Mass Ejection Episodes Research Articles

More Luminous Red Novae That Require Jets

In this paper, I study two intermediate luminosity optical transients (ILOTs), classified as luminous red novae (LRNe), and argue that their modeling with a common envelope evolution (CEE) without jets encounters challenges. LRNe are ILOTs powered by violent binary interaction. Although in the literature it is popular to assume a CEE is the cause of LRNe, I here repeat an old claim that many LRNe are powered by grazing envelope evolution (GEE) events; the GEE might end in a CEE or a detached binary system. I find that the LRN AT 2021biy might have continued to experience mass ejection episodes after its eruption and, therefore, might not have suffered a full CEE during the outburst. This adds to an earlier finding that a jetless model does not account for some of its properties. I find that a suggested jetless CEE model for the LRN AT 2019zhd does not reproduce its photosphere radius evolution. These results that challenge jetless models of two LRNe strengthen a previous claim that jets play major roles in powering ILOTs and shaping their ejecta and that, in many LRNe, the more compact companion launches the jets during a GEE.

Revealing Multiple Nested Molecular Outflows with Rotating Signatures in HH270mms1-A with ALMA

We present molecular line observations of the protostellar outflow associated with HH270mms1 in the Orion B molecular cloud with ALMA. The 12CO (J = 3−2) emissions show that the outflow velocity structure consists of four distinct components of low (≲10 km s−1), intermediate (∼10–25 km s−1) and high (≳40 km s−1) velocities in addition to the entrained gas velocity (∼25–40 km s−1). The high- and intermediate-velocity flows have well-collimated structures surrounded by the low-velocity flow. The chain of knots is embedded in the high-velocity flow or jet, which is the evidence of episodic mass ejections induced by time-variable mass accretion. We could detect the velocity gradients perpendicular to the outflow axis in both the low- and intermediate-velocity flows. We confirmed the rotation of the envelope and disk in the 13CO and C17O emission and found that their velocity gradients are the same as those of the outflow. Thus, we concluded that the velocity gradients in the low- and intermediate-velocity flows are due to the outflow rotation. Using observational outflow properties, we estimated the outflow launching radii to be 67.1–77.1 au for the low-velocity flow and 13.3–20.8 au for the intermediate-velocity flow. Although we could not detect the rotation in the jets due to the limited spatial resolution, we estimated the jet launching radii to be (2.36–3.14) × 10−2 au using the observed velocity of each knot. Thus, the jet is driven from the inner disk region. We could identify the launching radii of distinct velocity components within a single outflow with all the prototypical characteristics expected from recent theoretical works.

Left Ringing: Betelgeuse Illuminates the Connection between Convective Outbursts, Mode Switching, and Mass Ejection in Red Supergiants

Betelgeuse, the nearest red supergiant, dimmed to an unprecedented level in early 2020. The star emerged from this “Great Dimming” episode with its typical, roughly 400 days pulsation cycle halved, and a new dominant period of around 200 days. The dimming event has been attributed to a surface mass ejection, in which rising material drove shocks through the stellar atmosphere and expelled some material, partially obscuring the star as it formed molecules and dust. In this paper, we use hydrodynamic simulations to reveal the connections between Betelgeuse's vigorously convective envelope, the surface mass ejection, and the pulsation mode switching that ensued. An anomalously hot convective plume, generated rarely but naturally in the star's turbulent envelope, can rise and break free from the surface, powering an upwelling that becomes the surface mass ejection. The rising plume also breaks the phase coherence of the star's pulsation, causing the surface to keep expanding even as the deeper layers contract. This drives a switch from the 400 days fundamental mode of pulsation, in which the whole star expands and contracts synchronously, to the 200 days first overtone, where a radial node separates the interior and exterior of the envelope moving in opposite phase. We predict that the star's convective motions will damp the overtone oscillation and Betelgeuse will return to its previous, 400 days fundamental-mode pulsation in the next 5–10 yr. With its resolved surface and unprecedentedly detailed characterization, Betelgeuse opens a window to episodic surface mass ejection in the late-stage evolution of massive stars.

