γ Cas stars are a ∼1% minority among classical Be stars with hard (≥5−10 keV), but only moderately strong continuous thermal X-ray flux, and mostly very early-B spectral type. The X-ray flux has been ...suggested to originate from matter accelerated via magnetic disc-star interaction, by a rapidly rotating neutron star (NS) companion via the propeller effect, or by accretion onto a white dwarf (WD) companion. In view of the growing number of identified γ Cas stars and the only imperfect matches between these suggestions and the observations, alternative models should be pursued. Two of the three best-observed γ Cas stars, γ Cas itself and π Aqr , have a low-mass companion with low optical flux, whereas interferometry of BZ Cru is inconclusive. Binary-evolution models are examined for their ability to produce such systems. The OB+He-star stage of post-mass transfer binaries, which is otherwise observationally unaccounted, can potentially reproduce many observed properties of γ Cas stars. The interaction of the fast wind of helium stars with the circumstellar disc and/or with the wind of Be stars may give rise to the production of hard X-rays. While not modelling this process, it is shown that the energy budget is favourable, and that the wind velocities may lead to hard X-rays, as observed in γ Cas stars. Furthermore, the observed number of these objects appears to be consistent with the evolutionary models. Within the Be+He-star binary model, the Be stars in γ -Cas stars are conventional classical Be stars. They are encompassed by O-star+Wolf-Rayet systems towards higher mass, where no stable Be decretion discs exist, and by Be+sdO systems at lower mass, where the sdO winds may be too weak to cause the γ Cas phenomenon. In decreasing order of the helium-star mass, the descendants could be Be+black-hole, Be+NS, or Be+WD binaries. The interaction between the helium-star wind and the disc may provide new diagnostics of the outer disc.
Stars stripped of their hydrogen-rich envelope through interaction with a binary companion are generally not considered when accounting for ionizing radiation from stellar populations, despite the ...expectation that stripped stars emit hard ionizing radiation, form frequently, and live 10–100 times longer than single massive stars. We compute the first grid of evolutionary and spectral models specially made for stars stripped in binaries for a range of progenitor masses (2–20 M⊙) and metallicities ranging from solar to values representative for pop II stars. For stripped stars with masses in the range 0.3–7 M⊙, we find consistently high effective temperatures (20 000–100 000 K, increasing with mass), small radii (0.2–1 R⊙), and high bolometric luminosities, comparable to that of their progenitor before stripping. The spectra show a continuous sequence that naturally bridges subdwarf-type stars at the low-mass end and Wolf-Rayet-like spectra at the high-mass end. For intermediate masses we find hybrid spectral classes showing a mixture of absorption and emission lines. These appear for stars with mass-loss rates of 10−8−10−6 M⊙ yr−1, which have semi-transparent atmospheres. At low metallicity, substantial hydrogen-rich layers are left at the surface and we predict spectra that resemble O-type stars instead. We obtain spectra undistinguishable from subdwarfs for stripped stars with masses up to 1.7 M⊙, which questions whether the widely adopted canonical value of 0.47 M⊙ is uniformly valid. Only a handful of stripped stars of intermediate mass have currently been identified observationally. Increasing this sample will provide necessary tests for the physics of interaction, internal mixing, and stellar winds. We use our model spectra to investigate the feasibility to detect stripped stars next to an optically bright companion and recommend systematic searches for their UV excess and possible emission lines, most notably HeII λ4686 in the optical and HeII λ1640 in the UV. Our models are publicly available for further investigations or inclusion in spectral synthesis simulations.
The young cluster NGC 6231 (stellar ages ∼2-7 Myr) is observed shortly after star formation activity has ceased. Using the catalog of 2148 probable cluster members obtained from Chandra, VVV, and ...optical surveys (Paper I), we examine the cluster's spatial structure and dynamical state. The spatial distribution of stars is remarkably well fit by an isothermal sphere with moderate elongation, while other commonly used models like Plummer spheres, multivariate normal distributions, or power-law models are poor fits. The cluster has a core radius of 1.2 0.1 pc and a central density of ∼200 stars pc−3. The distribution of stars is mildly mass segregated. However, there is no radial stratification of the stars by age. Although most of the stars belong to a single cluster, a small subcluster of stars is found superimposed on the main cluster, and there are clumpy non-isotropic distributions of stars outside ∼4 core radii. When the size, mass, and age of NGC 6231 are compared to other young star clusters and subclusters in nearby active star-forming regions, it lies at the high-mass end of the distribution but along the same trend line. This could result from similar formation processes, possibly hierarchical cluster assembly. We argue that NGC 6231 has expanded from its initial size but that it remains gravitationally bound.
Aims.
We study the evolution of the rotation and the high energy X-ray, extreme ultraviolet (EUV), and Ly-
α
emission for F, G, K, and M dwarfs, with masses between 0.1 and 1.2
M
⊙
, and provide a ...freely available set of evolutionary tracks for use in planetary atmosphere studies.
Methods.
