Variable Stars Research Articles

Classification of Variable Star Light Curves with Convolutional Neural Network

The classification of variable stars is essential for understanding stellar evolution and dynamics. With the growing volume of light curve data from extensive surveys, there is a need for automated and accurate classification methods. Traditional methods often rely on manual feature extraction and selection, which can be time-consuming and less adaptable to large datasets. In this work, we present an approach using a convolutional neural network (CNN) to classify variable stars using only raw light curve data and their known periods, without the need for manual feature extraction or hand-selected data preprocessing. Our method utilizes phase-folding to organize the light curves and directly learns the variability patterns crucial for classification. Trained and tested on the Optical Gravitational Lensing Experiment (OGLE) dataset, our model demonstrates an average accuracy of 88% and an F1 score of 0.89 across five well-known classes of variable stars. We also compared our classification model with the Random Forest (RF) classifier and showed that our model gives better results across all of the classification metrics. By leveraging CNN, our approach does not need manual feature extraction and can handle diverse light curve shapes and sampling cadences. This automated, data-driven method offers a powerful tool for classifying variable stars, enabling efficient processing of large datasets from current and future sky surveys.

Measuring Sub-Kelvin Variations in Stellar Temperature with High-resolution Spectroscopy

The detection of stellar variability often relies on the measurement of selected activity indicators, such as coronal emission lines and nonthermal emissions. On the flip side, the effective stellar temperature is normally seen as one of the key fundamental parameters (with mass and radius) to understanding the basic physical nature of a star and its relation with its environment (e.g., planetary instellation). We present a novel approach for measuring disk-averaged temperature variations to sub-Kelvin accuracy inspired by algorithms developed for precision radial velocity (pRV). This framework uses the entire content of the spectrum, not just preidentified lines, and can be applied to existing data obtained with high-resolution spectrographs. We demonstrate the framework by recovering the known rotation periods and temperature modulation of Barnard star and AU Mic in data sets obtained in the infrared with SPIRou at CHFT and at optical wavelengths on ϵ Eridani with HARPS at ESO 3.6 m telescope. We use observations of the transiting hot Jupiter HD189733b, obtained with SPIRou, to show that this method can unveil the minute temperature variation signature expected during the transit event, an effect analogous to the Rossiter–McLaughlin effect but in temperature space. This method is a powerful new tool for characterizing stellar activity, and in particular temperature and magnetic features at the surfaces of cool stars, affecting both pRV and transit spectroscopic observations. We demonstrate this method in the context of high-resolution spectroscopy but it could be used at lower resolution.

Long-term Simultaneous Monitoring Observations of SiO and H2O Masers toward the Mira Variable WX Serpentis

We present the results from long-term simultaneous monitoring observations of SiO and H2O masers toward the Mira variable star WX Serpentis. This study has been conducted with 21 m single-dish radio telescopes of the Korean VLBI Network from 2009 June to 2021 June. Five maser lines were considered: SiO v = 1, 2, J = 1−0; SiO v = 1, J = 2−1, 3−2; and H2O 61,6–52,3 transitions, with the SiO maser lines distributed near the stellar velocity and the H2O maser exhibiting an asymmetric line profile with five to six peaked components. Intense H2O maser emissions suddenly appeared in 2019 September, indicating flaring. The intensity variations of SiO and H2O masers are strongly correlated with the optical light curve (OLC) of the central star, with individual phase lags; the phase lag of the H2O maser relative to the OLC is larger than that of the SiO masers. The consequent phase difference between the SiO masers and the H2O maser likely indicates that their formation regions and main driving mechanisms are different from each other. The SiO masers in WX Ser exhibit a dominant single-peak velocity distribution, similar to other Mira variable stars. However, the H2O maser displays distinct morphological features, showing a radial acceleration and preferential intensity dominance at blueshifted velocities. This suggests that the H2O maser clouds of WX Ser are moving outward, thereby developing an asymmetric outflow owing to nonuniform material ejection from the stellar atmosphere. The findings confirm that an initial asymmetric outflow structure emerged during the thermally pulsing asymptotic giant branch phase, specifically in the Mira variable star stage.

