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Register a new userThe Little Dippers: Transits of Star-grazing Exocomets?
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We describe EPIC 205718330 and EPIC 235240266, two systems identified in the K2 data whose light curves contain episodic drops in brightness with shapes and durations similar to those of the young dipper stars, yet shallower by ~1-2 orders of magnitude. These little dippers have diverse profile shapes with durations of ~0.5-1.0 days and depths of ~0.1-1.0% in flux; however, unlike most of the young dipper stars, these do not exhibit any detectable infrared excess indicative of protoplanetary disks, and our ground-based follow-up spectra lack any signatures of youth while indicating these objects as kinematically old. After ruling out instrumental and/or data processing artifacts as sources of the dimming events, we investigate possible astrophysical mechanisms based on the light curve and stellar properties. We argue that the little dippers are consistent with transits of star-grazing exocomets, and speculate that they are signposts of massive non-transiting exoplanets driving the close-approach orbits.
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We assess a physically feasible explanation for the low number of discovered (near-)grazing planetary transits through all ground and space based transit surveys. We performed simulations to generate the synthetic distribution of detectable planets based on their impact parameter, and found that a larger number of (near-)grazing planets should have been detected than have been detected. Our explanation for the insufficient number of (near-)grazing planets is based on a simple assumption that a large number of (near-)grazing planets transit host stars which harbor dark giant polar spot, and thus the transit light-curve vanishes due to the occultation of grazing planet and the polar spot. We conclude by evaluating the properties required of polar spots in order to make disappear the grazing transit light-curve, and we conclude that their properties are compatible with the expected properties from observations.
Some of the most scientifically valuable transiting planets are those that were already known from radial velocity (RV) surveys. This is primarily because their orbits are well characterized and they preferentially orbit bright stars that are the targets of RV surveys. The Transiting Exoplanet Survey Satellite ({it TESS}) provides an opportunity to survey most of the known exoplanet systems in a systematic fashion to detect possible transits of their planets. HD~136352 (Nu$^2$~Lupi) is a naked-eye ($V = 5.78$) G-type main-sequence star that was discovered to host three planets with orbital periods of 11.6, 27.6, and 108.1 days via RV monitoring with the HARPS spectrograph. We present the detection and characterization of transits for the two inner planets of the HD~136352 system, revealing radii of $1.482^{+0.058}_{-0.056}$~$R_oplus$ and $2.608^{+0.078}_{-0.077}$~$R_oplus$ for planets b and c, respectively. We combine new HARPS observations with RV data from Keck/HIRES and the AAT, along with {it TESS} photometry from Sector 12, to perform a complete analysis of the system parameters. The combined data analysis results in extracted bulk density values of $rho_b = 7.8^{+1.2}_{-1.1}$~gcm$^{-3}$ and $rho_c = 3.50^{+0.41}_{-0.36}$~gcm$^{-3}$ for planets b and c, respectively, thus placing them on either side of the radius valley. The combination of the multi-transiting planet system, the bright host star, and the diversity of planetary interiors and atmospheres means this will likely become a cornerstone system for atmospheric and orbital characterization of small worlds.
Exocomets are small bodies releasing gas and dust which orbit stars other than the Sun. Their existence was first inferred from the detection of variable absorption features in stellar spectra in the late 1980s using spectroscopy. More recently, they have been detected through photometric transits from space, and through far-IR/mm gas emission within debris disks. As (exo)comets are considered to contain the most pristine material accessible in stellar systems, they hold the potential to give us information about early stage formation and evolution conditions of extra Solar Systems. In the Solar System, comets carry the physical and chemical memory of the protoplanetary disk environment where they formed, providing relevant information on processes in the primordial solar nebula. The aim of this paper is to compare essential compositional properties between Solar System comets and exocomets. The paper aims to highlight commonalities and to discuss differences which may aid the communication between the involved research communities and perhaps also avoid misconceptions. Exocomets likely vary in their composition depending on their formation environment like Solar System comets do, and since exocomets are not resolved spatially, they pose a challenge when comparing them to high fidelity observations of Solar System comets. Observations of gas around main sequence stars, spectroscopic observations of polluted white dwarf atmospheres and spectroscopic observations of transiting exocomets suggest that exocomets may show compositional similarities with Solar System comets. The recent interstellar visitor 2I/Borisov showed gas, dust and nuclear properties similar to that of Solar System comets. This raises the tantalising prospect that observations of interstellar comets may help bridge the fields of exocomet and Solar System comets.
In contrast to photometric transits, whose peak signal occurs at mid-transit due to occultation of the brightest region of the disk, polarimetric transits provide a signal upon ingress and egress due to occultation of the polarized stellar limb. Limb polarization, the bright corollary to limb darkening, arises from the $90^circ$ scattering angle and low optical depth experienced by photons at the limb. In addition to the ratio $R_{rm p} / R_*$, the amplitude of a polarimetric transit is expected to be controlled by the strength and width of the stellar limb polarization profile, which depend on the scattering-to-total opacity ratio at the stellar limb. We present a short list of the systems providing the highest expected signal-to-noise ratio for detection of this effect, and we draw particular attention to HD 80606b. This planet is spin/orbit misaligned, has a three-hour ingress, and has a bright parent star, which make it an attractive target. We report on test observations of an HD 80606b ingress with the POLISH2 polarimeter at the Lick Observatory Shane 3-m telescope. We conclude that unmodeled telescope systematic effects prevented polarimetric detection of this event. We outline a roadmap for further refinements of exoplanet polarimetry, whose eventual success will require a further factor of ten reduction in systematic noise.
The Oort cloud (OC) probably formed more than 4$,$Gyr ago and has been moving with the Sun in the Galaxy since, exposed to external influences, most prominently to the Galactic tide and passing field stars. Theories suggest that other stars might posses exocomets distributed similarly to our OC. We study the erosion of the OC and the possibility for capturing exocomets during the encounters with such field stars. We carry out simulations of flybys, where both stars are surrounded by a cloud of comets. We measure how many exocomets are transferred to the OC, how many OCs comets are lost, and how this depends on the other stars mass, velocity and impact parameter. Exocomets are transferred to the OC only during relatively slow ($lesssim0.5,$km$,$s$^{-1}$) and close ($lesssim10^5,$AU) flybys and these are expected to be extremely rare. Assuming that all passing stars are surrounded by a cloud of exocomets, we derive that the fraction of exocomets in the OC has been about $10^{-5}$--$10^{-4}$. Finally we simulate the OC for the whole lifetime of the Sun, taking into account the encounters and the tidal effects. The OC has lost 25--65% of its mass, mainly due to stellar encounters, and at most 10% (and usually much less) of its mass can be captured. However, exocomets are often lost shortly after the encounter that delivers them, due to the Galactic tide and consecutive encounters.
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