We investigate the observational signatures of super-Earths (i.e., planets with. Earth-to-Neptune. mass), which are the most common type of exoplanet discovered to date, in their natal disks of gas and dust. Combining two-fluid global hydrodynamics simulations with a radiative transfer code, we calculate the distributions of gas and of submillimeter-sized dust in a disk perturbed by a super-Earth, synthesizing images in near-infrared scattered light and the millimeter-wave thermal continuum for direct comparison with observations. In low-viscosity gas (alpha (sic) 10(-4)), a super-Earth opens two annular gaps to either side of its orbit by the action of Lindblad torques. This double gap and its associated gas pressure gradients cause dust particles to be dragged by gas into three rings: one ring sandwiched between the two gaps, and two rings located at the gap edges farthest from the planet. Depending on the. system parameters, additional rings may manifest for a single planet. A double gap located at tens of au. from a host star in Taurus can be detected in the dust continuum by the Atacama Large Millimeter Array (ALMA) at an angular resolution of similar to 0".03 after two hours of integration. Ring and gap features persist in a variety of background disk profiles, last for thousands of orbits, and change their relative positions and dimensions depending on the speed and direction of planet migration. Candidate double gaps have been observed by ALMA in systems such as. HL Tau (D5 and D6) and TW Hya (at 37 and 43 au); we submit that each double gap is carved by one super-Earth in nearly inviscid gas.

We have imaged GM Aurigae with the Hubble Space Telescope, detected its disk in scattered light at 1400 and 1650 angstrom, and compared these with observations at 3300 angstrom, 5550 angstrom, 1.1 mu m, and 1.6 mu m. The scattered light increases at shorter wavelengths. The radial surface brightness profile at 3300 angstrom shows no evidence of the 24 au radius cavity that has been previously observed in submillimeter observations. Comparison with dust grain opacity models indicates that. the surface of the entire disk is populated with submicron grains. We have compiled a. spectral energy distribution from 0.1 mu m to 1 mm. and used it to constrain a model of the star + disk system that includes the submillimeter cavity using the Monte Carlo radiative transfer code by Barbara Whitney. The best-fit model image indicates that the cavity should be detectable in the F330W bandpass if the cavity has been cleared of both large and small dust grains, but we do not detect it. The lack of an observed cavity can be explained by the presence of submicron grains interior to the submillimeter cavity wall. We suggest one explanation for this that. could be due to a planet of mass <9 M-J interior to 24 au. A unique cylindrical structure is detected in the far-UV data from the Advanced Camera for Surveys/ Solar Blind Channel. It is aligned along the system semiminor axis, but does not resemble an accretion-driven jet. The structure is limb. brightened and extends 190 +/- 35 au above the disk midplane. The inner radius of the limb. brightening is 40 +/- 10 au, just beyond the submillimeter cavity wall.

Polaris the Cepheid has been observed for centuries, presenting surprises and changing our view of Cepheids and stellar astrophysics, in general. Specifically, understanding Polaris helps anchor the Cepheid Leavitt law, but the distance must be measured precisely. The recent debate regarding the distance to Polaris has raised questions about its role in calibrating the Leavitt law and even its evolutionary status. In this work, I present new stellar evolution models of Cepheids to compare with previously measured CNO abundances, period change and angular diameter. Based on the comparison, I show that Polaris cannot be evolving along the first crossing of the Cepheid instability strip and cannot have evolved from a rapidly-rotating main sequence star. As such, Polaris must also be at least 118 pc away and pulsates in the first overtone, disagreeing with the recent results of Turner et al. (2013, ApJ, 762, L8).

