I present low-resolution (Dl 6 A° ) follow-up spectroscopy of 370 Ha emitters (12 . r . 17) identified with IPHAS, in a 100 deg2 wide section of the Galactic plane that is located between ` = (120 ; 140 ) and b = ( 1 ; +4 ). Classical Be stars are found to be the most numerous group of the observed targets ( 60%). Sixty-eight classical Be stars have also been observed at higher spectral resolution (Dl 2 4 A° ) and S/N ratio, which allows spectral typing to an estimated precision of 1 sub-type. Colour excesses were measured via spectral energy distribution fitting of flux-calibrated data. I took care to remove the circumstellar contribution to the measured colour excess, using an established scaling to the Ha equivalent widths. In doing so, this method of correction was re-evaluated and modified to better suit the data at hand. Spectroscopic parallaxes were measured constraining the luminosity class via estimates of distances to main sequence A/F stars, which are found within a few arcminutes of each classical Be star on the sky. In order to probe the structure of the outer Galactic disc, I studied the spatial distribution of 63 out of 248 classical Be stars identified. Their cumulative distribution function with respect to the distance is statistically compatible both with a smooth exponential density profile and with a simple spiral arms representation. The distribution of reddenings of classical Be stars is compared with estimates of the total Galactic reddening along their sightlines. It is expected that the measured reddenings match the integrated Galactic values, for distant stars located outside the Galactic dust layer, or they are smaller than the asymptotic values if the stars are less distant. The outcome meets expectations, and lends support to the conclusion that the measured reddenings are determined to a precision of 10%. The sample of 248 objects doubles the number of known classical Be stars in this part of the Galactic plane. Unlike the pre-existing bright sample, the new objects are seen at large distances, between 2 – 8 kpc with typical E(B V) 0:9. Only four stars are members of known clusters. Ten classical Be stars are proposed to be well beyond the putative Outer Arm, at distances larger than 8 kpc. The large sample of stars, which has been identified here, is the result of a successful selection and analysis of classical Be stars that is offered for more exploitation in future. The proposition is that GAIA observations will use the present sample of classical Be stars as a new tracer of the Galactic disc.

As estrelas Be compreendem uma grande faixa de massas e temperaturas. Por definição, são objetos de tipo B com classe de luminosidade entre V e III que apresentam, ou apresentaram alguma vez, linhas de Balmer em emissão (eventualmente metais uma vez ionizados) e/ou linhas com padrões de absorção shell, possivelmente formadas em um envelope circunstelar. Embora se saiba há muito tempo que esses objetos são rodadores rápidos e que giram pelo menos 1,5 a 2 vezes mais rápido do que as estrelas B normais, ainda é incerto se esses objetos são ou não em média rodadores crticos, não obstante as recentes observações interferométricas de HD 10144 (Achernar) (uma estrela Be tpica) indicarem se tratar de um rodador crtico. Devido às suas altas taxas de rotação, as quais originam distorções geométricas e distribuições não uniformes de temperatura dependentes da latitude estelar, os valores de velocidade de rotação derivados por métodos clássicos são sistematicamente subestimados. Além disso, os efeitos da rotação, aliados à presença do envelope circunstelar, mascaram as condições fsicas desses objetos, resultando em diferenças significativas em seus estágios evolutivos na seqüência principal. Neste trabalho apresentamos os resultados do estudo espectroscópico de estrelas Be em duas vertentes. Na primeira tratamos o tema da formação e estrutura do envelope circunstelar de estrelas Be, através das análises de duas estrelas, HD 127972 ( Cen) e HD 10144 (Achernar). Nesse estudo identificamos e caracterizamos seus modos de pulsação, os quais se constituem em um possvel mecanismo de perda de massa e formação do envelope. Além disso, estudamos a estrutura de seus envelopes circunstelares através da modelagem de perfis de Balmer em emissão. Na segunda vertente quantificamos as condições fsicas de 141 estrelas de campo, onde 114 são de tipo Be e 27 estrelas são B normais. Nesse estudo, comparamos os estágios evolutivos desses objetos obtidos através de métodos clássicos com os estágios evolutivos corrigidos dos efeitos da rotação elevada. Concluimos que o \"fenômeno Be\" pode ocorrer em todas as fases da evolução estelar na seqüência principal. / Be stars encompass a large mass and temperature range. By definition, they are B-type objects with luminosity classes V to III that have, or have shown at least once, Balmer lines in emission (eventually single-ionized metals) and/or lines with shell absorption patterns possibly formed in a circumstellar envelope. Though it has long been known that these objects are fast rotators and that they rotate at least 1.5 to 2 times faster than normal B stars, it is still uncertain whether or not these objects are in average critical rotators, although recent interferometric observations on Achernar (a typical Be star) pointed it out to be a critical rotator. Due to their high rotation rates which originate geometrical distortions and non-uniform temperature distributions dependent on the stellar latitude, the rotation velocity values derived from classical methods are systematically underestimated. Moreover, the rotation effects allied to the continuum emission due to the presence of a circumstellar envelope disguise the physical conditions of these objects, resulting in significative differences of their main-sequence evolutionary stages. In this work we present the results of the spectroscopic study of Be stars in two approaches. In the first one we treat the subject of formation and structure of the circumstellar envelope of Be stars through the analyses of two stars, HD 127972 ( Cen) and HD 10144 (Achernar). In this study we identify and characterize their pulsation modes, which constitute in a possible mechanism of mass loss and envelope formation. Moreover we study the structure of their circumstellar envelopes through the modeling of Balmer profiles in emission. In the second approach we quantify the physical conditions of 141 field stars, where 114 are Be-type stars and 27 stars are normal B stars. In this study, we compared the evolutionary stages of these objects obtained through classical methods with evolutionary stages corrected for high rotation effects. We conclude that the \"Be phenomenon\" can occur at whatever stage of the stellar evolution on the main sequence.

