Comet-like Tail Research Articles

The Active Asteroids Citizen Science Program: Overview and First Results

We present the Citizen Science program Active Asteroids and describe discoveries stemming from our ongoing project. Our NASA Partner program is hosted on the Zooniverse online platform and launched on 2021 August 31, with the goal of engaging the community in the search for active asteroids—asteroids with comet-like tails or comae. We also set out to identify other unusual active solar system objects, such as active Centaurs, active quasi-Hilda asteroids (QHAs), and Jupiter-family comets (JFCs). Active objects are rare in large part because they are difficult to identify, so we ask volunteers to assist us in searching for active bodies in our collection of millions of images of known minor planets. We produced these cutout images with our project pipeline that makes use of publicly available Dark Energy Camera data. Since the project launch, roughly 8300 volunteers have scrutinized some 430,000 images to great effect, which we describe in this work. In total, we have identified previously unknown activity on 15 asteroids, plus one Centaur, that were thought to be asteroidal (i.e., inactive). Of the asteroids, we classify four as active QHAs, seven as JFCs, and four as active asteroids, consisting of one main-belt comet (MBC) and three MBC candidates. We also include our findings concerning known active objects that our program facilitated, an unanticipated avenue of scientific discovery. These include discovering activity occurring during an orbital epoch for which objects were not known to be active, and the reclassification of objects based on our dynamical analyses.

Lyα Absorption in a “Croissant-like” Heliosphere

Lyα absorption profiles have been used to detect astrospheres and heliospheric absorption from the hydrogen wall and heliotail. Using magnetohydrodynamic models of the heliosphere, we can compare simulated to observed Lyα profiles to probe the neutral hydrogen within and near the heliosphere. There is an ongoing controversy whether the heliosphere has a long “comet-like” tail or a short “croissant-like” tail. Here we present the first Lyα absorption investigation using a croissant-like heliosphere. With identical boundary conditions we compare the BU model, which presents a croissant-like tail, and the Moscow model, which presents a comet-like tail. The BU and Moscow models present nearly identical Lyα profiles toward nose targets (α Cen and 36 Oph). Differences in Lyα profiles are shown toward the tail target (HD 35296). Despite the shortened heliotail of the croissant model, significant downwind heliosheath absorption is seen, just 5% shallower and shifted by 4 km s−1. This implies that an extended tail model is not required to reproduce the heliosheath Lyα absorption observations. Finer observation gratings may be able to resolve this shift. Additionally, when using higher interstellar medium (ISM) neutral and plasma densities and lower magnetic field (∣B LISM∣ = 3.2 μG, α BV ≈ 40°) than in the Moscow model, we find better agreement with observed Lyα profiles. None of the models presented show agreement in all directions simultaneously. Furthermore, we show that for the ISM conditions with the least certainty (n p,LISM, n H,LISM, T LISM, B LISM), B LISM has the most significant effect on the structure of the hydrogen wall and Lyα profiles.

On the likely magnesium–iron silicate dusty tails of catastrophically evaporating rocky planets

ABSTRACT Catastrophically evaporating rocky planets provide a unique opportunity to study the composition of small planets. The surface composition of these planets can be constrained via modelling their comet-like tails of dust. In this work, we present a new self-consistent model of the dusty tails: we physically model the trajectory of the dust grains after they have left the gaseous outflow, including an on-the-fly calculation of the dust cloud’s optical depth. We model two catastrophically evaporating planets: KIC 1255 b and K2-22 b. For both planets, we find the dust is likely composed of magnesium–iron silicates (olivine and pyroxene), consistent with an Earth-like composition. We constrain the initial dust grain sizes to be ∼ 1.25–1.75 μm and the average (dusty) planetary mass-loss rate to be ∼ 3$\, M_{\oplus } \mathrm{Gyr^{-1}}$. Our model shows that the origin of the leading tail of dust of K2-22 b is likely a combination of the geometry of the outflow and a low radiation pressure force to stellar gravitational force ratio. We find the optical depth of the dust cloud to be a factor of a few in the vicinity of the planet. Our composition constraint supports the recently suggested idea that the dusty outflows of these planets go through a greenhouse effect–nuclear winter cycle, which gives origin to the observed transit depth time variability. Magnesium–iron silicates have the necessary visible-to-infrared opacity ratio to give origin to this cycle in the high mass-loss state.

