Context. A sample of 27 sources, cataloged as pre-main sequence stars by the Pico dos Dias Survey (PDS), is analyzed to investigate a possible contamination by post-AGB stars. The far-infrared excess due to dust present in the circumstellar envelope is typical of both categories: young stars and objects that have already left the main ...
NASA Astrophysics Data System (ADS)
During their pilgrimage towards Planetary Nebulae (PNe), dust-enshrouded post-AGB stars undergo a structural and perhaps chemical transformation. Many potentially responsible shaping mechanisms have been proposed, including an axisymmetric super wind, several types of binary interactions and magnetic confinement. During this critical phase, the central ...
either evolutionary models star:ing from post-AGB progenitors, or using lessrestrictive structural models, as shown in Figure 5,(Kawaler 1993). ...
NASA Website
To date, only a limited number of post-AGB stars are known throughout the Milky Way. If we look at possible members of the old Galactic populations - halo and thick disc - numbers get even smaller with only a handful of candidates known plus a small number of PNe. Most known post-AGB stars were selected from IR surveys, and thus a bias ...
Low- and intermediate-mass stars are one of the important dust sources in the interstellar medium (ISM) of galaxies. The compositions of dust ejected from these stars are likely to affect those in the ISM. We investigate dust in post-Asymptotic Giant Branch (AGB) stars, which are in a late evolutionary phase for low- and ...
This report summarizes activities carried out with support from the NASA Ultraviolet, Visible, and Gravitational Astrophysics Research and Analysis Program under Grant NAG 5-6821. The Principal Investigator is Howard E. Bond (Space Telescope Science Insti...
National Technical Information Service (NTIS)
Laboratory emission spectra of simple PAH molecules naphthalene, pyrene and chrysene are compared with the astronomical spectra of the unidentified 21 mu m emission feature. The correlation of the presence of this feature with other PAH features in carbon-rich post-AGB stars suggests that this feature is related to PAH molecules.
A typical high-resolution TEM (HRTEM) image of the large cages produced in. CO gas is shown in Fig. 3. In the case of the method A, a small amount of Ti ...
Purchase. Order. S-97228-E. Title: Beryllium .... 52961 is a pecufi_.r post-AGB star whose abundance anomalies are caused by a depletion ...
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... demonstrated when the IR spectra of gas-phase titanium carbide nanocrystals were measured in the laboratory and ... post-AGB) stars (57). The IR spectra of titanium carbide clusters were measured by IR-RE...
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Context: The transition from the asymptotic giant branch (AGB) to the planetary nebula (PN) phase is critical in the shaping of PNe. It has been suggested that the most asymmetric PNe are the descendants of massive AGB stars. Since these AGB stars are believed to have evolved into heavily obscured post-AGB stars ...
We propose UV spectroscopy of the four unique post-AGB stars of spectral type O{He} in order to understand the origin of their peculiar surface abundances. These stars are the only known amongst the hottest post-AGB stars {effective temperatures > 100,000 K} whose atmospheres are composed of ...
The spectra of the star BD+33 deg 2642 and its surrounding planetary nebula (PN) are analyzed. The stellar parameters are found to be Teff = 20,200 K, log g = 2.9; the stellar mass is 0.560 solar mass. In the stellar atmosphere, He is solar, C, N, O, Mg, Si are depleted by about 1 dex, and Fe is depleted by 2 dex relative to the sun. This abundance pattern is similar to that ...
The hydrogen-deficiency in extremely hot post-AGB stars of spectral class PG1159 is probably caused by a (very) late helium-shell flash or a AGB final thermal pulse that consumes the hydrogen envelope, exposing the usually-hidden intershell region. Thus, the photospheric element abundances of these stars allow to draw conclusions about ...
The hydrogen-deficiency in extremely hot post-AGB stars of spectral class PG 1159 is probably caused by a (very) late helium-shell flash or a AGB final thermal pulse that consumes the hydrogen envelope, exposing the usually-hidden intershell region. Thus, the photospheric elemental abundances of these stars allow us to draw conclusions ...
H-deficient post-AGB stars are the result of a (very) late thermal pulse (VLTP). The particular time of the He-shell flash determines the amount of H remaining in the stellar envelope. In case of a VLTP, H is mixed downward by the He-shell ignition and begins to burn as soon as it reaches depths hot enough. These so-called H-ingestion flashes cause ...
as an OH IR star. PPN also show us that the immediate precursors of OH IR stars are mimics: the rst emergence of 1612 MHz masers from a mimic's shell then ags its birth as an OH IR star. Prototype examples do not exhibit 1612 MHz masers; these are dubbed OH IR star color mimics", or mimics for short
We present the results of an unsupervised classification method applied on a set of 344 spectral energy distributions (SED) of post-AGB stars extracted from the Torun catalogue of Galactic post-AGB stars. This method aims to find a new unbiased method for post-AGB star classification based on the information ...
Stellar evolution models predict that the chemical composition in the atmosphere of stars in the post- AGB has been enhanced in C and N. The aim of this work is to use the planetary nebula NGC 7009 as a test of the stellar evolution models, by making simultaneous models of the nebula and its central star; the ...
Obscured by their circumstellar dusty envelopes post-AGB stars emit a large fraction of their energy in the infrared and thus, infrared sky surveys like IRAS were essential for discoveries of post-AGBs in the past. Now, with the AKARI infrared sky survey we can extend our knowledge about the late stages of stellar evolution. The long-term goal of our work ...
In this thesis, we present an LTE model atmosphere analyses of a group of early B-type postasymptotic giant branch (pAGB) stars. With initial masses ? 9M?, post-AGB stars form an important group of evolved stars and provide a unique opportunity to study stellar evolution almost on a human time-scale. ...
Context. The detected variety in chemistry and circumstellar shell morphology of the limited sample of Galactic post-asymptotic giant branch (AGB) stars is so large that there is no consensus yet on how the different objects are linked by evolutionary channels. The evaluation is complicated by the fact that their distances and hence luminosities remain largely unknown. Aims: ...
We present 10 and 20-?m spectroscopic observations of a sample of F- and G-type carbon-rich post-AGB objects, three of which were known to exhibit the unidentified emission feature at 21?m. We also find the F3I post-AGB object SAO 163075 to exhibit a (weak) 21-?m emission feature. We additionally obtained a 10-?m spectrum of IRAS 05341+0852, which has been ...
A unified evolutionary scheme that includes post-AGB systems, barium stars, symbiotics, and related systems, explaining their similarites as well as differences. Can we construct it? We compare these various classes of objects in order to construct a consistent picture. Special attention is given to the comparison of the barium pollution and symbiotic ...
) Fluorine in extremely hot post�AGB stars: evidence for nucleosynthesis # K. Werner 1 , T. Rauch 1,2 and J.S.A. Received xxx / Accepted xxx Abstract. We have discovered lines of highly ionized fluorine (F # and F location of the Galaxy with known fluorine ( 19 F) abundance (see e.g. Meynet & Arnould 2000
Hot post-AGB stars in globular clusters typically exhibit Fe abundances considerably lower than the cluster mean. This anomaly may reflect selective condensation of metals onto dust grains at the end of the AGB phase, a mechanism first suggested for cooler post-AGB stars with peculiar Fe abundances. To investigate ...
