The PDS 70 system has been subject to many studies in the past year following the discovery of two accreting planets in the gap of its circumstellar disk. Nevertheless, the mass accretion rate onto the star is still not well known. Here we determined the stellar mass accretion rate and its variability based on TESS and HARPS observations. The stellar light curve shows a strong signal with a 3.03±0.06 days period, which we attribute to stellar rotation. Our analysis of the HARPS spectra shows a rotational velocity of v sin i = 16.0 ± 0.5 km s −1 , indicating that the inclination of the rotation axis is 50 ± 8 degrees. This implies that the rotation axes of the star and its circumstellar disk are parallel within the measurement error. We apply magnetospheric accretion models to fit the profiles of the Hα line and derive mass accretion rates onto the star in the range of 0.6 − 2.2 × 10 −10 M yr −1 , varying over the rotation phase. The measured accretion rates are in agreement with those estimated from NUV fluxes using accretion shock models. The derived accretion rates are higher than expected from the disk mass and planets properties for the low values of the viscous parameter α suggested by recent studies, potentially pointing to an additional mass reservoir in the inner disk to feed the accretion, such as a dead zone. We find that the He I λ10830 line shows a blueshifted absorption feature, indicative of a wind. The mass-loss rate estimated from the line depth is consistent with an accretion-driven inner disk MHD wind.

Classical novae are shock-powered multi-wavelength transients triggered by a thermonuclear runaway on an accreting white dwarf. V1674 Her is the fastest nova ever recorded (time to declined by two magnitudes is t2 = 1.1 d) that challenges our understanding of shock formation in novae. We investigate the physical mechanisms behind nova emission from GeV γ-rays to cm-band radio using coordinated Fermi-LAT, NuSTAR, Swift and VLA observations supported by optical photometry. Fermi-LAT detected short-lived (18 h) 0.1-100 GeV emission from V1674 Her that appeared 6 h after the eruption began; this was at a level of (1.6 ± 0.4) × 10−6 photons cm−2 s−1. Eleven days later, simultaneous NuSTAR and Swift X-ray observations revealed optically thin thermal plasma shock-heated to kTshock = 4 keV. The lack of a detectable 6.7 keV Fe Kα emission suggests super-solar CNO abundances. The radio emission from V1674 Her was consistent with thermal emission at early times and synchrotron at late times. The radio spectrum steeply rising with frequency may be a result of either free-free absorption of synchrotron and thermal emission by unshocked outer regions of the nova shell or the Razin-Tsytovich effect attenuating synchrotron emission in dense plasma. The development of the shock inside the ejecta is unaffected by the extraordinarily rapid evolution and the intermediate polar host of this nova.

Much is known about the processes driving accretion from protoplanetary disks onto low-mass pre-main-sequence stars (T Tauri stars). Nevertheless, it is unclear how accretion stops. To determine the accretion properties and their relation to stellar properties and gain insight into the last stages of accretion, we present a detailed analysis of 24 low and possible accretors, previously identified using the He i λ10830 line. We model moderate-resolution Hα profiles of these stars using magnetospheric accretion flow models that account for the chromospheric contribution at the line center. Based on parameters derived from the fits of 20 stars that can be reproduced with the models, we find a power-law relation between the disk truncation radius and the mass accretion rate consistent with predictions from theory and simulations. Comparing the corotation and truncation radii, we find that most of our targets are accreting in the unstable regime and rule out the propeller as the main process stopping accretion. For the truncation radius to be the same as the magnetic radius, the dipole magnetic field and/or the efficiency parameter ξ need to be smaller than previously determined, suggesting that higher-order fields dominate in low accretion rates. Lastly, we determine that the lowest accretion rates that can be detected by Hα line modeling are (1–3) × 10−11 M ☉ yr−1 for M3 stars and (3–5) × 10−11 M ☉ yr−1 for K5 stars. These limits are lower than the observed accretion rates in our sample, suggesting that we have reached a physical lower limit. This limit, M ̇ ∼ 10 − 10 M ☉ yr − 1 , is consistent with EUV-dominated photoevaporation.

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