arXiv subjects

S. Geier

Publications and source records attributed to S. Geier.

At least 19 recordsLinked to original sources

arXiv2026

A 500 pc volume-limited sample of hot subluminous stars III. The short-period binary population

Hot subdwarf stars of spectral types O and B (sdO/B) in binaries form as products of substantial mass loss near the tip of the red giant branch and offer powerful constraints on binary-star evolutionary models. However, details of some formation channels remain missing. We present a comprehensive analysis of the short-period binary population in the 500 pc volume-limited sample of hot subdwarfs, enabled by precise Gaia DR3 parallaxes. Besides 45 known binaries within 500 pc, this work identifies 50 new single-lined radial-velocity variable systems, 34 with orbital solutions with periods between 0.07 and 22 days. We derive an overall short-period binary fraction of 34.7 (+2.8/-2.9)% for the full sample of 301 hot subdwarfs within 500 pc, corrected for orbital inclination and detection efficiency. The newly solved binaries predominantly occupy the 1-20 day range, a parameter space previously under-represented in the literature. We identify five new reflection-effect systems, three ellipsoidal modulation systems, a newly solved HW Vir system, and a new triple candidate. The sdB and sdO binary fractions are similar, while the sdOB class shows a fraction roughly half that of sdB/sdO stars, with periods over one day, suggesting a different evolutionary pathway. Underluminous hot subdwarfs below the canonical extreme horizontal branch show a binary fraction comparable to other sdB/sdO stars but host only white dwarf companions, which are more massive and mainly found at periods under one day. We provide the first volume-complete birthrate estimates for sub-populations, determining a Galactic merger rate of 2.5+-1.5x10^-5 yr^-1 for sdO/B binaries, explaining no more than ~12% of the eHe-sdO population. We also derive birthrates for two SN Ia progenitor channels, finding hot subdwarf binaries could account for up to 2.5 (+0.7/-0.5)% of the observed Galactic SN Ia rate.

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arXiv2026

Toward 3D orbits of wide sdO/B binaries I. Six composite systems spatially resolved with VLTI/GRAVITY

Hot subdwarf stars (sdO/Bs) are widely considered to be products of binary evolution. A significant fraction of them are found in long-period or wide binaries ($P>500$ d) with main sequence (MS) companions, likely resulting from a stable mass transfer episode where the MS companion stripped the hydrogen envelope of the sdO/B progenitor. Consequently, wide sdO/B binaries represent a key population in our pursuit of understanding stable mass transfer. They exhibit a modest range of orbital periods and eccentricities as revealed by long-term spectroscopic campaigns, though the component masses are not well constrained. In this Letter, we present the first long-baseline interferometry campaign to observe wide sdO/B + MS binaries and take the first step toward determining their 3-dimensional (3D) orbits and model-independent component masses. We target six composite sdO/B + MS systems with VLTI/GRAVITY and spatially resolve all of them. The projected physical separations range between $1-3$ au, with uncertainties between $1-8$%. When combined with complementary information from spectroscopic or astrometric observations, our precise measurements will be crucial to constrain 3D orbits for these systems. Additionally, we also identify a potential third component in BD+10 2357, although additional data will be necessary for confirmation. In light of continued spectroscopic monitoring and the imminent Gaia Data Release 4, we strongly encourage expanding the interferometric sample presented here to establish new, precise orbital and mass constraints for this key population of binary interaction products.

astro-ph.SR↗

arXiv2026

Evidence for neutron capture in heavy-metal hot subdwarfs: Far-UV spectroscopy of EC22536-5304 and LSIV-14 116

