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The Infrared Evolution of Dust in V838 Monocerotis

['C. E. Woodward', 'Minnesota Institute For Astrophysics', 'University Of Minnesota', 'Church Street Se', 'Minneapolis', 'Mn', 'Usa', 'Http', 'A. Evans', 'Astrophysics Group']

Date: 2022-01

The process of mass loss and dust condensation is unknown for mergers, and the synoptic study (temporal periods of several 100 s of days to 10 s of years) of V838 Mon may provide the constraints to confront observations with theoretical predictions. The nature and dynamic evolution of the dust that forms in the material ejected by a stellar merger is not well understood. Observations of V4332 Sgr (another proposed stellar merger) suggest that the grains have a alumina component (Banerjee et al. 2007, 2015), which would be consistent with the strong AlO features in oxygen-rich environments.

Figure 3. Continuum subtracted SOFIAFORCAST spectra of V838 Mon from 5.0–8.0 μm. A second-order Chebyshev polynomial fit using wavelengths from 5.2–5.6 μm, which are on the Rayleigh–Jeans tail of the stellar 2300 K blackbody, was used to determine the local continuum. Features are indicated by the arrows.

From 5.0–8.0 μm (Figure 1 (a)), the SED is a composite of emission from the Rayleigh–Jeans tail of the stellar 2300 K blackbody and emission from a cooler dust component. Superposed on the continuum there are a few suggestive emission features. Figure 3 shows the continuum subtracted residual emission, highlighting potential emission features. Features near 6.30 μm and 6.85 μm have been associated with water vapor emission (ν 2 bands) and formaldehyde (H 2 CO) in spectra of the dense circumstellar disk environments of T Tauri stars (Sargent et al. 2014 ). Higher spectral resolution observations with instruments like EXES (Richter et al. 2018 ) on SOFIA or MIRI on the James Webb Space Telescope (JWST) are necessary to confirm these identifications.

The 8–14 μm segment of the SED of V838 Mon is complex, being dominated by dust features, including a deep silicate absorption band centered near 10 μm, an amorphous alumina (Al 2 O 3 ) emission feature near ∼11.3 μm, and a 13 μm feature that may be evidence of high temperature spinel (MgAl 2 O 4 ; Posch et al. 1999 ; Zeidler et al. 2013 ). However, the measured FWHM of this latter feature (λ o = 13.53 μm) is of an order of ∼0.1 μm, which is much narrower than the bandwidth measurements of high temperature (300 ≲ T(K) ≲ 928) spinels (Table 8 in Zeidler et al. 2013 ). In addition, expected weaker 32 μm spinel emission bands are not evident in the SOFIA data. Measurement of the 10 μm feature depth, τ 9.7 is challenged by regions of poor atmospheric transmission in the SOFIA SED. A more detailed discussion and modeling of the SED is discussed below (Section 3.3 ).

The SOFIA spectra (Figure 1 ) exhibit interesting details regarding the SED of V838 Mon. At wavelengths longwards of 18 μm the spectra are devoid of any strong line emission from hydrogen, helium, or forbidden lines such as [O iv] 25.91 μm, which is a strong coolant present in the late evolution of novae when electron densities in the ejecta are less than ≃10 6 –10 7 cm −3 (Evans & Gehrz 2012 ; Helton et al. 2012 ; Gehrz et al. 2015 ). No molecular absorption bands or broad features from dust are evident. For example broad amorphous silicate dust emission near 18 μm or (Mg, Fe)O features near 19.5 μm (Posch et al. 2002 ) are not present.

The 2019 SOFIA SED is similar to that observed by Spitzer in 2008 only at wavelengths ≥18.0 μm as shown in Figure 4 (b). The flux density in the 9 to 14 μm region, Figure 4 (b) observed in 2019 by SOFIA has decreased by ∼38% and exhibits a distinct broad (Δλ ≃ 2.2 μm) emission band, a very distinct 10 μm absorption feature as illustrated in Figure 4 (c).

