Sazuna/UAT_keywords huggingface dataset:
Overfitting Affects the Reliability of Radial Velocity Mass Estimates of the V1298 Tau Planets. Mass, radius, and age measurements of young (≲100 Myr) planets have the power to shape our understanding of planet formation. However, young stars tend to be extremely variable in both photometry and radial velocity (RV) measurements, which makes constraining these properties challenging. The V1298 Tau system of four ~0.5 R J planets transiting a pre-main-sequence star presents an important, if stress-inducing, opportunity to observe and measure directly the properties of infant planets. Suárez Mascareño et al. published radial-velocity-derived masses for two of the V1298 Tau planets using a state-of-the-art Gaussian process regression framework. The planetary densities computed from these masses were surprisingly high, implying extremely rapid contraction after formation in tension with most existing planet-formation theories. In an effort to constrain further the masses of the V1298 Tau planets, we obtained 36 RVs using Keck/HIRES, and analyzed them in concert with published RVs and photometry. Through performing a suite of cross-validation tests, we found evidence that the preferred model of Suárez Mascareño et al. suffers from overfitting, defined as the inability to predict unseen data, rendering the masses unreliable. We detail several potential causes of this overfitting, many of which may be important for other RV analyses of other active stars, and recommend that additional time and resources be allocated to understanding and mitigating activity in active young stars such as V1298 Tau.

Keywords: Starspots, Radial velocity
Mind the Gap. I. Hα Activity of M Dwarfs Near the Partially/Fully Convective Boundary and a New Hα Emission Deficiency Zone on the Main Sequence. Since identifying the gap in the H-R Diagram (HRD) marking the transition between partially and fully-convective interiors, a unique type of slowly pulsating M dwarf has been proposed. These unstable M dwarfs provide new laboratories in which to understand how changing interior structures can produce potentially observable activity at the surface. In this work, we report the results of the largest high-resolution spectroscopic Hα emission survey to date spanning this transition region, including 480 M dwarfs observed using the CHIRON spectrograph at CTIO/SMARTS 1.5 m. We find that M dwarfs with Hα in emission are almost entirely found 0-0.5 mag above the top edge of the gap in the HRD, whereas effectively no stars in and below the gap show emission. Thus, the top edge of the gap marks a relatively sharp activity transition, and there is no anomalous Hα activity for stars in the gap. We also identify a new region at 10.3 &lt; M  G  &lt; 10.8 on the main sequence where fewer M dwarfs exhibit Hα emission compared to M dwarfs above and below this magnitude range. Careful evaluation of the results in the literature indicates that (1) rotation and Hα activity distributions on the main-sequence are closely related, and (2) fewer stars in this absolute magnitude range rotate in less than ~13 days than populations surrounding this region. This result suggests that the most massive fully-convective stars lose their angular momentum faster than both partially convective stars and less massive fully-convective stars.

Keywords: M dwarf stars, High resolution spectroscopy, Stellar activity, Hertzsprung Russell diagram, Stellar rotation
X-Ray Emission of Ultraviolet Variable Active Galactic Nucleus Candidates. We analyze the X-ray properties of a sample of 23 high-probability active galactic nucleus (AGN) candidates with ultraviolet variability identified in Wasleske et al. Using data from the Chandra X-ray Observatory and XMM-Newton Observatory, we that find 11/23 nuclei are X-ray detected. We use spectral energy distribution modeling to compute star formation rates and show that the X-ray luminosities are typically in excess of the X-ray emission expected from star formation by at least an order of magnitude. Interestingly, this sample shows a diversity of optical spectroscopic properties. We explore possible reasons for why some objects lack optical spectroscopic signatures of black hole activity while still being UV-variable and X-ray bright. We find that host galaxy stellar emission and obscuration from gas and dust are potential factors. We study where this sample falls on relationships such as α OX - L 2500 and L X - L IR and find that some of the sample falls outside the typical scatter for these relations, indicating that they differ from the standard quasar population. With the diversity of optical spectroscopic signatures and varying impacts of dust and stellar emissions on our sample, these results emphasize the strength of variability in selecting the most complete set of AGN, regardless of other host galaxy properties.

