Observations Of Variable Stars Research Articles

Photometric Analysis of the Cataclysmic Variable Star LT Eri

Abstract LT Eri is a long-period eclipsing cataclysmic variable star (CV). This study investigates its outbursts, negative superhumps (NSHs), tilted-disk precession, eclipse-depth variations, and observed minus calculated (O – C) behavior, using photometric data from the Transiting Exoplanet Survey Satellite (TESS), the Zwicky Transient Facility, the All-Sky Automated Survey for Supernovae, and the American Association of Variable Star Observers. Our analysis reveals an outburst period of 14.7(31) days, with variable periods ranging from 11.62(4) to 22.15(13) days. Both long and short outbursts were detected, lasting approximately 18 and 10 days, respectively. Analysis of the TESS photometry shows that the NSH period is updated to 0.16265161(7) days, persists during the outburst, and its amplitude is correlated with the outburst phase. A suspected signal of tilted-disk precession with a period of 3.5781(6) days is also detected. Additionally, the eclipse depth shows a periodic variation of 3.6270(9) days, closely matching the tilted-disk precession period; however, no biperiodic variation is found, as seen in the dwarf nova HS 2325+8205, suggesting that this phenomenon is not universal. An O – C analysis reveals a weak oscillation with a period of 3.5772(6) days. Our superposition of sine curves consistent with the NSH period and amplitude, along with mean eclipse curves, produces periodic O – C variations with a period of 3.6666(2) days. This indicates that the observed periodic variations in O – C, often around the disk precession period in CVs, may result from the beat between the NSHs and the eclipse rather than actual variations in the accretion disk.

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.

Identifying Light-curve Signals with a Deep-learning-based Object Detection Algorithm. II. A General Light-curve Classification Framework

Vast amounts of astronomical photometric data are generated from various projects, requiring significant effort to identify variable stars and other object classes. In light of this, a general, widely applicable classification framework would simplify the process of designing specific classifiers for various astronomical objects. We present a novel deep-learning framework for classifying light curves using a weakly supervised object detection model. Our framework identifies the optimal windows for both light curves and power spectra automatically, and zooms in on their corresponding data. This allows for automatic feature extraction from both time and frequency domains, enabling our model to handle data across different scales and sampling intervals. We train our model on data sets obtained from Kepler, TESS, and Zwicky Transient Facility multiband observations of variable stars and transients. We achieve an accuracy of 87% for combined variable and transient events, which is comparable to the performance of previous feature-based models. Our trained model can be utilized directly for other missions, such as the All-sky Automated Survey for Supernovae, without requiring any retraining or fine-tuning. To address known issues with miscalibrated predictive probabilities, we apply conformal prediction to generate robust predictive sets that guarantee true-label coverage with a given probability. Additionally, we incorporate various anomaly detection algorithms to empower our model with the ability to identify out-of-distribution objects. Our framework is implemented in the Deep-LC toolkit, which is an open-source Python package hosted on Github (https://github.com/ckm3/Deep-LC) and PyPI.

The deep eclipses of RW Aur revisited by long‐term photometric and spectroscopic monitoring

AbstractRW Aurigae is a young stellar system containing a classical T Tauri star as the primary component. It shows deep, irregular dimmings, first detected in 2010. At the University Observatory Jena, we carried out optical follow‐up observations. We performed multiband (BVRI) photometry of the system with the Cassegrain‐Teleskop‐Kamera II and the Schmidt‐Teleskop‐Kamera between September 2016 and April 2019, as well as spectroscopy, using the Fibre Linked ÉCHelle Astronomical Spectrograph between September 2016 and April 2018. We present the apparent photometry of RW Aur, which is consistent with and complementary to photometric data from the American Association of Variable Star Observers. The V‐band magnitude of RW Aur changed by up to three magnitudes during the timespan of our monitoring campaign. For the observing epochs 2016/2017 and 2017/2018 we report a decreasing brightness, while in the epoch 2018/2019 the system remained in a relatively constant bright state. In the color‐magnitude diagram, we see that RW Aur lies close to a track for grey extinction. The spectra show a decreasing equivalent width (EW) of the emission line for decreasing brightness, whereas the EW of the line increases, indicating an increased outflow activity during the obscuration. Both give further evidence for the favored theory that the obscurations are caused by a hot dusty wind emerging from the inner disk.

