scholarly journals Are Some Stellar Coronae Optically Thick?

1996 ◽  
Vol 152 ◽  
pp. 121-128
Author(s):  
C.J. Schrijver ◽  
G.H.J. van den Oord ◽  
R. Mewe ◽  
J.S. Kaastra
Keyword(s):  
Extreme Ultraviolet ◽  
Formal Solution ◽  
Emission Measure ◽  
Resonant Scattering ◽  
Heavy Elements ◽  
Volume Filling ◽  
Dominant Component ◽  
Weak Component ◽  
Cool Stars ◽  
Spurious Result

We discuss the coronal spectra of a sample of cool stars observed with the spectrometers of the Extreme Ultraviolet Explorer (EUVE). The emission measure distributions show (a) a relatively weak component between 0.1 MK and 1 MK, (b) a dominant component somewhere between 2 MK and 10 MK, and (c) in all cases but one a component in the formal solution at temperatures exceeding ≈ 20 MK. Where this hot tail is not associated with a real hot component, it is a spurious result reflecting a lowered line-to-continuum ratio, which, for instance, may be the result of a low abundance of heavy elements or of resonant scattering in some of the strongest coronal lines. We suggest that in Procyon’s corona photons in the strongest lines formed around a few million Kelvin undergo resonant scattering in a circumstellar medium, possibly a stellar wind. The flare spectrum of AU Mic suggests that resonant scattering may also occur in dense, hot flare plasmas. The electron densities of the 5–15 MK component are some three orders of magnitude higher than typical of the solar-like component around 2 MK; the volume filling factors of the hot components are therefore expected to be relatively small.

scholarly journals DEM Analyses with the Utrecht Codes

1996 ◽  
Vol 152 ◽  
pp. 553-560
Author(s):  
R. Mewe ◽  
G.H.J. van den Oord ◽  
C.J. Schrijver ◽  
J.S. Kaastra
Keyword(s):  
Emission Measure ◽  
Resonant Scattering ◽  
Continuum Emission ◽  
Second Derivative ◽  
Inversion Problem ◽  
Differential Emission Measure ◽  
The Past ◽  
Cool Stars ◽  
Ill Posed ◽  
Thin Plasma

We address the inversion problem of deriving the differential emission measure (DEM) distribution D(T) = nenHdV/d log T from the spectrum of an optically thin plasma. In the past we have applied the iterative Withbroe-Sylwester technique and the Polynomial technique to the analysis of EXOSAT spectra of cool stars, but recently we have applied the inversion technique discussed by Craig & Brown (1986) and Press et al. (1992) in the analysis of EUVE spectra of cool stars. The inversion problem-a Fredholm equation of the first kind-is ill-posed and solutions tend to show large, unphysical oscillations. We therefore apply a second-order regularization, i.e., we select the specific DEM for which the second derivative is as smooth as is statistically allowed by the data. We demonstrate the importance of fitting lines and continuum simultaneously, discuss the effect on the DEM of continuum emission at temperatures where no line diagnostics are available, and address possible ways to check various model assumptions such as abundances and photon destruction induced by resonant scattering.

scholarly journals Energetics of Hi-C EUV brightenings

2018 ◽  
Vol 615 ◽  
pp. A47 ◽  
Author(s):  
Srividya Subramanian ◽  
Vinay L. Kashyap ◽  
Durgesh Tripathi ◽  
Maria S. Madjarska ◽  
John G. Doyle
Keyword(s):  
High Resolution ◽  
Energy Loss ◽  
Extreme Ultraviolet ◽  
Thermal Structure ◽  
Emission Measure ◽  
Light Curves ◽  
Solar Dynamics Observatory ◽  
Differential Emission Measure ◽  
Dominant Mechanism ◽  
Solar Dynamics

We study the thermal structure and energetics of the point-like extreme ultraviolet (EUV) brightenings within a system of fan loops observed in the active region AR 11520. These brightenings were simultaneously observed on 2012 July 11 by the High-resolution Coronal (Hi-C) imager and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). We identified 27 brightenings by automatically determining intensity enhancements in both Hi-C and AIA 193 Å light curves. The energetics of these brightenings were studied using the Differential Emission Measure (DEM) diagnostics. The DEM weighted temperatures of these transients are in the range log T(K) = 6.2−6.6 with radiative energies ≈1024−25 ergs and densities approximately equal to a few times 109 cm−3. To the best of our knowledge, these are the smallest brightenings in EUV ever detected. We used these results to determine the mechanism of energy loss in these brightenings. Our analysis reveals that the dominant mechanism of energy loss for all the identified brightenings is conduction rather than radiation.

