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Effect of Supernovae on the Local Interstellar Material
Abstract
A range of astronomical data indicates that ancient supernovae created the galactic environment of the Sun and sculpted the physical properties of the interstellar medium near the heliosphere. In this paper we review the characteristics of the local interstellar medium that have been affected by supernovae. The kinematics, magnetic field, elemental abundances, and configuration of the nearest interstellar material support the view that the Sun is at the edge of the Loop I superbubble, which has merged into the low density Local Bubble. The energy source for the higher temperature X-ray emitting plasma pervading the Local Bubble is uncertain. Winds from massive stars and nearby supernovae, perhaps from the Sco-Cen Association, may have contributed radioisotopes found in the geologic record and galactic cosmic ray population. Nested supernova shells in the Orion and Sco-Cen regions suggest spatially distinct sites of episodic star formation. The heliosphere properties vary with the pressure of the surrounding interstellar cloud. A nearby supernova would modify this pressure equilibrium and thereby severely disrupt the heliosphere as well as the local interstellar medium.
... The LHB is the result of shock waves and stellar winds from ancient supernova explosions sweeping up gas and dust as they expand (Frisch & Dwarkadas 2017). Berghöfer & Breitschwerdt (2002) estimate that there have been 20 supernova explosions in this region of space within the last 10-20 Myr. ...
... The Loop I Superbubble results from explosions in the ScoCen association ∼15 Myr ago. If Loop I is a spherical feature, the Sun sits on or near its rim (Frisch 1990;Heiles 1998;Frisch & Dwarkadas 2017). Optical polarization and reddening data show that the eastern parts of Loop I, l = 3 − 60, b > 0, fall within 60-80 pc of the Sun (Frisch et al. 2011;Santos et al. 2011). ...
... Shocks from the multiple supernova and stellar winds that created the Local Bubble stir and mix the interstellar grains in the LHB. Large grains still associated with undisturbed or weakly disturbed dusty regions and shocked grains should both be found in the LHB (Frisch & Dwarkadas 2017). Dusty regions with large grains tend to be found at the characteristic distances usually thought of as the walls, but there is no hard division between regions. ...
The properties of dust in the interstellar medium (ISM) nearest the Sun are poorly understood because the low column densities of dust toward nearby stars induce little photometric reddening, rendering the grains largely undetectable. Stellar polarimetry offers one pathway to deducing the properties of this diffuse material. Here we present multi-wavelength aperture polarimetry measurements of seven bright stars chosen to probe interstellar polarization near the edge of the Local Hot Bubble (LHB) - an amorphous region of relatively low density interstellar gas and dust extending ~70-150 pc from the Sun. The measurements were taken using the HIgh Precision Polarimetric Instrument (HIPPI) on the 3.9-m Anglo-Australian Telescope. HIPPI is an aperture stellar polarimeter with a demonstrated sensitivity of 4.3 parts-per-million (ppm). Of the stars observed two are polarized to a much greater degree than the others; they have a wavelength of maximum polarization ($\lambda_{max}$) of ~550 $\pm$ 20 nm - similar to that of stars beyond the LHB - and we conclude that they are in the wall of the LHB. The remaining five stars have polarizations of ~70 to 160 ppm, of these four have a much bluer $\lambda_{max}$, ~350 $\pm$ 50 nm. Bluer values of $\lambda_{max}$ may indicate grains shocked during the evolution of the Loop I Superbubble. The remaining star, HD 4150 is not well fit by a Serkowski curve, and may be intrinsically polarized.
The direct measurements of interstellar matter by the Interstellar Boundary Explorer (IBEX) mission have opened a new and important chapter in our study of the interactions that control the boundaries of our heliosphere. Here we derive for the quantitative information about interstellar O flow parameters from IBEX low-energy neutral atom data for the first time. Specifically, we derive a relatively narrow four-dimensional parameter tube along which interstellar O flow parameters must lie. Along the parameter tube, we find a large uncertainty in interstellar O flow longitude, 76.°0 ±3.°4 from χ ² analysis and 76.°5 ±6.°2 from a maximum likelihood fit, which is statistically consistent with the flow longitude derived for interstellar He, 75.°6 ±1.°4. The best-fit O and He temperatures are almost identical at a reference flow longitude of 76°, which provides a strong indication that the local interstellar plasma near the Sun is relatively unaffected by turbulent heating. However, key differences include an oxygen parameter tube for the interstellar speed (relation between speed and longitude) that has higher speeds than those in the corresponding parameter tube for He, and an upstream flow latitude for oxygen that is southward of the upstream flow latitude for helium. Both of these differences are likely the result of enhanced filtration of interstellar oxygen due to its charge-exchange ionization rate, which is higher than that for helium. Furthermore, we derive an interstellar O density near the termination shock of 5.8-0.8+0.9 × 10⁻⁵ cm⁻³ that, within uncertainties, is consistent with previous estimates. Thus, we use IBEX data to probe the interstellar properties of oxygen. © 2016. The American Astronomical Society. All rights reserved..
Significance
Massive stars, which terminate their evolution in a cataclysmic explosion called a type-II supernova, are the nuclear engines of galactic nucleosynthesis. Among the elemental species known to be produced in these stars, the radioisotope ⁶⁰ Fe stands out: This radioisotope has no natural, terrestrial production mechanisms; thus, a detection of ⁶⁰ Fe atoms within terrestrial reservoirs is proof for the direct deposition of supernova material within our solar system. We report, in this work, the direct detection of live ⁶⁰ Fe atoms in biologically produced nanocrystals of magnetite, which we selectively extracted from two Pacific Ocean sediment cores. We find that the arrival of supernova material on Earth coincides with the lower Pleistocene boundary (2.7 Ma) and that it terminates around 1.7 Ma.
With the velocity vector and temperature of the pristine interstellar neutral (ISN) He recently obtained with high precision from a coordinated analysis summarized by McComas et al.2015b, we analyzed the IBEX observations of neutral He left out from this analysis. These observations were collected during the ISN observation seasons 2010---2014 and cover the region in the Earth's orbit where the Warm Breeze persists. We used the same simulation model and a very similar parameter fitting method to that used for the analysis of ISN He. We approximated the parent population of the Warm Breeze in front of the heliosphere with a homogeneous Maxwell-Boltzmann distribution function and found a temperature of $\sim 9\,500$ K, an inflow speed of 11.3 km s$^{-1}$, and an inflow longitude and latitude in the J2000 ecliptic coordinates $251.6^\circ$, $12.0^\circ$. The abundance of the Warm Breeze relative to the interstellar neutral He is 5.7\% and the Mach number is 1.97. The newly found inflow direction of the Warm Breeze, the inflow directions of ISN H and ISN He, and the direction to the center of IBEX Ribbon are almost perfectly co-planar, and this plane coincides within relatively narrow statistical uncertainties with the plane fitted only to the inflow directions of ISN He, ISN H, and the Warm Breeze. This co-planarity lends support to the hypothesis that the Warm Breeze is the secondary population of ISN He and that the center of the Ribbon coincides with the direction of the local interstellar magnetic field. The common plane for the direction of inflow of ISN gas, ISN H, the Warm Breeze, and the local interstellar magnetic field %includes the Sun and is given by the normal direction: ecliptic longitude $349.7^\circ \pm 0.6^\circ$ and latitude $35.7^\circ \pm 0.6^\circ$ in the J2000 coordinates, with the correlation coefficient of 0.85.
