Stellar Spectra Research Articles

Titanium abundances in late-type stars

Context. Rapidly growing datasets from stellar spectroscopic surveys are providing unprecedented opportunities to analyse the chemical evolution history of our Galaxy. However, spectral analysis requires accurate modelling of synthetic stellar spectra for late-type stars, for which the assumption of local thermodynamic equilibrium (LTE) has been shown to be insufficient in many cases. Errors associated with LTE can be particularly large for Ti I, which is susceptible to over-ionisation, particularly in metal-poor stars. Aims. The aims of this work are to study and quantify the 1D non-LTE effects on titanium abundances across the Hertzsprung-Russell diagram for a large sample of stars. Methods. A large grid of departure coefficients, βν, were computed on standard MARCS model atmospheres. The grid extends from 3000 K to 8000 K in Teff, −0.5 dex to +5.5 dex in log g, and −5.0 to +1.0 in [Fe/H], with non-LTE effects in this grid reaching up to 0.4 dex. This was used to compute abundance corrections that were subsequently applied to the LTE abundances of over 70 000 stars selected from the GALAH survey in addition to a smaller sample of literature Keck data for metal-poor dwarfs. Results. The non-LTE effects grow towards lower [Fe/H], lower log g, and higher Teff, with a minimum and maximum ΔA(Ti)Ti I of 0.02 and 0.19 in the GALAH sample. For metal-poor giants, the non-LTE modelling reduces the average ionisation imbalance (ΔI−II) from −0.11 dex to −0.01 dex at [Fe/H] = −1.7, and the enhancement in titanium abundances from Ti I lines results in a [Ti/Fe] versus [Fe/H] trend that more closely resembles the behaviour of Ti II at low metallicities. At higher metallicities, the results are limited by the precision of the GALAH DR3 LTE abundances and the effects are within the errors. For the most metal-poor dwarfs from the Keck sample, the average ionisation imbalance increases from −0.1 dex to +0.2 dex, a shortcoming that is consistent with previous 1D non-LTE studies and which we speculate could be related to 3D effects. Conclusions. Non-LTE effects on titanium abundances are significant. Neglecting them may alter our understanding of Galactic chemical evolution. We have made our grid of departure coefficients publicly available, with the caveat that the Ti abundances of metal-poor dwarfs need further study in 3D non-LTE.

Estimating the Atmospheric Parameters of Early-type Stars from the Chinese Space Station Telescope (CSST) Slitless Spectra Survey

The measurement of atmospheric parameters is fundamental for scientific research using stellar spectra. The Chinese Space Station Telescope (CSST), scheduled to be launched in 2024, will provide researchers with hundreds of millions of slitless spectra for stars during a 10 yr survey. And machine learning has unparalleled efficiency in processing large amounts of data compared to manual processing. Here we studied the stellar parameters of early-type stars (effective temperature T eff > 15,000 K) based on the design indicators of the CSST slitless spectrum and the machine learning algorithm, Stellar LAbel Machine. We used the Potsdam Wolf–Rayet (POWR) synthetic spectra library for cross validation. Then we tested the reliability of machine learning results by using the Next Generation Spectrum Library (NGSL) from Hubble Space Telescope observation data. In an ideal case for full-wavelength spectra without any noise, the average absolute value of the relative deviation is 2.7% (800 K) for T eff, and 3.5% (0.11 c.g.s) for surface gravity logg . We use the spectra with the impact of interstellar extinction (AV = 0, 0.5, 1, 1.5, 2 mag) and radial velocity (RV = −50, −30, 0, 30, 50 km s−1) from the POWR library as the test set. When RV = 0 km s−1 and AV = 0 mag, the average value and standard deviation for 3 wavelength ranges (2550–4050 Å (R = 287); 4050–6300 Å (R = 232); 6300–10000 Å (R = 207)) are –66 ± 3351 K, 550 ± 3536 K, and 356 ± 3616 K for T eff, and 0.004 ± 0.224 c.g.s, –0.024 ± 0.246 c.g.s, and 0.01 ± 0.212 c.g.s for logg . When using the observed data from NGSL as the testing samples, the deviation of T eff is less than 5% (1500 K), and the deviation of logg is less than 11% (0.33 c.g.s). In addition, we also test the influence of shifting of spectra on the parameters’ accuracy. The deviation of T eff for the case with a shift of 5 Å and 10 Å are 3.6% (1100 K) and 4.3% (1300 K), respectively; the deviation of logg are 4.2% (0.13 c.g.s) and 5.1% (0.15 c.g.s). These results demonstrate that we can obtain relatively accurate stellar parameters of a population of early-type stars with the CSST slitless spectra and a machine-learning method.

