In this thesis, an assessment is made of the value of optical CMDs as a useful diagnostic of the accretion properties of young stars. An analysis has been made of the phenomena that we observe and their effect on the position of stars in the CMD. Limitations and potential biases have been identified and evaluated. Variability causes some luminosity spread at a given colour in optical CMDs. A detailed characterisation of variability has been performed which places strong constraints on the magnitudes and the timescales on which the variability is seen. On timescales 15 minutes, almost no variability is detected (at levels greater than ≈ 0.2%) in the i band for a sample of ≈ 700 disc-bearing young stellar objects (YSOs). This suggests that the variability predicted by some accretion shock models is either very weak or not present. On hours to days timescales the optical variability in most stars is well described by a simple power law. The amplitude of the variability, a ∝ f−k, where f is the frequency of the variability in days. Disc-bearing and discless YSOs exhibit median values of k of 0.85 ± 0.02 and 0.95 ± 0.03 respectively, the uncertainity being the error on the median. The power law is valid up to a certain timescale (tmax) at which point the variability amplitude does not increase any further. tmax is found to be 1.50 ± 0.07 days and 1.41 ± 0.10 days for disc-bearing and discless stars respectively. Disc-bearing stars show greater variability amplitudes than the discless stars. However, it is notable that the variability timescale and power spectrum exponent are remarkably similar. This implies that the amplitude of the variability is driven by the physics of the underlying process, but that the timescales are instead driven by geometric effects. For disc-bearing stars, the highest amplitude variables are the accreting stars, which often appear to vary in the CMD along lines that correspond to changes in accretion luminosity. Four disc-bearing stars (approximately 0.5% of the disc-bearing sample) in Cep OB3b show extreme variability on timescales of years. Three (possible EXor candidates), show long-timescale changes that have a dramatic effect on their CMD position. However their small numbers mean that the overall impact on the CMDs of young associations is small. Variability on timescales of the rotational period and shorter adds uncertainty to age estimates of individual stars that are calculated by comparison with PMS models. Having provided a detailed description of variability and its impact on the CMD, it is clear that there are further significant mechanisms that affect the positions of YSOs in the CMD. I show that the spread in luminosity seen in the Orion Nebula Cluster and NGC 2264 could not be explained by accretion at rates of M ̇ ≥ 5 × 10−4 M⊙ yr−1 occurring within the protostellar phase of YSO evolution. Thus it appears that CMDs are not a useful diagnostic for study of the accretion histories of YSOs. The wavelength dependence of the extinction by dust within the inner regions of YSO discs is shown to differ from that seen in the ISM. Typically the wavelength dependence of the extinction is given by RV ≈5-8, compared with the value of RV ≈3.1 typical of the ISM. The interpretation is that grain growth has occurred. The location of this material within the ‘snow line’ implies that grains have coalesced rather than simply gaining an ice mantle. This is evidence for the beginning of planet formation. The effect of the high value of RV on the CMD is to add additional uncertainty of 0.1 mag to photometric measurements that have been corrected for the effects of extinction. Accretion luminosity is shown to be the dominant signal in the luminosity spread seen in CMDs of young associations. Stars which exhibit excess flux in the U band or Hα are displaced in CMD space. The accretion vector is shown to be a significant blueward shift in colour accompa- nied by a modest brightening in the g, g − i CMD. Accretion results in a luminosity spread as stars are displaced blueward below the PMS locus. This effect is not seen in non-accreting disc-bearing stars. Examination of the underlying excess luminosity spectrum for 15 accreting stars shows that the colour of the emission excess is not consistent across the sample. Thus to quantify the effect of accretion luminosity on CMD positions for individual stars, moderate resolution spectra are required with a large range in wavelength. This accretion luminosity may systematically bias estimates of PMS ages. A simple mitigation is to exclude accreting stars from age analysis. U band and Hα flux excesses are shown to vary independently by ≈ 1 dex on timescales shorter than the rotation period of the star. The relation between U band flux excess and veiling at 7000Å also appears to be variable. This implies that single epoch measurements of these parameters will add an uncertainty of ≈ 1 dex on accretion rates derived from them. Accretion rates derived from either U or Hα excess should be calculated from a mean of several photometric measurements, separated by significant fractions of the rotation period of the star. In most stars, the veiling at 7000Å is shown not to be a good measure for the calculation of the accretion rate. Despite providing a detailed characterisation of phenomena that influence the positions of YSOs in the CMD, there exists some residual luminosity spread at a given Teff that cannot be explained by variability on any timescale, extinction uncertainties or accretion luminosity. This residual spread should provide an opportunity to study an as-yet uncharacterised aspect of young stars.

