arXiv subjects

Jos de Bruijne

Publications and source records attributed to Jos de Bruijne.

14 recordsLinked to original sources

arXiv2026

Searching for Isolated Black Hole Candidates within 15 pc of the Solar System in Gaia DR3

Theoretical models predict that the Galaxy hosts $10^8$-$10^9$ black holes formed from the complete gravitational collapse of heavy stars and that most of these black holes are isolated, without any companion. Within 15 pc of the Solar System ($\sim 50$ ly), there may be a few black holes. If located inside one of the Local Interstellar Clouds - which occupy 5-20% of this local volume - an isolated black hole could produce detectable electromagnetic emission via accretion from the interstellar medium, given the capabilities of current or near-future observatories. However, precise predictions remain challenging due to large uncertainties in the expected accretion spectra. Outside these clouds, the accretion rate would be too low; according to our models, the resulting electromagnetic flux is well below the detection thresholds of current and near-future observational facilities. While astrometric detection via gravitational perturbation of nearby stars is conceivable, the local stellar density is too low for this method to be realistically successful. We have searched the Gaia DR3 catalog for candidate isolated black holes accreting from the interstellar medium and identified five sources. All candidates lie close to the Galactic plane, making them likely spurious astrometric solutions, for instance caused by unmodelled background sources (crowding) and/or unmodelled binarity. Our search for infrared and radio emission from these sources further suggests that they are unlikely to be black holes accreting from the interstellar medium.

astro-ph.HE↗

arXiv2025

Strömgren photometric metallicity map of the Magellanic Cloud stars using Gaia DR3--XP spectra

One important key in understanding a galaxy's evolution is to study the consequences of its past dynamical interactions that influenced its shape. By measuring the metallicity distribution of stellar populations with different ages, one can learn about these interactions. The Magellanic Clouds, being the nearest pair of interacting dwarf galaxies with a morphology characterised by different tidal and kinematic sub-structures as well as a vast range of stellar populations, represent an excellent place to study the consequences of dwarf-dwarf galaxy interactions and with their large host, the Milky Way. We aim to determine the metallicities ([Fe/H]) of red giant branch (old) and supergiant (young) stars covering the entire galaxies, estimate their radial metallicity gradients and produce homogeneous metallicity maps. We use the XP spectra from Gaia Data Release 3 to calculate synthetic Strömgren magnitudes from the application of the GaiaXPy tool and adopt calibration relations from the literature to estimate the metallicities. We present photometric metallicity maps for ~90,000 young stars and ~270,000 old stars within ~11 deg of the Small Magellanic Cloud and ~20 deg of the Large Magellanic Cloud from a homogeneous dataset. We find that the overall radial metallicity gradients decrease linearly in agreement with previous studies. We apply piecewise-regression fitting to derive the gradients within different radial regions. The overall metallicity gradients, traced by young and old stars, decrease from the centre to the outskirts of both galaxies. However, they show multiple breakpoints depicting regions following different and sometime opposite trends. These are associated to the structure of the galaxies, their history of star formation and chemical evolution but may be influenced by a low number of sources, especially at the centre (due to crowding) and in the outermost regions.

astro-ph.GA↗

arXiv2024

HD 7977 and its possible influence on Solar System bodies

In the latest Gaia third data release one can find extremely small proper motion components for the star HD 7977. This, together with the radial velocity measurement lead to the conclusion that this star passed very close to the Sun in the recent past. Such a very close approach of a one solar mass star must result in noticeable changes in the motion of all Solar System bodies, especially those on less tight orbits, namely long-period comets (LPCs) and Kuiper belt objects. We estimate and present these effects. Our current knowledge on the stellar surroundings of the Sun found in the latest Gaia catalogues allowed us to perform numerical integrations and prepare a list of potential stellar perturbers of LPCs. We use this list, made available in the StePPeD database. To study the past motion of LPCs under the simultaneous action of the Galactic potential and passing stars, we use precise original cometary orbits taken from the current CODE catalogue. We examine the reliability of the extremely small proper motion of HD 7977 concluding that this star can be an unresolved binary but according to the astrometry covering more than a century, the current Gaia results cannot be ruled out. We present the parameters of a very close passage of this star near the Sun. We also show examples of the strong influence of this passage on the past motion of some LPCs. We also discuss the possible influence of this perturber on other Solar System bodies. It is possible that 2.47 Myr ago the one solar mass star HD 7977 passed as close as one thousand au from the Sun. Such an event constitutes a kind of dynamical horizon for all studies of the past Solar System bodies' dynamics.

