Comparable Mass Research Articles

Planetary inward migration as the potential cause of GJ 504 's fast rotation and bright X-ray luminosity. New constraints from eROSITA

The discovery of an increasing variety of exoplanets in very close orbits around their host stars raised many questions about how stars and planets interact and to what extent host stars' properties may be influenced by the presence of close-by companions. Understanding how the evolution of stars is impacted by the interactions with their planets is indeed fundamental to disentangling their intrinsic evolution from star-planet-interaction (SPI) induced phenomena. In this context, GJ 504 is a promising candidate for a star that underwent strong SPI. Its unusually short rotational period (rm P_ rot ∼ 3.4 days), while being in contrast with what is expected of single-star models, could result from the inward migration of a close-by, massive companion (rm M_ pl ≥ 2 M_ J ), pushed towards its host by the action of tides. Moreover, its brighter emission in the X-ray luminosity may hint at a rejuvenation of the dynamo process sustaining the stellar magnetic field, which is a consequence of the SPI-induced spin-up. We aim to study the evolution of GJ 504 and establish whether by invoking the engulfment of a planetary companion we can better reproduce its rotational period and X-ray luminosity. We simulated the past evolution of the star by assuming two different scenarios: `star without close-by planet' and `star with close-by planet'. In the second scenario, we use our SPI code to investigate how the inward migration and eventual engulfment of a giant planet driven by stellar tides may spin-up the stellar surface and rejuvenate its dynamo. We compare our theoretical tracks with archival-rotational-period and X-ray data of GJ 504 collected from the all-sky surveys of the ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Russian Spektrum-Roentgen-Gamma mission (SRG). Despite the large uncertainty on the stellar age, we find that the second evolutionary scenario characterised by the inward migration of a massive planetary companion is in better agreement with the short rotational period and the bright X-ray luminosity of GJ 504; thus, it strongly favours the inward migration scenario over the one in which close-by planets have no tidal impact on the star.

Binary properties of the globular cluster 47 Tuc (NGC 104)

Spectroscopic observations of binary stars in globular clusters are essential to shed light on the poorly constrained period, eccentricity, and mass ratio distributions and to develop an understanding of the formation of peculiar stellar objects. 47 Tuc (NGC 104) is one of the most massive Galactic globular clusters, with a large population of blue stragglers and with many predicted but as-yet elusive stellar-mass black holes. This makes it an exciting candidate for binary searches. We present a multi-epoch spectroscopic survey of 47 Tuc with the VLT/MUSE integral field spectrograph to determine radial velocity variations for 21 699 stars. We find a total binary fraction in the cluster of (2.4 ± 1.0)%, consistent with previous photometric estimates, and an increased binary fraction among blue straggler stars, approximately three times higher than the cluster average. We find very few binaries with periods below three days, and none with massive dark companions. A comparison with predictions from state-of-the-art models shows that the absence of such short-period binaries and of binaries with massive companions is surprising, highlighting the need to improve our understanding of stellar and dynamical evolution in binary systems.

4FGL J1544.2−2554: A new spider pulsar candidate

Context. Spider pulsars are millisecond pulsars in tight binary systems in which a low-mass companion star is heated and ablated by the pulsar wind. Observations of these objects allow one to study stellar evolution with the formation of millisecond pulsars and the physics of superdense matter in neutron stars. However, spiders are rare due to difficulties related to their discovery when using typical radio search techniques. The Fermiγ-ray source 4FGL J1544.2−2554 was recently proposed as a pulsar candidate, and its likely X-ray and optical counterparts, with the galactic coordinates l ≈ 344.​​°76, b ≈ 22.​​°59 and the magnitude G ≈ 20.6, were found using the eROSITA and Gaia surveys. Aims. Our goals are to study whether the source is a new spider pulsar and to estimate its fundamental parameters. Methods. We performed the first optical time series multi-band photometry of the object. We used the Lomb-Scargle periodogram to search for its brightness periodicity and fitted its light curves with a model of direct heating of the binary companion by the pulsar wind. Results. The source shows a strong brightness variability with a period of ≈2.724 h and an amplitude of ≳2.5 mag, and its light curves have a single broad peak per period. These features are typical for spider pulsars. The curves are well fitted by the direct heating model, resulting in an orbit inclination of the presumed spider system of ≈83°, a companion mass of ≈0.1 M⊙, ‘day-side’ and ‘night-side’ temperatures of ≈7200 K and ≈3000 K, a Roche lobe filling factor of ≈0.65, and a distance of ≈2.1 kpc. Conclusions. Our findings suggest that 4FGL J1544.2−2554 is a spider pulsar. This result encourages the search for pulsar millisecond pulsations in radio and γ-rays to confirm its nature.