The young moving groups are collections of nearby (<200 pc), young (5-150 Myr) pre-main sequence stars; these stars offer us one of the best opportunities to characterise stellar multiplicity, sub-stellar phenomena, disc evolution and planet formation. Here we present results from a series of multiplicity studies aimed at producing comprehensive multiplicity statistics of the young moving groups. The aim was to compare the derived statistics of the young moving groups to other populations in order to investigate whether the abundance and properties of multiple systems are environment independent. We have combined high-resolution spectroscopy, AO-imaging and direct imaging to identify and characterise multiple systems across a huge range of orbital periods (1- 10e10 day). The observational techniques also allow us to constrain the abundance of multiple systems in these populations by calculating detection limits. We found many similarities (frequency of spectroscopic binaries; frequency, mass-ratio and physical separation of visual binaries) between the young moving groups and both younger and older regions, for multiple systems with physical separations smaller than 1000 au. We did, however, identify a significant number of new wide (>1000 au) companions. We reconciled the apparent excess of wide binary systems, when compared to the field population, by arguing that the wide systems are weakly bound and most likely decaying. By comparing the multiplicity statistics in one particular moving group we showed that the dynamical evolution of non-hierarchical protostars could lead to the population of wide binaries we can observe today. Our results indicate that the majority of low-mass stars form in small groups with 3 or 4 components that undergo significant dynamical evolution. The multiplicity properties of the young nearby moving groups are statistically similar to many other populations, supporting the environment-independent formation of multiple systems.

No description available.

We have surveyed 84 Class 0, Class I, and flat-spectrum protostars in mid-infrared [Si II], [Fe II], and [S I] line emission, and 11 of these in far-infrared [O I] emission. We use the results to derive their mass. outflow rates, (M) over dot(w). Thereby we observe a strong correlation of (M) over dot(w) with bolometric luminosity, and with the inferred mass accretion rates of the central objects, (M) over dot(a), which continues through the Class 0 range the trend observed in Class II young stellar objects. Along this trend from large to small mass. flow rates, the different classes of young stellar objects lie in the sequence Class 0-Class I/flat-spectrum-Class II, indicating that the trend is an evolutionary sequence in which (M) over dot(a) and (M) over dot(w) decrease together with increasing age, while maintaining rough proportionality. The survey results include two that. are key tests of magnetocentrifugal outflow-acceleration mechanisms: the distribution of the outflow/accretion branching ratio b = (M) over dot(w)/(M) over dot(a), and limits on the distribution of outflow speeds. Neither rules out any of the three leading outflow-acceleration, angular-momentum-ejection mechanisms, but they provide some evidence that disk winds and accretion-powered stellar winds (APSWs) operate in many protostars. An upper edge observed in the branching-ratio distribution is consistent with the upper bound of b = 0.6 found in models of APSWs, and a large fraction (31%) of the sample have a. branching ratio sufficiently small that only disk winds, launched on scales as large as several au, have been demonstrated to account for them.

Herbig Ae/Be (HAeBe) stars are the pre-main sequence progenitors of main sequence A and B stars, and are characterized observationally by strong emission in spectral lines and significant infra-red excess that results from their presence in dust-obscured regions. These stars are usually surrounded by a complex environment composed of gas and dust and often a significant stellar wind and a circumstellar disc. This complex circumstellar environment can have a significant affect on their spectral energy distributions, leading to large systematic uncertainties in determinations of their effective temperatures from photometric methods. In an attempt to improve temperature determinations for HAeBe stars, we have conducted an experiment to evaluate the potential of high-resolution spectra to constrain their atmospheric parameters. To this end, high-resolution (R~68 000) and low-resolution (R~1500) spectra obtained using the ESPaDOnS spectropolarimeter (at the Canada-France-Hawaii telescope) and the FORS1 spectropolarimeter (at the Very Large Telescope) have been used with an automatic spectrum fitting procedure. This procedure compares spectroscopic data to a grid of synthetic LTE, solar abundance spectra, spanning a range in effective temperature, surface gravity, and micro-turbulence. This analysis was applied to the spectra of a sample of twelve previously well-studied HAeBe stars. Our temperatures were found to be consistent with previously published values, while providing much lower uncertainties - in some cases about 5 times smaller. Numerous methods were investigated to obtain these quantitative uncertainties (chi-squared statistics, Bayesian analysis, Monte Carlo bootstrap method, individual temperature sensitive line region analysis). We conclude that our method can be used to efficiently and effectively obtain temperatures of HAeBe stars in addition to providing us with a characterization of the degree of departure of the spectrum from solar abundance, LTE photospheric models. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2009-02-13 10:23:04.9

