A white dwarf is most likely the end stage of a low mass star like our Sun, which results when the parent star consumes all the hydrogen in its core, thus bringing fusion to a halt. It is a dense and compact object, where the inward gravitational pull is balanced by the outward pressure arising due to the motion of its constituent degenerate electrons. The theory of non-magnetized and non-rotating white dwarfs was formulated extensively by S. Chandrasekhar in the 1930s, who also proposed a maximum possible mass for this objects, known as the Chandrasekhar limit (Chandrasekhar 1935)1. White dwarfs are believed to be the progenitors of extremely bright explosions called type Ia supernovae (SNeIa). SNeIa are extremely important and popular astronomical events, which are hypothesized to be triggered in white dwarfs having mass close to the famous Chandrasekhar limit ∼ 1.44M⊙. The characteristic nature of the variation of luminosity with time of SNeIa is believed to be powered by the decay of 56Ni to 56Co and, finally, to 56Fe. This feature, along with the consistent mass of the exploding white dwarf, is deeply linked with their utilization as “standard candles” for cosmic distance measurement. In fact, SNeIa measurements were instrumental in establishing the accelerated nature of the current expansion of the universe (Perlmutter et al. 1999). However, several recently observed peculiar SNeIa do not conform to this traditional explanation. Some of these SNeIa are highly over-luminous, e.g. SN 2003fg, SN 2006gz, SN 2007if, SN 2009dc (Howell et al. 2006; Scalzo et al. 2010), and some others are highly under-luminous, e.g. SN 1991bg, SN 1997cn, SN 1998de, SN 1999by, SN 2005bl (Filippenko et al. 1992; Taubenberger et al. 2008). The luminosity of the former group of SNeIa implies a huge Ni-mass (often itself super-Chandrasekhar), invoking highly super-Chandrasekhar white dwarfs, having mass 2.1 − 2.8M⊙, as their most plausible progenitors (Howell et al. 2006; Scalzo et al. 2010). On the other hand, the latter group produces as low as ∼ 0.1M⊙ of Ni (Stritzinger et al. 2006), which rather seem to favor sub-Chandrasekhar explosion scenarios. In this thesis, as the title suggests, we have endeavored to establish the existence of exotic, super- and sub-Chandrasekhar limiting mass white dwarfs, in order to explain the aforementioned peculiar SNeIa. This is an extremely important puzzle to solve in order to comprehensively understand the phenomena of SNeIa, which in turn is essential for the correct interpretation of the evolutionary history of the universe. Effects of magnetic field: White dwarfs have been observed to be magnetized, having surface fields as high as 105 − 109 G (Vanlandingham et al. 2005). The interior field of a white dwarf cannot be probed directly but it is quite likely that it is several orders of magnitude higher than the surface field. The theory of weakly magnetized white dwarfs has been investigated by a few authors, however, their properties do not starkly contrast with that of the non-magnetized cases (Ostriker & Hartwick 1968). In our venture to find a fundamental basis behind the formation of super-Chandrasekhar white dwarfs, we have explored in this thesis the impact of stronger magnetic fields on the properties of white dwarfs, which has so far been overlooked. We have progressed from a simplistic to a more rigorous, self-consistent model, by adding complexities step by step, as follows: • spherically symmetric Newtonian model with constant (central) magnetic field • spherically symmetric general relativistic model with varying magnetic field • model with self-consistent departure from spherical symmetry by general relativis-tic magnetohydrodynamic (GRMHD) numerical modeling. We have started by exploiting the quantum mechanical effect of Landau quanti-zation due to a maximum allowed equipartition central field greater than a critical value Bc = 4.414 × 1013 G. To begin with, we have carried out the calculations in a Newtonian framework assuming spherically symmetric white dwarfs. The primary ef-fect of Landau quantization is to stiffen the equation of state (EoS) of the underlying electron degenerate matter in the high density regime, and, hence, yield significantly super-Chandrasekhar white dwarfs having mass much & 2M⊙ (Das & Mukhopadhyay 2012a,b). Consequently, we have proposed a new mass limit for magnetized white dwarfs which may establish the aforementioned peculiar, over-luminous SNeIa as new standard candles (Das & Mukhopadhyay 2013a,b). We have furthermore predicted possible evo-lutionary scenarios by which super-Chandrasekhar white dwarfs could form by accretion on to a commonly observed magnetized white dwarf, by invoking the phenomenon of flux freezing, subsequently ending in over-luminous, super-Chandrasekhar SNeIa (Das et al. 2013). Before moving on to a more complex model, we have justified the assumptions in our simplistic model, in the light of various related physics issues (Das & Mukhopad-hyay 2014b), and have also clarified, and, hence, removed some serious misconceptions regarding our work (Das & Mukhopadhyay 2015c). Next, we have considered a more self-consistent general relativistic framework. We have obtained stable solutions of magnetostatic equilibrium models for white dwarfs pertaining to various magnetic field profiles, however, still in spherical symmetry. We have showed that in this framework, a maximum stable mass as high as ∼ 3.3M⊙ can be realized (Das & Mukhopadhyay 2014a). However, it is likely that the anisotropic effect due to a strong magnetic field may cause a deformation in the spherical structure of the white dwarfs. Hence, in order to most self-consistently take into account this departure from