Peculiar Stars Research Articles

Full Abundance Study of Two Newly Discovered Barium Giants

Barium (Ba) stars are chemically peculiar stars that show enhanced surface abundances of heavy elements produced by the slow-neutron-capture process, the so-called s-process. These stars are not sufficiently evolved to undergo the s-process in their interiors, so they are considered products of binary interactions. Ba stars form when a former Asymptotic Giant Branch (AGB) companion, which is now a white dwarf, pollutes them with s-process-rich material through mass transfer. This paper presents a detailed chemical characterization of two newly discovered Ba giants. Our main goal is to confirm their status as extrinsic s-process stars and explore potential binarity and white dwarf companions. We obtained high-resolution spectra with UVES on the Very Large Telescope to determine the chemical properties of the targets. We perform line-by-line analyses and measure 22 elements with an internal precision up to 0.04 dex. The binary nature of the targets is investigated through radial velocity variability and spectral energy distribution fitting. We found that both targets are enhanced in all the measured s-process elements, classifying our targets as Ba giants. This is the first time they are classified as such in the literature. Additionally, both stars present a mild enhancement in Eu, but less than in pure s-process elements, suggesting that the sources that polluted them were pure s-process sources. Finally, we confirmed that the two targets are RV variable and likely binary systems. The abundances in these two newly discovered polluted binaries align with classical Ba giants, providing observational constraints to better understand the s-process in AGB stars.

MEASNet. I. A Model for Barium Star Identification and s-process Abundance Estimation from LAMOST DR10 Low-resolution Survey

Barium stars are peculiar stars with enhanced slow neutron capture process (s-process) elements. Abundance analysis of them aids in better understanding the chemical evolution of the Milky Way. In this paper, we introduce a data-driven method named the memory-enhanced adaptive spectral network (MEASNet) to search for barium candidates in the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) low-resolution survey (LRS) and estimate the abundance of five s-process elements: Sr, Y, Ba, Ce, and Nd. MEASNet, trained using spectra from common stars in both LAMOST and the Galactic Archaeology with HERMES survey, showcases notable performance: for the classification task, precision = 98.22% and recall = 94.12%; in prediction, the mean absolute error for the seven elements range between 0.07 and 0.15 dex. After training, we apply the model to 4,083,003 stellar spectra from LAMOST DR10 LRS, successfully identifying 1,803,670 spectra of barium candidates ([Ba/Fe] ≥ 0.25 dex) along with their five s-process elemental abundances. The catalog enlarges the sample size, providing a wealth of data for further statistical analysis of the formation and evolution of barium stars. Meanwhile, this work highlights the potential value of MEASNet in star classification and abundance estimation, offering a strong reference for future data-driven models.

Asteroseismology of the mild Am δ Sct star HD 118660 : TESS photometry and modelling

Abstract We present the results of an asteroseismic study of HD 118660 (TIC 171729860), being a chemically peculiar (mild Am) star exhibiting δ Scuti (δ Sct) pulsations. It is based on the analysis of two sectors of time-series photometry from the space mission TESS and seismic modelling. It yielded the detection of 15 and 16 frequencies for TESS sectors 23 and 50, respectively. The identified pulsation modes include four radial (ℓ = 0) and five dipolar (ℓ = 1) ones. The radial modes are overtones with order n ranging from 3 and 6. Such high values of n are theoretically not expected for stars with the effective temperature of HD 118660 ($\rm T_{\rm eff}\approx 7550 \rm K$ ) located near the red edge of the δ Sct instability strip. To estimate the asteroseismic parameters, we have generated a grid of stellar models assuming a solar metallicity (Z = 0.014) and different values for the convective overshooting parameter (0.1 ≤ αov ≤ 0.3). We conclude that the analysis of the radial modes is insufficient to constrain αov and Z for δ Sct stars. The value for the equatorial velocity of HD 118660 derived from the seismic radius and the rotational frequency is consistent with values found in the literature.