Figure 4. Evolution of the infrared spectral energy distribution of V838 Mon. (a) The 2005 Spitzer IRS spectra (blue symbols) and the Spitzer 2008 IRS spectra (red symbols) show the slowly evolving spectral energy distribution, including the emergence of silicate emission bands, especially in the 10 μm region between the two epochs. (b) Comparison of the composite 2019 SOFIA 5–36 μm FORCAST spectra (blue symbols) and the 2008 Spitzer 10–40 μm IRS spectra (red symbols). Gaps in the SOFIA SED are due to noncontiguous spectral coverage of the FORCAST grisms. (c) Same as (b) but highlighting the the region between 9 and 14 μm in detail, which shows the marked change of structure in the 10 μm feature.

Spitzer spectra between 2005 and and 2008 show that the mid-IR SEDs is evolving as shown in Figure 4 (a). In 2005 the SED is smooth with little evidence for any broad dust emission features; a blackbody fit to the SED yields a dust temperature of T bb = 425 ± 1.2 K. No 10 μm feature is evident although it is difficult to draw a definite conclusion because the spectrum is saturated below 10 μm. Three years later, the SED has markedly evolved, broad emission features are present, and at long wavelengths (λ ≥ 20 μm) the SED has a contribution from a cooler dust component.

Grids of simple models that varied the relative ratios of these two grain components were constructed, adopting a Mathis–Rumpl–Nordsieck (Mathis et al. 1977 ) grain-size distribution, N(a) ∝ a −q with q = 3.5, and a grain-size range of 5.0 × 10 −3 ≤ a grain (μm) ≤ 2.5 × 10 −1 . The input radiation field was represented by a single 2300 K Planckian (blackbody) source commensurate with a L3 supergiant, having an effective temperature of 2300 K (Loebman et al. 2015 ). A spherical shell of dust was illuminated by this source, where the dust temperature at the shell inner boundary was set at 400 K having a dust density distribution described by a inverse power law (α = 2, assuming a constant wind scenario) with the shell extending 2.5 times the inner radius (Y = 2.5). Added to this was a disk illuminated the same source with the temperature at the outer disk+envelope boundary set to 25 K, with no accretion. The grids also comprised a range of optical depths, specified at 0.55 μm, varying in step size from 0.01–0.1 spanning 0.5 ≤ τ 0.55 ≤ 5.0. A bolometric flux (scaling factor) of 3.1 × 10 −11 W cm −2 was adopted (see Appendix discussion in Jurkic & Kotnik-Karuza 2012 ).

For the modeling two grain compositions were considered: silicate grains with optical properties described by Draine & Lee ( 1984 ); and amorphous, porous alumina (Begemann et al. 1997 ). These are bare grains, with no ice coatings. Typical spectral indicators for ice-mantled grains are not seen in V383Mon. The water ice feature at 3.05 μm (due to and O–H stretch mode) is not seen (see Figure 2 in Lynch et al. 2004 ) nor is the 6.02 μm H–O–H bending mode detected in archival Spitzer low-resolution IRS spectra (which are unsaturated at λ ≲ 8 μm) shortly after outburst. Inspection of the SOFIA spectra near 6.02 μm (Figure 3 ) also shows no signature of a broad ice absorption feature.

To characterize the observed changes due to dust formation and evolution in V838 Mon, we modeled the system using the radiative transfer code DUSTY-DISK, which is similar to the original DUSTY code (Ivezic & Elitzur 1995 ), but incorporates an additional disk component, appropriate for the case of V838 Mon.

Initial analysis of the mid-infrared observational data of V838 Mon suggest that the SED of the system at present can be explained as the sum of at least two components. The first is a cool central star at ∼2300 K which is likely the central remnant of the merger. The SED of this component behaves as a blackbody at 2300 K with a dusty envelope of modest optical depth such that the emergent radiation, modeled under assumptions of spherical geometry plus a disk, reasonably (in the χ2 sense) reproduces the SEDs. The second component is emission from dust in the disk+envelope.

