We report new Very Large Array high-resolution observations of the radio jet from the outbursting high-mass star S255IR NIRS 3. The images at 6, 10, and 22.2 GHz confirm the existence of a new lobe emerging to the SW and expanding at a mean speed of ∼285 km s−1, about half as fast as the NE lobe. The new data allow us to reproduce both the morphology and the continuum spectrum of the two lobes with the model already adopted in our previous studies. We conclude that in all likelihood both lobes are powered by the same accretion outburst. We also find that the jet is currently fading down, recollimating, and recombining.

Context. Accretion bursts from low-mass young stellar objects (YSOs) have been known for many decades. In recent years, the first accretion bursts of massive YSOs (MYSOs) have been observed. These phases of intense protostellar growth are of particular importance for studying massive star formation. Bursts of MYSOs are accompanied by flares of Class II methanol masers (here after masers), which are caused by an increase in exciting mid-infrared (MIR) emission. They can lead to long-lasting thermal afterglows of the dust continuum radiation visible at infrared (IR) and (sub)millimeter (hereafter (sub)mm) wavelengths. Furthermore, they might cause a scattered light echo. The G323.46-0.08 (here after G323) event, which shows all these features, extends the small sample of known MYSO bursts. Aims. Maser observations of the MYSO G323 show evidence of a flare, which was presumed to be caused by an accretion burst. This should be verified with IR data. We used time-dependent radiative transfer (TDRT) to characterize the heating and cooling time scales for eruptive MYSOs and to infer the main burst parameters. Methods. Burst light curves, as well as the pre-burst spectral energy distribution (SED) were established from archival IR data. The properties of the MYSO, including its circumstellar disk and envelope, were derived by using static radiative transfer modeling of pre-burst data. For the first time, TDRT was used to predict the temporal evolution of the SED. Observations with SOFIA/HAWC+were performed to constrain the burst energy from the strength of the thermal afterglow. Image subtraction and ratioing were applied to reveal the light echo. Results. The G323 accretion burst is confirmed. It reached its peak in late 2013/early 2014 with aKs-band increase of similar to 2.5 mag. BothKs-band and integrated maser flux densities follow an exponential decay. TDRT indicates that the duration of the thermal afterglowin the far-infrared (FIR) can exceed the burst duration by years. The latter was proved by SOFIA observations, which indicate a fluxincrease of (14.2 +/- 4.6)% at 70 mu m and (8.5 +/- 6.1)% at 160 mu m in 2022 (2 yr after the burst ended). A one-sided light echo emerged that was propagating into the interstellar medium. Conclusions. The burst origin of the G323 maser flare has been verified. TDRT simulations revealed the strong influence of the burstenergetics and the local dust distribution on the strength and duration of the afterglow. The G323 burst is probably the most energetic MYSO burst that has been observed so far. Within 8.4 yr, an energy of (0.9(-0.8)(+2.5))x10(47)erg was released. The short timescale points to the accretion of a compact body, while the burst energy corresponds to an accumulated mass of at least (7(-6)(+20))MJup and possibly even more if the protostar is bloated. In this case, the accretion event might have triggered protostellar pulsations, which give rise to the observed maser periodicity. The associated IR light echo is the second observed from a MYSO burst.

We report new Very Large Array high-resolution observations of the radio jet from the outbursting high-mass star S255IR~NIRS3. The images at 6, 10, and 22.2 GHz confirm the existence of a new lobe emerging to the SW and expanding at a mean speed of ~285 km/s, about half as fast as the NE lobe. The new data allow us to reproduce both the morphology and the continuum spectrum of the two lobes with the model already adopted in our previous studies. We conclude that in all likelihood both lobes are powered by the same accretion outburst. We also find that the jet is currently fading down, recollimating, and recombining.

