Browsing by Author "Cuntz, Manfred"

Now showing 1 - 4 of 4

Citation - WoS: 2
Citation - Scopus: 3
Chromospheric Activity in 55 Cancri - I. Results From Theoretical Wave Studies
(Oxford Univ Press, 2021) Fawzy, Diaa E.; Cuntz, Manfred
We present theoretical models of chromospheric heating for 55 Cancri, an orange dwarf of relatively low activity. Self-consistent, non-linear, and time-dependent ab initio numerical computations are pursued encompassing the generation, propagation, and dissipation of waves. We consider longitudinal waves operating among arrays of flux tubes as well as acoustic waves pertaining to non-magnetic stellar regions. Additionally, flux enhancements for the longitudinal waves are also taken into account as supplied by transverse tube waves. The Caii K fluxes are computed (multi-ray treatment) assuming partial redistribution as well as time-dependent ionization (TDI). The self-consistent treatment of TDI (especially for hydrogen) greatly impacts the atmospheric temperatures and electron densities (especially behind the shocks); it also affects the emergent Ca ii fluxes. Particularly, we focus on the influence of magnetic heating on the stellar atmospheric structure and the emergent Caii emission, as well as the impact of non-linearities. Our study shows that a higher photospheric magnetic filling factor entails a larger Caii emission; however, an increased initial wave energy flux (e.g. associated with mode coupling) is of little difference. Comparisons of our theoretical results with observations will be conveyed in forthcoming Paper II.
Citation - WoS: 2
Citation - Scopus: 2
Chromospheric Activity in 55 Cancri: Ii. Theoretical Wave Studies Versus Observations
(Oxford Univ Press, 2021) Cuntz, Manfred; Schroder, Klaus-Peter; Fawzy, Diaa E.; Ridden-Harper, Andrew R.
In this study, we consider chromospheric heating models for 55 Cancri in conjunction with observations. The theoretical models, previously discussed in , are self-consistent, non-linear, and time-dependent ab-initio computations encompassing the generation, propagation, and dissipation of waves. Our focus is the consideration of both acoustic waves and longitudinal flux tube waves amounting to two-component chromosphere models. 55 Cancri, a K-type orange dwarf, is a star of low activity, as expected by its age, which also implies a relatively small magnetic filling factor. The Caii K fluxes are computed (multi-ray treatment) assuming partial redistribution and time-dependent ionization. The theoretical Caii H+K fluxes are subsequently compared with observations. It is found that for stages of lowest chromospheric activity the observed Caii fluxes are akin, though not identical, to those obtained by acoustic heating, but agreement can be obtained if low levels of magnetic heating - consistent with the expected photospheric magnetic filling factor - are considered as an additional component; this idea is in alignment with previous proposals conveyed in the literature.
Citation - Scopus: 2
Evolution of Solar-Type Activity: Acoustic and Magnetic Energy Generation and Propagation in Beta Hydri (g2 Iv)
(Springer, 2023) Fawzy, Diaa E. E.; Cuntz, Manfred
We examine the acoustic and magnetic energy generation and propagation in )beta Hydri (G2 IV). The underlying motivation for this work is based on the solar, stellar, and galactic relevance of )beta Hyi (a star in the Southern hemisphere), which is readily understood as a prime example and proxy of the future Sun - thus allowing assessments and analyses of the secular decay of solar activity. Regarding the magnetic energy generation, we consider longitudinal flux tube waves. We also assess acoustic waves. For the acoustic wave energy flux, the difference between the results obtained for )beta Hyi and the Sun is significantly smaller than typically attained for main-sequence stars, which is largely due to the gravity-dependence of the acoustic energy generation. Furthermore, we study the height-dependent behavior of the magnetic energy flux for different magnetic filling factors corresponding to different flux tube spreadings. Finally, we comment on possible directions of future research.
Citation - WoS: 26
Citation - Scopus: 28
The Secret Lives of Cepheids: Delta Cep-The Prototype of a New Class of Pulsating X-Ray Variable Stars
(Iop Publishing Ltd, 2017) Engle, Scott G.; Guinan, Edward F.; Harper, Graham M.; Cuntz, Manfred; Evans, Nancy Remage; Neilson, Hilding R.; Fawzy, Diaa E.
From our Secret Lives of Cepheids program, the prototype Classical Cepheid, delta Cep, is found to be an X-ray source with periodic pulsation-modulated X-ray variations. This finding complements our earlier reported phasedependent FUV-UV emissions of the star that increase similar to 10-20 times with highest fluxes at similar to 0.90-0.95 phi, just prior to maximum brightness. Previously delta Cep. was found as potentially X-ray variable, using XMM-Newton observations. Additional phase-constrained data were secured with Chandra near X-ray emission peak, to determine if the emission and variability were pulsation-phase-specific to delta Cep. and not transient or due to a possible coronally active, cool companion. The Chandra data were combined with prior XMM-Newton observations, and were found to very closely match the previously observed X-ray behavior. From the combined data set, a similar to 4 increase in X-ray flux is measured, reaching a peak L-X = 1.7 x 10(29) erg s(-1) near 0.45 phi. The precise X-ray flux phasing with the star's pulsation indicates that the emissions arise from the Cepheid and not from a companion. However, it is puzzling that the maximum X-ray flux occurs similar to 0.5 phi (similar to 3 days) later than the FUV-UV maximum. There are several other potential Cepheid X-ray detections with properties similar to delta Cep, and comparable X-ray variability is indicated for two other Cepheids: beta Dor. and V473 Lyr. X-ray generating mechanisms in delta Cep. and other Cepheids are discussed. If additional Cepheids are confirmed to show phased X-ray variations, then delta Cep. will be the prototype of a new class of pulsation-induced X-ray variables.