Recurring Activity Discovered on Quasi-Hilda 2009 DQ118

We have discovered two epochs of activity on quasi-Hilda 2009 DQ118. Small bodies that display comet-like activity, such as active asteroids and active quasi-Hildas, are important for understanding the distribution of water and other volatiles throughout the solar system. Through our NASA Partner Citizen Science project, Active Asteroids, volunteers classified archival images of 2009 DQ118 as displaying comet-like activity. By performing an in-depth archival image search, we found over 20 images from UT 2016 March 8–9 with clear signs of a comet-like tail. We then carried out follow-up observations of 2009 DQ118 using the 3.5 m Astrophysical Research Consortium Telescope at Apache Point Observatory, Sunspot, New Mexico, USA and the 6.5 m Magellan Baade Telescope at Las Campanas Observatory, Chile. These images revealed a second epoch of activity associated with the UT 2023 April 22 perihelion passage of 2009 DQ118. We performed photometric analysis of the tail and find that it had a similar apparent length and surface brightness during both epochs. We also explored the orbital history and future of 2009 DQ118 through dynamical simulations. These simulations show that 2009 DQ118 is currently a quasi-Hilda and that it frequently experiences close encounters with Jupiter. We find that 2009 DQ118 is currently on the boundary between asteroidal and cometary orbits. Additionally, it has likely been a Jupiter family comet or Centaur for much of the past 10 kyr and will be in these same regions for the majority of the next 10 kyr. Since both detected epochs of activity occurred near perihelion, the observed activity is consistent with sublimation of volatile ices. 2009 DQ118 is currently observable until ∼mid-October 2023. Further observations would help to characterize the observed activity.

Active Asteroid 311P/PanSTARRS: Rotational Instability as the Origin of its Multitails?

The active asteroid 311P/PanSTARRS is one of the two targets of the planned Chinese asteroid exploration mission Tianwen-2. During 2013, 311P experienced several mass-loss events and exhibited multiple comet-like tails. Here we analyze the morphology and surface brightness of the tails to investigate the dust environment around the nucleus and the mechanism of activities. We enhance the features of the tails using image processing techniques to obtain information about the morphologies of the tails, and fit the processed images with syndyne–synchrone diagrams. The fitting results give estimations of the upper limits of the durations (2–8 days) of eruptions and the dust size ranges (0.006 –38.9 mm) in the tails. The results of surface photometry performed for each dust tail show that the brightness distribution index of each tail ranged from approximately −1.81 to 0 and the dust size distribution indices of 311P's tails ranged from −2.29 to −1.45. The quantity of particles in each tail ranged from 0.5 to 8 × 106 kg, which leads to a total dust-loss quantity of 3.0 × 107 kg and a mass-loss rate of 1.59 kg s−1. Sublimation, continuous impacts, or tidal forces of planets are excluded as explanations for 311P’s activities, and rotational instability remains a possible activation cause without strong evidence against it.

