Sazuna/UAT_keywords huggingface dataset:
Single-band VLBI Absolute Astrometry. The ionospheric path delay impacts single-band, very long baseline interferometry (VLBI) group delays, which limits their applicability for absolute astrometry. I consider two important cases: when observations are made simultaneously in two bands, but delays in only one band are available for a subset of observations; and when observations are made in one-band design. I developed optimal procedures of data analysis for both cases using Global Navigation Satellite System (GNSS) ionosphere maps, provided a stochastic model that describes ionospheric errors, and evaluated their impact on source position estimates. I demonstrate that the stochastic model is accurate at a level of 15%. I found that using GNSS ionospheric maps as is introduces serious biases in estimates of declination and I developed a procedure that almost eliminates them. I found serendipitously that GNSS ionospheric maps have multiplicative errors and have to be scaled by 0.85 in order to mitigate the declination bias. A similar scale factor was found in comparison of the vertical total electron content from satellite altimetry against GNSS ionospheric maps. I favor interpretation of this scaling factor as a manifestation of the inadequacy of the thin-shell model of the ionosphere. I showed that we are able to model the ionospheric path delay to the extent that no noticeable systematic errors emerge and we are able to assess adequately the contribution of the ionosphere-driven random errors on source positions. This makes single-band absolute astrometry a viable option that can be used for source position determination.

Keywords: Very long baseline interferometry, Radio astrometry, Earth ionosphere
L-dwarf Detection from SDSS Images using Improved Faster R-CNN. We present a data-driven approach to automatically detect L dwarfs from Sloan Digital Sky Survey (SDSS) images using an improved Faster R-CNN framework based on deep learning. The established L-dwarf automatic detection (LDAD) model distinguishes L dwarfs from other celestial objects and backgrounds in SDSS field images by learning the features of 387 SDSS images containing L dwarfs. Applying the LDAD model to the SDSS images containing 93 labeled L dwarfs in the test set, we successfully detected 83 known L dwarfs with a recall rate of 89.25% for known L dwarfs. Several techniques are implemented in the LDAD model to improve its detection performance for L dwarfs, including the deep residual network and the feature pyramid network. As a result, the LDAD model outperforms the model of the original Faster R-CNN, whose recall rate of known L dwarfs is 80.65% for the same test set. The LDAD model was applied to detect L dwarfs from a larger validation set including 843 labeled L dwarfs, resulting in a recall rate of 94.42% for known L dwarfs. The newly identified candidates include L dwarfs, late M and T dwarfs, which were estimated from color (i - z) and spectral type relation. The contamination rates for the test candidates and validation candidates are 8.60% and 9.27%, respectively. The detection results indicate that our model is effective to search for L dwarfs from astronomical images.

Keywords: L dwarfs, Astronomy data analysis
Equalizing the Pixel Response of the Imaging Photoelectric Polarimeter Onboard the IXPE Mission. The Gas Pixel Detector is a gas detector, sensitive to the polarization of X-rays, currently flying onboard the Imaging X-ray Polarimetry Explorer (IXPE)-the first observatory dedicated to X-ray polarimetry. It detects X-rays and their polarization by imaging the ionization tracks generated by photoelectrons absorbed in the sensitive volume, and then reconstructing the initial direction of the photoelectrons. The primary ionization charge is multiplied and ultimately collected on a finely pixellated ASIC specifically developed for X-ray polarimetry. The signal of individual pixels is processed independently and gain variations can be substantial, of the order of 20%. Such variations need to be equalized to correctly reconstruct the track shape, and therefore its polarization direction. The method to do such equalization is presented here and is based on the comparison between the mean charge of a pixel with respect to the other pixels for equivalent events. The method is shown to finely equalize the response of the detectors onboard IXPE, allowing a better track reconstruction and energy resolution, and can in principle be applied to any imaging detector based on tracks.