Most hot subdwarfs (sdO/B) are low-mass core-helium-burning stars formed through binary interaction. A subgroup of intermediate He-rich sdOBs shows extreme heavy-metal (Z>30) enrichments exceeding $10^4$ times solar, especially in Zr or Pb. We analyse the first ultraviolet spectra of the "heavy metal" subdwarfs LSIV-14 116 (Zr-rich) and EC22536-5304 (Pb-rich) to determine their abundance patterns and test nucleosynthesis models. Both stars show exceptionally rich heavy-element spectra dominated by ions in stages III-VI, many absent from standard line lists. We compiled literature energy levels, wavelengths, and oscillator strengths and implemented them in the SYNSPEC code. In addition, we computed new oscillator strengths for As III, Se III, Hf IV, and Tl IV. New photoionisation cross-sections for Pb III-VI enabled the first non-LTE models of multiply ionised Pb. In LSIV-14 116 we detect 16 light and 24 heavy metals (Ga-Bi); Br, Nb, Mo, Pd, In, Sb, Te, and Xe are measured in an sdO/B star for the first time. In EC22536-5304 13 light and 26 heavy metals are detected, including first detections of La, Ce, Pr, Nd, Er, Yb, Lu, Hf, Ta, W, Os, Pt, Hg, Tl, and Bi. LSIV-14 116 peaks at ~4.3 dex for Sr-Sn relative to solar, declining to 3.1 dex at Pb and 2.3 dex at Bi, whereas EC22536-5304 reaches 6.2 dex for Pb and 5.4 dex for Bi. Both stars are Fe-poor. The abundance patterns cannot be explained by atomic diffusion alone and retain a clear nucleosynthetic signature. EC22536-5304 closely matches predictions of i-process nucleosynthesis, providing strong evidence for i-process self-enrichment in hot subdwarfs. EC22536-5304 likely formed via Roche-lobe overflow, whereas LSIV-14 116 likely originated from the merger of two low-mass white dwarfs, which may explain differences in its enrichment pattern. These results suggest that heavy metals in other He-sdO/Bs may also be self-synthesised.

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arXiv2026

Stars on the ascending helium giant branch I. From white dwarf merger to helium giant: the evolutionary state of the rapidly rotating hot subdwarf HE 1518-0948

Hot subdwarf stars with masses above $0.8 M_\odot$ ascend the helium giant branch after the end of core helium burning, before entering the white dwarf cooling track or exploding as type Ib/c supernovae. Such massive helium stars are typically expected to form through the stripping of an intermediate mass star by a binary companion after which some hydrogen is still expected to be retained. However, the subclass of extreme helium rich hot subdwarfs (He-sdOs) shows no or very weak hydrogen traces, and their low binary fraction suggests that they are either created through single-star evolution triggered by a late hot flash in a low-mass red giant or the merger of two helium white dwarfs. Most He-sdOs are located close to the helium zero-age main sequence, while a small number exhibit much lower surface gravities, indicating inflated radii. Whether these objects are evolutionarily connected to the main He-sdO population remains unclear. In this work, we analyse the luminous, extreme helium-rich, low-surface-gravity sdO HE 1518-0948 (HE 1518) through a detailed spectroscopic study of optical and ultraviolet data. A comparison with evolutionary models indicates that HE 1518 is the product of a massive double helium white dwarf merger and is currently undergoing helium shell burning while ascending the helium giant branch. This makes HE 1518 one of only a few known objects located in this sparsely populated region of the Hertzsprung-Russell diagram. Such stars provide valuable laboratories for studying the evolution of massive hot subdwarfs beyond core helium burning, and their high luminosities allow them to be detected at large distances.

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arXiv2026

Discovery of a runaway star likely ejected by a Type Iax Supernova

Over the past decade, runaway stars have been identified, believed to originate either as surviving donors of Type Ia supernovae or as partially deflagrated accretors producing Type Iax supernovae. While the former have been extensively studied recently, the origins of the latter (also called LP 40-365 type stars) remain under-explored and therefore less well understood. So far seven such objects are known. In this paper, we report the discovery of a new LP 40-365 type runaway star, notably hotter than previously studied members of this class. Spectral analysis confirms that its atmosphere is neon- and oxygen-dominated, consistent with earlier analyses of other LP 40-365 type stars. Kinematic analysis indicates that the star has a high probability of being unbound from the Galaxy and was most likely ejected from the Galactic disk approximately 2.8 Myr ago with an ejection velocity exceeding 600 km/s. This result further emphasizes the discrepancy between the abundance yields and kick velocities predicted by white dwarf deflagration models and those observed in stars of LP 40-365 type, underscoring the need for a reassessment of such systems.

