Context.
Hyperactive comets are a small group of comets whose activity is higher than expected. They seem to emit more water than would normally be expected given the size of their nucleus. Comet ...46P/Wirtanen (hereafter, 46P) is among these objects of interest. Investigating its activity and composition evolution could provide clues about its origins and its formation region in the Solar nebulae.
Aims.
Given the exceptional close approach of comet 46P to the Earth in 2018, we aim to study the evolution of its activity and composition as a function of heliocentric distances before and after perihelion.
Methods.
We used both TRAPPIST telescopes to monitor the comet for almost a year with broad and narrow-band filters. We derived the production rates of five gaseous species (OH, NH, CN, C
3
, and C
2
) using a Haser model as well as the A(
θ
)f
ρ
dust proxy parameter. The comet was also observed with the two optical high-resolution spectrographs UVES and ESPRESSO, mounted on the 8-m ESO VLT, to measure the isotopic ratios of C and N, along with the oxygen forbidden-line ratios and the NH
2
ortho-to-para ratios.
Results.
Over nearly a year, we followed the rise and decline of the production rates of different species as well as the dust activity of 46P on both pre- and post-perihelion. Relative abundances with respect to CN and OH along the orbit of the comet show constant and symmetric abundance ratios and a typical coma composition. We determined the rotation period of the nucleus using high-cadence observations and long series of CN images on several nights. We obtained a value of (9.18±0.05) hours at perihelion. Using the high-resolution spectra of 46P coma, we derived C and N isotopic ratios of 100±20 and 150±30 as well as a green-to-red forbidden oxygen OI line ratio of 0.23±0.02. We measured a NH
2
ortho-to-para ratio of 3.31 ±0.03 and derived an ammonia ratio of 1.19±0.03, corresponding to a spin temperature of 27±1 K.
Conclusions.
Narrow-band observations show that comet 46P is a hyperactive comet for which 40% of its nucleus surface is active. It has a typical composition, similar to other normal comets; however, an asymmetric behavior with respect to perihelion has been seen in its activity, which is typical of seasonal effects. Photometric measurements show no evidence for a change in the rotation period of the nucleus during this apparition. High-resolution spectra show that 46P has typical NH
2
ortho-to-para, OI lines ratios, and C and N isotopic ratios.
Line-of-sight integration of emissions from planetary and cometary atmospheres is the Abel transform of the emission rate, under the spherical symmetry assumption. Indefinite integrals constructed ...from the Abel transform integral are useful for implementing remote sensing data analysis methods, such as the numerical inverse Abel transform. We propose analytical expressions obtained by a suitable, non-alternating, series development to compute those indefinite integrals. We establish expressions allowing absolute accuracy control of the convergence of these series and illustrate how this accuracy depends on the number of terms involved in the series computation. We compare the analytical method with numerical computation techniques, which are found to be sufficiently accurate as well. Inverse Abel transform fitting is then tested in order to establish that the expected emission rate profiles can be retrieved from the observation of both planetary and cometary atmospheres. We show that the method is robust, i.e. that it can be applied even when the properties of the observed atmosphere depart from the assumed ones, especially when Tikhonov regularization is included. A first application is conducted over observation of comet 46P/Wirtanen, showing some variability, possibly attributable to an evolution of the contamination by dust and icy grains.
•Tailored analytical and numerical tools are applied to Inverse Abel transform fitting.•Exponential dependence is included in atmospheric inverse Abel transform fitting.•The CN 387 nm emission rate profile of 46P/Wirtanen is obtained using TRAPPIST South.
Context.
In the age of JWST, temperate terrestrial exoplanets transiting nearby late-type M dwarfs provide unique opportunities for characterising their atmospheres, as well as searching for ...biosignature gases. In this context, the benchmark TRAPPIST-1 planetary system has garnered the interest of a broad scientific community.
Aims.
