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We have determined positions, proper motions, and parallaxes of 77 millisecond pulsars (MSPs) from ∼3 yr of MeerKAT radio telescope observations. Our timing and noise analyses enable us to measure 35 significant parallaxes (12 of them for the first time) and 69 significant proper motions. Eight pulsars near the ecliptic have an accurate proper motion in ecliptic longitude only. PSR J0955−6150 has a good upper limit on its very small proper motion (<0.4 mas yr−1). We used pulsars with accurate parallaxes to study the MSP velocities. This yields 39 MSP transverse velocities, and combined with MSPs in the literature (excluding those in Globular Clusters) we analyse 66 MSPs in total. We find that MSPs have, on average, much lower velocities than normal pulsars, with a mean transverse velocity of only 78(8) km s−1 (MSPs) compared with 246(21) km s−1 (normal pulsars). We found no statistical differences between the velocity distributions of isolated and binary MSPs. From Galactocentric cylindrical velocities of the MSPs, we derive 3D velocity dispersions of σρ, σϕ, σ z  = 63(11), 48(8), 19(3) km s−1. We measure a mean asymmetric drift with amplitude 38(11) km s−1, consistent with expectation for MSPs, given their velocity dispersions and ages. The MSP velocity distribution is consistent with binary evolution models that predict very few MSPs with velocities >300 km s−1 and a mild anticorrelation of transverse velocity with orbital period. [ABSTRACT FROM AUTHOR]

The ability to measure precise and accurate stellar effective temperatures (T eff) and surface gravities (log (g)) is essential in determining accurate and precise abundances of chemical elements in stars. Measuring log (g) from isochrones fitted to colour–magnitude diagrams of open clusters is significantly more accurate and precise compared to spectroscopic log (g). By determining the ranges of ages, metallicity, and extinction of isochrones that fit the colour–magnitude diagram, we constructed a joint probability distribution of T eff and log (g). The joint photometric probability shows the complex correlations between T eff and log (g), which depend on the evolutionary stage of the star. We show that by using this photometric prior while fitting spectra, we can acquire more precise spectroscopic stellar parameters and abundances of chemical elements. This reveals higher-order abundance trends in open clusters like traces of atomic diffusion. We used photometry and astrometry provided by the Gaia DR3 catalogue, Padova isochrones, and Galactic Archaeology with HERMES (GALAH) DR4 spectra. We analysed the spectra of 1979 stars in nine open clusters, using MCMC to fit the spectroscopic abundances of 26 elements, T eff, log (g), v mic, and v broad. We found that using photometric priors improves the accuracy of abundances and log (g), which enables us to view higher order trends of abundances caused by atomic diffusion in M67 and Ruprecht 147. [ABSTRACT FROM AUTHOR]

We use TESS 10-min full-frame images (Sectors 27–55) to study a sample of 1708 stars within 500 pc of the Sun that lie in a narrow colour range in the centre of the δ Scuti instability strip (0.29 < G BP − G RP < 0.31). Based on the Fourier amplitude spectra, we identify 848 δ Scuti stars, as well as 47 eclipsing or contact binaries. The strongest pulsation modes of some δ Scuti stars fall on the period–luminosity relation of the fundamental radial mode but many correspond to overtones that are approximately a factor of two higher in frequency. Many of the low-luminosity δ Scuti stars show a series of high-frequency modes with very regular spacings. The fraction of stars in our sample that show δ Scuti pulsations is about 70 per cent for the brightest stars (G < 8), consistent with results from Kepler. However, the fraction drops to about 45 per cent for fainter stars and we find that a single sector of TESS data only detects the lowest amplitude δ Scuti pulsations (around 50 ppm) in stars down to about G  = 9. Finally, we have found four new high-frequency δ Scuti stars with very regular mode patterns, and have detected pulsations in λ Mus that make it the fourth-brightest δ Scuti in the sky (G  = 3.63). Overall, these results confirm the power of TESS and Gaia for studying pulsating stars. [ABSTRACT FROM AUTHOR]

In this paper, we construct the circular velocity curve of the Milky Way out to ∼30 kpc, providing an updated model of the dark matter density profile. We derive precise parallaxes for 120 309 stars with a data-driven model, using APOGEE DR17 spectra combined with Gaia DR3, 2MASS , and WISE photometry. At outer galactic radii up to 30 kpc, we find a significantly faster decline in the circular velocity curve compared to the inner parts. This decline is better fit with a cored Einasto profile with a slope parameter |$0.91^{+0.04}_{-0.05}$| than a generalized Navarro–Frenk–White (NFW) profile. The virial mass of the best-fitting dark matter halo profile is only |$1.81^{+0.06}_{-0.05}\times 10^{11}$| M⊙, significantly lower than what a generalized NFW profile delivers. We present a study of the potential systematics, affecting mainly large radii. Such a low mass for the Galaxy is driven by the functional forms tested, given that it probes beyond our measurements. It is found to be in tension with mass measurements from globular clusters, dwarf satellites, and streams. Our best-fitting profile also lowers the expected dark matter annihilation signal flux from the galactic centre by more than an order of magnitude, compared to an NFW profile-fit. In future work, we will explore profiles with more flexible functional forms to more fully leverage the circular velocity curve and observationally constrain the properties of the Milky Way's dark matter halo. [ABSTRACT FROM AUTHOR]

Hydrogen-deficient stars include the cool R CrB variable (RCBs) and hydrogen-deficient carbon (HdCs) giants through extreme helium stars (EHes) to the very hot helium-rich subdwarfs (He-sdO and O(He) stars) and white dwarfs. With surfaces rich in helium, nitrogen, and carbon, their origins have been identified with the merger of two white dwarfs. Using Gaia to focus on the EHes, we aim to identify progenitor populations and test the evolution models. Gaia DR3 measurements and ground-based radial velocities have been used to compute Galactic orbits using galpy. Each orbit has been classified by population; EHe stars are found in all of the thin disc, thick disc, halo, and bulge, as are RCB, HdC, and He-sdO stars. Spectral energy distributions were constructed for all EHes, to provide angular diameters, and hence radii and luminosities. The EHes fall into two luminosity groups divided at |$L \approx 2500 \, {\rm L_{\odot }}$|⁠. This supports theory for the origin of EHes, and is the strongest confirmation so far in terms of luminosity. The lower luminosity EHes correspond well with the post-merger evolution of a double helium white dwarf binary. Likewise, the higher luminosity EHes match the post-merger evolution of a carbon/oxygen plus helium white dwarf binary. In terms of parent populations, current models predict that double white dwarf mergers should occur in all Galactic populations, but favour mergers arising from recent star formation (i.e. thin disc), whereas the statistics favour an older epoch (i.e. thick disc). [ABSTRACT FROM AUTHOR]

We investigate the spatial structure and dynamical properties of the young (∼60 Myr), richly populated open cluster NGC 6649 based on reliable cluster members from Gaia -DR3 data. The density-based spatial clustering of applications with noise is used to estimate membership probabilities for 25045 sample stars in the field of the cluster. More than 1000 likely cluster members down to G ∼ 20 mag can be identified based on the membership probabilities. The cluster members of NGC 6649 extend up to a radial distance of |${\sim} 21\,\mathrm{ arcmin}$| (12 pc), which is much larger than previously thought. We estimate a core radius of |$R_{\mathrm{ c}}=2\overset{^{\prime }}{.}38\pm 0\overset{^{\prime }}{.}06$| (1.4 pc) and a tidal radius of |$R_{\mathrm{ t}}=35\overset{^{\prime }}{.}85\pm 6\overset{^{\prime }}{.}64$| (20.9 pc) for the cluster, indicating that the cluster has formed a dense core and an extended halo. We find that mass segregation has taken place within the cluster. We also detect two small and sparse clusters (C1 and C2) around NGC 6649. C1 has recently been reported, but C2 is detected for the first time. The Gaussian mixture model clustering method is used to identify 184 and 60 members for C1 and C2, respectively. We find that NGC 6649, C1, and C2 are located at similar distances. Moreover, we estimate a small spatial separation of ∼11.6 pc between C1 and NGC 6649, which is close to or slightly smaller than the detectable radius of NGC 6649. This indicates that NGC 6649 and C1 are physically connected, and there may exist strong tidal interactions and mass transfer between the two clusters. C2 shows a significant lack of faint members (G > 17 mag), probably due to star evaporation and tidal stripping. Our results show that NGC 6649 may be the dominant cluster in a triple-cluster system (NGC 6649+C1+C2). [ABSTRACT FROM AUTHOR]

