A spectroscopic and photometric study of MSP companions in Galactic Globular Clusters

This Thesis is devoted to the study of the optical companions of Millisecond Pulsars in Galactic Globular Clusters (GCs) as a part of a large project started at the Department of Astronomy of the Bologna University, in collaboration with other institutions (Astronomical Observatory of Cagliari and Bologna, University of Virginia), specifically dedicated to the study of the environmental effects on passive stellar evolution in galactic GCs. Globular Clusters are very efficient “Kilns” for generating exotic object, such as Millisecond Pulsars (MSP), low mass X-ray binaries(LMXB) or Blue Straggler Stars (BSS). In particular MSPs are formed in binary systems containing a Neutron Star which is spun up through mass accretion from the evolving companion (e.g. Bhattacharia & van den Heuvel 1991). The final stage of this recycling process is either the core of a peeled star (generally an Helium white dwarf) or a very light almos exhausted star, orbiting a very fast rotating Neutron Star (a MSP). Despite the large difference in total mass between the disk of the Galaxy and the Galactic GC system (up a factor 103), the percentage of fast rotating pulsar in binary systems found in the latter is very higher. MSPs in GCs show spin periods in the range 1.3 ÷ 30ms, slowdown rates ˙P 1019 s/s and a lower magnetic field, respect to ”normal” radio pulsars, B 108 gauss . The high probability of disruption of a binary systems after a supernova explosion, explain why we expect only a low percentage of recycled millisecond pulsars respect to the whole pulsar population. In fact only the 10% of the known 1800 radio pulsars are radio MSPs. Is not surprising, that MSP are overabundant in GCs respect to Galactic field, since in the Galactic Disk, MSPs can only form through the evolution of primordial binaries, and only if the binary survives to the supernova explosion which lead to the neutron star formation. On the other hand, the extremely high stellar density in the core of GCs, relative to most of the rest of the Galaxy, favors the formation of several different binary systems, suitable for the recycling of NSs (Davies at al. 1998). In this thesis we will present the properties two millisecond pulsars companions discovered in two globular clusters, the Helium white dwarf orbiting the MSP PSR 1911-5958A in NGC 6752 and the second case of a tidally deformed star orbiting an eclipsing millisecond pulsar, PSR J1701-3006B in NGC6266

The VLT/NaCo large program to probe the occurrence of exoplanets and brown dwarfs at wide orbits II. Survey description, results, and performances

Context. Young, nearby stars are ideal targets for direct imaging searches for giant planets and brown dwarf companions. After the first-imaged planet discoveries, vast efforts have been devoted to the statistical analysis of the occurence and orbital distributions of giant planets and brown dwarf companions at wide (>= 5-6 AU) orbits. Aims. In anticipation of the VLT/SPHERE planet-imager, guaranteed-time programs, we have conducted a preparatory survey of 86 stars between 2009 and 2013 to identify new faint comoving companions to ultimately analyze the occurence of giant planets and brown dwarf companions at wide (10-2000 AU) orbits around young, solar-type stars. Methods. We used NaCo at VLT to explore the occurrence rate of giant planets and brown dwarfs between typically 0.1 and 8 ''. Diffraction-limited observations in H-band combined with angular differential imaging enabled us to reach primary star-companion brightness ratios as small as 10(-6) at 1.5 ''. Repeated observations at several epochs enabled us to discriminate comoving companions from background objects. Results. During our survey, twelve systems were resolved as new binaries, including the discovery of a new white dwarf companion to the star HD8049. Around 34 stars, at least one companion candidate was detected in the observed field of view. More than 400 faint sources were detected; 90% of them were in four crowded fields. With the exception of HD8049 B, we did not identify any new comoving companions. The survey also led to spatially resolved images of the thin debris disk around HD61005 that have been published earlier. Finally, considering the survey detection limits, we derive a preliminary upper limit on the frequency of giant planets for the semi-major axes of [10, 2000] AU: typically less than 15% between 100 and 500 AU and less than 10% between 50 and 500 AU for exoplanets that are more massive than 5 M-Jup and 10 M-Jup respectively, if we consider a uniform input distribution and a confidence level of 95%. Conclusions. The results from this survey agree with earlier programs emphasizing that massive, gas giant companions on wide orbits around solar-type stars are rare. These results will be part of a broader analysis of a total of similar to 210 young, solar-type stars to bring further statistical constraints for theoretical models of planetary formation and evolution.

