For a complete 12 μn flux-limited sample of 207 IRAS sources (F 12 ≥ 150 Jy, |b| ≥ 5°), the majority of which are AGB stars (∼87%), we have extracted light curves in seven infrared bands between 1.25 and 60 μm using the database of the Diffuse Infrared Background Experiment (DIRBE) instrument on the Cosmic Background Explorer (COBE) satellite. Using previous infrared surveys, we filtered these light curves to remove data points affected by nearby companions and obtained time-averaged flux densities and infrared colors, as well as estimates of their variability at each wavelength. In the time-averaged DIRBE color-color plots, we find clear segregation of semiregulars, Mira variables, carbon stars, OH/IR stars, and red giants without circumstellar dust (i.e., V-[12] < 5) and with little or no visual variation (ΔV < 0.1 mag). The DIRBE 1.25-25 μm colors become progressively redder and the variability in the DIRBE database increases along the oxygen-rich sequence nondusty slightly varying red giants→SRb/Lb→SRa→Mira→OH/IR and the carbon-rich SRb/Lb→Mira sequence. This supports previous assertions that these are evolutionary sequences involving the continued production and ejection of dust. The carbon stars are redder than their oxygen-rich counterparts for the same variability type, except in the F12/F25 ratio, where they are bluer. Of the 28 sources in the sample not previous noted to be variable, 18 are clearly variable in the DIRBE data, with amplitudes of variation of ∼0.9 mag at 4.9 μm and ∼0.6 mag at 12 μm, consistent with them being very dusty Mira-like variables. We also present individual DIRBE light curves of a few selected stars. The DIRBE light curves of the semiregular variable L2 Pup are particularly remarkable. The maxima at 1.25, 2.2, and 3.5 μm occur 10-20 days before those at 4.9 and 12 μm, and, at 4.9 and 12 μm, another maximum is seen between the two near-infrared maxima.

We have used the COBE DIRBE database to derive near- and mid-infrared light curves for a well-defined sample of 38 infrared-bright Mira variable stars and compared with optical data from the AAVSO. In general the 3.5 and 4.9 μm DIRBE bandpasses provide the light curves with the best signal-to-noise ratio (S/N), with S/N decreasing with wavelength at longer wavelengths. At 25 μm good light curves are only available for ∼10% of our stars, and at wavelengths ≥60 μm extracting high quality light curves is not possible. The amplitude of variability is typically less in the near-infrared than in the optical and less in the mid-infrared than in the near-infrared, with decreasing amplitude with increasing wavelength. On average there are 0.20 ± 0.01 mag variation at 1.25 μm and 0.14 ± 0.01 mag variation at 4.9 μm for each magnitude variation in V. The observed amplitudes are consistent with results of recent theoretical models of circumstellar dust shells around Mira variables. For a few stars in our sample we find clear evidence of time lags between the optical and near-infrared maxima of phase ∼0.05-0.13, with no lags in the minima. For three stars mid-infrared maximum appears to occur slightly before that in the near-infrared, but after optical maximum. We find three examples of secondary maxima in the rising portions of the DIRBE light curves, all of which have optical counterparts in the AAVSO data, supporting the hypothesis that they are due to shocks rather than newly formed dust layers. We find no conclusive evidence for rapid (hours to days) variations in the infrared brightnesses of these stars.

We have been intensely monitoring photometric variability in proto-planetary nebulae (PPNe) over the past 25 years and radial velocity variability over the past ten years. Pulsational variability has been obvious, in both the light and velocity, although the resulting curves are complex, with multiple periods and varying amplitudes. Observed periods range from 25 to 160 days, and the periods and amplitudes reveal evolutionary trends. We will present our observational results to date for approximately 30 PPNe, and discuss these results, including the search for period changes that might help constrain post-AGB evolutionary timescales.

