Characterization of Thermal Infrared Dust Emission and Refinements to the Nucleus Properties of Centaur 29P/Schwassmann-Wachmann 1

We present analyses of Spitzer observations of 29P/Schwassmann-Wachmann 1 using 16 $\mu$m IRS"blue"peak-up (PU) and 24 $\mu$m and 70 $\mu$m MIPS images obtained on UT 2003 November 23 and 24 that characterize the Centaur's large-grain (10-100 $\mu$m) dust coma during a time of non-outbursting"quiescent"activity. Estimates of $\epsilon f \rho$ for each band (16 $\mu$m (2600 $\pm$ 43 cm), 24 $\mu$m (5800 $\pm$ 63 cm), and 70 $\mu$m (1800 $\pm$ 900 cm)) follow the trend between nucleus size vs. $\epsilon f \rho$ that was observed for the WISE/NEOWISE comet ensemble. A coma model was used to derive a dust production rate in the range of 50-100 kg/s. For the first time, a color temperature map of SW1's coma was constructed using the 16 $\mu$m and 24 $\mu$m imaging data. With peaks at $\sim$ 140K, this map implies that coma water ice grains should be slowly sublimating and producing water gas in the coma. We analyzed the persistent 24 $\mu$m"wing"(a curved southwestern coma) feature at 352,000 km (90$''$) from the nucleus attributed by Stansberry et al. (2004) to nucleus rotation and instead propose that it is largely created by solar radiation pressure and gravity acting on micron sized grains. We performed coma removal to the 16 $\mu$m PU image in order to refine the nucleus' emitted thermal flux. A new application of the Near Earth Asteroid Thermal Model (NEATM; Harris 1998) at five wavelengths (5.730 $\mu$m, 7.873 $\mu$m, 15.80 $\mu$m, 23.68 $\mu$m, and 71.42 $\mu$m) was then used to refine SW1's effective radius measurement to $R = 32.3 \pm 3.1$ km and infrared beaming parameter to $\eta = 1.1 \pm 0.2$, respectively.

Note-a The position angle is measured counter clockwise from north through east.
µm images, suggesting that the same particles are being measured in both bandpasses. Overall, aside from the slight increase in dust emission on the south-southeast side of the coma, as indicated by the division of an azimuthal average enhanced image (Figure 1(b)), the coma is lacking any defining coma morphology. A faint linear feature can be seen from approximately the 1 o'clock to 7 o'clock positions.

Thermal Infrared Coma Analysis
Thermal infrared imaging of cometary dust comae allows for preferential probing of grain sizes on the order of microns and larger, such as those recorded with the IRS PU and MIPS, because smaller grains with 2πa/λ < 1, where a is the grain radius, are inefficient emitters in the infrared (see Hanner et al. 1994;Lisse et al. 1998Lisse et al. , 2004. Our Spitzer 16 µm, 24 µm, and 70 µm images were analysed to characterize the continuum emission created by µm-sized and larger grains in SW1's quiescent dust coma. We note that micron and sub-micron sized grains also contain silicate emission bands between ∼ 8 − 13 µm and at ∼ 20 µm, which probably contribute a few percent to the flux in the 24 µm images (see Schambeau et al. 2015, Figures 13 and 14). However, a detailed analysis of these emission features and their relatively minor impacts on the 24 µm imaging is beyond the scope of our current work.
In this section we take advantage of these thermal infrared images in combination with Spitzer's stable and well characterized point spread function (PSF) in order to accurately isolate SW1's dust coma flux contributions in each image. We assumed that the dominant grain sizes contributing to the detected flux in each band were approximately the size of their effective monochromatic bandpass wavelengths: 15.80 µm, 23.68 µm, and 71.42 µm (as used by e.g., Bauer et al. (2015Bauer et al. ( , 2017a).
