172 research outputs found
Pulsating hot O subdwarfs in ω Centauri: mapping a unique instability strip on the extreme horizontal branch
No full textpeer reviewedWe present the results of an extensive survey for rapid pulsators among Extreme Horizontal Branch (EHB) stars in ω Cen. The observations performed consist of nearly 100 h of time-series photometry for several off-centre fields of the cluster, as well as low-resolution spectroscopy for a partially overlapping sample. We obtained photometry for some 300 EHB stars, for around half of which we are able to recover light curves of sufficient quality to either detect or place meaningful non-detection limits for rapid pulsations. Based on the spectroscopy, we derive reliable values of log g, T[SUB]eff[/SUB] and log N(He) /N(H) for 38 targets, as well as good estimates of the effective temperature for another nine targets, whose spectra are slightly polluted by a close neighbour in the image. The survey uncovered a total of five rapid variables with multi-periodic oscillations between 85 and 125 s. Spectroscopically, they form a homogeneous group of hydrogen-rich subdwarf O stars clustered between 48 000 and 54 000 K. For each of the variables we are able to measure between two and three significant pulsations believed to constitute independent harmonic oscillations. However, the interpretation of the Fourier spectra is not straightforward due to significant fine structure attributed to strong amplitude variations. In addition to the rapid variables, we found an EHB star with an apparently periodic luminosity variation of ~2700 s, which we tentatively suggest may be caused by ellipsoidal variations in a close binary. Using the overlapping photometry and spectroscopy sample we are able to map an empirical ω Cen instability strip in log g - T[SUB]eff[/SUB] space. This can be directly compared to the pulsation driving predicted from the Montréal "second-generation" models regularly used to interpret the pulsations in hot B subdwarfs. Extending the parameter range of these models to higher temperatures, we find that the region where p-mode excitation occurs is in fact bifurcated, and the well-known instability strip between 29 000-36 000 K where the rapid subdwarf B pulsators are found is complemented by a second one above 50 000 K in the models. While significant challenges remain at the quantitative level, we believe that the same κ-mechanism that drives the pulsations in hot B subdwarfs is also responsible for the excitation of the rapid oscillations observed in the ω Cen variables. Intriguingly, the ω Cen variables appear to form a unique class. No direct counterparts have so far been found either in the Galactic field, nor in other globular clusters, despite dedicated searches. Conversely, our survey revealed no ω Cen representatives of the rapidly pulsating hot B subdwarfs found among the field population, though their presence cannot be excluded from the limited sample. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile (proposal IDs 083.D-0833, 386.D-0669, 087.D-0216 and 091.D-0791).The reduced spectra are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/589/A1</A
The Arizona-Montréal Spectroscopic Survey of hot subluminous stars
No full textContext . Hot subdwarf B (sdB) and O (sdO) type stars are evolved helium-burning objects that lost their hydrogen envelope before the helium flash when their progenitors were close to the tip of the red giant branch (RGB). They populate the extreme horizontal branch (EHB) in the Hertzsprung-Russell diagram (HRD). The mass distribution of canonical hot subdwarfs is expected to peak at the core mass required for helium ignition under degenerate conditions in the 0.45-0.5 M ⊙ range. However, non-degenerate helium ignition from intermediate-mass progenitors and non-canonical pathways, such as the merger of helium white dwarfs and delayed helium flashes, are also expected to contribute to the hot subdwarf population. Aims . Using high-quality, homogeneous spectra of 335 hot subluminous star candidates from the Arizona-Montréal Spectroscopic Survey, we aim to improve our understanding of the atmospheric and stellar properties of hot subdwarf stars. Our focus is on the mass distribution of the different types of hot subdwarfs and their connections to the various formation scenarios. Methods . We used large grids of model atmospheres to fit the observed spectra and derived their