Peculiar Stars Research Articles

Confident detection of doubly ionized thorium in the extreme Ap star CPD-62° 2717

ABSTRACT Despite the Universe containing primordial thorium (Th) of sufficient abundance to appear in stellar spectra, detection of Th has to date been tentative and based on just a few weak and blended lines. Here, we present convincing evidence not only for the first Th detection in a magnetic chemically peculiar Ap star but also for the first detection of Th iii in a stellar spectrum. CPD-62° 2717 was initially recognized as a highly magnetized Ap star due to resolved magnetically split lines captured in H-band spectra from the SDSS/APOGEE survey. The star was subsequently pinpointed as extraordinarily peculiar when careful inspection of the H-band line content revealed the presence of five lines of Th iii, none of which are detected in the other ∼1500 APOGEE-observed Ap stars. Follow-up with the VLT + UVES confirmed a similarly peculiar optical spectrum featuring dozens of Th iii lines, among other peculiarities. Unlike past claims of Th detection, and owing to high-resolution observations of the strong (∼8–12 kG) magnetic field of CPD-62° 2717, the detection of Th iii can in this case be supported by matches between the observed and theoretical magnetic splitting patterns. Comparison of CPD-62° 2717 to stars for which Th overabundances have been previously reported (e.g. Przybylski’s Star) indicates that only for CPD-62° 2717 is the Th detection certain. Along with the focus on Th iii, we use time series measurements of the magnetic field modulus to constrain the rotation period of CPD-62° 2717 to ∼4.8 yr, thus establishing it as a new example of a superslowly rotating Ap star.

A runaway T Tauri star leaving an extended trail

Aims.We address the problem of young stellar objects that are found too far away from possible star formation sites. Different mechanisms have been proposed before to explain this unexpected circumstance. The idea of high-velocity protostars is one of these mechanisms, although observational support is not always easy to obtain. We aim to shed light on this issue after the serendipitous discovery of a related stellar system.Methods.Following the inspection of archival infrared data, a peculiar anonymous star was found that apparently heads a long tail that resembles a wake-like feature. We conducted a multiwavelength analysis including photometry, astrometry, and spectroscopy. Together with theoretical physical considerations, this approach provided a reasonable knowledge of the stellar age and kinematic properties, together with compelling indications that the extended feature is indeed the signature of a high-velocity, or runaway, newborn star.Results.Our main result is the discovery of a low-mass young stellar object that fits the concept of a runaway T Tauri star that was hypothesized several decades ago. In this peculiar star, nicknamed UJT-1, the interaction of the stellar wind with the surrounding medium becomes extreme. Under reasonable assumptions, this unusual degree of interaction has the potential to encode the mass-loss history of the star on timescales of several ∼105years.

A New Catalog of Am-type Chemically Peculiar Stars Based on LAMOST

A total of about 21,600 Am candidates were detected with the MKCLASS code based on the low-resolution spectra of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope Data Releases 8 (v1.0), 9 (v0), and 10 (v0), which greatly expands the database of Am-type stars. By crossmatching the known catalogs of Am stars and our Am candidates with the AAVSO International Variable Star Index catalog, a catalog of the largest eclipsing Am binaries was obtained, which includes 754 binaries and provides a substantial sample with which to study Am stars. Fundamental information, including atmospheric parameters of the two kinds of candidates, are collected and listed in the catalogs, such as temperature, log g, and metallicity. We further carried out statistical analysis of the types of parameters. In our new catalog, there are some Am candidates with a temperature lower than 7000 K and some eclipsing Am binary candidates with a period of less than 1 day, which poses challenges to the slow rotation of stars classified as type Am observed in previous research. These candidates are significant and provide a great opportunity to explore the real relationship between Am-type stars’ peculiarity and slow stellar rotation. The Hertzsprung–Russell diagram of Am stars and the primary stars of eclipsing Am binary candidates indicate that a majority of the Am stars and almost all primary stars of binaries are in or around the main-sequence evolution stage.