Numerical Models of Planetary Nebulae with Different Episodes of Mass Ejection: The Particular Case of HuBi 1

We have studied the evolution of HuBi 1–like planetary nebulae, considering several stages of mass injection. We have carried out numerical ionization+1D hydrodynamics+atomic/ionic rate models with our code Coral1d to reproduce planetary nebulae that present multiple shells produced by different ejection events around the ionizing source. We are interested in comparing numerical simulations with Hα and [N ii] λ6584 emission structures and the position–velocity diagrams observed for HuBi 1. This object also has a phase where it has drastically decreased the injection of ionized photons ejected from the source. The result of these different stages of ejection is a nebula with intense [N ii] line emission in the inner part of the planetary nebula and extended H ii recombination line emission around the central zone. The model for HuBi 1 shows the capability of our code to explain the hydrodynamical and photoionization evolution in ionization nebulae. This is our first step with a 1D code to study these two physical phenomena at the same time.

ALMA Reveals Hidden Morphologies in the Molecular Envelope of VY Canis Majoris

The J = 2 → 1 transition of CO near 230 GHz and the J = 3 → 2 line of HCN at 265 GHz have been imaged in the envelope of the red hypergiant star, VY Canis Majoris (VY CMa), using the Atacama Large Millimeter Array (ALMA) with angular resolutions 0.″2–1.″5; single-dish data were added to provide sensitivity up to 30″. These images reveal a far more complex envelope, with previously unseen outflows extending 4″–9″ from the star. These new structures include an arc-like outflow with an angular separation of ∼9″ northeast from the stellar position (“NE Arc”), twin fingerlike features approximately 4″ to the north/northeast (“NE Extension”), and a roughly spherical region observed ∼7″ E of the star (“E Bubble”). The NE Arc appears to be decelerating from base (V LSR ∼ 7 km s−1) to tip (V LSR ∼ 18 km s−1), while the NE Extension is blueshifted with V LSR ∼ −7 km s−1. Among the new features, HCN is only detected in the NE Arc. In addition, known structures Arc 1, Arc 2, and NW Arc, as well as other features closer to the star, are closely replicated in CO, suggesting that the gas and dust are well mixed. The CO spectra are consistent with the kinematic picture of VY CMa derived from HST data. Arc 2, however, has added complexity. Preliminary results from CO suggest 12C/13C ∼ 22–38 across the envelope. The additional presence of at least three major episodic mass ejection events significantly broadens the current perspective of the envelope structure and mass-loss history of VY CMa.

Evidence for episodic and patchy mass ejection in the circumstellar envelope of AGB star R Leonis

ABSTRACT The morpho-kinematics of the circumstellar envelope of oxygen-rich AGB star R Leonis is probed using Atacama Large Millimeter/submillimeter Array observations of the emission of molecular lines, including in particular CO(2–1) and 29SiO(5–4). Evidence is found for an episode of enhanced mass-loss, a few centuries ago, that produced a broad expanding shell of mean radius ∼6 arcsec and mean radial expansion velocity ∼5.5 km s−1. Detailed scrutiny of its structure, as displayed by the emission of the CO(2–1) line, reveals strong inhomogeneity and patchy morphology. Evidence is also found, in particular from the morpho-kinematics of the emission of SiO, SO, and SO2 lines probing the close neighbourhood of the star, for distinct gas outflows covering broad solid angles in the south-eastern, south-western, and north-western quadrants, suggesting significant contribution of the convective cell granulation in defining the pattern of mass ejection. A study of relative molecular abundances in these outflows suggests that a local thermal equilibrium description applies approximately beyond ∼10 stellar radii from the centre of the star but not at the smaller angular separations where the SO and SO2 molecules are found to be confined. Near the stellar disc, masers of the vibrationally excited SiO lines are found to probe north-western parts of a layer of hot gas, consistent with the earlier observation of an asymmetric expanding shell within 1–2 stellar radii from the centre of the star. Globally, a picture dominated by episodic and patchy mass ejections is found to prevail.

