Accretion Outburst Research Articles

Collisional evolution of dust and water ice in protoplanetary discs during and after an accretion outburst

ABSTRACTMost protoplanetary discs are thought to undergo violent and frequent accretion outbursts, during which the accretion rate and central luminosity are elevated for several decades. This temporarily increases the disc temperature, leading to the sublimation of ice species as snowlines move outwards. In this paper, we investigate how an FUor-type accretion outburst alters the growth and appearance of dust aggregates at different locations in protoplanetary discs. We develop a model based on the Monte Carlo approach to simulate locally the coagulation and fragmentation of icy dust particles and investigate different designs for their structure and response to sublimation. Our main finding is that the evolution of dust grains located between the quiescent and outburst water snowlines is driven by significant changes in composition and porosity. The time required for the dust population to recover from the outburst and return to a coagulation/fragmentation equilibrium depends on the complex interplay of coagulation physics and outburst properties, and can take up to 4500 yr at 5 au. Pebble-sized particles, the building blocks of planetesimals, are either deprecated in water ice or completely destroyed, respectively, resulting in drier planetesimals or halting their formation altogether. When accretion outbursts are frequent events, the dust can be far from collisional equilibrium for a significant fraction of time, offering opportunities to track past outbursts in discs at millimetre wavelengths. Our results highlight the importance of including accretion outbursts in models of dust coagulation and planet formation.

Repeating Outbursts from the Young Stellar Object Gaia23bab (=SPICY 97589)

The light curve of Gaia23bab (=SPICY 97589) shows two significant (ΔG > 2 mag) brightening events, one in 2017 and an ongoing event starting in 2022. The source’s quiescent spectral energy distribution indicates an embedded (A V > 5 mag) pre-main-sequence star, with optical accretion emission and mid-infrared disk emission. This characterization is supported by the source’s membership in an embedded cluster in the star-forming cloud DOBASHI 1604 at a distance of 900 ± 45 pc. Thus, the brightening events are probable accretion outbursts, likely of EX Lup-type.

H2O masers and host environments of FU Orionis and EX Lupi type low-mass eruptive YSOs

AbstractThe FU Orionis (FUor) and EX Lupi (EXor) type objects are rare pre-main sequence low-mass stars undergoing accretion outbursts. Maser emission is widespread and is a powerful probe of mass accretion and ejection on small scales in star forming region. However, very little is known about the overall prevalence of water masers towards FUors/Exors. We present results from our survey using the Effelsberg 100-m telescope to observe the largest sample of FUors and EXors, plus additional Gaia alerted sources (with the potential nature of being eruptive stars), a total of 51 targets, observing the 22.2 GHz H2O maser, while simultaneously covering the NH3 23 GHz.

Time-dependent, long-term hydrodynamic simulations of the inner protoplanetary disk

Context. The spin evolution of young protostars, surrounded by an accretion disk, still poses problems for observations and theoretical models. In recent studies, the importance of the magnetic star-disk interaction for stellar spin evolution has been elaborated. The accretion disk in these studies, however, is only represented by a simplified model and important features are not considered. Aims. A more realistic representation of the accretion disk is indispensable for a better understanding of the star-disk interaction and the stellar spin evolution. The aim of this study is to investigate the influence of a hydrodynamic disk evolution on the stellar rotational period and vice versa during the accretion phase. Methods. We combined the implicit hydrodynamic TAPIR disk code with a stellar spin evolution model. The influence of stellar magnetic fields on the disk dynamics, the radial position of the inner disk radius, as well as the influence of stellar rotation on the disk were calculated self-consistently. Results. Within a defined parameter space, we can reproduce the majority of fast and slow rotating stars observed in young stellar clusters. Additionally, the back reaction of different stellar spin evolutionary tracks on the disk can be analyzed. Disks around fast rotating stars are located closer to the star. Consequently, the disk midplane temperature in the innermost disk region increases significantly compared to slow rotating stars. We can show the effects of stellar rotation on episodic accretion outbursts. The higher temperatures of disks around fast rotating stars result in more outbursts and a longer outbursting period over the disk lifetime. Conclusions. The combination of a long-term hydrodynamic disk and a stellar spin evolution model allows the inclusion of previously unconsidered effects such as the back-reaction of stellar rotation on the long-term disk evolution and the occurrence of accretion outbursts. However, a wider parameter range has to be studied to further investigate these effects. Additionally, a possible interaction between our model and a more realistic stellar evolution code (e.g., the MESA code) can improve our understanding of the stellar spin evolution and its effects on the pre-main sequence star.