Deep drilling in the time domain with DECam II: characterizing the light curves of candidates in the extragalactic fields

Abstract In this second paper on the DECam deep drilling field (DDF) program we release 2,020 optical gri-band light curves for transients and variables in the extragalactic COSMOS and ELAIS fields based on time series observations with a 3-day cadence from semester 2021A through 2023A. In order to demonstrate the wide variety of time domain events detected by the program and encourage others to use the data set, we characterize the sample by presenting a brief analysis of the light curve parameters such as time span, amplitude, and peak brightness. We also present preliminary light curve categorizations, and identify potential stellar variables, active galactic nuclei, tidal disruption events, supernovae (such as Type Ia, Type IIP, superluminous, and gravitationally lensed supernovae), and fast transients. Where relevant, the number of identified transients is compared to the predictions of the original proposal. We also discuss the challenges of analyzing DDF data in the context of the upcoming Vera C. Rubin Observatory and its Legacy Survey of Space and Time, which will include DDFs. Images from the DECam DDF program are available without proprietary period and the light curves presented in this work are publicly available for analysis.

A summary of instruments proposed for observing pulsating variables from the Mt. Abu Observatory

A summary of instruments proposed for observing pulsating variables from the Mt. Abu Observatory

LEAVES: An Expandable Light-curve Data Set for Automatic Classification of Variable Stars

With the increasing amount of astronomical observation data, it is an inevitable trend to use artificial intelligence methods for automatic analysis and identification of light curves for full samples. However, data sets covering all known classes of variable stars that meet all research needs are not yet available. There is still a lack of standard training data sets specifically designed for any type of light-curve classification, but existing light-curve training sets or data sets cannot be directly merged into a large collection. Based on the open data sets of the All-Sky Automated Survey for SuperNovae, Gaia, and Zwicky Transient Facility, we construct a compatible light-curve data set named LEAVES for automated recognition of variable stars, which can be used for training and testing new classification algorithms. The data set contains a total of 977,953 variable and 134,592 nonvariable light curves, in which the supported variables are divided into six superclasses and nine subclasses. We validate the compatibility of the data set through experiments and employ it to train a hierarchical random forest classifier, which achieves a weighted average F1-score of 0.95 for seven-class classification and 0.93 for 10-class classification. Experimental results prove that the classifier is more compatible than the classifier established based on a single band and a single survey, and has wider applicability while ensuring classification accuracy, which means it can be directly applied to different data types with only a relatively small loss in performance compared to a dedicated model.

The VSPEC Collection: A suite of utilities to model spectroscopic phase curves of 3D exoplanet atmospheres in the presence of stellar variability

We present the Variable Star PhasE Curve (VSPEC) Collection, a set of Python packages for simulating combined-light spectroscopic observations of 3-dimensional exoplanet atmospheres in the presence of stellar variability and inhomogeneity. VSPEC uses the Planetary Spectrum Generator’s Global Emission Spectra (PSG/GlobES) application along with a custom-built multi-component time-variable stellar model based on a user-defined grid of stellar photosphere models to produce spectroscopic light curves of the planet-host system. VSPEC can be a useful tool for modeling observations of exoplanets in transiting geometries (primary transit, secondary eclipse) as well as orbital phase curve measurements, and is built in a modular and flexible configuration for easy adaptability to new stellar and planetary model inputs. We additionally present a set of codes developed alongside the core VSPEC modules, including the stellar surface model generator vspec-vsm, the stellar spectral grid interpolation code GridPolator, and a Python interface for PSG, libpypsg.

