Total Ejected Mass Research Articles

High luminosity, slow ejecta and persistent carbon lines: SN 2009dc challenges thermonuclear explosion scenarios★

SN 2009dc shares similarities with normal Type Ia supernovae, but is clearly overluminous, with a (pseudo-bolometric) peak luminosity of log(L) = 43.47 [erg/s]. Its light curves decline slowly over half a year after maximum light, and the early-time near-IR light curves show secondary maxima, although the minima between the first and second peaks are not very pronounced. Bluer bands exhibit an enhanced fading after ~200 d, which might be caused by dust formation or an unexpectedly early IR catastrophe. The spectra of SN 2009dc are dominated by intermediate-mass elements and unburned material at early times, and by iron-group elements at late phases. Strong C II lines are present until ~2 weeks past maximum, which is unprecedented in thermonuclear SNe. The ejecta velocities are significantly lower than in normal and even subluminous SNe Ia. No signatures of CSM interaction are found in the spectra. Assuming that the light curves are powered by radioactive decay, analytic modelling suggests that SN 2009dc produced ~1.8 solar masses of 56Ni assuming the smallest possible rise time of 22 d. Together with a derived total ejecta mass of ~2.8 solar masses, this confirms that SN 2009dc is a member of the class of possible super-Chandrasekhar-mass SNe Ia similar to SNe 2003fg, 2006gz and 2007if. A study of the hosts of SN 2009dc and other superluminous SNe Ia reveals a tendency of these SNe to explode in low-mass galaxies. A low metallicity of the progenitor may therefore be an important pre-requisite for producing superluminous SNe Ia. We discuss a number of explosion scenarios, ranging from super-Chandrasekhar-mass white-dwarf progenitors over dynamical white-dwarf mergers and Type I 1/2 SNe to a core-collapse origin of the explosion. None of the models seem capable of explaining all properties of SN 2009dc, so that the true nature of this SN and its peers remains nebulous.

The Type Ib SN 1999dn: one year of photometric and spectroscopic monitoring★

Extensive optical and near-infrared (NIR) observations of the type Ib supernova 1999dn are presented, covering the first year after explosion. These new data turn this object, already considered a prototypical SNIb, into one of the best observed objects of its class. The light curve of SN 1999dn is mostly similar in shape to that of other SNeIb but with a moderately faint peak M_V=-17.2 mag). From the bolometric light curve and ejecta expansion velocities, we estimate that about 0.11 Msun of 56Ni were produced during the explosion and that the total ejecta mass was 4-6 Msun with a kinetic energy of at least 5x10^{51} erg. The spectra of SN 1999dn at various epochs are similar to those of other Stripped Envelope (SE) SNe showing clear presence of H at early epochs. The high explosion energy and ejected mass, along with the small flux ratio [CaII]/[OI] measured in the nebular spectrum, together with the lack of signatures of dust formation and the relatively high-metallicity environment point toward a single massive progenitor (M_ZAMS>=23-25 Msun) for SN 1999dn.

Early-stage ejecta velocity distribution for vertical hypervelocity impacts into sand

The ejecta dynamics during main-stage excavation flow in a cratering event have previously been well characterized, particularly for vertical impacts. In this experimental study, we present new results addressing the early-time, low-angle, high-speed component of the ejecta velocity distribution as a function of time for hypervelocity vertical impacts into sand. Although this regime represents a very small portion of total ejected mass in laboratory experiments, it comprises a greater percentage of growth for larger craters.