To aid in the analysis of comae morphology, it is useful to reference an idealized "canonical" coma, containing an isotropic and steady state emission of dust grains from the nucleus, with negligible dust grain fragmentation and solar radiation pressure. This canonical coma has a surface brightness profile following a 1/ρ behavior (where ρ is the skyplane projected conetocentric distance from the nucleus's position), and is assumed in the derivation of the often used Af ρ and f ρ parameters (A'Hearn et al. 1984;Kelley et al. 2013;Lisse et al. 2002) that are described in more detail in Section 3.1.2. In practice the assumptions used to derive f ρ break down for real comae, but its calculation provides a first order estimate of comae dust production behaviors. SW1 experienced quiescent activity for at least two months surrounding the UT 2003 Nov. epoch of Spitzer observations, based on Minor Planet shows an increased brightness in the south-west direction, possibly indicating preferential sunward emission.
Also present are a more compact curved feature initially directed towards the south-southwest, curving towards the south-east and a linear feature from 1 o'clock to 7 o'clock similar to that in the 16 µm image.
Center (MPC) reported magnitude measurements 2 , so the canonical coma assumption is reasonable for these observations.

16 µm and 24 µm Coma Morphology
The 16µm blue PU images ( Fig. 1) were obtained 1.3 days before the 24 µm MIPS images ( Fig.   2), which were obtained on UT 2003-11-24 15:05. The 16 µm image's coma did not display any clearly distinguishable large scale radial or azimuthal features in either the un-enhanced or enhanced images. A slight enhancement on the south-southeast through south-west side of the coma is detected in the division by azimuthal average and the 1/ρ-removed enhanced images (Figure 1(b) and (c)).
This is further confirmed in Figure 3, which displays radial surface-brightness profiles for position angles (PA) at 45 • spacings for the 16µm image. The radial profiles were generated by taking the median pixel value at a given radial position using 10 • wide wedges center on the indicated PA. For comparison, each PA plot includes a radial profile for a scaled STINYTIM generated point spread function (PSF; Krist (2006)) representing how a detection of SW1's bare nucleus would behave in the absence of a coma. A C/ρ n functional form was fit to the profiles for ρ values between 14 and 30 for each PA (i.e., beyond any significant influence from the nucleus point source contribution), where C is a scaling constant representing the peak coma flux near the nucleus and n is the power index of the coma's profile. The fitted profile power law indices are listed in Table 2. Profiles for PAs spanning from the south-through-west directions, approximately centered on the projected sunward direction, have nearly canonical 1/ρ coma profiles whereas profiles in the northeast have profile powers of approximately n = 2. This asymmetric profile behavior is consistent with preferential emission of grains in the sunward direction (south-west).  Table 2. For reference, included in each plot are two black lines representing a 1/ρ and 1/ρ 2 coma behavior. The "roller coaster" shaped profile for the PSF is the result of the Airy diffraction pattern of the space-based telescope.
The overall coma morphology as seen in the unenhanced 24 µm ( Fig. 2(a)) image similarly shows an increased brightness in the southwest direction. This is further confirmed by the division by an azimuthal average and 1/ρ-removed enhanced images. The rotational-shift-differenced enhanced image ( Fig. 2(d)) contains a curved wing feature that Stansberry et al. (2004) attribute to a rotating jet and from which they derived an ∼ 60 day rotation period for SW1's nucleus. Taking into consideration the great similarity in the 16 µm and 24 µm image morphology taken 1.3 days apart, and the relationship between the projected nucleus-Sun vector and the curved wing's structure suggests that this feature is possibly not the result of nucleus rotation, but is instead due to solar radiation pressure effects on micron sized dust grains emitted in the sunward direction being turned back to form the dust tail in the north-east direction (Farnham & Schleicher 2005;Li et al. 2014;Mueller et al. 2013). While the ∼ 60 day rotation period derived by the earlier work may in fact coincidentally be reflective of SW1 potentially possessing a long rotation period (Miles et al. 2016;Schambeau et al. 2017Schambeau et al. , 2019, we propose that this curved wing feature is not the result of a slowly rotating nucleus.
Interestingly, the wing would be symmetric around the skyplane projected nucleus-Sun axis for the case of isotropic emission from a localized nucleus surface area. Instead it is asymmetric, indicating a possible preferential direction for dust lofting from this source region.
