Low-mass Main-sequence Stars Research Articles

A study of the electrostatic properties of the interiors of low-mass stars: Possible implications for the observed rotational properties

Context. In the partially ionized material of stellar interiors, the strongest forces acting on electrons and ions are the Coulomb interactions between charges. The dynamics of the plasma as a whole depend on the magnitudes of the average electrostatic interactions and the average kinetic energies of the particles that constitute the stellar material. An important question is how these interactions of real gases are related to the observable stellar properties. Specifically, the relationships between rotation, magnetic activity, and the thermodynamic properties of stellar interiors are still not well understood. These connections are crucial for understanding and interpreting the abundant observational data provided by space-based missions, such as Kepler/K2 and TESS, and the future data from the PLATO mission. Aims. In this study, we investigate the electrostatic effects within the interiors of low-mass main sequence (MS) stars. Specifically, we introduce a global quantity, a global plasma parameter, which allows us to compare the importance of electrostatic interactions across a range of low-mass theoretical models (0.7 − 1.4 M⊙) with varying ages and metallicities. We then correlate the electrostatic properties of the theoretical models with the observable rotational trends on the MS. Methods. We use the open-source 1D stellar evolution code MESA to compute a grid of main-sequence stellar models. Our models span the log g − Teff space of a set of 66 Kepler main-sequence stars. Results. We identify a correlation between the prominence of electrostatic effects in stellar interiors and stellar rotation rates. The variations in the magnitude of electrostatic interactions with age and metallicity further suggest that understanding the underlying physics of the collective effects of plasma can clarify key observational trends related to the rotation of low-mass stars on the MS. These results may also advance our understanding of the physics behind the observed weakened magnetic braking in stars.

Angular momentum and chemical transport by azimuthal magnetorotational instability in radiative stellar interiors

Context. The transport of angular momentum and chemical elements within evolving stars remains poorly understood. Asteroseismic and spectroscopic observations of low-mass main sequence stars and red giants reveal that their radiative cores rotate orders of magnitude slower than classical predictions from stellar evolution models and that the abundances of their surface light elements are too small. Magnetohydrodynamic (MHD) turbulence is considered a primary mechanism to enhance the transport in radiative stellar interiors but its efficiency is still largely uncertain. Aims. We explore the transport of angular momentum and chemical elements due to azimuthal magnetorotational instability, one of the dominant instabilities expected in differentially rotating radiative stellar interiors. Methods. We employed 3D MHD direct numerical simulations in a spherical shell of unstratified and stably stratified flows under the Boussinesq approximation. The background differential rotation was maintained by a volumetric body force. We examined the transport of chemical elements using a passive scalar. Results. We provide evidence of magnetorotational instability for purely azimuthal magnetic fields in the parameter regime expected from local and global linear stability analyses. Without stratification and when the Reynolds number Re and the background azimuthal field strength are large enough, we observed dynamo action driven by the instability at values of the magnetic Prandtl number Pm in the range 0.6 − 1, which is the smallest ever reported in a global setup. When considering stable stratification at Pm = 1, the turbulence is transitional and becomes less homogeneous and isotropic upon increasing buoyancy effects. The transport of angular momentum occurs radially outward and is dominated by the Maxwell stresses when stratification is large enough. We find that the turbulent viscosity decreases when buoyancy effects strengthen and scales with the square root of the ratio of the reference rotation rate Ωa to the Brunt–Väisälä frequency N. The chemical turbulent diffusion coefficient scales with stratification similarly to the turbulent viscosity, but is lower in amplitude so that the transport of chemicals is slower than the one of angular momentum, in agreement with recent stellar evolution models of low-mass stars. Conclusions. We show that the transport induced by azimuthal magnetorotational instability scales somewhat slowly with stratification and may enforce rigid rotations of red giant cores on a timescale of a few thousand years. In agreement with recent stellar evolution models of low-mass stars, the instability transports chemical elements less efficiently than angular momentum.

