Color-magnitude Diagram Research Articles

Exploration of Stellar Variability in 20 s Cadence TESS Data

The introduction of TESS’s 20 s cadence mode has given us the opportunity to examine the variation of stellar flux on shorter timescales than ever before. While we find no previously undetected variability at these timescales, we find oscillations outside of the published instability strip in TIC 349902873. We also quantify the stellar variability on specific timescales using the combined differential photometric precision (CDPP). We developed a light-curve creation pipeline for 57 bright stars in TESS’ SCVZ across the HR diagram with extant spectra. From the light curves, we reproduced the scaling previously seen between CDPP and magnitude at all timescales in our data set. We also find a potential correlation between CDPP on the shortest timescales and surface gravity, which may be related to convection and oscillations and impact the relative detectability of exoplanet features.

The δ Scuti stars of the Cep–Her Complex – I. Pulsator fraction, rotation, asteroseismic large spacings, and the νmax relation

ABSTRACT We identify delta Scuti ($\delta$ Sct) pulsators amongst members of the recently discovered Cep–Her Complex using light curves from the Transiting Exoplanet Survey Satellite (TESS). We use Gaia colours and magnitudes to isolate a subsample of provisional Cep–Her members that are located in a narrow band on the colour–magnitude diagram compatible with the zero-age main sequence. The $\delta$ Sct pulsator fraction amongst these stars peaks at 100 per cent and we describe a trend of higher pulsator fractions for younger stellar associations. We use four methods to measure the frequency of maximum amplitude or power, $\nu _{\rm max}$, to minimize methodological bias, and we demonstrate their sound performance. The $\nu _{\rm max}$ measurements display a correlation with effective temperature, but with a scatter that is too large for the relation to be useful. We find two ridges in the $\nu _{\rm max}$–$T_{\rm eff}$ diagram, one of which appears to be the result of rapid rotation causing stars to pulsate in low-order modes. We measure the $\nu _{\rm max}$ values of $\delta$ Sct stars in four other clusters or associations of similar age (Trumpler 10, the Pleiades, NGC 2516, and Praesepe) and find similar behaviour with $T_{\rm eff}$. Using échelle diagrams, we measure the asteroseismic large spacing, $\Delta \nu$, for 70 stars, and find a correlation between $\Delta \nu$, rotation, and luminosity that allows rapid rotators seen at low inclinations to be distinguished from slow rotators. We find that rapid rotators are more likely than slow rotators to pulsate, but they do so with less regular pulsation patterns. We also investigate the reliability of Gaia’s vbroad measurement for A-type stars, finding that it is mostly accurate but underestimates $v\sin i$ for slow rotators ($v\sin i \lt 50$ km s−1) by 10–15 per cent.

Combined Gemini-South and HST photometric analysis of the globular cluster NGC 6558

Context. NGC 6558 is a low-galactic-latitude globular cluster projected in the direction of the Galactic bulge. Due to high reddening, this region presents challenges in deriving accurate parameters, which require meticulous photometric analysis. We present a combined analysis of near-infrared and optical photometry from multi-epoch high-resolution images collected with Gemini-South/GSAOI+GeMS (in the J and KS filters) and HST/ACS (in the F606W and F814W filters). Aims. We aim to refine the fundamental parameters of NGC 6558, utilising high-quality Gemini-South/GSAOI and HST/ACS photometries. Additionally, we intend to investigate its role in the formation of the Galactic bulge. Methods. We performed a meticulous differential reddening correction to investigate the effect of contamination from Galactic bulge field stars. To derive the fundamental parameters – age, distance, reddening, and the total-to-selective coefficient – we employed a Bayesian isochrone fitting. The results from high-resolution spectroscopy and RR Lyrae stars were implemented as priors. For the orbital parameters, we employed a barred Galactic mass model. Furthermore, we analysed the age-metallicity relation to contextualise NGC 6558 within the Galactic bulge’s history. Results. We studied the impact of two differential reddening corrections on the age derivation. When removing as much as possible of the Galactic bulge field star contamination, the isochrone fitting combined with synthetic colour-magnitude diagrams gives a distance of 8.41−0.10+0.11 kpc, an age of 13.0 ± 0.9 Gyr, and a reddening of E(B − V) = 0.34 ± 0.02. We derived a total-to-selective coefficient of RV = 3.2 ± 0.2 thanks to the simultaneous near-infrared–optical synthetic colour-magnitude diagram fitting, which, aside from errors, agrees with the commonly used value. The orbital parameters showed that NGC 6558 is confined within the inner Galaxy and it is not compatible with a bar-shape orbit, indicating that it is a bulge member. Assembling the old and moderately metal-poor ([Fe/H] ∼ −1.1) clusters in the Galactic bulge, we derived their age-metallicity relation with star formation starting at 13.6 ± 0.2 Gyr and effective yields of ρ = 0.05 ± 0.01 Z⊙. Conclusions. The old age derived for NGC 6558 is compatible with other clusters with similar metallicity and a blue horizontal branch in the Galactic bulge, which constitute the moderately metal-poor globular clusters. The age-metallicity relation shows that the starting age of star formation is compatible with the age of NGC 6558, and the chemical enrichment is ten times faster than the ex situ globular cluster branch.

