bluuebunny/crossref_metadata_2025 huggingface dataset:
A Galaxy Merging Sequence Traced by X-rays
Authors: Gao Yu , Wang Q. Daniel , Markowsky Ted A.
Published: July 2016

We are studying a sample of nearly 20 nearby (cz≲13,000 km s−1) IR-luminous interacting/merging galaxies observed with the ACIS in the Chandra archive (e.g., Gao et al. 2003; Ptak et al. 2003). Most galaxies in the sample are luminous infrared galaxies (LIGs) and nearly half of them are ultraluminous infrared galaxies (ULIGs) with bolometric luminosities comparable to QSOs. Based on multiwavelength data and numerical simulations of gas-rich galaxy mergers, we have attempted to arrange them in a merger sequence perspective and compared their broadband X-ray emission here with the optical images (mostly DSS).
HST Observations of the Toomre Sequence of Merging Galaxies
Authors: Rossa Jörn , Van Der Marel Roeland P. , Böker Torsten , Laine Seppo , Mihos J. Christopher , Hibbard John E. , Zabludoff Ann I.
Published: July 2016

We discuss our ongoing multi-instrument HST investigation of the nuclear regions of the 11 interacting and merging galaxies in the Toomre Sequence. We are studying the nuclear kinematics using STIS (G750M) spectra, the nuclear stellar populations using STIS (G430L) spectra, and the nuclear morphology using NICMOS and WFPC2 images. The results will provide new insight into the physical processes that operate during galaxy interactions.
HI and Hot Gas in the Outskirts of the M81 Group
Authors: Bureau M. , Walter F. , van Gorkom J. , Carignan C.
Published: July 2016

Results are presented from a wide area, high resolution HI synthesis survey of the outer regions of the nearby M81 group, where internal (galactic) and external (group-related) evolution processes can be studied simultaneously in great detail. The survey encompasses the star forming dwarf galaxies M81dwA, UGC4483, and HoII, where evidence of ram pressure stripping was recently discovered. The data do not reveal any intergalactic HI, but the outer parts of HoII are reminiscent of tidal tails. We argue however that those structures are equally consistent with the latest ram pressure models including cooling. The case for a hot intergalactic medium in this poor, spiral-only group is thus still open. The survey also puts tight constraints on possible counterparts to the local high velocity cloud population in an external group, reaching a 3σ column density of 1019 atom cm−2 and a 6σ limiting mass of 1.5 × 105M⊙.
A Tidal Dwarf Galaxy in the Hercules Cluster?
Authors: van Driel W. , Duc P.-A. , Amram P. , Bournaud F. , Balkowski C. , Cayatte V. , Dickey J. M. , Hernández H. , Iglesias-Páramo J. , O'Neil K. , Papaderos P. , Vílchez J.M.
Published: July 2016

A candidate Tidal Dwarf Galaxy, ce-61, was identified in the merger system IC 1182 in the Hercules supercluster. The multi-wavelength data we obtained so far do not prove, however, that it is kinematically detached from the IC 1182 system and gravitationally bound.
OPENING ADDRESS: The Magellanic Clouds: distance, structure, chemical composition
Authors: Feast M. W.
Published: July 2016

As an introduction to the symposium the related problems of the distances, structures and chemical compositions of the Magellanic Clouds are reviewed.
Carbon Monoxide in the Magellanic Clouds
Authors: Israel F. P. , De Graauw Th.
Published: July 2016

Molecular gas is a major constituent of the interstellar medium of all late-type galaxies. Virtually all of it is in the form of cold molecular hydrogen (H2) which today cannot be observed directly. However, the tracer molecule carbon monoxide (CO) (relative abundance 10−5) is easily detected. For the Magellanic Clouds (MCs), CO studies are of specific importance. The Clouds are rich in HI, and if we can establish the presence of significant amounts of H2 as well, this will influence our estimates of the global rate of star formation and its history. Complexes of presently quiescent molecular gas may betray regions primed for star formation, but not yet in action. Detailed studies of the HI, HII and H2 and young star content may provide estimates of star formation efficiences on scales of a kiloparsec.
The magnetic field strength in the Large Magellanic Cloud
Authors: Costa M.E. , McCulloch P. M. , Hamilton P. A.
Published: July 2016

We have measured a value of 4±5m--2rad for the rotation measure of the radio pulsar PSR0529-66 in the LMC and, after allowing for the dispersion and rotation measures of our Galaxy on the pulsar's line of sight, we deduce that the magnetic field strength in the LMC is in the range 0 to 5μGauss oriented away from the Sun.
Turbulence and stochastic star formation in the Large Magellanic Cloud
Authors: Feitzinger J. V. , Spicker J.
Published: July 2016

A recent turbulence analysis of the Large Magellanic Cloud (LMC) is summarized. New model calculations on stochastic self-propagating star formation taking into account the multi-phase structure of the interstellar medium are presented. The relationship between star formation and turbulence is discussed.
The Molonglo 843 MHz survey of the Magellanic Clouds
Authors: Turtle A. J. , Amy S. W.
Published: July 2016

A radio survey of the Magellanic Clouds (MCs) at 843 MHz has been completed and is being prepared for publication. The survey was made with the Molonglo Observatory Synthesis Telescope (MOST) operated by the Astrophysics Department, University of Sydney. This survey of the Clouds has a high angular resolution (45 arcsec) and a sensitivity of a few mJy/beam even for objects of large angular extent
Star formation in the Magellanic Clouds
Authors: Hyland A. R.
Published: July 2016

