P. Bouchet

Dust Production and Particle Acceleration in Supernova 1987A Revealed with ALMA

Supernova (SN) explosions are crucial engines driving the evolution of galaxies by shock heating gas, increasing the metallicity, creating dust, and accelerating energetic particles. In 2012 we used the Atacama Large Millimeter/Submillimeter Array to observe SN 1987A, one of the best-observed supernovae since the invention of the telescope. We present spatially resolved images at 450 mu m, 870 mu m, 1.4 mm, and 2.8 mm, an important transition wavelength range. Longer wavelength emission is dominated by synchrotron radiation from shock-accelerated particles, shorter wavelengths by emission from the largest mass of dust measured in a supernova remnant (>0.2 M-circle dot). For the first time we show unambiguously that this dust has formed in the inner ejecta (the cold remnants of the exploded stars core). The dust emission is concentrated at the center of the remnant, so the dust has not yet been affected by the shocks. If a significant fraction survives, and if SN 1987A is typical, supernovae are important cosmological dust producers.

The bolometric light curve of SN 1987A. I. Results from ESO and CTIO U to Q0 photometry

The UV, optical, and IR (UVOIR) bolometric luminosity curve of SN 1987A was derived from ESO, CTIO, and NASA Kuiper Airborne Observatory spectrophotometry for days 1-903 since outburst. It is found that the sum of this UVOIR flux and the high-energy flux predicted by models is consistent with the energy liberated by 0.071 solar mass of Co-56, with no need for additional energy sources for days 126-903 since outburst. By day 400, the flux at wavelegths larger than 5 microns was found to increase rapidly, and by day 650, the UVOIR flux shifted from the optical to a thermal IR source with a temperature of 200-300 K. The optical colors began to fade more rapidly at the time the FIR flux increased, consistent with dust formation local to the supernova. 61 refs.

Infrared and X-Ray Evidence for Circumstellar Grain Destruction by the Blast Wave of Supernova 1987A

Multiwavelength observations of supernova remnant 1987A show that its morphology and luminosity are rapidly changing at X-ray, optical, infrared (IR), and radio wavelengths as the blast wave from the explosion expands into the circumstellar equatorial ring, produced by mass loss from the progenitor star. The observed IR radiation arises from the interaction of dust grains that formed in mass outflow with the soft X-ray-emitting plasma component of the shocked gas. Spitzer Infrared Spectrograph spectra at 5-30 μm taken on day 6190 since the explosion show that the emission arises from ~1.1 × 10−6 M☉ of silicate grains radiating at a temperature of ~180−15+20 K. Subsequent observations on day 7137 show that the IR flux had increased by a factor of 2 while maintaining an almost identical spectral shape. The observed IR-to-X-ray flux ratio (IRX) is consistent with that of a dusty plasma with standard Large Magellanic Cloud dust abundances. IRX has decreased by a factor of ~2 between days 6190 and 7137, providing the first direct observation of the ongoing destruction of dust in an expanding supernova blast wave on dynamic timescales. Detailed models consistent with the observed dust temperature, the ionization timescale of the soft X-ray emission component, and the evolution of IRX suggest that the radiating silicate grains are immersed in a 3.5 × 106 K plasma with a density of (0.3–1) × 104 cm−3 and have a size distribution that is confined to a narrow range of radii between 0.023 and 0.22 μm. Smaller grains may have been evaporated by the initial UV flash from the supernova.

SN?1987A after 18 Years: Mid-Infrared Gemini and Spitzer Observations of the Remnant

