Unified line profiles for hydrogen perturbed by collisions with protons: satellites and asymmetries

We present new calculations of unified line profiles for hydrogen perturbed by collisions with protons. We report on new calculations of the potential energies and dipole moments which allow the evaluation of profiles for the lines of the Lyman series up to Lyman$\delta$ and the Balmer series up to Balmer10. Unified calculations only existed for the lines Lyman$\alpha$ to Lyman$\gamma$ and Balmer$\alpha$ including the H$_2^+$ quasi-molecule. These data are available as online material accompanying this paper and should be included in atmosphere models, in place of the Stark effect of protons, since the quasi-molecular contributions cause not only satellites, but large asymmetries that are unaccounted for in models that assume Stark broadening of electrons and protons are equal.Comment: 13 pages, 25 figures. Accepted for publication in MNRA

The sdA problem - II. Photometric and Spectroscopic Follow-up

Subdwarf A star (sdA) is a spectral classification given to objects showing H-rich spectra and sub-main sequence surface gravities, but effective temperature lower than the zero-age horizontal branch. Their evolutionary origin is an enigma. In this work, we discuss the results of follow-up observations of selected sdAs. We obtained time resolved spectroscopy for 24 objects, and time-series photometry for another 19 objects. For two targets, we report both spectroscopy and photometry observations. We confirm seven objects to be new extremely-low mass white dwarfs (ELMs), one of which is a known eclipsing star. We also find the eighth member of the pulsating ELM class.Comment: Accepted for publication in MNRAS. 19 pages, 30 figures, 6 table

Estrelas anãs brancas e subanãs no sloan digital sky survey

White dwarf stars are the final observable evolutionary state of over 95% of stars and also a common outcome in binary evolution. Therefore, studying white dwarfs is a powerful tool to understand both single and binary stellar evolution, local initial mass function, and post-main sequence mass loss, leading us to a better comprehension of the history of stellar formation and evolution of different stellar populations. In order to make this type of studies possible, a large and preferably complete sample of white dwarf stars, covering the whole range of physical parameters, is required. The simplest way to achieve that is to take advantage of data provided by large surveys. The Sloan Digital Sky Survey has already allowed the increase of the number of known white dwarf stars fivefold up to its data release 10. In this work, we extended the search for white dwarfs to the new objects in the data release 12, discovering 3 157 new white dwarfs and 1 349 new subdwarfs. For the first time, we have extended this search to log g < 6.5, corresponding to M < 0.3 M⊙. White dwarfs below this mass limit cannot be formed through single evolution within a Hubble time; however, if the star is part of a close binary system, the mass loss of the system may be so intense that the resulting white dwarf has mass below the single evolution limit. These objects are known as extremely-low mass white dwarfs (ELMs) They show Teff < 20 000 K and 5.0 . log g . 6.5 and spectra very similar to main sequence A stars. Less than a hundred of them are known, and most objects were discovered relying on biased selection criteria, that excluded cool (Teff < 9 000 K), lowermass (M . 0.15 M⊙) ELMs, making it difficult to validate the models and comprehend the properties of the ELMs as a class. We have identified thousands of objects whose physical properties, effective temperature and surface gravity, place them in the range of by-products of binary interaction such as the ELMs. We have called them sdAs, referring to their sub-main sequence log g and hydrogen dominated spectra. They seem to be composed of overlapping stellar populations, and we found that at least 7% are more likely ELMs or their precursors, the pre-ELMs, than main sequence stars. Obtaining time-resolved spectroscopy for 26 objects, we could confirm 15 to be in close binaries. One of them is also an eclipsing system, while another is a pulsator — the eighth member of the pulsating ELM class. Other six new pulsators were found as part of our follow-up, five of them in the vicinity of the ELM instability strip. With these results, we increase the population of ELMs by 20%, raising the fraction of cool ELMs from 4 to 20%, which is consistent with the predictions from the evolutionary models.Estrelas anãs brancas são o último estágio evolutivo observável de mais de 95% das estrelas e também um resultado comum na evolução de estrelas binárias. O estudo de anãs brancas é, portanto, uma ferramenta poderosa na compreensão da evolução de estrelas simples e binárias, da função de massa inicial local, e da perda de massa após a sequência principal, levando-nos a uma melhor compreensão do histórico de formação e evolução estelar de diferentes populações. Para que esses estudos sejam possíveis, é necessária uma amostra grande e preferencialmente completa de anãs brancas, cobrindo todo o intervalo de parâmetros físicos. A maneira mais simples de obter isso é utilizando dados de grandes projetos de mapeamento. O Sloan Digital Sky Survey já permitiu o aumento do número de anãs brancas conhecidas em cinco vezes até o data release 10. Neste trabalho, estendemos a busca por anãs brancas aos novos objetos no data release 12, descobrindo 3 157 novas anãs brancas e 1 349 novas subanãs. Pela primeira vez, estendemos essa busca para log g < 6.5, correspondente a M < 0.3 M⊙. Anãs brancas abaixo desse limite de massa não podem formar-se em um tempo de Hubble; contudo, se a estrela é parte de um sistema de binárias próximas, a perda de massa pode ser tão intensa que a anã branca resultante tem massa inferior ao limite por evolução simples. Esses objetos são chamados anãs brancas de massa extremamente-baixa (ELMs, do inglês extremely-low mass white dwarfs) Elas têm Teff < 20 000 K e 5.0 . log g . 6.5 e espectros muito similares a estrelas A de sequência principal. Menos de cem são conhecidas, e a maioria dos objetos foi descoberta tendo em conta um critério de seleção tendencioso, que excluiu ELMs frias (Teff < 9 000 K) e com mais baixa massa (M . 0.15 M⊙), tornando difícil verificar modelos e compreender as propriedades das ELMs como classe. Nós identificamos milhares de objetos cujas propriedades físicas, temperatura efetiva e log g, estão no intervalo de produtos de evolução binária, como as ELMs. Nós os chamamos de sdAs, referindo-nos ao seu log g que as coloca abaixo da sequência principal e seu espectro dominado por hidrogênio. As sdAs parecem conter populações estelares sobrepostas, e encontramos que 7% são mais compatíveis com (pre-)ELMs do que com objetos de sequência principal. Obtivemos espectroscopia resolvida temporalmente para 26 objetos e pudemos confirmar que 15 estão em binárias. Um objeto faz parte de um sistema eclipsante, enquanto outro é pulsante — o oitavo membro da classe de ELMs pulsantes. Outros seis objetos também apresentaram pulsações em nossas observações, cinco desses estão na vizinhança da faixa de instabilidade das ELMs. Com estes resultados, aumentamos a população de ELMs por um fator de 20%, elevando a fração de ELMs de 4 para 20%, um valor que é consistente com as previsões de modelos evolucionários