A Systematic Search for Redback and Black Widow Candidates Based on the 4FGL-DR3 Unassociated Sources and the Zwicky Transient Facility Data

Spider pulsars represent a unique subclass of radio millisecond pulsars in binaries, and are further categorized into black widows and redbacks according to the mass of the low-mass companion. These pulsars, observable across multiple wavelengths, exhibit periodic variability in optical. The discovery and study of additional spider-type pulsars are crucial for a fuller understanding the evolution of binary stars in close orbits and the recycling theory of millisecond pulsars. In this work, we systematically searched for spider pulsar binary systems using time-domain variability data from the Zwicky Transient Facility and unassociated gamma-ray sources from the Fermi 4FGL-DR3 catalog. We developed a time-domain data processing pipeline that employs the Lomb–Scargle periodogram algorithm. As a result, we identified a total of 194 ellipsoidal variables and irradiation-type binary stars. Further refinement using the Gaia Hertzsprung–Russell diagram resulted in a selection of 24 spider pulsar candidates. Incorporating the 4FGL 95% confidence error ellipse reduced the sample to 19 candidates. An additional filter using the Gaia color-reduced proper motion diagram yielded nine “gold sample” candidates. These newly identified spider pulsar candidates will guide future observational campaigns in radio, X-ray, and optical spectroscopy, aiding in the comprehensive validation of their nature.

Detection and characterization of giant planets with Gaia astrometry

ABSTRACT Astrometric observations with Gaia are expected to play a valuable role in future exoplanet surveys. With current data from Gaia’s third data release (DR3), we are sensitive to periods from less than 1 yr to more than 4 yr but, unlike radial velocity (R.V.) are not as restricted by the orbital inclination of a potential planet. The presence and potential properties of a companion affect the primary’s renormalized unit weight error (RUWE) making this a valuable quantity in the search for exoplanets. Using this value and the fitted astrometric tracks from Gaia, we use Bayesian inference to constrain the mass and orbital parameters of companions in known systems. Combining this with R.V. measurements, we show it is possible to independently measure mass and inclination and suggest HD 66141 b is a possible brown dwarf with maximum mass 23.9$^{+7.2}_{-6.4}$ $\mathrm{ M}_{\mathrm{J}}$. We show how this method may be applied to directly imaged planets in the future, using $\beta$-Pictoris c as an example but note that the host star is bright and active, making it difficult to draw clear conclusions. We show how the next Gaia data release, which will include epoch astrometry, will allow us to accurately constrain orbital parameters from astrometric data alone, revolutionizing future searches for exoplanets. Combining predicted observational limits on planet mass with theoretical distributions, we estimate the probability that a star with a given RUWE will host a detectable planet, which will be highly valuable in planning future surveys.

VLTI/GRAVITY Observations of AF Lep b: Preference for Circular Orbits, Cloudy Atmospheres, and a Moderately Enhanced Metallicity