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arXiv2026

The massive hot subdwarf binary LAMOST J065816.72+094343.1

Massive short-period binaries involving hot subdwarf stars (sdO/Bs) are rare but important to constraining pathways for binary star evolution. Moreover, some of the most promising candidate progenitor systems leading to Type Ia supernovae (SNe Ia) involve sdO/Bs. LAMOST J065816.72+094343.1 has been identified as such a candidate. To explore the nature and evolutionary future of LAMOST J065816.72+094343.1, we complemented archival spectroscopic data with additional time series spectra and high-resolution spectroscopy of the object. After combining these with photometric data, we determined the orbital parameters of the system and the mass of the companion. We solved the orbit of the system by analyzing 68 low- and medium-resolution spectra using state-of-the-art mixed local thermodynamic equilibrium (LTE) and non-LTE model atmospheres. Additionally, we gathered nine high-resolution spectra to determine atmospheric parameters and the projected rotational velocity of the sdOB. The inclination angle of the system was constrained assuming tidal synchronization of the sdOB, which was verified via analysis of the ellipsoidal variations in the TESS light curve. We determine LAMOSTJ065816.72+094343.1 to be a binary consisting of a massive $0.82 \pm 0.17 \mathrm{M}_{\odot}$ sdOB component with a $1.30^{+0.31}_{-0.26} \mathrm{M}_{\odot}$ unseen companion. Due to the companion's mass being very close to the Chandrasekhar mass limit and high for a white dwarf, it is unclear whether it is a white dwarf or a neutron star. We find the system to be in a close orbit, with a period of $P=0.31955193 \mathrm{d}$ and an inclination angle of $i = 49.6^{+5.2}_{-4.2} \mathrm{deg}$. While the exact nature of the companion remains unknown, we determine the system to either lead to a SN Ia or an intermediate mass binary pulsar, potentially after a phase as an intermediate-mass X-ray binary.

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arXiv2026

A 500 pc volume-limited sample of hot subluminous stars II. Atmospheric parameters, mass distribution, and kinematics

We present a quantitative spectroscopic and kinematic analysis of a volume-complete sample of hot subluminous stars within 500 pc of the Sun, assembled using accurate parallax measurements from Gaia Data Release 3 (DR3). In total, 3226 spectra of 253 hot subdwarf stars were analysed to derive atmospheric parameters (effective temperature, surface gravity, and helium abundance) and radial velocities. Spectral energy distributions (SEDs) combined with Gaia parallaxes were used to measure stellar radii, luminosities, and masses. The derived atmospheric parameters reveal a consistent alignment between sdB and sdO stars in the Kiel diagram when compared to theoretical evolutionary models. We identify a population (about 10%) of hot subdwarfs located below the 0.45 Msun zero-age EHB in both the Kiel and Hertzsprung-Russell diagrams, which likely originate from intermediate-mass progenitors (1.8-8 Msun). The overall mass distribution peaks at 0.48 pm 0.12 Msun, while hot subdwarfs below the EHB peak at 0.43 pm 0.10 Msun, supporting non- or semi-degenerate helium ignition characteristic of intermediate-mass stars. Interpolation of EHB and post-EHB tracks yields mass distributions consistent with those derived from SEDs and parallaxes. Assuming a mass range between 0.40 and 0.50 Msun, we find that the post-EHB birthrate is 2-3 times higher than the EHB birthrate, suggesting overestimated EHB lifetimes or contamination from additional formation channels. Our kinematic analysis shows that 86 pm 2% of the stars belong to the Galactic thin disk, with 13 pm 1% and 1 pm 1% associated with the thick disk and halo. The below-EHB population is found exclusively in the thin disk, the only Galactic component young enough to host intermediate-mass progenitors. Its absence from other large samples suggests that non-degenerate formation channels play a more prominent role in the Galactic disk.

