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.

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.

The weekly averaged near-infrared fluxes for 2652 stars were extracted from the cold and warm era all-sky maps of the Diffuse Infrared Background Experiment (DIRBE). Since the DIRBE program only archived the individual Calibrated Infrared Observations for the 10 month cold era mission, the weekly averaged fluxes were all that were available for the warm era. The steps required to extract stellar fluxes are described as are the adjustments that were necessary to correct the results for several systematic effects. The observations are at a cadence of once a week for 3.6 years (∼1300 days), providing continuous sampling on variable stars that span the entire period for the longest fundamental pulsators. The stars are divided into three categories: those with large amplitude of variability, smaller amplitude variables, and sources whose near-infrared brightness do not vary according to our classification criteria. We show examples of the results and the value of the added baseline in determining the phase lag between the visible and infrared.

To search for phase lags in the optical-infrared light curves of asymptotic giant branch stars, we have compared infrared data from the COBE DIRBE satellite with optical light curves from the AAVSO and other sources. We found 17 examples of phase lags between the times of infrared and optical maximum, and 4 stars with no observed lags. There is a clear difference between the Mira variables and the semiregular variables in the sample, with the maximum in the optical preceding that in the near-infrared in the Miras, while in most of the semiregular variables no lags are observed. Comparison to published theoretical models indicates that the phase lags in the Miras are due to strong titanium oxide absorption in the visual at stellar maximum, and suggests that Miras pulsate in the fundamental mode, while at least some semiregular variables are first-overtone pulsators. There is a clear optical-near-infrared phase lag in the carbon-rich Mira V CrB; this is likely due to C 2 and CN absorption variations in the optical.

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.