Context. Two main hypotheses for the origin of Galactic cosmic rays are the supernova and superbubble origin hypotheses. Aims. We analyse the evidence for the superbubble hypothesis provided by the measurements of the relativive abundances of isotopes of cobalt and nickel in the cosmic ray flux. Methods. We compare the measured upper limit on the abundance of 59Ni in the cosmic ray flux with the 59Ni abundance predictions of the most recent stellar evolution models. Non-detection of 59Ni in the cosmic ray flux has previously been attributed to a long time delay of the order of 105 yr between the moment of supernova explosion and the onset of particle acceleration process. This long time delay was considered as an argument in favour of the superbubble scenario. Results. We show that the recent calculation of the 59Ni yield of massive stars, which takes the initial mass range up to 120 solar masses into account and includes stellar rotation, results in prediction of low 59Ni abundance relative to its decay product 59Co. The predicted abundance is consistent with the upper bound on 59Ni abundance in the cosmic ray flux for the supernova parameters assumed. This result removes the necessity of decay of 59Ni in the time interval between the supernova explosion and the onset of acceleration process and restores the consistency of measurements of 59Ni/59Co abundances with the "supernova" hypothesis of the CR origin.
The Interstellar Boundary Explorer (IBEX) has been directly observing neutral atoms from the local interstellar medium for the last six years (2009-2014). This paper ties together the 14 studies in this Astrophysical Journal Supplement Series Special Issue, which collectively describe the IBEX interstellar neutral results from this epoch and provide a number of other relevant theoretical and observational results. Interstellar neutrals interact with each other and with the ionized portion of the interstellar population in the "pristine" interstellar medium ahead of the heliosphere. Then, in the heliosphere's close vicinity, the interstellar medium begins to interact with escaping heliospheric neutrals. In this study, we compare the results from two major analysis approaches led by IBEX groups in New Hampshire and Warsaw. We also directly address the question of the distance upstream to the pristine interstellar medium and adjust both sets of results to a common distance of ∼1000 AU. The two analysis approaches are quite different, but yield fully consistent measurements of the interstellar He flow properties, further validating our findings. While detailed error bars are given for both approaches, we recommend that for most purposes, the community use "working values" of ∼25.4 km s-1, ∼75.°7 ecliptic inflow longitude, ∼ -5.°1 ecliptic inflow latitude, and ∼7500 K temperature at ∼1000 AU upstream. Finally, we briefly address future opportunities for even better interstellar neutral observations to be provided by the Interstellar Mapping and Acceleration Probe mission, which was recommended as the next major Heliophysics mission by the NRC's 2013 Decadal Survey. © 2015. The American Astronomical Society. All rights reserved.
We investigate the directional distribution of heavy neutral atoms in the heliosphere by using heavy neutral maps generated with the IBEX-Lo instrument over three years from 2009 to 2011. The interstellar neutral (ISN) O&Ne gas flow was found in the first-year heavy neutral map at 601 keV and its flow direction and temperature were studied. However, due to the low counting statistics, researchers have not treated the full sky maps in detail. The main goal of this study is to evaluate the statistical significance of each pixel in the heavy neutral maps to get a better understanding of the directional distribution of heavy neutral atoms in the heliosphere. Here, we examine three statistical analysis methods: the signal-to-noise filter, the confidence limit method, and the cluster analysis method. These methods allow us to exclude background from areas where the heavy neutral signal is statistically significant. These methods also allow the consistent detection of heavy neutral atom structures. The main emission feature expands toward lower longitude and higher latitude from the observational peak of the ISN O&Ne gas flow. We call this emission the extended tail. It may be an imprint of the secondary oxygen atoms generated by charge exchange between ISN hydrogen atoms and oxygen ions in the outer heliosheath. © 2015. The American Astronomical Society. All rights reserved..
The winds of massive stars create large (>10 pc) bubbles around their progenitors. As these bubbles expand they encounter the interstellar coherent magnetic field which, depending on its strength, can influence the shape of the bubble. We wish to investigate if, and how much, the interstellar magnetic field can contribute to the shape of an expanding circumstellar bubble around amassive star.
We use the MPI-AMRVAC code to make magneto-hydrodynamical simulations of bubbles, using a single star model, combined with several different field strengths: B=5, 10, and 20 muG for the interstellar magnetic field. This covers the typical field strengths of the interstellar magnetic
fields found in the galactic disk and bulge. Furthermore, we present two simulations that include both a 5 muG interstellar magnetic field and a 10,000K interstellar medium and two different ISM densities to demonstrate how the magnetic field can combine with other external factors to influnece the morphology of the circumstellar bubbles.
Our results show that low magnetic fields, as found in the galactic disk, inhibit the growth of the circumstellar bubbles in the direction perpendicular to the field. As a result, the bubbles
become ovoid, rather than spherical. Strong interstellar fields, such as observed for the galactic bulge, can completely stop the expansion of the bubble in the direction perpendicular to the field, leading to the formation of a tube-like bubble. When combined with a warm, high-density ISM the bubble is greatly reduced in size, causing a dramatic change in the evolution of temporary features inside the bubble. The magnetic field of the interstellar medium can affect the shape of circumstellar bubbles. This effect may have consequences for the shape and evolution of circumstellar nebulae and supernova remnants, which are formed within the main wind-blown bubble.
We present radio continuum and polarization images of the North Polar Spur
(NPS) from the Global Magneto-Ionic Medium Survey (GMIMS) conducted with the
Dominion Radio Astrophysical Observatory 26-m Telescope. We fit polarization
angle versus wavelength squared over 2048 frequency channels from 1280 to 1750
MHz to obtain a Faraday Rotation Measure (RM) map of the NPS. Combining this RM
map with a published Faraday depth map of the entire Galaxy in this direction,
we derive the Faraday depth introduced by the NPS and the Galactic interstellar
medium (ISM) in front of and behind the NPS. The Faraday depth contributed by
the NPS is close to zero, indicating that the NPS is an emitting only feature.
The Faraday depth caused by the ISM in front of the NPS is consistent with zero
at b>50 degree, implying that this part of the NPS is local at a distance of
approximately several hundred parsecs. The Faraday depth contributed by the ISM
behind the NPS gradually increases with Galactic latitude up to b=44 degree,
and decreases at higher Galactic latitudes. This implies that either the part
of the NPS at b<44 degree is distant or the NPS is local but there is a sign
change of the large-scale magnetic field. If the NPS is local, there is then no
evidence for a large-scale anti-symmetry pattern in the Faraday depth of the
Milky Way. The Faraday depth introduced by the ISM behind the NPS at latitudes
b>50 degree can be explained by including a coherent vertical magnetic field.
The North Galactic Pole Rift (NGPR) is one of the few distinct neutral hydrogen clouds at high Galactic latitudes that have well-defined distances. It is located at the edge of the Local Cavity (LC) and provides an important test case for understanding the Local Hot Bubble (LHB), the presumed location for the hot diffuse plasma responsible for much of the observed keV emission originating in the solar neighborhood. Using data from the ROSAT All-Sky Survey and the Planck reddening map, we find the path length within the LC (LHB plus Complex of Local Interstellar Clouds) to be 98 ± 27 pc, in excellent agreement with the distance to the NGPR of 98 ± 6 pc. In addition, we examine another 14 directions that are distributed over the sky where the LC wall is apparently optically thick at keV. We find that the data in these directions are also consistent with the LHB model and a uniform emissivity plasma filling most of the LC.