Similar asymmetric curved-shapes features have long been seen in broadband visible imaging data of SW1 while undergoing major outbursts. Accounts of these coma morphologies have been reported in the early works of Jeffers (1956) and Roemer (1958). Whipple (1980) presents a detailed analysis of SW1's outburst coma morphology as detected over a 50 year baseline, resulting in the descriptive term of "ringtailed snorter" for this often seen curved shape feature. While it may at first seem appropriate to compare the outburst and quiescent coma morphologies, detailed analyses of SW1's dust coma while in both phases of activity (Hosek et al. 2013;Miles et al. 2016;Schambeau et al. 2017Schambeau et al. , 2019 have provided descriptions of the underlying processes ongoing in both phases of activity and that the two are different. The morphology of the 24 µm quiescent coma's wing may resemble that of SW1's outburst coma; however, it was produced by different mechanisms (i.e., slow, sustained dust lofting with expansion velocities in the range of 10-50 m/s while quiescent (Jewitt 1990) vs. impulsive short lived dust emission at high velocities in the 100-300 m/s range during major outbursts (Feldman 1995;Schambeau et al. 2017Schambeau et al. , 2019Trigo-Rodríguez et al. 2010).) The outer edge of the wing feature seen in the 24 µm image in the south-west direction ( Fig. 2 where, ρ g is the projected sky-plane turn back distance of the dust grains, γ is the angle between the initial direction of the dust grains and the sky-plane, β is the ratio of radiation pressure acceleration to acceleration due to solar gravity, α is the solar phase angle of the observations, and g = GM /R 2 H is the solar gravitational acceleration on the dust grains (G is the gravitational constant, M is the Sun's mass and R H is the heliocentric distance of the dust grains). We estimate a β value based on equations from Finson & Probstein (1968) and Fulle (2004): where C pr is a collection of constants equal to 3E /(8πcGM ), where E is the Sun's mean radiation.
The parameter Q pr is the scattering efficiency for radiation pressure for a dust grain of diameter d. Burns et al. (1979) provide a thorough description of Q pr and explain that a value of Q pr ≈ 1 is appropriate for the assumed d = 24 µm grains here. We use a value for the dust grain bulk density based on recent spacecraft visited comae in situ measurements: ρ d = 500 kg/m 3 (Fulle et al. 2016).
With these assumptions we arrive at an estimated value of β = 0.096. The exact value for γ of the dust grains most dominantly contributing to the wing feature is unknown. Most probably it is the result of dust grains emitted over a continuum of angles. For this reason we calculate the outflow velocity for a range of sky-plane projected dust grain angles: A similar radial surface brightness profile analysis for the 24 µm image is shown in Figure 4. The overall appearance of the coma morphology is similar to that seen in the 16 µm image, however the larger field of view (FOV) and higher S/N coma detection in the 24 µm image allows a more detailed investigation of the underlying processing ongoing within the dust coma. A change in slope of the profiles at a cometocentric distance of ∼ 130 for PAs between 0 -180 • is suggestive of possible ongoing fragmentation for larger grains out to a projected cometocentric distance of 520,000 km (i.e., ∼ 130 ). This view is supported by the coma profile's power law index being shallower than −1 interior to 520,000 km, suggesting an overabundance of dust grains interior to this projected distance when compared to a canonical steady-state dust emission. This behavior is possibly explained by a process of larger grains emitted from the nucleus and their subsequent fragmentation as they expand in the coma, or possibly from the decreasing size via sublimation of larger icy grains losing their volatile content. In Section 3.1.4 we discuss the possibility of icy grains in more detail. These larger (0.1 -1.0 mm) grain populations would not contribute significantly to the 24 µm coma cross section close to the nucleus because of its relative lack of surface area, but could still easily support the observed number density of 24 micron sized grains due to a fragmentation cascade (N.B. -as long as there are particles >> 24 µm in radius, they can always fragment/disrupt into many smaller particles and keep the observed particle size distribution (PSD) going) and thus maintain the coma's enhanced 24 µm surface brightness. profiles between 0 • -180 • contains a knee-shaped feature at ρ ∼ 130 (520,000 km) that is suggestive of a projected skyplane length for ongoing coma grain fragmentation and/or the projected turnback distance of dust grains from solar radiation pressure. Fitted power law indices corresponding to the yellow, red and orange curves are presented in Table 2. The location of the nucleus is indicated by the black circle in the center image. For reference, included in each plot are two black lines representing a 1/ρ and 1/ρ 2 coma behavior. The roller coaster shaped profile for the PSF is the result of the Airy diffraction pattern of the space-based telescope.
