Stellar Structure Research Articles

The stellar ‘Snake’ – II. The mass function

ABSTRACT We present a comprehensive investigation on the mass function (MF) of a snake-like stellar structure in the solar neighbourhood, building on our previous discovery. To ensure the reliability of the data, we reselect the member stars of the Stellar ‘Snake’ in the latest Gaia Data Release 3 using the same approach as the initial series of articles. We also precisely measure the physical parameters of the clusters within the Stellar Snake. In light of the high completeness of the member stars in the cluster regions, we develop a simulated model colour–magnitude diagram-based inference method to derive the mass function, binary fraction, and mass-ratio distribution of the clusters in the Stellar Snake. Notably, despite their similar ages and metallicity, we discover systematic variations in the MFs along the elongation direction of the Snake in the mass range of 0.5 to 2.0 M⊙. The ‘head’ of the Snake conforms to a canonical initial mass function with a power-law slope of α ∼ −2.3. Extending towards the ‘tail’, the MF becomes more top-light, indicating a deficiency of massive stars within these clusters. This result provides evidence for the delayed formation of massive stars in the clusters. Such clues give support to the hypothesis that the Stellar Snake constitutes as a hierarchically primordial structure.

Stability analysis of anisotropic stellar structures in [formula omitted] gravity

The article explores the dynamics and stability of compact stars in the field of f(R) gravity by investigating the empirical data of different significant compact star candidates. We used the corrections offered by Karmarkar and Tolman to examine the properties of these specific compact stars. For different values of the curvature term, we analyzed the behavior of material variables, energy conditions, equations of state parameters, anisotropy measurements, and the Tolman–Oppenheimer–Volkoff equation in the modeling of stellar structures We provided illustrations with their graphical representations that depict the configuration of the compact stars. For this purpose, we utilized known f(R)-gravity models to express these results as graphical representations to provide evidence supporting the authenticity and stability of the stars.

The EBLM Project– XI. Mass, radius, and effective temperature measurements for 23 M-dwarf companions to solar-type stars observed with CHEOPS

ABSTRACT Observations of low-mass stars have frequently shown a disagreement between observed stellar radii and radii predicted by theoretical stellar structure models. This ‘radius inflation’ problem could have an impact on both stellar and exoplanetary science. We present the final results of our observation programme with the CHaracterising ExOPlanet Satellite (CHEOPS) to obtain high-precision light curves of eclipsing binaries with low-mass stellar companions (EBLMs). Combined with the spectroscopic orbits of the solar-type companions, we can derive the masses, radii, and effective temperatures of 23 M-dwarf stars. We use the pycheops data analysis software to analyse their primary and secondary occultations. For all but one target, we also perform analyses with Transiting Exoplanet Survey Satellite (TESS) light curves for comparison. We have assessed the impact of starspot-induced variation on our derived parameters and account for this in our radius and effective temperature uncertainties using simulated light curves. We observe trends in inflation with both metallicity and orbital separation. We also observe a strong trend in the difference between theoretical and observational effective temperatures with metallicity. There is no such trend with orbital separation. These results are not consistent with the idea that the observed inflation in stellar radius combines with lower effective temperature to preserve the luminosity predicted by low-mass stellar models. Our EBLM systems provide high-quality and homogeneous measurements that can be used in further studies of radius inflation.

Structure and composition of Jupiter, Saturn, Uranus, and Neptune under different constraints and distortion due to rotation

ABSTRACT The radii of planets serve as significant constraints for their internal structure. Despite the complexity of planetary internal structure compared to stars, substantial advancements have been made in this field. The most critical uncertainties stem from the chemical composition and equation of state of planetary material. Using the MESA code, we construct rotating and non-rotating interior models for Jupiter and Saturn and sought to align these models to the observed radii. Rotation exerts a significant influence on their structures, distorting planetary, and stellar structures in distinct ways. Regarding gas planets’ structure, two pivotal uncertain parameters depend on a possible separation between hydrogen and helium in the protosolar disc gas due to unequal evaporation between these two gases. In an extreme scenario where only hydrogen is lost and no heavy elements or helium are lost, Jupiter and Saturn would have a core mass of zero. However, this approach fails to yield a solution for Uranus and Neptune. Instead, our models indicate that hydrogen and helium were likely lost together during the protosolar disc phase, resulting in core masses of approximately 40, 25, 14, and 12 M⊕ for Jupiter, Saturn, Neptune, and Uranus, respectively. These findings are highly compatible with the observed mass–radius relationship of exoplanets, as well as the seismic and Juno data for Jupiter’s near-surface temperature.

