We present simultaneous Hubble Space Telescope (HST) WFC3+Spitzer IRAC variability monitoring for the highly variable young (similar to 20 Myr) planetary-mass object PSO J318.5-22. Our simultaneous HST + Spitzer observations covered approximately two rotation periods with Spitzer and most of a rotation period with the HST. We derive a period of 8.6. +/-. 0.1 hr from the Spitzer light curve. Combining this period with the measuredvsinifor this object, we find an inclination of 56 degrees.2. +/-. 8 degrees.1. We measure peak-to-trough variability amplitudes of 3.4%. +/-. 0.1% for Spitzer Channel 2 and 4.4%-5.8% (typical 68% confidence errors of similar to 0.3%) in the near-IR bands (1.07-1.67 mu m) covered by the WFC3 G141 prism-the mid-IR variability amplitude for PSO J318.5-22 is one of the highest variability amplitudes measured in the mid-IR for any brown dwarf or planetary-mass object. Additionally, we detect phase offsets ranging from 200 degrees to 210 degrees (typical error of similar to 4 degrees) between synthesized near-IR light curves and the Spitzer mid-IR light curve, likely indicating depth-dependent longitudinal atmospheric structure in this atmosphere. The detection of similar variability amplitudes in wide spectral bands relative to absorption features suggests that the driver of the variability may be inhomogeneous clouds (perhaps a patchy haze layer over thick clouds), as opposed to hot spots or compositional inhomogeneities at the top-of-atmosphere level.

Disks around brown dwarfs (BDs) are excellent laboratories to study the first steps of planet formation in cold and low-mass disk conditions. The radial-drift velocities of dust particles in BD disks higher than in disks around more massive stars. Therefore, BD disks are expected to be more depleted in millimeter-sized grains compared to disks around T Tauri or Herbig Ae/Be stars. However, recent millimeter observations of BD disks revealed low millimeter spectral indices, indicating the presence of large grains in these disks and challenging models of dust evolution. We present 3 mm photometric observations carried out with the IRAM/Plateau de Bure Interferometer (PdBI) of three BD disks in the Taurus star-forming region, which have been observed with ALMA at 0.89 mm. The disks were not resolved and only one was detected with enough confidence (similar to 3.5 sigma) with PdBI. Based on these observations, we obtain the values and lower limits of the spectral index and find low values (alpha(mm) less than or similar to 3.0). We compare these observations in the context of particle trapping by an embedded planet, a promising mechanism to explain the observational signatures in more massive and warmer disks. We find, however, that this model cannot reproduce the current millimeter observations for BD disks, and multiple-strong pressure bumps globally distributed in the disk remain as a favorable scenario to explain observations. Alternative possibilities are that the gas masses in the BD disk are very low (similar to 2 x 10(-3) M-Jup) such that the millimeter grains are decoupled and do not drift, or fast growth of fluffy aggregates.

>Magister Scientiae - MSc / This thesis reports the results of a twofold study on the recently proposed phenomenon of 'stellar pulsations' in young brown dwarfs by the seminal study of Palla and Baraffe (2005) (PB05, thereafter). The PB05 study presents results of a non-adiabatic linear stability analysis showing that young brown dwarfs should become pulsationally unstable during the deuterium burning phase of their evolution. The PB05 calculations on which this prediction is based have already been applied in a number of ground and space-based observational campaigns aimed at searching for this newly proposed putative class of potential pulsators. However, despite their significance and implications, the theoretical calculations by PB05 have not yet, to date, been subjected to independent verification in a different computational framework. To achieve this, we have generated equilibrium brown dwarf models and performed non-adiabatic linear stability calculations similar to PB05 assuming their 'frozen-in convection' approximation and the relevant input physics. The calculations performed in this thesis show, in overall, that there is a good agreement between the results from our study and those in PB05. However, there seem to be significant differences for very low mass objects as pointed out in our comparative results. We attribute this difference to our different boundary conditions. Our outer boundary condition is equivalent to the Eddington approximation in the 3-D case (e.g see Unno and Spiegel (1966)), while PB05 use a combination of different atmospheric profiles as discussed in Chabriel and Baraffe (2000). The validity of the frozen-in assumption used by PB05, which is based on the argument that the convective time scales calculated for these objects are much less than the pulsation time scales, has not been investigated. In this thesis, we have invoked a time-dependent theory of convection similar to Kuhfuss (1986) and Stellingwerf (1982) which includes turbulent pressure, turbulent diffusion and turbulent viscosity to study the pulsations. We have also investigated the effects of varying a number of free parameters in the above theoretical models. Our results show that turbulent pressure dominates in driving the pulsations in young brown dwarfs yielding growth rates much higher than in the frozen-in scenario. This is a new result that requires further analysis. The perturbation in the convective flux is found to have a damping effect on the acoustic modes. Turbulent viscosity is found to lead to damping which increases with increase in the value of the turbulent viscosity parameter and is found to have very little effect on the fundamental mode pulsation periods. Variation in the turbulent diffusion parameter has a very small effect on the fundamental mode periods and e-folding times. As a side lobe, we have determined theoretical pulsation constants for the fundamental mode and calculated the period ratios for the fundamental mode to those of the first and second harmonics. We find values of pulsation constants falling within the theoretical values calculated for variable stars shown in Cox (1980). This is explained in relation to the terms that go into the theoretical formula discussed later in this thesis. We find a correlation between the period ratios and the BDs mass and argue that such plots of the period ratios vs mass of the BDs could be useful in constraining the masses, given known periods from observations.