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arXiv2025

SN 2021lwz: Another Exotic Luminous and Fast Evolving Optical Stripped Envelope Supernova ?

Current large-scale, high-cadence surveys, such as the ZTF, provide detections of new and rare types of transients and supernovae whose physical origins are not well understood. We investigate the nature of SN 2021lwz at a redshift $z=0.065$, an overluminous supernova (SN) of absolute magnitude, $M_{g} \sim -20.1$ AB, falling in the lower range of superluminous supernovae (SLSNe) luminosities, and discovered in a faint dwarf galaxy with an absolute magnitude of $M_{g} \simeq -14.5$ AB. SN 2021lwz is studied using optical spectroscopy, photometry and imaging linear polarimetry obtained during several follow-up campaigns. All the data are used to analyse and model the evolution of the explosion. Comparisons with other SNe of well known or rarer types are investigated. SN 2021lwz belongs to the rare class of rapidly evolving transients. The bolometric light curve rises in about $7$ days to a peak luminosity of about $5 \times 10^{43}$ erg/s, at a rate of 0.2 mag day$^{-1}$ close to the peak. Spectroscopy modelling reveals more similarities with a normal Type Ic-like SN than with a SLSN before peak, showing slightly broadened lines after peak. Light curve modelling shows that the Arnett model of the bolometric light curve using a radioactive source ($^{56}$ Ni) is not able to reasonably explain the light curve evolution. A magnetar model seems more appropriate, suggesting that the explosion of low ejecta mass ($M_{\rm ej} \sim 0.24 ~M_\odot$) took place in a low mass ($M \sim 10^{6.66}~M_\odot$) dwarf galaxy of specific star-formation rate about ten times larger than typical star-forming galaxies. In conclusion SN 2021lwz is an uncommon transient showing many similarities with several classes of transients, and with rare transients. It may be an interesting example pointing on how differences in ejecta mass and engine parameters could produce a wide range of engine-driven SESNe.

astro-ph.HE↗

arXiv2025

The twin red giant branch system BD+20 5391 A case study of low-mass double-core evolution

Understanding interactions of binary systems on the red giant branch is crucial to understanding the formation of compact stellar remnants such as helium-core white dwarfs (He-WDs) and hot subdwarfs. However, the detailed evolution of such systems, particularly those with nearly identical components, remains under-explored. We aim to analyse the double-lined spectroscopic binary system BD+20 5391, composed of two red giant stars, in order to characterise its orbital and stellar parameters and to constrain its evolution. Spectroscopic data were collected between 2020 and 2025 using the Ond\v{r}ejov Echelle Spectrograph and the Mercator \'Echelle Spectrograph. The time-resolved spectra were fitted with models to determine the radial velocity curve and derive the system's parameters. We then used the position of both stars in the Hertzsprung-Russell diagram to constrain the system's current evolutionary state, and we discuss potential outcomes of future interactions between the binary components. We find that the two stars in BD+20 5391 will likely initiate Roche lobe overflow (RLOF) simultaneously, leading to a double-core evolution scenario. The stars' helium core masses at RLOF onset will be almost identical, at 0.33 $\mathrm{M}_{\odot}$. This synchronised evolution suggests two possible outcomes: common envelope ejection, resulting in a short-period double He-WD binary, or a merger without envelope ejection. In the former case, the resulting double He-WD may merge later and form a hot subdwarf star. This study provides a valuable benchmark example for understanding the evolution of interacting red giant binaries, which will be discovered in substantial numbers in upcoming large-scale spectroscopic surveys.