UV Emitting Spicules

1980 ◽  
Vol 91 ◽  
pp. 199-201
Author(s):  
Giannina Poletto
Keyword(s):  
Coronal Hole ◽  
Extreme Ultraviolet ◽  
Emission Measure ◽  
Harvard College ◽  
Chromospheric Network ◽  
Harvard College Observatory ◽  
Ultraviolet Observations ◽  
Lower Limits

Extreme ultraviolet observations of the chromospheric network in a coronal hole obtained in 1973 by the Harvard College Observatory experiment aboard Skylab are analyzed. Upper and lower limits to the actual emission measure in UV spicules have been obtained, and the consistency of the derived values with the hypothesis that UV spicules are Hα spicules falling back after being heated is discussed.

scholarly journals 45. Stellar Classification

1985 ◽  
Vol 19 (1) ◽  
pp. 645-652
Author(s):  
V. Straižys ◽  
R. F. Garrison ◽  
R. A. Bell ◽  
M. Golay ◽  
A. Heck ◽  
...  
Keyword(s):  
Luminosity Function ◽  
Heavy Elements ◽  
Cool Stars ◽  
Nearby Stars

During the report period (1982–1984) several symposia, colloquia, and workshops in the field of stellar classification were held. These include: IAU Symposium No. 111 ”Calibration of Fundamental Stellar Quantities” (Como, May 24 29, 1984, ed. D. S. Hayes et al.), IAU Colloquium No. 76 “The Nearby Stars and the Stellar Luminosity Function” (Middletown, June 13-16, 1983, ed. A. G. Davis Philip and A. R. Upgren), IAU Colloquium No. 78 “Astronomy with Schmidt-Type Telescopes” (Asiago, August 30-September 2, 1983, ed. M. Capaccioli), the workshop “The MK Process and Stellar Classification” (Toronto, June 6-10, 1983, ed. R. F. Garrison), the Colloquium ”Cool Stars with Excess of Heavy Elements” (Strasbourg, July 3-6, 1984).

scholarly journals The Chemical Composition of Cool Stars: II - The Hydrogen Deficient Stars

1985 ◽  
Vol 87 ◽  
pp. 126-150
Author(s):  
David L. Lambert
Keyword(s):  
Chemical Composition ◽  
Spectral Type ◽  
Heavy Elements ◽  
Carbon Stars ◽  
Rr Lyrae Stars ◽  
Rr Lyrae ◽  
Cool Stars ◽  
Similarities And Differences ◽  
Lyrae Stars

AbstractThe chemical composition of the R Coronae Borealis and cool hydrogen deficient carbon stars is reviewed. Similarities and differences between these stars and the hot He stars are noted. Proposed origins for the hydrogen deficient stars are sketched. Recent claims that normal (spectral type N) cool carbon stars are hydrogen deficient are shown to be unfounded. Attention is drawn to the curious case of pop. II variables (RV Tauri, W Virginis, and RR Lyrae stars) whose atmospheres show striking deficiencies of heavy elements and may be hydrogen deficient.

scholarly journals Dissecting the EUV Spectrum of Capella

1996 ◽  
Vol 152 ◽  
pp. 105-112 ◽  
Author(s):  
Nancy S. Brickhouse
Keyword(s):  
Extreme Ultraviolet ◽  
Emission Measure ◽  
Continuous Distribution ◽  
Line Emission ◽  
Local Maximum ◽  
Abundance Ratio ◽  
Emission Lines ◽  
Cool Star ◽  
The Individual ◽  
The Continuum

Extreme ultraviolet spectra of Capella, obtained at various orbital phases over the past two years by the EUVE satellite, show strong emission lines from a continuous distribution of temperatures (~ 105 − 107.3 K). In addition to the strong He II λ303.8, the spectra are dominated by emission lines of highly ionized iron. Strong lines of Fe IX, XV, XVI, and XVIII–XXIV are used to construct emission measure distributions for the individual pointings, which show several striking features, including a minimum near 106 K and a local maximum at 106.8 K. Furthermore, intensities of the highest temperature lines (Te > 107 K) show variations (factors of 2–3) at different orbital phases, while the lower temperature Fe lines show variations of about 30% or less. The low variability of most of the strong low temperature features motivates a detailed analysis of the summed spectrum. With ~ 280 ks of total exposure time, we have measured over 200 emission features with S/N ≥ 3.0 in the summed spectrum. We report here initial results from the analysis of this spectrum. We can now identify lines of Fe VIII and X–XIV, as well as a number of electron density and abundance diagnostic lines.We also report here the first direct measurement of the continuum flux around ~ 100 Å in a cool star atmosphere with EUVE. The continuum flux can be predicted from the emission measure model based on Fe line emission, and demonstrates that the Fe/H abundance ratio is close to the solar photospheric value.

