Low-mass Stars Research Articles

A highly magnetized environment in a pulsar binary system.

Spider pulsars are millisecond pulsars in short-period (≲12-h) orbits with low-mass (~0.01-0.4 M⊙) companion stars. The pulsars ablate plasma from the companion star, causing time delays and eclipses of the radio emission from the pulsar. The magnetic field of the companion has been proposed to strongly influence both the evolution of the binary system1 and the eclipse properties of the pulsar emission2. Changes in the rotation measure (RM) have been seen in a spider system, implying that there is an increase in themagnetic field near the eclipse3. Here we report a diverse range of evidence for a highly magnetized environment in the spider system PSR B1744 - 24A4, located in the globular cluster Terzan 5. We observe semi-regular profile changes to the circular polarization, V, when the pulsar emission passes close to the companion. This suggests that there is Faraday conversion where the radio wave tracks a reversal in the parallel magnetic field and constrains the companion magnetic field, B (> 10 G). We also see irregular, fast changes in the RM at random orbital phases, implying that the magnetic strength of the stellar wind, B, is greater than 10 mG. There are similarities between the unusual polarization behaviour of PSR B1744 - 24A and some repeating fast radio bursts (FRBs)5-7. Together with the possible binary-produced long-term periodicity of two active repeating FRBs8,9, and the discovery of a nearby FRB in a globular cluster10, where pulsar binaries are common, these similarities suggest that a proportion of FRBs have binary companions.

Resolved imaging confirms a radiation belt around an ultracool dwarf

Radiation belts are present in all large-scale Solar System planetary magnetospheres: Earth, Jupiter, Saturn, Uranus and Neptune1. These persistent equatorial zones of relativistic particles up to tens of megaelectron volts in energy can extend further than ten times the planet’s radius, emit gradually varying radio emissions2–4 and affect the surface chemistry of close-in moons5. Recent observations demonstrate that very low-mass stars and brown dwarfs, collectively known as ultracool dwarfs, can produce planet-like radio emissions such as periodically bursting aurorae6–8 from large-scale magnetospheric currents9–11. They also exhibit slowly varying quiescent radio emissions7,12,13 hypothesized to trace low-level coronal flaring14,15 despite departing from empirical multiwavelength flare relationships8,15. Here we present high-resolution imaging of the ultracool dwarf LSR J1835 + 3259 at 8.4 GHz, demonstrating that its quiescent radio emission is spatially resolved and traces a double-lobed and axisymmetrical structure that is similar in morphology to the Jovian radiation belts. Up to 18 ultracool dwarf radii separate the two lobes, which are stably present in three observations spanning more than one year. For plasma confined by the magnetic dipole of LSR J1835 + 3259, we estimate 15 MeV electron energies, consistent with Jupiter’s radiation belts4. Our results confirm recent predictions of radiation belts at both ends of the stellar mass sequence8,16–19 and support broader re-examination of rotating magnetic dipoles in producing non-thermal quiescent radio emissions from brown dwarfs7, fully convective M dwarfs20 and massive stars18,21.

NGTS clusters survey – V. Rotation in the Orion star-forming complex

ABSTRACT We present a study of rotation across 30 square degrees of the Orion Star-forming Complex, following a ∼200 d photometric monitoring campaign by the Next Generation Transit Survey (NGTS). From 5749 light curves of Orion members, we report periodic signatures for 2268 objects and analyse rotation period distributions as a function of colour for 1789 stars with spectral types F0–M5. We select candidate members of Orion using Gaia data and assign our targets to kinematic sub-groups. We correct for interstellar extinction on a star-by-star basis and determine stellar and cluster ages using magnetic and non-magnetic stellar evolutionary models. Rotation periods generally lie in the range 1–10 d, with only 1.5 per cent of classical T Tauri stars or Class I/II young stellar objects rotating with periods shorter than 1.8 d, compared with 14 per cent of weak-line T Tauri stars or Class III objects. In period–colour space, the rotation period distribution moves towards shorter periods among low-mass (>M2) stars of age 3–6 Myr, compared with those at 1–3 Myr, with no periods longer than 10 d for stars later than M3.5. This could reflect a mass-dependence for the dispersal of circumstellar discs. Finally, we suggest that the turnover (from increasing to decreasing periods) in the period–colour distributions may occur at lower mass for the older-aged population: ∼K5 spectral type at 1–3 Myr shifting to ∼M1 at 3–6 Myr.