As estrelas Be compreendem uma grande faixa de massas e temperaturas. Por definição, são objetos de tipo B com classe de luminosidade entre V e III que apresentam, ou apresentaram alguma vez, linhas de Balmer em emissão (eventualmente metais uma vez ionizados) e/ou linhas com padrões de absorção shell, possivelmente formadas em um envelope circunstelar. Embora se saiba há muito tempo que esses objetos são rodadores rápidos e que giram pelo menos 1,5 a 2 vezes mais rápido do que as estrelas B normais, ainda é incerto se esses objetos são ou não em média rodadores crticos, não obstante as recentes observações interferométricas de HD 10144 (Achernar) (uma estrela Be tpica) indicarem se tratar de um rodador crtico. Devido às suas altas taxas de rotação, as quais originam distorções geométricas e distribuições não uniformes de temperatura dependentes da latitude estelar, os valores de velocidade de rotação derivados por métodos clássicos são sistematicamente subestimados. Além disso, os efeitos da rotação, aliados à presença do envelope circunstelar, mascaram as condições fsicas desses objetos, resultando em diferenças significativas em seus estágios evolutivos na seqüência principal. Neste trabalho apresentamos os resultados do estudo espectroscópico de estrelas Be em duas vertentes. Na primeira tratamos o tema da formação e estrutura do envelope circunstelar de estrelas Be, através das análises de duas estrelas, HD 127972 ( Cen) e HD 10144 (Achernar). Nesse estudo identificamos e caracterizamos seus modos de pulsação, os quais se constituem em um possvel mecanismo de perda de massa e formação do envelope. Além disso, estudamos a estrutura de seus envelopes circunstelares através da modelagem de perfis de Balmer em emissão. Na segunda vertente quantificamos as condições fsicas de 141 estrelas de campo, onde 114 são de tipo Be e 27 estrelas são B normais. Nesse estudo, comparamos os estágios evolutivos desses objetos obtidos através de métodos clássicos com os estágios evolutivos corrigidos dos efeitos da rotação elevada. Concluimos que o \"fenômeno Be\" pode ocorrer em todas as fases da evolução estelar na seqüência principal. / Be stars encompass a large mass and temperature range. By definition, they are B-type objects with luminosity classes V to III that have, or have shown at least once, Balmer lines in emission (eventually single-ionized metals) and/or lines with shell absorption patterns possibly formed in a circumstellar envelope. Though it has long been known that these objects are fast rotators and that they rotate at least 1.5 to 2 times faster than normal B stars, it is still uncertain whether or not these objects are in average critical rotators, although recent interferometric observations on Achernar (a typical Be star) pointed it out to be a critical rotator. Due to their high rotation rates which originate geometrical distortions and non-uniform temperature distributions dependent on the stellar latitude, the rotation velocity values derived from classical methods are systematically underestimated. Moreover, the rotation effects allied to the continuum emission due to the presence of a circumstellar envelope disguise the physical conditions of these objects, resulting in significative differences of their main-sequence evolutionary stages. In this work we present the results of the spectroscopic study of Be stars in two approaches. In the first one we treat the subject of formation and structure of the circumstellar envelope of Be stars through the analyses of two stars, HD 127972 ( Cen) and HD 10144 (Achernar). In this study we identify and characterize their pulsation modes, which constitute in a possible mechanism of mass loss and envelope formation. Moreover we study the structure of their circumstellar envelopes through the modeling of Balmer profiles in emission. In the second approach we quantify the physical conditions of 141 field stars, where 114 are Be-type stars and 27 stars are normal B stars. In this study, we compared the evolutionary stages of these objects obtained through classical methods with evolutionary stages corrected for high rotation effects. We conclude that the \"Be phenomenon\" can occur at whatever stage of the stellar evolution on the main sequence.

The brightness and proximity of many classical Be stars makes them perfect laboratories for studying the physics of astrophysical disks. They are also among the most popular targets for optical/IR interferometers, which are able to fully resolve their circumstellar disks, to which much of the recent progress in our understanding of these enigmatic objects is owed. The current consensus is that classical Be stars eject material from the stellar surface into Keplerian orbits, thus forming a disk, whose subsequent evolution is governed by turbulent viscosity, which is the basis of the so-called viscous decretion disk (VDD) model. Among the main results of the present work is arguably the best-constrained model of a particular Be star β CMi. The VDD predictions were confronted also with radio observations, which allowed for the first determination of the physical extent of a Be disk. This result subsequently led to the detection of a binary companion, which is truncating the disk by tidal forces. Extending the sample to include five more targets led to revealing a similar outer disk structure in all of them. The range of explanations includes the most plausible scenario, in which the truncation of Be disks by (unseen) companions is much more common than previously thought.

To rigorously study hot stars, we need to determine their properties as accurately as possible. This thesis focuses on improving the accuracy of parameters of two hot stars and contributes to the collection of stars with more known parameters. While photometric and spectroscopic data sets on hot stars δ Ori A and ω CMa are plentiful, the former is a triple star that suffers from having a faint spectrum of the secondary, which complicates its analysis, and the latter is a Be star, the origin, formation, and long-term variability of whose gaseous envelopes (veils) remain unexplained. Using mathematical techniques (Fourier transforms) and modelling software PHOEBE 1, properties of δ Ori A are determined, and conjecture on the presence of a circumstellar envelope in δ Ori A might be true by considering the shape changes of the Hα line. A conjecture that period changes of ω CMa correlate to the mass of the circumstellar envelopes seems to be proved true by considering variations of radial velocities, V/R ratio, and determining profile line asymmetry. 1