Using multifrequency very long baseline interferometry (VLBI) observations, we probe the jet size in the optically thick hard state jets of two black hole X-ray binary (BHXRB) systems, MAXI J1820+070 and V404 Cygni. Because of optical depth effects, the phase-referenced VLBI core positions move along the jet axis of the BHXRB in a frequency-dependent manner. We use this 'core shift' to constrain the physical size of the hard state jet. We place an upper limit of 0.3 au on the jet size measured between the 15 and 5 GHz emission regions of the jet in MAXI J1820+070, and an upper limit of 1.0 au between the 8.4 and 4.8 GHz emission regions of V404 Cygni. Our limit on the jet size in MAXI J1820+070 observed in the low-hard state is a factor of 5 smaller than the values previously observed in the high-luminosity hard state (using time lags between multifrequency light curves), thus showing evidence of the BHXRB jet scaling in size with jet luminosity. We also investigate whether motion of the radio-emitting region along the jet axis could affect the measured VLBI parallaxes for the two systems, leading to a mild tension with the parallax measurements of Gaia. Having mitigated the impact of any motion along the jet axis in the measured astrometry, we find the previous VLBI parallax measurements of MAXI J1820+070 and V404 Cygni to be unaffected by jet motion. With a total time baseline of 8 yr, due to having incorporated 14 new epochs in addition to the previously published ones, our updated parallax measurement of V404 Cygni is 0.450 ± 0.018 mas (2.226 ± 0.091 kpc). [ABSTRACT FROM AUTHOR]

We compile a catalogue of low-mass and high-mass X-ray binaries, some recently reported binaries that likely host a neutron star (NS) or a black hole (BH), and binary pulsars (a pulsar and a non-degenerated companion) that have measured systemic radial velocities (γ). Using Gaia and radio proper motions together with γ, we integrate their Galactic orbits and infer their post-supernova (post-SN) 3D peculiar velocities (⁠|$v_\mathrm{pec }^{z=0}$| at Galactic plane crossing); these velocities bear imprints of the natal kicks that compact objects received at birth. With the sample totalling 85 objects, we model the overall distribution of |$v_\mathrm{pec }^{z=0}$| and find a two-component Maxwellian distribution with a low- (⁠|$\sigma _v \approx 21\, \mathrm{km s^{-1}}$|⁠) and a high-velocity (⁠|$\sigma _v \approx 107\, \mathrm{km s^{-1}}$|⁠) component. A further comparison between distributions of binary subgroups suggests that binaries hosting high-mass donors/luminous companions mostly have |$v_\mathrm{pec }^{z=0}\lesssim 100\, \mathrm{km s^{-1}}$|⁠ , while binaries with low-mass companions exhibit a broader distribution that extends up to |$\sim 400\, \mathrm{km s^{-1}}$|⁠. We also find significant anticorrelations of |$v_\mathrm{pec }^{z=0}$| with binary total mass (M tot) and orbital period (P orb), at over 99 per cent confidence. Specifically, our fit suggests |$v_\mathrm{pec }^{z=0}\propto M_\mathrm{tot}^{-0.5}$| and |$v_\mathrm{pec }^{z=0}\propto P_\mathrm{orb}^{-0.2}$|⁠. Discussions are presented on possible interpretation of the correlations in the context of kinematics and possible biases. The sample should enable a range of follow-up studies on compact object binary kinematics and evolution. [ABSTRACT FROM AUTHOR]

To accurately link the radio and optical Celestial Reference Frames (CRFs) at optical bright end, i.e. with GaiaG -band magnitude |$\lesssim$| 13, increasing number and improving sky distribution of radio stars with accurate astrometric parameters from both Very Long Baseline Interferometry (VLBI) and Gaia measurements are mandatory. We selected two radio stars HD 199178 and AR Lacertae as the target for a pilot program for the frame link, using the Very Long Baseline Array at 15 GHz at six epochs spanning about 1 yr, to measure their astrometric parameters. The measured parallax of HD 199178 is 8.949 ± 0.059 mas and the proper motion is μ αcos δ = 26.393 ± 0.093 and μ δ = −0.950 ± 0.083 mas yr−1, while the parallax of AR Lac is 23.459 ± 0.094 mas and the proper motion is μ αcos δ = −51.906 ± 0.138 and μ δ = 46.732 ± 0.131 mas yr−1. Our VLBI measured astrometric parameters have accuracies about 4–5 times better than the corresponding historic VLBI measurements and comparable accuracies with those from Gaia , validating the feasibility of frame link using radio stars. With the updated astrometric parameters for these two stars, there is a ∼25 per cent reduction of the uncertainties on the Y -axis for both orientation and spin parameters. [ABSTRACT FROM AUTHOR]

We use the latest parallaxes measurements from Gaia DR3 to obtain a geometric calibration of the tip of the red giant branch (TRGB) in Cousins I magnitudes as a standard candle for cosmology. We utilize the following surveys: SkyMapper DR3, APASS DR9, ATLAS Refcat2, and Gaia DR3 synthetic photometry to obtain multiple zero-point calibrations of the TRGB magnitude, |$M_{I}^{TRGB}$|⁠. Our sample contains Milky Way halo stars at high galactic latitudes (| b | > 36) where the impact of metallicity, dust, and crowding are minimized. The magnitude of the TRGB is identified using Sobel edge detection, but this approach introduced a systematic offset. To address this issue, we utilized simulations with parsec isochrones and showed how to calibrate and remove this bias. Applying our method within the colour range where the slope of the TRGB is relatively flat for metal-poor halo stars (1.55 < (BP − RP) < 2.25), we find a weighted average |$M_{I}^{TRGB} = -4.042 \pm 0.041$| (stat) ±0.031 (sys) mag. A geometric calibration of the Milky Way TRGB has the benefit of being independent of other distance indicators and will help probe systematics in the local distance ladder, leading to improved measurements of the Hubble constant. [ABSTRACT FROM AUTHOR]