The emergence of a 10 μm dust feature was first observed in the 2008 Spitzer observations. Clearly, this silicate emission feature at ∼10 μm has arisen newly formed in the intervening 3 yr since 2005 (Figure 4). This 10 μm feature could arise from a combination of silicate dust (peaking at 9.7 μm) and alumina dust (peaking at 11.3 μm). More sophisticated RT modeling is required to robustly conclude whether the feature is composed purely of silicates or alumina or a combination of both, and to determine the constraints on the grain-size distribution power law.

Figure 5(a) shows the DUSTY-DISK models with range of silicate-to-alumina ratio mixes (Si:Alumina) with τ 0.55 = 1.50, which illustrate how variation in the dust grain components alter the the shape and structure of the model SED. The best fit to the shape of the 10 μm region at this epoch is one with Si:Alumina = 0.2:0.8, suggesting that alumina dust dominates at this epoch (2008). In order to account for additional emission longward of 20 μm a third component contributing to the overall model emission was necessary. This component is characterized by thermal continuum emission likely from dust with a T bb ≃ 170 K, as shown in Figure 5(b). This cooler component may be associated with the cool circumstellar material detected by ALMA (Kamiński et al. 2021). The sum of these three components gives a reasonable overall fit to the SED. However, the plateau between 13 and 15 μm in the observed Spitzer spectra could not be adequately reproduced by any combination of grain composition or size distributions.

Figure 5. DUSTY-DISK models of V838 Mon 2008 Spitzer spectrum. The Spitzer spectrum is depicted by the blue dots, while the dereddend optical and infrared photometry are the black squares. (a) Representative sample of grid models illustrating the effect on varying the silicate-to-alumna dust ratios at a fixed optical depth τ 0.55 = 1.50. (b) Best-fit model composite spectra (solid green line) that includes a grain mixture ratio Si:Alumina = 0.2:0.8, emission from 2300 K blackbody representing the stellar emission (red solid curve), and a third contribution to the composite SED from a ≃170 K blackbody (cyan line). The latter is necessary to account for the observed continuum emission at wavelengths ≳20 μm. The dereddended optical photometry is given by the black squares. Download figure: Standard image High-resolution image

The evolution of the 10 μm region of the SED of V838 Mon has continued and demonstrates that the chemistry of the circumstellar environment has changed over approximately the last decade. The 2019 SOFIA spectrum shows that the emission plateau from 13–15 μm has developed into a deep trough, the width of the broad 10 μm feature has narrowed (Δλ ≲ 2.0 μm) becoming more distinct, while an apparent absorption feature shortward of 10 μm is seen (Figure 4(c)). This absorption may be a signature of silicates or more likely an artifact caused by imperfect removal of the deep telluric feature that lies between 8 and 10 μm (Figure 1(b)). In other oxygen-rich environments, a deep 9.7 μm absorption feature is attributed to SiO materials and the depth of the feature indicates that silicates may now be the dominant grain component.

In other merger-system nova likes that have dusty circumstellar envelopes, such as V1309 Sco, the broad spectral feature at 9.7 μm is attributed to silicate grain solid-state absorption (Nicholls et al. 2013). Our models of V838 Mon require amorphous silicates and the observed SED suggests a dust absorption feature indicative of an optically thick circumstellar environment is present. Following the arguments discussed in Nicholls et al. (2013) an upper limit to the column density in V838 Mon can be derived from the observed depth of the 9.7 μm feature (upper limit of ∼2.4 × 10−12 W m−2) and the best-fit model continuum (∼4.6 × 10−12 W m−2) at the same wavelength, which yields an optical depth τ 9.7 ∼ 0.3. Using values of Q ext and Q scat for "astronomical silicate" taken from Draine (1985) and assuming amorphous silicate grains with radii a between 0.1 and 3.0 μm (the upper limit to a is set by the transition to a regime were the 9.7 μm feature is suppressed; see Figure 5 in Laor & Draine 1993) leads to a derived column density of between ∼8 × 108 cm−2 to 2 × 1010 cm−2.