An overview of the understanding of processes of star formation in the Magellanic Clouds, including a historical perspective, is given. The current status is reviewed with emphasis on evidence for sequential star formation in large associations, on optical and infrared discoveries of possible pre-main sequence objects in luminous young clusters and 30 Doradus. The history of star formation in both Clouds, and evidence for differences between the processes of star formation in the Clouds and the Milky Way are discussed.
HII regions and star formation in the Magellanic Clouds
Authors: Kennicutt Robert C.
Published: July 2016

The H II regions in the Magellanic Clouds provide an opportunity to characterize the global star formation properties of a galaxy at close range. They also provide a unique laboratory for testing empirical tracers of the massive star formation rates and initial mass functions in more distant galaxies, and for studying the dynamical interactions between massive stars and the interstellar medium. This paper discusses several current studies in these areas.
Similarities of the intermediate-age SMC star cluster NGC 152 with inner disk clusters in the Milky Way
Authors: Richtler T. , De Boer Klaas S. , Vallenari A. , Seggewiss W.
Published: July 2016

A colour-magnitude diagram (CMD) of the region containing the intermediate-age SMC globular cluster NGC 152 was published recently (Melcher & Richtler 1989). A particularly interesting feature of this CMD is the “clump” of He-core burning stars, which are predominantly field stars. A selection of stars near the cluster centre leads to the CMD shown in Figure 1. The vertical extension of the clump (explainable by the evolution of stars younger than 1 Gyr) is replaced by a “tilted horizontal branch” (we use this expression for lack of a better one). The age of NGC 152 is about 1.3 Gyr and the reddening is small; the metallicity is unknown but less than −0.6 dex, which is the mean metallicity of the young SMC population. The tilted HB can be reproduced in CMD simulations using the method developed by Vallenari et al. (1990), and thus can be considered as a normal feature of star clusters like NGC 152. It is evident also in other intermediate-age MC clusters like Kron 3 (Rich et al. 1984).
Line observations of the 30-Dor complex and N159A5 with the MPE imaging NIR spectrometer FAST
Authors: Krabbe A. , Storey J. , Rotaciuc V. , Drapatz S. , Genzel R.
Published: July 2016

Images with subarcsec spatial resolution in the light of near-infrared atomic (Bry) and molecular hydrogen H2 (S(1) v=1-0) emission lines were obtained for some extended, pointlike objects in the Large Magellanic Cloud (LMC) for the first time. We used the Max-Planck-Institut für extraterrestrische Physik (MPE) near-infrared array spectrometer FAST (image scale 0.8”/pix, spectral resolving power 950) at the ESO/MPI 2.2m telescope, La Silla. We present some results on the 30-Dor complex and N159A5.
Search for UHE gamma ray sources in the Magellanic Clouds
Authors: Bond I. A.
Published: July 2016

An experimental survey of ultra-high-energy γ-ray sources in the Magellanic Clouds is described. Results of the survey may be used to study the intergalactic magnetic field.
Charting cool carbon stars in the Large Magellanic Cloud
Authors: M.F. McCarthy S.J. , Blanco V. M.
Published: July 2016

We describe the preparation of charts and positions for 1035 cool red giant carbon stars discovered by the authors in the Large Magellanic Cloud (LMC). Not considered here are Small Magellanic Cloud (SMC) carbon stars, nor the M-type giants found in the LMC and SMC (Blanco et al. 1980, Blanco & McCarthy 1983). Charts of the C stars discovered are available in three formats: (a) 12 charts contain 186 C stars discovered in three central LMC regions (Blanco et al. 1980); (b) charts of C and late M stars in 52 LMC regions; these charts will not be published but are available from the authors (Blanco & McCarthy 1983, discussion); and (c) charts for 849 individual C stars in 49 regions of LMC (Blanco & McCarthy 1990).
A search for long period variables in the SMC
Authors: Moore G.K.G.
Published: July 2016

A search for long period variables in the Small Magellanic Cloud (SMC) was made using I plates from the U.K. Schmidt telescope. Variables were identified using a blink comparator built specially for this project and plates were digitised to determine periods and light curves for suspected variables. Over 300 new variables with periods in excess of 100 days were found, some variables having periods in excess of 1000 days.
Carbon and nitrogen in the cool supergiant A7 of the young globular cluster NGC 330 in the SMC
Authors: Barbuy B. , Milone A. , Spite M. , Spite F.
Published: July 2016

C2 and CN bands in the wavelength region λ 560 - 680 nm were used to obtain the carbon and nitrogen abundances in the supergiant star A7 of the young, metal-poor, globular cluster NGC 330 in the Small Magellanic Cloud (SMC).
Mass loss and evolution of massive stars in the Magellanic Clouds
Authors: Leitherer Claus , Langer Norbert
Published: July 2016

The structure and evolution of massive stars is significantly influenced by effects of chemical composition in a low-metallicity environment (as compared to the solar neighbourhood, SN), such as the Magellanic Clouds. A fundamental ingredient in evolutionary models is the stellar mass-loss rate M. Lower metal content decreases the mass-loss rates derived theoretically, which in turn affects the stellar evolution models. On the other hand, different evolutionary models predict different stellar parameters (especially L), which again influence M so that an iterative procedure is required to achieve self-consistency.
Measurement of the absolute proper motions of the Magellanic Clouds
Authors: Tucholke H.-J. , Hiesgen M.
Published: July 2016

