ALBERT

Description: Context. The Perseus cluster is one of the most massive nearby galaxy clusters and is fascinating in various respects. Though the galaxies in the central cluster region have been intensively investigated, an analysis of the galaxy population in a larger field is still outstanding. Aims. This paper investigates the galaxies that are brighter than B ≈ 20 within a field corresponding to the Abell radius of the Perseus cluster. Our first aim is to compile a new catalogue in a wide field around the centre of the Perseus cluster. The second aim of this study is to employ this catalogue for a systematic study of the cluster galaxy population with an emphasis on morphology and activity. Methods. We selected the galaxies in a 10 square degrees field of the Perseus cluster on Schmidt CCD images in B and Hα in combination with SDSS images. Morphological information was obtained both from the “eyeball” inspection and the surface brightness profile analysis. We obtained low-resolution spectra for 82 galaxies and exploited the spectra archive of SDSS and redshift data from the literature. Results. We present a catalogue of 1294 galaxies with morphological information for 90% of the galaxies and spectroscopic redshifts for 24% of them. We selected a heterogeneous sample of 313 spectroscopically confirmed cluster members and two different magnitude-limited samples with incomplete redshift data. These galaxy samples were used to derive such properties as the projected radial velocity dispersion profile, projected radial density profile, galaxy luminosity function, supermassive black hole mass function, total stellar mass, virial mass, and virial radius, to search for indications of substructure, to select active galaxies, and to study the relation between morphology, activity, density, and position. In addition, we present brief individual descriptions of 18 cluster galaxies with conspicuous morphological peculiarities.

Description: Aims. The rotational spectral modulation (spectro-photometric variability) of brown dwarfs is usually interpreted as a sign indicating the presence of inhomogeneous cloud covers in the atmosphere. This paper is aimed at exploring the role of temperature fluctuations in these spectral modulations. These fluctuations could naturally arise in a convective atmosphere impacted by such diabatic processes as complex chemistry, namely, a mechanism recently proposed to explain the L/T transition: CO/CH4 radiative convection. Methods. After exploring the observed spectral-flux ratios between different objects along the cooling sequence, we used the 1D radiative-convective code ATMO, with ad hoc modifications of the temperature gradient, to model the rotational spectral modulation of 2MASS 1821, 2MASS 0136, and PSO 318.5-22. We also explored the impact of CH4 abundance fluctuations on the spectral modulation of 2MASS 0136. Results. The spectral-flux ratio of different objects along the cooling sequence and the rotational spectral modulation within individual objects at the L/T transition have similar characteristics. This strongly suggests that the main parameter varying along the cooling sequence, namely, temperature, might play a key role in the rotational spectral modulations at the L/T transition. Modeling the spectral bright-to-faint ratio of the modulation of 2MASS 1821, 2MASS 0136, and PSO 318.5-22 shows that most spectral characteristics can be reproduced by temperature variations alone. Furthermore, the approximately anti-correlated variability between different wavelengths can be easily interpreted as a change in the temperature gradient in the atmosphere, which is a consequence we expect from CO/CH4 radiative convection as an explanation of the L/T transition. The deviation from an exact anti-correlation could then be interpreted as a phase shift similar to the hot-spot shift at different bandpasses in the atmospheres of hot Jupiters. Conclusions. Our results suggest that the rotational spectral modulation from cloud opacity and temperature variations are degenerate. If the nearly anti-correlated signal between different wavelengths is, indeed, a strong sign of a change in the temperature gradient, the detection of direct cloud spectral signatures, for instance, the silicate absorption feature at 10 μm, would help to confirm the presence of clouds and their contribution to spectral modulations (which does not exclude temperature variations or other mechanisms that may also be at play). Future studies considering the differences in the spectral modulation of objects with and without the silicate absorption feature may give us some insight into how to distinguish cloud-opacity fluctuations from temperature fluctuations.

