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The core values of red giant stars The core rotation rate of a star, a key indicator of its evolutionary state, cannot be measured directly because the core is inaccessible to direct observation. This paper presents a method for calculating core rotation in an evolved star. The Fourier spectra of brightness variations of four stars derived from Kepler spacecraft data were used to measure the rotational frequency splitting of the recently identified 'mixed modes' caused by rotation in red giant stars. The results suggest that the core of a red giant rotates at least ten times faster than the surface. When the core hydrogen is exhausted during stellar evolution, the central region of a star contracts and the outer envelope expands and cools, giving rise to a red giant. Convection takes place over much of the star’s radius. Conservation of angular momentum requires that the cores of these stars rotate faster than their envelopes; indirect evidence supports this 1 , 2 . Information about the angular-momentum distribution is inaccessible to direct observations, but it can be extracted from the effect of rotation on oscillation modes that probe the stellar interior. Here we report an increasing rotation rate from the surface of the star to the stellar core in the interiors of red giants, obtained using the rotational frequency splitting of recently detected ‘mixed modes’ 3 , 4 . By comparison with theoretical stellar models, we conclude that the core must rotate at least ten times faster than the surface. This observational result confirms the theoretical prediction of a steep gradient in the rotation profile towards the deep stellar interior 1 , 5 , 6 .

The dust shells of three intermediate-mass stars are observed to lie remarkably close to the photosphere and to be composed of unexpectedly large grains, consistent with mass loss from such stars occurring by means of ejection of this dust by photon scattering rather than as a result of radiation pressure. Last gasp of a red giant Towards the ends of their lives, intermediate-mass stars lose much of their mass in the form of gas and dust ejected in a slow, dense wind. The underlying processes driving these outflows are poorly understood, owing in part to difficulties in observing such ejected material. Norris et al . use an innovative technique that combines interferometric imaging with high-precision differential polarimetry to observe three red giants. Their images reveal circumstellar dust shells with remarkably small radii (less than two times the radius of the star), made up of unexpectedly large dust grains approximately 300 nanometres in radius. The authors suggest that these observations support a wind-driving model based on acceleration of dust grains by the scattering, rather than absorption, of starlight. An intermediate-mass star ends its life by ejecting the bulk of its envelope in a slow, dense wind 1 , 2 , 3 . Stellar pulsations are thought to elevate gas to an altitude cool enough for the condensation of dust 1 , which is then accelerated by radiation pressure, entraining the gas and driving the wind 2 , 4 , 5 . Explaining the amount of mass loss, however, has been a problem because of the difficulty of observing tenuous gas and dust only tens of milliarcseconds from the star. For this reason, there is no consensus on the way sufficient momentum is transferred from the light from the star to the outflow. Here we report spatially resolved, multiwavelength observations of circumstellar dust shells of three stars on the asymptotic giant branch of the Hertzsprung–Russell diagram. When imaged in scattered light, dust shells were found at remarkably small radii (less than about two stellar radii) and with unexpectedly large grains (about 300 nanometres in radius). This proximity to the photosphere argues for dust species that are transparent to the light from the star and, therefore, resistant to sublimation by the intense radiation field. Although transparency usually implies insufficient radiative pressure to drive a wind 6 , 7 , the radiation field can accelerate these large grains through photon scattering rather than absorption 8 —a plausible mass loss mechanism for lower-amplitude pulsating stars.

Acoustic oscillations in stars are sensitive to stellar interiors 1 . Frequency differences between overtone modes—large separations—probe stellar density 2 , whereas differences between low-degree modes—small separations—probe the sound-speed gradient in the energy-generating core of main-sequence Sun-like stars 3 , and hence their ages. At later phases of stellar evolution, characterized by inert cores, small separations are believed to lose much of their power to probe deep interiors and become proportional to large separations 4 , 5 . Here we present evidence of a rapidly evolving convective zone as stars evolve from the subgiant phase into red giants. By measuring acoustic oscillations in 27 stars from the open cluster M67, we observe deviations of proportionality between small and large separations, which are caused by the influence of the bottom of the convective envelope. These deviations become apparent as the convective envelope penetrates deep into the star during subgiant and red giant evolutions, eventually entering an ultradeep regime that leads to the red-giant-branch luminosity bump. The tight sequence of cluster stars, free of large spreads in ages and fundamental properties, is essential for revealing the connection between the observed small separations and the chemical discontinuities occurring at the bottom of the convective envelope. We use this sequence to show that combining large and small separations can improve estimations of the masses and ages of field stars well after the main sequence. Measuring acoustic oscillations in 27 stars within the M67 cluster presents evidence of a rapidly evolving convective zone as stars evolve from subgiants to red giants.

Asteroseismology probes the internal structures of stars by using their natural pulsation frequencies 1 . It relies on identifying sequences of pulsation modes that can be compared with theoretical models, which has been done successfully for many classes of pulsators, including low-mass solar-type stars 2 , red giants 3 , high-mass stars 4 and white dwarfs 5 . However, a large group of pulsating stars of intermediate mass—the so-called δ Scuti stars—have rich pulsation spectra for which systematic mode identification has not hitherto been possible 6 , 7 . This arises because only a seemingly random subset of possible modes are excited and because rapid rotation tends to spoil regular patterns 8 – 10 . Here we report the detection of remarkably regular sequences of high-frequency pulsation modes in 60 intermediate-mass main-sequence stars, which enables definitive mode identification. The space motions of some of these stars indicate that they are members of known associations of young stars, as confirmed by modelling of their pulsation spectra. The pulsation spectra of intermediate-mass stars (so-called δ Scuti stars) have been challenging to analyse, but new observations of 60 such stars reveal remarkably regular sequences of high-frequency pulsation modes.