spherical symmetry, we have constructed equilibrium models of strongly magnetized, static, white dwarfs in a general relativistic framework, first time in the literature to the best of our knowledge. In order to achieve this, we have modified the GRMHD code XNS (Pili et al. 2014), to apply it in the context of white dwarfs. Interestingly, we have found that signifi-cantly super-Chandrasekhar white dwarfs, in the range ∼ 1.7 − 3.4M⊙, are obtained for many possible field configurations, namely, poloidal, toroidal and mixed (Das & Mukhopadhyay 2015a). Furthermore, due to the inclusion of deformation caused by a strong magnetic field, super-Chandrasekhar white dwarfs are obtained for relatively lower central magnetic field strengths (∼ 1014 G) compared to that in the simplistic model — as correctly speculated in our first work of this series (Das & Mukhopadhyay 2012a). We have also found that although the characteristic deformation induced by a purely toroidal field is prolate, the overall shape remains quasi-spherical — justifying our earlier spherically symmetric assumption while constructing at least some models of strongly magnetized white dwarfs (Das & Mukhopadhyay 2014a). Indeed more accurate and extensive numerical analysis seems to have validated our analytical findings. Thus, very interestingly, our investigation has established that magnetized white dwarfs can indeed have mass that significantly exceeds the Chandrasekhar limit, irre-spective of the origin of the underlying magnetic effect — a discovery which is not only of theoretical importance, but also has a direct astrophysical implication in explaining the progenitors of the peculiar, over-luminous, super-Chandrasekhar SNeIa. Effects of modified Einstein’s gravity: A large array of models has been required to explain the peculiar, over- and under- luminous SNeIa. However, it is unlikely that nature would seek mutually antagonistic scenarios to exhibit sub-classes of apparently the same phenomena, i.e., triggering of thermonuclear explosions in white dwarfs. Hence, driven by the aim to establish a unification theory of SNeIa, we have invoked in the last part of this thesis a modification to Einstein’s theory of general relativity in white dwarfs. The validity of general relativity has been tested mainly in the weak field regime, for example, through laboratory experiments and solar system tests. However, the question remains, whether general relativity requires modification in the strong gravity regime, such as, the expanding universe, the region close to a black hole and neutron star. For instance, there is evidence from observational cosmology that the universe has undergone two epochs of cosmic acceleration, the theory behind which is not yet well understood. The period of acceleration in the early universe is known as inflation, while the current accelerated expansion is often explained by invoking a mysterious dark energy. An alternative approach to explain the mysteries of inflation and dark energy is to modify the underlying gravitational theory itself, as it conveniently avoids involving any exotic form of matter. Several modified gravity theories have been proposed which are extensions of Einstein’s theory of general relativity. A popular class of such theories is known as f (R) gravity (e.g. see de Felice & Tsujikawa 2010), where the Lagrangian density f of the gravitational field is an arbitrary function of the Ricci scalar R. In the context of astrophysical compact objects, so far, modified gravity theories have been applied only to neutron stars, which are much more compact than white dwarfs, in order to test the validity of such theories in the strong field regime (e.g. Cooney et al. 2010; Arapoˇglu et al. 2011). Moreover, a general relativistic correction itself does not seem to modify the properties of a white dwarf appreciably when compared to Newtonian calculations. Our venture of exploring modified gravity in white dwarfs in this thesis, is a first in the literature to the best of our knowledge. We have exploited the advantage that white dwarfs have over neutron stars, i.e., their EoS is well established. Hence, any change in the properties of white dwarfs can be solely attributed to the modification of the underlying gravity, unlike in neutron stars, where similar effects could be produced by invoking a different EoS. We have explored a popular, yet simple, model of f (R) gravity, known as the Starobinsky model (Starobinsky 1980) or R−squared model, which was originally pro-posed to explain inflation. Based on this model, we have first shown that modified gravity reproduces those results which are already explained in the paradigm of general relativity (and Newtonian framework), namely, low density white dwarfs in this context. This is a very important test of the modified gravity model and is furthermore necessary to constrain the underlying model parameter. Next, depending on the magnitude and sign of a single model parameter, we have not only obtained both highly super-Chandrasekhar and highly sub-Chandrasekhar limiting mass white dwarfs, but we have also established them as progenitors of the peculiar, over- and under-luminous SNeIa, respectively (Das & Mukhopadhyay 2015b). Thus, an effectively single underlying the-ory unifies the two apparently disjoint sub-classes of SNeIa, which have so far hugely puzzled astronomers. To summarize, in the first part of the thesis, we have established the enormous significance of magnetic fields in white dwarfs in revealing the existence of significantly super-Chandrasekhar white dwarfs. These super-Chandrasekhar white dwarfs could be ideal progenitors of the peculiar, over-luminous SNeIa, which can, hence, be used as new standard candles of cosmic distance measurements. In the latter part of the thesis, we have established the importance of a modified theory of Einstein’s gravity in revealing both highly super- and highly sub-Chandrasekhar limiting mass white dwarfs. We have furthermore demonstrated how such a theory can serve as a missing link between the peculiar, super- and sub-Chandrasekhar SNeIa. Thus, the significance of the current thesis lies in the fact that it not only questions the uniqueness of the Chandrasekhar mass-limit for white dwarfs, but it also argues for the need of a modified theory of Einstein’s gravity to explain astrophysical observations.