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

Hot subdwarfs are core-helium burning stars that show lower masses and higher temperatures than canonical horizontal branch stars. They are believed to be formed when a red giant suffers an extreme mass-loss episode. Binary interaction is suggested to be the main formation channel, but the high fraction of apparently single hot subdwarfs (up to 30%) has prompted single star formation scenarios to be proposed. If such formation scenarios without interaction were possible, that would also imply the existence of hot subdwarfs in wide binaries that have undergone no interaction. We probe the existence of these systems by analysing light curves from the Transiting Exoplanet Survey Satellite (TESS) for all known hot subdwarfs with a main sequence wide binary companion, and by searching for common proper motion pairs to spectroscopically confirmed hot subdwarfs. We find that (i) the companions in composite hot subdwarfs show short rotation periods when compared to field main sequence stars. They display a triangular-shaped distribution with a peak around 2.5 days, similar to what is observed for young open clusters. This observed distribution of rotation rates for the companions in known wide hot subdwarf binaries provides evidence of previous interaction causing spin-up. We also report (ii) a shortage of hot subdwarfs with candidate common proper motion companions, considering the frequency of such systems among progenitors. We identify only 16 candidates after probing 2938 hot subdwarfs with good astrometry. Out of those, at least six seem to be hierarchical triple systems, in which the hot subdwarf is part of an inner binary. These results suggest that binary interaction is always required for the formation of hot subdwarfs.Comment: 17 pages, 11 figures, 5 tables. Accepted for publication in A&

The sdA problem : III. New extremely low-mass white dwarfs and their precursors from Gaia astrometry

The physical nature of the sdA stars – cool hydrogen-rich objects with spectroscopic surface gravities intermediate between main-sequence and canonical-mass white dwarfs – has been elusive since they were found in Sloan Digital Sky Survey Data Release 12 spectra. The population is likely dominated by metal-poor A/F stars in the halo with overestimated surface gravities, with a small contribution of extremely low-mass white dwarfs and their precursors, i.e. ELMs and pre-ELMs. In this work, we seek to identify (pre-)ELMs with radii smaller than what is possible for main-sequence stars, allowing even for very low metallicity. We analyse 3891 sdAs previously identified in the Sloan Digital Sky Survey using Gaia DR2 data. Our Monte Carlo analysis supports that 90 of these are inconsistent with the main sequence. 37 lie close to or within the canonical white dwarf cooling sequence, while the remaining 53 lie between the canonical white dwarfs and main sequence, which we interpret as likely (pre-)ELMs given their spectral class. Of these, 30 pass more conservative criteria that allow for higher systematic uncertainties on the parallax, as well as an approximate treatment of extinction. Our identifications increase the number of known (pre-)ELMs by up to 50 per cent, demonstrating how Gaia astrometry can reveal members of the compact (pre-)ELM subpopulation of the sdA spectral class

