Infrared Observations of 2I/Borisov near Perihelion

Crimean amateur astronomer Gennady Borisov discovered the second interstellar object, 2I/Borisov, on 2019 August 30. With a cometary appearance, it was reported to the IAU minor planet center and (formerly) designated C/2019 Q4. However, as more observations and astrometric measurements came in, there was growing evidence that this comet has a hyperbolic orbit and could have an interstellar origin. Over the next 21 days, Guzik et al. (2019) pinned down the orbital elements of 2I/Borisov with 447 astrometric measurements. This led to an eccentricity of 3.38 ± 0.02 (now updated to 3.356 in JPL/HORIZONS as of 2020 July), hence confirming its interstellar nature. The most interesting feature of 2I/Borisov is that it showed an extended coma and a cometary tail in both g'- and r'-band imaging taken on 2019 September 10 (Guzik et al. 2019). Hence it is clear that this second-known interstellar interloper is a comet, contrary to the asteroid-like appearance of 'Oumuamua.

It is known that comets bear the primordial materials of the planetary disk. Thus, 2I/Borisov can help shed light on the chemical composition of other planetary systems, and whether exo-comets were formed and evolved differently. The advantage of 2I/Borisov is that it was discovered very early on, long before its perihelion (unlike 'Oumuamua; Meech et al. 2017). Hence, we could study this interstellar comet in great detail while it was still inbound to the Sun (Guzik et al. 2019), which triggered numerous follow-up campaigns. de León et al. (2019) obtained the first optical spectrum with the 10.4 m GTC telescope at La Palma observatory. Their spectrum, while featureless, showed that 2I/Borisov has the same spectral shape as D-type asteroids, which is common for comet nuclei in the solar system. Fitzsimmons et al. (2019) first detected CN emission in the optical spectrum of 2I/Borisov in 2019 September using the William Herschel Telescope and going bluer than the observations of de León et al. (2019). Weak C2 detections in 2019 September/October were first reported by Kareta et al. (2020), although they later joined Opitom et al. (2019) and concluded that both Multiple Mirror Telescope (MMT)/Large Binocular Telescope (LBT) and William Herschel Telescope (WHT) optical spectra were only able to put an upper limit on the C2 emission lines without a firm detection. The first definite detection of C2 emission lines was reported by Lin et al. (2020) at a later epoch, when 2I/Borisov was only 1 month away from perihelion. In addition to C2, Bannister et al. (2020) also reported the detection and rich emissions of NH2. While there have been attempts to indirectly infer the water production rates from CN and C2, McKay et al. (2020) were the first to use [O i] $6300\,\mathring{\rm A}$ to directly trace water production rates. In addition, Xing et al. (2020) employed UV narrowband imaging with Swift and inferred water production from OH gas. Another approach to directly detect water/water ice is through infrared spectra. However, both Bolin et al. (2020) and Yang et al. (2020) reported no sign of water ice in the infrared spectra of 2I/Borisov at early epochs. Nevertheless, as predicted by Fitzsimmons et al. (2019) and Bolin et al. (2020), 2I/Borisov was transitioning from CO-dominated activity to H₂O-dominated activity, hence further infrared spectroscopic observations are needed to study the H₂O activity and water ice (if any) on the nucleus of 2I/Borisov. We expect, as this interstellar comet comes closer to the Sun (perihelion at ∼2 au), that the surface activity would develop further, and the best chance to investigate its properties would be close to perihelion. Here we present infrared observations at the epochs of 2019 November 30 and December 7 UT.

Infrared imaging and spectroscopic observations of 2I/Borisov were carried out with FLAMINGOS-2 on the Gemini South Telescope, under the fast turnaround program GS-2019B-FT-207. The imaging component was conducted on 2019 November 30 UT. In the imaging mode, FLAMINGOS-2 covers a circular 61 field with an average pixel scale of 0179 per pixel with f/16. As infrared observations from ground-based telescopes are sky-background limited, we needed to use short exposure times to avoid detector saturation. We took 20 × 15 s dithered exposure with the Ks_G0804 filter, centered at 2.157 μm. The observations were carried out in nonsidereal tracking mode, with ephemerides gathered from JPL/HORIZONS.