Similar to the 16 µm image, the coma's profiles in 24 µm close to the projected sunward direction (PAs: 225 • , 270 • , and 315 • ) all have a single profile index close to −1. A possible explanation for this constant surface brightness could be a preferential sunward emission of dust grains.

f ρ Measurements and Dust Production Estimates
For this analysis we calculated the f ρ parameter Lisse et al. 2002), an often used proxy for dust production rates using infrared emission that is analogous to the Af ρ parameter for reflected dust flux in the visible (A'Hearn et al. 1984). While the assumed canonical dust coma used to derive f ρ is not valid for many comets, the utility of f ρ comes from it establishing a standard procedure for estimating comae dust production rates and allowing a relative comparison between individual comets.
The expression for f ρ used is where is the emissivity of the dust grains at wavelength λ, f is a filling factor expressing the fraction of the photometry aperture containing dust grains, ρ is the linear aperture radius centered on the nucleus which is being used to measure the flux, F th (λ) is the flux measured in the photometric aperture for wavelength λ, B(λ, T c ) is the Planck function calculated at the color temperature T c of the dust grains, and ∆ is the geocentric distance during the observation. Most probably this is the result of super-heated sub-µm sized amorphous carbon grains present in the dust coma (Hanner et al. 1997) and/or potentially from the many emission features present in the thermal infrared region (Markkanen & Agarwal 2019;Wooden 2002).
For F th (λ) we subtracted the nucleus' contribution to SW1's overall flux in each aperture based on the scaled PSFs found during the coma removal process presented in Section 3.2. Additionally, flux from background sources (some of them serendipitously detected asteroids) was removed by interpolating the dust coma behavior for regions around each background source. Figure 5 shows plots of the 16 and 24 µm measured spectral flux density values for an array of aperture radii along with their associated f ρ measurements for the three color temperature assumptions. Table 3 reports the measured flux and f ρ values along with their associated uncertainties for the largest photometry apertures used for each image. The 16 µm's nearly constant f ρ value for aperture radii larger than ∼ 5 indicates that the 3-D shape of the dust coma primarily contributing to this image maintains a nearly canonical spherical shape (Fink & Rubin 2012). On the other hand, the 24 µm f ρ profile has a slight positive slope indicating deviations from a canonical 1/ρ coma's expected aperture-independent constant value. The 24 µm slope behaviors support the possibility for an overabundance of 24 µm sized dust grains for larger cometocentric distances. The steep decrease for f ρ profiles for small apertures is an artifact of the coma's image being the convolution of the coma's intrinsic surface-brightness distribution with the telescope's PSF (e.g., the intrinsic surface-brightness is spread over a larger projected surface area by the convolution process resulting in a decrease in integrated flux for apertures smaller than the PSF).
To verify that the difference in aperture photometry for the coma between the 16 µm and 24 µm images is not the result of the local infrared background in each image, we compared coadded Widefield Infrared Survey Explorer (WISE; Wright et al. (2010)) backgrounds retrieved from the W3 (12 µm) and W4 (22 µm) intensity images downloaded from the NASA/IPAC Infrared Science Archive.
W3 and W4 coadded images centered on SW1's nucleus position during each epoch of imaging were compared and we found no significant differences that could explain the different photometry behaviors.
