Explore the vast range of titles available.

Internal stellar magnetic fields are inaccessible to direct observations, and little is known about their amplitude, geometry, and evolution. We demonstrate that strong magnetic fields in the cores of red giant stars can be identified with asteroseismology. The fields can manifest themselves via depressed dipole stellar oscillation modes, arising from a magnetic greenhouse effect that scatters and traps oscillation-mode energy within the core of the star. The Kepler satellite has observed a few dozen red giants with depressed dipole modes, which we interpret as stars with strongly magnetized cores. We find that field strengths larger than ∼105 gauss may produce the observed depression, and in one case we infer a minimum core field strength of ≈107 gauss.

Luminous blue variables are massive, evolved stars that exhibit large variations in luminosity and size on timescales from months to years, with high associated rates of mass loss 1 – 5 . In addition to this on-going variability, these stars exhibit outburst phases, during which their size increases and as a result their effective temperature decreases, typically to about 9,000 kelvin 3 , 6 . Outbursts are believed to be caused by the radiation force on the cooler, more opaque, outer layers of the star balancing or even exceeding the force of gravity, although the exact mechanisms are unknown and cannot be determined using one-dimensional, spherically symmetric models of stars because such models cannot determine the physical processes that occur in this regime 7 . Here we report three-dimensional simulations of massive, radiation-dominated stars, which show that helium opacity has an important role in triggering outbursts and setting the observed effective temperature during outbursts of about 9,000 kelvin. It probably also triggers the episodic mass loss at rates of 10 −7 to 10 −5 solar masses per year. The peak in helium opacity is evident in our three-dimensional simulations only because the density and temperature of the stellar envelope (the outer part of the star near the photosphere) need to be determined self-consistently with convection, which cannot be done in one-dimensional models that assume spherical symmetry. The simulations reproduce observations of long-timescale variability, and predict that convection causes irregular oscillations in the radii of the stars and variations in brightness of 10–30 per cent on a typical timescale of a few days. The amplitudes of these short-timescale variations are predicted to be even larger for cooler stars (in the outburst phase). This short-timescale variability should be observable with high-cadence observations. Three-dimensional simulations of the convective envelopes of massive stars suggest that it is the helium opacity that controls outbursts in luminous blue variable stars.

General relativity 1 predicts that short-orbital-period binaries emit considerable amounts of gravitational radiation. The upcoming Laser Interferometer Space Antenna 2 (LISA) is expected to detect tens of thousands of such systems 3 but few have been identified 4 , of which only one 5 is eclipsing—the double-white-dwarf binary SDSS J065133.338+284423.37, which has an orbital period of 12.75 minutes. Here we report the discovery of an eclipsing double-white-dwarf binary system, ZTF J153932.16+502738.8, with an orbital period of 6.91 minutes. This system has an orbit so compact that the entire binary could fit within the diameter of the planet Saturn. The system exhibits a deep eclipse, and a double-lined spectroscopic nature. We see rapid orbital decay, consistent with that expected from general relativity. ZTF J153932.16+502738.8 is a strong source of gravitational radiation close to the peak of LISA’s sensitivity, and we expect it to be detected within the first week of LISA observations, once LISA launches in approximately 2034. Observations of an eclipsing double-white-dwarf binary with an orbital period of 6.91 minutes that is decaying as predicted by general relativity are reported; once launched, the Laser Interferometer Space Antenna (LISA) should swiftly detect this binary.