Understanding the interstellar and potentially circumstellar extinction in the sight lines of classical T Tauri stars is an important ingredient for constructing reliable spectral energy distributions, which catalyze protoplanetary disk chemistry, for example. Therefore, some attempts of measuring A(V) toward individual stars have been made using partly different wavelength regimes and different underlying assumptions. We used strong lines of Ly alpha fluorescent H-2 and derived the extinction based on the assumption of optically thin transitions. We investigated a sample of 72 classical T Tauri stars observed with the Hubble Space Telescope in the framework of the ULLYSES program. We computed A(V) and R-V values for the 34 objects with sufficient data quality and an additionally A(V) value for the canonical R-V = 3.1 value. Our results agree largely with values obtained from optical data. Moreover, we confirm the degeneracy between A(V) and R(V )and present possibilities to break this. Finally, we discuss whether the assumption of optical thin lines is valid.

Context. Characterising stellar jet asymmetries is key to setting robust constraints on jet launching models and improving our understanding of the underlying mechanisms behind jet launching. Aims. We aim to characterise the asymmetric properties of the bipolar jet coming from the Classical T Tauri Star Th 28. Methods. We combined data from integral field spectroscopy with VLT/MUSE and high-resolution spectra from VLT/X-shooter to map the optical emission line ratios in both jet lobes. We carried out a diagnostic analysis of these ratios to compare the density, electron temperature, and ionisation fraction within both lobes. The mass accretion rate was derived from the emission lines at the source and compared with the mass outflow rate derived for both lobes, using the estimated densities and measured [O I]lambda 6300 and [S II]lambda 6731 luminosities. Results. The blue-shifted jet exhibits a significantly higher electron temperature and moderately higher ionisation fraction than the red-shifted jet. In contrast to previous studies, we also estimated higher densities, denoted as n(H), in the blue-shifted jet by a factor of similar to 2. These asymmetries are traced to within 1 '' (160 au) of the source in the line ratio maps. We find <(M)over dot>(acc) = 2.4 x 10(-7) M-circle dot yr(-1), with an estimated obscuration factor of similar to 54 due to grey scattering around the star. Estimated values of <(M)over dot>(out) range between 0.66 and 13.7 x 10(-9) M-circle dot yr(-1) in the blue-shifted jet and 5-9 x 10(-9) M-circle dot yr(-1) in the red-shifted jet. Conclusions. The emission line maps and diagnostic results suggest that the jet asymmetries originate close to the source and are likely to be intrinsic to the jet. Furthermore, the combined dataset offers access to a broad array of accretion tracers. In turn, this enables a more accurate estimation of the mass accretion rate, revealing a value of Macc that is higher by a factor >350 than would otherwise be determined.

Teegarden's star is a late-type M-dwarf planet host, typically showing only rather low levels of activity. In this paper we present an extensive characterisation of this activity at photospheric, chromospheric, and coronal levels. We specifically investigated TESS observations of Teegarden's star, which showed two very large flares with an estimated flare fluence between 1029 and 1032 erg comparable to the largest solar flares. We furthermore analysed nearly 300 CARMENES spectra and 11 ESPRESSO spectra covering all the usually used chromospheric lines in the optical from the Ca II H & K lines at 3930 & Aring; to the He I infrared triplet at 10 830 & Aring;. These lines show different behaviour: The He I infrared triplet is the only one absent in all spectra, some lines show up only during flares, and others are always present and highly variable. Specifically, the H alpha line is more or less filled in during quiescence; however, the higher Balmer lines are still observed in emission. Many chromospheric lines show a correlation with H alpha variability, which, in addition to stochastic behaviour, shows systematic behaviour on different timescales including the rotation period. Moreover, we found several flares and also report hints of an erupting prominence, which may have led to a coronal mass ejection. Finally, we present X-ray observations of Teegarden's star (i.e. a discovery pointing obtained with the Chandra observatory) and an extensive study with the XMM-Newton observatory, which observed two large flares. One of these showed clear signatures of the Neupert effect, suggesting the production of hard X-rays in the system.

