La détermination de l'extrémité inférieure de la fonction de masse initiale (FMI) prévoit de fortes contraintes sur les théories de la formation des étoiles. IC4665 est un amas d'´étoile jeune (30Myr) et il a situe 356pc de la Terre. L'extinction est Av~ 0.59 ± 0.15 mag. WIRCam Y, J, H et K observations ont été faites par le CFHT et a comprise 10 champs (de 1.1sq.deg totale) et deux zones de contrle de 20'x20' chacun. Diagrammes couleur/magnitude et couleur/couleur ont été utilisées pour comparer les candidats sélectionnées par les modèles BT-SETTL 30 et 50Myr. Les images CH4off et CH4on ont été obtenus avec CFHT/WIRCam plus 0.11 sq.deg. dans IC348. Naines-T ont ensuite été identifiés à partir de leur couleur de 1.69μm d'absorption du méthane et trois candidats nain-T ont été trouvée avec CH4on−CH4 >0.4 mag. Extinction a été estimée à Av~ 5 − 12 mag. Les comparaisons avec les naines-T modèles, et des diagrammes couleur/couleur et magnitude, rejeter 2 entre 3 candidats en raison de leur extrême z′ − J coleur. L'objet reste n'est pas considéré comme un nain avant l'amas en raison d'un argument de densité en nombre ou l'extinction forte Av~ 12 mag, ni d'être un champ de fond nain-T qui serait devrait être beaucoup plus faible. Les modèles et les schémas de donner cet objet un type T6 préliminaires spectrale. Avec un peu de la masse de Jupiter, ce jeune candidat nain-T est potentiellement parmi les plus jeunes, des objets de masse plus faible détectée dans une région de formation d'´étoiles `a ce jour. Sa fréquence est conforme à l'extrapolation du courant lognormal FMI estime `a au domaine de masse planétaire.

This thesis is an investigation into some properties of brown dwarfs using medium-resolution spectroscopy. In the first part of the thesis, I address the issue of parameter degeneracy in brown dwarfs. In the course of my analysis, I derive a gravity-sensitive spectral index which can be used, statistically at least, to differentiate populations of young objects from field dwarfs. The index is also capable of finding the difference between a population of ~1 Myr objects and a population of ~10 Myr objects and may be used to separate low-mass members from foreground and background objects in young clusters and associations. The second part of my thesis is an investigation into the major opacity sources in the atmospheres of late T dwarfs. I look particularly at CH4 and NH3 absorption features in the near-infrared spectra of these objects. In my analysis, I identify new absorption features produced by these molecules. I also correct features which had previously been wrongly identified. This has been made possible by the use of high quality data, together with a new CH4 synthetic line list, which is more complete at these temperatures than any previously available list.

Stars are born from dense cores of gas within molecular clouds. The exact nature of the connection between these gas cores and the stars they form is an important issue in the field of star formation. In this thesis I use numerical simulations of molecular clouds to trace the evolution of cores into stars. The CLUMPFIND method, commonly used to identify gas structures is tested. I find that the core boundaries it yields are unreliable, but in spite of this, the same profile is universally found for the mass function. To facilitate a more robust definition of a core, a modified clumpfind algorithm which uses gravitational potential instead of density is introduced. This allows the earliest fragmentation in a simulated molecular cloud to be identified. The first bound cores have a mass function that closely resembles the stellar IMF, but there is a poor correspondence between individual core masses and the stellar masses formed from them. From this, it is postulated that environmental factors play a significant part in a core’s evolution. This is particularly true for massive stars, as massive cores are prone to further fragmentation. In these simulations, massive stars are formed simultaneously with stellar clusters, and thus the evolution of one can affect the other. In particular, the global collapse of the forming cluster aids accretion by the precursors of the massive stars. By tracing the evolution of the massive stars, I find that most of the material accreted by them comes from diffuse gas, rather than from a well-defined stellar core.