The DESI Early Data Release white dwarf catalogue

Abstract The Early Data Release (EDR) of the Dark Energy Spectroscopic Instrument (DESI) comprises spectroscopy obtained from 2020 December 14 to 2021 June 10. White dwarfs were targeted by DESI both as calibration sources and as science targets and were selected based on Gaia photometry and astrometry. Here we present the DESI EDR white dwarf catalogue, which includes 2706 spectroscopically confirmed white dwarfs of which approximately 60 percnt have been spectroscopically observed for the first time, as well as 66 white dwarf binary systems. We provide spectral classifications for all white dwarfs, and discuss their distribution within the Gaia Hertzsprung–Russell diagram. We provide atmospheric parameters derived from spectroscopic and photometric fits for white dwarfs with pure hydrogen or helium photospheres, a mixture of those two, and white dwarfs displaying carbon features in their spectra. We also discuss the less abundant systems in the sample, such as those with magnetic fields, and cataclysmic variables. The DESI EDR white dwarf sample is significantly less biased than the sample observed by the Sloan Digital Sky Survey, which is skewed to bluer and therefore hotter white dwarfs, making DESI more complete and suitable for performing statistical studies of white dwarfs.

A Portrait of the Rotation of Ultra-cool Dwarfs Revealed by TESS

This study presents the results of a search for rotation signatures in 250 Gaia Data Release 3 ultra-cool dwarfs (UCDs) with TESS light curves (LCs). We identified 71 targets with unambiguous periodicities, of which 61 present rotation signatures and a single-source behavior, with periods between 0.133 and 5.81 days. Five UCDs show double-dip features, namely variations with two periods, one approximately double or half the other. The remaining 10 UCDs with unambiguous variability present a likely nonsingle behavior. We also found 20 UCDs showing complex behavior in their LCs, with noticeable fluctuations and irregular structure, with a few exhibiting apparent changes in their temporal structure. The remaining 159 targets show noisy LCs corresponding to low-amplitude signals, whose temporal variation cannot be easily identified. The distribution of the UCDs with rotation signature in a color–magnitude diagram points to a lack of rotating objects within about 11.5 < M G < 12.5 and G–G RP < 1.5, separating them into two regimes, one mainly composed of less massive late-M stars with P rot ≥ 1.0 day, and another mainly composed of more massive early-M stars with P rot < 1.0 day. It is important to emphasize that by separating stars into age intervals, one observes that UCDs with P rot ≥ 1.0 day tend to be located in regions of younger objects, and, in contrast, those with P rot < 1.0 day are mainly concentrated in regions of older objects. Whether this trend of stars contrasting the sample separation is physical or produced by observational biases is a question to be verified in future studies.