The physical mechanism leading to the formation of the blue loop in the Hertzsprung-Russell (HR) diagram is not satisfactorily explained by the evolutionary track of single stars. Rapid rotation and low metallicity drastically modify the internal structures and surface compositions of stars. Therefore, they provide a very significant pattern to investigate the evolutionary properties of the blue loop. In this paper, we mainly explore how rapid rotation and low metallicity have an important impact on the occurrence and extension of the blue loop. To this end, we implemented the rotating stellar evolution model, including the angular momentum transportation and chemical element mixing. We incorporated several initial rotational velocities and two characteristic metallicities in various models to explore the blue loop extension. The blue loop can occur when the hydrogen burning shell merges with the hydrogen--helium abundance discontinuity. We find that the blue loop extension strongly depends on the amplitude and gradient of the hydrogen--helium discontinuity. The hydrogen--helium discontinuity is created by the intermediate convective region or the convective dredge-up. A steeper hydrogen gradient in association with a greater amplitude of the hydrogen abundance discontinuity may favour a hotter star. Both the low metallicity and rapid rotation tend to restrain the development of the outer convective envelope and thus disfavour the occurrence and extension of the blue loop. There are three main reasons for this occurrence. Firstly, the helium core and its core potential can be enlarged by rotational mixing or low metallicity. Secondly, rapid rotation reduces the convective dredge-up depth in the star with $ Z=0.014$ and the mass extension of the intermediate convective region in the star with $ Z=0.0008$. Both of these phenomena lead to a reduction of the amplitude of the hydrogen abundance gradient. Thirdly, strong rotational mixing in the model (i.e. $ ini =350$ Km/s) with $ Z=0.0008$ reduces the energy generation rate from the hydrogen burning shell. Without bending towards higher effective temperature in the HR diagram, the additional helium brought near the H-burning shell associated with the larger He core can cause the star to expand towards becoming a red giant star directly after the core hydrogen burning. Rapid rotation and low metallicity tend to produce surface enrichment of the ratio of nitrogen to carbon and reduce the $ C$ left in the core; this has an important influence on the stellar compactness of the supernovae progenitor.

Using observations with e-MERLIN and the VLA, together with archival data from ALMA, we obtain high-resolution radio images of two binary YSOs: L1551 IRS 5 and L1551 NE, covering a wide range of frequencies from 5 - 336 GHz, and resolving emission from the radio jet on scales of only 15 au. By comparing these observations to those from a previous epoch, it is shown that there is a high degree of variability in the free-free emission from the jets of these sources. In particular, the northern component of L1551 IRS 5 shows a remarkable decline in flux density of a factor of 5, suggesting that the free-free emission of this source has almost disappeared. By fitting the spectra of the sources, the ionised mass-loss rates of the jets are derived and it is shown that there is significant variability of up to a factor of 6 on timescales of 20 years. Using radiative transfer modelling, we also obtained a model image for the jet of the southern component of L1551 IRS 5 to help study the inner region of the ionised high-density jet. The findings favour the X-wind model launched from a very small innermost region.

$Context.$ The YSO Th 28 possesses a highly collimated jet, which clearly exhibits an asymmetric brightness of its jet lobes at optical and NIR wavelengths. There may be asymmetry in the jet plasma parameters in opposite jet lobes (e.g. electron density, temperature, and outflow velocity). $Aims.$ We examined the Th 28 jet in a 3"x3" where the jet material is collimated and accelerated. Our goal is to map the morphology and determine its physical parameters to determine the physical origin of such asymmetries. $Methods.$ We present $JHK$-spectra of Th 28 obtained with the SINFONI on the (VLT, ESO) in June-July 2015. $Results.$ The [Fe II] emission originates in collimated jet lobes. Two new axial knots are detected at 1" in the blue lobe and 1".2 in the red lobe. The H$_2$ radiation is emitted from an extended region with a radius of $\gtrsim270$ au, which is perpendicular to the jet. The PV diagrams of the bright H$_2$ lines reveal faint H$_2$ emission along both jet lobes as well. The compact and faint H I emission (Pa$\beta$ and Br$\gamma$) comes from two regions, namely from a spherical region around the star and from the jet lobes. The size of the jet launching region is derived as 0".015 ($\sim$3 au at 185 pc), and the initial opening angle of the Th 28 jet is $\sim28^0$, which makes this jet substantially less collimated than most jets from other CTTs. $Conclusions.$ The emission in [Fe II], H$_2$, and H I lines suggests a morphology in which the ionised gas in the disc appears to be disrupted by the jet. The resolved disc-like H$_2$ emission most likely arises in the disc atmosphere from shocks caused by a radial uncollimated wind. The asymmetry of the [Fe II] photocentre shifts with respect to the jet source arises in the immediate vicinity of the driving source of Th28 and suggests that the observed brightness asymmetry is intrinsic as well.