AR Scorpii (AR Sco), the only-known radio-pulsing white dwarf binary, shows unusual pulsating emission at the radio, infrared, optical, and ultraviolet bands. To determine its astrometric parameters at the radio band independently, we conducted multi-epoch Very Long Baseline Interferometry (VLBI) phase-referencing observations with the European VLBI Network at 5 GHz and the Chinese VLBI Network plus the Warkworth 30-m telescope (New Zealand) at 8.6 GHz. By using the differential VLBI astrometry, we provide high-precision astrometric measurements on the parallax (⁠|$\pi =8.52_{-0.07}^{+0.04}$|  mas) and proper motion (⁠|$\mu _{\alpha }=9.48_{-0.07}^{+0.04}$|  mas yr−1, |$\mu _{\delta }=-51.32_{-0.38}^{+0.22}$|  mas yr−1). The new VLBI results agree with the optical Gaia astrometry. Our kinematic analysis reveals that the Galactic space velocities of AR Sco are quite consistent with that of both intermediate polars and polars. Combined with the previous tightest VLBI constraint on the size, our parallax distance suggests that the radio emission of AR Sco should be located within the light cylinder of its white dwarf. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the memberships, spatial structure, and dynamical state of the old open cluster Collinder 261 (8 Gyr) based on Gaia-DR3 data. We develop a robust clustering method to calculate membership probabilities for 24858 sample stars within 20′ of the cluster center. We can identify 2619 likely cluster members (including 54 blue stragglers) down to G ∼ 20 mag. We use a Monte Carlo simulation method to estimate a distance of 2909 ± 90 pc for the cluster based on 281 high-probability members (P > 0.9). We find that the detectable cluster radius (R cl) is at least 14′ (∼11.8 pc), which is much larger than previously thought. Based on the obtained cluster members, the core and tidal radii of the cluster are determined to be |$R_{\rm c}={3{^{\prime }_{.}}4}\pm {0.1}$| (∼2.9 ± 0.1 pc) and |$R_{\rm t}={19{^{\prime }_{.}}8}\pm {1{^{\prime }_{.}}9}$| (∼16.8 ± 1.6 pc), respectively. A concentration parameter of log (R t/ R c) ∼0.76 is determined, indicating that Collinder 261 has formed a clear core–halo structure, but its spatial structure is much looser than the old open clusters M 67 (4 Gyr) and NGC 188 (7 Gyr). We also find that the membership probabilities are suitable for analyzing the mass segregation effect in the cluster. The orbit parameters show that Collinder 261 is moving along a near-circular orbit (e ∼ 0.025) well inside the solar circle (R p ∼ 6.928 kpc, R a ∼ 7.285 kpc), and it can reach a maximum height of Z max ∼ 533 pc from the Galactic plane. [ABSTRACT FROM AUTHOR]

Nearby galaxies provide populations of stellar and non-stellar sources at a common distance and in quantifiable environments. All are observed through the Milky Way foreground, with varying degrees of contamination that depend on observed Galactic latitude and the distance and size of the target galaxy. This work uses Gaia Data Release 3 (DR3) to identify foreground sources via astrometric measurements and thus quantify foreground contamination for a large sample of nearby galaxies. There are approximately half a million Gaia  sources in the directions of 1401 galaxies listed in the Local Volume Galaxy catalogue (D < 11 Mpc), excluding the largest Local Group galaxies. About two-thirds of the Gaia  sources have astrometric properties consistent with foreground sources; these sources are brighter, redder, and less centrally concentrated than non-foreground sources. Averaged over galaxies, foreground sources make up 50 per cent of Gaia  sources at projected radius r 50 = 1.06 a 26, where a 26 is the angular diameter at the B = 26.5 isophote. Foreground sources make up 50 per cent of Gaia  sources at apparent magnitude m G , 50 = 20.50. This limit corresponds to the tip of the red giant branch absolute magnitude at D = 450 kpc, and to the globular cluster luminosity function peak absolute magnitude at 5 Mpc. Gaia  data provide a powerful tool for removing foreground contamination in stellar population studies of nearby galaxies, although Gaia  foreground removal will be incomplete beyond distances of 5 Mpc. [ABSTRACT FROM AUTHOR]

We present a sub-arcsec cross-match of Gaia Data Release 3 (DR3) against the INT Galactic Plane Surveys (IGAPS) and the United Kingdom Infrared Deep Sky Survey (UKIDSS). The resulting cross-match of Galactic Plane Surveys (XGAPS) provides additional precise photometry (U RGO, g, r, i , Hα, J, H , and K) to the Gaia photometry. In building the catalogue, proper motions given in Gaia DR3 are wound back to match the epochs of the IGAPS constituent surveys (INT Photometric HαSurvey of the Northern Galactic Plane, IPHAS, and the UV-Excess Survey of the Northern Galactic Plane, UVEX) and UKIDSS, ensuring high-proper motion objects are appropriately cross-matched. The catalogue contains 33 987 180 sources. The requirement of >3σ parallax detection for every included source means that distances out to 1–1.5 kpc are well covered. In producing XGAPS, we have also trained a Random Forest classifier to discern targets with problematic astrometric solutions. Selection cuts based on the classifier results can be used to clean colour-magnitude and colour–colour diagrams in a controlled and justified manner, as well as producing subsets of astrometrically reliable targets. We provide XGAPS as a 111 column table. Uses of the catalogue include the selection of Galactic targets for multi-object spectroscopic surveys as well as identification of specific Galactic populations. [ABSTRACT FROM AUTHOR]

We use the Very Long Baseline Array to conduct high precision astrometry of a sample of 33 compact, flat spectrum, variable radio sources in the direction of the Galactic plane (Becker et al. 2010). Although Becker et al. (2010) ruled out a few potential scenarios for the origin of the radio emission, the study could not rule out that these sources were black hole X-ray binaries (BHXBs). Most known BHXBs are first detected by X-ray or optical emission when they go into an outburst, leaving the larger quiescent BHXB population undiscovered. In this paper, we attempt to identify any Galactic sources amongst the Becker et al. (2010) sample by measuring their proper motions as a first step to finding quiescent BHXB candidates. Amongst the 33 targets, we could measure the proper motion of six sources. We find that G32.7193-0.6477 is a Galactic source and are able to constrain the parallax of this source with a 3σ significance. We found three strong Galactic candidates, G32.5898-0.4468, G29.1075-0.1546, and G31.1494-0.1727, based purely on their proper motions, and suggest that G29.1075-0.1546 is also likely Galactic. We detected two resolved targets for multiple epochs (G30.1038+0.3984 and G29.7161-0.3178). We find six targets are only detected in one epoch and have an extended structure. We cross-match our VLBA detections with the currently available optical, infrared, and X-ray surveys, and did not find any potential matches. We did not detect 19 targets in any VLBA epochs and suggest that this could be due to limited uv -coverage, drastic radio variability, or faint, extended nature of the sources. [ABSTRACT FROM AUTHOR]

A total of 3350 objects from the Sixth Catalog of Orbits of Visual Binary Stars (ORB6) are investigated to validate Gaia Early Data Release 3 (EDR3) parallaxes and provide mass estimates for the systems. We show that two-thirds of binaries with a separation of 0.2–0.5 arcsec are left without a parallax solution in EDR3. Special attention is paid to 521 pairs with the parallax known separately for each component. We find 16 entries that are deemed to be chance alignments of unrelated stars. First we show examples of high-confidence binary systems with significant differences in the reported parallaxes of their components. Next we conclude that the reported Gaia EDR3 parallax errors are underestimated, by at least a factor of 3, for sources with a large renormalized unit-weight error (RUWE). Parallaxes are needed to estimate stellar masses. Because nearly 30 per cent of ORB6 entries lack a 5- or 6-parameter solution in EDR3, we attempt to enrich the astrometric data. Distant companions of ORB6 entries are revealed in EDR3 by the analysis of stellar proper motions and Hipparcos parallaxes. Notably, in 28 cases, intrinsic EDR3 parallaxes of the binary components appear to be less reliable than the parallax of the outer companions. Gaia DR2, The Tycho-Gaia Astrometric Solution (TGAS) and Hipparcos parallaxes are used when EDR3 data are unavailable. A synthetic mass–luminosity relationship in the G band for main-sequence stars is obtained to provide mass estimates along with dynamical masses calculated via Kepler's third law. [ABSTRACT FROM AUTHOR]

