Measurements of the Ca ii infrared triplet emission lines of pre-main-sequence stars

We investigated the chromospheric activity of 60 pre-main-sequence (PMS) stars in four molecular clouds and five moving groups. It is considered that strong chromospheric activity is driven by the dynamo processes generated by stellar rotation. In contrast, several researchers have pointed out that the chromospheres of PMS stars are activated by mass accretion from their protoplanetary disks. In this study, the Ca ii infrared triplet (IRT) emission lines were investigated utilizing medium- and high-resolution spectroscopy. The observations were conducted with Nayuta/MALLS and Subaru/HDS. Additionally, archive data obtained by Keck/HIRES, VLT/UVES, and VLT/X-Shooter were used. The small ratios of the equivalent widths indicate that Ca ii IRT emission lines arise primarily in dense chromospheric regions. Seven PMS stars show broad emission lines. Among them, four PMS stars have more than one order of magnitude brighter emission line fluxes compared to the low-mass stars in young open clusters. The four PMS stars have a high mass accretion rate, which indicates that the broad and strong emission results from a large mass accretion. However, most PMS stars exhibit narrow emission lines. No significant correlation was found between the accretion rate and flux of the emission line. The ratios of the surface flux of the Ca ii IRT lines to the stellar bolometric luminosity, $R^{\prime }_{\rm IRT}$, of the PMS stars with narrow emission lines are as large as the largest $R^{\prime }_{\rm IRT}$ of the low-mass stars in the young open clusters. This result indicates that most PMS stars, even in the classical T Tauri star stage, have chromospheric activity similar to zero-age main-sequence stars.

AT2018kzr: the merger of an oxygen–neon white dwarf and a neutron star or black hole

ABSTRACT We present detailed spectroscopic analysis of the extraordinarily fast-evolving transient AT2018kzr. The transient’s observed light curve showed a rapid decline rate, comparable to the kilonova AT2017gfo. We calculate a self-consistent sequence of radiative transfer models (using tardis) and determine that the ejecta material is dominated by intermediate-mass elements (O, Mg, and Si), with a photospheric velocity of ∼12 000–14 500 $\rm {km}\, s^{-1}$. The early spectra have the unusual combination of being blue but dominated by strong Fe ii and Fe iii absorption features. We show that this combination is only possible with a high Fe content (3.5 per cent). This implies a high Fe/(Ni+Co) ratio. Given the short time from the transient’s proposed explosion epoch, the Fe cannot be 56Fe resulting from the decay of radioactive 56Ni synthesized in the explosion. Instead, we propose that this is stable 54Fe, and that the transient is unusually rich in this isotope. We further identify an additional, high-velocity component of ejecta material at ∼20 000–26 000 $\rm {km}\, s^{-1}$, which is mildly asymmetric and detectable through the Ca ii near-infrared triplet. We discuss our findings with reference to a range of plausible progenitor systems and compare with published theoretical work. We conclude that AT2018kzr is most likely the result of a merger between an ONe white dwarf and a neutron star or black hole. As such, it would be the second plausible candidate with a good spectral sequence for the electromagnetic counterpart of a compact binary merger, after AT2017gfo.

The Pristine Dwarf-Galaxy survey – II. In-depth observational study of the faint Milky Way satellite Sagittarius II

ABSTRACT We present an extensive study of the Sagittarius II (Sgr II) stellar system using MegaCam g and i photometry, narrow-band, metallicity-sensitive calcium H&K doublet photometry and Keck II/DEIMOS multiobject spectroscopy. We derive and refine the Sgr II structural and stellar properties inferred at the time of its discovery. The colour–magnitude diagram implies Sgr II is old (12.0 ± 0.5 Gyr) and metal poor. The CaHK photometry confirms the metal-poor nature of the satellite ([Fe/H] CaHK = −2.32 ± 0.04 dex) and suggests that Sgr II hosts more than one single stellar population ($\sigma _\mathrm{[FeH]}^\mathrm{CaHK} = 0.11^{+0.05}_{-0.03}$ dex). Using the Ca infrared triplet measured from our highest signal-to-noise spectra, we confirm the metallicity and dispersion inferred from the Pristine photometric metallicities ([Fe/H]spectro = −2.23 ± 0.05 dex, $\sigma _\mathrm{[Fe/H]}^\mathrm{spectro} = 0.10 ^{+0.06}_{-0.04}$ dex). The velocity dispersion of the system is found to be $\sigma _{v} = 2.7^{+1.3}_{-1.0} {\rm \, km \,\, s^{-1}}$ after excluding two potential binary stars. Sgr II’s metallicity and absolute magnitude (MV = −5.7 ± 0.1 mag) place the system on the luminosity–metallicity relation of the Milky Way dwarf galaxies despite its small size. The low but resolved metallicity and velocity dispersions paint the picture of a slightly dark-matter-dominated satellite ($M/L = 23.0^{+32.8}_{-23.0}$ M⊙ L$^{-1}_{\odot }$). Furthermore, using the Gaia Data Release 2, we constrain the orbit of the satellite and find an apocentre of $118.4 ^{+28.4}_{-23.7} {\rm \, kpc}$ and a pericentre of $54.8 ^{+3.3}_{-6.1} {\rm \, kpc}$. The orbit of Sgr II is consistent with the trailing arm of the Sgr stream and indicates that it is possibly a satellite of the Sgr dSph that was tidally stripped from the dwarf’s influence.

