TOI-4600b and c, originally identified by the Transiting Exoplanet Survey Satellite (TESS) and reported by I. Mireles et al. (2023), are a rare pair of transiting long-period giant planets (P_b=82.7 days, P_c=482.8 days) orbiting an early K dwarf. In this work, we refine the orbital parameters of the TOI-4600 system by combining new TESS photometry, ground-based transit follow-up, and radial velocity (RV) observations from MAROON-X. We obtain improved constraints on planetary masses and eccentricities, and update other parameters, such as the stellar age. For TOI-4600b, we measure a mass of M_p = 74.7^+4.7_-4.4 M_⊕ and an eccentricity of e=0.153^+0.020_-0.018, and M_p = 212.53^+13.26_-13.03 M_⊕ and e=0.219^+0.015_-0.018 for TOI-4600c. We find significant transit timing variations (TTV) in both planets, with semi-amplitudes of approximately 1 hr. We derive Transit Spectroscopy Metric values of 16.87 for TOI-4600b and 10.09 for TOI-4600c, indicating that both planets are promising JWST targets for studying the atmospheres of temperate and cold Jupiters, a relatively poorly characterized sample thus far. These updated parameters and TTV ephemerides are important for planning and interpreting future photometric, spectroscopic, and dynamical studies of the TOI-4600 system.

We present a new spectroscopic view of the brown dwarf population in the young star-forming region IC 1396 and investigate the impact of environment on low-mass star formation. We use deep optical photometry from Subaru-HSC to identify the candidate low-mass stars and brown dwarfs in the region. Our follow-up low-resolution spectroscopic survey with GTC-EMIR and IRTF-SpeX has identified 32 new members in the region with spectral types between M3 and M9, among which 25 are brown dwarfs with spectral types M6 or later. We use the BT-Settl atmospheric models to derive the effective temperatures of the members. Using a comprehensive catalogue of known members and candidates, we estimate the star to brown dwarf ratio for IC 1396 to be 5.0±0.4, for a mass range between 1-0.03 M_⊙. This ratio is largely consistent with measurements in other young clusters spanning a range of UV radiation fields and stellar densities, supporting formation scenarios in which the relative abundance of brown dwarfs is not strongly influenced by the local environmental conditions.

Establishing ages for young clusters is key for properly tracking the star formation history of a region. In this paper we investigate a new approach to estimating ages for young populations, based on the well-founded assumption that the initial mass function is the same throughout a star forming cloud. We trial this method for six young clusters in the Perseus star forming region. For all six clusters, we construct new member samples in a homogeneous way using Gaia DR3. We estimate masses by comparing 2MASS photometry to theoretical isochrones, including Monte Carlo simulations to propagate the errors. We compare the mass distributions of the clusters for a range of plausible ages, looking for a combination of ages that results in indistinguishable mass distributions across the region. We find the best fit for ages of 1 Myr for NGC1333+Autochthe, 2 Myr for IC348, 2-3 Myr for Heleus, 3-4 Myr for Mestor, 4-5 Myr for Electryon+Cynurus, and 5-8 Myr for Alcaeus. All other combinations of ages are ruled out by this criterion. The established age sequence is consistent with the relative ages inferred from disc fractions, and broadly aligns with the age sequence determined in previous studies using isochrone fitting. We suggest that this approach can be a useful complement and cross-check to established methods to estimate ages in young populations.

We present a homogeneous analysis of rotational variability and spot properties in young stellar objects across multiple star-forming regions observed by the Hunting Outbursting Young Stars (HOYS) project. From over 2000 candidate members, we identify 144 YSOs with robust periodic signals and well-constrained multi-band amplitudes. The sample has a median age of $\sim$1~Myr, effective temperatures of 3500--6500~K (masses $\sim$0.6--2~M$_\odot$), and is dominated by Class~2 objects, one third of which exhibit inner disc dust emission. The rotation period distribution is strongly bimodal, with 55 percent fast rotators ($P<5.5$~d) and 45 percent slow rotators. Fast rotators are predominantly inner disc-less, whereas slow rotators include both disc-bearing and disc-free systems, indicating that disc braking alone cannot explain the observed rotational states. We derive spot properties from multi-band amplitudes and find that, after correcting for observational biases, the intrinsic cold-spot coverage distribution of fast rotators is well described by an exponential function. This implies that small spot coverages are intrinsically much more common than large ones, consistent with stochastic magnetic flux emergence governing spot formation. In contrast, slow rotators show a pronounced deficit of small cold spots. After considering observational biases and alternative physical explanations, we conclude that small spots on slowly rotating YSOs have significantly shorter lifetimes. These results provide new evidence that magnetic surface structure and its evolution depend on stellar rotation, placing new empirical constraints on models of magnetic activity and angular momentum evolution in young stars.