Modelling of eclipsing binary systems with pulsating components and tertiary companions: BF Vel and RR Lep

This research paper presents a comprehensive analysis of RR Lep and BF Vel, two southern short-period, semi-detached, oscillating Algols (oEA stars) that have been shown to be triple systems. We determined the spectral types of the primary components and calculated the radial velocities from spectra observed with the Australian National University's 2.3 m telescope and Wide Field Spectrograph. The spectra of the Na I D doublet confirmed the presence of tertiary components that were apparent in the broadening function analyses. During primary eclipses chromospherical activity in their secondary components was apparent in their H$_ spectra; it was also apparent in the Na I D spectra of BF Vel with its almost complete annular eclipse. Ground-based telescopes were used for observations in several pass bands for photometric analyses. These observations were complemented by data from the Transiting Exoplanet Survey Satellite (TESS) mission, allowing us to model the light curves, followed by a detailed analysis of pulsations. Eclipse-timing variation (ETV) analyses of both systems were used to determine the most likely mechanisms modulating the orbital period. We found mass values of $M_1=2.9$ sun $ and $M_2=0.75$ M$_ sun $ for the components of RR Lep, along with $M_1=1.93$ M$_ sun $ and $M_2=0.97$ M$_ sun $ for those of BF Vel. By integrating information from photometry, spectroscopy, and ETV analysis, we found that tertiary components revolve around both systems. The primary star of RR Lep pulsates in 36 frequencies; of these, five were identified as independent modes, with the dominant one being 32.28 d$^ $. The pulsating component of BF Vel oscillates in 37 frequencies, with the frequency 46.73 d$^ $ revealed as the only independent mode. For both systems, a number of frequencies were found to be related to the orbital frequency. Their physical properties were compared with other oEA stars in mass-radius and Hertzsprung-Russell diagrams, while the pulsational properties of their delta Sct components were compared with currently known systems of this type within the orbital-pulsation period and $ g$-pulsation period diagrams.

Unresolved binary systems with white dwarfs in open star clusters

We invesigate unresolved binary systems with components of main sequence star (MS) and white dwarf (WD) in nine open clusters. These systems are located below and to the left of the main sequence at the colour-magnitude diagram. We compare the number of cluster stars, which have likely evolved into white dwarfs, with the number of candidates for unresolved binary systems with WD. The number of probable cluster members, lying below the main sequence, is generally less than the expected number of white dwarfs. The observations in the ultraviolet could detect WDs and unresolved binary WD+MS systems more confidently than the observations in the visible range.

Unveiling the quasar main sequence: illuminating the complexity of active galactic nuclei and their evolution

The Eigenvector 1 schema, or the main sequence of quasars, was introduced as an analogous scheme to the HR diagram that would allow us to understand the more complex, extended sources - active galactic nuclei (AGNs) that harbor accreting supermassive black holes. The study has spanned more than three decades and has advanced our knowledge of the diversity of Type-1 AGNs from both observational and theoretical aspects. The quasar main sequence, in its simplest form, is the plane between the FWHM of the broad Hβ emission line and the strength of the optical Fe ii emission to the Hβ. While the former allows the estimation of the black hole mass, the latter enables direct measurement of the metal content and traces the accretion rate of the AGN. Together, they allow us to track the evolution of AGN in terms of the activity of the central nuclei, its effect on the line-emitting regions surrounding the AGN, and their diversity making them suitable distance indicators to study the expansion of our Universe. This mini-review aims to provide (i) a brief history leading up to the present day in the study of the quasar main sequence, (ii) introduce us to the many possibilities to study AGNs with the main sequence as a guiding tool, and (iii) highlight some recent, exciting lines of researches at the frontier of this ever-growing field.

Kinematic study of the orion complex: Analysing the young stellar clusters from big and small structures

Abstract In this work, we analysed young stellar clusters with spatial and kinematic coherence in the Orion star-forming complex. For this study, we selected a sample of pre-main sequence candidates using parallaxes, proper motions and positions on the colour-magnitude diagram. After applying a hierarchical clustering algorithm in the 5D parameter space provided by Gaia DR3, we divided the recovered clusters into two regimes: Big Structures and Small Structures, defined by the number of detected stars per cluster. In the first regime, we found 13 stellar groups distributed along the declination axis in the regions where there is a high density of stars. In the second regime, we recovered 34 clusters classified into two types: 14 as small groups completely independent from the larger structures, including four candidates of new clusters, and 12 classified as sub-structures embedded within five larger clusters. Additionally, radial velocity data from APOGEE-2 and GALAH DR3 was included to study the phase space in some regions of the Orion complex. From the Big Structure regime, we found evidence of a general expansion in the Orion OB1 association over a common centre, giving a clue about the dynamical effects the region is undergoing. Likewise, in the Small Structure regime, the projected kinematics shows the ballistic expansion in the λ Orionis association and the detection of likely events of clusters’ close encounters in the OB1 association.

