Yakiv V. Pavlenko

WISEA J181006.18−101000.5 (WISE1810) is the nearest metal-poor ultracool dwarf to the Sun. It has a low effective temperature and has been classified as an extreme early-T subdwarf. However, methane, the characteristic molecule of the spectral class T, was not seen in the previous low-resolution spectrum. Using the 10.4 m Gran Telescopio Canarias, we collected a high-quality JHK-band intermediate-resolution R ≈ 5000 spectrum of WISE1810, in which 17 ± 6 ppm of methane is clearly detected, while carbon monoxide is absent. Based on a custom computed ATMO2020++ model, we estimated an effective temperature of 1000 ± 100 K, a high surface gravity of log g = 5.5 ± 0.5 dex, and a carbon abundance of [C/H] = −1.5 ± 0.2 dex, inferring [Fe/H] = −1.7 ± 0.2 dex. Potassium is not seen in our data, and the upper limits of the pseudoequivalent width of the J-band atomic lines are at least 25–60 times weaker than those measured from solar-metallicity early-T counterparts. We measured a heliocentric radial velocity of −83 ± 13 km s⁻¹, inferring that WISE1810 is more likely a thick-disk member.

WISEA J181006.18-101000.5 (WISE1810) is the nearest metal-poor ultracool dwarf to the Sun. It has a low effective temperature and has been classified as extreme early-T subdwarf. However, methane, the characteristic molecule of the spectral class T, was not seen in the previous low-resolution spectrum. Using the 10.4-m Gran Telescopio Canarias, we collected a high-quality JHK-band intermediate-resolution R~5000 spectrum of WISE1810, in which a 17+/-6 ppm of methane is clearly detected, while carbon monoxide is absent. Based on customly computed ATMO2020++ model, we estimated an effective temperature of 1000+/-100 K, a high surface gravity of log g = 5.5+/-0.5 dex, a carbon abundance [C/H]=-1.5+/-0.2 dex, inferring [Fe/H]=-1.7+/-0.2 dex. Potassium is not seen in our data, and the upper limits of pseudo-equivalent width of J-band atomic lines are at least 25 to 60 times weaker than those measured from solar-metallicity early-T counterparts. We measured a heliocentric radial velocity of -83+/-13 km/s, inferring that WISE1810 is more likely a thick disk member.

Eleven lines of promethium Pm II in the spectra of the magnetic peculiar A0 V star HD 25354 were analysed thoroughly in order to obtain information about how the short-lived element lines behave in spectra acquired on different observation dates. It has turned out that the intensity variations in time are typical for these lines with some of them being observed as emissive or absent at certain time intervals. The obtained list of Pm II lines in the spectra of the magnetic peculiar star HD 25354 has been employed to investigate promethium abundances in atmospheres of other stars, such as HR 465 (Teff = 111,840 K), HIP 13962 (Teff = 6,250 K) and BL 138 (Teff = 3,939 K). The investigation interval has been limited to the spectral range of 4000–5800 Å and oscillator strengths within –0.76 < log gf < 0.4.

In recent years, the topic of existence and exploration of exocomets has been gaining increasing attention. The asymmetrical decrease in the star’s brightness due to the passage of a comet-like object in front of the star was successfully predicted. It was subsequently confirmed on the basis of the light curves of stars observed by Kepler and TESS orbital telescopes. Since then, there have been successful attempts to fit the asymmetrical dips observed in the stars’ light curves utilizing a simple 1D model of an exponentially decaying optically thin dust tail. In this work, we propose fitting the photometric profiles of some known exocomet transits based on a Monte Carlo approach to build up the distribution of dust particles in a cometary tail. As the exocomet prototypes, we used the physical properties of certain Solar System comets belonging to the different dynamical groups and moving at heliocentric distances of 0.6 au, 1.0 au, 5.0 au, and 5.5 au. We obtained a good agreement between the observed and modeled transit light curves. We also show that the physical characteristics of dust particles, such as the particle size range, the power index of dust size distribution, the particle terminal velocity, and distance to the host star affect the shape of the transit light curve, while the dust productivity of the comet nucleus and the impact parameter influence its depth and duration. The estimated dust production rates of the transiting exocomets are at the level of the most active Solar System comets.