Multiple flares caused by mass ejection episodes during the advanced nebular phase of Nova Scuti 2019

ABSTRACT Our photometric and spectroscopic monitoring shows that starting with 2020 June 4, day +217 from optical maximum and well into its advanced nebular stage, Nova Scuti (Nova Sct) 2019 begun displaying a series of nine large amplitude flares (up to $\Delta m \sim 1.7$ mag), characterized by a rapid rise to peak (≤10 h) and a fast exponential decline (e-folding time ∼50 h). The time interval Δt between flares follows an ordered sequence, declining from 8.43 to 4.90 d, that safely allows to exclude that any other flare occurred without being recorded by the observations. When the sequence of flares was over by 2020 July 28 (day +271), Nova Sct 2019 slowed its overall decline rate from Δm = 0.0067 to 0.0027 mag d−1. The flares were caused by material expelled at high velocity (∼1000 km s−1) from the still burning white dwarf (WD). The cooler pseudo-photosphere forming at each flare in the expelled material, resulted in a recombination wave to spread through the original nova ejecta (at ∼170 au from the WD), quenching emission from [Fe x] and [Fe vii] and boosting that from lower ionization species. After each flare, once the small amount of expelled material had turned optically thin, the original nova ejecta resumed displaying [Fe x] and [Fe vii] emission lines, a fact that clearly proves the direct photoionization action exerted on the ejecta by the burning WD. While the other known flaring novae (V458 Vul, V4745 Sgr, and V5588 Sgr) presented the flares close to maximum brightness and with increasing Δt, Nova Sct 2019 is unique in having displayed them during the advanced nebular stage and with decreasing Δt.

Nonradial and nonpolytropic astrophysical outflows

Context. Recent observational evidence has shown that RY Tau may present two different outflow stages, a quiescent and a more active stage. We try to model this phenomenon. Aims. We have performed new 2.5D magnetohydrodynamical simulations of the possible accretion-outflow environment of RY Tau based on analytical solutions with the aim to reduce the relaxation time. Methods. We used the analytical self-similar solution that we used to model the RY Tau microjet as initial conditions. In the closed field line region of the magnetosphere, we reversed the direction of the flow and increased the accretion rate by increasing the density and velocity. We also implemented the heating rate and adjusted it according to the velocity of the flow. The accretion disk was treated as a boundary condition. Results. The simulations show that the stellar jet and the accreting magnetosphere attain a steady state in only a few stellar rotations. This confirms the robustness and stability of self-similar solutions. Additionally, two types of behavior were observed that are similar to the behavior observed in RY Tau. Either the steady stellar outflow and magnetospheric inflow are separated by a low static force-free region or the interaction between the stellar jet and the magnetospheric accretion creates episodic coronal mass ejections that originate from the disk and bounce back onto the star. Conclusions. The ratio of mass-loss rate to mass-accretion rate that coincides with the change in behavior observed in RY Tau lies within the range of ratios that have been measured during the period in which the initial microjet was analyzed.

Classifying Be Star Variability With TESS. I. The Southern Ecliptic

TESS photometry is analyzed for 430 classical Be stars observed in the first year of the mission. The often complex and diverse variability of each object in this sample is classified to obtain an understanding of the behavior of this class as a population. Ninety-seven percent of the systems are variable above the noise level, with timescales spanning nearly the entire range of what is accessible with TESS, from tens of minutes to tens of days. The variability seen with TESS is summarized as follows. Nearly every system contains multiple periodic signals in the frequency regime between about 0.5 and 4 day−1. One or more groups of closely spaced frequencies is the most common feature, present in 87% of the sample. Among the Be stars with brightening events that are characteristic of mass ejection episodes (18% of the full sample, or 31% of early-type stars), all have at least one frequency group, and the majority of these (83%) show a concurrent temporary amplitude enhancement in one or more frequency groups. About one-third (34%) of the sample is dominated by longer-term trends (timescales >2 day). Low-frequency stochastic signals are prominent in about 25% of the sample, with varying degrees of intensity. Higher-frequency signals (6 < f < 15 day−1) are sometimes seen (in 15% of the sample) and in most cases likely reflect p-mode pulsation. In rare cases (∼3%), even higher frequencies beyond the traditional p-mode regime (f > 15 day−1) are observed.