Slowly Rotating Peculiar Star BD00°1659 as a Benchmark for Stratification Studies in Ap/Bp Stars

We present the results of a self-consistent analysis of the magnetic silicon star BD+00°1659, based on its high-resolution spectra taken from the ESPaDOnS archive (R = 68,000). This narrow-lined star shows the typical high Si abundance and Si ii– iii anomaly, making it an ideal prototype for investigating the vertical distribution of Si and Fe in the stellar atmosphere. The derived abundances, ranging from helium to lanthanides, confirm the star’s classification as a silicon Bp spectral type. Silicon and iron are represented by lines of different ionisation stages (Fe i– iii, Si i– iii), indicating an ionisation imbalance interpreted as evidence of atmospheric stratification. Our stratification analysis reveals that there is a jump in iron and silicon abundances of 1.5 dex at atmospheric layers with an optical depth of logτ5000 = −0.85–−1.00. Non-LTE calculations for iron in this stratified atmosphere show minor non-LTE effects. Our results can be applied to studying the impact of stratification on the emergent flux in rapidly rotating Si stars with similar atmospheric parameters and abundance anomalies (for example, MX TrA), where direct stratification analysis is challenging due to line blending.

Long-term spectral variations of V Sagittae: Two spectroscopic states with red- or blue-shifted emission wings of H i and He ii

Abstract The spectral variation in the optical region of a peculiar variable star V Sagittae was monitored in the recent three decades. The main aim of this article is to report several interesting phenomena discovered in the monitoring; we briefly discuss interpretations of them. Emission lines of H i and He ii were decomposed into narrow components and wide wings. The radial velocities of the wings were related to the orbital motion of the hot primary, but did not reflect it directly. Widely red-shifted sinusoidal variations according to the orbital period were detected in 1993–2005 (e.g., $K = 195$ km s$^{-1}$ and $\gamma = +252$ km s$^{-1}$ for He ii 4686). Sporadic blue-shifts were also observed. The mode of the radial velocity variation drastically changed between 2005 and 2007. Red-shifted sinusoidal variations ($K = 224$ km s$^{-1}$ and $\gamma = +260$ km s$^{-1}$ idem) or blue-shifted ones ($K =206$ km s$^{-1}$ and $\gamma = -289$ km s$^{-1}$ idem) appeared alternately in 2007–2019. One red- or blue-shift state lasted several years. The radial velocities of O iii 5592 and the doublet C iv 5805 exhibited sinusoidal variations without large red- or blue-shifts (e.g., $K =215$ km s$^{-1}$ and $\gamma = -18$ km s$^{-1}$ for C iv 5805). The widths of emission lines He ii 5412 and C iv in the doublet 5805 suggest that helium and carbon ions contained nearly the same kinetic energy, because the former line was wider than the latter by a factor of about $\surd {3}$ on most spectra. It is probable that there was no large-scale mass motion in such a thermal-equilibrium-like condition. The C iv lines and He ii 5412 had roughly same widths, like the spectra of Wolf–Rayet stars, only three times in 2007–2019 on which there were likely large-scale mass outflows.

Magnetism in LAMOST CP stars observed by TESS

A thousand new magnetic candidate CP stars have been identified with LAMOST, of which sim 700 prime targets have had rotational modulation determined by TESS. We aim to check for the presence of a magnetic field in a subsample of these LAMOST CP stars, test the viability of the 5200 depression used to select the mCP candidates in the LAMOST survey as a reliable indicator of magnetism, and expand on the limited database of known magnetic hot stars. The sample includes some pulsators that would be valuable targets for magneto-asteroseismology. We selected sim 100 magnetic candidate LAMOST CP stars that present a depression at 5200 in their spectrum and that also display rotational modulation in their TESS photometric light curves. We obtained spectropolarimetric observations of 39 targets from this sample with ESPaDOnS at CFHT . We utilised the least squares deconvolution method to generate the mean profile of the Stokes V and I parameters, from which the longitudinal magnetic field strength for each target can be determined. For HD\,49198, we performed more in-depth analysis to determine the polar magnetic field strength and configuration. We detect fields in at least 36 of the 39 targets in our sample. This success rate in detecting a magnetic field (above 92<!PCT!>) is very high compared to the occurrence of magnetic fields in hot stars (sim 10<!PCT!>). Four of these newly discovered magnetic stars are magnetic pulsators. In particular, we detect the strongest field around a delta Scuti star discovered to date: a 12 kG dipolar field in HD\,49198. From our analysis, we conclude that using the 5200 depression displayed in the spectra in combination with rotational modulation in photometric data is a very reliable method of identifying magnetic candidates in this population of stars.