Keywords: X-ray astronomy, X-ray detectors, X-ray telescopes, Astronomical detectors, Calibration, X-ray observatories, Polarimeters, Polarimetry
Searching for Compact Object Candidates from LAMOST Time-domain Survey of Four K2 Plates. The time-domain (TD) surveys of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) yield high-cadence radial velocities, paving a new avenue to study binary systems including compact objects. In this work, we explore LAMOST TD spectroscopic data of four K2 plates and present a sample of six single-lined spectroscopic binaries that may contain compact objects. We conduct analyses using phase-resolved radial velocity measurements of the visible star to characterize each source and to infer the properties of invisible companion. By fitting the radial velocity curves for the six targets, we obtain accurate orbital periods, ranging from ~(0.6 to 6) days, and radial velocity semiamplitudes, ranging from ~(50 to 130) km s-1. We calculate the mass function of the unseen companions to be between 0.08 and 0.17 M ⊙. Based on the mass function and the estimated stellar parameters of the visible star, we determine the minimum mass of the hidden star. Three targets-J034813, J063350, and J064850-show ellipsoidal variability in the light curves from K2, ZTF, and TESS surveys. Therefore, we can put constraints on the mass of the invisible star using the ellipsoidal variability. We identify no X-ray counterparts for these targets except for J085120, of which the X-ray emission can be ascribed to stellar activity. We note that the nature of these six candidates is worth further characterization utilizing multiwavelength follow-up observations.

Keywords: Close binary stars, Radial velocity, Light curves, Compact objects
Variable Blue Straggler Stars in the Open Cluster NGC 6819 Observed in the Kepler "Superstamp" Field. NGC 6819 is an open cluster of age 2.4 Gyr that was in the NASA Kepler spacecraft's field of view from 2009 to 2013. The central part of the cluster was observed in a 200 × 200 pixel "superstamp" during these four years in 30 minute cadence photometry, providing a unique, high-precision, long time-series data set. The cluster contains "blue straggler" stars, i.e., stars on the main sequence above the cluster turnoff that should have left the main sequence to become red giants. We present light curves and pulsation frequency analyses derived from custom photometric reductions for five confirmed cluster members-four blue stragglers and one star near the main-sequence turnoff. Two of these stars show a rich spectrum of δ Scuti pulsation modes, with 236 and 124 significant frequencies identified, respectively, while two stars show mainly low-frequency modes, characteristic of γ Doradus variable stars. The fifth star, a known active X-ray binary, shows only several harmonics of two main frequencies. For the two δ Scuti stars, we use a frequency separation-mean density relation to estimate their mean densities, and then use these values along with their effective temperature to derive their stellar masses and radii. For the two stars showing low frequencies, we searched for period-spacing sequences that may be representative of gravity-mode or Rossby-mode sequences, but found no clear sequences. The common age for the cluster members, considered along with the frequencies, will provide valuable constraints for asteroseismic analyses, and may shed light on the origin of the blue stragglers.

Keywords: Delta Scuti variable stars, Pulsating variable stars, Blue straggler stars, Gamma Doradus variable stars, Asteroseismology, Stellar evolution
The Environments around W Serpentis Systems: Independent Limits on System Masses and Extended Envelopes. Information extracted from the Gaia Data Release 3 is used to examine the stellar contents within projected separations of 10 pc from eight close binary systems that are either classical W Serpentis systems or related objects. The goal is to search for remnant star clusters or moving groups with proper motions that are similar to those of the binaries. While some of the binary systems have proper motions that are distinct from those of the majority of stars within the search area, there is a tendency for W Ser stars to be accompanied by companions with separations on parsec or larger scales. At least three candidate companions are identified within the search area for each system, although in the majority of cases the numbers are much higher. Evidence is presented that SX Cas is near the center of a diffuse cluster. Color-magnitude diagrams of the groupings associated with the binaries are compared with isochrones, and the majority of the groupings are found to have ages ≥1 Gyr, indicating that they have an intermediate age. The masses of stars at the main-sequence turnoff of the groupings are estimated, and these provide insights into the initial mass of the donor star in each binary system. Images from the WISE All-Sky Survey are also used to search for circumsystem envelopes. Extended thermal emission is found around six systems in W2 (i.e., ~4.5 μm) images.

Keywords: Interacting binary stars
Zwicky Transient Facility and Globular Clusters: The Period-Luminosity and Period-Wesenheit Relations for SX Phoenicis Variables in the gri Band. SX Phoenicis (SXP) variables are short-period pulsating stars that exhibit a period-luminosity (PL) relation. We derived the gri-band PL and extinction-free period-Wesenheit (PW) relations, as well as the period-color and reddening-free period-Q-index relations for 47 SXP stars located in 21 globular clusters, using the optical light curves taken from Zwicky Transient Facility. These empirical relations were derived for the first time in the gri filters except for the g-band PL relation. We used our gi-band PL and PW relations to derive a distance modulus to Crater II dwarf spheroidal which hosts one SXP variable. Assuming that the fundamental and first-overtone pulsation mode for the SXP variable in Crater II, we found distance moduli of 20.03 ± 0.23 mag and 20.37 ± 0.24 mag, respectively, using the PW relation, where the latter is in excellent agreement with independent RR Lyrae based distance to Crater II dwarf galaxy.

