arXiv subjects

R. Napiwotzki

Publications and source records attributed to R. Napiwotzki.

At least 55 records · Page 3Linked to original sources

arXiv2011

Substellar companions and the formation of hot subdwarf stars

We give a brief review over the observational evidence for close substellar companions to hot subdwarf stars. The formation of these core helium-burning objects requires huge mass loss of their red giant progenitors. It has been suggested that besides stellar companions substellar objects in close orbits may be able to trigger this mass loss. Such objects can be easily detected around hot subdwarf stars by medium or high resolution spectroscopy with an RV accuracy at the km/s-level. Eclipsing systems of HW Vir type stick out of transit surveys because of their characteristic light curves. The best evidence that substellar objects in close orbits around sdBs exist and that they are able to trigger the required mass loss is provided by the eclipsing system SDSS J0820+0008, which was found in the course of the MUCHFUSS project. Furthermore, several candidate systems have been discovered.

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arXiv2011

The Magnetic and Metallic Degenerate G77-50

An accumulation of multi-epoch, high-resolution, optical spectra reveal that the nearby star G77-50 is a very cool DAZ white dwarf externally polluted by Mg, Fe, Al, Ca, and possibly Na, Cr, Mn. The metallic and hydrogen absorption features all exhibit multiple components consistent with Zeeman splitting in a B~120kG magnetic field. Ultraviolet through infrared photometry combined with trigonometric parallaxes yield Teff=5310K, M=0.60Msun, and a cooling age of 5.2Gyr. The space velocity of the white dwarf suggests possible membership in the Galactic thick disk, consistent with an estimated total age of 8.6Gyr. G77-50 is spectrally similar to G165-7 and LHS 2534; these three cool white dwarfs comprise a small group exhibiting both metals and magnetism. The photospheric metals indicate accretion of rocky debris similar to that contained in asteroids, but the cooling age implies a remnant planetary system should be stable. A possibility for G77-50 and similarly old, polluted white dwarfs is a recent stellar encounter that dynamically rejuvenated the system from the outside-in. Metal abundance measurements for these cooler white dwarfs have the potential to distinguish material originating in outer region planetesimals injected via fly-by. If common envelope evolution can generate magnetic fields in white dwarfs, then G77-50 and its classmates may have cannibalized an inner giant planet during prior evolution, with their metals originating in terrestrial bodies formed further out. Although speculative, this scenario can be ruled out if terrestrial planet formation is prohibited in systems where a giant planet has migrated to the inner region nominally engulfed during the post-main sequence.

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arXiv2010

Evolved solar systems in Praesepe

We have obtained near-IR photometry for the 11 Praesepe white dwarfs, to search for an excess indicative of a dusty debris disk. All the white dwarfs are in the DAZ temperature regime, however we find no indications of a disk around any white dwarf. We have, however determined that the radial velocity variable white dwarf WD0837+185 could have an unresolved T8 dwarf companion that would not be seen as a near-IR excess.

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arXiv2010

The Hyper-MUCHFUSS project: probing the Galactic halo with sdB stars

High-velocity stars in the Galactic halo, e.g. the so-called hyper-velocity stars (HVS), are important tracers of the properties of the dark matter halo, in particular its mass. Based on the SDSS DR6 spectral database a search for the fastest stars among hot subdwarfs (sdB) in the halo is carried out to identify HVS, unbound to the Galaxy, and bound population II stars in order to derive a lower limit to the halo mass. The radial velocity measurements were verified at several telescopes to exclude radial velocity variable stars. Out of 88 stars observed in the follow-up campaign 39 stars were found to have constant radial velocities. For twelve of them we measured a proper motion significantly different from zero and obtained spectroscopic distances from quantitative spectral analysis to construct the full 6D phase space information for a kinematical study. The programme sdBs can be distinguished into two kinematical groups, one (G1) with low Galactic rotation typical of halo stars and a second one (G2) with rapid retrograde motion. The G1 objects crossed the Galactic plane in the central bulge, whereas the G2 stars did in the outer Galactic disc. J1211+1437 (G2) is a HVS candidate, as it is unbound to the Galaxy if the standard Galactic potential is adopted. We conclude that in the ejection scenario G1 stars might have been formed via the slingshot mechanism that invokes acceleration by tidal interaction of a binary with the central supermassive black hole. The G2 stars, however, would originate in the outskirts of the Galactic disc and not in the central bulge. J1211+1437 is the first unbound subdwarf B star, for which we can rule out the slingshot mechanism. Alternatively, we may assume that the stars are old population II stars and therefore have to be bound. Then the kinematics of J1211+1437 set a lower limit of 2 x 10^12 Msun to the mass of the Galactic dark matter halo.

