LS IV-14 Research Articles

Gaia DR2 data and the evolutionary status of eight high-velocity hot post-AGB candidates

Abstract From Gaia DR2 data of eight high-velocity hot post-AGB candidates, LS 3593, LSE 148, LS 5107, HD 172324, HD 214539, LS IV −12 111, LS III +52 24, and LS 3099, we found that six of them have accurate parallaxes which made it possible to derive their distances, absolute visual magnitudes (MV) and luminosity (log L/L⊙). All the stars except LS 5107 have an accurate effective temperature (Teff) in the literature. Some of these stars are metal poor, and some of them do not have circumstellar dust shells. In the past, the distances of some stars were estimated to be 6 kpc, which we find to be incorrect. The accurate Gaia DR2 parallaxes show that they are relatively nearby, post-AGB stars. When compared with post-AGB evolutionary tracks we find their initial masses to be in the range 1 M⊙ to 2 M⊙. We find the luminosity of LSE 148 to be significantly lower than that of post-AGB stars, suggesting that this is a post-horizontal-branch star or post-early-AGB star. LS 3593 and LS 5107 are new high-velocity hot post-AGB stars from Gaia DR2.

Opacity modelling of heavy-metal hot subdwarfs: photoionization of Sr0, Y+, and Zr2 +

ABSTRACT Heavy-metal hot subdwarfs (sdB and sdO) represent a small group of stars with unusually high concentrations of trans-iron elements in their atmospheres, having abundances ${\sim } 10\, 000$ times solar. One example is LS IV-14 116, where a number of heavy-metal absorption lines of Sr ii, Y iii, and Zr iv have been observed in the optical band $4000\!-\!5000\, {\mathring{\rm A}}$. We use a fully relativistic Dirac Atomic R-Matrix Code (darc) to calculate photoionization cross-sections of Sr0, Y+, and Zr2 + from their ground state to the 20th excited level. We use the cross-sections and the oscillator strengths to simulate the spectrum of a hot subdwarf. We obtain complete sets of photoionization cross-sections for the three ions under study. We use these data to calculate the opacity of the stellar atmospheres of hot subdwarf stars, and show that for overabundances observed in some heavy-metal subdwarves, photoexcitation from zirconium, in particular, does contribute some back warming in the model.

Heavy metal enrichment in the intermediate He-sdOB pulsator Feige 46

The intermediate He-enriched hot subdwarf star Feige 46 was recently reported as the second member of the V366 Aqr (or He-sdOBV) pulsating class. Feige 46 is very similar to the prototype of the class, LS IV − 14°116, not only in terms of pulsational properties, but also in terms of atmospheric parameters and kinematic properties. LS IV − 14°116 is additionally characterized by a very peculiar chemical composition, with extreme overabundances of the trans-iron elements Ge, Sr, Y, and Zr. We investigate the possibility that the similarity between the two pulsators extends to their chemical composition. We retrieved archived optical and UV spectroscopic observations of Feige 46 and performed an abundance analysis using model atmospheres and synthetic spectra computed with TLUSTY and SYNSPEC. In total, we derived abundances for 16 elements and provide upper limits for four additional elements. Using absorption lines in the optical spectrum of the star we measure an enrichment of more than 10 000× solar for yttrium and zirconium. The UV spectrum revealed that strontium is equally enriched. Our results confirm that Feige 46 is not only a member of the now growing group of heavy metal subdwarfs, but also has an abundance pattern that is remarkably similar to that of LS IV − 14°116.

Discovery of a second pulsating intermediate helium-enriched sdOB star

We present the discovery of long-period, low-amplitude, g-mode pulsations in the intermediate He-rich hot subdwarf (sdOB) star Feige 46. So far, only one other He-enriched sdOB star (LS IV−14 ° 116) was known to exhibit such pulsations. From our ground-based light curves of Feige 46, we extracted five independent periodicities ranging from 2294 s to 3400 s. We fit our optical spectrum of the star with our grid of non-local thermodynamic equilibrium (NLTE) model atmospheres and derived the following atmospheric parameters: Teff = 36120 ± 230 K, log g = 5.93 ± 0.04, and log N(He)/N(H) = −0.32 ± 0.03 (formal fitting errors only). These parameters are very similar to those of LS IV−14 ° 116 and place Feige 46 well outside of the instability strip where the hydrogen-rich g-mode sdB pulsators are found. We used the Gaia parallax and proper motion of Feige 46 to perform a kinematic analysis of this star and found that it likely belongs to the Galactic halo population. This is most certainly an intriguing and interesting result given that LS IV−14 ° 116 is also a halo object. The mechanism responsible for the pulsations in these two peculiar objects remains unclear, but a possible scenario involves the ϵ-mechanism. Although they are the only two members in their class of variable stars, these pulsators appear to have more in common than just their pulsation properties.

