ABSTRACT
Planets orbiting intermediate- and low-mass stars are in jeopardy as their stellar hosts evolve to white dwarfs (WDs) because the dynamics of the planetary system changes due to the increase ...of the planet:star mass ratio after stellar mass-loss. In order to understand how the planet multiplicity affects the dynamical stability of post-main sequence (MS) systems, we perform thousands of N-body simulations involving planetary multiplicity as the variable and with a controlled physical and orbital parameter space:equal-mass planets; the same orbital spacing between adjacent planet’s pairs; and orbits with small eccentricities and inclinations. We evolve the host star from the MS to the WD phase following the system dynamics for 10 Gyr. We find that the fraction of dynamically active simulations on the WD phase for two-planet systems is $10.2^{+1.2}_{-1.0}$–$25.2^{+2.5}_{-2.2}$ ${{\rm per\,cent}}$ and increases to $33.6^{+2.3}_{-2.2}$–$74.1^{+3.7}_{-4.6}$ ${{\rm per\,cent }}$ for the six-planet systems, where the ranges cover different ranges of initial orbital separations. Our simulations show that the more planets the system has, the more systems become unstable when the star becomes a WD, regardless of the planet masses and range of separations. Additional results evince that simulations with low-mass planets (1, 10 M⊕) lose at most two planets, have a large fraction of systems undergoing orbit crossing without planet losses, and are dynamically active for Gyr time-scales on the WD’s cooling track. On the other hand, systems with high-mass planets (100, 1000 M⊕) lose up to five planets, preferably by ejections, and become unstable in the first few hundred Myr after the formation of the WD.
Context. The study of protoplanetary disc evolution and theories of planet formation has predominantly concentrated on solar- (and low-) mass stars since they host the majority of confirmed ...exoplanets. Nevertheless, the confirmation of numerous planets orbiting stars more massive than the Sun (up to ~3 M⊙) has sparked considerable interest in understanding the mechanisms involved in their formation, and thus in the evolution of their hosting protoplanetary discs. Aims. We aim to improve our knowledge of the evolution of the gaseous component of protoplanetary discs around intermediate-mass stars and to set the stage for future studies of planet formation around them. Methods. We study the long-term evolution of protoplanetary discs affected by viscous accretion and photoevaporation by X-ray and far-ultraviolet (FUV) photons from the central star around stars in the range of 1–3 M⊙, considering the effects of stellar evolution and solving the vertical structure equations of the disc. We explore the effect of different values of the viscosity parameter and the initial mass of the disc. Results. We find that the evolutionary pathway of protoplanetary disc dispersal due to photoevaporation depends on the stellar mass. Our simulations reveal four distinct evolutionary pathways for the gas component not reported before that are a consequence of stellar evolution and that likely have a substantial impact on the dust evolution, and thus on planet formation. As the stellar mass increases from one solar mass to ~1.5–2 M⊙, the evolution of the disc changes from the conventional inside-out clearing, in which X-ray photoevaporation generates inner holes, to a homogeneous disc evolution scenario where both inner and outer discs formed after a gap is opened by photoevaporation vanish over a similar timescale. As the stellar mass continues to increase, reaching ~2–3 M⊙, we identify a distinct pathway that we refer to as revenant disc evolution. In this scenario, the inner and outer discs reconnect after the gap opened. For the largest masses, we observe outside-in disc dispersal, in which the outer disc dissipates first due to a stronger FUV photoevaporation rate. Revenant disc evolution stands out as it is capable of extending the disc lifespan. Otherwise, the disc dispersal timescale decreases with increasing stellar mass except for low-viscosity discs.
We have computed new stellar evolution models that include the effects of rotation and magnetic torques under different hypotheses. The goal is to test whether a single star can sustain the ...rotational velocities needed in the envelope for magnetohydrodynamical(MHD) simulations to shape bipolar planetary nebulae (PNe) when high mass-loss rates take place. Stellar evolution models with main sequence masses of 2.5 and 5 M sub(middot in circle)and initial rotational velocities of 250 km s super(-1) have been followed through the PNe formation phase. We find that stellar cores have to be spun down using magnetic torques in order to reproduce the rotation rates observed for white dwarfs. During the asymptotic giant branch phase and beyond, the magnetic braking of the core has a practically null effect on increasing the rotational velocity of the envelope since the stellar angular momentum is efficiently removed by the wind. We have also tested the best possible case scenarios in rather non-physical contexts to give enough angular momentum to the envelope. We find that we cannot get the envelope of a single star to rotate at the speeds needed for MHD simulations to form bipolar PNe. We conclude that single stellar rotators are unlikely to be the progenitors of bipolar PNe under the current MHD model paradigm.
