Initial Characterization of Active Transitioning Centaur, P/2019 LD₂ (ATLAS), Using Hubble, Spitzer, ZTF, Keck, Apache Point Observatory, and GROWTH Visible and Infrared Imaging and Spectroscopy

We searched for serendipitous observations of P/2019 LD₂ made with the Zwicky Transient Facility survey mounted on the Palomar Observatory's 48 inch telescope (Bellm et al. 2019) in the ZTF archive (Masci et al. 2019). The ZTF archive possessed observations of P/2019 LD₂ made as far back as 2019 April 9 UTC, which we include up to 2019 November 8 UTC. The observations were made in the g and r bands in images consisting of 30 s exposures. Seeing conditions were typically between 15 and 25 and at air masses ranging from 1.4 to 2.6. A full list of observations of P/2019 LD₂ made by ZTF containing the viewing geometry and observing conditions is presented in Table 2.

Following the announcement of the appearance of activity of P/2019 LD₂² (then called 2019 LD₂), we triggered target-of-opportunity observations with the ARC 3.5 m telescope at Apache Point Observatory (APO) on 2019 September 7 UTC using the ARCTIC large-format optical CCD camera (Huehnerhoff et al. 2016). The camera was used in full-frame, quad amplifier readout, 2 × 2 binning mode, resulting in a pixel scale of 0228, and it was used with the g and r filters. In total, 14 g and r exposures were obtained, each 120 s long and in alternating order between the g and r filters. The telescope was tracked at the sky-motion rate of the comet of 86 hr⁻¹. The seeing was 14 and the air mass was 1.8 during the observations.

Observations of P/2019 LD₂ were made with the Spitzer Space Telescope (Spitzer) using the IRAC instrument (Fazio et al. 2004) on 2020 January 25–26 UTC (DDT program 14331, PI Bolin et al. 2019). The observations consisted of 11 Astronomical Observing Requests (AORs), each consisting of 80 × 12 s dithered frames and having a ∼0.44 hr duration for a total of 4.8 hr clock time. The frames where dithered in groups of 10, with each using a large cycling pattern. The sky at the location of P/2019 LD₂ during the Spitzer observations possessed a high density of stars due to its low −18° galactic latitude, so shadow observations were used to improve the sensitivity of the observations. Out of a total of 11 AORs, eight were focused on the observed P/2019 LD₂, for a total of 2.13 hr of on-source time. The remaining three AORs were shadow observations that were evenly spaced in the sky location covering the trajectory of P/2019 LD₂ during 2020 January 02:23:32–23:10:44 that P/2019 LD₂ was being observed. The target was centered in the 4.5 μm channel because this channel is sensitive to CO/CO₂ emission and also because the object was expected to be the brightest at this wavelength. The 4.5 μm IRAC channel has a spatial resolution of 12 pixel⁻¹. The data were reduced in a method as described in Fernández et al. (2013).

The Hubble Space Telescope was used to observe P/2019 LD₂ with General Observer's (GO) time on 2020 April 1 UTC (HST GO 16077, PI Bolin et al. 2020a). During the one orbit visit, five 380 s F350LP filter exposures were obtained with the UVIS2 array of the WFC3/UVIS camera (Dressel 2012) for a total of 1900 s of integration time over a single orbit. The F350LP filter has a central wavelength of 582 nm with an FWHM bandpass of 490 nm (Deustua et al. 2017). The instrument and filter combination of WFC3 and the F350LP filter provides a per-pixel resolution of 004 corresponding to 145 km at the topocentric distance of the comet. The comet was tracked nonsidereally according to its sky-plane rate of motion of 40'' hr⁻¹.

Multiband optical images of P/2019 LD₂ were obtained with the 1.0 m Telescope at the Mount Laguna Observatory (Smith & Nelson 1969) on 2020 May 17 UTC. Johnson–Cousins B, V, and R filters were used in combination with the E2V 42–40 CCD camera to obtain seven to nine 120 s exposures in each filter. The seeing conditions were 192, the air mass was 1.49, and sidereal tracking was used. This facility is a member of the GROWTH collaboration.