We report here the discovery and validation of two temperate super-Earths transiting LP 890-9 (TOI-4306, SPECULOOS-2), a relatively low-activity nearby (32 pc) M6V star. The inner planet, LP 890-9 b, was first detected by TESS (and identified as TOI-4306.01) based on four sectors of data. Intensive photometric monitoring of the system with the SPECULOOS Southern Observatory then led to the discovery of a second outer transiting planet, LP 890-9 c (also identified as SPECULOOS-2 c), previously undetected by TESS. The orbital period of this second planet was later confirmed by MuSCAT3 follow-up observations.
Methods.
We first inferred the properties of the host star by analyzing its Lick/Kast optical and IRTF/SpeX near-infrared spectra, as well as its broadband spectral energy distribution, and
Gaia
parallax. We then derived the properties of the two planets by modelling multi-colour transit photometry from TESS, SPECULOOS-South, MuSCAT3, ExTrA, TRAPPIST-South, and SAINT-EX. Archival imaging, Gemini-South/Zorro high-resolution imaging, and Subaru/IRD radial velocities also support our planetary interpretation.
Results.
With a mass of 0.118 ± 0.002
M
⊙
, a radius of 0.1556 ± 0.0086
R
⊙
, and an effective temperature of 2850 ± 75 K, LP 890-9 is the second-coolest star found to host planets, after TRAPPIST-1. The inner planet has an orbital period of 2.73 d, a radius of 1.320
−0.027
+0.053
R
⊕
, and receives an incident stellar flux of 4.09 ± 0.12
S
⊕
. The outer planet has a similar size of 1.367
−0.039
+0.055
R
⊕
and an orbital period of 8.46 d. With an incident stellar flux of 0.906 ± 0.026
S
⊕
, it is located within the conservative habitable zone, very close to its inner limit (runaway greenhouse). Although the masses of the two planets remain to be measured, we estimated their potential for atmospheric characterisation via transmission spectroscopy using a mass-radius relationship and found that, after the TRAPPIST-1 planets, LP 890-9 c is the second-most favourable habitable-zone terrestrial planet known so far (assuming for this comparison a similar atmosphere for all planets).
Conclusions.
The discovery of this remarkable system offers another rare opportunity to study temperate terrestrial planets around our smallest and coolest neighbours.
•Asteroid Apophis was found in a non-principal axis rotation state (tumbling).•Its dynamical parameters and a shape model were derived.•Its retrograde spin calls for a further assessment of its ...impact probability.•Recent estimates of asteroid nutational damping times were reviewed and applied.•Parameters of tumbling asteroids place constraints on their evolution and properties.
Our photometric observations of Asteroid (99942) Apophis from December 2012 to April 2013 revealed it to be in a state of non-principal axis rotation (tumbling). We constructed its spin and shape model and found that it is in a moderately excited Short Axis Mode (SAM) state with a ratio of the rotational kinetic energy to the basic spin state energy E/E0=1.024±0.013. (All quoted uncertainties correspond to 3σ.) The greatest and intermediate principal moments of inertia are nearly the same with I2/I3=0.965-0.015+0.009, but the smallest principal moment of inertia is substantially lower with I1/I3=0.61-0.08+0.11; the asteroid’s dynamically equivalent ellipsoid is close to a prolate ellipsoid. The precession and rotation periods are Pϕ=27.38±0.07h and Pψ=263±6h, respectively; the strongest observed lightcurve amplitude for the SAM case is in the 2nd harmonic of P1=Pϕ-1-Pψ-1-1=30.56±0.01h. The rotation is retrograde with the angular momentum vector’s ecliptic longitude and latitude of 250° and -75° (the uncertainty area is approximately an ellipse with the major and minor semiaxes of 27° and 14°, respectively). An implication of the retrograde rotation is a somewhat increased probability of the Apophis’ impact in 2068, but it is still very small with the risk level on the Palermo Scale remaining well below zero. Apophis is a member of the population of slowly tumbling asteroids. Applying the theory of asteroid nutational damping by Breiter et al. (Breiter, S., Rożek, A., Vokrouhlický, D. 2012. Mon. Not. R. Astron. Soc. 427, 755–769), we found that slowly tumbling asteroids predominate in the spin rate–size range where their estimated damping times are greater than about 0.2Gyr. The appearance that the PA/NPA rotators transition line seems to follow a line of constant damping time may be because there are two or more asteroid spin evolution mechanisms in play, or the factor of μQ (the elastic modulus times the quality factor) is not constant but it may decrease with decreasing asteroid size, which would oppose the trend due to decreasing collisional age or excitation time.
