, we conclude that large Ne abundance enhancements are a common feature of active stellar coronae.

On Stellar Coronae and Solar Active Regions

Based on Yohkoh Soft X-Ray Telescope (SXT) observations of the Sun near peak activity level obtained on 1992 January 6, we search for coronal structures that have emission measure distributions EM(T ) that match the observed stellar coronal emission measure distributions derived for the intermediate-activity stars v Eri (K2 V) and m Boo A (G8 V) from Extreme Ultraviolet Explorer spectro- scopic observations. We —nd that the temperatures of the peaks of the observed stellar distributions EM(T ), as well as their slopes in the temperature range are very similar to those 6.0 ( log T ( 6.5, obtained for the brightest of the solar active regions in the 1992 January 6 SXT images. The observed slopes correspond approximately to EM P T b with b D 4, which is much steeper than predicted by static, uniformly heated loop models. Plasma densities in the coronae of v Eri and m Boo A are also observed to be essentially the same as the plasma densities typical of solar active regions. These data provide the best observational support yet obtained for the hypothesis that solar-like stars up to the activity levels of v Eri (K2 V) and m Boo A are dominated by active regions similar to, though possibly considerably larger than, those observed on the Sun. The surface —lling factor of bright active regions needed to explain the observed stellar emission measures is approximately unity. We speculate on the scenario in which small-scale ii nano—ares ˇˇ dominate the heating of active regions up to activity levels similar to those of v Eri (K2 V) and m Boo A. At higher activity levels still, the interactions of the active regions themselves may lead to increasing —aring on larger scales that is responsible for heating plasma to the observed coronal temperatures of on very active stars. Observations of X-ray and T Z 107 K EUV light curves using more sensitive instruments than are currently available, together with determi- nations of plasma densities over the full range of coronal temperatures (106¨107 K and higher), will be important to con—rm —are heating hypotheses and to elicit further details concerning coronal structures at solar-like active region temperatures and the temperatures that characterize the most (T ( 5 ) 106 K) active stars (T Z 107 K). Subject headings: stars: coronaestars: individual (v Eridani, m Bootis) ¨ Sun: corona ¨ Sun: X-rays, gamma raysX-rays: stars

Stellar coronal abundances. 3: The solar first ionization potential effect determined from full-disk observation

In this paper we reanalayze the full-disk quiet-sun spectrum of Mallinovsky & Heroux (1973) with modern atomic data. The purposes of this are to check our atomic data and methods in other investigations using data from nearby stars obtained with the NASA Extreme Ultraviolet Explorer (EUVE) satellite, and to confirm that the solar first ionization potential (FIP) effect investigated by previous authors studying discrete solar regions is the same as that found in full-disk spectra. We recover the usual solar FIP effect of a coronal abundance enhancement of elements with a low FIP of a factor approximately 3-4 for lines formed at temperatures greater than approximately 10(exp 6) K. For lower temperatures, the FIP effect seems to be substantially smaller, in qualitative agreement with other data. Comparing our full-disk result with those from discrete solar structures suggest that the FIP effect is a function of altitude, with the lower temperature full-disk emission being dominated by the super-granulation network. We also compare the recent ionization balance of Arnaud & Raymond (1992) with that of Arnaud & Rothenflug (1985).