Keywords: Supermassive black holes, X-ray photometry, Active galaxies, Active galactic nuclei, Astrophysical black holes
Comprehensive Analysis of a Symbiotic Candidate V503 Her. V503 Her was previously proposed as an eclipsing symbiotic candidate based on photometric behavior and spectroscopic appearance indicating the composite optical spectrum. To investigate its nature, we analyzed long-term photometric observations covering 100 yr of its photometric history and new low-resolution optical spectroscopic data, supplemented with the multifrequency measurements collected from several surveys and satellites. Based on the analysis presented in this paper, we claim that V503 Her is not an eclipsing binary star. The optical and infrared wavelengths are dominated by a K-type bright giant with an effective temperature of 4500 K, luminosity of 1900 L ☉, and subsolar metallicity on the asymptotic giant branch showing semiregular complex multiperiodic pulsation behavior. V503 Her does not show the characteristics of strongly interacting symbiotic variables, but some pieces of evidence suggest that it could still be one of the "hidden" accreting-only symbiotic systems. However, the currently available data do not allow us to fully confirm or constrain the parameters of a possible companion.

Keywords: Evolved stars, Pulsating variable stars, Asymptotic giant branch stars, Eclipsing binary stars, Symbiotic binary stars
Performance of the Quasar Spectral Templates for the Dark Energy Spectroscopic Instrument. Millions of quasar spectra will be collected by the Dark Energy Spectroscopic Instrument (DESI), leading to a fourfold increase in the number of known quasars. High-accuracy quasar classification is essential to tighten constraints on cosmological parameters measured at the highest redshifts DESI observes (z &gt; 2.0). We present spectral templates for identification and redshift estimation of quasars in the DESI Year 1 data release. The quasar templates are comprised of two quasar eigenspectra sets, trained on spectra from the Sloan Digital Sky Survey. The sets are specialized to reconstruct quasar spectral variation observed over separate yet overlapping redshift ranges and, together, are capable of identifying DESI quasars from 0.05 &lt; z &lt; 7.0. The new quasar templates show significant improvement over the previous DESI quasar templates regarding catastrophic failure rates, redshift precision and accuracy, quasar completeness, and the contamination fraction in the final quasar sample.

Keywords: Cosmology, Observational cosmology, Redshift surveys, Quasars
The Radial Distribution and Excitation of H<SUB>2</SUB> around Young Stars in the HST-ULLYSES Survey. The spatial distribution and evolution of gas in the inner 10 au of protoplanetary disks form the basis for estimating the initial conditions of planet formation. Among the most important constraints derived from spectroscopic observations of the inner disk are the radial distributions of the major gas phase constituents, how the properties of the gas change with inner disk dust evolution, and how the chemical abundances and excitation conditions are influenced by the high-energy radiation from the central star. We present a survey of the radial distribution, excitation, and evolution of inner disk molecular hydrogen (H2) obtained as part of the Hubble Space Telescope-ULLYSES program. We analyze far-UV spectroscopy of 71 (63 accreting) pre-main-sequence systems in ULLYSES DR5 to characterize the H2 emission lines, H2 dissociation continuum emission, and major photochemical/disk evolution driving the UV emissions (Lyα, UV continuum, and C IV). We use the widths of the H2 emission lines to show that most fluorescent H2 arises between 0.1 and 1.4 au from the parent star, and show positive correlations of the average emitting radius with the accretion luminosity and with the dust disk mass. We find a strong correlation between H2 dissociation emission and both the accretion-dominated Lyα luminosity and the inner disk dust clearing, painting a picture where water molecules in the inner 3 au are exposed to and dissociated by strong Lyα emission as the opacity of the inner disk declines with time.