Optical and near-infrared spectroscopy of Nova V2891 Cygni: evidence for shock-induced dust formation

ABSTRACT We present multi-epoch optical and near-infrared observations of the highly reddened, Fe ii class slow Nova V2891 Cygni. The observations span 15 months since its discovery. The initial rapid brightening from quiescence, and the presence of an ∼35-d long pre-maximum halt, is well documented. The evidence that the current outburst of V2891 Cyg has undergone several distinct episodes of mass ejection is seen through time-varying P Cygni profiles of the O i 7773 Å line. A highlight is the occurrence of a dust formation event centred around approximately +273 d, which coincides with a phase of coronal line emission. The dust mass is found to be $\sim\!{0.83{-}1.25} \times 10^{-10}\ \mathrm{M}_{\odot }$. There is strong evidence to suggest that the coronal lines are created by shock heating rather than by photoionization. The simultaneous occurrence of the dust and coronal lines (with varying velocity shifts) supports the possibility that dust formation is shock induced. Such a route for dust formation has not previously been seen in a nova, although the mechanism has been proposed for dust formation in some core-collapse supernovae. Analysis of the coronal lines indicates a gas mass and temperature of 8.35–8.42 × 10−7 M⊙ and ∼(4.8–9.1) × 105 K, respectively, and an overabundance of aluminium and silicon. A Case B analysis of the hydrogen lines yields a mass of the ionized gas of (8.60 ± 1.73) × 10−5 M⊙. The reddening and distance to the nova are estimated to be E(B − V) = 2.21 ± 0.15 and d = 5.50 kpc, respectively.

The ALMA Survey of 70 μm Dark High-mass Clumps in Early Stages (ASHES). IV. Star Formation Signatures in G023.477

Abstract With a mass of ∼1000 M ⊙ and a surface density of ∼0.5 g cm−2, G023.477+0.114, also known as IRDC 18310-4, is an infrared dark cloud (IRDC) that has the potential to form high-mass stars and has been recognized as a promising prestellar clump candidate. To characterize the early stages of high-mass star formation, we have observed G023.477+0.114 as part of the Atacama Large Millimeter/submillimeter Array (ALMA) Survey of 70 μm Dark High-mass Clumps in Early Stages. We have conducted ∼1.″2 resolution observations with ALMA at 1.3 mm in dust continuum and molecular line emission. We have identified 11 cores, whose masses range from 1.1 to 19.0 M ⊙. Ignoring magnetic fields, the virial parameters of the cores are below unity, implying that the cores are gravitationally bound. However, when magnetic fields are included, the prestellar cores are close to virial equilibrium, while the protostellar cores remain sub-virialized. Star formation activity has already started in this clump. Four collimated outflows are detected in CO and SiO. H2CO and CH3OH emission coincide with the high-velocity components seen in the CO and SiO emission. The outflows are randomly oriented for the natal filament and the magnetic field. The position-velocity diagrams suggest that episodic mass ejection has already begun even in this very early phase of protostellar formation. The masses of the identified cores are comparable to the expected maximum stellar mass that this IRDC could form (8–19 M ⊙). We explore two possibilities on how IRDC G023.477+0.114 could eventually form high-mass stars in the context of theoretical scenarios.

Environments of evolved massive stars: evidence for episodic mass ejections

AbstractThe post-main sequence evolutionary path of massive stars comprises various transition phases, in which the stars shed large amounts of material into their environments. Our studies focus on two of them: B[e] supergiants and yellow hypergiants, for which we investigate the structure and dynamics within their environments. We find that each B[e] supergiant is surrounded by a unique set of rings or arc-like structures. These structures are either stable over time or they display high variability, including expansion and dilution. In contrast, yellow hypergiants are embedded in multiple shells of gas and dust. These objects are famous for their outburst activity. Moreover, the dynamics in their extended atmospheres imply an enhanced pulsation activity prior to outburst. The physical mechanism(s) leading to episodic mass ejections in these two types of stars is still uncertain. We propose that strange-mode instabilities, excited in the inflated envelopes of these objects, play a significant role.