The Factory and the Beehive. V. Chromospheric and Coronal Activity and Its Dependence on Rotation in Praesepe and the Hyades

Low-mass (≲1.2 M ⊙) main-sequence stars lose angular momentum over time, leading to a decrease in their magnetic activity. The details of this rotation–activity relation remain poorly understood, however. Using observations of members of the ≈700 Myr old Praesepe and Hyades open clusters, we aim to characterize the rotation–activity relation for different tracers of activity at this age. To complement published data, we obtained new optical spectra for 250 Praesepe stars, new X-ray detections for 10, and new rotation periods for 28. These numbers for Hyads are 131, 23, and 137, respectively. The latter increases the number of Hyads with periods by 50%. We used these data to measure the fractional Hα and X-ray luminosities, L Hα /L bol and L X/L bol, and to calculate Rossby numbers R o. We found that at ≈700 Myr almost all M dwarfs exhibit Hα emission, with binaries having the same overall color–Hα equivalent width distribution as single stars. In the R o–L Hα /L bol plane, unsaturated single stars follow a power law with index β = −5.9 ± 0.8 for R o > 0.3. In the R o–L X/L bol plane, we see evidence for supersaturation for single stars with R o ≲ 0.01, following a power law with index βsup=0.5−0.1+0.2 , supporting the hypothesis that the coronae of these stars are being centrifugally stripped. We found that the critical R o value at which activity saturates is smaller for L X/L bol than for L Hα /L bol. Finally, we observed an almost 1:1 relation between L Hα /L bol and L X/L bol, suggesting that both the corona and the chromosphere experience similar magnetic heating.

Identifying Quiescent Compact Objects in Massive Galactic Single-lined Spectroscopic Binaries

It is now clearly established that massive stars are predominantly found in multiple systems. While their period and eccentricity distributions are now well established across different metallicity regimes, the mass-ratio distribution has been mostly limited to double-lined spectroscopic binaries. The mass-ratio distribution therefore remains subject to significant uncertainties and open questions encompass the shape and extend of the companion mass-function towards its low-mass end and the nature of undetected companions in single-lined spectroscopic binaries, i.e. low-mass main-sequence stars, binary interaction products, compact objects, etc. We have conducted a large and systematic analysis of a sample of more than 80 single-line O-type spectroscopic binaries in the Milky Way and in the Large Magellanic Cloud using a sophisticated multi-technique, and multi-wavelength approach. We report on the developed methodology, the constraints obtained on the nature of SB1 companions, the distribution of O star mass-ratios at LMC metallicity and the occurrence of quiescent OB+black hole binaries.

The Formation of Blue Large-amplitude Pulsators from White-dwarf Main-sequence Star Mergers

Blue large-amplitude pulsators (BLAPs) are hot low-mass stars that show large-amplitude light variations likely due to radial oscillations driven by iron group opacities. Period changes provide evidence of both secular contraction and expansion among the class. Various formation histories have been proposed, but none are completely satisfactory. Zhang et al. proposed that the merger of a helium-core white dwarf with a low-mass main-sequence star (HeWD+MS) can lead to the formation of some classes of hot subdwarfs. We have analyzed these HeWD+MS merger models in more detail. Between helium-shell ignition and full helium-core burning, the models pass through the volume of luminosity–gravity–temperature space occupied by BLAPs. Periods of expansion and contraction associated with helium-shell flashes can account for the observed rates of period change. We argue that the HeWD+MS merger model provides at least one BLAP formation channel.

The merger of hard binaries in globular clusters as the primary channel for the formation of second-generation stars

ABSTRACT We have recently presented observational evidence which suggests that the origin of the second-generation (G2) stars in globular clusters (GCs) is due to the binary-mediated collision of primordial (G1) low-mass main-sequence (MS) stars. This mechanism avoids both the mass budget problem and the need of external gas for dilution. Here, we report on another piece of evidence supporting this scenario: (1) the fraction of MS binaries is proportional to the fraction of G1 stars in GCs and, at the same time, (2) the smaller the fraction of G1 stars is, the more deficient binaries of higher mass ratio (q>0.7) are. They are, on average, harder than their smaller mass-ratio counterparts due to higher binding energy at a given primary mass. Then (2) implies that (1) is due to the merging/collisions of hard binaries rather than to their disruption. These new results complemented by the present-day data on binaries lead to the following conclusions: (i) the mass-ratio distribution of binaries, particularly short-period ones, with low-mass primaries, MP < 1.5 M⊙, is strongly peaked close to q=1.0, whereas (ii) dynamical processes at high stellar density tend to destroy softer binaries and make hard (nearly) twin binaries to become even harder and favour their mergers and collisions. G2 stars formed this way gain mass that virtually doubles the primary one, 2MP, at which the number of G1 stars is approximately five times smaller than at MP according to the slope of a Milky Way-like initial mass function at MMS < 1.0 M⊙.