Observations of P/2019 LD₂ were made in g, r, i, and z filters by the 2 m Liverpool Telescope located at the Observatorio del Roque de los Muchachos on 2020 May 29 UTC. The IO:O wide-field camera was used with a 2 × 2 binning, providing a pixel scale of 03 (Steele et al. 2004). Two 30 s exposures were made per filter with the telescope tracking at the sidereal rate. The seeing conditions were 121, and the air mass was 1.75. Detrending of data was performed using the automated IO:O pipeline software (Steele et al. 2004). This facility is a member of the GROWTH collaboration.

Multiband B, V, and R imaging of P/2019 LD₂ was made by the 1 m Lulin Optical Telescope on 2020 June 23–27 UTC and 2020 July 10 UTC. The observations were made using the 2K × 2K SOPHIA camera with a pixel scale of 052 (Kinoshita et al. 2005). Exposure times of 90 s were where the telescope was tracked at the nonsidereal rate determined by the ephemeris of the comet. The seeing conditions of the observations were ∼15, and the air mass was ∼1.15.

A spectrum of P/2019 LD₂ was obtained using the Low-Resolution Imaging Spectrometer (LRIS; Oke et al. 1995) on the Keck I telescope on 2020 August 19 UTC (PI J. von Roestel, C272). The blue camera consisting of a 2 × 2K × 4K Marconi CCD array was used with the red camera consisting of a science-grade Lawrence Berkeley National Laboratory 2K × 4K CCD array. Both cameras have a spatial resolution of 0135 pixel⁻¹. We used the 560 nm dichroic with ∼50% transmission efficiency in combination with the 600/4000 grism for the blue camera, rebinned twice in both the spectral and spatial directions, and the 600/7500 grating for the red camera, rebinned once in the spectral direction and twice in the spatial direction, providing a spectral resolution of 0.8 nm and 0.5 nm, respectively, and a spatial resolution of 027. A total integration time of 300 s was used for the exposure and was obtained at air mass 18 in 085 seeing conditions. Both telluric correction and solar-analog stars were observed at air masses similar to that of P/2019 LD₂. Wavelength calibration was completed using the HgCdZn lamps for the blue camera and the ArNeXe lamps for the red camera. We used a local solar-analog star to remove the solar component from the spectrum of P/2019 LD₂. The LPipe spectroscopy reduction package was used to reduce the data (Perley 2019).

Serendipitous prediscovery observations of P/2019 LD₂ were obtained with ZTF on 2019 April 26 UTC consisting of three 30 s exposures in the r band. These prediscovery data of P/2019 LD₂ have been coadded into a composite image with an equivalent 90 s integration time presented in the top left panel of Figure 1. The comet has an extended appearance with a ∼20'' long tail with a position angle of ∼260° in the antisolar direction. ZTF obtained prediscovery detections of P/2019 LD₂ on 2019 April 9, 15, and 20, but the comet did not have a discernible extended appearance in these data.

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On 2019 September 7 UTC, the ARC 3.5 m was used to obtain 20 × 120 s exposures of P/2019 LD₂ in the r band. A composite median stack with an equivalent exposure time of 2400 s is presented in the top right panel of Figure 1. In the ARC 3.5 m images, the comet has a diffuse, nonstellar appearance. The tail is not easily defined in the ARC 3.5 m median stack, though the comet's extended appearance is enhanced in the opposite direction of the comet's orbital motion with a position angle of ∼230° and length of 5''.

The center panel of Figure 1 presents the appearance of P/2019 LD₂ in a median stack of five 380 s F350LP images with an equivalent integration time of 1900 s taken with HST/WFC3 on 2020 April 01 UTC. Cosmic rays have been removed from the composite image stack, with median interpolation of the surrounding pixels. The high-resolution composite HST stack was taken when the comet was at an orbit-plane angle of ∼−044 and had a tail with a length of ∼32'' limited by background structure caused by galaxies and sky noise opposite to the solar direction with a position angle of ∼250°. The ∼32''-long tail translates into a length of 6.2 × 10⁸ m given its topocentric distance of 5.02 au and a phase angle of 107. An enhanced version of the HST median composite stack normalized by the distance from the optocenter reveals a possible jet structure ∼1'' long, as seen in the bottom panel of Figure 1. We will discuss the implications of the comet's morphology from these observations for its dust properties in Section 3.3.