This paper includes a re-examination the coeval, multiple dwarf system Glies 569 in order to resolve the contradictory findings in the area of age. Absolute magnitudes in the f I and K bands are calculated, and are analyzed along with newly obtained dynamic mass estimates for the B components as well as other attributes of this system that have been well established in the literature. Ages are inferred by the placement of these objects on a variety of HR diagrams_ using the evolutionary models of Baraffe et al. These results strengthen the case for a possible third member of the Bab component of Gliese 569. The system's age is probably in the range of 100 to 500 million years; however, a more specific age estimate cannot yet be concluded. Because of the uncertainty associated with the dynamical mass estimates for the B component, the error bars associated with the age estimates are considerable. Finally, future research is suggested to better understand this hierarchical multiple system.

<p> Fragmentation during encounters between protostellar disks provides a possible scenario for the formation of substellar objects such as brown dwarfs and planets. A series of simulations of protostellar disk encounters were performed to investigate the fragmentation under different encounter parameters, and to characterize the properties of any resultant fragments. It was found that the initial disk minimum Toomre Q must satisfy Qini ;S 1.1 for the fragmentation to be induced by the encounters. Fragments of substellar mass can form via disk fragmentation, shock layer fragmentation and tidal tail fragmentation, and the effectiveness of each mechanism is closely related to the initial disk configuration. The fragmentation is also constrained by the relative encounter velocity since the number of fragments decreases quickly with increasing velocity. </p> <p> In comparing to previous studies of protostellar disk encounters it was also found that resolving both the local Jeans Mass during the encounter and the disks' vertical structure are critical to prevent artificial fragmentation and give the correct picture. Heating and cooling rates were estimated in both the optically thin and thick regimes. The comparison between the two indicates that during strong impacts the heating rate increases rapidly but is still comparable to the cooling rate, so the locally isothermal equation of state used in this study is an acceptable approximation. </p> <p> 32 clumps formed in various Qini = 0.9 disk-disk encounters were taken as the sample in an analysis of fragment properties and prospects for their further evolution. The results show that the clump masses are all less than the hydrogen burning mass limit ~ 0.075M0 , so the objects are substellar. Most of the clumps are of brown dwarf mass since the formation of planetary mass clumps is suppressed due to numerical resolution. The mass distribution is broadly consistent to the observed initial mass function in Pleiades. The clumps have highly flattened disk-like shapes and possess large spin angular momentum, which implies that young brown dwarfs may develop disks, jets, or planetary mass companions. About one third of the fragments are unbound to the stars and likely to form free floating brown dwarfs. Orbital analyses of the clumps which are bound to the stars show that there is a lack of close brown dwarf companions ( R < 3 AU), which is consistent to the observed "brown dwarf desert". Many of the orbits are highly eccentric and intersect with other orbits, so ejection of some clumps due to gravitational scattering is likely. Also, dispersion of gas during the encounter and the high spin angular momentum of the clumps may provide mechanisms other than ejection to prevent the clumps from accreting more mass, making the simulated clumps representative of the long term substellar mass function. </p> / Thesis / Master of Science (MSc)