Currently, we are measuring the absolute proper motions of the Magellanic Clouds relative to background galaxies, using plates taken with the ESO Schmidt Telescope. In spite of the small epoch difference of about 15 years, an accuracy of 0.5-1.0 milliarcsecs (mas) may be achieved using large numbers of stars and galaxies. Measurement and reduction procedures are presented; a preliminary solution for the absolute proper motion of the LMC from the measurements of three plates gives a result similar to the independent study of Jones et al. (1989).
The Analysis of AGB Star Photospheres
Authors: Smith Verne V.
Published: July 2016

The presence of excess 12C, 13C, 14N, Li, and the heavy s-process elements in the photospheres of AGB stars betray the operation of Eland 4He-burning, hot-bottom burning, and slow neutron-capture nucleosynthesis. Careful abundance analyses of these various elements provides insight into both nucleosynthesis events and mixing processes which occur deep within the AGB star. Recent results for the s-process are discussed.
Lithium and s-process enrichment in massive galactic AGB stars
Authors: García-Lario P. , D'Antona F. , Lub J. , Plez B. , Habing H.J.
Published: July 2016

We present the first results of a wide observational program intended to study the effects of hot-bottom burning and of the third dredge-up in the chemical evolution of the most luminous AGB stars in our Galaxy. The main goal is to provide new observational constraints to the models through the analysis of lithium and s-process element abundances in a carefully selected sample of galactic AGB stars. Our results indicate that, unlike observed in the Magellanic Clouds, the most massive AGB candidates in our Galaxy do not show any significant enrichment in s-process elements. In addition, only some of them are found to be Li-rich.
Infrared molecular spectra of carbon stars observed by the ISO SWS
Authors: Aoki W. , Tsuji T. , Ohnaka K.
Published: July 2016

We carried out observations of bright optical carbon stars with the ISO SWS, and detected the absorption features of CO, CS, CH, SiS and HCN in N-type and SC-type stars. The CH fundamental bands in 3-4 μm in N-type stars are stronger than those in SC-type stars. On the other hand, the first overtone bands of SiS at 6.6 μm were detected in WZ Cas (SC-type), while none of them were detected in N-type stars. These results can be explained well by the lower C/O ratio in SC-type stars than in N-type stars. The absorption features of HCN (v1, v2 + v3 and v1 − v2) are stronger in SC-type stars than in N-type stars. This fact can be explained by the cooler atmospheres of our SC-type stars due to the lower C/O ratio as well as due to the lower effective temperature, compared with the N-type stars in our sample. The CS bands and the band heads of the CO fundamental bands are weaker in the observed spectra than in those predicted by our model atmospheres. This result cannot be explained by the present model atmosphere and one possible explanation is the contribution by the emission of CO and CS in the outer atmosphere.
Laboratory Astrophysics of Circumstellar Dust
Authors: Henning Th.
Published: July 2016

This paper summarizes laboratory investigations of Stardust analogue materials with special emphasis on the spectroscopic behaviour of silicates and carbonaceous materials. The review will also deal with the identification of the observed features in the dust spectra of AGB stars.
ISOSWS Spectral Variations of Oxygen-Rich Miras
Authors: Creech-Eakman M. J. , Stencel R. E.
Published: July 2016

In conjunction with an extensive ground-based monitoring program of 32 oxygen-rich Mira variables, a subset of this sample has been monitored with phase using ISO's Short Wavelength Spectrometer (ISOSWS) and ground-based mid-infrared spectrometers. Some of the six sources will be presented here in 2.5–45 μm ISOSWS spectra and 7.5–13.5 μm ground-based spectra using CGS3. Discussion of the spectral features of SiO, CO, H2O, and silicates will be presented. These data are considered in the context of recent demonstrations of variations in the spectral features with phase of the Miras.
Infrared interferometry of circumstellar envelopes
Authors: Monnier John D.
Published: July 2016

This paper will review the technical progress of interferometric infrared observing techniques from the first 2-element interferometer 25 years ago to the 3+ element arrays now coming into service. To date, only the Infrared Spatial Interferometer (ISI) has published separate-element interferometric data on circumstellar dust shells in the infrared and many of these scientific results will be discussed. Speckle interferometry has also evolved significantly over the last few decades as slit-scanning techniques over single-pixel detectors have largely been replaced by fast-readout of large format detector arrays. Important near-infrared and mid-infrared results derived from speckle data will also be reviewed.

Until recently, two-dimensional information about circumstellar dust distributions has been sorely lacking, hence detections of dust shell asymmetries have been difficult and uncertain. New speckle observations using modern, 10-m class telescopes have yielded surprising results, demonstrating the importance of accurate closure phase information in interpreting interferometric data. These discoveries hopefully precursor those to be made from closure-phase imaging with the new generation of separate-element, interferometric arrays.
Imaging of Stellar Disks and Mass Loss Envelopes in Evolved Stars
Authors: Tuthill P.G. , Monnier J.D. , Danchi W.C.
Published: July 2016

