Long-period Variables Research Articles

Calibration of the JAGB method for the Magellanic Clouds and Milky Way from Gaia DR3, considering the role of oxygen-rich AGB stars

The JAGB method is a new way of measuring distances in the Universe with the use of asymptotic giant branch (AGB) that are situated in a selected region in a $J$ versus $J- colour--magnitude diagram (CMD), and relying on the fact that the absolute $J$ magnitude is (almost) constant. It is implicitly assumed in the method that the selected stars are carbon-rich AGB stars (carbon stars). However, as the sample selected to determine J $ is purely colour based, there can also be contamination by oxygen-rich AGB stars in principle. As the ratio of carbon-rich to oxygen-rich stars is known to depend on metallicity and initial mass, the star formation history and age--metallicity relation in a galaxy should influence the value of $M_ J $. The aim of this paper is to look at mixed samples of oxygen-rich and carbon-rich stars for the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Milky way (MW) using the Gaia catalogue of long-period variables (LPVs) as a basis. The advantage of this catalogue is that it contains a classification of O- and C-stars based on the analysis of Gaia Rp spectra. The LPV catalogue is correlated with data from the Two Micron All Sky Survey (2MASS) and samples in the LMC, SMC, and the MW are retrieved. Following methods proposed in the literature, we report the mean and median magnitudes of the selected sample using different colour and magnitude cuts and the results of fitting Gaussian and Lorentzian profiles to the luminosity function (LF). For the SMC and LMC, we confirm previous results in the literature. The LFs of the SMC and LMC JAGB stars are clearly different, yet it can be argued that the mean magnitude inside a selection box agrees at the 0.021 mag level. The results of our analysis of the MW sample are less straightforward. The contamination by O-rich stars is substantial for a classical lower limit of $(J- 1.3$, and becomes less than 10<!PCT!> only for $(J- 1.5$. The sample of AGB stars is smaller than for the MCs for two reasons. Nearby AGB stars (with potentially the best determined parallax) tend to be absent as they saturate in the 2MASS catalogue, and the parallax errors of AGB stars tend to be larger compared to non-AGB stars. Several approaches have been taken to improve the situation but finally the JAGB LF for the MW contains about 130 stars and the fit of Gaussian and Lorentzian profiles is essentially meaningless. The mean and median magnitudes are fainter than for the MC samples by about 0.4 mag which is not predicted by theory. We do not confirm the claim in the literature that the absolute calibration of the JAGB method is independent of metallicity up to solar metallicity. A reliable calibration of the JAGB method at (near) solar metallicity should await further data releases, or should be carried out in another environment.

Orbital lifetime design and optimization of CORBES based on lunisolar perturbation

In order to design the orbit for the COntellation of Radiation BElt Survey(CORBES) mission orbit that meets the mission requirements and deorbit constraints, this paper proposed a universal method to design optimal orbital parameters for Highly Elliptic Orbit(HEO). The impact of lunisolar perturbation on perigee altitude is evident, and once a portion of the orbital parameters is determined, the mathematical relationship between the amplitude of the long-period variation in perigee (referred to as “perigee variation”) and the orbital parameters to be designed can be described. The perigee variation exhibits a significant correlation with the orbital lifetime. The initial optimization of orbit parameters is conducted by considering the correlation between lunisolar perturbations and orbital lifetime. Based on this, the orbital parameter RAAN is designed using an improved Decimal Ant Colony Optimization(DACO), and the corresponding launch window is given. The simulation results and comparisons with previous methods validate the effectiveness and efficiency of the initial optimization and the improved DACO in providing optimal feasible solutions, taking into account the law of lunisolar perturbation on orbital lifetime. This method can be well applied to design and estimate the lifetime of orbits dominated by lunisolar perturbation, such as HEO and Geostationary Transfer Orbit(GTO).

