Near-infrared (NIR) spectra of the subluminous Type Ia supernova SN 1999by are presented which cover the time evolution from about 4 days before to 2 weeks after maximum light. Analysis of these data was accomplished through the construction of an extended set of delayed detonation (DD) models covering the entire range of normal to subluminous SNe Ia. The explosion, light curves (LC), and the time evolution of the synthetic spectra were calculated self-consistently for each model with the only free parameters being the initial structure of the white dwarf (WD) and the description of the nuclear burning front during the explosion. From these, one model was selected for SN 1999by by matching the synthetic and observed optical light curves, principly the rapid brightness decline. DD models require a minimum amount of burning during the deflagration phase which implies a lower limit for the 56 N i mass of about 0.1M ⊙ and consequently a lower limit for the SN brightness. The models which best match the optical light curve of SN 1999by were those with a 56

HST ACS images of the young SN remnant Cas A are used to explore the expansion and spatial distribution of its highest velocity debris. Proper motions of over 1800 outlying ejecta knots are reported. The distribution of transverse expansion velocities for these knots shows a striking bipolar asymmetry with the highest velocity knots confined to nearly opposing northeast and southwest `jets'. The jets appear kinematically and chemically distinct with respect to the remnant's highest velocity debris seen in other directions. Significant gaps in the spatial distribution of outlying ejecta lie in directions which are approximately perpendicular to the jets. Extrapolations of 9 month proper motions for all outer ejecta knots and a subsample of 72 bright and compact knots suggest explosion dates (assuming no knot deceleration) of 1662 +/- 27 and 1672 +/- 18, respectively. We find some evidence for non-uniform deceleration in different directions with knots located along the northwestern limb among the least decelerated ejecta suggesting a convergence date of 1681 +/-19. The remnant's central X-ray point source lies some $7''$ to the southeast of the estimated expansion center (PA = 169 deg) indicating a projected motion of ~350 km/s toward the middle of the broad southern outer ejecta knot gap.Comment: 13 pages, 5 figures, ApJ, in pres

We present observations of the Type Ia supernova 2003du obtained with the Hobby-Eberly Telescope and report the detection of a high-velocity component in the Ca ii infrared triplet near 8000 8, similar to features previously observed in SN 2000cx and SN 2001el. This feature exhibits a large expansion velocity (%18,000 km s À1 ), which is nearly constant between À7 and +2 days relative to maximum light and disappears shortly thereafter. Other than this feature, the spectral evolution and light curve of SN 2003du resemble those of a normal SN Ia. We consider a possible origin for this high-velocity Ca ii line in the context of a self-consistent spherical delayed-detonation model for the supernova. We find that the Ca ii feature can be caused by a dense shell formed when circumstellar material of solar abundance is overrun by the rapidly expanding outermost layers of the SN ejecta. Model calculations show that the optical and infrared spectra are remarkably unaffected by the circumstellar interaction and the resulting shell. In particular, no hydrogen lines are detectable in either absorption or emission after the phase of dynamic interaction. The only qualitatively different features in the model spectra are the strong, highvelocity feature in the Ca ii IR triplet around 8000 8 and a somewhat weaker O i feature near 7,300 8. The Doppler shift and time evolution of these features provides an estimate for the amount of accumulated matter (decreasing Doppler shift with increasing shell mass) and also an indication of the mixing within the dense shell. For high shell masses (%5 ; 10 À2 M ), the high-velocity component of the Ca ii line merges with the photospheric line forming a broad feature. A cutoff of the blue wings of strong, unblended lines (particularly the Si ii feature at about 6,000 8) may also be observable for larger shell masses. The model SN Ia light curves are little effected except at very early times when the shell is partially optically thick because of Thomson scattering, resulting in larger (BÀV ) colors by up to 0.3 mag. We apply these diagnostic tools to SN 2003du and infer that about 2 ; 10 À2 M of solar abundance material may have accumulated in a shell prior to the observations. Furthermore, in this interpretation, the early light-curve data imply that the circumstellar material was originally very close to the progenitor system, perhaps from an accretion disk, Roche lobe, or common envelope. Because of the observed confinement of Ca ii in velocity space and the lack of ongoing interaction inferred from the light curve, the matter cannot be placed in the outer layers of the exploding white dwarf star or related to a recent period of high mass loss in the progenitor system prior to the explosion. We note that the signatures of circumstellar interaction could be rather common in Type Ia supernovae and may have eluded discovery because optical spectra often do not extend significantly beyond 7500 8.