A rich hydrocarbon chemistry and high C to O ratio in the inner disk around a very low-mass star

Carbon is an essential element for life but how much can be delivered to young planets is still an open question. The chemical characterization of planet-forming disks is a crucial step in our understanding of the diversity and habitability of exoplanets. Very low-mass stars (less than 0.2 M⊙) are interesting targets because they host a rich population of terrestrial planets. Here we present the James Webb Space Telescope detection of abundant hydrocarbons in the disk of a very low-mass star obtained as part of the Mid-InfraRed Instrument mid-INfrared Disk Survey (MINDS). In addition to very strong and broad emission from C2H2 and its 13C12CH2 isotopologue, C4H2, benzene and possibly CH4 are identified, but water, polycyclic aromatic hydrocarbons and silicate features are weak or absent. The lack of small silicate grains indicates that we can look deep down into this disk. These detections testify to an active warm hydrocarbon chemistry with a high C/O ratio larger than unity in the inner 0.1 astronomical units (AU) of this disk, perhaps due to destruction of carbonaceous grains. The exceptionally high C2H2/CO2 and C2H2/H2O column density ratios indicate that oxygen is locked up in icy pebbles and planetesimals outside the water iceline. This, in turn, will have important consequences for the composition of forming exoplanets. Highly abundant hydrocarbons in a very low-mass star’s disk are detected using the JWST. This unique chemical composition is probably due to the destruction of carbon grains, and the resulting high gaseous C/O ratio may have a profound impact on the composition of growing exoplanets.

Detectability of subsolar mass neutron stars through a template bank search

We study the detectability of gravitational-wave signals from sub-solar mass binary neutron star systems by the current generation of ground-based gravitational-wave detectors. We find that finite size effects from large tidal deformabilities of the neutron stars and lower merger frequencies can significantly impact the sensitivity of the detectors to these sources. By simulating a matched-filter based search using injected binary neutron star signals with tidal deformabilities derived from physically motivated equations of state, we calculate the reduction in sensitivity of the detectors. We conclude that the loss in sensitive volume can be as high as $78.4 \%$ for an equal mass binary system of chirp mass $0.17 \, \textrm{M}_{\odot}$, in a search conducted using binary black hole template banks. We use this loss in sensitive volume, in combination with the results from the search for sub-solar mass binaries conducted on data collected by the LIGO-Virgo observatories during their first three observing runs, to obtain a conservative upper limit on the merger rate of sub-solar mass binary neutron stars. Since the discovery of a low-mass neutron star would provide new insight into formation mechanisms of neutron stars and further constrain the equation of state of dense nuclear matter, our result merits a dedicated search for sub-solar mass binary neutron star signals.

The nearby astrometric-spectroscopic binary star Hip 68682

The nearby astrometric-spectroscopic binary star Hip 68682 has an orbital period of ∼ 9.88 yr. The dynamical state (component masses and kinematic parameters) of this system is redetermined by fitting all the observational data available to us, including the radial velocity data (RVD), the relative position data (RPD), and the Hipparcos Intermediate Astrometric Data (HIAD). The precision-weighted sum of squared residuals calculated from our results is significantly smaller than the previous ones. With the simulated data based on Chinese Space Station Telescope (CSST), the relative orbit could be improved further.Based on the known apparent magnitude information and the fitted parallax 60.59 ± 0.36 mas, we derive the absolute V magnitudes of the two components as 5.21 mag and 8.91 mag, respectively. The component masses are determined to be 0.946 ± 0.024M⊙ and 0.526 ± 0.014M⊙. These improved dynamical masses are helpful to constrain the stellar empirical mass–luminosity relation (MLR) for low mass stars. However, significant improvement of mass–luminosity relation for this mass region still needs more data.