Application of Convolutional Neural Networks to time domain astrophysics. 2D image analysis of OGLE light curves

In recent years the amount of publicly available astronomical data has increased exponentially, with a remarkable example being large-scale multiepoch photometric surveys. This wealth of data poses challenges to the classical methodologies commonly employed in the study of variable objects. As a response, deep learning techniques are increasingly being explored to effectively classify, analyze, and interpret these large datasets. In this paper we use two-dimensional histograms to represent Optical Gravitational Lensing Experiment (OGLE) phasefolded light curves as images. We use a Convolutional Neural Network (CNN) to classify variable objects within eight different categories (from now on labels): Classical Cepheid (CEP), RR Lyrae (RR), Long Period Variable (LPV), Miras (M), Ellipsoidal Binary (ELL), Delta Scuti (DST), Eclipsing Binary (E), and spurious class with Incorrect Periods (Rndm). We set up different training sets to train the same CNN architecture in order to characterize the impact of the training. The training sets were built from the same source of labels but different filters and balancing techniques were applied. Namely: Undersampling (U), Data Augmentation (DA), and Batch Balancing (BB). The best performance was achieved with the BB approach and a training sample size of sim 370000 stars. Regarding computational performance, the image representation production rate is of sim 76 images per core per second, and the time to predict is sim 60$\ s $ per star. The accuracy of the classification improves from sim 92<!PCT!>, when based only on the CNN, to sim 98<!PCT!> when the results of the CNN are combined with the period and amplitude features in a two step approach. This methodology achieves comparable results with previous studies but with two main advantages: the identification of miscalculated periods and the improvement in computational time cost.

Non-radial oscillations mimicking a brown dwarf orbiting the cluster giant NGC 4349 No. 127

Context. Several evolved stars have been found to exhibit long-period radial velocity variations that cannot be explained by planetary or brown dwarf companions. Non-radial oscillations caused by oscillatory convective modes have been put forth as an alternative explanation, but no modeling attempt has yet been undertaken. Aims. We provide a model of a non-radial oscillation, aiming to explain the observed variations of the cluster giant NGC 4349 No. 127. The star was previously reported to host a brown dwarf companion, but whose existence was later refuted in the literature. Methods. We reanalyzed 58 archival HARPS spectra of the intermediate-mass giant NGC 4349 No. 127. We reduced the spectra using the SERVAL and RACCOON pipelines, acquiring additional activity indicators. We searched for periodicity in the indicators and correlations between the indicators and radial velocities. We further present a simulation code able to produce synthetic HARPS spectra, incorporating the effect of non-radial oscillations, and compare the simulated results to the observed variations. We discuss the possibility that non-radial oscillations cause the observed variations. Results. We find a positive correlation between chromatic index and radial velocity, along with closed-loop Lissajous-like correlations between radial velocity and each of the spectral line shape indicators (full width at half maximum, and contrast of the cross-correlation function and differential line width). Simulations of a low-amplitude, retrograde, dipole (l = 1, m = 1), non-radial oscillation can reproduce the observed behavior and explain the observables. Photometric variations below the detection threshold of the available ASAS-3 photometry are predicted. The oscillation and stellar parameters are largely in agreement with the prediction of oscillatory convective modes. Conclusions. The periodic variations of the radial velocities and activity indicators, along with the respective phase shifts, measured for the intermediate-mass cluster giant NGC 4349 No. 127, can be explained by a non-radial oscillation.

The Complex Star Formation History of the Halo of NGC 5128 (Cen A)

NGC 5128 (Cen A) is the nearest giant elliptical galaxy and one of the brightest extragalactic radio sources in the sky, boasting a prominent dust lane and jets emanating from its nuclear supermassive black hole. In this paper, we construct the star formation history (SFH) of two small fields in the halo of NGC 5128: a northeastern field (Field 1) at a projected distance of ∼18.8 kpc from the center, and a southern field (Field 2) ∼9.9 kpc from the center. Our method is based on identifying long-period variable (LPV) stars that trace their sibling stellar population and hence historical star formation due to their high luminosity and strong variability; we identified 395 LPV stars in Field 1 and 671 LPV stars in Field 2. Even though the two fields are ∼28 kpc apart on opposite sides from the center, they show similar SFHs. In Field 1, the star formation rate (SFR) increased significantly around t ∼ 800 Myr and t ∼ 3.8 Gyr and in Field 2, the SFR increased considerably around t ∼ 800 Myr, t ∼ 3.8 Gyr, and t ∼ 6.3 Gyr, where t is the lookback time. The increase in SFR ∼800 Myr ago agrees with previous suggestions that the galaxy experienced a merger around that time. The SFH reconstructed from LPV stars supports a scenario in which multiple episodes of nuclear activity lead to episodic jet-induced star formation. While there is no catalog of LPV stars for the central part of NGC 5128, applying our method to the outer regions (for the first time in a galaxy outside the Local Group) has enabled us to put constraints on the complex evolution of this cornerstone galaxy.