Keywords: Globular star clusters, Sky surveys, Wide-field telescopes, Distance indicators, SX Phoenicis variable stars
A Survey for High-redshift Gravitationally Lensed Quasars and Close Quasar Pairs. I. The Discoveries of an Intermediately Lensed Quasar and a Kiloparsec-scale Quasar Pair at z   5. We present the first results from a new survey for high-redshift (z ≳ 5) gravitationally lensed quasars and close quasar pairs. We carry out candidate selection based on the colors and shapes of objects in public imaging surveys, then conduct follow-up observations to confirm the nature of high-priority candidates. In this paper, we report the discoveries of J0025-0145 (z = 5.07), which we identify as an intermediately lensed quasar, and J2329-0522 (z = 4.85), which is a kiloparsec-scale close quasar pair. The Hubble Space Telescope (HST) image of J0025-0145 shows a foreground lensing galaxy located 0.″6 away from the quasar. However, J0025-0145 does not exhibit multiple lensed images of the quasar, and we identify J0025-0145 as an intermediate lensing system (a lensing system that is not multiply imaged but has a significant magnification). The spectrum of J0025-0145 implies an extreme Eddington ratio if the quasar luminosity is intrinsic, which could be explained by a large lensing magnification. The HST image of J0025-0145 also indicates a tentative detection of the quasar host galaxy in the rest-frame UV, illustrating the power of lensing magnification and distortion in studies of high-redshift quasar host galaxies. Object J2329-0522 consists of two resolved components with significantly different spectral properties and a lack of lensing galaxy detection under subarcsecond seeing. We identify it as a close quasar pair, which is the highest confirmed kiloparsec-scale quasar pair to date. We also report four lensed quasars and quasar pairs at 2 &lt; z &lt; 4 and discuss possible improvements to our survey strategy.

Keywords: Double quasars, Gravitational lensing, Quasars
Contemporaneous Observations of Hα Luminosities and Photometric Amplitudes for M Dwarfs. While many M dwarfs are known to have strong magnetic fields and high levels of magnetic activity, we are still unsure about the properties of their starspots and the origin of their magnetic dynamos. Both starspots and chromospheric heating are generated by the surface magnetic field; they produce photometric variability and Hα emission, respectively. Connecting brightness variations to magnetic activity therefore provides a means to examine M-dwarf magnetism. We survey 30 M dwarfs previously identified as fast rotating stars (P rot &lt; 10 days). We present time-series optical photometry from the Transiting Exoplanet Survey Satellite (TESS) and contemporaneous optical spectra obtained using the Ohio State Multi-Object Spectrograph (OSMOS) on the 2.4 m Hiltner telescope at MDM Observatory in Arizona. We measure rotation periods and photometric amplitudes from TESS light curves using Gaussian Processes. From the OSMOS spectra, we calculate the equivalent width of Hα, and L Hα /L bol. We find a weak positive correlation between Hα luminosity and the semiamplitude, R var ( $p={0.005}_{-0.005}^{+0.075}$ ). We also observe short-term variability (between 20 and 45 minutes) in Hα equivalent widths and possible enhancement from flares consistent to recent literature.