The PAU survey: photometric calibration of narrow band images

ABSTRACT The physics of the accelerating Universe (PAU) camera is an optical narrow band and broad band imaging instrument mounted at the prime focus of the William Herschel Telescope. We describe the image calibration procedure of the PAU survey data. We rely on an external photometric catalogue to calibrate our narrow band data using stars that have been observed by both data sets. We fit stellar templates to the stellar broad-band photometry of the Sloan Digital Sky Survey and synthesize narrow band photometry that we compare to the PAUS narrow band data to determine their calibration. Consequently, the PAUS data are in the AB system as inherited from its reference calibrator. We do several tests to check the performance of the calibration. We find it self-consistent when comparing repeated observations of the same objects, with a good overall accuracy to the AB system which we estimate to be at the 2 per cent precision level and no significant trends as a function of narrow band filter or wavelength. Repeated observations allow us to build a spatial map of the illumination pattern of the system. We also check the wavelength dependence of the calibration comparing to stellar spectra. We find that using only blue stars reduces the effects of variations in the stellar template fitting to broad-band colours, improving the overall precision of the calibration to around 1 per cent and its wavelength uniformity. The photometric redshift performance obtained with the PAUS data attests to the validity of our calibration to reach the PAUS science goals.

KIC 4150611: A quadruply eclipsing heptuple star system with a g-mode period-spacing pattern. Eclipse modelling of the triple and spectroscopic analysis.

KIC 4150611 is a high-order multiple composed of a triple system.\ It comprises: (1) a F1V primary (Aa) that is eclipsed on a 94.2d period by a tight 1.52d binary composed of two dim K/M dwarfs (Ab1 and Ab2), which also eclipse each other; (2) an 8.65d eccentric, eclipsing binary composed of two G stars (Ba and Bb); and (3) another faint eclipsing binary composed of two stars of unknown spectral type (Ca and Cb). In addition to its many eclipses, the system is an SB3 spectroscopic multiple (Aa, Ba, and Bb), and the primary (Aa) is a hybrid pulsator that exhibits high amplitude pressure and gravity modes. In aggregate, this richness in physics offers an excellent opportunity to obtain a precise physical characterisation of some of the stars in this system. In this work we aim to characterise the F1V primary by modelling its complex eclipse geometry and disentangled stellar spectra in preparation for follow-up work that will focus on its pulsations. We employed a novel photometric analysis of the complicated eclipse geometry of Aa to obtain the orbital and stellar properties of the triple. We acquired 51 TRES spectra at the Fred L. Whipple Observatory, calculating radial velocities and orbital elements of Aa (SB1) and the B binary (SB2). These spectra and radial velocities were used to perform spectral disentangling for Aa, Ba, and Bb. Spectral modelling was applied to the disentangled spectrum of Aa to obtain atmospheric properties. From our eclipse modelling we obtain precise stellar properties of the triple, including the mass ratios ($M_ Aa /(M_ Ab1 +M_ Ab2 Ab1 /M_ Ab2 the separation ratio Aab /a_ Ab1Ab2 0.01$), orbital periods Aab Ab1Ab2 and stellar radii ( R R R Via radial velocity fitting and spectral disentangling, we find orbital elements for Aa, Ba, and Bb that are in excellent agreement with each other and with previous results in the literature. Spectral modelling on the disentangled spectrum of Aa provides constraints on the effective temperature eff K), surface gravity (log$(g) = 4.14 0.18$ dex), micro-turbulent velocity micro rotation velocity ($v i = 127 and metallicity M/H

Oxygen and Air Density Retrieval Method for Single-Band Stellar Occultation Measurement

The stellar occultation technique is capable of atmospheric trace gas detection using the molecule absorption characteristics of the stellar spectra. In this paper, the non-iterative and iterative retrieval methods for oxygen and air density detection by stellar occultation are investigated. For the single-band average transmission data in the oxygen 761 nm A-band, an onion-peeling algorithm is used to calculate the effective optical depth of each atmospheric layer, and then the optical depth is used to retrieve the oxygen number density. The iteration method introduces atmospheric hydrostatic equilibrium and the ideal gas equation of state, and it achieves a more accurate retrieval of the air density under the condition of a priori temperature deviation. Finally, this paper analyzes the double solution problem in the iteration process and the ideas to improve the problem. This paper provides a theoretical basis for the development of a new type of atmospheric density detection method.