Via Machinae 2.0: Full-sky, model-agnostic search for stellar streams in Gaia DR2

ABSTRACT We present an update to Via Machinae, an automated stellar stream-finding algorithm based on the deep learning anomaly detector ANODE. Via Machinae identifies stellar streams within Gaia, using only angular positions, proper motions, and photometry, without reference to a model of the Milky Way potential for orbit integration or stellar distances. This new version, Via Machinae 2.0, includes many improvements and refinements to nearly every step of the algorithm, that altogether result in more robust and visually distinct stream candidates than our original formulation. In this work, we also provide a quantitative estimate of the false positive rate of Via Machinae 2.0 by applying it to a simulated Gaia-mock catalogue based on galaxia, a smooth model of the Milky Way that does not contain substructure or stellar streams. Finally, we perform the first full-sky search for stellar streams with Via Machinae 2.0, identifying 102 streams at high significance within the Gaia Data Release 2, of which only 10 have been previously identified. While follow-up observations for further confirmation are required, taking into account the false positive rate presented in this work, we expect approximately 90 of these stream candidates to correspond to real stellar structures.

Automated eccentricity measurement from raw eclipsing binary light curves with intrinsic variability

Eclipsing binary systems provide the opportunity to measure the fundamental parameters of their component stars in a stellar-model-independent way. This makes them ideal candidates for testing and calibrating theories of stellar structure and (tidal) evolution. Large photometric (space) surveys provide a wealth of data for both the discovery and the analysis of these systems. Even without spectroscopic follow-up there is often enough information in their photometric time series to warrant analysis, especially if there is an added value present in the form of intrinsic variability, such as pulsations. Our goal is to implement and validate a framework for the homogeneous analysis of large numbers of eclipsing binary light curves, such as the numerous high-duty-cycle observations from space missions like TESS. The aim of this framework is to be quick and simple to run and to limit the user's time investment when obtaining, amongst other parameters, orbital eccentricities. We developed a new and fully automated methodology for the analysis of eclipsing binary light curves with or without additional intrinsic variability. Our method includes a fast iterative pre-whitening procedure that results in a list of extracted sinusoids that is broadly applicable for purposes other than eclipses. After eclipses are identified and measured, orbital and stellar parameters are measured under the assumption of spherical stars of uniform brightness. We tested our methodology in two settings: a set of synthetic light curves with known input and the catalogue of Kepler eclipsing binaries. The synthetic tests show that we can reliably recover the frequencies and amplitudes of the sinusoids included in the signal as well as the input binary parameters, albeit to varying degrees of accuracy. Recovery of the tangential component of eccentricity is the most accurate and precise. Kepler results confirm a robust determination of orbital periods, with 80.5<!PCT!> of periods matching the catalogued ones. We present the eccentricities for this analysis and show that they broadly follow the theoretically expected pattern as a function of the orbital period. Our analysis methodology is shown to be capable of analysing large numbers of eclipsing binary light curves with no user intervention, and in doing so provide a basis for a further in-depth analysis of systems of particular interest as well as for statistical analysis at the sample level. Furthermore, the computational performance of the frequency analysis, extracting hundreds of sinusoids from Kepler light curves in a few hours, demonstrates its value as a tool for a field like asteroseismology.