Using the Gemini South 8 m telescope, we obtained high-resolution 11.7 and 18.3 μm mid-IR images of SN 1987A on day 6526 since the explosion. All the emission arises from the equatorial ring. Nearly contemporaneous spectra obtained at 5-38 μm with the Spitzer Space Telescope show that this is thermal emission from silicate dust that condensed out in the red giant wind of the progenitor star. The dust temperature is 166 K, and the emitting dust mass is 2.6 × 10-6 M☉. Comparison of the Gemini 11.7 μm image with Chandra X-ray images, HST UV-optical images, and ATCA radio synchrotron images shows generally good correlation across all wavelengths. If the dust resides in the diffuse X-ray-emitting gas then it is collisionally heated. The IR emission can then be used to derive the plasma temperature and density, which were found to be in good agreement with those inferred from the X-rays. Alternatively, the dust could reside in the dense UV-optical knots and be heated by the radiative shocks that are propagating through the knots. In either case the dust-to-gas mass ratio in the CSM around the supernova is significantly lower than that in the general interstellar medium of the LMC, suggesting either a low condensation efficiency in the wind of the progenitor star or the efficient destruction of the dust by the SN blast wave. Overall, we are witnessing the interaction of the SN blast wave with its surrounding medium, creating an environment that is rapidly evolving at all wavelengths.

X-ray illumination of the ejecta of supernova 1987A

When a massive star explodes as a supernova, substantial amounts of radioactive elements—primarily 56Ni, 57Ni and 44Ti—are produced. After the initial flash of light from shock heating, the fading light emitted by the supernova is due to the decay of these elements. However, after decades, the energy powering a supernova remnant comes from the shock interaction between the ejecta and the surrounding medium. The transition to this phase has hitherto not been observed: supernovae occur too infrequently in the Milky Way to provide a young example, and extragalactic supernovae are generally too faint and too small. Here we report observations that show this transition in the supernova SN 1987A in the Large Magellanic Cloud. From 1994 to 2001, the ejecta faded owing to radioactive decay of 44Ti as predicted. Then the flux started to increase, more than doubling by the end of 2009. We show that this increase is the result of heat deposited by X-rays produced as the ejecta interacts with the surrounding material. In time, the X-rays will penetrate farther into the ejecta, enabling us to analyse the structure and chemistry of the vanished star.

Evolution and Geometry of Hot Spots in Supernova Remnant 1987A

We present ground-based near-infrared imaging and Hubble Space Telescope optical imaging and spectroscopy of the interaction between the ejecta of SN 1987A and its equatorial circumstellar ring. This interaction has made a transition, from emission originating in just a few hot spots at restricted locations in position angle around the ring, to a collision producing optical emission over a nearly continuous distribution, with few breaks larger than 45°. The centroids of the first three spots are measured to move at 2000-3000 km s-1, which we interpret as a lower limit of the velocity of the forward blast front. Multiwavelength light curves of the spots show that they do not evolve uniformly and change significantly on timescales as short as 1 month; in particular, the first spot shows a significant break in its light curve. Implications of observed delays between spot appearances are discussed, which leads to a generalized model of hot spot evolution and suggests that the early appearance of the first hot spot is explained by its inward radial position and a fairly uniform forward blast wave, rather than extraordinary physical circumstances. Data further suggest that the forward blast is reaching the bulk of the inner ring material to the east, the density of which appears higher than elsewhere in the ring. We study the ring geometry, finding evidence suggestive of an intrinsic ellipticity of 0.95, and find lower and upper distance limits of 47.9 ± 0.92 and 54.4 ± 2.1 kpc, respectively.

Evidence for Type Ia Supernova Diversity from Ultraviolet Observations with the Hubble Space Telescope

We present ultraviolet (UV) spectroscopy and photometry of four Type Ia supernovae (SNe 2004dt, 2004ef, 2005M, and 2005cf) obtained with the UV prism of the Advanced Camera for Surveys on the Hubble Space Telescope. This data set provides unique spectral time series down to 2000 A. Significant diversity is seen in the near-maximum-light spectra (~2000-3500 A) for this small sample. The corresponding photometric data, together with archival data from Swift Ultraviolet/Optical Telescope observations, provide further evidence of increased dispersion in the UV emission with respect to the optical. The peak luminosities measured in the uvw1/F250W filter are found to correlate with the B-band light-curve shape parameter Δm 15(B), but with much larger scatter relative to the correlation in the broadband B band (e.g., ~0.4 mag versus ~0.2 mag for those with 0.8 mag 3σ), being brighter than normal SNe Ia such as SN 2005cf by ~0.9 mag and ~2.0 mag in the uvw1/F250W and uvm2/F220W filters, respectively. We show that different progenitor metallicity or line-expansion velocities alone cannot explain such a large discrepancy. Viewing-angle effects, such as due to an asymmetric explosion, may have a significant influence on the flux emitted in the UV region. Detailed modeling is needed to disentangle and quantify the above effects.

