arXiv subjects

S. Geier

Publications and source records attributed to S. Geier.

At least 91 records · Page 5Linked to original sources

arXiv2017

Spectroscopic twin to the hypervelocity sdO star US 708 and three fast sdB stars from the Hyper-MUCHFUSS project

Important tracers for the dark matter halo of the Galaxy are hypervelocity stars (HVSs), which are faster than the local escape velocity of the Galaxy and their slower counterparts, the high-velocity stars in the Galactic halo. Such HVSs are believed to be ejected from the Galactic Centre (GC) through tidal disruption of a binary by the super-massive black hole (Hills mechanism). The Hyper-MUCHFUSS survey aims at finding high-velocity potentially unbound hot subdwarf stars. We present the spectroscopic and kinematical analyses of a He-sdO as well as three candidates among the sdB stars using optical Keck/ESI and VLT (Xshooter, FORS) spectroscopy. Proper motions are determined by combining positions from early-epoch photographic plates with those derived from modern digital sky surveys. The Galactic rest frame velocities range from 203 km\,s$^{-1}$ to 660 km\,s$^{-1}$, indicating that most likely all four stars are gravitationally bound to the Galaxy. With $T_\text{eff}=47000$ K and a surface gravity of $\log g = 5.7$, SDSS J205030.39$-$061957.8 (J2050) is a spectroscopic twin of the hypervelocity He-sdO US 708. As for the latter, the GC is excluded as a place of origin based on the kinematic analysis. Hence, the Hills mechanism can be excluded for J2050. The ejection velocity is much more moderate ($385\pm79$ km\,s$^{-1}$) than that of US 708 ($998\pm68$ km\,s$^{-1}$). The binary thermonuclear supernova scenario suggested for US 708 would explain the observed properties of J2050 very well without pushing the model parameters to their extreme limits, as required for US 708. Accordingly, the star would be the surviving donor of a type Ia supernova. Three sdB stars also showed extreme kinematics; one could be a HVS ejected from the GC, whereas the other two could be ejected from the Galactic disk through the binary supernova mechanism. Alternatively, they might be extreme halo stars.

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arXiv2016

Close binary progenitors and ejected companions of thermonuclear supernovae

Hot subdwarf stars (sdO/Bs) are evolved core helium-burning stars with very thin hydrogen envelopes, which can be formed by common envelope ejection. Close sdB binaries with massive white dwarf (WD) companions are potential progenitors of thermonuclear supernovae type Ia (SN~Ia). We discovered such a progenitor candidate as well as a candidate for a surviving companion star, which escapes from the Galaxy. More candidates for both types of objects have been found by crossmatching known sdB stars with proper motion and light curve catalogues. We found 72 sdO/B candidates with high Galactic restframe velocities, 12 of them might be unbound to our Galaxy. Furthermore, we discovered the second-most compact sdB+WD binary known. However, due to the low mass of the WD companion, it is unlikely to be a SN\,Ia progenitor.

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arXiv2016

The population of hot subdwarf stars studied with Gaia I. The catalogue of known hot subdwarf stars

In preparation for the upcoming all-sky data releases of the Gaia mission we compiled a catalogue of known hot subdwarf stars and candidates drawn from the literature and yet unpublished databases. The catalogue 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 colours, published atmospheric parameters, radial velocities and light curve variability information. Using several different techniques we removed outliers and misclassified objects. By matching this catalogue with astrometric and photometric data from the Gaia mission, we will develop selection criteria to construct a homogeneous, magnitude-limited all-sky catalogue of hot subdwarf stars based on Gaia data.

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arXiv2016

Space Telescope and Optical Reverberation Mapping Project VI: reverberating Disk Models for NGC 5548

We conduct a multiwavelength continuum variability study of the Seyfert 1 galaxy NGC 5548 to investigate the temperature structure of its accretion disk. The 19 overlapping continuum light curves (1158 to 9157 angstroms) combine simultaneous HST , Swift , and ground-based observations over a 180 day period from 2014 January to July. Light-curve variability is interpreted as the reverberation response of the accretion disk to irradiation by a central time-varying point source. Our model yields the disk inclination, i, temperature T1 at 1 light day from the black hole, and a temperature-radius slope, alpha. We also infer the driving light curve and find that it correlates poorly with both the hard and soft X-ray light curves, suggesting that the X-rays alone may not drive the ultraviolet and optical variability over the observing period. We also decompose the light curves into bright, faint, and mean accretion-disk spectra. These spectra lie below that expected for a standard blackbody accretion disk accreting at L/LEdd = 0.1