Analysis of integral field spectroscopy observations of the planetary nebula Hen 2-108 and its central star

ABSTRACT The study of planetary nebulae provides important constraints for many aspects of stellar and Galactic evolution. Hen 2-108 is a poorly known planetary nebula with a slight elliptical morphology and a peculiar central star (CS), which has defied classification. In this work, we present the first detailed integral field spectroscopic study of the planetary nebula Hen 2-108 and its CS. We provide spatially resolved flux maps for important emission lines, as well as diagnostic maps of extinction and electronic density and temperature. Physical conditions and chemical abundances were also calculated from the integrated spectrum. The analysis was also performed with the code satellite that uses a distinct strategy to evaluate physical and chemical properties. Both satellite and traditional procedure give consistent results, showing some variation in physical and chemical properties. We detect and measure a number of faint heavy element recombination lines from which we find a significant abundance discrepancy factor for O/H, and possibly for N/H. Pseudo-3D photoionization models were used to assist in the interpretation with results supporting the low-ionization nature of this nebula, indicating a CS with Teff = 40 kK and a shell structure. The spectrum of the CS has been analysed with a detailed model for expanding atmospheres to infer stellar parameters, finding that it is a [Of/WN8] type with T* = 41.5 kK, making it a new addition to a small set (∼20) of rare objects.

A Possible Surviving Companion of the SN Ia in the Galactic SNR G272.2-3.2

We use the Gaia EDR3 to explore the Galactic supernova remnant (SNR) G272.2-3.2, produced by the explosion of a Type Ia supernova (SN Ia) about 7500 yr ago, to search for a surviving companion. From the abundances in the SNR ejecta, G272.2-3.2 is a normal SN Ia. The Gaia parallaxes allow us to select the stars located within the estimated distance range of the SNR, and the Gaia proper motions allow us to study their kinematics. From the Gaia EDR3 photometry, we construct the H-R diagram of the selected sample, which we compare with the theoretical predictions for the evolution of possible star companions of SNe Ia. We can discard several proposed types of companions by combining kinematics and photometry. We can also discard hypervelocity stars. We focus our study on the kinematically most peculiar star, Gaia EDR3 5323900215411075328 (hereafter MV-G272), an 8.9σ outlier in proper motion. It is of M1–M2 stellar type. Its trajectory on the sky locates it at the center of the SNR, 6000–8000 yr ago, a unique characteristic among the sample. Spectra allow a stellar parameter determination and a chemical abundance analysis. In conclusion, we have a candidate to be the surviving companion of the SN Ia that resulted in SNR G272.2-3.2. It is supported by its kinematical characteristics and its trajectory within the SNR. This opens the possibility of a single-degenerate scenario for an SN Ia with an M-type dwarf companion.

The Behavior of Bp He-weak Stars in the Far UV—Paper 2: HD 125823

The analysis of seventeen spectra of the Bp He-weak star HD 125823 recorded by the SWP camera on board the International Ultraviolet Explorer reveals no variations of the far-ultraviolet spectral energy distribution of this star over a time interval of three years. However line variations are present in the range 1400 up to 1600 Å which can be partly attributed to variations of lines of Si ii and Si iii. This confirms that silicon is not distributed in a uniform manner over the surface but the horizontal gradients of the silicon abundance are probably not large.

A candidate magnetic helium-core white dwarf in the globular cluster NGC 6397

ABSTRACT We report a peculiar variable blue star in the globular cluster (GC) NGC 6397, using Hubble Space Telescope optical imaging. Its position in the colour–magnitude diagrams and its spectrum are consistent with this star being a helium-core white dwarf (He WD) in a binary system. The optical light curve shows a periodicity at 18.5 h. We argue that this periodicity is due to the rotation of the WD and possibly due to magnetic spots on the surface of the WD. This would make this object the first candidate magnetic He WD in any GC, and the first candidate magnetic WD in a detached binary system in any GC and one of the few He WDs with a known rotation period and of magnetic nature. Another possibility is that this system is an He WD in a binary system with another WD or another degenerate object, which would make this object one of the few candidate non-accreting double-degenerate binaries in any GC.

