As part of the search for planets around evolved stars, we can understand planet populations around significantly higher-mass stars than the Sun on the main sequence. This population is difficult to study any other way, particularly with radial-velocities since these stars are too hot and rotate too fast to measure precise velocities. Here we estimate stellar parameters for all of the giant stars from the EXPRESS project, which aims to detect planets orbiting evolved stars, and study their occurrence rate as a function of stellar mass. We analyse high resolution echelle spectra of these stars, and compute the atmospheric parameters by measuring the equivalent widths for a set of iron lines, using an updated method implemented during this work. Physical parameters are computed by interpolating through a grid of stellar evolutionary models, following a procedure that carefully takes into account the post-MS evolutionary phases. Probabilities of the star being in the red giant branch (RBG) or the horizontal branch (HB) are estimated from the derived distributions. Results: We find that, out of 166 evolved stars, 101 of them are most likely in the RGB phase, while 65 of them are in the HB phase. The mean derived mass is 1.41 and 1.87 Msun for RGB and HB stars, respectively. To validate our method, we compared our results with interferometry and asteroseismology studies. We find a difference in the radius with interferometry of 1.7%. With asteroseismology, we find 2.4% difference in logg, 1.5% in radius, 6.2% in mass, and 11.9% in age. Compared with previous spectroscopic studies, and find a 0.5% difference in Teff, 1% in logg, and 2% in [Fe/H]. We also find a mean mass difference with respect to the EXPRESS original catalogue of 16%. We show that the method presented here can greatly improve the estimates of the stellar parameters for giant stars compared to what was presented previously.
We have used the Habitable Zone Planet Finder (HPF) to gather high resolution, high signal-to-noise near-infrared spectra of 13 field red horizontal-branch (RHB) stars, one open-cluster giant, and one very metal-poor halo red giant. The HPF spectra cover the 0.81$-$1.28 \micron\ wavelength range of the $zyJ$ bands, filling in the gap between the optical (0.4$-$1.0~\micron) and infrared (1.5$-$2.4~\micron) spectra already available for the program stars. We derive abundances of 17 species from LTE-based computations involving equivalent widths and spectrum syntheses, and estimate abundance corr…
The heavy mass loss observed in evolved asymptotic giant branch (AGB) stars is usually attributed to a two-stage process: atmospheric levitation by pulsation-induced shock waves, followed by radiative acceleration of newly formed dust grains. The dust transfers momentum to the surrounding gas through collisions and thereby triggers a general outflow. Radiation-hydrodynamical models of M-type AGB stars suggest that these winds can be driven by photon scattering -- in contrast to absorption -- on Fe-free silicate grains of sizes 0.1--1\,$μ$m. In this paper we study photometric constraints for wi…
This work is part of an ongoing effort aiming at identifying the actual wind-drivers among the dust species observed in circumstellar envelopes. In particular, we focus on the interplay between a strong stellar radiation field and the dust formation process. To identify critical properties of potential wind-driving dust species we use detailed radiation-hydrodynamical models which include a parameterized dust description, complemented by simple analytical estimates to help with the physical interpretation of the numerical results. The adopted dust description is constructed to mimic different …
Stellar winds observed in asymptotic giant branch (AGB) stars are usually attributed to a combination of stellar pulsations and radiation pressure on dust. Shock waves triggered by pulsations propagate through the atmosphere, compressing the gas and lifting it to cooler regions, which create favourable conditions for grain growth. If sufficient radiative acceleration is exerted on the newly formed grains through absorption or scattering of stellar photons, an outflow can be triggered. Strong candidates for wind-driving dust species in M-type AGB stars are magnesium silicates (Mg$_2$SiO$_4$ and…
We investigate the effects of coherent isotropic continuum scattering on the formation of spectral lines in local thermodynamic equilibrium (LTE) using 3D hydrodynamical and 1D hydrostatic model atmospheres of red giant stars. Continuum flux levels, spectral line profiles and curves of growth for different species are compared with calculations that treat scattering as absorption. Photons may escape from deeper, hotter layers through scattering, resulting in significantly higher continuum flux levels beneath a wavelength of 5000 A. The magnitude of the effect is determined by the importance of…
We calculated an atmospheric grid for hot mini-Neptune and giant exoplanets, that links astrophysical observable parameters- orbital distance and stellar type- with the chemical atmospheric species expected. The grid can be applied to current and future observations to characterize exoplanet atmospheres and serves as a reference to interpret atmospheric retrieval analysis results. To build the grid, we developed a 1D code for calculating the atmospheric thermal structure and link it to a photochemical model that includes disequilibrium chemistry (molecular diffusion, vertical mixing and photoc…
Can the Long-Term Impact of Stellar M-Dwarf Flares Alter the Spectral Features of a Giant Gaseous Exoplanet?