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arXiv2010

Massive unseen companions to hot faint underluminous stars from SDSS (MUCHFUSS) - Analysis of seven close subdwarf B binaries

The project Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS (MUCHFUSS) aims at finding hot subdwarf stars with massive compact companions like massive white dwarfs (M > 1.0 Msun), neutron stars or stellar mass black holes. The existence of such systems is predicted by binary evolution theory and recent discoveries indicate that they exist in our Galaxy. First results are presented for seven close binary sdBs with short orbital periods ranging from 0.21 d to 1.5 d. The atmospheric parameters of all objects are compatible with core helium-burning stars. The companions are most likely white dwarfs. In one case the companion could be shown to be a white dwarf by the absence of light-curve variations. However, in most cases late type main sequence stars cannot be firmly excluded. Comparing our small sample with the known population of close sdB binaries we show that our target selection method aiming at massive companions is efficient. The minimum companion masses of all binaries in our sample are high compared to the reference sample of known sdB binaries.

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arXiv2010

A white dwarf companion to the relativistic pulsar PSR J1141-6545

Pulsars with compact companions in close eccentric orbits are unique laboratories for testing general relativity and alternative theories of gravity. Moreover, they are excellent targets for future gravitational wave experiments like LISA and they are also highly important for understanding the equation of state of super-dense matter and the evolution of massive binaries. Here we report on optical observations of the 1.02 M_sun companion to the pulsar PSR J1141-6545. We detect an optical counterpart with apparent magnitudes V=25.08(11) and R=24.38(14), consistent with the timing position of the pulsar. We demonstrate that our results are in agreement with a white dwarf companion. However the latter is redder than expected and the inferred values are not consistent with the theoretical cooling tracks, preventing us from deriving the exact age. Our results confirm the importance of the PSR J1141-6545 system for gravitational experiments.

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arXiv2010

Anyone out there? Galactic Halo Post-AGB stars

We present results of a survey of post-asymptotic giant branch stars (post-AGBs) at high galactic latitude. To date, few post-AGB stars are known throughout the Galaxy and the number of known members of the older populations like the galactic halo is even smaller. This study looks at the number of post-AGB stars which are produced using different synthetic population methods and compare the results with observations. The resulting synthetic populations are compared to observational results from a complete and studied subsample from the photographic Palomar-Green (PG) survey (with high resolution spectroscopic follow-up for post-AGB candidates) and the SDSS spectroscopic database. The results show only two candidate post-AGB stars in a complete subsample of the PG survey spanning 4200 square degrees and one in the SDSS database. We discuss and explore any observational biases which may cause the result. If found to be truely representative of the halo population, one can expect the majority of Population II stars to fail to ascend the AGB and evolve through other evolutionary channels such as the extended horizontal branch.

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arXiv2010

Search for the coolest white dwarfs in the Galaxy

A number of so-called ultra-cool white dwarfs have been detected in different surveys so far. However, based on anecdotal evidence it is believed that most or all of these ultra-cool white dwarfs are low-mass products of binary evolution and thus not representative for the oldest white dwarfs. Their low mass causes relatively high luminosity making them the first cool white dwarfs detected in relatively shallow surveys. Deeper observations are needed for the oldest, high mass white dwarfs with the longest cooling times. We report results of an ongoing project that combines deep IR and optical data. This combination plus proper motion information will allow an unambiguous identification of very cool white dwarfs, since the spectral energy distributions are very different from other types of stellar objects. The atmospheric parameters that can be derived from the spectral energy distributions together with the proper motions inferred from the IR data can be used to construct the white dwarf luminosity functions for the thick disc and halo populations. From these we will be able to test the early star formation history and initial mass function of the first stellar populations.