Predictions for the hydrogen-free ejecta of pulsational pair-instability supernovae

Present and upcoming time-domain astronomy efforts, in part driven by gravitational-wave follow-up campaigns, will unveil a variety of rare explosive transients in the sky. Here, we focus on pulsational pair-instability evolution, which can result in signatures that are observable with electromagnetic and gravitational waves. We simulated grids of bare helium stars to characterize the resulting black hole (BH) masses together with the ejecta composition, velocity, and thermal state. We find that the stars do not react “elastically” to the thermonuclear ignition in the core: there is not a one-to-one correspondence between pair-instability driven ignition and mass ejections, which causes ambiguity as to what is an observable pulse. In agreement with previous studies, we find that for initial helium core masses of 37.5 M⊙ ≲ MHe, init ≲ 41 M⊙, corresponding to carbon-oxygen core masses 27.5 M⊙ ≲ MCO ≲ 30.1 M⊙, the explosions are not strong enough to affect the surface. With increasing initial helium core mass, they become progressively stronger causing first large radial expansion (41 M⊙ ≲ MHe, init ≲ 42 M⊙, corresponding to 30.1 M⊙ ≲ MCO ≲ 30.8 M⊙) and, finally, also mass ejection episodes (for MHe, init ≳ 42 M⊙, or MCO ≳ 30.8 M⊙). The lowest mass helium core to be fully disrupted in a pair-instability supernova is MHe, init ≃ 80 M⊙, corresponding to MCO ≃ 55 M⊙. Models with MHe, init ≳ 200 M⊙ (MCO ≳ 114 M⊙) reach the photodisintegration regime, resulting in BHs with masses of MBH ≳ 125 M⊙. Although this is currently considered unlikely, if BHs from these models form via (weak) explosions, the previously-ejected material might be hit by the blast wave and convert kinetic energy into observable electromagnetic radiation. We characterize the hydrogen-free circumstellar material from the pulsational pair-instability of helium cores by simply assuming that the ejecta maintain a constant velocity after ejection. We find that our models produce helium-rich ejecta with mass of 10−3 M⊙ ≲ MCSM ≲ 40 M⊙, the larger values corresponding to the more massive progenitor stars. These ejecta are typically launched at a few thousand km s−1 and reach distances of ∼1012 − 1015 cm before the core-collapse of the star. The delays between mass ejection events and the final collapse span a wide and mass-dependent range (from subhour to 104 years), and the shells ejected can also collide with each other, powering supernova impostor events before the final core-collapse. The range of properties we find suggests a possible connection with (some) type Ibn supernovae.

The ISM local to the runaway star WR16

ABSTRACT Massive stars leave their imprint on the interstellar medium as they radiate their energy and undergo episodes of mass ejection throughout their lives. In this paper, we analyse the case of the Wolf–Rayet star WR16 combining archival multiwavelength data with new molecular observations obtained with the Atacama Submillimeter Telescope Experiment (ASTE). Our results suggest that during the main-sequence phase, WR16 swept up the surrounding gas creating a molecular structure (which we call Component 1) which also contains very cold dust observed in the infrared band. In a subsequent stage of evolution, as an LBV, the star underwent mass eruptions that were later overrun by the fast winds of the current WR phase. The final result is the round nebula revealed by the optical and IR images, and the molecular clumps detected. We have also computed the peculiar velocity of WR16 using Gaia data and, accordingly, confirm it as a runaway star. We propose that several features observed in different wavelengths can be explained under a bow-shock scenario linked to the high velocity of WR16.