Light curves for 10 R Coronae Borealis stars for longer than a century: secular evolution, dip statistics, and a general model for the shape of isolated light-curve dips

ABSTRACT R Coronae Borealis stars (RCBs) are cool supergiants that display non-periodic deep dips in brightness. Recently, a group of ‘Hot RCB stars’ has been discovered to be fast evolving across the HR (Hertzsprung-Russell) diagram, as these stars leave the RCB region, with brightness changes at the rate of ∼1 mag per century. Perhaps cool RCB stars can also be seen evolving, either increasing in temperature as they evolve to become Hot RCB stars, or perhaps increasing in luminosity as the stars arrive at the RCB region. To seek these changes, the only possible method is to extract archival data going back more than a century, looking for the brightness changes associated with the evolution. I have measured and extracted 323 464 mag (mostly from the Harvard plates and from the American Association of Variable Star Observers) for 10 cool RCB stars, all with over a century for the light curves, all consistently calibrated to a modern magnitude system. For times away from any dips, these light curves are flat to within the typical uncertainty of ±0.10 mag per century. That is, I see no significant evolution. I also have collected a large data base of light-curve dips and their properties. From this, the light curves for all the well-observed isolated dips have the same shape, featuring a flat slope for the few days immediately after the minima. Further, I derive a general model for the shape of the light curve for all isolated RCB dips, with a simple equation accurately describing the observed recovery to maximum light.

Evolution in pulsating variable stars: Long term, and inter-cycle

Abstract We used machine learning techniques to predict period changes in variable stars using noisy and sparse time series data, while inferring underlying physics and generalizing predictions about cycle-to-cycle variations. Our focus was on Mira variables, a well-known class of pulsating stars. Pre-processing data from Mira, R Andromedae, U Orionis, and Chi Cygni, obtained from the American Association of Variable Star Observers (AAVSO), we predicted luminosity magnitude uncertainty and classified pulsation states. Employing various classification and regression algorithms, along with feature engineering, we aimed to generalize predictions. We created a generalized dataset with collective averaged data points, limiting our analysis to a common time duration. Linear regression models yielded no successful predictions, but Decision Tree and KNN regressors accurately predicted luminosity magnitude errors, indicative of variation over time. Feature engineering successfully aided regression and classification of pulsating star states. After hyper parameter tuning using Bayesian Neural networks, we achieved a classification accuracy of 0.8 and 0.94 for the KNN classifier, respectively, in classifying pulsation states of Mira variables. The regression model achieved an R2 score of 0.98. Our work provides a foundation for developing tools to analyze various pulsating star variables, including Cepheids, RR Lyrae, and Delta Scuti variables, as well as other astrophysical data. These techniques demonstrate impressive performance with time series datasets.

Miras as a distance indicator in the CSST, JWST, and Gaia era

Mira variables are important distance indicators owing to the clear period–luminosity relation at infrared wavelengths. We have compiled a sample of 343 Galactic Miras selected from the American Association of Variable Star Observers database and Gaia Data Release 3. We used V-band observations from the American Association of Variable Star Observers to generate template light curves and derived mean magnitudes in the near-infrared JHK band from the Two Micron All-Sky Survey. Based on the Gaia-Two Micron All-Sky Survey diagram and the Gaia-LPV2 catalog, we classified 299 O-rich and 44 C-rich Miras. We fitted the Galactic period–luminosity relations for O-rich Miras in the near-infrared JHK band and estimated slopes (–2.74 ± 0.32, –3.14 ± 0.30, and –3.50 ± 0.25) and zero points (–5.58 ± 0.06 mag, –6.40 ± 0.06 mag, and –6.84 ± 0.05 mag) defined at logP (days) = 2.30, separately. Although the uncertainties of our derived Galactic PLRs for Miras are still larger than those of other galaxies, the multi-band sky survey conducted using the China Space Survey Telescope could help build more reliable and accurate PLRs for Miras, particularly in a near-infrared y band (927–1,080 nm).

