Accretion Variability Research Articles

A Multiwavelength, Multiepoch Monitoring Campaign of Accretion Variability in T Tauri Stars from the ODYSSEUS Survey. III. Optical Spectra* * Based on observations collected at the European Southern Observatory under ESO program 106.20Z8.

Classical T Tauri Stars (CTTSs) are highly variable stars that possess gas- and dust-rich disks from which planets form. Much of their variability is driven by mass accretion from the surrounding disk, a process that is still not entirely understood. A multiepoch optical spectral monitoring campaign of four CTTSs (TW Hya, RU Lup, BP Tau, and GM Aur) was conducted along with contemporaneous Hubble Space Telescope (HST) UV spectra and ground-based photometry in an effort to determine accretion characteristics and gauge variability in this sample. Using an accretion flow model, we find that the magnetospheric truncation radius varies between 2.5 and 5 R ⋆ across all of our observations. There is also significant variability in all emission lines studied, particularly Hα, Hβ, and Hγ. Using previously established relationships between line luminosity and accretion, we find that, on average, most lines reproduce accretion rates consistent with accretion shock modeling of HST spectra to within 0.5 dex. Looking at individual contemporaneous observations, however, these relationships are less accurate, suggesting that variability trends differ from the trends of the population and that these empirical relationships should be used with caution in studies of variability.

Highly variable magmatic accretion at the ultraslow-spreading Gakkel Ridge

Crustal accretion at mid-ocean ridges governs the creation and evolution of the oceanic lithosphere. Generally accepted models1–4 of passive mantle upwelling and melting predict notably decreased crustal thickness at a spreading rate of less than 20 mm year−1. We conducted the first, to our knowledge, high-resolution ocean-bottom seismometer (OBS) experiment at the Gakkel Ridge in the Arctic Ocean and imaged the crustal structure of the slowest-spreading ridge on the Earth. Unexpectedly, we find that crustal thickness ranges between 3.3 km and 8.9 km along the ridge axis and it increased from about 4.5 km to about 7.5 km over the past 5 Myr in an across-axis profile. The highly variable crustal thickness and relatively large average value does not align with the prediction of passive mantle upwelling models. Instead, it can be explained by a model of buoyant active mantle flow driven by thermal and compositional density changes owing to melt extraction. The influence of active versus passive upwelling is predicted to increase with decreasing spreading rate. The process of active mantle upwelling is anticipated to be primarily influenced by mantle temperature and composition. This implies that the observed variability in crustal accretion, which includes notably varied crustal thickness, is probably an inherent characteristic of ultraslow-spreading ridges.

A Multiwavelength, Multiepoch Monitoring Campaign of Accretion Variability in T Tauri Stars from the ODYSSEUS Survey. II. Photometric Light Curves

Classical T Tauri stars (CTTSs) are young, low-mass stars that accrete material from their surrounding protoplanetary disk. To better understand accretion variability, we conducted a multiepoch, multiwavelength photometric monitoring campaign of four CTTSs, TW Hya, RU Lup, BP Tau, and GM Aur, in 2021 and 2022, contemporaneous with Hubble Space Telescope UV and optical spectra. We find that all four targets display significant variability in their light curves, generally on days-long timescales (but in some cases year-to-year), often due to periodicity associated with stellar rotation and to stochastic accretion variability. There is a strong connection between mass accretion and photometric variability in all bands, but the relationship varies per target and epoch. Thus, photometry should be used with caution as a direct measure of accretion in CTTSs.

The Santa Barbara Binary−disk Code Comparison

We have performed numerical calculations of a binary interacting with a gas disk, using 11 different numerical methods and a standard binary−disk setup. The goal of this study is to determine whether all codes agree on a numerically converged solution and to determine the necessary resolution for convergence and the number of binary orbits that must be computed to reach an agreed-upon relaxed state of the binary−disk system. We find that all codes can agree on a converged solution (depending on the diagnostic being measured). The zone spacing required for most codes to reach a converged measurement of the torques applied to the binary by the disk is roughly 1% of the binary separation in the vicinity of the binary components. For our disk model to reach a relaxed state, codes must be run for at least 200 binary orbits, corresponding to about a viscous time for our parameters, 0.2(a 2Ω B /ν) binary orbits, where ν is the kinematic viscosity. The largest discrepancies between codes resulted from the dimensionality of the setup (3D vs. 2D disks). We find good agreement in the total torque on the binary between codes, although the partition of this torque between the gravitational torque, orbital accretion torque, and spin accretion torque depends sensitively on the sink prescriptions employed. In agreement with previous studies, we find a modest difference in torques and accretion variability between 2D and 3D disk models. We find cavity precession rates to be appreciably faster in 3D than in 2D.