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arXiv2016

The extended High A(V) Quasar Survey: Searching for dusty absorbers toward mid-infrared selected quasars

We present the results of a new spectroscopic survey for dusty intervening absorption systems, particularly damped Ly$α$ absorbers (DLAs), towards reddened quasars. The candidate quasars are selected from mid-infrared photometry from the Wide-field Infrared Survey Explorer combined with optical and near-infrared photometry. Out of 1073 candidates, we secure low-resolution spectra for 108 using the Nordic Optical Telescope on La Palma, Spain. Based on the spectra, we are able to classify 100 of the 108 targets as quasars. A large fraction (50 %) is observed to have broad absorption lines (BALs). Moreover, we find 6 quasars with strange breaks in their spectra, which are not consistent with regular dust reddening. Using template fitting we infer the amount of reddening along each line of sight ranging from A(V)$\approx$0.1 mag to 1.2 mag (assuming an SMC extinction curve). In four cases, the reddening is consistent with dust exhibiting the 2175Å feature caused by an intervening absorber, and for two of these, a MgII absorption system is observed at the best-fit absorption redshift. In the rest of the cases, the reddening is most likely intrinsic to the quasar. We observe no evidence for dusty DLAs in this survey. However, the large fraction of BAL quasars hampers the detection of absorption systems. Out of the 50 non-BAL quasars only 28 have sufficiently high redshift to detect Ly$α$ in absorption.

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arXiv2016

Determining the fraction of reddened quasars in COSMOS with multiple selection techniques from X-ray to radio wavelengths

The sub-population of quasars reddened by intrinsic or intervening clouds of dust are known to be underrepresented in optical quasar surveys. By defining a complete parent sample of the brightest and spatially unresolved quasars in the COSMOS field, we quantify to which extent this sub-population is fundamental to our understanding of the true population of quasars. By using the available multiwavelength data of various surveys in the COSMOS field, we built a parent sample of 33 quasars brighter than $J=20$ mag, identified by reliable X-ray to radio wavelength selection techniques. Spectroscopic follow-up with the NOT/ALFOSC was carried out for four candidate quasars that had not been targeted previously to obtain a 100\% redshift completeness of the sample. The population of high $A_V$ quasars (HAQs), a specific sub-population of quasars selected from optical/near-infrared photometry, is found to contribute $21\%^{+9}_{-5}$ of the parent sample. The full population of bright spatially unresolved quasars represented by our parent sample consists of $39\%^{+9}_{-8}$ reddened quasars defined by having $A_V>0.1$, and $21\%^{+9}_{-5}$ of the sample having $E(B-V)>0.1$ assuming the extinction curve of the Small Magellanic Cloud. We show that the HAQ selection works well for selecting reddened quasars, but some are missed because their optical spectra are too blue to pass the $g-r$ color cut in the HAQ selection. This is either due to a low degree of dust reddening or anomalous spectra. We find that the fraction of quasars with contributing light from the host galaxy is most dominant at $z \lesssim 1$. At higher redshifts the population of spatially unresolved quasars selected by our parent sample is found to be representative of the full population at $J<20$ mag. This work quantifies the bias against reddened quasars in studies that are based solely on optical surveys.

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arXiv2016

Serendipitous discovery of a projected pair of QSOs separated by 4.5 arcsec on the sky

We present the serendipitous discovery of a projected pair of quasi-stellar objects (QSOs) with an angular separation of $Δθ=4.50$ arcsec. The redshifts of the two QSOs are widely different: one, our programme target, is a QSO with a spectrum consistent with being a narrow line Seyfert 1 AGN at $z=2.05$. For this target we detect Lyman-$α$, \ion{C}{4}, and \ion{C}{3]}. The other QSO, which by chance was included on the spectroscopic slit, is a Type 1 QSO at a redshift of $z=1.68$, for which we detect \ion{C}{4}, \ion{C}{3]} and \ion{Mg}{2}. We compare this system to previously detected projected QSO pairs and find that only about a dozen previously known pairs have smaller angular separation.

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arXiv2016

Hot subdwarf stars and their connection to thermonuclear supernovae

Hot subdwarf stars (sdO/Bs) are evolved core helium-burning stars with very thin hydrogen envelopes, which can be formed by common envelope ejection. Close sdB binaries with massive white dwarf (WD) companions are potential progenitors of thermonuclear supernovae type Ia (SN Ia). We discovered such a progenitor candidate as well as a candidate for a surviving companion star, which escapes from the Galaxy. More candidates for both types of objects have been found by crossmatching known sdB stars with proper motion and light curve catalogues. The Gaia mission will provide accurate astrometry and light curves of all the stars in our hot subdwarf sample and will allow us to compile a much larger all-sky catalogue of those stars. In this way we expect to find hundreds of progenitor binaries and ejected companions.