The hydrogen-deficiency in extremely hot post-AGB stars of spectral class PG1159 is probably caused by a (very) late helium-shell flash or a AGB final thermal pulse that consumes the hydrogen envelope, exposing the usually-hidden intershell region. Thus, the photospheric element abundances of these stars allow us to draw conclusions ...
The transition from Asymptotic Giant Branch star to Planetary Nebula is short-lived and mysterious. Though it lasts only a few thousand years, it is thought to be the time when the asymmetries observed in subsequent phases arise. During this epoch, the star is shrouded in thick clouds of dust and molecular gas; infrared observations are needed to reveal ...
We propose to obtain IRS spectra in the SH and LH mode of a sample of carefully selected binary post-AGB stars. Through detailed observing campaigns, we have recently established that these binaries are surrounded by a Keplerian dusty disc. The ISO-SWS spectra of only the few brightest examples, have shown that concerning mass, kinematics, mineralogy and ...
We have analysed optical spectra of the carbon-rich post-AGB star IRAS 22272+5435. The circumstellar components of molecular lines of CN (Red System) and C2 (Phillips and Swan bands) were identified and measured. The mechanism of excitation of the rotational levels of the ground electronic states of CN and C2 was analysed with a non-LTE code. An empirical ...
The hydrogen-deficiency in extremely hot post-AGB stars of spectral class PG1159 is probably caused by a (very) late helium-shell flash or a AGB final thermal pulse that consumes the hydrogen envelope, exposing the usually-hidden intershell region. Thus, the photospheric elemental abundances of these stars allow to draw conclusions ...
We present COSAS (CO Survey of late AGB Stars), a project to map and analyze the 12CO J = 1-0 and J = 2-1 line emission in a representative sample of circumstellar envelopes around AGB and post-AGB stars. The survey was undertaken with the aim of investigating small- and large-scale morphological and kinematical properties of the molecular environment ...
H-alpha emission is ubiquitous in our Galaxy. It traces ionised gas of assorted nebulae such as HII regions, planetary nebulae, Wolf-Rayet nebulae, and supernova remnants. It is a strong signature of active stars, interacting binaries, very massive stars (especially supergiants, Luminous Blue Variables and Wolf-Rayet stars), Be ...
We present accurate radial velocity measurements on the pulsating extremely iron-deficient post-AGB object HD 52961 and the RV Tauri star EN TrA (HD 131356) proving them to be binaries. Our long-term photometric monitoring campaign shows that the RV Tauri photometric class ``b'' phenomenon in HD 52961 is due to variable circumstellar extinction during ...
We have observed at high spectral resolution (R~50000) the 4.6 ?m region of binary post-AGB objects with circumbinary dust disks. Prototype objects are HR 4049 and the Red Rectangle. For most objects our infrared spectra reveal the presence of CO vibration-rotation lines and in some objects H2O vibration-rotation lines. The most extreme of these objects is HR 4049 where the ...
The shaping of planetary and protoplanetary nebulae {PNe and PPNe} is probably the most exciting yet least understood problem in the late evolution of 1-8 solar mass stars. An increasing number of astronomers believe that fast jet-like winds ejected in the PPN phase are responsible for carving out the diverse shapes in the dense envelopes of the Asymptotic Giant Branch {AGB} ...
The R Coronae Borealis (RCB) stars are rare hydrogen-deficient carbon-rich supergiants, all apparently single stars which are consistent with being post-AGB stars. RCB stars undergo massive declines of up to 8 mag due to the formation of carbon dust at irregular intervals. The mechanism of dust ...
We present mid-infrared spectra from evolved sources in the Large and Small Magellanic Clouds (LMC and SMC) observed with the Infrared Spectrograph on the Spitzer Space Telescope. Many targets in the project were selected based on their mid-infrared colors, with little additional information available, and in this sample, we observed more carbon stars than originally ...
Here we present the first result from the COSAS (CO Survey of late AGB Stars) program (P.I. A. Castro-Carrizo), a J=1-0 and J=2-1 line emission mapping survey of a statistically representative sample of circumstellar envelopes around AGB and post-AGB stars. This mapping survey has been carried out to investigate the small and large scale morphological and ...
We explore the realm of post-asymptotic giant branch (post-AGB) stars from a theoretical viewpoint, by constructing a synthetic population of transition objects, proto-planetary nebulae, planetary nebula nuclei (PNNs), and post-planetary nebula objects. We use the Monte Carlo procedure to filter out the populations according to a given set of assumptions. ...
H1504+65 is a young white dwarf with Teff=200 000 K and is the hottest post-AGB star ever analysed with detailed model atmospheres. Chandra LETG+HRC-S spectra have revealed the richest X-ray absorption line spectrum recorded from a stellar photosphere to date. The line forming regions in this extremely hot photosphere produce many transitions in absorption ...
Thermally pulsing asymptotic giant branch (AGB) stars suffer mass loss which leads to the formation of a circumstellar shell of gas and dust. At the end of the AGB phase, mass-loss stops and the circumstellar shell begins to drift away from the star. If the velocity of the AGB wind has been relatively constant, then dust furthest from the ...
We cross-identified AKARI-Hipparcos and AKARI-2MASS combined catalogs with the following astronomical catalogs using a tolerance radius of 3 arcsec: (1) S-type stars, i.e., stars with surface carbon-to-oxygen number ratio (C/O) close to unity: Stephenson (1984, Cat. , 1990AJ....100..569S), who list 1,412 sources; (2) ...
Using a radiative transfer code (DUSTY) parameters of the circumstellar dust shells of 15 hot post-AGB stars have been derived. Combining the optical, near and far-infrared (ISO, IRAS) data of the stars, we have reconstructed their spectral energy distributions (SEDs) and estimated the dust temperatures, mass loss rates, angular radii ...
Context: Abundance analysis of post-AGB objects as probes of AGB nucleosynthesis. Aims: A detailed photospheric abundance study is performed on the carbon-rich post-AGB candidate MACHO 47.2496.8 in the LMC. Methods: High-resolution, high signal-to-noise ESO VLT-UVES spectra of MACHO 47.2496.8 are analysed by performing detailed spectrum synthesis modelling using ...
Infrared spectra of a large sample of (ultra)compact HII regions (UCHII), Herbig Ae/Be stars, post-AGB stars and PNe are obtained with the ISO-SWS spectrograph on board ISO. These data show a wealth of infrared emission features attributed to Polycyclic Aromatic Hydrocarbons (PAHs). Here we focus on three topics, i.e. the new detection ...
We present a detailed chemical analysis on the basis of high-resolution, high signal-to-noise optical spectra of two post-AGB objects IRAS 22223+4327 and IRAS 04296+3429. Both display the unidentified 21 mu m feature in their IR-spectra. The analysis is performed using HR 1865 (F0Ib) and HR 1017 (F5Iab) as reference stars. The spectroscopic indicators ...
Hydrogen-deficient post-AGB stars have experienced a late helium-shell flash that mixes the hydrogen-rich envelope and the helium-rich intershell (between hydrogen- and helium-burning shell). The amount of hydrogen remaining in the stellar envelope depends on the particular moment when this late thermal pulse occurs. Previous spectral analyses of ...