Within this thesis, we present an extended multiwavelength analysis of the rich massive Galactic star-forming complex G305. We have focused our attention on studying the both the embedded massive star-forming population within G305, while also identifying the intermediate-, to lowmass content of the region also. Though massive stars play an important role in the shaping and evolution of their host galaxies, the physics of their formation still remains unclear. We have therefore set out to studying the nature of star formation within this complex, and also identify the impact that such a population has on the evolution of G305. We firstly present a Herschel far-infrared study towards G305, utilising PACS 70, 160 μm and SPIRE 250, 350, and 500 μm observations from the Hi-GAL survey of the Galactic plane. The focus of this study is to identify the embedded massive star-forming population within G305, by combining far-infrared data with radio continuum, H2O maser, methanolmaser,MIPS, and Red MSX Source survey data available from previous studies. From this sample we identify some 16 candidate associations are identified as embedded massive star-forming regions, and derive a two-selection colour criterion from this sample of log(F70/F500)! 1 and log(F160/F350)! 1.6 to identify an additional 31 embedded massive star candidates with no associated starformation tracers. Using this result, we are able to derive a star formation rate (SFR) of 0.01 - 0.02 M! yr−1. Comparing this resolved star formation rate, to extragalactic star formation rate tracers (based on the Kennicutt-Schmidt relation), we find the star formation activity is underestimated by a factor of !2 in comparison to the SFR derived from the YSO population. By next combining data available from 2MASS and VVV, Spitzer GLIMPSE and MIPSGAL, MSX, and Herschel Hi-GAL, we are able to identify the low-, to intermediate-mass YSOs present within the complex. Employing a series of stringent colour selection criteria and fitting reddened stellar atmosphere models, we are able remove a significant amount of contaminating sources from our sample, leaving us with a highly reliable sample of some 599 candidate YSOs. From this sample, we derive a present-day SFR of 0.005±0.001M! yr−1, and find the YSOmass function (YMF) of G305 to be significantly steeper than the standard Salpeter-Kroupa IMF. We find evidence of mass segregation towards G305, with a significant variation of the YMF both with the active star-forming region, and the outer region. The spatial distribution, and age gradient, of our 601 candidate YSOs also seem to rule out the scenario of propagating star formation within G305, with a more likely scenario of punctuated star formation over the lifetime of the complex.

We study the galaxy populations in 74 Sunyaev-Zeldovich effect selected clusters from the South Pole Telescope survey, which have been imaged in the science verification phase of the Dark Energy Survey. The sample extends up to z similar to 1.1 with 4 x 10(14)M(circle dot) <= M-200 <= 3 x 10(15)M(circle dot). Using the band containing the 4000 angstrom break and its redward neighbour, we study the colour-magnitude distributions of cluster galaxies to similar to m(*) + 2, finding that: (1) The intrinsic rest frame g - r colour width of the red sequence (RS) population is similar to 0.03 out to z similar to 0.85 with a preference for an increase to similar to 0.07 at z = 1, and (2) the prominence of the RS declines beyond z similar to 0.6. The spatial distribution of cluster galaxies is well described by the NFW profile out to 4R(200) with a concentration of c(g) = 3.59(-0.18)(+0.20), 5.37(-0.24)(+0.27) and 1.38(-0.19)(+0.21) for the full, the RS and the blue non-RS populations, respectively, but with similar to 40 per cent to 55 per cent cluster to cluster variation and no statistically significant redshift or mass trends. The number of galaxies within the virial region N-200 exhibits a mass trend indicating that the number of galaxies per unit total mass is lower in the most massive clusters, and shows no significant redshift trend. The RS fraction within R-200 is (68 +/- 3) per cent at z = 0.46, varies from similar to 55 per cent at z = 1 to similar to 80 per cent at z = 0.1 and exhibits intrinsic variation among

We present measurements of the clustering of galaxies as a function of their stellar mass in the Baryon Oscillation Spectroscopic Survey. We compare the clustering of samples using 12 different methods for estimating stellar mass, isolating the method that has the smallest scatter at fixed halo mass. In this test, the stellar mass estimate with the smallest errors yields the highest amplitude of clustering at fixed number density. We find that the PCA stellar masses of Chen et al. clearly have the tightest correlation with halo mass. The PCA masses use the full galaxy spectrum, differentiating them from other estimates that only use optical photometric information. Using the PCA masses, we measure the large-scale bias as a function of M-* for galaxies with logM(*) >= 11.4, correcting for incompleteness at the low-mass end of our measurements. Using the abundance matching ansatz to connect dark matter halo mass to stellar mass, we construct theoretical models of b(M-*) that match the same stellar mass function but have different amounts of scatter in stellar mass at fixed halo mass, sigma(logM*). Using this approach, we find sigma(logM*) = 0.18(+0.01) (-0.02). This value includes both intrinsic scatter as well as random errors in the stellar masses. To partially remove the latter, we use repeated spectra to estimate statistical errors on the stellar masses, yielding an upper limit to the intrinsic scatter of 0.16 dex.