A radius valley between migrated steam worlds and evaporated rocky cores

AbstractThe radius valley (or gap) in the observed distribution of exoplanet radii, which separates smaller super-Earths from larger sub-Neptunes, is a key feature that theoretical models must explain. Conventionally, it is interpreted as the result of the loss of primordial hydrogen and helium (H/He) envelopes atop rocky cores. However, planet formation models predict that water-rich planets migrate from cold regions outside the snowline towards the star. Assuming water to be in the form of solid ice in their interior, many of these planets would be located in the radius gap contradicting observations. Here we use an advanced coupled formation and evolution model that describes the planets’ growth and evolution starting from solid, moon-sized bodies in the protoplanetary disk to mature Gyr-old planetary systems. Employing new equations of state and interior structure models to treat water as vapour mixed with H/He, we naturally reproduce the valley at the observed location. The model results demonstrate that the observed radius valley can be interpreted as the separation of less massive, rocky super-Earths formed in situ from more massive, ex situ, water-rich sub-Neptunes. Furthermore, the occurrence drop at larger radii, the so-called radius cliff, is matched by planets with water-dominated envelopes. Our statistical approach shows that the synthetic distribution of radii quantitatively agrees with observations for close-in planets, but only if low-mass planets initially containing H/He lose their atmosphere due to photoevaporation, which populates the super-Earth peak with evaporated rocky cores. Therefore, we provide a hybrid theoretical explanation of the radius gap and cliff caused by both planet formation (orbital migration) as well as evolution (atmospheric escape).

Predicting the heaviest black holes below the pair instability gap

ABSTRACT Traditionally, the pair instability (PI) mass gap is located between 50 and 130 M⊙, with stellar mass black holes (BHs) expected to ‘pile up’ towards the lower PI edge. However, this lower PI boundary is based on the assumption that the star has already lost its hydrogen (H) envelope. With the announcement of an ‘impossibly’ heavy BH of 85 M⊙ as part of GW 190521 located inside the traditional PI gap, we realized that blue supergiant (BSG) progenitors with small cores but large hydrogen envelopes at low metallicity (Z) could directly collapse to heavier BHs than had hitherto been assumed. The question of whether a single star can produce such a heavy BH is important, independent of gravitational wave events. Here, we systematically investigate the masses of stars inside the traditional PI gap by way of a grid of 336 detailed mesa stellar evolution models calculated across a wide parameter space, varying stellar mass, overshooting, rotation, semiconvection, and Z. We evolve low Z stars in the range 10−3 < Z/Z⊙ < ZSMC, making no prior assumption regarding the mass of an envelope, but instead employing a wind mass-loss recipe to calculate it. We compute critical carbon–oxygen and helium core masses to determine our lower limit to PI physics, and we provide two equations for Mcore and Mfinal that can also be of use for binary population synthesis. Assuming the H envelope falls into the BH, we confirm the maximum BH mass below PI is MBH ≃ 93.3 M⊙. Our grid allows us to populate the traditional PI gap, and we conclude that the distribution of BHs above the traditional boundary is not solely due to the shape of the initial mass function, but also to the same stellar interior physics (i.e. mixing) that which sets the BH maximum.

Multi-scale magnetic field investigation of the M-dwarf eclipsing binary CU Cancri

We aim to characterise the magnetic field of the eclipsing binary CU Cancri, which consists of two M-dwarf components. The determination of the magnetic field parameters of this target enables comparisons with both observations of similar stars and theoretical predictions of the magnetic field strength in CU Cnc. The target therefore provides an excellent opportunity to test our understanding of the generation of magnetic fields in low-mass stars and its impact on stellar structure. We used spectropolarimetric observations obtained with ESPaDOnS at the CFHT to investigate the magnetic properties of CU Cnc. To improve the signal, we used least-squares deconvolution (LSD) to create average line profiles. From these LSD profiles, we extracted information about the radial velocities of the components, significantly expanding the number of radial velocity measurements available and allowing for a determination of the orbital parameters. Stokes $V$ LSD profiles were used with Zeeman Doppler imaging to obtain the large-scale magnetic field structures of the two components. We also used detailed polarised radiative transfer modelling to investigate the small-scale fields, by Zeeman-splitting magnetically sensitive Ti I lines in non-polarised spectra. We obtain both the small- and large-scale magnetic field properties of the two components. The large-scale fields are dominantly poloidal, and both components have an average strength of approximately 100\,G. This analysis of the large-scale fields likely suffers from some amount of hemisphere degeneracy due to the high inclination of the target, which would cause the large-scale field strength of the components to be underestimated. Both components also show unusual magnetic field configurations compared to stars with similar parameters: the primary is weakly axisymmetric (sim \,10\,<!PCT!>), and the secondary has a strong toroidal contribution (sim \,20\,<!PCT!>). The small-scale fields are significantly stronger, 3.1 and 3.6\,kG for the primary and secondary, respectively. This measurement is in excellent agreement with surface field strength predictions for CU Cnc from magneto-convective stellar evolution models. These results indicate that magnetic fields could play a significant role in the radius inflation due to convective inhibition.