Kinematics and dynamics of the Galactic bar revealed by Gaia long-period variables

ABSTRACT We use low-amplitude long period variable (LA-LPV) candidates in Gaia DR3 to trace the kinematics and dynamics of the Milky Way bar. LA-LPVs, like other LPVs, are intrinsically bright and follow a tight period–luminosity relation, but unlike e.g. Mira variables, their radial velocity measurements are reliable due to their smaller pulsation amplitudes. We supplement the Gaia astrometric and radial velocity measurements with distance moduli assigned using a period–luminosity relation to acquire full 6D phase space information. The assigned distances are validated by comparing to geometric distances and StarHorse distances, which shows biases less than $\sim 5~{{\rm per\ cent}}$. Our sample provides an unprecedented panoramic picture of the inner Galaxy with minimal selection effects. We map the kinematics of the inner Milky Way and find a significant kinematic signature corresponding to the Galactic bar. We measure the pattern speed of the Galactic bar using the continuity equation and find $\Omega _{\rm b}=34.1\pm 2.4$ km s$^{-1}$ kpc$^{-1}$. We develop a simple robust and potential-independent method to measure the dynamical length of the bar using only kinematics and find $R_{\rm b}\sim 4.0$ kpc. We validate both measurements using N-body simulations. Assuming knowledge of the gravitational potential of the inner Milky Way, we analyse the orbital structure of the Galactic bar using orbital frequency ratios. The $x_1$ orbits are the dominant bar-supporting orbital family in our sample. Amongst the selected bar stars, the $x_1 v_1$ or ‘banana’ orbits constitute a larger fraction ($\sim 15~{{\rm per\ cent}}$) than other orbital families in the bar, implying that they are the dominant family contributing to the Galactic X-shape, although contributions from other orbital families are also present.

An Automated Catalog of Long Period Variables using Infrared Lightcurves from Palomar Gattini-IR

Long Period Variables (LPVs) are stars with periods of several hundred days, representing the late, dust-enshrouded phase of stellar evolution in low to intermediate mass stars. In this paper, we present a catalog of 154,755 LPVs using near-IR lightcurves from the Palomar Gattini-IR (PGIR) survey. PGIR has been surveying the entire accessible northern sky (δ > −28°) in the J-band at a cadence of 2–3 days since 2018 September, and has produced J-band lightcurves for more than 60 million sources. We used a gradient-boosted decision tree classifier trained on a comprehensive feature set extracted from PGIR lightcurves to search for LPVs in this data set. We developed a parallelized and optimized code to extract features at a rate of ∼0.1 s per lightcurve. Our model can successfully distinguish LPVs from other stars with a true positive rate of 95%. Cross-matching with known LPVs, we find 70,369 (∼46%) new LPVs in our catalog.

OH and H2O Maser Activities Among Long-period Variables

The distribution within an infrared color–magnitude diagram of variable red giants known to have circumstellar masers is reported. Stars with H2O-maser sources occur within a color and period domain of (J − K s ) > 1.15 and P > 199 days, respectively, often in the absence of OH masers. By contrast, OH masers are mostly detected among cooler red giants with (J − K s ) > 1.35 and P > 316 days, and can coexist with H2O-maser sources. The results suggest that red giants must evolve to absolute magnitudes brighter than MKs=−6.8 prior to enabling H2O-maser emission, and possibly expand beyond a certain size before triggering OH masers.