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arXiv2010

Ejection velocities of high Galactic latitude runaway stars

We estimate the distribution of ejection velocities for the known population of high galactic latitude runaway stars. The initial sample is a collection of 174 early-type stars selected from the literature. The stars are first classified according to their evolutionary status in order to obtain a homogeneous sample of 96 genuine main sequence stars. Their present velocities and flight times are then estimated using proper motion data from various astrometric catalogues (including Tycho-2, UCAC2, and USNO-B) and the ejection velocities are computed by tracing their orbits back in time, based on a galactic potential. The potential used is constructed from a mass density model chosen to fit the most recent observational constraints. We find evidence for two different populations of runaway stars: a "high" velocity population, with a maximum ejection velocity of about 400 - 500 km/s, and a "low" velocity population, with a maximum ejection velocity of about 300 km/s. We argue that the observed limit of 500 km/s and the bimodality of the observed ejection velocity distribution are natural consequences of the so-called Binary Ejection Mechanism. We discuss the connection between the "high" velocity population and the so-called hypervelocity stars, showing how previously studied hypervelocity stars are consistent with the results obtained. We also find that some stars that were once thought to be best explained as being formed in the Halo are compatible with a runaway hypothesis once proper motions are included in the analysis. However, three stars in the selected sample appear to be inconsistent with ejection from the galactic disc. Possible scenarios are discussed, including a possible formation in the galactic halo.

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arXiv2010

Discovery of a T dwarf + white dwarf binary system

We present the discovery of the first T dwarf + white dwarf binary system LSPM 1459+0857AB, confirmed through common proper motion and spectroscopy. The white dwarf is a high proper motion object from the LSPM catalogue that we confirm spectroscopically to be a relatively cool (Teff=5535+\-45K) and magnetic (B~2MG) hydrogen-rich white dwarf, with an age of at least 4.8Gyrs. The T dwarf is a recent discovery from the UKIRT Infrared Deep Sky Survey (ULAS 1459+0857), and has a spectral type of T4.5+\-0.5 and a distance in the range 43-69pc. With an age constraint (inferred from the white dwarf) of >4.8Gyrs we estimate Teff=1200-1500K and logg=5.4-5.5 for ULAS 1459+0857, making it a benchmark T dwarf with well constrained surface gravity. We also compare the T dwarf spectra with the latest LYON group atmospheric model predictions, which despite some shortcomings are in general agreement with the observed properties of ULAS 1459+0857. The separation of the binary components (16,500-26,500AU, or 365 arcseconds on the sky) is consistent with an evolved version of the more common brown dwarf + main-sequence binary systems now known, and although the system has a wide separation, it is shown to be statistically robust as a non spurious association. The observed colours of the T dwarf show that it is relatively bright in the z band compared to other T dwarfs of similar type, and further investigation is warranted to explore the possibility that this could be a more generic indicator of older T dwarfs. Future observations of this binary system will provide even stronger constraints on the T dwarf properties, and additional systems will combine to give a more comprehensively robust test of the model atmospheres in this temperature regime.

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arXiv2010

IPHAS extinction distances to Planetary Nebulae

The determination of reliable distances to Planetary Nebulae (PNe) is one of the major limitations in the study of this class of objects in the Galaxy. The availability of new photometric surveys such as IPHAS covering large portions of the sky gives us the opportunity to apply the "extinction method" to determine distances of a large number of objects. The technique is applied to a sample of 137 PNe located between -5 and 5 degrees in Galactic latitude, and between 29.52 and 215.49 degrees in longitude. The characteristics of the distance-extinction method and the main sources of errors are carefully discussed. The data on the extinction of the PNe available in the literature, complemented by new observations, allow us to determine extinction distances for 70 PNe. A comparison with statistical distance scales from different authors is presented.

