Teses de Doutorado PPGA

PARÂMETROS ATMOSFÉRICOS E CAMPOS MAGNÉTICOS DE ESTRELAS ANÃS M OBSERVADAS PELO LEVANTAMENTO APOGEE

2025-01-13T14:53:52Z

PARÂMETROS ATMOSFÉRICOS E CAMPOS MAGNÉTICOS DE ESTRELAS ANÃS M OBSERVADAS PELO LEVANTAMENTO APOGEE CARNEIRO WANDERLEY, FABIO CUNHA, KATIA This thesis presents a spectroscopic analysis of M dwarf stars using high-resolution (R$\sim$22.500) near-infrared (1,514--1,696 $\mu$m) spectra from the SDSS/APOGEE survey. Our methodology adopts spectrum synthesis with LTE MARCS model atmospheres, along with the APOGEE DR17 line list, and the radiative transfer code Turbospectrum. We studied 48 M dwarf stars from the Hyades open cluster and obtained a median metallicity of [M/H]=0.09$\pm$0.03 dex, in good agreement with optical results for Hyades red-giants, and which is chemically homogeneous within the abundance uncertainties. We determined for these stars a median radius inflation between 1.6$\pm$2.3$\%$ and 2.4$\pm$2.3$\%$, which can be explained by a stellar spot coverage of up to 40$\%$. We also derived stellar magnetic fields for a sample containing 62 M-dwarf members of the Pleiades open cluster (with effective temperatures between 3400 K $\lesssim$ T$_{\rm eff} \lesssim$4000 K), using Fe I lines that are sensitive to magnetic fields. These calculations used the radiative transfer code Synmast, and a methodology based on Monte Carlo and Markov Chain (MCMC). The obtained mean magnetic fields vary between $\sim$1.0 and $\sim$4.2 kG, with most of the stars of the sample being in the saturated regime of magnetic fields. The studied M dwarfs present a median radius inflation ranging from +3.0$\%$ to +7.0$\%$, depending on the isochrone model selected. There is a positive correlation between magnetic field strength and radius inflation, as well as with stellar spot coverage, correlations that together indicate that stellar spots generated by strong magnetic fields might be the mechanism that drives radius inflation in these stars. We also derived atmospheric parameters for a sample containing 34 planet-hosting M dwarf stars. The oxygen abundances and metallicities obtained for the studied M dwarf stars show that [O/M] versus [M/H] are in good agreement with galactic evolution models for the solar neighborhood. We derived planetary radii for 47 exoplanets orbiting these stars. Most of the studied planets are Super-Earths; the planet radius distribution has a peak at 1.2 –- 1.4 R$_{\oplus}$, followed by a decline for larger planetary radii, and a drop at 1.8 –- 2.0 R$_{\oplus}$, corresponding to the radius valley. The slope of the radius valley with orbital periods and insolation shows that planetesimal impacts may be the dominant mechanism for the creation of the radius valley for this sample. We separated our sample of exoplanets into multi-planetary systems, and systems containing only one detected planet. We identified that the first group shows a positive linear relation between stellar metallicities and planetary radii, while the latter presents two regimes, with Earth-sized and Super-Earths planets orbiting stars with [M/H]$<$0, and planets of different sizes, including sub-Neptunes orbiting stars with higher metallicities. Finally, we found that the exoplanets members of multi-planetary systems of the sample orbit on average lower-metallicity stars ($<$[M/H]$>$=$-$0.29±0.16) than single exoplanets ($<$[M/H]$>$=$-$0.02±0.18). We also derive mean magnetic fields for 29 M dwarf stars of this sample. Differently from what we found for the Pleiades cluster, the mean magnetic fields of this sample are in the unsaturated regime, presenting values ranging between $\sim$0.2 to $\sim$1.5 kG. We studied habitability for 43 exoplanets that orbit these stars and found that only the exoplanets Kepler-186f and TOI-700d are inside their respective habitable zones, with the other studied exoplanets presenting equilibrium temperatures and insolation levels too high to maintain liquid water on the surface. We evaluated the minimum planetary magnetic field that would be necessary for the exoplanets to maintain a present-day Earth magnetosphere and found values for Kepler-186f and TOI-700d of respectively 0.65 and 3.02 G. Considering a young Earth magnetosphere of 3.4 Gyr ago when there was already life on Earth, the minimum planetary magnetic fields are respectively 0.05 and 0.24 G. These results suggest that these two exoplanets might be able to protect their atmospheres from stellar winds driven by stellar magnetic fields, and are very interesting for habitability studies. Observatorio Nacional Tese