Moving groups are associations of stars which originated from the same star forming region. These groups are typically young (< 200 Myr) since they have not dissipated into the galactic field population. Over the last 15 years, roughly 10 such moving groups have been found with distances < 150 pc (7 with distances < 100 pc), each with a unique velocity and position. This work first investigates the likelihood to resolve star from two moving groups (AB Doradus and Beta Pictoris) using high spacial resolution optical interferrometry and found 5 AB Doradus stars and 1 Beta Pictoris star with declinations > -30 could be spacially resolved. To more deeply characterize individual groups, we used the 2.7m telescope at the McDonald Observatory to observe 10 proposed AB Doradus stars and 5 proposed Octans-Near stars (3 probable members, 2 possible) with high resolution (R ~60,000) optical spectroscopy. Each group is characterized in three ways: (1) Chemical analysis to determine the homogeneity among members, (2) Kinematic traceback to determine the origin, and (3) Isochrone fitting to determine the age. We find the 8 stars in our AB Doradus sample are chemically homogeneous with [M/H] = -0.03 ± 0.06 dex, traceback to an age of 125 Myr, and the stars in this mass range are on the main sequence. The two deviants are a metal rich, potentially younger member and a metal poor, young star likely not associated with AB Doradus. In our Octans-Near sample, we find the 3 probable members have [M/H] = -0.06 ± 0.11, the stars do not trace back to a common origin, and the probable members are on the main sequence. In addition to these tests, we found that the probable members are slightly more lithium depleted than the Pleiades, implying an age between 125 and 200 Myr. Finally, we investigate systematic trends in fundamental stellar parameters from the use of different techniques. Preliminary results find differences in temperatures between interferrometric and spectroscopic techniques to be a function of temperature with a interferrometric temperatures being cooler by an average of 36 ± 115 K. We also calculated the chemical abundances as a function of condensation temperature for our moving group sample and predict 2 stars in AB Doradus could represent the initial star forming environment and discuss the implications for planet hosting stars in nearby moving groups. This updated characterization technique allows for a deeper understanding of the moving group environment. As future, high precision instruments emerge in astronomy (Jame Webb Space Telescope, GAIA, 30m class telescopes), moving groups are ideal targets since these associations will help us understand star forming regions, stellar evolution at young ages, constrain stellar evolutionary models, and identify planetary formation and evolution mechanisms.

This study consisted of an optical photometric and spectroscopic analysis on a 7′ 7′ field around the Southern high mass star forming region RCW 34. A previous study on RCW 34 in the NIR discov- ered many deeply embedded young stellar objects which were suspected to be T Tauri stars and which justified further investigation. The data used in this study consisted of three sets, the first two are photometric and spectroscopic data sets which were obtained during the first two weeks of February 2002. A third data set of spectroscopic observations was obtained by the author during the second week of 2011 of selected candidates using results from the NIR study and from the photometric data sets. All of the spectroscopy was conducted with the long slit spectrograph on the 1.9-m telescope and the photometry with DANDICAM on the 1.0-m telescope at the South African Astronomical Observatory (SAAO) in Sutherland. Objectives accomplished in the course of this study were to understand, ob- tain, reduce and interpret photometric and long slit spectroscopic CCD images. From the photometric results 57 stars showed excess blue emission on a colour-colour diagram which could be generated by circumstellar matter. The spectroscopic study showed 5 stars that showed H emission and 2 with strong Li absorption lines which confirm the suspicions of the NIR study about T Tauri stars in the region. All of the stars from the spectroscopic study in 2011 were identified as low-mass K or M type stars. Using colour-magnitude diagrams it was possible to see that the majority of the stars in the cluster are low-mass pre-main sequence stars. The stars matching between the optical and NIR filters were plotted on NIR colour-colour diagrams showing that the 5 stars that had H emission lines also had NIR colours characteristic to T Tauri stars. Out of the 5 stars that showed H emission, 2 were found to be classical T Tauris and three were found to be weak line T Tauris. / Thesis (MSc (Space Physics))--North-West University, Potchefstroom Campus, 2013