Homogeneous J, H, K, L photometry of a sample of BL Lac objects

Homogeneous J, H, K, L, photometry of 42 BL Lacertae objects is presented. The observations cover a period of 3 yr with typical time interval of 6 months. We show that the near-IR nonthermal emission of all the objects but two is well described by a single power law (f ν ∞ν −α ) after accounting for host galaxy contribution. In spite of large flux variability, changes of spectral index are small and rare. In the few cases where significant variations of α are observed, no evidence of correlation with the flux level is found.

Oblateness, radius, and mean stratospheric temperature of Neptune from the 1985 August 20 occultation

Abstract The occultation of a bright ( K ∼6) infrared star by Neptune revealed a central flash at two stations and provided accurate measurements of the limb position at these and several additional stations. We have fitted this data ensemble with a general model of an oblate atmosphere to deduce the oblateness e and equatorial radius a 0 of Neptune at the 1-μbar pressure level, and the position angle p n of the projected spin axis. The results are e =0.0209±0.0014, a 0 =25269±10 km, p n =20.1°±1°. Parameters derived from fitting to the limb data alone are in excellent agreement with parameters derived from fitting to central flash data alone (E. Lellouch, W.B. Hubbard, B. Sicardy, F. Vilas, and P. Bouchet, 1986, Nature 324, 227–231) , and the principal remaining source of uncertainty appears to be the Neptune-centered declination of the Earth at the time of occultation. As an alternative to the methane absorption model proposed by Lellouch et al ., we explain an observed reduction in the central flash intensity by a decrease in temperature from 150 to 135°K as the pressure rises from 1 to 400 μbar. Implications of the oblateness results for Neptune interior models are briefly discussed.

EVOLUTION OF THE FU ORIONIS OBJECT BBW 76

We have carried out a long-term photometric and spectroscopic monitoring program of the southern FU Orionis–type object BBW 76 spanning the period from 1982 to 1997. BBW 76 has the same radial velocity as the small cloud toward which it is projected, and for which a kinematic distance of about 1.8 kpc has been derived. We have determined a large reddening of E(B-V) ~ 0.7 for BBW 76. Optical and infrared spectra show the change toward later spectral type with increasing wavelength characteristic of FU Orionis stars and indicative of a hot luminous disk. High-resolution echelle spectra of BBW 76 show P Cygni profiles with extended blueshifted absorption troughs at the Hα and sodium lines from a neutral, supersonic wind. Comparison of such spectra obtained at six different epochs between 1985 and 1997 reveals major changes in these Hα and sodium line profiles. For a period of 10 years from 1985, the massive absorption troughs diminished in extent and depth, until by 1994 they had all but disappeared, while at the same time the blueshifted emission peak in the Hα line increased markedly in strength. However, when observed in 1997, the absorption had increased again and the emission had diminished. We interpret this in terms of an extended period during which accretion through a circumstellar disk decreased, with a resulting decrease in wind production. But the increased activity by 1997 shows that this is not a constant decay and that the star was not about to revert to its presumably original T Tauri stage. We monitored the star with optical photometry from 1983 to 1994, during which period it decreased almost monotonically in brightness by 0.2 mag in V. Infrared J, H, and K photometry from 1983 to 1991 shows a period of monotonic fading between 1984 and 1988, followed by more irregular behavior. In a search of the Harvard plate archives we have found a plate from the year 1900 on which BBW 76 is seen at approximately its present brightness, certainly not 2 mag brighter as expected if the optical decline between 1983 and 1994 had persisted during the whole century. Also, a plate taken for the Franklin-Adams charts in 1927 again shows BBW 76 at approximately the same brightness. This historical light curve makes BBW 76 the FU Orionis star with the longest-documented period in a high state. Overall, the observations suggest that BBW 76 is virtually identical to the prototype of its class, FU Orionis itself, in all respects except that BBW 76 has not shown the regular fading that FU Orionis has displayed after its eruption in 1936. This may be due to continued replenishment of the circumstellar accretion disk.