Solar and stellar photospheric abundances

Understanding the formation and evolution of our galaxy, the Milky Way, has been an ongoing process, which with the development of large-scale surveys has picked up considerable pace. Together with these new surveys, pipelines have been constructed which allow for the rapid and automatic processing of this wealth of new data. These codes are able to turn raw data files into tables of stellar parameters and chemical abundances in far less time than if they were analysed by hand. The results from these surveys open new windows on to the history of our galaxy and other disk galaxies. In this thesis, we present the development of a new pipeline, the STellAR Parameter AND Abundances pipeline (STARPANDA), which is able to rapidly derive stellar parameters, CNO abundances and other elemental abundances by utilising measurements of spectral features in both observed and synthetic spectra. We take the observed spectra, synthetic spectra and line lists employed by the APOGEE survey and produce new values for the stellar parameters, CNO abundances and Al abundances of the APOGEE stars. We then compare our results with those achieved by the APOGEE pipeline.

Elemental abundances in late-type stars are of interest in several ways: they determine the location of the stars in the HR diagram and therefore their ages, as well as the atmospheric structure in their middle and upper photospheres. Especially in the case of chromospherically active late-type stars the question arises to what degree the upper photosphere is influenced by the nearby chromosphere. Analysing S/N ∼ 200 and Δλ/λ ∼ 20 000 data, we found a mean metallicity index [M/H] = −0.2 for programme K and M field stars based on an analysis of spectra in the region 5500–9000 Å. We also found that the Ca i 6162-Å transition is a potential surface gravity indicator for K-type stars. For the chromospheric activity interval 4.4 < log FMgii < 6.6 we did not find any chromospheric activity impact on photospheric and upper photospheric transitions. With the derived metallicity, we confirmed the Li abundance from our previous paper and thus its dependence on the Mg ii chromospheric activity index. The nature of the spectrum for the active M-type star Gl 896A is explained by pure rotation of 14 km s−1. As far as the lithium—rotation relation is concerned, the spectrum of Gl 517 is rotationally broadened as well, by 12 km s−1, and the Li abundance is the second highest in our sample of stars. However, there is no link between very high Li abundance, 2.2 dex, in the K dwarf star Gl 5 and stellar rotation.

Context. The comparison of coronal and photospheric abundances in cool stars is an essential question to resolve. In the Sun an enhancement of the elements with low first ionization potential (FIP) is observed in the corona with respect to the photosphere. Stars with high levels of activity seem to show a depletion of elements with low FIP when compared to solar standard values; however, the few cases of active stars in which photospheric values are available for comparison lead to confusing results, and an enlargement of the sample is mandatory for solving this longstanding problem. Aims. We calculate in this paper the photospheric and coronal abundances of two well known active binary systems, AR Psc and AY Cet, to get further insight into the complications of coronal abundances. Methods. Coronal abundances of 9 elements were calculated by means of the reconstruction of a detailed emission measure distribution, using a line-based method that considers the lines from different elements separately. Photospheric abundances of 8 elements were calculated using high-resolution optical spectra of the stars. Results. The results once again show a lack of any FIP-related effect in the coronal abundances of the stars. The presence of metal abundance depletion (MAD) or inverse FIP effects in some stars could stem from a mistaken comparison to solar photospheric values or from a deficient calculation of photospheric abundances in fast-rotating stars. Conclusions. The lack of FIP fractionation seems to confirm that Alfven waves combined with pondermotive forces are dominant in the corona of active stars.

The Galactic centre hosts a crowded, dense nuclear star cluster with a half-light radius of 4 pc. Most of the stars in the Galactic centre are cool late-type stars, but there are also >100 hot early-type stars in the central parsec of the Milky Way. These stars are only 3-8 Myr old. Our knowledge of the number and distribution of early-type stars in the Galactic centre is incomplete. Only a few spectroscopic observations have been made beyond a projected distance of 0.5 pc of the Galactic centre. The distribution and kinematics of early-type stars are essential to understand the formation and growth of the nuclear star cluster. We cover the central >4pc^2 of the Galactic centre using the integral-field spectrograph KMOS. We extracted more than 1,000 spectra from individual stars and identified early-type stars based on their spectra. Our data set contains 114 bright early-type stars: 6 have narrow emission lines, 23 are Wolf-Rayet stars, 9 stars have featureless spectra, and 76 are O/B type stars. Our wide-field spectroscopic data confirm that the distribution of young stars is compact, with 90% of the young stars identified within 0.5 pc of the nucleus. We identify 24 new O/B stars primarily at large radii. We estimate photometric masses of the O/B stars and show that the total mass in the young population is >12,000M_sun. The O/B stars all appear to be bound to the Milky Way nuclear star cluster, while less than 30% belong to the clockwise rotating disk. The central concentration of the early-type stars is a strong argument that they have formed in situ. A large part of the young O/B stars is not on the disk, which either means that the early-type stars did not all form on the same disk or that the disk is dissolving rapidly. [abridged]