Chemically peculiar stars on the pre-main sequence

Context. The chemically peculiar (CP) stars of the upper main sequence are defined by spectral peculiarities that indicate unusual elemental abundance patterns in the presence of diffusion in the calm, stellar atmospheres. Some of them have a stable local magnetic field of up to several kiloGauss. The pre-main-sequence evolution of these objects is still a mystery and contains many open questions. Aims. We identify CP stars on the pre-main sequence to determine possible mechanisms that lead to the occurrence of chemical peculiarities in the (very) early stages of stellar evolution. Methods. We identified likely pre-main-sequence stars by fitting the spectral energy distributions. The subsequent analysis using stellar spectra and photometric time series helped us to distinguish between CP and non-CP stars. Additionally, we compared our results to the literature to provide the best possible quality assessment. Results. Out of 45 candidates, about 70% seem to be true CP stars or CP candidates. Furthermore, 9 sources appear to be CP stars on the pre-main sequence, and all are magnetic. We finally report a possible CP2 star that is also a pre-main-sequence star and was not previously in the literature. Conclusions. The evolution of the peculiarities seems to be related to the (strong) magnetic fields in these CP2 stars.

New ACV variables discovered in the Zwicky Transient Facility survey

The manifestation of surface spots on magnetic chemically peculiar (mCP) stars is most commonly explained by the atomic diffusion theory, which requires a calm stellar atmosphere and only moderate rotation. While very successful and well described, this theory still needs to be revised and fine-tuned to the observations. Our study aims to enlarge the sample of known photometrically variable mCP stars (ACV variables) to pave the way for more robust and significant statistical studies. We derive accurate physical parameters for these objects and discuss our results in the framework of the atomic diffusion theory. We studied 1314 candidate ACV variables that were selected from the Zwicky Transient Factory catalogue of periodic variables based on light curve characteristics. We investigated these objects using photometric criteria, a colour-magnitude diagram, and spectroscopic data from the LAMOST and Gaia missions to confirm their status as ACV variables. We present a sample of 1232 new ACV variables, including information on distance from the Sun, mass, fractional age on the main sequence, fraction of the radius between the zero-age and terminal-age main sequence, and the equatorial velocity and its ratio to the critical velocity. Our results confirm that the employed selection process is highly efficient for detecting ACV variables. We have identified 38 stars with $v_ equ $). This challenges current theories that cannot explain the occurrence of such fast-rotating mCP stars.

The Abundance of Chlorine in the Atmosphere of HD 87240

High resolution high signal-to-noise UVES spectra of the late B-type chemically peculiar star, HD 87240, a member of the NGC 3114 open cluster, have been synthesized to measure the abundance of chlorine, an element which is rarely found in late B-type stars. Sixteen lines of Cl ii are found and the mean chlorine overabundance is about 2.1 dex higher than the solar abundance, which is slightly lower than the chlorine excess in HR 1094, another chlorine-rich late B-type star.

New evidence of binarity in young α-rich turn-off and subgiant stars: fast rotation and strong magnetic activity

ABSTRACT Young α-rich (YAR) stars within the old Galactic thick disc exhibit a dual characteristic of relative youth determined with asteroseismology and abundance enhancement in α elements measured from high-resolution spectroscopy. The youth origin of YAR stars has been proposed to be binary evolution via mass transfer or stellar mergers. If that is the case, YAR stars should spin rapidly and thus be magnetically active, because they are mass and angular momentum gainers. In this study, to seek this binary footprint, we select YAR stars on the main-sequence turn-off or the subgiant branch (MSTO-SGB) from APOGEE DR17, whose ages and projected rotation velocities (vsin i) can be precisely measured. With APOGEE vsin i and LAMOST spectra, we find that YAR stars are indeed fast rotators and magnetically active. In addition, we observe low [C/N] ratios and high Gaia RUWE in some YAR stars, suggesting that these MSTO-SGB stars probably have experienced mass transfer from red-giant companions. Our findings underscore that magnetic activity can serve as a valuable tool for probing the binary evolution for other chemically peculiar stars, such as red giants with lithium anomalies and carbon-enhanced metal-poor stars.