Minute-Cadence Observations of the LAMOST Fields with the TMTS: V. Machine Learning Classification of TMTS Catalogues of Periodic Variable Stars

Abstract Periodic variables are always of great scientific interest in astrophysics. Thanks to the rapid advancement of modern large-scale time-domain surveys, the number of reported variable stars has experienced substantial growth for several decades, which significantly deepened our comprehension of stellar structure and binary evolution. The Tsinghua University-Ma Huateng Telescopes for Survey (TMTS) has started to monitor the LAMOST sky areas since 2020, with a cadence of 1 minute. During the period from 2020 to 2022, this survey has resulted in densely sampled light curves for ~30,000 variables of the maximum powers in the Lomb-Scargle periodogram above the 5σ threshold. In this paper, we classified 11,638 variable stars into 6 main types using XGBoost and Random Forest classifiers with accuracies of 98.83% and 98.73%, respectively. Among them, 5301 (45.55%) variables are newly discovered, primarily consisting of δ Scuti stars, demonstrating the capability of TMTS in searching for short-period variables. We cross-matched the catalogue with Gaia’s second Data Release (DR2) and LAMOST’s seventh Data Release (DR7) to obtain important physical parameters of the variables. We identified 5504 δ Scuti stars (including 4876 typical δ Scuti stars and 628 high-amplitude δ Scuti stars), 5899 eclipsing binaries (including EA-, EB- and EW-type) and 226 candidates of RS Canum Venaticorum. Leveraging the metal abundance data provided by LAMOST and the Galactic latitude, we discovered 8 candidates of SX Phe stars within the class of ”δ Scuti stars”. Moreover, with the help of Gaia color-magnitude diagram, we identified 9 ZZ ceti stars.

Distinguishing Oceans of Water from Magma on Mini-Neptune K2-18b

Mildly irradiated mini-Neptunes have densities potentially consistent with them hosting substantial liquid-water oceans (“Hycean” planets). The presence of CO2 and simultaneous absence of ammonia (NH3) in their atmospheres has been proposed as a fingerprint of such worlds. JWST observations of K2-18b, the archetypal Hycean, have found the presence of CO2 and the depletion of NH3 to <100 ppm; hence, it has been inferred that this planet may host liquid-water oceans. In contrast, climate modeling suggests that many of these mini-Neptunes, including K2-18b, may likely be too hot to host liquid water. We propose a solution to this discrepancy between observation and climate modeling by investigating the effect of a magma ocean on the atmospheric chemistry of mini-Neptunes. We demonstrate that atmospheric NH3 depletion is a natural consequence of the high solubility of nitrogen species in magma at reducing conditions; precisely the conditions prevailing where a thick hydrogen envelope is in communication with a molten planetary surface. The magma ocean model reproduces the present JWST spectrum of K2-18b to ≲3σ, suggesting this is as credible an explanation for current observations as the planet hosting a liquid-water ocean. Spectral areas that could be used to rule out the magma ocean model include the >4 μm region, where CO2 and CO features dominate: magma ocean models suggest a systematically lower CO2/CO ratio than estimated from free-chemistry retrieval, indicating that deeper observations of this spectral region may be able to distinguish between oceans of liquid water and magma on mini-Neptunes.