Accurate, Precise, and Physically Self-consistent Ages and Metallicities for 400,000 Solar Neighborhood Subgiant Branch Stars

Age is the most difficult fundamental stellar parameter to infer for isolated stars. While isochrone-based ages are in general imprecise for both main-sequence dwarfs and red giants, precise isochrone-based ages can be obtained for stars on the subgiant branch transitioning from core to shell hydrogen burning. We synthesize Gaia Data Release 3–based distance inferences, multiwavelength photometry from the ultraviolet to the mid-infrared, and three-dimensional extinction maps to construct a sample of 289,759 solar-metallicity stars amenable to accurate, precise, and physically self-consistent age inferences. Using subgiants in the solar-metallicity open clusters NGC 2682 (i.e., M67) and NGC 188, we show that our approach yields accurate and physically self-consistent ages and metallicities with median statistical precisions of 8% and 0.06 dex, respectively. The inclusion of systematic uncertainties resulting from nonsingle or variable stars results in age and metallicity precisions of 9% and 0.12 dex, respectively. We supplement this solar-metallicity sample with an additional 112,062 metal-poor subgiants, including over 3000 stars with [Fe/H] ≲ −1.50, 7% age precisions, and apparent Gaia G-band magnitudes G < 14. We further demonstrate that our inferred metallicities agree with those produced by multiplexed spectroscopic surveys. As an example of the scientific potential of this catalog, we show that the solar neighborhood star formation history has three components at ([Fe/H], τ/Gyr) ≈ (+0.0, 4), (+0.2, 7), and a roughly linear sequence in age–metallicity space beginning at ([Fe/H], τ/Gyr) ≈ (+0.2, 7) and extending to (−0.5, 13). Our analyses indicate that the solar neighborhood includes stars on disk-like orbits even at the oldest ages and lowest metallicities accessible by our samples.

Revisiting the Period Change and Frequency Analysis of a Variable Star SZ Lyncis

This study aims to investigate the period change of the variable star SZ Lyncis. The observations were conducted at the Regional Observatory for the Public, Nakhon Ratchasima (ROP-NMA), operated by the National Astronomical Research Institute of Thailand (NARIT), on January 8th-9th and February 19th-20th 2024. The data was acquired using the CCD camera with a B-filter mount on the 0.7-m reflecting telescope. We report the period change of 1.37(±0.08)´10-12 day cycle-1. Analysis of the O-C diagram revealed the presence of a binary companion with a semi-amplitude of 0.0052(±0.0002) days, the projected semi-major axis of 0.90(±0.04) AU, the eccentricity of 0.25(±0.09), and the orbital period of 1184.1(±1.5) days and the mass function of 0.07(±0.01) M⨀. We also identified the time-series light curve data using the Discrete Fourier transform (DFT) in the Period04 package and obtained results of four frequencies. The main frequency of 8.29630(±0.00002) cycle day-1 corresponds to a period of 0.1205357(±0.0000003) days. These results contribute to our understanding of the pulsation properties of SZ Lyncis and its binary system.

Detecting and sizing the Earth with PLATO: A feasibility study based on solar data

