Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userEC 22536-5304: SALT identifies a new lead-rich intermediate helium subdwarf
76
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
SALT spectra of the helium-rich hot subdwarf EC22536-5304 show strong absorption lines of triply-ionized lead. Analysis of the HRS spectrum and a follow-up SALT/RSS spectrum show EC22536-5304 to have surface properties (temperature, gravity, helium/hydrogen ratio) similar to other heavy-metal subdwarfs. With a lead overabundance of 4.8 dex relative to solar, EC22536-5304 is the most lead-rich intermediate helium subdwarf discovered so far.
rate research
Read More
The hydrogen-deficient star DY Cen has been reported as an R CrB-type variable, an extreme helium star (with some hydrogen), and as a single-lined spectroscopic binary. It has been associated with a dramatic change in visual brightness and colour corresponding to a change in effective temperature ($T_{mathrm eff}$) of some 20,000 K in the last century. To characterize the binary orbit and $T_{mathrm eff}$ changes more precisely, new high-resolution spectroscopy has been obtained with SALT. The previous orbital period is not confirmed; previous measurements may have been confused by the presence of pulsations. Including data from earlier epochs (1987, 2002, and 2010), self-consistent spectral analyses from all four epochs demonstrate an increase in $T_{mathrm eff}$ from 18,800 to 24,400 K between 1987 and 2015. Line profiles demonstrate that the surface rotation has increased by a factor two over the same interval. This is commensurate with the change in $T_{mathrm eff}$ and an overall contraction. Rotation will exceed critical if contraction continues. The 1987 spectrum shows evidence of a very high abundance of the s-process element strontium. The very rapid evolution, non-negligible surface hydrogen and high surface strontium point to a history involving a very late thermal pulse. Observations over the next thirty years should look for a decreasing pulsation period, reactivation of R CrB-type activity as the star seeks to shed angular momentum and increasing illumination by emission lines from nebular material ejected in the past.
A medium- and high-resolution spectroscopic survey of helium-rich hot subdwarfs is being carried out using the Southern African Large Telescope (SALT). Objectives include the discovery of exotic hot subdwarfs and of sequences connecting chemically-peculiar subdwarfs of different types. The first phase consists of medium-resolution spectroscopy of over 100 stars selected from low-resolution surveys. This paper describes the selection criteria, and the observing, classification and analysis methods. It presents 107 spectral classifications on the MK-like Drilling system and 106 coarse analyses ($T_{rm eff}, log g, log y$) based on a hybrid grid of zero-metal non-LTE and line-blanketed LTE model atmospheres. For 75 stars, atmospheric parameters have been derived for the first time. The sample may be divided into 6 distinct groups including the classical `helium-rich sdO stars with spectral types (Sp) sdO6.5 - sdB1 (74) comprising carbon-rich (35) and carbon-weak (39) stars, very hot He-sdOs with Sp $lesssim$ sdO6 (13), extreme helium stars with luminosity class $lesssim 5$ (5), intermediate helium-rich subdwarfs with helium class 25 -- 35 (8), and intermediate helium-rich subdwarfs with helium class $10 - 25$ (6). The last covers a narrow spectral range (sdB0 -- sdB1) including two known and four candidate heavy-metal subdwarfs. Within other groups are several stars of individual interest, including an extremely metal-poor helium star, candidate double-helium subdwarf binaries, and a candidate low-gravity He-sdO star.
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.
About 1% of giant stars have been shown to have large surface Li abundances, which is unexpected according to standard stellar evolution models. Several scenarios for lithium production have been proposed, but it is still unclear why these Li-rich giants exist. A missing piece in this puzzle is the knowledge of the exact stage of evolution of these stars. Using low-and-high-resolution spectroscopic observations, we have undertaken a survey of lithium-rich giants in the Kepler field. In this letter, we report the finding of the first confirmed Li-rich core-helium-burning giant, as revealed by asteroseismic analysis. The evolutionary timescales constrained by its mass suggest that Li-production most likely took place through non-canonical mixing at the RGB-tip, possibly during the helium flash.
We announce the discovery of a new eclipsing hot subdwarf B + M dwarf binary, EC 10246-2707, and present multi-colour photometric and spectroscopic observations of this system. Similar to other HW Vir-type binaries, the light curve shows both primary and secondary eclipses, along with a strong reflection effect from the M dwarf; no intrinsic light contribution is detected from the cool companion. The orbital period is 0.1185079936 +/- 0.0000000009 days, or about three hours. Analysis of our time-series spectroscopy reveals a velocity semi-amplitude of K_1 = 71.6 +/- 1.7 km/s for the sdB and best-fitting atmospheric parameters of Teff = 28900 +/- 500 K, log g = 5.64 +/- 0.06, and log[N(He)/N(H)] = -2.5 +/- 0.2. Although we cannot claim a unique solution from modeling the light curve, the best-fitting model has an sdB mass of 0.45 Msun and a cool companion mass of 0.12 Msun. These results are roughly consistent with a canonical-mass sdB and M dwarf separated by a ~ 0.84 Rsun. We find no evidence of pulsations in the light curve and limit the amplitude of rapid photometric oscillations to < 0.08%. Using 15 years of eclipse timings, we construct an O-C diagram but find no statistically significant period changes; we rule out |P-dot| > 7.2 x 10^(-12). If EC 10246-2707 evolves into a cataclysmic variable, its period should fall below the famous CV period gap.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