Bimodal star formation in simulations of strongly magnetized giant molecular clouds

ABSTRACT We present the results of a set of radiation magnetohydrodynamic simulations of turbulent molecular clouds in which we vary the initial strength of the magnetic field within a range (1 ≲ μ ≲ 5) consistent with observations of local giant molecular clouds (GMCs). We find that as we increase the strength of the magnetic field, star formation transitions from unimodal (the baseline case, μ = 5, with a single burst of star formation and Salpeter IMF) to bimodal. This effect is clearest in the most strongly magnetized GMC (μ = 1): a first burst of star formation with duration, intensity, and IMF comparable to the baseline case, is followed by a second star formation episode in which only low-mass stars are formed. Overall, due to the second burst of star formation, the strongly magnetized case results in a longer star formation period and a higher efficiency of star formation. The second burst is produced by gas that is not expelled by radiative feedback, instead remains trapped in the GMC by the large-scale B-field, producing a nearly one-dimensional flow of gas along the field lines. The trapped gas has a turbulent and magnetic topology that differs from that of the first phase and strongly suppresses gas accretion onto protostellar cores, reducing their masses. We speculate that this star formation bimodality may be an important ingredient to understand the origin of multiple stellar populations observed in massive globular clusters.

Survey of $$\hbox {H}{\varvec{\alpha }}$$ emission-line stars in the star-forming region IC 5070

Actively accreting young stellar objects show H$\alpha$ emission line in their spectra. We present the results of survey for H$\alpha$ emission-line stars in the star-forming region IC~5070 taken with 2-m Himalyan {\it Chandra} Telescope. Based on the H$\alpha$ slitless spectroscopy data, we identified 131 emission-line stars in $\sim$ 0.29 square degrees area of the IC~5070 region. Using Gaia Early Data Release 3, we estimated the mean proper motion and parallax of the emission-line stars. We also estimated the mean distance and reddening toward the region using the emission-line stars, that are $\sim$ 833 pc and $\sim$2 mag, respectively. By examining the locations of these stars in the color-magnitude diagrams constructed using Gaia and PanSTARRS1 data, we found that a majority of the H$\alpha$ emitters are young low-mass ($<$ 1.5 $M\odot$) stars. We also compared our catalog of emission-line stars with the available young stellar catalogs and found that most of them are Class~{II}/ flat spectrum sources with the spectral type ranging from K to M. Based on the proper motion/ parallax values and locations on the color-magnitude diagrams, about 20 emission-line stars are flagged as non-members. The relative proper motion of the emission-line stars with respect to the ionizing source suggest the possibility of the `rocket effect' scenario in the remnant cloud (BRC~31).

Unmasking strange dwarfs with gravitational-wave observations

The advent of space-based gravitational-wave detectors like the Laser Interferometer Space Antenna will allow us to observe signals, most of which are expected to be emitted by white-dwarf binaries. Among these systems another kind of compact objects could be hidden, postulated by Glendenning, Kettner, and Weber in 1995, containing a small core made of strange quark matter surrounded by layers of hadronic matter reaching densities much higher than in white dwarfs. These so-called strange dwarfs cannot be easily distinguished from white dwarfs through electromagnetic observations alone; their outermost envelopes are expected to have the same composition and their radii are quite similar (except for low masses). However, future measurements of the tidal deformability through gravitational-wave observations could provide a new way to reveal their existence. In this paper, we revisit the structure and the tidal deformability of strange dwarfs in full general relativity taking into account the possible crystallization of their envelope. Unlike most previous studies, we do not describe the hadronic layers using the equation of state of Baym, Pethick, and Sutherland, which was originally designed for the outer crust of a neutron star. The conditions leading to such heavy elements in gravitational-core collapse supernova explosions are not expected to be met during the stellar evolution leading to the formation of strange dwarfs. Instead, we consider the same light elements as found in white dwarfs but we take into account electron captures by nuclei and possibly pycnonuclear fusions that may be triggered by compression. We find that the radius of strange dwarfs is systematically smaller than that of their white dwarf relatives. These deviations can be large for very low-mass stars but mostly lie within the current observational uncertainties for typical white dwarf masses. On the other hand, the observable tidal deformability coefficient ${\mathrm{\ensuremath{\Lambda}}}_{2}$ is strongly reduced in comparison to white dwarfs and the deviation will be potentially measurable by future space-based gravitational-wave detectors.

Enhanced YSO population in Serpens

Serpens molecular cloud is one of the most active sites of ongoing star formation at a distance of about 300 pc, and hence, is very well-suited for studies of young low-mass stars and sub-stellar objects. In this paper, for the Serpens star-forming region, we found potential members of the Young Stellar Objects (YSOs) population from the Gaia DR3 data and study their kinematics and distribution. We compiled a catalog of 656 YSOs from available catalogs ranging from X-ray to the infrared. We use this as a reference set and cross-match it to find 87 Gaia DR3 member stars to produce a control sample with revised parameters. We queried the DR3 catalog with these parameters and found 1196 stars. We then applied three different density-based machine learning algorithms (DBSCAN, OPTICS and HDBSCAN) to this sample and found potential YSOs. The three clustering algorithms identified a common set of 822 YSO members from Gaia DR3 in this region. We also classified these objects using 2MASS and WISE data to study their distribution and the progress of star-formation in Serpens.