The Behavior of He-weak in the Far-UV—Paper I: HD 5737

The analysis of 28 spectra of the bright B6 He-weak star HD 5737 recorded by the SWP camera on board the International Ultraviolet Explorer reveals one instance of marginal variation of the far-ultraviolet flux of this star from 1300 up to 1700 Å. All the other large aperture spectra show a remarkably constant spectral energy distribution. Over the 6 yr of the IUE mission, HD 5737 has experienced at least two episodes of very strong absorption in the C iv resonance lines at 1548.19 and 1550.77 Å,

Evidence of extra mixing in field giants as traced by the lithium and carbon isotope ratio

Context.Although not predicted by standard stellar evolution, the surface abundance of light elements, such as lithium (Li), carbon, and nitrogen, changes during the red giant branch (RGB) as a result of extra mixing. This is usually associated with thermohaline mixing acting after the RGB bump. Peculiar Li-enriched RGB stars might also be related to either enhanced mixing or pollution from external sources.Aims.We measure the Li abundance and carbon isotopic ratio12C/13C in a sample of 166 field red giants with −0.3 ≤ [Fe/H] ≤ 0.2, targeted by the EXPRESS radial velocity program to analyze the effects of extra mixing.Methods.We measured the abundances with spectral synthesis using high-quality spectra. Multiple-epoch observations needed for exoplanet detection were used to decrease the effects of telluric contamination in12C/13C measurements.Results.Due to the prevalence of upper limits, the Li abundance pattern is complicated to interpret, but the comparison between RGB and core He-burning giants shows effects of mixing consistent with thermohaline. The most Li-enriched giant in the sample, classified as a RGB star close to the RGB bump, has low12C/13C. Given that the12C/13C should not be affected by planet engulfment, this does not seem to be the source of the high Li. There is a decreasing correlation between mass and12C/13C in the RGB and an increasing correlation in the horizontal branch, which, once again, is consistent with thermohaline mixing. Our data also show a correlation between12C/13C and [Fe/H]. There is no evident impact of binarity either on Li or on12C/13C.Conclusions.Our sample shows behavior consistent with additional mixing acting after the RGB bump. The12C/13C adds new clues which can be used to describe extra mixing, and it could well be the best tool to study mixing in giants. Additional measurements of12C/13C in field stars would greatly improve our ability to compare data with models and understand mixing mechanisms.

BD +30°549: a young helium-weak silicon star in the NGC 1333 star-forming region

ABSTRACT We present results of the spectroscopic study of the chemically peculiar star BD +30°549, which is a bona fide member of the young NGC 1333 star-forming region. We found that the star possesses negligible rotation and a helium-weak spectroscopic pattern with strongly enhanced Si ii and Si iii lines. The fundamental parameters of the star, Teff = 13 100 K and log (L/L⊙) = 2.1, indicate that its age is about 2.7 Myr and that its position on the Hertzsprung–Russell diagram is at the end of the pre-main-sequence evolutionary track, close to the zero-age main sequence. Abundance analysis reveals the modest deficit of almost all elements, with the exception of Si , Fe , Ca and P, which are overabundant. We performed non-local thermodynamic equilibrium (non-LTE) calculations for Si ii/Si iii, Mg ii and Ca ii in order to check the influence of departures from LTE on line formation. Non-LTE calculations lead to much better reproduction of individual silicon line profiles, but do not completely remove the abundance discrepancy between Si ii and Si iii lines. We also investigate the effects of possible chemical stratification in BD +30°549. We suspect that the ‘Si ii/Si iii anomaly’ observed in the BD +30°549 spectrum arises under the combined action of the vertical and horizontal abundance gradients and non-LTE effects. We suppose that the evolutionary status and phenomena observed in BD +30°549 indicate that conditions favourable for the selective diffusion and formation of the surface chemical composition peculiarities (slow rotation and stabilization of the atmosphere) could already be built up at the pre-main-sequence phase.