We report the discovery of a long-lasting burst of disk accretion in Cha J11070768-7626326 (Cha 1107-7626), a young, isolated, 5-10 MJupiter object. In spectra taken with XSHOOTER at ESO's Very Large Telescope as well as NIRSpec and MIRI on the James Webb Space Telescope, the object transitions from quiescence in 2025 April-May to a strongly enhanced accretion phase in 2025 June-August. The line flux changes correspond to a 6-8-fold increase in the mass accretion rate, reaching 10-7 MJupiteryr-1, the highest measured in a planetary-mass object. During the burst, the H alpha line develops a double-peaked profile with redshifted absorption, as observed in stars and brown dwarfs undergoing magnetospheric accretion. The optical continuum increases by a factor of 3-6; the object is similar to 1.5-2 mag brighter in the R band during the burst. Mid-infrared continuum fluxes rise by 10%-20%, with clear changes in the hydrocarbon emission lines from the disk. We detect water vapour emission at 6.5-7 mu m, which were absent in quiescence. By the end of our observing campaign, the burst was still ongoing, implying a duration of at least 2 months. A 2016 spectrum also shows high accretion levels, suggesting that this object may undergo recurring bursts. The observed event is inconsistent with typical variability in accreting young stars and instead matches the duration, amplitude, and line spectrum of an EXor-type burst, making Cha1107-7626 the first substellar object with evidence of a potentially recurring EXor burst.

ABSTRACT Protoplanetary discs have been found around free-floating objects with masses comparable to those of giant planets. The frequency and properties of these discs around planetary-mass objects are still debated. Here, we present ultradeep mid-infrared images for the young cluster IC348, obtained through stacking of time-series images from Spitzer. We measure fluxes at 3.6 and 4.5 $\mu$m for known free-floating planetary-mass objects (FFPMOs, spectral type M9 or later) in this cluster. By comparing the observed infrared spectral energy distributions with photospheric templates, we identify six planetary-mass objects with discs, plus three, which may or may not have a disc. This corresponds to a disc fraction of $46\pm ^{13}_{12}$ per cent. The disc fraction among planetary-mass objects is comparable to more massive brown dwarfs. We show the disc fraction among FFPMOs as a function of age, demonstrating that these objects retain discs for several million years, similar to low-mass stars and brown dwarfs.

NGC 2261 is a reflection nebula illuminated by the young star R Monocerotis. Objects moving near the star occasionally cast shadows on the nebula, giving rise to its alternative name: Hubble's Variable Nebula. For 7 yr since Spring 2017 robotic telescopes have been used to compile a roughly twice-weekly record of changes in the object. The results, over 1000 images at separate epochs, have been compiled into a movie. This shows that, as well as the large scale but infrequent variability for which it is famous, the nebula is continually traversed by low level 'ripples' of light and dark. These record changes in the light output from R Mon and analysis of their progress indicates that the reflecting material takes the form of a thin (<3 x 10(16) cm) screen whose shape resembles a half paraboloid, rooted at the star and bowed towards us. The brightness of the screen in Herschel far-infrared maps indicates a density n(H) > 1.7 x 10(5) cm(-3) and CO observations show the material is moving towards us at a few km s(-1) relative to the rest cloud, consistent it with being a dense shell of material displaced by R Mon's outflow. The results demonstrate the value of studying such objects in the time domain, and are a glimpse of what will be achieved by instruments like the Zwicky Transient Facility and Vera Rubin Observatory.