The space weather around the exoplanet GJ 436b

Context. The space environment in which planets are embedded mainly depends on the host star and impacts the evolution of the planetary atmosphere. The quiet M dwarf GJ 436 hosts a close-in hot Neptune which is known to feature a comet-like tail of hydrogen atoms that escaped from its atmosphere due to energetic stellar irradiation. Understanding such star-planet interactions is essential to shed more light on planet formation and evolution theories, in particular the scarcity of Neptune-sized planets below a 3 d orbital period, also known as the ‘Neptune desert’. Aims. We aimed to characterise the stellar environment around GJ 436, which requires accurate knowledge of the stellar magnetic field. The latter is studied efficiently with spectropolarimetry, since it is possible to recover the geometry of the large-scale magnetic field by applying tomographic inversion on time series of circularly polarised spectra. Methods. We used spectropolarimetric data collected in the optical domain with Narval in 2016 to compute the longitudinal magnetic field, examine its periodic content via Lomb-Scargle periodogram and Gaussian process regression analysis, and finally reconstruct the large-scale field configuration by means of Zeeman-Doppler imaging. Results. We found an average longitudinal field of −12 G and a stellar rotation period of 46.6 d using a Gaussian process model and 40.1 d using Zeeman-Doppler imaging, which are both consistent with the literature. The Lomb-Scargle analysis did not reveal any significant periodicity. The reconstructed large-scale magnetic field is predominantly poloidal, dipolar, and axisymmetric, with a mean strength of 16 G. This is in agreement with magnetic topologies seen for other stars of a similar spectral type and rotation rate.

Simulated Gravity Field Estimation for the Main Belt Comet 133P/Elst-Pizarro Based on a Satellite-to-satellite Tracking Mode

Most asteroids and comets are formed in the early stages of the solar system and therefore contain a wealth of information about their birth. The asteroid exploration mission planned in the coming years by China will likely target the celestial body named 133P/Elst-Pizarro (estimated diameter of about 4 km). The orbit of this asteroid stays within the asteroid belt, but nevertheless, it displays a comet-like dust tail. In this study, we used differential tracking data between two simulated probes and the data from an Earth station to estimate 133P gravity field model. This observation mode is similar to how the gravity field was estimated for large celestial objects in the GRAIL and GRACE missions, but here the object is the very small 133P asteroid. We compared the estimated gravity fields obtained for 133P from the satellite-to-satellite combined with the Earth-based two-way range-rate observation mode, with only the Earth-based two-way range rate mode. The results show that the accuracy of the low-degree (4 degree and order) estimate of the gravity field is improved by one order of magnitude by using the satellite-to-satellite combined with the Earth-based two-way range-rate observation mode with respect to the Earth-only tracking. Furthermore, another order of magnitude improvement in the gravity field solution is gained by decreasing the orbit altitude from 12 to 8 km.

Large-scale structures in the stellar wind of fast-rotating stars spawned by the presence of Earth-like planets

ABSTRACT Forming planets around young, fast-rotating solar-like stars are exposed to an intense X-ray/extreme ultraviolet radiation field and strongly magnetized stellar winds, as a consequence of the high magnetic activity of these stars. Under these conditions, Earth-like exoplanets may experience a rapid loss of their primordial hydrogen atmospheres, resulting in atmosphereless rocky obstacles for the stellar winds. The interaction of stellar winds with those planets leads to the formation of potentially observable structures due to the formation of large-scale magnetic field and density disturbances in the vicinity of these planets, such as bow shocks, induced magnetospheres, and comet-like tails. In this work, we study the interaction between the stellar winds of active, fast-rotating solar-like stars in the superfast-magnetosonic regime with Earth-like, unmagnetized, tenuous atmosphere, planetary obstacles through numerical three-dimensional simulations using the pluto magnetohydrodynamical code. The properties of AB Doradus, a nearby young star with a small rotation period (0.51 d) and a strong flaring activity, have been used to parametrize this early wind state. Bow shock and induced magnetosphere formation are characterized through the Alfvénic Mach number MA of the wind, for different stellar wind configurations. Large bow shocks, up to an extension of ∼7.0 planetary radii, are found for low-MA winds. The general increase of density, temperature, and magnetic field in these large-scale structures formed around planets may result in potentially detectable spectral signatures.

Comet-like Activity Discovered on Quasi-Hilda Asteroid 2009 DQ118

We report evidence of comet-like activity on asteroid 2009 DQ118, a quasi-Hilda object near the 3:2 interior mean-motion resonance with Jupiter. This discovery was made by volunteers as part of the Active Asteroids Citizen Science project, a NASA Partner hosted on the Zooniverse platform. Follow-up archival searches yielded over 20 images of 2009 DQ118 exhibiting a comet-like tail on UT 2016 March 8 and 9 acquired with the Dark Energy Camera on the Blanco 4 m telescope at the Cerro Tololo Inter-American Observatory, Chile. These images were taken when 2009 DQ118 was near its 2016 perihelion passage. 2009 DQ118 will next reach perihelion on UT 2023 April 22; hence, the next several months are an excellent time to observe 2009 DQ118 to search for a second epoch of activity for this object.