Context. Growing observational evidence indicates that the accretion process leading to star formation may occur in an episodic way, through accretion outbursts revealed in various tracers. This phenomenon has also now been detected in association with a few young massive (proto)stars (>8 M⊙), where an increase in the emission has been observed from the IR to the centimetre domain. In particular, the recent outburst at radio wavelengths of S255IR NIRS 3 has been interpreted as due to the expansion of a thermal jet, fed by part of the infalling material, a fraction of which has been converted into an outflow. Aims. We wish to follow up on our previous study of the centimetre and millimetre continuum emission from the outbursting massive (proto)star S255IR NIRS 3 and confirm our interpretation of the radio outburst, based on an expanding thermal jet. Methods. The source was monitored for more than 1 yr in six bands from 1.5 GHz to 45.5 GHz with the Karl G. Jansky Very Large Array, and, after an interval of ~1.5 yr, it was imaged with the Atacama Large Millimeter/submillimeter Array at two epochs, which made it possible to detect the proper motions of the jet lobes. Results. The prediction of our previous study is confirmed by the new results. The radio jet is found to expand, while the flux, after an initial exponential increase, appears to stabilise and eventually decline, albeit very slowly. The radio flux measured during our monitoring is attributed to a single lobe, expanding towards the NE. However, starting from 2019, a second lobe has been emerging in the opposite direction, probably powered by the same accretion outburst as the NE lobe, although with a delay of at least a couple of years. Flux densities measured at frequencies higher than 6 GHz were satisfactorily fitted with a jet model, whereas those below 6 GHz are clearly underestimated by the model. This indicates that non-thermal emission becomes dominant at long wavelengths. Conclusions. Our results suggest that thermal jets can be a direct consequence of accretion events, when yearly flux variations are detected. The formation of a jet lobe and its early expansion appear to have been triggered by the accretion event that started in 2015. The end of the accretion outburst is also mirrored in the radio jet. In fact, ~1 yr after the onset of the radio outburst, the inner radius of the jet began to increase, at the same time the jet mass stopped growing, as expected if the powering mechanism of the jet is quenched. We conclude that our findings strongly support a tight connection between accretion and ejection in massive stars, consistent with a formation process involving a disk-jet system similar to that of low-mass stars.

HD163296 is a Herbig Ae star which drives a bipolar knotty jet with a total length of ~6000au. Strong evidence exists that the disk of HD163296 harbors planets. Studies have shown that the presence of companions around jet-driving stars could affect the morphology of the jets. This includes a `wiggling' of the jet axis and a periodicity in the positions of the jet knots. In this study we investigate the morphology (including the jet width and axis position) and proper motions of the HD163296 jets, and use our results to better understand the whole system.This study is based on optical integral-field spectroscopy observations obtained with VLT/MUSE in 2017. Using spectro-images and position velocity diagrams extracted from the MUSE data cube, we investigated the number and positions of the jet knots. A comparison was made to X-Shooter data collected in 2012 and the knot proper motions were estimated. The jet width and jet axis position with distance from the star were studied from the extracted spectro-images. We observe the merging of knots and identify two previously undetected knots. Measurements of the jet axis position reveal a similar pattern of deviation in all forbidden emission lines along the first 20 arc seconds of the jets. This result is interpreted as being due to asymmetric shocks and not due to a wiggling of the jet axis. The number of new knots detected and their positions challenge the 16-year knot ejection periodicity proposed in prior studies, arguing for a more complicated jet system than was previously assumed. We use the non-detection of a jet axis wiggling to rule out companions with a mass $>$0.1~\Msun\ and orbits between 1~au and 35~au. Any object inferred at these distances using other methods must be a brown dwarf or planet, otherwise it would have impacted the jet axis position. Both the precession and orbital motion scenarios are considered.