We present the 0.6–12 μ m spectrum of Cha 1107-7626, a 6–10 Jupiter-mass free-floating object in the ∼2 Myr-old Chamaeleon-I star-forming region, from observations with the NIRSpec and MIRI instruments on board the James Webb Space Telescope. We confirm that Cha 1107-7626 is one of the lowest-mass objects known to harbor a dusty disk with infrared excess emission at wavelengths beyond 4 μ m. Our NIRSpec data and prior ground-based observations provide strong evidence for ongoing accretion through hydrogen recombination lines. In the mid-infrared spectrum, we detect unambiguously emission lines caused by methane (CH _4 ) and ethylene (C _2 H _4 ) in its circumsubstellar disk. Our findings mean that Cha 1107-7626 is by far the lowest-mass object with hydrocarbons observed in its disk. The spectrum of the disk looks remarkably similar to that of ISO-ChaI 147, a very low-mass star with a carbon-rich disk that is 10–20 times more massive than Cha 1107-7626. The hydrocarbon lines can be accounted for with a model assuming gas temperatures of a few hundred kelvin in the inner disk. The obvious similarities between the spectra of a low-mass star and a planetary-mass object indicate that the conditions in the inner disks can be similar across a wide range of central object masses.

Free-floating planetary-mass objects (FFPMOs) are known to harbor disks at young ages. Here, we present 1–13 μ m spectra for eight young FFPMOs with masses of 5–10 M _Jup (at ages of 1–5 Myr), using the NIRSpec and MIRI instruments on the James Webb Space Telescope. We derive fundamental properties of these targets, and find spectral types of M9.5 to L4, with effective temperatures of 1600–1900 K. The photospheric spectra of our targets show a clear diversity at similar temperatures, especially in the 3–5 μ m range, unaccounted for by existing atmospheric models. We find a silicate absorption feature in the photosphere of one of our targets, the first such detection in very young FFPMOs, indicating silicate clouds in their cool atmospheres. Six of our objects show mid-infrared excess emission above the photosphere, as well as silicate emission features, demonstrating the presence of disks. The shape and strength of the latter features constitute strong evidence of grain growth and crystallization, similar to what is seen in more massive brown dwarfs and stars. We also detect emission lines from hydrocarbon molecules in the disks of several targets. These are the lowest-mass isolated objects found so far with silicate and hydrocarbon emission features arising in their disks. The presence of disks and their characteristics point to the potential for the formation of rocky companions around free-floating planetary-mass objects.

The Hunting Outbursting Young Stars (HOYS) project performs long-term, optical, multi-filter, high cadence monitoring of 25 nearby young clusters and star forming regions. Utilising Gaia DR3 data we have identified about 17000 potential young stellar members in 45 coherent astrometric groups in these fields. Twenty one of them are clear young groups or clusters of stars within one kiloparsec and they contain 9143 Gaia selected potential members. The cluster distances, proper motions and membership numbers are determined. We analyse long term (about 7yr) V, R, and I-band light curves from HOYS for 1687 of the potential cluster members. One quarter of the stars are variable in all three optical filters, and two thirds of these have light curves that are symmetric around the mean. Light curves affected by obscuration from circumstellar materials are more common than those affected by accretion bursts, by a factor of 2-4. The variability fraction in the clusters ranges from 10 to almost 100 percent, and correlates positively with the fraction of stars with detectable inner disks, indicating that a lot of variability is driven by the disk. About one in six variables shows detectable periodicity, mostly caused by magnetic spots. Two thirds of the periodic variables with disk excess emission are slow rotators, and amongst the stars without disk excess two thirds are fast rotators - in agreement with rotation being slowed down by the presence of a disk.

ABSTRACT We present spot properties on 32 periodic young stellar objects in IC 5070. Long term, ∼5 yr, light curves in the V, R, and I-bands are obtained through the HOYS (Hunting Outbursting Young Stars) citizen science project. These are dissected into 6 months long slices, with 3 months oversampling, to measure 234 sets of amplitudes in all filters. We fit 180 of these with reliable spot solutions. Two thirds of spot solutions are cold spots, the lowest is 2150 K below the stellar temperature. One third are warm spots that are above the stellar temperature by less than ∼2000 K. Cold and warm spots have maximum surface coverage values of 40 per cent, although only 16 per cent of warm spots are above 20 per cent surface coverage as opposed to 60 per cent of the cold spots. Warm spots are most likely caused by a combination of plages and low-density accretion columns, most common on objects without inner disc excess emission in K − W2. Five small hot spot solutions have <3 per cent coverage and are 3000–5000 K above the stellar temperature. These are attributed to accretion, and four of them occur on the same object. The majority of our objects are likely to be accreting. However, we observe very few accretion hot spots as either the accretion is not stable on our time-scale or the photometry is dominated by other features. We do not identify cyclical spot behaviour on the targets. We additionally identify and discuss a number of objects that have interesting amplitudes, phase changes, or spot properties.