We compare the surface-brightness profile of P/2019 LD₂ to the simulated surface-brightness profile of a G2 field star WFC3 point-spread function (PSF) assuming the use of the F350LP filter using the TinyTim software (Krist et al. 2011), as seen in Figure 2. The radial profiles of both P/2019 LD₂ and the simulated stellar G2V source are computed by azimuthally averaging concentric apertures centered on the optocenter separated by the pixel scale allowed by WFC3 using the F350LP filter. The normalized surface-brightness profile of P/2019 LD₂ between 024 and 12 was fit to the functional form of Σ ∝ θ^m, where Σ is the surface brightness and θ is the distance from the optocenter in pixels, resulting in a radial profile slope of m ∼ −1.71. We note that the radial profile slope is steeper than the typical −1 to −1.5 radial profile slope of comets with an isotopic coma in a steady state. The steeper radial profile slope of P/2019 LD₂ compared to comets with isotopic coma may be an independent indication of the comet's evolving dust production rate (Jewitt & Meech 1987).

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The fitted 024–12 radial profile of P/2019 LD₂ was convolved with the synthetic G2V PSF and subtracted from the measured radial profile of P/2019 LD₂ to calculate a equivalent nucleus brightness of V = 22.6 ± 0.04 assuming a m_V-m_F350LP ∼ 0.1 (Bolin et al. 2020b). We assume the following phase function for determining the absolute magnitude of the nucleus, H:

$$\begin{eqnarray}&&H=V-5\,{\mathrm{log}}_{10}({r}_{h}{\rm{\Delta }})-{\rm{\Phi }}(\alpha )\end{eqnarray} \tag{ 2 }$$

where r_h, Δ, and α are the heliocentric distance, topocentric distance, and phase angle of the comet as listed in Table 1 for the 2020 April 1 UTC observation. Here, Φ(α) = 0.04α, where we assume a phase coefficient of 0.04 in magnitudes/degree, resulting in H = 15.53 ± 0.05. The true phase coefficient of P/2019 LD₂ is unknown, so our uncertainty on the measured value of H is considered a lower limit.

From our measured value of H, we calculate the light-scattering cross section, C, of P/2019 LD₂ in km² using the following function:

$$\begin{eqnarray}&&C=1.5\times {10}^{6}\,{p}_{v}^{-1}\,{10}^{-0.4H}\end{eqnarray} \tag{ 3 }$$

where p_v is the albedo of the nucleus, assumed to be ∼0.08, the typical albedo measured for Centaurs (Bauer et al. 2013), resulting in C = 11.15 ± 0.42 km². Converting our measured cross section to a radius using r = (C/π)², we obtain a radius of ∼1.8 km, comparable to the radius estimates of P/2019 LD₂ based on unresolved photometry and the nondetection of P/2019 LD₂ from ground-based observations (Schambeau et al. 2020). We note that this is a radius estimate based on a single observation and represents a size assuming a spheroidal shape. Significant deviations from a spheroidal shape, such as a bilobal (Nesvorný et al. 2018) or elongated shape (Bolin et al. 2018; Hanuš et al. 2018) as has been observed for other comet-like bodies, may require additional observations to be made of P/2019 LD₂ to accurately determine its size.

Using the combination of our ground-based observations with the ARC 3.5 m taken on 2019 September 7 UTC, the MLO 1.0 m on 2020 May 27 UTC, the LT on 2020 May 29 UTC, and the Lulin Optical Observatory on 2020 July 10 UTC, we have calculated the mean colors of P/2019 LD₂ using 10,000 km photometric apertures of g–r = 0.60 ± 0.03, r–i = 0.18 ± 0.05, and i–z = 0.01 ± 0.07. The filter configuration and viewing geometry of our observations are presented in Table 1. The equivalent angular size of the 10,000 km used in our photometric calculations ranged from 32 to 37, with the seeing during observations ranging from 12 to 19. We used the color transformations from Jordi et al. (2006) to convert the BVR Johnson–Cousins photometry of P/2019 LD₂ from the MLO 1.0 m and LT to the Sloan Digital Sky Survey (SDSS) system.