RINGS DIVERSITY AROUND SMALL SOLAR SYSTEM BODIES: DISCOVERIES AND DETECTION LIMITS

2025-01-13T14:48:27Z

RINGS DIVERSITY AROUND SMALL SOLAR SYSTEM BODIES: DISCOVERIES AND DETECTION LIMITS PEREIRA, CHRYSTIAN LUCIANO BRAGA RIBAS, FELIPE The first detection of a ring system through stellar occultation, a significant milestone in the history of planetary science, occurred in 1977. Then, astronomers unexpectedly discovered Uranus’s rings while studying the planet’s atmosphere. This discovery was followed by a similar one in 1984 when Neptune’s rings were detected as arcs using the same method. A decade ago, a ring system was identified around the Centaur object (10199) Chariklo, marking the first instance of such a feature in a small body beyond the giant planets. This discovery suggested that ring systems might be more common among small Solar System bodies than previously believed, as evidenced by subsequent findings around the dwarf planet Haumea and the indications of rings or cometary material around the Centaur (2060) Chiron. Considering that some authors suggest cometary activity might be related to the presence of rings, we sounded the vicinity of small bodies in the outer solar system, using the stellar occultation technique to search for signs of confined material. A stellar occultation happens when an object is observed blocking a distant star’s light during a certain time interval. Observing the star’s light before, during, and after the occultation lets us gather detailed information about the object’s astrometric position, size, shape, atmosphere, and ring systems. In terms of spatial resolution, this highly precise method can reveal fine details that are otherwise difficult to detect from ground-based observations, making it invaluable for studying distant and faint Small Solar System Objects. The Centaurs Objects (60558) 174P/Echeclus, 29P/Schwassman-Wachamnn 1 (29P), and (2060) Chiron exhibited regular outbursts, making them ideal targets for the search for confined material. We first examined light curves from stellar occultations of the active Centaur Echeclus observed between 2019 and 2021. While no features indicative of surrounding material were detected, strong detection limits were established, and the physical properties of Echeclus were derived. We observed the first stellar occultation by 29P in December 2022 and were able to find indications of confined material symmetrically distributed around the object. Occultation observations of Chiron were conducted in 2018, 2019, and 2022 to characterize its structures over time. The 2019 event, being the first multi-chord observation of this Centaur, allowed us to constrain its tri-axial dimensions. The 2022 observations confirmed previously proposed structures and revealed significant property variations, such as width and optical depth. A recent stellar occultation of Chiron in 2023 is still under analysis, but the high-resolution data obtained at Observatório do Pico dos Dias may solve ongoing questions about Chiron’s environment. The greatest finding came after discovering a ring beyond the Roche limit around the Trans- Neptunian Object (50000) Quaoar. We conducted an observation campaign to further characterize this ring in 2022. Our analysis revealed the presence of a second ring around Quaoar, much closer to the main body but also located beyond the Roche limit. This thesis presents the methods developed to search for, characterize, and place upper limits on detecting confined material around SSSO. We present observational evidence that material ejections alone are not capable of forming rings. Due to the diversity of the confined structures’ location in the Solar System and their properties presented here, it is clearer that the formation of rings around small bodies can only result from a combination of factors, such as (primordial) collisions or ejection processes, allied with a favorable dynamical environment around the main body. Our findings and physical properties of the studied objects are presented in association with the respective research papers. Observatorio Nacional Tese