The sdA problem - III. New extremely low-mass white dwarfs and their precursors from Gaia astrometry

The physical nature of the sdA stars---cool hydrogen-rich objects with spectroscopic surface gravities intermediate between main sequence and canonical mass white dwarfs---has been elusive since they were found in Sloan Digital Sky Survey Data Release 12 spectra. The population is likely dominated by metal-poor A/F stars in the halo with overestimated surface gravities, with a small contribution of extremely low-mass white dwarfs and their precursors, i.e., ELMs and pre-ELMs. In this work, we seek to identify (pre-)ELMs with radii smaller than is possible for main sequence stars, allowing even for very low metallicity. We analyse 3891 sdAs previously identified in the Sloan Digital Sky Survey using Gaia DR2 data. Our Monte Carlo analysis supports that 90 of these are inconsistent with the main sequence. 37 lie close to or within the canonical white dwarf cooling sequence, while the remaining 53 lie between the canonical white dwarfs and main sequence, which we interpret as likely (pre-)ELMs given their spectral class. Of these, 30 pass more conservative criteria that allow for higher systematic uncertainties on the parallax, as well as an approximate treatment of extinction. Our identifications increase the number of known (pre-)ELMs by up to 50 per cent, demonstrating how Gaia astrometry can reveal members of the compact (pre-)ELM subpopulation of the sdA spectral class.Comment: 13 pages, 19 figures, 1 table. Accepted for publication in MNRA

Spectroscopic classification and Gaia DR2 parallaxes of new nearby white dwarfs among selected blue proper motion stars

AIMS: With our low-resolution spectroscopic observing program for selected blue proper motion stars, we tried to find new white dwarfs (WDs) in the solar neighbourhood. METHODS: We used the LSPM catalogue with a lower proper motion limit of 150mas/yr and the UCAC2 for proper motions down to about 90mas/yr. The LSPM and UCAC2 photometry was combined with Two Micron All Sky Survey (2MASS) near-infrared (NIR) photometry. Targets selected according to their blue optical-to-NIR and NIR colours were observed mainly at Calar Alto. The spectra were classified by comparison with a large number of already known comparison objects, including WDs, simultaneously observed within our program. Gaia DR2 parallaxes and colours were used to confirm or reject spectroscopic WD candidates and to derive improved effective temperatures. RESULTS: We found ten new WDs at distances between 24.4pc and 79.8pc, including six hot DA WDs: GD 221 (DA2.0), HD 166435 B (DA2.2), GD 277 (DA2.2), 2MASS J19293865+1117523 (DA2.4), 2MASS J05280449+4105253 (DA3.6), and 2MASS J05005185-0930549 (DA4.2). The latter is rather bright (G~12.6) and with its Gaia DR2 parallax of ~14mas it appears overluminous by about 3mag compared to the WD sequence in the Gaia DR2 colour-magnitude diagram. It may be the closest extremely low mass (ELM) WD to the Sun. We further classified 2MASS J07035743+2534184 as DB4.1. With its distance of 25.6pc it is the second nearest known representative of its class. With GD 28 (DA6.1), LP 740-47 (DA7.5), and LSPM J1919+4527 (DC10.3) three additional cool WDs were found. Gaia DR2 parallaxes showed us that four of our candidates but also two previously supposed WDs (WD 1004+665 and LSPM J1445+2527) are in fact distant Galactic halo stars with high tangential velocities. Among our rejected WD candidates, we identified a bright (G=13.4mag) G-type carbon dwarf, LSPM J0937+2803, at a distance of 272pc.Comment: 14 pages, 13 figures, accepted for publication in Astronomy and Astrophysic

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

White dwarfs carry information on the structure and evolution of the Galaxy, especially through their luminosity function and initial-to-final mass relation. Very cool white dwarfs provide insight into the early ages of each population. Examining the spectra of all stars with $3\sigma$ proper motion in the Sloan Digital Sky Survey Data Release 14, we report the classification for 20 088 spectroscopically confirmed white dwarfs, plus 415 hot subdwarfs, and 311 cataclysmic variables. We obtain Teff, log g and mass for hydrogen atmosphere white dwarf stars (DAs), warm helium atmosphere white dwarfs (DBs), hot subdwarfs (sdBs and sdOs), and estimate photometric Teff for white dwarf stars with continuum spectra (DCs). We find 15793 sdAs and 447 dCs between the white dwarf cooling sequence and the main sequence, especially below Teff= 10000 K; most are likely low-mass metal-poor main sequence stars, but some could be the result of interacting binary evolution.Comment: 18 pages, 13 figure