End-to-end simulations of exoplanet detection with multiplexed volume Bragg gratings

Context. Direct imaging of exoplanets greatly enhances our ability to spectroscopically detect molecules in their atmospheres, but requires both excellent angular and spectral resolution. A highly promising approach uses multiplexed transmission gratings to achieve compressed sensing, but the published theoretical calculations are not realistic enough to determine whether the approach is feasible. Aims. We aim to determine the performance of recovering exoplanet signals of an instrument with multiplexed volume Bragg gratings in a full, detailed numerical simulation, specifically examining any effects caused by multiplexing several gratings. Methods. Our end-to-end simulation includes realistic stellar and planetary spectra, a closed-loop adaptive optics simulation, and rigorous coupled wave analysis to model the multiplexed gratings. We chose 20 passbands around expected methane absorption features that were optically combined using 20 gratings. Results. We find that exoplanet signals can be recovered down to contrasts of 10−6−10−7, without the addition of a coronagraph. Multiplexing a larger number of gratings improves the deepest recovered contrast, if photon noise is the dominant noise source. When residuals from our simple post-processing approach are the largest noise source, a larger number of multiplexed gratings decreases the performance, due to the stacking of the PSFs at different wavelengths. This is an artifact of our data reduction approach. Conclusions. We conclude that multiplexed Bragg gratings are a viable method to look for exoplanets with a compressed sensing approach, and additional gains may be made in post-processing.

M3DIS - A grid of 3D radiation-hydrodynamics stellar atmosphere models for stellar surveys. I. Procedure, validation, and the Sun

Large-scale stellar surveys, such as SDSS-V, 4MOST, WEAVE, and PLATO, require accurate atmospheric models and synthetic spectra of stars for accurate analyses of fundamental stellar parameters and chemical abundances. The primary goal of our work is to develop a new approach to solve radiation-hydrodynamics (RHD) and generate model stellar spectra in a self-consistent and highly efficient framework. We build upon the Copenhagen legacy RHD code, the MULTI3D non-local thermodynamic equilibrium (NLTE) code, and the DISPATCH high-performance framework. The new approach allows us to calculate 3D RHD models of stellar atmospheres on timescales of a few thousand CPU hours and to perform subsequent spectrum synthesis in local thermodynamic equilibrium (LTE) or NLTE for the desired physical conditions within the parameter space of FGK-type stars. We compare the 3D RHD solar model with other available models and validate its performance against solar observations, including the centre-to-limb variation of intensities and key solar diagnostic lines of H and Fe. We show that the performance of the new code allows to overcome the main bottleneck in 3D NLTE spectroscopy and enables calculations of multi-dimensional grids of synthetic stellar observables for comparison with modern astronomical observations.

The Effect of Starspots on Spectroscopic Age and Mass Estimates of Nonaccreting T Tauri Stars in the Taurus–Auriga Star-forming Region

Accurate age and mass determinations for young pre-main-sequence stars are made challenging by the presence of large-scale starspots. We present results from a near-infrared spectroscopic survey of 10 T-Tauri Stars in Taurus-Auriga that characterize spot-filling factors and temperatures, the resulting effects on temperature and luminosity determinations, and the consequences for inferred stellar masses and ages. We constructed composite models of spotted stars by combining BTSettl-CIFIST synthetic spectra of atmospheres to represent the spots and the photosphere along with continuum emission from a warm inner disk. Using a Markov Chain Monte Carlo algorithm, we find the best-fit spot and photospheric temperatures, spot-filling factors, as well as disk-filling factors. This methodology allowed us to reproduce the 0.75–2.40 μm stellar spectra and molecular feature strengths for all of our targets, disentangling the complicated multicomponent emission. For a subset of stars with multiepoch observations spanning an entire stellar rotation, we correlate the spectral variability and changes in the filling factors with rotational periods observed in K2 and AAVSO photometry. Combining spot-corrected effective temperatures and Gaia distances, we calculate luminosities and use the Stellar Parameters of Tracks with Starspots models to infer spot-corrected masses and ages for our sample of stars. Our method of accounting for spots results in an average increase of 60% in mass and a doubling in age with respect to traditional methods using optical spectra that do not account for the effect of spots.

