Transit timing variation signature of planet migration: the case of K2-24

The convergent migration of two planets in a gaseous disc can lead to mean motion resonance (MMR) capture. In addition, pairs of planets in or near MMRs are known to produce strong transit timing variations (TTVs). In this paper, we study the impact of disc-induced migrations on the TTV signal of pairs of planets that enter a resonant configuration. We show that disc-induced migration creates a correlation between the amplitude and the period of the TTVs. We study the case of K2-24, a system of two planets whose period ratio indicates that they are in or near the 2:1 MMR, with non-zero eccentricities and large-amplitude TTVs. We show that a simple disc-induced migration cannot reproduce the observed TTVs. Moreover, we propose a formation scenario in which the capture in resonance during migration in a disc with strong eccentricity damping is followed by eccentricity excitation during the dispersal of the disc. This is assisted by a third planet whose presence has been suggested by radial velocity observations. This scenario accounts for the eccentricities of the two planets and their period ratio, and it accurately reproduces the amplitude and period of the TTVs. It allows for a unified view of the formation and evolution history of K2-24, from disc-induced migration to its currently observed properties.

Extreme kinematic misalignment in IllustrisTNG galaxies: the origin, structure, and internal dynamics of galaxies with a large-scale counterrotation

ABSTRACT We provide an in-deep analysis of 25 galaxies with substantial counterrotation from IllustrisTNG100 simulations in the stellar mass range 2×109−3×1010 M⊙. The counterrotation is a result of an external gas infall ≈2–8 Gyr ago. The infall leads to the removal of pre-existing gas, which is captured and mixed together with the infalling component. This mixture ends up in the counterrotating gaseous disc where ${\approx}90{{\ \rm per\ cent}}$ of counterrotating stars formed in-situ. During the early phases of the infall, gas can be found in extended structures which, in some galaxies, are similar to (nearly-) polar ring-like components. We suggest that the AGN activity does not cause the counterrotation, although it is efficiently triggered by the retrograde gas infall, and it correlates well with the misaligned component appearance. We also find the vertical-to-radial velocity dispersion ratio above unity implying the importance of misalignment in shaping the velocity ellipsoids.

Formation of satellites in circumplanetary discs generated by disc instability

ABSTRACT We investigated the formation and evolution of satellite systems in a cold, extended circumplanetary disc (CPD) around a 10MJupiter gas giant, which was formed by gravitational instability at 50 au from its star. The disc parameters were from a 3D global smoothed particle hydrodynamics simulation. We used a population synthesis approach, where we placed satellite embryos in this disc, and let them accrete mass, migrate, collide until the gaseous disc is dissipated. In each run, we changed the initial dust-to-gas ratio, dispersion- and refilling time-scales within reasonable limits, as well as the number of embryos and their starting locations. We found that most satellites have mass similar to the Galilean ones, but very few can reach a maximum of 3MEarth due to the massive CPD. Large moons are often form as far as 0.5Rdisc. The migration rate of satellites are fast, hence during the disc lifetime, an average of 10MEarth worth of moons will be engulfed by the planet, increasing greatly its metallicity. We also investigated the effect of the planet’s semimajor axis on the resulting satellite systems by rescaling our model. This test revealed that for the discs closer to the star, the formed moons are lighter, and a larger amount of satellites are lost into the planet due to the even faster migration. Finally, we checked the probability of detecting satellites like our population, which resulted in a low number of ≤ 3 per cent even with upcoming powerful telescopes like E-ELT.

Rapid filamentary accretion as the origin of extended thin discs

ABSTRACT Galactic outflows driven by stellar feedback are crucial for explaining the inefficiency of star formation in galaxies. Although strong feedback can promote the formation of galactic discs by limiting star formation at early times and removing low angular momentum (AM) gas, it is not understood how the same feedback can result in diverse objects such as elliptical galaxies or razor thin spiral galaxies. We investigate this problem using cosmological zoom-in simulations of two galaxies forming within 1012 M⊙ haloes with almost identical mass accretion histories and halo spin parameters. However, the two resulting galaxies end up with very different bulge-to-disc ratios at z = 0. At z > 1.5, the two galaxies feature a surface density of star formation ΣSFR ≃ 10 M⊙ yr−1 kpc−2, leading to strong outflows. After the last starburst episode, both galaxies feature a dramatic gaseous disc growth from 1 to 5 kpc during 1 Gyr, a decisive event we dub ‘the Grand Twirl’. After this event, the evolutionary tracks diverge strongly, with one galaxy ending up as a bulge-dominated galaxy, whereas the other ends up as a disc-dominated galaxy. The origins of this dichotomy are the AM of the accreted gas, and whether it adds constructively to the initial disc angular momentum. The build-up of this extended disc leads to a rapid lowering of ΣSFR by over two orders of magnitude with ΣSFR ≲ 0.1 M⊙ yr−1 kpc−2, in remarkable agreement with what is derived from Milky Way stellar populations. As a consequence, supernovae explosions are spread out and cannot launch galactic outflows anymore, allowing for the persistence of a thin, gently star-forming, extended disc.