The astrophysical parameters of chemically peculiar stars from automatic methods

Context. The chemically peculiar (CP) stars of the upper main sequence are excellent astrophysical laboratories for investigating the diffusion, mass loss, rotational mixing, and pulsation in the presence and absence of a stable local magnetic field. For this, we need a homogeneous set of parameters, such as effective temperature (Teff) and surface gravity (log g), to locate the stars in the Hertzsprung-Russell diagram so that we can then estimate the mass, radius, and age. Aims. In recent years, the results of several automatic pipelines have been published; these use various techniques and data sets, including Teff and log g values for millions of stars. Because CP stars are known to have flux anomalies, these astrophysical parameters must be tested for their reliability and usefulness. If the outcome is positive, these can be used to analyse the new and faint CP stars published recently. Methods. I compared published Teff and log g values of a set of CP stars, which are mostly based on high-resolution spectroscopy, with values from four automatic pipeline approaches. In doing so, I searched for possible correlations and offsets. Results. I present a detailed statistical analysis of a comparison between the ‘standard’ and published Teff and log g values. The accuracy depends on the presence of a magnetic field and the spectral type of the CP subgroups. However, I obtain standard deviations of between 2% and 20%. Conclusions. Considering the statistical errors, the astrophysical parameters from the literature can be used for CP stars, although caution is advised for magnetic CP stars.

Seventeen 2 Micron All Sky Survey (2MASS) Hypervelocity Stars (HVS) from Gaia DR3

As part of an ongoing search for hypervelocity stars I found seventeen two micron all sky survey sources with Gaia G magnitudes less than 16.0 and radial velocities less than −600 km s−1. All these stars are brighter in the K band when compared with their V and G magnitudes. Ten of these (including three carbon stars) are long period variable stars of Mira type. One is a relatively nearby high proper motion star and one is a very high galactic latitude chemically peculiar metal-poor star. It may be a galactic halo star. One star is a Kepler red giant, two stars may be cluster members and two are in the star-forming region (probably YSOs). It is not clear how these stars acquired such high radial velocities. Further study of these seventeen stars is needed.

TESS Exploration of Targets Investigated for the Nainital-Cape Survey Project

The Nainital-Cape Survey was initiated more than two decades ago aiming to search for and study the pulsational variability in two subclasses of chemically peculiar (CP) stars, namely the Ap and Am stars. In this paper, we present the TESS photometry of 4 targets out of the 381 sample stars observed under the survey, which were not studied before using TESS data. Our results suggest that HD34060 is a rotational variable, HD 25487 is of eclipsing nature, HD 15550 exhibits pulsational variability while HD48953 is a non-variable star. The diverse variability detected in the studied sources places important constraints for the study of the internal structure and evolution of the CP stars in the presence of surface inhomogeneity, magnetic field, rotation and pulsation.

A Morphokinematic Study of the Enigmatic Emission Nebula NGC 6164/5 Surrounding the Magnetic O-type Star HD 148937