Two type Ic supernovae in low-metallicity, dwarf galaxies: diversity of explosions

We present BVRI photometry and optical spectroscopy of two Type Ic supernovae SN 2007bg and SN 2007bi discovered in wide-field, non-targeted surveys and associated with sub-luminous blue dwarf galaxies. Neither SNe 2007bg nor 2007bi were found in association with an observed GRB, but are found to inhabit similar low-metallicity environments as GRB associated supernovae. The radio-bright SN 2007bg is hosted by an extremely sub-luminous galaxy of magnitude MB = -12.4+/-0.6 mag with an estimated oxygen abundance of 12+log(O/H) = 8.18+/-0.17. The lightcurve of SN 2007bg displays one of the fastest post-maximum decline rates of all broad-lined Type Ic supernovae known to date and, when combined with its high expansion velocities, a high kinetic energy to ejected mass ratio (E_K/Mej ~ 2.7). We show that SN 2007bi is possibly the most luminous Type Ic known, reaching a peak magnitude of MR ~ 21.3 mag and displays a remarkably slow decline, following the radioactive decay rate of 56Co to 56Fe throughout the course of its observed lifetime. From a simple model of the bolometric light curve of SN 2007bi we estimate a total ejected 56Ni mass of M_Ni = 3.5 - 4.5 solar masses, the largest 56Ni mass measured in the ejecta of a supernova to date. There are two models that could explain the high luminosity and large ejected 56Ni mass. One is a pair-instability supernova (PISN) which has been predicted to occur for massive stars at low metallicities. We measure the host galaxy metallicity of SN 2007bi to be 12 + log(O/H) = 8.15+/-0.15 which is somewhat high to be consistent with the PISN model. An alternative is the core-collapse of a C+O star of 20 - 40 solar masses which is the core of a star of originally 50 - 100 solar masses. (Abridged)

Fragmentation model dependence of collision cascades

Mass depletion of bodies through successive collisional disruptions (i.e., collision cascade) is one of the most important processes in the studies of the asteroids belt, the Edgeworth-Kuiper belt, debris disks, and planetary formation. The collisional disruption is divided into two types, i.e., catastrophic disruption and cratering. Although some studies of the collision cascades neglected the effect of cratering, it is unclear which type of disruption makes a dominant contribution to the collision cascades. In the present study, we construct a simple outcome model describing both catastrophic disruption and cratering, which has some parameters characterizing the total ejecta mass, the mass of the largest fragment, and the power-law exponent of the size distribution of fragments. Using this simple outcome model with parameters, we examine the model dependence of the mass depletion time in collision cascades for neglect of coalescence of colliding bodies due to high collisional velocities. We find the cratering collisions are much more effective in collision cascades than collisions with catastrophic disruption in a wide region of the model parameters. It is also found that the mass depletion time in collision cascades is mainly governed by the total ejecta mass and almost insensitive to the mass of the largest fragment and the power-law exponent of fragments for a realistic parameter region. The total ejecta mass is usually determined by the ratio of the impact energy divided by the target mass (i.e. Q-value) to its threshold value Q D ★ for catastrophic disruption, as well as in our simple model. We derive a mass depletion time in collision cascades, which is determined by Q D ★ of the high-mass end of collision cascades. The mass depletion time derived with our model would be applicable to debris disks and planetary formation.

Energy balance of the Deep Impact experiment

We present results on the energy balance of the Deep Impact experiment based on analysis of 180 infrared spectra of the ejecta obtained by the Deep Impact spacecraft. We derive an output energy of 16.5 (+9.1/−4.1) GJ. With an input energy of 19.7 GJ, the error bars are large enough so that there may or may not be a balance between the kinetic energy of the impact and that of outflowing materials. Although possible, no other source of energy other than the impactor or the Sun is needed to explain the observations. Most of the energy (85%) goes into the hot plume in the first few seconds, which only represents a very small fraction (<0.01%) of the total ejected mass. The hot plume contains 190 (+263/−71) kg of H 2O, 1.6 ± 0.5 kg of CO 2, 8.2 (+11.3/3.1) kg of CO (assuming a CO/H 2O ratio of 4.3%), 27.9 (+25.0/−8.9) kg of organic material and 255 ± 128 kg of dust, while the ejecta contains ∼10 7 kg of materials. About 12% of the energy goes into the ejecta (mostly water) and 3% to destroy the impactor. Volatiles species other than H 2O (CO 2, CO or organic molecules) contribute to <7% of the energy balance. In terms of physical processes, 68% of the energy is used to accelerate grains (kinetic energy), 16% to heat them, 6% to sublimate or melt them and 10% (upper limit) to break and compress dust and/or water ice aggregates into small micron size particles. For the hot plume, we derive a dust/H 2O ratio of 1.3 (+1.9/−1.0), a CO 2/H 2O ratio of 0.008 (+0.009/−0.006), an organics/H 2O ratio of 0.15 (+0.29/−0.11) and an organics/dust ratio of 0.11 (+0.30/−0.07). This composition refers to the impact site and is different from that of the bulk nucleus, consistent with the idea of layers of different composition in the nucleus sub-surface. Our results emphasize the importance of laboratory impact experiments to understand the physical processes involved at such a large scale.