Betelgeuse as a Merger of a Massive Star with a Companion

We investigate the merger between a 16M ⊙ star, on its way to becoming a red supergiant (RSG), and a 4M ⊙ main-sequence companion. Our study employs three-dimensional hydrodynamic simulations using the state-of-the-art adaptive mesh refinement code Octo-Tiger. The initially corotating binary undergoes interaction and mass transfer, resulting in the accumulation of mass around the companion and its subsequent loss through the second Lagrangian point (L2). The companion eventually plunges into the envelope of the primary, leading to its spin-up and subsequent merger with the helium core. We examine the internal structural properties of the post-merger star, as well as the merger environment and the outflow driven by the merger. Our findings reveal the ejection of approximately ∼0.6 M ⊙ of material in an asymmetric and somewhat bipolar outflow. We import the post-merger stellar structure into the MESA stellar evolution code to model its long-term nuclear evolution. In certain cases, the post-merger star exhibits persistent rapid equatorial surface rotation as it evolves in the H–R diagram toward the observed location of Betelgeuse. These cases demonstrate surface rotation velocities of a similar magnitude to those observed in Betelgeuse, along with a chemical composition resembling that of Betelgeuse. In other cases, efficient rotationally induced mixing leads to slower surface rotation. This pioneering study aims to model stellar mergers across critical timescales, encompassing dynamical, thermal, and nuclear evolutionary stages.

Asteroseismic Inversions for Internal Sound Speed Profiles of Main-sequence Stars with Radiative Cores

The theoretical oscillation frequencies of even the best asteroseismic models of solar-like oscillators show significant differences from observed oscillation frequencies. Structure inversions seek to use these frequency differences to infer the underlying differences in stellar structure. While used extensively to study the Sun, structure inversion results for other stars have so far been limited. Applying sound speed inversions to more stars allows us to probe stellar theory over a larger range of conditions, as well as look for overall patterns that may hint at deficits in our current understanding. To that end, we present structure inversion results for 12 main-sequence solar-type stars with masses between 1 and 1.15 M ⊙. Our inversions are able to infer differences in the isothermal sound speed in the innermost 30% by radius of our target stars. In half of our target stars, the structure of our best-fit model fully agrees with the observations. In the remainder, the inversions reveal significant differences between the sound speed profile of the star and that of the model. We find five stars where the sound speed in the core of our stellar models is too low and one star showing the opposite behavior. For the two stars in which our inversions reveal the most significant differences, we examine whether changing the microphysics of our models improves them and find that changes to nuclear reaction rates or core opacities can reduce, but do not fully resolve, the differences.

Constraining stellar and orbital co-evolution through ensemble seismology of solar-like oscillators in binary systems

Context. Binary systems constitute a valuable astrophysics tool for testing our understanding of stellar structure and evolution. Systems containing at least one oscillating component are interesting in this regard because asteroseismology offers independent parameters for the oscillating component that aid in the analysis. Systems of particular interest include those with known inclinations. With ∼0.8 million binary candidates, the two-body orbit catalog (TBO) of Gaia Data Release 3 (DR3) substantially increases the number of known binaries and the quality of the astrometric data available for them. Aims. To enlarge the sample of these astrophysically valuable benchmark objects, we searched for new binary system candidates identified in the Gaia DR3 TBO, for which one component has a detection of solar-like oscillations reported in the literature. Methods. We cross-matched the TBO, the full non-single star (NSS) and eclipsing binary catalogs from Gaia DR3 with catalogs of confirmed solar-like oscillators in the main-sequence and red-giant phase from the NASA Kepler mission and stars in the Southern Continuous Viewing Zone of NASA TESS. The wealth of seismic information is used to characterize the oscillating primary. To test the completeness and robustness of the values reported in the TBO catalog, we performed a similar analysis on stars of the Ninth Catalog of Spectroscopic Binary Orbits (SB9). Results. The analysis of the SB9 reveals an overall completeness factor for the Gaia TBO catalog of up to ∼30% providing reliable orbital parameters for ≥90% of the systems below Porb, SB9 ≲ 250 d. We obtained new 954 unique binary system candidates from Gaia DR3, which host solar-like oscillators, of which we found 45 stars in binary candidates to be on the main sequence and 909 in the red giant phase. Additionally, we report 918 oscillators in potentially long-periodic systems. We present the seismic properties of the full sample and test whether the reported orbital periods are physically possible. For 146 giants, the evolutionary state has been determined from their mixed-mode period spacing, showing a clear trend to long periodic and less eccentric systems in the advanced phases of stellar evolution. Two new eclipsing binary systems, hosting a red-giant primary were found. For another 146 systems hosting oscillating stars, the values for the orbital inclination were found in the TBO. Of 181 TBO candidate systems observed multiple times with APOGEE, 149 (82%) are confirmed as binaries from radial-velocity (RV) measurement. Conclusions. We conclude that the grand majority of the orbital elements reported in the TBO catalog are physically reasonable and realistic. This finding increases the number included in the sample of known solar-like oscillators in binary systems by an order of magnitude. The large fraction of confirmed binaries from APOGEE RV measurements indicates that the TBO catalog is robust. We suggest that due to instrumental noise, the seismically inferred masses and radii of stars observed with the TESS satellite and with an excess of oscillation power of νmax ≲ 30 μHz could be significantly overestimated. The differences in the distributions of the orbital period and eccentricity are due to the accumulative effect of the equilibrium tide acting in these evolved binary systems.