Main-sequence companions to white dwarfs – II. The age–activity–rotation relation from a sample of Gaia common proper motion pairs

ABSTRACT Magnetic activity and rotation are related to the age of low-mass main-sequence stars. To further constrain these relations, we study a sample of 574 main-sequence stars members of common proper motion pairs with white dwarfs, identified thanks to Gaia astrometry. We use the white dwarfs as age indicators, while the activity indexes and rotational velocities are obtained from the main-sequence companions using standard procedures. We find that stars older than 5 Gyr do not display H α nor Ca ii H&K emission unless they are fast rotators due to tidal locking from the presence of unseen companions and that the rotational velocities tend to decrease over time, thus supporting the so-called gyrochronology. However, we also find moderately old stars (≃2–6 Gyr) that are active presumably because they rotate faster than they should for their given ages. This indicates that they may be suffering from weakened magnetic braking or that they possibly evolved through wind accretion processes in the past. The activity fractions that we measure for all stars younger than 5 Gyr range between ≃10 and 40 per cent. This is line with the expectations, since our sample is composed of F, G, K, and early M stars, which are thought to have short (<2 Gyr) activity lifetimes. Finally, we observe that the H α fractional luminosities and the $R^{\prime }_\mathrm{HK}$ indexes for our sample of (slowly rotating) stars show a spread (−4 >log(LH α/Lbol); log($R^{\prime }_\mathrm{HK}$) > −5) typically found in inactive M stars or weakly active/inactive F, G, K stars.

Photometry and astrometry with JWST – III. A NIRCam-Gaia DR3 analysis of the open cluster NGC 2506

ABSTRACT In the third paper of this series aimed at developing the tools for analysing resolved stellar populations using the cameras on board of the James Webb Space Telescope (JWST), we present a detailed multiband study of the 2 Gyr Galactic open cluster NGC 2506. We employ public calibration data sets collected in multiple filters to: (i) derive improved effective Point Spread Functions (ePSFs) for 10 NIRCam filters; (ii) extract high-precision photometry and astrometry for stars in the cluster, approaching the main sequence (MS) lower mass of ∼0.1 M⊙; and (iii) take advantage of the synergy between JWST and Gaia DR3 to perform a comprehensive analysis of the cluster’s global and local properties. We derived a MS binary fraction of ∼57.5 per cent, extending the Gaia limit (∼0.8 M⊙) to lower masses (∼0.4 M⊙) with JWST. We conducted a study on the mass functions (MFs) of NGC 2506, mapping the mass segregation with Gaia data, and extending MFs to lower masses with the JWST field. We also combined information on the derived MFs to infer an estimate of the cluster present-day total mass. Lastly, we investigated the presence of white dwarfs (WDs) and identified a strong candidate. However, to firmly establish its cluster membership, as well as that of four other WD candidates and of the majority of faint low-mass MS stars, further JWST equally deep observations will be required. We make publicly available catalogues, atlases, and the improved ePSFs.

Rotational Stalling Ends when Main-sequence Core Temperatures are at a Minimum

Low-mass stars spin-down over time due to angular momentum loss from magnetized stellar winds. This spin-down is interrupted by an epoch of stalling where stars halt their spin-down for 0.1–2 Gyr. Duration of the stalling epoch is mass-dependent, with higher mass stars spending less time stalled than the lowest mass stars. We demonstrate that the mass-dependent age at which stars resume spinning down correlates with the age where low-mass stars are predicted to achieve their minimum core temperature on the main sequence. This suggests that the stalling epoch is commensurate with a phase in low-mass main-sequence star evolution where stars develop a small convective core and their convective envelopes are growing in mass.