The CARMENES search for exoplanets around M dwarfs

Chromospheric modeling of observed differences in stellar activity lines is imperative to fully understand the upper atmospheres of late-type stars. We present one-dimensional parametrized chromosphere models computed with the atmosphere code PHOENIX using an underlying photosphere of 3500 K. The aim of this work is to model chromospheric lines of a sample of 50 M2–3 dwarfs observed in the framework of the CARMENES, the Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Echelle Spectrographs, exoplanet survey. The spectral comparison between observed data and models is performed in the chromospheric lines of Na I D2, Hα, and the bluest Ca II infrared triplet line to obtain best-fit models for each star in the sample. We find that for inactive stars a single model with a VAL C-like temperature structure is sufficient to describe simultaneously all three lines adequately. Active stars are rather modeled by a combination of an inactive and an active model, also giving the filling factors of inactive and active regions. Moreover, the fitting of linear combinations on variable stars yields relationships between filling factors and activity states, indicating that more active phases are coupled to a larger portion of active regions on the surface of the star.

The CARMENES search for exoplanets around M dwarfs

Context. The Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Echelle Spectrographs (CARMENES) survey is searching for Earth-like planets orbiting M dwarfs using the radial velocity method. Studying the stellar activity of the target stars is important to avoid false planet detections and to improve our understanding of the atmospheres of late-type stars. Aims. In this work we present measurements of activity indicators at visible and near-infrared wavelengths for 331 M dwarfs observed with CARMENES. Our aim is to identify the activity indicators that are most sensitive and easiest to measure, and the correlations among these indicators. We also wish to characterise their variability. Methods. Using a spectral subtraction technique, we measured pseudo-equivalent widths of the He I D3, Hα, He I λ10833 Å, and Pa β lines, the Na I D doublet, and the Ca II infrared triplet, which have a chromospheric component in active M dwarfs. In addition, we measured an index of the strength of two TiO and two VO bands, which are formed in the photosphere. We also searched for periodicities in these activity indicators for all sample stars using generalised Lomb-Scargle periodograms. Results. We find that the most slowly rotating stars of each spectral subtype have the strongest Hα absorption. Hα is correlated most strongly with He I D3, whereas Na I D and the Ca II infrared triplet are also correlated with Hα. He I λ10833 Å and Paβ show no clear correlations with the other indicators. The TiO bands show an activity effect that does not appear in the VO bands. We find that the relative variations of Hα and He I D3 are smaller for stars with higher activity levels, while this anti-correlation is weaker for Na I D and the Ca II infrared triplet, and is absent for He I λ10833 Å and Paβ. Periodic variation with the rotation period most commonly appears in the TiO bands, Hα, and in the Ca II infrared triplet.

Spectrally resolved helium absorption from the extended atmosphere of a warm Neptune-mass exoplanet

Stellar heating causes atmospheres of close-in exoplanets to expand and escape. These extended atmospheres are difficult to observe because their main spectral signature—neutral hydrogen at ultraviolet wavelengths—is strongly absorbed by interstellar medium. We report the detection of the near-infrared triplet of neutral helium in the transiting warm Neptune-mass exoplanet HAT-P-11b by using ground-based, high-resolution observations. The helium feature is repeatable over two independent transits, with an average absorption depth of 1.08 ± 0.05%. Interpreting absorption spectra with three-dimensional simulations of the planet’s upper atmosphere suggests that it extends beyond 5 planetary radii, with a large-scale height and a helium mass loss rate of ≲3 × 105 grams per second. A net blue-shift of the absorption might be explained by high-altitude winds flowing at 3 kilometers per second from day to night-side.

HD 189733 b: bow shock or no shock?

Context. Hot Jupiters are surrounded by extended atmospheres of neutral hydrogen. Observations have provided evidence for in-transit hydrogen Hα absorption as well as variable pre-transit absorption signals. These have been interpreted in terms of a bow shock or an accretion stream that transits the host star before the planet. Aims. We test the hypothesis of planetary-related Hα absorption by studying the time variability of the Hα and stellar activity-sensitive calcium lines in high-resolution TIGRE (Telescopio Internacional de Guanajuato Robótico Espectroscópico) spectra of the planet host HD 189733. Methods. In the framework of an observing campaign spanning several months, the host star was observed several times per week randomly sampling the orbital phases of the planet. We determine the equivalent width in the Hα and Ca IRT(calcium infrared triplet) lines, and subtract stellar rotationally induced activity from the Hα time series via its correlation with the IRT evolution. The residuals are explored for significant differences between the pre-, in-, and out-of-transit phases. Results. We find strong stellar rotational variation with a lifetime of about 20–30 days in all activity indicators, but the corrected Hα time series exhibits no significant periodic variation. We exclude the presence of more than 6.2 mÅ pre-transit absorption and 5.6 mÅ in-transit absorption in the corrected Hα data at a 99% confidence level. Conclusions. Previously observed Hα absorption signals exceed our upper limits, but they could be related to excited atmospheric states. The Hα variability in the HD 189733 system is dominated by stellar activity, and observed signals around the planetary transit may well be caused by short-term stellar variability.

A large catalog of young active RAVE stars in the Solar neighborhood

The catalog of 38,000 chromospherically active RAVE dwarfs represents one of the largest samples of young active solar-like and later-type single field stars in the Solar neighbourhood. It was established from the unbiased magnitude limited RAVE Survey using an unsupervised stellar classification algorithm based merely on stellar fluxes (Ca II infrared triplet). Using a newly-calibrated age-activity relation, ~15,000 active stars are estimated to be younger than 1 Gyr. Almost 2000 stars are presumably younger than ~100 Myr and possibly still in the pre-main sequence phase, the latter being supported by their significant offset from the main sequence in the NUV − V versus J − K space. 16,000 stars from the sample have positional and velocity vectors available (using TGAS parallaxes and proper motions and radial velocities from RAVE).