The number of known periodic variables has grown rapidly in recent years. Thanks to its large field of view and faint limiting magnitude, the Zwicky Transient Facility (ZTF) offers a unique opportunity to detect variable stars in the northern sky. Here, we exploit ZTF Data Release 2 (DR2) to search for and classify variables down to r ∼ 20.6 mag. We classify 781,602 periodic variables into 11 main types using an improved classification method. Comparison with previously published catalogs shows that 621,702 objects (79.5%) are newly discovered or newly classified, including ∼700 Cepheids, ∼5000 RR Lyrae stars, ∼15,000 δ Scuti variables, ∼350,000 eclipsing binaries, ∼100,000 long-period variables, and about 150,000 rotational variables. The typical misclassification rate and period accuracy are on the order of 2% and 99%, respectively. 74% of our variables are located at Galactic latitudes, . This large sample of Cepheids, RR Lyrae, δ Scuti stars, and contact (EW-type) eclipsing binaries is helpful to investigate the Galaxy’s disk structure and evolution with an improved completeness, areal coverage, and age resolution. Specifically, the northern warp and the disk’s edge at distances of 15–20 kpc are significantly better covered than previously. Among rotational variables, RS Canum Venaticorum and BY Draconis-type variables can be separated easily. Our knowledge of stellar chromospheric activity would benefit greatly from a statistical analysis of these types of variables.

Context. The hierarchical structure formation model predicts that stellar halos should form, at least partly, via mergers. If this was a predominant formation channel for the Milky Way's halo, imprints of this merger history in the form of moving groups or streams should also exist in the vicinity of the Sun. Aims. We study the kinematics of halo stars in the Solar neighbourhood using the very recent first data release from the Gaia mission, and in particular the TGAS dataset, in combination with data from the RAVE survey. Our aim is to determine the amount of substructure present in the phase-space distribution of halo stars that could be linked to merger debris. Methods. To characterise kinematic substructure, we measured the velocity correlation function in our sample of halo (lowmetallicity) stars. We also studied the distribution of these stars in the space of energy and two components of the angular momentum, in what we call "integrals of motion" space. Results. The velocity correlation function reveals substructure in the form of an excess of pairs of stars with similar velocities, well above that expected for a smooth distribution. Comparison to cosmological simulations of the formation of stellar halos indicates that the levels found are consistent with the Galactic halo having been built solely via accretion. Similarly, the distribution of stars in the space of integrals of motion is highly complex. A strikingly high fraction (from 58% up to more than 73%) of the stars that are somewhat less bound than the Sun are on (highly) retrograde orbits. A simple comparison to Milky Way-mass galaxies in cosmological hydrodynamical simulations suggests that less than 1% have such prominently retrograde outer halos. We also identify several other statistically significant structures in integrals of motion space that could potentially be related to merger events.

The Milky Way's neutral hydrogen (HI) disk is warped and flared 1, 2 . However, a dearth of accurate HI-based distances has thus far prevented the development of an accurate Galactic disk model. Moreover, the extent to which our Galaxy's stellar and gas disk morphologies are mutually consistent is also unclear. Classical Cepheids, primary distance indicators with distance accuracies of 3-5% 3 , offer a unique opportunity to develop an intuitive and accurate three-dimensional picture. Here, we establish a robust Galactic disk model based on 1339 classical Cepheids. We provide strong evidence that the warp's line of nodes is not oriented in the Galactic Center-Sun direction. Instead, it subtends a mean angle of 17.5 • ± 1 • (formal) ±3 • (systematic) and exhibits a leading spiral pattern. Our Galaxy thus follows Briggs' rule for spiral galaxies 4 , which suggests that the origin of the warp is associated with torques forced by the massive inner disk 5 . The stellar disk traced by Cepheids follows the gas disk in terms of their amplitudes; the stellar disk extends to at least 20 kpc 6,7 . This morphology provides a crucial, updated map for studies of the kinematics and archaeology of the Galactic disk.We have compiled samples of classical Cepheids from the Wide-field Infrared Survey Explorer (WISE) catalogue of periodic variables 8 (our 'WISE Cepheid sample') as well as from a number of optical surveys (collectively referred to as our 'optical Cepheid sample'). We will discuss both samples separately, because the catalogues' optical and infrared passbands are characterised by significantly different photometric and extinction sensitivities. Highly accurate Cepheid distances can be estimated using their well-established wavelength-dependent period-luminosity relations. To mitigate the influence of extinction in the Galactic plane and of photometric uncertainties at infrared wavelengths, we adopted the 'infrared multi-passband optimal distance method' 9 to determine accurate Cepheid distances. Contaminants, including Type-II Cepheids, long-period eclipsing binaries and quasi-periodic variables were removed using Gaia Data Release 2 parallaxes 10 .Cepheids located in areas centered on the Magellanic Clouds were also excluded. Careful sample selection resulted in a tally of 2330 classical Cepheids for further analysis.Distances were converted to 3D XY z and spherical Rφz coordinates by adopting a reference frame centered on the Galactic Center and a solar Galactocentric distance R 0 = 8.0 kpc. Here, φ is the Galactocentric angle in the anticlockwise direction (aligned with the disk's rotation axis) with respect to the solar position (φ = 0 • ). Since Gaia parallaxes are reliable within ∼5 kpc, we only selected Cepheids within the volume R < 20, |z| < 2 kpc to avoid significant contamination by Type-II Cepheids. Our downselection included 1459 Cepheids with distances accurate to <5%, corresponding to a distance modulus standard deviation <0.108 mag. Cepheids located clearly away from the best-fitting warp model (∆ > 1 kp...