We present AKARI/IRC observations of a sample of six extremely red IRAS sources, of which three are variable OH/IR stars and the rest are early post-AGB stars. The OH/IR stars show a red continuum with the expected strong 10 ?m silicate absorption feature, while the post-AGB stars show an even ...
On June 17-19, 2009 a workshop took place in Rochester, NY, USA, to define key problem areas in the field of planetary nebulae and the evolution of intermediate mass stars. The overarching aim was to determine groups of projects that could be achieved with current and next generation of observational and computational facilities. A second aim was to seed medium and large ...
The post-asymptotic-giant-branch (post-AGB) or pre-planetary nebulae (PPN) phase of stellar evolution plays an important role in the dynamical shaping of PNe. While the interacting stellar winds model has achieved tremendous success in the explaining of the 1-D structures (shells, crowns, haloes) of PNe, the origins of arcs, rings, and multipolar structures remain a mystery. ...
We have identified a strong P Cygni feature (? 973) in the far-UV spectra of some very hot (Teff ? 85, 000 K) central stars of planetary nebulae (CSPN) and PG 1159 stars as originating from NeVII. Using stellar atmosphere models, we reproduce this feature as it is observed in the spectra of hydrogen-deficient CSPN, as well as in PG 1159 ...
We performed an iron abundance determination of the hot, hydrogen deficient post-AGB star PG1159-035, which is the prototype of the PG1159 spectral class and the GW Vir pulsators, and of two related objects (PG1520+525, PG1144+005), based on the first detection of Fe VIII lines in stellar photospheres. In another PG1159 star, ...
The paper contains 4 catalogues that are archived electronically: The first and most important one is the catalogue of 70 high probability and 1422 candidate optically bright post-AGB stars in the LMC. For this, we performed a cross-correlation with three optical catalogues and data from 2MASS and SAGE. The estimated luminosity and some cross-correlation ...
We present K-band integral field spectroscopy of the bipolar post-asymptotic giant branch (post-AGB) object IRAS 18276-1431 (OH 17.7-2.0) using SINFONI on the VLT. This allows us to image both the continuum and molecular features in this object from 1.95 to 2.45 ?m with a spatial resolution down to 70 mas and a spectral resolution of �5000. We detect a range of H2 ...
The evolution of (single) low and intermediate-mass stars from the main sequence to the post-AGB phase is reviewed. These stars evolve through core H and He-burning before entering the asymptotic giant branch (AGB) phase. During the AGB, recurrent mixing events may occur that can significantly change the composition of the envelope, with enrichments in ...
We present the results of a long-term (15 year) photometric monitoring program of 30 post-AGB stars. They are classified as proto-planetary nebulae (PPNs), objects in the evolutionary transition between the AGB and planetary nebula phases. They all show variability, and periods have been determined for most of these. However, the variability is not ...
State-of-the-art non-LTE spectral analysis requires high-resolution and high-S/N observations of strategic metal lines in order to achieve reliable photospheric parameters like, e.g., effective temperature, surface gravity, and element abundances.Hot stars with effective temperatures higher than about 40 000 K exhibit their metal-line spectrum arising from highly ionized ...
Energy Citations Database
The 19F(p, ?)16O reaction is an important fluorine destruction channel in the proton-rich outer layers of asymptotic giant branch (AGB) stars and it might also play a role in hydrogen-deficient post-AGB star nucleosynthesis. So far, available direct measurements do not reach the energy region of astrophysical interest (E cm <~ 300 ...
We present a detailed dust radiative transfer model for the circumstellar envelope of the 21 micron post-AGB object, HD 56126 taking into account the dust composition. In order to achieve this we have developed a model for the optical characteristics of various carbon-rich compounds, including hydrogenated amorphous carbon, magnesium sulfide and titanium carbide. We have ...
Context: The influence of binarity on the late stages of stellar evolution remains an open issue. Aims: While the first binary post-AGB stars were serendipitously discovered, the distinct characteristics of their spectral energy distribution (SED) allowed us to launch a more systematic search for binaries. We selected post-AGB ...
Jun 6, 2007 ... Artist's rendering of the planet orbiting the star Gliese 436 (Lynette ... "A" type stars - ones that have nearly completed their life cycle ...
The history of mass loss and chemical enrichment is inscribed on the nebulae surrounding post-AGB stars. Of these objects the most prominent record is the circumnuclear region of the Egg Nebula. We present changes in structure and surface brightness of CRL2688 derived from several pairs of Hubble Space Telescope images taken between 1995.5 and 2009.6 in ...
H 1504+65 is a young white dwarf with an effective temperature of 200 000 K and is the hottest post-AGB star ever analysed with detailed model atmospheres. Chandra LETG+HRC-S spectra have revealed the richest X-ray absorption line spectrum recorded from a stellar photosphere to date. The line forming regions in this extremely hot photosphere produce many ...
Pre-Planetary Nebulae (PPNe) are believed to represent a relatively short, intermediate evolutionary phase in the evolution of AGB stars to Planetary Nebulae (PNe). Our unbiased, high-resolution imaging surveys with HST of young PNe and PPNe show very strong morphological similarities between these classes. We therefore extend our morphological scheme for PPN classification to ...
We report on results of astrometric observations of H2O masers in the pre-planetary nebula candidate source IRAS 19312-1950 with the VLBI Exploration of Radio Astrometry (VERA), yielding an annual parallax, ? = 263�47?as, and a secular motion, (??, ??) = (-2.61�0.47, -6.73�0.14) [mas yr-1], for IRAS 19312+1950. We then derived a heliocentric distance of D = 3.80+0.83-0.58 kpc, a location in ...
We present the near infrared photometry of a new sample of 225 IRAS sources, many of them previously unidentified in the literature, selected because their far infrared colours are similar to those shown by known planetary nebulae. The results obtained are used to establish the main source of near infrared emission. Combining this information with the far infrared IRAS data and a few additional ...
The final evolutionary stage of a low mass stellar object is a complex phase which is still poorly understood. In this thesis we contribute to a better understanding of the nucleosynthesis and dredge-up phenomena that occur in such objects during their ascent on the AGB by means of a detailed study of high-resolution optical spectra of post-AGB objects. In the first four chapters we mainly focus ...
Stars on the asymptotic giant branch (AGB) produce dust in their circumstellar shells. The nature of the dust-forming environment is influenced by the evolution of the stars, in terms of both chemistry and density, leading to an evolution in the nature of the dust that is produced. Carbon-rich AGB stars are known to produce silicon ...
We present an analysis of our extensive data-set on IRAS 08544-4431. It is the first object we discuss of our newly defined sample of stars, selected for their position in the ``RV Tauri'' box in the IRAS [12]-[25], [25]-[60] two-color diagram. Moreover, our selection criteria included an observed excess in the L-band, indicative of a dusty disc. The SED of IRAS 08544-4431 ...
We report optical long-slit spectra and direct imaging (ground-based and with the Hubble Space Telescope (HST)) of the pre-planetary nebula M 2-56 obtained at different epochs. The optical nebula is composed of shock-excited material distributed in two pairs of nested lobes with different sizes and surface brightness. The compact, bright inner lobes (ILs) have an angular size of ~1farcs5 � 1'' ...