Ground-based observability of Dimorphos DART impact ejecta: photometric predictions

ABSTRACT The Double Asteroid Redirection Test (DART) is a NASA mission intended to crash a projectile on Dimorphos, the secondary component of the binary (65803) Didymos system, to study its orbit deflection. As a consequence of the impact, a dust cloud will be be ejected from the body, potentially forming a transient coma- or comet-like tail on the hours or days following the impact, which might be observed using ground-based instrumentation. Based on the mass and speed of the impactor, and using known scaling laws, the total mass ejected can be roughly estimated. Then, with the aim to provide approximate expected brightness levels of the coma and tail extent and morphology, we have propagated the orbits of the particles ejected by integrating their equation of motion, and have used a Monte Carlo approach to study the evolution of the coma and tail brightness. For typical power-law particle size distribution of index –3.5, with radii rrmin = 1 μm and rmax = 1 cm, and ejection speeds near 10 times the escape velocity of Dimorphos, we predict an increase of brightness of ∼3 magnitudes right after the impact, and a decay to pre-impact levels some 10 d after. That would be the case if the prevailing ejection mechanism comes from the impact-induced seismic wave. However, if most of the ejecta is released at speeds of the order of ≳100 m s−1, the observability of the event would reduce to a very short time span, of the order of 1 d or shorter.

The Posttransit Tail of WASP-107b Observed at 10830 Å

Abstract Understanding the effects of high-energy radiation and stellar winds on planetary atmospheres is vital for explaining the observed properties of close-in exoplanets. Observations of transiting exoplanets in the triplet of metastable helium lines at 10830 Å allow extended atmospheres and escape processes to be studied for individual planets. We observed one transit of WASP-107b with NIRSPEC on Keck at 10830 Å. Our observations, for the first time, had significant posttransit phase coverage, and we detected excess absorption for over an hour after fourth contact. The data can be explained by a comet-like tail extending out to ∼7 planet radii, which corresponds to roughly twice the Roche lobe radius of the planet. Planetary tails are expected based on three-dimensional simulations of escaping exoplanet atmospheres, particularly those including the interaction between the escaped material and strong stellar winds, and have been previously observed at 10830 Å in at least one other exoplanet. With both the largest midtransit absorption signal and the most extended tail observed at 10830 Å, WASP-107b remains a keystone exoplanet for atmospheric escape studies.

The effects of magnetic fields on observational signatures of atmospheric escape in exoplanets: Double tail structures

ABSTRACTUsing 3D radiative MHD simulations and Lyman-α transit calculations, we investigate the effect of magnetic fields on the observational signatures of atmospheric escape in exoplanets. Using the same stellar wind, we vary the planet’s dipole field strength (Bp) from 0 to 10G. For Bp < 3G, the structure of the escaping atmosphere begins to break away from a comet-like tail following the planet (Bp = 0), as we see more absorbing material above and below the orbital plane. For Bp ≥ 3G, we find a ‘dead-zone’ around the equator, where low velocity material is trapped in the closed magnetic field lines. The dead-zone separates two polar outflows where absorbing material escapes along open field lines, leading to a double tail structure, above and below the orbital plane. We demonstrate that atmospheric escape in magnetized planets occurs through polar outflows, as opposed to the predominantly night-side escape in non-magnetized models. We find a small increase in escape rate with Bp, though this should not affect the time-scale of atmospheric loss. As the size of the dead-zone increases with Bp, so does the line centre absorption in Lyman-α, as more low-velocity neutral hydrogen covers the stellar disc during transit. For Bp < 3G the absorption in the blue wing decreases, as the escaping atmosphere is less funnelled along the line of sight by the stellar wind. In the red wing (and for Bp > 3G in the blue wing) the absorption increases caused by the growing volume of the magnetosphere. Finally we show that transits below and above the mid-disc differ caused by the asymmetry of the double tail structure.