An atlas of resolved spectral features in the transmission spectrum of WASP-189 b with MAROON-X

Exoplanets in the ultra-hot Jupiter regime provide an excellent laboratory for testing the impact of stellar irradiation on the dynamics and chemical composition of gas giant atmospheres. In this study, we observed two transits of the ultra-hot Jupiter WASP-189 b with MAROON-X/Gemini-North to probe its high-altitude atmospheric layers, using strong absorption lines. We derived posterior probability distributions for the planetary and stellar parameters by calculating the stellar spectrum behind the planet at every orbital phase during the transit. This was used to correct the Rossiter–McLaughlin imprint on the transmission spectra. Using differential transmission spectroscopy, we detect strong absorption lines of Ca+, Ba+, Na, Hα, Mg, Fe, and Fe+, providing an unprecedented and detailed view of the atmospheric chemical composition. Ca+ absorption is particularly well suited for analysis through time-resolved narrow-band spectroscopy, owing to its transition lines formed in high-altitude layers. The spectral absorption lines show no significant blueshifts that would indicate high-altitude day-to-night winds, and further analysis is needed to investigate the implications for atmospheric dynamics. These high signal-to-noise observations provide a benchmark data set for testing high-resolution retrievals and the assumptions of atmospheric models. We also simulate observations of WASP-189 b with ANDES/ELT, and show that ANDES will be highly sensitive to the individual absorption lines of a myriad of elements and molecules, including TiO and CO.

Hydrologic cycle weakening in hothouse climates.

The hydrologic cycle has wide impacts on the ocean salinity and circulation, carbon and nitrogen cycles, and the ecosystem. Under anthropogenic global warming, previous studies showed that the intensification of the hydrologic cycle is a robust feature. Whether this trend persists in hothouse climates, however, is unknown. Here, we show in climate models that mean precipitation first increases with rising surface temperature, but the precipitation trend reverses when the surface is hotter than ~320 to 330 kelvin. This nonmonotonic phenomenon is robust to the cause of warming, convection scheme, ocean dynamics, atmospheric mass, planetary rotation, gravity, and stellar spectrum. The weakening occurs because of the existence of an upper limitation of outgoing longwave emission and the continuously increasing shortwave absorption by H2O and is consistent with atmospheric dynamics featuring the strong increase of atmospheric stratification and marked reduction of convective mass flux. These results have wide implications for the climate evolutions of Earth, Venus, and potentially habitable exoplanets.

Measuring the Chemodynamics and Ages of the M32 and M110 Dwarf Galaxies with APOGEE

We present a full-spectrum-fitting analysis of the central kinematics and chemistry of the Andromeda dwarf satellite galaxies M32 and M110. We use an Markov Chain Monte Carlo routine to fit high-resolution, near-infrared, integrated-light spectra from APOGEE with empirical simple stellar population templates constructed from individual APOGEE stellar spectra. This yields the best-fitting mean radial velocity, velocity dispersion, metallicity, α abundance, and age for each spectrum. In general, our results are consistent with literature values where available, and we explore possible reasons where offsets are measured. This study was presented in a poster at the 243rd meeting of the American Astronomical Society in 2024 January.