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arXiv2025

Where are the binaries? -- Searching for blue horizontal-branch stars in binary systems in the inner Galactic Halo

Blue horizontal-branch (BHB) stars are evolved low-mass objects that have completed their core hydrogen burning main-sequence (MS) stage and have lost significant mass during the red giant phase culminating in the helium flash. We determine the fraction of BHBs in binary systems over a wide range of separations in the inner Galactic Halo to constrain mass-loss mechanisms and evolutionary pathways. Using a catalog of 22,336 BHB candidates from Gaia DR3, we analysed radial velocity variations found in spectra (263 spectra of 89 targets) acquired using the Ondrejov Echelle spectrograph at the Astronomical Institute of the Czech Academy of Sciences together with archival spectra from the Ultraviolet and Visual Echelle Spectrograph (UVES) and Fiber-fed Extended Range Optical Spectrograph (FEROS). We searched for wide common proper motion pairs, binary candidates with enhanced astrometric noise, and binaries with astrometric orbital solutions in Gaia DR3. Archival light curves from Gaia DR3 and the Zwicky Transient Facility (ZTF) were checked for binary induced variations. Synthetic SEDs and binary detection probabilities were modelled to account for selection effects. We find a binary fraction of <2.2%, far lower than the rates for their main-sequence (MS) and red-giant branch (RGB) progenitors (30-50%). This suggests that BHBs are either not descendants of binary systems, or that existing companions do not survive the BHB formation process. The negligible binary fraction implies single-star evolution could dominate BHB formation, contrasting with EHB stars where binarity is critical. Our results challenge models of mass loss on the RGB and highlight the need for alternative mechanisms.

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arXiv2025

Ancient, eclipsing, tidally-locked: A blue lurker progenitor in the population of extreme-velocity star candidates

Many extreme velocity candidate stars have been found based on \textit{Gaia} astrometry, but need spectroscopic confirmation. We select late-type hypervelocity star (HVS) candidates from the \textit{Gaia} DR3 catalog with a $1\sigma$ lower limit of the tangential velocity of 800 km\,s$^{-1}$. J1903-0023, one of the brightest targets, stands out as high priority candidate for follow-up spectroscopy using the X-shooter instrument at ESO-VLT. We determine its atmospheric parameters and abundances utilizing synthetic spectral grids and a global $\chi^2-$minimization procedure, and its stellar parameters with the help of evolutionary tracks and the spectral energy distribution. The star shows variability in its light curve and follow-up spectroscopy confirms that the star is radial-velocity variable. The spectroscopic distance of J1903-0023 is lower than that based on the parallax, indicating that the star is not a hypervelocity binary star but bound to the Galaxy. The star turned out to be of spectral type F, very similar to the extreme-velocity star J0725-2351, which we analyse in the same way as the target. Apparently, both stars are very metal poor and old halo main-sequence (sdF) stars with masses slightly below the halo turn-off mass, and share the low metallicity ([Fe/H]=-2.3,-2.6) and strong alpha enhancement ([$\alpha$/Fe]$\sim0.44$). While J0725-2351 is non-rotating ($v\sin\,i<3$\,km\,s$^{-1}$), J1903-0023 is a fast rotator ($v\sin i=42.3\pm2.0$ km\,s$^{-1}$). The Gaia, and ZTF light curves show an eclipse at a 1.179 day period, similar to the rotation period of J1903-0023. We therefore conclude that J1903-0023 is a high-velocity tidally-synchronised binary most likely with a metal-poor M dwarf companion.