The Low-Frequency Array (LOFAR) Long-Baseline Calibrator Survey (LBCS) was conducted between 2014 and 2019 in order to obtain a set of suitable calibrators for the LOFAR array. In this paper we present the complete survey, building on the preliminary analysis published in 2016 which covered approximately half the survey area. The final catalogue consists of 30006 observations of 24713 sources in the northern sky, selected for a combination of high low-frequency radio flux density and flat spectral index using existing surveys (WENSS, NVSS, VLSS, and MSSS). Approximately one calibrator per square degree, suitable for calibration of $\geq$ 200 km baselines is identified by the detection of compact flux density, for declinations north of 30 degrees and away from the Galactic plane, with a considerably lower density south of this point due to relative difficulty in selecting flat-spectrum candidate sources in this area of the sky. Use of the VLBA calibrator list, together with statistical arguments by comparison with flux densities from lower-resolution catalogues, allow us to establish a rough flux density scale for the LBCS observations, so that LBCS statistics can be used to estimate compact flux densities on scales between 300 mas and 2 arcsec, for sources observed in the survey. The LBCS can be used to assess the structures of point sources in lower-resolution surveys, with significant reductions in the degree of coherence in these sources on scales between 2 arcsec and 300 mas. The LBCS survey sources show a greater incidence of compact flux density in quasars than in radio galaxies, consistent with unified schemes of radio sources. Comparison with samples of sources from interplanetary scintillation (IPS) studies with the Murchison Widefield Array (MWA) shows consistent patterns of detection of compact structure in sources observed both interferometrically with LOFAR and using IPS.

[abridged] The International LOFAR Telescope is an interferometer with stations spread across Europe. With baselines of up to ~2,000 km, LOFAR has the unique capability of achieving sub-arcsecond resolution at frequencies below 200 MHz, although this is technically and logistically challenging. Here we present a calibration strategy that builds on previous high-resolution work with LOFAR. We give an overview of the calibration strategy and discuss the special challenges inherent to enacting high-resolution imaging with LOFAR, and describe the pipeline, which is publicly available, in detail. We demonstrate the calibration strategy by using the pipeline on P205+55, a typical LOFAR Two-metre Sky Survey (LoTSS) pointing. We perform in-field delay calibration, solution referencing to other calibrators, self-calibration, and imaging of example directions of interest in the field. For this specific field and these ionospheric conditions, dispersive delay solutions can be transferred between calibrators up to ~1.5 degrees away, while phase solution transferral works well over 1 degree. We demonstrate a check of the astrometry and flux density scale. Imaging in 17 directions, the restoring beam is typically 0.3 x 0.2 although this varies slightly over the entire 5 square degree field of view. We achieve ~80 to 300 $\mu$Jy/bm image rms noise, which is dependent on the distance from the phase centre; typical values are ~90 $\mu$Jy/bm for the 8 hour observation with 48 MHz of bandwidth. Seventy percent of processed sources are detected, and from this we estimate that we should be able to image ~900 sources per LoTSS pointing. This equates to ~3 million sources in the northern sky, which LoTSS will entirely cover in the next several years. Future optimisation of the calibration strategy for efficient post-processing of LoTSS at high resolution (LoTSS-HR) makes this estimate a lower limit.