Metastable Helium Absorptions with 3D Hydrodynamics and Self-consistent Photochemistry. I. WASP-69b, Dimensionality, X-Ray and UV Flux Level, Spectral Types, and Flares

Abstract The metastable helium (He*) lines near 10830 Å are ideal probes of atmospheric erosion—a common phenomenon of close-in exoplanet evolution. A handful of exoplanet observations yielded well-resolved He* absorption features in transits, yet they were mostly analyzed with 1D isothermal models prescribing mass-loss rates. This work devises 3D hydrodynamics coevolved with ray-tracing radiative transfer and nonequilibrium thermochemistry. Starting from the observed stellar/planetary properties with reasonable assumptions about the host’s high-energy irradiation, we predict from first principles the mass-loss rate, the temperature and ionization profiles, and 3D outflow kinematics. Our simulations well reproduce the observed He* line profiles and light curves of WASP-69b. We further investigate the dependence of He* observables on simulation conditions and host radiation. The key findings are as follows: (1) Simulations reveal a photoevaporative outflow (∼0.55 M ⊕ Gyr−1 ≃ 1011 g s−1) for WASP-69b without a prominent comet-like tail, consistent with the symmetric transit shape. (2) 3D simulations are mandatory for hydrodynamic features, including Coriolis force, advection, and kinematic line broadening. (3) EUV (>13.6 eV) photons dominate photoevaporative outflows and populate He* via recombination; FUV is also detrimental by destroying He*; X-ray plays a secondary role. (4) K stars hit the sweet spot of EUV/far-UV balance for He* line observation, while G and M stars are also worthy targets. (5) Stellar flares create characteristic responses in the He* line profiles.

Grazing, non-transiting disintegrating exoplanets observed with the planned Ariel space observatory

Disintegrating/evaporating rocky exoplanets can be observed not only as transiting planets, but also in a grazing, non-transiting regime, where the solid body of the planet does not transit, but part of the comet-like tail can transit. In this case the forward scattering on the escaping particles is the dominant process, which amplifies the photometric signal of the parent star detected by the observer. The change in the flux is small, about 10^-3 (1000 ppm) at the best properties of the planetary system, but if the observation is enough precise, the detection is possible. The planned Ariel space observatory is designed to achieve a stability of < 100 ppm (the goal is 10 ppm) over the temporal bandwidth of the transit, typically less than 10 hours. In this case study we took the disintegrating exoplanet Kepler-1520b and changed the orbital properties of the system to get a grazing, non-transiting orbit scenario, and investigated, how different particle sizes, species, Ariel observational channels, and other factors affect the amplitude of the forward-scattering peak, and the detectability of the scattering event. Our most important result is that the forward-scattering amplitude is not sensitive to the dust composition, but is very sensitive to the particle size, observational channel, and other factors. These factors can reduce mainly the detectability of 1-micron grains. 0.1-micron grains will be detectable at short wavelengths. 0.01-micron grains generate long and very small forward scattering amplitude, which is below the detection limit. Based on our results we can conclude that using Ariel will be possible to detect and investigate not only transiting, but also grazing, non-transiting disintegrating exoplanets based on the forward scattering. From the viewpoint of such objects the big advantage of Ariel will be the possibility of multiwavelength observations.

