We present a quantitative, empirically based argument that at least some Class I sources are lowmass, pre-main-sequence stars surrounded by spatially extended envelopes of dusty gas. The source luminosity arises principally from stellar gravitational contraction, as in optically visible pre-mainsequence stars that lack such envelopes. We base our argument on the fact that some Class I sources in Orion and other star-forming regions have been observed by Spitzer to be periodic variables in the mid-infrared, and with periods consistent with T Tauri rotation rates. Using a radiative transfer code, we construct a variety of dust envelopes surrounding rotating, spotted stars, to see if an envelope that produces a Class I SED at least broadly matches the observed modulations in luminosity. Acceptable envelopes can either be spherical or flattened, and may or may not have polar cavities. The key requirement is that they have a modest equatorial optical depth at the Spitzer waveband of 3.6 µm, typically τ 3.6 ≈ 0.6. The total envelope mass, based on this limited study, is at most about 0.1 M ⊙ , less than a typical stellar mass. Future studies should focus on the dynamics of the envelope, to determine whether material is actually falling onto the circumstellar disk.

The majority of binary star systems that host exoplanets will spend the first portion of their lives within a star-forming cluster that may drive dynamical evolution of the binary-planet system. We perform numerical simulations of S-type planets, with masses and orbital architecture analogous to the Solar system’s 4 gas giants, orbiting within the influence of a 0.5 M⊙ binary companion. The binary-planet system is integrated simultaneously with an embedded stellar cluster environment. ∼10 per cent of our planetary systems are destabilized when perturbations from our cluster environment drive the binary periastron toward the planets. This destabilization occurs despite all of our systems being initialized with binary orbits that would allow stable planets in the absence of the cluster. The planet-planet scattering triggered in our systems typically results in the loss of lower mass planets and the excitement of the eccentricities of surviving higher mass planets. Many of our planetary systems that go unstable also lose their binary companions prior to cluster dispersal and can therefore masquerade as hosts of eccentric exoplanets that have spent their entire histories as isolated stars. The cluster-driven binary orbital evolution in our simulations can also generate planetary systems with misaligned spin-orbit angles. This is typically done as the planetary system precesses as a rigid disk under the influence of an inclined binary, and those systems with the highest spin-orbit angles should often retain their binary companion and possess multiple surviving planets.