Interstellar Boundary Explorer (IBEX) measurements from 2009-2010 identified a set of possible solutions with very tight coupling between the interstellar He inflow longitude, latitude, speed, and temperature. The center of this allowable parameter space suggested that the heliosphere could be moving more slowly and in a slightly different direction with respect to the interstellar medium than indicated by earlier Ulysses observations. In this study we examine data from 2012-2014 and compare results from an analytic analysis and a detailed computer model. For observations where the IBEX spacecraft pointing is near the ecliptic plane, the latest measurements indicate a different portion of IBEX's four-dimensional tube of possible parameters—one that is more consistent with the Ulysses flow direction and speed, but with a much higher temperature. Together, the current combined IBEX/Ulysses values we obtain are V ISM∞ ~ 26 km s-1, λISM∞ ~ 75°, βISM∞ ~ -5°, and T He∞ ~ 7000-9500 K. These indicate that the heliosphere is in a substantially warmer region of the interstellar medium than thought from the earlier Ulysses observations alone, and that this warmer region may be roughly isothermal. However, measurements taken when IBEX was pointing ~5° south of the ecliptic are inconsistent with this solution and suggest a slower speed, lower temperature, and flow direction similar to IBEX's prior central values. IBEX measures much deeper into the tails of the distributions of the inflowing interstellar material than Ulysses did and these observations indicate that the heliosphere's interstellar interaction is likely far more complex and interesting than previously appreciated.
High energy resolution spectroscopy of the 1.8 MeV radioactive decay line of
26Al with the SPI instrument on board the INTEGRAL satellite has recently
revealed that diffuse 26Al has large velocities in comparison to other
components of the interstellar medium in the Milky Way. 26Al shows Galactic
rotation in the same sense as the stars and other gas tracers, but reaches
excess velocities up to 300 km/s. We investigate if this result can be
understood in the context of superbubbles, taking into account the statistics
of young star clusters and H I supershells, as well as the association of young
star clusters with spiral arms. We derive energy output and 26Al mass of star
clusters as a function of the cluster mass via population synthesis from
stellar evolutionary tracks of massive stars. [...] We link this to the size
distribution of HI supershells and assess the properties of likely
26Al-carrying superbubbles. 26Al is produced by star clusters of all masses
above about 200 solar masses, roughly equally contributed over a logarithmic
star cluster mass scale, and strongly linked to the injection of feedback
energy. The observed superbubble size distribution cannot be related to the
star cluster mass function in a straight forward manner. In order to avoid that
the added volume of all superbubbles exceeds the volume of the Milky Way,
individual superbubbles have to merge frequently. If any two superbubbles
merge, or if 26Al is injected off-centre in a bigger HI supershell we expect
the hot 26Al-carrying gas to obtain velocities of the order of the typical
sound speed in superbubbles, about 300 km/s before decay. [...]
The production factor, or broad band averaged cross-section, for solar wind
charge-exchange with hydrogen producing emission in the ROSAT 1/4 keV (R12)
band is $3.8\pm0.2\times10^{-20}$ count degree$^{-2}$ cm$^4$. This value is
derived from a comparison of the Long-Term (background) Enhancements in the
ROSAT All-Sky Survey with magnetohysdrodynamic simulations of the
magnetosheath. This value is 1.8 to 4.5 times higher than values derived from
limited atomic data, suggesting that those values may be missing a large number
of faint lines. This production factor is important for deriving the exact
amount of 1/4 keV band flux that is due to the Local Hot Bubble, for planning
future observations in the 1/4 keV band, and for evaluating proposals for
remote sensing of the magnetosheath. The same method cannot be applied to the
3/4 keV band as that band, being composed primarily of the oxygen lines, is far
more sensitive to the detailed abundances and ionization balance in the solar
wind. We also show, incidentally, that recent efforts to correlate XMM-Newton
observing geometry with magnetosheath solar wind charge-exchange emission in
the oxygen lines have been, quite literally, misguided. Simulations of the
inner heliosphere show that broader efforts to correlate heliospheric solar
wind charge-exchange with local solar wind parameters are unlikely to produce
useful results.
A recently discovered filament of polarized starlight that traces a coherent
magnetic field is shown to have several properties that are consistent with an
origin in the outer heliosheath of the heliosphere: (1) The magnetic field that
provides the best fit to the polarization position angles is directed within
6.7+-11 degrees of the observed upwind direction of the flow of interstellar
neutral helium gas through the heliosphere. (2) The magnetic field is ordered;
the component of the variation of the polarization position angles that can be
attributed to magnetic turbulence is small. (3) The axis of the elongated
filament can be approximated by a line that defines an angle of 80+/-14 degrees
with the plane that is formed by the interstellar magnetic field vector and the
vector of the inflowing neutral gas (the "BV" plane). We propose that this
polarization feature arises from aligned interstellar dust grains in the outer
heliosheath where the interstellar plasma and magnetic field are deflected
around the heliosphere. The proposed outer heliosheath location of the
polarizing grains requires confirmation by modeling grain-propagation through
three-dimensional MHD heliosphere models that simultaneously calculate torques
on asymmetric dust grains interacting with the heliosphere.
Aims: We present the first high-resolution non-equilibrium
ionization simulation of the joint evolution of the Local Bubble (LB)
and Loop I superbubbles in the turbulent supernova-driven interstellar
medium (ISM). The time variation and spatial distribution of the Li-like
ions Civ, Nv, and Ovi inside the LB are studied in detail.
Methods: This work uses the parallel adaptive mesh refinement code
EAF-PAMR coupled to the newly developed atomic and molecular plasma
emission module E(A+M)PEC, featuring the time-dependent calculation of
the ionization structure of H through Fe, using the latest revision of
solar abundances. The finest AMR resolution is 1 pc within a grid that
covers a representative patch of the Galactic disk (with an area of 1
kpc2 in the midplane) and halo (extending up to 10 kpc above
and below the midplane). Results: The evolution age of the LB is
derived by the match between the simulated and observed absorption
features of the Li-like ions Civ, Nv, and Ovi. The modeled LB current
evolution time is bracketed between 0.5 and 0.8 Myr since the last
supernova reheated the cavity in order to have N(Ovi) < 8 ×
1012 cm-2, log [N(Civ)/N(Ovi)] < -0.9 and log
[N(Nv)/N(Ovi)] < -1 inside the simulated LB cavity, as found in
Copernicus, IUE, GHRS-IST and FUSE observations.
Neutral hydrogen atoms that travel into the heliosphere from the local interstellar medium (LISM) experience strong effects due to charge exchange and radiation pressure from resonant absorption and re-emission of Lyα. The radiation pressure roughly compensates for the solar gravity. As a result, interstellar hydrogen atoms move along trajectories that are quite different than those of heavier interstellar species such as helium and oxygen, which experience relatively weak radiation pressure. Charge exchange leads to the loss of primary neutrals from the LISM and the addition of new secondary neutrals from the heliosheath. IBEX observations show clear effects of radiation pressure in a large longitudinal shift in the peak of interstellar hydrogen compared with that of interstellar helium. Here, we compare results from the Lee et al. interstellar neutral model with IBEX-Lo hydrogen observations to describe the distribution of hydrogen near 1 AU and provide new estimates of the solar radiation pressure. We find over the period analyzed from 2009 to 2011 that radiation pressure divided by the gravitational force (μ) has increased slightly from μ = 0.94 ± 0.04 in 2009 to μ = 1.01 ± 0.05 in 2011. We have also derived the speed, temperature, source longitude, and latitude of the neutral H atoms and find that these parameters are roughly consistent with those of interstellar He, particularly when considering the filtration effects that act on H in the outer heliosheath. Thus, our analysis shows that over the period from 2009 to 2011, we observe signatures of neutral H consistent with the primary distribution of atoms from the LISM and a radiation pressure that increases in the early rise of solar activity.