Frequency Analysis of High Amplitude δ Scuti Star V593 Lyr by American Association of Variable Stars Observers International Database

The new results of light variability and frequency analysis of a high amplitude δ Scuti star (HADS) V593 Lyr were reported. δ Scuti-type variable stars are fascinating short-period stars with luminosity classes from III to V and spectral types from A0 to F6. Their location on the Hertzsprung-Russell (H-R) diagram is in the classical cepheid instability strip. The photometric data in the V filter and clear filter of V593 Lyr were collected from the American Association of Variable Stars Observers (AAVSO) international database. Discrete Fourier transformation and pre-whitening methods were used in the Period04 package to examine time-series light curve data. During 15 observational nights, we obtained 28 times of new light maxima magnitude variability and significant pulsation frequencies for the star. The study found that the fundamental frequency of V593 Lyr is 9.7894180 ± 0.0000002 d−1, corresponding to the main pulsation period of about 0.1021511 d and detected its second to fifth harmonics with a signal-to-noise ratio (S/N) of more than four. A study of the pulsation frequencies and period change of HADS stars can estimate their pulsation modes and evolution. Moreover, HADS stars are suitable targets for understanding asteroseismology, which is the subject that applies a star's pulsations to investigate its internal structure, and the high amplitude stars provide reference points for calculating cosmic distances in the Universe.

A Critical Comment on “Can Solar Cycle 25 Be a New Dalton Minimum?”

The sunspot number is the most used solar-activity index to study the behavior of solar activity. In this work, we reproduce the methodology of Coban, Raheem, and Cavus (Solar Phys. 296, 156, 2021) using a long short-term memory model with daily data from the American Association of Variable Star Observers (AAVSO) to predict the maximum amplitude of Solar Cycle 25. We have also used that same methodology with daily values from the official sunspot number (Version 2) of the Sunspot Index and Long-term Solar Observations (SILSO). The objective of this work is to analyze if the predictions obtained from that methodology agree with the observed values available for the current Solar Cycle 25. Thus, we conclude that the predictions are not reproducing well the behavior of the Solar Cycle 25 in its rising phase. Moreover, contrary to the previous prediction, no minor peak occurred in February 2022, and we also conclude that it seems unlikely that the combination of the solar-activity level of Solar Cycle 24 and 25 constitutes a new Dalton-type Minimum, such as Coban, Raheem, and Cavus (2021) proposed.

A survey for variable young stars with small telescopes: VI – Analysis of the outbursting Be stars NSW 284, gaia 19eyy, and VES 263

ABSTRACT This paper is one in a series reporting results from small telescope observations of variable young stars. Here, we study the repeating outbursts of three likely Be stars based on long-term optical, near-infrared, and mid-infrared photometry for all three objects, along with follow-up spectra for two of the three. The sources are characterized as rare, truly regularly outbursting Be stars. We interpret the photometric data within a framework for modelling light-curve morphology, and find that the models correctly predict the burst shapes, including their larger amplitudes and later peaks towards longer wavelengths. We are thus able to infer the start and end times of mass loading into the circumstellar discs of these stars. The disc sizes are typically 3 – 6 times the areas of the central star. The disc temperatures are ∼40 per cent, and the disc luminosities are ∼10 per cent of those of the central Be star, respectively. The available spectroscopy is consistent with inside-out evolution of the disc. Higher excitation lines have larger velocity widths in their double-horned shaped emission profiles. Our observations and analysis support the decretion disc model for outbursting Be stars.