SN 2005cs in M51 - II. Complete evolution in the optical and the near-infrared

We present the results of the one-year long observational campaign of the type II plateau SN 2005cs, which exploded in the nearby spiral galaxy M51 (the Whirlpool galaxy). This extensive data set makes SN 2005cs the best observed low-luminosity, ^(56)Ni-poor type II plateau event so far and one of the best core-collapse supernovae ever. The optical and near-infrared spectra show narrow P-Cygni lines characteristic of this SN family, which are indicative of a very low expansion velocity (about 1000 km s^(−1) ) of the ejected material. The optical light curves cover both the plateau phase and the late-time radioactive tail, until about 380 d after core-collapse. Numerous unfiltered observations obtained by amateur astronomers give us the rare opportunity to monitor the fast rise to maximum light, lasting about 2 d. In addition to optical observations, we also present near-infrared light curves that (together with already published ultraviolet observations) allow us to construct for the first time a reliable bolometric light curve for an object of this class. Finally, comparing the observed data with those derived from a semi-analytic model, we infer for SN 2005cs a ^(56)Ni mass of about 3 × 10^(−3) M⊙, a total ejected mass of 8–13 M⊙ and an explosion energy of about 3 × 10^(50) erg .

A ballistics analysis of the Deep Impact ejecta plume: Determining Comet Tempel 1's gravity, mass, and density

In July of 2005, the Deep Impact mission collided a 366 kg impactor with the nucleus of Comet 9P/Tempel 1, at a closing speed of 10.2 km s −1. In this work, we develop a first-order, three-dimensional, forward model of the ejecta plume behavior resulting from this cratering event, and then adjust the model parameters to match the flyby-spacecraft observations of the actual ejecta plume, image by image. This modeling exercise indicates Deep Impact to have been a reasonably “well-behaved” oblique impact, in which the impactor–spacecraft apparently struck a small, westward-facing slope of roughly 1 / 3 – 1 / 2 the size of the final crater produced (determined from initial ejecta plume geometry), and possessing an effective strength of not more than Y ¯ = 1 – 10 kPa . The resulting ejecta plume followed well-established scaling relationships for cratering in a medium-to-high porosity target, consistent with a transient crater of not more than 85–140 m diameter, formed in not more than 250–550 s, for the case of Y ¯ = 0 Pa (gravity-dominated cratering); and not less than 22–26 m diameter, formed in not less than 1–3 s, for the case of Y ¯ = 10 kPa (strength-dominated cratering). At Y ¯ = 0 Pa , an upper limit to the total ejected mass of 1.8 × 10 7 kg ( 1.5 – 2.2 × 10 7 kg ) is consistent with measurements made via long-range remote sensing, after taking into account that 90% of this mass would have stayed close to the surface and then landed within 45 min of the impact. However, at Y ¯ = 10 kPa , a lower limit to the total ejected mass of 2.3 × 10 5 kg ( 1.5 – 2.9 × 10 5 kg ) is also consistent with these measurements. The expansion rate of the ejecta plume imaged during the look-back phase of observations leads to an estimate of the comet's mean surface gravity of g ¯ = 0.34 mm s −2 (0.17–0.90 mm s −2), which corresponds to a comet mass of m t = 4.5 × 10 13 kg ( 2.3 – 12.0 × 10 13 kg ) and a bulk density of ρ t = 400 kg m −3 (200–1000 kg m −3), where the large high-end error is due to uncertainties in the magnitude of coma gas pressure effects on the ejecta particles in flight.