ON THE COSMIC ACCELERATION AND MATTER CLUSTERING IN MODIFIED f(R) GRAVITY MODELS

2025-01-13T14:39:42Z

ON THE COSMIC ACCELERATION AND MATTER CLUSTERING IN MODIFIED f(R) GRAVITY MODELS NOGUEIRA RIBEIRO, BRUNO WESLEY BARTOLOME BERNUI LEO, ARMANDO itulo original da tese: ON THE COSMIC ACCELERATION AND MATTER CLUSTERING IN MODIFIED f(R) GRAVITY MODELS Understanding the origin of the current accelerated expansion of the universe is one of the greatest challenges in modern cosmology and particle physics. In the context of the standard cosmological model, the ΛCDM model, this acceleration is attributed to the cosmological constant Λ, or, equivalently, to the energy density of the quantum vacuum, which is recognized as the simplest form of dark energy. It turns out that, whether through a cosmological constant or more general forms of dark energy, this explanation for the origin of recent cosmic acceleration has some internal shortcomings. On the one hand, within the standard view of cosmology, two issues involving the cosmological constant concern both particle physicists and cosmologists: (i) the vacuum energy density measured via cosmological observations is astoundingly smaller than that estimated by quantum field theory (up to 120 orders of magnitude, depending on the theory used); and (ii) the normalized vacuum and matter energy densities have curiously close values today, which appears to be a cosmic coincidence. On the other hand, it should be possible to observe dark energy directly, since it corresponds to approximately 70% of the total energy density of the universe according to current observations. However, no dark energy has been observed directly at a fundamental level (in particle physics) to date. All evidence of an energetic component exerting negative pressure on the universe comes from indirect cosmological observations. In order to avoid the issues inherent to a cosmological constant, or to more general forms of dark energy, some alternative scenarios based on a suitable modification of General Relativity, the current theory of gravity, have been proposed. This is the case with the f(R) theories of gravity, which generalize gravitation by replacing the term R − 2Λ in the Einstein-Hilbert Lagrangian by a general function of the Ricci scalar, R. These theories are known to properly explain cosmic acceleration as an effect of the spacetime geometry, instead of an exotic form of dark energy. They are also conformally equivalent to Einstein’s theory with the addition of an extra degree of freedom in the gravitational sector, the scalaron, a canonical scalar field whose potential is uniquely determined by the Ricci scalar curvature, R. In this thesis, we study the cosmological viability of three f(R) models, namely, the Appleby-Battye, Hu-Sawicki, and Starobinsky models. We first derive the equations of motion for each model and numerically solve them for important parameters of the cosmological background: the Hubble rate H(z) and the equation of state wDE(z). Since the cosmological background is highly degenerate, we proceed to the perturbative level by numerically solving the differential equations related to the matter density contrast δm(z) and the normalized growth rate at the physical scale of 8Mpc/h, [fσ8](z), for each model. Next, we perform MCMC statistical analyses and constrain the free parameters of the Appleby-Battye and Hu-Sawicki models by considering three cosmological datasets: H(z) measurements from the cosmic chronometer method, [fσ8](z) from redshift-space distortion observations, and type Ia supernovae mB(z) measurements from Pantheon+ and SH0ES collaborations. Our results are consistent with those reported in the literature for the cosmological parameters, such as the Hubble constant (H0), the normalized matter density (Ωm,0), the variance of matter fluctuations at the scale of 8Mpc/h (σ8,0), and the absolute magnitude (MB), in both cases. Additionally, our best-fit model parameters were: b = 2.28 +6.52 −0.55 (SNe Ia data alone) and b = 2.18 +5.41 −0.55 (SNe+CC+RSD data combination), encompassing General Relativity (b ≫ 1) at 2σ CL, for the Appleby-Battye model, and μ = 77.0 +18.0 −56.0 (SNe Ia data alone) and μ = 93.0 +41.0 −55.0 (SNe+CC+RSD data combination), which excludes General Relativity (μ = 0) at 2σ CL, for the Hu-Sawicki model. Finally, the Akaike information criterion penalized both the Appleby-Battye and Hu-Sawicki models due to each having an additional independent parameter compared to the flat-ΛCDM reference model: ∆AIC = 0.716 and ∆AIC = 132.969, respectively. However, our results show that the Appleby-Battye model exhibits an AIC value very close to that of the flat-ΛCDM model (∆AIC ∼ 0.7), making it a competitive alternative to the standard model in describing the accelerated expansion and growth of structures in the universe, but without requiring any exotic dark energy. Observatorio Nacional Tese

COSMOLOGICAL APPLICATIONS OF FAST RADIO BURSTS

2025-01-10T17:09:10Z

COSMOLOGICAL APPLICATIONS OF FAST RADIO BURSTS LEMOS PORCIUNCULA ALVES, THAIS SOUZA DE ALCANIZ, JAILSON Fast radio bursts (FRBs) are a new class of high-energy transient events with short duration within the radio frequency range from a few hundred to a few thousand MHz. Although the physical mechanism responsible for these events is still in debate, the larger value of the observed dispersion measure ($\mathrm{DM}$) above that of the Milk Way contribution suggests an extragalactic or cosmological origin for the FRBs. By identifying the origin of the burst, it is possible to measure the redshift directly and can be combined with $\mathrm{DM}$ to study cosmology. For instance, from the $\mathrm{DM}-z$ relation one can test the weak equivalence principle and constrain the cosmological parameters, such as the fraction of baryon mass in the intergalactic medium ($f_{\mathrm{IGM}}$) and the Hubble constant. Since the first discovered FRB in 2007 by the Parkes telescope, almost one thousand events have been detected by new survey telescopes. However, only a few FRBs in the literature are well localized (with redshift of the host galaxy), and this number is not large enough to perform robust statistical analysis in a cosmological scenario. The other issues in FRB analyses are: (i) the $f_{\mathrm{IGM}}$ is strongly degenerated with the cosmological parameters and is not well constrained; (ii) the poor modeling of the large variance in the $\mathrm{DM}$ due to inhomogeneities in the cosmic electrons density; (iii) and lastly, the limited knowledge of the host galaxy contribution. In this context, in the first part of this Thesis, we discuss in detail the main astrophysical features of FRB. In the second part, dedicated to the cosmological application of FRBs, we present a cosmological model-independent method to estimate the $f_{\mathrm{IGM}}$ and host galaxy contribution by combining FRBs with localized host galaxy and supernovae type Ia dataset. We use the current FRBs observational data and then we explore how future surveys will improve these parameters estimation by simulating the FRBs data from Monte Carlo simulation method. In the second part, we test our physical theories by searching for a space-time variation of the fundamental constants. In particular, we use the dispersion measure of FRBs to investigate a possible redshift evolution of the fine-structure constant ($\alpha$), considering the runaway dilaton scenario. Using a cosmological model-independent method, we derive new expressions for $\mathrm{DM}$ dependence concerning the fine-structure constant. Observatorio Nacional Tese