Photometric Observations of Nine High Mass X-Ray Binaries and Analysis of Potential Periodicities and Variations

Beginning with American Association of Variable Star Observers Alert Notice 348 (2007 April 6) and being continually extended and updated through 2020 April 17 with Alert Notice 377, Dr. Gordon Sarty requested continuing assistance with his ongoing radial velocity observations of High Mass X-Ray Binaries. As of Alert Notice 377, nine targets were identified. These are: 1H 1936+541, RX J2030.5+4751, V2175 Cyg, V1008 Cep, V831 Cas, RX J0440.9+4431/LS V+44 17, V420 Aur, HD 249179, and SAX J0635+0533. While these Alert Notices prioritized radial velocity observations with the objective of determining orbital periods of these binary systems, optical photometry can also reveal other periods and variations due to mass transfer, pulsations, rotations, spin rates, and other effects. This Research Note will address in more detail the observation and analysis of one of these nine High Mass X-Ray Binaries, specifically 1H 1936+541.

Photometric variability of high amplitude δ Scuti stars using AAVSO international database: V2455 Cyg and GP And

This research aims to study photometric variability and Fourier analysis of high amplitude δ Scuti stars (HADS), which are interesting short-period variable stars with spectral types between A2V and F0V. They are located in the area of the classical cepheid instability strip, which crosses the main-sequence (MS) on the Hertzsprung-Russell (H-R) diagram. The CCD photometric V-magnitude data of the selected HADS stars were acquired based on the AAVSO international database (American Association of Variable Stars Observers), which is the most comprehensive digital variable star database. Two interesting targets V2455 Cyg and GP And were selected for the study. The discrete Fourier transformation was used to analyze time-series light curves. The pulsating properties of the selected HADS stars were analyzed using the Period04 astronomical program. We obtained each star’s times of new light maxima, magnitude changes, and pulsation frequencies. The main frequency of V2245 Cyg was determined to be 10.614995 ± 0.000002 d−1 and the main frequency of GP And is 12.709253 ± 0.000001 d−1. V2245 Cyg and GP And are high amplitude δ Scuti stars. The study of the pulsation frequencies and period change of HADS stars can estimate their pulsation modes and their evolution. Moreover, HADS stars are very important as standard candles that can be used to measure galactic distances.

η Geminorum: an eclipsing semiregular variable star orbited by a companion surrounded by an extended disc

ABSTRACT We report 11 yr of spectroscopic monitoring of the M-type asymptotic giant branch star η Gem, a semiregular variable and a known spectroscopic binary with a period of 8.2 yr. We combine our radial velocities with others from the literature to provide an improved spectroscopic orbital solution, giving a period of 2979 d, which we then use to predict past times of eclipse. We examine archival photometry from amateur variable star observers, and other sources, and find many instances of dimmings that occurred at the right time. This confirms previous indications that the system is eclipsing, and it now ranks among those with the longest known periods. No secondary eclipses are seen. The ∼0.4 mag eclipses that last about 5 months are much too deep to be produced by a stellar companion. We propose instead that the companion is surrounded by a large disc that is at least 1.5 au in diameter but is likely larger. We predict the centre of the next eclipse will occur on New Year’s Day, 2029.

Fermi LAT Gamma-ray Detection of the Recurrent Nova RS Ophiuchi during its 2021 Outburst

We report the Fermi LAT γ-ray detection of the 2021 outburst of the symbiotic recurrent nova RS Ophiuchi. In this system, unlike classical novae from cataclysmic binaries, the ejecta from the white dwarf form shocks when interacting with the dense circumstellar wind environment of the red giant companion. We find the LAT spectra from 50 MeV to ∼20–23 GeV, the highest-energy photons detected in some subintervals, are consistent with π 0-decay emission from shocks in the ejecta as proposed by Tatischeff & Hernanz for its previous 2006 outburst. The LAT light curve displayed a fast rise to its peak >0.1 GeV flux of ≃6 × 10−6 ph cm−2 s−1 beginning on day 0.745 after its optically constrained eruption epoch of 2021 August 8.50. The peak lasted for ∼1 day and exhibited a power-law decline up to the final LAT detection on day 45. We analyze the data on shorter timescales at early times and found evidence of an approximate doubling of emission over ∼200 minutes at day 2.2, possibly indicating a localized shock-acceleration event. Comparing the data collected by the American Association of Variable Star Observers, we measured a constant ratio of ∼ 2.8 × 10−3 between the γ-ray and optical luminosities except for a ∼5×smaller ratio within the first day of the eruption likely indicating attenuation of γ rays by ejecta material and lower high-energy proton fluxes at the earliest stages of the shock development. The hard X-ray emission due to bremsstrahlung from shock-heated gas traced by the Swift-XRT 2–10 keV light curve peaked at day ∼6, later than at GeV and optical energies. Using X-ray derived temperatures to constrain the velocity profile, we find the hadronic model reproduces the observed >0.1 GeV light curve.