North-PHASE: studying periodicity, hot spots, accretion stability, and early evolution in young stars in the Northern hemisphere

ABSTRACT We present the overview and first results from the North-PHASE Legacy Survey, which follows six young clusters for five years, using the 2 deg$^2$ FoV of the JAST80 telescope from the Javalambre Observatory (Spain). North-PHASE investigates stellar variability on time-scales from days to years for thousands of young stars distributed over entire clusters. This allows us to find new YSO, characterize accretion, and study inner disc evolution within the cluster context. Each region (Tr 37, Cep OB3, IC 5070, IC 348, NGC 2264, and NGC 1333) is observed in six filters (SDSS griz, u band, and J0660, which covers H$\alpha$), detecting cluster members as well as field variable stars. Tr 37 is used to prove feasibility and optimize the variability analysis techniques. In Tr 37, variability reveals 50 new YSO, most of them proper motion outliers. North-PHASE independently confirms the youth of astrometric members, efficiently distinguishes accreting and non-accreting stars, reveals the extent of the cluster populations along Tr37/IC 1396 bright rims, and detects variability resulting from rotation, dips, and irregular bursts. The proper motion outliers unveil a more complex star formation history than inferred from Gaia alone, and variability highlights previously hidden proper motion deviations in the surrounding clouds. We also find that non-YSO variables identified by North-PHASE cover a different variability parameter space and include long-period variables, eclipsing binaries, RR Lyr, and $\delta$ Scuti stars. These early results also emphasize the power of variability to complete the picture of star formation where it is missed by astrometry.

Mid-infrared Period–Luminosity Relations of Gaia DR3 Long Period Variables

Long period variable (LPV) stars are very promising distance indicators in the infrared bands. We selected asymptotic giant branch (AGB) stars in the Large and Small Magellanic Cloud (LMC and SMC) from the Gaia Data Release 3 LPV catalog, and classified them into oxygen-rich (O-rich) and carbon-rich (C-rich) AGB stars. Using the Wide-field Infrared Survey Explorer database, we determined the W1- and W2-band period–luminosity relations (PLRs) for each pulsation-mode sequence of AGB stars. The dispersion of the PLRs of O-rich AGB stars in sequences C′ and C is relatively small, around 0.14 mag. The PLRs of LMC and SMC are consistent in each sequence. In the W2 band, the PLR of large-amplitude C-rich AGB stars is steeper than that of small-amplitude C-rich AGB stars, due to their more circumstellar dust. By two methods, we find that some PLR sequences of O-rich AGB stars in the LMC are dependent on metallicity. The coefficients of the metallicity effect are β = –0.533 ± 0.213 mag dex1 and β = –0.767 ± 0.158 mag dex1 for sequence C in W1 and W2 bands, respectively. The significance of the metallicity effect in W1 band for the four sequences is 2.2−3.5σ. Both of these imply that distance measurements using O-rich Mira may need to take the metallicity effect into account.

Comprehensive Analysis on GPS Carrier Phase under Various Cutoff Elevation Angles and Its Impact on Station Coordinates’ Repeatability