The solar cycle has a profound influence on the solar wind (SW) interaction with the local interstellar medium (LISM) on more than one timescales. Also, there are substantial differences in individual solar cycle lengths and SW behavior within them. The presence of a slow SW belt, with a variable latitudinal extent changing within each solar cycle from rather small angles to 90°, separated from the fast wind that originates at coronal holes substantially affects plasma in the inner heliosheath (IHS)—the SW region between the termination shock (TS) and the heliopause (HP). The solar cycle may be the reason why the complicated flow structure is observed in the IHS by Voyager 1. In this paper, we show that a substantial decrease in the SW ram pressure observed by Ulysses between the TS crossings by Voyager
1 and 2 contributes significantly to the difference in the heliocentric distances at which these crossings occurred. The Ulysses spacecraft is the source of valuable information about the three-dimensional and time-dependent properties of the SW. Its unique fast latitudinal scans of the SW regions make it possible to create a solar cycle model based on the spacecraft in situ measurements. On the basis of our analysis of the Ulysses data over the entire life of the mission, we generated time-dependent boundary conditions at 10 AU from the Sun and applied our MHD-neutral model to perform a numerical simulation of the SW-LISM interaction. We analyzed the global variations in the interaction pattern, the excursions of the TS and the HP, and the details of the plasma and magnetic field distributions in the IHS. Numerical results are compared with Voyager data as functions of time in the spacecraft frame. We discuss solar cycle effects which may be reasons for the recent decrease in the TS particles (ions accelerated to anomalous cosmic-ray energies) flux observed by Voyager
1.
The ribbon observed by the Interstellar Boundary Explorer (IBEX) mission is a narrow, ~20° wide feature that stretches across much of the sky in the global flux of energetic neutral atoms from the outer heliosphere. The ribbon remains an enigma despite its persistence after 3 years of IBEX observations and after almost a dozen theories that attempt to explain it. While each theory that has been posed has its strengths, each one also contradicts IBEX observations or demonstrates significant flaws in internal consistency. Here, we present a new theory that is different than any of the existing ideas and yet accounts for many of the key observations. We argue that the ribbon could be produced by a spatial region in the local interstellar medium where newly ionized atoms are temporarily contained through increased rates of scattering by locally generated waves in the electromagnetic fields. The particles in the ribbon are created predominantly from neutralized solar wind and neutralized pickup ions from inside the solar wind termination shock.
We continue our numerical analysis of the morphological and energetic influence of massive stars on their ambient interstellar medium for a 35 Msolar star that evolves from the main-sequence through red supergiant and Wolf-Rayet phases, until it ultimately explodes as a supernova. We find that structure formation in the circumstellar gas during the early main-sequence evolution occurs as in the 60 Msolar case but is much less pronounced because of the lower mechanical wind luminosity of the star. On the other hand, since the shell-like structure of the H II region is largely preserved, effects that rely on this symmetry become more important. At the end of the stellar lifetime 1% of the energy released as Lyman continuum radiation and stellar wind has been transferred to the circumstellar gas. From this fraction 10% is kinetic energy of bulk motion, 36% is thermal energy, and the remaining 54% is ionization energy of hydrogen. The sweeping up of the slow red supergiant wind by the fast Wolf-Rayet wind produces remarkable morphological structures and emission signatures, which are compared with existing observations of the Wolf-Rayet bubble S308, whose central star has probably evolved in a manner very similar to our model star. Our model reproduces the correct order of magnitude of observed X-ray luminosity, the temperature of the emitting plasma, and the limb brightening of the intensity profile. This is remarkable, because current analytical and numerical models of Wolf-Rayet bubbles fail to consistently explain these features. A key result is that almost the entire X-ray emission in this stage comes from the shell of red supergiant wind swept up by the shocked Wolf-Rayet wind rather than from the shocked Wolf-Rayet wind itself as hitherto assumed and modeled. This offers a possible solution to what is called the ``missing wind problem'' of Wolf-Rayet bubbles.
Early-type stars blow bubbles in the interstellar medium. The radii of the bubbles are typically 30 pc. Typical conditions in their interiors are temperatures of about 1 million K and densities of about 0.01 per cu cm. The dense shell of swept-up interstellar gas that surrounds them is likely to trap the ionization front and may also have an outer layer of H2. The column density of O VI in the interior is in accord with observations by the Copernicus ultraviolet telescope.
The analysis of wind bubbles and superbubbles in Paper I is extended to
the case of power-law energy injection [Lin(t) ∝
tηin-1] in a medium with a power-law density distribution
[ρα(r) ∝ r-κρ]. As
before, the wind velocity is assumed to be constant, so that the energy
injection rate is proportional to the mass injection rate,
Lin(t) ∝ Mṡin(t). The shock
in the ambient medium is assumed not to accelerate which requires
κρ ≤ 3 - ɛin. The evolution
is followed from the free-expansion stage, in which the mass of the wind
dominates the swept- up mass, through the self-similar stage, to the
stage in which the bubble is confined by the pressure of the ambient
medium. As in Paper I, winds may be divided into slow and fast,
depending on the importance of radiative losses at the transition from
the free-expansion to the self-similar stage. Slow winds generally
follow the radiative sequence of bubble evolution, which depends on the
value of ɛin: At the transition from the
free-expansion stage, the bubble is radiative both the shock in the wind
and the shock in the ambient medium are radiative. If
ɛin <(5 - κρ)/(3 -
κρ) and if the wind velocity is not too low, the
bubble evolves from a radiative bubble to a partially radiative bubble,
in which the cooling time of the hot shocked wind is less than the age
of the bubble but longer than the crossing time; in this case most of
the volume of the bubble is filled by the small fraction of the shocked
wind that has not yet cooled. The partially radiative bubble further
evolves to an adiabatic bubble if ηin < (1 +
κρ)/(2 - κρ). The evolution
of a bubble blown by a fast wind generally follows the adiabatic
sequence of bubble evolution, which does not depend on
ηin, and the shocked wind remains adiabatic unless there
is additional mass injection into the bubble. [If the ambient density
decreases sufficiently steeply, κρ > (14 -
3ηin) (6 + ηin); however, slow winds are
adiabatic and fast winds are radiative.] Bubbles blown by either slow
winds or fast winds eventually expand to the point that the pressure of
the ambient medium is significant, and become pressure-confined bubbles.
We have obtained characteristic evolutionary time scales, as well as the
equation of motion for both the swept-up gas and the wind shock in each
evolutionary stage. The special case of a bubble blown in a preexisting
wind is discussed in Appendix A. Self-similar bubbles are discussed in
Appendix B. If the ambient medium has a finite mass, as in the case of a
galactic disk, for example, then the evolution of the bubble is arrested
when it reaches a scale height; the possible evolutionary sequences are
categorized.
Measurements of the abundances of cosmic-ray 59Ni and 59Co are reported from the Cosmic-Ray Isotope Spectrometer (CRIS) on the Advanced Composition Explorer. These nuclides form a parent-daughter pair in a radioactive decay which can occur only by electron capture. This decay cannot occur once the nuclei are accelerated to high energies and stripped of their electrons. The CRIS data indicate that the decay of 59Ni to 59Co has occurred, leading to the conclusion that a time longer than the 7.6 × 104 yr half-life of 59Ni elapsed before the particles were accelerated. Such long delays indicate the acceleration of old, stellar or interstellar material rather than fresh supernova ejecta. For cosmic-ray source material to have the composition of supernova ejecta would require that these ejecta not undergo significant mixing with normal interstellar gas before ~105 yr has elapsed.