The Deep Impact oblique impact cratering experiment

The Deep Impact probe collided with 9P Tempel 1 at an angle of about 30° from the horizontal. This impact angle produced an evolving ejecta flow field very similar to much smaller scale oblique-impact experiments in porous particulate targets in the laboratory. Similar features and phenomena include a decoupled vapor/dust plume at the earliest times, a pronounced downrange bias of the ejecta, an uprange “zone of avoidance” (ZoA), heart-shaped ejecta ray system (cardioid pattern), and a conical (but asymmetric) ejecta curtain. Departures from nominal cratering evolution, however, provide clues on the nature of the impact target. These departures include: fainter than expected flash at first contact, delayed emergence of the self-luminous vapor/dust plume, uprange-directed plume, narrow early-time uprange ray followed by a late-stage uprange plume, persistence of ejecta asymmetries (and the uprange ZoA) throughout the approach sequence, emergence of a downrange ZoA at late times, detachment of uprange curved rays, very long lasting non-radial ejecta rays, and high-angle ejecta plume lasting over the entire encounter. The first second of crater formation most closely resembles the consequences of a highly porous target, while later evolution indicates that the target may be layered as well. Experiments also reveal that impacts into highly porous targets produce a vapor/dust plume directed back up the incoming trajectory. This uprange plume is attributed to cavitation within a narrow penetration funnel. The observed lateral expansion speed of the initial vapor plume downrange provides an estimate for the total vaporized mass equal to ∼ 5 m p (projectile masses) of water ice or 6 m p of CO 2. The downrange plume speed is consistent with the gas expansion added to the downrange horizontal component of the DI probe. Based on high-speed spectroscopy of experimental impacts, the observed delay in brightening of the DI plume may be the result of delayed condensation of carbon, in addition to silicates. Observed molecular species in the initial self-luminous vapor plume likely represent recombination products from completely dissociated target materials. The crater produced by the impact can be estimated from Earth-based observations of total ejected mass to be 130–220 m in diameter. This size range is consistent with a 220 m-diameter circular feature at the point of impact visible in highly processed, deconvolved HRI images. The final crater, however, may resemble an inverted sombrero-hat, with a deep central pit surrounded by a shallow excavation crater. Excavated distal material observed from the Earth was likely from the upper few meters contrasted with ballistic ejecta observed from the DI flyby, which included deep materials (10–30 m) within the diffuse plume above the crater and shallower (5–10 m) materials within the ejecta curtain.

Keck and European Southern Observatory Very Large Telescope View of the Symmetry of the Ejecta of the XRF/SN 2006aj

Nebular-phase spectra of SN 2006aj, which was discovered in coincidence with X-ray flash 060218, were obtained with Keck in 2006 July and the Very Large Telescope in 2006 September. At the latter epoch spectropolarimetry was also attempted, yielding an upper limit of ~ 2% for the polarization. The spectra show strong emission lines of [OI] and MgI], as expected from a Type Ic supernova, but weak CaII lines. The [FeII] lines that were strong in the spectra of SN 1998bw are much weaker in SN 2006aj, consistent with the lower luminosity of this SN. The outer velocity of the line-emitting ejecta is ~ 8000 km/s in July and ~ 7400 km/s in September, consistent with the relatively low kinetic energy of expansion of SN 2006aj. All emission lines have similar width, and the profiles are symmetric, indicating that no major asymmetries are present in the ejecta at the velocities sampled by the nebular lines (v < 8000 km/s), except perhaps in the innermost part. The spectra were modelled with a non-LTE code. The mass of 56Ni required to power the emission spectrum is ~ 0.20 Msun, in excellent agreement with the results of early light curve modelling. The oxygen mass is ~ 1.5 Msun, again much less than in SN 1998bw but larger by ~ 0.7 Msun than the value derived from the early-time modelling. The total ejected mass is ~ 2 Msun below 8000 km/s. This confirms that SN 2006aj was only slightly more massive and energetic than the prototypical Type Ic SN 1994I, but also indicates the presence of a dense inner core, containing ~ 1 Msun of mostly oxygen and carbon. The presence of such a core is inferred for all broad-lined SNe Ic. This core may have the form of an equatorial oxygen-dominated region, but it is too deep to affect the early light curve and too small to affect the late polarization spectrum.