Archival images of the progenitor system of supernova SN 2011fe are so sensitive that the presence of luminous red giants or most helium stars is directly ruled out. Identification of a supernova companion Supernova 2011fe in the Pinwheel galaxy, discovered by the Palomar Transient Factory on 24 August 2011, is the brightest type Ia supernova that's been seen from Earth for many years. Type Ia supernovae are thought to result from a thermonuclear explosion of an accreting white dwarf in a binary system, but little is known of the precise nature of the companion star and the physical properties of the progenitor system. Two new reports of observations of SN 2011fe narrow down the range of possibilities for the mystery companion. Nugent et al . present some of the earliest data ever obtained from a type Ia supernova. They find that the exploding star was probably a carbon–oxygen white dwarf, and conclude from the lack of an early shock that the companion may have been a main sequence star. Li et al . analysed pre-discovery images in the Hubble Space Telescope archives and find that no object was visible before the explosion. That rules out luminous red giants and the vast majority of helium stars as the mass-donating companion to an exploding white dwarf. Type Ia supernovae are thought to result from a thermonuclear explosion of an accreting white dwarf in a binary system 1 , 2 , but little is known of the precise nature of the companion star and the physical properties of the progenitor system. There are two classes of models 1 , 3 : double-degenerate (involving two white dwarfs in a close binary system 2 , 4 ) and single-degenerate models 5 , 6 . In the latter, the primary white dwarf accretes material from a secondary companion until conditions are such that carbon ignites, at a mass of 1.38 times the mass of the Sun. The type Ia supernova SN 2011fe was recently detected in a nearby galaxy 7 . Here we report an analysis of archival images of the location of SN 2011fe. The luminosity of the progenitor system (especially the companion star) is 10–100 times fainter than previous limits on other type Ia supernova progenitor systems 8 , 9 , 10 , allowing us to rule out luminous red giants and almost all helium stars as the mass-donating companion to the exploding white dwarf.

Suppression of dipolar oscillation modes by strong magnetic fields in the cores of intermediate-mass red giant stars reveals that powerful magnetic dynamos were very common in the previously convective cores of these stars. Core magnetic fields in intermediate-mass stars Stellar magnetic fields are present on the surfaces and in the immediate surroundings of stars such as the Sun, and it has been conjectured that magnetic fields also exist deep within stars, where they may have a major effect on stellar evolution. Dennis Stello et al . report observations of dipolar oscillation modes of 3,600 intermediate-mass red giant stars that suggest the presence of strong internal magnetic fields in 60 per cent of the sample. About 20 per cent of the sample show mode suppression from strong magnetic fields in the cores, but this fraction is a strong function of mass. Strong core fields only occur in red giants above 1.1 solar masses. This result demonstrates that strong magnetic fields in stars are much more common than previously thought. Magnetic fields play a part in almost all stages of stellar evolution 1 . Most low-mass stars, including the Sun, show surface fields that are generated by dynamo processes in their convective envelopes 2 , 3 . Intermediate-mass stars do not have deep convective envelopes 4 , although 10 per cent exhibit strong surface fields that are presumed to be residuals from the star formation process 5 . These stars do have convective cores that might produce internal magnetic fields 6 , and these fields might survive into later stages of stellar evolution, but information has been limited by our inability to measure the fields below the stellar surface 7 . Here we report the strength of dipolar oscillation modes for a sample of 3,600 red giant stars. About 20 per cent of our sample show mode suppression, by strong magnetic fields in the cores 8 , but this fraction is a strong function of mass. Strong core fields occur only in red giants heavier than 1.1 solar masses, and the occurrence rate is at least 50 per cent for intermediate-mass stars (1.6–2.0 solar masses), indicating that powerful dynamos were very common in the previously convective cores of these stars.

