arXiv subjects

Stephan Geier

Publications and source records attributed to Stephan Geier.

At least 37 records · Page 2Linked to original sources

arXiv2021

Looking into the cradle of the grave: J22564-5910, a young post-merger hot subdwarf?

We present the discovery of J22564-5910, a new type of hot subdwarf (sdB) which shows evidence of gas present in the system and has shallow, multi-peaked hydrogen and helium lines which vary in shape over time. All observational evidence points towards J22564-5910 being observed very shortly after the merger phase that formed it. Using high-resolution, high signal-to-noise spectroscopy, combined with multi-band photometry, Gaia astrometry, and TESS light curves, we aim to interpret these unusual spectral features. The photometry, spectra and light curves are all analyzed, and their results are combined in order to support our interpretation of the observations: the likely presence of a magnetic field combined with gas features around the sdB. Based on the triple-peaked H lines, the magnetic field strength is estimated and, by using the shellspec code, qualitative models of gas configurations are fitted to the observations. All observations can either be explained by a magnetic field of ~650 kG which enables the formation of a centrifugal magnetosphere, or a non-magnetic hot subdwarf surrounded by a circumstellar gas disk/torus. Both scenarios are not mutually exclusive and both can be explained by a recent merger. J22564-5910 is the first object of its kind. It is a rapidly spinning sdB with gas still present in the system. It is the first post-merger star observed this early after the merger event, and as such is very valuable system to test merger theories. If the magnetic field can be confirmed, it is not only the first magnetic sdB, but it hosts the strongest magnetic field ever found in a pre-white dwarf object. Thus, it could represent the long-sought for immediate ancestor of strongly magnetic WDs.

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arXiv2021

Mysterious, Variable, and Extremely Hot: White Dwarfs Showing Ultra-High Excitation Lines I. Photometric Variability

About 10% of all stars exhibit absorption lines of ultra-high excited (UHE) metals (e.g. OVIII) in their optical spectra when entering the white dwarf cooling sequence. The recent discovery of a both spectroscopic and photometric variable UHE white dwarf led to the speculation that the UHE lines might be created in a shock-heated circumstellar magnetosphere. We investigate (multi-band) light curves from several ground- and space-based surveys of all 16 currently known UHE white dwarfs (including one newly discovered) and eight white dwarfs that show only the HeII line problem, as both phenomena are believed to be connected. We find that $75^{+8}_{-13}$% of the UHE white dwarfs, and $75^{+9}_{-19}$% of the HeII line problem white dwarfs are significantly photometrically variable, with periods ranging from 0.22d to 2.93d and amplitudes from a few tenth to a few hundredth mag. The high variability rate is in stark contrast to the variability rate amongst normal hot white dwarfs (we find $9^{+4}_{-2}$%), marking UHE and HeII line problem white dwarfs as a new class of variable stars. The period distribution of our sample agrees with both the orbital period distribution of post-common envelope binaries and the rotational period distribution of magnetic white dwarfs if we assume that the objects in our sample will spin-up as a consequence of further contraction. The lack of increasing photometric amplitudes towards longer wavelengths, as well as the non-detection of optical emission lines arising from the highly irradiated face of a hypothetical secondary in the optical spectra of our stars, makes it seem unlikely that an irradiated late type companion is the origin of the photometric variability. Instead, we believe that spots on the surfaces of these stars and/or geometrical effects of circumstellar material might be responsible. (abridged)

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arXiv2020

Alone but not lonely: Observational evidence that binary interaction is always required to form hot subdwarf stars