Direct imaging observations are biased toward wide-separation, massive companions that have degenerate formation histories. Although the majority of exoplanets are expected to form via core accretion, most directly imaged exoplanets have not been convincingly demonstrated to follow this formation pathway. We obtained new interferometric observations of the directly imaged giant planet AF Lep b with the VLTI/GRAVITY instrument. We present three epochs of ∼50 μas relative astrometry and the K-band spectrum of the planet for the first time at a resolution of R = 500. Using only these measurements, spanning less than 2 months, and the Hipparcos-Gaia Catalogue of Accelerations, we are able to significantly constrain the planet’s orbit; this bodes well for interferometric observations of planets discovered by Gaia DR4. Including all available measurements of the planet, we infer an effectively circular orbit (e < 0.02, 0.07, and 0.13 at 1σ, 2σ, and 3σ, respectively) in spin–orbit alignment with the host and measure a dynamical mass of M p = 3.75M Jup ± 0.5M Jup. Models of the spectrum of the planet show that it is metal-rich ([M/H] = 0.75 ± 0.25), with a C/O abundance encompassing the solar value. This ensemble of results shows that the planet is consistent with core accretion formation.

A multi-technique detection of an eccentric giant planet around the accelerating star HD 57625

The synergy between different detection methods is a key asset in exoplanetology that allows the precise characterization of detected exoplanets and robust constraints even in the case of a non-detection. The interplay between imaging, radial velocities and astrometry has recently produced significant advancements in exoplanetary science. We report a first result of an ongoing survey performed with SHARK-NIR, the new high-contrast near-infrared imaging camera at the Large Binocular Telescope, in parallel with LBTI/LMIRCam in order to detect planetary companions around stars with a significant proper motion anomaly. We focus on HD\,57625, a F8 star for which we determine a $ Ga age, exhibiting significant astrometric acceleration and for which archival radial velocities indicate a previously undetected massive long-period companion. We analysed the imaging data we collected with SHARK-NIR and LMIRCam in synergy with the available public SOPHIE radial velocity time series and Hipparcos-Gaia proper motion anomaly. With this joint multi-technique analysis, we characterised the companion causing the astrometric and radial velocity signals. The imaging observations result in a non-detection, indicating the companion to be in the substellar regime. This is confirmed by the synergic analysis of archival radial velocity and astrometric measurements resulting in the detection of HD\,57625\,b, a planetary companion with an orbital separation of and an eccentricity of HD\,57625\,b joins the small but growing population of giant planets on outer orbits with a true mass determination provided by the synergic usage of multiple detection methods. This again proves the importance of a multi-technique analysis in providing a robust characterization of planetary companions.

A GTC spectroscopic study of three spider pulsar companions: Line-based temperatures, a new face-on redback, and improved mass constraints

Abstract We present GTC-OSIRIS phase-resolved optical spectroscopy of three compact binary MSPs, or ‘spiders’: PSR J1048+2339, PSR J1810+1744, and (for the first time) PSR J1908+2105. For the companion in each system, the temperature is traced throughout its orbit, and radial velocities are measured. The radial velocities are found to vary with the absorption features used when measuring them, resulting in different radial velocity curve semi-amplitudes: for J1048 (Kmetals, red = 344 ± 4 km s−1, Kmetals, blue = 372 ± 3 km s−1) and, tentatively, for J1810 (KBalmer = 448 ± 19 km s−1, Kmetals = 491 ± 32 km s−1). With existing inclination constraints, this gives the neutron star (NS) and companion masses MNS = 1.50–2.04 M⊙ and M2 = 0.32–0.40 M⊙ for J1048, and MNS &amp;gt; 1.7 M⊙ and M2 = 0.05–0.08 M⊙ for J1810. For J1908, we find an upper limit of K2 &amp;lt; 32 km s−1, which constrains its mass ratio q = M2/MNS &amp;gt; 0.55 and inclination i &amp;lt; 6.0○, revealing the previously misunderstood system to be the highest mass ratio, lowest inclination redback yet. This raises questions for the origins of its substantial radio eclipses. Additionally, we find evidence of asymmetric heating in J1048 and J1810, and signs of metal enrichment in J1908. We also explore the impact of inclination on spectroscopic temperatures, and demonstrate that the temperature measured at quadrature (φ = 0.25, 0.75) is essentially independent of inclination, and thus can provide additional constraints on photometric modelling.

The Companion Mass Distribution of Post Common Envelope Hot Subdwarf Binaries: Evidence for Boosted and Disrupted Magnetic Braking?