Description: The hot sub-Saturn-mass exoplanet HAT-P-12b is an ideal target for transmission spectroscopy because of its inflated radius. We observed one transit of the planet with the multi-object double spectrograph (MODS) on the Large Binocular Telescope (LBT) with the binocular mode and obtained an atmosphere transmission spectrum with a wavelength coverage of ~0.4–0.9 μm. The spectrum is relatively flat and does not show any significant sodium or potassium absorption features. Our result is consistent with the revised Hubble Space Telescope (HST) transmission spectrum of a previous work, except that the HST result indicates a tentative detection of potassium. The potassium discrepancy could be the result of statistical fluctuation of the HST dataset. We fit the planetary transmission spectrum with an extensive grid of cloudy models and confirm the presence of high-altitude clouds in the planetary atmosphere. The fit was performed on the combined LBT and HST spectrum, which has an overall wavelength range of 0.4–1.6 μm. The LBT/MODS spectrograph has unique advantages in transmission spectroscopy observations because it can cover a wide wavelength range with a single exposure and acquire two sets of independent spectra simultaneously.

Description: Giant exoplanets on wide orbits have been directly imaged around young stars. If the thermal background in the mid-infrared can be mitigated, then exoplanets with lower masses can also be imaged. Here we present a ground-based mid-infrared observing approach that enables imaging low-mass temperate exoplanets around nearby stars, and in particular within the closest stellar system, α Centauri. Based on 75–80% of the best quality images from 100 h of cumulative observations, we demonstrate sensitivity to warm sub-Neptune-sized planets throughout much of the habitable zone of α Centauri A. This is an order of magnitude more sensitive than state-of-the-art exoplanet imaging mass detection limits. We also discuss a possible exoplanet or exozodiacal disk detection around α Centauri A. However, an instrumental artifact of unknown origin cannot be ruled out. These results demonstrate the feasibility of imaging rocky habitable-zone exoplanets with current and upcoming telescopes.

Description: Context. Large sub-Neptunes are uncommon around the coolest stars in the Galaxy and are rarer still around those that are metal-poor. However, owing to the large planet-to-star radius ratio, these planets are highly suitable for atmospheric study via transmission spectroscopy in the infrared, such as with JWST. Aims. Here we report the discovery and validation of a sub-Neptune orbiting the thick-disk, mid-M dwarf star TOI-2406. The star’s low metallicity and the relatively large size and short period of the planet make TOI-2406 b an unusual outcome of planet formation, and its characterisation provides an important observational constraint for formation models. Methods. We first infer properties of the host star by analysing the star’s near-infrared spectrum, spectral energy distribution, and Gaia parallax. We use multi-band photometry to confirm that the transit event is on-target and achromatic, and we statistically validate the TESS signal as a transiting exoplanet. We then determine physical properties of the planet through global transit modelling of the TESS and ground-based time-series data. Results. We determine the host to be a metal-poor M4 V star, located at a distance of 56 pc, with properties Teff = 3100 ± 75 K, M* = 0.162 ± 0.008M⊙, R* = 0.202 ± 0.011R⊙, and [Fe∕H] = −0.38 ± 0.07, and a member of the thick disk. The planet is a relatively large sub-Neptune for the M-dwarf planet population, with Rp = 2.94 ± 0.17R⊕ and P= 3.077 d, producing transits of 2% depth. We note the orbit has a non-zero eccentricity to 3σ, prompting questions about the dynamical history of the system. Conclusions. This system is an interesting outcome of planet formation and presents a benchmark for large-planet formation around metal-poor, low-mass stars. The system warrants further study, in particular radial velocity follow-up to determine the planet mass and constrain possible bound companions. Furthermore, TOI-2406 b is a good target for future atmospheric study through transmission spectroscopy. Although the planet’s mass remains to be constrained, we estimate the S/N using amass-radius relationship, ranking the system fifth in the population of large sub-Neptunes, with TOI-2406 b having a much lower equilibrium temperature than other spectroscopically accessible members of this population.