Fast Neutrino Cooling in the Accreting Neutron Star MXB 1659-29

Modeling of crust heating and cooling across multiple accretion outbursts of the low mass X-ray binary MXB 1659-29 indicates that the neutrino luminosity of the neutron star core is consistent with direct Urca (dUrca) reactions occurring in ∼1% of the core volume. We investigate this scenario with neutron star models that include a detailed equation of state parametrized by the slope of the nuclear symmetry energy L, and a range of neutron and proton superfluid gaps. We find that the predicted neutron star mass depends sensitively on L and the assumed gaps. We discuss which combinations of superfluid gaps reproduce the inferred neutrino luminosity. Larger values of L ≳ 80 MeV require superfluidity to suppress dUrca reactions in low mass neutron stars, i.e., the proton or neutron gap is sufficiently strong and extends to high enough density. However, the largest gaps give masses near the maximum mass, making it difficult to accommodate colder neutron stars. The heat capacities of our models span the range from fully paired to fully unpaired nucleons meaning that long-term observations of core cooling could distinguish between models. As a route to solutions with a larger emitting volume, which could provide a more natural explanation for the inferred neutrino luminosity, we discuss the possibility of alternative, less efficient, fast cooling processes in exotic cores. To be consistent with the inferred neutrino luminosity, such processes must be within a factor of ∼1000 of dUrca. We discuss the impact of future constraints on neutron star mass, radius, and the density dependence of the symmetry energy.

Diagnosing FU Ori-like Sources: The Parameter Space of Viscously Heated Disks in the Optical and Near-infrared

FU Ori-type objects (FUors) are decades-long outbursts of accretion onto young stars that are strong enough to viscously heat disks so that the disk outshines the central star. We construct models for FUor objects by calculating emission components from a steady-state viscous accretion disk, a passively-heated dusty disk, magnetospheric accretion columns, and the stellar photosphere. We explore the parameter space of the accretion rate and stellar mass M * to investigate implications on the optical and near-infrared spectral energy distribution and spectral lines. The models are validated by fitting to multiwavelength photometry of three confirmed FUor objects, FU Ori, V883 Ori, and HBC 722, and then comparing the predicted spectra to the observed optical and infrared spectra. The brightness ratio between the viscous disk and the stellar photosphere, η, provides an important guide for identifying viscous accretion disks, with η = 1 (“transition line”) and η = 5 (“sufficient dominance line”) marking turning points in diagnostics, evaluated here in the near-infrared. These turning points indicate the emergence and complete development of FUor-characteristic strong CO absorption, weak metallic absorption, the triangular spectral continuum shape in the H band, and location in color–magnitude diagrams. Lower M * and higher imply larger η; for M * = 0.3 M ⊙, η = 1 corresponds to yr−1 and η = 5 to yr−1. The “sufficient dominance line” also coincides with the expected accretion rate where accreting material directly reaches the star. We discuss implications of the models on extinction diagnostics, FUor brightening timescales, viscous disks during initial protostellar growth, and eruptive young stellar objects associated with FUors.

Accretion Burst Echoes as Probes of Protostellar Environments and Episodic Mass Assembly

Protostars likely accrete material at a highly time-variable rate, but measurements of accretion variability from the youngest protostars are rare, as they are still deeply embedded within their envelopes. Submillimeter/millimeter observations can trace the thermal response of dust in the envelope to accretion luminosity changes, allowing variations in the accretion rate to be quantified. In this paper, we present contemporaneous submillimeter/millimeter light curves of variable protostars in Serpens Main, as observed by the ALMA Atacama Compact Array (ACA), the Submillimeter Array (SMA), and the James Clerk Maxwell Telescope (JCMT). The most recent outburst of EC 53 (V371 Ser), an ∼18 month periodic variable, is well sampled in the SMA and JCMT observations. The SMA light curve of EC 53 is observed to peak weeks earlier and exhibit a stronger amplitude than at the JCMT. Stochastic variations in the ACA observations are detected for SMM 10 IR, with an amplitude a factor of ∼2 greater than that seen by the JCMT. We develop a toy model of the envelope response to accretion outbursts to show that EC 53's light curves are plausibly explained by the delay associated with the light travel time across the envelope and the additional dilution of the JCMT response, due to the incorporation of cold envelope material in the beam. The larger JCMT beam can also wash out the response to rapid variations, which may be occurring for SMM 10 IR. Our work thus provides a valuable proof of concept for the use of submillimeter/millimeter observations as a means of probing both the underlying accretion luminosity variations and the protostellar environment.