Abstract We measure the mass distribution of main-sequence (MS) companions to hot subdwarf B stars (sdBs) in post-common envelope binaries (PCEBs). We carried out a spectroscopic survey of 14 eclipsing systems (“HW Vir binaries”) with orbital periods of 3.8 &lt; P orb &lt; 12 hr, resulting in a well-understood selection function and a near-complete sample of HW Vir binaries with G &lt; 16. We constrain companion masses from the radial velocity curves of the sdB stars. The companion mass distribution peaks at M MS ≈ 0.15 M ⊙ and drops off at M MS &gt; 0.2 M ⊙, with only two systems hosting companions above the fully convective limit. There is no correlation between P orb and M MS within the sample. A similar drop-off in the companion mass distribution of white dwarf (WD) + MS PCEBs has been attributed to disrupted magnetic braking (MB) below the fully convective limit. We compare the sdB companion mass distribution to predictions of binary evolution simulations with a range of MB laws. Because sdBs have short lifetimes compared to WDs, explaining the lack of higher-mass MS companions to sdBs with disrupted MB requires MB to be boosted by a factor of 20–100 relative to MB laws inferred from the rotation evolution of single stars. We speculate that such boosting may be a result of irradiation-driven enhancement of the MS stars’ winds. An alternative possibility is that common envelope evolution favors low-mass companions in short-period orbits, but the existence of massive WD companions to sdBs with similar periods disfavors this scenario.

Investigating 39 Galactic Wolf-Rayet stars with VLTI/GRAVITY

Context. Wolf-Rayet stars (WRs) represent one of the final evolutionary stages of massive stars and are thought to be the immediate progenitors of stellar-mass black holes. Their multiplicity characteristics form an important anchor point in single and binary population models for predicting gravitational-wave progenitors. Recent spectroscopic campaigns have suggested incompatible multiplicity fractions and period distributions for N- and C-rich Galactic WRs (WNs and WCs) at both short and long orbital periods, in contradiction with evolutionary model predictions. Aims. In this work, we employed long-baseline infrared interferometry to investigate the multiplicity of WRs at long periods and explored the nature of their companions. We present a magnitude-limited (K &lt; 9; V &lt; 14) survey of 39 Galactic WRs, including 11 WN, 15 WC, and 13 H-rich WN (WNh) stars. Methods. We used the K-band instrument GRAVITY at the Very Large Telescope Interferometer (VLTI) in Chile. The sensitivity of GRAVITY at spatial scales of ∼1 to 200 milliarcseconds and flux contrast of 1% allowed an exploration of periods in the range 102 − 105 d and companions down to ∼5 M⊙. We carried out a companion search for all our targets, with the aim of either finding wide companions or calculating detection limits. We also explored the rich GRAVITY dataset beyond a multiplicity search to look for other interesting properties of the WR sample. Results. We detected wide companions with VLTI/GRAVITY for only four stars in our sample: WR 48, WR 89, WR 93, and WR 115. Combining our results with spectroscopic studies, we arrived at observed multiplicity fractions of fobsWN = 0.55 ± 0.15, fobsWC = 0.40 ± 0.13, and fobsWNh = 0.23 ± 0.12. The multiplicity fractions and period distributions of WNs and WCs are consistent in our sample. For single WRs, we placed upper limits on the mass of potential companions down to ∼5 M⊙ for WNs and WCs, and ∼7 M⊙ for WNh stars. In addition, we also found other features in the GRAVITY dataset, such as (i) a diffuse extended component contributing significantly to the K-band flux in over half the WR sample; (ii) five known spectroscopic binaries resolved in differential phase data, which constitutes an alternative detection method for close binaries; and (iii) spatially resolved winds in four stars: WR 16, WR 31a, WR 78, and WR 110. Conclusions. Our survey reveals a lack of intermediate- (a few hundred days) and long- (a few years to decades) period WR systems. The 200d peak in the period distributions of WR+OB and BH+OB binaries predicted by Case B mass-transfer binary evolution models is not seen in our data. The rich companionship of their O-type progenitors in this separation range suggests that the WR progenitor stars expand and interact with their companions, most likely through unstable mass transfer, resulting in either a short-period system or a merger.

