We present an exquisite 30 minute cadence Kepler (K2) light curve of the Type Ia supernova (SN Ia) 2018oh (ASASSN-18bt), starting weeks before explosion, covering the moment of explosion and the subsequent rise, and continuing past peak brightness. These data are supplemented by multi-color Panoramic Survey Telescope (Pan-STARRS1) and Rapid Response System 1 and Cerro Tololo Inter-American Observatory 4 m Dark Energy Camera (CTIO 4-m DECam) observations obtained within hours of explosion. The K2 light curve has an unusual twocomponent shape, where the flux rises with a steep linear gradient for the first few days, followed by a quadratic rise as seen for typical supernovae (SNe)Ia. This "flux excess" relative to canonical SNIa behavior is confirmed in our i-band light curve, and furthermore, SN 2018oh is especially blue during the early epochs. The flux excess peaks 2.14±0.04 days after explosion, has a FWHM of 3.12±0.04 days, a blackbody temperature of T 17, 500 9,000 11,500 =-+ K, a peak luminosity of 4.3 0.2 10 erg s 37 1 ´-, and a total integrated energy of 1.27 0.01 10 erg 43 ´. We compare SN 2018oh to several models that may provide additional heating at early times, including collision with a companion and a shallow concentration of radioactive nickel. While all of these models generally reproduce the early K2 light curve shape, we slightly favor a companion interaction, at a distance of ∼2 10 cm 12 based on our early color measurements, although the exact distance depends on the uncertain viewing angle. Additional confirmation of a companion interaction in future modeling and observations of SN 2018oh would provide strong support for a single-degenerate progenitor system.

Supernova (SN) 2018oh (ASASSN-18bt) is the first spectroscopically confirmed Type Ia supernova (SN Ia) observed in the Kepler field. The Kepler data revealed an excess emission in its early light curve, allowing us to place interesting constraints on its progenitor system. Here we present extensive optical, ultraviolet, and nearinfrared photometry, as well as dense sampling of optical spectra, for this object. SN 2018oh is relatively normal in its photometric evolution, with a rise time of 18.3±0.3 days and Δm 15 (B)=0.96±0.03 mag, but it seems to have bluer B−V colors. We construct the "UVOIR" bolometric light curve having a peak luminosity of 1.49×10 43 erg s −1 , from which we derive a nickel mass as 0.55±0.04 M e by fitting radiation diffusion models powered by centrally located 56 Ni. Note that the moment when nickel-powered luminosity starts to emerge is +3.85 days after the first light in the Kepler data, suggesting other origins of the early-time emission, e.g., mixing of 56 Ni to outer layers of the ejecta or interaction between the ejecta and nearby circumstellar material or a nondegenerate companion star. The spectral evolution of SN 2018oh is similar to that of a normal SN Ia but is characterized by prominent and persistent carbon absorption features. The CII features can be detected from the early phases to about 3 weeks after the maximum light, representing the latest detection of carbon ever recorded in an SN Ia. This indicates that a considerable amount of unburned carbon exists in the ejecta of SN 2018oh and may mix into deeper layers.

We present space-based ultraviolet/optical photometry and spectroscopy with the Swift Ultra-Violet/Optical Telescope and Hubble Space Telescope, respectively, along with ground-based optical photometry and spectroscopy and near-infrared spectroscopy of supernova SN 2017erp. The optical light curves and spectra are consistent with a normal Type Ia supernova (SN Ia). Compared to previous photometric samples in the near-ultraviolet (NUV), SN 2017erp has colors similar to the NUV-red category after correcting for Milky Way and host dust reddening. We find the difference between SN 2017erp and the NUV-blue SN 2011fe is not consistent with dust reddening alone but is similar to the SALT color law, derived from rest-frame UV photometry of higher redshift SNe Ia. This chromatic difference is dominated by the intrinsic differences in the UV and only a small contribution from the expected dust reddening. Differentiating the two can have important consequences for determining cosmological distances with rest-frame UV

Blue large-amplitude pulsators (BLAPs) represent a new and rare class of hot pulsating stars with unusually large amplitudes and short periods. Up to now, only 21 confirmed BLAPs have been identified from more than one billion monitored stars, including a group with pulsation period longer than 20 min (classical BLAPs; [1]) and the other group with pulsation period below 10 min (high-gravity BLAPs; [2]). The evolutionary path that could give rise to such kinds of stellar configurations is unclear. Here we report on a comprehensive study of the peculiar BLAP discovered by the Tsinghua University -Ma Huateng Telescopes for Survey (TMTS) [3], TMTS J035143.63+584504.2 (TMTS-BLAP-1); it is the first confirmed BLAP located within the "period gap" between high-gravity and classical BLAPs. This new BLAP has an 18.9 min pulsation period, and is similar to the classical BLAPs with an extended helium-enriched envelope. In particular, the long-term monitoring data of TMTS-BLAP-1 reveal that this pulsating star has an unusually large rate of period change, Ṗ /P = 2.3 × 10 −6 yr −1 . Such a significant and positive value challenges the scenarios of both the helium-core pre-white-dwarf and the core-helium burning [1, 2, 4-6]), but is consistent with that derived from shell-helium burning. The particular pulsation period and unusual rate of period change indicate that TMTS-BLAP-1 is at a short-lived (∼ 10 6 yr) phase of shell-helium ignition before the stable shell-helium burning; in other words, TMTS-BLAP-1 is going through a "Hertzsprung gap" of hot subdwarfs. This demonstrates that hot subdwarf stars have a similar evolutionary fate as main-sequence stars; some subdwarfs can leave their "main-sequence" stage and appear as diverse pulsating variables in distinct regions of the Hertzsprung-Russell diagram, thereby opening a new window for explorations of rare pulsating stars that evolved from stripped-envelope stars.The minute-cadence observations by Tsinghua University -Ma Huateng Telescopes for Survey (TMTS) enable the search of variable stars with periods shorter than 1 hr. TMTS J035143.63+584504.2 is such a newly discovered short-period variable, having an 18.9 min period and a peak-to-peak amplitude up to ∼ 0.3 mag in white light [3]. Follow-up observations with SNOVA,