The 70 µm image's low S/N surface brightness coma detection did not allow a similar radial profile analysis. Instead, we report in Table 3  We use the measured f ρ values to estimate dust production rates during the Spitzer imaging where a is the radius of the grains, ρ d is the density of the grains, and v is the radial velocity of the grains lofted from the nucleus' surface. For our calculations we assumed that the diameter of the pressure. While it is likely that larger grains will have slower radial velocities than smaller grains, we adopt the same value for each band, due to the observational uncertainties of the measurements.
We use a value for the dust emissivity of = 0.95. Estimated dust production rates are presented in Table 3.
We  Note-a The radius of the sky-plane projected photometry aperture.
them to develop an empirical expression relating an expected thermal dust activity for an individual comet based on its nucleus size: where D N is the nucleus diameter in km and N (0, 0.25) is a Gaussian distribution with mean of 0 and variance of 0.25. In Figure 6, we plotted measurements from both infrared surveys, the empirical expression developed by Bauer et al. (2017a), and SW1's measurements from this work.
As Figure 6 shows, Equation 5   Reports of SW1's dust production rate as derived from visible observations during periods of quiescent activity indicate a typical mass loss rate for sub-micron sized grains on the order of 1 -50 kg/s. We arrived at these typical quiescent dust production rates using reported Af ρ measurements from Trigo-Rodríguez et al. (2010) and Hosek et al. (2013), but here we use a value of grain density ρ d = 500 kg/m 3 in order to be consistent with our f ρ derived dust production rates. We note that these dust production rates are upper limits due to their calculated Af ρ values containing nucleus flux contributions. When compared to the estimated dust production rates as derived from the Spitzer data, which have nucleus flux contributions removed, the estimated dust production rates for grains in the range of 16 µm to 70 µm have a higher mass loss rate (Table 3) than the sub-micron sized coma (< 1 µm grains). It would be interesting to see if this trend of higher mass loss rate for the tens of micron sized grains is also seen during periods of major dust coma outburst (i.e., is the bulk of SW1's outburst mass loss coming from grains that are on the order of 10s of microns to 100 microns or from sub-micron sized grains), enabling investigations of the quiescent vs. outburst comae activity mechanisms.

Coma Modeling
Another approach to determine the dust production rate is to model the thermal emission of an ensemble of particles defined by its size distribution. We used the model described in Bockelée 3) a matrix of amorphous carbon with inclusions of crystalline ice. For mixture 1) the carbon/olivine mass ratio is 1, a value consistent with the organic mass fraction measured in comet 67P dust particles (Bardyn et al. 2017). Mixtures 2) and 3) have the same ice fraction in mass of ∼ 45%, but have different optical properties.
Other parameters set in the model are the dust maximum size a max and the dust velocity as a function of particle size, described as varying ∝ a −0.5 , with a value of 60 m/s for 10-µm particles.
The maximum liftable size from the surface of SW1's nucleus is estimated to be a max = 250 µm, for a CO-driven activity restricted to a spherical segment with half-angle of 45 • and a total CO production rate of 4 × 10 28 s −1 , assuming our nucleus radius estimate of 32.3 km (Section 3.2) and a nucleus density of 500 kg/m 3 (V. Zakharov, personal communication, see Zakharov et al. 2018Zakharov et al. , 2021 The model was applied to simulate the flux density in a 9" FOV radius at 16, 24 and 70 µm, for comparison with Spitzer data. Simulations were made for a minimum dust particle size a min in the range 0.5-50µm and size indices in the range 2.5-4.6. These two parameters have indeed a strong influence on the dust thermal spectrum, with, e.g., a larger contribution from small particles for low  colors, respectively. We only show results for ice-carbon mixtures 2) and 3), since results for carbonsilicate mixture 1) are similar to those obtained for mixture 3). For mixtures 1) (not shown) and 3), the orange and blue domains overlap for a min = 2-5 µm, whereas no overlapping is observed for mixture 2) for any set of (a min ,β). Grains made of mixture 2) are hotter than other mixtures for sizes below 30µm (Fig. 8), and this explains the different infrared spectra. and blue, respectively. The assumed maximum particle size is a max = 250 µm.