Keywords: Protoplanetary disks, Planet formation, Circumstellar gas
Newly Discovered Wolf-Rayet Stars in M31. The evolved massive star populations of the Local Group galaxies are generally thought to be well understood. However, recent work has suggested that the Wolf-Rayet (WR) content of M31 may have been underestimated. We therefore began a pilot project to search for new WRs in M31 and to reexamine the completeness of our previous WR survey, finished almost a decade prior. Our improved imaging data and spectroscopic follow-up confirmed 19 new WRs across three small fields in M31. These newly discovered WRs are generally fainter than the previously known sample due to slightly increased reddening as opposed to intrinsic faintness. From these findings, we estimate that there are another ~60 WRs left to be discovered in M31; however, the overall ratio of WN-type (nitrogen-rich) to WC-type (carbon-rich) WRs remains unchanged with our latest additions to the M31 WR census. We are in the process of extending this pilot WR survey to include the rest of M31, and a more complete population will be detailed in our future work.

Keywords: Evolved stars, Stellar populations, Massive stars, Wolf-Rayet stars, Andromeda Galaxy
Transmission Spectroscopy of the Lowest-density Gas Giant: Metals and a Potential Extended Outflow in HAT-P-67b. Extremely low-density exoplanets are tantalizing targets for atmospheric characterization because of their promisingly large signals in transmission spectroscopy. We present the first analysis of the atmosphere of the lowest-density gas giant currently known, HAT-P-67b. This inflated Saturn-mass exoplanet sits at the boundary between hot and ultrahot gas giants, where thermal dissociation of molecules begins to dominate atmospheric composition. We observed a transit of HAT-P-67b at high spectral resolution with CARMENES and searched for atomic and molecular species using cross-correlation and likelihood mapping. Furthermore, we explored potential atmospheric escape by targeting Hα and the metastable helium line. We detect Ca II and Na I with significances of 13.2σ and 4.6σ, respectively. Unlike in several ultrahot Jupiters, we do not measure a day-to-night wind. The large line depths of Ca II suggest that the upper atmosphere may be more ionized than models predict. We detect strong variability in Hα and the helium triplet during the observations. These signals suggest the possible presence of an extended planetary outflow that causes an early ingress and late egress. In the averaged transmission spectrum, we measure redshifted absorption at the ~3.8% and ~4.5% level in the Hα and He I triplet lines, respectively. From an isothermal Parker wind model, we derive a mass-loss rate of $\dot{M}\sim {10}^{13}\,{\rm{g}}\,{{\rm{s}}}^{-1}$ and an outflow temperature of T ~ 9900 K. However, due to the lack of a longer out-of-transit baseline in our data, additional observations are needed to rule out stellar variability as the source of the Hα and He signals.

Keywords: High resolution spectroscopy, Exoplanet atmospheric composition , Hot Jupiters, Exoplanet atmospheric dynamics, Exoplanet atmospheric evolution
UV Spectral Characterization of Low-mass Stars with AstroSat UVIT for Exoplanet Applications: The Case Study of HIP 23309. Characterizing rocky exoplanet atmospheres is a key goal of exoplanet science, but interpreting such observations will require understanding the stellar ultraviolet (UV) irradiation incident on the planet from its host star. Stellar UV mediates atmospheric escape, photochemistry, and planetary habitability, and observations of rocky exoplanets can only be understood in the context of the UV spectral energy distribution (SED) of their host stars. Particularly important are SEDs from observationally favorable but poorly understood low-mass M-dwarf stars, which are the only plausible targets for rocky planet atmospheric characterization for the next 1-2 decades. In this work, we explore the utility of AstroSat UltraViolet Imaging Telescope (UVIT) for the characterization of the UV SEDs of low-mass stars. We present observations of the nearby M0 star HIP 23309 in the far-UV (FUV) and near-UV (NUV) gratings of UVIT. Our FUV spectra are consistent with contemporaneous Hubble Space Telescope (HST) data and our NUV spectra are stable between orbits, suggesting UVIT is a viable tool for the characterization of the SEDs of low-mass stars. We apply our measured spectra to simulations of photochemistry and habitability for a hypothetical rocky planet orbiting HIP 23309 and elucidate the utility and limitations of UVIT in deriving UV SEDs of M-dwarf exoplanet hosts. Our work validates UVIT as a tool to complement HST in the characterization of exoplanet host stars and carries implications for its successor missions like INSIST.