SRON, Netherlands Institute for Space Research（荷兰空间研究所）；Institute of Astronomy and Astrophysics, Academia Sinica（中央研究院天文及天文物理研究所）；Leiden Observatory, Leiden University（莱顿大学莱顿天文台）
The atmospheres of gaseous giant exoplanets orbiting close to their parent stars (hot Jupiters) have been probed for nearly two decades. They allow us to investigate the chemical and physical properties of planetary atmospheres under extreme irradiation conditions. Previous observations of hot Jupiters as they transit in front of their host stars have revealed the frequent presence of water vapour and carbon monoxide in their atmospheres; this has been studied in terms of scaled solar composition under the usual assumption of chemical equilibrium. Both molecules as well as hydrogen cyanide wer…
The common-envelope (CE) phase is an important stage in the evolution of binary stellar populations. The most common way to compute the change in orbital period during a CE is to relate the binding energy of the envelope of the Roche-lobe filling giant to the change in orbital energy. Especially in population-synthesis codes, where the evolution of millions of stars must be computed and detailed evolutionary models are too expensive computationally, simple approximations are made for the envelope binding energy. In this study, we present accurate analytic prescriptions based on detailed stella…
We compare the abundances of various chemical species as derived with 3D hydrodynamical and classical 1D stellar atmosphere codes in a late-type giant characterized by T_eff=3640K, log g = 1.0, [M/H] = 0.0. For this particular set of atmospheric parameters the 3D-1D abundance differences are generally small for neutral atoms and molecules but they may reach up to 0.3-0.4 dex in case of ions. The 3D-1D differences generally become increasingly more negative at higher excitation potentials and are typically largest in the optical wavelength range. Their sign can be both positive and negative, an…
We report the discovery of planetary companions orbiting four low-luminosity giant stars with M$_\star$ between 1.04 and 1.39 M$_\odot$. All four host stars have been independently observed by the EXoPlanets aRound Evolved StarS (EXPRESS) program and the Pan-Pacific Planet Search (PPPS). The companion signals were revealed by multi-epoch precision radial velocities obtained during nearly a decade. The planetary companions exhibit orbital periods between $\sim$ 1.2 and 7.1 years, minimum masses of m$_{\rm p}$sini $\sim$ 1.8-3.7 M$_{jup}$ and eccentricities between 0.08 and 0.42. Including these…
In this work we have used 3D hydrodynamical (CO5BOLD) and 1D hydrostatic (LHD) stellar atmosphere models to study the importance of convection and horizontal temperature inhomogeneities in stellar abundance work related to late-type giants. We have found that for a number of key elements, such as Na, Mg, Si, Ca, Ti, Fe, Ni, Zn, Ba, Eu, differences in abundances predicted by 3D and 1D models are typically minor (< 0.1 dex) at solar metallicity. However, at [M/H] = -3 they become larger and reach to -0.5...-0.8 dex. In case of neutral atoms and fixed metallicity, the largest abundance difference…
In an atmosphere, a cloud condensation region is characterized by a strong vertical gradient in the abundance of the related condensing species. On Earth, the ensuing gradient of mean molecular weight has relatively few dynamical consequences because N$_2$ is heavier than water vapor, so that only the release of latent heat significantly impacts convection. On the contrary, in an hydrogen dominated atmosphere (e.g. giant planets), all condensing species are significantly heavier than the background gas. This can stabilize the atmosphere against convection near a cloud deck if the enrichment in…