Interferometric imaging of evolved stars has been performed in the near-IR at the Keck Observatory revealing diffraction-limited structure at unprecedented spatial resolution. By studying the hottest regions of the circumstellar dust shells of a number of bright prototypical objects, we have revealed complex and anisotropic structures in the inner regions where dust nucleation occurs. We find that dramatic dust plumes and clumps dominate the morphology of the circumstellar environment at a scale of a few stellar radii for extreme carbon stars such as IRC+10216 and CIT 6. Furthermore our observations have yielded near-IR diameters and images of the stellar photospheres of nearby red giant and supergiant stars. With the additional benefit of multi-epoch observations, we have begun to investigate the time-evolution of our enshrouded objects as dust is created and accelerated away from the nucleation zone. Armed with such a powerful new set of observational constraints, we are able to address contemporary problems such as the origin of structure in PNe, the physics of mass loss, and the dynamical properties of Mira pulsation.
AGB circumstellar envelopes: molecular observations
Authors: Bujarrabal V.
Published: July 2016

Due to the low excitation requirements and easy observation from the ground, molecular line observations probably constitute our main source of empirical knowledge on circumstellar envelopes (CSEs) around AGB stars. The CO rotational transitions, the most intense ‘thermal’ lines, are efficiently used to determine the total gas content and its spatial distribution in wide samples of objects. Thermal emission from other molecules is mainly useful in order to study their abundances and the chemical reactions taking place in CSEs. Maser lines are easily observed due to their high intensity and the flux distribution in very compact spatial spots and narrow profile spikes, characteristic of the exponential amplification; however the data interpretation is difficult due to the intricate pumping processes. The most important maser lines (of SiO, H2O and OH) arise from very different regions, which allows the study of various components of the CSEs. We will focus here on SiO masers.
Link between Mass-loss and Variability Type for AGB Stars?
Authors: Ivezić Željko , Knapp Gillian R.
Published: July 2016

We find that AGB stars separate in the 25–12 vs. 12-K color-color diagram according to their chemistry (O, S vs. C) and variability type (Miras vs. SRb/Lb). While discrimination according to the chemical composition is not surprising, the separation of Miras from SRb/Lb variables is unexpected.

We show that “standard” steady-state radiatively driven models provide excellent fits to the color distribution of Miras of all chemical types. However, these models are incapable of explaining the dust emission from O-rich SRb/Lb stars. The models can be altered to fit the data by postulating different optical properties for silicate grains, or by assuming that the dust temperature at the inner envelope radius is significantly lower (300–400 K) than typical condensation temperatures (800–1000 K), a possibility which is also supported by the detailed characteristics of LRS data. While such lower temperatures are required only for O- and S-rich SRb/Lb stars, they are also consistent with the colors of C-rich SRb/Lb stars.

The absence of hot dust for SRb/Lb stars can be interpreted as a recent (order of 100 yr) decrease in the mass-loss rate. The distribution of O-rich SRb/Lb stars in the 25–12 vs. K-12 color-color diagram shows that the mass-loss rate probably resumes again, on similar time scales. It cannot be ruled out that the mass-loss rate is changing periodically on such time scales, implying that the stars might oscillate between the Mira and SRb/Lb phases during their AGB evolution as proposed by Kerschbaum et al. (1996). Such a possibility appears to be supported by recent HST images of the Egg Nebula obtained by Sahai et al. (1997), the discovery of multiple CO winds reported by Knapp et al. (1998), and long-term visual light-curve changes detected for some stars by Mattei (1998).
Evolutionary connection between C-rich AGB stars and C-rich central stars of PNe
Authors: Parthasarathy M.
Published: July 2016

On the basis of C/O ratios the evolutionary connection between C-rich AGB/post-AGB stars and C-rich central stars of PNe is discussed. The proportion of carbon-rich post-AGB stars is comparable to that of carbon-rich PNe. The post-AGB stars with 21μm emission are found to be over abundant in carbon and s-process elements indicating that they have gone through the third dredge-up and carbon star phase on the AGB. As these stars evolve to higher temperature they will form carbon-rich PNe with carbon-rich central stars. Analysis of C/O ratios of PNe suggests that 68 percent of PNe have C/O > 1 indicating that the progenitors have gone through the third dredge-up and carbon-rich phase on the AGB. The Type I PNe can be divided into two groups. Group I Type I PNe are those with C/O < 1 indicating that their massive progenitors have gone through the third dredge-up and hot bottom burning on the AGB. Group II Type I PNe are those with C/O > 1. Their progenitors have gone through third dredge-up only and may not have experienced hot bottom burning. Carbon stars with silicate dust and or carbon-rich post-AGB stars (such as post-AGB stars with 21μm emission) may be the progenitors of PNe with [WC] central stars. The evolutionary sequence seems to be: carbon-rich AGB → carbon-rich post-AGB → [WC late type] → [WC early type] → [WC-PG 1159] → PG 1159.
Mass loss and AGB evolution in extra-galactic stellar populations
Authors: Zijlstra Albert A.
Published: July 2016

The effect of metallicity on the AGB mass loss is reviewed. Observations have mainly been limited to the Magellanic Clouds but are observationally feasible throughout the Local Group. Expansion velocities are predicted to depend strongly on Z but the peak mass-loss rates appear not to. The Mira PL relation shows no evidence for a Z dependence, giving a powerful potential distance indicator. I derive a distance modulus to the LMC of m − M = 18.63 ± 0.09 as well as a bright calibration for the Horizontal Branch in globular clusters. Finally, the predicted initial-final mass relation at low Z is shown to give rise to higher mass remnants. This would result in an increased supernova rate in young, low-Z populations, such as found in the early Universe.
Systematic study of AGB stars in the intermediate-age globular clusters in the Magellanic Clouds
Authors: Tanabé T. , Nishida S. , Nakada Y. , Onaka T. , Glass I. S. , Sauvage M.
Published: July 2016