The little-studied southern Galactic globular cluster NGC 5986 is remarkable in containing two very luminous A-F-type stars. Membership of the two stars in the cluster is confirmed by their radial velocities and by Str�mgren photometry showing very low surface gravities. These stars are candidate objects lying on ...
Intermediate mass stars (0.8-8.0 Msolar) eventually evolve on the H-R diagram, up the asymptotic giant branch (AGB). The intensive mass loss which characterizes the AGB produces a circumstellar shell of dust and neutral gas. At the end of the AGB, mass loss virtually stops and the circumstellar shell begins to drift away from the star. At the same time the ...
Scanning of the POSS R--band plates of the northern hemisphere, completed recently at the STScI, together with the scans of the ``Quick-V'' plates taken for the HST Guide Star Catalogue, make possible the determination of proper motions of large numbers of stars in selected--target or survey modes. In the first mode, we have been obtaining motions for ...
We present a spectral atlas of the post-main-sequence population of the most massive Galactic globular cluster, ?Centauri. Spectra were obtained of more than 1500 stars selected as uniformly as possible from across the (B, B - V) colour-magnitude diagram of the proper motion cluster member candidates of van Leeuwen et al. The spectra were obtained with the 2dF multifibre ...
We present the classification of 197 point sources observed with the Infrared Spectrograph in the SAGE-Spec Legacy programme on the Spitzer Space Telescope. We introduce a decision-tree method of object classification based on infrared spectral features, continuum and spectral energy distribution shape, bolometric luminosity, cluster membership and variability information, which is used to ...
We present the classification of 197 point sources observed with the Infrared Spectrograph in the SAGE-Spec Legacy program on the Spitzer Space Telescope. We introduce a decision-tree method of object classification based on infrared spectral features, continuum and spectral energy distribution shape, bolometric luminosity, cluster membership, and variability information, which is used to classify ...
Detached, symbiotic binaries are generally assumed to interact via Bondi-Hoyle-Littleton (BHL) wind accretion. However, the accretion rates and outflow geometries that result from this mass-transfer mechanism cannot adequately explain the observations of the nearest and best studied symbiotic binary, Mira, or the formation of some post-AGB binaries, e.g. barium ...
We present 10- and 20-?m images of IRAS 19500-1709 taken with the mid-infrared camera, OSCIR, mounted on the Gemini North Telescope. An extended circumstellar envelope is detected, with the N-band image indicating an elongation in a north-east-south-west direction. We use a dust radiation transport code to fit the spectral energy distribution from ultraviolet to submillimetre wavelengths, with a ...
We present a catalog of compact sources detected in the MIPSGAL 24 microns survey data. These small (< 1 arcminute) rings, bubbles, disks or shells are pervasive through the entire Galactic plane in the mid-infrared. Over 400 such sources have been identified from visual inspection of the MIPSGAL mosaic images. Their distribution is approximately uniform in Galactic latitude and longitude, and ...
The visually brightest members of Population II are post-AGB (PAGB) stars evolving through spectral types F and A. The aim of this proposal is to find such PAGB stars in the halos of three galaxies that lie just outside the Local Group: Sextans A, NGC 3109, and NGC 5237. The importance of PAGB stars is: (1) they can probe the structure ...
We have conducted an LTE abundance analysis for SAO 40039, a warm post-AGB star whose spectrum is known to show surprisingly strong He I lines for its effective temperature and has been suspected of being H-deficient and He-rich. High-resolution optical spectra are analyzed using a family of model atmospheres with different He/H ratios. Atmospheric ...
The surface abundances of extreme helium (EHe) and R Coronae Borealis (RCB) stars are discussed in terms of the merger of a carbon-oxygen white dwarf with a helium white dwarf. The model is expressed as a linear mixture of the individual layers of both constituent white dwarfs, taking account of the specific evolution of each star. In developing this ...
The surface abundances of extreme helium (EHe) and R Coronae Borealis (RCB) stars are discussed in terms of a model for their origin in the merger of a carbon-oxygen white dwarf with a helium white dwarf. The model is expressed as a linear mixture of the individual layers of both constituent white dwarfs, taking account of the specific evolution of each ...
The visually brightest members of Population II are post-AGB (PAGB) stars evolving through spectral types F and A. The aim of this proposal is to find such PAGB stars in the halos of Sextans A and B (two galaxies just outside the Local Group) and of NGC 4236 (a nearly edge-on spiral in the M81 Group). The importance of these stars is: ...
Lithium and zirconium abundances (the latter taken as representative of s-process enrichment) are determined for a large sample of massive Galactic O-rich AGB stars, for which high-resolution optical spectroscopy has been obtained (R� 40 000{-}50 000). This was done by computing synthetic spectra based on classical hydrostatic model atmospheres for cool ...
By tracing the distribution of cool dust in the extended envelopes of post-asymptotic giant branch stars and (proto)-planetary nebulae ((P)PNe), we aim to recover, or constrain, the mass-loss history experienced by these stars in their recent past. The Far-Infrared Surveyor (FIS) instrument on board the AKARI satellite was used to obtain far-infrared maps ...
We request a small investment of 24 minutes of Spitzer time, to obtain four IRAC observations of epsilon Aurigae. A naked eye object located near Capella, epsilon Aurigae is the eclipsing binary star with the longest known orbital period, showing a single long duration (~2 yr) eclipse every 27.1 yr. For much of the last 150 years, the nature of the eclipsing object defied ...
Post-asymptotic giant branch (post-AGB) stars are key objects for the study of the dramatic morphological changes of low- to intermediate-mass stars on their evolution from the AGB towards the planetary nebula stage. There is growing evidence that binary interaction processes may very well have a determining role in the shaping process ...
We present 2-45 mu m spectra of a large sample of carbon-rich evolved stars in order to study the ``30'' mu m feature. We find the ``30'' mu m feature in a wide range of sources: low mass loss carbon stars, extreme carbon-stars, post-AGB objects and planetary nebulae. We extract the profiles from the sources by ...
We present the first results of a biased survey of sub-millimetre and FIR CO transitions towards the circumstellar envelopes of AGB and post-AGB stars. This Herschel Guaranteed Time programme uses the HIFI instrument to collect the emission of two CO transitions (J=5-4 and J=9-8) in 74 sources, as well as the J=14-13 transition in the 10 most intense or intriguing sources of ...
The full understanding of final stellar evolution phases is a fundamental request to properly investigate the Universe at any temporal and spatial scale. While the theoretical scenarios of H- and He-burning have been deeply investigated in the last 30 years, the modelling of stellar evolution beyond the core-helium burning phase and the related nucleosynthesis still present problems related to the ...
We performed an iron abundance determination of the hot, hydrogen deficient post-AGB star PG 1159 - 035, which is the prototype of the PG 1159 spectral class and the GW Vir pulsators, and of two related objects (PG 1520 + 525, PG 1144 + 005), based on the first detection of Fe viii lines in stellar photospheres. In another PG 1159 ...
