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Register a new userThe origin of the Local Bubble
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Added by
Jes\\'us Ma\\'iz-Apell\\'aniz
Publication date
2001
fields
Physics
and research's language is
English
Authors
Jesus Maiz-Apellaniz
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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 Scorpius-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.
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We present a new unbiased search and analysis of all B stars in the solar neighbourhood (within a volume of 400 pc diameter) using the Arivel data base to track down the remains of the OB associations, which hosted the supernovae responsible for the Local Bubble in the interstellar gas. We find after careful dereddening and by comparison with theoretical isochrones, that besides the Upper Scorpius the Upper Centaurus Lupus and Lower Centaurus Crux subgroups are the youngest stellar associations in the solar neighbourhood with ages of 20 to 30 Myr, in agreement with previous work. In search for the ``smoking gun of the origin of the Local Bubble, we have traced the paths of the associations back into the past and found that they entered the present bubble region 10 to 15 Myr ago. We argue that the Local Bubble began to form then and estimate that 14 to 20 supernovae have gone off since. It is shown that the implied energy input is sufficient to excavate a bubble of the presently observed size.
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.
The Solar neighborhood is the closest and most easily studied sample of the Galactic interstellar medium, an understanding of which is essential for models of star formation and galaxy evolution. Observations of an unexpectedly intense diffuse flux of easily-absorbed 1/4 keV X rays, coupled with the discovery that interstellar space within ~100 pc of the Sun is almost completely devoid of cool absorbing gas led to a picture of a local cavity filled with X-ray emitting hot gas dubbed the local hot bubble. This model was recently upset by suggestions that the emission could instead be produced readily within the solar system by heavy solar wind ions charge exchanging with neutral H and He in interplanetary space, potentially removing the major piece of evidence for the existence of million-degree gas within the Galactic disk. Here we report results showing that the total solar wind charge exchange contribution is 40% +/- 5% (stat) +/- 5% (sys) of the 1/4 keV flux in the Galactic plane. The fact that the measured flux is not dominated by charge exchange supports the notion of a million-degree hot bubble of order 100 pc extent surrounding the Sun.
Zehavi et al. (1998) have suggested that the Hubble flow within 70/h Mpc may be accelerated by the existence of a void centered on the Local Group. Its underdensity would be ~20 %, which would result in a local Hubble distortion of about 6.5 %. We have combined the peculiar velocity data of two samples of clusters of galaxies, SCI and SCII, to investigate the amplitude of Hubble distortions to 200/h Mpc. Our results are not supportive of that conclusion. The amplitude of a possible distortion in the Hubble flow within 70/h Mpc in the SCI+SCII merged data is 0.010pm0.022. The largest, and still quite marginal, geocentric deviation from smooth Hubble flow consistent with that data set is a shell with (Delta H)/H =0.027pm0.023, centered at hd = 101 Mpc and extending over some 30/h Mpc. Our results are thus consistent with a Hubble flow that, on distances in excess of about 50/h Mpc, is remarkably smooth.
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 critical 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.
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