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.

We use photometry from the Kepler Mission to study oscillations in γ Do radus stars. Some stars show remarkably clear sequences of g modes and we use period échelle diagrams to measure period spacings and identify rotationally split multiplets with ℓ = 1 and ℓ = 2. We find small deviations from regular period spacings that arise from the gradient in the chemical composition just outside the convective core. We also find stars for which the period spacing shows a strong linear trend as a function of period, consistent with relatively rapid rotation. Overall, the results indicate it will be possible to apply asteroseismology to a range of γ Dor stars.

Kepler-based asteroseismology sorts red giants NASA's Kepler mission has been remarkably productive in its primary role, that of discovering and characterizing extrasolar planets. It does this indirectly, by monitoring the brightness of many thousands of main sequence stars in search of periodic fluctuations caused by planets crossing the face of the stars. But the high-precision photometry involved is also ideal for studying the stars themselves. Bedding et al . have used Kepler data to probe the internal structure of red giants. Their detailed measurements of the gravity modes in the cores of these stars allow them to distinguish between those burning hydrogen in a shell around a relatively inactive core and those burning helium in the core. Red giants are evolved stars that have exhausted the supply of hydrogen in their cores and instead burn hydrogen in a surrounding shell. Once a red giant is sufficiently evolved, the helium in the core also undergoes fusion. However, it is difficult to distinguish between the two groups. Asteroseismology offers a way forward. This study reports observations of gravity-mode period spacings in red giants using high precision photometry obtained by the Kepler spacecraft. It is found that the stars fall into two clear groups, making it possible to distinguish unambiguously between hydrogen-shell-burning stars and those that are also burning helium. Red giants are evolved stars that have exhausted the supply of hydrogen in their cores and instead burn hydrogen in a surrounding shell 1 , 2 . Once a red giant is sufficiently evolved, the helium in the core also undergoes fusion 3 . Outstanding issues in our understanding of red giants include uncertainties in the amount of mass lost at the surface before helium ignition and the amount of internal mixing from rotation and other processes 4 . Progress is hampered by our inability to distinguish between red giants burning helium in the core and those still only burning hydrogen in a shell. Asteroseismology offers a way forward, being a powerful tool for probing the internal structures of stars using their natural oscillation frequencies 5 . Here we report observations of gravity-mode period spacings in red giants 6 that permit a distinction between evolutionary stages to be made. We use high-precision photometry obtained by the Kepler spacecraft over more than a year to measure oscillations in several hundred red giants. We find many stars whose dipole modes show sequences with approximately regular period spacings. These stars fall into two clear groups, allowing us to distinguish unambiguously between hydrogen-shell-burning stars (period spacing mostly ∼50 seconds) and those that are also burning helium (period spacing ∼100 to 300 seconds).

The Minerva-Australis telescope array is a facility dedicated to the follow-up, confirmation, characterization, and mass measurement of planets orbiting bright stars discovered by the Transiting Exoplanet Survey Satellite (TESS)-a category in which it is almost unique in the Southern Hemisphere. It is located at the University of Southern Queensland's Mount Kent Observatory near Toowoomba, Australia. Its flexible design enables multiple 0.7 m robotic telescopes to be used both in combination, and independently, for high-resolution spectroscopy and precision photometry of TESS transit planet candidates. Minerva-Australis also enables complementary studies of exoplanet spin-orbit alignments via Doppler observations of the Rossiter-McLaughlin effect, radial velocity searches for nontransiting planets, planet searches using transit timing variations, and ephemeris refinement for TESS planets. In this first paper, we describe the design, photometric instrumentation, software, and science goals of Minerva-Australis, and note key differences from its Northern Hemisphere counterpart, the Minerva array. We use recent transit observations of four planets, WASP-2b, WASP-44b, WASP-45b, and HD 189733b, to demonstrate the photometric capabilities of Minerva-Australis.

Stars hosting hot Jupiters are often observed to have high obliquities, whereas stars with multiple coplanar planets have been seen to have low obliquities. This has been interpreted as evidence that hot-Jupiter formation is linked to dynamical disruption, as opposed to planet migration through a protoplanetary disk. We used asteroseismology to measure a large obliquity for Kepler-56, a red giant star hosting two transiting coplanar planets. These observations show that spin-orbit misalignments are not confined to hot-Jupiter systems. Misalignments in a broader class of systems had been predicted as a consequence of torques from wide-orbiting companions, and indeed radial velocity measurements revealed a third companion in a wide orbit in the Kepler-56 system.

A star expands to become a red giant when it has fused all the hydrogen in its core into helium. If the star is in a binary system, its envelope can overflow onto its companion or be ejected into space, leaving a hot core and potentially forming a subdwarf B star 1 – 3 . However, most red giants that have partially transferred envelopes in this way remain cool on the surface and are almost indistinguishable from those that have not. Among ~7,000 helium-burning red giants observed by NASA’s Kepler mission, we use asteroseismology to identify two classes of stars that must have undergone considerable mass loss, presumably due to stripping in binary interactions. The first class comprises about seven underluminous stars with smaller helium-burning cores than their single-star counterparts. Theoretical models show that these small cores imply the stars had much larger masses when ascending the red giant branch. The second class consists of 32 red giants with masses down to 0.5  M ⊙ , whose implied ages would exceed the age of the universe had no mass loss occurred. The numbers are consistent with binary statistics, and our results open up new possibilities to study the evolution of post-mass-transfer binary systems. Using asteroseismology to analyse 7,000 helium-burning red giants observed by NASA’s Kepler mission results in the separation of two classes of stars that must have undergone considerable mass loss, presumably due to stripping in binary interactions.