We present results of numerical simulations carried out with a two-dimensional radiation hydrodynamics code in order to study the impact of massive stars on their surrounding interstellar medium. This first paper deals with the evolution of the circumstellar gas around an isolated 60 M☉ star. The interaction of the photoionized H II region with the stellar wind bubble forms a variety of interesting structures like shells, clouds, fingers, and spokes. These results demonstrate that complex structures found in H II regions are not necessarily relics from the time before the gas became ionized but may result from dynamical processes during the course of the H II region evolution. We have also analyzed the transfer and deposit of the stellar wind and radiation energy into the circumstellar medium until the star explodes as a supernova. Although the total mechanical wind energy supplied by the star is negligible compared to the accumulated energy of the Lyman continuum photons, the kinetic energy imparted to the circumstellar gas over the star's lifetime is 4 times higher than for a comparable windless simulation. Furthermore, the thermal energy of warm photoionized gas is lower by some 55%. Our results document the necessity to consider both ionizing radiation and stellar winds for an appropriate description of the interaction of OB stars with their circumstellar environment.
We report the abundances of neon isotopes in the Galactic cosmic rays (GCRs) using data from the Cosmic Ray Isotope Spectrometer (CRIS) aboard the Advanced Composition Explorer (ACE). These abundances have been measured for seven energy intervals over the energy range of 84 ≤ E/M ≤ 273 MeV nucleon-1. We have derived the 22Ne/20Ne ratio at the cosmic-ray source using the measured 21Ne, 19F, and 17O abundances as "tracers" of secondary production of the neon isotopes. Using this approach, the 22Ne/20Ne abundance ratio that we obtain for the cosmic-ray source is 0.387 ± 0.007(statistical) ± 0.022(systematic). This corresponds to an enhancement by a factor of 5.3 ± 0.3 over the 22Ne/20Ne ratio in the solar wind. This cosmic-ray source 22Ne/20Ne ratio is also significantly larger than that found in anomalous cosmic rays, solar energetic particles, most meteoritic samples of matter, and interplanetary dust particles. We compare our ACE CRIS data for neon and refractory isotope ratios, and data from other experiments, with recent results from two-component Wolf-Rayet (W-R) models. The three largest deviations of GCR isotope ratios from solar system ratios predicted by these models, 12C/16O, 22Ne/20Ne, and 58Fe/56Fe, are indeed present in the GCRs. In fact, all of the isotope ratios that we have measured are consistent with a GCR source consisting of about 80% material with solar system composition and about 20% W-R material. Since W-R stars are evolutionary products of OB stars, and most OB stars exist in OB associations that form superbubbles, the good agreement of these data with W-R models suggests that superbubbles are the likely source of at least a substantial fraction of GCRs.
The Interstellar Boundary Explorer (IBEX) observes a remarkable feature, the IBEX ribbon, which has energetic neutral atom (ENA) flux over a narrow region ~20° wide, a factor of 2-3 higher than the more globally distributed ENA flux. Here, we separate ENA emissions in the ribbon from the distributed flux by applying a transparency mask over the ribbon and regions of high emissions, and then solve for the distributed flux using an interpolation scheme. Our analysis shows that the energy spectrum and spatial distribution of the ribbon are distinct from the surrounding globally distributed flux. The ribbon energy spectrum shows a knee between ~1 and 4 keV, and the angular distribution is approximately independent of energy. In contrast, the distributed flux does not show a clear knee and more closely conforms to a power law over much of the sky. Consistent with previous analyses, the slope of the power law steepens from the nose to tail, suggesting a weaker termination shock toward the tail as compared to the nose. The knee in the energy spectrum of the ribbon suggests that its source plasma population is generated via a distinct physical process. Both the slope in the energy distribution of the distributed flux and the knee in the energy distribution of the ribbon are ordered by latitude. The heliotail may be identified in maps of globally distributed flux as a broad region of low flux centered ~44°W of the interstellar downwind direction, suggesting heliotail deflection by the interstellar magnetic field.
The Orion star-forming region is the nearest active high-mass star-forming region and has created a large superbubble, the Orion-Eridanus superbubble. Recent work by Ochsendorf et al. (2015) has extended the accepted boundary of the superbubble. We fit Kompaneets models of superbubbles expanding in exponential atmospheres to the new, larger shape of the Orion-Eridanus superbubble. We find that this larger morphology of the superbubble is consistent with the evolution of the superbubble being primarily controlled by expansion into the exponential Galactic disk ISM if the superbubble is oriented with the Eridanus side farther from the Sun than the Orion side. Unlike previous Kompaneets model fits that required abnormally small scale heights for the Galactic disk (<40 pc), we find morphologically consistent models with scale heights of 80 pc, similar to that expected for the Galactic disk.
Cosmic rays from a nearby supernova
Supernova explosions produce unstable isotopes, spreading them through space in the form of cosmic rays. Binns et al. used NASA's Advanced Composition Explorer spacecraft to search for previously undetected traces of ⁶⁰ Fe in cosmic rays passing through the solar system. Seventeen years of observations detected just 15 ⁶⁰ Fe nuclei—a small but statistically significant number. Because ⁶⁰ Fe is radioactive, with a half-life of 2.6 million years, these nuclei must have formed relatively recently in a nearby supernova. The most likely candidates are massive stars in the Scorpius-Centaurus association.
Science , this issue p. 677
Cassini detects interstellar dust grains
The interstellar medium contains an array of small solid particles known as dust grains. Altobelli et al. used the dust analyzer on the Cassini probe to detect 36 interstellar dust grains as they passed by Saturn, and they measured the grains' elemental abundances. The results show that, remarkably, these grains lack carbon-bearing compounds and have been homogenized in the interstellar medium into silicates with iron inclusions.
Science , this issue p. 312
The rate of supernovae in our local Galactic neighbourhood within a distance of about 100 parsecs from Earth is estimated to be one every 2-4 million years, based on the total rate in the Milky Way (2.0 ± 0.7 per century). Recent massive-star and supernova activity in Earth's vicinity may be traced by radionuclides with half-lives of up to 100 million years, if trapped in interstellar dust grains that penetrate the Solar System. One such radionuclide is (60)Fe (with a half-life of 2.6 million years), which is ejected in supernova explosions and winds from massive stars. Here we report that the (60)Fe signal observed previously in deep-sea crusts is global, extended in time and of interstellar origin from multiple events. We analysed deep-sea archives from all major oceans for (60)Fe deposition via the accretion of interstellar dust particles. Our results reveal (60)Fe interstellar influxes onto Earth at 1.5-3.2 million years ago and at 6.5-8.7 million years ago. The signal measured implies that a few per cent of fresh (60)Fe was captured in dust and deposited on Earth. Our findings indicate multiple supernova and massive-star events during the last ten million years at distances of up to 100 parsecs.
The solar wind emanating from the Sun interacts with the local interstellar medium (LISM), forming the heliosphere. Hydrogen energetic neutral atoms (ENAs) produced by the solar-interstellar interaction carry important information about plasma properties from the boundaries of the heliosphere, and are currently being measured by NASA's Interstellar Boundary Explorer (IBEX). IBEX observations show the existence of a "ribbon" of intense ENA emission projecting a circle on the celestial sphere that is centered near the local interstellar magnetic field (ISMF) vector. Here we show that the source of the IBEX ribbon as a function of ENA energy outside the heliosphere, uniquely coupled to the draping of the ISMF around the heliopause, can be used to precisely determine the magnitude (2.93 ± 0.08 μG) and direction (22728 ± 069, 3462 ± 045 in ecliptic longitude and latitude) of the pristine ISMF far (~1000 AU) from the Sun. We find that the ISMF vector is offset from the ribbon center by ~83 toward the direction of motion of the heliosphere through the LISM, and their vectors form a plane that is consistent with the direction of deflected interstellar neutral hydrogen, thought to be controlled by the ISMF. Our results yield draped ISMF properties close to that observed by Voyager 1, the only spacecraft to directly measure the ISMF close to the heliosphere, and give predictions of the pristine ISMF that Voyager 1 has yet to sample.