We propose deep far and near-UV images of 15 Galactic globular clusters (GCs) as a follow up to programs by our group in Cycles 1 and 3. The final database will be the largest homogeneous dataset ever produced on the UV photometric properties of Galactic GCs and will allow a full characterization of their hot stellar contents. Image size and resolution are such that excellent photometry outside ...
We propose to collect deep far and near-UV images of 35 Galactic globular clusters (GCs). This will be the largest homogeneous dataset ever produced on the UV photometric properties of Galactic GCs and will allow a full characterization of their hot stellar contents. For all the targets, horizontal branch (HB) stars will be clearly resolved, so that the data will allow a ...
We summarize some of the results we obtained using detailed evolutionary calculations that take into account diffusion and mass loss to revisit the question of the carbon pollution observed in the atmospheres of DQ white dwarfs and in some DB stars. Our basic premise is that gravitational settling of C and O is slowed down by dying residual winds in the hot PG1159 phase and it ...
We have observed a small sample of hot post-asymptotic giant branch (AGB) stars with the Infrared Array Camera (IRAC) and the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. The stars were selected from the literature on the basis of their far-infrared (IR) excess (i.e., post-AGB candidates) and B spectral type (i.e., ...
Cycling of matter between the ISM and stars drives the evolution of a galaxy's visible matter and its emission characteristics. To understand this recycling, the SAGE legacy project has surveyed the Large Magellanic Cloud with IRAC and MIPS to study the physical processes of the ISM, the formation of new stars and the injection of mass by evolved ...
This paper describes the design principles for a CCD-based photometric system that is highly optimized for ground-based measurement of the size of the Balmer jump in stellar energy distributions. It is shown that, among ultraviolet filters in common use, the Thuan-Gunn u filter is the most efficient for this purpose. This filter is combined with the standard Johnson-Kron-Cousins B, V, and I ...
We are constructing comprehensive grids of CMFGEN synthetic spectra, suitable for the analysis of spectroscopic data of post-Asymptotic Giant Branch (post-AGB) objects. We present initial results for Hydrogen deficient Central Stars of Planetary Nebulae (CSPN) migrating along the constant luminosity branch of the Hertzsprung-Russell diagram. These grids ...
The imaging of young planetary and pre-planetary nebulae (PNe and PPNe) with unprecedented high angular resolution and dynamic range using the Hubble Space Telescope (HST), has shown that almost all of these objects are highly aspherical, with complex multipolar morphologies. These data have forced radical changes in our understanding of the mass-loss processes during late stellar evolution, ...
Using far-infrared observations obtained from the Infrared Space Observatory (ISO), we have discovered extremely large dust shells around two post-AGB stars (the Egg Nebula and AFGL 618; Speck, Meixner & Knapp 2001). These circumstellar shells contain the fossil record of their previous AGB mass loss. The radial profiles of these dust shells suggest ...
HD 179821 is an enigmatic evolved star that possesses characteristics of both a post-asymptotic giant branch (post-AGB) star and a yellow hypergiant, and there has been no evidence that unambiguously defines its nature. These two hypotheses are products of an indeterminate distance, presumed to be 1 kpc or 6 kpc. We have obtained the ...
Aims: Eclipses and occultations of post-AGB stars provide a powerful method of exploring the near-stellar environment, including close companions and circumstellar debris disks. Only six eclipsing systems and one dust-occultation system are currently known. New cases are important for our understanding of binary evolution during the AGB mass-loss phase. Methods: We study the ...
Recent evidence of a young progenitor population for many TypeIa supernovae (SNeIa) raises the possibility that evolved intermediate-mass progenitor stars may be detected in pre-explosion images. NGC 1316, a radio galaxy in the Fornax cluster, is a prolific producer of SNeIa, with four detected since 1980. We analyse Hubble Space Telescope (HST) pre-explosion images of the ...
We present combined radiative transfer and H2O ice formation calculations for the dusty envelopes of oxygen-rich evolved stars. We study the effects of various (circum-)stellar parameters on the spectral energy distribution of these stars, their infrared spectral water ice features at 3 ?m and in the 30-100 ?m region, and the properties of (water ice on) ...
A search for Lambda Bootis stars in two OB associations, Orion OB1 and Lacerta OB1, has been carried out. One Lambda Bootis star has been discovered in the Orion OB1 association. In addition, nine A-type stars which appear to be slightly metal weak have been found in the two associations. The Lambda Bootis ...
Interstellar dust is constantly being processed and recycled, from the time it forms in the outer atmospheres of evolved tars, to its injection into the ISM, to its accumulation in molecular clouds, and formation of planetary systems. Much grain formation occurs in the atmospheres of low-to-intermediate mass stars (0.8 ? M ? 8 M?) near the end of the asymptotic giant branch ...
Symbiotic stars are long-period interacting binaries composed of an evolved cool giant and a hot and luminous companion - in most cases a wind-accreting post-AGB star - surrounded by an ionized nebula. The nature of the cool giant plays a key role in the symbiotic phenomenon: it constrains the size of the binary, which must have enough ...
Dust disks have been detected around white dwarfs (WDs), and they appear to bifurcate into (1) small dust disks around cool WDs that are replenished by tidally disrupted asteroids, and (2) large dust disks around hot WDs that have been suggested to be generated by collisions among Kuiper-Belt-like objects. We have made a Spitzer MIPS 24 (micron) survey of 72 hot WDs and found IR excess for 7, all ...
We present the results of proper-motion measurements of the dust shell structure in the Egg Nebula (AFGL 2688, CRL 2688, V1610 Cyg), based on the archived two-epoch data at 2 m taken with the Hubble Space Telescope.We measured the amount of motion of local structures in the nebula by determining their relative shifts over an interval of 5.5 yr. The dynamical age of the nebula is found to be ...
Four-color (uvby) photoelectric observations have yielded provisional periods for three peculiar A-type stars. For 108 Aquarii, the period is 3.73d. For HD 184905, the period is either 1.84d or 2.17d, and for theta 1 Microscopii the most probable periods ...
Essentially all planetary nebulae {PNs} are aspherical, whereas the mass-loss envelopes of AGB stars are strikingly spherical. Our previous SNAPshot surveys of a morphologically unbiased sample of pre-planetary nebulae {PPNs} - objects in transition between the AGB and PN evolutionary phases - show that roughly half our observed targets are resolved, with bipolar or multipolar ...
Pre-Planetary nebulae (PPNs) - objects in transition between the Asymptotic Giant Branch (AGB) and planetary nebula (PN) evolutionary phases - hold the key to our understanding of these very late stages of stellar evolution for intermediate mass stars. Observationally, it is well-established that PNs show a dazzling variety of elliptical and bipolar morphologies, whereas AGB ...
We present H? images of two objects, He 2-47 and M1-37, obtained during a Hubble Space Telescope imaging survey of young planetary nebulae (PNs) selected on the basis of their low-excitation characteristics. The two objects show a highly aspherical morphology, characterized by multiple lobes distributed around the central star. Such a morphology has never been seen before in ...
We present the results of proper-motion measurements of the dust shell structure in the Egg Nebula (AFGL 2688, CRL 2688, V1610 Cyg), based on the archived two-epoch data at 2 ?m taken with the Hubble Space Telescope. We measured the amount of motion of local structures in the nebula by determining their relative shifts over an interval of 5.5 yr. The dynamical age of the nebula is found to be ...