Post asymptotic giant branch (post-AGB) stars, central stars of planetary nebulae (CSPNe) and planetary nebulae (PNe) are important phases of stellar evolution as the material they feedback is the seed of subsequent star formation in a galaxy. The majority of low and intermediate mass stars are expected to evolve through these channels, however, it is uncertain how many actually do, and at what rate. The Galactic halo, with its older population, provides a direct test of evolutionary models for low mass stars. Birthrate estimates of PNe are uncertain and worse still, are in contradiction with accepted white dwarf (WD) birthrate estimates. Much of the uncertainty stems from the lack of complete samples and poorly determined distance estimates. New surveys such as the Sloan Digital Sky Survey (SDSS), Galaxy Evolutionary Ex- plorer (GALEX) and the INT Photometric H® Survey (IPHAS) have discovered many new PNe and have observed the far edges of the Galaxy. Improved methods of determining distances to CSPNe are presented here, using model atmospheres, evolutionary tracks and high resolution reddening maps utilising these revolution- ary surveys. Locating the CSPN is non-trivial particularly for evolved PNe, as they are ex- tended with their central star often displaced from the centre of the nebula. There- fore, photometric criteria are required to locate the CSPN in the nebula’s field. Synthetic photometry of the CSPNe is derived from spectral energy distributions (SEDs) computed from a grid of model atmospheres covering the parameter range of CSPNe. The SEDs are convolved with filter transmission curves to compute synthetic magnitudes for a given photometric system which are then calibrated with standard stars and WDs. A further project borne out of a search for luminous central stars of faint PNe, resulted in a systematic search for post-AGB stars in the Galactic halo. In this work, new candidate halo post-AGB stars are discovered from a search through the SDSS spectroscopic database. Combined with previously identified halo post- AGB stars, including the results of a sub-sample from the Palomar-Green (PG) survey, the number of observed and predicted populations are compared. The number of observed post-AGB candidates shows a remarkable deficit to expecta- tions. A survey within a subset of the photometric database of SDSS supports the findings of the PG and SDSS spectroscopic surveys. These findings provide strong evidence for a lack of post-AGB stars in the Galac- tic halo and thick disc. A plausible explanation is that a large fraction of stars in these old, metal-poor populations are evolving via alternative channels. The implications of such a result are far reaching with knock on effects for stellar evolutionary theory, galactic evolution and extragalactic redshift estimates.

Dans cette thèse, j'analyse des observations spectropolarimétriques d'étoiles froides évoluées dans tous les paramètres de Stokes ($I$, $Q$, $U$ et $V$).L'étude des spectres polarisés circulairement de trois étoiles supergéantes rouges (RSG) m'a permis de détecter un faible champ magnétique (de l'ordre de 1 Gauss) à leur surface ainsi que de mettre en évidence l'influence de la forte polarisation linéaire de ces étoiles sur la mesure du champ magnétique.Cette forte polarisation linéaire est d'origine non magnétique et est reliée à la polarisation du continu dont l'observabilité résulte de la présence de structures brisants la symétrie cylindrique du disque stellaire.Après une analyse en profondeur de la polarisation du continu des étoiles RSG, j'utilise un modèle analytique spectropolarimétrique permettant de remonter à la position sur le disque de ces structures, en bon accord avec des observations interférométriques.Les étoiles variables pulsantes montrent elles aussi des signaux polarisés linéairement intenses dont l'origine est la polarisation intrinsèque (où de résonance) associée à certaines raies spectrales.Pour ces étoiles, les gradients de vitesses, introduits par les ondes de chocs se propageants dans leur atmosphère, amplifient la polarisation intrinsèque des raies, un mécanisme d'éclaircissement Doppler, bien connu dans le cas du Soleil, qui offre de nouvelles perspectives dans l'étude de ces objets. / In this thesis work, full Stokes ($I$, $Q$, $U$ et $V$) spectropolarimetric observations of cool evolved stars have been analysed.I have measured a weak magnetic field at the surface of three well known red supergiant stars. Furthermore, I have shown that the strong level of linear polarisation, measured for these stars, can lead to some ambiguity in the measurement of magnetic fields.This strong, unmagnetic, linear polarisation originate from depolarisation of the continuum which, in turn, is only detectable because of symmetry-breaking effects on the stellar disk.I study in some details the continuum polarisation of RSG stars and I present a way of mapping symmetry-breaking strucutres through the mean of a spectropolarimetric model. Good agreement with interferometric observations is reached.From their side, pulsating variable stars also show strong linearly polarised features in their spectra. These features are indeed due to resonant scattering polarisation associated to metallic lines, a phenomenon reminiscent of the second solar spectrum.For these stars, the velocity gradients, pertaining to the shock waves propagating through their atmosphere, enhance the intrinsic polarisation of the lines. This Doppler brightening effect, well known in the solar case, offer a new innovative method for the analysis of these stars.