Pulsational instabilities driven by the ∈ mechanism in hot pre-horizontal branch stars

Context. The ∈ mechanism is a self-excitation mechanism of stellar pulsations that acts in regions inside the star where nuclear burning takes place. It has been shown that the ∈ mechanism can excite pulsations in models of hot pre-horizontal branch stars before they settle into the stable helium core-burning phase. Moreover, it has been shown that this mechanism could explain the shortest periods of LS IV-14°116, a mild He-sdBV star. Aims. We aim to study the ∈ mechanism in stellar models appropriate for hot pre-horizontal branch stars to predict their pulsational properties and the instability domain in the log g − log Teff plane. Methods. We performed detailed computations of non-adiabatic non-radial pulsations on stellar models during the helium subflashes just before the helium-core burning phase. Computations were carried out for different values of initial helium composition, metallicity, and envelope mass at the moment of helium flash. Results. We find an instability domain of long-period gravity modes due to the ∈ mechanism in the log g − log Teff plane at roughly 22 000 K ≲ Teff ≲ 50 000 K and 4.67 ≲ log g ≲ 6.15. Consequently, we find instabilities due to the ∈ mechanism on pre-extreme horizontal branch stellar models (Teff ≳ 22 000 K), but not on pre-blue horizontal branch stellar models (Teff ≲ 21 000 K). The periods of excited modes range between ~200 and ~2000 s. Comparison with the three known pulsating He-rich subdwarfs shows that ∈ mechanism can excite gravity modes in stars with similar surface properties (He abundances, log g, log Teff), but in our models it is only able to excite modes in the range of the shortest observed periods. Conclusions. We predict a new instability strip for hot-subdwarf stars of which LS IV-14°116 could be the first inhabitant. Based on simple estimates we expect 1 to 10 stars in the current samples of hot-subdwarf stars to be pulsating by the ∈ mechanism. Our results could constitute a theoretical basis for future searches of pulsators in the Galactic field.

Pulsational instabilities in hot pre-horizontal branch stars

The ϵ mechanism is a self-excitation mechanism of pulsations which acts on the regions where nuclear burning takes place. It has been shown that the ϵ mechanism can excite pulsations in models of hot helium-core flash, and that the pulsations of LS IV-14· 116, a He-enriched hot subdwarf star, could be explained that way. We aim to study the ϵmechanism effects on models of hot pre-horizontal branch stars and determine, if possible, a domain of instability in the log g — log T eff plane. We compute non-adiabatic non-radial pulsations on such stellar models, adopting different values of initial chemical abundances and mass of the hydrogen envelope at the time of the main helium flash. We find an instability domain of long-period (400 s ≲ P ≲ 2500 s) g -modes for models with 22000K ≲ Teff ≲ 50000K and 4.67 ≲ log g ≲ 6.15.

Time resolved spectroscopy and photometry of three little known bright cataclysmic variables: LS IV [formula omitted]08° 3, HQ Monocerotis and ST Chamaeleontis

As part of a project to better characterize comparatively bright but so far little studied cataclysmic variables in the southern hemisphere, we have obtained spectroscopic and photometric data of the nova-like variables LS IV −08° 3 and HQ Mon, and of the Z Cam type dwarf nova ST Cha. The spectra of all systems are as expected for their respective types. We derive improved orbital ephemeris of LS IV −08° 3 and map its accretion disk in the light of the Hα emission using Doppler tomography. We find that the emission has a two component origin, arising in the outer parts of the accretion disk and possibly on the illuminated face of the secondary star. The light curve of LS IV −08° 3 exhibits a low level of flickering and indications for a modulation on the orbital period. Spectroscopy of HQ Mon suggests an orbital period of ≈ 5h.15 which is incompatible with previous (uncertain) estimates. The light curves show the typical low scale flickering of UX UMa type nova-like systems, superposed upon variations on longer time scales. During one night a modulation with a period of ≈ 41m is observed, visible for at least 4 hours. However, it does not repeat itself in other nights. A spectroscopic orbital period of ≈ 5h.5 is derived for ST Cha. A previously suspected period of 6h.8 (or alternatively 9h.6), based on historical photographic photometry is incompatible with the spectroscopic period. Moreover, we show that our new as well as previous photometry does not contain evidence for the quoted photometric period.