A Multiwavelength, Multiepoch Monitoring Campaign of Accretion Variability in T Tauri Stars from the ODYSSEUS Survey. I. HST Far-UV and Near-UV Spectra

The classical T Tauri star (CTTS) stage is a critical phase of the star and planet formation process. In an effort to better understand the mass accretion processes, which can dictate future stellar evolution and planet formation, a multiepoch, multiwavelength photometric and spectroscopic monitoring campaign of four CTTSs (TW Hya, RU Lup, BP Tau, and GM Aur) was carried out in 2021 and 2022/2023 as part of the Outflows and Disks around Young Stars: Synergies for the Exploration of ULLYSES Spectra program. Here we focus on the Hubble Space Telescope (HST) UV spectra obtained by the HST Director’s Discretionary Time UV Legacy Library of Young Stars as Essential Standards (ULLYSES) program. Using accretion shock modeling, we find that all targets exhibit accretion variability, varying from short increases in accretion rate by up to a factor of 3 within 48 hr to longer decreases in accretion rate by a factor of 2.5 over the course of 1 yr. This is despite the generally consistent accretion morphology within each target. Additionally, we test empirical relationships between accretion rate and UV luminosity and find stark differences, showing that these relationships should not be used to estimate the accretion rate for an individual target. Our work reinforces that future multiepoch and simultaneous multiwavelength studies are critical in our understanding of the accretion process in low-mass star formation.

The Evolution of Accreting Binaries: From Brown Dwarfs to Supermassive Black Holes

Circumbinary accretion occurs throughout the universe, from the formation of stars and planets to the aftermath of major galactic mergers. We present an extensive investigation of circumbinary accretion disks, studying circular binaries with mass ratios (q ≡ M 2/M 1) from 0.01 to 1 and at each mass ratio probing the effects of disk thickness and viscosity. We study disks with aspect ratios H/r ∈ {0.1, 0.05, 0.03} and vary both the magnitude and spatial dependence of viscosity. Although thin accretion disks have previously been found to promote rapid inspirals of equal-mass binaries, we find that gravitational torques become weaker at lower mass ratios and most binaries with 0.01 ≤ q ≤ 0.04 outspiral, which may delay the coalescence of black hole binaries formed from minor mergers and cause high-mass exoplanets to migrate outward. However, in a number of cases, the disks accreting onto binaries with mass ratios ∼0.07 fail to develop eccentric modes, leading to extremely rapid inspirals. Variability in black hole accretion correlates with disk eccentricity, and we observe variability above the ∼10% level even for mass ratios of 0.01. We demonstrate that the spatial dependence of the viscosity (e.g., α vs. constant ν) significantly affects the degree of preferential accretion onto the secondary, resolving discrepancies between previous studies. Colder circumbinary disks remain eccentric even at q ∼ 0.01 and sustain deep, asymmetric cavities.

Reliable Identification of Binary Supermassive Black Holes from Rubin Observatory Time-domain Monitoring

Periodic signatures in time-domain observations of quasars have been used to search for binary supermassive black holes (SMBHs). These searches, across existing time-domain surveys, have produced several hundred candidates. The general stochastic variability of quasars, however, can masquerade as a false-positive periodic signal, especially when monitoring cadence and duration are limited. In this work, we predict the detectability of binary SMBHs in the upcoming Rubin Observatory Legacy Survey of Space and Time (LSST). We apply computationally inexpensive sinusoidal curve fits to millions of simulated LSST Deep Drilling Field light curves of both single, isolated quasars and binary quasars. The period and phase of simulated binary signals can generally be disentangled from quasar variability. Binary amplitude is overestimated and poorly recovered for two-thirds of potential binaries due to quasar accretion variability. Quasars with strong intrinsic variability can obscure a binary signal too much for recovery. We also find that the most luminous quasars mimic current binary candidate light curves and their properties: The false-positive rates are 60% for these quasars. The reliable recovery of binary period and phase for a wide range of input binary LSST light curves is promising for multi-messenger characterization of binary SMBHs. However, pure electromagnetic detections of binaries using photometric periodicity with amplitude greater than 0.1 mag will result in samples that are overwhelmed by false positives. This paper represents an important and computationally inexpensive way forward for understanding the true and false-positive rates for binary candidates identified by Rubin.