Central stars of planetary nebulae are low-mass stars on the brink of their final evolution towards white dwarfs. Because of their surface temperature of above 25,000 K their UV radiation ionizes the surrounding material, which was ejected in an earlier phase of their evolution. Such fluorescent circumstellar gas is called a "Planetary Nebula". About one-tenth of the Galactic central stars are hydrogen-deficient. Generally, the surface of these central stars is a mixture of helium, carbon, and oxygen resulting from partial helium burning. Moreover, most of them have a strong stellar wind, similar to massive Pop-I Wolf-Rayet stars, and are in analogy classified as [WC]. The brackets distinguish the special type from the massive WC stars. Qualitative spectral analyses of [WC] stars lead to the assumption of an evolutionary sequence from the cooler, so-called late-type [WCL] stars to the very hot, early-type [WCE] stars. Quantitative analyses of the winds of [WC] stars became possible by means of computer programs that solve the radiative transfer in the co-moving frame, together with the statistical equilibrium equations for the population numbers. First analyses employing models without iron-line blanketing resulted in systematically different abundances for [WCL] and [WCE] stars. While the mass ratio of He:C is roughly 40:50 for [WCL] stars, it is 60:30 in average for [WCE] stars. The postulated evolution from [WCL] to [WCE] however could only lead to an increase of carbon, since heavier elements are built up by nuclear fusion. In the present work, improved models are used to re-analyze the [WCE] stars and to confirm their He:C abundance ratio. Refined models, calculated with the Potsdam WR model atmosphere code (PoWR), account now for line-blanketing due to iron group elements, small scale wind inhomogeneities, and complex model atoms for He, C, O, H, P, N, and Ne. Referring to stellar evolutionary models for the hydrogen-deficient [WC] stars, Ne and N abundances are of particular interest. Only one out of three different evolutionary channels, the VLTP scenario, leads to a Ne and N overabundance of a few percent by mass. A VLTP, a very late thermal pulse, is a rapid increase of the energy production of the helium-burning shell, while hydrogen burning has already ceased. Subsequently, the hydrogen envelope is mixed with deeper layers and completely burnt in the presence of C, He, and O. This results in the formation of N and Ne. A sample of eleven [WCE] stars has been analyzed. For three of them, PB 6, NGC 5189, and [S71d]3, a N overabundance of 1.5% has been found, while for three other [WCE] stars such high abundances of N can be excluded. In the case of NGC 5189, strong spectral lines of Ne can be reproduced qualitatively by our models. At present, the Ne mass fraction can only be roughly estimated from the Ne emission lines and seems to be in the order of a few percent by mass. Furthermore, using a diagnostic He-C line pair, the He:C abundance ratio of 60:30 for [WCE] stars is confirmed. Within the framework of the analysis, a new class of hydrogen-deficient central stars has been discovered, with PB 8 as its first member. Its atmospheric mixture resembles rather that of the massive WNL stars than of the [WC] stars. The determined mass fractions H:He:C:N:O are 40:55:1.3:2:1.3. As the wind of PB 8 contains significant amounts of O and C, in contrast to WN stars, a classification as [WN/WC] is suggested. / Zentralsterne Planetarischer Nebel sind massearme Sterne kurz vor ihrer finalen Entwicklung zu Weißen Zwergen. Aufgrund ihrer Oberflächentemperatur von über 25 000 K sind sie in der Lage, durch Abstrahlung von UV-Licht das sie umgebende Material, welches in einer vorigen Phase ihrer Entwicklung abgestoßen wurde, zu ionisieren. Das solchermaßen zum Leuchten angeregte Gas bezeichnet man als Planetarischen