In Figure 9, we show dust production rates derived from the 24 µm flux density using the (a min ,β) parameters that provide T 16/24 values consistent with the measured value, i.e., those defining the orange region in Fig. 7. For mixtures 1) and 3) with matrices of amorphous carbon, the range is 50-200 kg/s. The low end is obtained for the highest (a min ,β) values (= (5µm, 4.1-4.4)), that is a steep size distribution where 5-10 µm grains dominate the infrared emission. For size distributions with a min = 4-5 µm, the dust production rates deduced from the 24 and 70-µm fluxes are consistent, and in the range 50-100 kg/s. However, this is not the case for size distributions with small a min values (and consequently low β values, Fig. 7), for which 70-µm derived dust production rates are by of crystalline ice), the dust production rate inferred from the 24 µm flux is between 130-240 kg/s ( Fig. 9). The values derived from the 70 µm flux are more than 2 times lower for all sets of (a min ,β) parameters. This is an expected result since for this composition, the model fails in reproducing both the T 16/24 and T 24/70 values. The dust production rates derived with model parameters leading to a satisfactory fit to data (50-100 kg/s) are in overall agreement with those estimated in Section 3.1.2 using a simple approach.
The Mie-scattering model shows that measuring dust fluxes at several wavelengths in the thermal IR can provide constraints on the particle size distribution and thermal properties. The obtained results are here limited due to the low SNR of the 70 µm dust coma flux. A flaw in the present analysis is Figure 9. Dust production rates derived from the 24-µm flux density measured in a 9" FOV radius, using (a min ,β) parameters providing color temperature T 16/24 values consistent with the measured value of 129 ± 5 K. The range of production rate values for a given minimum size reflects the range of β values fulfilling the requirement, and the uncertainty in the 24-µm flux. Results for mixtures 1, 2, and 3 are shown in blue, turquoise and red, respectively.

Coma Color Temperature Map
In Figure 10, a color temperature map of the coma based on the 16 µm and 24 µm images is shown. This was generated by using the spectral flux density values of the coma after removal of flux contributions from the nucleus; the procedure of nucleus vs. coma flux contributions is described in Section 3.2 for the 16 µm image, and in our earlier work (Schambeau et al. 2015) for the 24 µm  Overall, the general trend is a decreasing color temperature with increasing projected distance away from the nucleus. The eastern half of the coma has a higher temperature than the western side by ∼ 20 degrees. The interpretation of this behavior is uncertain based on the current Spitzer imaging data.
We mention here plausible explanations for these color temperature behaviors based on properties of the dust coma. One possible explanation can be a population of relatively smaller grains on the eastern side of the coma composing the tail that are less efficient at radiating their stored thermal energy. Another possibility is that the western side of the coma has a higher abundance of sub-micron sized grains, resulting in an enhanced 24 µm emission above that of an ideal blackbody due to the silicate emission bands around 20 µm. The overall impact of this behavior would be a slightly lower color temperature for the western side of the coma. Future modeling efforts may be able to select between the combination of processes driving the observed color temperature, but are beyond the scope of this current work.
Using the color temperature as a proxy for the approximate dust grain temperatures and the results of Beer et al. (2006) indicates that for grain sizes on the order of tens of microns, as we have here for the 16 µm and 24 µm images, the grains have a dust mass fraction for water ice (X, where X = 1 for pure water ice) in the range of 25-50%, with smaller grains having a higher ice content.
We calculated the expected lifetimes for the water ice content of assumed spherical icy grains with diameters equal to 16 µm, 24 µm, and 70 µm and dust mass fractions X 16 = 0.5, X 24 = 0.40, X 70 = 0.25 (Beer et al. 2006;Lien 1990;Mukai 1986). The lifetimes of the water ice content of the grains is respectively: 112 days, 154 days, and 373 days. For these estimated lifetimes we have ignored the increased temperatures of grains as their sizes decrease due to the ongoing water ice sublimation, so our derived lifetimes are estimated upper limits.