Previous studies have shown that thermal feedback from protostars plays a key role in the process of low-mass star formation. In this thesis, we explore the effects of protostellar heating on the formation of stellar clusters. We describe new methods for modelling protostellar accretion luminosities and protostellar evolution in calculations of star formation. We then present results of a series of numerical simulations of stellar cluster formation which include these effects, and examine their impact. We begin by investigating the dependence of stellar properties on the initial density of molecular clouds. We find that the dependence of the median stellar mass on the initial density of the cloud is weaker than the dependence of the thermal Jeans mass when radiative effects are included. We suggest that including protostellar accretion luminosities and protostellar evolution may weaken this dependence further, and may account for the observed invariance of the median stellar mass in Galactic star-forming regions. Next, we investigate the effects of including accretion feedback from sink particles on the formation of small stellar groups. We find that including accretion feedback in calculations suppresses fragmentation even further than calculations that only include radiative transfer within the gas. Including feedback also produces a higher median stellar mass, which is insensitive to the sink particle accretion radius used. Finally, we compare calculations of small stellar clusters which model the evolution of protostars using a live stellar model with those which use a fixed stellar structure. We find that the dynamics of the clusters are primarily determined by the accretion luminosities of protostars, but that the relative effects of protostellar evolution depend on the accretion rate and advection of energy into the protostar. We also demonstrate how such calculations may be used to study the properties of young stellar populations.

Most proposed regulation methods do not take advantage of the state information of the underlying scheduler, resulting in a waste of resources. We propose a dynamic regulation approach in which the regulation function is modulated by both the tagged stream's characteristics and the state information fed-back from the scheduler. The transmission speed of a regulator is accelerated when too much traffic has been sent to the scheduler by the other regulators or when the scheduler's queue is empty. As a result, the mean delay of the traffic can be reduced and the scheduler's throughput can be increased. Since no complicated computation is involved, our approach is suitable for the use in high-speed networks.

Using data from the SCUBA Legacy Catalogue (850 µm) and Spitzer (3.6 - 70 µm), we explore dense cores in the Ophiuchus, Taurus, Perseus, Serpens, and Orion molecular clouds. In particular, we focus on identifying which cores host young stars while others remain starless. Understanding the nature of star formation and the influence of local environment will give us insight into several key properties, such as the origin of stellar mass. Here, we present starless and protostellar core mass functions (CMFs) for the five clouds. We develop a new method to discriminate starless from protostellar cores, using Spitzer colours and positions. We found best-fit slopes to the high-mass end of −1.26±0.20, −1.22±0.06, −0.95±0.20, and −1.85±0.53 for Ophiuchus, Taurus, Perseus, and Orion, respectively. We were unable to fit a slope to our fifth cloud, Serpens. Broadly, these slopes are consistent with the −1.35 power-law seen in the Salpeter IMF, but suggest some differences. We examined a variety of trends between these CMF shapes and their parent cloud properties, potentially finding a correlation between the high-mass slope and temperature. We also attempt to predict what future surveys with SCUBA-2 will detect in each of our clouds.

Population synthesis models (PSMs) are fundamental tools to study the star formation history and IMF of unresolved stellar populations using spectral features. This work presents a new set of PSMs constructed using theoretical isochrones and two state-ofthe- art synthetic spectral libraries. The BT-Settl and Munari libraries were chosen for their ability to predict the observed values of Lick-type and IMF-sensitive indices in individual stars of the solar neighbourhood. The BT-Settl library was used to sample the cool main sequence stars and the Munari library for the rest of the evolutionary phases. The PSMs cover a range of metallicities with [Fe/H]= 0, -1.31 and -1.81 for scaled-solar and α-enhanced metal mixtures. The models were used to study the behaviour of the IMF indices defined in the literature and the results are in good agreement with what other PSMs have determined. The PSMs in this work predict a strong degeneracy between age, metallicity and IMF. I used the models to study which are the main evolutionary phases contributing to each IMF-sensitive index and found that most indices reach their final integrated values before the turn off. The post-main sequence stars contribute mainly to the continuum of these bands. Uncertainties in the the effective temperature of the isochrones can affect IMF estimates. The PSMs were applied to extragalactic globular clusters (GCs) and early-type galaxies (ETGs) using data from the literature. I determined the ages, metallicities and IMFs of these systems using index combinations in the optical and infrared. I explored how the morphology of the Horizontal Branch (HB) and dynamical evolution (which are key uncertainties in the modelling of GCs) can affect the IMF predictions. In a population with a Milky Way IMF, dynamical evolution can make the IMF indices mimic a bottom-light IMF. HB morphology has no impact on the IMF estimates at low [Fe/H]. In the IMF index-index diagrams for GCs, the results are significantly affected by the unknown sodium abundances of these systems. Using the PSMs in this work the best index combination to determine the IMF is CaH1 and TiO2. The ETGs and the [Fe/H]=0 GCs appear to have a bottom-heavy IMF with x ~ 3:0. These results are discussed in the work.