Detection of CN gas in Interstellar Object 2I/Borisov (1909.12144v2)

Abstract: The detection of Interstellar Objects passing through the Solar System offers the promise of constraining the physical and chemical processes involved in planetary formation in other extrasolar systems. While the effect of outgassing by 1I/2017 U1 ('Oumuamua) was dynamically observed, no direct detection of the ejected material was made. The discovery of the active interstellar comet 2I/Borisov means spectroscopic investigations of the sublimated ices is possible for this object. We report the first detection of gas emitted by an interstellar comet via the near-UV emission of CN from 2I/Borisov at a heliocentric distance of $r$ = 2.7 au on 2019 September 20. The production rate was found to be Q(CN) = $(3.7\pm0.4)\times10^{24}$ s$^{-1}$, using a simple Haser model with an outflow velocity of 0.5 km s$^{-1}$. No other emission was detected, with an upper limit to the production rate of C$_2$ of $4\times10^{24}$ s$^{-1}$. The spectral reflectance slope of the dust coma over $3900$ \AA\ $< \lambda< 6000$ \AA \ is steeper than at longer wavelengths, as found for other comets. Broad band $R_c$ photometry on 2019 September 19 gave a dust production rate of $Af\rho=143\pm10$ cm. Modelling of the observed gas and dust production rates constrains the nuclear radius to $0.7-3.3$ km assuming reasonable nuclear properties. Overall, we find the gas, dust and nuclear properties for the first active Interstellar Object are similar to normal Solar System comets.

• The paper presents the first spectroscopic detection of CN gas in 2I/Borisov, with a production rate of 3.7×10^24 s⁻¹.
• It employs the Haser model and an outflow velocity of 0.5 km/s to analyze the spectroscopic data and differentiate CN from other emissions like C₂.
• The analysis constrains the nucleus size to 0.7–3.3 km and highlights compositional similarities with carbon-chain depleted solar system comets.

Detection of CN Gas in Interstellar Object 2I/Borisov

In this paper, the authors present their findings on the detection and analysis of CN gas in the interstellar object 2I/Borisov, marking the first spectroscopic observation of gas emissions from an interstellar comet. The detection of CN in this exotic interloper provides empirical data that contribute to the understanding of the chemical properties of cometary nuclei from beyond our solar system.

The authors conducted observations using the 4.2 m William Herschel Telescope equipped with the ISIS spectrograph. Their analysis of spectroscopic data revealed a significant presence of CN gas, characterized by a production rate of $Q(CN) = (3.7\pm0.4)\times10^{24}$ s$^{-1}$, observed at a heliocentric distance of 2.7 au. The paper employs a Haser model to interpret the spectroscopic data, applying an outflow velocity of 0.5 km s$^{-1}$.

It is worth noting that no significant emissions of other gases, such as C$_2$, were detected, with an upper limit placed at $Q(C_2) \leq 4 \times 10^{24}$ s$^{-1}$. This absence suggests the possibility of differing compositional properties compared to classical solar system comets.

The authors modeled the dust and gas production to constrain the nuclear radius of 2I/Borisov to a range between 0.7 and 3.3 km, assuming characteristics similar to those of typical solar system nuclei. The dust properties inferred from broad band photometry indicate a steep reflectance slope at shorter wavelengths, aligning with observations of solar system comets and suggesting a commonality in dust grain properties.

The results hold several implications. Practically, the research enhances our methodologies for spectroscopy and modeling of interstellar bodies, potentially aiding in future studies of such objects. Theoretically, these findings align with the heterogeneous nature of cometary nuclei postulated in solar system formation models. The comparison of the CN production rates and the lack of C$_2$ suggest that 2I/Borisov could be representative of a subset of comets within our own solar system, specifically those exhibiting carbon-chain depletion.

Looking forward, the authors propose that follow-up observations at different spectral bands, including infrared, would be crucial for a comprehensive understanding of the volatiles present. Furthermore, these observations could shed light on sublimation characteristics and provide a deeper insight into the evolutionary paths of interstellar objects. The comparative paper of such objects could eventually contribute to developing a cosmic taxonomy of small bodies, advancing our understanding of planetary formation processes across different stellar systems.