We present deep XMM–Newton European Photon Imaging Camera observations of the Wolf–Rayet (WR) bubble NGC 6888 around the star WR 136. The complete X-ray
mapping of the nebula confirms the distribution of the hot gas in three maxima spatially associated with the caps and north-west
blowout hinted at by previous Chandra observations. The global X-ray emission is well described by a two-temperature optically thin plasma model (T1 = 1.4 × 106 K, T2 = 8.2 × 106 K) with a luminosity of LX = 7.8 × 1033 erg s−1 in the 0.3–1.5 keV energy range. The rms electron density of the X-ray-emitting gas is estimated to be ne = 0.4 cm−3. The high-quality observations presented here reveal spectral variations within different regions in NGC 6888, which allowed
us for the first time to detect temperature and/or nitrogen abundance inhomogeneities in the hot gas inside a WR nebula. One
possible explanation for such spectral variations is that the mixing of material from the outer nebula into the hot bubble
is less efficient around the caps than in other nebular regions.
Interstellar He represents a key sample of interstellar matter that, due to its high first ionization potential, survives the journey from beyond our solar system's heliospheric boundaries to Earth. Ongoing analysis of interstellar neutral (ISN) He atoms by the Interstellar Boundary Explorer (IBEX) has resulted in a growing sophistication in our understanding of the local interstellar flow. A key feature of the IBEX observations near perihelion of the ISN trajectories is a narrow "tube" of approximately degenerate interstellar parameters. These degenerate solutions provide a tightly coupled relationship between the interstellar flow longitude and latitude, speed, and temperature. However, the IBEX analysis resulting in a specific solution for the inflow longitude, inflow speed, temperature, and inflow latitude was accompanied by a sizeable uncertainty along the parameter tube. Here, we use the three-step method to find the interstellar parameters: (1) the ISN He peak rate in ecliptic longitude uniquely determines a relation (as part of the tube in parameter space) between the longitude and the speed of the He ISN flow at infinity; (2) the ISN He peak latitude (on the great circle swept out in each spin) is compared to simulations to derive unique values for and along the parameter tube; and (3) the angular width of the He flow distributions as a function of latitude is used to derive the interstellar He temperature. For simulated peak latitudes, we use a relatively new analytical tool that traces He atoms from beyond the termination shock into the position of IBEX and incorporates the detailed response function of IBEX-Lo. By varying the interstellar parameters along the IBEX parameter tube, we find the specific parameters that minimize the chi-square difference between observations and simulations. The new computational tool for simulating neutral atoms through the integrated IBEX-Lo response function makes no assumptions or expansions with respect to the spin-axis pointing or frame of reference. Thus, we are able to move beyond closed-form approximations and utilize observations of interstellar He during the complete five year period from 2009 to 2013 when the primary component of interstellar He is most prominent. Chi-square minimization of simulations compared to observations results in a He ISN flow longitude of 75.°6 ± 1.°4, latitude of -5.°12 ± 0.°27, speed of 25.4 ± 1.1 km s-1, and temperature of 8000 ± 1300 K, where the uncertainties are related and apply along the IBEX parameter tube. This paper also provides documentation for a new release of ISN data and associated model runs. © 2015. The American Astronomical Society. All rights reserved.
Interstellar polarization at optical-to-infrared wavelengths is known to arise from asymmetric dust grains aligned with the magnetic field. This effect provides a potentially powerful probe of magnetic field structure and strength if the details of the grain alignment can be reliably understood. Theory and observations have recently converged on a quantitative, predictive description of interstellar grain alignment based on radiative processes. The development of a general, analytical model for this radiative alignment torque (RAT) theory has allowed specific, testable predictions for realistic interstellar conditions. We outline the theoretical and observational arguments in favor of RAT alignment, as well as reasons the "classical" paramagnetic alignment mechanism is unlikely to work, except possibly for the very smallest grains. With further detailed characterization of the RAT mechanism, grain alignment and polarimetry promise to not only better constrain the interstellar magnetic field but also provide new information on the dust characteristics.
The Local Leo Cold Cloud (LLCC, at a distance of 11-24 pc) was studied in its relation to the Local Hot Bubble (LHB) and the result suggested that much of the observed keV emission in that direction originates in front of the cloud. This placed a strong constraint on the distribution of X-ray emission within the LHB and called into question the assumption of a uniform distribution of X-ray emitting plasma within the Local Cavity. However, recent work has quantified the contribution of heliospheric solar wind charge exchange (SWCX) emission to the diffuse X-ray background measured by the ROSAT All-Sky Survey (RASS) at keV, and led to the consistency of pressure measurements between the LHB and the local cloud component of the complex of local interstellar clouds (CLICs) surrounding the Sun. In this paper we revisit the LLCC and improve the previous analysis by using higher resolution RASS data, a serendipitous ROSAT pointed observation, a rigorous treatment of the band-averaged X-ray absorption cross section, and models for the heliospheric and magnetospheric SWCX contributions. We find that the foreground emission to the cloud is in excess of the expected heliospheric (interplanetary plus near Earth) SWCX contribution but that it is marginally consistent with the range of possible LHB plasma path lengths between the LLCC and the CLICs given the currently understood plasma emissivity.
The Orion-Eridanus superbubble is the prototypical superbubble due to its
proximity and evolutionary state. Here, we provide a synthesis of recent
observational data from WISE and Planck with archival data, allowing to draw a
new and more complete picture on the history and evolution of the
Orion-Eridanus region. We discuss the general morphological structures and
observational characteristics of the superbubble, and derive quantitative
properties of the gas- and dust inside Barnard's Loop. We reveal that Barnard's
Loop is a complete bubble structure which, together with the lambda Ori region
and other smaller-scale bubbles, expands within the Orion-Eridanus superbubble.
We argue that the Orion-Eridanus superbubble is larger and more complex than
previously thought, and that it can be viewed as a series of nested shells,
superimposed along the line of sight. During the lifetime of the superbubble,
HII region champagne flows and thermal evaporation of embedded clouds
continuously mass-load the superbubble interior, while winds or supernovae from
the Orion OB association rejuvenate the superbubble by sweeping up the material
from the interior cavities in an episodic fashion, possibly triggering the
formation of new stars that form shells of their own. The steady supply of
material into the superbubble cavity implies that dust processing from interior
supernova remnants is more efficient than previously thought. The cycle of
mass-loading, interior cleansing, and star formation repeats until the
molecular reservoir is depleted or the clouds have been disrupted. While the
nested shells come and go, the superbubble remains for tens of millions of
years.
Soft X-ray intensity at 0.89 keV along the North Polar Spur is shown to
follow the extinction law due to the interstellar gas in the Aquila Rift by
analyzing the ROSAT archival data, which proves that the NPS is located behind
the rift. The Aquila-Serpens molecular clouds, where the X-ray optical depth
exceeds unity, are shown to have a mean LSR velocity of v=7.33 +/- 1.94 km/s,
corresponding to a kinematic distance of r=0.642 +/- 0.174 kpc. Assuming a
shell structure, a lower limit of the distance to NPS is derived to be 1.01 +/-
0.25 kpc, with the shell center being located farther than 1.1 kpc. Based on
the distance estimation, we argue that the NPS is a galactic halo object.