The Heavy Metal Survey: Star Formation Constraints and Dynamical Masses of 21 Massive Quiescent Galaxies at z = 1.3–2.3

In this paper, we present the Heavy Metal Survey, which obtained ultradeep medium-resolution spectra of 21 massive quiescent galaxies at 1.3 < z < 2.3 with Keck/LRIS and MOSFIRE. With integration times of up to 16 hr per band per galaxy, we observe numerous Balmer and metal absorption lines in atmospheric windows. We successfully derive spectroscopic redshifts for all 21 galaxies, and for 19 we also measure stellar velocity dispersions (σ v ), ages, and elemental abundances, as detailed in an accompanying paper. Except for one emission-line active galactic nucleus, all galaxies are confirmed as quiescent through their faint or absent Hα emission and evolved stellar spectra. For most galaxies exhibiting faint Hα, elevated [N ii]/Hα suggests a non-star-forming origin. We calculate dynamical masses (M dyn) by combining σ v with structural parameters obtained from the Hubble Space Telescope COSMOS(-DASH) survey and compare them with stellar masses (M *) derived using spectrophotometric modeling, considering various assumptions. For a fixed initial mass function (IMF), we observe a strong correlation between M dyn/M * and σ v . This correlation may suggest that a varying IMF, with high-σ v galaxies being more bottom heavy, was already in place at z ∼ 2. When implementing the σ v -dependent IMF found in the cores of nearby early-type galaxies and correcting for biases in our stellar mass and size measurements, we find a low scatter in M dyn/M * of 0.14 dex. However, these assumptions result in unphysical stellar masses, which exceed the dynamical masses by 34%. This tension suggests that distant quiescent galaxies do not simply grow inside-out into today’s massive early-type galaxies and the evolution is more complicated.

The impact of nebular Lyman-Continuum on ionizing photons budget and escape fractions from galaxies

ABSTRACT Several Lyman Continuum (Ly C) emitters have been detected so far, but their observed ionizing spectra sometimes differ from attenuated stellar spectra predicted by stellar population synthesis modelling. This discrepancy may be due to a significant contribution of Ly C nebular emission. We aim to quantify the importance this emission in Ly C leakers: its contribution to the ionizing photons budget, and to measurements of Ly C escape. To estimate the nebular contribution to the Ly C spectra of galaxies, we run photoionization models with Cloudy for a range of Binary Population and Spectral Synthesis templates, varying the column density of the surrounding gas, from density-bounded (NH i = 1016 cm−2) to ionization-bounded (NH i = 1019 cm−2) regimes. In the limits of very optically thin (fesc = 1), or thick configurations (fesc = 0), there is no nebular contribution to the emergent Ly C spectra. This contribution matters only at intermediate Ly C opacities (0 &amp;lt; fesc &amp;lt; 1), where it alters the shape of the Ly C spectrum chromatically, so that escape fractions estimates are highly sensitive to the wavelength range over which they are calculated. We propose a formula to estimate integrated escape fractions using Fλ700/Fλ1100 flux ratios, since this wavelength range is not affected by nebular emission. Regarding simulations, the boost of hydrogen ionizing photons escaping galaxies is inversely proportional to the stellar escape fractions, but since typical simulated escape fractions are low, Ly C photons escape is important. Nebular Ly C is a non-negligible additional source of ionizing photons from galaxies, whose contribution has been overlooked so far in observations and in cosmic reionization simulations.

Autonomous disentangling for spectroscopic surveys

ABSTRACT A suite of spectroscopic surveys is producing vast sets of stellar spectra with the goal of advancing stellar physics and Galactic evolution by determining their basic physical properties. A substantial fraction of these stars are in binary systems, but almost all large-survey modelling pipelines treat them as single stars. For sets of multi-epoch spectra, spectral disentangling is a powerful technique to recover or constrain the individual components’ spectra of a multiple system. So far, this approach has focused on small samples or individual objects, usually with high-resolution ($R \gtrsim 10.000$) spectra and many epochs ($\gtrsim 8$). Here, we present a disentangling implementation that accounts for several aspects of few-epoch spectra from large surveys: that vast sample sizes require automatic determination of starting guesses; that some of the most extensive spectroscopic surveys have a resolution of only $\approx 2000$; that few epochs preclude unique orbit fitting; that one needs effective regularization of the disentangled solution to ensure resulting spectra are smooth. We describe the implementation of this code and show with simulated spectra how well spectral recovery can work for hot and cool stars at $R \approx 2000$. Moreover, we verify the code on two established binary systems, the ‘Unicorn’ and ‘Giraffe’. This code can serve to explore new regimes in survey disentangling in search of massive stars with massive dark companions, for example, the $\gtrsim 200\,000$ hot stars of the SDSS-V survey.