A study of carbon-rich post-AGB stars in the Milky Way to understand the production of carbonaceous dust from evolved stars

Context. Knowledge of the Gaia, DR3 parallaxes of Galactic post-asymptotic giant branch (AGB) stars makes it possible to exploit these objects as tracers of AGB evolution, nucleosynthesis, and dust production as well as to use them to shed new light on still poorly known physical processes experienced by AGB stars. Aims. The goal of this study is to reconstruct the evolution and the dust formation processes during the final AGB phases of a sample of carbon-rich, post-AGB Galactic stars, with particular attention to the determination of the past mass-loss history. Methods. We study the IR excess of Galactic sources classified as post-AGB single stars by means of dust formation modelling where dust grains form and grow in a static wind and expand from the surface of the star. The method is applied to various evolutionary stages of the final AGB phase of stars with different masses and metallicities. The results from a spectral energy distribution (SED) fitting are used to infer information on mass loss, efficiency of dust formation, and wind dynamics. Results. The detailed analysis of the SED of the sources investigated, which included the derivation of the luminosities and the dust properties, allows us to confirm previous results, mostly based on the surface chemical composition, that most of the investigated sources descend from low-mass (M &lt; 1.5 M⊙) progenitors that reached the C-star stage. Metal-poor carbon stars are characterised by higher IR excesses with respect to their more metal-rich counterparts of similar luminosity due to a higher surface carbon-to-oxygen excess. This work confirms previous conclusions based on a limited sample of carbon-rich post-AGB objects in the Magellanic Clouds, namely that more luminous stars descending from higher-mass progenitors are generally more opaque due to shorter evolutionary timescales that place the dust shell closer to the central object. Through the study of the dynamics of the outflow and results from stellar evolution modelling, we find that the mass-loss rate at the tip of the AGB phase of metal-rich low-mass carbon stars is approximately 1#x2212;1.5 × 10−5 M⊙ yr−1, whereas in the metal-poor domain Ṁ ∼ 4 − 5 × 10−5 M⊙ yr−1 is required. These results indicate the need for an upwards revision of the theoretical mass-loss rates of low-mass carbon stars in the available literature, which in turn require a revised determination of carbon dust yields by AGB stars.

Massive Protostellar Disks as a Hot Laboratory of Silicate Grain Evolution

Typical accretion disks around massive protostars are hot enough for water ice to sublimate. We here propose to utilize the massive protostellar disks for investigating the collisional evolution of silicate grains with no ice mantle, which is an essential process for the formation of rocky planetesimals in protoplanetary disks around lower-mass stars. We, for the first time, develop a model of massive protostellar disks that includes the coagulation, fragmentation, and radial drift of dust. We show that the maximum grain size in the disks is limited by collisional fragmentation rather than by radial drift. We derive analytic formulae that produce the radial distribution of the maximum grain size and dust surface density in the steady state. Applying the analytic formulae to the massive protostellar disk of GGD27-MM1, where the grain size is constrained from a millimeter polarimetric observation, we infer that the silicate grains in this disk fragment at collision velocities above ≈10 m s−1. The inferred fragmentation threshold velocity is lower than the maximum grain collision velocity in typical protoplanetary disks around low-mass stars, implying that coagulation alone may not lead to the formation of rocky planetesimals in those disks. With future measurements of grain sizes in massive protostellar disks, our model will provide more robust constraints on the sticking property of silicate grains.