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arXiv2010

The orbital period of V458 Vulpeculae, a post double common-envelope nova

We present time-resolved optical spectroscopy of V458 Vulpeculae (Nova Vul 2007 No. 1) spread over a period of 15 months starting 301 days after its discovery. Our data reveal radial velocity variations in the HeII λ5412 and HeII λ4686 emission lines. A period analysis of the radial velocity curves resulted in a period of 98.09647 \pm 0.00025 min (0.06812255 \pm 0.00000017 d) which we identify with the orbital period of the binary system. V458 Vul is therefore the planetary nebula central binary star with the shortest period known. We explore the possibility of the system being composed of a relatively massive white dwarf (M1 \gsim 1.0 Msun) accreting matter from a post-asymptotic giant branch star which produced the planetary nebula observed. In this scenario, the central binary system therefore underwent two common-envelope episodes. A combination of previous photoionisation modelling of the nebular spectra, post-asymptotic giant branch evolutionary tracks and the orbital period favour a mass of M2 \sim 0.6 Msun for the donor star. Therefore, the total mass of the system may exceed the Chandrasekhar mass, which makes V458 Vul a Type Ia supernova progenitor candidate.

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arXiv2010

Hot subdwarf stars in close-up view I. Rotational properties of subdwarf B stars in close binary systems and nature of their unseen companions

The origin of hot subdwarf B stars (sdBs) is still unclear. About half of the known sdBs are in close binary systems for which common envelope ejection is the most likely formation channel. Little is known about this dynamic phase of binary evolution. Due to the tidal influence of the companion in close binary systems, the rotation of the primary becomes synchronised to its orbital motion. In this case it is possible to constrain the mass of the companion, if the primary mass, its projected rotational velocity as well as its surface gravity are known. For the first time we measured the projected rotational velocities of a large sdB binary sample from high resolution spectra. We analysed a sample of 51 sdB stars in close binaries, 40 of which have known orbital parameters comprising half of all such systems known today. Synchronisation in sdB binaries is discussed both from the theoretical and the observational point of view. The masses and the nature of the unseen companions could be constrained in 31 cases. The companions to seven sdBs could be clearly identified as late M stars. One binary may have a brown dwarf companion. The unseen companions of nine sdBs are white dwarfs with typical masses. The mass of one white dwarf companion is very low. In eight cases the companion mass exceeds 0.9 solar masses, four of which even exceed the Chandrasekhar limit indicating that they may be neutron stars. Even stellar mass black holes are possible for the most massive companions. The distribution of the inclinations of the systems with low mass companions appears to be consistent with expectations, whereas a lack of high inclinations becomes obvious for the massive systems. We show that the formation of such systems can be explained with common envelope evolution and present an appropriate formation channel including two phases of unstable mass transfer and one supernova explosion.

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arXiv2010

Binaries discovered by the SPY project V. GD 687 - a massive double degenerate binary progenitor that will merge within a Hubble time

Aims. The ESO SN Ia Progenitor Survey (SPY) aims at finding merging double degenerate binaries as candidates for supernova type Ia (SN Ia) explosions. A white dwarf merger has also been suggested to explain the formation of rare types of stars like R CrB, extreme helium or He sdO stars. Here we present the hot subdwarf B binary GD 687, which will merge in less than a Hubble time. Methods. The orbital parameters of the close binary have been determined from time resolved spectroscopy. Since GD 687 is a single-lined binary, the spectra contain only information about the subdwarf primary and its orbit. From high resolution spectra the projected rotational velocity was derived. Assuming orbital synchronisation, the inclination of the system and the mass of the unseen companion were constrained. Results. The derived inclination is $i=39.3^{+6.2}_{-5.6}\,^{\circ}$. The mass $M_{\rm 2}=0.71_{-0.21}^{+0.22}\,M_{\rm \odot}$ indicates that the companion must be a white dwarf, most likely of C/O composition. This is only the fourth case that an sdB companion has been proven to be a white dwarf unambiguously. Its mass is somewhat larger than the average white dwarf mass, but may be as high as $0.93\,M_{\rm \odot}$ in which case the total mass of the system comes close to the Chandrasekhar limit. Conclusions. GD 687 will evolve into a double degenerate system and merge to form a rare supermassive white dwarf with a mass in excess of solar. A death in a sub-Chandrasekhar supernova is also conceivable.