Constraining the Presence of Companion Planets in Hot Jupiter Planetary Systems Using Transit-timing Variation Observations from TESS

The presence of another planetary companion in a transiting exoplanet system can impact its transit light curve, leading to sinusoidal transit-timing variations (TTV). By utilizing both χ 2 and rms analysis, we have combined the TESS observation data with an N-body simulation to investigate the existence of an additional planet in the system and put a limit on its mass. We have developed CMAT, an efficient and user-friendly tool for fitting transit light curves and calculating TTV with a theoretical period, based on which we can give a limit on its hidden companion’s mass. We use 260 hot Jupiter systems from the complete TESS data set to demonstrate the use of CMAT. Our findings indicate that, for most systems, the upper mass limit of a companion planet can be restricted to several Jupiter masses. This constraint becomes stronger near resonance orbits, such as the 1:2, 2:1, 3:1, and 4:1 mean-motion resonance, where the limit is reduced to several Earth masses. These findings align with previous studies suggesting that a lack of companion planets with resonance in hot Jupiter systems could potentially support the high-eccentricity migration theory. Additionally, we observed that the choice between χ 2 or rms method does not significantly affect the upper limit on companion mass; however, χ 2 analysis may result in weaker restrictions but is statistically more robust compared to rms analysis in most cases.

Population of giant planets around B stars from the first part of the BEAST survey

Exoplanets form from circumstellar protoplanetary disks whose fundamental properties (notably their extent, composition, mass, temperature, and lifetime) depend on the host star properties, such as their mass and luminosity. B stars are among the most massive stars and their protoplanetary disks test extreme conditions for exoplanet formation. This paper investigates the frequency of giant planet companions around young B stars (median age of 16 Myr) in the Scorpius-Centaurus (Sco-Cen) association, the closest association containing a large population of B stars. We systematically searched for massive exoplanets with the high-contrast direct imaging instrument SPHERE using the data from the BEAST survey, which targets a homogeneous sample of young B stars from the wide Sco-Cen association. We derived accurate detection limits in the case of non-detections. We found evidence in previous papers for two substellar companions around 42 stars. The masses of these companions are straddling the sim 13 Jupiter mass deuterium burning limit, but their mass ratio with respect to their host star is close to that of Jupiter. We derived a frequency of such massive planetary-mass companions around B stars of $11_ $&lt;!PCT!&gt;, accounting for the survey sensitivity. The discoveries of substellar companions b and B happened after only a few stars in the survey had been observed, raising the possibility that massive Jovian planets might be common around B stars. However, our statistical analysis shows that the occurrence rate of such planets is similar around B stars and around solar-type stars of a similar age, while B-star companions exhibit low mass ratios and a larger semi-major axis.

Blazar S 0528+134 is possibly the most powerful emitter in the Universe, including in the range of gravitational waves

Aims. A new method of determining the parameters of close binary systems of supermassive black holes (SMBHs) and the level of gravitational waves (GWs) on the Earth’s surface are proposed. Methods. Data are presented from long-term monitoring of possibly the most powerful emitter in the Universe, S 0528+134, at five frequencies in the radio range from 4.8 GHz to 37 GHz, obtained by the RT-22 radio telescope of the Radio Astronomy Laboratory CrAO in Simeiz, the RT-26 radio telescope at Michigan Observatory, and the 40 m radio telescope of the Owens Valley Radio Observatory. Results. The dynamics of powerful flares that have occurred in the object since its discovery in a sky survey in 1970 were considered. The main physical characteristics of binary SMBHs located in the central regions of the system were obtained. These data were used to find the masses of the SMBH companions, the parameters of their orbits, the energy reserve of the system, and the lifetime of the object before the SMBHs’ merger. The level of GWs on the Earth’s surface was determined and the possibility of their detection by International Pulsar Timing Array (IPTA) GW detectors was considered.