Modelling hydrogen-deficient carbon stars in mesa – the effects of total mass and mass ratio

ABSTRACT Hydrogen-deficient carbon (HdC) stars are rare, low-mass, chemically peculiar, supergiant variables believed to be formed by a double white dwarf (DWD) merger, specifically of a carbon/oxygen- (CO-) and a helium-white dwarf (He-WD). They consist of two subclasses – the dust-producing R Coronae Borealis (RCB) variables and their dustless counterparts the dustless HdCs (dLHdCs). Additionally, there is another, slightly cooler set of potentially related carbon stars, the DY Persei type variables which have some, but not conclusive, evidence of hydrogen-deficiency. Recent works have begun to explore the relationship between these three classes of stars, theorizing that they share an evolutionary pathway (a DWD merger) but come from different binary populations, specifically different total masses ($M_\text{tot}$) and mass ratios (q). In this work, we use the mesa modelling framework that has previously been used to model RCB stars and vary the merger parameters, $M_\text{tot}$ and q, to explore how those parameters affect the abundances, temperatures, and luminosities of the resultant post-merger stars. We find that lower $M_\text{tot}$ and larger q’s both decrease the luminosity and temperatures of post-merger models to the region of the Hertzsprung–Russell diagram populated by the dLHdCs. These lower $M_\text{tot}$ and larger q models also have smaller oxygen isotopic ratios ($^{16}$O/$^{18}$O) which is consistent with recent observations of dLHdCs compared to RCBs. None of the models generated in this work can explain the existence of the DY Persei type variables, however this may arise from the assumed metallicity of the models.

Exploring the dynamical evolution of Cepheid multiplicity in star clusters and its implications for B-star multiplicity at birth

Classical Cepheid variable stars provide a unique probe of binary evolution in intermediate-mass stars over the course of several tens to hundreds of Myr. In addition, understanding binary evolution with the inclusion of cluster dynamics is desirable for obtaining a more complete picture of these stars, especially as they play a vital role in distance determinations. We studied the binary and multiple properties of Cepheids, assuming that all mid-B stars form in binaries inside star clusters. We also estimated the birth multiplicity of mid-B stars by comparing the observed multiplicity statistics of Cepheids with models based on particular assumptions. The clusters were modelled with the code, including synthetic stellar and binary evolutionary tracks. The Cepheids were identified from their position on the Hertzsprung-Russell diagram. The dynamical cluster environment results in a higher binary fraction among the Cepheids that remain in star clusters ($ 60$&lt;!PCT!&gt;) than among the Cepheids which have escaped to the field ($ 35$&lt;!PCT!&gt;). The fraction of Cepheids in triples ($ 30$&lt;!PCT!&gt; and $ 10$&lt;!PCT!&gt; in clusters and field, respectively) follows the same trend. In clusters, the binary, triple, and multiple fraction decreases with increasing cluster mass. More massive clusters have binaries of shorter orbital periods than lower mass clusters and field Cepheids. Mergers are very common with $ 30$&lt;!PCT!&gt; of mid-B stars not evolving to Cepheids because of the interaction with their companion. Approximately $40$ &lt;!PCT!&gt; of Cepheids have merged with their companion, and the merger event impacts stellar evolution, so that $ 25$&lt;!PCT!&gt; of all Cepheids occur at an age by more than $40$&lt;!PCT!&gt; different than what would be expected from their mass and the current cluster age; the expected age of Cepheids can differ from the age of their host cluster. Our models predict that one in five Cepheids is the result of a merger between stars with mass below the lower mass limit for Cepheids; in clusters, these objects occur substantially later than expected from their mass. Approximately $10$&lt;!PCT!&gt; of binary Cepheids have a different companion from the zero-age main sequence (ZAMS) one, and $ 3$ to $5$&lt;!PCT!&gt; of all Cepheids have a compact companion ($ 0.15$ &lt;!PCT!&gt; of all Cepheids are accompanied by a black hole). The binary fraction derived from our simulations (42&lt;!PCT!&gt;) underestimates the observed binary Cepheid fraction by approximately a factor of 2. This suggests that the true multiplicity fraction of B-stars at birth could be substantially larger than unity and, thus, that mid-B stars may typically form in triple and higher order systems.