What governs the spin distribution of very young &lt; 1 Myr low-mass stars

Context. The origin of the stellar spin distribution at young ages is still unclear. Even in very young clusters (∼1 Myr), a significant spread is observed in rotational periods ranging from ≲1 to ∼10 days. Aims. We study the parameters that might govern the spin distribution of low-mass stars (≲1.0 M⊙) during the first million years of their evolution. Methods. We compute the evolution and rotational periods of young stars, using the MESA code, starting from a stellar seed, and take protostellar accretion, stellar winds, and magnetic star–disk interaction into account. Furthermore, we add a certain fraction of the energy of accreted material into the stellar interior as additional heat and combine the resulting effects on stellar evolution with the stellar spin model. Results. For different combinations of parameters, stellar periods at an age of 1 Myr range between 0.6 days and 12.9 days. Thus, during the relatively short time period of 1 Myr, a significant amount of stellar angular momentum can already be removed by the interaction between the star and its accretion disk. The amount of additional heat added into the stellar interior, the accretion history, and the presence of a disk and stellar winds have the strongest impact on the stellar spin evolution during the first million years. The slowest stellar rotations result from a combination of strong magnetic fields, a large amount of additional heat, and effective winds. The fastest rotators combine weak magnetic fields and ineffective winds or result from a small amount of additional heat added to the star. Scenarios that could lead to such configurations are discussed. Different initial rotation periods of the stellar seed, on the other hand, quickly converge and do not affect the stellar period at all. Conclusions. Our model matches up to 90% of the observed rotation periods in six young clusters (≲3 Myr). Based on these intriguing results, we were motivated to combine our model with a hydrodynamic disk evolution code to self-consistently include several important aspects, such as episodic accretion events, magnetic disk winds, and internal and external photoevaporation. This combined model could replace the widely used disk-locking model during the lifetime of the accretion disk, and could provide valuable insights into the origin of the rotational period distribution of young clusters.

Seismological Studies of Pulsating DA White Dwarfs Observed with the Kepler Space Telescope and K2 Campaigns 1–8

AllS single stars that are born with masses up to 8.5–10 M ⊙ will end their lives as white dwarf (WD) stars. In this evolutionary stage, WDs enter the cooling sequence, where the stars radiate away their thermal energy and are basically cooling. As these stars cool, they reach temperatures and conditions that cause the stars to pulsate. Using differential photometry to produce light curves, we can determine the observed periods of pulsation from the WD. We used the White Dwarf Evolution Code (WDEC) to calculate a grid of over one million models with various temperature, stellar mass, and mass of helium and hydrogen layers and calculated their theoretical pulsation periods. In this paper, we describe our approach to WD asteroseismology using WDEC models, and we present seismological studies for 29 observed DAVs in the Kepler and K2 data sets, 25 of which have never been analyzed using these observations and 19 of which have never been seismically analyzed in any capacity before. Learning about the internal structure of WDs places important constraints on the WD cooling sequence and our overall understanding of stellar evolution for low-mass stars.

The molecular environment of the solar-type protostar IRAS 16293–2422

Context. Studying the physical and chemical processes leading to the formation of low-mass stars is crucial for understanding the origin of our Sun and the Solar System. In particular, analyzing the emission and absorption lines from molecules to derive their spatial distribution in the envelopes of young stellar objects is a fundamental tool to obtain information on the kinematics and chemistry at the very early stages of star formation. Aims. In this work we aim to examine in detail the spatial structures and molecular abundances of material surrounding the very well-known low-mass binary protostar IRAS 16293-2422 and the prestellar core 16293E, which are embedded in the Lynds 1689 N dark cloud. This analysis is performed to obtain information on the physical and chemical properties of these young objects and their interaction with the molecular outflows present across the region. Methods. We have used the LAsMA heterodyne array installed on the Atacama Pathfinder Experiment (APEX) 12 meter submillimeter telescope to image a region of about 0.12 × 0.12 pc2 around IRAS 16293-2422 and 16293E and to study their molecular environment covering 45.6 GHz in a frequency range from 277 GHz to 375 GHz. We have also used the APEX FLASH+ receiver to observe and search for molecular lines in a frequency range between 476 GHz to 493 GHz. Results. We have identified 144 transitions from 36 molecular species, including isotopologues. This is the first time that such a large number of species have been mapped at large scales simultaneously in this region. The maps reveal the envelope to have a complex morphology around the cloud cores and the emission peaks known as E1, E2, W1, W2, and HE2, including the outflow structure arising from IRAS 16293-2422. Using several transitions of para-H2CO, we have derived new lower limits for the kinetic temperatures toward IRAS 16293-2422 and the surrounding emission peaks. Based on these temperatures, new column densities for all detected species were derived around the cloud cores and all emission peaks using the radiative transfer codes CLASS-Weeds, CASSIS, and RADEX. We derived H2 volume densities in Lynds 1689 N based on ortho-H2CO transitions with different upper level energies, varying between 5 × 106 cm−3 and 63 K at IRAS 16293-2422 to values on the order of 1 × 106 cm−3 and 35 K at the other emission peaks. Conclusions. Our new observations further confirm the scenario of an outflow arising from IRAS 16293-2422 interacting with the prestellar core 16293E. This is inferred from the velocity and linewidth gradient shown by several deuterated species closer to the outflow-core interaction region in 16293E. We observe a large-scale velocity gradient across the molecular cloud which coincides with the rotation of the envelope around IRAS 16293-2422 reported previously in the literature. A comparison with JCMT SCUBA-2 450 μm dust continuum maps and our data suggests that emission peak W2 may be related to a colder dust source rather than a shocked region. The newly derived column densities and temperatures for different species, combined with the molecular spatial distribution in all sources, indicate clear chemical differences between the protostellar source, the prestellar core and the shocked positions as a result of the diverse physical conditions at different locations in this region.