The masses of open star clusters and their tidal tails and the stellar initial mass function

Context. Unresolved binaries have a strong influence on the observed parameters of stellar clusters (SCs). Aims. We quantify this influence and compute the resulting mass underestimates and stellar mass function (MF). Methods. N-body simulations of realistic SCs were used to investigate the evolution of the binary population in a SC and its tidal tails. Together with an empirically gauged stellar mass-luminosity relation, the results were then used to determine how the presence of binaries changes the photometric mass and MF of the SC and its tails as deduced from observations. Results. Tail 1 (T1), which is the tidal tail caused by gas expulsion, contains a larger fraction of binaries than both the SC and Tail 2 (T2), which forms after gas expulsion. Additionally, T1 has a larger velocity dispersion. Using the luminosity of an unresolved binary, an observer would underestimate its mass. This bias sensitively depends on the companion masses due to the structure of the stellar mass-luminosity relation. Combining the effect of all binaries in the simulation, the total photometric mass of the SC is underestimated by 15%. Dark objects (black holes and neutron stars) increase the difference between the real and observed mass of the SC further. For both the SC and the tails, the observed power-law index of the MF between a stellar mass of 0.3 and 0.7 M⊙ is smaller by up to 0.2 than the real one, the real initial mass function (IMF) being steeper by this amount. This difference is larger for stars with a larger velocity dispersion or binary fraction. Conclusions. Since the stars formed in SCs are the progenitors of the Galactic field stars, this work suggests that the binary fractions of different populations of stars in the Galactic disc will differ as a function of the velocity dispersion. However, the direction of this correlation is currently unclear, and a complete population synthesis will be needed to investigate this effect. Variations in the binary fractions of different clusters can lead to perceived variations of the deduced stellar MFs.

Photometric study for the short period contact binary V724 And

We present the new light curve synthesis and orbital period change analysis of V724 And. We found that it is a W-subtype contact binary system with a mass ratio of q = 2.31 and a fill-out factor of f = 10.8%. The weak O’Connell effect can be explained by a cool star-spot on the more massive component star. By analyzing orbital period changes, it is found that the target displays a secular period decrease superimposed on a cyclic oscillation. The orbital period decreases at a rate of dp/dt = −4.13 × 10−8 d yr−1, which can be explained by the mass transfer from the more massive one to the less massive component. The new discovery suggests that the semi-amplitude and period of the cyclic variation are A = 0.0013 d and P = 10.54 yr, respectively, which implies that V724 And is a possibly triple system. The minimum mass of the tertiary companion is estimated as M3min = 0.054 M⊙, which plays an important role during the formation and evolution of the central binary system.

The Smallest Scale of Hierarchy Survey (SSH)

We present new deep, wide-field imaging data from the Large Binocular Telescope (LBT) in g and r bands from the Smallest Scale of Hierarchy Survey (SSH) that reveal previously undetected tidal features and stellar streams in the outskirts of six dwarf irregular galaxies (NGC 5238, UGC 6456, UGC 6541, UGC 7605, UGC 8638, and UGC 8760) with stellar masses in the range 1.2 × 107 M⊙ to 1.4 × 108 M⊙. The six dwarfs are located 1-2 Mpc away from large galaxies, which implies that the observed distortions are unlikely to be due to tidal effects from a nearby, massive companion. At the distances of ~3-4 Mpc at which the dwarfs lie, the identified tidal features are all resolved into individual stars in the LBT images and appear to consist of a population older than 1–2 Gyr. This excludes the possibility that they result from irregular and asymmetric star formation episodes that are common in gas-rich dwarf galaxies. The most plausible explanation is that we witness the hierarchical merging assembly of these dwarfs with their satellite populations. This scenario is also supported by the peculiar morphology and disturbed velocity field of their HI component. From the SSH sample, we estimate that a fraction of ~13% of the late-type dwarfs show signs of merging with satellites. This is in agreement with other recent independent studies and theoretical predictions within the ΛCDM cosmological framework.