The Global Asteroseismology Project Proof of Concept: Asteroseismology of Massive Stars with Continuous Ground-based Observations

Massive (≳8M ⊙) stars are the progenitors of many astrophysical systems, yet key aspects of their structure and evolution are poorly understood. Asteroseismology has the potential to solve these open puzzles; however, sampling both the short period pulsations and long period beat patterns of massive stars poses many observational challenges. Ground-based single-site observations require years or decades to discern the main oscillation modes. Multisite campaigns were able to shorten this time span, but have not been able to scale up to population studies on sample of objects. Space-based observations can achieve both continuous sampling and observe large numbers of objects; however, most lack the multiband data that is often necessary for mode identification and removing model degeneracies. Here, we develop and test a new ground-based observational strategy for discerning and identifying the main oscillation modes of a massive star in a few months, in a way that can be scaled to large samples. We do so using the Las Cumbres Observatory—a unique facility consisting of robotic, homogeneous telescopes operating as a global network, overcoming most of the challenges of previous multisite efforts, but presenting new challenges which we tailor our strategy to address. This work serves as the proof of concept for the Global Asteroseismology Project, which aims to move massive star asteroseismology from single-objects to bulk studies, unleashing its full potential in constraining stellar structure and evolution models. This work also demonstrates the ability of the Las Cumbres Observatory to perform multisite continuous observations for various science goals.

Modelling stellar variability in archival HARPS data: I - Rotation and activity properties with multidimensional Gaussian processes

ABSTRACT Although instruments for measuring the radial velocities (RVs) of stars now routinely reach sub-metre per second accuracy, the detection of low-mass planets is still very challenging. The rotational modulation and evolution of spots and/or faculae can induce variations in the RVs at the level of a few m s–1 in Sun-like stars. To overcome this, a multidimensional Gaussian Process framework has been developed to model the stellar activity signal using spectroscopic activity indicators together with the RVs. A recently published computationally efficient implementation of this framework, S + LEAF 2, enables the rapid analysis of large samples of targets with sizeable data sets. In this work, we apply this framework to HARPS observations of 268 well-observed targets with precisely determined stellar parameters. Our long-term goal is to quantify the effectiveness of this framework to model and mitigate activity signals for stars of different spectral types and activity levels. In this first paper in the series, we initially focus on the activity indicators (S-index and Bisector Inverse Slope), and use them to (a) measure rotation periods for 49 slow rotators in our sample, (b) explore the impact of these results on the spin-down of middle-aged late F, G, and K stars, and (c) explore indirectly how the spot to facular ratio varies across our sample. Our results should provide valuable clues for planning future RV planet surveys such as the Terra Hunting Experiment or the PLATO ground-based follow-up observations programme, and help fine-tune current stellar structure and evolution models.