In this thesis I present the spectroscopic analysis of a large sample of L and T dwarfs, in order to constrain the sub-stellar initial mass function and formation history. The main points I tried to address are the development of a better spectral type to distance calibration and of a better spectral type to effective temperature calibration, and the identification of a statistically complete sample of brown dwarf to be used to measure the luminosity function, and therefore to constrain the initial mass function and formation history. To achieve the first goal I conducted the spectroscopic follow-up of brown dwarfs from the PARallaxes of Southern Extremely Cool objects (PARSEC) program. This is a large astrometric campaign to measure the parallaxes and proper motions of 120 L and T dwarfs in the southern hemisphere. I combined the astrometric results with the near infra-red spectra I obtained using the Ohio State Infra-Red Imager/Spectrometer (OSIRIS) on the Southern Astrophysical Research telescope (SOAR). That allowed me to investigate the nature of some unresolved binaries and common proper motion companion in the sample, as well as sub-dwarfs candidates, and potential members of young moving groups. Combining the spectra with the astrometric information and the available photometry I derived the bolometric luminosity and effective temperature for the targets, and determined a new polynomial conversion between spectral type and effective temperature of a brown dwarfs. This is a fundamental step to compare the results of empirical observations to numerical simulations of the sub-stellar luminosity function. Once refined the type to temperature calibration, I measured the luminosity function. In order to do so my collaborators and I have selected a sample of 250 brown dwarfs candidates from the United Kingdom Deep Infra-red Sky Survey (UKIDSS) Large Area Survey (LAS) and followed them up with the echelle spectrograph X-shooter on the Very Large Telescope. I present in this thesis the results of the observations of 196 of the brown dwarfs candidates. Using the X-shooter spectra I determined their spectral types, and I identified a number of unresolved binary candidates and peculiar objects. One of the peculiar objects in the sample, ULAS J222711 004547, turned out to be the reddest brown dwarf observed so far, and I therefore proceeded to analyse further its spectrum. Applying a de-reddening technique to its spectrum suggests that the most likely reason for its redness is an excess of dust in its photosphere, and that can account for the differences seen between objects of similar spectral type. By comparing the results of the spectroscopic campaign to numerical simulations, I found that it is currently impossible to constrain robustly the initial mass function and formation history of sub-stellar objects, because of our limited knowledge of the binary fraction among brown dwarfs. The sample of binary candidates identified in this thesis can be used to place a better constraint on the binary fraction, but in order to do that the candidates need to be followed-up via high resolution imaging or radial velocity monitoring to confirm their binary nature.

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.