In recent years, thanks to new facilities such as LOFAR capable of sensitive observations, much work has been done on the detection of stellar radio emission at low frequencies. Such emission has commonly been shown to be coherent emission, generally attributed to electron-cyclotron maser emission, and has usually been detected from main-sequence M dwarfs. Here we report the first detection of coherent emission at low frequencies from T Tauri stars, which are known to be associated with high levels of stellar activity. Using LOFAR, we have detected several bright radio bursts at 150 MHz from two weak-line T Tauri stars: KPNO-Tau 14 and LkCa 4. All of the bursts have high brightness temperatures ($10^{13} - 10^{14}\ \mathrm{K}$) and high circular polarization fractions (60 - 90 \%), indicating that they must be due to a coherent emission mechanism. This could be either plasma emission or electron-cyclotron maser (ECM) emission. Due to the exceptionally high brightness temperatures seen in at least one of the bursts ($\geq 10^{14}\ \mathrm{K}$), and the high circular polarization levels, it seems unlikely that plasma emission could be the source and so ECM is favoured as the most likely emission mechanism. Assuming this is the case, the required magnetic field in the emission regions would be 40 - 70 G. We determine that the most likely method of generating ECM emission is plasma co-rotation breakdown in the stellar magnetosphere. There remains the possibility, however, it could be due to an interaction with an orbiting exoplanet.

Context. Th 28 is a Classical T Tauri star in the Lupus 3 cloud that drives an extended bipolar jet. Previous studies of the inner jet identified signatures of rotation around the outflow axis, a key result for theories of jet launching. Thus this is an important source in which to investigate the poorly understood jet launching mechanism. Aims. In this study we investigate the morphology and kinematics of the Th 28 micro-jets, with the aim of characterising their structure and outflow activity, using optical integral-field spectroscopy observations obtained with VLT/MUSE. Methods. We use spectro-imaging and position–velocity maps to investigate the kinematic and morphological features of the jet and to obtain a catalogue of emission lines in which the jet is visible. A Lucy-Richardson deconvolution procedure is used to differentiate the structure of the inner micro-jet region in selected emission lines. Spatial profiles extracted perpendicular to the jet axis are fitted to investigate the jet width, opening angle, and the evolution of the jet axis. Results. We confirm the previously identified knot HHW2 within the red-shifted jet and identify three additional knots in each lobe for the first time. We also find [O III]λ5007 emission from the blue-shifted micro-jet, including the knot closest to the star. Proper motions for the innermost knots on each side are estimated to be 0′′.35 yr−1 and 0′′.47 yr−1 for the red- and blue-shifted jets, respectively. Based on this we show that new knots are ejected on an approximate timescale of 10–15 yr. Gaussian fitting to the jet axis centroids shows a point-symmetric wiggle within the inner portion of both micro-jets, indicating precession of the jet. We use the jet shape to measure a precession period of 8 yr, with a half-opening angle β < 0.6°. This precession may provide an alternative explanation for the rotation signatures previously reported. Conclusions. We find that these parameters are compatible with precession due to a brown dwarf companion orbiting at a separation of ≤0.3 au. Further observations with higher spatial resolution may help to clarify the source of this precession.

We present SOFIA/FIFI-LS observations of three Class 0 and one Class I outflows (Cep E, HH 1, HH 212, and L1551 IRS5) in the far-infrared [O I]63mum and [O I]145mum transitions. Spectroscopic [O I]63mum maps enabled us to infer the spatial extent of warm, low-excitation atomic gas within these protostellar outflows. If proper shock conditions prevail, the instantaneous mass-ejection rate is directly connected to the [O I]63mum luminosity. In order to unravel evolutionary trends, we analysed a set of 14 Class 0/I outflow sources that were spatially resolved in the [O I]63mum emission. We compared these data with a sample of 72 Class 0/I/II outflow sources that have been observed with Herschel (WISH, DIGIT, WILL, GASPS surveys) without spatially resolving the [O I]63mum line.