astro-ph.SR↗

arXiv2023

Strömgren photometric metallicity of the Small Magellanic Cloud stars using Gaia DR3-XP spectra

Observational studies have identified several sub-structures in different regions of the Magellanic Clouds, the nearest pair of interacting dwarf satellites of the Milky Way. By studying the metallicity of the sources in these sub-structures, we aim to shed light on the possible origin of these sub-structures. Spectroscopic metallicities exist only for a few thousand sources, mostly giant stars located in specific regions of the galaxies. These metallicities come from different instruments at various spectral resolutions, and systematic uncertainties hamper comparisons and draw firm conclusions about their origin. The third data release of \textit{Gaia} has provided us with $\sim$ 0.17 million XP spectra of the different stellar populations in the SMC alone as faint as $\sim$ 18 mags in the G band, which are spread across $\sim$ 10$^\circ$ from the SMC centre. We aim to determine the metallicities of these sources based on synthetic Strömgren photometry derived from XP spectra and produce a high-resolution metallicity map of the SMC. Our metallicity gradient estimate of the SMC turns out to be --0.062 $\pm$ 0.009 dex/deg. This is comparable with the previous estimates, which also validate our method of metallicity estimation. We aim to apply this method to other stellar populations and to the LMC to create a high-resolution metallicity map of the Magellanic Clouds.

astro-ph.GA↗

arXiv2021

Constraining the initial conditions of NGC 2264 using ejected stars found in Gaia DR2

Fast, ejected stars have been found around several young star-forming regions, such as the Orion Nebula Cluster (ONC). These ejected stars can be used to constrain the initial density, spatial and kinematic substructure when compared to predictions from $N$-body simulations. We search for runaway and slower walkaway stars using $Gaia$ DR2 within 100 pc of NGC 2264, which contains subclustered regions around higher-mass OB-stars (S Mon, IRS 1 and IRS 2). We find five runaways and nine walkaways that trace back to S Mon and six runaways and five walkaways that trace back to IRS 1/2 based on their 3D-kinematics. We compare these numbers to a range of $N$-body simulations with different initial conditions. The number of runaways/walkaways is consistent with initial conditions with a high initial stellar density ($\sim$10 000 M$_{\odot}$ pc$^{-3}$), a high initial amount of spatial substructure and either a subvirial or virialised ratio for all subclusters. We also confirm the trajectories of our ejected stars using the data from $Gaia$ EDR3, which reduces the number of runaways from IRS 1/2 from six to four but leaves the number of runaways from S Mon unchanged. The reduction in runaways is due to smaller uncertainties in the proper motion and changes in the parallax/distance estimate for these stars in $Gaia$ EDR3. We find further runaway/walkaway candidates based on proper motion alone in $Gaia$ DR2, which could increase these numbers once radial velocities are available. We also expect further changes in the candidate list with upcoming $Gaia$ data releases.

astro-ph.SR↗

arXiv2020

Benfords law in the Gaia universe

Benfords law states that for scale- and base-invariant data sets covering a wide dynamic range, the distribution of the first significant digit is biased towards low values. This has been shown to be true for wildly different datasets, including financial, geographical, and atomic data. In astronomy, earlier work showed that Benfords law also holds for distances estimated as the inverse of parallaxes from the ESA Hipparcos mission. We investigate whether Benfords law still holds for the 1.3 billion parallaxes contained in the second data release of Gaia (Gaia DR2). In contrast to previous work, we also include negative parallaxes. We examine whether distance estimates computed using a Bayesian approach instead of parallax inversion still follow Benfords law. Lastly, we investigate the use of Benfords law as a validation tool for the zero-point of the Gaia parallaxes.

astro-ph.GA↗

arXiv2020

Runaway and walkaway stars from the ONC with Gaia DR2

Theory predicts that we should find fast, ejected (runaway) stars of all masses around dense, young star-forming regions. $N$-body simulations show that the number and distribution of these ejected stars could be used to constrain the initial spatial and kinematic substructure of the regions. We search for runaway and slower walkaway stars within 100 pc of the Orion Nebula Cluster (ONC) using $Gaia$ DR2 astrometry and photometry. We compare our findings to predictions for the number and velocity distributions of runaway stars from simulations that we run for 4 Myr with initial conditions tailored to the ONC. In $Gaia$ DR2, we find 31 runaway and 54 walkaway candidates based on proper motion, but not all of these are viable candidates in three dimensions. About 40 per cent are missing radial velocities, but we can trace back 9 3D-runaways and 24 3D-walkaways to the ONC, all of which are low/intermediate-mass (<8 M$_{\odot}$). Our simulations show that the number of runaways within 100 pc decreases the older a region is (as they quickly travel beyond this boundary), whereas the number of walkaways increases up to 3 Myr. We find fewer walkaways in $Gaia$ DR2 than the maximum suggested from our simulations, which may be due to observational incompleteness. However, the number of $Gaia$ DR2 runaways agrees with the number from our simulations during an age of $\sim$1.3-2.4 Myr, allowing us to confirm existing age estimates for the ONC (and potentially other star-forming regions) using runaway stars.