Searching for Barium Stars from the LAMOST Spectra Using the Machine-learning Method: I

Barium stars are chemically peculiar stars that exhibit enhancement of s-process elements. Chemical abundance analysis of barium stars can provide crucial clues for the study of the chemical evolution of the Galaxy. The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) has released more than 6 million low-resolution spectra of FGK-type stars by Data Release 9, which can significantly increase the sample size of barium stars. In this paper, we used machine-learning algorithms to search for barium stars from low-resolution spectra of LAMOST. We have applied the Light Gradient Boosting Machine (LGBM) algorithm to build classifiers of barium stars based on different features, and build predictors for determining [Ba/Fe] and [Sr/Fe] of barium candidates. The classification with features in the whole spectrum performs best: for the sample with strontium enhancement, Precision = 97.81% and Recall = 96.05%; for the sample with barium enhancement, Precision = 96.03% and Recall = 97.70%. In prediction, [Ba/Fe] estimated from Ba ii line at 4554 Å has smaller dispersion than that from Ba ii line at 4934 Å: MAE4554 Å = 0.07, σ 4554 Å = 0.12. [Sr/Fe] estimated from Sr ii line at 4077 Å performs better than that from Sr ii line at 4215 Å: MAE4077 Å = 0.09, σ 4077 Å = 0.16. A comparison of the LGBM and other popular algorithms shows that LGBM is accurate and efficient in classifying barium stars. This work demonstrated that machine learning can be used as an effective means to identify chemically peculiar stars and determine their elemental abundance.

Diffusion velocity of metals in Ap star atmospheres: the effect of ambipolar diffusion of H

ABSTRACT Ambipolar diffusion of hydrogen is effective in the atmosphere of stars when hydrogen is partially ionized. This is precisely the case for Ap stars whose atmospheres are stable enough to allow atomic diffusion in building metals stratification. Now, ambipolar diffusion of H induces an additional term to the diffusion velocity usually considered for metals. For some chemical elements, this may significantly affect the diffusion flux, and so, their abundance distribution. This effect decreases for high effective temperatures since protons density becomes very dominant compared to neutral H (that is partial ionization of H may be neglected), and so, this process has been neglected in recent models of abundance distributions in Bp stars. In this study, we evaluate for many metals the role of ambipolar diffusion in view of future detailed modelling of atomic diffusion for chemically peculiar stars with $T_{\rm eff}\lesssim 12\, 000$ K. We discuss a possible effect that we name the ambipolar flooding.

Cool, Luminous, and Highly Variable Stars in the Magellanic Clouds. II. Spectroscopic and Environmental Analysis of Thorne–Żytkow Object and Super-AGB Star Candidates

In previous work, we identified a population of 38 cool and luminous variable stars in the Magellanic Clouds and examined 11 in detail in order to classify them as either Thorne–Żytkow objects (TŻOs; red supergiants with a neutron star cores) or super-asymptotic giant branch (sAGB) stars (the most massive stars that will not undergo core collapse). This population includes HV 2112, a peculiar star previously considered in other works to be either a TŻO or high-mass asymptotic giant branch (AGB) star. Here we continue this investigation, using the kinematic and radio environments and local star formation history of these stars to place constraints on the age of the progenitor systems and the presence of past supernovae. These stars are not associated with regions of recent star formation, and we find no evidence of past supernovae at their locations. Finally, we also assess the presence of heavy elements and lithium in their spectra compared to red supergiants. We find strong absorption in Li and s-process elements compared to RSGs in most of the sample, consistent with sAGB nucleosynthesis, while HV 2112 shows additional strong lines associated with TŻO nucleosynthesis. Coupled with our previous mass estimates, the results are consistent with the stars being massive (∼4–6.5 M ⊙) or sAGB (∼6.5–12 M ⊙) stars in the thermally pulsing phase, providing crucial observations of the transition between low- and high-mass stellar populations. HV 2112 is more ambiguous; it could either be a maximally massive sAGB star, or a TŻO if the minimum mass for stability extends down to ≲13 M ⊙.