Our visible measured colors of P/2019 LD₂ are reddish to neutral in the ∼480 nm to ∼910 nm wavelength range covered by our filters, which is consistent with the measured colors of other active solar system comets as presented in Figure 3. For comparison purposes only, we have included the colors of inactive objects in Figure 3. We note that the measured colors of P/2019 LD₂ from our observations are somewhat bluer than the colors of active and inactive Centaurs measured by Jewitt (2015), though this may be due to the longer wavelength coverage of our observations, which go as far as ∼910 nm, compared to the shorter visible-wavelength observations of Jewitt (2015).

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Images from each of the Spitzer DDT program 14331 AORs 1, 4, 6, 9, and 10 that were used to take images of P/2019 LD₂ from 2020 January 25 2:23 to 23:11 UTC were reduced using the reduction methods described in Fernández et al. (2013). Images obtained during each of these five AORs using the 4.5 μm channel were coadded to form a single composite image with an equivalent exposure time of 948 s for each AOR.

P/2019 LD₂ was located in a crowded star field at a galactic latitude of ∼−18°. Therefore, due to the imperfections in the shadowing technique, we used an aperture size with an angular width of 324, equivalent to 10,000 km at the topocentric distance of 4.26 au of the comet from Spitzer. We obtain an on-source flux density for P/2019 LD₂ of 35.6 ± 2.8 μJy at 4.5 μm using the average of the five photometry measurements from the composite images made from each of the AORs 1, 4, 6, 9, and 10. The comet has a slightly extended appearance of more than ∼24 as seen in Figure 4, and an aperture correction was applied to the flux density measurement. The flux from the nucleus assuming a kilometer-scale nucleus radius as measured in Section 3.1 is ∼0.1 μJy, less than 1% of the total flux.

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We present the Afρ based on the Spitzer/IRAC photometry of P/2019 LD₂ using the Afρ definition of A'Hearn et al. (1984), which is a quantity in units of length, in this case centimeters, that corrects the comet's brightness with respect to heliocentric distance, geocentric distance, aperture size, solar spectrum, and filter wavelength. The values Afρ are normalized to 0° phase angle, A(0°)fρ, using the Halley–Marcus cometary phase function defined by Schleicher & Bair (2011). Assuming that the entirety of the flux in the 4.5 μm Spitzer/IRAC observations is from dust in local thermal equilibrium, we calculate A(0°)fρ = 334 cm. This is strictly an upper limit on A(0°)fρ, due to the possible contribution of gas in the measured flux of the comet.

If we assume that the entirety of the flux from the comet is from CO emission and that the gas speed is 0.5 km s⁻¹, we measure a gas production rate of 1.6 ± 0.1 × 10²⁷ mol s⁻¹, which is similar to the results of Kareta et al. (2020b). For CO₂, assuming that the entirety of the flux is due to gas emission, we obtain a gas production rate of 1.4 ± 0.1 × 10²⁶ mol s⁻¹, which is comparable to the CO₂ measured for comets observed in the MIR at similar heliocentric distances (Reach et al. 2013; Bauer et al. 2015).

Using our archival observations of P/2019 LD₂ from ZTF taken between 2019 April 9 UTC and 2019 November 8 UTC, we have plotted the equivalent r magnitudes of P/2019 LD₂ as a function of time since the perihelion date of 2020 April 10 UTC, (T − T_p), measured with equivalent 20,000 km apertures and presented in the top panel of Figure 5 with observational details in Table 2. These observations include data taken in the g band, which have been corrected to an equivalent r-band magnitude using our g–r color estimate for P/2019 LD₂ of ∼0.6. The 20,000 km aperture was equivalent to an angular size of 54 on 2019 November 8 UTC, when the comet had a geocentric distance of 5.14 au and an angular size of 7.52'', and when the comet had a geocentric distance of 3.67 au on 2019 July 1 UTC. The measured local seeing in the ZTF images at the time of observation ranged between 16 and 39 with a median seeing value of 20. Using only the r-band photometry, the data show a secular brightening trend of the comet as the comet approached opposition on 2019 June 24.42 UTC and was increasing in brightness by 1.6 ± 0.2 × 10⁻² mags/day. After leaving opposition, the comet showed an asymmetrical secular fading trend of 0.4 ± 0.1 × 10⁻² mags/day compared to the preopposition brightening trend.