When processing the carrier phase, the global navigation satellite system (GNSS) grants the highest precision for geodetic measurements. The analysis centers (ACs) from the International GNSS Service (IGS) provide different data such as precise clock data, precise orbits, reference frame, ionosphere and troposphere data, as well as other geodetic products. Each individual AC has its own strategy for delivering the abovementioned products, with one of the key elements being the cutoff elevation angle. Typically, this angle is arbitrarily chosen using generic values without studying the impact of this choice on the obtained results, in particular when very precise positions are considered. This article addresses this issue. To this end, the article has two key sections, and the first is to evaluate the impact of using the two different cutoff elevation angles that are most widely used: (a) 3 degrees cutoff and (b) 10 degrees cutoff elevation angle. This analysis is completed in two major parts: (i) the analysis of the root mean square (RMS) for the carrier phase and (ii) the analysis of the station position in terms of repeatability. The second key section of the paper is a comprehensive carrier phase analysis conducted by adopting a new approach using a mean of the 25-point average RMS (A-RMS) and the single-point RMS and using an ionosphere-free linear combination. By using the ratio between the 25-point average RMS and the single-point RMS we can define the type of scatter that dominates the phase solution. The analyzed data span a one-year period. The tested GNSS stations belong to the EUREF Permanent Network (EPN) and the International GNSS Service (IGS). These comprise 55 GNSS stations, of which only 23 GNSS stations had more than 95% data availability for the entire year. The RMS and A-RMS are analyzed in conjunction with the precipitable water vapor (PWV), which shows clear signs of temporal correlation. Of the 23 GNSS stations, three stations show an increase of around 50% of the phase RMS when using a 3° cutoff elevation angle, and only four stations have a difference of 5% between the phase RMS when using both cutoff elevation angles. When using the A-RMS, there is an average improvement of 37% of the phase scatter for the 10° cutoff elevation angle, whereas for the 3° cutoff elevation angle, the improvement is around 33%. Based on studying this ratio, four stations indicate that the scatter is dominated by the stronger-than-usual dominance of long-period variations, whereas the others show short-term noise. In terms of station position repeatability, the weighted root mean square (WRMS) is used as an indicator, and the results between the differences of using a 3° and 10° cutoff elevation angle strategy show a difference of −0.16 mm for the North component, −0.21 mm for the East component and a value of −0.75 mm for the Up component, indicating the importance of using optimal cutoff angles.

The M33 synoptic stellar survey. III. Miras and LPVs in griJHKS

ABSTRACT We present the results of a search for Miras and long-period variables (LPVs) in M33 using griJHKS archival observations from the Canada–France–Hawaii Telescope. We use multiband information and machine learning techniques to identify and characterize these variables. We recover ∼1300 previously discovered Mira candidates and identify ∼13 000 new Miras and LPVs. We detect for the first time a clear first-overtone pulsation sequence among Mira candidates in this galaxy. We use O-rich, fundamental-mode Miras in the LMC and M33 to derive a distance modulus for the latter of μ = 24.629 ± 0.046 mag.

High mass function ellipsoidal variables in the Gaia Focused Product Release: searching for black hole candidates in the binary zoo

The recent Gaia Focused Product Release contains radial velocity time-series for more than 9,000 Gaia long-period photometric variables. Here we search for binary systems with large radial velocity amplitudes to identify candidates with massive, unseen companions. Eight targets have binary mass function f(M)>1M⊙, three of which are eclipsing binaries. The remaining five show evidence of ellipsoidal modulations. We fit spectroscopic orbit models to the Gaia radial velocities, and fit the spectral energy distributions of three targets. For the two systems most likely to host dark companions, J0946 and J1640, we use PHOEBE to fit the ASAS-SN light curves and Gaia radial velocities. The derived companion masses are >3M⊙, but the high Galactic dust extinctions towards these objects limit our ability to rule out main sequence companions or subgiants hotter than the photometric primaries. These systems are similar to other stellar-mass black hole impostors, notably the Unicorn (V723 Mon) and the Giraffe (2M04123153+6738486). While it is possible that J1640 and J0946 are similar examples of stripped giant star binaries, high-resolution spectra can be used to determine the nature of their companions.

Variability in Protoplanetary Nebulae. X. Multiyear Periods as an Indicator of Potential Binaries

New observations are presented of four evolved objects that display long, multiyear variations in their light curves. These are interpreted as good evidence of their binary nature, with the modulation caused by the barycenter motion of the evolved star resulting in a periodic obscuration by a circumbinary disk. Although protoplanetary nebulae (PPNe) commonly possess bipolar nebulae, which are thought to be shaped by a binary companion, there are very few PPNe in which a binary companion has been found. Three of the objects in this study appear to be PPNe, IRAS 07253−2001, 08005−2356, and 17542−0603, with long periods of 5.2, 6.9, and 8.2 yr, respectively. The binary nature of IRAS 08005−2356 has recently been confirmed by a radial velocity study. Two samples, one of PPNe and the other of post-AGB star candidates, are investigated for further evidence on how common is a long-period light-curve variation. Both samples suggest such light-curve variations are not common. The fourth object, IRAS 20056+1834 (QY Sge), is an obscured RV Tau variable of the RVb subclass, with a long period of 3.9 yr and pulsation periods of 102.9 and 51.5 days. The period of this object is seen to vary by 2%. Evidence is presented for a recent mass ejection in IRAS 17542−0603.