Morphodynamics and depositional architecture of mid‐channel bars in large Amazonian rivers

AbstractLarge rivers are characterized by large water discharges, high suspended sediment fluxes and low slope, and typically display multiple channels that are separated by large complex bars. The most common channel style found in these rivers is characterized by the alternating presence of single and multiple channel threads. Mid‐channel compound bars separate a main deeper channel from shallower secondary channels, leading to low‐order braiding and low to moderate sinuosity. Despite the importance of this type of river for global terrestrial sediment transport and the interpretation of ancient fluvial successions, integrated depositional models for large multi‐channel rivers are still not fully developed. This paper interprets the channel morphodynamics and depositional architecture of such large rivers by investigating the distinctive features of their laterally‐accreting mid‐channel bars in the Solimões–Amazonas River. This is achieved by examination of temporal series of satellite images, quantification of bedforms using multibeam echosounding surveys, analysis of shallow seismic and ground penetrating radar surveys of selected areas, and on‐site field observations and sample collections. Such mid‐channel bars produce directional variability in planform scroll bar accretion of up to 180°. As these bars – and their associated channels – evolve, they become progressively shallower, due to the bifurcation of the two channels and the resultant partitioning of discharge that leads to a gradual reduction in transport capacity within the outer channel. The preserved successions of this process in the alluvial plain are characterized by fining‐upward trends, with larger cross‐strata sets at mid‐depths and internal erosional surfaces marking distinct cycles of bar development with potentially differing palaeocurrent trends. The new depositional models proposed for these mid‐channel bars and channels constitute a basis for the recognition of large multi‐channel river deposits in the ancient rock record.

Ejection Patterns in the DG Tau Jet over the Last 40 yr: Insights into Mass Accretion Variability* * Based on observations obtained at the international Gemini Observatory, a program of NSF's NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation on behalf of the Gemini Observatory

We aim to clarify the link between mass accretion and ejection by analyzing DG Tau’s jet observations from optical and near-infrared data spanning 1984–2019, alongside photometric variations between 1983 and 2015. We classified 12 moving knot groups among 17 total knot groups based on their constant proper motions and comparable radial velocities. A strong correlation emerges between the deprojected flow velocities of the knots and the photometric magnitudes of DG Tau. From 1983 to 1995, as the deprojected ejection velocities surged from ∼273 ± 15 to ∼427 ± 16 km s−1, the photometric magnitudes (V) concurrently brightened from 12.3 to 11.4. Notably, when DG Tau became brighter than 12.2 in the V band, its (B − V) color shifted bluer than its intrinsic color range of K5–M0. During this period, the launching point of the jet in the protoplanetary disk moved closer to 0.06 au from the star in 1995. Following a V magnitude drop from 11.7 to 13.4 in 1998, the star may have experienced significant extinction due to a dust wall created by the disk wind during the ejection of the high-velocity knot in 1999. Since then, the magnitude became fainter than 12.2, the (B − V) and (V − R) colors became redder, and the deprojected velocities consistently remained below 200 km s−1. The launching point of the jet then moved away to ∼0.45 au by 2008. The prevailing factor influencing photometric magnitude appears to be the active mass accretion causing the variable mass ejection velocities.

Eccentric binaries in retrograde discs

ABSTRACT Modern numerical hydrodynamics tools have recently enabled detailed examinations of binaries accreting from prograde circumbinary discs. These have reframed the current understanding of binary-disc interactions and disc driven orbital evolution. We present the first full-domain grid-based hydrodynamics simulations of equal-mass, eccentric binaries accreting from retrograde circumbinary discs. We study binary eccentricities that span e = 0.0 to e = 0.8 continuously, and explore the influence of retrograde accretion on the binary orbital response, disc morphology, and observational properties. We find that, at all eccentricities, retrograde accretion shrinks the binary semimajor axis and pumps its eccentricity leading to the previously identified possibility of highly eccentric mergers. Contrary to past studies and models, we observe gravitational forces to dominate the binary’s orbital evolution as opposed to the physical accretion of mass and momentum. Retrograde accretion variability also differs strongly from prograde solutions. Preeminently, binaries with e > 0.55 reveal a unique two-period, double-peaked accretion signature that has not previously been identified. We additionally find evidence for the emergence of retrograde Lindblad resonances at large eccentricities in accordance with predictions from linear theory. Our results suggest that some astrophysical binaries for which retrograde accretion is possible will experience factors-of-a-few times faster orbital decay than in prograde discs and will have their eccentricities pumped beyond the limits found from prograde solutions. Such effects could lead to rapid inward migration for some young stellar binaries, the detection of highly eccentric LISA mergers, and the tentatively observed turnover at the low-frequency end of the gravitational wave background.