A Multi-epoch, Multiwavelength Study of the Classical FUor V1515 Cyg Approaching Quiescence

Historically, FU Orionis-type stars are low-mass, pre-main-sequence stars. The members of this class experience powerful accretion outbursts and remain in an enhanced accretion state for decades or centuries. V1515 Cyg, a classical FUor, started brightening in the 1940s and reached its peak brightness in the late 1970s. Following a sudden decrease in brightness, it stayed in a minimum state for a few months, then started brightening for several years. We present the results of our ground-based photometric monitoring complemented with optical/near-infrared spectroscopic monitoring. Our light curves show a long-term fading with strong variability on weekly and monthly timescales. The optical spectra show P Cygni profiles and broad blueshifted absorption lines, common properties of FUors. However, V1515 Cyg lacks the P Cygni profile in the Ca ii 8498 Å line, a part of the Ca infrared triplet, formed by an outflowing wind, suggesting that the absorbing gas in the wind is optically thin. The newly obtained near-infrared spectrum shows the strengthening of the CO bandhead and the FeH molecular band, indicating that the disk has become cooler since the last spectroscopic observation in 2015. The current luminosity of the accretion disk dropped from the peak value of 138 L ⊙ to about 45 L ⊙, suggesting that the long-term fading is also partly caused by the dropping of the accretion rate.

Constraining equation-of-state groups from g-mode asteroseismology

ABSTRACT Buoyancy-restored modes inside neutron stars depend sensitively on both the microphysical (e.g. composition and entropy gradients) and macrophysical (e.g. stellar mass and radius) properties of the star. Asteroseismology efforts for g modes are therefore particularly promising avenues for recovering information concerning the nuclear equation of state. In this work, it is shown that the overall low-temperature g-space consists of multiple groups corresponding to different classes of equation of state (e.g. hadronic versus hybrid). This is in contrast to the case of pressure-driven modes, for example, which tend to follow a universal relation regardless of microphysical considerations. Using a wide library of currently viable equations of state, perturbations of static, stratified stars are calculated in general relativity to demonstrate in particular how g-space groupings can be classified according to the mean mass density, temperature, central speed of sound, and tidal deformability. Considering present and future observations regarding gravitational waves, accretion outbursts, quasi-periodic oscillations, and precursor flashes from gamma-ray bursts, it is shown how one might determine which group the g modes belong to.

A likely flyby of binary protostar Z CMa caught in action

Close encounters between young stellar objects in star forming clusters are expected to dramatically perturb circumstellar disks. Such events are witnessed in numerical simulations of star formation, but few direct observations of ongoing encounters have been made. Here we report sub-0".1 resolution Atacama Large Millimeter Array (ALMA) and Jansky Very Large Array (JVLA) observations towards the million year old binary protostar Z CMa in dust continuum and molecular line emission. A point source ~4700 au from the binary has been discovered at both millimeter and centimeter wavelengths. It is located along the extension of a ~2000 au streamer structure previously found in scattered light imaging, whose counterpart in dust and gas emission is also newly identified. Comparison with simulations shows signposts of a rare flyby event in action. Z CMa is a "double burster", as both binary components undergo accretion outbursts, which may be facilitated by perturbations to the host disk by flybys.

Millimeter-sized Dust Grains Surviving the Water-sublimating Temperature in the Inner 10 au of the FU Ori Disk

Abstract Previous observations have shown that the ≲10 au, ≳400 K hot inner disk of the archetypal accretion outburst young stellar object, FU Ori, is dominated by viscous heating. To constrain dust properties in this region, we have performed radio observations toward this disk using the Karl G. Jansky Very Large Array in 2020 June–July, September, and November. We also performed complementary optical photometric monitoring observations. We found that the dust thermal emission from the hot inner disk mid-plane of FU Ori has been approximately stationary and the maximum dust grain size is ≳1.6 mm in this region. If the hot inner disk of FU Ori, which is inward of the 150–170 K water snowline, is turbulent (e.g., corresponding to a Sunyaev & Shakura viscous α t ≳ 0.1), or if the actual maximum grain size is still larger than the lower limit we presently constrain, then as suggested by the recent analytical calculations and the laboratory measurements, water-ice-free dust grains may be stickier than water-ice-coated dust grains in protoplanetary disks. Additionally, we find that the free–free emission and the Johnson B- and V-band magnitudes of these binary stars were brightening in 2016–2020. The optical and radio variability might be related to the dynamically evolving protostellar- or disk-accretion activities. Our results highlight that the hot inner disks of outbursting objects are important laboratories for testing models of dust grain growth. Given the active nature of such systems, to robustly diagnose the maximum dust grain sizes, it is important to carry out coordinated multiwavelength radio observations.