HD 148937 is a peculiar massive star (Of?p) with a strong magnetic field (1 kG). The hourglass-shaped emission nebula NGC 6164/5 surrounds this star. This nebula is presumed to originate from episodic mass-loss events of the central O-type star, but the detailed formation mechanism is not yet well understood. Grasping its three-dimensional structure is essential to uncovering the origin of this nebula. Here we report the high-resolution multiobject spectroscopic observations of NGC 6164/5 using the GIRAFFE on the 8.2 m Very Large Telescope. Integrated intensity maps constructed from several spectral lines delineate well the overall shape of this nebula, such as the two bright lobes and the inner gas region. The position–velocity diagrams show that the two bright lobes are found to be redshifted and blueshifted, respectively, while the inner region has multiple layers. We consider a geometric model composed of a bilateral outflow harboring nitrogen-enriched knots and expanding inner shells. Its spectral features are then simulated by using a Monte Carlo radiative transfer technique for different sets of velocities. Some position–velocity diagrams from simulations are very similar to the observed ones. According to the model that best reproduces the observational data, the two bright lobes and the nitrogen-enriched knots are moving away from HD 148937 at about 120 km s−1. Their minimum kinematic age is estimated to be about 7500 yr. We discuss possible formation mechanisms of this nebula in the context of binary interaction.

Period and Surface Spot Analysis of Magnetically Chemically Peculiar Stars with CoRoT Data

Bu çalışmada, manyetik özellikli kimyasal sıra dışı (mCP) HD 171586 ve HD 170973 yıldızlarının CoRoT uyudu gözlem verilerine dayalı fotometrik analizleri gerçekleştirildi. Periyot analizlerinde Period04 (Lenz ve Breger 2005) ve Lomb- Scargle (Lomb 1976, Scargle 1982) programları kullanılarak yıldızların dönme periyotları belirlendi. İncelenen manyetik özellikli A tayf türüne sahip kimyasal sıra dışı iki yıldızın oldukça duyarlı, kesintisiz, uzun zaman aralığını kapsayan gözlemsel verilerinden tespit edilen fotometrik ışık değişimleri değerlendirilerek, Dorren (Dorren 1987) ve M acula (Kipping 2012) programları ile yüzey element bolluk dağılımları ilk kez incelendi. HD 171586 ve HD 170973 yıldızları için, sırasıyla, üç ve iki sıcak lekeye uyumlu yüzey haritalı modellerin uygun olduğu anlaşıldı.

Doppler imaging of a southern ApSi star HD 152564

ABSTRACT We present the results of the spectroscopic study of a chemically peculiar star HD 152564. Using medium-resolution (R = 37 000) observations obtained with the high-resolution fibre échelle spectrograph mounted on the South African Large Telescope, we determined atmospheric parameters Teff = 11 950 ± 200 K and log g = 3.6 ± 0.2 dex. Abundance analysis revealed mild deficiency of the light elements and an overabundance of up to ∼2 dex of metals, with the greatest excess for silicon. With these characteristics, HD 152564 is a typical member of the silicon subgroup of Ap stars. The rotational modulation of the light curve and line profiles of HD 152564 are typical for the inhomogeneous surface distribution of elements in its atmosphere. We performed multi-element Doppler imaging of the surface of HD 152564. Abundance maps constructed for He, O, Mg, Si, and Fe revealed the concentration of these elements in a sequence of equatorial spots as well as in the circumpolar rings. The photometric maximum of the light curve coincided with the visibility of two most overabundant silicon spots. Abundances determined from the different ionization stages of Fe and Si show clear evidence for vertical stratification of these elements in the atmosphere of HD 152564. Meanwhile, the horizontal distribution of silicon reconstructed from the lines of different ionization stages and excitation energies appears to be identical with increasing average abundance deeper in atmosphere.

TESS Cycle 2 observations of roAp stars with 2-min cadence data

ABSTRACT We present the results of a systematic search of the Transiting Exoplanet Survey Satellite (TESS) 2-min cadence data for new rapidly oscillating Ap (roAp) stars observed during the Cycle 2 phase of its mission. We find seven new roAp stars previously unreported as such and present the analysis of a further 25 roAp stars that are already known. Three of the new stars show multiperiodic pulsations, while all new members are rotationally variable stars, leading to almost 70 per cent (22) of the roAp stars presented being α2 CVn-type variable stars. We show that targeted observations of known chemically peculiar stars are likely to overlook many new roAp stars, and demonstrate that multiepoch observations are necessary to see pulsational behaviour changes. We find a lack of roAp stars close to the blue edge of the theoretical roAp instability strip, and reaffirm that mode instability is observed more frequently with precise, space-based observations. In addition to the Cycle 2 observations, we analyse TESS data for all-known roAp stars. This amounts to 18 further roAp stars observed by TESS. Finally, we list six known roAp stars that TESS is yet to observe. We deduce that the incidence of roAp stars amongst the Ap star population is just 5.5 per cent, raising fundamental questions about the conditions required to excite pulsations in Ap stars. This work, coupled with our previous work on roAp stars in Cycle 1 observations, presents the most comprehensive, homogeneous study of the roAp stars in the TESS nominal mission, with a collection of 112 confirmed roAp stars in total.