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In addition to photometry, we present the Afρ based on the ZTF photometry of P/2019 LD₂ as implemented by Mommert et al. (2019). The values Afρ are normalized to 0° phase angle, A(0°)fρ, and are plotted against the second Y-axis in the top panel of Figure 5 and presented in Table 2. Values of constant A(0°)fρ are plotted for reference in the top panel of Figure 5. The value of A(0°)fρ rises consistently over the span of our observations, resulting in asymmetry in the brightness of P/2019 LD₂ as it passed through opposition on 2019 June 24 UTC (T − T_p = −291 days). Before opposition, between T − T_p = −370 and −312, the error-weighted mean of A(0°)fρ = 93.9 ± 11.8 cm, and between T − T_p = −312 and −291, A(0°)fρ = 113.1 ± 8.57 cm. After opposition, the error-weighted mean of A(0°)fρ between T − T_p = −291 and −211 equaled 136.2 ± 9.7 cm, and from T − T_p = −211 and −50, A(0°)fρ = 188.8 ± 25.8 cm. The range of A(0°)fρ from 85 cm to 200 cm is consistent with the observed Afρ range of comets, which ranges from 1 to 10,000 cm (A'Hearn et al. 1995).

In addition to calculating the A(0°)fρ of P/2019 LD₂, we calculate the value of C for each of the equivalent r-band magnitudes in Table 2 using Equation (3), which are plotted in the bottom panel of Figure 5. A linear function is fit to these data, resulting in a fitted slope parameter value of dC/dt = 0.94 ± 0.06 km²/day. The change in phase angle over the time of our observations is modest, as seen in Table 2, so variations in the phase function used to calculate C should have a minimal effect on the estimate of the uncertainty of our measured slope parameter. Extrapolating backward in time beyond the range of our data results in C = 0 km² at ∼135 days before 2019 April 9 UTC, the date of our first photometry data point, or on 2018 November 24 UTC, during which P/2019 LD₂ had a heliocentric distance of ∼4.8 au, when water ice begins to sublimate (Lisse et al. 2019).

The dimensionless ratio of solar radiation and gravitational forces is defined by β (Burns et al. 1979):

$$\begin{eqnarray}&&\beta =\displaystyle \frac{2{L}_{0}{r}_{H}^{2}}{{g}_{\odot }(1\mathrm{au}){t}^{2}}\end{eqnarray} \tag{ 4 }$$

where L₀ is the length of dust travel, in this case, the observed length of the tail of P/2019 LD₂ of 6.2 × 10⁸ m; r_H is the heliocentric distance; g_⊙(1au) is the gravitational acceleration toward the Sun at 1 au equal to 6.0 × 10⁻³ m/s²; and t is the time of particle release. Assuming a mean value of r_H ∼ 4.6 au, L₀ = 6.2 × 10⁸ m, the length of the tail estimated from the 2020 April 1 UTC HST/WFC3 images, and t = 4.3 × 10⁷ s, the time between the 2020 April 1 UTC HST/WFC3 observations and the estimated start of the activity, we calculate a value of β = 2.4 × 10³. Making the assumption that the dust particles are dielectric spheres (Bohren & Huffman 1983), we find that the reciprocal of our estimated value of β translates into a particle size, $\bar{a}$, of ∼400 μm. However, we caution that this may be an upper limit on the dust size because of the limitations of our tail length measurement by the contamination of background galaxies in the HST/WFC3 images due to variations in the activity of P/2019 LD₂ affecting our estimate of t based on the backward extrapolation of the photometric data.