The spectroscopic component was conducted on 2019 December 7 UT. Based on previous studies of solar system comets, e.g., 17P/Holmes (Yang et al. 2009) and 67P/Churyumov–Gerasimenko (Barucci et al. 2016), features of water ice are prominent at 1.5 and 2.0 μm. We therefore employed the moderate resolution grism HK, along with the HK bandpass filter, to cover a wavelength range of 1.3–2.5 μm. We used a 2 pixel slit (036), resulting in a resolving power of R ∼ 1250 at the center of the spectral range and an average R ∼900 throughout the covered wavelengths. The slit was aligned with the parallactic angle to mitigate atmospheric differential refraction. To better remove the sky emission, the spectroscopic observations were conducted with ABBA offsets along the slit. Each slit position had an integration time of 300 s, which resulted in a total integration time of 20 minutes on source. In addition to the target, we also observed HIP 55051 as a telluric star. The telluric star observations used the same setup and offset as the main target, but with an integration time of 10 s at each slit position. We also took Ar arc lamp observations for wavelength calibration. The 2I spectroscopic observations were carried out in nonsidereal tracking mode, with ephemerides gathered from JPL/HORIZONS. The telluric star observations were carried out in sidereal tracking mode. An observation log can be found in Table 1.

3.1. Imaging

The Ks-band images were reduced using the Image Reduction and Analysis Facility (IRAF).⁴ We first constructed sky flats from the science frame. Since we have 20 exposures, we group them into four subgroups to have corresponding sky flats and sky images. Since each image is dithered by 15'' in random directions, we first median-combined all five images within each subgroup to get rid of sources and construct sky flats. We then flat fielded all five images in the same subgroup with the corresponding sky flats. After flat fielding, we then combined all five images within the subgroups to construct the sky image. Since the images were dithered, we could obtain sky images without stars and galaxies by median combining. We then subtracted the corresponding sky images from all five images in the same subgroup. After sky subtraction, our target, 2I/Borisov, was clearly visible in single exposures. Nevertheless, to obtain deeper imaging, we stacked all 20 single exposures. In order to do so, we use Source Extractor (Bertin & Arnouts 1996) to find the centroid of 2I/Borisov on each single exposure, and shifted and added the 20 single exposures. We note that at first glance, there does not appear to be apparent coma in our infrared imaging. This is due to a combination of the bright sky background and our short exposure times, resulting in a lack of sensitivity in our imaging to detect the fainter coma. Nevertheless the coma is faintly visible in Figure 1 and a more detailed analysis of the comet surface brightness profile in Section 3 indicates the detection of coma. We also stacked the single exposures aligned using the background stars to reveal the motion of 2I/Borisov on the sky (see Figure 1, right-hand panel). We provide the astrometry of 2I/Borisov in our 20 exposures in Table 2, which can be used to investigate nongravitational acceleration when combined with additional high-precision astrometry from other facilities.

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Table 2.  Astrometry of 2I/Borisov from FLAMINGOS-2 Ks-band imaging on 2019 November 30

HH:MM:SS [UT]	R.A. (J2000)	Decl. (J2000)
07:56:40	11:14:28.206	−12:03:30.94
07:57:19	11:14:28.254	−12:03:32.32
07:57:58	11:14:28.304	−12:03:33.18
07:58:36	11:14:28.345	−12:03:34.22
07:59:13	11:14:28.406	−12:03:36.01
07:59:56	11:14:28.422	−12:03:36.97
08:00:35	11:14:28.481	−12:03:38.91
08:01:17	11:14:28.540	−12:03:39.58
08:01:57	11:14:28.582	−12:03:40.75
08:02:35	11:14:28.635	−12:03:42.14
08:03:15	11:14:28.660	−12:03:44.35
08:03:57	11:14:28.699	−12:03:44.36
08:04:35	11:14:28.742	−12:03:45.87
08:05:16	11:14:28.819	−12:03:46.89
08:05:55	11:14:28.845	−12:03:48.56
08:06:34	11:14:28.945	−12:03:49.06
08:07:13	11:14:28.958	−12:03:51.69
08:07:56	11:14:29.035	−12:03:52.97
08:08:35	11:14:29.065	−12:03:53.96
08:09:14	11:14:29.093	−12:03:54.73

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In order to estimate the photometry of 2I/Borisov, we first calibrated the photometric zero-point by comparing the background stars with the Two Micron All-Sky Survey (2MASS) catalog. The uncertainty in the measured zero-point is 0.03 mag, which we treat as the systematic uncertainty. We then measure the photometry of 2I/Borisov on the comet-centered deep stack. With an aperture size of 68, corresponding to a projected radius of 10,000 km on 2019 November 30 UT, we found the comet to be 16.22 ± 0.01(statistic) ± 0.03(systematic) mag in the Ks band.

