84 research outputs found
Destruction of Interstellar Dust in Evolving Supernova Remnant Shock Waves
Supernova generated shock waves are responsible for most of the destruction
of dust grains in the interstellar medium (ISM). Calculations of the dust
destruction timescale have so far been carried out using plane parallel steady
shocks, however that approximation breaks down when the destruction timescale
becomes longer than that for the evolution of the supernova remnant (SNR)
shock. In this paper we present new calculations of grain destruction in
evolving, radiative SNRs. To facilitate comparison with the previous study by
Jones et al. (1996), we adopt the same dust properties as in that paper. We
find that the efficiencies of grain destruction are most divergent from those
for a steady shock when the thermal history of a shocked gas parcel in the SNR
differs significantly from that behind a steady shock. This occurs in shocks
with velocities >~ 200 km/s for which the remnant is just beginning to go
radiative. Assuming SNRs evolve in a warm phase dominated ISM, we find dust
destruction timescales are increased by a factor of ~2 compared to those of
Jones et al. (1996), who assumed a hot gas dominated ISM. Recent estimates of
supernova rates and ISM mass lead to another factor of ~3 increase in the
destruction timescales, resulting in a silicate grain destruction timescale of
~2-3 Gyr. These increases, while not able resolve the problem of the discrepant
timescales for silicate grain destruction and creation, are an important step
towards understanding the origin, and evolution of dust in the ISM.Comment: 30 pages, 8 figures, accepted for publication in the Astrophysical
Journa
Trajectories and Distribution of Interstellar Dust Grains In The Heliosphere
The solar wind carves a bubble in the surrounding interstellar medium (ISM) known as the heliosphere. Charged interstellar dust grains (ISDG) encountering the heliosphere may be diverted around the heliopause or penetrate it depending on their charge-to-mass ratio. We present new calculations of trajectories of ISDG in the heliosphere, and the dust density distributions that result. We include up-to-date grain charging calculations using a realistic UV radiation field and full three-dimensional magnetohydrodynamic fluid + kinetic models for the heliosphere. Models with two different (constant) polarities for the solar wind magnetic field (SWMF) are used, with the grain trajectory calculations done separately for each polarity. Small grains a gr 0.01 μm are completely excluded from the inner heliosphere. Large grains, a gr 1.0 μm, pass into the inner solar system and are concentrated near the Sun by its gravity. Trajectories of intermediate size grains depend strongly on the SWMF polarity. When the field has magnetic north pointing to ecliptic north, the field de-focuses the grains resulting in low densities in the inner heliosphere, while for the opposite polarity the dust is focused near the Sun. The ISDG density outside the heliosphere inferred from applying the model results to in situ dust measurements is inconsistent with local ISM depletion data for both SWMF polarities but is bracketed by them. This result points to the need to include the time variation in the SWMF polarity during grain propagation. Our results provide valuable insights for interpretation of the in situ dust observations from Ulysses
Photoionization of Galactic Halo Gas by Old Supernova Remnants
We present new calculations on the contribution from cooling hot gas to the
photoionization of warm ionized gas in the Galaxy. We show that hot gas in
cooling supernova remnants (SNRs) is an important source of photoionization,
particularly for gas in the halo. We find that in many regions at high latitude
this source is adequate to account for the observed ionization so there is no
need to find ways to transport stellar photons from the disk. The flux from
cooling SNRs sets a floor on the ionization along any line of sight. Our model
flux is also shown to be consistent with the diffuse soft X-ray background and
with soft X-ray observations of external galaxies.
We consider the ionization of the clouds observed towards the halo star HD
93521, for which there are no O stars close to the line of sight. We show that
the observed ionization can be explained successfully by our model EUV/soft
X-ray flux from cooling hot gas. In particular, we can match the H alpha
intensity, the S++/S+ ratio, and the C+* column. From observations of the
ratios of columns of C+* and either S+ or H0, we are able to estimate the
thermal pressure in the clouds. The slow clouds require high (~10^4 cm^-3 K)
thermal pressures to match the N(C+*)/N(S+) ratio. Additional heating sources
are required for the slow clouds to maintain their ~7000 K temperatures at
these pressures, as found by Reynolds, Hausen & Tufte (1999).Comment: AASTeX 5.01; 34 pages, 2 figures; submitted to Astrophysical Journa
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