The discovery and characterization of free-floating planetary-mass objects (FFPMOs) is fundamental to our understanding of star and planet formation. Here we report results from an extremely deep spectroscopic survey of the young star cluster NGC1333 using Near-InfraRed Imager and Slitless Spectrograph (NIRISS) wide field slitless spectroscopy on the James Webb Space Telescope. The survey is photometrically complete to K similar to 21, and includes useful spectra for objects as faint as K similar to 20.5. The observations cover 19 known brown dwarfs, for most of which we confirm spectral types using NIRISS spectra. We discover six new candidates with L-dwarf spectral types that are plausible planetary-mass members of NGC1333, with estimated masses between 5 and 15 M Jup. One, at similar to 5 M Jup, shows clear infrared excess emission and is a good candidate to be the lowest-mass object known to have a disk. We do not find any objects later than mid-L spectral type (M less than or similar to 4 M Jup). The paucity of Jupiter-mass objects, despite the survey's unprecedented sensitivity, suggests that our observations reach the lowest-mass objects that formed like stars in NGC1333. Our findings put the fraction of FFPMOs in NGC1333 at similar to 10% of the number of cluster members, significantly more than expected from the typical log-normal stellar mass function. We also search for wide binaries in our images and report a young brown dwarf with a planetary-mass companion.

We report on a search for variability in the young brown dwarf SST1624 ( ∼M7 spectral type, M∼0.05M⊙), previously found to feature an expanding gaseous shell and to undergo quasi-spherical mass loss. We find no variability on timescales of 1-6,hours. Specifically, on these timescales, we rule out the presence of a period with amplitude >1%. A photometric period in that range would have been evidence for either pulsation powered by Deuterium burning or rotation near breakup. However, we see a 3% decrease in the K-band magnitude between two consecutive observing nights (a 10, σ result). There is also clear evidence for variations in the WISE lightcurves at 3.6 and 4.5 μm on timescales of days, with a tentative period of about 6,d (with a plausible range between 3 and 7,d). The best explanation for the variations over days is rotational modulation due to spots. These results disfavour centrifugal winds driven by fast rotation as mechanism for the mass loss, which, in turn, makes the alternative scenario – a thermal pulse due to Deuterium burning – more plausible.

ABSTRACTWe present measurements of spot properties on 31 young stellar objects, based on multiband data from the HOYS (Hunting Outbursting Young Stars) project. On average the analysis for each object is based on 270 data points during 80 d in at least three bands. All the young low-mass stars in our sample show periodic photometric variations. We determine spot temperatures and coverage by comparing the measured photometric amplitudes in optical bands with simulated amplitudes based on atmosphere models, including a complete error propagation. 21 objects in our sample feature cool spots, with spot temperatures 500–2500 K below the stellar effective temperature (Teff), and a coverage of 0.05–0.4. Six more have hot spots, with temperatures up to 3000 K above Teff and coverage below 0.15. The remaining four stars have ambiguous solutions or are AA Tau-type contaminants. All of the stars with large spots (i.e. high coverage >0.1) are relatively cool with Teff < 4500 K, which could be a result of having deeper convection zones. Apart from that, spot properties show no significant trends with rotation period, infrared excess, or stellar properties. Most notably, we find hot spots in stars that do not show K − W2 infrared excess, indicating the possibility of accretion across an inner disc cavity or the presence of plage.

We report on a sensitive infrared search for disks around isolated young planetary-mass objects (PMOs) in the NGC 1333 cluster, by stacking 70 Spitzer/IRAC frames at 3.6 and 4.5 μ m. Our coadded images go >2.3 mag deeper than single-epoch frames, and cover 50 brown dwarfs, 15 of which have M9 or later spectral types. Spectral types >M9 correspond to masses in the giant-planet domain, i.e., near or below the deuterium-burning limit of 0.015 M ⊙ . Five of the 12 PMOs show definitive evidence of excess, implying a disk fraction of 42%, albeit with a large statistical uncertainty given the small sample. Comparing with measurements for higher-mass objects, the disk fraction does not decline substantially with decreasing mass in the substellar domain, consistent with previous findings. Thus, free-floating PMOs have the potential to form their own miniature planetary systems. We note that only one of the six lowest-mass objects in NGC 1333, with spectral type L0 or later, has a confirmed disk. Reviewing the literature, we find that the lowest-mass free-floating objects with firm disk detections have masses ∼0.01 M ⊙ (or ∼10 M Jup ). It is not clear yet whether even lower-mass objects harbor disks. If not, it may indicate that ∼10 M Jup is the lower-mass limit for objects that form like stars. Our disk-detection experiment on deep Spitzer images paves the way for studies with JWST at longer wavelengths and higher sensitivity, which will further explore disk prevalence and formation of free-floating PMOs.