Context. Nearby galaxies are ideal places to study metallicity gradients in detail and their time evolution. Aims. We analyse the spatial distribution of metals in M 33 using a new sample and the ...literature data on H ii regions, and constrain a model of galactic chemical evolution with H ii region and planetary nebula (PN) abundances. Methods. We consider chemical abundances of a new sample of H ii regions complemented with previous data sets. We compared H ii region and PN abundances obtained with a common set of observations taken at MMT. With an updated theoretical model, we followed the time evolution of the baryonic components and chemical abundances in the disk of M 33, assuming that the galaxy is accreting gas from an external reservoir. Results. From the sample of H ii regions, we find that i) the 2D metallicity distribution has an off-centre peak located in the southern arm; ii) the oxygen abundance gradients in the northern and southern sectors, as well as in the nearest and farthest sides, are identical within the uncertainties, with slopes around -0.03-4 dex kpc-1; iii) bright giant H ii regions have a steeper abundance gradient than the other H ii regions; iv) H ii regions and PNe have O/H gradients very close within the errors; v) our updated evolutionary model is able to reproduce the new observational constraints, as well as the metallicity gradient and its evolution. Conclusions. Supported by a uniform sample of nebular spectroscopic observations, we conclude that i) the metallicity distribution in M 33 is very complex, showing a central depression in metallicity probably due to observational bias; ii) the metallicity gradient in the disk of M 33 has a slope of -0.037 ± 0.009 dex kpc-1 in the whole radial range up to ~8 kpc, and -0.044 ± 0.009 dex kpc-1 excluding the central kpc; iii) there is little evolution in the slope with time from the epoch of PN progenitor formation to the present.
Standard stellar evolution theory does not predict existence of Li-rich giant stars. Several mechanisms for Li-enrichment have been proposed to operate at certain locations inside some stars. The ...actual mechanism operating in real stars is still unknown. Using the sample of 348 stars from the Penn State -- Torun Centre for Astronomy Planet Search, for which uniformly determined atmospheric parameters are available, with chemical abundances and rotational velocities presented here, we investigate various channels of Li enrichment in giants. We also study Li-overabundant giants in more detail in search for origin of their peculiarities. Our results do not point out to one specific Li-enrichment mechanism operating in our sample of giants. On the contrary, in some cases, we cannot identify fingerprints of any of known scenarios. We show, however, that the four most Li-rich giants in our sample either have low-mass companions or have radial velocity variations at the level of ~100m ssup -1, which strongly suggests that the presence of companions is an important factor in the Li-enrichment processes in giants.
We present a study of 16 planetary nebulae (PNe) where fullerenes have been detected in their Spitzer Space Telescope spectra. Among the 16 PNe studied, we present the first detection of C sub(60) ...(and possibly also C sub(70)) fullerenes in the PN M 1-60 as well as of the unusual ~6.6, 9.8, and 20 mum features (attributed to possible planar C sub(24)) in the PN K 3-54. We do not find a metallicity dependence on the estimated fullerene abundances. The observed C sub(60) intensity ratios in the Galactic sources confirm our previous finding in the MCs that the fullerene emission is not excited by the UV radiation from the central star. With the data at hand, we suggest that the most likely explanation for the formation of fullerenes and graphene precursors in PNe is that these molecular species are built from the photochemical processing of a carbonaceous compound with a mixture of aromatic and aliphatic structures similar to that of hydrogenated amorphous carbon dust.
High-resolution observations of the extended atmospheres of asymptotic giant branch (AGB) stars can now directly be compared to the theories that describe stellar mass loss. Using Atacama Large ...Millimeter/submillimeter Array (ALMA) high angular resolution (30 × 42 mas) observations, we have for the first time resolved stellar rotation of an AGB star, R Dor. We measure an angular rotation velocity of ωR sin i = (3.5 ± 0.3) × 10−9 rad s−1, which indicates a rotational velocity of |υrot sin i| = 1.0 ± 0.1 km s−1 at the stellar surface (R* = 31.2 mas at 214 GHz). The rotation axis projected on the plane of the sky has a position angle Φ = 7 ± 6°. We find that the rotation of R Dor is two orders of magnitude faster than expected for a solitary AGB star that will have lost most of its angular momentum. Its rotational velocity is consistent with angular momentum transfer from a close companion. As a companion has not been directly detected, we suggest R Dor has a low-mass, close-in companion. The rotational velocity approaches the critical velocity, set by the local sound speed in the extended envelope, and is thus expected to affect the mass-loss characteristics of R Dor.