Gas kinematics of key prebiotic molecules in GV Tau N revealed with an ALMA, PdBI, and Herschel synergy

ABSTRACT A large effort has been made to detect warm gas in the planet formation zone of circumstellar discs using space and ground-based near-infrared facilities. GV Tau N, the most obscured component of the GV Tau system, is an outstanding source, being one of the first targets detected in HCN and the only one detected in CH4 so far. Although near-infrared observations have shed light on its chemical content, the physical structure and kinematics of the circumstellar matter remained unknown. We use interferometric images of the HCN 3→2 and 13CO 3→2 lines, and far-IR observations of 13CO, HCN, CN, and H2O transitions to discern the morphology, kinematics, and chemistry of the dense gas close to the star. These observations constitute the first detection of H2O towards GV Tau N. Moreover, ALMA high spatial resolution (∼ 7 au) images of the continuum at 1.1 mm and the HCN 3→2 line resolve different gas components towards GV Tau N, a gaseous disc with R∼25 au, an ionized jet, and one (or two) molecular outflows. The asymmetric morphology of the gaseous disc shows that it has been eroded by the jet. All observations can be explained if GV Tau N is binary, and the primary component has a highly inclined individual disc relative to the circumbinary disc. We discuss the origin of the water and the other molecules emission according to this scenario. In particular, we propose that the water emission would come from the disrupted gaseous disc and the molecular outflows.

The GRAVITY young stellar object survey

Context. The inner regions of the discs of high-mass young stellar objects (HMYSOs) are still poorly known due to the small angular scales and the high visual extinction involved. Aims. We deploy near-infrared spectro-interferometry to probe the inner gaseous disc in HMYSOs and investigate the origin and physical characteristics of the CO bandhead emission (2.3–2.4 μm). Methods. We present the first GRAVITY/VLTI observations at high spectral (ℛ = 4000) and spatial (mas) resolution of the CO overtone transitions in NGC 2024 IRS 2. Results. The continuum emission is resolved in all baselines and is slightly asymmetric, displaying small closure phases (≤8°). Our best ellipsoid model provides a disc inclination of 34° ±1°, a disc major axis position angle (PA) of 166° ± 1°, and a disc diameter of 3.99 ± 0.09 mas (or 1.69 ± 0.04 au, at a distance of 423 pc). The small closure phase signals in the continuum are modelled with a skewed rim, originating from a pure inclination effect. For the first time, our observations spatially and spectrally resolve the first four CO bandheads. Changes in visibility, as well as differential and closure phases across the bandheads are detected. Both the size and geometry of the CO-emitting region are determined by fitting a bidimensional Gaussian to the continuum-compensated CO bandhead visibilities. The CO-emitting region has a diameter of 2.74±0.070.08 mas (1.16 ± 0.03 au), and is located in the inner gaseous disc, well within the dusty rim, with inclination and PA matching the dusty disc geometry, which indicates that both dusty and gaseous discs are coplanar. Physical and dynamical gas conditions are inferred by modelling the CO spectrum. Finally, we derive a direct measurement of the stellar mass of M* ∼ 14.7−3.6+2 M⊙ by combining our interferometric and spectral modelling results.