We performed systematic infrared observations of the intermediate-age Magellanic Clouds clusters NGC 419, NGC 1783 and NGC 1978. Mid-infrared stars discovered in NGC 419 and NGC 1978 are very red and must be undergoing intense mass loss (comparable to superwinds). They are probably carbon stars but do not seem to show any FIR excesses. Three optically visible carbon stars as well as (at least) 2 near-infrared carbon stars observed with ISOPHOT show 60 μm excesses which may indicate mass loss in the past. It seems that the MIR stars are fainter than the AGB tip luminosity and that their Mbols are close to those of the transition luminosity from M type to C stars. Therefore, these MIR stars may not be in the final stage of the AGB phase. This may suggest that AGB stars lose mass heavily at some other time, possibly during the transition from M type to C stars.
The Distance and Orientation of the Galactic Bulge from Mira Variables: A Preliminary Report
Authors: Catchpole R. M. , Whitelock P. A. , Feast M. W. , Hughes S.M.G. , Alard C. , Olivier E.
Published: July 2016

Near-infrared, JHKL, observations of 595 Mira variables in two fields on either side of the centre of our Galaxy, confirm that the Bulge is not spherically symmetric about its axis of rotation, but is elongated so that the part to the east of the centre is closer to us. The shape of the Bulge about its axis of rotation is not uniquely defined by these data, but the shape that deviates least from circular symmetry has an axis ratio xo/yo = 1.7, with a major axis at an angle, θ = 58° ± 7, to the plane of the sky, for a galactic centre distance, R0 = 9.4 ± 0.5 kpc. This is based on an assumed scale length in galactic coordinates of bo = 375 pc and lo/bo = 2.0.
Old Structure – Young Stars The Stellar Content of the Centre of the LMC Bar
Authors: Linde P. , Ardeberg A. , Gustafsson B.
Published: July 2016

The history of star formation and chemical evolution are studied in the LMC Bar centre with the HST PC and WFC and uvby photometry. Using dedicated image processing, we secured high spatial resolution and photometric quality. We present colour magnitude diagrams (CMDs) from PC and WFC y and b data. The PC provides a CMD close to complete to V = 23.5, the WFC contributes favourable statistics on brighter stars. We find a population of stars seemingly around or younger than 0.2 Gyr comprising around 30% of the total amount of stars. Star formation seems to have decreased 3-0.2 Gyr ago. Older populations are aged 3–9 Gyr. Stars older than 10 Gyr seem rare if not absent. Our CMD morphology and, especially, our uvby metallicity index give [Me/H] close to −0.4, with older stars more and younger stars less metal poor.
Carbon Stars in the Sagittarius Dwarf Galaxy
Authors: Whitelock Patricia , Menzies John , Irwin Mike , Feast Michael
Published: July 2016

Coordinates are listed for 26 carbon stars which are radial velocity members of the Sagittarius Dwarf Galaxy (SDG). Their near-infrared properties indicate that most are on an extended Asymptotic Giant Branch (AGB), and the total number of such stars in the SDG is estimated at about 100. At least one-third of the C stars studied are large amplitude, i.e. Mira, variables; these are the first Miras to be found in a dSph galaxy. Surprisingly, the most luminous C star is not a Mira, but a low amplitude variable.

A comparison of the properties of C stars in the Bulge and in the SDG indicates that the two groups cannot be drawn from the same population. It is therefore highly unlikely that the Bulge C stars are, contrary to previous suggestions, related in any way to the SDG.
New Photometric Surveys of the Fornax Dwarf Spheroidal Galaxy
Authors: Smecker-Hane T.A. , Stetson P. B. , Hesser J. E.
Published: July 2016

We report on our new photometric surveys of star fields in the Fornax dwarf spheroidal (dSph) galaxy designed to investigate its star formation and chemical evolution histories.
Stellar Population of the Decoupled Nucleus in M31
Authors: Sil'chenko O.K. , Burenkov A. N. , Vlasyuk V. V.
Published: July 2016

Spectral and imaging results for the central region of M31 are presented. An analysis of absorption-index radial profiles involving Mg, Fe, and Ca lines reveals that the unresolved core of M31 is distinct from the inner bulge in having a higher metallicity; of the two nuclei of M31, it is the fainter in visible light and is located exactly in the dynamical center of the galaxy (and dynamically decoupled) which is chemically distinct. By including the Balmer absorption line Hβ in the analysis, we have disentangled metallicity and age effects; a stellar age difference by a factor of three is found between the nucleus and the bulge, the nucleus being younger. A morphological analysis of CCD images has shown the presence of an inner stellar-gaseous disk with a radius of some 80 pc which is embedded into the bulge.
The Application of Artificial Neural Networks: A Catalog of Spectral Indices
Authors: Gulati R. K. , Bravo A. , Padilla G. , Altamirano R. L.
Published: July 2016

Artificial neural networks have been applied to a catalog of spectral indices based on low-resolution spectra in the wavelength regions 3820–4500 Å and 4780–5450 Å. We classify the spectral indices using supervised neural networks. This project is in continuation of the previous efforts by one of us, applying new tools to determine basic properties of stars. We envisage the use of such methods for stellar contents studies of local stellar systems for which spectroscopic surveys are underway.
Applications of Multi-object Spectroscopy – A large Sample of Carbon Stars in the LMC
Authors: Cannon Russell , Offer Alison , Hatzidimitriou Despina , Croke Barry , Morgan David
Published: July 2016