Sensitivity of the lower edge of the pair-instability black hole mass gap to the treatment of time-dependent convection

ABSTRACT Gravitational-wave detections are now probing the black hole (BH) mass distribution, including the predicted pair-instability mass gap. These data require robust quantitative predictions, which are challenging to obtain. The most massive BH progenitors experience episodic mass ejections on time-scales shorter than the convective turnover time-scale. This invalidates the steady-state assumption on which the classic mixing length theory relies. We compare the final BH masses computed with two different versions of the stellar evolutionary code $\tt{MESA}$: (i) using the default implementation of Paxton et al. (2018) and (ii) solving an additional equation accounting for the time-scale for convective deceleration. In the second grid, where stronger convection develops during the pulses and carries part of the energy, we find weaker pulses. This leads to lower amounts of mass being ejected and thus higher final BH masses of up to ∼$5\, \mathrm{M}_\odot$. The differences are much smaller for the progenitors that determine the maximum mass of BHs below the gap. This prediction is robust at $M_{\rm BH, max}\simeq 48\, \mathrm{M}_\odot$, at least within the idealized context of this study. This is an encouraging indication that current models are robust enough for comparison with the present-day gravitational-wave detections. However, the large differences between individual models emphasize the importance of improving the treatment of convection in stellar models, especially in the light of the data anticipated from the third generation of gravitational-wave detectors.

Can we detect aurora in exoplanets orbiting M dwarfs?

ABSTRACT New instruments and telescopes, such as SPIRou, CARMENES, and Transiting Exoplanet Survey Satellite (TESS), will increase manyfold the number of known planets orbiting M dwarfs. To guide future radio observations, we estimate radio emission from known M dwarf planets using the empirical radiometric prescription derived in the Solar system, in which radio emission is powered by the wind of the host star. Using solar-like wind models, we find that the most promising exoplanets for radio detections are GJ 674 b and Proxima b, followed by YZ Cet b, GJ 1214 b, GJ 436 b. These are the systems that are the closest to us (&amp;lt;10 pc). However, we also show that our radio fluxes are very sensitive to the unknown properties of winds of M dwarfs. So, which types of winds would generate detectable radio emission? In a ‘reverse engineering’ calculation, we show that winds with mass-loss rates $\dot{M} \gtrsim \kappa _{\rm sw} /u_{\rm sw}^3$ would drive planetary radio emission detectable with present-day instruments, where usw is the local stellar wind velocity and κsw is a constant that depends on the size of the planet, distance, and orbital radius. Using observationally constrained properties of the quiescent winds of GJ 436 and Proxima Cen, we conclude that it is unlikely that GJ 436 b and Proxima b would be detectable with present-day radio instruments, unless the host stars generate episodic coronal mass ejections. GJ 674 b, GJ 876 b, and YZ Cet b could present good prospects for radio detection, provided that their host stars’ winds have $\dot{M} u_{\rm sw}^{3} \gtrsim 1.8\times 10^{-4} \, {\rm M}_\odot \,{\rm yr}^{-1}\, ({\rm km\,s^{-1}})^{3}$.

On the Nature of the Compact Sources in IRAS 16293–2422 Seen at Centimeter to Submillimeter Wavelengths

We present multi-epoch continuum observations of the Class 0 protostellar system IRAS 16293–2422 taken with the Very Large Array (VLA) at multiple wavelengths between 7 mm and 15 cm (41 GHz down to 2 GHz), as well as single-epoch Atacama Large Millimeter/submillimeter Array continuum observations covering the range from 0.4 to 1.3 mm (700 GHz down to 230 GHz). The new VLA observations confirm that source A2 is a protostar driving episodic mass ejections, and reveal the complex relative motion between A2 and A1. The spectrum of component B can be described by a single power law (Sν ∝ ν2.28) over the entire range from 3 to 700 GHz (10 cm down to 0.4 mm), suggesting that the emission is entirely dominated by dust even at λ = 10 cm. Finally, the size of source B appears to increase with frequency up to 41 GHz, remaining roughly constant (at 039 ≡ 55 au) at higher frequencies. We interpret this as evidence that source B is a dusty structure of finite size that becomes increasingly optically thick at higher frequencies until, in the millimeter regime, the source becomes entirely optically thick. The lack of excess free–free emission at long wavelengths, combined with the absence of high-velocity molecular emission, indicates that source B does not drive a powerful outflow, and might indicate that source B is at a particularly early stage of its evolution.