Hot subdwarfs are core-helium burning stars that show lower masses and higher temperatures than canonical horizontal branch stars. They are believed to be formed when a red giant suffers an extreme mass-loss episode. Binary interaction is suggested to be the main formation channel, but the high fraction of apparently single hot subdwarfs (up to 30%) has prompted single star formation scenarios to be proposed. If such formation scenarios without interaction were possible, that would also imply the existence of hot subdwarfs in wide binaries that have undergone no interaction. We probe the existence of these systems by analysing light curves from the Transiting Exoplanet Survey Satellite (TESS) for all known hot subdwarfs with a main sequence wide binary companion, and by searching for common proper motion pairs to spectroscopically confirmed hot subdwarfs. We find that (i) the companions in composite hot subdwarfs show short rotation periods when compared to field main sequence stars. They display a triangular-shaped distribution with a peak around 2.5 days, similar to what is observed for young open clusters. This observed distribution of rotation rates for the companions in known wide hot subdwarf binaries provides evidence of previous interaction causing spin-up. We also report (ii) a shortage of hot subdwarfs with candidate common proper motion companions, considering the frequency of such systems among progenitors. We identify only 16 candidates after probing 2938 hot subdwarfs with good astrometry. Out of those, at least six seem to be hierarchical triple systems, in which the hot subdwarf is part of an inner binary. These results suggest that binary interaction is always required for the formation of hot subdwarfs.

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arXiv2020

Confirming new white dwarf-ultracool dwarf binary candidates

We present the results of a study to discover prospective new white dwarf-L dwarf binaries as identified by their near-infrared excesses in the UKIDSS catalogue. We obtained optical spectra to validate the white dwarf nature for 22 of the candidate primary stars, confirming ten as white dwarfs and determining their effective temperatures and gravities. For all ten white dwarfs we determined that the near-infrared excess was indeed indicative of a cool companion. Six of these are suggestive of late M dwarf companions, and three are candidate L dwarf companions, with one straddling the M$-$L boundary. We also present near-infrared spectra of eight additional candidate white dwarf-ultracool dwarf binaries where the white dwarf primary had been previously confirmed. These spectra indicate one candidate at the M$-$L boundary, three potential L dwarf companions, and one suspected M dwarf, which showed photometric variability on a $\sim$6 hour period, suggesting the system may be close. Radial velocity follow up is required to confirm whether these systems are close, or widely separated.

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arXiv2020

A stripped helium star in the potential black hole binary LB-1

The recently claimed discovery of a massive ($M_\mathrm{BH}=68^{+11}_{-13}\,M_\odot$) black hole in the Galactic solar neighborhood has led to controversial discussions because it severely challenges our current view of stellar evolution. A crucial aspect for the determination of the mass of the unseen black hole is the precise nature of its visible companion, the B-type star LS V+22 25. Because stars of different mass can exhibit B-type spectra during the course of their evolution, it is essential to obtain a comprehensive picture of the star to unravel its nature and, thus, its mass. To this end, we study the spectral energy distribution of LS V+22 25 and perform a quantitative spectroscopic analysis that includes the determination of chemical abundances for He, C, N, O, Ne, Mg, Al, Si, S, Ar, and Fe. Our analysis clearly shows that LS V+22 25 is not an ordinary main sequence B-type star. The derived abundance pattern exhibits heavy imprints of the CNO bi-cycle of hydrogen burning, that is, He and N are strongly enriched at the expense of C and O. Moreover, the elements Mg, Al, Si, S, Ar, and Fe are systematically underabundant when compared to normal main-sequence B-type stars. We suggest that LS V+22 25 is a stripped helium star and discuss two possible formation scenarios. Combining our photometric and spectroscopic results with the Gaia parallax, we infer a stellar mass of $1.1\pm0.5\,M_\odot$. Based on the binary system's mass function, this yields a minimum mass of $2-3\,M_\odot$ for the compact companion, which implies that it may not necessarily be a black hole but a massive neutron- or main sequence star. The star LS V+22 25 has become famous for possibly having a very massive black hole companion. However, a closer look reveals that the star itself is a very intriguing object. Further investigations are necessary for complete characterization of this object.