Nebel. Etwa ein Zehntel der galaktischen Zentralsterne sind wasserstoffarm. Im Allgemeinen besteht die Oberfläche dieser Zentralsterne aus einer Mischung der Elemente Helium, Kohlenstoff und Sauerstoff, welche z.T. durch Heliumbrennen erzeugt wurden. Die meisten dieser Sterne haben darüberhinaus einen starken Sternwind, ähnlich den massereichen Pop-I-Wolf-Rayet-Sternen und werden in Analogie zu diesen als [WC] klassifiziert, wobei die eckigen Klammern der Unterscheidung von den massereichen WC-Sternen dienen. Qualitative Analysen der Spektren von [WC]-Sternen lassen eine Entwicklungssequenz dieser Sterne von kühleren sogenannten late-type [WC]-Sternen (kurz [WCL]) zu sehr heißen, early-type [WC]-Sternen (kurz [WCE]) vermuten. Mithilfe von Computerprogrammen, die den Strahlungstransport im mitbewegten Beobachtersystem zusammen mit den statistischen Gleichungen der Besetzungszahlen der Ionen im Sternwind rechnen können, wurden quantitative Untersuchungen der Winde von [WC]-Sternen möglich. Erste Analysen mit Modellen ohne Eisenlinien ergaben dabei systematisch unterschiedliche Häufigkeiten für [WCL]- und [WCE]-Sterne. Während sich für [WCL]-Sterne ein Verhältnis der Massenanteile von He:C von etwas 40:50 ergab, fand man für die [WCE]-Sterne ein mittleres Verhältnis von 60:30 für die He:C-Massenanteile. Dabei sollte die Entwicklung von [WCL] nach [WCE] innerhalb einer sehr kurzen Zeit durch Aufheizung infolge der Kontraktion der Hülle erfolgen und nicht mit einer wesentlichen Abnahme der Kohlenstoffhäufigkeit bei gleichzeitiger Zunahme der Heliumhäufigkeit an der Oberfläche einhergehen. Im Rahmen der vorgelegten Arbeit wird untersucht, ob sich mittels verbesserter Modelle für die Atmosphären von [WC]-Sternen das He:C-Häufigkeitsverhältnis der [WCE]-Sterne bestätigt. Elaboriertere Modelle, welche vom Potsdamer WR-Modelatmosphären-Code (PoWR) berechnet werden können, berücksichtigen Line-Blanketing aufgrund von Elementen der Eisengruppe, kleinskalige Windinhomogenitäten und die Elemente He, C, O, H, P, N und Ne. Unter Bezug auf Sternentwicklungsmodelle, die die Ursache der Wasserstoffunterhäufigkeit von [WC]-Sternen erklären, sind insbesondere die Neon- und Stickstoff-Häufigkeiten interessant. Von den drei möglichen Entwicklungskanälen für [WC]-Sterne führt lediglich das VLTP-Szenario zu einer Stickstoff-Überhäufigkeit von einigen Prozent bezogen auf die Masse. Bei einem VLTP, einem very late thermal pulse, handelt es sich um einen plötzlichen, starken Anstieg der Energieproduktion in der helium-brennenden Schale, während das Wasserstoffbrennen bereits zum Erliegen gekommen ist. Infolge eines VLTPs wird sämtlicher Wasserstoff kurz nach dem thermischen Puls in tiefere Schichten gemischt und in Anwesenheit von C, He und O verbrannt. Infolgedessen wird N und auch Ne erzeugt. Bei der Analyse von elf [WCE]-Sternen wurden für drei von ihnen, PB 6, NGC 5189 und [S71d]3, Stickststoffmassenanteile von 1,5 % bestimmt, während für drei andere Sterne solche hohen Stickstoffhäufigkeiten ausgeschlossen werden können. Für NGC 5189 gelang außerdem die qualitative Reproduktion der beobachteten, starken Ne-Spektrallinien mittels unserer Modelle. Zur Zeit lässt sich aus der Stärke der Ne-Emissionslinien der Ne-Massenanteil leider nur abschätzen, er scheint aber im Bereich einiger Prozent zu liegen. Mittels eines diagnostischen He-C-Linienpaares konnte das He:C-Massenverhältnis von 60:30 für [WCE]-Sterne bestätigt werden. Als Ergebnis der Analyse von PB 8 postulieren wir eine neue Klasse von wasserstoffarmen Zentralsternen, die in ihrer Elementzusammensetzung eher an massereiche WNL-Sterne als an [WC]-Sterne erinnern. Die ermittelten Massenanteile H:He:C:N:O betragen 40:55:1.3:2:1.3, der Wind von PB 8 enthält daher im Unterschied zu WN-Sternen signifikante Mengen von O und C. Es wird daher eine Klassifizierung als [WN/WC] vorgeschlagen.

