We present the discovery of seven new proplyds (i.e., sources surrounded by cometary H alpha emission characteristic of offset ionization fronts (IFs)) in NGC 1977, located about 30' north of the Orion Nebula Cluster (ONC) at a distance of similar to 400 pc. Each of these proplyds is situated at projected distances 0.04-0.27 pc from the B1V star 42 Orionis (c Ori), which is the main source of UV photons in the region. In all cases the IFs of the proplyds are clearly pointing toward the common ionizing source, 42 Ori, and six of the seven proplyds clearly show tails pointing away from it. These are the first proplyds to be found around a B star, with previously known examples instead being located around O stars, including those in the ONC around theta(1) Ori C. The radii of the offset IFs in our proplyds are between similar to 200 and 550 au; two objects also contain clearly resolved central sources that we associate with disks of radii 50-70 au. The estimated strength of the FUV radiation field impinging on the proplyds is around 10-30 times less than that incident on the classic proplyds in the ONC. We show that the observed proplyd sizes are however consistent with recent models for FUV photoevaporation in relatively weak FUV radiation fields.

We conduct a multiwavelength continuum variability study of the Seyfert 1 galaxy NGC 5548 to investigate the temperature structure of its accretion disk. The 19 overlapping continuum light curves (1158 angstrom to 9157 angstrom) combine simultaneous Hubble Space Telescope, Swift, and ground-based observations over a 180 day period from 2014 January to July. Light-curve variability is interpreted as the reverberation response of the accretion disk to irradiation by a central time-varying point source. Our model yields the disk inclination i = 36 degrees +/- 10 degrees, temperature T-1= (44 +/- 6) x 10(3) K at 1 light day from the black hole, and a temperature-radius slope (T proportional to r(-alpha)) of alpha = 0.99 +/- 0.03. We also infer the driving light curve and find that it correlates poorly with both the hard and soft X-ray light curves, suggesting that the X-rays alone may not drive the ultraviolet and optical variability over the observing period. We also decompose the light curves into bright, faint, and mean accretion-disk spectra. These spectra lie below that expected for a standard blackbody accretion disk accreting at L/L-Edd = 0.1.

We report the large effort that is producing comprehensive high-level young star cluster (YSC) catalogs for a significant fraction of galaxies observed with the Legacy ExtraGalactic UV Survey (LEGUS) Hubble treasury program. We present the methodology developed to extract cluster positions, verify their genuine nature, produce multiband photometry (from NUV to NIR), and derive their physical properties via spectral energy distribution fitting analyses. We use the nearby spiral galaxy NGC 628 as a test case for demonstrating the impact that LEGUS will have on our understanding of the formation and evolution of YSCs and compact stellar associations within their host galaxy. Our analysis of the cluster luminosity function from the UV to the NIR finds a steepening at the bright end and at all wavelengths suggesting a dearth of luminous clusters. The cluster mass function of NGC 628 is consistent with a power-law distribution of slopes similar to-2 and a truncation of a few times 10(5) M-circle dot. After their formation, YSCs and compact associations follow different evolutionary paths. YSCs survive for a longer time frame, confirming their being potentially bound systems. Associations disappear on timescales comparable to hierarchically organized star-forming regions, suggesting that they are expanding systems. We find massindependent cluster disruption in the inner region of NGC 628, while in the outer part of the galaxy there is little or no disruption. We observe faster disruption rates for low mass (<= 10(4) M-circle dot) clusters, suggesting that a massdependent component is necessary to fully describe the YSC disruption process in NGC 628.

O espectro no núcleo da galáxia Seyfert 1 NGC 7469 apresenta linhas de emissão intensas e largas, com assimetria para o vermelho no caso das linhas de Balmer do HI, e para o azul no caso de linhas proibidas. / The nuclear spectrum of the Seyfert 1 galaxy NGC 7469 exhibits intense and broad emission lines: the HI Balmer lines areasymmetric to the blue.

