Maria Kun

We report the discovery of a nearby FU Ori–type outburst (FUor), PR Ori B, in the L1641 cluster of the Orion star-forming region. The high-amplitude variability was first identified in the NEOWISE (3–5 μ m) photometry of the unresolved PR Ori binary system. Long-term, resolved optical photometric monitoring demonstrates that PR Ori B is the driver of a Δ G = 5 mag outburst, while PR Ori A has remained constant over the past 20 yr. The near-IR spectrum of PR Ori B changes from a late K-type spectral type during quiescence to a viscously heated disk during outburst, including deep absorption in ¹² CO and H ₂ O bands. The optical spectrum also exhibits features that are commonly associated with FUors, including P Cygni profiles in Na i D lines and absorption in the Ca ii infrared triplet. The luminosity of the outburst ( L _acc ∼ 30–40 L _⊙ ) is similar to that commonly observed in FUors. The comparison of Spitzer/Infrared Spectrograph and Very Large Telescope/VISIR spectroscopy shows some evidence of silicate crystallization during the outburst. PR Ori B is one of the closest and brightest FUors discovered over the past few years, only one magnitude fainter than the archetype of the class FU Ori. The proximity and brightness will allow for future high angular resolution observations to probe the physics of the inner disk and to evaluate changes in the disk due to the increased luminosity.

Gaia21bja is a Gaia alerted young stellar object (YSO) that exhibits at least seven quasiperiodic brightenings over a 20 yr long light curve with durations of 1.5–2 yr and amplitudes up to ∼1.7 mag in the Gaia G band. We analyze its optical and near-infrared photometry and spectra taken using the Infrared Telescope Facility and Very Large Telescope in its faint and bright states in order to characterize its physical properties. A Lomb–Scargle periodogram analysis results in the most significant period of 916 ± 77 days. We derived the stellar parameters as R _⋆ = 0.78 ± 0.04 R _⊙ , L _⋆ = (4.5 ± 0.3) × 10 ⁻² L _⊙ , and M _⋆ = 0.16 ± 0.03 M _⊙ . The spectra taken during the burst are dominated by emission lines and are similar to those of EX Lupi-type eruptive young stars (EXors). We found that the accretion luminosity and mass accretion rate increased by a factor of 5.5–6 during the burst. Based on this, and the quasiperiodic bursts, we suggest that Gaia21bja is an eruptive YSO, and is most consistent with the “Periodic” category of the Outbursting YSOs Catalogue.

Context . Among known young stellar objects (YSOs), those exhibiting the most dramatic increases in brightness due to sudden increase in mass accretion rate are eruptive young stars. Gaia20dsk is one of the Gaia-alerted young star candidates that has displayed a double, nonperiodic brightening resembling that of other young eruptive stars. Aims . The goal of this work is to determine the physical and accretion properties of Gaia20dsk to confirm its classification as an eruptive young star. Methods . We combined publicly available optical and near-infrared (NIR) photometry with our X-shooter optical/NIR spectrum. In our analysis, we examined the optical and IR light curves from the bursts, reviewing the color-magnitude diagrams across different bands, reporting the detection of emission lines, and providing estimates of the star’s accretion rates during the burst. Results . The optical light curve shows two major and one brief brightening events with an maximum amplitude of ∼ 1.8 mag in the last five years. A classification based on spectral index indicates that Gaia20dsk is a flat-spectrum star. The X-shooter spectrum exhibit emission lines characteristic of accreting low-to-intermediate-mass young stars, displaying features typical of MNor-type objects. The mass accretion rate is between (0.5−1.8) × 10 ⁻⁶ M _⊙ /yr. Conclusions . Gaia20dsk is an eruptive YSO that exhibits photometric features similar to those of MNors, including its characteristic brightening amplitude and burst duration, along with similar spectroscopic features and accretion rates.