Free-floating (or rogue) planets are planets that are liberated (or ejected) from their host systems. Although simulations predict their existence in substantial numbers, direct observational evidence for free-floating planets with masses below ~5 MJup is still lacking. Several cycle-1 observing programs with JWST aim to hunt for them in four different star-forming clusters. These surveys are designed to be sensitive to masses of 1-15 MJup (assuming a hot-start formation), which corresponds to spectral types of early L to late T for the ages of these clusters. If the existing simulations are not wide off the mark, we show here that the planned programs are likely to find up to 10-20 giant rogue planets in moderate density clusters like NGC1333 or IC348, and several dozen to ~100 in high-density regions like NGC2024 and the Orion Nebula Cluster. These numbers correspond to 1-5% of the total cluster population; they could be substantially higher if stars form multiple giant planets at birth. In contrast, the number of free-floating brown dwarfs, formed from core collapse ("like stars") is expected to be significantly lower, only about 0.25% of the number of stars, or 1-7 for the clusters considered here. Below 10 MJup that number drops further by an order of magnitude. We also show that the planned surveys are not at risk of being significantly contaminated by field brown dwarfs in the foreground or background, after spectroscopic confirmation. Taken together, our results imply that if a population of L and T dwarfs were to be found in these JWST surveys, it is expected to be predominantly made up of rogue planets.

We present the wide field slitless spectroscopy mode of the NIRISS instrument on the James Webb Space Telescope. This mode employs two orthogonal low-resolution (resolving power ≈150) grisms in combination with a set of six blocking filters in the wavelength range 0.8–2.3 μm to provide a spectrum of almost every source across the field-of-view. When combined with the low background, high sensitivity and high spatial resolution afforded by the telescope, this mode will enable unprecedented studies of the structure and evolution of distant galaxies. We describe the performance of the as-built hardware relevant to this mode and expected imaging and spectroscopic sensitivity. We discuss operational and calibration procedures to obtain the highest quality data. As examples of the observing mode usage, we present details of two planned Guaranteed Time Observations programs: The Canadian NIRISS Unbiased Cluster Survey and The NIRISS Survey for Young Brown Dwarfs and Rogue Planets.

We present first results from the Fleming Survey, a high-cadence search for variable stars in the northern Galactic Plane, conducted with the James Gregory Telescope in St Andrews (UK). After observing twelve fields, each over a timespan of 3–5 hr and with a cadence of 1 min, we identify 8 variable stars with clear periodic flux modulation in our lightcurves. Two of these are deemed to be δ Scuti pulsators, five are most likely eclipsing binaries. For most of our discoveries, the variability is known in the literature, but the exact period and shape of the lightcurve is not. This mini-survey demonstrates that high-cadence observations with small telescopes are a useful complement to ongoing and planned massive variability surveys.

We use photometric and kinematic data from Gaia DR2 to explore the structure of the star forming region associated with the molecular cloud of Perseus. Apart from the two well known clusters, IC 348 and NGC 1333, we present five new clustered groups of young stars, which contain between 30 and 300 members, named Autochthe, Alcaeus, Heleus, Electryon and Mestor. We demonstrate these are co-moving groups of young stars, based on how the candidate members are distributed in position, proper motion, parallax and colour-magnitude space. By comparing their colour-magnitude diagrams to isochrones we show that they have ages between 1 and 5 Myr. Using 2MASS and WISE colours we find that the fraction of stars with discs in each group ranges from 10 to 50 percent. The youngest of the new groups is also associated with a reservoir of cold dust, according to the Planck map at 353 GHz. We compare the ages and proper motions of the five new groups to those of IC 348 and NGC 1333. Autochthe is clearly linked with NGC 1333 and may have formed in the same star formation event. The seven groups separate roughly into two sets which share proper motion, parallax and age: Heleus, Electryon, Mestor as the older set, and NGC 1333, Autochthe as the younger set. Alcaeus is kinematically related to the younger set, but at a more advanced age, while the properties of IC 348 overlap with both sets. All older groups in this star forming region are located at higher galactic latitude.