Aims. We report the results of a novel determination of the solar oxygen abundance using spatially resolved observations and inversions. We seek to derive the photospheric solar oxygen abundance with a method that is robust against uncertainties in the model atmosphere. Methods. We use observations with spatial resolution obtained at the Vacuum Tower Telescope to derive the oxygen abundance at 40 different spatial positions in granules and intergranular lanes. We first obtain a model for each location by inverting the Fe I lines with the NICOLE inversion code. These models are then integrated into a hierarchical Bayesian model that is used to infer the most probable value for the oxygen abundance that is compatible with all the observations. The abundance is derived from the [O I] forbidden line at 6300 Å taking into consideration all possible nuisance parameters that can affect the abundance. Results. Our results show good agreement in the inferred oxygen abundance for all the pixels analyzed, demonstrating the robustness of the analysis against possible systematic errors in the model. We find a slightly higher oxygen abundance in granules than in intergranular lanes when treated separately (log(ϵO) = 8.83 ± 0.02 vs. log(ϵO) = 8.76 ± 0.02), which is a difference of approximately 2-σ. This tension suggests that some systematic errors in the model or the radiative transfer still exist but are small. When taking all pixels together, we obtain an oxygen abundance of log(ϵO) = 8.80 ± 0.03, which is compatible with both granules and lanes within 1-σ. The spread of results is due to both systematic and random errors.

We present a well-calibrated EUV spectrum of a solar coronal bright point observed with the Extreme Ultraviolet Normal Incidence Spectrograph (EUNIS) sounding rocket instrument on 2006 April 12. The coronal bright point brightened around 06:30 UT during a period of emerging magnetic flux and remained bright at least until the rocket flight around 18:12 UT, while the magnetic flux merged and canceled. Density-sensitive line intensity ratios yield mutually consistent coronal electron densities (Ne in cm−3) of log Ne ≈ 9.4. The differential emission measure (DEM, in cm−5 K−1) curve derived from the spectrum yields a peak of log DEM ≈ 20.70 at log T ≈ 6.15 and a local minimum of log DEM ≈ 20.15 at log T ≈ 5.35. Photospheric (not coronal) element abundances are required to achieve equality and consistency in the DEM derived from lines of Mg V, Mg VI, Mg VII, and Ca VII (with a low first ionization potential, or FIP) and lines from Ne IV and Ne V (with a high FIP) formed at transition region temperatures. The bright point's photospheric abundance is likely produced by reconnection-driven chromospheric evaporation, a process that is not only central to existing bright point models, but also consistent with measurements of relative Doppler velocities.

Context.Identifying past wet merger activity in galaxies has been a longstanding issue in extragalactic formation history studies.Gaia’s 6D kinematic measurements in our Milky Way (MW) have vastly extended the possibilities for Galactic archaeology, leading to the discovery of a multitude of early mergers in the MW’s past. As recent work has established a link between younger globular clusters (GCs; less than about 10–11 Gyr old) and wet galaxy merger events, the MW provides an ideal laboratory for testing which GC properties can be used to trace extragalactic galaxy formation histories.Aims.To test the hypothesis that GCs trace wet mergers, we relate the measured GC age distributions of the MW and three nearby galaxies, M 31, NGC 1407, and NGC 3115, to their merger histories and interpret the connection with wet mergers through an empirical model for GC formation.Methods.The GC ages of observed galaxies are taken from a variety of studies to analyze their age distributions side-by-side with the model. For the MW, we additionally cross-match the GCs with their associated progenitor host galaxies to disentangle the connection to the GC age distribution. For the modeled GCs, we take galaxies with similar GC age distributions as observed to compare their accretion histories with those inferred through observations.Results.We find that the MW GC age distribution is bimodal, mainly caused by younger GCs (10–11 Gyr old associated with Gaia-Sausage/Enceladus (GSE) and in part by unassociated high-energy GCs. The GSE GC age distribution also appears to be bimodal. We propose that the older GSE GCs (12–13 Gyr old) were accreted together with GSE, while the younger ones formed as a result of the merger. For the nearby galaxies, we find that clear peaks in the GC age distributions coincide with active early gas-rich merger phases. Even small signatures in the GC age distributions agree well with the expected wet formation histories of the galaxies inferred through other observed tracers. From the models, we predict that the involved cold gas mass can be estimated from the number of GCs found in the formation burst.Conclusions.Multimodal GC age distributions can trace massive wet mergers as a result of GCs being formed through them. From the laboratory of our own MW and nearby galaxies we conclude that the ages of younger GC populations of galaxies can be used to infer the wet merger history of a galaxy.

