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Register a new userSpitzer observations of V4332 Sagittarii: Detection of Alumina Dust
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D.P.K. Banerjee
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We present broad-band 24, 70 and 160 micron photometry, 5-35 micron and 55-90 micron spectra of the eruptive variable V4332 Sgr from Spitzer observations. The distinguishing feature of the 5-35 micron spectrum is an unusually broad absorption feature near 10 micron at the position generally associated with silicate-rich dust. Through radiative transfer modeling, we show that this broad feature cannot arise from silicates alone but requires the inclusion of alumina (Al2O3) as a dust condensate. The case for including Al2O3 is strengthened further by the presence of the AlO radical, a potentially important molecule in forming Al2O3. The present detection indicates that porous alumina manifests itself through a broadening of the 9.7 micron silicate feature and additionally displays, on the shoulder of the silicate feature, a component at ~11.5 micron. We discuss how further observations of V4332 Sgr may have the potential of verifying some general predictions of the dust condensation process.
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An eruptive nova-like event took place in 1994 in the stellar-merger candidate V4332 Sgr. Following the eruption, dust consisting of refractory silicate rich dust grains containing a significant component of AlO bonding was formed sometime between 1998 and 2003. Observations using Spitzer between 2005 and 2009 show significant changes in the 10 micron silicate stretch feature. There is a deepening of the 10 micron silicate stretch as well as the development of a feature between about 13 and 20 microns consistent with a blend of the MgO and FeO stretching features and the O-Si-O bending mode of increasingly ordered silicate dust. Near-infrared observations show the presence of AlO and water vapor in the outflow in 2003, 2004 and 2005: the AlO has significantly decreased in spectra obtained in 2014 while the water vapor remains largely unchanged. An attempt is made to correlate these observations and understand the significance of these changes using DUSTY modeling. The observations appear consistent with the kinetically-controlled, condensation of highly under-oxidized SiO/AlO/Fe/Mg dust grains in the outflow followed by the continuous evolution of the initial condensate due to thermal annealing and oxidation of the dust via reaction with ambient O, OH and H2O in the expanding, cooling shell. Periodic monitoring of this dust shell over the mid-infrared spectral range could yield useful information on the evolution of under-oxidized silicate condensates exposed to hot water vapor in more conventional circumstellar environments.
We present four MIPS (24 micron) and two IRAC (3.6, 4.5, 5.8, and 8.0 micron) Spitzer observations of the newly discovered Tremendous Outburst Amplitude Dwarf nova (TOAD) Var Her 04 during decline from super-outburst. The four MIPS observations span 271 days and the two IRAC observations span 211 days. Along the line-of-sight to Var Her 04, there is a foreground M-star within 1arcsec of the variable; as a result, all of the Spitzer photometry presented in this paper is a blend of the foreground M-star and Var Her 04. We estimate the quiescent level of the TOAD to be $Delta V=4-5$ magnitudes below that of the M-star. Based upon the spectral energy distribution and the 2MASS colors, we find the M-star to be an M3.5V dwarf at a distance of 80-130 pc. Based upon its outburst amplitude and quiescent apparent magnitude, we estimate the distance to Var Her 04 to be 200-400 pc, suggesting that the line-of-sight foreground star is physically unrelated to the cataclysmic variable. All of the Spitzer photometry is consistent with the photospheric emission of the line-of-sight M3.5V star, except for one 24 micron observation obtained after the variable re-brightened. This 24 micron flux density is 75 $mu$Jy ($4sigma$) above the preceding and following MIPS observations. We tentatively suggest that the mid-infrared brightening of 75 $mu$Jy may be associated with a dust formation event in the super-outburst ejecta. Assuming a dust temperature of 100-400 K, we have estimated the amount of dust required. We find $10^{-13}-10^{-11}$ M$_odot$ of dust is needed, consistent with amounts of mass ejection in TOADs expected during super-outburst, and possibly making TOADs important contributors to the recycling of the interstellar medium.
We present optical and infrared spectroscopy of V1309 Sco, an object that erupted in 2008 in a stellar-merger event. During the outburst, V1309 Sco displayed characteristics typical of red transients, a class of objects similar to V838 Mon. Our observations were obtained in 2009 and 2012, i.e. months and years after the eruption of V1309 Sco, and illustrate severe changes in the remnant, mainly in its circumstellar surroundings. In addition to atomic gas observed in earlier epochs, we identified molecular bands of TiO, VO, H$_2$O, ScO, AlO, and CrO. The infrared bands of CrO we analyse are the first astronomical identification of the features. Over the whole period covered by our data, the remnant was associated with a cool ($lesssim$1000 K) outflow with a terminal velocity of about 200 km/s. Signatures of warmer atomic gas, likely to be still dissipating the energy of the 2008 outburst, dramatically decreased their brightness between 2009 and 2012. In addition, the source of optical continuum disappeared sometime before 2012, likely owing to the formation of new dust. The final stage of V1309 Scos evolution captured by our spectra is an object remarkably similar to an older red transient, V4332 Sgr. In addition to providing a detailed view on the settling of the eruptive object, the observations presented here reinforce the conclusion that all the Galactic red transients are a manifestation of the same phenomenon, i.e. a stellar merger. The late spectra of V1309 Sco also suggest peculiarities in the chemical composition of the remnant, which still need to be explored.
Far-infrared Spitzer observations of elliptical galaxies are inconsistent with simple steady state models of dust creation in red giant stars and destruction by grain sputtering in the hot interstellar gas at T ~ 10^7 K. The flux at 24 microns correlates with optical fluxes, suggesting that this relatively hot dust is largely circumstellar. But fluxes at 70 and 160 microns do not correlate with optical fluxes. Elliptical galaxies with similar L_B have luminosities at 70 and 160 microns (L_70 and L_160) that vary over a factor ~ 100, implying an additional source of dust unrelated to that produced by ongoing local stellar mass loss. Neither L_70/L_B nor L_160/L_B correlate with the stellar age or metallicity. Optical line fluxes from warm gas at T ~ 10^4 K correlate weakly with L_70 and L_160, suggesting that the dust may be responsible for cooling this gas. Many normal elliptical galaxies have emission at 70 microns that is extended to 5-10 kpc. Extended far-infrared emission with sputtering lifetimes of ~10^8 yrs is difficult to maintain by mergers with gas-rich galaxies. Instead, we propose that this cold dust is buoyantly transported from reservoirs of dust in the galactic cores which is supplied by mass loss from stars in the core. Intermittent energy outbursts from AGNs can drive the buoyant outflow.
We present Spitzer observations of the unusual variable V838 Monocerotis. Extended emission is detected around the object at 24, 70 and 160um. The extended infrared emission is strongly correlated spatially with the HST optical light echo images taken at a similar epoch. We attribute this diffuse nebulosity to be from an infrared light echo caused by reprocessed thermal emission from dust heated by the outward-propagating radiation from the 2002 eruption. The detection of an IR light echo provides an opportunity to estimate the mass in dust of the echo material and hence constrain its origin. We estimate the dust mass of the light echo to be on the order of a solar mass - thereby implying the total gas plus dust mass to be considerably more - too massive for the echo material to be the ejecta from previous outburst/mass-losing events. This is therefore suggestive that a significant fraction of the matter seen through the light echo is interstellar in origin. Unresolved emission at 24 and 70um is also seen at the position of the central star possibly indicating the presence of hot dust freshly condensed in the outburst ejecta.
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