Context. (1) Ceres is the largest body in the main asteroid belt and one of the most intriguing objects in the solar system, in part because of the discovery of water outgassing by the Herschel Space ...Observatory (HSO) and its still-debated origin. Ceres was the target of NASA’s Dawn spacecraft for 3.5 yr, which achieved a detailed characterization of the dwarf planet. The possible influence of the local flux of solar energetic particles (SEP) on the production of a Cerean exosphere and water vapor has been suggested, in addition to the sublimation of water ice that depends on the temperature, meaning the heliocentric distance. Aims. We used the opportunity of both the perihelion passage of (1) Ceres in April 2018, and the presence of Dawn in its vicinity (for measuring the SEP flux in real time) to check the influence of heliocentric distance and SEP flux on water outgassing. Methods. We searched for OH emission lines near the limb of Ceres in the near-UV with the UVES spectrograph mounted on the 8-m ESO Very Large Telescope. Two spectra were recorded when Ceres was close to its perihelion, in February 2018, and with Dawn spacecraft orbiting Ceres. It was possible to simultaneously measure energetic particles around Ceres at the time of our observations. Results. Our observations did not permit detection of OH emission lines to a very high sensitivity level. This level is estimated to correspond to a global water production rate of QH2O ∽ 2 × 1026 $Q_{\textrm{H}_2O}\,{\sim}\,2\,{\times}\,10^{26}$ QH2O ~ 2 × 1026 molecules s−1, similar to the water production rate derived from HSO observations. The solar energetic particles flux measured around Ceres was negligible at the time of these observations. Conclusions. Our observations support the idea that heliocentric distance (i.e., the sublimation of water ice) does not play a major role in the water emission from Ceres. This production rate could be either related to SEP events or to other mechanisms, possibly of endogenic origin.
We report observations of the recently discovered, nearby late-M dwarf WISE J072003.20-084651.2. With high-resolution optical and near-infrared spectroscopy, we measure a stable radial velocity of ...+83.8 + or - 0.3 km s super(-1), indicative of old disk kinematics and consistent with the angular separation of the possible companion. We measure a projected rotational velocity of v sin i = 8.0 + or - 0.5 km s super(-1) and find evidence of low-level variabilty (~1.5%) in a 13 day TRAPPIST light curve, but cannot robustly constrain the rotational period. We also observe episodic changes in brightness (1%-2%) and occasional flare bursts (4%-8%) with a 0.8% duty cycle, and order-of-magnitude variations in H alpha line strength. Combined, these observations reveal WISE J0720-0846 to be an old, very low-mass binary whose components straddle the hydrogen burning minimum mass, and whose primary is a relatively rapid rotator and magnetically active.
Abstract
We report the discovery of WASP-166b, a super-Neptune planet with a mass of 0.1 MJup (1.9 MNep) and a bloated radius of 0.63 RJup. It transits a V = 9.36, F9V star in a 5.44-d orbit that is ...aligned with the stellar rotation axis (sky-projected obliquity angle λ = 3 ± 5 deg). Variations in the radial-velocity measurements are likely the result of magnetic activity over a 12-d stellar rotation period. WASP-166b appears to be a rare object within the ‘Neptune desert’.
Abstract
The relative rarity of giant planets around low-mass stars compared with solar-type stars is a key prediction from the core-accretion planet formation theory. In this paper we report on the ...discovery of four gas giant planets that transit low-mass late K and early M dwarfs. The planets HATS-74Ab (TOI 737b), HATS-75b (TOI 552b), HATS-76b (TOI 555b), and HATS-77b (TOI 730b) were all discovered from the HATSouth photometric survey and follow-up using TESS and other photometric facilities. We use the new ESPRESSO facility at the VLT to confirm systems and measure their masses. We find that these planets have masses of 1.46 ± 0.14
M
J, 0.491 ± 0.039
M
J, 2.629 ± 0.089
M
J, and
1.374
−
0.074
+
0.100
M
J, respectively, and radii of 1.032 ± 0.021
R
J, 0.884 ± 0.013
R
J, 1.079 ± 0.031
R
J, and 1.165 ± 0.021
R
J, respectively. The planets all orbit close to their host stars with orbital periods ranging from 1.7319 days to 3.0876 days. With further work, we aim to test core-accretion theory by using these and further discoveries to quantify the occurrence rate of giant planets around low-mass host stars.