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arXiv2019

EVR-CB-001: An evolving, progenitor, white dwarf compact binary discovered with the Evryscope

We present EVR-CB-001, the discovery of a compact binary with an extremely low mass ($.21 \pm 0.05 M_{\odot}$) helium core white dwarf progenitor (pre-He WD) and an unseen low mass ($.32 \pm 0.06 M_{\odot}$) helium white dwarf (He WD) companion. He WDs are thought to evolve from the remnant helium-rich core of a main-sequence star stripped during the giant phase by a close companion. Low mass He WDs are exotic objects (only about .2$\%$ of WDs are thought to be less than .3 $M_{\odot}$), and are expected to be found in compact binaries. Pre-He WDs are even rarer, and occupy the intermediate phase after the core is stripped, but before the star becomes a fully degenerate WD and with a larger radius ($\approx .2 R_{\odot}$) than a typical WD. The primary component of EVR-CB-001 (the pre-He WD) was originally thought to be a hot subdwarf (sdB) star from its blue color and under-luminous magnitude, characteristic of sdBs. The mass, temperature ($T_{\rm eff}=18,500 \pm 500 K$), and surface gravity ($\log(g)=4.96 \pm 0.04$) solutions from this work are lower than values for typical hot subdwarfs. The primary is likely to be a post-RGB, pre-He WD contracting into a He WD, and at a stage that places it nearest to sdBs on color-magnitude and $T_{\rm eff}$-$\log(g)$ diagrams. EVR-CB-001 is expected to evolve into a fully double degenerate, compact system that should spin down and potentially evolve into a single hot subdwarf star. Single hot subdwarfs are observed, but progenitor systems have been elusive.

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arXiv2019

PG 1610$+$062: a runaway B star challenging classical ejection mechanisms

Hypervelocity stars are rare objects, mostly main-sequence (MS) B stars, traveling so fast that they will eventually escape from the Milky Way. Recently, it has been shown that the popular Hills mechanism, in which a binary system is disrupted via a close encounter with the supermassive black hole at the Galactic center, may not be their only ejection mechanism. The analyses of Gaia data ruled out a Galactic center origin for some of them, and instead indicated that they are extreme disk runaway stars ejected at velocities exceeding the predicted limits of classical scenarios (dynamical ejection from star clusters or binary supernova ejection). We present the discovery of a new extreme disk runaway star, PG 1610$+$062, which is a slowly pulsating B star bright enough to be studied in detail. A quantitative analysis of spectra taken with ESI at the Keck Observatory revealed that PG 1610$+$062 is a late B-type MS star of 4-5 $M_\odot$ with low projected rotational velocity. Abundances (C, N, O, Ne, Mg, Al, Si, S, Ar, and Fe) were derived differentially with respect to the normal B star HD 137366 and indicate that PG 1610$+$062 is somewhat metal rich. A kinematic analysis, based on our spectrophotometric distance (17.3 kpc) and on proper motions from Gaia's second data release, shows that PG 1610$+$062 was probably ejected from the Carina-Sagittarius spiral arm at a velocity of $550\pm40$ km s$^{-1}$, which is beyond the classical limits. Accordingly, the star is in the top five of the most extreme MS disk runaway stars and is only the second among the five for which the chemical composition is known.

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arXiv2019

White dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 14

White dwarfs carry information on the structure and evolution of the Galaxy, especially through their luminosity function and initial-to-final mass relation. Very cool white dwarfs provide insight into the early ages of each population. Examining the spectra of all stars with $3σ$ proper motion in the Sloan Digital Sky Survey Data Release 14, we report the classification for 20 088 spectroscopically confirmed white dwarfs, plus 415 hot subdwarfs, and 311 cataclysmic variables. We obtain Teff, log g and mass for hydrogen atmosphere white dwarf stars (DAs), warm helium atmosphere white dwarfs (DBs), hot subdwarfs (sdBs and sdOs), and estimate photometric Teff for white dwarf stars with continuum spectra (DCs). We find 15793 sdAs and 447 dCs between the white dwarf cooling sequence and the main sequence, especially below Teff= 10000 K; most are likely low-mass metal-poor main sequence stars, but some could be the result of interacting binary evolution.