astro-ph.SR↗

arXiv2025

Asteroid sizes determined with thermophysical model and stellar occultations

Context. The sizes of many asteroids, especially slowly rotating, low-amplitude targets, remain poorly constrained due to selection effects. These biases limit the availability of high-quality data, leaving size estimates reliant on spherical shape assumptions. Such approximations introduce significant uncertainties propagating, e.g. into density determinations or thermophysical and compositional studies, affecting our understanding of asteroid properties. Aims. This work targets poorly studied main-belt asteroids, most of which lacked shape models. Using only high-quality dense light curves, thermal IR observations (incl. WISE), and stellar occultations, we aimed to produce reliable shape models and scale them via two independent techniques, allowing size comparison. We conducted two campaigns to obtain dense photometric light curves and to acquire multi-chord stellar occultations. Methods. Shape and spin models were reconstructed using lightcurve inversion. Sizes were determined by (1) thermophysical modeling with the Convex Inversion Thermophysical Model (CITPM), optimizing spin and shape models to visible lightcurve and IR data, and (2) scaling shape models with stellar occultations. Results. We obtained precise sizes and shape models for 15 asteroids. CITPM- and occultation-derived sizes agree within 5% in most cases, demonstrating the modeling's reliability. Larger discrepancies usually stem from incomplete occultation chord coverage. The study also gives insights into surface properties incl. albedo, roughness and thermal inertia. Conclusions. Using high-quality data and an advanced TPM integrating thermal and visible data with shape adjustment enabled precise size estimates comparable to those from multi-chord stellar occultations. We resolved major inconsistencies in previous size estimates, providing solid input for future studies on asteroid densities and surfaces.

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arXiv2024

Basaltic quasi-mini-moon: Characterizing 2024 PT5 with the 10.4 m Gran Telescopio Canarias and the Two-meter Twin Telescope

Context. Small bodies in Earth-like orbits, the Arjunas, are good targets for scientific exploration, and space mining or in situ resource utilization (ISRU) trials as they enable low-cost missions. The subset of these objects that experience recurrent temporarily captured flyby or orbiter episodes are among the best ranked in terms of accessibility. Only a handful of objects are known to have engaged in such a dynamical behavior. Finding and characterizing more of them may help to expand scientific and commercial research activities in space over the next few decades. Asteroid 2024 PT5 is a recent finding that shows dynamical traits in common with this group. Aims. Here we investigate the orbital context of 2024 PT5 and its spectral and rotational properties. Methods. We studied the short-term orbital evolution of 2024 PT5 using direct N-body simulations. We identified its spectral class from the visible reflectance spectrum and used photometric observations to derive its rotational properties. Observational data were obtained with the OSIRIS camera spectrograph at the 10.4 m Gran Telescopio Canarias and the Two-meter Twin Telescope. Results. Asteroid 2024 PT5 experiences recurrent co-orbital engagements and episodes in which it has negative geocentric orbital energy while inside a geocentric distance under three Hill radii, which we call quasi-mini-moon events. Its visible spectrum is consistent with that of lunar-like silicates. Photometric data suggest a rotation period under 1 h. Conclusions. The discovery of 2024 PT5 confirms that events resembling temporary captures are relatively frequent and involve objects larger than a few meters, suitable as accessible targets for scientific research activities and demonstrating ISRU technologies.

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arXiv2024

Eruptive mass loss less than a year before the explosion of superluminous supernovae: I. The cases of SN 2020xga and SN 2022xgc