Revealing the kinematic puzzle of the AGN host NGC 3884: optical integral field spectroscopy unravels stellar and gas motions

ABSTRACT We used optical integral field spectroscopy to analyse the stellar and gas properties of the inner 1.4 kpc radius of NGC 3884, a low-luminosity active galactic nucleus (AGN) host. The observations were performed with Gemini Multi-Object Spectrograph (GMOS)-Integral Field Unit at a seeing of ∼0.85 arcsec (475 pc at the galaxy) that allowed us to map the stellar and gas emission structure and kinematics, for the first time in this galaxy. The stellar motions are consistent with rotation in a disc, with the kinematic position angle (PA) ranging from approximately 0° within 500 pc to 20° beyond 1 kpc, consistent with the photometric PA. We detected extended ionized and neutral gas emission throughout most of the GMOS field of view, with three kinematic components: (i) a disc component with a kinematic PA similar to that of the stars beyond ∼670 pc from the nucleus; (ii) a twist in the PA of up to 60° at a smaller radii that we attribute to gas inflow towards the nucleus; and (iii) an outflow detected as broad components to the emission lines (σ ∼ 250–400 km s−1), with a maximum mass outflow rate of 0.25 ± 0.15 M⊙ yr−1 and a kinetic power corresponding to 0.06 per cent of the AGN bolometric luminosity, possibly being powerful enough to suppress star formation in the galaxy. The observed gas kinematics thus reveals both inflows and outflows in ionized gas.

Prospects for Connections to the Outer Halo of the Milky Way with LSST

Over its ten-year mission, Rubin Observatory's Legacy Survey of Space and Time will gather enough time series data to identify variable stars much fainter than previously observed. This makes ideal for finding RR Lyrae stars and mapping previously unexplored regions of our Galaxy's stellar halo. This project creates several mock data sets from both simulations and known satellite properties to model what might be found. We use these to examine the prospects for making connections between stellar structures already known around the Galaxy (satellites, streams and shells) and newly discovered stars. Such connections could be used to place constraints on the mass distribution in the outer regions of the Galaxy’s dark matter halo. Our results are encouraging, suggesting: that tens of percent of the known structures should have counterparts in the outer halo; and that several entirely new unbound structures could be discovered.

Spectropolarimetry of Type II supernovae

The explosion processes of supernovae (SNe) are imprinted in their explosion geometries. The recent discovery of several highly aspherical core-collapse SNe is significant, and studying these is regarded as being crucial in order to understand the underlying explosion mechanism. Here, we study the intrinsic polarization of 15 hydrogen-rich core-collapse SNe and explore the relation between polarization and the photometric and spectroscopic properties of these objects. Our sample shows diverse properties of the continuum polarization. Most SNe show a low degree of polarization at early phases but a sudden rise to ∼1% at certain points during the photospheric phase followed by a slow decline during the tail phase, with a constant polarization angle. The variation in the timing of peak polarization values implies diversity in the explosion geometry: some SNe have aspherical structures only in their helium cores, while in other SNe such structures reach out to a significant part of the outer hydrogen envelope with a common axis from the helium core to the hydrogen envelope. Other SNe show high polarization from early phases and a change in polarization angle around the middle of the photospheric phase. This implies that the ejecta are significantly aspherical out to the outermost layer and have multi-directional aspherical structures. Exceptionally, the Type IIL SN 2017ahn shows low polarization at both the photospheric and tail phases. Our results show that the timing of the polarization rise in Type IIP SNe is likely correlated with their brightness, velocity, and the amount of radioactive Ni produced: brighter SNe with faster ejecta velocity and a larger 56Ni mass have more extended aspherical explosion geometries. In particular, there is a clear correlation between the timing of the polarization rise and the explosion energy; that is, the explosion asphericity is proportional to the explosion energy. This implies that the development of a global aspherical structure, such as a jet, might be the key for the realisation of an energetic SN in the mechanism of SN explosions.

Multiwavelength View of Massive Binaries

The high luminosity of massive, early-type stars drives strong stellar winds through line scattering of the star’s continuum radiation. Their momenta contribute substantially to the dynamics and energetics of the ambient interstellar medium in galaxies. The detailed multiwavelength study of massive O-type and Wolf-Rayet binaries is essential to explore the hydrodynamics of the shocks formed in the stellar outflows and wind structure. Further, deep analysis of some of the interesting phenomena like particle acceleration and dust formation associated with hot stars’ winds provides a global view of stellar outflows. In this context, a few massive binaries have been explored using photometric and spectroscopic measurements in different wavebands. This paper highlights important insights gained from investigating massive binaries with several ground and space-based facilities.