PENELLOPE

HM Lup is a young M-type star that accretes material from a circumstellar disk through a magnetosphere. Our aim is to study the inner disk structure of HM Lup and to characterize its variability. We used spectroscopic data from HST/STIS, X-shooter, and ESPRESSO taken in the framework of the ULLYSES and PENELLOPE programs, together with photometric data from TESS and AAVSO. The 2021 TESS light curve shows variability typical for young stellar objects of the “accretion burster” type. The spectra cover the temporal evolution of the main burst in the 2021 TESS light curve. We compared the strength and morphology of emission lines from different species and ionization stages. We determined the mass accretion rate from selected emission lines and from the UV continuum excess emission at different epochs, and we examined its relation to the photometric light curves. The emission lines in the optical spectrum of HM Lup delineate a temperature stratification along the accretion flow. While the wings of the H I and He I lines originate near the star, the lines of species such as Na I, Mg I, Ca I, Ca II, Fe I, and Fe II are formed in an outer and colder region. The shape and periodicity of the 2019 and 2021 TESS light curves, when qualitatively compared to predictions from magnetohydrodynamic models, suggest that HM Lup was in a regime of unstable ordered accretion during the 2021 TESS observation due to an increase in the accretion rate. Although HM Lup is not an extreme accretor, it shows enhanced emission in the metallic species during this high accretion state that is produced by a density enhancement in the outer part of the accretion flow.

Hydrodynamical simulations of circumbinary accretion: balance between heating and cooling

ABSTRACT Hydrodynamical interaction in circumbinary discs (CBDs) plays a crucial role in various astrophysical systems, ranging from young stellar binaries to supermassive black hole binaries in galactic centres. Most previous simulations of binary-disc systems have adopted locally isothermal equation of state. In this study, we use the grid-based code Athena++ to conduct a suite of two-dimensional viscous hydrodynamical simulations of circumbinary accretion on a Cartesian grid, resolving the central cavity of the binary. The gas thermodynamics is treated by thermal relaxation towards an equilibrium temperature (based on the constant − β cooling ansatz, where β is the cooling time in units of the local Keplerian time). Focusing on equal mass, circular binaries in CBDs with (equilibrium) disc aspect ratio H/R = 0.1, we find that the cooling of the disc gas significantly influences the binary orbital evolution, accretion variability, and CBD morphology, and the effect depends sensitively on the disc viscosity prescriptions. When adopting a constant kinematic viscosity, a finite cooling time (β ≳ 0.1) leads to a binary inspiral as opposed to an outspiral and the CBD cavity becomes more symmetric. When adopting a dynamically varying α-viscosity, binary inspiral only occurs within a narrow range of cooling time (corresponding to β around 0.5).

Physical conditions for dust grain alignment in Class 0 protostellar cores

Context. The polarized dust emission observed in Class 0 protostellar cores at high angular resolution with ALMA has raised several concerns about the grain alignment conditions in these regions. Aims. We aim to study the role of the radiation field in grain alignment mechanisms that occur in the interior (≤1000 au) of Class 0 protostars. Methods. We produced synthetic observations of the polarized dust emission from a magnetohydrodynamic model of protostellar formation using the POLARIS dust radiative transfer tool, which includes dust alignment with radiative torque alignment (RAT). We tested how the polarized dust emission from the model core depends on the irradiation conditions in the protostellar envelope by varying the radiation due to accretion luminosity propagating from the central protostellar embryo throughout the envelope. The level of grain alignment efficiency obtained in the radiative transfer models was then compared to (sub)millimeter ALMA dust polarization observations of Class 0 protostars. Results. Our radiative transfer calculations have a central irradiation that reproduces the protostellar luminosities typically observed toward low- to intermediate-mass protostars, as well as super-paramagnetic grains and grains ≥10 µm, which are required to bring the dust grain alignment efficiencies of the synthetic observations up to the observed levels. We discuss the characteristics timescales of the grain alignment physics together with the radiative torque disruption (RATD) of grains and the typical time variability of accretion occurring in Class 0 protostellar cores. In our model, during an accretion burst or a steady-state phase of high luminosity from the protostellar embryo, RATD could have enough time to disrupt the largest grains in irradiated regions. Finally, in high-luminosity conditions (with L★ ≥ 20 L⊙ in our model), we find that the alignment of grains with respect to the anisotropic component of the radiation field (k-RAT) could drive inefficient alignment for grains ≳10 µm. However, given the high grain alignment efficiency observed in protostellar envelopes, large grains are most likely aligned with the magnetic field and thus potentially subject to rotational disruption, depending on their tensile strength. Conclusions. Our radiative transfer calculations show that irradiation plays an important role in the mechanisms that dictate the size range of aligned grains in Class 0 protostars. Regions of the envelope that are preferentially irradiated harbor strong polarized dust emission but can be affected by the rotational disruption of dust grains, thus controlling the population of the largest aligned grains. Episodes of high luminosity could affect grain alignment and trigger grain disruption mechanisms.