Parallel programming models are quite challenging and emerging topic in the parallel computing era. These models allow a developer to port a sequential application on to a platform with more number of processors so that the problem or application can be solved easily. Adapting the applications in this manner using the Parallel programming models is often influenced by the type of the application, the type of the platform and many others. There are several parallel programming models developed and two main variants of parallel programming models classified are shared and distributed memory based parallel programming models. The recognition of the computing applications that entail immense computing requirements lead to the confrontation of the obstacle regarding the development of the efficient programming models that bridges the gap between the hardware ability to perform the computations and the software ability to support that performance for those applications [25][9]. And so a better programming model is needed that facilitates easy development and on the other hand porting high performance. To answer this challenge this thesis confines and compares four different shared memory based parallel programming models with respect to the development time of the application under a shared memory based parallel programming model to the performance enacted by that application in the same parallel programming model. The programming models are evaluated in this thesis by considering the data parallel applications and to verify their ability to support data parallelism with respect to the development time of those applications. The data parallel applications are borrowed from the Dense Matrix dwarfs and the dwarfs used are Matrix-Matrix multiplication, Jacobi Iteration and Laplace Heat Distribution. The experimental method consists of the selection of three data parallel bench marks and developed under the four shared memory based parallel programming models considered for the evaluation. Also the performance of those applications under each programming model is noted and at last the results are used to analytically compare the parallel programming models. Results for the study show that by sacrificing the development time a better performance is achieved for the chosen data parallel applications developed in Pthreads. On the other hand sacrificing a little performance data parallel applications are extremely easy to develop in task based parallel programming models. The directive models are moderate from both the perspectives and are rated in between the tasking models and threading models. / From this study it is clear that threading model Pthreads model is identified as a dominant programming model by supporting high speedups for two of the three different dwarfs but on the other hand the tasking models are dominant in the development time and reducing the number of errors by supporting high growth in speedup for the applications without any communication and less growth in self-relative speedup for the applications involving communications. The degrade of the performance by the tasking models for the problems based on communications is because task based models are designed and bounded to execute the tasks in parallel without out any interruptions or preemptions during their computations. Introducing the communications violates the purpose and there by resulting in less performance. The directive model OpenMP is moderate in both aspects and stands in between these models. In general the directive models and tasking models offer better speedup than any other models for the task based problems which are based on the divide and conquer strategy. But for the data parallelism the speedup growth however achieved is low (i.e. they are less scalable for data parallel applications) are equally compatible in execution times with threading models. Also the development times are considerably low for data parallel applications this is because of the ease of development supported by those models by introducing less number of functional routines required to parallelize the applications. This thesis is concerned about the comparison of the shared memory based parallel programming models in terms of the speedup. This type of work acts as a hand in guide that the programmers can consider during the development of the applications under the shared memory based parallel programming models. We suggest that this work can be extended in two different ways: one is from the developer‘s perspective and the other is a cross-referential study about the parallel programming models. The former can be done by using a similar study like this by a different programmer and comparing this study with the new study. The latter can be done by including multiple data points in the same programming model or by using a different set of parallel programming models for the study. / C/O K. Manoj Kumar; LGH 555; Lindbloms Vägan 97; 37233; Ronneby. Phone no: 0738743400 Home country phone no: +91 9948671552

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.