The age of the universe from nuclear chronometers

An overview is presented of the current situation regarding radioactive dating of the matter of which our Galaxy is comprised. A firm lower bound on the age from nuclear chronometers of ≈9–10 Gyr is entirely consistent with age determinations from globular clusters and white dwarf cooling histories. The reasonable assumption of an approximately uniform nucleosynthesis rate yields an age for the Galaxy of 12.8 ± 3 Gyr, which again is consistent with current determinations from other methods.

No surviving evolved companions of the progenitor of SN 1006

Type Ia supernovae are thought to occur when a white dwarf made of carbon and oxygen accretes sufficient mass to trigger a thermonuclear explosion(1). The accretion could be slow, from an unevolved (main-sequence) or evolved (subgiant or giant) star(2,3) (the single-degenerate channel), or rapid, as the primary star breaks up a smaller orbiting white dwarf(3,4) (the double-degenerate channel). A companion star will survive the explosion only in the single-degenerate channel(5). Both channels might contribute to the production of type Ia supernovae(6,7), but the relative proportions of their contributions remain a fundamental puzzle in astronomy. Previous searches for remnant companions have revealed one possible case for SN 1572 (refs 8, 9), although that has been questioned(10). More recently, observations have restricted surviving companions to be small, main-sequence stars(11-13), ruling out giant companions but still allowing the single-degenerate channel. Here we report the results of a search for surviving companions of the progenitor of SN 1006 (ref. 14). None of the stars within 4 arc minutes of the apparent site of the explosion is associated with the supernova remnant, and we can firmly exclude all giant and subgiant stars from being companions of the progenitor. In combination with previous results, our findings indicate that fewer than 20 per cent of type Ia supernovae occur through the single-degenerate channel.

Chandra and Suzaku observations of the Be/X-ray star HD110432

We present an analysis of a pointed 141 ks Chandra high-resolution transmission gratings observation of the Be X-ray emitting star HD110432, a prominent member of the γ Cas analogs. This observation represents the first high-resolution spectrum taken for this source as well as the longest uninterrupted observation of any γ Cas analog. The Chandra light curve shows a high variability but its analysis fails to detect any coherent periodicity up to a frequency of 0.05 Hz. Hardness ratio versus intensity analyses demonstrate that the relative contributions of the [1.5-3] Å, [3-6] Å, and [6-16] Å energy bands to the total flux change rapidly in the short term. The analysis of the Chandra High Energy Transmission Grating (HETG) spectrum shows that, to correctly describe the spectrum, three model components are needed. Two of those components are optically thin thermal plasmas of different temperatures (kT ≈ 8-9 and 0.2-0.3 keV, respectively) described by the models vmekal or bvapec. The Fe abundance in each of these two components appears equal within the errors and is slightly subsolar with Z ≈ 0.75 Z ☉. The bvapec model better describes the Fe L transitions, although it cannot fit well the Na XI Lyα line at 10.02 Å, which appears to be overabundant. Two different models seem to describe well the third component. One possibility is a third hot optically thin thermal plasma at kT = 16-21 keV with an Fe abundance Z ≈ 0.3 Z ☉, definitely smaller than for the other two thermal components. Furthermore, the bvapec model describes well the Fe K shell transitions because it accounts for the turbulence broadening of the Fe XXV and Fe XXVI lines with a v turb ≈ 1200 km s–1. These two lines, contributed mainly by the hot thermal plasma, are significantly wider than the Fe Kα line whose FWHM < 5 mÅ is not resolved by Chandra. Alternatively, the third component can be described by a power law with a photon index of Γ = 1.56. In either case, the Chandra HETG spectrum establishes that each one of these components must be modified by distinct absorption columns. The analysis of a noncontemporaneous 25 ks Suzaku observation shows the presence of a hard tail extending up to at least 33 keV. The Suzaku spectrum is described with the sum of two components: an optically thin thermal plasma at kT ≈ 9 keV and Z ≈ 0.74 Z ☉, and a very hot second plasma with kT ≈ 33 keV or, alternatively, a power law with photon index of Γ = 1.58. In either case, each one of the two components must be affected by different absorption columns. Therefore, the kT = 8-9 keV component is definitely needed while the nature of the harder emission cannot be unambiguously established with the present data sets. The analysis of the Si XIII and S XV He-like triplets present in the Chandra spectrum points to a very dense (ne ~ 1013 cm–3) plasma located either close to the stellar surface (r < 3R *) of the Be star or, alternatively, very close (r ~ 1.5R WD) to the surface of a (hypothetical) white dwarf companion. We argue, however, that the available data support the first scenario.