Follow-up Timing of 12 Pulsars Discovered in Commensal Radio Astronomy FAST Survey

We present phase-connected timing ephemerides, polarization pulse profiles, and Faraday rotation measurements of 12 pulsars discovered by the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in the Commensal Radio Astronomy FAST Survey. The observational data for each pulsar span at least 1 yr. Among them, PSR J1840+2843 shows subpulse drifting, and five pulsars are detected to exhibit pulse nulling phenomena. PSR J0640−0139 and PSR J2031−1254 are isolated millisecond pulsars (MSPs) with stable spin-down rates ( Ṗ ) of 4.8981(6) × 10−20 s s−1 and 6.01(2) × 10−21 s s−1, respectively. Additionally, one pulsar (PSR J1602−0611) is in a neutron star−white dwarf (WD) binary system with an 18.23-day orbit and a companion of ≤0.65 M ⊙. PSR J1602−0611 has a spin period, companion mass, and orbital eccentricity that are consistent with the theoretical expectations for MSP−helium WD (He WD) systems. Therefore, we believe that it might be an MSP−He WD binary system. The locations of PSR J1751−0542 and PSR J1840+2843 on the P−Ṗ diagram are beyond the traditional death line. This indicates that FAST has discovered some low- Ė pulsars, contributing new samples for testing pulsar radiation theories. We estimated the distances of these 12 pulsars based on NE2001 and YMW16 electron density models, and our work enhances the data set for investigating the electron density model of the Galaxy.

SPH modelling of AGB wind morphology in hierarchical triple systems and a comparison to observation of R Aql

Context. Complex asymmetric 3D structures are observed in the outflows of evolved low- and intermediate-mass stars, and are believed to be shaped through the interaction of companions that remain hidden within the dense wind. One example is the AGB star R Aql, for which ALMA (Atacama Large Millimeter Array) observations have revealed complex wind structures that might originate from a higher-order stellar system. Aims. We investigate how triple systems can shape the outflow of asymptotic giant branch (AGB) stars and characterise the different wind structures that form. For simplicity, we solely focus on co-planar systems in a hierarchical, stable orbit, consisting of an AGB star with one relatively close companion, and another one at a large orbital separation. Methods. We modelled a grid of hierarchical triple systems including a wind-launching AGB star, with the smoothed-particle- hydrodynamic PHANTOM code. We varied the outer companion mass, the AGB wind velocity, and the orbital eccentricities to study the impact of these parameters on the wind morphology. To study the impact of adding a triple companion, we additionally modelled and analysed a small grid of binary sub-systems, for comparison. To investigate if R Aql could be shaped by a triple system, we postprocessed one of our triple models with a radiative transfer routine, and compared this to data of the ALMA ATOMIUM programme. Results. The characteristic wind structures resulting from a hierarchical triple system are the following. A large two-edged spiral wake results behind the outer companion star. This structure lies on top of the spiral structure formed by the close binary, which is itself affected by the orbital motion around the system’s centre of mass, such that it resembles a snail-shell pattern. This dense inner spiral pattern interacts with, and strongly impacts, the spiral wake of the outer companion, resulting in a wave pattern on the outer edge of this spiral wake. The higher the mass of the outer companion, the larger the density enhancement and the more radially compressed the outer spiral. Lowering the wind velocity has a similar effect, and additionally results in an elongation of the global wind morphology. Introducing eccentricity in the inner and outer orbit of the hierarchical system results in complex phase-dependent wind-companion interactions, and consequently in asymmetries in the inner part of the wind and the global morphology, respectively. From the comparison of our models to the observations of R Aql, we conclude that this circumstellar environment might be shaped by a similar system to the ones modelled in this work, but an elaborate study of the observational data is needed to better determine the orbital parameters and characteristics of the central system. Conclusions. The modelled outflow of an AGB star in a co-planar hierarchical systems is characterised by a large-scale spiral wake with a wavey outer edge, attached to the outer companion, on top of a compact inner spiral pattern that resembles a snail-shell pattern.