Since the identi cation of proto-planetary nebulae (PPNe) as transition objects between the asymptotic giant branch stars and planetary nebulae more than two decades ago, astronomers have attempted to characterise these exciting objects. Today many questions still elude a conclusive answer, partly due to the sheer diversity observed within this small subset of stellar objects, and partly due to the low numbers detected. Fortunately, many of these objects display a rich spectrum of emission/absorption lines that can be used as diagnostics for these nebulae. This dissertation presents a study of six PPNe using the relatively new (at NIR wavelengths) integral eld spectroscopy technique. This method has allowed the investigation of distinct regions of these nebulae, and in certain cases the application of magneto-hydrodynamic shock models to the data. The goal of this research has been to investigate the evolution of PPNe by detailed examination of a small sample of objects consisting of a full range of evolutionary types. Near-IR ro-vibrational lines were employed as the primary tool to tackle this problem. In all six sources the 1!0S(1) line is used to map the spatial extent of the H2. In three of these objects the maps represent the rst images of their H2 emission nebulae. In the case of the earliest-type object (IRAS 14331-6435) in this sample, the line map gives the rst image of its nebula at any wavelength. In the only M-type object (OH 231.8+4.2) in the sample, high-velocity H2 is detected in discrete clumps along the edges of the bipolar out ow, while a possible ring of slower moving H2 is found around the equatorial region. This is the rst detection of H2 in such a late-type object but due its peculiarities, it is possibly not representative of what is expected of M-type objects. In IRAS 19500-1709, an intermediate-type object, the line map shows the H2 emission to originate in clumpy structures along the edges of a bipolar shell/out ow. The remaining three objects have all been the subject of previous studies but in each case new H2 lines are detected in this work along with other emission lines (Mg ii, Na i &amp; CO). In the case of IRAS 16594-4656, MHD shock models have been used to determine the gas density and shock velocity. Two new python modules/classes have been written. The rst one to deal with the data cubes, extract ux measurements, rebin regions of interest, and produce line maps. The second class allows the easy calculation of many important parameters, for example, excitation temperatures, column density ratio values, extinction estimates from several line-pairs, column density values, and total mass of the H2. The class also allows the production of input les for the shock tting procedure, and simulated shocks for testing this tting process. A new framework to t NIR shock models to data has been developed, employing Monte Carlo techniques and the extensive computing cluster at the University of Hertfordshire (UH). This method builds on the approach used by many other authors, with the added advantages that this framework provides a method of correctly sampling the shock model parameter space, and providing error estimates on the model t. Using this approach, data from IRAS 16594-4656 have been successfully modelled using the shock models. A full description of this class of stellar objects from such a small sample is not possible due to their diverse nature. Although H2 was detected across the full spectral vi range of post-AGB objects, the phase at which H2 emission begins is still not clear. The only M-type object in this work is a peculiar object and may not be representative of a typical post-AGB star. The H2 PPNe appear to be located at lower Galactic latitudes (b 20 ) than the total PPNe population, possibly pointing to an above average mass and hence younger age of these objects.

We present an ongoing, five-year systematic search for extragalactic infrared transients, dubbed SPIRITS-SPitzer InfraRed Intensive Transients Survey. In the first year, using Spitzer/IRAC, we searched 190 nearby galaxies with cadence baselines of one month and six months. We discovered over 1958 variables and 43 transients. Here, we describe the survey design and highlight 14 unusual infrared transients with no optical counterparts to deep limits, which we refer to as SPRITEs (eSPecially Red Intermediate-luminosity Transient Events). SPRITEs are in the infrared luminosity gap between novae and supernovae, with [4.5] absolute magnitudes between -11 and -14 (Vega-mag) and [3.6]-[4.5] colors between 0.3 mag and 1.6 mag. The photometric evolution of SPRITEs is diverse, ranging from < 0.1 mag yr(-1) to > 7 mag yr(-1). SPRITEs occur in star-forming galaxies. We present an indepth study of one of them, SPIRITS 14ajc in Messier 83, which shows shock-excited molecular hydrogen emission. This shock may have been triggered by the dynamic decay of a non-hierarchical system of massive stars that led to either the formation of a binary or a protostellar merger.