We present the COBE DIRBE Point Source Catalog, an all-sky catalog containing infrared photometry in 10 infrared bands from 1.25 to 240 μm for 11,788 of the brightest near and mid-infrared point sources in the sky. Since DIRBE had excellent temporal coverage (100-1900 independent measurements per object during the 10 month cryogenic mission), the Catalog also contains information about variability at each wavelength, including amplitudes of variation observed during the mission. Since the DIRBE spatial resolution is relatively poor (0°.7), we have carefully investigated the question of confusion and have flagged sources with infrared-bright companions within the DIRBE beam. In addition, we filtered the DIRBE light curves, for data points affected by companions outside of the main DIRBE beam but within the "sky" portion of the scan. At high Galactic latitudes (|b| > 5°), the Catalog contains essentially all of the unconfused sources with flux densities greater than 90, 60, 60, 50, 90, and 165 Jy at 1.25, 2.2, 3.5, 4.9, 12, and 25 μm, respectively, corresponding to magnitude limits of approximately 3.1, 2.6, 1.7, 1.3, -1.3, and -3.5. At longer wavelengths and in the Galactic plane, the completeness is less certain because of the large DIRBE beam and possible contributions from extended emission. The Catalog also contains the names of the sources in other catalogs, their spectral types, variability types, and whether or not the sources are known OH/IR stars. We discuss a few remarkable objects in the Catalog, including the extremely red object OH 231.8+4.2 (QX Pup), an asymptotic giant branch star in transition to a protoplanetary nebula, which has a DIRBE 25 μm amplitude of 0.29 ± 0.07 mag.

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.

We report ALMA Cycle 2 observations of 230 GHz (1.3 mm) dust continuum emission, and (CO)-C-12, (CO)-C-13, and (CO)-O-18 J = 2-1 line emission, from the Upper Scorpius transitional disk [PZ99] J160421.7-213028, with an angular resolution of similar to 0''.25 (35 au). Armed with these data and existing H-band scattered light observations, we measure the size and depth of the disk's central cavity, and the sharpness of its outer edge, in three components: sub-mu m-sized "small" dust traced by scattered light, millimeter-sized "big" dust traced by the millimeter continuum, and gas traced by line emission. Both dust populations feature a cavity of radius similar to 70 au that is depleted by factors of at least 1000 relative to the dust density just outside. The millimeter continuum data are well explained by a cavity with a sharp edge. Scattered light observations can be fitted with a cavity in small dust that has either a sharp edge at 60 au, or an edge that transitions smoothly over an annular width of 10 au near 60 au. In gas, the data are consistent with a cavity that is smaller, about 15 au in radius, and whose surface density at 15 au is 10(3 +/- 1) times smaller than the surface density at 70 au; the gas density grades smoothly between these two radii. The CO isotopologue observations rule out a sharp drop in gas surface density at 30 au or a double-drop model, as found by previous modeling. Future observations are needed to assess the nature of these gas and dust cavities (e.g., whether they are opened by multiple as-yet-unseen planets or photoevaporation).

High-contrast imaging instruments such as GPI and SPHERE are discovering gap structures in protoplanetary disks at an ever faster pace. Some of these gaps may be opened by planets forming in the disks. In order to constrain planet formation models using disk observations, it is crucial to find a robust way to quantitatively back out the properties of the gap-opening planets, in particular their masses, from the observed gap properties, such as their depths and widths. Combining 2D and 3D hydrodynamics simulations with 3D radiative transfer simulations, we investigate the morphology of planet-opened gaps in near-infrared scattered-light images. Quantitatively, we obtain correlations that directly link intrinsic gap depths and widths in the gas surface density to observed depths and widths in images of disks at modest inclinations under finite angular resolution. Subsequently, the properties of the surface density gaps enable us to derive the disk scale height at the location of the gap h, and to constrain the quantity M-p(2)/alpha, where Mp is the mass of the gap-opening planet and a characterizes the viscosity in the gap. As examples, we examine the gaps recently imaged by VLT/SPHERE, Gemini/GPI, and Subaru/HiCIAO in HD 97048, TW Hya, HD 169142, LkCa. 15, and RX J1615.3-3255. Scale heights of the disks and possible masses of the gap-opening planets are derived assuming each gap is opened by a single planet. Assuming a = 10(-3), the derived planet masses in all cases are roughly between 0.1 and 1M(J).