Candidate Runaway Stars in the Cluster NGC 2180

The relative proper motions and tangential velocities of four stars from the cluster NGC 2180 are investigated using Gaia DR3 data. Our approach is to subtract the mean proper motion of the fields (the stars within a 1” radius around each target star) and the cluster from the motion of the target stars to calculate the relative proper motions with respect to their field and the cluster, respectively. From the relative proper motions of the target stars with respect to the field stars and the distance to the center of the cluster, we estimated the relative tangential velocities of our target stars. We use the astrometric excess noise information to filter the field stars from star-like objects, such as unresolved binaries. To eliminate foreground contaminations of the field stars used to figure out the relative proper motions of the target stars, we made a parallax cut, Color Magnitude Diagram (CMD) cut, and proper motion (PM) selection. The flight times of the target stars to the cluster NGC 2180 (kinematic ages) are calculated using angular separations from the cluster and their relative proper motions with respect to the cluster. All of the target stars have kinematic ages less than the age of the cluster. The stars, ID1 and ID3 have and with respect to their fields, which corresponds to the relative velocities respectively. These two stars could be runaways from the cluster because they have flight times from the cluster agreed with late ejection from the cluster and have larger proper. The remaining two stars, ID 2 and ID 4 have and with respect to the fields, which corresponds to the relative velocities and respectively, and could not be runaways as they have smaller proper motions, although their flight times to the cluster agreed with late ejection from the cluster.

Twenty-three new Heartbeat Star systems discovered based on TESS data

ABSTRACT Heartbeat stars (HBSs) are ideal astrophysical laboratories to study the formation and evolution of binary stars in eccentric orbits and the internal structural changes of their components under strong tidal action. We discover 23 new HBSs based on TESS photometric data. The orbital parameters, including orbital period, eccentricity, orbital inclination, argument of periastron, and epoch of periastron passage of these HBSs, are derived by using a corrected version of Kumar et al.'s model based on the Markov Chain Monte Carlo (MCMC) method. The preliminary results show that these HBSs have orbital periods in the range from 2.7 to 20 d and eccentricities in the range from 0.08 to 0.70. The eccentricity-period relation of these objects shows a positive correlation between eccentricity and period and also shows the existence of orbital circularization. The Hertzsprung–Russell diagram shows that the HBSs are not all located in a particular area. The distribution of the derived parameters suggests a selection bias within the TESS survey towards HBSs with shorter periods. These objects are a very useful source to study the structure and evolution of eccentricity orbit binaries and to extend the TESS HBS catalog.

A New Catalog of 100,000 Variable TESS A-F Stars Reveals a Correlation between δ Scuti Pulsator Fraction and Stellar Rotation

δ Scuti variables are found at the intersection of the classical instability strip and the main sequence on the Hertzsprung–Russell diagram. With space-based photometry providing millions of light curves of A-F type stars, we can now probe the occurrence rate of δ Scuti pulsations in detail. Using the 30 minutes cadence light curves from NASA's Transiting Exoplanet Survey Satellite's first 26 sectors, we identify variability in 103,810 stars within 5–24 cycles per day down to a magnitude of T = 11.25. We fit the period–luminosity relation of the fundamental radial mode for δ Scuti stars in the Gaia G band, allowing us to distinguish classical pulsators from contaminants for a subset of 39,367 stars. Out of this subset, over 15,918 are found on or above the expected period–luminosity relation. We derive an empirical red edge to the classical instability strip using Gaia photometry. The center where the pulsator fraction peaks at 50%–70%, combined with the red edge, agrees well with previous work in the Kepler field. While many variable sources are found below the period–luminosity relation, over 85% of sources inside of the classical instability strip derived in this work are consistent with being δ Scuti stars. The remaining 15% of variables within the instability strip are likely hybrid or γ Doradus pulsators. Finally, we discover strong evidence for a correlation between pulsator fraction and spectral line broadening from the Radial Velocity Spectrometer on board the Gaia spacecraft, confirming that rotation has a role in driving pulsations in δ Scuti stars.

