The Infrared Evolution of Dust in V838 Monocerotis

The FORCAST scientific data products were retrieved from the Infrared Processing and Analysis Center (IPAC) Infrared Science Archives (IRSA) after standard pipeline processing and flux calibration was performed (for details see Clarke et al. 2015). Computed atmospheric transmission models for the flight altitudes (which are contained in the data products) were used to mask out grism data points in wavelength regions where the transmission was less than 70%. Spectra with grisms G063 and the G111 were obtained on two separate and distinct flight missions. However, the spectral energy distributions (SEDs) did not vary in shape or average intensity between the flights (i.e., the source was not detected to be varying on a timescale of ≲72 hr) and the difference in the spectral calibration were within the pipeline CALERR (systematic) uncertainties. Hence these data were averaged into a single spectrum for each grating. Figure 1 presents panels for each individual grating segment, spanning their respective spectral free range, to illustrate details of the observed SED.

Images of V838 Mon were obtained on two different nights (see Table 1). Azimuthally averaged radial profiles of V838 Mon in each filter exhibited some evidence of extended emission beyond the point-spread function (PSF) of point sources observed with FORCAST under optimal telescope jitter performance in each filter. ¹² The mean full width half maximum (FWHM) of the azimuthally averaged radial profiles of the V838 Mon image data was 300 ±027 (i.e., 3.91 ±0.35 pixels). Centroiding on the photocenter of V838 Mon, photometry in an effective circular aperture of diameter of 1075 (i.e., a photometric aperture equivalent to ≃3 × the observed source FWHM), with a background aperture annulus of inner radius 12 pixels (922) and outer radius of 17 pixels (1301) was performed on the Level 3 pipeline coadded (*.COA) image data products using the Aperture Photometry Tool (APT v2.4.7; Laher et al. 2012). Sky-annulus median subtraction (ATP Model B as described in Laher et al. 2012) was used in the computation of the source intensity. The random source intensity uncertainty was computed using a depth of coverage value equivalent to the number of coadded image frames. The calibration factors (and associated uncertainties) applied to the resultant aperture sums were included in the Level 3 data distribution and were derived from the weighted average calibration observations of α Cet or α Tau. The resultant SOFIA photometry is presented in Table 2.

Table 2. V838 Mon Photometry ^a

	Flux	Flux
Filter	Density
(μm)	(Jy)	(×10−12 W m−2)
7.7	5.912 ± 0.059	2.302 ± 0.023
11.2	19.416 ± 0.074	5.197 ± 0.020
19.7	38.454 ± 0.088	5.852 ± 0.013
31.5	26.161 ± 0.077	2.490 ± 0.007
37.1	17.527 ± 0.266	1.416 ± 0.021

Note.

^a Measured in a circular aperture with a diameter of 1075 centroided on the photocenter of V838 Mon in each SOFIA FORCAST image at a given filter.

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Figure 2 provides 3072 × 3072 postage-stamp gray-scale images with superimposed surface brightness contours. Generally the images are point like, although the 7.7 through 19.7 μm images are slightly elongated at low surface brightness, with a position angle (PA; East of North) of ∼56°. This elongation may be associated with bipolar lobes of dust emission, perpendicular to the flattened-disk (derived major axis size 23 mas at 8 μm and 70 mas at 13 μm) structure interferometrically detected from 8–13 μm by Chesneau et al. (2014), which has a major axis PA of −10°. Given the FORCAST platescale and the beam FWHM (which has telescope jitter effects), SOFIA would not be able to directly detect such a structure even at λ ≥ 30 μm. However, the SOFIA elongation in the low-surface-brightness emission is similar in the position angle to that of the SiO maser emission channel velocity maps observed after 2018 November 20 (Ortiz-León et al. 2020). The 11.2 μm SOFIA image also has a secondary source (likely a background source) 1260 to the southwest of V838 Mon with a flux density (in a 1075 diameter aperture) of 1.23 ± 0.07 Jy.

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To study the long-term spectral evolution of the circumstellar material, reduced archival Spitzer Infrared Spectrograph (IRS; Houck et al. 2004) high-resolution spectra (optimal difference extraction of nod1 and nod2) of V838 Mon also were retrieved from the Combined Atlas of Sources with Spitzer IRS Spectra ¹³ (Lebouteiller et al. 2015, 2011) from high-spectral resolution observations conducted on 2005 March 17.6139 (AORKey 10523136) and 2008 December 10.0133 (AORKey 2543355).

Subsequent to outburst and initial decline of the light curve toward quiescence, optical BVIR photometry of V838 Mon acquired since JD 245 7648.91 (2016 Sept 17) through JD 245 933.53 (2021 April 26) in the AAVSO ¹⁴ database (Kafka 2020) show that there has been little (≲0.5 mag) change in the light curve; it has remained essentially flat at all bands.

V838 Mon was observed with The Nordic Optical Telescope's near-infrared camera and spectrograph (NOTCam), using itshigh-resolution camera (0079 per pixel⁻¹; Abbott et al. 2000) the broadband filters J, H, and K_s filters on 2020 March 02, JD 245 8911.43715 (the midpoint of the observations), a few months after the 2019 SOFIA flights. Photometric calibration was performed using three 2MASS stars in the field of view of the images and standard fields at a similar airmass observed just before the target. Standard infrared imaging reduction techniques using IRAF ¹⁵ and apertures photometry (70 circular diameter). The error (in magnitudes) is dominated by the uncertainty in the calibration stars.

The observed near-infrared photometry, J = 6.59±0.05, H = 5.55±0.05, K_s = 4.76±0.05, and AAVSO photometry from JD 245 8879.565 of B 15.65 ± 0.07, V = 13.31 ± 0.04, R = 11.47 ± 0.03, and I = 9.47 ± 0.01 were dereddened adopting an E(B − V) = 0.87 (see Loebman et al. 2015) with a standard galactic extinction curve (Rieke & Lebofsky 1985). The dereddened photometric data are used later in the analysis to constrain the SED at short wavelengths.

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 ≃10⁶–10⁷ 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 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 (Al₂O₃) emission feature near ∼11.3 μm, and a 13 μm feature that may be evidence of high temperature spinel (MgAl₂O₄; 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).

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 (ν₂ bands) and formaldehyde (H₂CO) 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.

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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.

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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).

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.

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.

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).

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.

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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).

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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 Al₂O₃ 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_mO_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).