RV surveys of evolved stars allow us to probe a higher stellar mass range compared to main-sequence samples. Differences between the planet populations can be caused by either the differing stellar mass or stellar evolution. To properly disentangle the effects of both, the planet population around giant stars needs to be characterized as accurately as possible. Our goal is to investigate the giant planet occurrence rate around evolved stars and determine its dependence on stellar mass, metallicity, and orbital period. We combine data from the Lick, EXPRESS, and PPPS giant star surveys, yieldin…
We investigate the role of convection in the formation of atomic and molecular lines in the atmosphere of a red giant star. For this purpose we study the formation properties of spectral lines that belong to a number of astrophysically important tracer elements, including neutral and singly ionized atoms, and molecules. We focus our investigation on a prototypical red giant located close to the red giant branch (RGB) tip (Teff=3660K, logg=1.0, [M/H]=0.0). We used two types of model atmospheres, 3D hydrodynamical and classical 1D, calculated with the CO5BOLD and LHD stellar atmosphere codes, re…
The titanium abundances of late-type stars are important tracers of Galactic formation history. However, abundances inferred from Ti I and Ti II lines can be in stark disagreement in very metal-poor giants. Departures from local thermodynamic equilibrium (LTE) have a large impact on the minority neutral species and thus influences the ionisation imbalance, but satisfactory non-LTE modelling for both dwarfs and giants has not been achieved in previous literature. The reliability of titanium abundances is reassessed in benchmark dwarfs and giants using a new non-LTE model and one-dimensional (1D…
High spatial resolution techniques have given valuable insights into the mass loss mechanism of AGB stars, which presumably involves a combination of atmospheric levitation by pulsation-induced shock waves and radiation pressure on dust. Observations indicate that Al$_2$O$_3$ condenses at distances of about 2 stellar radii or less, prior to the formation of silicates. Al$_2$O$_3$ grains are therefore prime candidates for producing the scattered light observed in the close vicinity of several M-type AGB stars, and they may be seed particles for the condensation of silicates at lower temperature…
Chemical compositions of giant planets provide a means to constrain how and where they form. Traditionally, super-stellar elemental abundances in giant planets were thought to be possible due to accretion of metal-rich solids. Such enrichments are accompanied by oxygen-rich compositions (i.e. C/O below the disc's value, assumed to be solar, C/O=0.54). Without solid accretion the planets are expected to have sub-solar metallicity, but high C/O ratios. This arises because the solids are dominated by oxygen-rich species, e.g. H$_2$O and CO$_2$, which freeze out in the disk earlier than CO, leavin…
We present detailed chemical compositions of 10 red giant star members of the Galactic (open) cluster NGC 752, derived from high-resolution (R $\approx$ 60,000), high signal-to-noise ($S/N$ $\geq$ 140) spectra. We confirmed cluster memberships by measuring the stellar radial velocities, and by deriving model atmosphere parameters ($T_{eff}$, logg, [Fe/H] and $ξ_{t}$) from equivalent widths of Fe I, Fe II, Ti I, and Ti II lines. The metallicity we obtained for NGC 752 ([Fe/H]=$-$0.02$\pm$0.05) is in good agreement with previous studies. We derived abundances of $α$ (Mg, Si, Ca), light odd-Z (Na…
Rotation plays a key role in stellar structure and its evolution. Through transport processes which induce rotational mixing of chemical species and the redistribution of angular momentum, internal stellar rotation influences the evolutionary tracks in the Hertzsprung-Russell diagram. In turn, evolution influences the rotational properties. Therefore, information on the rotational properties of the deep interior would help to better understand the stellar evolution. However, as the internal rotational profile cannot be measured directly, it remains a major unknown leaving this important aspect…