The evolution and formation of dwarf galaxies has great importance to our knowledge of cosmological history from the Big Bang through the present day structure we observe in our local universe. Dwarf galaxies are believed to be the "building blocks" of larger galaxies, which implies that interactions and mergers of these small systems must have occurred frequently in the early universe. There is a population of starburst dwarf irregular (dIm) galaxies that seem to have characteristics indicative of interactions or mergers. One of these dIm galaxies is the nearby post-starburst NGC 1569. This dissertation project explores the stellar and gas kinematics of NGC 1569 as well as examines a deep neutral Hydrogen (HI) map made using the Robert C. Byrd Green Bank Telescope (GBT). From these observations, this dissertation analyzes the evolution of NGC 1569 by understanding the three-dimensional shape of this dIm system for the first time. The structure of dIm galaxies is an important fundamental, physical property necessary to understand the evolution and formation of these common systems. However, the intrinsic shape of dIm galaxies remains controversial. Projected minor-to-major axis ratios provide insucient data to determine the shapes of dIm galaxies. Fortunately, there is another method by which accurate structures can be measured. The stellar velocity dispersion, coupled with the maximum rotational velocity derived from HI observations, gives a measure of how kinematically hot a system is, and, therefore, indicates its structure. In this dissertation, we present the stellar kinematics, including the stellar velocity dispersion, of NGC 1569 obtained using the Kitt Peak National Observatory (KPNO) Mayall 4-m+Echelle spectrograph. These data are combined with an in depth analysis of high resolution HI data and a discussion of the nature of this starburst dwarf system. The dissertation concludes with a deep HI map of NGC 1569 and three of its nearest neighbors in the IC 342 galaxy group. Extended HI structures are observed in this map and are likely associated with NGC 1569. However, distinguishing if these structures are from an interaction or a merger is not possible and hydrodynamic simulations are needed. These simulations are for future work.

White dwarf stars are the burnt out remnants that remain after a star like the Sun has completed its nuclear evolution. In such a star there are no remaining nuclear energy sources, so the star evolves by simply radiating its stored thermal energy out into space. This may seem rather uninteresting, but in fact there is a wealth of physical phenomena that occur during this part of a star's life - from getting kicked at birth, to neutrino emission in early life, to some interesting high density physics, through to functioning as precise clocks that can provide an age for some of the oldest know stars in the Universe. Some of these phases will be illustrated with detailed observations taken recently with the Hubble Space Telescope.

Through the cosmic matter cycle, the chemical evolution of the Milky Way is imprinted in the elemental abundance patterns of late-type stars (spectral types F to K). Due to their long lifetimes ( 1 Hubble time), these stars are of particular importance when it comes to studying the build-up of elements during the early times of our Galaxy. The chemical composition of the atmospheric layers of such stars is believed to resemble the gas from which they were formed. However, recent observations in globular clusters seem to contradict this assumption. The observations indicate that processes are at work that alter the surface compositions in these stars. The combined effect of processes responsible for an exchange of material between the stellar interior and atmosphere during the main sequence lifetime of the star, is referred to as atomic diffusion. Yet, the extent to which these processes alter surface abundances is still debated. By comparing abundances in unevolved and evolved stars all drawn from the same stellar population, any surface abundance anomalies can be traced. The anomalies, if found, can be compared to theoretical predictions from stellar structure models including atomic diffusion. Globular clusters provide stellar populations suitable to conduct such a comparison. In this thesis, the results of three independent analyses of two globular clusters, NGC 6752 and M4, at different metallicities are presented. The comparison between observations and models yields constraints on the models and finally a better understanding of the physical processes at work inside stars.

Universiẗat, Diss., 1999--München.

No description available.