Context.
With an estimated diameter in the 320–350 km range, (704) Interamnia is the fifth largest main belt asteroid and one of the few bodies that fills the gap in size between the four largest ...bodies with
D
> 400 km (Ceres, Vesta, Pallas and Hygiea) and the numerous smaller bodies with diameter ≤200 km. However, despite its large size, little is known about the shape and spin state of Interamnia and, therefore, about its bulk composition and past collisional evolution.
Aims.
We aimed to test at what size and mass the shape of a small body departs from a nearly ellipsoidal equilibrium shape (as observed in the case of the four largest asteroids) to an irregular shape as routinely observed in the case of smaller (
D
≤ 200 km) bodies.
Methods.
We observed Interamnia as part of our ESO VLT/SPHERE large program (ID: 199.C-0074) at thirteen different epochs. In addition, several new optical lightcurves were recorded. These data, along with stellar occultation data from the literature, were fed to the All-Data Asteroid Modeling algorithm to reconstruct the 3D-shape model of Interamnia and to determine its spin state.
Results.
Interamnia’s volume-equivalent diameter of 332 ± 6 km implies a bulk density of
ρ
= 1.98 ± 0.68 g cm
−3
, which suggests that Interamnia – like Ceres and Hygiea – contains a high fraction of water ice, consistent with the paucity of apparent craters. Our observations reveal a shape that can be well approximated by an ellipsoid, and that is compatible with a fluid hydrostatic equilibrium at the 2
σ
level.
Conclusions.
The rather regular shape of Interamnia implies that the size and mass limit, under which the shapes of minor bodies with a high amount of water ice in the subsurface become irregular, has to be searched among smaller (
D
≤ 300 km) less massive (
m
≤ 3 × 10
19
kg) bodies.
Context.
The largest asteroids in the Koronis family (sizes ≥25 km) have very peculiar rotation state properties, with the retrograde- and prograde-rotating objects being distinctly different. A ...recent re-analysis of observations suggests that one of the asteroids formerly thought to be retrograde-rotating, 208 Lacrimosa, in reality exhibits prograde rotation, yet other properties of this object are discrepant with other members this group.
Aims.
We seek to understand whether the new spin solution of Lacrimosa invalidates the previously proposed model of the Koronis large members or simply reveals more possibilities for the long-term evolutionary paths, including some that have not yet been explored.
Methods.
We obtained additional photometric observations of Lacrimosa, and included thermal and occultation data to verify its new spin solution. We also conducted a more detailed theoretical analysis of the long-term spin evolution to understand the discrepancy with respect to the other prograde-rotating large Koronis members.
Results.
We confirm and substantiate the previously suggested prograde rotation of Lacrimosa. Its spin vector has an ecliptic longitude and latitude of (
λ
,
β
) = (15° ± 2°, 67° ± 2°) and a sidereal rotation period
P
= 14.085734 ± 0.000007 h. The thermal and occultation data allow us to calibrate a volume equivalent size of
D
= 44 ± 2 km of Lacrimosa. The observations also constrain the shape model relatively well. Assuming uniform density, the dynamical ellipticity is Δ = 0.35 ± 0.05. Unlike other large prograde-rotating Koronis members, Lacrimosa spin is not captured in the Slivan state. We propose that Lacrimosa differed from this group in that it had initially slightly larger obliquity and longer rotation period. With those parameters, it jumped over the Slivan state instead of being captured and slowly evolved into the present spin configuration. In the future, it is likely to be captured in the Slivan state corresponding to the proper (instead of forced) mode of the orbital plane precession in the inertial space.
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