TOI-5375 B: A Very Low Mass Star at the Hydrogen-burning Limit Orbiting an Early M-type Star* *Based on observations obtained with the Hobby–Eberly Telescope (HET), which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Ludwig-Maximillians-Universitaet Muenchen, and Georg-August Universitaet Goettingen. The HET is named in honor of its principal benefactors, William P. Hobby

The Transiting Exoplanet Survey Satellite (TESS) mission detected a companion orbiting TIC 71268730, categorized it as a planet candidate, and designated the system TOI-5375. Our follow-up analysis using radial-velocity data from the Habitable-zone Planet Finder, photometric data from Red Buttes Observatory, and speckle imaging with NN-EXPLORE Exoplanet Stellar Speckle Imager determined that the companion is a very low mass star near the hydrogen-burning mass limit with a mass of 0.080 ± 0.002M ☉ (83.81 ± 2.10M J ), a radius of (1.0841), and brightness temperature of 2600 ± 70 K. This object orbits with a period of 1.721553 ± 0.000001 days around an early M dwarf star (0.62 ± 0.016M ☉). TESS photometry shows regular variations in the host star’s TESS light curve, which we interpreted as an activity-induced variation of ∼2%, and used this variability to measure the host star’s stellar rotation period of days. The TOI-5375 system provides tight constraints on stellar models of low-mass stars at the hydrogen-burning limit and adds to the population in this important region.

How tidal waves interact with convective vortices in rapidly rotating planets and stars

Context. The dissipation of tidal inertial waves in planetary and stellar convective regions is one of the key mechanisms that drive the evolution of star–planet and planet–moon systems. This dissipation is particularly efficient for young low-mass stars and gaseous giant planets, which are rapid rotators. In this context, the interaction between tidal inertial waves and turbulent convective flows must be modelled in a realistic and robust way. In the state-of-the-art simulations, the friction applied by convection on tidal waves is commonly modeled as an effective eddy viscosity. This approach may be valid when the characteristic length scales of convective eddies are smaller than those of the tidal waves. However, it becomes highly questionable in the case where tidal waves interact with potentially stable large-scale vortices such as those observed at the poles of Jupiter and Saturn. The large-scale vortices are potentially triggered by convection in rapidly-rotating bodies in which the Coriolis acceleration forms the flow in columnar vortical structures along the direction of the rotation axis. Aims. We investigate the complex interactions between a tidal inertial wave and a columnar convective vortex. Methods. We used a quasi-geostrophic semi-analytical model of a convective columnar vortex, which is validated by numerical simulations. First, we carried out linear stability analysis using both numerical and asymptotic Wentzel–Kramers–Brillouin–Jeffreys (WKBJ) methods. We then conducted linear numerical simulations of the interactions between a convective columnar vortex and an incoming tidal inertial wave. Results. The vortex we consider is found to be centrifugally stable in the range –Ωp ≤ Ω0 ≤ 3.62Ωp and unstable outside this range, where Ω0 is the local rotation rate of the vortex at its center and Ωp is the global planetary (stellar) rotation rate. From the linear stability analysis, we find that this vortex is prone to centrifugal instability with perturbations with azimuthal wavenumbers m = {0,1, 2}, which potentially correspond to eccentricity, obliquity, and asynchronous tides, respectively. The modes with m &gt; 2 are found to be neutral or stable. The WKBJ analysis provides analytic expressions of the dispersion relations for neutral and unstable modes when the axial (vertical) wavenumber is sufficiently large. We verify that in the unstable regime, an incoming tidal inertial wave triggers the growth of the most unstable mode of the vortex. This would lead to turbulent dissipation. For stable convective columns, the wave-vortex interaction leads to the mixing of momentum for tidal inertial waves while it creates a low-velocity region around the vortex core and a new wave-like perturbation in the form of a progressive wave radiating in the far field. The emission of this secondary wave is the strongest when the wavelength of the incoming wave is close to the characteristic size (radius) of the vortex. Incoming tidal waves can also experience complex angular momentum exchanges locally at critical layers of stable vortices. Conclusions. The interaction between tidal inertial waves and large-scale coherent convective vortices in rapidly-rotating planets (stars) leads to turbulent dissipation in the unstable regime and complex behaviors such as mixing of momentum and radiation of new waves in the far field or wave-vortex angular momentum exchanges in the stable regime. These phenomena cannot be modeled using a simple effective eddy viscosity.