The detection of the luminous, blue progenitor system of the type Iax supernova 2012Z suggests that this supernova was the explosion of a white dwarf accreting material from a helium-star companion. A type Iax supernova progenitor SN 2012Z, discovered in the Lick Observatory Supernova Search on 29 January 2012, is a type Iax supernova. Sometimes referred to as 'mini supernovae', these are initially spectroscopically similar to some type-Ia supernovae but diverge with time and are much less energetic and fainter. It is not clear what triggers a type Iax explosion. This paper reports the detection of a progenitor in deep observations of NGC 1309, the host galaxy of SN 2012Z, obtained with the Hubble Space Telescope and including the location of the supernova before its explosion. Its optical properties and similarity to the progenitor of the helium nova V445 Puppis suggest that SN 2012Z was probably an explosion of a white dwarf accreting from a helium-star companion. Type Iax supernovae are stellar explosions that are spectroscopically similar to some type Ia supernovae at the time of maximum light emission, except with lower ejecta velocities 1 , 2 . They are also distinguished by lower luminosities. At late times, their spectroscopic properties diverge from those of other supernovae 3 , 4 , 5 , 6 , but their composition (dominated by iron-group and intermediate-mass elements 1 , 7 ) suggests a physical connection to normal type Ia supernovae. Supernovae of type Iax are not rare; they occur at a rate between 5 and 30 per cent of the normal type Ia rate 1 . The leading models for type Iax supernovae are thermonuclear explosions of accreting carbon–oxygen white dwarfs that do not completely unbind the star 8 , 9 , 10 , implying that they are ‘less successful’ versions of normal type Ia supernovae, where complete stellar disruption is observed. Here we report the detection of the luminous, blue progenitor system of the type Iax SN 2012Z in deep pre-explosion imaging. The progenitor system's luminosity, colours, environment and similarity to the progenitor of the Galactic helium nova V445 Puppis 11 , 12 , 13 suggest that SN 2012Z was the explosion of a white dwarf accreting material from a helium-star companion. Observations over the next few years, after SN 2012Z has faded, will either confirm this hypothesis or perhaps show that this supernova was actually the explosive death of a massive star 14 , 15 .

Observations of an event (several energetic eruptions leading to a terminal explosion that is surprisingly hydrogen-rich) with the spectrum of a supernova do not match with other observations of supernovae. A very unusual supernova Thousands of 'core-collapse' supernovae have been observed over the past 15 years, with common observational elements such as hydrogen absorption lines that slow over time and a single light-curve peak or luminosity that plateaus for around 100 days before declining. Iair Arcavi and colleagues report observations of the supernova iPTF14hls, which does not display the usual elements. Its light curve has multiple peaks and extends over 600 days. They conclude that the properties could be explained by ejection of several tens of solar masses of gas a few hundred days before the explosion, but there is no viable explanation for how this occurred. Although multiple pre-supernova eruptions are predicted by the pulsational pair instability, that model is inconsistent with the energetics involved here and the continued presence of hydrogen absorption lines with no decrease in velocity. Every supernova so far observed has been considered to be the terminal explosion of a star. Moreover, all supernovae with absorption lines in their spectra show those lines decreasing in velocity over time, as the ejecta expand and thin, revealing slower-moving material that was previously hidden. In addition, every supernova that exhibits the absorption lines of hydrogen has one main light-curve peak, or a plateau in luminosity, lasting approximately 100 days before declining 1 . Here we report observations of iPTF14hls, an event that has spectra identical to a hydrogen-rich core-collapse supernova, but characteristics that differ extensively from those of known supernovae. The light curve has at least five peaks and remains bright for more than 600 days; the absorption lines show little to no decrease in velocity; and the radius of the line-forming region is more than an order of magnitude bigger than the radius of the photosphere derived from the continuum emission. These characteristics are consistent with a shell of several tens of solar masses ejected by the progenitor star at supernova-level energies a few hundred days before a terminal explosion. Another possible eruption was recorded at the same position in 1954. Multiple energetic pre-supernova eruptions are expected to occur in stars of 95 to 130 solar masses, which experience the pulsational pair instability 2 , 3 , 4 , 5 . That model, however, does not account for the continued presence of hydrogen, or the energetics observed here. Another mechanism for the violent ejection of mass in massive stars may be required.