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arXiv2016

Space Telescope and Optical Reverberation Mapping Project. IV. Anomalous behavior of the broad ultraviolet emission lines in NGC 5548

During an intensive Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS) UV monitoring campaign of the Seyfert~1 galaxy NGC 5548 performed from 2014 February to July, the normally highly correlated far-UV continuum and broad emission-line variations decorrelated for ~60 to 70 days, starting ~75 days after the first HST/COS observation. Following this anomalous state, the flux and variability of the broad emission lines returned to a more normal state. This transient behavior, characterised by significant deficits in flux and equivalent width of the strong broad UV emission lines, is the first of its kind to be unambiguously identified in an active galactic nucleus reverberation mapping campaign. The largest corresponding emission-line flux deficits occurred for the high-ionization collisionally excited lines, C IV and Si IV(+O IV]), and also He II(+O III]), while the anomaly in Ly-alpha was substantially smaller. This pattern of behavior indicates a depletion in the flux of photons with E_{\rm ph} > 54 eV, relative to those near 13.6 eV. We suggest two plausible mechanisms for the observed behavior: (i) temporary obscuration of the ionizing continuum incident upon BLR clouds by a moving veil of material lying between the inner accretion disk and inner BLR, perhaps resulting from an episodic ejection of material from the disk, or (ii) a temporary change in the intrinsic ionizing continuum spectral energy distribution resulting in a deficit of ionizing photons with energies > 54 eV, possibly due to a transient restructuring of the Comptonizing atmosphere above the disk. Current evidence appears to favor the latter explanation.

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arXiv2016

Space Telescope and Optical Reverberation Mapping Project. III. Optical Continuum Emission and Broad-Band Time Delays in NGC 5548

We present ground-based optical photometric monitoring data for NGC 5548, part of an extended multi-wavelength reverberation mapping campaign. The light curves have nearly daily cadence from 2014 January to July in nine filters (\emph{BVRI} and \emph{ugriz}). Combined with ultraviolet data from the \emph{Hubble Space Telescope} and \emph{Swift}, we confirm significant time delays between the continuum bands as a function of wavelength, extending the wavelength coverage from 1158\,Å to the $z$ band ($\sim\!9160$\,Å). We find that the lags at wavelengths longer than the {\it V} band are equal to or greater than the lags of high-ionization-state emission lines (such as He\,{\sc ii}\,$λ1640$ and $λ4686$), suggesting that the continuum-emitting source is of a physical size comparable to the inner broad-line region (BLR). The trend of lag with wavelength is broadly consistent with the prediction for continuum reprocessing by an accretion disk with $τ\propto λ^{4/3}$. However, the lags also imply a disk radius that is 3 times larger than the prediction from standard thin-disk theory, assuming that the bolometric luminosity is 10\% of the Eddington luminosity ($L = 0.1L_{\rm Edd}$). Using optical spectra from the Large Binocular Telescope, we estimate the bias of the interband continuum lags due to BLR emission observed in the filters. We find that the bias for filters with high levels of BLR contamination ($\sim\! 20\%$) can be important for the shortest continuum lags, and likely has a significant impact on the {\it u} and {\it U} bands owing to Balmer continuum emission.

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arXiv2016

RV variable, hot post-AGB stars from the MUCHFUSS project - Classification, atmospheric parameters, formation scenarios

In the course of the MUCHFUSS project we have recently discovered four radial velocity (RV) variable, hot (Teff $\approx$ 80,000 - 110,000 K) post-asymptotic giant branch (AGB) stars. Among them, we found the first known RV variable O(He) star, the only second known RV variable PG 1159 close binary candidate, as well as the first two naked (i.e., without planetary nebula (PN)) H-rich post-AGB stars of spectral type O(H) that show significant RV variations. We present a non-LTE spectral analysis of these stars along with one further O(H)-type star whose RV variations were found to be not significant. We also report the discovery of an far-infrared excess in the case of the PG 1159 star. None of the stars in our sample displays nebular emission lines, which can be explained well in terms of a very late thermal pulse evolution in the case of the PG 1159 star. The "missing" PNe around the O(H)-type stars seem strange, since we find that several central stars of PNe have much longer post-AGB times. Besides the non-ejection of a PN, the occurrence of a late thermal pulse, or the re-accretion of the PN in the previous post-AGB evolution offer possible explanations for those stars not harbouring a PN (anymore). In case of the O(He) star J0757 we speculate that it might have been previously part of a compact He transferring binary system. In this scenario, the mass transfer must have stopped after a certain time, leaving behind a low mass close companion that could be responsible for the extreme RV shift of 107.0 $\pm$ 22.0 km/s measured within only 31 min.