We present photometric and spectroscopic observations of SN 2020xga and SN 2022xgc, two hydrogen-poor superluminous supernovae (SLSNe-I) at $z = 0.4296$ and $z = 0.3103$, respectively, which show an additional set of broad Mg II absorption lines, blueshifted by a few thousands kilometer second$^{-1}$ with respect to the host galaxy absorption system. Previous work interpreted this as due to resonance line scattering of the SLSN continuum by rapidly expanding circumstellar material (CSM) expelled shortly before the explosion. The peak rest-frame $g$-band magnitude of SN 2020xga is $-22.30 \pm 0.04$ mag and of SN 2022xgc is $-21.97 \pm 0.05$ mag, placing them among the brightest SLSNe-I. We used high-quality spectra from ultraviolet to near-infrared wavelengths to model the Mg II line profiles and infer the properties of the CSM shells. We find that the CSM shell of SN 2020xga resides at $\sim 1.3 \times 10^{16}~\rm cm$, moving with a maximum velocity of $4275~\rm km~s^{-1}$, and the shell of SN 2022xgc is located at $\sim 0.8 \times 10^{16}~\rm cm$, reaching up to $4400~\rm km~s^{-1}$. These shells were expelled $\sim 11$ and $\sim 5$ months before the explosions of SN 2020xga and SN 2022xgc, respectively, possibly as a result of luminous-blue-variable-like eruptions or pulsational pair instability (PPI) mass loss. We also analyzed optical photometric data and modeled the light curves, considering powering from the magnetar spin-down mechanism. The results support very energetic magnetars, approaching the mass-shedding limit, powering these SNe with ejecta masses of $\sim 7-9~\rm M_\odot$. The ejecta masses inferred from the magnetar modeling are not consistent with the PPI scenario pointing toward stars $> 50~\rm M_\odot$ He-core; hence, alternative scenarios such as fallback accretion and CSM interaction are discussed.

astro-ph.HE↗

arXiv2024

A spectroscopic and kinematic survey of fast hot subdwarfs

Hot subdwarfs (sdO/B) are the stripped helium cores of red giants formed by binary interactions. Close hot subdwarf binaries with massive white dwarf companions have been proposed as possible progenitors of thermonuclear supernovae type Ia (SN Ia). If the supernova is triggered by stable mass transfer from the helium star, the companion should survive the explosion and should be accelerated to high velocities. The hypervelocity star US 708 is regarded as the prototype for such an ejected companion. To find more of those objects we conducted an extensive spectroscopic survey. Candidates for such fast stars have been selected from the spectroscopic database of the Sloan Digital Sky Survey (SDSS) and several ground-based proper motion surveys. Follow-up spectroscopy has been obtained with several 4m- to 10m-class telescopes. Combining the results from quantitative spectroscopic analyses with space-based astrometry from \textit{Gaia} Early Data Release 3 (EDR3) we determined the atmospheric and kinematic parameters of 53 fast hot subdwarf stars. None of these stars is unbound to the Galaxy, although some have Galactic restframe velocities close to the Galactic escape velocity. 21 stars are apparently single objects, which crossed the Galactic disc within their lifetimes in the sdO/B stage and could be regarded as potential candidates for the SN Ia ejection scenario. However, the properties of the full sample are more consistent with a pure old Galactic halo population. We therefore conclude that the fast sdO/B stars we found are likely to be extreme halo stars.

astro-ph.SR↗

arXiv2024

The BlackGEM telescope array I: Overview

The main science aim of the BlackGEM array is to detect optical counterparts to gravitational wave mergers. Additionally, the array will perform a set of synoptic surveys to detect Local Universe transients and short time-scale variability in stars and binaries, as well as a six-filter all-sky survey down to ~22nd mag. The BlackGEM Phase-I array consists of three optical wide-field unit telescopes. Each unit uses an f/5.5 modified Dall-Kirkham (Harmer-Wynne) design with a triplet corrector lens, and a 65cm primary mirror, coupled with a 110Mpix CCD detector, that provides an instantaneous field-of-view of 2.7~square degrees, sampled at 0.564\arcsec/pixel. The total field-of-view for the array is 8.2 square degrees. Each telescope is equipped with a six-slot filter wheel containing an optimised Sloan set (BG-u, BG-g, BG-r, BG-i, BG-z) and a wider-band 440-720 nm (BG-q) filter. Each unit telescope is independent from the others. Cloud-based data processing is done in real time, and includes a transient-detection routine as well as a full-source optimal-photometry module. BlackGEM has been installed at the ESO La Silla observatory as of October 2019. After a prolonged COVID-19 hiatus, science operations started on April 1, 2023 and will run for five years. Aside from its core scientific program, BlackGEM will give rise to a multitude of additional science cases in multi-colour time-domain astronomy, to the benefit of a variety of topics in astrophysics, such as infant supernovae, luminous red novae, asteroseismology of post-main-sequence objects, (ultracompact) binary stars, and the relation between gravitational wave counterparts and other classes of transients

astro-ph.IM↗

arXiv2024

A 500 pc volume-limited sample of hot subluminous stars I. Space density, scale height, and population properties

