Near-Infrared Spectroscopy of the Cassiopeia A and Kepler Supernova Remnants

Near-infrared spectra (0.95 - 2.4 micron) of the Cassiopeia A and Kepler supernova remnants (SNRs) are presented. Low-dispersion (R = 700) spectra were obtained for five bright fast-moving ejecta knots (FMKs) at two locations on the main shell and for three bright circumstellar knots (QSFs) near the southwest rim of Cas A. The main shell FMKs in Cas A exhibit a sparse near-infrared spectrum dominated by [S II] 1.03 micron emission with a handful of other, fainter emission lines. Among these are two high-ionization silicon lines, [Si VI] 1.96 micron and [Si X] 1.43 micron, which have been detected in AGNs and novae but never before in a supernova remnant. The near-infrared spectra of circumstellar QSFs in Cas A show a much richer spectrum, with strong He I 1.083 micron emission and over a dozen bright [Fe II] lines. Observed [Fe II] line ratios indicate electron densities of 5 - 9 * 10^4 cm^-3 in the QSFs. The Cas A QSF data are quite similar to the observed spectrum of a bright circumstellar knot along the northwest rim of the Kepler SNR, which also shows strong He I and [Fe II] emission with a measured electron density of 2.5 - 3 * 10^4 cm^-3. Finally, we present J- and K-band images of Cas A. The K-band image shows faint diffuse emission which has no optical or mid-infrared counterpart but is morphologically similar to radio continuum maps and may be infrared synchrotron radiation

Hubble Space Telescope WFPC2 Imaging of Cassiopeia A

The young galactic supernova remnant Cassiopeia A was imaged with Wide Field Planetary Camera 2 (WFPC2) aboard the Hubble Space Telescope through filters selected to capture the complete velocity range of the remnant\u27s main shell in several emission lines. The primary lines detected, along with the specific WFPC2 filters used, were [O III] λλ4959, 5007 (F450W), [N II] λ6583 (F658N), [S II] λλ6716, 6731 + [O II] λλ7319, 7330 + [O I] λλ6300, 6364 (F675W), and [S III] λλ9069, 9532 (F850LP). About three-quarters of the remnant\u27s 4\u27 diameter main shell was imaged with all four filters in three WFPC2 pointings, with most remaining shell regions imaged in just the F675W filter via three additional pointings. Considerable detail is observed in the reverse-shocked ejecta with typical knot scale lengths of 02–04 (1–2 × 1016 cm). Both bright and faint emission features appear highly clumped with little in the way of a smooth, diffuse emission component detected. Strong differences in [S III] and [O III] line intensities, indicating chemical abundance differences, are also seen, particularly in knots located along the bright northern limb and near the base of the northeastern jet. A line of curved overlapping filaments in the remnant\u27s northwestern rim appears to mark the location of the remnant\u27s reverse shock front in this region. The morphology of some finger-like ejecta structures elsewhere suggest cases in which the reverse shock front is encountering the remnant\u27s clumped ejecta. Large velocity shears (1000 km s-1), possibly associated with the formation of these Rayleigh-Taylor–like features, are found in the line profiles of several emission lines (e.g., [S III] λλ9069, 9532 and [Cl II] λ8679) in ground-based, optical spectra of knots near the remnant\u27s center. The [N II] images of the remnant\u27s circumstellar knots (QSFs) reveal them to be 01–06 thick knots and filaments, often with diffuse edges facing away from the center of expansion. Three-color composite images of the whole remnant and certain sections, along with individual filter enlargements of selected regions of the bright optical shell, are presented and discussed

Carbon Monoxide in type II supernovae

Infrared spectra of two type II supernovae 6 months after explosion are presented. The spectra exhibit a strong similarity to the observations of SN 1987A and other type II SNe at comparable epochs. The continuum can be fitted with a cool black body and the hydrogen lines have emissivities that are approximately those of a Case B recombination spectrum. The data extend far enough into the thermal region to detect emission by the first overtone of carbon monoxide. The molecular emission is modeled and compared with that in the spectra of SN 1987A. It is found that the flux in the CO first overtone is comparable to that found in SN 1987A. We argue that Carbon Monoxide forms in the ejecta of all type II SNe during the first year after explosion.Comment: 6 pages, 6 figures, accepted for publications in A&

Dust Formation in Very Massive Primordial Supernovae

At redshift z>5 Type II supernovae (SNII) are the only known dust sources with evolutionary timescales shorter than the Hubble time. We extend the model of dust formation in the ejecta of SNII by Todini & Ferrara (2001) to investigate the same process in pair-instability supernovae (PISN), which are though to arise from the explosion of the first, metal free, very massive (140-260 Msun) cosmic stars. We find that 15%-30% of the PISN progenitor mass is converted into dust, a value >10 times higher than for SNII; PISN dust depletion factors (fraction of produced metals locked into dust grains) range between 0.3 and 0.7. These conclusions depend very weakly on the mass of the PISN stellar progenitor, which instead affects considerably the composition and size distribution. For the assumed temperature evolution, grain condensation starts 150-200 days after the explosion; the dominant compounds for all progenitor masses are SiO2 and Mg2SiO4 while the contribution of amorphous carbon and magnetite grains grows with progenitor mass; typical grain sizes range between 0.001 and a few 0.1 micron and are always smaller than 1 micron. We give a brief discussion of the implications of dust formation for the IMF evolution of the first stars, cosmic reionization and the intergalactic medium.Comment: 10 pages, 8 figures, accepted for publication in MNRA

Detection of CO and Dust Emission in Near-Infrared Spectra of SN 1998S

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

Low Carbon Abundance in Type Ia Supernovae

We investigate the quantity and composition of unburned material in the outer layers of three normal Type Ia supernovae (SNe Ia): 2000dn, 2002cr and 20 04bw. Pristine matter from a white dwarf progenitor is expected to be a mixture of oxygen and carbon in approximately equal abundance. Using near-infrared (NIR, 0.7-2.5 microns) spectra, we find that oxygen is abundant while carbon is severely depleted with low upper limits in the outer third of the ejected mass. Strong features from the OI line at rest wavelength = 0.7773 microns are observed through a wide range of expansion velocities approx. 9,000 - 18,000 km/s. This large velocity domain corresponds to a physical region of the supernova with a large radial depth. We show that the ionization of C and O will be substantially the same in this region. CI lines in the NIR are expected to be 7-50 times stronger than those from OI but there is only marginal evidence of CI in the spectra and none of CII. We deduce that for these three normal SNe Ia, oxygen is more abundant than carbon by factors of 100 - 1,000. MgII is also detected in a velocity range similar to that of OI. The presence of O and Mg combined with the absence of C indicates that for these SNe Ia, nuclear burning has reached all but the extreme outer layers; any unburned material must have expansion velocities greater than 18,000 km/s. This result favors deflagration to detonation transition (DD) models over pure deflagration models for SNe Ia.Comment: accepted for publication in Ap