Helium in the Extended Atmosphere of the Warm Superpuff TOI-1420b

Superpuffs are planets with exceptionally low densities (ρ ≲ 0.1 g cm−3) and core masses (M c ≲ 5M ⊕). Many lower-mass (M p ≲ 10M ⊕) superpuffs are expected to be unstable to catastrophic mass loss via photoevaporation and/or boil-off, whereas the larger gravitational potentials of higher-mass (M p ≳ 10M ⊕) superpuffs should make them more stable to these processes. We test this expectation by studying atmospheric loss in the warm, higher-mass superpuff TOI-1420b (M = 25.1M ⊕, R = 11.9R ⊕, ρ = 0.08 g cm−3, T eq = 960 K). We observed one full transit and one partial transit of this planet using the metastable helium filter on Palomar/WIRC and found that the helium transits were 0.671% ± 0.079% (8.5σ) deeper than the TESS transits, indicating an outflowing atmosphere. We modeled the excess helium absorption using a self-consistent 1D hydrodynamics code to constrain the thermal structure of the outflow given different assumptions for the stellar XUV spectrum. These calculations then informed a 3D simulation, which provided a good match to the observations with a modest planetary mass-loss rate of 1010.82 g s−1 ( Mp/Ṁ≈70 Gyr). Superpuffs with M p ≳ 10M ⊕, like TOI-1420b and WASP-107b, appear perfectly capable of retaining atmospheres over long timescales; therefore, these planets may have formed with the unusually large envelope mass fractions they appear to possess today. Alternatively, tidal circularization could have plausibly heated and inflated these planets, which would bring their envelope mass fractions into better agreement with expectations from core-nucleated accretion.

Research on the Optimal Design of the High-stability Optical System for Atmospheric Spectra in Transit Observation

Capturing the characteristics of exoplanetary atmospheres (CEA) through transit spectroscopy (TS) holds profound implications for our understanding of planetary formation and evolution. However, TS, the method employed for detecting CEA, indirectly extracts these characteristics from the subtle variations in stellar spectra during the transit process, necessitating a high level of observational stability in optical instrumentation. To mitigate observational errors in spectral energy within the optical system, this dissertation delves into the optimal design of a high-stability optical system tailored for atmospheric spectra in transit observations. Initially, a theoretical model of transit signal-to-noise ratios (S/Ns) catered to the EAC retrievals is formulated based on transit observation strategies. Subsequently, the optimal parameters and design approach for the optical system are explored through an analysis of the optical factors influencing S/N. Leveraging an observation simulator for optical instruments, the detection feasibility of the optimized optical system for capturing CEA is validated.

Data-driven Discovery of Diffuse Interstellar Bands with APOGEE Spectra

Data-driven models of stellar spectra are useful tools to study nonstellar information, such as the diffuse interstellar bands (DIBs) caused by intervening interstellar material. Using ∼55,000 spectra of ∼17,000 red clump stars from the APOGEE DR16 data set, we create second-order polynomial models of the continuum-normalized flux as a function of stellar parameters (T eff, logg , [Fe/H], [α/Fe], and age). The model and data show good agreement within uncertainties across the APOGEE wavelength range, although many regions reveal residuals that are not in the stellar rest-frame. We show that many of these residual features—having average extrema at the level of ∼3% in stellar flux on average—can be attributed to incompletely removed spectral lines from the Earth’s atmosphere and DIBs from the interstellar medium (ISM). After removing most of the remaining contamination from Earth’s sky, we identify 84 absorption features not seen in unreddened sightlights that have <50% probability of being noise artifacts—with 25 of these features having <5% probability of being noise artifacts—including all 10 previously known DIBs in the APOGEE wavelength range. Because many of these features occur in the wavelength windows that APOGEE uses to measure chemical abundances, note that characterization and removal of this nonstellar contamination establish an important step in reaching the precision required for chemical tagging experiments. Proper characterization of these features will benefit Galactic ISM science and the currently ongoing Milky Way Mapper program of Sloan Digital Sky Survey V, which relies on the APOGEE spectrograph.