We present the first volume-limited sample of spectroscopically confirmed hot subluminous stars out to 500 pc, defined using the accurate parallax measurements from the {\em Gaia} space mission data release 3 (DR3). The sample comprises a total of 397 members, with 305 ($\sim 77\%$) identified as hot subdwarf stars, including 83 newly discovered systems. Of these, we observe that 178 ($\sim58\%$) are hydrogen-rich sdBs, 65 are sdOBs ($\sim 21\%$), 32 are sdOs ($\sim 11\%$), and 30 are He-sdO/Bs ($\sim 10\%$). Among them, 48 ($\sim 16\%$) exhibit an infrared excess in their spectral energy distribution fits, suggesting a composite binary system. The hot subdwarf population is estimated to be 90\% complete, assuming that most missing systems are these composite binaries located within the main sequence (MS) in the \emph{Gaia} colour-magnitude diagram (CMD). The remaining sources in the sample include cataclysmic variables (CVs), blue horizontal branch stars (BHBs), hot white dwarfs (WDs), and MS stars. We derived the mid-plane density $\rho_{0}$ and scale height $\mathrm{h}_{z}$ for the non-composite hot subdwarf star population using a hyperbolic sechant profile (sech$^2$). The best-fit values are $\rho_{0}\,=\,5.17\pm 0.33 \times10^{-7}$ stars/pc$^{3}$ and $\mathrm{h}_{z} = 281 \pm 62$ pc. When accounting for the composite-colour hot subdwarfs and their estimated completeness, the mid-plane density increases to $\rho_{0}\,=\,6.15^ {+1.16}_{-0.53} \times10^{-7}$ stars/pc$^{3}$. This corrected space density is an order of magnitude lower than predicted by population synthesis studies, supporting previous observational estimates.

astro-ph.SR↗

arXiv2024

Dynamics of 2023 FW14, the second L4 Mars trojan, and a physical characterization using the 10.4 m Gran Telescopio Canarias

Context. Known Mars trojans could be primordial small bodies that have remained in their present-day orbits for the age of the Solar System. Their orbital distribution is strongly asymmetric; there are over a dozen objects at the L5 point and just one at L4, (121514) 1999 UJ7. Most L5 trojans appear to form a collision-induced asteroid cluster, known as the Eureka family. Asteroid 2023 FW14 was recently discovered and it has a robust orbit determination that may be consistent with a Mars trojan status. Aims. Our aim is determine the nature and dynamical properties of 2023 FW14. Methods. We carried out an observational study of 2023 FW14 to derive its spectral class using the OSIRIS camera spectrograph at the 10.4 m Gran Telescopio Canarias. We investigated its possible trojan resonance with Mars using direct N-body simulations. Results. The reflectance spectrum of 2023 FW14 is not compatible with the olivine-rich composition of the Eureka family; it also does not resemble the composition of the Moon, although (101429) 1998 VF31 does. The Eureka family and 101429 are at the L5 point. The spectrum of 2023 FW14 is also different from two out of the three spectra in the literature of the other known L4 trojan, 121514, which are of C-type. The visible spectrum of 2023 FW14 is consistent with that of an X-type asteroid, as is the third spectrum of 121514. Our calculations confirm that 2023 FW14 is the second known L4 Mars trojan although it is unlikely to be primordial; it may remain in its present-day tadpole path for several million years before transferring to a Mars-crossing orbit. It might be a fragment of 121514, but a capture scenario seems more likely. Conclusions. The discovery of 2023 FW14 suggests that regular Mars-crossing asteroids can be captured as temporary Mars trojans.

astro-ph.EP↗