In a companion paper by Koposov et al., RR Lyrae from Gaia Data Release 2 are used to demonstrate that stars in the Orphan stream have velocity vectors significantly misaligned with the stream track, suggesting that it has received a large gravitational perturbation from a satellite of the Milky Way. We argue that such a mismatch cannot arise due to any realistic static Milky Way potential and then explore the perturbative effects of the Large Magellanic Cloud (LMC). We find that the LMC can produce precisely the observed motiontrack mismatch and we therefore use the Orphan stream to measure the mass of the Cloud. We simultaneously fit the Milky Way and LMC potentials and infer that a total LMC mass of 1.38 +0.27 −0.24 × 10 11 M is required to bend the Orphan Stream, showing for the first time that the LMC has a large and measurable effect on structures orbiting the Milky Way. This has far-reaching consequences for any technique which assumes that tracers are orbiting a static Milky Way. Furthermore, we measure the Milky Way mass within 50 kpc to be 3.80 +0.14 −0.11 × 10 11 M . Finally, we use these results to predict that, due to the reflex motion of the Milky Way in response to the LMC, the outskirts of the Milky Way's stellar halo should exhibit a bulk, upwards motion.

We use astrometry, broad-band photometry and variability information from the Data Release 2 of ESA's Gaia mission (GDR2) to identify members of the Orphan Stream (OS) across the whole sky. The stream is traced above and below the celestial equator and in both Galactic hemispheres, thus increasing its visible length to ∼ 210 • equivalent to ∼ 150 kpc in physical extent. Taking advantage of the large number of RR Lyrae stars in the OS, we extract accurate distances and proper motions across the entire stretch of the tidal debris studied. As delineated by the GDR2 RR Lyrae, the stream exhibits two prominent twists in its shape on the sky which are accompanied by changes in the tangential motion. We complement the RR Lyrae maps with those created using GDR2 Red Giants and the DECam Legacy Survey Main Sequence Turn-Off stars. The behavior of the OS track on the sky is consistent across all three tracers employed. We detect a strong non-zero motion in the across-stream direction for a substantial portion of the stream. Such a misalignment between the debris track and the streaming velocity cannot be reproduced in a static gravitational potential and signals an interaction with a massive perturber.

We present results of our large scale, optical, multi-epoch photometric survey of ∼ 180 square degrees across the Orion OB1 association, complemented with extensive follow up spectroscopy. Our focus is mapping and characterizing in an uniform way the off-cloud, low-mass, pre-main sequence populations. We report 2064, mostly K and M-type, confirmed T Tauri members. Most (59%) are located in the OB1a subassociation, 27% are projected onto the OB1b subassociation, and the remaining 14% are located within the confines of the A and B molecular clouds. There is significant structure in the spatial distribution of the young stars. We characterize two new clusterings of T Tauri stars, the HD 35762 and HR 1833 groups, both located in the OB1a subassociation, not far form the 25 Ori cluster. We also confirm two stellar overdensities in OB1b, I and II, containing a total of 231 T Tauri stars. A ∼ 2 deg wide halo of young stars surrounds the Orion Nebula Cluster; the north and south parts corresponding to the low-mass populations of NGC 1977 and NGC 1980, respectively. There is indication of two populations of young stars in the OB1b region, located at two different distances, which may be due to the OB1a subassociation overlapping on front of OB1b. The various groups and regions can be ordered in an age sequence that agrees with the long standing picture of star formation starting in Orion OB1a some 10-15 Myr ago.We use the strength of the Hα line in emission, combined with characteristics of IR excesses and optical variability, to define a new type of T Tauri star, the C/W class, stars we propose may be nearing the end of their accretion phase, in an evolutionary state between that of Classical T Tauri and Weak-lined T Tauri stars. The evolution of the ensemble-wide equivalent width of the Li Iλ6707 line shows the depletion of Li with a timescale of 8.5 Myr. The decline of the accretion fraction, from ∼ 2 − 10 Myr, implies an accretion e-folding timescale of 2.1 Myr, consistent with previous studies. Finally, we use the median amplitude of the V -band variability in each type of star, to show the decline of stellar activity, from the accreting Classical T Tauri stars to the population of least active field dwarfs.

We present the results of a survey of the low mass star and brown dwarf population of the 25 Orionis group. Using optical photometry from the CIDA Deep Survey of Orion, near IR photometry from the Visible and Infrared Survey Telescope for Astronomy and low resolution spectroscopy obtained with Hectospec at the MMT, we selected 1246 photometric candidates to low mass stars and brown dwarfs with estimated masses within 0.02 < ∼ M/M ⊙ < ∼ 0.8 and spectroscopically confirmed a sample of 77 low mass stars as new members of the cluster with a mean age of ∼7 Myr. We have obtained a system initial mass function of the group that can be well described by either a Kroupa power-law function with indices α 3 = −1.73 ± 0.31 and α 2 = 0.68 ± 0.41 in the mass ranges 0.03 M/M ⊙ 0.08 and 0.08 M/M ⊙ 0.5 respectively, or a Scalo log-normal function with coefficients m c = 0.21 +0.02 −0.02 and σ = 0.36 ± 0.03 in the mass range 0.03 M/M ⊙ 0.8. From the analysis of the spatial distribution of this numerous candidate sample, we have confirmed the East-West elongation of the 25 Orionis group observed in previous works, and rule out a possible southern extension of the group. We find that the spatial distributions of low mass stars and brown dwarfs in 25 Orionis are statistically indistinguishable. Finally, we found that the fraction of brown dwarfs showing IR excesses is higher than for low mass stars, supporting the scenario in which the evolution of circumstellar discs around the least massive objects could be more prolonged.