Gaia Focused Product Release: Radial velocity time series of long-period variables

Context. The third Gaia Data Release (DR3) provided photometric time series of more than 2 million long-period variable (LPV) candidates. Anticipating the publication of full radial-velocity data planned with Data Release 4, this Focused Product Release (FPR) provides radial-velocity time series for a selection of LPV candidates with high-quality observations. Aims. We describe the production and content of the Gaia catalog of LPV radial-velocity time series, and the methods used to compute the variability parameters published as part of the Gaia FPR. Methods. Starting from the DR3 catalog of LPV candidates, we applied several filters to construct a sample of sources with high-quality radial-velocity measurements. We modeled their radial-velocity and photometric time series to derive their periods and amplitudes, and further refined the sample by requiring compatibility between the radial-velocity period and at least one of the G, GBP, or GRP photometric periods. Results. The catalog includes radial-velocity time series and variability parameters for 9614 sources in the magnitude range 6 ≲ G/mag ≲ 14, including a flagged top-quality subsample of 6093 stars whose radial-velocity periods are fully compatible with the values derived from the G, GBP, and GRP photometric time series. The radial-velocity time series contain a mean of 24 measurements per source taken unevenly over a duration of about three years. We identify the great majority of the sources (88%) as genuine LPV candidates, with about half of them showing a pulsation period and the other half displaying a long secondary period. The remaining 12% of the catalog consists of candidate ellipsoidal binaries. Quality checks against radial velocities available in the literature show excellent agreement. We provide some illustrative examples and cautionary remarks. Conclusions. The publication of radial-velocity time series for almost ten thousand LPV candidates constitutes, by far, the largest such database available to date in the literature. The availability of simultaneous photometric measurements gives a unique added value to the Gaia catalog.

VVV-WIT-12 and Its Fashionable Nebula: A 4 yr Long-period Young Stellar Object with a Light Echo?

We report the serendipitous discovery of VVV-WIT-12, an unusual variable source that seems to induce variability in its surrounding nebula. The source belongs to the rare objects that we call WITs (short for What Is This?) discovered within the VISTA Variables in the Vía Láctea (VVV) survey. VVV-WIT-12 was discovered during a pilot search for light echoes from distant supernovae in the Milky Way using the near-IR images of the VVV survey. This source has an extremely red spectral energy distribution, consistent with a very reddened (A V ∼ 100 mag) long-period variable star (P ∼ 1525 days). Furthermore, it is enshrouded in a nebula that changes brightness and color with time, apparently in sync with the central source variations. The near-IR light curve and complementary follow-up spectroscopy observations are consistent with a variable young stellar object illuminating its surrounding nebula. In this case the source periodic variation along the cycles produces an unprecedented light echo in the different regions of the nebula.

Atmospheric Planetary Waves at Ionospheric Heights Measured at the Moscow Observatory (IZMIRAN)

The variations of the ionospheric current in the lower ionosphere and the plasma density of the upper ionosphere have been studied based on the monitoring of the horizontal component of the geomagnetic field induction and high-frequency sounding of the critical frequency of the ionosphere F-layer at the Moscow Observatory. It is shown that in the spectra of time variations of the geomagnetic field and the critical frequency of the F2 layer in the range of planetary waves in winter, there are both harmonics associated with solar activity and harmonics corresponding to quasi-5, 10, and 16-day planetary waves. Involvement of geomagnetic field registration data for a twenty-year time interval (from 2001 to 2020) made it possible to identify more subtle effects, namely, the harmonics associated with the modulation effect of longer-period variations and tidal effects were identified.