Context. Classical T Tauri stars are newly formed, low-mass stars, which may display both periodic and random variations in their brightness. These systems are surrounded by a circumstellar disk, from which material falls onto the stellar surface. The interaction between the star and the circumstellar disk is time dependent, leading to short- or long-term physical changes in the physical environment, and hence variability of the system. Aims. DR Tau is a highly variable young star. By compiling a large dataset with high-cadence photometric, and high-resolution spectroscopic observations, we aim to examine the short- and long-term variability of the system, and identify the underlying physical mechanisms. Methods. We combined multifilter ground-based optical, near-infrared, and space-based mid-infrared (Spitzer Space Telescope) monitoring observations from 2009, 2017, and 2021 with high-cadence optical Kepler K2 and TESS light curves. We complemented our photometric dataset with spectropolarimetric monitoring observations obtained with the CFHT/ESPaDOnS instrument in 2016, which provided high-resolution data at optical wavelengths. Results. Our results reveal that DR Tau exhibits stochastic photometric variability not only on daily, but also on hourly timescales, with a peak-to-peak amplitude of 1.4 mag probably originating from accretion related variations. Our ground-based multifilter photometry shows that the shape of the light curves are similar at all wavelengths, although the amplitude of the variability decreases with increasing wavelength. This trend toward the infrared wavelengths suggests that part of the disk may be optically thick and invariable. In addition to this, the high-cadence Kepler and TESS data allowed us to carry out a detailed period analysis. The spectroscopic analysis showed that the Hα line presents the most complex line profile with several components, but the significance of the components changes over time. This suggests the presence and variation of both accretion flow and wind. Broad and narrow components can be clearly distinguished in the He I and the Ca II lines, which suggests a contribution from both the accretion flow and the post-shock region. The CFHT/ESPaDOnS data suggest that the strength of the longitudinal magnetic field varies between 400 and 1800 G. Conclusions. DR Tau exhibits a high level of photometric and spectroscopic variability on both short and long timescales, which is caused by the combination of accretion, wind, stellar activity, and obscuration by circumstellar matter. Furthermore, the significance of the physical mechanisms that cause the observed variability changes over time.

Episodic accretion is a fundamental process in the build-up of the stellar mass. EX Lupi-type eruptive young stars (EXors) represent one of the main types of episodic accretion. We study the recently discovered EXor Gaia23bab during its 2023 outburst. We obtained optical and near-infrared photometry and spectroscopy to probe the variation of the physical properties of Gaia23bab during its recent outburst. We also collected archival photometry to study a previous outburst of the star. We used several accretion tracers, including the Ca ii triplet, He i, and various hydrogen lines from the Paschen and Brackett series, to measure the accretion rate during the outburst. The accretion rate is consistent with ∼2.0 × 10⁻⁷ M _⊙ yr⁻¹. Comparing the line fluxes of the hydrogen Brackett series to predictions of Case B theory suggests excitation temperatures of 5000–10,000 K and electron densities of 10⁹–10¹⁰ cm⁻³. Comparison to the predictions of a model for T Tauri stars revealed that the fluxes of the Balmer series are consistent with temperatures of 5000–12,500 K and a hydrogen density of 10⁸ cm⁻³, while the fluxes of the Paschen series are consistent with temperatures in the range between 10,000 and 12,500 K and a hydrogen density of 10¹¹ cm⁻³. The derived temperatures and densities confirm that Gaia23bab is a prototypical EXor, not only due to its accretion rate, but also based on the best-fit temperatures and densities revealed by the detected hydrogen lines.

Context. We analyse multi-colour photometric and spectroscopic observations of the young stellar object (YSO) Gaia20bdk. Aims. We aim to investigate the exact nature of the eruptive phenomenon that the star has been undergoing since 2018. Methods. We used public-domain archival photometry to characterise the quiescent phase and to establish the major physical parameters of the progenitor. We used our own optical and infrared (IR) photometry and spectroscopy, along with data from the public domain, to study the outburst. Results. Gaia20bdk is a member of the Sharpless 2-301 star-forming region, at a distance of 3.3 kpc. The progenitor is a rather massive 2.7 ± 0.5 M_⊙, G7-type Class I young star, with an effective temperature of 5300₋₃₀₀⁺⁵⁰⁰ K and bolometric luminosity of 11 ± 2 L_⊙. The optical and IR photometric and spectroscopic data obtained during the outburst reveal a variety of signatures commonly found in classical FU Ori-type stars (FUors). Our disc modelling gives a bolometric luminosity of 100 − 200 L_⊙ and mass accretion rate of 1 − 2 × 10⁻⁵ M_⊙ yr⁻¹, also confirming the object’s FUor classification. Further monitoring is necessary to track the light changes, accretion rate, and spectral variations, as well as to understand the mechanisms behind the disc flickering.

