arXiv subjects

D. P. Huenemoerder

Publications and source records attributed to D. P. Huenemoerder.

22 records · Page 2Linked to original sources

arXiv2008

Geometry Diagnostics of a Stellar Flare from Fluorescent X-rays

We present evidence of Fe fluorescent emission in the Chandra HETGS spectrum of the single G-type giant HR 9024 during a large flare. In analogy to solar X-ray observations, we interpret the observed Fe K$α$ line as being produced by illumination of the photosphere by ionizing coronal X-rays, in which case, for a given Fe photospheric abundance, its intensity depends on the height of the X-ray source. The HETGS observations, together with 3D Monte Carlo calculations to model the fluorescence emission, are used to obtain a direct geometric constraint on the scale height of the flaring coronal plasma. We compute the Fe fluorescent emission induced by the emission of a single flaring coronal loop which well reproduces the observed X-ray temporal and spectral properties according to a detailed hydrodynamic modeling. The predicted Fe fluorescent emission is in good agreement with the observed value within observational uncertainties, pointing to a scale height $\lesssim 0.3$\rstar. Comparison of the HR 9024 flare with that recently observed on II Peg by Swift indicates the latter is consistent with excitation by X-ray photoionization.

astro-ph↗

arXiv2005

An Assessment of the Fe XVIII and Fe XIX Line Ratios from the Chandra Grating Observations of Capella

Observations of Fe XVIII and Fe XIX X-ray, EUV, and FUV line emission, formed at the peak of Capella's (alpha Aurigae) emission measure distribution and ubiquitous in spectra of many cool stars and galaxies, provide a unique opportunity to test the robustness of Fe XVIII and Fe XIX spectral models. The Astrophysical Plasma Emission Code (APEC) is used to identify over 35 lines from these two ions alone, and to compare model predictions with spectra obtained with the Chandra Low Energy Transmission Grating and High Energy Transmission Grating Spectrometers, the Far Ultraviolet Spectroscopic Explorer, and the Extreme Ultraviolet Explorer. Some flux discrepancies larger than factors of two are found between observations of Fe XVIII and Fe XIX lines and predictions by APEC and other models in common usage. In particular the X-ray resonance lines for both ions are stronger than predicted by all models relative to the EUV resonance lines. The multiwavelength observations demonstrate the importance of including dielectronic recombination and proton impact excitation, and of using accurate wavelengths in spectral codes. These ions provide important diagnostic tools for 10^7 K plasmas currently observed with Chandra, XMM-Newton, and FUSE.

astro-ph↗

arXiv2001

X-Ray Plasma Diagnostics of Stellar Winds in Young Massive Stars

High resolution X-ray spectra of very young massive stars opened a new chapter in the diagnostics and understanding of the properties of stellar wind plasmas. Observations of several very young early type stars in the Orion Trapezium demonstrated that the conventional model of shock heated plasmas in stellar winds is not sufficient to explain the observed X-ray spectra. Detailed X-ray line diagnostics revealed extreme temperatures in some of the candidates as well as evidence for high plasma densities. It is also evident from high resolution spectra of more conventional early type stars, that not all show such extreme characteristics. However, the fact that some of the stars show hot and dense components and some do not requires more understanding of the physical processes involved in stellar wind emissions. The Orion Trapezium stars distinguish themselves from all the others by their extreme youth. By comparing the diverse spectral properties of theta Ori A and theta Ori E with those of theta Ori C, we further demonstrate that X-ray spectral properties of very young massive stars are far from understood.

astro-ph↗

arXiv2000

High Resolution X-Ray Spectra of Capella: Initial Results from the Chandra High Energy Transmission Grating Spectrometer

High resolution spectra of the active binary Capella (G8 III + G1 III) covering the energy range 0.4-8.0 keV (1.5-30 Angstroms) show a large number of emission lines, demonstrating the performance of the HETGS. A preliminary application of plasma diagnostics provides information on coronal temperatures and densities. Lines arising from different elements in a range of ionization states indicate that Capella has plasma with a broad range of temperatures, from log T = 6.3 to 7.2, generally consistent with recent results from observations with the Extreme Ultraviolet Explorer (EUVE) and the Advanced Satellite for Cosmology and Astrophysics (ASCA). The electron density is determined from He-like O VII lines, giving the value N_e=10^10 cm^-3 at T_e=2*10^6 K; He-like lines formed at higher temperatures give only upper limits to the electron density. The density and emission measure from O VII lines together indicate that the coronal loops are significantly smaller than the stellar radius.

astro-ph↗