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arXiv2015

Quantitative spectral analysis of the sdB star HD 188112: a helium-core white dwarf progenitor

HD 188112 is a bright (V = 10.2 mag) hot subdwarf B (sdB) star with a mass too low to ignite core helium burning and is therefore considered as a pre-extremely low mass (ELM) white dwarf (WD). ELM WDs (M $\le$ 0.3 Msun) are He-core objects produced by the evolution of compact binary systems. We present in this paper a detailed abundance analysis of HD 188112 based on high-resolution Hubble Space Telescope (HST) near and far-ultraviolet spectroscopy. We also constrain the mass of the star's companion. We use hybrid non-LTE model atmospheres to fit the observed spectral lines and derive the abundances of more than a dozen elements as well as the rotational broadening of metallic lines. We confirm the previous binary system parameters by combining radial velocities measured in our UV spectra with the already published ones. The system has a period of 0.60658584 days and a WD companion with M $\geq$ 0.70 Msun. By assuming a tidally locked rotation, combined with the projected rotational velocity (v sin i = 7.9 $\pm$ 0.3 km s$^{-1}$) we constrain the companion mass to be between 0.9 and 1.3 Msun. We further discuss the future evolution of the system as a potential progenitor of a (underluminous) type Ia supernova. We measure abundances for Mg, Al, Si, P, S, Ca, Ti, Cr, Mn, Fe, Ni, and Zn, as well as for the trans-iron elements Ga, Sn, and Pb. In addition, we derive upper limits for the C, N, O elements and find HD 188112 to be strongly depleted in carbon. We find evidence of non-LTE effects on the line strength of some ionic species such as Si II and Ni II. The metallic abundances indicate that the star is metal-poor, with an abundance pattern most likely produced by diffusion effects.

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arXiv2015

Why do hot subdwarf stars pulsate?

Hot subdwarf B stars (sdBs) are the stripped cores of red giants located at the bluest extension of the horizontal branch. Several different kinds of pulsators are found among those stars. The mechanism that drives those pulsations is well known and the theoretically predicted instability regions for both the short-period p-mode and the long-period g-mode pulsators match the observed distributions fairly well. However, it remains unclear why only a fraction of the sdB stars pulsate, while stars with otherwise very similar parameters do not show pulsations. From an observers perspective I review possible candidates for the missing parameter that makes sdB stars pulsate or not.

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arXiv2015

Hot subdwarfs in the Galactic halo - Tracers of prominent events in late stellar evolution

Hot subdwarf stars (sdO/Bs) are the stripped cores of red giants located at the bluest extension of the horizontal branch. They constitute the dominant population of UV-bright stars in old stellar environments and are most likely formed by binary interactions. We perform the first systematic, spectroscopic analysis of a sample of those stars in the Galactic halo based on data from SDSS. In the course of this project we discovered 177 close binary candidates. A significant fraction of the sdB binaries turned out to have close substellar companions, which shows that brown dwarfs and planets can significantly influence late stellar evolution. Close hot subdwarf binaries with massive white dwarf companions on the other hand are good candidates for the progenitors of type Ia supernovae. We discovered a hypervelocity star, which not only turned out to be the fastest unbound star known in our Galaxy, but also the surviving companion of such a supernova explosion.

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arXiv2015

Phase resolved spectroscopy and Kepler photometry of the ultracompact AM CVn binary SDSS J190817.07+394036.4