Astronomical control on organic matter enrichment of lacustrine mudstones in the first member of the Late Cretaceous Qingshankou Formation, the Songliao Basin, NE China

Astronomical forcing has an important effect on organic matter enrichment in marine mudstones, but this effect remains poorly constrained in lacustrine mudstones. Here, we investigate the astronomical control of organic matter enrichment in lacustrine black mudstone from the first member of the Qingshankou Formation in the Songliao Basin. Detailed mineralogical, lithological, geochemical, and cyclostratigraphic studies were carried out. The results show that there are high-frequency variations in the organic matter content in the mudstones of the Qingshankou Formation. The upper and lower sections are further divided according to the organic matter content level and the variation frequency. The organic matter content in the lower section is high, up to 6.08 wt%, with long-period variation and individual cycles around 10 m. The organic matter content in the upper section is relatively low, the highest organic matter content is only 2.91 wt%, with short-period variation and individual cycles mostly less than 5 m. Cyclostratigraphic studies indicate that significant astronomical signals are recorded in the mudstone stratigraphy. 100 kyr short eccentricity and 38 kyr obliquity strongly control the variation of organic matter content in the lower and upper sections, respectively. Organic matter enrichment has a closely synchronized phase relationship with paleoproductivity, preservation conditions, and dilution indicators. Astronomical cycles influence the depositional conditions of organic matter enrichment by controlling climate change, which eventually forces organic matter enrichment. At high values of eccentricity and obliquity, enhanced insolation, monsoon, precipitation, and river runoff caused lake expansion, increased input of terrestrial nutrients, and increased bottom water reduction conditions, which are conducive to organic matter enrichment. On the contrary, the depositional conditions for organic matter enrichment become poor.

An innovative tool for automating classification of stellar variability through nonlinear data analytics

Though Classical Cepheids, δ-Scuti, Eclipsing Binary, Long-Period variables, and RRLyraes are abundant in most of the clusters, automating the classification of the objects faces challenges. Since the rate at which the data has been getting accumulated is enormous, this automation of classification is paramount for carrying out appropriate analysis of the objects depending on the class it belongs to. Our results prove that the proposed tool for automating stellar classification not only reduces misclassification by up to 94.79% (in case of classification between multimode subclass of δ-Scuti and Mira subclass of Long-Period variables) but also improves reliability by as high as 78.35% (in case of conventionally misclassified pair of RRab subclass of RRLyrae and Fundamental Mode subclass of Classical Cepheids). Our random forest model has achieved a cross-validation accuracy of 0.88 with conventional statistical parameters coupled with tools of Nonlinear Dynamical Theory. It has achieved the highest precision and recalls for Long-Period variables of the Mira subclass (i.e., 0.99 & 0.99) and the lowest for Eclipsing Binary of subclass contact (i.e., 0.81 & 0.77). A positive improvement in accuracy rate by 7.3% is observed when compared with a model based on a conventional statistical platform. This proves the significance of introducing the proposed tools in devising an automated classification model for stellar variables.

Variable stars in the field of the Galactic bulge globular cluster NGC 6522

We present a comprehensive analysis of the variable stars projected on the field of the Galactic bulge globular cluster NGC 6522, offering valuable insights into their characteristics. Using proper motions from Gaia-DR3, we aim to distinguish between field stars and true cluster members. For an accurate color-magnitude diagram of the member stars, we produced a differential reddening map. We detect and discuss the peculiarities of variable stars of the types RR Lyrae, type II Cepheids, Long Period Variables (LPV) and eclipsing binaries, whose light curves are available through the OGLE III and IV databases. Notably, we explore the variable V24, which shows a prominent phase modulation resulting from period changes in a time scale of 1100 days. The variable stars among the cluster members serve as indicators of the cluster metallicity and distance; these determinations are based on their light curves. With the Fourier light curve decomposition of three RRc stars, we have derived the following cluster parameters: the metallicity in the spectroscopic scale text{[Fe/H]}_{mathrm{UVES}}=-1.16 pm 0.09; and the mean distance D=8.77 pm 0.16 kpc.