Near-infrared spectra (0.95 -- 2.4 micron) of the peculiar Type IIn supernova 1998S in NGC 3877 from 95 to 355 days after maximum light are presented. K-band data taken at days 95 and 225 show the presence of the first overtone of CO emission near 2.3 micron, which is gone by day 355. An apparent extended blue wing on the CO profile in the day 95 spectrum could indicate a large CO expansion velocity (~2000 -- 3000 km/s). This is the third detection of infrared CO emission in nearly as many Type II supernovae studied, implying that molecule formation may be fairly common in Type II events, and that the early formation of molecules in SN 1987A may be typical rather than exceptional. Multi-peak hydrogen and helium lines suggest that SN 1998S is interacting with a circumstellar disk, and the fading of the red side of this profile with time is suggestive of dust formation in the ejecta, perhaps induced by CO cooling. Continuum emission that rises towards longer wavelengths (J -> K) is seen after day 225 with an estimated near-infrared luminosity >~ 10^40 erg/s. This may be related to the near-infrared excesses seen in a number of other supernovae. If this continuum is due to free-free emission, it requires an exceptionally shallow density profile. On the other hand, the shape of the continuum is well fit by a 1200 +- 150 K blackbody spectrum possibly due to thermal emission from dust. Interestingly, we observe a similar 1200 K blackbody-like, near-infrared continuum in SN 1997ab, another Type IIn supernova at an even later post-maximum epoch (day 1064+). A number of dust emission scenarios are discussed, and we conclude that the NIR dust continuum is likely powered by the interaction of SN 1998S with the circumstellar medium.Comment: 38 Pages, 12 Figures, Submitted to The Astronomical Journa

We present mid-infrared (5.2Y15.2 m) spectra of the Type Ia supernovae (SNe Ia) 2003hv and 2005df observed with the Spitzer Space Telescope. These are the first observed mid-infrared spectra of thermonuclear supernovae, and show strong emission from fine-structure lines of Ni, Co, S, and Ar. The detection of Ni emission in SN 2005df 135 days after the explosion provides direct observational evidence of high-density nuclear burning forming a significant amount of stable Ni in a SN Ia. The SN 2005df Ar lines also exhibit a two-pronged emission profile, implying that the Ar emission deviates significantly from spherical symmetry. The spectrum of SN 2003hv also shows signs of asymmetry, exhibiting blueshifted [Co iii], which matches the blueshift of [ Fe ii ] lines in nearly coeval near-infrared spectra. Finally, local thermodynamic equilibrium abundance estimates for the yield of radioactive 56 Ni give M56 Ni % 0:5 M , for SN 2003hv, but only M56 Ni % 0:13Y0:22 M for the apparently subluminous SN 2005df, supporting the notion that the luminosity of SNe Ia is primarily a function of the radioactive 56 Ni yield. The observed emission-line profiles in the SN 2005df spectrum indicate a chemically stratified ejecta structure, which matches the predictions of delayed detonation ( DD) models, but is entirely incompatible with current three-dimensional deflagration models. Furthermore, the degree that this layering persists to the innermost regions of the supernova is difficult to explain even in a DD scenario, where the innermost ejecta are still the product of deflagration burning. Thus, while these results are roughly consistent with a delayed detonation, it is clear that a key piece of physics is still missing from our understanding of the earliest phases of SN Ia explosions. Subject headingg s: supernovae: general -supernovae: individual (SN 2003hv, SN 2005df )