Three bright molecular line sources in G333 have recently been shown to exhibit signatures of infall. We describe a molecular line radiative transfer modelling process which is required to extract the infall signature from Mopra and Nanten2 data. The observed line profiles differ greatly between individual sources but are reproduced well by variations upon a common unified model where the outflow viewing angle is the most significant difference between the sources. The models and data together suggest that the observed properties of the high-mass star-forming regions such as infall, turbulence…
We present molecular line imaging observations of three massive molecular outflow sources, G333.6-0.2, G333.1-0.4, and G332.8-0.5, all of which also show evidence for infall, within the G333 giant molecular cloud (GMC). All three are within a beam size (36 arcseconds) of IRAS sources, 1.2-mm dust clumps, various masing species and radio continuum-detected HII regions and hence are associated with high-mass star formation. We present the molecular line data and derive the physical properties of the outflows including the mass, kinematics, and energetics and discuss the inferred characteristics …
The composition and structure of the upper atmospheres of Extrasolar Giant Planets (EGPs) are affected by the high-energy spectrum of their host stars from soft X-rays to EUV. This emission depends on the activity level of the star, which is primarily determined by its age. We focus upon EGPs orbiting K- and M-dwarf stars of different ages. XUV spectra for these stars are constructed using a coronal model. These spectra are used to drive both a thermospheric model and an ionospheric model, providing densities of neutral and ion species. Ionisation is included through photo-ionisation and elect…
We have gathered near-infrared $zyJ$-band high resolution spectra of nearly 300 field red giant stars with known lithium abundances in order to survey their \species{He}{i} $λ$10830 absorption strengths. This transition is an indicator of chromospheric activity and/or mass loss in red giants. The majority of stars in our sample reside in the red clump or red horizontal branch based on their $V-J,M_V$ color-magnitude diagram and their Gaia \teff, \logg\ values. Most of our target stars are Li-poor in the sense of having normally low Li abundances, defined here as \eps{Li}~$<$~1.25. Over 90\% of…
Lithium rich stars in globular clusters are rare. In fact, only 14 have been found so far, in different evolutionary phases from dwarfs to giants. Different mechanisms have been proposed to explain this enhancement, but it is still an open problem. Using spectra collected within the Gaia-ESO Survey, obtained with the GIRAFFE spectrograph at the ESO Very Large Telescope, we present the discovery of the first Li-rich star in the cluster NGC 1261, the second star known in the red giant branch bump phase. The star shows an extreme Li overabundance of A(Li)_LTE=3.92\pm0.14, corresponding to A(Li)_N…
We study the abundances of complex carbon-bearing molecules in the oxygen-rich dust- forming atmospheres of Brown Dwarfs and giant gas planets. The inner atmospheric re- gions that form the inner boundary for thermochemical gas-phase models are investigated. Results from Drift-phoenix atmosphere simulations, which include the feedback of phase- non-equilibrium dust cloud formation on the atmospheric structure and the gas-phase abun- dances, are utilised. The resulting element depletion leads to a shift in the carbon-to-oxygen ratio such that several hydrocarbon molecules and cyanopolycyanopoly…
The amount of dust present in circumstellar disks is expected to steadily decrease with age due to the growth from micron-sized particles to planetesimals and planets. Mature circumstellar disks, however, can be observed to contain significant amounts of dust and possess high dust-to-gas ratios. Using HD 163296 as our case study, we explore how the formation of giant planets in disks can create the conditions for collisionally rejuvenating the dust population, halting or reversing the expected trend. We combine N-body simulations with statistical methods and impact scaling laws to estimate the…