This study consisted of an optical photometric and spectroscopic analysis on a 7′ 7′ field around the Southern high mass star forming region RCW 34. A previous study on RCW 34 in the NIR discov- ered many deeply embedded young stellar objects which were suspected to be T Tauri stars and which justified further investigation. The data used in this study consisted of three sets, the first two are photometric and spectroscopic data sets which were obtained during the first two weeks of February 2002. A third data set of spectroscopic observations was obtained by the author during the second week of 2011 of selected candidates using results from the NIR study and from the photometric data sets. All of the spectroscopy was conducted with the long slit spectrograph on the 1.9-m telescope and the photometry with DANDICAM on the 1.0-m telescope at the South African Astronomical Observatory (SAAO) in Sutherland. Objectives accomplished in the course of this study were to understand, ob- tain, reduce and interpret photometric and long slit spectroscopic CCD images. From the photometric results 57 stars showed excess blue emission on a colour-colour diagram which could be generated by circumstellar matter. The spectroscopic study showed 5 stars that showed H emission and 2 with strong Li absorption lines which confirm the suspicions of the NIR study about T Tauri stars in the region. All of the stars from the spectroscopic study in 2011 were identified as low-mass K or M type stars. Using colour-magnitude diagrams it was possible to see that the majority of the stars in the cluster are low-mass pre-main sequence stars. The stars matching between the optical and NIR filters were plotted on NIR colour-colour diagrams showing that the 5 stars that had H emission lines also had NIR colours characteristic to T Tauri stars. Out of the 5 stars that showed H emission, 2 were found to be classical T Tauris and three were found to be weak line T Tauris. / Thesis (MSc (Space Physics))--North-West University, Potchefstroom Campus, 2013

The notion that low- to intermediate-mass young stellar objects (YSOs) gain mass at a constant rate during the early stages of their evolution appears to be challenged by observations of YSOs suffering sudden increases of the rate at which they gain mass from their circumstellar discs. Also, this idea that stars spend most of their lifetime with a low accretion rate and gain most of their final mass during short-lived episodes of high accretion bursts, helps to solve some long-standing problems in stellar evolution. The original classification of eruptive variables divides them in two separate subclasses known as FU Orionis stars (FUors) and EX Lupi stars (EXors). In this classical view FUors are at an early evolutionary stage and are still gaining mass from their parent envelopes, whilst EXors are thought to be older objects only surrounded by an accretion disc. The problem with this classical view is that it excludes younger protostars which have higher accretion rates but are too deeply embedded in circumstellar matter to be observed at optical wavelengths. Optically invisible protostars have been observed to display large variability in the near-infrared. These and some recent discoveries of new eruptive variables, show characteristics that can be attributed to both of the optically-defined subclasses of eruptive variables. The new objects have been proposed to be part of a new class of eruptive variables. However, a more accepted scenario is that in fact the original classes only represent two extremes of the same phenomena. In this sense eruptive variability could be explained as arising from one physical mechanism, i.e. unsteady accretion, where a variation in the parameters of such mechanism can cause the different characteristics observed in the members of this class. With the aim of studying the incidence of episodic accretion among young stellar objects, and to characterize the nature of these eruptive variables we searched for high amplitude variability in two multi-epoch infrared surveys: the UKIDSS Galactic Plane Survey (GPS) and the Vista Variables in the Via Lactea (VVV). In order to further investigate the nature of the selected variable stars, we use photometric information arising from public surveys at near- to farinfrared wavelengths. In addition we have performed spectroscopic and photometric follow-up for a large subset of the samples arising from GPS and VVV. We analyse the widely separated two-epoch K-band photometry in the 5th, 7th and 8th data releases of the UKIDSS Galactic Plane Survey. We find 71 stars with _K > 1 mag, including 2 previously known OH/IR stars and a Nova. Even though the mid-plane is mostly excluded from the dataset, we find the majority (66%) of our sample to be within known star forming regions (SFRs), with two large concentrations in the Serpens OB2 association (11 stars) and the Cygnus-X complex (27 stars). The analysis of the multi-epoch K-band photometry of 2010-2012 data from VVV covering the Galactic disc at |b| < 1◦ yields 816 high amplitude variables, which include known variables of different classes such as high mass X-ray binaries, Novae and eclipsing binaries among others. Remarkably, 65% of the sample are found concentrated towards areas of star formation, similar to the results from GPS. In both surveys, sources in SFRs show spectral energy distributions (SEDs) that support classification as YSOs. This indicates that YSOs dominate the Galactic population of high amplitude infrared variable stars at low luminosities and therefore likely dominate the total high amplitude population. Spectroscopic follow-up allows us to confirm the pre-main sequence nature of several GPS and VVV Objects. Most objects in both samples show spectroscopic signatures that can be attributed to YSOs undergoing high states of accretion, such as veiling of photospheric features and CO emission, or show FUor-like spectra. We also find a large fraction of objects with 2.12 μm H2 emission that can be explained as arising from shock-excited emission caused by molecular outflows. Whether these molecular outflows are related to outbursts events cannot be confirmed from our data. Adding the GPS and VVV spectroscopic results, we find that between 6 and 14 objects are new additions to the FUor class from their close resemblance to the near-infrared spectra of FUors, and at least 23 more objects are new additions to the eruptive variable class. For most of these we are unable to classify them into any of the original definitions for this variable class. In any case, we are adding up to 37 new stars to the eruptive variable class which would double the current number of known objects. We note that most objects are found to be deeply embedded optically invisible stars, thus increasing the number of objects belonging to this subclass by a much larger factor. In general, objects in our samples which are found to be likely eruptive variable stars show a mixture of characteristics that can be attributed to both of the optically-defined classes. This agrees well with the recent discoveries in the literature. Finally, we are able to derive a first rough estimate on the incidence of episodic accretion among class I YSOs in the star-forming complex G305. We find that _ 9% of such objects are in a state of high accretion. This number is in agreement with previous theoretical and observational estimates among class I YSOs.