The Decoupling of Binaries from Their Circumbinary Disks

We have investigated, both analytically and numerically, accreting supermassive black hole binaries as they inspiral due to gravitational radiation to elucidate the decoupling of binaries from their disks and inform future multimessenger observations of these systems. Our numerical studies evolve equal-mass binaries from initial separations of 100 GM/c until merger, resolving scales as small as ∼0.04 GM/c, where M is the total binary mass. Our simulations accurately capture the point at which the orbital evolution of each binary decouples from that of its circumbinary disk, and precisely resolve the flow of gas throughout the inspiral. We demonstrate analytically and numerically that timescale-based predictions overestimate the binary separations at which decoupling occurs by factors of ∼3, and illustrate the utility of a velocity-based decoupling criterion. High-viscosity (ν ≳ 0.03 GM/c) circumbinary systems decouple late (a b ≲ 15 GM/c) and have qualitatively similar morphologies near merger to circumbinary systems with constant binary separations. Lower-viscosity circumbinary disks decouple earlier and exhibit qualitatively different accretion flows, which lead to precipitously decreasing accretion onto the binary. If detected, such a decrease may unambiguously identify the host galaxy of an ongoing event within a LISA error volume. We illustrate how accretion amplitude and variability evolve as binaries gradually decouple from their circumbinary disks, and where decoupling occurs over the course of binary inspirals in the LISA band. We show that, even when dynamically negligible, gas may leave a detectable imprint on the phase of gravitational waves.

Meridional circulation driven by planetary spiral wakes in radiative and magnetized protoplanetary discs

ABSTRACT We study a Jupiter-mass planet formation for the first time in radiative magnetohydrodynamics (MHD) simulations and compare it with pure hydrodynamical simulations, and also with different isothermal configurations. We found that the meridional circulation is the same in every set-up. The planetary spiral wakes drive a vertical stirring inside the protoplanetary disc and the encounter with these shock fronts also helps in delivering gas vertically on to the Hill sphere. The accretion dynamics are unchanged: the planet accretes vertically, and there is outflow in the mid-plane regions inside the Hill sphere. We determined the effective α-viscosity generated in the disc by the various angular momentum loss mechanisms, which showed that magnetic fields produce high turbulence in the ideal MHD limit, and grows from α ∼ 10−2.5 up to ∼10−1.5 after the planet spirals develop. In the HD simulations, the planetary spirals contribute to α ∼ 10−3, making this a very important angular momentum transport mechanism. Due to the various α values in the different set-ups, the gap opening is different in each case. In the radiative MHD set-ups, the high turbulent viscosity prevents gap opening, leading to a higher Hill mass, and no clear dust trapping regions. While the Hill accretion rate is $10^{-6}~ \rm {M_{Jup}\,yr^{-1}}$ in all set-ups, the accretion variability is orders of magnitude higher in radiative runs than in isothermal ones. Finally, with higher resolution runs, the magnetorotational instability started to be resolved, changing the effective viscosity and increasing the heating in the disc.