The application of multi-object instruments to nearby galaxies permits the spectroscopy of large samples of faint stars. Such data are needed to determine the kinematics and metallicity distributions of stellar systems, to complement the excellent photometric data now becoming available. Together, these data should reveal the star formation and evolutionary histories of Local Group galaxies. The Two-degree Field (2dF) instrument on the Anglo-Australian Telescope is described and some initial results are presented for a sample of 700 carbon stars in the Large Magellanic Cloud. The velocity precision is more than adequate for kinematic studies, while the application of Principal Component Analysis techniques promises to yield an objective and quantitative classification scheme for carbon stars. This may lead to a better understanding of the carbon stars themselves, as well as enhancing their use as tracers of the intermediate-age and old stellar populations. Another application of multi-fibre spectroscopy, to a sample of main sequence stars in the globular cluster 47 Tuc, illustrates the possibility of combining the data for many faint stars to yield composite spectra with relatively high signal-to-noise ratio.
Prospects for Studying the Local Group with the Subaru Telescope
Authors: Sekiguchi K.
Published: July 2016

Subaru Telescope is an 8.3-m diameter optical-infrared new-generation telescope under construction on Mauna Kea, Hawaii. The telescope is expected to be operational by early 1999. The current status of its construction and the plans for its instrumentation are presented. The design specifications of the telescope and its seven common user instruments are described, with emphasis on their application to studies of the Local Group.
The VIIth Catalogue of Galactic Wolf-Rayet stars
Authors: van der Hucht Karel A.
Published: July 2016

The VIIth catalogue of Galactic Population I Wolf-Rayet stars provides improved coordinates, spectral types and b, v, K photometry of known WR stars, and it adds 62 new WR stars to the previous WR catalogue. The present census of Galactic WR stars stands at 218, including 117 WN stars, 87 WC stars, 11 WN/WC stars and 3 WO stars.
Spectroscopic analysis of OB stars in Cygnus OB2
Authors: Corral Luis , Herrero Artemio , Villamariz M.R. (Charo) , Martín Eduardo L.
Published: July 2016

We obtain the stellar parameters of eleven OB stars in Cyg OB2 using model atmospheres. Each of the analysed stars shows an enhanced Helium abundance. We releate this is to the young agae of the OB association and/or the low rotational velocity of the stars.
The nature of NaSt1 from Keck spectroscopy
Authors: Crowther Paul A. , Smith Linda J.
Published: July 2016

The availability of 8–10m telescopes now permit the routine observation of faint, heavily reddened peculiar emission line objects, which may provide new information on stellar evolution. One such object is NaSt1 (WR 122, IRAS 18497+0056), discovered by Nassau & Stephenson (1963) who proposed a WR classification because of its strong emission line spectrum. Massey & Conti (1983) identified NaSt1 as a peculiar WN10 star. However, subsequent work has argued against a genuine WR identification, favouring either B[e], O[e] or Ofpe/WN9 spectral types from optical and IR spectroscopy (e.g., van der Hucht et al. 1997). NaSt1 has recently received renewed attention, principally because it is extremely bright in the IR (K = 6.5 mag). Indeed, NaSt1 is currently used as a late WN-type IR spectral standard, despite its uncertain nature.
Optical spectroscopic variability of possible Wolf-Rayet binaries
Authors: Flores Aaron , Cardona Octavio , Koenigsberger Gloria
Published: July 2016

There are a number of Wolf-Rayet stars which, based on their emission line profile variations, were proposed to be WR+cc systems (Firmani et al. 1980; Cherepashchuk & Aslanov 1984). However, recent intensive monitoring with different observational techniques, has given rise to alternative explanations for the variability in these WR stars including the presence of a disk, rotating jet-like structures (Vreux et al. 1992), and corotating interaction regions (St-Louis et al. 1995).
The runaway supergiant HD 188209 (O9.5Iab): a binary or a pulsating star?
Authors: Israelian Garik , Herrero Artemio , Santolaya-Rey E. , Kaufer A. , Musaev F. , Galazutdinov G. , Galeev A.
Published: July 2016

We report radial velocity studies of photospheric absorption lines from spectral time series of the late O-type runaway supergiant HD 188209. Radial velocity variations with a quasi-period ∼ 2 days have been detected in high-resolution echelle spectra and most probably indicate that the supergiant is pulsating. Night-to-night variations in the position and strength of the central emission reversal of the Hα profile have been observed. The fundamental parameters of the star have been derived using state-of-the-art plane-parallel and unified non-LTE model atmospheres, these last including the mass-loss rate. The binary nature of this star is not suggested either from Hipparcos photometry or from radial-velocity curves.
Discrepancies between observational and theoretical parameters for three O-type binaries
Authors: Penny Laura R. , Gies Douglas R. , Bagnuolo W.G.
Published: July 2016

Here we present fundamental parameters (masses, spectral classifications, rotational velocities) obtained from a combination of spectroscopic and photometric orbital solutions for the O-type binary systems HD 152248, HD 149404, and HD 135240. We also present estimated masses derived from theoretical single star evolutionary tracks for these same stars. In all cases the evolutionary masses are larger than those derived from observations.
On the so-called WO stars
Authors: Polcaro V. Francesco , Norci Laura , Rossi Corinne , Viotti Roberto
Published: July 2016