We have used NASA's TESS mission to study catalogued δ Scuti stars. We examined TESS light curves for 434 stars, including many for which few previous observations exist. We found that 62 are not δ Scuti pulsators, with most instead showing variability from binarity. For the 372 δ Scuti stars, we provide a catalogue of the period and amplitude of the dominant pulsation mode. Using Gaia DR3 parallaxes, we place the stars in the period–luminosity (P–L) diagram and confirm previous findings that most stars lie on a ridge that corresponds to pulsation in the fundamental radial mode, and that many others fall on a second ridge that is a factor two shorter in period. This second ridge is seen more clearly than before, thanks to the revised periods and distances. We demonstrate the value of the P–L diagram in distinguishing δ Scuti stars from short-period RR Lyrae stars, and we find several new examples of high-frequency δ Scuti stars with regular sequences of overtone modes, including XX Pyx and 29 Cyg. Finally, we revisit the sample of δ Scuti stars observed by Kepler and show that they follow a tight period–density relation, with a pulsation constant for the fundamental mode of Q  = 0.0315 d. [ABSTRACT FROM AUTHOR]

Westerlund 1 (Wd 1) is one of the most massive young star clusters in the Milky Way. Although relevant for star formation and evolution, its fundamental parameters are not yet very well constrained. We aim to derive an accurate distance and provide constraints on the cluster age. We used the photometric and astrometric information available in the Gaia Early Data Release 3 (Gaia -EDR3) to infer its distance of 4.06 |$^{+0.36}_{-0.34}$|  kpc. Modelling of the eclipsing binary system W36, reported in Paper II , led to the distance of 4.03 ± 0.25 kpc, in agreement with the Gaia -EDR3 distance and, therefore, validating the parallax zero-point correction approach appropriate for red objects. The weighted average distance based on these two methods results in d wd1  =  4.05 ± 0.20 kpc (m − M   =  13.04 |$^{+0.11}_{-0.12}$|  mag), which has an unprecedented accuracy of 5 per cent. Using the Binary Population and Spectral Synthesis (BPASS) models for the Red Supergiants with solar abundance, we derived an age of 10.7 ± 1 Myr, in excellent agreement with recent work by Beasor & Davies (10.4 |$^{+1.3}_{-1.2}$|  Myr) based on MIST evolutionary models. In Paper II , W36B was reported to be younger than 7.1 Myr, supporting recent claims of a temporal spread of several Myrs for the star-forming process within Wd 1 instead of a single monolithic starburst episode scenario. [ABSTRACT FROM AUTHOR]

We examine the prospects for measurement of the Hubble parameter H 0 via observation of the secular parallax of other galaxies due to our own motion relative to the cosmic microwave background rest frame. Peculiar velocities make distance measurements to individual galaxies highly uncertain, but a survey sampling many galaxies can still yield a precise H 0 measurement. We use both a Fisher information formalism and simulations to forecast errors in H 0 from such surveys, marginalizing over the unknown peculiar velocities. The optimum survey observes ∼102 galaxies within a redshift z max = 0.06. The required errors on proper motion are comparable to those that can be achieved by Gaia and future astrometric instruments. A measurement of H 0 via parallax has the potential to shed light on the tension between different measurements of H 0. [ABSTRACT FROM AUTHOR]

This paper presents the first census of Galactic post-asymptotic giant branch (post-AGB) stars in the Hertzsprung–Russell (HR) diagram. We combined Gaia DR3 parallax-based distances with extinction corrected integrated fluxes and derived luminosities for a sample of 185 stars that had been proposed to be post-AGB stars in the literature. The luminosities allow us to create an HR diagram containing the largest number of post-AGB candidate objects to date. A significant fraction of the objects fall outside the typical luminosity range as covered by theoretical evolutionary post-AGB tracks as well as observed for planetary nebula central stars. These include massive evolved supergiants and lower luminosity objects. Here, we highlight the fact that one-third of the post-AGB candidates are underluminous and we identify these with the recently recognized class of post-red giant branch objects thought to be the result of binary evolution. [ABSTRACT FROM AUTHOR]

Understanding the exact extent and content of tidal tails of open clusters provides useful clues on how field stars populate the Milky Way. We reanalyse, using Gaia EDR3 data, the tails around the open cluster NGC 752. Compared to previous analyses, we look at a much wider region around the cluster and use first the convergent point method, coupled with a clustering analysis using dbscan. We find that the cluster, located 433 pc away and well described by a Plummer profile, has very long and asymmetric tails, extending more than 260 pc on the sky (from tip to tip) – four times larger than previously thought – and contains twice as many stars. Numerical models computed with petar serve as a guide and confirm our analysis. The tails follow the predictions from the models, but the trailing tail appears slightly distorted, possibly indicating that the cluster had a complicated history of galactic encounters. Applying an alternative method to the newly developed compact convergent point method, we potentially trace the cluster's tidal tails to their full extent, covering several thousands of parsecs and more than 1000 stars. Our analysis therefore opens a new window on the study of open clusters, whose potential will be fully unleashed with future Gaia data releases. [ABSTRACT FROM AUTHOR]

We examine the capacity to identify binary systems from astrometric deviations alone. We apply our analysis to the Gaia eDR3 and DR2 data, specifically the Gaia Catalogue of Nearby Stars. We show we must renormalize (R)UWE over the local volume to avoid biasing local observations, giving a local unit weight error (LUWE). We use the simple criterion of LUWE>2, along with a handful of quality cuts to remove likely contaminants, to identify unresolved binary candidates. We identify 22 699 binary candidates within 100 pc of the Sun (just under 10 per cent of sources in this volume). We find an astrometric binary candidate fraction of around 20 per cent for giant stars, 10 per cent on the main sequence and lower than 1 per cent for white dwarfs. We also look for Variability Induced Movers, by computing the correlation between photometric variability and astrometric noise – and show that VIMs may dominate the binary population of sub-Solar mass MS stars. We discuss the possibility and limitations of identifying non-luminous massive companions from astrometry alone, but find that our method is insensitive to these. Finally, we compare the astrometric deviations of MS binaries to the simulated sample from paper I, which show excellent agreement, and compute the astrometric candidate binary fraction as a function of absolute magnitude. [ABSTRACT FROM AUTHOR]

We examine the capacity to identify binary systems from astrometric errors and deviations alone. Until the release of the fourth Gaia data release, we lack the full astrometric time-series that the satellite records, but as we show can still infer the presence of binaries from the best-fitting models, and their error, already available. We generate a broad catalogue of simulated binary systems within 100 pc, and examine synthetic observations matching the Gaia survey's scanning law and astrometric data processing routine. We show how the unit weight error (UWE) and proper motion anomaly (PMA) vary as a function of period, and the properties of the binary. Both UWE and PMA peak for systems with a binary period close to the time baseline of the survey. Thus UWE can be expected to increase or remain roughly constant as we observe the same system over a longer baseline, and we suggest UWE eDR 3 > 1.25 and ΔUWE/UWE eDR 3 > −0.25 as criteria to select astrometric binaries. For stellar binaries, we find detectable significant astrometric deviations for 80–90 per cent of our simulated systems in a period range from months to decades. We confirm that for systems with periods less than the survey's baseline the observed UWE scales ∝ ϖ (parallax), a (semimajor axis), and |$\Delta =\frac{|q-l|}{(1+q)(1+l)}$|⁠ , where q and l are the mass and light ratio, respectively, with a modest dependence on viewing angle. For longer periods the signal is suppressed by a factor of roughly ∝ P −2 (period). PMA is largest in orbits with slightly longer periods but obeys the same approximate scaling relationships. [ABSTRACT FROM AUTHOR]