M17 MIR: A Massive Protostar with Multiple Accretion Outbursts *

Abstract We report the discovery of a massive protostar M17 MIR embedded in a hot molecular core in M17. The multiwavelength data obtained during 1993–2019 show significant mid-IR (MIR) variations, which can be split into three stages: the decreasing phase during 1993.03–mid-2004, the quiescent phase from mid-2004 to mid-2010, and the rebrightening phase from mid-2010 until now. The variation of the 22 GHz H2O maser emission, together with the MIR variation, indicates an enhanced disk accretion rate onto M17 MIR during the decreasing and rebrightening phases. Radiative transfer modeling of the spectral energy distributions of M17 MIR in the 2005 epoch (quiescent) and 2017 epoch (accretion outburst) constrains the basic stellar parameters of M17 MIR, which is an intermediate-mass protostar (M * ∼ 5.4 M ⊙) with M ̇ acc ∼ 1.1 × 10 − 5 M ⊙ yr − 1 in the 2005 epoch and M ̇ acc ∼ 1.7 × 10 − 3 M ⊙ yr − 1 in the 2017 epoch. The enhanced M ̇ acc during outburst induces the luminosity outburst ΔL ≈ 7600 L ⊙. In the accretion outburst, a larger stellar radius is required to produce M ̇ acc consistent with the value estimated from the kinematics of H2O masers. M17 MIR shows two accretion outbursts (Δt ∼ 9–20 yr) with outburst magnitudes of about 2 mag, separated by a 6 yr quiescent phase. The accretion outburst occupies 83% of the time over 26 yr. The accretion rate in outburst is variable with amplitude much lower than the contrast between quiescent and outburst phases. The extreme youth of M17 MIR suggests that minor accretion bursts are frequent in the earliest stages of massive star formation.

Constraining the properties of dense neutron star cores: the case of the low-mass X-ray binary HETE J1900.1–2455

ABSTRACT Measuring the time evolution of the effective surface temperature of neutron stars can provide invaluable information on the properties of their dense cores. Here, we report on a new Chandra observation of the transient neutron star low-mass X-ray binary HETE J1900.1–2455, which was obtained ≈2.5 yr after the end of its ≈10-yr long accretion outburst. The source is barely detected during the observation, collecting only six net photons, all below 2 keV. Assuming that the spectrum is shaped as a neutron star atmosphere model, we perform a statistical analysis to determine a 1σ confidence upper range for the neutron star temperature of ≈30–39 eV (for an observer at infinity), depending on its mass, radius, and distance. Given the heat injected into the neutron star during the accretion outburst, estimated from data provided by all-sky monitors, the inferred very low temperature suggests that the core either has a very high heat capacity or undergoes very rapid neutrino cooling. While the present data do not allow us to disentangle these two possibilities, both suggest that a significant fraction of the dense core is not superfluid/superconductor. Our modelling of the thermal evolution of the neutron star predicts that it may still cool further, down to a temperature of ≃15 eV. Measuring such a low temperature with a future observation may provide constraints on the fraction of baryons that is paired in the stellar core.

A Study of the Photometric and Spectroscopic Variations of the Prototypical FU Orionis–type Star V1057 Cyg

Abstract Among the low-mass pre-main sequence stars, a small group called FU Orionis–type objects (FUors) are notable for undergoing powerful accretion outbursts. V1057 Cyg, a classical example of an FUor, went into outburst around 1969–1970, after which it faded rapidly, making it the fastest-fading FUor known. Around 1995, a more rapid increase in fading occurred. Since that time, strong photometric modulations have been present. We present nearly 10 yr of source monitoring at Piszkéstető Observatory, complemented with optical/NIR photometry and spectroscopy from the Nordic Optical Telescope, Bohyunsan Optical Astronomy Observatory, Transiting Exoplanet Survey Satellite, and Stratospheric Observatory for Infrared Astronomy. Our light curves show continuation of significant quasi-periodic variability in brightness over the past decade. Our spectroscopic observations show strong wind features, shell features, and forbidden emission lines. All of these spectral lines vary with time. We also report the first detection of [S ii], [N ii], and [O iii] lines in the star.