The Deep Impact oblique impact cratering experiment

The Deep Impact probe collided with 9P Tempel 1 at an angle of about 30° from the horizontal. This impact angle produced an evolving ejecta flow field very similar to much smaller scale oblique-impact experiments in porous particulate targets in the laboratory. Similar features and phenomena include a decoupled vapor/dust plume at the earliest times, a pronounced downrange bias of the ejecta, an uprange “zone of avoidance” (ZoA), heart-shaped ejecta ray system (cardioid pattern), and a conical (but asymmetric) ejecta curtain. Departures from nominal cratering evolution, however, provide clues on the nature of the impact target. These departures include: fainter than expected flash at first contact, delayed emergence of the self-luminous vapor/dust plume, uprange-directed plume, narrow early-time uprange ray followed by a late-stage uprange plume, persistence of ejecta asymmetries (and the uprange ZoA) throughout the approach sequence, emergence of a downrange ZoA at late times, detachment of uprange curved rays, very long lasting non-radial ejecta rays, and high-angle ejecta plume lasting over the entire encounter. The first second of crater formation most closely resembles the consequences of a highly porous target, while later evolution indicates that the target may be layered as well. Experiments also reveal that impacts into highly porous targets produce a vapor/dust plume directed back up the incoming trajectory. This uprange plume is attributed to cavitation within a narrow penetration funnel. The observed lateral expansion speed of the initial vapor plume downrange provides an estimate for the total vaporized mass equal to ∼ 5 m p (projectile masses) of water ice or 6 m p of CO 2. The downrange plume speed is consistent with the gas expansion added to the downrange horizontal component of the DI probe. Based on high-speed spectroscopy of experimental impacts, the observed delay in brightening of the DI plume may be the result of delayed condensation of carbon, in addition to silicates. Observed molecular species in the initial self-luminous vapor plume likely represent recombination products from completely dissociated target materials. The crater produced by the impact can be estimated from Earth-based observations of total ejected mass to be 130–220 m in diameter. This size range is consistent with a 220 m-diameter circular feature at the point of impact visible in highly processed, deconvolved HRI images. The final crater, however, may resemble an inverted sombrero-hat, with a deep central pit surrounded by a shallow excavation crater. Excavated distal material observed from the Earth was likely from the upper few meters contrasted with ballistic ejecta observed from the DI flyby, which included deep materials (10–30 m) within the diffuse plume above the crater and shallower (5–10 m) materials within the ejecta curtain.

A ballistics analysis of the Deep Impact ejecta plume: Determining Comet Tempel 1's gravity, mass, and density