The PLAnetary Transits and Oscillations of stars (PLATO) mission will observe the same area of the sky continuously for at least two years in an effort to detect transit signals of an Earth-like planet orbiting a solar-like star. We aim to study how short-term solar-like variability caused by oscillations and granulation would affect PLATO's ability to detect and size Earth if PLATO were to observe the Solar System itself. We also compare different approaches to mitigate noise caused by short-term solar-like variability and perform realistic transit fitting of transit signals in PLATO-like light curves. We injected Earth-like transit signals onto real solar data taken by the Helioseismic and Magnetic Imager (HMI) instrument on board the Solar Dynamics Observatory (SDO). We isolated short-term stellar variability in the HMI observations by removing any variability with characteristic timescales longer than five hours using a smooth Savitzky-Golay filter. We then added a noise model for a variety of different stellar magnitudes computed by assuming an observation by all 24 normal cameras. We first compared four different commonly used treatments of correlated noise in the time domain by employing them in a transit fitting scheme. We then tried to recover pairs of transit signals using an algorithm similar to the transit least squares algorithm. Finally, we performed transit fits using realistic priors on planetary and stellar parameters and assessed how accurately the pair of two injected transits was recovered. We find that short-term solar-like variability affects the correct retrieval of Earth-like transit signals in PLATO data. Variability models accounting for variations with typical timescales at the order of one hour are sufficient to mitigate these effects. We find that when the limb-darkening coefficients of the host star are properly constrained, the impact parameter does not negatively affect the detectability of a transit signal or the retrieved transit parameters, except for high values ($b &gt; 0.8$). For bright targets (8.5 - 10.5 mag), the transit signal of an Earth analogue can reliably be detected in PLATO data. For faint targets a detection is still likely, though the results of transit search algorithms have to be verified by transit-fitting algorithms to avoid false positive detections being flagged. For bright targets (V-mag leq 9.5), the radius of an Earth-like planet orbiting a solar-like star can be correctly determined at a precision of 3&lt;!PCT!&gt; or less, assuming that at least two transit events are observed and the characteristics of the host star are well understood.

Polarization position angle standard stars: a reassessment of θ and its variability for seventeen stars based on a decade of observations

ABSTRACT Observations of polarization position angle ($\theta$) standards made from 2014 to 2023 with the High Precision Polarimetric Instrument (HIPPI) and other HIPPI-class polarimeters in both hemispheres are used to investigate their variability. Multiband data were first used to thoroughly recalibrate the instrument performance by bench-marking against carefully selected literature data. A novel co-ordinate difference matrix (CDM) approach – which combines pairs of points – was then used to amalgamate monochromatic ($g^\prime$ band) observations from many observing runs and re-determine $\theta$ for 17 standard stars. The CDM algorithm was then integrated into a fitting routine and used to establish the impact of stellar variability on the measured position angle scatter. The approach yields variability detections for stars on long time-scales that appear stable over short runs. The best position angle standards are $\ell$ Car, o Sco, HD 154445, HD 161056, and $\iota ^1$ Sco, which are stable to $\le$0.123$^\circ$. Position angle variability of 0.27–0.82$^\circ$, significant at the 3$\sigma$ level, is found for 5 standards, including the Luminous Blue Variable HD 160529 and all but one of the other B/A-type supergiants (HD 80558, HD 111613, HD 183143, and 55 Cyg), most of which also appear likely to be variable in polarization magnitude (p) – there is no preferred orientation for the polarization in these objects, which are all classified as $\alpha$ Cygni variables. Despite this we make six key recommendations for observers – relating to data acquisition, processing and reporting – that will allow them to use these standards to achieve $\lt $ 0.1$^\circ$ precision in the telescope position angle with similar instrumentation, and allow data sets to be combined more accurately.

Fundamental Parameters of a Binary System Consisting of a Red Dwarf and a Compact Star

TIC 157365951 has been classified as a δ Scuti type by the International Variable Star Index. Through the spectra from Large Sky Area Multi-Object Fiber Spectroscopic Telescope and its light curve, we further discovered that it is a binary system. This binary system comprises a red-dwarf star and a compact star. Through the spectral energy distribution fitting, we determined the mass of the red dwarf star as M 1 = 0.31 ± 0.01M ⊙ and its radius as R 1 = 0.414 ± 0.004R ⊙. By fitting the double-peaked Hα emission, we derived the mass ratio of q = 1.76 ± 0.04, indicating a compact star mass of M 2 = 0.54 ± 0.01M ⊙. Using Phoebe to model the light curve and radial velocity curve for the detached binary system, we obtained a red dwarf star mass of M 1 = 0.29 ± 0.02M ⊙, a radius of R 1 = 0.39 ± 0.04R ⊙, and a Roche–lobe filling factor of f = 0.995 ± 0.129, which is close to the f = 1 expected for a semidetached system. The Phoebe model gives a compact star mass M 2 = 0.53 ± 0.05M ⊙. Constraining the system to be semidetached gives M 1 = 0.34 ± 0.02M ⊙, R 1 = 0.41 ± 0.01R ⊙, and M 2 = 0.62 ± 0.03M ⊙. The consistency of the models is encouraging. The value of the Roche–lobe filling factor suggests that there might be ongoing mass transfer. The compact star mass is as massive as a typical white dwarf.