Water maser variability in a high-mass YSO outburst

Context. Clarifying the relationship between mass accretion and ejection history is one of the key issues in understanding high-mass star formation processes. Aims. We aim to investigate the possible relationship between the mass accretion burst event in mid-June 2015 and the jet ejection in the high-mass protostar S255 NIRS 3. Methods. The Very Long Baseline Interferometer (VLBI) monitoring observations of the 22 GHz H2O masers were carried out using VLBI Exploration of Radio Astrometry (VERA) to reveal the 3D velocity and spatial structure of the outflow/jet traced by the H2O masers in S255 NIRS 3. In addition, we conducted follow-up observations of the submillimeter continuum and the 321 GHz H2O masers with the Atacama Large Millimeter/submillimeter Array (ALMA) at Band 7. Results. We successfully measured the proper motions of the 22 GHz H2O masers associated with a bipolar outflow. The structure is almost the same as was observed in 2005 and 2010. The expansion velocity of the blueshifted bow shock traced by the 22 GHz H2O masers was measured to be 28 km s−1, corresponding to a dynamical timescale of 60 yr. The direction of the maser outflow is slightly tilted compared with the radio jet, which could suggest a more recent ejection episode during the accretion burst event. The total flux density of the 22 GHz H2O masers gradually increases from the beginning of the VLBI monitoring in early 2017 and becomes almost constant in subsequent single-dish monitoring in 2018. The brightening of the H2O masers is more prominent in the northeast outflow lobe. For the first time, we revealed extended H2O maser emission at 22 GHz in a star-forming region, which is partly resolved out by VERA and even by the most extended Very Large Array (VLA) configurations. We find that the flux variation of such an extended component is similar to that of the unresolved maser emission. The ALMA Band 7 continuum emission did not show significant variations compared with the previous observations performed five months before. We mapped the 321 GHz H2O masers in S255 NIRS 3 providing the fourth example, for this maser, of the spatial distribution in a high-mass star-forming region. Conclusions. We conclude that the bow shock structure traced by the 22 GHz H2O maser features is unlikely to originate at the interface between the radio jet powered by the recent accretion outburst and the surrounding medium. The brightening of the 22 GHz H2O masers could be due to radiative excitation by photons form the (declining) infrared (IR) outburst escaping along the cavity created by the newly ejected material. The lower ratio of the 22 GHz/321 GHz maser luminosity in the blueshifted bow shock suggests a temperature ( >1000 K), higher than for the other maser features in this region.

Optical and ultraviolet pulsed emission from an accreting millisecond pulsar

Millisecond spinning, low magnetic field neutron stars are believed to attain their fast rotation in a 0.1-1 Gyr-long phase during which they accrete matter endowed with angular momentum from a low-mass companion star. Despite extensive searches, coherent periodicities originating from accreting neutron star magnetospheres have been detected only at X-ray energies and in ~10% of the presently known systems. Here we report the detection of optical and ultraviolet coherent pulsations at the X-ray period of the transient low mass X-ray binary system SAX J1808.4-3658, during an accretion outburst that occurred in August 2019. At the time of the observations, the pulsar was surrounded by an accretion disc, displayed X-ray pulsations and its luminosity was consistent with magnetically funneled accretion onto the neutron star. Current accretion models fail to account for the luminosity of both optical and ultraviolet pulsations; these are instead more likely driven by synchro-curvature radiation in the pulsar magnetosphere or just outside of it. This interpretation would imply that particle acceleration can take place even when mass accretion is going on, and opens up new perspectives in the study of coherent optical/UV pulsations from fast spinning accreting neutron stars in low-mass X-ray binary systems.

The evolution of a circumplanetary disc with a dead zone

ABSTRACT We investigate whether the regular Galilean satellites could have formed in the dead zone of a circumplanetary disc. A dead zone is a region of weak turbulence in which the magnetorotational instability is suppressed, potentially an ideal environment for satellite formation. With the grid-based hydrodynamic code fargo3d, we examine the evolution of a circumplanetary disc model with a dead zone. Material accumulates in the dead zone of the disc leading to a higher total mass and but a similar temperature profile compared to a fully turbulent disc model. The tidal torque increases the rate of mass transport through the dead zone leading to a steady-state disc with a dead zone that does not undergo accretion outbursts. We explore a range of disc, dead zone, and mass inflow parameters and find that the maximum mass of the disc is around $0.001 M_{\rm J}$. Since the total solid mass of such a disc is much lower, we find that there is not sufficient material in the disc for in situ formation of the Galilean satellites and that external supplement is required.