scholarly journals EUVE Spectra of Coronae and Flares

1996 ◽  
Vol 152 ◽  
pp. 81-88 ◽  
Author(s):  
Carole Jordan
Keyword(s):  
Short Wavelength ◽  
Extreme Ultraviolet ◽  
Emission Measure ◽  
Wavelength Region ◽  
Late Type ◽  
Short Wavelength Region ◽  
Plane Parallel ◽  
Parallel Geometry ◽  
Measure Distribution

Following a summary of early solar EUV spectroscopy the spectra of some late-type stars obtained with the Extreme Ultraviolet Explorer (EUVE) are briefly surveyed. Some transitions which are not included in current emissivity codes but could lead to numerous weak lines, and an apparent continuum in the EUVE short wavelength region, are discussed. The importance of the geometry adopted when interpreting the emission measure distribution is stressed, since radial factors can lead to an apparent emission measure distribution gradient that is steeper than the value of 3/2 expected in plane parallel geometry.

scholarly journals The 3D solar corona Cycle 24 rising phase from SDO/AIA tomography

2011 ◽  
Vol 7 (S286) ◽  
pp. 238-241
Author(s):  
Federico A. Nuevo ◽  
Alberto M. Vásquez ◽  
Richard A. Frazin ◽  
Zhenguang Huang ◽  
Ward B. Manchester
Keyword(s):  
Solar Corona ◽  
Extreme Ultraviolet ◽  
Emission Measure ◽  
Source Surface ◽  
Solar Dynamics Observatory ◽  
Differential Emission Measure ◽  
Height Range ◽  
Field Source ◽  
Solar Dynamics ◽  
Cycle 24

AbstractWe recently extended the differential emission measure tomography (DEMT) technique to be applied to the six iron bands of the Atmospheric Imaging Assembly (AIA) instrument aboard the Solar Dynamics Observatory (SDO). DEMT products are the 3D reconstruction of the coronal emissivity in the instrument's bands, and the 3D distribution of the local differential emission measure, in the height range 1.0 to 1.25 R⊙. We show here derived maps of the electron density and temperature of the inner solar corona during the rising phase of solar Cycle 24. We discuss the distribution of our results in the context of open/closed magnetic regions, as derived from a global potential field source surface (PFSS) model of the same period. We also compare the results derived with SDO/AIA to those derived with the Extreme UltraViolet Imager (EUVI) instrument aboard the Solar TErrestrial RElations Observatory (STEREO).

scholarly journals The 3D solar minimum with differential emission measure tomography

2011 ◽  
Vol 7 (S286) ◽  
pp. 123-133
Author(s):  
Alberto M. Vásquez ◽  
Richard A. Frazin ◽  
Zhenguang Huang ◽  
Ward B. Manchester ◽  
Paul Shearer
Keyword(s):  
Extreme Ultraviolet ◽  
Emission Measure ◽  
Three Dimensional ◽  
Differential Emission Measure ◽  
Height Range ◽  
Heliospheric Observatory ◽  
Temperature Distributions ◽  
Imaging Telescope ◽  
Electron Density And Temperature ◽  
Comparative Results

AbstractDifferential emission measure tomography (DEMT) makes use of extreme ultraviolet (EUV) image series to deliver two products: a) the three-dimensional (3D) reconstruction of the coronal emissivity in the instrumental bands, and b) the 3D distribution of the local differential emission measure (LDEM). The LDEM allows, in turn, construction of 3D maps of the electron density and temperature distribution. DEMT is being currently applied to the space-based EUV imagers, allowing reconstruction of the inner corona in the height range 1.00 to 1.25 R⊙. In this work we applied DEMT to different Carrington Rotations corresponding to the last two solar Cycle minima. To reconstruct the 2008 minimum we used data taken by the Extreme UltraViolet Imager (EUVI), on board the Solar TErrestrial RElations Observatory (STEREO) spacecraft, and to reconstruct the 1996 minimum we used data taken by the Extreme ultraviolet Imaging Telescope (EIT), on board the Solar and Heliospheric Observatory (SOHO). We show here comparative results, discussing the observed 3D density and temperature distributions in the context of global potential magnetic field extrapolations. We also compare the DEMT results with other observational and modeling efforts of the same periods.

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