In this thesis we address fundamental questions about what constitutes and limits an HI disc, probing the distribution of neutral gas in the outer parts of galaxies. We use a subsample of galaxies observed as part of the THINGS survey to investigate the HI extent of spiral galaxy discs. We revisit previous work on the extent of HI discs, showing the limitations set by insufficient linear resolution. We then exploit the high spatial and velocity resolution combined with good sensitivity of THINGS to investigate where the atomic gas discs end and what might shape their edges. We find that the atomic gas surface density across most of the disc is constant at 5 – 10 x 10^20 atoms/cm^2 and drops sharply at large radius. The general shape of the HI distribution is well described by a Sérsic-type function with a slope index, n = 0.14 - 0.22 and characteristic radius ri. We propose a new column density threshold of 5 x 10^19 atoms/cm^2 to define the extent of the gas disc. This limit is well within reach of modern instruments and is at the level where disc gas becomes susceptible to ionisation by an extragalactic radiation field. We argue that at this level the HI column density has decreased to one tenth of that across the inner disc and that by going to yet lower column density the disc is unlikely to grow by more than 10% in radius. The HI column density at which the radial profiles turn over is too high for it to be caused by ionisation by an extragalactic UV field and we postulate that the HI extent is set by how galaxy discs form. Ionisation by extragalactic radiation will only play a rôle at column densities below 5 x 10^19 atoms/cm^2, if any. To study the crucial relation between observed edges and how closely these reproduce the intrinsic distribution of gas through our interferometric measurements, we created an ensemble of models based on four radial density distributions. We conclude that the observed edges in spiral galaxies faithfully reflect their intrinsic shape. Only in very specific cases of highly inclined (>75º) and/or large vertical scaleheight discs do we see strong deviations from the intrinsic surface density of the observed shape of the edges in spiral galaxies. In the case of NGC 3198 we concluded that there is no significant difference in the radial profiles obtained with either constant or exponentially increasing vertical gas distributions, when scaleheights are not higher than 1 kpc at the outskirts of the disc. We infer an upper limit to the scaleheight of NGC 3198 of 2 kpc. To address the distribution of neutral gas at larger scales, we study an HI rich, giant LSB galaxy, NGC 765. We present HI spectral line and radio-continuum VLA data, complemented by optical and Chandra X-ray maps. NGC 765 has the largest HI-to-optical ratio known to date of any spiral galaxy and one of the largest known HI discs in absolute size with a diameter of ~ 240 kpc measured at a surface density of 2 x 10^19 atoms/cm^2. We derive a total HI mass of M_HI = 4.7 x 10^10 M_sun, a dynamical mass of M_dyn = 5.1 x 10^11 M_sun and an HI mass to luminosity ratio of M_HI/L_B = 1.6, making it the nearest and largest “crouching giant”. Optical images reveal evidence of a central bar with tightly wound low-surface brightness spiral arms extending from it. Radio-continuum (L_1.4 GHz = 1.3 x 10^21 W/Hz) and X-ray (L_x ~ 1.7 x 10^40 erg/s) emission is found to coincide with the optical core of the galaxy, compatible with nuclear activity powered by a low-luminosity AGN. We may be dealing with a galaxy that has retained in its current morphology traces of its formation history. In fact, it may still be undergoing some accretion, as evidenced by the presence of HI clumps the size (< 10 kpc) and mass (10^8 -10^9 M_sun) of small (dIrr) galaxies in the outskirts of its HI disc and by the presence of two similarly sized companions. In an exploration of future work, we engaged in a study of the edges in the HI discs of dwarf irregular galaxies, their parameterisation and simulation. A collection of simulations were created based on the dwarf galaxy NGC 2366, similar to what was done for the larger spiral galaxies, showing that line-of-sight column densities are affected for discs with inclinations higher than 60º. Five out of eleven of the dwarfs from THINGS which are less inclined than 60º were analysed and parameterised with our Sérsic-type function. Their discs have average central column densities spread evenly from log_10 NHI = 20.7 atoms/cm^2 to log_10 NHI =21.3 atoms/cm^2. Their radial decline is shallower (slope index peaks around n ~ 0.3) than for spirals. The up-coming Local Irregular That Trace Local Extremes (LITTLE) THINGS project, will likely enlarge the number of local dwarf irregular (dIm) galaxies to which this type of analysis can be applied and for which these preliminary results verified.

