![]() | ![]() |
Astron. Astrophys. 346, 831-842 (1999) 2. Young supernova remnantsStudies of supernovae from the moment of the explosion up until
they dissolve and merge with the general interstellar medium has, in
general, been guided by observational opportunities as combined with
model interpretations; the interplay of different physical processes,
which vary spatially and rapidly within a young supernova remnant,
cannot be disentangled from measurements alone. Observational windows
are (from low to high energy radiation): Radio maps from electrons
synchrotron-emitting in magnetic-field structures around the
shocked-gas region; infrared emission from cool and hot dust within
dense clumps embedded in the supernova remnant; optical line emission
from the edges of dense material embedded in the remnant's hot plasma;
X-ray emission from the hot, ionized, gas that has been shocked by the
forward blastwave and by the reverse shock traveling inward,
respectively; and Here we are mainly interested in the thermal history of the radioactive 44Ti after it has been ejected in a supernova explosion. 44Ti as well as other iron-group elements are synthesized during the very early moments of the explosion of a massive star in the layers adjacent to the nascent compact object (a neutron star or black hole). Therefore, observations of iron can be used to trace also 44Ti. However, not many good cases are known so far. SN 1987A is certainly the best studied case. The large Doppler
shifts of iron lines observed in the early spectra require that iron
moves with velocities much higher than some of the hydrogen which
cannot be explained by spherically symmetric explosion models but
indicates that it is not homogeneously distributed in an expanding
shell but is found in clumpy structures. This conclusion is also
supported by the unexpectedly early detection of X- and
44Ti Recently, a second Galactic 44Ti source has been reported (Iyudin et al. 1998). Its alignment with an also recently discovered X-ray remnant (Aschenbach 1998) suggests that it is a very young and most nearby supernova remnant, with an age around 680y and a distance of 200pc only. This object still provides a puzzle because of the absence of radio and optical emission expected for such a nearby supernova. Yet, if confirmed, it will provide a unique opportunity for the study of supernova-produced 44Ti; in which case it is of some interest to see if the modified decay rate of ionized 44Ti addressed in our paper could also be important. The 44Ti found in young supernova remnants is probably
formed during the The evolution of young supernova remnants has been modeled extensively, and the spherically-symmetric explosion into homogeneous interstellar surroundings appears well-understood (McKee and Truelove 1995). A free expansion phase is followed by an adiabatic blastwave phase, where interaction with surrounding material produces outward and inward moving shock waves leading to bright X-ray and radio emission, yet being unimportant for the energetics of the remnant. Later phases of significant slowing-down and radiative losses of the expanding remnant lie beyond the early phase where 44Ti still decays. Young remnants such as Cas A are generally understood as being somewhere intermediate between free expansion and the second phase, commonly called "Sedov-Taylor" phase. The evolution of such idealized young supernova remnants during the
ejecta-dominated stage ( Although the general appearance of supernova remnant images in the radio and in X-rays is that of a large-scale shell configuration as expected from the above model, additional prominent clumpy structures appear in some cases (e.g., Anderson & Rudnick 1995; Koralesky et al. 1998). This suggests that the model outlined so far is an oversimplification as far as details are concerned. The gross radio and X-ray emissivity may not be very sensitive to such discrepancy, tracing the electron component in the vicinity of the shock region, but the bulk ejected mass may be inadequately represented by the inferred electron densities and temperatures (e.g., Koralesky et al. 1998). Thus, for the Cas A remnant, prominent structures have been studied in their respective forms of optical knots and filaments, "quasi-stationary flocculi", "fast-moving knots", and "fast-moving flocculi" (e.g., van den Bergh & Kamper 1983; Reed et al. 1995; Peimbert & van den Bergh 1971; Chevalier & Kirshner 1977, 1978, 1979; Reynoso et al. 1997; Lagage et al. 1996). There is overwhelming evidence for dense structures embedded in tenuous material within the entire remnant, and even outside the blastwave shock radius. Obviously the explosion itself produces fragments of material, seen now as fast-moving knots with their abundance patterns supporting an ejecta origin. These clumps might carry heavier elements preferentially as suggested by observations of fast-moving Fe clumps early in SN explosions, e.g. in SN 1987A (e.g., Nomoto et al. 1994, Wooden 1997 and references therein), but a connection between the instabilities and clumpiness early after the supernova explosion and the fragments and "bullets" seen in the remnants has not been established yet. Given all these uncertainties in the evolution of supernova
remnants we do not attempt to model a specific object, such as
Cas A, in detail here. We rather shall investigate by means of an
admittedly very simple model, varying its parameters within reasonable
limits, the potential effects of ionization on the 44Ti
abundance estimates obtained from ![]() ![]() ![]() ![]() © European Southern Observatory (ESO) 1999 Online publication: June 17, 1999 ![]() |