3.2. Spectroscopy

The moderate-resolution spectra were reduced using RAF (see footnote 4) and the Gemini IRAF package.⁵ We first subtracted corresponding dark images from flat, target, telluric star, and Ar arc lamp spectral images. We then constructed a normalized flat image, and divided the target and telluric star spectral images by the normalized flat image. It is recommended to divide the arc images by the normalized flat to improve the wavelength solution, which we did accordingly. We then used the arc image to find the wavelength solution, and applied it to both the target and telluric star spectral images to find the corresponding wavelengths. After wavelength calibration, we extracted 1D spectra for both the target and the telluric star. We then divided the target spectrum by the telluric star spectrum for telluric correction. According to the Gemini spectroscopic standards webpage,⁶ our telluric star, HIP 55051, is an early type B1V star, which can also be used to correct for relative flux variations due to atmospheric transmission and scattering. We multiplied the telluric corrected target spectrum with a blackbody profile with an effective temperature of T_eff = 25,600 K (corresponding to a B1V star, see Cox 2000). In order to derive the reflectance of 2I/Borisov, we divided the infrared spectrum by a solar analog, HD 76151, taken from the IRTF Spectral Library⁷ (Rayner et al. 2009). We then normalized the reflectance spectrum to 2.2 μm. The result is shown in Figure 2 with the unbinned data in red dots and the binned (every 20 element) data in a black line. Notably, the reflectance spectrum (binned data) shows a negative slope at −2.2 ± 0.5% per ${10}^{3}\,\mathring{\rm A}$, contrary to the infrared spectra of Yang et al. (2020) obtained at earlier epochs. We note that when comparing the IRTF reference spectrum of HD 76151 with a reference quiet-Sun solar spectrum by Meftah et al. (2018), there appears to be a smaller slope of approximately −0.9% per 1000 Å. This suggests an uncertainty on our derived slope of 2I spectrum due to the uncertainty in the reference solar spectrum being at least  1%/1000 Å, implying that our spectrum of 2I is probably consistent with a neutral spectrum. However, our spectrum of 2I at perihelion still appears to be less red then reported by Yang et al. (2020). We note that the possible absorption feature at 2 μm is relatively narrow when compared to the water ice absorption feature in other comets, e.g., 17P/Holmes (Yang et al. 2009) and 67P/Churyumov–Gerasimenko (Barucci et al. 2016). Furthermore, the 1.5 μm water ice absorption feature is not present, and there is a narrow and deep artifact around 1.8 μm. All of the above imply an imperfect sky subtraction that results in the narrow 2 μm artifact rather than a real detection of water absorption.

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Our infrared imaging of 2I/Borisov may appear different from that seen in optical imaging, in particular nucleus dominated and dust deficient. However, this is due to a combination of a bright sky background and our short exposure times, leading to a lack of sensitivity to the faint coma. Nevertheless, the infrared imaging can provide an upper limit for the nucleus size. Here we follow a procedure similar to Jewitt et al. (2020) and Bolin et al. (2020) to estimate the nucleus size of 2I/Borisov. First, we need to measure the photometry of the cometary nucleus. This is particularly important for comets when they are active and the nucleus can be embedded in the coma. Indeed, when we checked our Ks-band image of 2I/Borisov, it was elongated compared to field stars. We therefore attempted to compare the 2I/Borisov image to a point-spread function (PSF) measured from a bright star in the field. We used Galfit (Peng et al. 2002, 2010) to fit the 2I/Borisov Ks-band image with a given PSF, which returned a poor fit. The results can be found in Figure 3. This suggests that the Ks-band image of 2I/Borisov cannot be regarded as a simple point source. To extract the photometry of the nucleus, Jewitt et al. (2020) adopted an aperture of 02, while Bolin et al. (2020) employed an aperture of 048. As the Hubble Space Telescope (HST) observation provides a better resolution, and hence a more accurate morphology of the nucleus, and is closer to our FLAMINGOS-2 observations in time, we therefore followed Jewitt et al. (2020) and used an aperture of 02 on the FLAMINGOS-2 deep stacked Ks-band image. This results in photometry of the nucleus of 20.84 ± 0.06 mag.