Quasi-periodic eruptions from mildly eccentric unstable mass transfer in galactic nuclei

ABSTRACT We propose that the recently observed quasi-periodic eruptions (QPEs) in galactic nuclei are produced by unstable mass transfer due to Roche lobe overflow of a low-mass main-sequence star in a mildly eccentric (e ∼ 0.5) orbit. We argue that the QPE emission is powered by circularization shocks, but not directly by black hole (BH) accretion. Our model predicts the presence of a time-steady accretion disc that is bolometrically brighter than the time-averaged QPE luminosity, but primarily emits in the extreme-ultraviolet. This is consistent with the quiescent soft X-ray emission detected in between the eruptions in eROSITA QPE1, QPE2, and GSN 069. Such accretion discs have an unusual νLν ∝ ν12/7 optical spectrum. The lifetime of the bright QPE phase, 102–103 yr, is set by mass-loss triggered by ram-pressure interaction between the star and the accretion disc fed by the star itself. We show that the stellar orbits needed to explain QPEs can be efficiently created by the Hills breakup of tight stellar binaries provided that (i) the stellar binary orbit is tidally hardened before the breakup due to diffusive growth of the f-mode amplitude and (ii) the captured star’s orbit decays by gravitational wave emission without significant orbital angular momentum diffusion (which is the case for low-mass BHs, MBH ≲ 106 M⊙). We conclude by discussing the implications of our model for hyper-velocity stars, extreme mass ratio inspirals, repeating partial TDEs, and related stellar phenomena in galactic nuclei.

Modeling fast radio burst heating a main-sequence companion star in a close binary using MESA

The exact origin of fast radio bursts (FRBs) remains a mystery. The repeating fast radio burst source, FRB20200120E, was discovered in a globular cluster containing old stellar populations. Yang (2021) suggested that this FRB might be in close binaries with low-mass main-sequence (MS) stars. They analytically investigated the observational consequences caused by the heating of FRB radio radiation onto the low-mass MS companion star in a close binary, suggesting that the radio radiation emitted by FRB could make the MS companion star more luminous and detectable in future multi-wavelength follow-up observations for a Galactic FRB. We revisited the study of Yang (2021) by numerically modeling the detailed process of FRB heating onto an MS companion with 1D stellar evolution code Modules for Experiments in Stellar Astrophysics (MESA). Our results are consistent with the trends derived from the analytical model of Yang (2021), except that the typical re-emission luminosities of our main sequence (MS) models, caused by the heating from FRBs, are generally dimmer by about two orders of magnitude compared to his findings, and our models have a longer re-emission timescale. This may indicate that the searches of the optical transients caused by the radio radiation heating companion star are more likely to be successful within a distance of 0.3 Mpc.

Specific Heat of Electron Plasma Waves.

Collective modes in a plasma, like phonons in a solid, contribute to a material's equation of state and transport properties, but the long wavelengths of these modes are difficult to simulate with today's finite-size quantum simulation techniques. A simple Debye-type calculation of the specific heat of electron plasma waves is presented, yielding up to 0.05k/e^{-} for warm dense matter (WDM), where thermal and Fermi energies are near 1 Ry=13.6 eV. This overlooked energy reservoir is sufficient to explain reported compression differences between theoretical hydrogen models and shock experiments. Such an additional specific heat contribution refines our understanding of systems passing through the WDM regime, such as the convective threshold in low-mass main-sequence stars, white dwarf envelopes, and substellar objects; WDM x-ray scattering experiments; and the compression of inertial confinement fusion fuels.

Why the observed spin evolution of older-than-solar-like stars might not require a dynamo mode change

ABSTRACT The spin evolution of main-sequence stars has long been of interest for basic stellar evolution, stellar ageing, stellar activity, and consequent influence on companion planets. Observations of older-than-solar late-type main-sequence stars have been interpreted to imply that a change from a dipole-dominated magnetic field to one with more prominent higher multipoles might be necessary to account for the data. The spin-down models that lead to this inference are essentially tuned to the Sun. Here, we take a different approach that considers individual stars as fixed points rather than just the Sun. We use a time-dependent theoretical model to solve for the spin evolution of low-mass main-sequence stars that includes a Parker-type wind and a time-evolving magnetic field coupled to the spin. Because the wind is exponentially sensitive to the stellar mass over radius and the coronal base temperature, the use of each observed star as a separate fixed point is more appropriate and, in turn, produces a set of solution curves that produces a solution envelope rather than a simple line. This envelope of solution curves, unlike a single line fit, is consistent with the data and does not unambiguously require a modal transition in the magnetic field to explain it.