Keywords: Stellar rotation, Stellar spectral lines, Low mass stars, Stellar activity
A Catalog of Nearby Accelerating Star Candidates in Gaia DR3. We describe a new catalog of accelerating star candidates with Gaia G ≤ 17.5 mag and distances d ≤ 100 pc. Designated as the Gaia Nearby Accelerating Star Catalog (GNASC), it contains 29,684 members identified using a supervised machine-learning algorithm trained on the Hipparcos-Gaia Catalog of Accelerations (HGCA), Gaia Data Release 2, and Gaia Early Data Release 3. We take advantage of the difference in observation timelines between the two Gaia catalogs and information about the quality of the astrometric modeling based on the premise that acceleration will correlate with astrometric uncertainties. Catalog membership is based on whether constant proper motion over three decades can be ruled out at high confidence (greater than 99.9%). Test data suggest that catalog members each have a 68% likelihood of true astrometric acceleration; subsets of the catalog perform even better, with the likelihood exceeding 85%. We compare the GNASC with Gaia Data Release 3 and its table of stars for which acceleration is detected at high confidence based on precise astrometric fits. Our catalog, derived without this information, captures over 96% of the sources in the table that meet our selection criteria. In addition, the GNASC contains bright, nearby candidates that were not in the original Hipparcos survey, including members of known binary systems as well as stars with companions yet to be identified. It thus extends the HGCA and demonstrates the potential of the machine-learning approach for discovering hidden partners of nearby stars in future astrometric surveys.

Keywords: Multiple stars, Binary stars, Substellar companion stars
Modulation of the Blazhko Cycle in LS Her. We present analysis of the RR Lyrae star, LS Her, and confirm the previously reported modulation to its Blazhko cycles. We performed Fourier analysis on two sectors (Sector 24 and 25) of data from the Transiting Exoplanet Survey Satellite (TESS) spanning 53 days. We find LS Her to have a primary pulsation period of 0.2308 day and a Blazhko period of 12.7 days in keeping with previously reported results. We also identified sideband frequencies around the Blazhko multiplets suggesting the Blazhko cycle is modulated on a timescale of 112 days. Analysis of the Blazhko effect using the TESS data clearly shows a changing amplitude and phase throughout the four Blazhko cycles. We compared our modeled results, which were based on our TESS frequency analysis, to TESS data (Sector 51) taken ~700 days later and found our modulation model was not a good representation of the data. We then coupled our TESS analysis with the modulation frequency results from Wils et al. and found excellent agreement with the Sector 51 data. To further test this result we obtained ground-based, V-magnitude observations of LS Her in the summer of 2022. This data also showed excellent agreement with our coupled modulation model. We have verified that LS Her is a Blazhko star with a modulated Blazhko period of 109 days, stability over the 862 days of observations, and possible stability lasting over 15 yr. We discuss the ramifications of the modulation for other Blazhko stars that show Blazhko effect changes over time.

Keywords: RRc variable stars, RR Lyrae variable stars, Blazhko effect
Coronal X-Ray Emission from Nearby, Low-mass, Exoplanet Host Stars Observed by the MUSCLES and Mega-MUSCLES HST Treasury Survey Projects. The high-energy X-ray and ultraviolet (UV) radiation fields of exoplanet host stars play a crucial role in controlling the atmospheric conditions and the potential habitability of exoplanets. Major surveys of the X-ray/UV emissions from late-type (K and M spectral types) exoplanet hosts have been conducted by the Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary systems (MUSCLES) and Mega-MUSCLES Hubble Space Telescope Treasury programs. These samples primarily consist of relatively old, "inactive," low-mass stars. In this paper we present results from X-ray observations of the coronal emission from these stars obtained using the Chandra X-ray Observatory, the XMM-Newton Observatory, and the Neil Gehrels Swift Observatory. The stars effectively sample the coronal activity of low-mass stars over a wide range of masses and ages. The vast majority (21 of 23) of the stars are detected and their X-ray luminosities measured. Short-term flaring variability is detected for most of the fully convective (M ≤ 0.35 M ☉) stars but not for the more massive M dwarfs during these observations. Despite this difference, the mean X-ray luminosities for these two sets of M dwarfs are similar, with more massive (0.35 M ☉ ≤ M ≤ 0.6 M ☉) M dwarfs at ~5 × 1026 erg s-1 compared to ~2 × 1026 erg s-1 for fully convective stars older than 1 Gyr. Younger, fully convective M dwarfs have X-ray luminosities between 3 and 6 × 1027 erg s-1. The coronal X-ray spectra have been characterized and provide important information that is vital for the modeling of the stellar EUV spectra.