The Palomar Transient Factory (PTF) is a wide-field experiment designed to investigate the optical transient and variable sky on time scales from minutes to years. PTF uses the CFH12k mosaic camera, with a field of view of7.9 deg2 7.9 deg 2 and a plate scale of1″ pixel-1 1 ″     pixel - 1 , mounted on the Palomar Observatory 48 inch Samuel Oschin Telescope. The PTF operation strategy is devised to probe the existing gaps in the transient phase space and to search for theoretically predicted, but not yet detected, phenomena, such as fallback supernovae, macronovae, .Ia supernovae, and the orphan afterglows of gamma-ray bursts. PTF will also discover many new members of known source classes, from cataclysmic variables in their various avatars to supernovae and active galactic nuclei, and will provide important insights into understanding galactic dynamics (through RR Lyrae stars) and the solar system (asteroids and near-Earth objects). The lessons that can be learned from PTF will be essential for the preparation of future large synoptic sky surveys like the Large Synoptic Survey Telescope. In this article we present the scientific motivation for PTF and describe in detail the goals and expectations for this experiment.

Multi-instrument detection of a nearby type 1a supernova shows that the exploding star was probably a carbon–oxygen white dwarf star in a binary system with a main-sequence companion. Identification of a supernova companion Supernova 2011fe in the Pinwheel galaxy, discovered by the Palomar Transient Factory on 24 August 2011, is the brightest type Ia supernova that's been seen from Earth for many years. Type Ia supernovae are thought to result from a thermonuclear explosion of an accreting white dwarf in a binary system, but little is known of the precise nature of the companion star and the physical properties of the progenitor system. Two new reports of observations of SN 2011fe narrow down the range of possibilities for the mystery companion. Nugent et al . present some of the earliest data ever obtained from a type Ia supernova. They find that the exploding star was probably a carbon–oxygen white dwarf, and conclude from the lack of an early shock that the companion may have been a main sequence star. Li et al . analysed pre-discovery images in the Hubble Space Telescope archives and find that no object was visible before the explosion. That rules out luminous red giants and the vast majority of helium stars as the mass-donating companion to an exploding white dwarf. Type Ia supernovae have been used empirically as ‘standard candles’ to demonstrate the acceleration of the expansion of the Universe 1 , 2 , 3 even though fundamental details, such as the nature of their progenitor systems and how the stars explode, remain a mystery 4 , 5 , 6 . There is consensus that a white dwarf star explodes after accreting matter in a binary system, but the secondary body could be anything from a main-sequence star to a red giant, or even another white dwarf. This uncertainty stems from the fact that no recent type Ia supernova has been discovered close enough to Earth to detect the stars before explosion. Here we report early observations of supernova SN 2011fe in the galaxy M101 at a distance 7 from Earth of 6.4 megaparsecs. We find that the exploding star was probably a carbon–oxygen white dwarf, and from the lack of an early shock we conclude that the companion was probably a main-sequence star. Early spectroscopy shows high-velocity oxygen that slows rapidly, on a timescale of hours, and extensive mixing of newly synthesized intermediate-mass elements in the outermost layers of the supernova. A companion paper 8 uses pre-explosion images to rule out luminous red giants and most helium stars as companions to the progenitor.

The Palomar Transient Factory (PTF) is a fully-automated, wide-field survey aimed at a systematic exploration of the optical transient sky. The transient survey is performed using a new 8.1 square degree camera installed on the 48 inch Samuel Oschin telescope at Palomar Observatory; colors and light curves for detected transients are obtained with the automated Palomar 60 inch telescope. PTF uses 80% of the 1.2 m and 50% of the 1.5 m telescope time. With an exposure of 60 s the survey reaches a depth of m g ′  ≈ 21.3 m g ′ ≈ 21.3 and m R  ≈ 20.6 m R ≈ 20.6 (5σ, median seeing). Four major experiments are planned for the five-year project: (1) a 5 day cadence supernova search; (2) a rapid transient search with cadences between 90 s and 1 day; (3) a search for eclipsing binaries and transiting planets in Orion; and (4) a 3π sr deep H-alpha survey. PTF provides automatic, real-time transient classification and follow-up, as well as a database including every source detected in each frame. This paper summarizes the PTF project, including several months of on-sky performance tests of the new survey camera, the observing plans, and the data reduction strategy. We conclude by detailing the first 51 PTF optical transient detections, found in commissioning data.