GJ 436b and the stellar wind interaction: simulations constraints using Ly α and H α transits

ABSTRACT The GJ 436 planetary system is an extraordinary system. The Neptune-sized planet that orbits the M3 dwarf revealed in the Ly α line an extended neutral hydrogen atmosphere. This material fills a comet-like tail that obscures the stellar disc for more than 10 h after the planetary transit. Here, we carry out a series of 3D radiation hydrodynamic simulations to model the interaction of the stellar wind with the escaping planetary atmosphere. With these models, we seek to reproduce the ${\sim}56{{\ \rm per\ cent}}$ absorption found in Ly α transits, simultaneously with the lack of absorption in H α transit. Varying the stellar wind strength and the EUV stellar luminosity, we search for a set of parameters that best fit the observational data. Based on Ly α observations, we found a stellar wind velocity at the position of the planet to be around [250–460] km s−1 with a temperature of [3–4] × 105 K. The stellar and planetary mass-loss rates are found to be 2 × 10−15 M⊙ yr−1 and ∼[6–10] × 109 g s−1, respectively, for a stellar EUV luminosity of [0.8–1.6] × 1027 erg s−1. For the parameters explored in our simulations, none of our models present any significant absorption in the H α line in agreement with the observations.

Constraining the Bulk Composition of Disintegrating Exoplanets Using Combined Transmission Spectra from JWST and SPICA

Abstract Disintegrating planets are ultrashort-period exoplanets that appear to have a comet-like dust tail. They are commonly interpreted as low-mass planets whose solid surface is evaporating, and whose tails are made of recondensing minerals. Transmission spectroscopy of the dust tails could thus allow us to directly probe the elementary compositions of these planets. Previous work already investigated the feasibility of such observations using the James Webb Space Telescope (JWST) mid-infrared instrument. In this study, we explore if one can obtain a strong constrain on the tail composition by adding spectroscopy at longer wavelengths using the Space Infrared Telescope for Cosmology and Astrophysics (SPICA) mid-infrared instrument. We use a simple model for the spatial distribution of the dust tails and produce their synthetic transmission spectra assuming various dust compositions. We find that combined infrared spectra from JWST and SPICA will allow us to diagnose various components of the dust tails. JWST will be able to detect silicate and carbide absorption features with a feature-to-noise ratio of ≳3 in the tail transmission spectrum of a disintegrating planet located within 100 pc from the Earth, with a transit depth deeper than 0.5%. SPICA can distinguish between Fe- and Mg-bearing crystalline silicates for planets at ≲100 pc with a transit depth of ≳2%. Transit searches with current and future space telescopes (e.g., TESS and PLATO) will provide ideal targets for such spectroscopic observations.

Microbial transfers from Venus to Earth.

The possibility that the clouds of Venus are habitats for microorganisms has been discussed for several decades. Over the past two decades evidence to support this point of view has grown with new data from space probes and space exploration. In this article we argue that microorganisms are likely to be widely present in the clouds of Venus, and may under certain conditions have a ready route to Earth. Such transfers could occur by the action of the solar wind that leads to expulsion of parts of the atmosphere laden with microorganisms. The expelled material forms a comet-like tail in the antisolar direction and during inferior conjunctions of Venus could lead to injections of bacteria and other microorganisms onto the Earth. In situations of very low sunspot activity as now prevails, with a consequent weakening of the magnetopause this flux of microbes will be considerably enhanced. The inferior conjunction of 4 June 2020 together with the prevailing deep minimum in the sunspot cycle provides a combination of circumstances that is particularly favorable to such a process.

The Fate of Asymptotic Giant Branch Winds in Massive Galaxies and the Intracluster Medium

Abstract Winds from asymptotic giant branch (AGB) stars not only provide mass and energy return, but also produce dust grains in massive elliptical galaxies. Due to the fast stellar velocity, the wind is thought to form a comet-like tail, similar to Mira in the Local Bubble. Many massive elliptical galaxies and cluster centrals host extended dusty cold filaments. We carry out both analytical and numerical studies of the interaction between an AGB wind and the surrounding hot gas. We find that the cooling time of the tail is inversely proportional to the ambient pressure. In the absence of cooling, or in low-pressure environments (e.g., the outskirts of elliptical galaxies), AGB winds are quickly mixed into the hot gas, and all the AGB winds have a similar appearance and head-to-tail ratio. In high-pressure environments, such as the Local Bubble and the central regions of massive elliptical galaxies, some of the gas in the mixing layer between the stellar wind and the surrounding hot gas can cool efficiently and cause the tail to become longer. Our simulated tail of Mira itself has a similar length and velocity to that observed, and appears similar to the simulated AGB tail in the central regions of massive galaxies. While confirmation awaits future studies, we speculate that instead of thermal instability, the induced condensation at the mixing layer of AGB winds may be the origin of cold filaments in massive galaxies and galaxy clusters. This naturally explains the existence of dust and polycyclic aromatic hydrocarbon in the filaments.