Context. Debris discs have often been described as gas-poor discs as the gas-to-dust ratio is expected to be considerably lower than in primordial, protoplanetary discs. However, recent observations ...have confirmed the presence of a non-negligible amount of cold gas in the circumstellar (CS) debris discs around young main-sequence stars. This cold gas has been suggested to be related to the outgassing of planetesimals and cometary-like objects. Aims. The goal of this paper is to investigate the presence of hot gas in the immediate surroundings of the cold-gas-bearing debris-disc central stars. Methods. High-resolution optical spectra of all currently known cold-gas-bearing debris-disc systems, with the exception of β Pic and Fomalhaut, have been obtained from La Palma (Spain), La Silla (Chile), and La Luz (Mexico) observatories. To verify the presence of hot gas around the sample of stars, we have analysed the Ca II H&K and the Na I D lines searching for non-photospheric absorptions of CS origin, usually attributed to cometary-like activity. Results. Narrow, stable Ca II and/or Na I absorption features have been detected superimposed to the photospheric lines in 10 out of the 15 observed cold-gas-bearing debris-disc stars. Features are found at the radial velocity of the stars, or slightly blue- or red-shifted, and/or at the velocity of the local interstellar medium (ISM). Some stars also present transient variable events or absorptions extended towards red wavelengths (red wings). These are the first detections of such Ca II features in 7 out of the 15 observed stars. Although an ISM origin cannot categorically be excluded, the results suggest that the stable and variable absorptions arise from relatively hot gas located in the CS close-in environment of the stars. This hot gas is detected in at least ~80%, of edge-on cold-gas-bearing debris discs, while in only ~10% of the discs seen close to face-on. We interpret this result as a geometrical effect, and suggest that the non-detection of hot gas absorptions in some face-on systems is due to the disc inclination and likely not to the absence of the hot-gas component. This gas is likely released in physical processes related in some way to the evaporation of exocomets, evaporation of dust grains, or grain-grain collisions close to the central star.
Context. We present the latest result of the TAPAS project that is devoted to intense monitoring of planetary candidates that are identified within the PennState-Torun planet search. Aims. We aim to ...detect planetary systems around evolved stars to be able to build sound statistics on the frequency and intrinsic nature of these systems, and to deliver in-depth studies of selected planetary systems with evidence of star-planet interaction processes. Methods. The paper is based on precise radial velocity measurements: 13 epochs collected over 1920 days with the Hobby-Eberly Telescope and its High-Resolution Spectrograph, and 22 epochs of ultra-precise HARPS-N data collected over 961 days. Results. We present a warm-Jupiter (T sub(eq)= 1350 K, m sub(2) sin i= 5.4 + or - 0.4 M sub(J)) companion with an orbital period of 26.468 days in a circular (e= 0.036) orbit around a giant evolved (logg= 3.11 + or - 0.09, R= 6.26 + or - 0.86 R sub(middot in circle)) star with Mlow *= 1.87 + or - 0.17 M sub(middot in circle). This is the most massive and oldest star found to be hosting a close-in giant planet. Its proximity to its host (a= 0.21 au) means that the planet has a 13.9 + or - 2.0% probability of transits; this calls for photometric follow-up study. Conclusions. This massive warm Jupiter with a near circular orbit around an evolved massive star can help set constraints on general migration mechanisms for warm Jupiters and, given its high equilibrium temperature, can help test energy deposition models in hot Jupiters.
Hydrogen depleted environments are considered an essential requirement for the formation of fullerenes. The recent detection of C60 and C70 fullerenes in what was interpreted as the hydrogen-poor ...inner region of a post-final helium shell flash planetary nebula (PN) seemed to confirm this picture. Here, we present strong evidence that challenges the current paradigm regarding fullerene formation, showing that it can take place in circumstellar environments containing hydrogen. We report the simultaneous detection of polycyclic aromatic hydrocarbons (PAHs) and fullerenes toward C-rich and H-containing PNe belonging to environments with very different chemical histories such as our own Galaxy and the Small Magellanic Cloud. We suggest that PAHs and fullerenes may be formed by the photochemical processing of hydrogenated amorphous carbon. These observations suggest that modifications may be needed to our current understanding of the chemistry of large organic molecules as well as the chemical processing in space.
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