HD 60431, the CP2 star with the shortest rotational period

Context. CP2 stars show periodic photometric, spectroscopic, and magnetic variations with the rotational period. They are generally slow rotators, with rotational periods exceeding half a day, except for the late B-type star HD 60431, which has an unusually short rotational period of 0.4755 days. As slow rotation is deemed a necessary criterion for the establishment of chemical peculiarities, this characteristic renders HD 60431 a special object that might offer valuable insight into, and constraints on, the formation and evolution of CP2 stars. Aims. The aims of our study are to analyse the light variability, derive atmospheric abundances, and determine the detailed physical parameters of HD 60431 to confirm its status as the CP2 star with the shortest known rotational period, with special emphasis on the rotational period evolution. Methods. Photometric indices and high-resolution spectroscopy were employed to derive the physical parameters, evolutionary status, and atmospheric abundances of our target star. A light variability study was carried out using combined sets of photometric data from ground- and space-based facilities. A circularly polarised spectrum was employed to check the presence of a longitudinal magnetic field in the star. Results. With an age of only 10 Myr, an effective temperature of Teff = 13 000 ± 300 K, surface gravity log g = 4.10 ± 0.10, radius R = 1.97 ± 0.09 R⊙, and mass M = 3.1 ± 0.1 M⊙, HD 60431 is situated close to the zero age main sequence; it is a member of the open cluster NGC 2547 in the Vela OB2 complex. We confirm its status as a classical late B-type CP2 star showing strong overabundances of Mg (1.8 dex), Si (1.9 dex), Ca (1.6 dex), Ti (2.2 dex), and Fe (1.8 dex). No conclusive evidence for the presence of a strong magnetic field was found in the available spectroscopic data. The light curve of HD 60431 has remained constant over the last four decades. The available photometric time series data confirm the short rotational period and indicate a slight secular increase in the rotational period of Ṗ = 2.36(19) × 10−10 = 7.5(6) ms yr−1. The following quadratic ephemeris has been derived: HJDmin(E) = 2 459 212.969 35 + 0. d475 516 64 E + 5. d62 × 10−11E2. Conclusions. HD 60431 is indeed the CP2 star with the shortest known rotational period. Theory needs to explain the establishment and maintenance of chemical peculiarities in such a young and fast-rotating object. Our results furthermore raise the question of whether period variability on timescales significantly shorter than stellar evolution is inherent to all magnetic chemically peculiar stars.

Pulsation of chemically peculiar stars

AbstractThis paper reviews our current knowledge about pulsating chemically peculiar (CP) stars. CP stars are slowly rotating upper main-sequence objects, efficiently employing diffusion in their atmospheres. They can be divided into magnetic and non-magnetic objects. Magnetic activity significantly influence their pulsational characteristics. Only a handful of magnetic, classical pulsating objects are now known. The only exceptions are about 70 rapidly oscillating Ap stars, which seem to be located within a very tight astrophysical parameter space. Still, many observational and theoretical efforts are needed to understand all important physical aspects and their interrelationships. The most important steps to reach these goals are reviewed.

The Small Separation A-star Companion Population: First Results with CHARA/MIRC-X

We present preliminary results from our long-baseline interferometry (LBI) survey to constrain the multiplicity properties of intermediate-mass A-type stars within 80 pc. Previous multiplicity studies of nearby stars exhibit orbital separation distributions well fitted with a lognormal with peaks >15 au, increasing with primary mass. The A-star multiplicity survey of De Rosa et al., sensitive beyond 30 au but incomplete below 100 au, found a lognormal peak around 390 au. Radial velocity surveys of slowly rotating, chemically peculiar Am stars identified a significant number of very close companions with periods ≤5 days, ∼0.1 au, a result similar to surveys of O- and B-type primaries. With the improved performance of LBI techniques, we can probe these close separations for normal A-type stars where other surveys are incomplete. Our initial sample consists of 27 A-type primaries with estimated masses between 1.44 and 2.49 M ⊙ and ages 10–790 Myr, which we observed with the MIRC-X instrument at the CHARA Array. We use the open-source software CANDID to detect five companions, three of which are new, and derive a companion frequency of over mass ratios of 0.25–1.0 and projected separations of 0.288–5.481 au. We find a probability of 10−6 that our results are consistent with extrapolations based on previous models of the A-star companion population over the mass ratios and separations sampled. Our results show the need to explore these very close separations to inform our understanding of stellar formation and evolution processes.

Magnetic Fields of Chemically Peculiar and Related Stars. VIII. Main Results of 2021 and Near-Future Prospects

Abstract—We present a review of the papers written in the research field of stellar magnetism in 2021. The instruments, observation, and data analysis methods, photometry, spectroscopy, and spectropolarimetry results are considered. We present new data on magnetic fields of chemically peculiar stars and other objects of various types.