The R Coronae Borealis (RCB) stars are an enigmatic class of hydrogen-deficient supergiant stars, which along with the companion classes of Hydrogen-deficient Carbon (HdC) stars and Extreme Helium (EHe) stars, have been touted as being a result of mergers of low mass carbon-oxygen (CO) and helium (He) white dwarfs. Such mergers of white dwarfs are expected to be the genesis of several interesting stellar objects such as Type Ia supernovae, neutron stars and AM CVn stars, amongst others. The RCBs, HdCs and EHes are mostly near-solar mass single stars, which along with having predominantly helium atmospheres that are extremely exhausted in hydrogen and rich in carbon, are also host to some extraordinary nuclear isotopic ratios. The RCBs and EHes have 12C/13C >= 100, enhancements of up to 3 orders in fluorine compared to solar and significant amounts of s-process elements. The most outstanding characteristic of RCBs is that they, along with the HdCs, have the lowest O-isotopic ratios measured in any star in the Universe viz., 16O/18O ~ 1-10. We perform nucleosynthesis calculations with conditions found in the three-dimensional hydrodynamic simulations of CO and He WD mergers and compare the nuclear yields thus obtained with those measured in the surfaces of RCB stars. We do not find an agreement between the calculated yields and the measured ones and thus conclude that RCBs are not formed immediately after the merger of the white dwarfs. This leads us to surmise that the surface chemical composition of RCBs may be due to the result of nuclear processes occuring in a longer evolutionary period following the merger. To this end, we first construct chemical compositions of the merged white dwarfs based on the results of the hydrodynamic simulations. We then impose these compositions on homogeneous, spherically symmetric, one-dimensional stellar models and evolve these models through the giant phase of RCBs. Along with convection zones that develop in the stellar envelope, we induce a continuous envelope mixing profile that is meant to represent processes related to rotation in these merged objects. We then analyse the nuclear yields from the surface of these models and compare them with those of RCBs. Our models achieve the aforementioned striking characteristics of RCBs, viz., the low O-isotopic ratios, high C-isotopic ratios, high fluorine and s-process element enhancments. Along with these, for the first time, we have reproduced simultaneously, the range in observations of almost all the other elements measured in RCBs. Moreover, our one-dimensional models also place useful constraints on so far unexplored three-dimensional processes, thus providing directives for future studies about them. / Graduate

Ultraviolet spectra obtained with Hubble Space Telescope (HST) of two Mira-type variable stars, R Leo and R Hya, are presented, along with analysis providing information on their outer atmospheres. These high-dispersion spectra were taken with the Goddard High Resolution Spectrograph (HRS) in two spectral regions: 2320-2368 Å to record the C II] (UV0.01) multiplet and 2785-2835 Å to obtain the Mg II h and k lines. The R Hya spectrum was obtained at visual light phase 0.26 and shows a Mg II spectrum that is very clean, showing clear evidence for the overlying circumstellar absorption from Fe I (UV3) and Mn I (UV1) over the k line. The fluoresced Fe I (UV44) feature at 2824 Å is plainly visible in this spectrum, whereas past International Ultraviolet Explorer (IUE) observations of Mira variables at high dispersion were unable to record this feature. Remarkably, the newly identified fluoresced Fe I (UV45) feature near 2807 Å is seen in this spectrum. Until now, this line has been seen only in cool carbon stars with HST/HRS. This line is pumped by the thin C II] (UV0.01) emission line at 2325.5 Å. Two of the strongest C II] (UV0.01) lines near 2325 Å are plainly seen in this spectrum. This region of the spectrum, however, is dominated by the Si II] (UV0.01) line near 2335 Å, in contrast to that observed in the carbon stars and the non-Mira oxygen-rich red giant stars. Very weak Mg II lines are seen in the R Leo spectrum at phase 0.12. At this phase, these lines are typically absent in IUE spectra. Velocity shifts of emission features in the UV spectra of Mira variables are consistent with previously published hydrodynamic models of these stars. These velocities indicate, however, that the C II] (UV0.01) emission lines are not formed in the same atmospheric layers as the Mg II emission. The electron density deduced from the C II] (UV0.01) multiplet is ∼109 cm-3. Finally, the temperature-density structure of the semi-regular variable carbon stars is similar to the oxygen-rich Mira variables-both are hydrodynamic in nature; however, the carbon stars macroscopic velocity fields are not identical to the Mira stars in the atmosphere layers between the Mg II emission region and the circumstellar shell.