Keywords: M dwarf stars, Exoplanet atmospheric composition , Ultraviolet telescopes, Astrobiology, Spectral energy distribution, Habitable planets
Comparing Apples with Apples: Robust Detection Limits for Exoplanet High-contrast Imaging in the Presence of Non-Gaussian Noise. Over the past decade, hundreds of nights have been spent on the world's largest telescopes to search for and directly detect new exoplanets using high-contrast imaging (HCI). Thereby, two scientific goals are of central interest: first, to study the characteristics of the underlying planet population and distinguish between different planet formation and evolution theories. Second, to find and characterize planets in our immediate solar neighborhood. Both goals heavily rely on the metric used to quantify planet detections and nondetections. Current standards often rely on several explicit or implicit assumptions about noise. For example, it is often assumed that the residual noise after data postprocessing is Gaussian. While being an inseparable part of the metric, these assumptions are rarely verified. This is problematic as any violation of these assumptions can lead to systematic biases. This makes it hard, if not impossible, to compare results across data sets or instruments with different noise characteristics. We revisit the fundamental question of how to quantify detection limits in HCI. We focus our analysis on the error budget resulting from violated assumptions. To this end, we propose a new metric based on bootstrapping that generalizes current standards to non-Gaussian noise. We apply our method to archival HCI data from the NACO instrument at the Very Large Telescope and derive detection limits for different types of noise. Our analysis shows that current standards tend to give detection limits that are about one magnitude too optimistic in the speckle-dominated regime. That is, HCI surveys may have excluded planets that can still exist.

Keywords: Direct imaging, Bootstrap, High angular resolution, Astrostatistics
Variability of Known Exoplanet Host Stars Observed by TESS. Both direct and indirect methods of exoplanet detection rely upon detailed knowledge of the potential host stars. Such stellar characterization allows for accurate extraction of planetary properties, as well as contributing to our overall understanding of exoplanetary system architecture. In this analysis, we examine the photometry of 264 known exoplanet host stars (harboring 337 planetary companions) that were observed during the Transiting Exoplanet Survey Satellite (TESS) Prime Mission. We identify periodic signatures in the lightcurves of these stars and make possible connections to stellar pulsations and their rotation periods, and compare the stellar variability to the published planetary orbital periods. From these comparisons, we quantify the effects of stellar variability on exoplanet detection, confirming that exoplanets detection is biased toward lower variability stars, but larger exoplanets dominate the population of exoplanets around variable stars. Exoplanet detection methods represented among these systems are distinct between stellar spectral types across the main sequence, though notable outliers exist. In addition, biases present in both the sourced data from TESS and the host star selection process, which strongly influences the representation of both stellar and planetary characteristics in the final populations. We also determine whether the host stars photometric variability affects or mimics the behavior or properties of the system's planets. These results are discussed in the context of how the behavior of the host star is responsible for how we observe exoplanet characteristics, most notably their radii and atmospheric properties, and how the activity may alter our measurements or impact the evolution of planetary properties.

Keywords: Periodic variable stars, Hertzsprung Russell diagram, Stellar properties, Exoplanets, Exoplanet systems, Stellar activity, Photometry, Exoplanet detection methods, Time series analysis
The Unexpected Optical and Ultraviolet Variability of the Standard Star α Sex (HD 87887). The analysis of the available Transiting Exoplanet Survey Satellite (TESS) light curves of α Sex (HD 87887) reveals low-frequency pulsations with a period of about 9.1 hr in this spectroscopic A0 III standard star. The IUE observations in 1992 December reveal large flux variations both in the far-UV and in the mid-UV, which are accompanied by variations of the brightness in the V band recorded by the the Fine Error Sensor on board IUE. The ultraviolet variability could be due to an eclipse by an hitherto undetected companion of smaller radius, possibly 2.5 R ⊙, but this needs confirmation by further monitoring possibly with TESS. An abundance determination yields solar abundances for most elements. Only carbon and strontium are underabundant and titanium, vanadium, and baryum mildly overabundant. Identification is provided for most of the lines absorbing more than 2% in the optical spectrum of α Sex. Stellar evolution modeling shows that α Sex is near the terminal-age main sequence, and its mass, radius, and age are estimated to be M = 2.57 ± 0.32 M ⊙, R = 3.07 ± 0.90 R ⊙, A = 385 ± 77 Myr, respectively.