Triple trouble with PSR J1618−3921: Mass measurements and orbital dynamics of an eccentric millisecond pulsar

Context. PSR J1618−3921 is one of five known millisecond pulsars (MSPs) in eccentric orbits (eMPSs) located in the Galactic plane, whose formation is poorly understood. Earlier studies of these objects revealed significant discrepancies between observations and predictions from standard binary evolution scenarios of pulsar-helium white dwarf (HeWD) binaries, especially in the case of PSR J0955−6150, for which mass measurements ruled out most eMSP formation models. Aims. We aim to measure the masses of the pulsar and its companion, and constrain the orbital configuration of PSR J1618−3921. This facilitates understanding similarities among eMSPs and could offer hints as to their formation mechanism. Methods. We conducted observations with the L-band receiver of the MeerKAT radio telescope and the UWL receiver of the Parkes Murriyang radio telescope between 2019 and 2021. These data were added to archival Parkes and Nançay observations. We performed a full analysis on this joint data set with a timing baseline of 23 years. We also used the data from recent observations to give a brief account of the emission properties of J1618−3921, including a rotating vector model (RVM) fit of the linear polarisation position angle of the pulsar. Results. From the timing analysis, we measure a small but significant proper motion of the pulsar. The long timing baseline allowed for a highly significant measurement of the rate of advance of periastron of ω̇ = (0.00145±0.00010)°yr−1. Despite the tenfold improvement in timing precision from MeerKAT observations, we can only report a low-significance detection of the orthometric Shapiro delay parameters, h3 = 2.70−1.47+2.07 μs and ς = 0.68−0.09+0.13. Under the assumption of the validity of general relativity (GR), the self-consistent combination of these three parameters leads to mass estimates of the total and individual masses in the binary of Mtot = 1.42−0.19+0.20 M⊙, Mc = 0.20−0.03+0.11 M⊙, and Mp = 1.20−0.20+0.19 M⊙. We detect an unexpected change in the orbital period of Ṗb = −2.26−0.33+0.35 × 10−12, that is an order of magnitude larger and carries an opposite sign to what is expected from the Galactic acceleration and the Shklovskii effect, which are a priori the only non-negligible contributions expected for Ṗb. We also detect a significant second derivative of the spin frequency, f̈. The RVM fit reveals a viewing angle of ζ = (111 ± 1)°. Furthermore, we report an unexpected, abrupt change in the mean pulse profile in June 2021 of unknown origin. Conclusions. We propose that the anomalous Ṗb and f̈ we measure for J1618−3921 indicate an additional varying acceleration due to a nearby mass. The J1618−3921 binary system is likely part of a hierarchical triple, but with the third component much farther away than the outer component of the MSP in a triple star system, PSR J0337+1715. This finding suggests that at least some eMSPs might have formed in triple star systems. Although the uncertainties are large, the binary companion mass is consistent with the Pb − MWD relation, which has been verified for circular HeWD binaries and also for the two HeWDs in the PSR J0337+1715 system. Future regular observations with the MeerKAT telescope will, due to the further extension of the timing baseline, improve the measurement of Ṗb and f̈. This will help us further understand the nature of this system, and perhaps improve our understanding of eMSPs in general.

Tidal Spin-up of Subdwarf B Stars

Hot subdwarf B (sdB) stars are stripped helium-burning stars that are often found in close binaries, where they experience strong tidal interactions. The dissipation of tidally excited gravity waves alters their rotational evolution throughout the sdB lifetime. While many sdB binaries have well-measured rotational and orbital frequencies, there have been few theoretical efforts to accurately calculate the tidal torque produced by gravity waves. In this work, we directly calculate the tidal excitation of internal gravity waves in realistic sdB stellar models and integrate the coupled spin–orbit evolution of sdB binaries. We find that for canonical sdB (M sdB = 0.47 M ⊙) binaries, the transitional orbital period below which they could reach tidal synchronization in the sdB lifetime is ∼0.2 day, with weak dependence on the companion masses. For low-mass sdBs (M sdB = 0.37 M ⊙) formed from more massive progenitor stars, the transitional orbital period becomes ∼0.15 day. These values are very similar to the tidal synchronization boundary (∼0.2 day) evident from observations. We discuss the dependence of tidal torques on stellar radii, and we make predictions for the rapidly rotating white dwarfs formed from synchronized sdB binaries.