Unstable Mass Transfer from a Main-sequence Star to a Supermassive Black Hole and Quasiperiodic Eruptions

We discuss the formation and evolution of systems composed of a low-mass (M ⋆ ≲ 4 M ⊙) main-sequence star orbiting a 105–107 M ⊙ supermassive black hole with an orbital period of order ∼hours and a mild eccentricity (e ≈ 0.1–0.2), episodically shedding mass at each pericenter passage. We argue that the resulting mass transfer is likely unstable, with Roche lobe overflow initially driven by gravitational-wave emission, but then being accelerated by the star’s expansion in response to its mass loss, undergoing a runaway process. We show that such systems are naturally produced by two-body gravitational encounters within the inner parsec of a galaxy, followed by gravitational-wave circularization and inspiral from initially highly eccentric orbits. We argue that such systems can produce recurring flares similar to the recently identified class of X-ray transients known as quasiperiodic eruptions, observed at the centers of a few distant galaxies.

Primordial black holes capture by stars and induced collapse to low-mass stellar black holes

ABSTRACT Primordial black holes in the asteroid-mass window, which might constitute all the dark matter, can be captured by stars when they traverse them at low enough velocity. After being placed on a bound orbit during star formation, they can repeatedly cross the star if the orbit happens to be highly eccentric, slow down by dynamical friction, and end up in the stellar core. The rate of these captures is highest in haloes of high dark matter density and low velocity dispersion, when the first stars form at redshift z ∼ 20. We compute this capture rate for low-metallicity stars of 0.3–$1\, {\rm M_{\odot }}$, and find that a high fraction of these stars formed in the first dwarf galaxies would capture a primordial black hole, which would then grow by accretion up to a mass that may be close to the total star mass. We show the capture rate of primordial black holes does not depend on their mass over this asteroid-mass window, and should not be much affected by external tidal perturbations. These low-mass stellar black holes could be discovered today in low-metallicity, old binary systems in the Milky Way containing a surviving low-mass main-sequence star or a white dwarf, or via gravitational waves emitted in a merger with another compact object. No mechanisms in standard stellar evolution theory are known to form black holes below the Chandrasekhar mass, so detecting a low-mass black hole would fundamentally impact our understanding of stellar evolution, dark matter, and the early Universe.

Population synthesis of AX J1745.6−2901 X-ray nova-type binaries with rapidly decreasing orbital periods

ABSTRACT The neutron star low-mass X-ray binary (LMXB) AX J1745.6−2901 was detected with an anomalously fast decrease of its orbital period. The decreasing rate of the orbital period exceeds the contribution of all processes extracting angular momentum from the binary star in the standard model. Using the scenario machine code, we conducted a population synthesis study of X-ray novae with neutron stars to investigate a possible formation and evolution of such binaries. Such close LMXBs should experience a preceding common envelope stage, in which the magnetic fields of the low-mass main-sequence donor stars can be dramatically amplified. Our calculations show that the magnetic stellar wind of the optical companion can efficiently extract angular momentum from the binary systems, and produce the observed orbital-period derivatives of AX J1745.6−2901 and black hole LMXBs. The estimated values of the required magnetic field induction are the following: Bd ≈ 400 G (AX J1745.6−2901), Bd ≈ 1500 G (KV UMa), Bd ≈ 400 G (A0620−00) and Bd ≈ 1800 G (Nova Muscae). We successfully reproduced the current observational abundance of such anomalous neutron star X-ray novae, and computed the appropriate value of the parameter of magnetic braking λMSW (0.8−0.6 for Roche lobe filling stars and 0.4−0.15 for binaries with partial Roche lobe filling).

Evidence for the Late Arrival of Hot Jupiters in Systems with High Host-star Obliquities

It has been shown that hot Jupiters systems with massive, hot stellar primaries exhibit a wide range of stellar obliquities. On the other hand, hot Jupiter systems with low-mass, cool primaries often have stellar obliquities close to zero. Efficient tidal interactions between hot Jupiters and the convective envelopes present in lower-mass main-sequence stars have been a popular explanation for these observations. If this explanation is accurate, then aligned systems should be older than misaligned systems. Likewise, the convective envelope mass of a hot Jupiter’s host star should be an effective predictor of its obliquity. We derive homogeneous stellar parameters—including convective envelope masses—for hot Jupiter host stars with high-quality sky-projected obliquity inferences. Using a thin-disk stellar population’s Galactic velocity dispersion as a relative age proxy, we find that hot Jupiter host stars with larger-than-median obliquities are older than hot Jupiter host stars with smaller-than-median obliquities. The relative age difference between the two populations is larger for hot Jupiter host stars with smaller-than-median fractional convective envelope masses and is significant at the 3.6σ level. We identify stellar mass, not convective envelope mass, as the best predictor of stellar obliquity in hot Jupiter systems. The best explanation for these observations is that many hot Jupiters in misaligned systems arrived in the close proximity of their host stars long after their parent protoplanetary disks dissipated. The dependence of observed age offset on convective envelope mass suggests that tidal realignment contributes to the population of aligned hot Jupiters orbiting stars with convective envelopes.