Assuming our estimated particle size of ∼400 μm, we estimate the total mass loss over the duration of our ZTF observations between 2019 April 9 UTC and 2019 November 8 UTC as M = 4/3 $\rho \bar{a}{\rm{\Delta }}C$, where ΔC is the difference between the cross sections at the start and end of our observations and is equal to 220 km². Assuming a dust density of 1 kg m⁻³ (McDonnell et al. 1986), we obtain a total mass loss over the time span of our observations of ∼10⁸ kg. Adopting our estimated value of dC/dt ∼ 1 km²/day, we obtain a mass-loss rate using $\dot{M}=4/3\rho \bar{a}$ dC/dt ∼ 5 × 10⁵ kg/day.

To estimate the fraction of active area of P/2019 LD₂, A, we take the ratio between the mass-loss rate and the equilibrium mass sublimation flux at 4.6 au, f_s = 1.4 × 10⁻⁵ kg m⁻² (Jewitt et al. 2015), where A ∼ 0.4 km². Thus, ∼10% of P/2019 LD₂'s surface is active assuming our inferred size radius of 1.8 km from Section 3.1, comparable to the active surface area measured for Jupiter-family comets (Fernández et al. 1999). An alternative assumption is that P/2019 LD₂ has a 100% active area, setting a lower limit to its radius of 0.2 km.

In addition, we use the perpendicular profile of P/2019 LD₂ taken with the high-resolution images from HST/WFC3 to estimate the out-of-plane distribution of dust with a minimum of projection effects as the Earth passed through the projected orbital plane of P/2019 LD₂ with a projected orbital plane angle of only 04 on 2020 April 1 UTC. We measured the FWHM along the direction perpendicular to the tail's profile as a function of distance from the optocenter, ℓ_T, between 0 and ∼2'' in increments of 012 slices, as plotted in Figure 6.

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Neglecting projection effects, the FWHM of the tail gradually widened with ℓ_T and was fit to the function

$$\begin{eqnarray}&&\mathrm{FWHM}={V}_{\perp }{\left(\displaystyle \frac{8{{\ell }}_{{\rm{T}}}}{{g}_{\odot }}\right)}^{\tfrac{1}{2}}\end{eqnarray} \tag{ 5 }$$

from Jewitt et al. (2014), where V_⊥ is the component of the ejection velocity perpendicular to the orbital plane, equal to ∼1 m s⁻¹. We estimate that the perpendicular component of the ejection velocity scales with V_⊥ ∼ 1 m s⁻¹ ${(\bar{a}/{\bar{a}}_{0})}^{-1/2}$ where ${\bar{a}}_{0}=400$ μm (Jewitt et al. 2014).

The spectrum of P/2019 LD₂ was extracted using a 74-wide region centered on the peak of the continuum's brightness. We compute the normalized reflectance spectrum of P/2019 LD₂ taken with Keck/LRIS on 2020 August 19 UTC in the wavelength range between 400 nm and 1000 nm by dividing the P/2019 LD₂ spectrum by our solar-analog spectrum and normalizing to unity at 550 nm. The resulting spectrum indicates a reddish to neutral coma color for P/2019 LD₂, as seen in Figure 7. We measured a slope of ∼16%/100 nm between 480 and 760 nm and a flatter spectrum between 760 nm and 900 nm consistent with the photometric colors taken by LT on 2020 May 29 plotted over the LRIS spectra in Figure 7 for reference. Our spectrum shows no sign of C₂ emission in the 505 nm to 522 nm range (Farnham et al. 2000) in the highlighted range in Figure 7.

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We set an upper limit on the C₂ gas production of P/2019 LD₂ using the mean V-band continuum flux density of P/2019 LD₂ using its measured 550 nm flux, flux_V = 1.52 × 10⁻¹⁵ erg cm⁻² s⁻¹ nm⁻¹. The fractional 1σ continuum statistical uncertainty of our P/2019 LD₂ spectrum in the range spanning 505 nm to 522 nm is 0.01, corresponding to a 3σ flux density${}_{{{\rm{C}}}_{2}}=2.13\times {10}^{-17}$ erg cm⁻² s⁻¹ nm⁻¹ including a correction of 0.6 for slit losses. The 3σ upper limit to the flux in the 17 nm width of the C₂ band is ${F}_{{{\rm{C}}}_{2}}\,\leqslant$ 3.62 × 10⁻¹⁶ erg cm⁻² s⁻¹. The 3σ upper limit on the number of C₂ molecules projected within the 74 × 10 spectroscopic slit assuming that the coma is optically thin is