Estudamos as bandas de hidrocarbonetos aromáticos policíclicos (PAHs), linhas iônicas e contínuo de uma amostra composta por 98 galáxias com núcleo ativo (AGNs) e 88 galáxias Starburst (SB) usando espectros na região do infravermelho médio (MIR) observados com o telescópio espacial Spitzer. A forma do contínuo dessas galáxias aumenta para comprimentos de ondas maiores (_ 15μm) e segue uma distribuição de corpo-negro com temperaturas T_150 - 300K. As bandas de PAHs em 6.2, 7.7, 8.6, 11.3, 12.7μm e linhas em emissão de [Si ii] 34.8μm, [Ar ii] 6.9μm, [S iii] 18.7 e 33.4μm são detectadas em todas as galáxias SB e em _80% das galáxias Seyfert 2. Considerando apenas os PAHs em 7.7μm, 11.3μm e 12.7μm, encontramos que elas estão presentes em 80% das Seyfert 1, enquanto que apenas _50% delas apresentam as bandas de PAHs em 6.2μm e 8.6μm. As razões das bandas de PAHs neutros para ionizados (6.2μm/7.7μm×11.3μm/7.7μm) foram comparados com modelos teóricos e mostraram que as moléculas de PAHs em AGNs são maiores (> 180 átomos de carbono) que nas galáxias SB, além disso, os AGNs possuem alta fração de PAHs ionizados, enquanto que nas galáxias SB os PAHs tem baixo grau de ionização. Os valores da razão 7.7μm/11.3μm são aproximadamente constantes com o aumento de [Ne iii] 15.5μm/[Ne ii] 12.8μm, indicando que a fração das bandas de PAHs ionizados para neutros não depende da dureza do campo de radiação. Entretanto, as larguras equivalentes de ambos os PAHs diminuem com [Ne iii]/[Ne ii], sugerindo que as moléculas de PAHs, ionizadas (7.7μm) ou neutras (11.3μm), podem ser destruídas com o aumento da dureza do campo de radiação. Analisamos espectros com alta resolução espacial na banda N, observados com o Thermal- Region Camera Spectrograph (T-ReCS) e Michelle instalados nos telescópios Gemini, e comparamos com dados do Spitzer. Encontramos que AGNs com formação estelar circum-nuclear possuem bandas de PAHs e AGNs com intensa emissão de raio-X apresentam profunda absorção de silicato em 9.7μm. Também vemos que os espectros observados com o Gemini tem o mesmo comportamento que aqueles observados com o Spitzer. Entretanto, as bandas de PAHs nos dados do Spitzer são mais intensas que nos dados do Gemini. Além disso, existe emissão de PAHs próximo do núcleo ativo da NGC1808 (_ 26 pc), sugerindo que essas moléculas poderiam sobreviver próximas do AGN. Também realizamos um estudo com espectro no MIR, das galáxias Seyfert 2 NGC3281 e Mrk 3 classificadas como Compton-thick, obtidos com o T-ReCS e Michelle. Ambos espectros apresentam absorção de silicato em 9.7μm e linhas em emissão do [S iv] 10.5μm e [Ne ii] 12.7μm, porém, os espectros dessas galáxias não apresentam bandas de PAHs. Inferimos que a extin¸c˜ao no visual dessas gal´axias ´e AV _83mag (NGC3281) e AV _5.5mag (Mrk 3). Descrevemos os espectros nucleares dessas galáxias usando modelos de torus formados por nuvens. Os resultados sugerem que o núcleo dessas galáxias possui uma estrutura toroidal composta por poeira. Entretanto, as propriedades físicas do torus de NGC3281 e Mrk 3 são muito diferentes. Em NGC3281 o torus tem um raio de R0 _11 pc, 10 nuvens no equador, _V =40mag cada e estaríamos olhando na direção do equador (i = 60_). Na Mrk 3 o torus tem R0 _34 pc, 14 nuvens com _V =30mag e i = 90_. Usando estes modelos determinamos os valores da densidade colunar de hidrogênio (NH > 1024 cm−2) que são similares as inferidas a partir de dados do raio-X, que classificaram as galáxias NGC3281 e Mrk 3 como fontes Compton-thick. Este fato pode indicar que o material que absorve luz em raio-X também pode ser o responsável pela absorção em 9.7μm, além disso, também mostra uma forte evidência que a poeira de silicato, responsável por essa absorção, está localizada no torus. / We study polycyclic aromatic hydrocarbon (PAH) bands, ionic emission lines, and