Exploitation du potentiel sismique des étoiles naines blanches

Le but de cette thèse est d’explorer le potentiel sismique des étoiles naines blanches pulsantes, et en particulier celles à atmosphères riches en hydrogène, les étoiles ZZ Ceti. La technique d’astérosismologie exploite l’information contenue dans les modes normaux de vibration qui peuvent être excités lors de phases particulières de l’évolution d’une étoile. Ces modes modulent le flux émergent de l’étoile pulsante et se manifestent principalement en termes de variations lumineuses multi-périodiques. L’astérosismologie consiste donc à examiner la luminosité d’étoiles pulsantes en fonction du temps, afin d’en extraire les périodes, les amplitudes apparentes, ainsi que les phases relatives des modes de pulsation détectés, en utilisant des méthodes standards de traitement de signal, telles que des techniques de Fourier. L’étape suivante consiste à comparer les périodes de pulsation observées avec des périodes générées par un modèle stellaire en cherchant l’accord optimal avec un modèle physique reconstituant le plus fidèlement possible l’étoile pulsante. Afin d’assurer une recherche optimale dans l’espace des paramètres, il est nécessaire d’avoir de bons modèles physiques, un algorithme d’optimisation de comparaison de périodes efficace, et une puissance de calcul considérable. Les périodes des modes de pulsation de modèles stellaires de naines blanches peuvent être généralement calculées de manière précise et fiable sur la base de la théorie linéaire des pulsations stellaires dans sa version adiabatique. Afin de définir dans son ensemble un modèle statique de naine blanche propre à l’analyse astérosismologique, il est nécessaire de spécifier la gravité de surface, la température effective, ainsi que différents paramètres décrivant la disposition en couche de l’enveloppe. En utilisant parallèlement les informations obtenues de manière indépendante (température effective et gravité de surface) par la méthode spectroscopique, il devient possible de vérifier la validité de la solution obtenue et de restreindre de manière remarquable l’espace des paramètres. L’exercice astérosismologique, s’il est réussi, mène donc à la détermination précise des paramètres de la structure globale de l’étoile pulsante et fournit de l’information unique sur sa structure interne et l’état de sa phase évolutive. On présente dans cette thèse l’analyse complète réussie, de l’extraction des fréquences à la solution sismique, de quatre étoiles naines blanches pulsantes. Il a été possible de déterminer les paramètres structuraux de ces étoiles et de les comparer remarquablement à toutes les contraintes indépendantes disponibles dans la littérature, mais aussi d’inférer sur la dynamique interne et de reconstruire le profil de rotation interne. Dans un premier temps, on analyse le duo d’étoiles ZZ Ceti, GD 165 et Ross 548, afin de comprendre les différences entre leurs propriétés de pulsation, malgré le fait qu’elles soient des étoiles similaires en tout point, spectroscopiquement parlant. L’analyse sismique révèle des structures internes différentes, et dévoile la sensibilité de certains modes de pulsation à la composition interne du noyau de l’étoile. Afin de palier à cette sensibilité, nouvellement découverte, et de rivaliser avec les données de qualité exceptionnelle que nous fournissent les missions spatiales Kepler et Kepler2, on développe une nouvelle paramétrisation des profils chimiques dans le coeur, et on valide la robustesse de notre technique et de nos modèles par de nombreux tests. Avec en main la nouvelle paramétrisation du noyau, on décroche enfin le ”Saint Graal” de l’astérosismologie, en étant capable de reproduire pour la première fois les périodes observées à la précision des observations, dans le cas de l’étude sismique des étoiles KIC 08626021 et de GD 1212.

The crowded magnetosphere of the post-common-envelope binary QS Virginis

We present high-speed photometry and high-resolution spectroscopy of the eclipsing post-common-envelope binary QS Virginis (QS Vir). Our Ultraviolet and Visual Echelle Spectrograph (UVES) spectra span multiple orbits over more than a year and reveal the presence of several large prominences passing in front of both the M star and its white dwarf companion, allowing us to triangulate their positions. Despite showing small variations on a time-scale of days, they persist for more than a year and may last decades. One large prominence extends almost three stellar radii from the M star. Roche tomography reveals that the M star is heavily spotted and that these spots are long-lived and in relatively fixed locations, preferentially found on the hemisphere facing the white dwarf. We also determine precise binary and physical parameters for the system. We find that the 14 220 ± 350 K white dwarf is relatively massive, 0.782 ± 0.013 M⊙, and has a radius of 0.010 68 ± 0.000 07 R⊙, consistent with evolutionary models. The tidally distorted M star has a mass of 0.382 ± 0.006 M⊙ and a radius of 0.381 ± 0.003 R⊙, also consistent with evolutionary models. We find that the magnesium absorption line from the white dwarf is broader than expected. This could be due to rotation (implying a spin period of only ˜700 s), or due to a weak (˜100 kG) magnetic field, we favour the latter interpretation. Since the M star's radius is still within its Roche lobe and there is no evidence that it is overinflated, we conclude that QS Vir is most likely a pre-cataclysmic binary just about to become semidetached.

Searching for swept-up hydrogen and helium in the late-time spectra of 11 nearby Type Ia supernovae

The direct detection of a stellar system that explodes as a Type Ia supernova (SN Ia) has not yet been successful. Various indirect methods have been used to investigate SN Ia progenitor systems but none have produced conclusive results. A prediction of single-degenerate models is that H- (or He-) rich material from the envelope of the companion star should be swept up by the SN ejecta in the explosion. Seven SNe Ia have been analysed to date looking for signs of H-rich material in their late-time spectra and none were detected. We present results from new late-time spectra of 11 SNe Ia obtained at the Very Large Telescope using XShooter and FORS2. We present the tentative detection of Hα emission for SN 2013ct, corresponding to ∼0.007 M_⊙ of stripped/ablated companion star material (under the assumptions of the spectral modelling). This mass is significantly lower than expected for single-degenerate scenarios, suggesting that >0.1 M_⊙ of H-rich is present but not observed. We do not detect Hα emission in the other 10 SNe Ia. This brings the total sample of normal SNe Ia with non-detections (<0.001–0.058 M_⊙) of H-rich material to 17 events. The simplest explanation for these non-detections is that these objects did not result from the explosion of a CO white dwarf accreting matter from a H-rich companion star via Roche lobe overflow or symbiotic channels. However, further spectral modelling is needed to confirm this. We also find no evidence of He-emission features, but models with He-rich companion stars are not available to place mass limits.

Predicting polarization signatures for double-detonation and delayed-detonation models of Type Ia supernovae

Calculations of synthetic spectropolarimetry are one means to test multidimensional explosion models for Type Ia supernovae. In a recent paper, we demonstrated that the violent merger of a 1.1 and 0.9 M⊙ white dwarf binary system is too asymmetric to explain the low polarization levels commonly observed in normal Type Ia supernovae. Here, we present polarization simulations for two alternative scenarios: the sub-Chandrasekhar mass double-detonation and the Chandrasekhar mass delayed-detonation model. Specifically, we study a 2D double-detonation model and a 3D delayed-detonation model, and calculate polarization spectra for multiple observer orientations in both cases. We find modest polarization levels (<1 per cent) for both explosion models. Polarization in the continuum peaks at ∼0.1–0.3 per cent and decreases after maximum light, in excellent agreement with spectropolarimetric data of normal Type Ia supernovae. Higher degrees of polarization are found across individual spectral lines. In particular, the synthetic Si II λ6355 profiles are polarized at levels that match remarkably well the values observed in normal Type Ia supernovae, while the low degrees of polarization predicted across the O I λ7774 region are consistent with the non-detection of this feature in current data. We conclude that our models can reproduce many of the characteristics of both flux and polarization spectra for well-studied Type Ia supernovae, such as SN 2001el and SN 2012fr. However, the two models considered here cannot account for the unusually high level of polarization observed in extreme cases such as SN 2004dt.

Three-dimensional simulations of gravitationally confined detonations compared to observations of SN 1991T

The gravitationally confined detonation (GCD) model has been proposed as a possible explosion mechanism for Type Ia supernovae in the single-degenerate evolution channel. It starts with ignition of a deflagration in a single off-centre bubble in a near-Chandrasekhar-mass white dwarf. Driven by buoyancy, the deflagration flame rises in a narrow cone towards the surface. For the most part, the main component of the flow of the expanding ashes remains radial, but upon reaching the outer, low-pressure layers of the white dwarf, an additional lateral component develops. This causes the deflagration ashes to converge again at the opposite side, where the compression heats fuel and a detonation may be launched. We first performed five three-dimensional hydrodynamic simulations of the deflagration phase in 1.4 M⊙ carbon/oxygen white dwarfs at intermediate-resolution (2563computational zones). We confirm that the closer the initial deflagration is ignited to the centre, the slower the buoyant rise and the longer the deflagration ashes takes to break out and close in on the opposite pole to collide. To test the GCD explosion model, we then performed a high-resolution (5123 computational zones) simulation for a model with an ignition spot offset near the upper limit of what is still justifiable, 200 km. This high-resolution simulation met our deliberately optimistic detonation criteria, and we initiated a detonation. The detonation burned through the white dwarf and led to its complete disruption. For this model, we determined detailed nucleosynthetic yields by post-processing 106 tracer particles with a 384 nuclide reaction network, and we present multi-band light curves and time-dependent optical spectra. We find that our synthetic observables show a prominent viewing-angle sensitivity in ultraviolet and blue wavelength bands, which contradicts observed SNe Ia. The strong dependence on the viewing angle is caused by the asymmetric distribution of the deflagration ashes in the outer ejecta layers. Finally, we compared our model to SN 1991T. The overall flux level of the model is slightly too low, and the model predicts pre-maximum light spectral features due to Ca, S, and Si that are too strong. Furthermore, the model chemical abundance stratification qualitatively disagrees with recent abundance tomography results in two key areas: our model lacks low-velocity stable Fe and instead has copious amounts of high-velocity 56Ni and stable Fe. We therefore do not find good agreement of the model with SN 1991T.

Exploitation du potentiel sismique des étoiles naines blanches

Le but de cette thèse est d’explorer le potentiel sismique des étoiles naines blanches pulsantes, et en particulier celles à atmosphères riches en hydrogène, les étoiles ZZ Ceti. La technique d’astérosismologie exploite l’information contenue dans les modes normaux de vibration qui peuvent être excités lors de phases particulières de l’évolution d’une étoile. Ces modes modulent le flux émergent de l’étoile pulsante et se manifestent principalement en termes de variations lumineuses multi-périodiques. L’astérosismologie consiste donc à examiner la luminosité d’étoiles pulsantes en fonction du temps, afin d’en extraire les périodes, les amplitudes apparentes, ainsi que les phases relatives des modes de pulsation détectés, en utilisant des méthodes standards de traitement de signal, telles que des techniques de Fourier. L’étape suivante consiste à comparer les périodes de pulsation observées avec des périodes générées par un modèle stellaire en cherchant l’accord optimal avec un modèle physique reconstituant le plus fidèlement possible l’étoile pulsante. Afin d’assurer une recherche optimale dans l’espace des paramètres, il est nécessaire d’avoir de bons modèles physiques, un algorithme d’optimisation de comparaison de périodes efficace, et une puissance de calcul considérable. Les périodes des modes de pulsation de modèles stellaires de naines blanches peuvent être généralement calculées de manière précise et fiable sur la base de la théorie linéaire des pulsations stellaires dans sa version adiabatique. Afin de définir dans son ensemble un modèle statique de naine blanche propre à l’analyse astérosismologique, il est nécessaire de spécifier la gravité de surface, la température effective, ainsi que différents paramètres décrivant la disposition en couche de l’enveloppe. En utilisant parallèlement les informations obtenues de manière indépendante (température effective et gravité de surface) par la méthode spectroscopique, il devient possible de vérifier la validité de la solution obtenue et de restreindre de manière remarquable l’espace des paramètres. L’exercice astérosismologique, s’il est réussi, mène donc à la détermination précise des paramètres de la structure globale de l’étoile pulsante et fournit de l’information unique sur sa structure interne et l’état de sa phase évolutive. On présente dans cette thèse l’analyse complète réussie, de l’extraction des fréquences à la solution sismique, de quatre étoiles naines blanches pulsantes. Il a été possible de déterminer les paramètres structuraux de ces étoiles et de les comparer remarquablement à toutes les contraintes indépendantes disponibles dans la littérature, mais aussi d’inférer sur la dynamique interne et de reconstruire le profil de rotation interne. Dans un premier temps, on analyse le duo d’étoiles ZZ Ceti, GD 165 et Ross 548, afin de comprendre les différences entre leurs propriétés de pulsation, malgré le fait qu’elles soient des étoiles similaires en tout point, spectroscopiquement parlant. L’analyse sismique révèle des structures internes différentes, et dévoile la sensibilité de certains modes de pulsation à la composition interne du noyau de l’étoile. Afin de palier à cette sensibilité, nouvellement découverte, et de rivaliser avec les données de qualité exceptionnelle que nous fournissent les missions spatiales Kepler et Kepler2, on développe une nouvelle paramétrisation des profils chimiques dans le coeur, et on valide la robustesse de notre technique et de nos modèles par de nombreux tests. Avec en main la nouvelle paramétrisation du noyau, on décroche enfin le ”Saint Graal” de l’astérosismologie, en étant capable de reproduire pour la première fois les périodes observées à la précision des observations, dans le cas de l’étude sismique des étoiles KIC 08626021 et de GD 1212.

Recherche et analyse d'étoiles naines blanches chimiquement stratiées dans le Sloan Digital Sky Survey

Ce mémoire présente une recherche détaillée et une analyse des étoiles naines blanches hybrides chimiquement stratifiées dans le Sloan Digital Sky Survey (SDSS). Une seule étoile stratifiée, PG 1305-017, était connue avant notre recherche. L'objectif principal est de confirmer l'existence de plusieurs nouvelles étoiles stratifiées. Pour ce faire, il a fallu dans un premier temps développer une nouvelle génération de modèles d'atmosphère à partir de ceux de Bergeron et al. (1991) et Tremblay & Bergeron (2009). Nous y avons ajouté l'opacité de toutes les raies d'hélium et les calculs nécessaires pour tenir compte de la stratification chimique de l'atmosphère, où une mince quantité d’hydrogène flotte en équilibre diffusif au-dessus d’une enveloppe massive d’hélium. En parallèle, nous avons aussi calculé des modèles standards, chimiquement homogènes. Ensuite, nous avons sélectionné des naines blanches chaudes (Teff > 30,000 K) de type spectral hybride (traces d'hélium et d'hydrogène) parmi les ~38,000 naines blanches répertoriées dans le SDSS. Un total de 52 spectres d'étoile a été retenu dans notre échantillon final. La technique spectroscopique, c'est-à-dire l'ajustement des raies spectrales des modèles sur un spectre observé, a été appliquée à toutes les étoiles de notre échantillon. Nous avons ainsi mesuré la température effective, la gravité de surface et la composition chimique de l'atmosphère de ces étoiles. Par l'ajustement simultané de modèles stratifiés et homogènes, nous avons aussi pu déterminer si les étoiles étaient stratifiées ou non. Nous identifions ainsi 14 naines blanches stratifiées. Nous tirons de ces résultats plusieurs conclusions sur les processus physiques expliquant la présence d'hélium dans l'atmosphère.

Recherche et analyse d'étoiles naines blanches chimiquement stratiées dans le Sloan Digital Sky Survey

Ce mémoire présente une recherche détaillée et une analyse des étoiles naines blanches hybrides chimiquement stratifiées dans le Sloan Digital Sky Survey (SDSS). Une seule étoile stratifiée, PG 1305-017, était connue avant notre recherche. L'objectif principal est de confirmer l'existence de plusieurs nouvelles étoiles stratifiées. Pour ce faire, il a fallu dans un premier temps développer une nouvelle génération de modèles d'atmosphère à partir de ceux de Bergeron et al. (1991) et Tremblay & Bergeron (2009). Nous y avons ajouté l'opacité de toutes les raies d'hélium et les calculs nécessaires pour tenir compte de la stratification chimique de l'atmosphère, où une mince quantité d’hydrogène flotte en équilibre diffusif au-dessus d’une enveloppe massive d’hélium. En parallèle, nous avons aussi calculé des modèles standards, chimiquement homogènes. Ensuite, nous avons sélectionné des naines blanches chaudes (Teff > 30,000 K) de type spectral hybride (traces d'hélium et d'hydrogène) parmi les ~38,000 naines blanches répertoriées dans le SDSS. Un total de 52 spectres d'étoile a été retenu dans notre échantillon final. La technique spectroscopique, c'est-à-dire l'ajustement des raies spectrales des modèles sur un spectre observé, a été appliquée à toutes les étoiles de notre échantillon. Nous avons ainsi mesuré la température effective, la gravité de surface et la composition chimique de l'atmosphère de ces étoiles. Par l'ajustement simultané de modèles stratifiés et homogènes, nous avons aussi pu déterminer si les étoiles étaient stratifiées ou non. Nous identifions ainsi 14 naines blanches stratifiées. Nous tirons de ces résultats plusieurs conclusions sur les processus physiques expliquant la présence d'hélium dans l'atmosphère.

A Brown Dwarf Mass Donor in an Accreting Binary

A long-standing and unverified prediction of binary star evolution theory is the existence of a population of white dwarfs accreting from substellar donor stars. Such systems ought to be common, but the difficulty of finding them, combined with the challenge of detecting the donor against the light from accretion, means that no donor star to date has a measured mass below the hydrogen burning limit. We applied a technique that allowed us to reliably measure the mass of the unseen donor star in eclipsing systems. We were able to identify a brown dwarf donor star, with a mass of 0.052 ± 0.002 solar mass. The relatively high mass of the donor star for its orbital period suggests that current evolutionary models may underestimate the radii of brown dwarfs.

Cetacean Notes. I. Sei and Rorqual Whales on the Mississippi Coast, a Correction. II. A Dwarf Sperm Whale in Mississippi Sound and Its Helminth Parasites

I. Gunter and Christmas (1973) described the events leading to the stranding of a baleen whale on Ship Island, Mississippi, in 1968, giving the species as Balaenopteru physalus, the Rorqual. Unfortunately the identification was in error, but fortunately good photographs were shown. The underside of the tail was a splotched white, but there was no black margin. The specimen also had fewer throat and belly grooves than the Rorqual, as a comparison with True’s (1904) photograph shows. Dr. James Mead (in litt.) pointed out that the animal was a Sei Whale, Balaenoptera borealis. This remains a new Mississippi record and according to Lowery’s (1974) count, it is the fifth specimen reported from the Gulf of Mexico. The stranding of a sixth Sei Whale on Anclote Keys in the Gulf, west of Tarpon Springs, Florida on 30 May 1974, was reported in the newspapers and by the Smithsonian Institution (1974). II. Gunter, Hubbs and Beal (1955) gave measurements on a Pygmy Sperm Whale, Kogia breviceps, which stranded on Mustang Island on the Texas coast and commented upon the recorded variations of proportional measurements in this species. Then according to Raun, Hoese and Moseley (1970) these questions were resolved by Handley (1966), who showed that a second species, Kogia simus, the Dwarf Sperm Whale, is also present in the western North Atlantic. Handley’s argument is based on skull comparisons and it seems to be rather indubitable. According to Raun et al. (op. cit.), the stranding of a species of Kogia on Galveston Island recorded by Caldwell, Ingles and Siebenaler (1960) was K. simus. They also say that Caldwell (in litt.) had previously come to the same conclusion. Caldwell et al. also recorded another specimen from Destin, Florida, which is now considered to have been a specimen of simus. The known status of these two little sperm whales in the Gulf is summarized by Lowery (op. cit.).