We develop a physical rotational evolution model constrained by observed rotation distributions in young stellar clusters. Using rotation, X-ray, EUV, and Ly-
α
measurements, we derive empirical relations for the dependences of high energy emission on stellar parameters. Our description of X-ray evolution is validated using measurements of X-ray distributions in young clusters.
Results.
A star’s X-ray, EUV, and Ly-
α
evolution is determined by its mass and initial rotation rate, with initial rotation being less important for lower mass stars. At all ages, solar mass stars are significantly more X-ray luminous than lower mass stars and stars that are born as rapid rotators remain highly active longer than those born as slow rotators. At all evolutionary stages, habitable zone planets receive higher X-ray and EUV fluxes when orbiting lower mass stars due to their longer evolutionary timescales. The rates of flares follow similar evolutionary trends with higher mass stars flaring more often than lower mass stars at all ages, though habitable zone planets are likely influenced by flares more when orbiting lower mass stars.
Conclusions.
Our results show that single decay laws are insufficient to describe stellar activity evolution and highlight the need for a more comprehensive description based on the evolution of rotation that also includes the effects of short-term variability. Planets at similar orbital distances from their host stars receive significantly more X-ray and EUV energy over their lifetimes when orbiting higher mass stars. The common belief that M dwarfs are more X-ray and EUV active than G dwarfs is justified only when considering the fluxes received by planets with similar effective temperatures, such as those in the habitable zone.
Abstract
It is unknown whether or not low-mass stars can form at low metallicity. While theoretical simulations of Population III (Pop III) star formation show that protostellar disks can fragment, ...it is impossible for those simulations to discern if those fragments survive as low-mass stars. We report the discovery of a low-mass star on a circular orbit with orbital period
P
= 34.757 ± 0.010 days in the ultra metal-poor (UMP) single-lined spectroscopic binary system 2MASS J18082002–5104378. The secondary star 2MASS J18082002–5104378 B has a mass
M
2
=
0.14
−
0.01
+
0.06
M
⊙
, placing it near the hydrogen-burning limit for its composition. The 2MASS J18082002–5104378 system is on a thin disk orbit as well, making it the most metal-poor thin disk star system by a considerable margin. The discovery of 2MASS J18082002–5104378 B confirms the existence of low-mass UMP stars and its short orbital period shows that fragmentation in metal-poor protostellar disks can lead to the formation and survival of low-mass stars. We use scaling relations for the typical fragment mass and migration time along with published models of protostellar disks around both UMP and primordial composition stars to explore the formation of low-mass Pop III stars via disk fragmentation. We find evidence that the survival of low-mass secondaries around solar-mass UMP primaries implies the survival of solar-mass secondaries around Pop III primaries with masses
10
M
⊙
≲
M
*
≲
100
M
⊙
. If true, this inference suggests that solar-mass Pop III stars formed via disk fragmentation could survive to the present day.
We extract light curves for 4554 objects with 9 < G < 19 in the K2 superstamp observations of the globular cluster M4, including 3784 cluster members, and search for variability. Among cluster-member ...objects, we detect 66 variables, of which 52 are new discoveries. Among objects not belonging to the cluster, we detect 24 variables, of which 20 are new discoveries. We additionally discover 57 cluster-member suspected variables, 10 cluster-non-member suspected variables, and four variables with ambiguous cluster membership. Our light curves reach sub-millimagnitude precision for the cluster horizontal branch, permitting us to detect asteroseismic activity in six horizontal branch stars outside the instability strip and one inside the strip but with only ∼1 mmag amplitude variability. Nineteen additional stars along the red giant branch also have detected asteroseismic variability. Several eclipsing binaries are found in the cluster, including a 4.6 day detached eclipsing binary and an EW-class eclipsing binary, as well as an EW with uncertain cluster membership and three other candidate EWs. A 22 day detached eclipsing binary is also found outside the cluster. We identify a candidate X-ray binary that is a cluster member with quiescent and periodic ∼20 mmag optical variability. We also obtain high-precision light curves for 10 of the previously known RR Lyrae variables in the cluster and identify one as a candidate Blazhko variable with a Blazhko period in excess of 78 days. We make our light curves publicly available.
Context.
Binary evolution can result in fast-rotating stars through accretion of angular momentum during mass-transfer phases. These fast-rotating stars are predicted to be observable as Be stars. ...Current models struggle to produce a satisfactory description of Be star populations, even though numerous pieces of observational evidence indicate that the accretion process might be the dominant Be formation channel.
Aims.
Given the distinct uncertainties in detailed binary evolution calculations, we investigate a rigorous and model-independent upper limit for the production of Be stars through binary interaction and aim to confront this limit with observations of Be stars in young star clusters.
Methods.
Using extreme assumptions, we calculate the number ratio of post-interaction to pre-interaction binary systems in a coeval population. This ratio describes an upper limit to Be star formation through mass transfer. A detailed comparison is made between our derived upper limit and relevant observations of Be stars, which allows us to probe several aspects of binary star physics.