The Behavior of Bp He-weak Stars in the Far UV—Paper XIV: HD 143699

The analysis of seven spectra of the B6 He-weak star HD 143699 recorded by the SWP camera on board the International Ultraviolet Explorer reveals variations of the far ultraviolet spectral energy distribution shortwards of 1600 Å in spectra collected from 1994 April 12 to July 9. These variations do not seem to reflect rotational modulation. They could be caused by pulsation with the recently derived pulsation period close to 4 hr. The resonance line of Si ii at 1526.71 Å clearly shows an interstellar component. There is no evidence of C iv absorption at 1550 Å in any of the spectra.

A self-synthesized origin for heavy metals in hot subdwarf stars

Context. A number of He-rich hot subdwarf stars present high abundances for trans-iron elements, such as Sr, Y, Zr, and Pb. Diffusion processes are important in hot subdwarf stars and it is generally believed that the high abundances of heavy elements in these peculiar stars are due to the action of radiative levitation. However, during the formation of He-rich hot subdwarf stars, hydrogen can be ingested into the convective zone driven by the He-core flash. It is known that episodes of protons being ingested into He-burning convective zones can lead to neutron-capture processes and the formation of heavy elements. Aims. In this work, we aim to explore, for the first time, whether neutron-capture processes can occur in late He-core flashes taking place in the cores of the progenitors of He-rich hot subdwarfs. We aim to explore the possibility of a self-synthesized origin for the heavy elements observed in some He-rich hot subdwarf stars. Methods. We computed a detailed evolutionary model for a stripped red-giant star using a stellar evolution code with a nuclear network comprising 32 isotopes. Then we post-processed the stellar models in the phase of helium and hydrogen burning using a post-processing nucleosynthesis code with a nuclear network of 1190 species, which allowed us to follow the neutron-capture processes in detail. Results. We find the occurrence of neutron-capture processes in our model, with neutron densities reaching a value of ∼5 × 1012 cm−3. We determined that the trans-iron elements are enhanced in the surface by 1 to 2 dex, as compared to initial compositions. Moreover, the relative abundance pattern [Xi/Fe] produced by neutron-capture processes closely resembles those observed in some He-rich hot subdwarf stars, hinting at a possible self-synthesized origin for the heavy elements in these stars. Conclusions. We conclude that intermediate neutron-capture processes can occur during a proton ingestion event in the He-core flash of stripped red-giant stars. This mechanism offers a natural channel for the production of the heavy elements observed in certain He-rich hot subdwarf stars.

Abundances of Iron-peak Elements in the B8 Mn Star HD 110073

The synthesis of spectral intervals in the optical range allows the determination of LTE abundances of several iron-peak elements for the chemically peculiar B8 Mn star HD 110073. Titanium, chromium and manganese are found to be overabundant by factors of 19, 4 and 250 times their respective solar abundances. Iron and nickel are deficient by factors of 0.25 and 0.10. Weak lines of Cr ii formed deep in the atmosphere yield lower chromium abundances than strong lines do by a factor of 2. This supports that chromium is probably stratified vertically in the atmosphere of HD 110073. Several weak flat-bottomed lines which could be caused by vertical stratification of the abundances are present throughout the spectra. The new abundance determinations differ from previous determinations partly because the atomic data have changed. A non-uniform distribution of the elements over the surface of HD 110073 could also account for variation of the lines with time.