The presence of grains containing water ice has been inferred by the increased emissivity at longer wavelengths as derived from the modeling of SW1's Spitzer IRS spectrum (Schambeau et al. 2015 These measured production rates are the same order of magnitude as what would be produced by the sublimation of icy grains if we use the dust-to-ice mass fractions as constrained from their color temperature and the dust production rates derived from f ρ. As a first order estimate of the coma's Q H 2 O due to the sublimation of icy grains, we calculated the production rate that would be produced from sublimation of the water ice content of icy grains following the dust production rates presented in Table 3. Assuming that all of the water ice content for individual grains is fully sublimated we arrive at an estimate range of Q H 2 O ∼ (1 -3)×10 27 molecules/s, supporting the argument that the measured water production rates may be explained by a non-nuclear source of icy grains in the coma.

Nucleus Spectral Flux Density Measurements and a new NEATM
To obtain nucleus photometry measurements from the blue PU images, the flux from SW1's coma was modeled and removed. We used a well-established coma modeling technique (Fernandez 1999;Lamy & Toth 1995;Lisse et al. 1999) for this procedure, where the azimuthal coma behavior is measured in regions outside of significant contribution from the nucleus' PSF in order to generate a synthetic coma model. The model coma's flux contribution is then subtracted from the observations resulting in an approximately bare-nucleus residual image. The residual image is then used to scale an STINYTIM generated PSF (Krist 2006) to represent the nucleus's total flux. The reader is referred to our previous work (Schambeau et al. 2015) for a detailed description of this procedure.
The coma modeling and removal procedure was applied to each of the PU images resulting in six independent nucleus photometry measurements from six images at an effective 15.8 µm wavelength.
The individual color corrected measurements are: 84.1, 85.0, 85.0, 87.5, 89.6, and 88.4 mJy, with a typical uncertainty of ± 7 mJy. The final measurement used for thermal modeling analysis was taken as the average of the individual measurements: 86 ± 2 mJy, with the stated 1-σ uncertainty being the standard deviation of the six measurements. Figure 11 shows the new 15.8 µm measurement plotted along with the other four Spitzer nucleus photometry values that we reported earlier (Schambeau et al. 2015). We also plot the best-fitting 4band thermal model (NEATM, Harris (1998)) that we used in the earlier work to extract the nucleus' effective radius R = 30.2 +3.7 −2.9 km and beaming parameter η = 0.99 0.26 −0.19 . A re-fit using the now five spectral flux density measurements produces a nucleus size estimate and infrared beaming parameter that are slightly larger, but within the 1-sigma uncertainties of the earlier results: R = 32.3 ± 3.1 km and η = 1.1 ± 0.2. We propose these new values be used in future investigation of SW1 in lieu of our earlier analysis (Schambeau et al. 2015), because of the reduced uncertainty due to modeling with five, rather than four points. For our new NEATM analysis similar assumptions as those used for our previous work and for (e.g.) SEPPCoN  were used: bolometric bond albedo A = 0.012 (assuming a visible-wavelength geometrical albedo p = 0.04 and phase integral relation q = 0.290 + 0.684G, (Harris & Lagerros 2002), emissivity = 0.95, and slope parameter G = 0.05.

SUMMARY AND CONCLUSIONS
A more detailed analysis of November 2003 Spitzer observations of SW1 (Schambeau et al. 2015) is presented, which incorporates 16µm data for the first time, and significantly improves characterization of the Centaur's tens of microns dust coma during a period of quiescent activity.
The 16 µm blue PU images were remarkably symmetric with evidence for an ∼ 70 percent coma enhancement in the south-southeast direction, which may be reflective of tail formation. The 16 µm coma's morphology indicated preferential sunward emission of dust grains. No signs of grain fragmentation were indicated by the data within the image FOV (273,000 × 386,000 km).
Re-analysis of the 24 µm images reveal a large scale coma morphology of increased brightness in the southwest direction, consistent with preferential sunward emission. These data also show a more compact wing feature initially directed toward the south-southwest to a projected cometocentric distance of 352,000 km (90 ) and curving toward the southeast. This feature has previously been interpreted to be due to the nucleus' rotation, but we propose instead that this is the result of solar radiation pressure effects and gravity on micron sized dust grains that were emitted in the sunward direction and were turned back to form a dust tail. Further analysis of this feature is encouraged.