Results.
We find that in coeval populations, binary interaction can at most account for one-third of all main-sequence stars being Be stars. Near the cluster turn-off region, this limit appears to be realised in the clusters studied. Away from the turn-off, a good fit to the observed Be fraction as a function of mass is obtained by applying simple assumptions about which systems undergo unstable mass-transfer produces.
Conclusions.
We find that assuming distinct physics, binary evolution alone can in principle match the high numbers of Be stars that are observed in open clusters. Whether the required binary physics is realised in nature remains to be investigated.
The runaway collision scenario is one of the most promising mechanisms to explain the formation of intermediate-mass black holes (IMBHs) in young dense star clusters. On the other hand, the massive ...stars that participate in the runaway collisions lose mass by stellar winds. In this paper, we discuss new N-body simulations of massive (6.5 × 104 M⊙) star clusters, in which we added upgraded recipes for stellar winds and supernova explosion at different metallicity. We follow the evolution of the principal collision product (PCP), through dynamics and stellar evolution, till it forms a stellar remnant. At solar metallicity, the mass of the final merger product spans from few solar masses up to ∼30 M⊙. At low metallicity (0.01–0.1 Z⊙) the maximum remnant mass is ∼250 M⊙, in the range of IMBHs. A large fraction (∼0.6) of the PCPs are not ejected from the parent star cluster and acquire stellar or black hole (BH) companions. Most of the long-lived binaries hosting a PCP are BH–BH binaries. We discuss the importance of this result for gravitational wave detection.
The era of large transient surveys, gravitational-wave observatories, and multi-messenger astronomy has opened up new possibilities for our understanding of the evolution and final fate of massive ...stars. Most massive stars are born in binary or higher-order multiple systems and exchange mass with a companion star during their lives. In particular, the progenitors of a large fraction of compact-object mergers, and Galactic neutron stars (NSs) and black holes (BHs) have been stripped of their envelopes by a binary companion. Here, we study the evolution of single and stripped binary stars up to core collapse with the stellar evolution code M
ESA
and their final fates with a parametric supernova (SN) model. We find that stripped binary stars can have systematically different pre-SN structures compared to genuine single stars and thus also different SN outcomes. These differences are already established by the end of core helium burning and are preserved up to core collapse. Consequently, we find that Case A and B stripped stars and single and Case C stripped stars develop qualitatively similar pre-SN core structures. We find a non-monotonic pattern of NS and BH formation as a function of CO core mass that is different in single and stripped binary stars. In terms of initial mass, single stars of ≳35
M
⊙
all form BHs, while this transition is only at about 70
M
⊙
in stripped stars. On average, stripped stars give rise to lower NS and BH masses, higher explosion energies, higher kick velocities, and higher nickel yields. Within a simplified population-synthesis model, we show that our results lead to a significant reduction in the rates of BH–NS and BH–BH mergers with respect to typical assumptions made on NS and BH formation. Therefore, our models predict lower detection rates of such merger events with for example the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) than is often considered. Further, we show how certain features in the NS–BH mass distribution of single and stripped stars relate to the chirp-mass distribution of compact object mergers. Further implications of our findings are discussed with respect to the missing red-supergiant problem, a possible mass gap between NSs and BHs, X-ray binaries, and observationally inferred nickel masses from Type Ib/c and IIP SNe.
Zeeman-Doppler imaging studies have shown that the magnetic fields of T Tauri stars can be significantly more complex than a simple dipole and can vary markedly between sources. We collect and ...summarize the magnetic field topology information obtained to date and present Hertzsprung-Russell (H-R) diagrams for the stars in the sample. Intriguingly, the large-scale field topology of a given pre-main-sequence (PMS) star is strongly dependent upon the stellar internal structure, with the strength of the dipole component of its multipolar magnetic field decaying rapidly with the development of a radiative core. Using the observational data as a basis, we argue that the general characteristics of the global magnetic field of a PMS star can be determined from its position in the H-R diagram. Moving from hotter and more luminous to cooler and less luminous stars across the PMS of the H-R diagram, we present evidence for four distinct magnetic topology regimes. Stars with large radiative cores, empirically estimated to be those with a core mass in excess of ~40% of the stellar mass, host highly complex and dominantly non-axisymmetric magnetic fields, while those with smaller radiative cores host axisymmetric fields with field modes of higher order than the dipole dominant (typically, but not always, the octupole). Fully convective stars above > ~0.5 M sub(middot in circle) appear to host dominantly axisymmetric fields with strong (kilo-Gauss) dipole components. Based on similarities between the magnetic properties of PMS stars and main-sequence M-dwarfs with similar internal structures, we speculate that a bistable dynamo process operates for lower mass stars (<, ~0.5 M sub(middot in circle) at an age of a few Myr) and that they will be found to host a variety of magnetic field topologies. If the magnetic topology trends across the H-R diagram are confirmed, they may provide a new method of constraining PMS stellar evolution models.
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