Magnetically gated accretion model: Application to short bursts in the intermediate polar V1223 Sgr

Context. Some intermediate polars (IPs) show outbursts that are much shorter than those observed in normal dwarf novae, and their origin remains unclear. Aims. We examine the case of V1223 Sgr, an intermediate polar that showed a short outburst in 1984, and compare the outburst characteristics with the predictions of the magnetospheric gating model. Methods. We extracted outburst profiles from the American Association of Variable Star Observers (AAVSO) archival data. We used our code to compute the time-dependent evolution of an accretion disc truncated by the white dwarf magnetic field, using a simple description of the interaction between the disc and the magnetic field, as in D’Angelo & Spruit (2010, MNRAS, 406, 1208). Results. We find that V1223 Sgr underwent a series of short outbursts, with a rise lasting for typically two to three hours, and a slightly longer decay. When applied to IPs, the model by D’Angelo & Spruit (2010, MNRAS, 406, 1208) accounts well for the observed outburst duration and intensity. However, we confirm that the model outcome depends sensitively on the assumptions of the rather poorly constrained model. We also searched the AAVSO database for short outbursts in other IPs, identifying individual short outbursts in FO Aqr, TV Col, NY Lup, and EI UMa, but no series such as those observed in V1223 Sgr. We also found a superoutburst, followed by a reflare in CTCV J2056−3014. Conclusions. Although the magnetic–gating accretion instability model is clearly responsible for the series of V1223 Sgr short outbursts and most probably for similar events in other IPs, the model describing this process needs improvement, in particular concerning the interaction between the magnetic field of the white dwarf and the accretion disc. This difficult task might benefit from further comparison of the model outcome with additional observations that show good time coverage and time resolution.

Optical Photometry of WR140 as the Dust Formed During the 2016 Periastron Passage

Abstract The colliding wind binary WR140 produces dust in its shocked gas every periastron passage. While the infrared light curve is very repeatable, there are noticeable changes every cycle in the optical time-series photometry. In the phases following periastron, there are optical dips in the light curve that were postulated to be caused by localized clumps in the dust produced in our line of sight. We report on the B- and V-band light curves that were recorded by the American Association of Variable Star Observers after the 2016 periastron event and briefly discuss comparisons to geometric models of the dust production to infer that these features are likely caused by localized dust clumps in the new dust shell.

The activity of the dwarf nova RU Pegasi with rapidly changing outburst types

ABSTRACT RU Pegasi is a dwarf nova (DN) of the U Geminorum type. This analysis of its long-term optical activity uses data from the data base of the American Association of Variable Star Observers (AAVSO). It concentrates on investigating the properties of the individual outbursts and the time evolution of the ensemble of these events. No significant irradiation of the disc by the white dwarf was detected. In the interpretation, a variable steepness of the rising branches of the individual outbursts shows that the start of outbursts of RU Peg can occur at various distances from the disc centre without remarkable changes of the outburst recurrence time TC. The disc overflow of the inflowing mass stream, varying with time, could contribute to the changes in the starting position of the heating front, and hence the variations of the outburst types. A typical length of TC was 90 d. The segments of the relatively stable length of TC were accompanied by the primarily little variable and small values of the fluence (the energy radiated in the optical band in the individual outbursts). Jumps of TC, accompanied by the big scatter of the fluences, sometimes replaced them. In the interpretation, a combination of variations of TC with the unstable properties of the outbursts, including an unstable mass transfer rate between the components, shows the influence of several mechanisms on the state of the disc in various time segments.