Identification of Z Cam’s Historical Counterpart: The Quest for an Ancient Nova

Abstract In recent astronomical discussions, attempts have been made to link the known dwarf nova Z Cam to historical celestial events, particularly the ”guest star” phenomenon reported in China in 77 BCE. Despite other suggestions and the problems with regard to the location of the event in 77 BCE, its identification with Z Cam is used in the Variable Star IndeX (VSX) of the AAVSO and in several research papers that aim to derive knowledge on the evolution of cataclysmic variables. Through the reconstruction of the super-constellation of the Purple Palace in the Han Dynasty, we found that Z Cam is actually located outside this enclosure, contradicting the records of the 77 BCE guest star being ”within the Purple Palace”. With newly found text versions of the guest star in 77 BCE, we narrowed down the position given therein. Combined with a new analysis of accompanying divination text leads to the conclusion that this guest star was actually a comet. Finally, through meticulous examination and comparison, we conclude that the guest star of 369 CE appears the most plausible candidate for Z Cam’s historical counterpart, aligning with both textual evidence and modern astronomical observations.

TESS Detections of Pulsations in Of?p Stars in the Magellanic Clouds

We used Transiting Exoplanet Survey Satellite observations to search for pulsations in six known Of?p stars in the Magellanic Clouds. We find evidence for pulsational variability in three Of?p stars: UCAC4 115-008604, OGLE SMC-SC6 237339, and AzV 220. Two of them, UCAC4 115-008604 and OGLE SMC-SC6 237339, have been reported to possess kG-order magnetic fields. The obtained results are important to constrain and improve stellar evolution models.

On the membership of variable stars in galactic globular clusters: The Oosterhoff gap

On the membership of variable stars in galactic globular clusters: The Oosterhoff gap

Variable Stars in M31 Stellar Clusters from the Panchromatic Hubble Andromeda Treasury

Variable stars in stellar clusters can offer key constraints on stellar evolution and pulsation models, utilizing estimates of host cluster properties to constrain stellar physical parameters. We present a catalog of 86 luminous (F814W < 19) variable stars in M31 clusters identified by mining the archival Panchromatic Hubble Andromeda Treasury (PHAT) survey using a combination of statistical analysis of sparse PHAT light curves and difference imaging. We determine the evolutionary phases and initial masses of these variable stars by matching them with theoretical isochrones generated using host cluster properties from the literature. We calculate the probability of PHAT photometry being blended due to the highly crowded nature of cluster environments for each cluster-variable star, using these probabilities to inform our level of confidence in the derived properties of each star. Our 86 cluster-variable stars have initial masses between 0.8 and 67 M ⊙. Their evolutionary phases span the main sequence, more evolved hydrogen- and helium-burning phases, and the post–asymptotic giant branch. We identify numerous candidate variable star types: RV Tauri variables, red supergiants, and slowly pulsating B-type supergiants, along with Wolf–Rayet stars, α Cygni and Mira variables, a classical Cepheid, and a possible superasymptotic giant. We characterize 12 cluster-variable stars at higher confidence based on their difference image quality and lower blending probability. Ours is the first systematic study of variable stars in extragalactic stellar clusters leveraging the superior resolution of the Hubble Space Telescope and demonstrating the unique power of stellar clusters in constraining the fundamental properties of variable stars.