atmospheric parameters: effective temperature ( T eff ), surface gravity, and helium abundance. The model grids were further utilized to fit the spectral energy distribution of each star and the Gaia parallax was used to compute the stellar parameters radius, luminosity, and mass. Results . Our spectroscopic sample mostly consists of H-rich sdBs and sdOs, but also contains 41 He-rich sdOs. Additionally, the sample includes 11 intermediate-helium stars and 19 horizontal branch objects with T eff > 14 kK. We detected the presence of helium stratification in six sdB stars with T eff around 30 kK, making them good candidates for also showing 3 He enrichment in their atmospheres. Our sdB distribution along the EHB shows a gap near 33 kK, visible in both the Kiel (log g-T eff ) diagram and HRD, corroborating previous observations and predictions. The mass distributions of H-rich sdBs and sdOs are similar and centered around 0.47 M ⊙ , consistent with the canonical formation scenario of helium ignition under degenerate conditions. Among the H-rich hot subdwarfs, we found no difference between the mass distributions of close binaries and apparently single stars. The He-sdOs have a significantly wider mass distribution than their H-rich counterparts, with an average mass of about 0.78 M ⊙ . In the HRD, the He-sdOs lie on the theoretical helium main sequence for masses between 0.6 and 1 M ⊙ . This strongly favors a merger origin for these He-rich objects. We identified a small number of candidate low-mass (<0.45 M ⊙ ) sdBs located below the EHB that might have originated from more massive progenitors. These low-mass sdBs preferentially show low helium abundances. Finally, we identified more than 80 pulsating stars in our sample and found that they fall into well-defined p - and g -mode instability regions
Observations of the Ultraviolet-bright Star Barnard 29 in the Globular Cluster M13 (NGC 6205)
No full textWe have analyzed spectra from FUSE, COS, GHRS, and Keck HIRES of the UV-bright star Barnard 29 in M13 (NGC 6205). By comparing the photospheric abundances derived from multiple ionization states of C, N, O, Si, and S, we infer an effective temperature Teff = 21,400 ± 400 K. Balmer-line fits yield a surface gravity log g = 3.10 ± 0.03. We derive photospheric abundances of He, C, N, O, Mg, Al, Si, P, S, Cl, Ar, Ti, Cr, Fe, Ni, and Ge. Barnard 29 exhibits an abundance pattern typical of the first-generation stars in M13, enhanced in oxygen and depleted in aluminum. An underabundance of C and an overabundance of N suggest that the star experienced nonconvective mixing on the red giant branch (RGB). We see no evidence of significant chemical evolution since the star left the RGB; in particular, it did not undergo third dredge-up. Previous workers found that the star’s farUV spectra yield an iron abundance about 0.5 dex lower than its optical spectrum, but the iron abundances derived from all of our spectra are consistent with the cluster value. We attribute this difference to our use of model atmospheres without microturbulence, which is ruled out by careful fits to optical absorption features. We derive a mass M*/Me = 0.45–0.55 and luminosity log(L L */ ) = 3.26–3.35. Comparison with stellar-evolution models suggests that Barnard 29 evolved from a zero-age horizontal branch star of mass M*/Me between 0.50 and 0.55, near the boundary between the extreme and blue horizontal branches.Fil: Dixon, William V.. Space Telescope Science Institute; Estados UnidosFil: Chayer, Pierre. Space Telescope Science Institute; Estados UnidosFil: Reid, I. Neill. Space Telescope Science Institute; Estados UnidosFil: Miller Bertolami, Marcelo Miguel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; Argentin
Radiative levitation and opacity driving: The potential of hot subdwarf pulsators for testing diffusion and other competing processes in stars
No full textInternational audienceThe hot and compact subdwarf (sdB and sdO) stars harbor three classes of nonradial pulsators: the rapid p-mode EC14026-type sdB pulsators discovered in 1997, the slow g-mode sdB pulsators known as the V1093 Her variables found in 2003, and the rapid sdO pulsators identified in 2006. The oscillations in these stars are singular in that the driving mechanism a classical kappa effect triggered by the Z-bump, a region where partial ionisation of heavy elements from the iron group significantly increases the gas opacity is truly effective only through local abundance enhancements of such elements (in particular of iron itself ) caused by microscopic diffusion. They are the only pulsating stars, among those currently known across the HR diagram, that show oscillations so tightly linked to these diffusive phenomena. In this paper, we present a short review of our present understanding of the various components that drive pulsations in subdwarf stars and we outline the remarkable potential of these stars as laboratories where diffusion and other competing processes may be tested