I have used blue near-infrared colours to select a group of UKIDSS T dwarfs with spectral types later than T4. From amongst these I identify two kinematic halo T-dwarf candi- dates. Blue near-infrared colours have been attributed to collisionally-induced hydrogen absorption, which is enhanced by either high surface gravity or low metallicity. Proper motions are measured and distances estimated, allowing the determination of tangential velocities. U and V components are estimated for our objects by assuming Vrad = 0. From this, ULAS J0926+0835 is found to have U = 62 kms−1 and V = −140 kms−1 and ULAS J1319+1209 is found to have U = 192 kms−1 and V = −92 kms−1. These values are consistent with potential halo membership. However, surprisingly, these are not the bluest objects in the selection. The bluest is ULAS J1233+1219, with J −K = −1.16±0.07, and surprisingly this object is found to have thin disc-like U and V . Our sample also contains Hip 73786B, which I find to be a companion to the star Hip 73786. Hip 73786 is a metal- poor star, with [Fe/H]= −0.3 ± 0.1 and is located at a distance of 19±0.7 pc. U, V,W space velocity components are calculated for Hip 73786A and B, finding that U = −48±7 kms−1, V = −75 ± 4 kms−1 and W = −44 ± 8 kms−1. From the properties of the pri- mary, Hip 73786B is found to be at least 1.6Gyr old. As a metal poor object, Hip 73786B represents an important addition to the sample of known T dwarf benchmarks. Using mid-infrared data from WISE, I also identify T dwarfs with abnormally-red H − W2 and consider possible causes for their extreme colours. In particular I exam- ine three prominent examples of this phenomenon, ULAS J1416+1348B, 2MASS J0939- 2448 and BD+01o 2920B. A plot of spectral type against MW2-magnitude suggests that ULAS J1416+1348B is potentially an unresolved binary, similar to 2MASS J0939-2448. However, the plot also indicates that BD+01o 2920B is not an unresolved binary. I also present new FIRE spectroscopy for ULAS J1416+1348B and 2MASS J0939-2448. These data show that ULAS J1416+1348B has a similar shape to the Y -band spectrum to that of BD+01o 2920B, thus suggesting that the two objects have a similar metallicity, whereas 2MASS J0939-2448 appears to be a more metal-rich object. Using a new parallactic dis- tance, I derive a luminosity of (6.9±0.7)×1020W for ULAS J1416+1348B. I also find a radial velocity of −39 ± 1 kms−1 for this object. The agreement between this and that of the L dwarf SDSS J1416+1348A confirms that these two objects are physically-associated. I also present a set of simulated unresolved binaries; the colours of these systems do not appear to redden significantly with the addition of cooler companions. From this, I suggest that the colours of ULAS J1416+1348B and BD+01o 2920B cannot be solely attributed to any possible unresolved companions; for these two objects, composition and/or surface gravity must be playing a substantial role. Consideration of model predictions provides extra evidence for this argument, showing as it does that high log g and low metallicity can redden H − W2 colours by as much as »0.5mag as compared to a high-metallicity and low log g object of the same effective temperature. I also present kinematics and photometry for several more new candidate low-metallicity T dwarfs. Spectra are also presented, where available. In addition I provide new follow-up JHK spectroscopy for ULAS J0926+0835, ULAS J1233+1219 and ULAS J1319+1209. These new spectra allow full JHK-based spectral typing for these objects.

The disk mass is among the most important input parameter for every planet formation model to determine the number and masses of the planets that can form. We present an ALMA 887 mu m survey of the disk population around objects from similar to 2 to 0.03 M-circle dot in the nearby similar to 2 Myr old Chamaeleon I star-forming region. We detect thermal dust emission from 66 out of 93 disks, spatially resolve 34 of them, and identify two disks with large dust cavities of about 45 au in radius. Assuming isothermal and optically thin emission, we convert the 887 mu m flux densities into dust disk masses, hereafter M-dust. We find that the M-dust-M* relation is steeper than linear and of the form M-dust proportional to (M*)(1.3-1.9), where the range in the power-law index reflects two extremes of the possible relation between the average dust temperature and stellar luminosity. By reanalyzing all millimeter data available for nearby regions in a self-consistent way, we show that the 1-3 Myr old regions of Taurus, Lupus, and Chamaeleon. I share the same M-dust-M* relation, while the 10 Myr old Upper. Sco association has a steeper relation. Theoretical models of grain growth, drift, and fragmentation reproduce this trend and suggest that disks are in the fragmentation-limited regime. In this regime millimeter grains will be located closer in around lower-mass stars, a prediction that can be tested with deeper and higher spatial resolution ALMA observations.

Recent photometric studies have revealed that surface spots that produce flux variations are present on virtually all L and T dwarfs. Their likely magnetic or dusty nature has been a much-debated problem, the resolution to which has been hindered by paucity of diagnostic multi-wavelength observations. To test for a correlation between magnetic activity and photometric variability, we searched for Ha emission among eight L3-T2 ultra-cool dwarfs with extensive previous photometric monitoring, some of which are known to be variable at 3.6 mu m or 4.5 mu m. We detected Ha only in the non-variable T2 dwarf 2MASS J12545393-0122474. The remaining seven objects do not show Ha emission, even though six of them are known to vary photometrically. Combining our results with those for 86 other L and T dwarfs from the literature show that the detection rate of Ha emission is very high (94%) for spectral types between L0 and L3.5 and much smaller (20%) for spectral types. >= L4, while the detection rate of photometric variability is approximately constant (30%-55%) from L0 to T8 dwarfs. We conclude that chromospheric activity, as evidenced by H alpha emission, and large-amplitude photometric variability are not correlated. Consequently, dust clouds are the dominant driver of the observed variability of ultra-cool dwarfs at spectral types, at least as early as L0.