The concept of stars being tidally ripped apart and consumed by a massive black hole (MBH) lurking in the center of a galaxy first captivated theorists in the late 1970s. The observational evidence for these rare but illuminating phenomena for probing otherwise dormant MBHs first emerged in archival searches of the soft X-ray ROSAT All-Sky Survey in the 1990s, but has recently accelerated with the increasing survey power in the optical time domain, with tidal disruption events (TDEs) now regarded as a class of optical nuclear transients with distinct spectroscopic features. Multiwavelength observations of TDEs have revealed panchromatic emission, probing a wide range of scales, from the innermost regions of the accretion flow to the surrounding circumnuclear medium. I review the current census of 56 TDEs reported in the literature, and their observed properties can be summarized as follows:
▪ | The optical light curves follow a power-law decline from peak that scales with the inferred central black hole mass as expected for the fallback rate of the stellar debris, but the rise time does not. | ||||
▪ | The UV-optical and soft X-ray thermal emission come from different spatial scales, and their intensity ratio has a large dynamic range and is highly variable, providing important clues as to what is powering the two components. | ||||
▪ | They can be grouped into three spectral classes, and those with Bowen fluorescence line emission show a preference for a hotter and more compact line-emitting region, whereas those with only Heii emission lines are the rarest class. |
We provide a nonspecialist overview of the current state of understanding of the structure and origin of our Solar System's transneptunian region (often called the Kuiper Belt), highlighting perspectives on planetesimal formation, planet migration, and the contextual relationship with protoplanetary disks. We review the dynamical features of the transneptunian populations and their associated differences in physical properties. We describe aspects of our knowledge that have advanced in the past two decades and then move on to current issues of research interest (which thus still have unclear resolution).
▪ | The current transneptunian population consists of both implanted and primordial objects. | ||||
▪ | The primordial (aka cold) population is a largely unaltered remnant of the population that formed in situ. | ||||
▪ | The reason for the primordial cold population's current outer edge is unexplained. | ||||
▪ | The large semimajor-axis population now dynamically detached from Neptune is critical for understanding the Solar System's history. | ||||
▪ | Observational constraints on the number and orbits of distant objects remain poor. |