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arXiv2019

Spectral analysis of the binary nucleus of the planetary nebula Hen 2-428 - first results

Identifying progenitor systems for the double-degenerate scenario is crucial to check the reliability of type Ia super-novae as cosmological standard candles. Santander-Garcia et al. (2015) claimed that Hen 2-428 has a double-degenerate core whose combined mass significantly exceeds the Chandrasekhar limit. Together with the short orbital period (4.2 hours), the authors concluded that the system should merge within a Hubble time triggering a type Ia supernova event. Garcia-Berro et al. (2016) explored alternative scenarios to explain the observational evidence, as the high mass conclusion is highly unlikely within predictions from stellar evolution theory. They conclude that the evidence supporting the supernova progenitor status of the system is premature. Here we present the first quantitative spectral analysis of Hen 2-428 which allows us to derive the effective temperatures, surface gravities and helium abundance of the two CSPNe based on state-of-the-art, non-LTE model atmospheres. These results provide constrains for further studies of this particularly interesting system.

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arXiv2018

A $Gaia$ Data Release 2 catalogue of white dwarfs and a comparison with SDSS

We present a catalogue of white dwarf candidates selected from the second data release of $Gaia$ (DR2). We used a sample of spectroscopically confirmed white dwarfs from the Sloan Digital Sky Survey (SDSS) to map the entire space spanned by these objects in the $Gaia$ Hertzsprung-Russell diagram. We then defined a set of cuts in absolute magnitude, colour, and a number of $Gaia$ quality flags to remove the majority of contaminating objects. Finally, we adopt a method analogous to the one presented in our earlier SDSS photometric catalogues to calculate a probability of being a white dwarf ($P_{\mathrm{WD}}$) for all $Gaia$ sources which passed the initial selection. The final catalogue is composed of $486\,641$ stars with calculated $P_{\mathrm{WD}}$ from which it is possible to select a sample of $\simeq 260\,000$ high-confidence white dwarf candidates in the magnitude range $8 7000$ K, at high Galactic latitudes ($|b|>20^{\circ}$). However, the completeness drops at low Galactic latitudes, and the magnitude limit of the catalogue varies significantly across the sky as a function of $Gaia$'s scanning law. We also provide the list of objects within our sample with available SDSS spectroscopy. We use this spectroscopic sample to characterise the observed structure of the white dwarf distribution in the H-R diagram.

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arXiv2018

Revealing the True Nature of Hen 2-428

The nucleus of Hen 2-428 is a short orbital period (4.2 h) spectroscopic binary, whose status as potential supernovae type Ia progenitor has raised some controversy in the literature. We present preliminary results of a thorough analysis of this interesting system, which combines quantitative non-local thermodynamic equilibrium spectral modelling (non-LTE), radial velocity analysis,multi-band light curve fitting, and state-of-the art stellar evolutionary calculations. Importantly, we find that the dynamical system mass that is derived by using all available He II lines does not exceed the Chandrasekhar mass limit. Furthermore, the individual masses of the two central stars are too small to lead to an SN Ia in case of a dynamical explosion during the merger process.

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arXiv2018

Discovery of two bright DO-type white dwarfs

We discovered two bright DO-type white dwarfs, GALEXJ053628.3+544854 (J0536+5448) and GALEX231128.0+292935(J2311+2929), which rank among the eight brightest DO-type white dwarfs known. Our non-LTE model atmosphere analysis reveals effective temperatures and surface gravities of $T_{\mathrm{eff}}=80000\pm4600\,\mathrm{K}$ and $\log \mathrm{g}=8.25\pm0.15$ for J0536+5448 and $T_{\mathrm{eff}}=69400\pm900\,\mathrm{K}$ and $\log \mathrm{g}=7.80\pm0.06$ for J2311+2929. The latter shows a significant amount of carbon in its atmosphere ($C=0.003^{+0.005}_{-0.002}$, by mass), while for J0536+5448 we could derive only an upper limit of $C<0.003$. Furthermore, we calculated spectroscopic distances for the two stars and found a good agreement with the distances derived from the Gaia parallaxes.