DECam Multimessenger Astrophysics Pipeline. I. From Raw Data to Single-exposure Candidates

We introduce a pipeline that performs rapid image subtraction and source selection to detect transients, with a focus on identifying gravitational-wave optical counterparts using the Dark Energy Camera (DECam). In this work, we present the pipeline steps from processing raw data to identification of astrophysical transients on individual exposures. We process DECam data and build difference images using the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) Science Pipelines software, and we use flags and principal component analysis to select transients on a per-exposure basis, without associating the results from different exposures. Those candidates will be sent to brokers for further classification and alert distribution. We validate our pipeline using archival exposures that cover various types of objects, and the tested targets include a kilonova (GW170817), supernovae, stellar flares, variable stars (in a resolved galaxy or the Milky Way Bulge), and serendipitous objects. Overall, the data processing produces clean light curves that are comparable with published results, demonstrating the photometric quality of our pipeline. Real transients can be well selected by our pipeline when sufficiently bright (signal-to-noise ratio ≳15). This pipeline is intended to serve as a tool for the broader research community. Although this pipeline is designed for DECam, our method can be easily applied to other instruments and future LSST observations.

Photometric Study of ZZ Mic Variable Star

Abstract We report on the periodicity determination and O-C study of ZZ Mic, a short-period variable star. We used photometric data obtained on October 16, 2015 at Bosscha Observatory using the STEVia telescope (diameter 11 inches) and CCD ST8-XME. We also used 51 light curve data archives from the American Association of Variable Star Observers (AAVSO) database and 33 time of maxima records from various publications. The whole data set ranges from 1960 to 2015. We determine that ZZ Mic has a pulsation period of 0.067179 days with first overtone period of 0.053678 days. We fit the O-C diagram with a polynomial function we determine that the pulsation period of ZZ Mic has been decreasing at a rate of 8.28014 × 10−9 days per year. Using a known period-radius relation, we obtain the radius of ZZ Mic is 1.705 solar radii.

The YMDB catalog: Young massive detached binaries for the determination of high-precision absolute stellar parameters

Context. Massive stars play a crucial role in the cosmic dynamics and chemical evolution of galaxies. Despite their significance, our understanding of their evolution and properties remains limited. An accurate determination of stellar parameters, such as the mass and radius, is essential for advancing our knowledge. Detached eclipsing binaries (DEBs) are particularly valuable for these determinations due to the minimal interaction between their stellar components, allowing for precise measurements. Aims. This study aims to introduce the Young Massive Detached Binary (YMDB) catalog, designed to address the gap in the high-precision absolute parameter determination for young massive stars. By focusing on DEBs within the spectral range O9-B1, this catalog seeks to provide a reliable database for future astronomical studies and improve our understanding of massive star evolution. Methods. We conducted a photometric analysis of 87 young massive stars in detached eclipsing systems using TESS light curves (LCs) that were processed through a custom pipeline. This analysis involved determining the amplitude of magnitude variations, orbital periods, times of minima, eccentricities, and the presence of apsidal motion and heartbeat phenomena. A thorough literature review was performed to obtain MK spectral classifications. We performed our own spectral classification of 19 systems to support the sample where a new classification was lacking or inconclusive. Results. The analysis identified 20 previously unreported binary systems, with 13 newly recognized as variable stars. Among the 87 stars examined, 30 are confirmed as YMDB members, and 25 are candidates pending spectral classification. The exclusion of the remaining 32 stars is attributed to unsuitable spectral types or their nondetached binary nature. Notable findings include the identification of new LC classifications, eccentricities in 13 systems, and heartbeat phenomena in several targets. Conclusions. The YMDB catalog offers a resource of high-quality LCs and reliable stellar classifications, serving as a valuable tool for the astronomical community.

Modern Methodologies in Machine Learning–Based Classification of Variable Stars

Modern Methodologies in Machine Learning–Based Classification of Variable Stars