The striking broad emission line spectroscopic appearance of Wolf-Rayet (WR) stars has long defied analysis, owing to the extreme physical conditions within their line- and continuum-forming regions. Recently, model atmosphere studies have advanced sufficiently to enable the determination of stellar temperatures, luminosities, abundances, ionizing fluxes, and wind properties. The observed distributions of nitrogen- (WN) and carbon (WC)-sequence WR stars in the Milky Way and in nearby star-forming galaxies are discussed; these imply lower limits to progenitor masses of ∼25, 40, and 75 M⊙ for hydrogen-depleted (He-burning) WN, WC, and H-rich (H-burning) WN stars, respectively. WR stars in massive star binaries permit studies of wind-wind interactions and dust formation in WC systems. They also show that WR stars have typical masses of 10–25 M⊙, extending up to 80 M⊙ for H-rich WN stars. Theoretical and observational evidence that WR winds depend on metallicity is presented, with implications for evolutionary models, ionizing fluxes, and the role of WR stars within the context of core-collapse supernovae and long-duration gamma-ray bursts.

Within the central 10 pc of our Galaxy lies a dense cluster of stars, the nuclear star cluster. This cluster forms a distinct component of our Galaxy. Nuclear star clusters are common objects and are detected in ~ 75% of nearby galaxies. It is, however, not fully understood how nuclear star clusters form. The Milky Way nuclear star cluster is the closest of its kind. At a distance of only 8 kpc we can spatially resolve its stellar populations and kinematics much better than in external galaxies. This makes the Milky Way nuclear star cluster the perfect local reference object for understanding the structure and assembly history of nuclear star clusters in general. There are of the order of 107 stars within the central 10 pc of the Galactic center. Most of these stars are several Gyr old late-type stars. However, there are also more than 100 hot early-type stars in the central parsec of the Milky Way, with ages of only a few Myr. Beyond a projected distance of 0.5 pc of the Galactic center, the density of young stars was largely unknown, since only very few spectroscopic observations existed so far. We covered the central &gt;4 pc2 (0.75 sq.arcmin) of the Galactic center using the integral-field spectrograph KMOS (VLT). We extracted more than 1,000 spectra from individual stars and identified &gt;20 new early-type stars based on their spectra. We studied the spatial distribution of the different populations and their kinematics to put constraints on the assembly history of the Milky Way nuclear star cluster.

Galaxies act like engines converting gas into stars, which in turn produce the matter around us, laying the foundations of life. Studying the composition of this interstellar gas informs our understanding of how star formation proceeds, while also providing insight into the structure and evolution of our own Galaxy and the Universe. Research has found a strong connection between star formation and the molecular gas within galaxies. However, the most easily accessed molecular observables, such as carbon monoxide (CO) emission, only probe low-density gas in extragalactic systems, allowing us to scratch the surface of the star-forming structures. Molecules which trace denser gas, such as hydrogen cyanide (HCN), are more challenging to observe but probe the immediate sites of star formation. In this thesis I analyze data from the first survey mapping the entire star forming disk of a sample of nearby galaxies in a suite of dense gas tracers (EMPIRE), to understand the interplay between dense gas and a wide range of galactic environments, distinct from the Milky Way. By studying the content of this dense gas in nearby galaxies I find surprising results: systematic variations of the dense gas fraction and its efficiency to form stars within and among galaxies. While more dense gas is located in regions of high interstellar pressure, this dense gas is less efficient to form stars. To characterize the dense gas, I also explore how changes in the optical depth can affect the effective gas densities where the dense gas tracers emit, a new measurement in the disks normal star-forming galaxies. To better understand the bulk, lower-density molecular gas out of which the dense gas eventually forms, I also analyze CO line ratios to constrain carbon isotope abundances. I find the abundance to vary systematically within galaxy disks, likely due to strong fractionation effects. The results obtained in this thesis support a model where star formation depends strongly on host galaxy and the local galactic environment in the disk.

