Likely Transiting Exocomets Detected by Kepler

Abstract: We present the first good evidence for exocomet transits of a host star in continuum light in data from the Kepler mission. The Kepler star in question, KIC 3542116, is of spectral type F2V and is quite bright at K_p = 10. The transits have a distinct asymmetric shape with a steeper ingress and slower egress that can be ascribed to objects with a trailing dust tail passing over the stellar disk. There are three deeper transits with depths of ~0.1% that last for about a day, and three that are several times more shallow and of shorter duration. The transits were found via an exhaustive visual search of the entire Kepler photometric data set, which we describe in some detail. We review the methods we use to validate the Kepler data showing the comet transits, and rule out instrumental artefacts as sources of the signals. We fit the transits with a simple dust-tail model, and find that a transverse comet speed of ~35-50 km/s and a minimum amount of dust present in the tail of ~10^16 g are required to explain the larger transits. For a dust replenishment time of ~10 days, and a comet lifetime of only ~300 days, this implies a total cometary mass of > 3 x 10^17 g, or about the mass of Halley's comet. We also discuss the number of comets and orbital geometry that would be necessary to explain the six transits detected over the four years of Kepler prime-field observations. Finally, we also report the discovery of a single comet-shaped transit in KIC 11084727 with very similar transit and host-star properties.

• The paper’s main contribution is the identification and confirmation of exocomet transits by analyzing six asymmetric events in Kepler data.
• The study employed a dust-tail model yielding comet speeds of 35–50 km/s and estimating a dust mass of around 10^16 g per transit.
• The findings imply that exocomet transits may be common around F-type stars, paving the way for future photometric and spectroscopic investigations.

Overview of "Likely Transiting Exocomets Detected by Kepler"

This paper, authored by S. Rappaport et al., presents compelling evidence for the detection of transiting exocomets in the data from the Kepler mission. The primary target of this study, KIC 3542116, is an F2V-type star, whose light curve exhibited six asymmetric transit events, interpreted as exocomet transits due to the distinctive asymmetric profile characterized by a steep ingress and a more extended egress. This morphology can be attributed to the presence of a trailing dust tail.

Key Findings

• Detection and Validation: The detection was achieved through a meticulous visual examination of the entire Kepler photometric dataset. The six transits were rigorously validated to rule out instrumental artifacts and confirmed through supplementary imaging and spectral analyses.
• Transit Modeling: A simple dust-tail model was fitted to the observed transits. This modeling yielded transverse comet speeds of 35-50 km/s for the largest transits and necessitated a dust mass in the tail of approximately $10^{16}$ g to explain the transit depths.
• Cometary Dynamics and Mass: The study inferred a dust replenishment time of around ten days and a comet lifetime of approximately 300 days, leading to an estimated total mass for the comet of $\gtrsim 3 \times 10^{17}$ g, comparable to Halley's comet.
• Additional Discovery: A single transit event with similar characteristics was observed for another star, KIC 11084727, suggesting that such phenomena might not be rare.

Implications and Future Directions

The identification of these transit signals as likely exocomet events opens new avenues for understanding minor body populations in extrasolar systems. Traditionally, the detection sensitivity has been skewed towards large exoplanetary bodies; however, this methodology highlights the potential of using space-based photometric data, such as that from Kepler, to study smaller cometary bodies.

The confirmed asymmetric transit profiles align with models of comet dust tails, potentially providing insights into the compositional and dynamic properties of these bodies. Given the high-speed transit signature, there is a suggestion that these comets may possess highly elongated orbital paths. Further spectroscopic studies may reveal whether materials composing these cometary bodies are consistent with known populations in our solar system, and may cement the link between photometric and spectroscopic observations of comet transits.

From a theoretical perspective, these findings motivate a re-examination of dynamical models of cometary body injection into stellar grazing orbits, potentially involving mechanisms like the Kozai-Lidov effect or secular resonances driven by distant planets. Additionally, this study raises questions regarding the preferential occurrence of such transits around F-type stars and whether observational biases or intrinsic stellar properties may enhance these detections.

Conclusion

The study presented by Rappaport et al. makes a significant contribution to the growing body of work on minor bodies in exoplanetary systems, demonstrating how detailed analysis of space-based photometric datasets can reveal previously underexplored phenomena like exocomet transits. These findings pave the way for future investigations into the prevalence, composition, and dynamics of cometary bodies within and beyond our solar system, and suggest practical observation strategies for upcoming missions such as TESS and JWST to further elucidate this fascinating aspect of extrasolar planetary systems.