We present new luminosity–metallicity (LZ ; |$M_{V}\!-\!\rm [Fe/H]$| and |$M_{G}\!-\!\rm [Fe/H]$|⁠) relations and, for the first time, empirical, Gaia three-band (G ,  G BP, G RP) period-Wesenheit-metallicity (PWZ) relations of RR Lyrae stars (RRLs) derived using a hierarchical Bayesian approach and new accurate parallaxes published for these variables in the Gaia Early Data Release 3 (EDR3). In a previous study, we obtained Bayesian hierarchically derived LZ relations from a sample of about four hundred Milky Way field RRLs with G -band light curves and trigonometric parallaxes published in the G aia Data Release 2 (DR2), using V mean magnitudes, metallicities, absorptions, and pulsation periods available in the literature. We now extend that study in two directions. First, we update our previous results using trigonometric parallaxes from Gaia EDR3 and incorporate the Bayesian analysis of a first empirical PWZ relation derived using those field RRLs with G ,  G BP and G RP time-series photometry available in Gaia DR2. Secondly, we use Bayesian inference to derive LZ relations and empirical PW Gaia three-band relations from 385 RRLs belonging to 15 Milky Way globular clusters (GC) with literature-compiled spectroscopic metallicities ranging from −0.36 to −2.39 dex and prior distances extending from 2.2 to 41.2 kpc. From the samples of RRLs analysed in this paper we infer a mean Gaia EDR3 zero-point offset of −0.028 mas with median values ranging from −0.033 (LZ and PWZ models for field stars) to −0.024 mas (LZ model in the V band for GC RRLs). [ABSTRACT FROM AUTHOR]

The main goal of pulsar timing array experiments is to detect correlated signals such as nanohertz-frequency gravitational waves. Pulsar timing data collected in dense monitoring campaigns can also be used to study the stars themselves, their binary companions, and the intervening ionized interstellar medium. Timing observations are extraordinarily sensitive to changes in path-length between the pulsar and the Earth, enabling precise measurements of the pulsar positions, distances and velocities, and the shapes of their orbits. Here we present a timing analysis of 25 pulsars observed as part of the Parkes Pulsar Timing Array (PPTA) project over time spans of up to 24 yr. The data are from the second data release of the PPTA, which we have extended by including legacy data. We make the first detection of Shapiro delay in four Southern pulsars (PSRs J1017−7156, J1125−6014, J1545−4550, and J1732−5049), and of parallax in six pulsars. The prominent Shapiro delay of PSR J1125−6014 implies a neutron star mass of Mp = 1.5 ± 0.2 M⊙ (68 per cent credibility interval). Measurements of both Shapiro delay and relativistic periastron advance in PSR J1600−3053 yield a large but uncertain pulsar mass of |$M_p = 2.06^{+0.44}_{-0.41}$|  M⊙ (68 per cent credibility interval). We measure the distance to PSR J1909−3744 to a precision of 10 lyr, indicating that for gravitational wave periods over a decade, the pulsar provides a coherent baseline for pulsar timing array experiments. [ABSTRACT FROM AUTHOR]

We use the data from Gaia   Early Data Release 3 (EDR3) to study the kinematic properties of Milky Way globular clusters. We measure the mean parallaxes and proper motions (PM) for 170 clusters, determine the PM dispersion profiles for more than 100 clusters, uncover rotation signatures in more than 20 objects, and find evidence for radial or tangential PM anisotropy in a dozen richest clusters. At the same time, we use the selection of cluster members to explore the reliability and limitations of the Gaia  catalogue itself. We find that the formal uncertainties on parallax and PM are underestimated by |$10{-}20{{\ \rm per\ cent}}$| in dense central regions even for stars that pass numerous quality filters. We explore the spatial covariance function of systematic errors, and determine a lower limit on the uncertainty of average parallaxes and PM at the level 0.01 mas and 0.025 mas yr |$^{-1}$|  , respectively. Finally, a comparison of mean parallaxes of clusters with distances from various literature sources suggests that the parallaxes for stars with |$G>13$| (after applying the zero-point correction suggested by Lindegren et al.) are overestimated by |$\sim 0.01\pm 0.003$|  mas. Despite these caveats, the quality of Gaia   astrometry has been significantly improved in EDR3 and provides valuable insights into the properties of star clusters. [ABSTRACT FROM AUTHOR]

We revisit the subject of Be candidate stars towards the Magellanic Clouds, previously studied by the authors using SPM4 proper motions. We obtain Gaia DR2 parallaxes and proper motions for 2357 and 994 LMC and SMC Be candidates, respectively. Parallaxes and proper motions versus colour V − I easily reveal the presence of the redder galactic contaminant foreground, as concluded in our previous work, but this time we do find a few red Be candidates consistent with being true Magellanic objects. A membership assessment to each Magellanic Cloud is done for each Be candidate, based on the distribution of their parallaxes and proper motions. From a compilation of published catalogues of spectroscopically confirmed emission line stars,we find that 41 (LMC) and 56 (SMC) of the Be candidates have shown the H α line in emission. Near-infrared IRSF JHKs magnitudes are obtained for about 70 per cent of the Be candidates with Gaia DR2 astrometric data. Mid-infrared SAGE IRAC 3.6, 4.5, 5.8, and 8.0 |$\mu$| m magnitudes are obtained for about 85 per cent as well. After discarding possible Herbig Ae/Be stars, one LMC and three SMC B emission line confirmed stars show significantly redder optical, near- and mid-infrared colours than what has been typically measured for Classical Be stars – they are called red Be stars. Taking into account that classical Be stars do not show these red colours, further studies about these four stars are needed in order to establish their true nature and explain the observed red excess. [ABSTRACT FROM AUTHOR]

Due to the high quality of Gaia DR2 stellar parallaxes and proper motions, we can study the Cygnus OB2 region up to G = 17.5 mag in circular region centred at (l, b) = (79.8°, +0.8°) of radius 1°. We detect four overdensities in the Vector Point Diagram. We apply a parametric model of the proper motion distribution and determine their mean proper motions, identify their members, and estimate their distances by analysing the parallaxes of the most probable members. We confirm that one of these regions is Cygnus OB2, whose mean proper motion is (μαcosδ, μδ) = (−2.71 ± 0.02, −4.24 ± 0.02) |$\mathrm{mas\, yr}^{-1}$|⁠. We identify 2767 astrometric members at an estimated distance of 1683 ± 5 pc. A detailed comparison of these members with 333 photometric and spectroscopic Cygnus OB2 members shows that 33 stars of the last group are not members, 16 of them are O-B stars. 300 members in common lead to estimate a distance to the association of 1669 ± 6 pc. One of the remaining overdensities is the open cluster UCB585 for which we find eight more members, located at ∼1460 pc. The other two groups count 179 and 188 proper motion members which are distributed throughout the region and situated at a distance of ∼1280 pc. The estimated distances of these four groups suggest the existence of different substructures in the Cygnus OB2 region along the line of sight. [ABSTRACT FROM AUTHOR]

Precise and accurate measurements of distances to Galactic X-ray binaries (XRBs) reduce uncertainties in the determination of XRB physical parameters. We have cross-matched the XRB catalogues of Liu, van Paradijs & van den Heuvel to the results of Gaia  Data Release 2. We identify 86 XRBs with a Gaia  candidate counterpart, of which 32 are low-mass X-ray binaries (LMXBs) and 54 are high-mass X-ray binaries (HMXBs). Distances to Gaia  candidate counterparts are, on average, consistent with those measured by Hipparcos and radio parallaxes. When compared to distances measured by Gaia  candidate counterparts, distances measured using Type I X-ray bursts are systematically larger, suggesting that these bursts reach only 50 per cent of the Eddington limit. However, these results are strongly dependent on the prior assumptions used for estimating distance from the Gaia  parallax measurements. Comparing positions of Gaia  candidate counterparts for XRBs in our sample to positions of spiral arms in the Milky Way, we find that HMXBs exhibit mild preference for being closer to spiral arms; LMXBs exhibit mild preference for being closer to interarm regions. LMXBs do not exhibit any preference for leading or trailing their closest spiral arm. HMXBs exhibit a mild preference for trailing their closest spiral arm. The lack of a strong correlation between HMXBs and spiral arms may be explained by star formation occurring closer to the mid-point of the arms, or a time delay between star formation and HMXB formation manifesting as a spatial separation between HMXBs and the spiral arm where they formed. [ABSTRACT FROM AUTHOR]