Context. Gravitational waves from black-hole (BH) merging events have revealed a population of extra-galactic BHs residing in short-period binaries with masses that are higher than expected based on most stellar evolution models – and also higher than known stellar-origin black holes in our Galaxy. It has been proposed that those high-mass BHs are the remnants of massive metal-poor stars. Aims. Gaia astrometry is expected to uncover many Galactic wide-binary systems containing dormant BHs, which may not have been detected before. The study of this population will provide new information on the BH-mass distribution in binaries and shed light on their formation mechanisms and progenitors. Methods. As part of the validation efforts in preparation for the fourth Gaia data release (DR4), we analysed the preliminary astrometric binary solutions, obtained by the Gaia Non-Single Star pipeline, to verify their significance and to minimise false-detection rates in high-mass-function orbital solutions. Results. The astrometric binary solution of one source, Gaia BH3, implies the presence of a 32.70 ± 0.82 M_⊙ BH in a binary system with a period of 11.6 yr. Gaia radial velocities independently validate the astrometric orbit. Broad-band photometric and spectroscopic data show that the visible component is an old, very metal-poor giant of the Galactic halo, at a distance of 590 pc. Conclusions. The BH in the Gaia BH3 system is more massive than any other Galactic stellar-origin BH known thus far. The low metallicity of the star companion supports the scenario that metal-poor massive stars are progenitors of the high-mass BHs detected by gravitational-wave telescopes. The Galactic orbit of the system and its metallicity indicate that it might belong to the Sequoia halo substructure. Alternatively, and more plausibly, it could belong to the ED-2 stream, which likely originated from a globular cluster that had been disrupted by the Milky Way.

Context. Strongly lensed quasars are fundamental sources for cosmology. TheGaiaspace mission covers the entire sky with the unprecedented resolution of 0.18″ in the optical, making it an ideal instrument to search for gravitational lenses down to the limiting magnitude of 21. Nevertheless, the previousGaiaData Releases are known to be incomplete for small angular separations such as those expected for most lenses.Aims. We present the Data Processing and Analysis Consortium GravLens pipeline, which was built to analyse allGaiadetections around quasars and to cluster them into sources, thus producing a catalogue of secondary sources around each quasar. We analysed the resulting catalogue to produce scores that indicate source configurations that are compatible with strongly lensed quasars.Methods. GravLens uses the DBSCAN unsupervised clustering algorithm to detect sources around quasars. The resulting catalogue of multiplets is then analysed with several methods to identify potential gravitational lenses. We developed and applied an outlier scoring method, a comparison between the average BP and RP spectra of the components, and we also used an extremely randomised tree algorithm. These methods produce scores to identify the most probable configurations and to establish a list of lens candidates.Results. We analysed the environment of 3 760 032 quasars. A total of 4 760 920 sources, including the quasars, were found within 6″ of the quasar positions. This list is given in theGaiaarchive. In 87% of cases, the quasar remains a single source, and in 501 385 cases neighbouring sources were detected. We propose a list of 381 lensed candidates, of which we identified 49 as the most promising ones. Beyond these candidates, the associate tables in this Focused Product Release allow the entire community to explore the uniqueGaiadata for strong lensing studies further.

Young protostars that undergo episodic accretion can provide insight into the impact on their circumstellar environments while matter is accreted from the disk onto the protostar. IRAS 22343+7501 is a four-component protostar system with one of those being a fading outbursting protostar, which was obseved with the Karl G. Jansky Very Large Array (VLA) and is referred to as L1251 VLA 6. Given the rarity of young stellar objects undergoing this type of accretion, L1251 VLA 6 can elucidate the fading phase of the post-outburst process. Here, we examine structure in the disk around L1251 VLA 6 at frequencies of 10 and 33 GHz with the VLA. We model the disk structure using Markov Chain Monte Carlo (MCMC). This method is then combined with a parametric ray-tracing code to generate synthetic model images of an axisymmetric disk, allowing us to characterize the radial distribution of dust in the system. The results of our MCMC fit show that the most probable values for the mass and radius are consistent with values typical of Class I objects. We find that the total mass of the disk is 0.070 − 0.2 + 0.031 M ☉ and investigate the conditions that could cause the accretion outburst. We conclude that the eruption is not caused by gravitational instability and consider alternative explanations and trigger mechanisms.