We have discovered a large number of circular and elliptical shells at 24µm around luminous central sources with the MIPS instrument on-board the Spitzer Space Telescope. Our archival follow-up effort has revealed 90% of these circumstellar shells to be previously unknown. The majority of the shells is only visible at 24µm, but many of the central stars are detected at multiple wavelengths from the midto the near-IR regime. The general lack of optical counterparts, however, indicates that these sources represent a population of highly obscured objects. We obtained optical and near-IR spectroscopic observations of the central stars and find most of these objects to be massive stars. In particular, we identify a large population of sources that we argue represents a narrow evolutionary phase, closely related or identical to the LBV stage of massive stellar evolution.

Classical novae are runaway thermonuclear burning events on the surfaces of accreting white dwarfs in close binary star systems, sometimes appearing as new naked-eye sources in the night sky 1 . The standard model of novae predicts that their optical luminosity derives from energy released near the hot white dwarf which is reprocessed through the ejected material [2][3][4][5] . Recent studies with the Fermi Large Area Telescope have shown that many classical novae are accompanied by gigaelectronvolt γ-ray emission 6, 7 . This emission likely originates from strong shocks, providing new insights into the properties of nova outflows and allowing them to be used as laboratories to study the unknown efficiency of particle acceleration in shocks. Here we report γ-ray and optical observations of the Milky Way nova ASASSN-16ma, which is among the brightest novae ever detected in γ-rays. The γ-ray and optical light curves show a remarkable correlation, implying that the majority of the optical light comes from reprocessed emission from shocks rather than the white dwarf 8 . The ratio of γ-ray to optical flux in ASASSN-16ma directly constrains the acceleration efficiency of non-thermal particles to be ∼ 0.005, favouring hadronic models for the γ-ray emission 9 .The need to accelerate particles up to energies exceeding 100 gigaelectronvolts provides compelling evidence for magnetic field amplification in the shocks.ASASSN-16ma (a.k.a. PNV J18205200−2822100, Nova Sgr 2016d, and V5856 Sgr) is an optical transient source in the constellation Sagittarius, discovered by the All Sky Automated Survey for SuperNovae (ASAS-SN 10 ), on 25.02 October 2016 UT 11 (a corresponding Modified Julian Day of MJD 57686.02) and identified as a normal classical nova with optical spectroscopy 12, 13 .The optical light curve of the nova after its discovery shows three distinct phases (Figure 1). In Phase I, the nova slowly rose to m V ∼ 8 mag over two weeks. It then showed a rapid brightening by a factor of ∼ 10 over just two days (Phase II), reaching a naked-eye peak visual magnitude of 5.4 (MJD 57700). This was followed by a relatively stable decline lasting for several weeks (Phase III; see Figure 1 and Methods).Immediately following the optical peak, our Fermi target-of-opportunity (ToO) observation detected strong γ-ray emission from the nova with a very high photon flux of F ph,γ ≈ 2 10 −6 ph cm −2 s −1 (Methods). The γ-ray emission faded rapidly over the next nine days, with only marginal γ-ray detections in the following week. This is among the fastest-evolving γ-ray light curves seen to date from a nova. The optical and γ-ray light curves are tightly correlated, declining at the same rate and showing a simultaneous dip in the emission around MJD 57705 (Figure 1). The ratio of the γ-ray to optical luminosity (∼ 0.002) remains constant while the γ-rays are detectable (Figure 1; see also Supplementary Information, SI hereafter).The clear correlation between the γ-ray and optical light in ASASSN-16ma leads us to reconsider the standard mod...

We present HST spectroscopy for 45 cataclysmic variables (CVs), observed with HST/COS and HST/STIS. For 36 CVs, the white dwarf is recognisable through its broad Ly α absorption profile and we measure the white dwarf effective temperatures (T eff ) by fitting the HST data assuming log g = 8.35, which corresponds to the average mass for CV white dwarfs ( 0.8 M ). Our results nearly double the number of CV white dwarfs with an accurate temperature measurement. We find that CVs above the period gap have, on average, higher temperatures ( T eff 23 000 K) and exhibit much more scatter compared to those below the gap ( T eff 15 000 K). While this behaviour broadly agrees with theoretical predictions, some discrepancies are present: (i) all our new measurements above the gap are characterized by lower temperatures (T eff 16 000-26 000 K) than predicted by the present-day CV population models (T eff 38 000-43 000 K); (ii) our results below the gap are not clustered in the predicted narrow track and exhibit in particular a relatively large spread near the period minimum, which may point to some shortcomings in the CV evolutionary models. Finally, in the standard model of CV evolution, reaching the minimum period, CVs are expected to evolve back towards longer periods with mean accretion ratesṀ 2 × 10 −11 M yr −1 , corresponding to T eff 11 500 K. We do not unambiguously identify any such system in our survey, suggesting that this major component of the predicted CV population still remains elusive to observations.

The Kilodegree Extremely Little Telescope (KELT) has been surveying more than 70% of the celestial sphere for nearly a decade. While the primary science goal of the survey is the discovery of transiting, large-radii planets around bright host stars, the survey has collected more than 10 6 images, with a typical cadence between 10 − 30 minutes, for more than 4 million sources with apparent visual magnitudes in the approximate range 7 < V < 13. Here we provide a catalog of 52,741 objects showing significant large-amplitude fluctuations likely caused by stellar variability and 62,229 objects identified with likely stellar rotation periods. The detected variability ranges in rms-amplitude from 3 mmag to 2.3 mag, and the detected periods range from ∼0.1 days to 2000 days. We provide variability upper limits for all other ∼4 million sources. These upper limits are principally a function of stellar brightness, but we achieve typical 1σ sensitivity on 30-minute timescales down to ∼5 mmag at V ∼ 8, and down to ∼43 mmag at V ∼ 13. We have matched our catalog to the TESS Input catalog and the AAVSO Variable Star Index to precipitate the follow up and classification of each source. The catalog is maintained as a living database on the Filtergraph visualization portal at the URL https://filtergraph.com/kelt_vars.