We use the large homogeneous sample of late M dwarfs, M7–M9.5, of Ahmed & Warren (2019) matched to Gaia DR2, to measure the relation between absolute magnitude and spectral type, and to infer the multiplicity fraction of the population, and the distribution of mass ratios in the binary systems. Binaries are identified photometrically as overluminous sources. In order to define a sample that is unbiased with respect to multiplicity we use distance limits that are a function of G − J colour to define a volume-complete sample of 2706 systems. The G − J colours are very precise, with random errors all less than 0.02. We measure absolute magnitudes MJ that are on average 0.5 mag brighter than previous determinations. We find evidence that the discrepancies arise from differences in spectral types in different samples. The measured binary fraction is |$16.5\pm 0.8{{\ \rm per\ cent}}$|⁠ , of which |$98{{\ \rm per\ cent}}$| are unresolved: both values are consistent with results of previous studies. The distribution of excess flux in the binaries, compared to the singles, is used to infer the mass ratio distribution f (q)∝ q γ, where q = M s/ M p. We infer a very steep distribution over this spectral range, with γ > 10 (⁠|$99{{\ \rm per\ cent}}$| probability). This says that unresolved ultracool M dwarf binaries reside almost exclusively in equal mass systems, and implies that the spectral types of the unresolved binaries match to with 0.5 spectral subtypes. The intrinsic scatter in absolute magnitude MJ for ultracool M dwarfs at fixed G − J colour is measured to be 0.21 mag. [ABSTRACT FROM AUTHOR]

XTE J1810−197 (J1810) was the first magnetar identified to emit radio pulses, and has been extensively studied during a radio-bright phase in 2003–2008. It is estimated to be relatively nearby compared to other Galactic magnetars, and provides a useful prototype for the physics of high magnetic fields, magnetar velocities, and the plausible connection to extragalactic fast radio bursts. Upon the rebrightening of the magnetar at radio wavelengths in late 2018, we resumed an astrometric campaign on J1810 with the Very Long Baseline Array , and sampled 14 new positions of J1810 over 1.3 yr. The phase calibration for the new observations was performed with two-phase calibrators that are quasi-colinear on the sky with J1810, enabling substantial improvement of the resultant astrometric precision. Combining our new observations with two archival observations from 2006, we have refined the proper motion and reference position of the magnetar and have measured its annual geometric parallax, the first such measurement for a magnetar. The parallax of 0.40 ± 0.05 mas corresponds to a most probable distance |$2.5^{\, +0.4}_{\, -0.3}$|  kpc for J1810. Our new astrometric results confirm an unremarkable transverse peculiar velocity of ≈200  |$\rm km~s^{-1}$| for J1810, which is only at the average level among the pulsar population. The magnetar proper motion vector points back to the central region of a supernova remnant (SNR) at a compatible distance at ≈70 kyr ago, but a direct association is disfavoured by the estimated SNR age of ∼3 kyr. [ABSTRACT FROM AUTHOR]

There remain significant uncertainties in the origin and evolution of black holes in binary systems, in particular regarding their birth sites and the influence of natal kicks. These are long-standing issues, but their debate has been reinvigorated in the era of gravitational wave detections and the improving precision of astrometric measurements. Using recent and archival characterization of Galactic black hole X-ray binaries (BHXBs), we report here an apparent anticorrelation between P orb (system orbital periods) and scatter in |$z$| (elevation above the Galactic plane). The absence of long-period sources at high |$z$| is not an obvious observational bias, and two possible explanatory scenarios are qualitatively explored: (1) a disc origin for BHXBs followed by natal kicks producing the scatter in |$z$|⁠ , with only the tightest binaries preferentially surviving strong kicks; and (2) a halo origin, with P orb shortening through dynamical interactions in globular clusters (GCs). For the latter case, we show a correspondence in |$z$| -scatter between BHXBs and the GCs with most compact core radii of <0.1 pc. However, the known absence of outbursting BHXB transients within Galactic GCs remains puzzling in this case, in contrast to the multitude of known GC neutron star XRBs. These results provide an interesting observational constraint for any black hole binary evolutionary model to satisfy. [ABSTRACT FROM AUTHOR]

We study the stellar kinematic properties and spatial distribution of the association Cyg OB3 using precise astrometric data from Gaia DR2. All known O- and B-type stars in Cyg OB3 region with positions, parallaxes, and proper motions available are included, comprising a total of 41 stars. The majority of stars are found to be concentrated at a heliocentric distance of 2.0 ± 0.3 kpc. The mean peculiar velocity of the sample after removing Galactic rotation and solar motion is ∼22 km s−1, dominated by the velocity component towards the Galactic centre. The relative position and velocity of the black hole X-ray binary Cyg X-1 with respect to the association suggest that Cyg OB3 is most likely its parent association. The peculiar kinematic properties of some of the stars are revealed and are suggestive of past stellar encounters. The sample includes a previously known runaway star HD 227018, and its high peculiar velocity of ∼50 km s−1 is confirmed with Gaia. We estimated the velocities of stars relative to the association and the star HD 225577 exhibits peculiar velocity smaller than its velocity relative to the association. The star has lower value of proper motion than the rest of the sample. The results suggest a slowly expanding nature of the association, which is supported by the small relative speeds (<20 km s−1) with respect to the association for a majority of the sample stars. [ABSTRACT FROM AUTHOR]

Near-infrared spectroscopic surveys have uncovered a population of short-period, blended-light spectral binaries composed of low-mass stars and brown dwarfs. These systems are amenable to orbit determination and individual mass measurements via astrometric monitoring. Here, we present first results of a multiyear campaign to obtain high-precision absolute astrometry for spectral binaries using the Gemini-South and Gemini-North GMOS imagers. We measure the complete astrometric orbits for two systems: 2M0805+48 and 2M1059−21. Our astrometric orbit of 2M0805+48 is consistent with its 2-yr radial velocity orbit determined previously and we find a mass of |$66^{+5}_{-14} M_\mathrm{Jup}$| for its T5.5 companion. For 2M1059−21, we find a 1.9-yr orbital period and a mass of |$67^{+4}_{-5} M_\mathrm{Jup}$| for its T3.5 companion. We demonstrate that sub-milliarcsecond absolute astrometry can be obtained with both GMOS imagers and that this is an efficient avenue for confirming and characterizing ultracool binary systems. [ABSTRACT FROM AUTHOR]

It is imperative to map the strength and distribution of feedback in galaxies to understand how feedback affects galactic ecosystems. H2O masers act as indicators of energy injection into the interstellar medium. Our goal is to measure the strength and distribution of feedback traced by water masers in the Milky Way. We identify optical counterparts to H2O masers discovered by the HOPS survey. The distribution and luminosities of H2O masers in the Milky Way are determined using parallax measurements derived from the second Gaia Data Release. We provide evidence of a correlation between evolved stars, as traced by H2O masers, and the spiral structure of the Milky Way, suggesting a link between evolved stars and the Galactic environment. [ABSTRACT FROM AUTHOR]