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arXiv2018

SHOTGLAS I: The ultimate spectroscopic census of extreme horizontal branch stars in $ω$ Centauri

The presence of extreme horizontal branch (EHB) and blue hook stars in some Galactic globular clusters (GGCs) constitutes one of the remaining mysteries of stellar evolution. In this paper, we focus on $ω$ Cen, a peculiar, massive GGC that hosts multiple stellar populations. We use non-LTE model atmospheres to derive atmospheric parameters (Teff, log g and N(He)/N(H)) and spectroscopic masses for 152 EHB stars in the cluster. This constitutes the largest spectroscopic sample of EHB stars ever analyzed in a GGC and represents ~20% of the EHB population of $ω$ Cen. We also search for close binaries among these stars based on radial velocity variations. Our results show that the EHB population of $ω$ Cen is divided into three spectroscopic groups that are very distinct in the Teff - helium abundance plane. The majority of our sample consists of sdOB stars that have roughly solar or super-solar atmospheric helium abundances. It is these objects that constitute the blue hook at $V >$ 18.5 mag in the $ω$ Cen color-magnitude diagram. Interestingly, the helium-enriched sdOBs do not have a significant counterpart population in the Galactic field. Another major difference between the EHB stars in $ω$ Cen and the field is the fraction of close binaries. From our radial velocity survey we identify two binary candidates and estimate an EHB close binary fraction of ~5% in $ω$ Cen. This low fraction is in line with findings for other GGCs, but in sharp contrast to the situation in the field, where around 50% of the sdB stars reside in close binaries. Finally, the mass distribution derived is very similar for all three spectroscopic groups, however the average mass (0.38 Msun) is lower than that expected from stellar evolution theory. While this mass conundrum has previously been noted for EHB stars in $ω$ Cen, it so far appears to be unique to that cluster.

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arXiv2017

Meet the family - the catalog of known hot subdwarf stars

In preparation for the upcoming all-sky data releases of the Gaia mission, we compiled a catalog of known hot subdwarf stars and candidates drawn from the literature and yet unpublished databases. The catalog contains 5613 unique sources and provides multi-band photometry from the ultraviolet to the far infrared, ground based proper motions, classifications based on spectroscopy and colors, published atmospheric parameters, radial velocities and light curve variability information. Using several different techniques, we removed outliers and misclassified objects. By matching this catalog with astrometric and photometric data from the Gaia mission, we will develop selection criteria to construct a homogeneous, magnitude-limited all-sky catalog of hot subdwarf stars based on Gaia data. As first application of the catalog data, we present the quantitative spectral analysis of 280 sdB and sdOB stars from the Sloan Digital Sky Survey Data Release 7. Combining our derived parameters with state-of-the-art proper motions, we performed a full kinematic analysis of our sample. This allowed us to separate the first significantly large sample of 78 sdBs and sdOBs belonging to the Galactic halo. Comparing the properties of hot subdwarfs from the disk and the halo with hot subdwarf samples from the globular clusters $ω$ Cen and NGC 2808, we found the fraction of intermediate He-sdOBs in the field halo population to be significantly smaller than in the globular clusters.

