V. H. Tyne

Infrared spectroscopy of Nova Cassiopeiae 1993 — IV. A closer look at the dust

Nova Cassiopeiae 1993 (V705 Cas) was an archetypical dust-forming nova. It displayed a deep minimum in the visual light curve, and spectroscopic evidence for carbon, hydrocarbon and silicate dust. We report the results of fitting the infrared (IR) spectral energy distribution (SED) with the DUSTY code, which we use to determine the properties and geometry of the emitting dust. The emission is well described as originating in a thin shell whose dust has a carbon:silicate ratio of 2:1 by number (∼ 1.26:1 by mass) and a relatively flat size distribution. The 9.7- and 18-μm silicate features are consistent with freshly condensed dust and, while the lower limit to the grain size distribution is not well constrained, the largest grains have dimensions ∼0.06 μm; unless the grains in V705 Cas were anomalously small, the sizes of grains produced in nova eruptions may previously have been overestimated in novae with optically thick dust shells. Laboratory work by Grishko & Duley may provide clues to the apparently unique nature of nova unidentified infrared (UIR) features.

The Spitzer Infrared Spectrometer view of V4334 Sgr (Sakurai's Object)

We present an observation of the very late thermal pulse object V4334 Sgr (Sakurai’s Object) with the Infrared Spectrometer (IRS) on the Spitzer Space Telescope. The emission from 5‐ 38 μm is dominated by the still-cooling dust shell. A number of features are seen in absorption against the dust shell, which we attribute to HCN and polyyne molecules. We use these features to determine the 12 C/ 13 C ratio for the absorbing gas to be ∼3.2 +3.2; this implies that, despite the H-content of the molecules, the hydrocarbon-bearing gas must have originated in material produced in the very late thermal pulse. We see no evidence of emission lines, despite the recently-reported optical and radio observations that suggest the effective temperature of the stellar remnant is rising.

Silicate Dust in the Environment of RS Ophiuchi following the 2006 Eruption

We present further Spitzer Space Telescope observations of the recurrent nova RS Ophiuchi, obtained over the period 208-430 days after the 2006 eruption. The later Spitzer IRS data show that the line emission and free-free continuum emission reported earlier is declining, revealing incontrovertible evidence for the presence of silicate emission features at 9.7 and 18 μm. We conclude that the silicate dust survives the hard radiation impulse and shock blast wave from the eruption. The existence of the extant dust may have significant implications for understanding the propagation of shocks through the red giant wind and likely wind geometry.

CO bands in V4334 Sgr (Sakurai's Object): The

We present the results of our analysis of a high resolution (R ~ 30000) infrared spectrum of V4334 Sgr (Sakurais Object) around the first overtone CO bands, obtained in 1998 July. The 12CO and 13CO bands are well-resolved, and we compute synthetic hydrogen-deficient model atmosphere spectra to determine the 12C/13C ratio. We find 12C/13C ~ 4 +/-1, consistent with the interpretation of V4334 Sgr as an object that has undergone a very late thermal pulse.

\mathsf{^{12}}

Infrared spectroscopy and photometry have revealed the remarkableevolution of Sakurais Object from 1996 to the present. A cooling,carbon-rich photospheric spectrum was observable from 1996 to 1998.Considerable changes occured in 1998 as the continuum reddened due toabsorption and emission by newly formed dust located outside thephotosphere. In addition, a strong and broad helium 1.083 μm P Cygniline developed, signifying the acceleration of an outer envelope ofmaterial to speeds as high as 1000 km s-1. At the same time thephotosphere of the central star remained quiescent. By 1999 thephotosphere was virtually completely obscured by the dust and the heliumemission line was the only detectable spectral feature remaining in the1–5 μm band. In 2000 emission by dust has become even more dominant,as the envelope continues to expand and cool and the helium line weakens.

C/

We present UKIRT UIST spectra of Sakurai’s Object (= V4334 Sgr) showing CO fundamentalband absorption features around 4.7 µm. The line centres are at a heliocentric radial velocity of −170 ± 30 km s −1 . The number and relative strengths of the lines indicate a CO gas temperature of 400 ± 100 K and a CO column density of 7 +3 −2 × 10 17 cm −2 . The gas was moving away from the central star at an average speed of ∼290 ± 30 km s −1 in 2003 September. The lines appeared sometime between mid-1999 (well after the opaque dust shell formed) and mid-2000, and may have been somewhat more blueshifted initially than they are now. The observed CO velocity and temperature indicate the continued presence of a fast wind in the object, previously seen in the He I 1.083-µm line beginning just prior to massive dust formation, and more recently in atomic and ionized lines. The dust continuum is consistent with a temperature of 350 ± 30 K, indicating continued cooling of the shell. The similar CO temperature suggests that the bulk of the CO absorption occurs just outside the dust continuum surface.

\mathsf{^{13}}

We present a search for 1 2 CO and SiO emission in V838 Mon. We derive upper limits on the 1 2 CO and SiO emission, and on the total mass of the circumstellar material. Using an excitation temperature of 1000 K, and a likely distance of 6 kpc, the non-detection of 1 2 CO emission leads to a 3σ upper limit on the mass of gas around the central star to be 10 - 2 M O ..

C ratio

We report the results of monitoring of V4334 Sgr (Sakurai’s Object) at 450µm and 850µm with scuba on the James Clerk Maxwell Telescope. The flux density at both wavelengths has increased dramatically since 2001, and is consistent with continued cooling of the dust shell in which Sakurai’s Object is still enshrouded, and which still dominates the near-infrared emission. Assuming that the dust shell is optically thin at sub-millimetre wavelengths and optically thick in the near-infrared, the sub-millimetre data imply a mass-loss rate during 2003 of �3.4 ± 0.2 × 10 −5 M⊙ yr −1 for a gas-todust ratio of 75. This is consistent with the evidence from 1 5µm observations that the mass-loss is steadily increasing.

The Infrared Evolution of Sakurai's Object

Nova V705 Cas (1993) was an archetypical dust‐forming nova. It displayed a deep minimum in the visual light curve, and spectroscopic evidence for carbon, hydrocarbon and silicate dust. We report preliminary results of work in progress, in which we use the dusty code to determine the properties of the dust. Our results have implications for the rate of grain growth in the ejecta, and for the origin of the UIR features; the latter likely arise in hydrogenated amorphous carbon grains rather than in free‐flying PAH molecules.