A Monte Carlo Method for Evaluating Empirical Gyrochronology Models and Its Application to Wide Binary Benchmarks

Accurate stellar ages are essential for our understanding of the star formation history of the Milky Way and Galactic chemical evolution, as well as to constrain exoplanet formation models. Gyrochronology, a relationship between stellar rotation and age, appears to offer a reliable age indicator for main-sequence (MS) stars over the mass range of approximately 0.6–1.3 M ⊙. Those stars lose their angular momentum due to magnetic braking and as a result their rotation speeds decrease with age. Although current gyrochronology relations have been fairly well tested for young MS stars with masses greater than 1 M ⊙, primarily in young open clusters, insufficient tests exist for older and lower mass MS stars. Binary stars offer the potential to expand and fill in the range of ages and metallicity over which gyrochronology can be empirically tested. In this paper, we demonstrate a Monte Carlo approach to evaluate gyrochronology models using binary stars. As examples, we used five previously published wide binary pairs. We also demonstrate a Monte Carlo approach to assess the precision and accuracy of ages derived from each gyrochronology model. For the traditional Skumanich models, the age uncertainties are σ age/age = 15%–20% for stars with B − V = 0.65 and σ age/age = 5%–10% for stars with B − V = 1.5 and rotation period P ≤ 20 days.

Magnetic dynamos in white dwarfs – III. Explaining the occurrence of strong magnetic fields in close double white dwarfs

ABSTRACT The origin of strong ($\stackrel{\gt }{\scriptstyle \sim }1\,\mathrm{ MG}$) magnetic fields in white dwarfs has been a puzzle for decades. Recently, a dynamo mechanism operating in rapidly rotating and crystallizing white dwarfs has been suggested to explain the occurrence rates of strong magnetic fields in white dwarfs with close low-mass main-sequence star companions. Here, we investigate whether the same mechanism may produce strong magnetic fields in close double white dwarfs. The only known strongly magnetic white dwarf that is part of a close double white dwarf system, the magnetic component of NLTT 12758, is rapidly rotating and likely crystallizing and therefore the proposed dynamo mechanism represents an excellent scenario for the origin of its magnetic field. Presenting a revised formation scenario for NLTT 12758, we find a natural explanation for the rapid rotation of the magnetic component. We furthermore show that it is not surprising that strong magnetic fields have not been detected in all other known double white dwarfs. We therefore conclude that the incidence of magnetic fields in close double white dwarfs supports the idea that a rotation- and crystallization-driven dynamo plays a major role in the generation of strong magnetic fields in white dwarfs.

Jets in common envelopes: a low-mass main-sequence star in a red giant

ABSTRACTWe present small-scale 3D hydrodynamical simulations of the evolution of a 0.3 M⊙ main-sequence (MS) star that launches two perpendicular jets within the envelope of a 0.88 M⊙ red giant (RG). Based on previous large-scale simulations, we study the dynamics of the jets either when the secondary star is grazing, when it has plunged-in, or when it is well within the envelope of the RG (in each stage for ∼11 d). The dynamics of the jets through the common envelope (CE) depend on the conditions of the environment as well as on their powering. In the grazing stage and the commencement of the plunge self-regulated jets need higher efficiencies to break out of the envelope of the RG. Deep inside the CE, on the time-scales simulated, jets are choked independently of whether they are self-regulated or constantly powered. Jets able to break out of the envelope of the RG in large-scale simulations, are choked in our small-scale simulations. The accreted angular momentum on to the secondary star is not large enough to form a disc. The mass accretion on to the MS star is 1–10 per cent of the Bondi–Hoyle–Littleton rate (∼10−3–10−1 M⊙ yr−1). High-luminosity emission, from X-rays to ultraviolet and optical, is expected if the jets break out of the CE. Our simulations illustrate the need for inclusion of more realistic accretion and jet models in the dynamical evolution of the CEs.