Due to their production sites, as well as to how they are processed and destroyed in stars, the light elements are excellent tools to investigate a number of crucial issues in modern astrophysics: from stellar structure and non-standard processes at work in stellar interiors to age dating of stars; from pre-main sequence evolution to the star formation histories of young clusters and associations and to multiple populations in globular clusters; from Big Bang nucleosynthesis to the formation and chemical enrichment history of the Milky Way Galaxy and its populations, just to cite some relevant examples. In this paper, we focus on lithium, beryllium, and boron (LiBeB) and on carbon, nitrogen, and oxygen (CNO). LiBeB are rare elements, with negligible abundances with respect to hydrogen; on the contrary, CNO are among the most abundant elements in the Universe, after H and He. Pioneering observations of light-element surface abundances in stars started almost 70 years ago and huge progress has been achieved since then. Indeed, for different reasons, precise measurements of LiBeB and CNO are difficult, even in our Sun; however, the advent of state-of-the-art ground- and space-based instrumentation has allowed the determination of high-quality abundances in stars of different type, belonging to different Galactic populations, from metal-poor halo stars to young stars in the solar vicinity and from massive stars to cool dwarfs and giants. Noticeably, the recent large spectroscopic surveys performed with multifiber spectrographs have yielded detailed and homogeneous information on the abundances of Li and CNO for statistically significant samples of stars; this has allowed us to obtain new results and insights and, at the same time, raise new questions and challenges. A complete understanding of the light-element patterns and evolution in the Universe has not been still achieved. Perspectives for further progress will open up soon thanks to the new generation instrumentation that is under development and will come online in the coming years.

We model dust evolution in Milky Way-like galaxies by post-processing the IllustrisTNG cosmological hydrodynamical simulations in order to predict dust-to-gas ratios and grain size distributions. We treat grain-size-dependent dust growth and destruction processes using a 64-bin discrete grain size evolution model without spatially resolving each galaxy. Our model broadly reproduces the observed dust–metallicity scaling relation in nearby galaxies. The grain size distribution is dominated by large grains at z ≳ 3 and the small-grain abundance rapidly increases by shattering and accretion (dust growth) at z ≲ 2. The grain size distribution approaches the so-called MRN distribution at z ∼ 1, but a suppression of large-grain abundances occurs at z &amp;lt; 1. Based on the computed grain size distributions and grain compositions, we also calculate the evolution of the extinction curve for each Milky Way analogue. Extinction curves are initially flat at z &amp;gt; 2, and become consistent with the Milky Way extinction curve at z ≲ 1 at $1/\lambda \lt 6~\rm{\mu m}^{-1}$. However, typical extinction curves predicted by our model have a steeper slope at short wavelengths than is observed in the Milky Way. This is due to the low-redshift decline of gas-phase metallicity and the dense gas fraction in our TNG Milky Way analogues that suppresses the formation of large grains through coagulation.

In this paper we quantify the gas accretion rate from minor mergers onto star-forming galaxies in the Local Universe using HI observations of 148 nearby spiral galaxies (WHISP sample). We developed a dedicated code that iteratively analyses HI data-cubes, finds dwarf gas-rich satellites around larger galaxies and estimates an upper limit to the gas accretion rate. We found that 22% of the galaxies have at least one detected dwarf companion. We made the very stringent assumption that all satellites are going to merge in the shortest possible time transferring all their gas to the main galaxies. This leads to an estimate of the maximum gas accretion rate of 0.28 solar masses per year, about five times lower than the average SFR of the sample. Given the assumptions, our accretion rate is clearly an overestimate. Our result strongly suggests that minor mergers do not play a significant role in the total gas accretion budget in local galaxies.

The main difficulties encountered in determining a photospheric solar iron abundance are considered and a way is shown to circumvent most of them. Branching fractions of weak Fe II lines are measured from a hollow-cathode discharge to derive accurate transition probabilities from a previously determined lifetime. These transition probabilities — together with accurate equivalent widths from the solar spectrum — are subsequently used to calculate a photospheric iron abundance. The choice of suitable lines results in an abundance value nearly independent of any assumption concerning the temperature model and line-broadening parameters.

HMXB, ULX and star formation