Imaging of I Zw 18 by JWST

Context. The blue compact dwarf (BCD) galaxy I Zw 18 is one of the most metal-poor (Z ∼ 3% Z⊙) star-forming galaxies known in the local Universe. Since its discovery, the evolutionary status of this system has been at the center of numerous debates within the astronomical community. Aims. We aim to probe and resolve the stellar populations of I Zw 18 in the near-IR using JWST/NIRCam’s unprecedented imaging spatial resolution and sensitivity. Additionally, our goal is to derive the spatially resolved star formation history (SFH) of the galaxy within the last 1 Gyr, and to provide constraints for older epochs. Methods. We used the point spread function fitting photometry package DOLPHOT to measure positions and fluxes of point sources in the F115W and F200W filters’ images of I Zw 18, acquired as part of the JWST GTO ID 1233 (PI: Meixner). Furthermore, to derive I Zw 18’s SFH, we applied a state-of-the-art color-magnitude diagram (CMD) fitting technique (SFERA 2.0), using two independent sets of stellar models: PARSEC-COLIBRI and MIST. Results. Our analysis of I Zw 18’s CMD reveal three main stellar populations: one younger than ∼30 Myr, mainly associated with the northwest star-forming (SF) region; an intermediate-age population (∼100 − 800 Myr), associated with the southeast SF region; and a red and faint population, linked to the underlying halo of the galaxy, older than 1 Gyr and possibly as old as 13.8 Gyr. The main body of the galaxy shows a very low star formation rate (SFR) of ~10-4M⊙ yr−1 between 1 and 13.8 Gyr ago. In the last billion years, I Zw 18 shows an increasing trend, culminating in two strong bursts of SF around ∼10 and ∼100 Myr ago. Notably, I Zw 18 Component C mimics the evolution of the main body, but with lower SFRs on average. Conclusions. Our results confirm that I Zw 18 is populated by stars of all ages, without any major gaps. Thus, I Zw 18 is not a truly young galaxy, but rather a system characterized by an old underlying stellar halo, in agreement with what has been found in other BCDs by similar studies. The low SF activity exhibited at epochs older than 1 Gyr is in agreement with the “slow cooking" dwarf scenario proposed in the literature, and could have contributed to its low metal content. The galaxy is now experiencing its strongest episode of star formation (∼ 0.6 M⊙ yr−1) mainly located in the northwest region. A recent gravitational interaction between the main body and Component C is the most likely explanation for this starburst.

Benchmarking the effective temperature scale of red giant branch stellar models: The case of the metal-poor halo giant HD 122563

Context. There is plenty of evidence in the literature of significant discrepancies between the observations and models of metal-poor red giant branch stars, in particular regarding the effective temperature Teff scale. Aims. We revisit the benchmark star HD 122563 using the most recent observations from Gaia Data Release 3, to investigate if these new constraints may help in resolving this discrepancy. Methods. We review the most recent spectroscopic determinations of the metallicity [Fe/H] of HD 122563, and provide a new assessment of its fundamental parameters, specifically, bolometric luminosity, Teff, surface gravity, plus a photometric determination of its metal content. Using these constraints, we compare the position of the star in the Hertzsprung-Russell (H–R) diagram with various recent sets of stellar evolution tracks. Results. The H-R diagram analysis reveals a significant disagreement between observed and theoretical Teff values, when adopting the most recent spectroscopic estimate of [Fe/H]. On the other hand, by using the photometric determination of [Fe/H], some of the selected sets of stellar tracks appear in fair agreement with observations. The sets with discrepant Teff can be made to agree with observations either by modifying the prescription adopted to calculate the models’ outer boundary conditions, and/or by reducing the adopted value of the mixing length parameter with respect to the solar-calibration. Conclusions. A definitive assessment of whether the Teff scale of metal-poor stellar red giant branch models is consistent with observations requires a more robust determination of the fundamental parameters of HD 122563 and also a larger sample of calibrators. From the theoretical side, it is crucial to minimise the current uncertainties in the treatment (boundary conditions, temperature gradient) of the outer layers of stellar models with convective envelopes.