The high-resolution (R ~ 600) Spitzer/IRS spectrum of the bipolar protoplanetary nebula (PN) IRAS 17423-1755 is presented in order to clarify the dominant chemistry (C-rich versus O-rich) of its circumstellar envelope as well as to constrain its evolutionary stage. The high-quality Spitzer/IRS spectrum shows weak 9.7 ?m absorption from amorphous silicates. This confirms for the first time the ...
We have observed an evolved star with a rare combination of spectral features, MSX SMC 029, in the Small Magellanic Cloud (SMC) using the low-resolution modules of the Infrared Spectrograph on the Spitzer Space Telescope. A cool dust continuum dominates the spectrum of MSX SMC 029. The spectrum also shows both emission from polycyclic aromatic hydrocarbons (PAHs) and ...
We study the optical spectrum of the exciting B[e] star Hen 2-90 based on new high-resolution observations that cover the innermost 2�� of this object whose total extent is more than 3��. Our investigation is split in two parts, (i) a qualitative study of the presence of the numerous emission lines and classification of their line profiles, which indicate a ...
In this paper we introduce an extension to the MK system of spectral classification that allows the precise classification of pre-main-sequence (PMS) A-type stars, including most Herbig Ae stars. This classification scheme characterizes the star by a standard MK type, the presence and strength of emission and/or ...
Context. The first Japanese infrared astronomical satellite, AKARI, has completed an All-Sky Survey at mid- to far-infrared wavelengths with higher spatial resolutions and sensitivities than the previous survey with Infrared Astronomical Satellite (IRAS). Aims: We search for new T Tauri star (TTS) candidates with the mid-infrared (MIR) part of the AKARI All-Sky Survey at 9 ...
-Newton observation of Altair and study the coronal X-ray properties of a late A-type star for the first time that resembles those of low activity stars of later spectral type. Cool plasma at low density produces weak X-ray, Stars: activity, Stars: coronae, X-rays: stars ...
We present high angular resolution observations of the HC3N J = 5-4 line from the Egg nebula, which is the archetype of proto-planetary nebulae (PPNs). We find that the HC3N emission in the approaching and receding portion of the envelope traces a clumpy hollow shell, similar to that seen in normal carbon-rich envelopes. Near the systemic velocity, the hollow shell is fragmented into several large ...
Aims: In this paper we study the Spitzer and TIMMI2 infrared spectra of post-AGB disc sources, both in the Galaxy and the LMC. Using the observed infrared spectra we determine the mineralogy and dust parameters of the discs, and look for possible differences between the Galactic and extragalactic sources. Methods: Modelling the full spectral range observed allows us to determine the dust species ...
Collimated jets are one of the most intriguing, yet poorly understood phenomena in astrophysics. Jets have been found in a wide variety of object classes which include AGNs, YSOs, massive X-ray binaries {e.g. SS433}, black hole X-ray transients, symbiotic stars, supersoft X-ray sources, and finally, planetary and preplanetary nebulae {PNs & PPNs}. In the case of PNs and ...
We present a study of low- and high-resolution ultraviolet, high-resolution optical CAT/CES spectra and ultraviolet, optical and infrared photometry of the peculiar supergiant HD101584. From the photometry we learn that the ultraviolet and optical energy distribution cannot be fitted in a consistent way and we need a model in which the UV and optical energy distribution are formed by different ...
RESUMEN G. Haro descubri6 que una estrella debll V = 13.11, B-V = -0.15) y de alta latitud galactica presenta emisi6n en Ha, tiene tipo espectral BO-Bi y tiene Ia apariencia de una estrella Be de poblaci6n I. Silo anterior fuera cierto seri dificil explicar su posici6n a Z = 19.7 kpc considerando que se form6 cerca del piano galactico hace solo 1-2 x . Las alas en absorci6n, fuera de los centros ...
As part of our continuing search for peculiar A-type stars, especially ? Bootis stars, in open clusters of all ages, we have obtained classification spectra of 130 late B, A, and early F-type stars in 12 intermediate-age open clusters, including NGC 1039, 2281, 2548, 6633, 7039, 7063, 7092, and 7209, IC 4665, IC ...
Among the twenty A-type stars within a distance of 20 pc from the Sun there are several pairs and a triple composed in each case of stars having similar space motions. The probability of the occurrence of so many such moving systems in this sample is extr...
The ? Bootis stars are a spectroscopic subgroup of chemically peculiar Population I A-type stars. They show significant underabundances of Fe-peak elements and solar abundances of lighter elements. Theoretical explanations include surface phenomena and binarity. We compared the pulsating ? Bootis star HD 210111 ...
Context. The AKARI, a Japanese infrared space mission, has performed an All-Sky Survey in six infrared-bands from 9 to 180 ?m with higher spatial resolutions and better sensitivities than IRAS. Aims: We investigate the mid-infrared (9 and 18 ?m) point source catalog (PSC) obtained with the infrared camera (IRC) onboard AKARI, in order to understand the infrared nature of the known objects and to ...
The study of intermediate mass, evolved stars is undergoing renewed interest due to recent observational and theoretical results suggesting that binarity i fundamental for shaping post-Asymptotic Giant Branch and Planetary Nebula outflows. Despite extensive research, the physical mechanism responsible for transitioning from a spherical Asymptotic Giant Branch (AGB) ...
Cameron, A.G.W., 1971: Nuclear processes associated with peculiar A-type stars. Publ. Astron. Soc. Pacific , 83, 495-591, doi:10.1086/129175. ...
We present a detailed study of the circumstellar envelope of the post-asymptotic giant branch ``21 mu m object'' HD 56126. We build a detailed dust radiative transfer model of the circumstellar envelope in order to derive the dust composition and mass, and the mass-loss history of the star. To model the emission of the dust we use amorphous carbon, hydrogenated amorphous ...
Observations of the N=1-0 and N=2-1 transitions of 12CN and its 13C isotopomer at 1.2 and 3 mm have been conducted towards both molecular clouds and the circumstellar envelopes of various types of stars, using the Kitt Peak 12 m antenna and the Sub-Millimeter Telescope (SMT) of the Arizona Radio Observatory. These observations have been used to establish 12C/13C ratios in ...
Aims: We investigate the magnetic dichotomy between Ap/Bp and other A-type stars by carrying out a deep spectropolarimetric study of Am and HgMn stars. Methods: Using the NARVAL spectropolarimeter at the T�lescope Bernard Lyot (Observatoire du Pic du Midi, France), we obtained high-resolution circular polarisation spectroscopy of 12 ...
We present far-infrared spectra, ? = 52-93 ?m, obtained with the Spitzer Space Telescope in the spectral energy distribution mode of its MIPS instrument, of a representative sample of the most luminous compact far-infrared sources in the Large Magellanic Cloud (LMC). These include carbon stars, OH/IR asymptotic giant branch (AGB) stars, ...
ABS>A number of A-type stars suspected to have weak metallic lines were selected on Curtis Schmidt objective-prism plates, and four-color photometry was obtained for seven of these objects. The photometry confirms that the majority of the stars are in fact metal deficient, and it appears that such stars can be selected with ...