Interestingly, analysis of the 24 µm surface brightness radial profiles shows a noticeable change of slope at ∼ 520,000 km cometocentric distance at positions angles ∼ 0 through 180 degrees. This change in slope is consistent with the projected distance to the outer edge of the curved feature. We used measurements of this turning-back point of the curved feature to estimate a dust grain outflow velocity in the range of 50−270 m/s depending on the ejection direction of grains.
Additionally, for the first time, we compare the WISE/NEOWISE and SEPPCoN  derived f ρ measurements and see agreement between the two surveys, strengthening the argument for the empirically derived relationship's application as a predictor of cometary comae.

Schambeau et al.
A coma model (Bockelée-Morvan et al. 2017) was used to constrain the coma's dust grain size distribution and mass loss rate. The model was constrained by 9 radius aperture photometry measurements of 16 µm, 24 µm, and 70 µm coma flux density. Models with a dust grain composition of a matrix of amorphous carbon with inclusions of (1) amorphous olivine or (2) crystalline water ice were in agreement with the Spitzer data. The two models had similar ranges for the best-fit grain size distributions: power-law index β ranging from 4.1 to 4.4, minimum grain size a min ranging from 4 µm to 5 µm, and maximum grain radius a max = 250 µm. The dust production rates derived with model parameters leading to a satisfactory fit to data (50-100 kg/s) are in overall agreement with those estimated using the measured f ρ values.
Using the 16 µm and 24 µm images we constructed a coma color-temperature map, which also peaks at ∼ 140 K, decreasing with increasing cometocentric distance, and an east-to-west asymmetry with the eastern coma being ∼ 20 degrees higher. This behavior is the result of a particle size distribution of grains of varying compositions. Future analyses of these data are encouraged to better constrain SW1's large grain coma environment.
We used the 140 K color temperature as a plausible physical temperatures for individual grains. This assumption is supported by our earlier analysis of the IRS spectrum (Schambeau et al. 2015). Using the dust production rates measured here we estimated a H 2 O production rate from the sublimation of icy coma grains: Q H 2 O ∼ (1 -3)×10 27 molecules/s. This range agrees with other measurements of SW1's water production rate (Bockelée-Morvan & et al. 2021;Ootsubo et al. 2012) Coma modeling and its removal from the IRS blue PU imaging data at 16 µm were used, along with measurements at other infrared wavelengths, to produce a nucleus radius of R = 32.3 ± 3.1 km for SW1, which is within 1-σ of and has smaller uncertainties than prior measurements using Spitzer data (Schambeau et al. 2015;Stansberry et al. 2008Stansberry et al. , 2004. This analysis also yields a slightly higher NEATM derived beaming parameter (η = 1.1 ± 0.2). The size of SW1 places it on the smaller end of the currently-known Centaur size distribution (Bauer et al. 2013;Duffard et al. 2014;Lellouch et al. 2013), but on the larger end for small bodies with known cometary activity Stansberry et al. 2008). With the refined nucleus size estimate presented here, we encourage future modeling efforts to better understand the bound inner coma environment of SW1.
The Centaur SW1's large size among active objects, in combination with its orbital history that indicates it has not spent a significant amount of time interior to Jupiter (Sarid et al. 2019), positions it as a high-priority target for future observational and in situ investigations to better understand moderately sized and relatively pristine planetesimals to better understand the period of thermal evolution experienced while in the gateway transition from Centaur to JFC. We encourage the community to undertake new observations of SW1 and also for any currently existing and planned new observations to be listed on the SW1 observing campaign website: wirtanen.astro.umd.edu/29P/29P obs.shtml.
Additionally, we provide here links to the following resources emphasizing the importance of continued observations of SW1 and best practices for new observations: (1) the call for observations from Womack et al. (2020) and (2)