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arXiv2009

The chemical composition of TS 01, the most oxygen-deficient planetary nebula. AGB nucleosynthesis in a metal-poor binary star

The planetary nebula TS 01 (also called PN G 135.9+55.9 or SBS 1150+599A), with its record-holding low oxygen abundance and its double degenerate close binary core (period 3.9 h), is an exceptional object located in the Galactic halo. We have secured observational data in a complete wavelength range in order to pin down the abundances of half a dozen elements in the nebula. The abundances are obtained via detailed photoionization modelling taking into account all the observational constraints (including geometry and aperture effects) using the pseudo-3D photoionization code Cloudy_3D. The spectral energy distribution of the ionizing radiation is taken from appropriate model atmospheres. Both stellar components contribute to the ionization: the ``cool'' one provides the bulk of hydrogen ionization, and the ``hot'' one is responsible for the presence of the most highly charged ions, which explains why previous attempts to model the nebula experienced difficulties. The nebular abundances of C, N, O, and Ne are found to be respectively, 1/3.5, 1/4.2, 1/70, and 1/11 of the Solar value, with uncertainties of a factor 2. Thus the extreme O deficiency of this object is confirmed. The abundances of S and Ar are less than 1/30 of Solar. Standard models of stellar evolution and nucleosynthesis cannot explain the abundance pattern observed in the nebula. To obtain an extreme oxygen deficiency in a star whose progenitor has an initial mass of about 1 msun requires an additional mixing process, which can be induced by stellar rotation and/or by the presence of the close companion. We have computed a stellar model with initial mass of 1 msun, appropriate metallicity, and initial rotation of 100 kms, and find that rotation greatly improves the agreement between the predicted and observed abundances.

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arXiv2009

A survey for post common-envelope binary stars using GALEX and SDSS photometry

We report the first results of our programme to obtain multi-epoch radial velocity measurements of stars with a strong far-UV excess to identify post common-envelope binaries (PCEBs). The targets have been identified using optical photometry from SDSS DR4, ultraviolet photometry from GALEX GR2 and proper motion information from SDSS DR5. We have obtained spectra at two or more epochs for 36 targets. Three of our targets show large radial velocity shifts (>50km/s) on a timescale of hours or days and are almost certainly PCEBs. For one of these targets (SDSS J104234.77+644205.4) we have obtained further spectroscopy to confirm that this is a PCEB with an orbital period of 4.74h and semi-amplitude K =165 km/s. Two targets are rapidly rotating K-dwarfs which appear to show small radial velocity shifts and have strong Ca II H+K emission lines. These may be wind-induced rapidly rotating (WIRRing) stars. These results show that we can use GALEX and SDSS photometry to identify PCEBs that cannot be identified using SDSS photometry alone, and to identify new WIRRing stars. A more comprehensive survey of stars identified using the methods developed in this paper will lead to a much improved understanding of common envelope evolution.

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arXiv2009

High-resolution UVES/VLT spectra of white dwarfs observed for the ESO SN Ia Progenitor Survey. III. DA white dwarfs

The ESO Supernova Ia Progenitor Survey (SPY) took high-resolution spectra of more than 1000 white dwarfs and pre-white dwarfs. About two thirds of the stars observed are hydrogen-dominated DA white dwarfs. Here we present a catalog and detailed spectroscopic analysis of the DA stars in the SPY. Atmospheric parameters effective temperature and surface gravity are determined for normal DAs. Double-degenerate binaries, DAs with magnetic fields or dM companions, are classified and discussed. The spectra are compared with theoretical model atmospheres using a chi^2 fitting technique. Our final sample contains 615 DAs, which show only hydrogen features in their spectra, although some are double-degenerate binaries. 187 are new detections or classifications. We also find 10 magnetic DAs (4 new) and 46 DA+dM pairs (10 new).

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arXiv2009

The UV-Excess Survey of the Northern Galactic Plane (UVEX)

The UV-Excess Survey of the Northern Galactic Plane images a 10x185 degree wide band, centered on the Galactic Equator using the 2.5m Isaac Newton Telescope in four bands (U,g,r,HeI5875) down to ~21st-22nd magnitude (~20th in HeI5875). The setup and data reduction procedures are described. Simulations of the colours of main-sequence stars, giant, supergiants, DA and DB white dwarfs and AM CVn stars are made, including the effects of reddening. A first look at the data of the survey (currently 30% complete) is given.

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