mid-infrared (MIR) continuum properties in a sample of 98 active galactic nucleus (AGNs) and 88 Starburst (SB) with Spitzer spectra. The continuum rises steeply for longer wavelengths ( 15μm) and follow a warm blackbody distribution of T 150 - 300K. The brightest PAH spectral bands (6.2, 7.7, 8.6, 11.3, and 12.7μm) and the forbidden emission lines [Si ii] 34.8μm, [Ar ii] 6.9μm [S iii] 18.7 and 33.4μm were detected in all the SB and in 80% of the Seyfert 2s. Considering only the PAH bands at 7.7μm, 11.3μm, and 12.7μm, we find that they are present in 80% of the Seyfert 1s, but only half of them shows the 6.2μm and 8.6μm PAH bands. The observed intensity line ratios for neutral and ionized PAHs (6.2μm/7.7μm×11.3μm/7.7μm) were compared to theoretical ratios, showing that AGNs have higher ionization fraction and larger PAH molecules (> 180 carbon atoms) than SB galaxies. The ratios between the ionized (7.7μm) and the neutral PAH bands (8.6μm and 11.3μm) are distributed over different ranges in AGNs and SB galaxies, suggesting that these ratios depend on the ionization fraction, and on the hardness of the radiation field. The ratio between the 7.7μm and 11.3μm bands is nearly constant with the increase of [Ne iii] 15.5μm/[Ne ii] 12.8μm, indicating that the fraction of ionized to neutral PAH bands does not depend on the hardness of the radiation field. The equivalent widths of both PAH features show the same dependence (strongly decreasing) with [Ne iii]/[Ne ii], suggesting that the PAH molecules, emitting either ionized (7.7μm) or neutral (11.3μm) bands, may be destroyed with the increase of the hardness of the radiation field. We investigate Seyfert galaxies that are powered by starburst and AGN emission using N band high resolution spectra taken with the Thermal-Region Camera Spectrograph (T-ReCS) and Michelle at the Gemini South and North telescopes. Also, Spitzer observations including emission of the nucleus and most of the host galaxy are compared with Gemini spectra of the nuclear region. We find that AGNs with circum-nuclear star formation show stronger PAHs than those with hard X-ray emission, the latter presenting deep silicate features at 9.7μm. We also find that Gemini spectra follow the same feature observed in Spitzer spectra. However, Spitzer data show stronger PAH bands than Gemini’s. In addition, we find PAH bands close ( 26 pc) to the NGC1808 active nucleus, suggesting that these molecules survive near an AGN. We also analyse MIR spectra of the Compton-thick Seyfert 2 galaxies NGC3281 and Mrk 3, obtained with T-ReCS and Michelle. Both spectra present silicate absorption at 9.7μm, as well as [S iv] 10.5μm and [Ne ii] 12.7μm ionic lines, but with no evidence of PAH emission. We find that the nuclear optical extinctions are AV 83mag (NGC3281) and AV 5.5mag (Mrk 3). We describe their nuclear spectra with a clumpy torus model. However, the torus physical properties in NGC3281 and Mrk 3 are very different. While in NGC3281 the torus has a radius of R0 11 pc, 10 clouds in the equatorial radius with optical depth of V =40mag each, and we would be looking in the direction of the torus equatorial radius (i = 60 ), in Mrk 3 it has R0 34 pc, 14 clouds in the equator with each cloud having V =30mag. In addition, according to these models, Mrk 3 torus would be “edge-on” with observer angle equal i = 90. Using silicate absorption modeling with a clumpy torus model we also retrieve the values of hydrogen column density (NH > 1024 cm−2) that classify NGC3281 and Mrk 3 as Compton-thick sources from X-ray data. Our findings indicate that the X-ray absorbing column densities, which classify NGC3281 and Mrk 3 as a Compton-thick sources, may also be responsible for the absorption at 9.7μm, providing strong evidence that the silicate dust responsible for this absorption is located in the AGN torus.