Context. Class II methanol masers are signposts of massive young stellar objects (MYSOs). Recent evidence shows that flares of these masers are driven by MYSO accretion bursts. Thus, maser monitoring can be used to identify such bursts which are hard to discover otherwise. Infrared observations reveal burst-induced changes in the spectral energy distribution (first and foremost a luminosity increase), which provide valuable information on a very intense phase of high-mass star formation. Aims. In mid-January 2019, flaring of the 6.7 GHz CH3OH maser (hereafter maser) of the MYSO G358.93-0.03 (hereafter G358) was reported. The international maser community initiated an extensive observational campaign which revealed extraordinary maser activity and yielded the detection of numerous new masering transitions. Interferometric imaging with the Atacama Large Millimeter/submillimeter Array and the Submillimeter Array resolved the maser emitting core of the star forming region and proved the association of the masers with the brightest continuum source (MM1), which hosts a hot molecular core. These observations, however, failed to detect a significant rise in the (sub)millimeter dust continuum emission. Therefore, we performed near-infrared (NIR) and far-infrared (FIR) observations to prove or disprove whether the CH3OH flare was driven by an accretion burst. Methods. NIR imaging with the Gamma-Ray Burst Optical/Near-infrared Detector has been acquired and integral-field spectroscopy with the Field-Imaging Far-Infrared Line Spectrometer (FIFI-LS) aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA) was carried out on two occasions to detect possible counterparts to the (sub)millimeter sources and compare their photometry to archival measurements. The comparison of pre-burst and burst spectral energy distributions is of crucial importance to judge whether a substantial luminosity increase, caused by an accretion burst, is present and if it triggered the maser flare. Radiative transfer modeling of the spectral energy distribution (SED) of the dust continuum emission at multiple epochs provides valuable information on the bursting MYSO. Results. The FIR fluxes of MM1 measured with FIFI-LS exceed those from Herschel significantly, which clearly confirms the presence of an accretion burst. The second epoch data, taken about 16 months later, still show increased fluxes. Our radiative transfer modeling yielded major burst parameters and suggests that the MYSO features a circumstellar disk which might be transient. From the pre-burst, burst, and post-burst SEDs, conclusions on heating and cooling time-scales could be drawn. Circumstances of the burst-induced maser relocation have been explored. Conclusions. The verification of the accretion burst from G358 is another confirmation that Class II methanol maser flares represent an alert for such events. Thus, monitoring of these masers greatly enhances the chances of identifying MYSOs during periods of intense growth. The few events known to date already indicate that there is a broad range in burst strength and duration as well as environmental characteristics. The G358 event is the shortest and least luminous accretion burst known to date. According to models, bursts of this kind occur most often.

ABSTRACT It is now a widely held view that, in their formation and early evolution, stars build up mass in bursts. The burst mode of star formation scenario proposes that the stars grow in mass via episodic accretion of fragments migrating from their gravitationally unstable circumstellar discs, and it naturally explains the existence of observed pre-main-sequence bursts from high-mass protostars. We present a parameter study of hydrodynamical models of massive young stellar objects (MYSOs) that explores the initial masses of the collapsing clouds (Mc = 60–$200\, \rm M_{\odot }$) and ratio of rotational-to-gravitational energies (β = 0.005–0.33). An increase in Mc and/or β produces protostellar accretion discs that are more prone to develop gravitational instability and to experience bursts. We find that all MYSOs have bursts even if their pre-stellar core is such that β ≤ 0.01. Within our assumptions, the lack of stable discs is therefore a major difference between low- and high-mass star formation mechanisms. All our disc masses and disc-to-star mass ratios Md/M⋆ > 1 scale as a power law with the stellar mass. Our results confirm that massive protostars accrete about $40\, -\, 60{{\ \rm per\ cent}}$ of their mass in the burst mode. The distribution of time periods between two consecutive bursts is bimodal: there is a short duration ($\sim 1\, -\, 10~\rm yr$) peak corresponding to the short, faintest bursts and a long-duration peak (at $\sim 10^{3}\, -\, 10^{4} \rm yr$) corresponding to the long, FU-Orionis-type bursts appearing in later disc evolution, i.e. around $30\, \rm kyr$ after disc formation. We discuss this bimodality in the context of the structure of massive protostellar jets as potential signatures of accretion burst history.