Magnetic Activity of Millions of G-type Stars Based on the LAMOST DR10 Low-resolution Spectral and TESS Light-curve Surveys and the Future CSST Survey

The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) data release 10 (DR10) provides over 3 million G-type stellar spectra, which are important for the study of chromospheric activity of solar-like stars. We have used the iSpec program to perform spectral subtraction on the G-type stellar spectra obtained from LAMOST DR10 with a signal-to-noise ratio greater than 10. We have calculated the magnetic activity of G-type stars using Hα lines and analyzed the Hα variation. Among the more than 3 million spectra from more than 2.3 million stars, a total of 220,000 stars show excess chromospheric activity. There were a total of about 480,000 stars with repeated observations. About 390,000 stars were found to exhibit variations in the Hα line, and 14,000 stars showed changes in their radial velocity. By using the Gaia data, we determined the distances of these stars above the Galactic disk. We first concluded that the fraction of G-type active stars decreases with increasing distance above the Galactic disk. By using the Transiting Exoplanet Survey Satellite light curves, we obtained the effective fluctuation range of the light curve caused by the starspot and confirmed that there was a positive correlation between the starspot and chromospheric activity. We also concluded that RHα′ tends to be stable for Rossby number (Ro) < 0.13 and that RHα′ decreases as Ro increases in the region Ro ≥ 0.13.

StellarGAN: Classifying Stellar Spectra with Generative Adversarial Networks in SDSS and APOGEE Sky Surveys

Extracting precise stellar labels is crucial for large spectroscopic surveys like the Sloan Digital Sky Survey (SDSS) and APOGEE. In this paper, we report the newest implementation of StellarGAN, a data-driven method based on generative adversarial networks (GANs). Using 1D operators like convolution, the 2D GAN is modified into StellarGAN. This allows it to learn the relevant features of 1D stellar spectra without needing labels for specific stellar types. We test the performance of StellarGAN on different stellar spectra trained on SDSS and APOGEE data sets. Our result reveals that StellarGAN attains the highest overall F1-score on SDSS data sets (F1-score = 0.82, 0.77, 0.74, 0.53, 0.51, 0.61, and 0.55, for O-type, B-type, A-type, F-type, G-type, K-type, and M-type stars) when the signal-to-noise ratio (S/N) is low (90% of the spectra have an S/N < 50), with 1% of labeled spectra used for training. Using 50% of the labeled spectral data for training, StellarGAN consistently demonstrates performance that surpasses or is comparable to that of other data-driven models, as evidenced by the F1-scores of 0.92, 0.77, 0.77, 0.84, 0.84, 0.80, and 0.67. In the case of APOGEE (90% of the spectra have an S/N < 500), our method is also superior regarding its comprehensive performance (F1-score = 0.53, 0.60, 0.56, 0.56, and 0.78 for A-type, F-type, G-type, K-type, and M-type stars) with 1% of labeled spectra for training, manifesting its learning ability out of a limited number of labeled spectra. Our proposed method is also applicable to other types of data that need to be classified (such as gravitational-wave signals, light curves, etc.).

A reanalysis of the LHS 1140 b atmosphere observed with the Hubble Space Telescope

ABSTRACT The super-Earth LHS 1140 b is an interesting target for atmospheric observations since it is close to the habitable zone of its star and falls in the gap of the radius distribution of small exoplanets, in the region thought to correspond to the transition between planets with and without atmospheres. Observations of the primary transit with WFC3 onboard of the Hubble Space Telescope (HST) revealed a modulation in the planet transmission spectrum compatible with the presence of water; however, this modulation may be also due to stellar activity-related phenomena. Here, we present a detailed analysis of the WFC3/HST observations to probe the nature of this modulation and to understand if it can be attributable to the presence of unocculted spots on the stellar surface. Our analysis strongly suggests that LHS1140 is a rather quiet star with subsolar [Fe/H] and enriched in α elements. Therefore, we rule out the possibility that the planetary spectrum is affected by the presence of spots and faculae. This analysis shows the importance of a proper modelling of the stellar spectrum when analysing transit observations. Finally, we modelled the planetary atmosphere of LHS1140 b to retrieve its atmospheric composition. However, the low resolution and the narrow spectral range of HST observations prevented us from definitively determining whether the spectral features are attributable to the presence of water or of other molecules in the planetary atmosphere.