Keywords: M dwarf stars, K dwarf stars, Stellar x-ray flares, Planet hosting stars
Disks around Young Planetary-mass Objects: Ultradeep Spitzer Imaging of NGC 1333. We report on a sensitive infrared search for disks around isolated young planetary-mass objects (PMOs) in the NGC 1333 cluster, by stacking 70 Spitzer/IRAC frames at 3.6 and 4.5 μm. Our coadded images go &gt;2.3 mag deeper than single-epoch frames, and cover 50 brown dwarfs, 15 of which have M9 or later spectral types. Spectral types &gt;M9 correspond to masses in the giant-planet domain, i.e., near or below the deuterium-burning limit of 0.015 M ⊙. Five of the 12 PMOs show definitive evidence of excess, implying a disk fraction of 42%, albeit with a large statistical uncertainty given the small sample. Comparing with measurements for higher-mass objects, the disk fraction does not decline substantially with decreasing mass in the substellar domain, consistent with previous findings. Thus, free-floating PMOs have the potential to form their own miniature planetary systems. We note that only one of the six lowest-mass objects in NGC 1333, with spectral type L0 or later, has a confirmed disk. Reviewing the literature, we find that the lowest-mass free-floating objects with firm disk detections have masses ~0.01 M ⊙ (or ~10 M Jup). It is not clear yet whether even lower-mass objects harbor disks. If not, it may indicate that ~10 M Jup is the lower-mass limit for objects that form like stars. Our disk-detection experiment on deep Spitzer images paves the way for studies with JWST at longer wavelengths and higher sensitivity, which will further explore disk prevalence and formation of free-floating PMOs.

Keywords: Brown dwarfs
Catching Tidal Dwarf Galaxies at a Later Evolutionary Stage with ALFALFA. We present deep optical imaging and photometry of four objects classified as "Almost-Dark" galaxies in the Arecibo Legacy Fast Arecibo L-band Feed Array (ALFALFA) survey because of their gas-rich nature and extremely faint or missing optical emission in existing catalogs. They have H I masses of 107-109 M ⊙ and distances of ~9-100 Mpc. Observations with the WIYN 3.5 m telescope and One Degree Imager reveal faint stellar components with central surface brightnesses of ~24-25 $\mathrm{mag}\,{\mathrm{arcsec}}^{-2}$ in the g band. We also present the results of H I synthesis observations with the Westerbork Synthesis Radio Telescope. These Almost-Dark galaxies have been identified as possible tidal dwarf galaxies (TDGs) based on their proximity to one or more massive galaxies. We demonstrate that AGC 229398 and AGC 333576 likely have the low dark matter content and large effective radii representative of TDGs. They are located much farther from their progenitors than previously studied TDGs, suggesting they are older and more evolved. AGC 219369 is likely dark matter dominated, while AGC 123216 has a dark matter content that is unusually high for a TDG, but low for a normal dwarf galaxy. We consider possible mechanisms for the formation of the TDG candidates such as a traditional major merger scenario and gas ejection from a high-velocity flyby. Blind H I surveys like ALFALFA enable the detection of gas-rich, optically faint TDGs that can be overlooked in other surveys, thereby providing a more complete census of the low-mass galaxy population and an opportunity to study TDGs at a more advanced stage of their life cycle.

Keywords: Dwarf irregular galaxies, Galaxy interactions, Galaxy evolution, Galaxy photometry
Magnetic Fields in the Horsehead Nebula. We present the first polarized dust emission measurements of the Horsehead Nebula, obtained using the POL-2 polarimeter on the Submillimetre Common-User Bolometer Array 2 (SCUBA-2) camera on the James Clerk Maxwell Telescope. The Horsehead Nebula contains two submillimeter sources: a photodissociation region (PDR; SMM1) and a starless core (SMM2). We see well-ordered magnetic fields in both sources. We estimated plane-of-sky magnetic field strengths of 56 ± 9 and 129 ± 21 μG in SMM1 and SMM2, respectively, and obtained mass-to-flux ratios and Alfvén Mach numbers of less than 0.6, suggesting that the magnetic field can resist gravitational collapse and that magnetic pressure exceeds internal turbulent pressure in these sources. In SMM2, the kinetic and gravitational energies are comparable to one another, but less than the magnetic energy. We suggest a schematic view of the overall magnetic field structure in the Horsehead Nebula. Magnetic field lines in SMM1 appear to have been compressed and reordered during the formation of the PDR, while the likely more-embedded SMM2 may have inherited its field from that of the pre-shock molecular cloud. The magnetic fields appear to currently play an important role in supporting both sources.

Keywords: Molecular clouds, Polarimetry, Star formation, Submillimeter astronomy, Magnetic fields