A blue depression is found in the spectra of M dwarfs from 4000 to 4500 Å. This depression shows an increase towards lower temperatures, though it is particularly sensitive to gravity and metallicity. It is the single strongest and most sensitive feature in the optical spectra of M dwarfs. The depression appears as centred on the neutral calcium resonance line at 4227 Å and leads to nearby features being weaker by about two orders of magnitude than predicted. We consider a variety of possible causes for the depression, including temperature, gravity, metallicity, dust, damping constants, and atmospheric stratification. We also consider relevant molecular opacities that might be the cause identifying AlH, SiH, and NaH in the spectral region. However, none of these solutions are satisfactory. In the absence of a more accurate determination of the broadening of the calcium line perturbed by molecular hydrogen, we find a promising empirical fit using a modified Lorentzian line profile for the calcium resonance line. Such fits provide a simplistic line-broadening description for this calcium resonance line and potentially other unmodelled resonance lines in cool high-pressure atmospheres. Thus, we claim that the most plausible cause of the blue depression in the optical spectra of M dwarfs is a lack of appropriate treatment of line broadening for atomic calcium. The broad wings of the calcium resonance line develop at temperatures below about 4000 K and are analogous to the neutral sodium and potassium features, which dominate the red optical spectra of L dwarfs.

A quantitative analysis of the spectrum of HD 108564 is performed. It is a star of the main sequence of spectral class K5V, the atmosphere of which is depl

The recently computed ExoMol line lists for isotopologues of AlH are used to analyse the blue spectrum (4000–4500 Å) of Proxima Cen (M5.5 V). Comparison of the observed and computed spectra enables the identification of a large number of ²⁷AlH lines of the A ¹Π –X ¹Σ⁺ band system: The spectral range covering 1-0, 0-0, and 1-1 bands are dominated by clearly resolved AlH lines. We reveal the diffuse nature of transitions close to the dissociation limit which appears in the form of increasingly wider (up to 5 Å) and shallower (up to the continuum confusion limit) AlH line profiles. The predicted wavelengths of AlH diffuse lines are systematically displaced. The effect of broadening by predissociation states on the line profiles is included by increasing the radiative damping rate by up to 5 orders of magnitude. We determine empirical values of damping rates for a number of the clean 0-0 Q-branch transitions by comparing the observed and synthetic stellar spectra. We find excellent agreement between our damping rates and lifetimes available in the literature. A comparison of ²⁷Al¹H ExoMol and REALH spectra shows that the observed spectrum is better described by the ExoMol line list. A search for ²⁶Al¹H lines in the Proxima Cen spectrum does not reveal any notable features; giving an upper limit of ²⁷Al¹H /²⁶Al¹H >100.

Aims. The aim of our work is to analyze the light curves of β Pic which were recently observed by the Transiting Exoplanet Survey Satellite (TESS) in sectors 32, 33, and 34 by searching for the signatures of exocomet transits. Methods. We processed the β Pic light curves from the Mikulski Archive for Space Telescopes database, applying the frequency analysis to remove harmonic signals due to the star’s pulsations, and we used a simple 1D model to fit the profiles of the events that were found. Results. We recovered events previously found by other authors in sectors 5 and 6 and found five new distinct aperiodic dipping events with asymmetric shapes resembling the expected profiles due to the passage of a comet-like body across the star disk. These dips are rather shallow, with the flux drop at a level of 0.03% and a duration of less than 1 day. No periodic transits were found in the sectors investigated. Conclusions. The depth and duration of the identified dips are similar to the recently discovered transits in the β Pic light curves from sector 5 of the TESS observations as well as to those found in the light curves of KIC 354116 and KIC 1108472 from the Kepler database. This indicates that aperiodic shallow dips are not likely an exceptional phenomenon, at least for the β Pic system.