$$\begin{eqnarray}&&{N}_{{mol}}=\displaystyle \frac{4\pi {{\rm{\Delta }}}^{2}{F}_{{{\rm{C}}}_{2}}}{g({r}_{h})}\end{eqnarray} \tag{ 6 }$$

where g(r_h) is the fluorescence efficiency factor of the C₂ spectral band at r_h where g(1 au) = 2.2 × 10⁻¹⁴ erg s⁻¹ radical⁻¹ (A'Hearn 1982), which results in N_mol ≤ 1.27 × 10²⁷ molecules.

We apply the assumptions of the Haser model (Haser 1957) to determine a coarse 3σ upper limit on the production rate of C₂. The Haser model uses two length scales, the "parent" molecule species length scale, L_P, and the "daughter" molecule species length scale, L_D, to describe the distribution of the radicals. For C₂ at an r_h of 4.594 au, L_P = 5.3 × 10⁵ km and L_D = 2.5 × 10⁶ km (Cochran 1985). In addition, we assume the speed of the molecular gas is 0.5 km s⁻¹, which is used to determine the residence time of the molecules in the projected slit (Combi et al. 2004). Using these assumptions with the Haser model, we find the 3σ upper limit to the gas production rate ${Q}_{{C}_{2}}\,\lesssim$ 7.5 × 10²⁴ mol s⁻¹, which is a similar limit to the measured ${Q}_{{C}_{2}}$ of other solar system comets at similar heliocentric distances (Feldman et al. 2004) and to the results of Licandro et al. (2020). Scaling our measured spectroscopic upper limit on the ${Q}_{{C}_{2}}$ gas production rate to a OH gas production rate using the median ratio of C₂ to hydroxyl production rate for solar system comets (A'Hearn et al. 1995) results in an estimated spectroscopic upper limit of Q_OH ≲ 2.4 × 10²⁷ mol s⁻¹ and a mass-loss rate in water of ${{dM}}_{{H}_{2}O}/{dt}$ ≲ 80 kg s⁻¹.

In order to investigate the long-term dynamics of 2019 LD₂, we simulated 27,000 clones of its orbit. The clone set is created by using 1000 three-dimensional locations using the positional uncertainties. The velocity uncertainties are accounted for by creating 27 clones in the three-dimensional velocity space at each positional location for a total of 27,000 clones. Orbital six-vectors were generated with uncertainties from the JPL Horizons Orbit Solution dated 2020 May 20 at 00:43:28 and set for the 2019 September 15 00:00 UTC epoch. In addition, we use the major gravitational components of the solar system (Sun, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune). The simulations were conducted using REBOUND (Rein & Liu 2012) with the hybrid MERCURIUS integrator (Rein et al. 2019). We ran two sets of integrations, a short-term set, integrated for 100 yr, and a long-term set for 1 × 10⁷ yr. For each simulation set, we use a time-step size of 0.025 yr. For the long-term integrations, we output every 1000 yr and analyze the time at which the clones escape the solar system (distance from the Sun larger than 1000 au).

The short-term integrations replicate the previous work of Steckloff et al. (2020) and Hsieh et al. (2021). In the simulations, we find that the clones entered the Jovian region approximately 2.37 yr ago and are ejected from the region 8.70 yr in the future (see the top left panel of Figure 8 for a clone example). As was found previously, 2019 LD₂ transitions from a Centaur, with an approximate semimajor axis of 8.6 au, to a Jupiter-family comet with a semimajor axis of 6.2 au, spending 11.0712 yr in the Jovian region (semimajor axis 5.2 au). In the long-term simulations (1 × 10⁷ yr), we find that one-half of the clones escape the solar system in 3.4 × 10⁵ yr and 78.8% of the clones escape the solar system within the first 1 × 10⁶ yr, as seen in the bottom right panel of Figure 8. After ∼3.8 Myrs, 95% of the P/2019 LD₂ clones have escaped.

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