Sirius, the seventh-nearest stellar system, is a visual binary containing the metallic-line A1. V star Sirius. A, the brightest star in the sky, orbited in a 50.13. year period by Sirius B, the brightest and nearest white dwarf (WD). Using images obtained over nearly two decades with the Hubble Space Telescope (HST), along with photographic observations covering almost 20 years and nearly 2300 historical measurements dating back to the 19th century, we determine precise orbital elements for the visual binary. Combined with the parallax and the motion of the A component, these elements yield dynamical masses of 2.063 +/- 0.023 M circle dot and 1.018 +/- 0.011 M circle dot for Sirius. A and B, respectively. Our precise HST astrometry rules out third bodies orbiting either star in the system, down to masses of similar to 15-25 M-Jup. The location of Sirius. B in the Hertzsprung-Russell diagram is in excellent agreement with theoretical cooling tracks for WDs of its dynamical mass, and implies a cooling age of similar to 126 Myr. The position of Sirius. B on the mass-radius plane is also consistent with WD theory, assuming a carbon-oxygen core. Including the pre-WD evolutionary timescale of the assumed progenitor, the total age of Sirius B is about 228 +/- 10 Myr. We calculated evolutionary tracks for stars with the dynamical mass of Sirius A, using two independent codes. We find it necessary to assume a slightly subsolar metallicity, of about 0.85 Z circle dot, to fit its location on the luminosity-radius plane. The age of Sirius. A based on these models is about 237-247. Myr, with uncertainties of +/- 15 Myr, consistent with that of the WD companion. We discuss astrophysical puzzles presented by the Sirius system, including the probability that the two stars must have interacted in the past, even though there is no direct evidence for this and the orbital eccentricity remains high.

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.

On 2014 December 9.61, the All-sky Automated Survey for SuperNovae (ASAS-SN or "Assassin") discovered ASASSN-141p just similar to 2 days after first light using a global array of 14 cm diameter telescopes. ASASSN-141p went on to become a bright supernova (V = 11.94 mag), second only to SN 2014J for the year. We present prediscovery photometry (with a detection less than a day after first light) and ultraviolet through near-infrared photometric and spectroscopic data covering the rise and fall of ASASSN-141p for more than 100 days. We find that ASASSN-141p had a broad light curve (Delta m(15) (B) = 0.80 +/- 0.05), a B-band maximum at 2457015.82 +/- 0.03, a rise time of 16.941(-0.10)(+0.11) days, and moderate host-galaxy extinction (E (B - V)host = 0.33 +/- 0.06). Using ASASSN-141p, we derive a distance modulus for NGC 4666 of mu = 30.8 +/- 0.2, corresponding to a distance of 14.7 +/- 1.5 Mpc. However, adding ASASSN-141p to the calibrating sample of Type Ia supernovae still requires an independent distance to the host galaxy. Finally, using our early-time photometric and spectroscopic observations, we rule out red giant secondaries and, assuming a favorable viewing angle and explosion time, any nondegenerate companion larger than 0.34 RG(circle dot).

Non-radial pulsations have been identified in a number of accreting white dwarfs in cataclysmic variables. These stars offer insight into the excitation of pulsation modes in atmospheres with mixed compositions of hydrogen, helium, and metals, and the response of these modes to changes in the white dwarf temperature. Among all pulsating cataclysmic variable white dwarfs, GW Librae stands out by having a well-established observational record of three independent pulsation modes that disappeared when the white dwarf temperature rose dramatically following its 2007 accretion outburst. Our analysis of Hubble Space Telescope (HST) ultraviolet spectroscopy taken in 2002, 2010, and 2011, showed that pulsations produce variations in the white dwarf effective temperature as predicted by theory. Additionally in 2013 May, we obtained new HST/Cosmic Origin Spectrograph ultraviolet observations that displayed unexpected behaviour: besides showing variability at a parts per thousand integral 275 s, which is close to the post-outburst pulsations detected with HST in 2010 and 2011, the white dwarf exhibits high-amplitude variability on an a parts per thousand integral 4.4 h time-scale. We demonstrate that this variability is produced by an increase of the temperature of a region on white dwarf covering up to a parts per thousand integral 30 per cent of the visible white dwarf surface. We argue against a short-lived accretion episode as the explanation of such heating, and discuss this event in the context of non-radial pulsations on a rapidly rotating star.