In July of 2005, the Deep Impact mission collided a 366 kg impactor with the nucleus of Comet 9P/Tempel 1, at a closing speed of 10.2 km s −1. In this work, we develop a first-order, three-dimensional, forward model of the ejecta plume behavior resulting from this cratering event, and then adjust the model parameters to match the flyby-spacecraft observations of the actual ejecta plume, image by image. This modeling exercise indicates Deep Impact to have been a reasonably “well-behaved” oblique impact, in which the impactor–spacecraft apparently struck a small, westward-facing slope of roughly 1 / 3 – 1 / 2 the size of the final crater produced (determined from initial ejecta plume geometry), and possessing an effective strength of not more than Y ¯ = 1 – 10 kPa . The resulting ejecta plume followed well-established scaling relationships for cratering in a medium-to-high porosity target, consistent with a transient crater of not more than 85–140 m diameter, formed in not more than 250–550 s, for the case of Y ¯ = 0 Pa (gravity-dominated cratering); and not less than 22–26 m diameter, formed in not less than 1–3 s, for the case of Y ¯ = 10 kPa (strength-dominated cratering). At Y ¯ = 0 Pa , an upper limit to the total ejected mass of 1.8 × 10 7 kg ( 1.5 – 2.2 × 10 7 kg ) is consistent with measurements made via long-range remote sensing, after taking into account that 90% of this mass would have stayed close to the surface and then landed within 45 min of the impact. However, at Y ¯ = 10 kPa , a lower limit to the total ejected mass of 2.3 × 10 5 kg ( 1.5 – 2.9 × 10 5 kg ) is also consistent with these measurements. The expansion rate of the ejecta plume imaged during the look-back phase of observations leads to an estimate of the comet's mean surface gravity of g ¯ = 0.34 mm s −2 (0.17–0.90 mm s −2), which corresponds to a comet mass of m t = 4.5 × 10 13 kg ( 2.3 – 12.0 × 10 13 kg ) and a bulk density of ρ t = 400 kg m −3 (200–1000 kg m −3), where the large high-end error is due to uncertainties in the magnitude of coma gas pressure effects on the ejecta particles in flight.

Simulated X‐Ray Emission for a Runaway Model of Kepler's Supernova Remnant

We present two-dimensional numerical simulations of a model for Kepler's supernova remnant (SNR) carried out with the YGUAZU-A code. Following previous studies, we have assumed that the peculiar shape of this young remnant arises as a consequence of the interaction of the SNR blast wave with the bow shock formed by the wind of its high velocity progenitor. Furthermore, from our numerical results we have obtained synthetic X-ray emission maps, which can be directly compared with recent and previous observations of this SNR. Our models show that a nice fit with respect to the X-ray morphology and luminosity is obtained for a SN progenitor with mass-loss rate of 5 x 10(-5) M-circle dot yr(-1), an ambient medium density of 10(-2) cm(-3), an initial explosion energy of 8 x 10(50) ergs, and a total ejected mass within 1.4-2.5 M-circle dot. In our simulations, parameters typical of a young population progenitor have not been considered. This model also predicts a similar to 0.3% yearly decrease in the total X-ray luminosity, which is consistent with observed values. The parameters employed in our runs correspond to a Type Ia supernova. Based on our simulations, we find that the expansion rate increases after the SNR blast wave overruns the bow shock, and we discuss whether this can explain the observed difference between the expansion rates measured from sequences of radio and X-ray images. (Less)

Ejecta velocity distribution for impact cratering experiments on porous and low strength targets

Impact cratering experiments on porous targets with various compressive strength ranging from ∼0.5 to ∼250 MPa were carried out in order to investigate the relationship between the ejecta velocity, and material strength or porosity of the target. A spherical alumina projectile (diameter ∼1 mm) was shot perpendicularly into the target surface with velocity ranging from 1.2 to 4.5 km/s (nominal 4 km/s), using a two-stage light-gas gun. The ejecta velocity was estimated from the fall point distance of ejecta. The results show that there are in fact a large fraction of ejecta with very low velocities when the material strength of the target is small and the porosity is high. As an example, in the case of one specific target (compressive strength ∼0.5 MPa and porosity 43%), the amount of ejecta with velocities lower than 1 m/s is about 40% of the total mass. The average velocity of the ejecta decreases with decreasing material strength or increasing the porosity of the target. Moreover, in our experiments, the ejecta velocity distributions normalized to total ejecta mass seem to be mainly dependent on the material strength of the target, and not so greatly on the porosity. We also compare our experimental results with those of Gault et al. [1963. Spray ejected from the lunar surface by meteoroid impact. NASA Technical Note D-1767] and Housen [1992. Crater ejecta velocities for impacts on rocky bodies. LPSC XXIII, 555–556] for the ejecta velocity distribution using Housen's nondimensional scaling parameter. The ejecta velocity distributions of our experiments are lower than those of Gault et al. [1963. Spray ejected from the lunar surface by meteoroid impact. NASA Technical Note D-1767] and Housen [1992. Crater ejecta velocities for impacts on rocky bodies. LPSC XIII, 555–556].