Dust settling instability in protoplanetary discs

ABSTRACT The streaming instability (SI) has been extensively studied in the linear and non-linear regimes as a mechanism to concentrate solids and trigger planetesimal formation in the mid-plane of protoplanetary discs. A related dust settling instability (DSI) applies to particles while settling towards the mid-plane. The DSI has previously been studied in the linear regime, with predictions that it could trigger particle clumping away from the mid-plane. This work presents a range of linear calculations and non-linear simulations, performed with fargo3d, to assess conditions for DSI growth. We expand on previous linear analyses by including particle size distributions and performing a detailed study of the amount of background turbulence needed to stabilize the DSI. When including binned size distributions, the DSI often produces converged growth rates with fewer bins than the standard SI. With background turbulence, we find that the most favourable conditions for DSI growth are weak turbulence, characterized by α ≲ 10−6 with intermediate-sized grains that settle from one gas scale height. These conditions could arise during a sudden decrease in disc turbulence following an accretion outburst. Ignoring background turbulence, we performed a parameter survey of local 2D DSI simulations. Particle clumping was either weak or occurred slower than particles settle. Clumping was reduced by a factor of 2 in a comparison 3D simulation. Overall, our results strongly disfavour the hypothesis that the DSI significantly promotes planetesimal formation. Non-linear simulations of the DSI with different numerical methods could support or challenge these findings.

Unexpected late-time temperature increase observed in the two neutron star crust-cooling sources XTE J1701−462 and EXO 0748−676

Transient low-mass X-ray binaries (LMXBs) that host neutron stars (NSs) provide excellent laboratories for probing the dense matter physics present in NS crusts. During accretion outbursts in LMXBs, exothermic reactions may heat the NS crust, disrupting the crust-core equilibrium. When the outburst ceases, the crust cools to restore thermal equilibrium with the core. Monitoring this cooling evolution allows us to probe the dense matter physics in the crust. Properties of the deeper crustal layers can be probed at later times after the end of the outburst. We report on the unexpected late-time temperature evolution (≳2000 days after the end of their outbursts) of two NSs in LMXBs, XTE J1701−462 and EXO 0748−676. Although both these sources exhibited very different outbursts (in terms of duration and the average accretion rate), they exhibit an unusually steep decay of ∼7 eV in the observed effective temperature (occurring in a time span of ∼700 days) around ∼2000 days after the end of their outbursts. Furthermore, they both showed an even more unexpected rise of ∼3 eV in temperature (over a time period of ∼500–2000 days) after this steep decay. This rise was significant at the 2.4σ and 8.5σ level for XTE J1701−462 and EXO 0748−676, respectively. The physical explanation for such behaviour is unknown and cannot be straightforwardly be explained within the cooling hypothesis. In addition, this observed evolution cannot be well explained by low-level accretion either without invoking many assumptions. We investigate the potential pathways in the theoretical heating and cooling models that could reproduce this unusual behaviour, which so far has been observed in two crust-cooling sources. Such a temperature increase has not been observed in the other NS crust-cooling sources at similarly late times, although it cannot be excluded that this might be a result of the inadequate sampling obtained at such late times.

The Planetary Luminosity Problem: “Missing Planets” and the Observational Consequences of Episodic Accretion

Abstract The high occurrence rates of spiral arms and large central clearings in protoplanetary disks, if interpreted as signposts of giant planets, indicate that gas giants commonly form as companions to young stars (<few Myr) at orbital separations of 10–300 au. However, attempts to directly image this giant planet population as companions to more mature stars (>10 Myr) have yielded few successes. This discrepancy could be explained if most giant planets form by “cold start,” i.e., by radiating away much of their formation energy as they assemble their mass, rendering them faint enough to elude detection at later times. In that case, giant planets should be bright at early times, during their accretion phase, and yet forming planets are detected only rarely through direct imaging techniques. Here we explore the possibility that the low detection rate of accreting planets is the result of episodic accretion through a circumplanetary disk. We also explore the possibility that the companion orbiting the Herbig Ae star HD 142527 may be a giant planet undergoing such an accretion outburst.