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FU Ori-type stars (FUors) are young stellar objects (YSOs) experiencing luminosity outbursts by a few orders of magnitude, which last for ∼102 yr. A dozen of FUors are known up to date, but many more currently quiescent YSOs could have experienced such outbursts in the last ∼103 yr. To find observational signatures of possible past outbursts, we utilize andes, radmc-3d code as well as casa ALMA simulator to model the impact of the outburst on the physical and chemical structure of typical FU Ori systems and how it translates to the molecular lines' fluxes. We identify several combinations of molecular lines that may trace past FU Ori objects both with and without envelopes. The most promising outburst tracers from an observational perspective are the molecular flux combinations of the N2H+ J  = 3–2, C18O J  = 2–1, H2CO |$(J_{\rm K_a, K_c}) = 4_{04}-3_{03}$|⁠ , and HCN J  = 3–2 lines. We analyse the processes leading to molecular flux changes and show that they are linked with either thermal desorption or enhanced chemical reactions in the molecular layer. Using observed CO, HCN, N2H+, and H2CO line fluxes from the literature, we identify ten nearby disc systems that might have undergone FU Ori outbursts in the past ∼103 yr: [MGM2012] 556, [MGM2012] 371, and [MGM2012] 907 YSOs in L1641, Class II protoplanetary discs around CI Tau, AS 209, and IM Lup and transitional discs DM Tau, GM Aur, LkCa 15, and J1640-2130. [ABSTRACT FROM AUTHOR]

We show the first SPHERE/IRDIS and IFS data of the CO-rich debris disc around HD 131488. We use N-body simulations to model both the scattered light images and the spectral energy distribution of the disc in a self-consistent way. We apply the Henyey–Greenstein approximation, Mie theory, and the Discrete Dipole Approximation to model the emission of individual dust grains. Our study shows that only when gas drag is taken into account can we find a model that is consistent with scattered light as well as thermal emission data of the disc. The models suggest a gas surface density of 2 × 10−5 M⊕ au−2 which is in agreement with estimates from ALMA observations. Thus, our modelling procedure allows us to roughly constrain the expected amount of gas in a debris disc without actual gas measurements. We also show that the shallow size distribution of the dust leads to a significant contribution of large particles to the overall amount of scattered light. The scattering phase function indicates a dust porosity of ∼0.2...0.6 which is in agreement with a pebble pile scenario for planetesimal growth. [ABSTRACT FROM AUTHOR]

Gaia21bty, a pre-main-sequence star that previously had shown aperiodic dips in its light curve, underwent a considerable Δ G ≈ 2.9 mag brightening that occurred over a few months between 2020 October and 2021 February. The Gaia light curve shows that the star remained near maximum brightness for about 4–6 months, and then started slowly fading over the next 2 yr, with at least three superimposed ∼1 mag sudden rebrightening events. Whereas the amplitude and duration of the maximum is typical for EX Lupi-type stars, optical and near-infrared spectra obtained at the maximum are dominated by features which are typical for FU Ori-type stars (FUors). Modelling of the accretion disc at the maximum indicates that the disc bolometric luminosity is 43 L⊙ and the mass accretion rate is 2.5 × 10−5 M⊙ yr−1, which are typical values for FUors even considering the large uncertainty in the distance (⁠|$1.7_{-0.4}^{+0.8}$| kpc). Further monitoring is necessary to understand the cause of the quick brightness decline, the rebrightening, and the other post-outburst light changes, as our multicolour photometric data suggest that they could be caused by a long and discontinuous obscuration event. We speculate that the outburst might have induced large-scale inhomogeneous dust condensations in the line of sight leading to such phenomena, whilst the FUor outburst continues behind the opaque screen. [ABSTRACT FROM AUTHOR]

Classical T Tauri stars (CTTSs) are young stellar objects that accrete materials from their accretion disc influenced by their strong magnetic field. The magnetic pressure truncates the disc at a few stellar radii and forces the material to leave the disc plane and fall onto the stellar surface by following the magnetic field lines. However, this global scheme may be disturbed by the presence of a companion interacting gravitationally with the accreting component. This work is aiming to study the accretion and the magnetic field of the tight eccentric binary DQ Tau, composed of two equal-mass (∼ 0.6 M⊙) CTTSs interacting at different orbital phases. We investigated the variability of the system using a high-resolution spectroscopic and spectropolarimetric monitoring performed with ESPaDOnS at the CFHT. We provide the first ever magnetic field analysis of this system, the Zeeman–Doppler imaging revealed a stronger magnetic field for the secondary than the primary (1.2 and 0.5 kG, respectively), but the small-scale fields analysed through Zeeman intensification yielded similar strengths (about 2.5 kG). The magnetic field topology and strengths are compatible with the accretion processes on CTTSs. Both components of this system are accreting, with a change of the main accretor during the orbital motion. In addition, the system displays a strong enhancement of the mass accretion rate at periastron and apastron. We also discovered, for the first time in this system, the apsidal motion of the orbital ellipse. [ABSTRACT FROM AUTHOR]

The nature of the gas in CO-rich debris discs remains poorly understood, as it could either be a remnant from the earlier Class II phase or of secondary origin, driven by the destruction of icy planetesimals. The aim of this paper was to elucidate the origin of the gas content in the debris discs via various simple molecules that are often detected in the less-evolved Class II discs. We present millimetre molecular line observations of nine circumstellar discs around A-type stars: four CO-rich debris discs (HD 21997, HD 121617, HD 131488, HD 131835) and five old Herbig Ae protoplanetary discs (HD 139614, HD 141569, HD 142666, HD 145718, HD 100453). The sources were observed with the Atacama Large Millimeter/submillimeter Array (ALMA) in Bands 5 and 6 with 1–2 arcsec resolution. The Herbig Ae discs are detected in the CO isotopologues, CN, HCN, HCO+, C2H, and CS lines. In contrast, only CO isotopologues are detected in the debris discs, showing a similar amount of CO to that found in the Herbig Ae protoplanetary discs. Using chemical and radiative transfer modelling, we show that the abundances of molecules other than CO in debris discs are expected to be very low. We consider multiple sets of initial elemental abundances with various degrees of H2 depletion. We find that the HCO+ lines should be the second brightest after the CO lines, and that their intensities strongly depend on the overall CO/H2 ratio of the gas. However, even in the ISM-like scenario, the simulated HCO+ emission remains weak as required by our non-detections. [ABSTRACT FROM AUTHOR]