The rise of silicates also is supported by the shape and strength of the broad 10 μm band emission. This speculation is confirmed by DUSTY-DISK modeling of the 2019 SOFIA composite spectra, as shown in Figure 6. Models that best reproduce the 10 μm feature are those where the Si:Alumina ratio is now at least 50:50, with a slight decrease in the optical depth to τ 0.55 = 1.44. A cooler third component with T bb = 125 K and a wavelength dependent emissivity ∝ λ−2 at wavelengths ≳10 μm are thought to be present. The observed spectral evolution indicates that processing of the dust in V838 Mon is occurring, perhaps similar to that in the environs of V1309 Sco (Nicholls et al. 2013).

Figure 6. DUSTY-DISK models of V838 Mon SOFIA2019 spectrum. The SOFIA spectra are depicted by the blue dots, while the dereddend optical and infrared photometry are the black squares. The SOFIA photometry (Table 2) is indicated by the orange squares. Reasonable fit to the observed spectrum is achieved with a silicate-to-alumina ratio of order 50:50, optical depth τ 0.55 = 1.44. The model composite spectra (solid green line) includes model emission from the grain mixture, a 2300 K blackbody representing the stellar emission (red solid curve), and requires a third contribution to the composite SED from a ≃125 K emissivity modified ( λ ∝ λ−2) blackbody (cyan line). The latter contribution is necessary to account for the observed continuum emission at wavelengths ≳20 μm. Download figure: Standard image High-resolution image

Clearly, there is a temporal evolution of this 10 μm feature with the relative strengths of the silicate and alumina components evolving with time in a manner consistent with the chaotic silicate hypothesis of mineral condensation described by Nuth & Hecht (1990) or other models based on thermodynamically controlled evolution (Speck et al. 2000). The model fits (Figures 5 and 6) while not totally satisfactory, do permit two possible interpretations. First, there was a significant amount of alumina in the 10 μm feature when this feature first developed. Modeling suggests that the Si:Alumina ratio was ≳0.5. This would be observational evidence which supports the prediction that alumina should be the first dust condensate in an O-rich environment. Evidence for this prediction is rarely offered because most objects studied (e.g., Miras, AGB stars, etc.) are millions of years old. In the present case one is seeing this event happen in freshly condensed dust and almost in real time. The temporal sequence of dust evolution in V383Mon may be a rare validation of mineralogical condensation sequences (Nuth & Hecht 1990) occurring in O-rich environments, perhaps only seen before in V4332 Sgr (Banerjee et al. 2007).

Alternatively, one could conclude that the Sil:Alumina ratio changed between 2008 and 2019. It appears that the silicate fraction within the dust population has increased by ≳30%. The increase of silicate with time, at the expense of alumina, can be explained as follows (e.g., Nuth & Hecht 1990; Stencel et al. 1990): in the initial stages, the higher reduction of Al with respect to Si leads to the preferential formation of Al–O bonds at the expense of Si–O bonds. This implies that the infrared bands of alumina associated with the Al–O stretching mode should be prominent early in the formation of the chaotic silicates. However, as the Al atoms become fully oxidized, the higher abundance of Si will make the 9.7 μm band associated with Si–O bonds dominate.

The presence of alumina as a dust component is not surprising. V838 Mon is known to exhibit strong photospheric B-X infrared bands of AlO in the infrared (Banerjee & Ashok 2002; Evans et al. 2003; Lynch et al. 2004), which are present even today (as seen in the recent SOFIA spectra discussed herein). Aluminum oxide is likely to play a significant role in the route to Al 2 O 3 formation. LTE calculations by Gail & Sedlmayr (1999) show that any possible nucleation species that can go on to form dust around stars should begin with a monomer with exceptionally high bond energy. The AlO monomer satisfies this criterion and is thus a favored candidate to lead to the formation of larger Al m O n clusters that serve as nucleation sites for the formation of other grains or to alumina grains themselves by homogeneous nucleation.

We have not considered in our model the effect of ongoing processes such as annealing of the dust or grain growth. Annealing of silicate grains can change the optical constants of the grain significantly, as shown by the study of Hallenbeck et al. (2000) and this can result in changes in the shape and peak of the silicate profile. This point becomes relevant when comparing the evolution of dust features across different epochs. The physics of grain growth in the expanding ejecta of novae, where the radiation field may be similar to the early conditions in V838 Mon, is explored by Shore & Gehrz (2004).

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