A Candidate Neutron Star Associated with Galactic Center Supernova Remnant Sagittarius A East

We present imaging and spectral studies of the supernova remnant (SNR) Sagittarius (Sgr) A East from deep observations with Chandra. The spatially resolved spectral analysis of Sgr A East reveals the presence of a two-temperature thermal plasma (kT ~ 1 and 5 keV) near the center of the SNR. The central region is dominated by emission from highly ionized Fe-rich ejecta. We estimate a conservative upper limit to the total Fe ejecta mass of the SNR, MFe < 0.27 M☉. Comparisons with standard SN nucleosynthesis models suggest that this Fe mass limit is consistent with a Type II SN explosion for the origin of Sgr A East. On the other hand, the soft X-ray emission extending toward the north of the SNR can be described by a single-temperature (kT ~ 1.3 keV) thermal plasma with normal chemical composition. This portion of the SNR is thus X-ray emission from the heated interstellar medium rather than the metal-rich stellar ejecta. We point out that a hard pointlike source CXOGC J174545.5-285829 (the so-called cannonball) at the northern edge of the SNR shows X-ray characteristics unusual among other Galactic center sources. The morphological, spectral, and temporal characteristics of this source suggest identification as a high-velocity neutron star. Based on the suggested Type II origin for the SNR Sgr A East and the proximity between the two, we propose that CXOGC J174545.5-285829 is a high-velocity neutron star candidate, born from the core-collapse SN that also created the SNR Sgr A East.

Supernova Light-Curve Models for the Bump in the Optical Counterpart of X-Ray Flash 030723

XRF 030723 is the first X-ray flash (XRF) to show in its optical light curve (LC) a bump that has been interpreted as the signature of a supernova (SN). After subtracting the afterglow component from the observed optical LC of the XRF counterpart, the properties of the putative SN are constrained by means of synthetic LCs of core-collapse SNe. For the redshift range $z \sim 0.3$ -- 1, all possible models require a rather small mass of synthesized \Nifs, i.e. $M$(\Nifs) $\sim$ 0.01 -- 0.3 $\Msun$. The models used to describe the energetic SNe Ic associated with gamma-ray bursts (SNe 1998bw and 2003dh) are too massive for the observed LC. If the relation between ejected \Nifs mass and total ejecta mass established from models of various Type Ic SNe also holds for the putative SN in XRF 030723, the ejecta mass is constrained to be $\sim$ 1 -- 3 $\Msun$ and the kinetic energy $\lsim 1\times 10^{52}$ erg. This corresponds to a progenitor with $15\Msun \lesssim M_{\rm MS}\lesssim 25\Msun$. The SN therefore appears to have properties intermediate between a normal SN Ic like SN 1994I and a more energetic object like SN 2002ap.