Characterizing Long-term Optical Variability Properties of γ-Ray-bright Blazars

Abstract Optical observations of a sample of 12 γ-ray-bright blazars from four optical data archives—American Association of Variable Star Observers, Small and Moderate Aperture Research Telescope System, Catalina, and Steward Observatory—are compiled to create densely sampled light curves spanning more than a decade. As a part of the blazar multiwavelength studies, several methods of analysis, e.g., flux distribution and rms–flux relation, are performed on the observations with the aim to compare the results with the similar ones in the γ-ray band presented in Bhatta & Dhital. It is found that, similar to the γ-ray band, blazars display significant variability in the optical band that can be characterized with lognormal flux distribution and a power-law dependence of rms on flux. It could be an indication of a possible inherent linear rms–flux relation, yet the scatter in the data does not allow to rule out other possibilities. When comparing variability properties in the two bands, the blazars in the γ-rays are found to exhibit stronger variability with a steeper possible linear rms–flux relation and a flux distribution that is more skewed toward higher fluxes. The cross-correlation study shows that except for source 3C 273, the overall optical and the γ-ray emission in the sources are highly correlated, suggesting a cospatial existence of the particles responsible for both the optical and γ-ray emission. Moreover, sources S5 0716+714, Mrk 421, Mrk 501, PKS 1424-418, and PKS 2155-304 revealed possible evidence of quasiperiodic oscillations in the optical emission with the characteristic timescales, which are comparable to those in the γ-ray band detected in our previous work.

A New Catalog of Asymptotic Giant Branch Stars in Our Galaxy

Abstract We present a new catalog of 11,209 O-rich asymptotic giant branch (AGB) stars and 7172 C-rich AGB stars in our Galaxy, identifying more AGB stars in the bulge component and considering more visual carbon stars. For each object, we cross-identify the Infrared Astronomical Satellite (IRAS), AKARI, Midcourse Space Experiment, Wide-field Infrared Survey Explorer (WISE), Two-Micron All-Sky Survey, and American Association of Variable Star Observers counterparts. We present the new catalog in two parts: one is based on the IRAS PSC for brighter or more isolated objects; the other one is based on the ALLWISE source catalog for less bright objects or objects in crowded regions. We present various infrared two-color diagrams (2CDs) for the sample stars. We find that the theoretical dust shell models can roughly explain the observations of AGB stars on the various IR 2CDs. We investigate IR properties of SiO and OH maser emission sources in the catalog. For Mira variables in the sample stars, we find that the IR colors get redder for longer pulsation periods. We also study infrared variability of the sample stars using the WISE photometric data in the last 12 yr: the ALLWISE multiepoch data and the Near-Earth Object WISE Reactivation 2021 data release. We generate light curves using the WISE data at W1 and W2 bands and compute the Lomb–Scargle periodograms for all of the sample stars. From the WISE light curves, we have found useful variation parameters for 3710 objects in the catalog, for which periods were either known or unknown in previous works.

A study of the long-term activity of five intermediate polars with accretion discs

ABSTRACT Intermediate polars (IPs) are cataclysmic variables with mildly magnetized white dwarfs (WDs). This analysis of the long-term optical activity of five examples of IPs with accretion discs used data from the Catalina Real-time Transient Survey, Digital Access to a Sky Century @ Harvard (DASCH) and the American Association of Variable Star Observers (AAVSO). It is shown that each of these IPs had their most preferred value of absolute magnitude Mopt, even if it significantly varied on the superorbital time-scale. The values of Mopt of these IPs were in the zone of thermal-viscous instability (TVI) of the disc most of the time. The properties of a series of outbursts of V426 Oph can be explained by an intermittently operating TVI. The activity of TV Col and DW Cnc is interpreted as caused by a gradually variable mass inflow rate from the secondary to a cool disc. The mass transfer rate from the secondary varied on a well-determined time-scale. It is shown that Mopt of EI UMa, close to the peaks of outbursts of non-magnetic dwarf novae, fluctuated on the time-scale of days; it also produced short flares, ascribed to the bursts of matter from the donor. HY Leo, with a presumably cool disc, fluctuated between its high and low states. A temporary brightening from an extended low state is ascribed to a short, intense burst of matter from the donor to the remaining cool disc or torus.