We used data from the WMAP satellite at 23, 33 and 41 GHz to study the
diffuse polarised emission over the entire sky. This emission is due to
synchrotron radiation and it originates mostly from filamentary structures with
well-ordered magnetic fields. Some of these structures have been known for
decades in radio continuum maps: the 'radio loops', with the North Polar Spur
being the most studied. The origin of these filaments is not clear and there
are many filaments that are visible for the first time with these polarisation
data. We have identified 11 filaments and studied their observational
properties. We find that the polarisation spectral indices, averaged over 18
regions in the sky is $\beta = -3.06 \pm 0.02$, which is consistent with
synchrotron radiation, although there are significant variations in $\beta$
over the sky ($\Delta\beta\approx0.2$). The polarisation fraction of some of
the filaments can be as high as 40%, which is a signature of a well ordered
magnetic field.
We explore the link between the large-scale filaments and the local ISM,
using the model of an expanding shell in the vicinity of the Sun. We compared
the observed polarisation angles with the predictions from the model and found
good agreement over most of the sky. We also studied the level of contamination
added by the diffuse filaments to the CMB E- and B-modes power spectra. We find
that the power measured at low $\ell$ of the B-mode spectrum is $\sim10$ times
larger than the power measured after masking the filaments from the maps. We
conclude that, even though these filaments present low radio brightness, a
careful removal will be necessary for future all-sky CMB polarisation analysis.
A recent XMM–Newton observation has revealed diffuse X-ray emission inside the nebula NGC 2359 around the Wolf–Rayet star WR 7. Taking advantage
of an improved point-source rejection and background subtraction, and a detailed comparison of optical and X-ray morphology,
we have reanalysed these X-ray observations. Our analysis reveals diffuse X-ray emission from a blowout and the presence of
emission at energies from 1.0 to 2.0 keV. The X-ray emission from NGC 2359 can be described by an optically thin plasma emission
model, but contrary to previous analysis, we find that the chemical abundances of this plasma are similar to those of the
optical nebula, with no magnesium enhancement, and that two components at temperatures T1 = 2 × 106 K and T2 = 5.7 × 107 K are required. The estimated X-ray luminosity in the 0.3–2.0 keV energy range is LX = 2 × 1033 erg s−1. The averaged rms electron density of the X-ray-emitting gas (ne ≲ 0.6 cm−3) reinforces the idea of mixing of material from the outer nebula into the hot bubble.
Many studies of the Loop I magnetic superbubble place the Sun at the edges of the bubble. One recent study models the polarized radio continuum of Loop I as two magnetic shells with the Sun embedded in the rim of the "S1" shell. If the Sun is in such a shell, it should be apparent in both the local interstellar magnetic field and the distribution of nearby interstellar material. The properties of these subshells are compared to the interstellar magnetic field and the distribution of interstellar Fe+ and Ca+ within ~55 pc of the Sun. Although the results are not conclusive, the interstellar magnetic field direction obtained from polarized stars within ~30 pc is consistent with the interstellar magnetic field direction of the S1 shell. The distribution of nearby interstellar Fe+ with log N(Fe+) < 12.5 cm–2 is described equally well by a uniform distribution or an origin in spherical shell-like features. Higher column densities of Fe+ (log N(Fe+)>12.5 cm–2) tend to be better described by the path length of the sightline through the S1 and S2 subshells. Column densities of the recombinant ion Ca+ are found to increase with the strength of the interstellar radiation field, rather than with star distance or total pathlength through the two magnetic subshells. The ion Ca+ cannot be used to trace the distribution of local interstellar gas unless the spatial variations in the radiation field are included in the calculation of the ionization balance, in addition to possible abundance variations. The result is that a model of Loop I as composed of two spherical magnetic subshells remains a viable description of the distribution of nearby low density interstellar medium, but is not yet proven.
Among all known young nearby neutron stars, we search for the neutron star
that was born in the same supernova event that formed the Antlia supernova
remnant (SNR). We also look for a runaway star that could have been the former
companion to the neutron star (if it exists) and then got ejected due to the
same supernova. We find the pulsar PSR J0630-2834 to be the best candidate for
a common origin with the Antlia SNR. In that scenario the SNR is ~1.2 Myr old
and is presently located at a distance of ~138 pc. We consider the runaway star
HIP 47155 a former companion candidate to PSR J0630-2834. The encounter time
and place is consistent with both stars being ejected from the Antlia SNR. We
measured the radial velocity of HIP 47155 as 32.42 +/- 0.70km/s.
A 3-D MHD simulation for the heliosphere resulting from the interaction of the solar wind with the interstellar medium is carried out. The response of the heliospheric structure to the 11-year solar cycle variations is analyzed in detail. It is shown that (1) the MHD structure is different from the HD structure, especially the density pattern of the heliosheath in the upstream direction, and that (2) the heliosheath is in a mixed state containing various plasma remnants which come from inner heliosphere as high-speed/low-speed solar winds at their respective solar-cycle phases, hence the heliospheric response time is much longer than the 11-year solar cycle time: the structure quickly changes with time long before it would reach a stastionary structure at each solar cycle phase. It is also shown that (3) the magnetic-field strucure in the heliosheath during the solar cycle variations involves shell-like neutral sheets extending over a wide latitudinal range from the equator to the heliopause boundary, in addition to the well-known ecliptic sheet. These new sheets are produced when the polarity of the solar magnetic field reverses at the solar maximum.
X-ray emission due to charge transfer collisions between heavy solar wind ions and neutrals has been predicted to exist both in the heliosphere and in the geocorona. The heliospheric X-ray emission can account for roughly half of the observed soft X-ray background intensity. It was also suggested that temporal variations in the heliospheric and geocoronal soft X-ray intensities will result from solar wind variations. In this paper, a simple model of the charge exchange X-ray emission mechanism is combined with measured solar wind parameters as a function of time and used to generate predictions of the temporal variation of the X-ray intensity observed at Earth for the time periods 1990–1993 and 1996–1998. Measured solar wind proton fluxes are also directly compared with the “long-term enhancement” part of the soft X-ray background measured by the Röntgen Satellite (ROSAT). A significant positive correlation exists, which supports the existence of X-ray emission associated with the solar wind interaction with either interstellar neutrals and/or with geocoronal neutral hydrogen.
Direct sampling of neutral interstellar (NIS) atoms by the Interstellar
Boundary Explorer (IBEX) can potentially provide a complementary method for
studying element abundances in the Local Interstellar Cloud and processes in
the heliosphere interface.}{We set the stage for abundance-aimed in-depth
analysis of measurements of NIS He, Ne, and O by IBEX and determine systematic
differences between abundances derived from various calculation methods and
their uncertainties.}{Using a model of ionization rates of the NIS species in
the heliosphere, based on independent measurements of the solar wind and solar
EUV radiation, we develop a time-dependent method of calculating the survival
probabilities of NIS atoms from the termination shock (TS) of the solar wind to
IBEX. With them, we calculate densities of these species along the Earth's
orbit and simulate the fluxes of NIS species as observed by IBEX. We study
pairwise ratios of survival probabilities, densities and fluxes of NIS species
at IBEX to calculate correction factors for inferring the abundances at
TS.}{The analytic method to calculate the survival probabilities gives
acceptable results only for He and Ne during low solar activity. For the
remaining portions of the solar cycle, and at all times for O, a fully time
dependent model should be used. Electron impact ionization is surprisingly
important for NIS O. Interpreting the IBEX observations using the time
dependent model yields the LIC Ne/O abundance of $0.16\pm40%$. The uncertainty
is mostly due to uncertainties in the ionization rates and in the NIS gas flow
vector.}{The Ne/He, O/He and Ne/O ratios for survival probabilities, local
densities, and fluxes scaled to TS systematically differ and thus an analysis
based only on survival probabilities or densities is not recommended, except
the Ne/O abundance for observations at low solar activity.