We have demonstrated by experiment that, in an energetic environment, a simple polycyclic aromatic hydrocarbon (PAH) such as naphthalene will undergo chemical reactions that produce a wide array of more complex species (an aggregate). For a stellar wind of a highly evolved star (post-asymptotic giant branch [post-AGB]), this process would be in addition to ...
PubMed
The oxygen isotopic mapping by ion microprobe of the deuterium-rich acid insoluble organic macromolecule extracted from the Murchison meteorite revealed the presence of micrometersized silica-rich grains having extreme 18O/16O and 17O/16O ratios ( ~ 10-1). Such extreme ratios have previously been reported only once : in CO2 from the circumstellar enveloppe of the binary ...
Since the IRAS discovery of Vega's large thermal infrared excess over the expected photospheric flux, dust disks have been found around a large fraction of main-sequence A-type stars. With the detection of gas giant planets orbiting Alpha PsA, Beta Pic, and HR 8799, we now have evidence that A-type stars can host planetary systems. ...
Stellar population synthesis (SPS) provides the link between the stellar and dust content of galaxies and their observed spectral energy distributions. In the present work, we perform a comprehensive calibration of our own flexible SPS (FSPS) model against a suite of data. These data include ultraviolet, optical, and near-IR photometry, surface brightness fluctuations, and integrated spectra of ...
IRC+10216 is the second brightest non-Solar System object in the IR sky, second only to eta Carinae. At 5 microns {its peak flux} it is so bright that most ground-based IR detectors are unable to image it. We {Skinner, Meixner & Bobrowksy, In Preparation} have just obtained WFPC2 snapshot images of it. Briefly, we find that at 5000A it is so faint that we see only a faint blob, but at 814W we ...
We have identified 17 A-type stars in the Galactic Plane that have mid-infrared (mid-IR) excesses at 8 {mu}m. From observed colors in the (r' - H{alpha}) - (r' - i') plane, we first identified 23,050 early A-type main-sequence (MS) star candidates in the Isaac Newton Photometric H-Alpha ...
In the search of extraterrestrial life it was speculated about the existence of a Circumstellar Habitable Zone (CHZ) around stars at which inside planets are suitable for life. Different researches show that the stars types at which CHZ support life are the stars K, G, and F with the type G as the more probable. However in a recent ...
We analyse the light curves of over 9000 A-F stars in the first public release of Kepler data and examine the noise properties in constant or nearly constant stars. For the A stars, we find a correlation between the excess power in certain frequency regions and the effective temperature which may be due to granulation. The majority of ...
New diffraction-limited speckle images of the Red Rectangle in the wavelength range 2.1-3.3 mu m with angular resolutions of 44-68 mas (Tuthill et al. \\cite{Tuthill_etal2002}) and previous speckle images at 0.7-2.2 mu m (Osterbart et al. \\cite{Osterbart_etal1997}; Men'shchikov et al. \\cite{Men'shchikov_etal1998}) revealed well-resolved bright bipolar outflow lobes and long X-shaped spikes ...
for the factors and idiosyncratic terms for the errors. B�ank�ovi et al. (1981, 1983) introduced an iteration the so-called adding-up property of B�ank�ovi et al. (1981), except if all the n time series had the same of their standard deviations as weights w1, . . . , wn in the objective function (4). In B�ank�ovi et al. (1981
ms exposures (60 ,free gas transfer, while the sample was observed from below with bin 2) were taken- and Ted Salmon for their generosity with reagents. We also thank Lisa Cameron, Aaron Groen, Ted Salmon�414. 15. Miyamoto, D., Perlman, Z., Burbank, K., Groen, A., and Mitchi- son, T. The kinesin Eg5 drives
We report multi-epoch, spectroscopy and imaging of the pPN M 2-56, a unique, key object that enables witnessing the current, variable post-AGB wind activity at its core and tracking the rapid evolution of the circumstellar material shocked by short-lived, post-AGB mass ejections. Some of the results from our work tep[fully described in][]{san10} include ...
The study of detached eclipsing binaries is one of the most powerful ways to investigate the properties of individual stars and stellar systems. We present preliminary masses, radii and effective temperatures for the eclipsing binary WW Aurigae, which is composed of two metallic-lined A-type stars. We also reanalyse the data on HD ...
We have decided to address the problem of how abundances and peculiarities change during main sequence evolution. We have setup a program to measure the atmospheric abundance patterns from tens of A-type star members of clusters of different ages, and compare the results with theory predictions. In this paper we present the overall project and we focus on the results obtained ...
As the chemical abundance values of normal A stars overlap those of incipient and moderate Am stars, we examine the abundance anomalies of the Am and superficially normal A stars together. We study the results of 17 consistently performed studies by Adelman and his associates who used Dominion Astrophysical Observatory long camera ...
Archival Spitzer Infrared Array Camera (IRAC) and MIPS observations of the Large Magellanic Cloud (LMC) have been used to search for young stellar objects (YSOs). We have carried out independent aperture photometry of these data and merged the results from different passbands to produce a photometric catalog. To verify our methodology we have also analyzed the data from the SAGE and SWIRE Legacy ...
Aims:We aim to provide observational constraints on diffusion models that predict peculiar chemical abundances in the atmospheres of Am stars. We also intend to check if chemical peculiarities and slow rotation can be explained by the presence of a weak magnetic field. Methods: We have obtained high resolution, high signal-to-noise ratio spectra of eight previously-classified ...
We report the detection of N V 1239 A transition region emission in HST/Goddard High Resolution Spectrograph spectra of the A7 V stars, alpha Aql and alpha Cep. Our observations provide the first direct evidence of 1--3 x 105 K material in the atmospheres of normal A-type stars. For both stars, and for the mid ...
We present 24 ?m Spitzer MIPS photometric observations of the ~50 Myr open cluster IC 2391. Thirty-four cluster members ranging in spectral type from B3 to M5 were observed in the central square degree of the cluster. Excesses indicative of debris disks were discovered around one A star, six FGK stars, and possibly one M dwarf. For the cluster members ...
We present Spitzer MIPS observations at 24 {mu}m of 37 solar-type stars in the Pleiades and combine them with previous observations to obtain a sample of 71 stars. We report that 23 stars, or 32% +- 6.8%, have excesses at 24 {mu}m at least 10% above their photospheric emission. We compare our results with studies of debris disks in ...
Several ? Sct stars were photometrically monitored with the satellite CoRoT and observed in high-resolution spectroscopy from ground. We present here the preliminary results of the abundance analysis of several potential CoRoT targets. Moreover, new insights are given about the problem of the huge number of detected frequencies by comparing ? Sct stars ...
I wondered why there are still slowly rotating (V < 120 km s{sup -1}) normal A0-A9 field stars when such stars should have become Ap or Am stars by a diffusion mechanism. My first guess was that this was related to an unusually high binary frequency, but the frequency turned out to be normal, as shown in a paper in preparation. ...
We report on UBVRI CCD photometry that has been obtained over the course of the past seven years for a sample of over 1600 metal-poor (MP), field horizontal-branch (FHB), and other A-type candidates selected from two large objective-prism surveys, the HK survey of Beers and collaborators and the Hamburg/ESO stellar survey of Christlieb and colleagues. These ...