Iron-rich Ejecta in the Supernova Remnant DEM L71

Chandra X-ray observations of DEM L71, a supernova remnant (SNR) in the Large Magellanic Cloud (LMC), reveal a clear double shock morphology consisting of an outer blast wave shock surrounding a central bright region of reverse-shock heated ejecta. The abundances of the outer shock are consistent with LMC values, while the ejecta region shows enhanced abundances of Si, Fe, and other species. However, oxygen is not enhanced in the ejecta; the Fe/O abundance ratio there is more than 5 times the solar ratio. Based on the relative positions of the blast wave shock and the contact discontinuity in the context of SNR evolutionary models, we determine a total ejecta mass of approximately 1.5 solar masses. Ejecta mass estimates based on emission measures derived from spectral fits are subject to considerable uncertainty due to lack of knowledge of the true contribution of hydrogen continuum emission. Maximal mass estimates, i.e., assuming no hydrogen, result in 1.5 solar masses of Fe and 0.24 solar masses of Si. Under the assumption that an equal quantity of hydrogen has been mixed into the ejecta, we estimate 0.8 solar masses of Fe and 0.12 solar masses of Si. These characteristics support the view that in DEM L71 we see Fe-rich ejecta from a Type Ia SN several thousand years after explosion.

Critical-point phase separation in laser ablation of conductors.

Laser ablation due to an ultrashort laser pulse on a massive aluminum target was investigated by means of a one-dimensional fluid code. Clear separation between the ablated matter and the unablated target is seen to occur through spinodal decomposition involving thermodynamic instabilities near the critical point of aluminum. The code also shows that the end of the ablation process is preceded by the ejection of droplets, which form about 15% of the total ejected mass.

The spectrum of Nova Casssiopeia 1995

We observed the spectrum of Nova Cas 1995 using the 2.16-m telescope (dispersion 50 Å/mm) over the years 1995–1997. The spectra of 1995 had as main characteristic features Fe II and H I emission lines, all showing the P Cygni profile. The spectra of 1996 mainly had emission in H I and He II. The spectra of 1997 showed strong emission in [Fe X], [Fe VII] and [O III], showing that the nova had entered the nebular stage. Using the observed continuum we estimated the effective temperature of the ejected mass as follows: 21300 K on 1995-10-14, 9700 K on 1996-10-26, 10200 K on 1997-12-16. Using the measured line flux the total ejected mass was estimated to be 4.7 × 10 −5 M ⊙, and the helium abundance, N He N H = 0.047 .

Aspherical Explosion Models for SN 1998bw/GRB 980425

The recent discovery of the unusual supernova SN 1998bw and its apparent correlation with the γ-ray burst GRB 980425 has raised new issues concerning both γ-ray bursts and supernovae. Although the spectra of SN 1998bw resemble those of Type Ic supernovae (SNe Ic), there are distinct differences. At early times the expansion velocities inferred by the Doppler shift of (unidentified) absorption features were very high, and SN 1998bw was unusually bright and red at maximum light (Galama et al.). These distinctions make SN 1998bw a candidate of a hypernova, with explosion energies exceeding supernovae by a factor in excess of 10. We present an alternative picture that allows SN 1998bw to have an explosion energy and ejecta mass consistent with core-collapse supernovae, although at the bright end of the typical range. We specifically propose that all SNe Ic are significantly asymmetric and that SN 1998bw is a SN Ic that is distinguished principally by being viewed close to the symmetry axis. We investigate the hypothesis that SNe Ic and SN 1998bw are the results of aspherical explosions along the rotational axis of basically spherical, nondegenerate C/O cores of massive stars. Light curves for aspherical explosions are computed assuming an ellipsoidal geometry for the ejecta. Guided by the polarization observations of normal SN Ic and related events, we assume an axis ratio of 2 near maximum light. The evolution of the isodensity contours with time is discussed. We show that the light curve of SN 1998bw may be understood with an explosion energy of 2 × 1051 ergs, a total ejecta mass of 2 M☉, and a mass of 56Ni of 0.2 M☉ if it is observed at a large angle (≥60°) with respect to the plane of symmetry. In this picture, the high expansion velocities are a direct consequence of an aspherical explosion mechanism which, in turn, produces oblate isodensity contours. Prolate isodensity contours are ruled out. This interpretation suggests that the fundamental core-collapse explosion process itself is strongly asymmetric.