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We then convert the apparent magnitude m_K to absolute magnitude H_K by taking into account the distance and the phase of 2I/Borisov as

$$\begin{eqnarray}&&{H}_{K}={m}_{K}-2.5{\mathrm{log}}_{10}({r}_{H}^{2}{{\rm{\Delta }}}^{2})-f(\alpha ).\end{eqnarray} \tag{ 1 }$$

Here r_H and Δ are the heliocentric and geocentric distances respectively, in astronomical units, of 2I/Borisov on 2019 December 7 UT. $f(\alpha )$ is the phase function given the phase angle α. Here we adopt a linear phase function $f(\alpha )=\beta \alpha$, with a nominal β of 0.04 mag per degree. With the absolute magnitude H_K, the radius of the nucleus r_n (in meter) can be inferred by (Jewitt 1991)

$$\begin{eqnarray}&&{r}_{n}=\displaystyle \frac{1.496\,\times \,{10}^{11}}{\sqrt{p}}\,\times \,{10}^{0.2({m}_{\odot }-{H}_{K})}.\end{eqnarray} \tag{ 2 }$$

Here p is the geometric albedo of the comet and ${m}_{\odot }$ is the magnitude of Sun, both in the H band. Unlike in the optical, there has not been much study of comet nucleus albedos in the infrared, so we adopt p = 0.07, which was the value for comet 10P/Tempel 2 (Tokunaga et al. 1992). From the Ks-band imaging, we estimate the radius of the comet nucleus to be 0.58 km,⁸ which is consistent with the value of Jewitt et al. (2020). However, we should note that this estimate can only serve as an upper limit, and the true nucleus radius can be only be revealed with exquisite spatial resolution imaging and more detailed modeling with space-based facilities, e.g., by Jewitt et al. (2020).

As there has been indirect evidence tracing water in 2I/Borisov (McKay et al. 2020), and the cometary activity was transitioning from CO-dominant volatile to H₂O-dominant volatile as it became closer to the Sun (Fitzsimmons et al. 2019; Bolin et al. 2020), it is imperative to search for direct evidence of water ice in the spectrum of 2I/Borisov. The infrared spectrum of 2I/Borisov, while close to the perihelion, does not show any sign of water ice, especially at 1.5 and 2 μm. This is consistent with the results of Yang et al. (2020) drawn from spectra at earlier epochs. As indicated in Yang et al. (2020), ground-based observations rarely reveal water ice in infrared spectra when comets are closer than 2.5 au from the Sun (Protopapa et al. 2018) due to the short lifetime of water ice. The only case when water ice has been detected within 2.5 au from the Sun was comet 103P/Hartley 2 with in situ observations (Protopapa et al. 2014) that could probe the inner most region of the coma. Nevertheless, we note that by comparing our spectra close to perihelion with spectra taken at earlier epochs by Yang et al. (2020), we can see the slope of the spectra are decreasing; in our case, the near perihelion spectrum even exhibits a negative slope of 2.2% per $1000\,\mathring{\rm A}$. We note that the decreasing slope can be caused by an increased abundance of pure water ice or hydrated minerals (Yang et al. 2009), which is likely as the cometary activity is transitioning from CO-dominant volatile to H₂O-dominant volatile, and the comet is close to the perihelion. Further observations as 2I/Borisov passes perihelion, especially at larger distances, will be pivotal for revealing water ice signatures.

We have conducted infrared observations of 2I/Borisov near perihelion. Our findings are as follows.

• 1.  
  Our infrared imaging does not show an apparent coma or cometary tail at first glance. However, this is due to our lack of sensitivity to the faint coma, particularly due to the bright sky background in the ground-based infrared observations and our relatively short exposure times.
• 2.  
  From the infrared imaging, we can infer the upper limit of the nucleus size, one week prior to perihelion, to be 0.58 km from Ks-band photometry, comparable to the results derived from HST imaging by Jewitt et al. (2020).
• 3.  
  The infrared spectrum of 2I/Borisov near perihelion does not show signs of water ice absorption at 1.5 and 2 μm. Nevertheless, the slope of the spectra have changed to a negative sign, hinting at a higher abundance of water ice compared to spectroscopic observations taken at earlier epochs.

We are indebted to the anonymous referee, whose comments greatly improved the manuscript. We thank Jennifer Karr for comments on the manuscript. We are grateful to the staff at the Gemini Telescope, especially Dr. Morten Andersen and Dr. Hwihyun Kim, for supporting our observations. Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), Ministério da Ciência, Tecnologia e Inovação (Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea).