Chemical fingerprints of He-sdO stars

The chemical composition of helium-rich hot subluminous O stars plays an important role to understand and model their formation history. We present a spectroscopic analysis of four He-sdO stars,CD-31∘ 4800, [CW83] 0904- 02, LSS 1274 and LS IV +10∘ 9. The analysis is based on archival optical and UV high-resolution spectra. We used Tlusty200/Synspec48 to compute line blanketed non-LTE model atmospheres and their corresponding synthetic spectra and derive the atmospheric parameters as well as the abundances of the most prominent elements. All stars have helium-dominated atmospheres with hardly any hydrogen and temperatures between 42000 K and 47000 K while their surface gravity spans between log g = 5.4 and 5.7. CD-31∘ 4800 shows an enrichment of nitrogen and the characteristic pattern of hydrogen burning via the CNO-cycle, while the rest of the elements have about the solar abundance. This points to the slow merger of two helium white dwarfs as the most likely origin for this system. The other three stars are enriched in carbon, nitrogen and neon while their intermediate mass element’s abundance scatters around the solar value. They were possibly formed in the deep mixing late hot flasher scenario.Facultad de Ciencias Astronómicas y Geofísica

Evolutionäre Einflüsse auf die chemische Zusammensetzung von He-sdO Sternen

Helium rich hot subdwarf O stars (He-sdO stars) are known to exist in three flavors, depending on the enrichment of carbon and/or nitrogen in their atmospheres. It is believed that the chemical composition can be seen as a direct link to their evolutionary history. To examine on this link, a sample of twelve He-sdO stars was analyzed. To model the stellar atmosphere as accurately as possible, all available elements were included in NLTE conditions. Despite hydrogen and helium, the atmospheres contained carbon, nitrogen, oxygen, neon, magnesium, aluminum, silicon, phosphorus, sulfur, iron and nickel. In a first step, the atmospheric parameters of each star were measured. To do so, different grids of model atmospheres with a different metal content were calculated and extensively tested and compared to previous analyses. It turned out that a pure hydrogen/helium grid is not a good choice for atmospheric examinations of He-sdO stars as the changes in temperature stratification are tremendous when compared to a fully blanketed model. However, these differences could not be reproduced to the same extend when fitting line profiles. In this work, a HHeCNO grid was mainly used. This grid offers the best compromise between computational effort and accuracy of stratification reproduction and only shows minor temperature differences compared to a fully blanketed model. To determine the abundances of all elements included in the atmosphere, archival optical and UV spectra were used and a global fit of individual lines was applied to one element after the other iteratively. To get a first idea of the abundances, models without iron and nickel were computed using opacity distribution functions and applying them to optical data. After including iron and nickel in the opacity sampling mode, all elements were re-fitted using optical and UV data. Afterwards, photometric measurements were compared with the spectral energy distribution resulting from the calculated atmosphere to check for discrepancies and get an idea of the distance and the reddening assuming the canonical mass for He-sdOs of 0.47 solar masses. It turned out that there are basically two groups of He-sdO stars in the sample, those enriched in nitrogen and those enriched in carbon and nitrogen. The carbon-rich stars were found to belong to the same class as the carbon/nitrogen rich ones, at least for the stars in this sample. For the nitrogen-rich subsample, the slow merger of two He-core White Dwarfs seems the most plausible scenario as it reproduces the observed abundance pattern best. A cutback is that none of the N-rich stars shows any significant rotation as it would be expected from a merger scenario. The deep-mixing hot flasher scenario succeeds in explaining the CN-rich stars of the sample. Small deviations between model and measurements for certain elements can be explained by deficiencies in the modeling process of the stellar evolution. As some of the CN-rich stars show significant rotation it was tested if they can be explained by the composite merger scenario, but no matches were found. A cooperation with Astroserver.org was initialized to analyze the fasted star known so far, the He-sdO US708. This was necessary as the available UV COS spectra were of too poor quality to apply the usual fitting method to the data. In the Astroserver approach all available spectra are fitted together and all included elements are varied at the same time in each iteration step. Of special interest were the iron and nickel contents of the stellar atmosphere as they allow to draw direct conclusion on how US708 was accelerated to such a high speed. The common theory claims the ejection by a double detonation of a C/O White Dwarf in a supernova Type Ia. The supernova ejecta would hit US708 and could be seen in its atmosphere. Indeed, a significant enrichment of iron and nickel, trace elements for a SN Ia, was found compared to a He-sdO from the same N-rich class and with similar atmospheric parameters. This would be in principle a direct evidence for the supernova ejection theory, as well as the high rotational velocity. But as an ejection from the Galactic disk would require too extreme masses, not in accordance with evolutionary tracks for this type of star, an alternative scenario in which US708 is in fact a pre-He WD ejected from a very close binary system in the Galactic Halo by a supernova Ia, is proposed.Es wird angenommen, dass sich Helium reiche heiße Unterzwerge des Spektraltypes O (He-sdOs) in drei Klassen einteilen lassen, je nachdem wie viel Kohlenstoff und oder/Stickstoff sich in ihren Atmosphären findet. Wahrscheinlich lässt sich die chemische Zusammensetzung direkt mit ihrer Entstehungsgeschichte verknüpfen. Um diese Verbindung zu untersuchen wurden zwölf He-sdOs analysiert. Um ihre Atmosphären so gut wie möglich zu modellieren wurde alle Elemente (H, He, C, N, O, Ne, Mg, Al, Si, P, S, Fe, Ni) unter NLTE Bedingungen eingebaut. Als erstes wurden die atmosphärischen Parameter der Sterne bestimmt. Hierzu wurden Modellgitter von Atmosphären mit einer unterschiedlichen Metallhäufigkeit erstellt und ausgiebig getestet indem ihre Resultate mit früheren Analysen der selben Sterne verglichen wurde . Es zeigte sich dass ein reines H/He Gitter unzureichend ist da die Temperaturschichtung der Sternatmosphäre, im Vergleich zu einer solchen mit allen oben genannten Elementen, nicht exakt genug reproduziert werden kann. Beim Fitten einzelner Linien zeigte sich dass die Unterschiede bezüglich der effektiven Temperatur kleiner sind als es die Temperaturschichtung vermuten lässt. In dieser Arbeit wurde Großteils ein HHeCNO Gitter verwendet, welches einen guten Kompromiss zwischen Genauigkeit und Rechenaufwand bietet und nur geringe Abweichungen der efffektiven Temperatur im Vergleich zu einem Modell mit allen verfügbaren Elementen aufweist. Um die Metallhäufigkeiten zu bestimmen wurden optische und UV Spektren aus verschiedenen Archiven benutzt und mit einem globalen Fit individueller Linien für jedes einzelne Element untersucht. In einem ersten Schritt wurden Modelle ohne Eisen und Nickel auf optische Daten angewendet um einen ersten Eindruck der Häufigkeiten zu bekommen. Nach dem Hinzufügen von Eisen und Nickel wurden alle Elementhäufigkeiten erneut gefittet, dieses Mal unter Berücksichtigung optischer sowie UV Daten. Danach wurden photometrische Messungen mit einer aus den Berechnungen resultierenden spektralen Energieverteilung verglichen um etwaige Diskrepanzen zu entdecken und um einen Eindruck für die vorhandene Rötung und die Entfernung zu bekommen. Entgegen der Annahme wurden in der untersuchten Probe nur Stickstoff- und Kohlenstoff/Stickstoffreiche Sterne gefunden, keine mit alleiniger Kohlenstoffanreicherung. Für die stickstoffreichen Sterne erscheint die langsame Verschmelzung zwei weißer Zwerge mit Heliumkern das wahrscheinlichste Entstehungsszenario zu sein, da es die gemessenen Häufigkeiten richtig vorhersagt. Allerdings zeigt keiner dieser Sterne eine signifikante Rotation wie man sie nach der Verschmelzung zweier Sterne erwarten würde. Für die CN-reichen Sterne scheint das deep-mixing hot flasher Szenario die beste Lösung zu sein. Kleinere Abweichungen zwischen Modell und Beobachtung lassen sich durch Unzulänglichkeiten der Modellierung erklären. Da einige der Sterne rotieren wurde getestet ob eine Kombination aus langsamer und schneller Verschmelzung zweier He-WDs eine passende Erklärung liegen könnte, was aber nicht der Fall war. Um den schnellsten bisher bekannten He-sdO, US708, zu analysieren wurde eine Kooperation mit Astroserver.org gestartet, da die vorhandenen COS UV Spektren von zu schlechter Qualität waren um sie mit der sonst angewandten Methode zu analysieren. Das Astroserver Programm kann alle verfügbaren Elemente gleichzeitig unter Berücksichtigung aller Spektren, sowohl optischer als auch UV Natur, fitten. Von besonderem Interesse waren die Häufigkeiten von Eisen und Nickel da sich mit ihrer Hilfe direkt Rückschlüsse auf den Beschleunigungsmechanismus von US708 ziehen lassen. Die gängige Theorie beschreibt die Beschleunigung als das Ergebnis einer double detonation Supernova Ia, deren Ausstoß US708 trifft und in dessen Atmosphäre nachzuweisen wären. In der Tat wurde eine signifikante Anreicherung von Eisen und Nickel im Vergleich zu einem anderen stickstoffreichen He-sdO mit ähnlichen atmosphärischen Parametern gefunden. Dies bedeutet, in Kombination mit der hohen Rotationsgeschwindigkeit von US708 das die Möglichkeit der Beschleunigung durch eine Supernova Explosion sehr wahrscheinlich ist. Aber da eine Beschleunigung aus der Galaktischen Scheibe zu extreme Massen benötigen würde, welche nicht im Einklang mit den gängigen Entwicklungspfaden für solche Sterne sind, wurde eine alternative Theorie aufgestellt. Diese besagt dass US708 ein pre-He Weißer Zwerg ist der durch eine Supernova Ia Explosion aus einem sehr engen Doppelsternsystem im Galaktischen Halo geschossen wurde

Chemical fingerprints of He-sdO stars

The chemical composition of helium-rich hot subluminous O stars plays an important role to understand and model their formation history. We present a spectroscopic analysis of four He-sdO stars,CD-31° 4800, [CW83] 0904- 02, LSS 1274 and LS IV +10° 9. The analysis is based on archival optical and UV high-resolution spectra. We used Tlusty200/Synspec48 to compute line blanketed non-LTE model atmospheres and their corresponding synthetic spectra and derive the atmospheric parameters as well as the abundances of the most prominent elements. All stars have helium-dominated atmospheres with hardly any hydrogen and temperatures between 42000 K and 47000 K while their surface gravity spans between log g = 5.4 and 5.7. CD-31° 4800 shows an enrichment of nitrogen and the characteristic pattern of hydrogen burning via the CNO-cycle, while the rest of the elements have about the solar abundance. This points to the slow merger of two helium white dwarfs as the most likely origin for this system. The other three stars are enriched in carbon, nitrogen and neon while their intermediate mass element’s abundance scatters around the solar value. They were possibly formed in the deep mixing late hot flasher scenario

Use of Fondaparinux Off-Label or Approved Anticoagulants for Management of Heparin-Induced Thrombocytopenia.

BACKGROUND Life-threatening heparin-induced thrombocytopenia (HIT) is treated with the alternative nonheparin anticoagulants argatroban, lepirudin, or danaparoid. Frequently, the pentasaccharide fondaparinux is used off-label. OBJECTIVES The authors sought to investigate the safety and efficacy of the different anticoagulants for treating HIT. METHODS In a national, multicenter registry study, hospitalized patients who were diagnosed with HIT, an at least intermediate clinical HIT-risk (4Ts score ≥4 points), and received treatment with ≥1 dose of the aforementioned anticoagulants were included. Main outcome measures were the incidences of HIT-specific complications (thromboembolic venous/arterial events, amputations, recurrent/persistent thrombocytopenia, skin lesions) and bleedings. RESULTS Of 195 patients, 46 (23.6%), 4 (2.1%), 61 (31.3%), and 84 (43.1%) had been treated first-line with argatroban, lepirudin, danaparoid, and fondaparinux, respectively. The composite endpoint of HIT-specific complications (thromboembolic events, amputation, skin necrosis) occurred in 11.7% of patients treated with approved alternative anticoagulation and in 0.0% of fondaparinux-treated patients. The all-cause in-hospital mortality rates were 14.4% during approved alternative anticoagulation and 0.0% during fondaparinux treatment. Bleeding complications occurred in alternatively anticoagulated patients and in fondaparinux-treated patients in 6.3% and 4.8%, respectively. Post hoc analysis of clinical and laboratory features confirmed "true" HIT in at least 74 of 195 (38.0%) patients; 35 of 74 (47.3%) were treated with fondaparinux. CONCLUSIONS Fondaparinux is effective and safe in suspected acute HIT; no HIT-specific complications occurred in the fondaparinux-treated patients, even among those with a high clinical HIT probability. Further data from randomized controlled trials are urgently needed because lepirudin was recalled from the market; danaparoid access has been limited and is not approved in the United States; and argatroban is contraindicated in patients with impaired liver function, and activated partial thromboplastin time confounding may interfere with monitoring. (Retrospective Registry of Patients With Acute Heparin-induced Thrombocytopenia Type II; NCT01304238)