Calibration Target Selection for the SPARCS Satellite

The Star–Planet Activity Research CubeSat (SPARCS) is an upcoming NASA mission to understand the impact of low-mass stellar radiation on exoplanets. SPARCS will obtain photometric observations of low-mass stars in two bands: 153–171 nm (Far-UV; FUV) and 258–308 nm (Near-UV; NUV). SPARCS’ absolute calibration will be derived by observing White Dwarfs (WDs). WD selected as SPARCS calibrators should vary ≤7%. Here we present a study of UV variability of potential WD calibrators, based on Galaxy Evolution Explorer and HST/UVIS photometric observations. We collected data on a sample of 29 WDs from the Mikulski Archive for Space Telescopes and identified 8 potential SPARCS calibration targets in the NUV and 5 in the FUV, for a total of 10 distinct targets. Statistical analysis shows variability <3% in both bands. For some stars, the intrinsic astrophysical variability is smaller than these limits.

3D hydrodynamics simulations of internal gravity waves in red giant branch stars

ABSTRACT We present the first 3D hydrodynamics simulations of the excitation and propagation of internal gravity waves (IGWs) in the radiative interiors of low-mass stars on the red giant branch (RGB). We use the ppmstar explicit gas dynamics code to simulate a portion of the convective envelope and all the radiative zone down to the hydrogen-burning shell of a $1.2\, {\rm M}_{\odot }$ upper RGB star. We perform simulations for different grid resolutions (7683, 15363, and 28803), a range of driving luminosities, and two different stratifications (corresponding to the bump luminosity and the tip of the RGB). Our RGB tip simulations can be directly performed at the nominal luminosity, circumventing the need for extrapolations to lower luminosities. A rich, continuous spectrum of IGWs is observed, with a significant amount of total power contained at high wavenumbers. By following the time evolution of a passive dye in the stable layers, we find that IGW mixing in our simulations is weaker than predicted by a simple analytical prescription based on shear mixing and not efficient enough to explain the missing RGB extra mixing. However, we may be underestimating the efficiency of IGW mixing given that our simulations include a limited portion of the convective envelope. Quadrupling its radial extent compared to our fiducial set-up increases convective velocities by up to a factor 2 and IGW velocities by up to a factor 4. We also report the formation of a $\sim 0.2\, H_P$ penetration zone and evidence that IGWs are excited by plumes that overshoot into the stable layers.

An Estimate of the Distance to AN Ser using an RR Lyrae Period–Luminosity Relationship

The relationship between the period of a Cepheid variable star and its absolute magnitude—the Period-Luminosity, or Leavitt Law—can be used with a measurement of apparent magnitude to measure the star’s distance. RR Lyrae stars are evolved low-mass stars with shorter periods and lower luminosities than Cepheids, but they are particularly useful for estimating distances to the oldest stellar clusters. We observed the brightness of an RR Lyrae star over time using the robotic telescopes of Las Cumbres Observatory to determine its period and apparent magnitude in four bands. We used that information and theoretical period–luminosity relationships for RR Lyrae stars to determine the distance to AN Ser to be 876 ± 25 pc. Our measured distance is smaller than that modeled by GAIA DR3 via stellar parallax, BP/RP spectra, and G-band photometry (959 pc).