Este trabalho tem como objetivo principal a determinação da distância de estrelas pré-sequência principal em regiões de formação estelar próximas. A determinação precisa da distância individual das estrelas é necessária para obter os principais parâmetros físicos de cada estrela e para investigar a estrutura da Galáxia. Em particular, investigamos as regiões de formação estelar de Lupus e Ophiuchus que contém uma das associações mais ricas em estrelas T Tauri. A grande maioria das estrelas pré-sequência principal nessas regiões não foi observada pelo satélite Hipparcos devido à sua magnitude e também não têm paralaxe trigonométrica medida a partir do solo devido à distância em que se encontram. O procedimento aqui empregado para a obter a distância individual das estrelas baseia-se na estratégia de ponto de convergência e utiliza dados de movimento próprio e velocidade radial. Desenvolvemos uma nova versão do método de ponto de convergência que permite simultaneamente determinar a posição do ponto de convergência e selecionar os membros de um moving group. Partindo dos dados de movimento próprio e o novo método aqui desenvolvido investigamos as propriedades cinemáticas e realizamos uma análise de pertinência das estrelas em cada região estudada o que nos permitiu identificar um moving group com 114 estrelas em Lupus e 55 estrelas em Ophiuchus. Calculamos a distância para cada membro do grupo usando velocidades radiais publicadas, que foram complementadas com novas observações, e a velocidade espacial do moving grup para as estrelas com velocidade radial não conhecida. Calculamos as paralaxes com precisão de 1-2~mas o que implica em um erro relativo médio de 25% nas distâncias obtidas. Finalmente, investigamos as propriedades dos diversos subgrupos e a estrutura tridimensional dos complexos de nuvens em Lupus e Ophiuchus, concluindo que existem efeitos de profundidade importantes. Utilizamos os novos resultados de distância para obter os parâmetros físicos (luminosidade, massa e idade) das estrelas e o diagrama-HR de cada região de formação estelar considerada, confirmando a distribuição de idade diferente das duas subclasses de estrelas T Tauri. Os resultados aqui obtidos representam um primeiro passo no sentido de melhor entender a estrutura das regiões de formação estelar e os estágios iniciais da formação de estrelas e planetas. / The main objective of this work is to determine the distance of pre-main sequence stars in nearby star-forming regions. A precise determination of the distance to individual stars is required to accurately determine the main physical parameters of each star and the structure of the Galaxy. Here we investigate the Lupus and Ophiuchus star-forming regions that contain one of the richest associations of T Tauri stars. Most pre-main sequence stars in these regions were neither observed by the Hipparcos satellite due to their magnitude nor have any trigonometric parallax measured from the ground due to their distance. The procedure that we use here to derive the distance to individual stars is based on the convergent point strategy and makes full use of proper motion and radial velocity data. We developed a new version of the convergent point search method that simultaneously determines the convergent point position and selects the most likely members of a moving group. Based on proper motion data and our new method we investigate the kinematic properties and perform a membership analysis of the stars in each star-forming region considered that allows us to identify a moving group with 114 stars in Lupus and 55 stars in Ophiuchus. We calculate the distance of each group member using published radial velocities, which we supplemented with new measurements, and the spatial velocity of the moving group for the remaining stars with unknown radial velocity. We derived parallaxes with accuracies of 1-2 mas yielding the average relative error of 25% on the distances. Finally, we investigate the properties of the various subgroups and the three dimensional structure of the Lupus and Ophiuchus cloud complex and conclude that significant depth effects exist. We use the new distances to refine the physical parameters (luminosity, mass and age) of stars and the HR-diagram for each star-forming region considered confirming the different age distribution of the two T Tauri subclasses. These results represent a first step towards better understanding the structure of star-forming regions and the early stages of star and planet formation.