A-type stars have both a shallow near-surface zone of fast convection that can excite acoustic modes and a deep zone of core convection whose properties may be studied through asteroseismology. Many A stars also exhibit large magnetic spots as they rotate. We have explored the properties of core convection in rotating ...
We present an analysis of ?5 years of Lick Observatory radial velocity measurements targeting a uniform sample of 31 intermediate-mass (IM) subgiants (1.5 ? M?/M ? ? 2.0) with the goal of measuring the occurrence rate of Jovian planets around (evolved) A-type stars and comparing the distributions of their orbital and physical characteristics to those of ...
In order to reveal the star-forming history of the molecular cloud complex we studied the intermediate mass stellar population in the Cepheus Flare region. (Paper I dealt with the distance and the young stellar object candidates of the region.) Correlating the IRAS Point Source Catalogue and Faint Source Catalogue positions with those of 1214 B8-A8 and 1760 F0-F5 type ...
We have studied the CO emission from protoplanetary nebulae (PPNe). Our sample is composed of 37 objects and includes, we think, all well identified PPNe detected in CO, together with the two yellow hypergiants emitting in CO and one young PN. We present a summary of the existing CO data, including accurate new observations of the 12CO and 13CO J=1-0 and J=2-1 lines in 16 objects. We identify in ...
We present the results of 3--D simulations of core convection dynamos within A-type stars of 2 solar masses, at a range of rotation rates. The inner 30% by radius of such stars are considered in our calculations, with the spherical domain thereby encompassing the convective core and some of the surrounding radiative envelope. We ...
We present the results of 3-D nonlinear simulations of magnetic dynamo action by core convection within A-type stars of 2 M? with a range of rotation rates. We consider the inner 30% by radius of such stars, with the spherical domain thereby encompassing the convective core and a portion of the surrounding radiative envelope. The ...
We have carried out 3-D simulations of core convection and dynamo activity within A-type stars of two solar masses at a range of rotation rates. Our models consider the inner 30% by radius of such stars, thus capturing the entire convective core and a portion of the surrounding radiative envelope within the spherical computational ...
Star-forming events in starbursts and normal galaxies have a direct impact on the global stellar content of galaxies. These events create numerous compact clusters where stars are produced in great number. These stars eventually end up in the star field background where they are smoothly distributed. However, ...
A common explanation for the observed X-ray emission of A-type stars is the presence of a hidden late-type companion. While this assumption can be shown to be correct in some cases, a number of lines of evidence suggests that low-mass companions cannot be the correct cause for the observed activity in all cases. A model explains the X-ray emission for magnetic Ap/Bp ...
We have completed an extensive search for stars hosting terrestrial planet zone dust by cross-correlating the Tycho-2 and IRAS catalogs. Near-infrared to far-infrared excess emission has been discovered towards a 10-20 Myr old, A-type member of the Upper-Centaurus-Lupus association. The hot dust component ( 750 K) in combination with the high fractional ...
We present an analysis of ~5 years of Lick Observatory radial velocity measurements targeting a uniform sample of 31 intermediate-mass (IM) subgiants (1.5 lsim M */M sunlsim 2.0) with the goal of measuring the occurrence rate of Jovian planets around (evolved) A-type stars and comparing the distributions of their orbital and physical characteristics to ...
We present an analysis of approx5 years of Lick Observatory radial velocity measurements targeting a uniform sample of 31 intermediate-mass (IM) subgiants (1.5 approx< M{sub *}/M{sub sun}approx< 2.0) with the goal of measuring the occurrence rate of Jovian planets around (evolved) A-type stars and comparing the distributions of their orbital ...
We have identified a star in the Wide Angle Search for Planets (WASP) archive photometry with an unusual light curve due to the total eclipse of a small, hot star by an apparently normal A-type star and with an orbital period of only 0.668 d. From an analysis of the WASP light curve together with V-band and IC-band ...
We have searched for interstellar or circumstellar absorption lines in the center of Ca II K towards bright A-type stars that are mostly within 80 pc of the Sun. Narrow absorption features are found in about 30 % of the 28 normal main-sequence A stars and 18 metal-deficient lambda Bootis stars studied. We have ...
Aims:A sample of v sin i of B9 to F2-type main sequence single stars has been built from highly homogeneous {v sin i} parameters determined for a large sample cleansed of objects presenting the Am and Ap phenomenon as well as of all known binaries. The aim is to study the distributions of rotational velocities in the mass range of A-type stars for normal ...
We have performed abundance analysis of two slowly rotating, late A-type stars, HD 32115 (HR 1613) and HD 37594 (HR 1940), based on obtained echelle spectra covering the spectral range 4000-9850 �. These spectra allowed us to identify an extensive line list for 31 chemical elements, the most complete to date for A-type stars. Two ...
Massive stars are crucial building blocks of galaxies and the universe, as production sites of heavy elements and as stirring agents and energy providers through stellar winds and supernovae. The field of magnetic massive stars has seen tremendous progress in recent years. Different perspectives�ranging from direct field measurements over dynamo theory ...
Massive stars are crucial building blocks of galaxies and the universe, as production sites of heavy elements and as stirring agents and energy providers through stellar winds and supernovae. The field of magnetic massive stars has seen tremendous progress in recent years. Different perspectives - ranging from direct field measurements over dynamo theory ...
The pseudoluminosity effect in the metallic line A-type stars found by Abt and Morgan (1976) is confirmed in a random sample of 27 Am stars. From a morphological study of their spectra in the wavelength interval 3850-4400 A at a reciprocal dispersion of 66 A/mm, revised spectral types are given on the MK system for their K-line and ...
In an attempt of clarifying the connection between the photospheric abundance anomalies and the stellar rotation as well as of exploring the nature of ``normal A'' stars, the abundances of seven elements (C, O, Si, Ca, Ti, Fe, and Ba) and the projected rotational velocity for 46 A-type field stars were determined by applying the ...
In 2009, the rapidly oscillating peculiar A-type (roAp) star 33 Lib was the target of an intense observing campaign, combining ground-based spectroscopy with space photometry obtained with the Microvariability and Oscillation of STars (MOST) satellite. We collected 780 spectra using the Echelle Spectro Polarimetric Device for the ...
We present the results of a medium-resolution spectroscopic survey of 43 field horizontal-branch (FHB) candidates carried out near the south galactic pole, selected from the original list of FHB candidates compiled by Beers et al. (2007). The observation list includes only stars classified as �high-probability� candidates, according to their 2MASS infrared colours. ...
We present the results of three-dimensional simulations of core convection within A-type stars of 2 Msolar, at a range of rotation rates. We consider the inner 30% by radius of such stars, thereby encompassing the convective core and some of the surrounding radiative envelope. We utilize our anelastic spherical harmonic code, which ...
A common explanation for the observed X-ray emission of A-type stars is the presence of a hidden late-type companion. While this hypothesis can be shown to be correct in some cases, there is also evidence suggesting that low-mass companions cannot be the proper cause for the observed X-ray activity in all cases. Babel and Montmerle (1997) presented a theoretical framework to ...