Most binaries are undetected. Astrometric reductions of a system using the assumption that the object moves like a single point mass can be biased by unresolved binary stars. The discrepancy between the centre of mass of the system (which moves like a point mass) and the centre of light (which is what we observe) introduces additional motion. We explore the extent to which binary systems affect single object model fit to astrometric data. This tells us how observations are diluted by binaries and which systems cause the largest discrepancies – but it also allows us to make inferences about the binarity of populations based on observed astrometric error. By examining a sample of mock observations, we show that binaries with periods close to 1 yr can mimic parallax and thus bias distance measurements, while long-period binaries can introduce significant apparent proper motion. While these changes can soak up some of the error introduced by the binary, the total deviation from the best-fitting model can be translated into a lower limit on the on-sky separation of the pair. Throughout, we link these predictions to data from the Gaia satellite, while leaving the conclusions generalizable to other surveys. [ABSTRACT FROM AUTHOR]

The event rate and the efficiency of mass estimation for free-floating planet (FFP) microlensing events were determined from the simulation of the simultaneous parallax observations by Euclid, WFIRST , and LSST. The stellar population from the Besançon Galactic model toward (l, b) = (1°, −1.°75) was applied to our 3D microlensing model, and 30 000 parallax observations were simulated for each following FFP lens masses: Jupiter mass, Neptune mass, and Earth mass assuming the population of one FFP per star. The interstellar dust, unresolved stellar background, nearby star blending was modelled. A signal-to-noise limit considering a finite source effect determined the event detectability. The Euclid – WFIRST combination yielded 30.7 Jupiter-mass FFPs during two 30-d periods per year in parallax observation. The parallax event rate decreases to 3.9 FFPs for Earth-mass planets. The mass estimation from the parallax light curve allowed recovery of FFP masses to within a factor of 2 for 20–26 per cent of cases. The Euclid –LSST combination yielded 34.5 Jupiter-mass FFPs down to 0.5 Earth-mass FFPs for the same periods and the mass is recovered to within a factor of 2 in 20–40 per cent of cases. The event rate will be normalized by the unknown FFP abundance to recover the number of expected detections. [ABSTRACT FROM AUTHOR]

The systematic offset of Gaia parallaxes has been widely reported with Gaia 's second data release, and it is expected to persist in future Gaia data. In order to use Gaia parallaxes to infer distances to high precision, we develop a hierarchical probabilistic model to determine the Gaia parallax zero-point offset along with the calibration of an empirical model for luminosity of red clump stars by combining astrometric and photometric measurements. Using a cross-matched sample of red clump stars from the Apache Point Observatory Galactic Evolution Experiment and Gaia Data Release 2 (DR2), we report the parallax zero-point offset in DR2 to be |$\varpi _0 = -48 \pm 1\,\mu \text{as}$|⁠. We infer the red clump absolute magnitude to be −1.622 ± 0.004 in Ks , 0.435 ± 0.004 in G , −1.019 ± 0.004 in J , and −1.516 ± 0.004 in H. The intrinsic scatter of the red clump is ∼0.09 mag in J, H , and Ks , or ∼ 0.12 mag in G. We tailor our models to accommodate more complex analyses such as investigating the variations of the parallax zero-point with each source's observed magnitude, observed colour, and sky position. In particular, we find fluctuations of the zero-point across the sky to be of order or less than a few 10s of |$\mu \text{as}$|⁠. [ABSTRACT FROM AUTHOR]

Using the Very Long Baseline Array and the European Very Long Baseline Interferometry Network, we have made a precise measurement of the radio parallax of the black hole X-ray binary MAXI J1820+070, providing a model-independent distance to the source. Our parallax measurement of (0.348 ± 0.033) mas for MAXI J1820+070 translates to a distance of (2.96 ± 0.33) kpc. This distance implies that the source reached (15 ± 3) per cent of the Eddington luminosity at the peak of its outburst. Further, we use this distance to refine previous estimates of the jet inclination angle, jet velocity, and the mass of the black hole in MAXI J1820+070 to be (63 ± 3)°, (0.89 ± 0.09)  c , and (9.2 ± 1.3) M⊙, respectively. [ABSTRACT FROM AUTHOR]

We use very long baseline interferometry to measure the proper motions of three black hole X-ray binaries (BHXBs). Using these results together with data from the literature and Gaia DR2 to collate the best available constraints on proper motion, parallax, distance, and systemic radial velocity of 16 BHXBs, we determined their three-dimensional Galactocentric orbits. We extended this analysis to estimate the probability distribution for the potential kick velocity (PKV) a BHXB system could have received on formation. Constraining the kicks imparted to BHXBs provides insight into the birth mechanism of black holes (BHs). Kicks also have a significant effect on BH–BH merger rates, merger sites, and binary evolution, and can be responsible for spin–orbit misalignment in BH binary systems. 75 per cent of our systems have potential kicks |$\gt 70\, \rm {km\,s^{-1}}$|⁠. This suggests that strong kicks and hence spin–orbit misalignment might be common among BHXBs, in agreement with the observed quasi-periodic X-ray variability in their power density spectra. We used a Bayesian hierarchical methodology to analyse the PKV distribution of the BHXB population, and suggest that a unimodal Gaussian model with a mean of 107 |$\pm \,\,16\, \rm {km\,s^{-1}}$| is a statistically favourable fit. Such relatively high PKVs would also reduce the number of BHs likely to be retained in globular clusters. We found no significant correlation between the BH mass and PKV, suggesting a lack of correlation between BH mass and the BH birth mechanism. Our python code allows the estimation of the PKV for any system with sufficient observational constraints. [ABSTRACT FROM AUTHOR]

We have established a mixture model approach to derive the parallax of the Milky Way globular clusters. It avoids the problem of cluster membership determination and provides a completely independent astrometrical solution by purely using the parallax data. This method is validated with simulated clusters of Pancino et al.. We have resolved 120 real globular clusters by the mixture model using parallaxes of the second data release of Gaia. They construct the largest direct parallax sample up to now. In comparison with other direct parallax results based on cluster members, including 75 clusters of Gaia Collaboration, our method presents its accuracy, especially for some particular clusters. A systematic offset of −27.6 ± 1.7 μas, together with a scatter of 22.8 ± 1.3 μas is found in comparison with other indirect parallax measurements. They are consistent with the global value and the variation of the zero-point of current Gaia parallaxes. Distances of several specific nearby globular clusters are discussed while the closest ones can reach high precisions, even taking the systematic error into account. [ABSTRACT FROM AUTHOR]

We derive Bayesian distances for all stars in the radial velocity sample of Gaia DR2, and use the statistical method of Schönrich, Binney & Asplund to validate the distances and test the Gaia parallaxes. In contrast to other methods, which rely on special sources, our method directly tests the distances to all stars in our sample. We find clear evidence for a near-linear trend of distance bias f with distance s , proving a parallax offset δp. On average, we find |${\delta _{\rm p}}= -0.054 \, {\rm mas}$| (parallaxes in Gaia DR2 need to be increased) when accounting for the parallax uncertainty underestimate in the Gaia set (compared to |${\delta _{\rm p}}= -0.048 \, {\rm mas}$| on the raw parallax errors), with negligible formal error and a systematic uncertainty of about |$0.006 \, {\rm mas}$|⁠. The value is in concordance with results from asteroseismic measurements, but differs from the much lower bias found on quasar samples. We further use our method to compile a comprehensive set of quality cuts in colour, apparent magnitude, and astrometric parameters. Lastly, we find that for this sample δp appears to strongly depend on σp (when including the additional |$0.043 \, {\rm mas}$|⁠) with a statistical confidence far in excess of 10σ and a proportionality factor close to 1, though the dependence varies somewhat with σp. Correcting for the σp dependence also resolves otherwise unexplained correlations of the offset with the number of observation periods |${n_{\rm vis}}$| and ecliptic latitude. Every study using Gaia DR2 parallaxes/distances should investigate the sensitivity of its results on the parallax biases described here and – for fainter samples – in the DR2 astrometry paper. [ABSTRACT FROM AUTHOR]