Spatial Variability in Vertical Accretion and Carbon Sequestration in Salt Marsh Soils of an Urban Estuary

Spatial Variability in Vertical Accretion and Carbon Sequestration in Salt Marsh Soils of an Urban Estuary

On the Role of Dynamical Cooling in the Dynamics of Circumbinary Disks

Hydrodynamical interactions between binaries and circumbinary disks (CBDs) play an important role in a variety of astrophysical systems, from young stellar binaries to supermassive black hole binaries. Previous simulations of CBDs have mostly employed locally isothermal equations of state. We carry out 2D viscous hydrodynamic simulations of CBDs around equal-mass, circular binaries, treating the gas thermodynamics by thermal relaxation toward equilibrium temperature (the constant-β cooling ansatz, where β is the cooling time in units of the local Keplerian time). As an initial study, we use the grid-based code Athena++ on a polar grid, covering an extended disk outside the binary co-orbital region. We find that with a longer cooling time, the accretion variability is gradually suppressed, and the morphology of the CBD becomes more symmetric. The disk also shows evidence of hysteresis behavior depending on the initial conditions. Gas cooling also affects the rate of angular momentum transfer between the binary and the CBD, where given our adopted disk thickness and viscosity (H/r ∼ 0.1 and α ∼ 0.1), the binary orbit expands while undergoing accretion for most β values between 0 and 4.0 except over a narrow range of intermediate β values. The validity of using a polar grid excising the central domain is also discussed.

Accretion variability in RU Lup

Context. The process of accretion in classical T Tauri stars (CTTSs) has been observed to vary on different timescales. Studying this variability is vital to understanding a star’s evolution and provides insight into the complex processes at work within, including sources of the veiling present. Understanding the dichotomy between continuum veiling and emission line veiling is integral to accurately measuring the amount of veiling present in stellar spectra. Aims. Here, 15 roughly consecutive nights of optical spectroscopic data from the spectropolarimeter ESPaDOnS are utilised to characterise the short-term accretion activity in the CTTS, RU Lup, and investigate its relationship with the veiling in the Li I 6707 Å absorption line. Methods. The accretion-tracing H I Balmer series emission lines were studied and used to obtain the accretion luminosity (Lacc) and mass accretion rate (Ṁacc) for each night, which vary by a factor of ∼2 between the brightest and dimmest nights. We also measured the veiling using multiple photospheric absorption lines (Na I 5688 Å, Mn I 6021 Å, and Li I 6707 Å) for each night. Results. We find the Li I 6707 Å line provides measurements of veiling that produce a strong, positive correlation with Lacc in the star. When corrected for Li depletion, the average veiling measured in the Li I 6707 Å line is rLi I, avg ∼ 3.25 ± 0.20, which is consistent with the other photospheric lines studied (ravg ∼ 3.28 ± 0.65). Conclusions. We measured short timescale variability in the accretion luminosity and mass accretion rate that are intrinsic and not due to geometric effects. As the forbidden line emission we observe ([O I] 6300 Å and [S II] 6730 Å) remains remarkably constant over our epochs, it is clear that the variations in the mass accretion rate are too short to have an effect on these outflow tracers. Upon comparing the changes in veiling and accretion luminosity, we find a strong, positive correlation. This study provides an example of how this correlation can be used as a tool to determine whether a measured variability is due to extinction or an intrinsic change in accretion. As the determination of veiling is an independent process from measuring Lacc, their relationship allows further exploration of accretion phenomena in young stars.

Monitoring accretion rate variability in the Orion Nebula Cluster with the Wendelstein Wide Field Imager

Context. The understanding of the accretion process has a central role in the understanding of star and planet formation. Aims. We aim to test how accretion variability influences previous correlation analyses of the relation between X-ray activity and accretion rates, which is important for understanding the evolution of circumstellar disks and disk photoevaporation. Methods. We monitored accreting stars in the Orion Nebula Cluster from November 24, 2014, until February 17, 2019, for 42 epochs with the Wendelstein Wide Field Imager in the Sloan Digital Sky Survey u′g′r′ filters on the 2 m Fraunhofer Telescope on Mount Wendelstein. Mass accretion rates were determined from the measured ultraviolet excess. The influence of the mass accretion rate variability on the relation between X-ray luminosities and mass accretion rates was analyzed statistically. Results. We find a typical interquartile range of ∼0.3 dex for the mass accretion rate variability on timescales from weeks to ∼2 yr. The variability has likely no significant influence on a correlation analysis of the X-ray luminosity and the mass accretion rate observed at different times when the sample size is large enough. Conclusions. The observed anticorrelation between the X-ray luminosity and the mass accretion rate predicted by models of photoevaporation-starved accretion is likely not due to a bias introduced by different observing times.

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.