The frequency of gaseous debris discs around white dwarfs

ABSTRACT A total of 1–3 per cent of white dwarfs are orbited by planetary dusty debris detectable as infrared emission in excess above the white dwarf flux. In a rare subset of these systems, a gaseous disc component is also detected via emission lines of the Ca ii 8600 Å triplet, broadened by the Keplerian velocity of the disc. We present the first statistical study of the fraction of debris discs containing detectable amounts of gas in emission at white dwarfs within a magnitude and signal-to-noise ratio limited sample. We select 7705 single white dwarfs spectroscopically observed by the Sloan Digital Sky Survey (SDSS) and Gaia with magnitudes g ≤ 19. We identify five gaseous disc hosts, all of which have been previously discovered. We calculate the occurrence rate of a white dwarf hosting a debris disc detectable via Ca ii emission lines as $0.067\, \pm \, ^{0.042}_{0.025}$ per cent. This corresponds to an occurrence rate for a dusty debris disc to have an observable gaseous component in emission as 4 ± $_{2}^{4}$ per cent. Given that variability is a common feature of the emission profiles of gaseous debris discs, and the recent detection of a planetesimal orbiting within the disc of SDSS J122859.93+104032.9, we propose that gaseous components are tracers for the presence of planetesimals embedded in the discs and outline a qualitative model. We also present spectroscopy of the Ca ii triplet 8600 Å region for 20 white dwarfs hosting dusty debris discs in an attempt to identify gaseous emission. We do not detect any gaseous components in these 20 systems, consistent with the occurrence rate that we calculated.

Making bright giants invisible at the Galactic Centre

ABSTRACT Current observations of the Galactic Centre (GC) seem to display a core-like distribution of bright stars from ∼5 arcsec inwards. On the other hand, we observe young, massive stars at the GC, with roughly 20–50 per cent of them in a disc, mostly in the region where the bright giants appear to be lacking. In a previous publication we put the idea forward that the missing stars are deeply connected to the presence of this disc. The progenitor of the stellar disc is very likely to have been a gaseous disc that at some point fragmented and triggered star formation. This caused the appearance of overdensity regions in the disc that had high enough densities to ensure stripping large giants of their atmospheres and thus rendering them very faint. In this paper, we use a stellar evolution code to derive the properties that a red giant would display in a colour–magnitude diagram, as well as a non-linearity factor required for a correct estimate of the mass loss. We find that in a very short time-scale, the red giants leave their standard evolutionary track. The non-linearity factor has values that not only depend on the properties of the clumps, but also on the physical conditions of the giant stars, as we predicted analytically. According to our results, envelope stripping works, moving stars on a short time-scale from the giant branch to the white dwarf stage, thus rendering them invisible to observations.

Do bulges stop stars forming?

ABSTRACT In this paper, we use the Herschel Reference Survey to make a direct test of the hypothesis that the growth of a stellar bulge leads to a reduction in the star formation efficiency of a galaxy (or conversely a growth in the gas-depletion time-scale) as a result of the stabilization of the gaseous disc by the gravitational field of the bulge. We find a strong correlation between star formation efficiency and specific star formation rate in galaxies without prominent bulges and in galaxies of the same morphological type, showing that there must be some other process besides the growth of a bulge that reduces the star formation efficiency in galaxies. However, we also find that galaxies with more prominent bulges (Hubble types E to Sab) do have significantly lower star formation efficiencies than galaxies with later morphological types, which is at least consistent with the hypothesis that the growth of a bulge leads to the reduction in the star formation efficiency. The answer to the question in the title is therefore yes and no: bulges may reduce the star formation efficiency in galaxies but there must also be some other process at work. We also find that there is a significant but small difference in the star formation efficiencies of galaxies with and without bars, in the sense that galaxies with bars have slightly higher star formation efficiencies.

Peculiar motions of the gas at the centre of the barred galaxy UGC 4056

We derive the circular velocity curves of the gaseous and stellar discs of UGC 4056, a giant barred galaxy with an active galactic nucleus (AGN). We analyse UGC 4056 using the 2D spectroscopy obtained within the framework of the Mapping Nearby Galaxies at APO (MaNGA) survey. Using images and the colour index g − r from the Sloan Digital Sky Survey (SDSS), we determined the tilt of the galaxy, which allows us to conclude that the galaxy rotates clockwise with trailing spiral arms. We found that the gas motion at the central part of the UGC 4056 shows peculiar features. The rotation velocity of the gaseous disc shows a bump within around three kiloparsecs while the rotation velocity of the stellar disc falls smoothly to zero with decreasing galactocentric distance. We demonstrate that the peculiar radial velocities in the central part of the galaxy may be caused by the inflow of the gas towards the nucleus of the galaxy. The unusual motion of the gas takes place at the region with the AGN-like radiation and can be explained by the gas response to the bar potential.