The Wolf-Rayet WO spectral sequence was introduced by Barlow & Hummer (1982) for four Pop I WR stars with very strong O VI 381 nm doublet emission. At the time, the evolution of very massive stars was poorly known, and their classification was based on phenomenological grounds. Recently, efforts to improve the WO spectral subtype classification are increasing (Kingsburgh et al. 1995; Crowther et al. 1998).
On the perspectives of using XMM to study fundamental parameters of early-type stars
Authors: Rauw Gregor , van der Hucht Karel A. , Mewe Rolf , Güdel Manuel , Vreux Jean-Marie , Gosset Eric , Schmutz Werner , Stevens Ian R.
Published: July 2016

Although substantial progress has been achieved since the discovery of X-ray emission from early-type stars with the EINSTEIN satellite, several crucial aspects of this phenomenon are still not fully understood. Considerable breakthroughs in this field are expected from observations with the X-ray Multi-Mirror satellite (XMM) due for launch in early 2000. XMM is the second cornerstone mission of the ESA Horizon 2000 science programme (see Lumb et al. 1996 and references therein for an overall description of the satellite). XMM offers a large effective area over a wide range of energies and its instrumentation provides simultaneously non-dispersive spectroscopic imaging (EPIC - European Photon Imaging Camera), medium-resolution dispersive spectroscopy (RGS - Reflection Grating Spectrometer) and optical-UV imaging (OM - Optical Monitor).
Revised stellar parameters of Wolf-Rayet stars
Authors: Schmutz Werner , De Marco Orsola
Published: July 2016

We discuss the reliability of the stellar parameters derived from spectroscopic analyses with atmosphere models. We follow the revisions of the stellar parameters of a WN4b star through improvements of the model codes carried out in the last ten years. In order to judge the reliability of the current results we have started to analyze binaries for which the mass of the WR star is known and therefore its luminosity can be derived from the theoretical mass-luminosity relation. Along this line, we present preliminary results from work in progress on the binary γ2 Velorum.
γ2 Velorum revisited
Authors: De Marco Orsola , Schmutz Werner , Koesterke Lars , Hamann Wolf-Rainer
Published: July 2016

We have carried out a full spectroscopic analysis of the WC8 star in the γ2 Vel binary (WR11). Through the binary radial-velocity curve the mass of the WR star can be determined, and hence its luminosity can be derived through the mass-luminosity relation for WR stars. This can be compared to the luminosity from the spectroscopic analysis. We find that the standard modeling methods underestimate the luminosity of the WR star.
Spectroscopic analysis of O-type stars in the upper part of the HR diagram
Authors: Herrero Artemio , Villamariz M.R. (Charo)
Published: July 2016

We present observations and analyses of stars earlier than 06. The analyses are done with nlte planeparallel, hydrostatic model atmospheres. The effect of line-blocking for the determination of the stellar parameters is studied, and it is concluded that it must be taken into account for very hot stars. It is also shown that planeparallel models cannot fit properly the spectra of very hot stars. Thus, we perform new analyses with Unified Model Atmospheres, which result necessary for the reproduction of the spectra from the very upper part of the HR diagram.
Abundances of Wolf-Rayet stars as metallicity tracers
Authors: Machado Maria Auxiliadora Delgado
Published: July 2016

We present here the first results of a grid of helium-carbon-oxygen models for Wolf-Rayet stars of the carbon sequence (WC). The models were calculated with a numerical code that assumes the basic considerations of the standard model.
Spectral variability versus radial-velocity changes in WR 1
Authors: Niedzielski Andrzej
Published: July 2016

We discuss the possibility that WR 1 (HD 4004) is a binary system.
Hydrodynamics of ring nebulae: magnetic vs. non-magnetic hydro-models
Authors: García-Segura Guillermo , Langer Norbert , Różyczka Michał , Franco José , Mac Low Mordecai-Mark
Published: July 2016

The formation of ring nebulae and their shapes is studied by numerical simulations. The old problem of the origin of the nebular shape is addressed. Two proposed mechanisms are discussed: stellar rotation and stellar magnetic fields, or a combination of both. Two and three-dimensional hydrodynamical and magneto-hydrodynamical simulations indicate that these processes are equally efficient in driving elliptical nebulae such as those surrounding Wolf-Rayet stars, like NGC 6888 around WR136. However, bipolar nebulae are better reproduced by rotation.
Non-thermal radio emission in Wolf-Rayet stars: is binarity a pre-requisite?
Authors: Dougherty Sean M.
Published: July 2016

Radio observations of Wolf-Rayet stars currently available in the literature are examined to determine whether binarity is a common feature of WR systems with non-thermal emission. Among 24 stars with observed spectral index values, seven are definite non-thermal emitters, and six others possibly have composite thermal/non-thermal spectra. Stellar companions have been identified in 71% of the non-thermal emitters, strongly supporting a link between non-thermal emission and binarity.
Discovery of 50 000 K shocked gas in NGC 6888
Authors: Dufour Reginald J. , Buckalew Brent
Published: July 2016

We report the results of a re-analysis of KPNO-IRS spectroscopy of 30 locations in the prototypical WR shell nebula NGC 6888 (WN6 central star). The spectra covered a 3500–7200 Å wavelength range with R ≃ 360 effective spectral resolution. Electron temperatures for a majority of the locations were derived from the [N II] λλ(6548+6583)/5755 and the [O III] λλ(4959+5007)/4363 emission line ratios. Te([N II]) ≃ 8000 ± 2400 K for eleven locations in the north and west parts of the shell and Te([N II]) ≃ 13 000 ± 4000 K for five locations in the southwest part. By contrast, the [O III] temperatures show great variation with location, ranging from 12000 K to 55000 K, with the highest value being that for an [O III] filament located to the north and outside of the ensemble of knots defining the northern extent of the mass loaded ejecta from the star.