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arXiv2016

PTF1 J082340.04+081936.5: A hot subdwarf B star with a low mass white dwarf companion in an 87 minute orbit

We present the discovery of the hot subdwarf B star (sdB) binary PTF1 J082340.04+081936.5. The system has an orbital period P$_{\rm orb}=87.49668(1)$ min (0.060761584(10) days), making it the second-most compact sdB binary known. The lightcurve shows ellipsoidal variations. Under the assumption that the sdB primary is synchronized with the orbit, we find a mass $M_{\rm sdB}=0.45^{+0.09}_{-0.07}$ M$_\odot$, a companion white dwarf mass $M_{\rm WD}=0.46^{+0.12}_{-0.09}$ M$_\odot$ and a mass ratio $q = \frac{M_{\rm WD}}{M_{\rm sdB}}=1.03^{+0.10}_{-0.08}$. The future evolution was calculated using the MESA stellar evolution code. Adopting a canonical sdB mass of $M_{\rm sdB}=0.47$ M$_\odot$, we find that the sdB still burns helium at the time it will fill its Roche lobe if the orbital period was less than 106 min at the exit from the last common envelope phase. For longer common envelope exit periods the sdB will have stopped burning helium and turned into a C/O white dwarf at the time of contact. Comparing the spectroscopically derived log(g) and $T_{\rm eff}$ with our MESA models, we find that an sdB model with a hydrogen envelope mass of $5\times10^{-4} M_\odot$ matches the measurements at a post-common envelope age of 94 Myr, corresponding to a post-common envelope orbital period of 109 min which is close to the limit to start accretion while the sdB is still burning helium.

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arXiv2016

An extremely fast halo hot subdwarf star in a wide binary system

New spectroscopic observations of the halo hyper-velocity star candidate SDSS J121150.27+143716.2 ($V=17.92$ mag) revealed a cool companion to the hot subdwarf primary. The components have a very similar radial velocity and their absolute luminosities are consistent with the same distance, confirming the physical nature of the binary, which is the first double-lined hyper-velocity candidate. Our spectral decomposition of the Keck/ESI spectrum provided an sdB+K3V pair, analogous to many long-period subdwarf binaries observed in the Galactic disk. We found the subdwarf atmospheric parameters: $T_{\rm eff}=30\,600\pm500$ K, $\log{g}=5.57\pm0.06$ cm s$^{-2}$ and He abundance $\log(n{\rm He}/n{\rm H})=-3.0\pm0.2$. Oxygen is the most abundant metal in the hot subdwarf atmosphere, and Mg and Na lines are the most prominent spectral features of the cool companion, consistent with a metallicity of $[{\rm Fe}/{\rm H}]=-1.3$. The non-detection of radial velocity variations suggest the orbital period to be a few hundred days, in agreement with similar binaries observed in the disk. Using the SDSS-III flux calibrated spectrum we measured the distance to the system $d=5.5\pm0.5$ kpc, which is consistent with ultraviolet, optical, and infrared photometric constraints derived from binary spectral energy distributions. Our kinematic study shows that the Galactic rest-frame velocity of the system is so high that an unbound orbit cannot be ruled out. On the other hand, a bound orbit requires a massive dark matter halo. We conclude that the binary either formed in the halo or it was accreted from the tidal debris of a dwarf galaxy by the Milky Way.

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arXiv2015

Stellar archaeology with Gaia: the Galactic white dwarf population

Gaia will identify several 1e5 white dwarfs, most of which will be in the solar neighborhood at distances of a few hundred parsecs. Ground-based optical follow-up spectroscopy of this sample of stellar remnants is essential to unlock the enormous scientific potential it holds for our understanding of stellar evolution, and the Galactic formation history of both stars and planets.

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arXiv2012

Hot subluminous Stars: Highlights from the MUCHFUSS and Kepler missions

Research into hot subdwarf stars is progressing rapidly. We present recent important discoveries. First we review the knowledge about magnetic fields in hot subdwarfs and highlight the first detection of a highly-magnetic, helium-rich sdO star. We briefly summarize recent discoveries based on Kepler light curves and finally introduce the closest known sdB+WD binary discovered by the MUCHFUSS project and discuss its relevance as progenitor of a double-detonation type Ia supernova.

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