Efecto de diferentes ésteres de estradiol utilizados para sincronizar la ovulación sobre el porcentaje de preñez a la IATF en vacas Angus
No full textThe estradiol cypionate (ECP) allows animals inseminated at fixed time (FTAI) performing three enclosures. Recently there have been varying results, therefore, the objective of this work was to evaluate the use of ECP administered at device removal upon pregnancy rate after FTAI. Two trials were conducted (E1 and E2), using 273 lactating beef cows. On day 0 cows received a progesterone intravaginal device plus EB. On day 8, the intravaginal device was removed, cows received a luteolytic dose of PGF and were randomized to receive either ECP at 0 h or EB at 24 h. On day 10, 50-53 h after device removal, fixed-time artificial insemination was performed using frozen/thawed semen of proven fertility bulls (EI: GS y SE; EII: M y ME) and two inseminators. Pregnancy diagnosis was performed on day 40 by ultrasonografy. Pregnancy rates were not different between treatments (ECP: 55,17%; EB:58,59 %), inseminators (T: 54,79%; A: 59,06%) and bull (E1: GS: 55,3% y SE: 60,0%; E2: M:51,5% y SE: 59,1%). Interactions were not significant (P>0,05). In conclusion, according to our finding, estradiol cypionate could be used instead of estradiol benzoate to synchronize ovulation without affecting the pregnancy rate at fixed timed artificial insemination.El Cipionato de estradiol (CPE) permite inseminar animales a tiempo fijo (IATF) realizando tres encierres. Recientemente han surgido resultados variables, por lo tanto, el objetivo del presente trabajo fue evaluar el uso del CPE administrado al retirar un dispositivo intravaginal sobre el porcentaje de preñez a la IATF. Se realizaron 2 ensayos (EI y EII), utilizando 273 vacas con cría. El día 0 se colocó un dispositivo más BE. El día 8 se retiró el dispositivo, se administró prostaglandina, y los animales recibieron aleatoriamente, CPE en ese momento o BE 24 h posteriores. El día 10, 50-53 h post retiro, se realizó IATF utilizando semen de toros de probada fertilidad (EI: GS y SE; EII: M y ME) y dos inseminadores. El día 40 se realizó diagnóstico de gestación mediante ultrasonografía. No se observaron efectos del tratamiento (CPE: 55,17%; BE: 58,59%), inseminador (T: 54,79%; A: 59,06%), toro (E1: GS:55,3% y SE: 60,0%; E2: M: 51,5% y SE: 59,1%) y de sus interacciones sobre el porcentaje de preñez (P>0,05). Se concluye que en las condiciones del presente trabajo se puede utilizar CPE al retirar un dispositivo en lugar de BE a las 24 h sin afectar el porcentaje de preñez
Radiative levitation and opacity driving: The potential of hot subdwarf pulsators for testing diffusion and other competing processes in stars
No full textInternational audienceThe hot and compact subdwarf (sdB and sdO) stars harbor three classes of nonradial pulsators: the rapid p-mode EC14026-type sdB pulsators discovered in 1997, the slow g-mode sdB pulsators known as the V1093 Her variables found in 2003, and the rapid sdO pulsators identified in 2006. The oscillations in these stars are singular in that the driving mechanism a classical kappa effect triggered by the Z-bump, a region where partial ionisation of heavy elements from the iron group significantly increases the gas opacity is truly effective only through local abundance enhancements of such elements (in particular of iron itself ) caused by microscopic diffusion. They are the only pulsating stars, among those currently known across the HR diagram, that show oscillations so tightly linked to these diffusive phenomena. In this paper, we present a short review of our present understanding of the various components that drive pulsations in subdwarf stars and we outline the remarkable potential of these stars as laboratories where diffusion and other competing processes may be tested
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