Keywords: A giant stars
Autoencoding Galaxy Spectra. I. Architecture. We introduce the neural network architecture SPENDER as a core differentiable building block for analyzing, representing, and creating galaxy spectra. It combines a convolutional encoder, which pays attention to up to 256 spectral features and compresses them into a low-dimensional latent space, with a decoder that generates a restframe representation, whose spectral range and resolution exceeds that of the observing instrument. The decoder is followed by explicit redshift, resampling, and convolution transformations to match the observations. The architecture takes galaxy spectra at arbitrary redshifts and is robust to glitches like residuals of the skyline subtraction, so that spectra from a large survey can be ingested directly without additional preprocessing. We demonstrate the performance of SPENDER by training on the entire spectroscopic galaxy sample of SDSS-II; show its ability to create highly accurate reconstructions with substantially reduced noise; perform deconvolution and oversampling for a superresolution model that resolves the [O II] doublet; introduce a novel method to interpret attention weights as proxies for important spectral features; and infer the main degrees of freedom represented in the latent space. We conclude with a discussion of future improvements and applications.

Keywords: Galaxy spectroscopy, Astronomy data modeling
Autoencoding Galaxy Spectra. II. Redshift Invariance and Outlier Detection. We present an unsupervised outlier detection method for galaxy spectra based on the spectrum autoencoder architecture SPENDER, which reliably captures spectral features and provides highly realistic reconstructions for SDSS galaxy spectra. We interpret the sample density in the autoencoder latent space as a probability distribution, and identify outliers as low-probability objects with a normalizing flow. However, we found that the latent-space position is not, as expected from the architecture, redshift invariant, which introduces stochasticity into the latent space and the outlier detection method. We solve this problem by adding two novel loss terms during training, which explicitly link latent-space distances to data-space distances, preserving locality in the autoencoding process. Minimizing the additional losses leads to a redshift-invariant, nondegenerate latent-space distribution with clear separations between common and anomalous data. We inspect the spectra with the lowest probability and find them to include blends with foreground stars, extremely reddened galaxies, galaxy pairs and triples, and stars that are misclassified as galaxies. We release the newly trained SPENDER model and the latent-space probability for the entire SDSS-I galaxy sample to aid further investigations.

Keywords: Astrostatistics, Galaxies, Spectroscopy
Discovery of a New Local Group Dwarf Galaxy Candidate in UNIONS: Boötes V. We present the discovery of Boötes V, a new ultra-faint dwarf galaxy (UFD) candidate. This satellite is detected as a resolved overdensity of stars during an ongoing search for new Local Group dwarf galaxy candidates in the UNIONS photometric data set. It has a physical half-light radius of ${26.9}_{-5.4}^{+7.5}$ pc, a V-band magnitude of -4.5 ± 0.4 mag, and resides at a heliocentric distance of approximately 100 kpc. We use Gaia DR3 astrometry to identify member stars, characterize the systemic proper motion, and confirm the reality of this faint stellar system. The brightest star in this system was followed up using Gemini GMOS-N long-slit spectroscopy and is measured to have a metallicity of [Fe/H] = -2.85 ± 0.10 dex and a heliocentric radial velocity of v  r  = 5.1 ± 13.4 km s-1. Boötes V is larger (in terms of scale radius), more distant, and more metal-poor than the vast majority of globular clusters. It is likely that Boötes V is an UFD, though future spectroscopic studies will be necessary to definitively classify this object.

Keywords: Local Group, Broad band photometry, Milky Way stellar halo, Dwarf galaxies