Discovery of extremely lead-rich subdwarfs: does heavy metal signal the formation of subdwarf B stars?

Hot subdwarfs represent a group of low-mass helium-burning stars formed through binary-star interactions and include some of the most chemically-peculiar stars in the Galaxy. Stellar evolution theory suggests that they should have helium-rich atmospheres but, because radiation causes hydrogen to diffuse upwards, a majority are extremely helium poor. Questions posed include: when does the atmosphere become chemically stratified and at what rate? The existence of several helium-rich subdwarfs suggests further questions; are there distinct subgroups of hot subdwarf, or do hot subdwarfs change their surface composition in the course of evolution? Recent analyses have revealed remarkable surface chemistries amongst the helium-rich subgroup. In this paper, we analyse high-resolution spectra of nine intermediate helium-rich hot subdwarfs. We report the discovery that two stars, HE 2359-2844 and HE 1256-2738, show an atmospheric abundance of lead which is nearly ten thousand times that seen in the Sun. This is measured from optical Pb IV absorption lines never previously seen in any star. The lead abundance is ten to 100 times that measured in normal hot subdwarf atmospheres from ultraviolet spectroscopy. HE 2359-2844 also shows zirconium and yttrium abundances similar to those in the zirconium star LS IV-14 116. The new discoveries are interpreted in terms of heavily stratified atmospheres and the general picture of a surface chemistry in transition from a new-born helium-rich subdwarf to a normal helium-poor subdwarf.

ON THE CHALLENGING VARIABILITY OF LS IV-14°116: PULSATIONAL INSTABILITIES EXCITED BY THE ϵ-MECHANISM

We investigate the pulsation driving mechanism responsible for the long-period photometric variations observed in LS IV-14{\deg}116, a subdwarf B star showing a He-enriched atmospheric composition. To this end, we perform detailed nonadiabatic pulsation computations over fully evolutionary post He-core-flash stellar structure models, appropriate for hot subdwarf stars at evolutionary phases previous to the He-core burning stage. We found that the variability of LS IV-14{\deg}116 can be attributed to nonradial g-mode pulsations excited by the {\epsilon}-mechanism acting in the He-burning shells that appear before the star settles on the He-core burning stage. Even more interestingly, our results show that LS IV-14{\deg}116 could be the first known pulsating star in which the {\epsilon}-mechanism of mode excitation is operating. Last but not least, we find that the period range of destabilized modes is sensitive to the exact location of the burning shell, something that might help to distinguish between the different evolutionary scenarios proposed for the formation of this star.

An extremely peculiar hot subdwarf with a 10 000-fold excess of zirconium, yttrium and strontium

Helium-rich subdwarf B (He-sdB) stars represent a small group of low-mass hot stars with luminosities greater than those of conventional subdwarf B stars, and effective temperatures lower than those of subdwarf O stars. By measuring their surface chemistry, we aim to explore the connection between He-sdB stars, He-rich sdO stars and normal sdB stars. LS IV-14 116 is a relatively intermediate He-sdB star, also known to be a photometric variable. High-resolution blue-optical spectroscopy was obtained with the Anglo-Australian Telescope. Analysis of the spectrum shows LS IV-14 116 to have effective temperature Teff = 34 000 +/- 500 K, surface gravity log g = 5.6 +/- 0.2, and surface helium abundance nHe = 0.16 +/- 0.03 by number. This places the star slightly above the standard extended horizontal branch, as represented by normal sdB stars. The magnesium and silicon abundances indicate the star to be metal poor relative to the Sun. A number of significant but unfamiliar absorption lines were identified as being due to germanium, strontium, yttrium and zirconium. After calculating oscillator strengths (for Ge, Y and Zr), the photospheric abundances of these elements were established to range from 3 dex (Ge) to 4 dex (Sr, Y and Zr) above solar. The most likely explanation is that these overabundances are caused by radiatively-driven diffusion forming a chemical cloud layer in the photosphere. It is conjectured that this cloud formation could be mediated by a strong magnetic field.