{\it Kepler} satellite photometry and phase-resolved spectroscopy of the ultracompact AM CVn type binary SDSS J190817.07+394036.4 are presented. The average spectra reveal a variety of weak metal lines of different species, including silicon, sulphur and magnesium as well as many lines of nitrogen, beside the strong absorption lines of neutral helium. The phase-folded spectra and the Doppler tomograms reveal an S-wave in emission in the core of the He I 4471 Å\,absorption line at a period of $P_{\rm orb}=1085.7\pm2.8$\,sec identifying this as the orbital period of the system. The Si II, Mg II and the core of some He I lines show an S-wave in absorption with a phase offset of $170\pm15^\circ$ compared to the S-wave in emission. The N II, Si III and some helium lines do not show any phase variability at all. The spectroscopic orbital period is in excellent agreement with a period at $P_{\rm orb}=1085.108(9)$\,sec detected in the three year {\it Kepler} lightcurve. A Fourier analysis of the Q6 to Q17 short cadence data obtained by {\it Kepler} revealed a large number of frequencies above the noise level where the majority shows a large variability in frequency and amplitude. In an O-C analysis we measured a $\vert\dot{P}\vert\sim1.0\,$x$\,10^{-8}\,$s\,s$^{-1}$ for some of the strongest variations and set a limit for the orbital period to be $\vert\dot{P}\vert<10^{-10}$s\,s$^{-1}$. The shape of the phase folded lightcurve on the orbital period indicates the motion of the bright spot. Models of the system were constructed to see whether the phases of the radial velocity curves and the lightcurve variation can be combined to a coherent picture. However, from the measured phases neither the absorption nor the emission can be explained to originate in the bright spot.

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arXiv2015

Optical spectroscopy of candidate Alpha Persei white dwarfs

As part of an investigation into the high mass end of the initial mass-final mass relation we performed a search for new white dwarf members of the nearby (172.4 pc), young (80-90 Myr) $α$ Persei open star cluster. The photometric and astrometric search using the UKIRT Infrared Deep Sky Survey and SuperCOSMOS sky surveys discovered 14 new white dwarf candidates. We have obtained medium resolution optical spectra of the brightest 11 candidates using the William Herschel Telescope and confirmed that while 7 are DA white dwarfs, 3 are DB white dwarfs and one is an sdOB star, only three have cooling ages within the cluster age, and from their position on the initial mass-final mass relation, it is likely none are cluster members. This result is disappointing, as recent work on the cluster mass function suggests that there should be at least one white dwarf member, even at this young age. It may be that any white dwarf members of $α$ Per are hidden within binary systems, as is the case in the Hyades cluster, however the lack of high mass stars within the cluster also makes this seem unlikely. One alternative is that a significant level of detection incompleteness in the legacy optical image survey data at this Galactic latitude has caused some white dwarf members to be overlooked. If this is the case, Gaia will find them.

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arXiv2015

Candidate hypervelocity stars of spectral type G and K revisited

Hypervelocity stars (HVS) move so fast that they are unbound to the Galaxy. When they were first discovered in 2005, dynamical ejection from the supermassive black hole (SMBH) in the Galactic Centre (GC) was suggested as their origin. The two dozen HVSs known today are young massive B stars, mostly of 3-4 solar masses. Recently, 20 HVS candidates of low mass were discovered in the Segue G and K dwarf sample, but none of them originates from the GC. We embarked on a kinematic analysis of the Segue HVS candidate sample using the full 6D phase space information based on new proper motion measurements. Their orbital properties can then be derived by tracing back their trajectories in different mass models of our Galaxy. We present the results for 14 candidate HVSs, for which proper motion measurements were possible. Significantly lower proper motions than found in the previous study were derived. Considering three different Galactic mass models we find that all stars are bound to the Galaxy. We confirm that the stars do not originate from the GC. The distribution of their proper motions and radial velocities is consistent with predictions for runaway stars ejected from the Galactic disk by the binary supernova mechanism. However, their kinematics are also consistent with old disk membership. Moreover, most stars have rather low metallicities and strong $α$-element enrichment as typical for thick disk and halo stars, whereas the metallicity of the three most metal-rich stars could possibly indicate that they are runaway stars from the thin disk. One star shows halo kinematics.

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arXiv2015

Hot subdwarfs - Small stars marking important events in stellar evolution

Hot subdwarfs are considered to be the compact helium cores of red giants, which lost almost their entire hydrogen envelope. What causes this enormous mass loss is still unclear. Binary interactions are invoked and a significant fraction of the hot subdwarf population is indeed found in close binaries. In a large project we search for the close binary sdBs with the most and the least massive companions. Significantly enhancing the known sample of close binary sdBs we performed the first comprehensive study of this population. Triggered by the discovery of two sdB binaries with close brown dwarf companions in the course of this project, we were able to show that the interaction of stars with substellar companions is an important channel to form sdB stars. Finally, we discovered a unique and very compact binary system consisting of an sdB and a massive white dwarf, which qualifies as progenitor candidate for a supernova type Ia. In addition to that, we could connect those explosions to the class of hypervelocity hot subdwarf stars, which we consider as the surviving companions of such events. Being the stripped cores of red giants, hot subdwarfs turned out to be important markers of peculiar events in stellar evolution ranging all the way from star-planet interactions to the progenitors of stellar explosions used to measure the expansion of our Universe.

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