Are Am stars and hot-Jupiter planets related?

Context. Metallic-lined A stars (Am stars) are often components of short-period binary systems, where tidal interactions would result in low rotational velocities and help to develop the chemical peculiarities observed. However, the origin of single Am stars and Am stars that belong to wide binary systems is unclear. Aims. There is very recent evidence of an Am star hosting a likely synchronized hot brown dwarf (hot BD) and other possible Am stars hosting hot Jupiter planets. Following literature suggestions, we wonder if these hot low-mass companions could play a role in the development of an Am star, that is to say, if they could help to mitigate the “single Am” problem. Methods. We carried out a detailed abundance determination via spectral synthesis of 19 early-type stars hosting hot BDs and hot Jupiter planets, in order to determine the possible presence of Am stars in this sample. The abundances were determined iteratively for 25 different species by fitting synthetic spectra using the SYNTHE program together with local thermodynamic equilibrium (LTE) ATLAS12 model atmospheres. The abundances of C I, O I, and Mg I were corrected by non-LTE effects. The complete chemical patterns of the stars were then compared to those of Am stars and other chemically peculiar stars. Results. We studied a sample of 19 early-type stars, seven of them hosting hot BDs and 12 of them hosting hot Jupiter planets. We detected four Am stars in our sample (KELT-19A, KELT-17, HATS-70, and TOI-503) and two possible Am stars (TOI-681 and HAT-P-69). In particular, we detected the new Am star HATS-70, which hosts a hot BD, and we ruled out this class for the hot Jupiter host, WASP-189. Both stars show a different composition than previously reported. For the first time, we estimated the incidence of Am stars among stars hosting hot BDs (50−75%) and among stars hosting hot Jupiters (20−42%). The incidence of Am stars hosting hot BDs has shown to be higher than the frequency of Am stars in general. This would imply that the presence of hot BDs could play a role in the development of Am stars and possibly help to mitigate the single Am problem, which is different for the case of hot Jupiter planets. Notably, these results also indicate that the search for hot BDs may benefit from the targeting of single Am stars or Am stars in wide binary systems. We encourage the analysis of additional early-type stars hosting hot companions in order to improve the significance of the initial trends found here.

Features of the Behavior of Magnetic Chemically Peculiar Stars on the Main Sequence. III

Features of the Behavior of Magnetic Chemically Peculiar Stars on the Main Sequence. III

High-resolution spectroscopic analysis of four new chemically peculiar stars

ABSTRACT We present detailed chemical compositions of four stars on the first-ascent red giant branch that are classified as chemically peculiar, but lack comprehensive analyses at high spectral resolution. For BD+03°2688, HE 0457−1805, HE 1255−2324, and HE 2207−1746, we derived metallicities [Fe/H] = −1.21, −0.19, −0.31, and −0.55, respectively, indicating a range in Galactic population membership. In addition to atmospheric parameters, we extracted elemental abundances for 28 elements, including the evolutionary-sensitive CNO group and 12C/13C ratios. Novel results are also presented for the heavy elements tungsten and thallium. All four stars have very large enhancements of neutron-capture elements, with high [La/Eu] ratios indicating enrichments from the slow neutron capture (s-process). To interpret these abundances, all indicative of [s/Fe] >1.0, we compared our results with data from literature, as well as with predictions from the Monash and fruity s-process nucleosynthesis models. BD+03°2688, HE 1255−2324, and HE 2207−1746 show C/O >1, while HE 0457−1805 has C/O <1. Since HE 0457−1805 and HE 1255−2324 are binary stars, their peculiarities are attributable to mass transfer. We identified HE 0457−1805 as a new barium giant star, and HE 1255−2324 as a new CH star, in fact a higher metallicity analogue CEMP-r/s star; the single object reported in literature so far with similar characteristics is the barium star HD 100503 ([Fe/H] = −0.72). A systematic monitoring is needed to confirm the binary nature of BD+03°2688 and HE 2207−1746, which are probably CH stars.