A hybrid envelope-stripping mechanism for massive stars from supernova nebular spectroscopy

When nuclear fuel in the core of a massive star with a zero-age main-sequence mass $M_{\rm ZAMS} \gtrsim 8M_\odot$ is exhausted, the central part of the iron or magnesium core collapses and forms a neutron star or a black hole. At the same time, the material above the collapsing core is rapidly ejected, leading to a stripped-envelope supernova (SESN) explosion if the outer hydrogen envelope of the star was removed before its explosion. The envelope is presumably stripped either via strong stellar winds or due to mass transfer to a companion star in a close binary orbit. It is not clear which process is dominant, and whether different mechanisms are at work for different classes of SESNe; type IIb, Ib, and Ic SNe in order of increasing degree of envelope stripping. In this work, a new analysis of late-time nebular spectra of SESNe is presented, which is more sensitive to differences in the core structure than early-phase spectral analysis. The results show that the progenitors of SNe IIb and Ib are indistinguishable except for the residual amount of hydrogen envelope while the progenitors of SNe Ic are not only deficient in hydrogen and helium, but are also distinctly more massive than SNe IIb and Ib. These findings strongly suggest that more than one mechanism is responsible for the removal of the outer hydrogen envelope and the deeper helium layer, with the former most likely due to binary interaction, and the latter involving a mass-dependent process such as strong stellar winds or episodic pre-explosion mass ejection.

In Search of Recent Disruption of (3200) Phaethon: Model Implication and Hubble Space Telescope Search

Abstract Near-Earth asteroid (3200) Phaethon is notable for its association with a strong annual meteor shower, the Geminids, indicative of one or more episodes of mass ejection in the past. The mechanism of Phaethon’s past activity is not yet understood. Here, we present a Hubble Space Telescope (HST) search of meter-sized fragments in the vicinity of Phaethon, carried out during Phaethon’s historic approach to Earth in mid-December of 2017. Numerical simulations conducted to guide HST’s pointing also show that the dynamical evolution of Phaethon-originated particles is quick, as ejected materials take no longer than ∼250 years to spread to the entire orbit of Phaethon. Our search was completed down to a 4 m class limit (assuming Phaethon-like albedo) and was expected to detect 0.035% of particles ejected by Phaethon in the past several decades. The negative result of our search capped the total mass loss of Phaethon over the past few dozen orbits to be 1012 kg at the 3σ level, taking the best estimates of size power-law from meteor observations and spacecraft data. Our result also implies a millimeter-sized dust flux of within 0.1 au of Phaethon, suggesting that any Phaethon-bound mission is unlikely to encounter dense dust clouds.

High-entropy ejections from magnetized proto-neutron star winds: implications for heavy element nucleosynthesis

Although initially thought to be promising for production of the r-process nuclei, standard models of neutrino-heated winds from proto-neutron stars (PNSs) do not reach the requisite neutron-to-seed ratio for production of the lanthanides and actinides. However, the abundance distribution created by the r-, rp-, or νp-processes in PNS winds depends sensitively on the entropy and dynamical expansion time-scale of the flow, which may be strongly affected by high magnetic fields. Here, we present results from magnetohydrodynamic simulations of non-rotating neutrino-heated PNS winds with strong dipole magnetic fields from 1014 to 1016 G, and assess their role in altering the conditions for nucleosynthesis. The strong field forms a closed zone and helmet streamer configuration at the equator, with episodic dynamical mass ejections in toroidal plasmoids. We find dramatically enhanced entropy in these regions and conditions favourable for third-peak r-process nucleosynthesis if the wind is neutron-rich. If instead the wind is proton-rich, the conditions will affect the abundances from the νp-process. We quantify the distribution of ejected matter in entropy and dynamical expansion time-scale, and the critical magnetic field strength required to affect the entropy. For B ≳1015 G, we find that ≳10−6 M⊙ and up to ∼10−5 M⊙ of high-entropy material is ejected per highly magnetized neutron star birth in the wind phase, providing a mechanism for prompt heavy element enrichment of the universe. Former binary companions identified within (magnetar-hosting) supernova remnants, the remnants themselves, and runaway stars may exhibit overabundances. We provide a comparison with a semi-analytic model of plasmoid eruption and discuss implications and extensions.