astro-ph.SR↗

arXiv2019

XO-7 b: A transiting hot Jupiter with a massive companion on a wide orbit

Transiting planets orbiting bright stars are the most favorable targets for follow-up and characterization. We report the discovery of the transiting hot Jupiter XO-7 b and of a second, massive companion on a wide orbit around a circumpolar, bright, and metal rich G0 dwarf (V = 10.52, $T_{\rm eff} = 6250 \pm 100 \; \rm K$, $\rm[Fe/H] = 0.432 \pm 0.057 \; \rm dex$). We conducted photometric and radial velocity follow-up with a team of amateur and professional astronomers. XO-7 b has a period of $ 2.8641424 \pm 0.0000043$ days, a mass of $0.709 \pm 0.034 \; \rm M_{\rm J}$, a radius of $1.373 \pm 0.026 \; \rm R_{\rm J}$, a density of $0.340 \pm 0.027 \; \rm g \, {cm}^{-3}$, and an equilibrium temperature of $1743 \pm 23 \; \rm K$. Its large atmospheric scale height and the brightness of the host star make it well suited to atmospheric characterization. The wide orbit companion is detected as a linear trend in radial velocities with an amplitude of $\sim100 \; \rm m \, {s}^{-1}$ over two years, yielding a minimum mass of $4 \; \rm M_{\rm J}$; it could be a planet, a brown dwarf, or a low mass star. The hot Jupiter orbital parameters and the presence of the wide orbit companion point towards a high eccentricity migration for the hot Jupiter. Overall, this system will be valuable to understand the atmospheric properties and migration mechanisms of hot Jupiters and will help constrain the formation and evolution models of gas giant exoplanets.

astro-ph.EP↗

arXiv2019

Dynamical evolution of star-forming regions: III. Unbound stars and predictions for Gaia

We use $N$-body simulations to probe the early phases of the dynamical evolution of star-forming regions and focus on mass and velocity distributions of unbound stars. In this parameter space study, we vary the initial virial ratio and degree of spatial and kinematic substructure and analyse the fraction of stars that become unbound in two different mass classes (above and below 8 M$_{\odot}$). We find that the fraction of unbound stars differs depending on the initial conditions. After 10 Myr, in initially highly subvirial, substructured simulations, the high-mass and lower-mass unbound fractions are similar at $\sim$23 per cent. In initially virialised, substructured simulations, we find only $\sim$16 per cent of all high-mass stars are unbound, whereas $\sim$37 per cent of all lower-mass stars are. The velocity distributions of unbound stars only show differences for extremely different initial conditions. The distributions are dominated by large numbers of lower-mass stars becoming unbound just above the escape velocity of $\sim$3 km s$^{-1}$ with unbound high-mass stars moving faster on average than lower-mass unbound stars. We see no high-mass runaway stars (velocity > 30 km s$^{-1}$) from any of our initial conditions and only an occasional lower-mass runaway star from initially subvirial/substructured simulations. In our simulations, we find a small number of lower-mass walkaway stars (with velocity 5-30 km s$^{-1}$) from all of our initial conditions. These walkaway stars should be observable around many nearby star-forming regions with Gaia.