Asteroseismology provides fundamental stellar parameters independent of distance, but subject to systematics under calibration. Gaia DR2 has provided parallaxes for a billion stars, which are offset by a parallax zero-point (ϖzp). Red Clump (RC) stars have a narrow spread in luminosity, thus functioning as standard candles to calibrate these systematics. This work measures how the magnitude and spread of the RC in the Kepler field are affected by changes to temperature and scaling relations for seismology, and changes to the parallax zero-point for Gaia. We use a sample of 5576 RC stars classified through asteroseismology. We apply hierarchical Bayesian latent variable models, finding the population-level properties of the RC with seismology, and use those as priors on Gaia parallaxes to find ϖzp. We then find the position of the RC, using published values for ϖzp. We find a seismic temperature-insensitive spread of the RC of |${\sim }0.03\, \rm mag$| in the 2MASS K band and a larger and slightly temperature-dependent spread of |${\sim }0.13\, \rm mag$| in the Gaia G band. This intrinsic dispersion in the K band provides a distance precision of |${\sim } 1{{\ \rm per\ cent}}$| for RC stars. Using Gaia data alone, we find a mean zero-point of |$-41\pm 10\, \mu \rm as$|⁠. This offset yields RC absolute magnitudes of −1.634 ± 0.018 in K and 0.546 ± 0.016 in G. Obtaining these same values through seismology would require a global temperature shift of |${\sim }-70\, \mathrm{K}$|⁠ , which is compatible with known systematics in spectroscopy. [ABSTRACT FROM AUTHOR]

We have examined the period–luminosity (P–L) relation for δ Scuti stars using Gaia DR2 parallaxes. We included 228 stars from the catalogue of Rodríguez et al. as well as 1124 stars observed in the 4-yr Kepler mission. For each star, we considered the dominant pulsation period, and used DR2 parallaxes and extinction corrections to determine absolute V magnitudes. Many stars fall along a sequence in the P–L relation coinciding with fundamental-mode pulsation, while others pulsate in shorter period overtones. The latter stars tend to have higher effective temperatures, consistent with theoretical calculations. Surprisingly, we find an excess of stars lying on a ridge with periods half that of the fundamental. We suggest this may be due to a 2:1 resonance between the third or fourth overtone and the fundamental mode. [ABSTRACT FROM AUTHOR]

We study the fraction of stars in and around the δ Scuti instability strip that are pulsating, using Gaia DR2 parallaxes to derive precise luminosities. We classify a sample of over 15 000 Kepler A and F stars into δ Sct and non-δ Sct stars, paying close attention to variability that could have other origins. We find that 18 per cent of the δ Sct stars have their dominant frequency above the Kepler long-cadence Nyquist frequency (periods < 1 h), and 30 per cent have some super-Nyquist variability. We analyse the pulsator fraction as a function of effective temperature and luminosity, finding that many stars in the δ Sct instability strip do not pulsate. The pulsator fraction peaks at just over 70 per cent in the middle of the instability strip. The results are insensitive to the amplitude threshold used to identify the pulsators. We define a new empirical instability strip based on the observed pulsator fraction that is systematically hotter than theoretical strips currently in use. The stellar temperatures, luminosities, and pulsation classifications are provided in an online catalogue. [ABSTRACT FROM AUTHOR]

We report on a first census of Galactic black hole X-ray binary (BHXRB) properties with the second data release (DR2) of Gaia, focusing on dynamically confirmed and strong candidate BH transients. Gaia DR2 provides five-parameter astrometric solutions including position, parallax and proper motion for 11 out of a sample of 24 systems. Distance estimates are tested with parallax inversion as well as Bayesian inference. We derive an empirically motivated characteristic scalelength of L   =  2.17 ± 0.12 kpc for this BHXRB population to infer distances based upon an exponentially decreasing space density prior. Geometric DR2 parallaxes provide new, independent distance estimates, but the faintness of this population in quiescence results in relatively large fractional distance uncertainties. Despite this, DR2 estimates generally agree with the distances found in the literature. The most discrepant case is BW Cir, for which detailed studies of the donor star have suggested a distant location at ≳25 kpc. The large parallax and relatively high proper motion measured by DR2 instead indicate significantly smaller distances, suggesting that the source might be amongst the nearest of XRBs. However, both larger and smaller distances create problems for the interpretation of the source, and follow-up data are required to resolve its true nature. DR2 also provides a first distance estimate to one source, MAXI J1820+070, and novel proper motion estimates for seven sources. Peculiar velocities relative to Galactic rotation exceed ≈ 50 km s−1 for the bulk of the sample, with a median system kinetic energy of peculiar motion of ∼ 5 × 1047 erg. BW Cir could be a new high-velocity BHXRB if its astrometry is confirmed. A putative anticorrelation between peculiar velocity and BH mass is found, as expected in mass-dependent BH kick formation channels, but this trend remains weak in the DR2 data. [ABSTRACT FROM AUTHOR]

For the first time, we have explored the spatial substructure of the Cygnus OB2 association using parallaxes from the recent second Gaia data release. We find significant line-of-sight substructure within the association, which we quantify using a parametrized model that reproduces the observed parallax distribution. This inference approach is necessary due to the non-linearity of the parallax distance transformation and the asymmetry of the resulting probability distribution. Using a Markov Chain Monte Carlo ensemble sampler and an unbinned maximum likelihood test, we identify two different stellar groups superposed on the association. We find the main Cygnus OB2 group at ∼1760 pc, further away than recent estimates have envisaged, and a foreground group at ∼1350 pc. We also calculate individual membership probabilities and identify outliers as possible non-members of the association. [ABSTRACT FROM AUTHOR]

We report on a comparison of spectroscopic analyses for hydrogen (DA) and helium atmosphere (DB) white dwarfs with Gaia Data Release 2 (DR2) parallaxes and photometry. We assume a reddening law and a mass–radius relation to connect the effective temperatures (T eff) and surface gravities (log  g) to masses and radii. This allows the comparison of two largely independent sets of fundamental parameters for 7039 DA and 521 DB stars with high-quality observations. This subset of the Gaia white dwarf sample is large enough to detect systematic trends in the derived parameters. We find that spectroscopic and photometric parameters generally agree within uncertainties when the expectation of a single star is verified. Gaia allows the identification of a small systematic offset in the temperature scale between the two techniques, as well as confirming a small residual high-mass bump in the DA mass distribution around 11 000–13 000 K. This assessment of the accuracy of white dwarf fundamental parameters derived from Gaia is a first step in understanding systematic effects in related astrophysical applications such as the derivation of the local stellar formation history, initial-to-final mass relation, and statistics of evolved planetary systems. [ABSTRACT FROM AUTHOR]

We present new results from the Parallaxes of Southern Extremely Cool dwarfs program to measure parallaxes, proper motions and multiepoch photometry of L and early T dwarfs. The observations were made on 108 nights over the course of 8 yr using the Wide Field Imager on the ESO 2.2m telescope. We present 118 new parallaxes of L and T dwarfs of which 52 have no published values and 24 of the 66 published values are preliminary estimates from this program. The parallax precision varies from 1.0 to 15.5 mas with a median of 3.8 mas. We find evidence for two objects with long term photometric variation and 24 new moving group candidates. We cross-match our sample to published photometric catalogues and find standard magnitudes in up to 16 pass-bands from which we build spectral energy distributions and H–R diagrams. This allows us to confirm the theoretically anticipated minimum in radius between stars and brown dwarfs across the hydrogen burning minimum mass. We find the minimum occurs between L2 and L6 and verify the predicted steep dependence of radius in the hydrogen burning regime and the gentle rise into the degenerate brown dwarf regime. We find a relatively young age of ∼2  Gyr from the kinematics of our sample. [ABSTRACT FROM AUTHOR]