Physical Characterization of Active Asteroid (6478) Gault

Abstract Main belt asteroid (6478) Gault has been dynamically linked with two overlapping asteroid families: Phocaea, dominated by S-type asteroids, and Tamara, dominated by low-albedo C-types. This object has recently become an interesting case for study after images obtained in late 2018 revealed that it was active and displaying a comet-like tail. Previous authors have proposed that the most likely scenarios to explain the observed activity on Gault were rotational excitation or merger of near-contact binaries. Here we use new photometric and spectroscopic data of Gault to determine its physical and compositional properties. Lightcurves derived from the photometric data showed little variation over three nights of observations, which prevented us from determining the rotation period of the asteroid. Using WISE observations of Gault and the near-Earth Asteroid Thermal Model (NEATM) we determined that this asteroid has a diameter &lt;6 km. Near-infrared spectroscopic data obtained with the Infrared Telescope Facility showed a spectrum similar to that of S-complex asteroids, and a surface composition consistent with H chondrite meteorites. These results favor a compositional affinity between Gault and asteroid (25) Phocaea, and rules out a compositional link with the Tamara family. From the spectroscopic data we found no evidence of fresh material that could have been exposed during the outburst episodes.

Search for gas from the disintegrating rocky exoplanet K2-22b

Context. The red dwarf star K2-22 is transited every 9.14 h by an object which is best explained by being a disintegrating rocky exoplanet featuring a variable comet-like dust tail. While the dust is thought to dominate the transit light curve, gas is also expected to be present, either from being directly evaporated off the planet or by being produced by the sublimation of dust particles in the tail. Aims. Both ionized calcium and sodium have large cross-sections, and although present at low abundance, exhibit the strongest atomic absorption features in comets. We therefore also identify these species as the most promising tracers of circumplanetary gas in evaporating rocky exoplanets and search for them in the tail of K2-22 b to constrain the gas-loss and sublimation processes in this enigmatic object. Methods. We observed four transits of K2-22 b with X-shooter on the Very Large Telescope operated by ESO to obtain time series of intermediate-resolution (R ~ 11 400) spectra. Our analysis focussed on the two sodium D lines (588.995 and 589.592 nm) and the Ca+ triplet (849.802, 854.209, and 866.214 nm). The stellar calcium and sodium absorption was removed using the out-of-transit spectra. We searched for planet-related absorption in the velocity rest frame of the planet, which changes from approximately −66 to +66 km s−1 during the transit. Results. Since K2-22 b exhibits highly variable transit depths, we analysed the individual nights and their average. By injecting signals we reached 5σ upper limits on the individual nights that range from 11–13% and 1.7–2.0% for the sodium and ionized calcium absorption of the tail, respectively. Night 1 was contaminated by its companion star so we considered weighted averages with and without Night 1 and quote conservative 5σ limits without Night 1 of 9 and 1.4%, respectively. Assuming their mass fractions to be similar to those in the Earth’s crust, these limits correspond to scenarios in which 0.04 and 35% of the transiting dust is sublimated and observed as absorbing gas. However, this assumes the gas to be co-moving with the planet. We show that for the high irradiation environment of K2-22 b, sodium and ionized calcium could be quickly accelerated to 100s of km s−1 owing to radiation pressure and entrainment by the stellar wind, making these species much more difficult to detect. No evidence for such possibly broad and blue-shifted signals are seen in our data. Conclusions. Future observations aimed at observing circumplanetary gas should take into account the possible broad and blue-shifted velocity field of atomic and ionized species.