astro-ph.SR↗

arXiv2018

A Gaia study of the Hyades open cluster

We present a study of the membership of the Hyades open cluster, derive kinematically-modelled parallaxes of its members, and study the colour-absolute magnitude diagram of the cluster. We use Gaia DR1 Tycho-Gaia Astrometric Solution (TGAS) data complemented by Hipparcos-2 data for bright stars not contained in TGAS. We supplement the astrometric data with radial velocities collected from a dozen literature sources. By assuming that all cluster members move with the mean cluster velocity to within the velocity dispersion, we use the observed and the expected motions of the stars to determine individual cluster membership probabilities. We subsequently derive improved parallaxes through maximum-likelihood kinematic modelling of the cluster. This method has an iterative component to deal with 'outliers', caused for instance by double stars or escaping members. Our method extends an existing method and supports the mixed presence of stars with and without radial velocities. We find 251 candidate members, 200 of which have a literature radial velocity, and 70 of which are new candidate members with TGAS astrometry. The cluster is roughly spherical in its centre but significantly flattened at larger radii. The observed colour-absolute magnitude diagram shows a clear binary sequence. The kinematically-modelled parallaxes that we derive are a factor ~1.7 / 2.9 more precise than the TGAS / Hipparcos-2 values and allow to derive an extremely sharp main sequence. This sequence shows evidence for fine-detailed structure which is elegantly explained by the full spectrum turbulence model of convection.

astro-ph.GA↗

arXiv2018

On the accuracy of mass measurement for microlensing black holes as seen by Gaia and OGLE

We investigate the impact of combining Gaia astrometry from space with precise, high cadence OGLE photometry from the ground. For the archival event OGLE3-ULENS-PAR-02, which is likely a black hole, we simulate a realistic astrometric time-series of Gaia measurements and combine it with the real photometric data collected by the OGLE project. We predict that at the end of the nominal 5 years of the Gaia mission, for the events brighter than $G\approx15.5$ mag at the baseline, caused by objects heavier than 10 $M_{\odot}$, it will be possible to unambiguously derive masses of the lenses, with accuracy between a few to 15 per cent. We find that fainter events ($G<17.5$) can still have their lens masses determined, provided that they are heavier than 30 $M_{\odot}$. We estimate that the rate of astrometric microlensing events caused by the stellar-origin black holes is $\approx 4 \times 10^{-7} \, \rm yr^{-1}$, which implies, that after 5 years of Gaia operation and $\approx 5 \times 10^6$ bright sources in Gaia, it will be possible to identify few such events in the Gaia final catalogues.

astro-ph.SR↗

arXiv2012

Astrometric determination of white dwarf radial velocities with Gaia?

Usually, the determination of radial velocities of stars relies on the shift of spectral lines by the Doppler effect. Russel & Atkinson (1931) and Oort (1932) already noted that due to the large proper motion and parallax of the white dwarf (WD) van Maanen 2, a determination of the perspective acceleration of the proper motion would provide a direct astrometric determination of the radial velocity which is independent of the gravitational redshift. If spectroscopic redshift measurements of Halpha and Hbeta NLTE cores exist, a purely astrometric determination would allow disentangling the gravitational redshift from the Doppler shift. The best instrument for measuring the tiny perspective acceleration is the Gaia satellite of the European Space Agency, aiming at absolute astrometric measurements of one billion stars down to 20th magnitude with unprecedented accuracy. At 15th magnitude, the predicted angular accuracy of Gaia is about 20 micro-arcseconds. In this article, we estimate whether it is possible to measure the radial velocity of WDs astrometrically by the exchange of proper motion into radial velocity during the 5-year mission of the satellite or by combining Hipparcos data with the position measurements at the beginning of the Gaia mission (the Hundred-Thousand-Proper-Motion project HTPM).

astro-ph.SR↗

arXiv2001

Direct determination of quasar redshifts

We present observations of 11 quasars, selected in the range z = 2.2-4.1, obtained with ESA's Superconducting Tunnel Junction (STJ) camera on the WHT. Using a single template quasar spectrum, we show that we can determine the redshifts of these objects to about 1%. A follow-up spectroscopic observation of one QSO for which our best-fit redshift (z = 2.976) differs significantly from the tentative literature value (z ~ 2.30) confirms that the latter was incorrect.

astro-ph↗

arXiv2000

Young Stellar Groups, Runaway Stars, and Pulsars

Milli-arcsecond astrometry provided by Hipparcos and by radio observations makes it possible to retrace the orbits of nearby runaway stars and pulsars with sufficient accuracy to identify their parent stellar cluster or association. For two cases it is even possible to deduce the specific formation scenario. The runaway star zeta Oph and PSR J1932+1059 are the result of a supernova explosion which took place 1 Myr ago in a massive binary in the Upper Scorpius association. The pulsar received a kick velocity of about 350 km/s in this event. The runaway stars mu Col and AE Aur and the isolated eccentric massive binary iota Ori result from a binary-binary encounter, most likely inside the Trapezium cluster, 2.5 Myr ago. Future astrometric missions such as DIVA, FAME and in particular GAIA will allow extension of these studies to a significant fraction of the Galactic disk, and will provide new constraints on the formation and evolution of massive stars.

astro-ph↗