Discovery of 226 [formula omitted] Scuti and [formula omitted] Doradus Stars near NGC 6871 with TESS

We present the discovery of 269 pulsating variable stars of δ Scuti, γ Doradus, and Maia types in the vicinity of the open cluster NGC 6871, using data from the Transiting Exoplanet Survey Satellite (TESS). Our small-scale regional survey centered on the δ Scuti star V1821 Cyg in the open cluster NGC 6871, covering a radius of one degree. The results include a remarkable total of 1512 newly classified variable stars, comprising the following categories: 105 δ Scuti stars, 121 γ Doradus stars, 50 Maia variables, 198 eclipsing binary systems, with 12 exhibiting pulsating or rotating components, 500+ rotating variable stars, and dozens of other types. Out of 1512 newly discovered variable stars, 108 are confirmed members of NGC 6871 with a membership probability exceeding 50%. Notably, dedicated Fourier analyses were applied to eight representative stars from the newly discovered variables. Among these, one star exhibits a rich and complex pulsation spectrum characterized by amplitude variations in dominant pulsations. To contextualize the new pulsators, we plotted them in the Hertzsprung–Russell diagrams alongside the largest known group of class member stars. Surprisingly, both δ Scuti and γ Doradus stars occupy nearly the same region in the diagrams, hinting at a potential unified pulsation mechanism. This study contributes valuable insights into the variability census of NGC 6871 and sheds light on the pulsation behavior of different stellar types. Further investigations into the physical properties and evolutionary status of these stars are warranted.

Atmospheric heating and magnetism driven by 22Ne distillation in isolated white dwarfs

The origin of atmospheric heating in the cool, magnetic white dwarf GD 356 remains unsolved nearly 40 years after its discovery. This once idiosyncratic star with Teff ≈ 7500 K, yet Balmer lines in Zeeman-split emission is now part of a growing class of white dwarfs exhibiting similar features, and which are tightly clustered in the HR diagram suggesting an intrinsic power source. This paper proposes that convective motions associated with an internal dynamo can power electric currents along magnetic field lines that heat the atmosphere via Ohmic dissipation. Such currents would require a dynamo driven by core 22Ne distillation, and would further corroborate magnetic field generation in white dwarfs by this process. The model predicts that the heating will be highest near the magnetic poles, and virtually absent toward the equator, in agreement with observations. This picture is also consistent with the absence of X-ray or extreme ultraviolet emission, because the resistivity would decrease by several orders of magnitude at the typical coronal temperatures. The proposed model suggests that i) DAHe stars are mergers with enhanced 22Ne that enables distillation and may result in significant cooling delays; and ii) any mergers that distill neon will generate magnetism and chromospheres. The predicted chromospheric emission is consistent with the two known massive DQe white dwarfs.

The VISCACHA Survey – XI. Benchmarking SIESTA: a new synthetic CMD fitting code

ABSTRACT We present a novel code, named SIESTA (Statistical matchIng between rEal and Synthetic sTellar popuLations), designed for performing statistical isochrone fitting to colour–magnitude diagrams (CMDs) of single stellar populations by leveraging comparisons between the observed stellar distribution and predictions from synthetic populations, simulated on top of a grid of isochrones. These synthetic populations encompass determinant factors such as the cluster’s initial mass function (IMF), the presence of non-resolved binaries, as well as the expected photometric errors, and observational completeness (or the observed luminosity function). Employing Markov Chain Monte Carlo within a Bayesian framework, SIESTA allows for the determination of a cluster’s age, metallicity, distance, colour excess, and binary fraction (with masses exceeding a certain ratio). In this study, we rigorously benchmark the SIESTA code utilizing synthetic populations and evaluate its performance against observations from the VISCACHA Survey in the Small Magellanic Cloud, focusing on five star clusters: Lindsay 114, NGC 152, Lindsay 91, Lindsay 113, and NGC 121. These clusters were chosen for their diverse age range, spanning from 0.04 to 10 Gyr. Our findings demonstrate the capability of the SIESTA code to accurately represent the observed CMDs of these clusters. Furthermore, we compare the results obtained with SIESTA to previous characterizations of these clusters, highlighting the consistency between the derived metallicity and spectroscopic determinations from various sources.

Skynet Cluster Pro Plus: An Interactive Web Application for Stellar Astronomy Education

Cluster Pro Plus is an interactive web-based application built upon the Skynet Plotting Tools infrastructure to facilitate learning of concepts in stellar evolution by analysing star clusters. With the ability to remove field stars and fit isochrones to photometric data displayed interactively on an HR diagram, introductory astronomy students can use this tool to estimate any star cluster’s age, metallicity, distance, reddening, and number of stars, and compare their results to published values.