To investigate if magnetic fields are present in Wolf–Rayet stars, we selected a few stars in the Galaxy and one in the Large Magellanic Cloud (LMC). We acquired low-resolution spectropolarimetric observations with the European Southern Observatory FORS 2 (FOcal Reducer low dispersion Spectrograph) instrument during two different observing runs. During the first run in visitor mode, we observed the LMC Wolf–Rayet star BAT99 7 and the stars WR 6, WR 7, WR 18, and WR 23 in our Galaxy. The second run in service mode was focused on monitoring the star WR 6. Linear polarization was recorded immediately after the observations of circular polarization. During our visitor observing run, the magnetic field for the cyclically variable star WR 6 was measured at a significance level of 3.3σ (〈Bz〉 = 258 ± 78 G). Among the other targets, the highest value for the longitudinal magnetic field, 〈Bz〉 = 327 ± 141 G, was measured in the LMC star BAT99 7. Spectropolarimetric monitoring of the star WR 6 revealed a sinusoidal nature of the 〈Bz〉 variations with the known rotation period of 3.77 d, significantly adding to the confidence in the detection. The presence of the rotation-modulated magnetic variability is also indicated in our frequency periodogram. The reported field magnitude suffers from significant systematic uncertainties at the factor of 2 level, in addition to the quoted statistical uncertainties, owing to the theoretical approach used to characterize it. Linear polarization measurements showed no line effect in the stars, apart from WR 6. BAT99 7, WR 7, and WR 23 do not show variability of the linear polarization over two nights.

<p>A major fraction of stars will pass through a short period of dramatic events in their final evolutionary stage. Low- to intermediate-mass stars, studied here, are stripped of their outer parts in a slow massive wind. This mass loss reshapes both the star and the surrounding medium. The formation of the wind is a consequence of the non-linear interaction of a number of physical processes. Stellar pulsations and efficient dust formation are examples of such key processes. Time-dependent theoretical models, in combination with observations, are useful tools for understanding these winds.</p><p>The main object of this thesis has been the physical improvement of a theoretical wind model. Here the coupling between the dust and gas in the wind is studied in further detail, allowing drift. The methods that have been developed earlier to describe the micro-physical interaction are overviewed and summarized. Previously dust has often been assumed to move at the same velocity as gas. New time-dependent wind models are presented where grain drift has been treated self-consistently. Specifically, the coupling between dust and gas in the wind has been modeled more realistically, with descriptions of both the modified momentum and energy balances, and drift dependent dust formation. The results of these new ``drift models'' have been compared with the results of non-drift models. </p><p>A general result of the study is that the effects of drift are significant and difficult to predict if a simple analytical theory is used. It has been found that dust in drift models tends to accumulate in certain dense regions, an accumulation that was not possible without drift. Moreover the new models show an increased variability in the wind structure. The use of drift in dust formation tends to markedly increase the produced dust. Some sets of model parameters lead to a wind without including drift, but a corresponding wind does not form when drift is included -- and vice versa. The effects of drift are important and can probably not be ignored in realistic models.</p>

A major fraction of stars will pass through a short period of dramatic events in their final evolutionary stage. Low- to intermediate-mass stars, studied here, are stripped of their outer parts in a slow massive wind. This mass loss reshapes both the star and the surrounding medium. The formation of the wind is a consequence of the non-linear interaction of a number of physical processes. Stellar pulsations and efficient dust formation are examples of such key processes. Time-dependent theoretical models, in combination with observations, are useful tools for understanding these winds. The main object of this thesis has been the physical improvement of a theoretical wind model. Here the coupling between the dust and gas in the wind is studied in further detail, allowing drift. The methods that have been developed earlier to describe the micro-physical interaction are overviewed and summarized. Previously dust has often been assumed to move at the same velocity as gas. New time-dependent wind models are presented where grain drift has been treated self-consistently. Specifically, the coupling between dust and gas in the wind has been modeled more realistically, with descriptions of both the modified momentum and energy balances, and drift dependent dust formation. The results of these new ``drift models'' have been compared with the results of non-drift models. A general result of the study is that the effects of drift are significant and difficult to predict if a simple analytical theory is used. It has been found that dust in drift models tends to accumulate in certain dense regions, an accumulation that was not possible without drift. Moreover the new models show an increased variability in the wind structure. The use of drift in dust formation tends to markedly increase the produced dust. Some sets of model parameters lead to a wind without including drift, but a corresponding wind does not form when drift is included -- and vice versa. The effects of drift are important and can probably not be ignored in realistic models.