Discovery of pulsation in a helium-rich subdwarf B star

Helium-rich subdwarf B (He-sdB) stars form a very small inhomogeneous group of subluminous stars showing varying degrees of helium enrichment. They have been found in the field of our galaxy as well as in globular clusters. Here we report the first discovery of pulsation in a He-sdB star – LS IV-14°116. Two pulsation periods can be clearly identified (1950 and 2900 s) and are more likely to be due to high-order non-radial g -mode oscillations than to radial or non-radial p -modes.

Cyclic and secular variation in the temperatures and radii of extreme helium stars

The ultraviolet properties of 17 extreme helium stars have been examined using 150 IUE spectra. Combining short-wave and long-wave image pairs and using a grid of hydrogen-deficient model atmospheres and a χ2 minimization procedure, 70 measurements of effective temperature (Teff), angular diameters (θ) and interstellar extinction were obtained. In most cases, these were in good agreement with previous measurements, but there are some ambiguities in the case of the hotter stars, where the solutions for Teff and become degenerate, and in the case of the cooler stars with large , where the total flux is no longer dominated by the ultraviolet. The behaviour of 12 helium stars was examined over an interval exceeding 10 yr. The surfaces of four stars (HD 168476, HD 160641, BD −9°4395 and BD −1°3438) were found to be heating at rates between 20 and 120 K yr−1, in remarkable agreement with theoretical predictions. This result provides the first direct evidence that extreme helium stars are helium shell-burning stars of up to ∼0.9 M⊙contracting towards the white dwarf sequence. Low-luminosity helium stars do not show a detectable contraction, also in agreement with theory, although one, BD +10°2179, may be expanding. The short-term behaviour of three variable helium stars (PV Tel variables: HD 168476, BD +1°4381, LS IV −1°2) was examined over a short interval in 1995. All three showed changes in Teff and θ on periods consistent with previous observations. Near-simultaneous radial velocity (v) measurements were used to establish the total change in radius, with some reservations concerning the adopted periods. Subsequently, measurements of the stellar radii and distances could be derived. With Teff and surface gravities established previously, stellar luminosities and masses were thus obtained directly from observation. In the case of HD 168476, the mass is Assuming a similar gravity for LS IV −1°2 based on its neutral helium line profiles, its mass becomes The θ amplitude for BD +1°4381 appears to be overestimated by the IUE measurements and leads to a nonsensical result. These first direct measurements of luminous extreme helium star masses agree well with previous estimates from stellar structure and pulsation theory.

Spectrophotometry of extreme helium stars - Ultraviolet fluxes and effective temperatures

view Abstract Citations (58) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Spectrophotometry of extreme helium stars : ultraviolet fluxes and effective temperatures. Drilling, J. S. ; Schonberner, D. ; Heber, U. ; Lynas-Gray, A. E. Abstract Ultraviolet flux distributions are presented for the extremely helium rich stars BD +10 deg 2179, HD 124448, LSS 3378, BD -9 deg 4395, LSE 78, HD 160641, LSIV -1 deg 2, BD 1 deg 3438, HD 168476, MV Sgr, LS IV-14 deg 109 (CD -35 deg 11760), LSII +33 deg 5 and BD +1 deg 4381 (LSIV +2 deg 13) obtained with the International Ultraviolet Explorer (IUE). Broadband photometry and a newly computed grid of line blanketed model atmospheres were used to determine accurate angular diameters and total stellar fluxes. The resultant effective temperatures are in most cases in satisfactory agreement with those based on broadband photometry and/or high resolution spectroscopy in the visible. For two objects, LSII +33 deg 5 and LSE 78, disagreement was found between the IUE observations and broadband photometry: the colors predict temperatures around 20,000 K, whereas the UV spectra indicate much lower photospheric temperatures of 14,000 to 15,000 K. The new temperature scale for extreme helium stars extends to lower effective temperatures than that of Heber and Schoenberner (1981) and covers the range from 8,500 K to 32,000 K. Previously announced in STAR as N83-24433 Publication: The Astrophysical Journal Pub Date: March 1984 DOI: 10.1086/161786 Bibcode: 1984ApJ...278..224D Keywords: Astronomical Spectroscopy; B Stars; Stellar Spectrophotometry; Stellar Temperature; Ultraviolet Spectra; Atmospheric Models; Iue; Stellar Atmospheres; Stellar Color; Stellar Spectra; Astronomy full text sources ADS | data products SIMBAD (17) MAST (1) INES (1)

LS IV-14 109 and BD +1 4381, two new hydrogen-deficient A-type stars.

LS IV-14 109 and BD +1 4381, two new hydrogen-deficient A-type stars.