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تسجيل مستخدم جديدReport from Sessions 1 and 3, including the Local Bubble Debate
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Robin L. Shelton
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This report summarizes the discussions in the Session 1 and Session 3 groups which met to discuss the questions: ``What Physical Processes Drive the Multiphase Interstellar Medium in the Local Bubble?, and ``What are the Energy and Pressure Balances in the Local Bubble? Most of our understanding of the Local Bubble has come from soft X-ray observations, but recent appreciation of the importance of solar wind charge exchange (SWCX) reactions has shown that the heliosphere produces some fraction of the soft X-rays that were previously ascribed to the Local Bubble. Some astronomers suggest that the SWCX X-rays rather than Local Bubble emission could explain most of the locally produced X-rays. Our discussions, therefore, also included a debate concerning the Local Bubbles existence.
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The Sun is embedded in the so-called Local Bubble (LB) -- a cavity of hot plasma created by supernova explosions and surrounded by a shell of cold, dusty gas. Knowing the local distortion of the Galactic magnetic field associated with the LB is criti
cal for the modeling of interstellar polarization data at high Galactic latitudes. In this his paper, we relate the structure of the Galactic magnetic field on the LB scale to three-dimensional (3D) maps of the local interstellar medium (ISM). First, we extracted the geometry of the LB shell, its inner surface, in particular from 3D dust extinction maps of the local ISM. We expanded the shell inner surface in spherical harmonics, up to a variable maximum multipole degree, which enabled us to control the level of complexity for the modeled surface. Next, we applied an analytical model for the ordered magnetic field in the shell to the modeled shell surface. This magnetic field model was successfully fitted to the textit{Planck} 353~GHz dust polarized emission maps over the Galactic polar caps. For each polar cap, the direction of the mean magnetic field derived from dust polarization (together with the prior that the field points toward longitude $90^circ pm 90^circ$) is found to be consistent with the Faraday spectra of the nearby diffuse synchrotron emission. Our work presents a new approach to modeling the local structure of the Galactic magnetic field. We expect our methodology and our results to be useful both in modeling the local ISM as traced by its different components and in modeling the dust polarized emission, which is a long-awaited input for studies of the polarized foregrounds for cosmic microwave background.
The Sun is located in a low-density region of the interstellar medium partially filled with hot gas that is the likely result of several nearby supernova explosions within the last 10 Myr. Here we use astrometric data to show that part of the Scorpiu
s-Centaurus OB association was located closer to the present position of the Sun 5-7 Myr ago than today. Evolutionary synthesis models indicate that the association must have experienced ~20 supernova explosions in the last 10-12 Myr, a prediction that is supported by the detection of four or five runaway stars escaping from it. The ~6 SNe produced by the Lower Centaurus Crux subgroup are likely responsible for the creation of the Local Bubble.
DXL (Diffuse X-rays from the Local Galaxy) is a sounding rocket mission designed to quantify and characterize the contribution of Solar Wind Charge eXchange (SWCX) to the Diffuse X-ray Background and study the properties of the Local Hot Bubble (LHB)
. Based on the results from the DXL mission, we quantified and removed the contribution of SWCX to the diffuse X-ray background measured by the ROSAT All Sky Survey (RASS). The cleaned maps were used to investigate the physical properties of the LHB. Assuming thermal ionization equilibrium, we measured a highly uniform temperature distributed around kT=0.097 keV+/-0.013 keV (FWHM)+/-0.006 keV (systematic). We also generated a thermal emission measure map and used it to characterize the three-dimensional (3D) structure of the LHB which we found to be in good agreement with the structure of the local cavity measured from dust and gas.
We have measured the diagonal conductivity in the microwave regime of an ultrahigh mobility two dimensional electron system. We find a sharp resonance in Re[sigma_{xx}] versus frequency when nu > 4 and the partial filling of the highest Landau level,
nu^*, is ~ 1/4 or 3/4 and temperatures < 0.1 K. The resonance appears for a range of nu^* from 0.20 to 0.37 and again from 0.62 to 0.82. the peak frequency, f_{pk} changes from ~ 500 to ~ 150 as nu^* = 1/2 is approached. This range of f_{pk} shows no dependence on nu where the resonance is observed. The quality factor, Q, of the resonance is maximum at ~ nu^* = 0.25 and 0.74. We interpret the resonance as due to a pinning mode of the bubble phase crystal.
A hundred years ago (1920) in the auditorium of the Smithsonian Institutions U.S. National Museum there were two lectures under the auspices of the George Ellery Hale Lecture series, what has come to be called the Great Debate. In the debate, Harlow
Shapley and Heber Curtis argued over the Scale of the Universe. Curtis argued that the Universe is composed of many galaxies like our own and they are relatively small. Shapley argued that the Universe was composed of only one big Galaxy. In Shapleys model, our Sun was far from the center of this great island Universe.
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