In a deep ocean ferromanganese crust an excess of 60Fe radioactivity was measured by means of high sensitivity accelerator mass spectrometry. The enhanced concentrations measured in the first two of three layers (corresponding to a time span of 0-2.8 Myr and 3.7-5.9 Myr, respectively) suggest the deposition of supernova produced 60Fe on earth. There is even a weak indication that the flux into the crust was higher about 5 Myr ago.
A least squares fit of line-of-sight velocities of interstellar gas
observed in nearby stars' spectra to the homogeneous flow model is
performed. The ionized matter seems to flow in accordance with Crutcher
(1982) model, while the neutral hydrogen flow is in accordance with this
model for lines of sight to distant stars only and not for very nearby
stars. On the other hand, the flow of nearby interstellar hydrogen seems
to follow the flow derived from analysis of the backscattered solar
radiation.
This review concerns itself with Loop I (the North Polar Spur), an
angularly immense feature of the high latitude galactic continuum radio
emission. Observations of relevance are presented ranging in wavelength
from the radio region to γ-rays. The many theories for the origin
of the feature are considered. Special attention is paid to the
hypothesis that the object is a superova remnant, at its closest less
than 100 pc from the sun. The possibility that Loop I may have a major
influence on our local interstellar medium is mentioned.
Recent analysis of COMPTEL data has revealed an extremely close correlation between 53 GHz microwave free-free and 1.8 MeV gamma-ray line emission. While microwave free-free emission arises from the ionized interstellar medium, 1.8 MeV gamma rays are emitted during the radioactive decay of 26Al. We argue that the close correlation can only be understood if massive stars (M20 M☉) are at the origin of Galactic 26Al. Based on the measured proportionality factor, we estimate the 26Al yield of an "equivalent O7 V star" to be (1.0±0.5)×10−4 M☉. Using an estimate for the total Galactic Lyman continuum luminosity of Q=3.5×1053 photons s-1, we derive the Galactic 26Al mass to be 3.1±0.9 M☉. The mass estimate is compared to theoretical nucleosynthesis predictions for 26Al from core-collapse supernovae and Wolf-Rayet stars. We circumvent the problem of using a weakly constrained star formation rate for this comparison by determining the star formation rate self-consistently from our models, using the Galactic Lyman continuum luminosity. The effects of mass loss and metallicity are considered, and the uncertainties of predicted 26Al production rates due to poorly known initial mass limits for the candidate sources are discussed. Assuming solar metallicity throughout the entire Galaxy, we predict a Galactic 26Al mass of 1.6±0.3 M☉, of which ~60% is produced by core-collapse supernovae, while ~40% originates from Wolf-Rayet stars. Taking the Galactic metallicity gradient into account increases the Galactic 26Al mass to 2.2±0.4 M☉, consistent with the observed value. The increase mainly arises from enhanced production by Wolf-Rayet stars in the metal-rich inner Galaxy; these contribute ~60% of the Galactic 26Al budget. We predict that the metallicity gradient should produce an inner-to-outer Galaxy intensity contrast of ~30% between 1.8 MeV and Galactic free-free emission, which should be observable by the future gamma-ray spectrometer SPI on the International Gamma-Ray Astrophysics Laboratory (INTEGRAL).
Nearby supernova explosions—within a few tens of pc of the solar system—have become a subject of intense scrutiny, due to the discovery of live undersea 60Fe from an event 2.8 Myr ago. A key open question concerns the delivery of supernova ejecta to the Earth, in particular penetration of the heliosphere by the supernova remnant (SNR). We present the first systematic numerical hydrodynamical study of the interaction between a supernova blast and the solar wind. Our simulations explore dynamic pressure regimes that are factors ≥10 above those in other studies of the heliosphere under exotic conditions, for supernovae exploding at a range of distances through different interstellar environments, and interacting with solar winds of varying strengths. Our results are qualitatively consistent with the structure of the contemporary heliosphere modeled by previous work, but compressed to within the inner solar system. We demonstrate that key characteristics of the resulting heliospheric structure follow simple scaling laws that can be understood in terms of pressure-balance arguments, and which are in agreement with previous work. Our models show that a 10 pc supernova event, incident on a solar-wind outflow with the mean observed properties, compresses the heliopause to just beyond 1 AU. We also demonstrate scenarios where the supernova remnant compresses the heliopause to within 1 AU, in which cases supernova material will be directly deposited on Earth. Since 8 pc marks the nominal "kill radius" for severe biosphere damage, any extinction-level events should have left terrestrial deposits of supernova debris. We conclude with a brief discussion of the effect of our approximations and the impact of additional physics.
We present new Goddard High-Resolution Spectrograph (GHRS) echelle observations of the high ionization lines of Si IV, C IV, and N V toward HD 119608, a halo star at d = 4.1 kpc behind the Loop I and IV supernova remnants. Absorption along the path to HD 119608 makes it possible to study energetic processes that may result in the flow of gas into the Galactic halo. The data have a resolution (FWHM) of ≈ 3.5 km s⁻¹ and S/N ratios of 30:1-50:1. The integrated high ion column densities log N = 13.57 ± 0.02, 14.48 ± 0.06, and 13.45 ± 0.07 for Si IV, C IV, and N V, respectively, imply a factor of 2-4 enhancement in the amount of highly ionized gas compared to average sight lines through the Galactic disk and halo. The integrated high ion column density ratios, N(C IV)/N(Si IV) = 8.1 ± 1.1 and N(C IV)/N(N V) = 10.7 ± 2.1, are also several times larger than normal. These high ion results suggest the absorption is influenced by passage of the sight line through the center of Loop IV. The HD 119608 C IV absorption profile has a bimodal velocity structure indicative of an expanding shell; we tentatively derive an expansion velocity of 16 km s⁻¹ for Radio Loop IV. A detailed analysis of the high ion profile structure indicates that multiple types of highly ionized gas with a range of properties exist along this sight line.
We also reexamine the high ionization properties of the QSO 3C 273 sight line using new intermediate-resolution (FWHM ≈ 20 km s⁻¹) GHRS data. We obtain log N = 14.49 ± 0.03 and 13.87 ± 0.06 for C IV and N V, respectively. The C IV column density, which is 0.12 dex smaller than earlier estimates, leads to somewhat smaller ionic ratios than previously determined. We find N(C IV)/N(Si IV) = 5.1 ± 0.6 and N(C IV)/N(N V) = 4.2 ± 0.6. However, as for HD 119608, the high ion column densities toward 3C 273 are larger than normal by factors of 2-4. The 3C 273 high ion absorption profiles are much broader than those seen toward HD 119608 and other sight lines near the center of Loop IV. The larger line widths may result because the sight line passes through the turbulent edge of Loop IV as well as the X-ray and radio continuum emission regions of the North Polar Spur.
We have compiled a list of the highest quality IUE and GHRS high ion measurements available for interstellar sight lines through the disk and halo and find the following median averaged results: N(C IV)/N(Si IV) = 3.8 ± 1.9 and N(C IV)/N(N V) = 4.0 ± 2.4. These ratios are lower than those found for four Loop IV sight lines. We suggest a model for the production of highly ionized gas in Loop IV in which the contributions from turbulent mixing layers and conductive interfaces/SNR bubbles to the total high ion column densities are approximately equal. Much of the high ion absorption toward HD 119608 and 3C 273 may occur within a highly fragmented medium within the remnant or the outer cavity walls of the remnant.