Using a combination of spectral ‘arithmetic’, shock- and photoionization models, we demonstrate that NGC 6888 is permeated by a strong hot wind-shocked gaseous medium (which extends outside the mass-loaded ejected material in the north and west) that contaminates the spectrum of any given location and must be removed to determine realistic Te, Ne, and abundances for the ejected material in the observed knots. For some knots, the effects are such that the abundances obtained via naive empirical methods were found to be several times too small (particularly for O/H) than in reality. We present a few illustrative examples and discuss the implications the results have on abundances previously derived for NGC 6888, but more importantly, what these results imply about the origin of temperature fluctuations in other types of nebulae which may be influenced by stellar winds (i.e., planetary nebulae and H II regions).
Selective absorption in γ2 Velorum
Authors: Lloyd Chris , Stickland David J.
Published: July 2016

Detailed analysis of the phase-dependent line-profile variations of IUE observations γ2 Velorum it is found that selective absorption by the wind is the dominant source of variation in the UV. Excess emission from the shock zone also contributes but this too is selectively absorbed.
Kinematical study of the wind-blown bubble NGC 6888
Authors: Manchado Arturo , García-Segura Guillermo , Chu You-Hua
Published: July 2016

A detailed kinematical analysis of wind-blown bubble ring nebula NGC 6888 swept-up by the wind of the Wolf-Rayet star WR 136, combining high resolution echelle [O III] spectra, high resolution HST imaging and high resolution ground base imaging, is presented. The kinematics is compared to new theoretical models for the dynamics of WR bubbles based on a three-winds scenario. The actual visible nebula is formed by the red supergiant wind swept-up by the fast WR wind. We conclude that the intringuing filamentary morphology, mainly formed by clumps, tails and ripples are explained by the actuation of Vishniac instabilities on the swept-up shell during the expansion of a red supergiant wind, as predicted by the gas dynamical simulations. The kinematics confirm that NGC 6888 is actually on a breakout process at the NW side, as derived from the blowouts. The blowouts are formed by shocked gas which is directly interacting with the cavity formed by the main sequence wind. The fact that these blowouts are bounded by shocks suggests that the cooling is efficient in a main sequence bubble during the evolution.
Heat conduction and colliding winds in Wolf-Rayet binaries
Authors: Motamen Simin M. , Walder Rolf , Folini Doris
Published: July 2016

We numerically investigate the impact of heat conduction by thermal electrons on the physics of colliding winds by means of a 1D model problem. Compared to ideal flows, the hot plasma has significantly lower temperatures and has both a high- and low-density component. Consequently, the shocks in WR binaries become generally more radiative and the emitted X-ray spectrum is softer, shows more emission lines, and is a composite of the emission from high- and low density regions.
Hot stars in the Galactic Center
Authors: Eckart Andreas , Ott Thomas , Genzel Reinhard , Lutz Dieter
Published: July 2016

The central parsec of our Galaxy is powered by a cluster of young massive hot stars which formed a few million years ago. Within that cluster the seven most luminous (L >105.75 L⊙) and moderately hot (T < 104.5 K) blue supergiants contribute half of the ionizing luminosity of that region. These stars probably formed when a dense cloud fell into the center < 107 years ago, was highly compressed there, and became gravitationally unstable. Over six years of high spatial resolution, near-infrared imaging and spectroscopy have made it possible to carry out a detailed investigation of the stars in the central cluster and its enclosed mass. As one result of a detailed variability study of the central cluster stars we found that the bright He I star IRS 16SW is a short-period variable with a period of ∼9.72 days. It is most likely an eclipsing binary with a lower mass limit of 100 solar masses. Line of sight velocities and proper motions have been measured for these hot stars (as well as ∼200 other stars) down to separations of less than five light days from the compact radio source Sgr A* at the dynamic center of the Milky Way. These confirmed measurements imply the presence of a central dark mass of 2.6 × 106 solar masses. The dark mass at the center of the Milky Way is currently the most compelling case for a massive black hole. Simple physical considerations show that this dark mass cannot consist of a stable cluster of stars, stellar remnants, substellar condensations or a degenerate gas of elementary particles but that at least 103 to 105 solar masses must be in the form of a massive black hole associated with Sgr A* itself.
Super star clusters in the Galactic Center as revealed by HST-NICMOS
Authors: Figer Donald F. , Kim Sungsoo S. , Morris Mark , Serabyn Eugene
Published: July 2016

The three massive clusters in the Galactic Center are not only the most massive young clusters in the Galaxy, but they harbor more Wolf-Rayet stars than any other starburst region in the Local Group. An understanding of their stellar content will be valuable for extending models to starburst regions in other galaxies. We present HST-NICMOS images, luminosity functions, and color-magnitude diagrams of two of these: the Quintuplet and Arches clusters. The images allow the detection of stars over 6 magnitudes fainter than ever before and reveal previously undetected multiple star systems. For the first time, we clearly identify the main sequence in the Quintuplet cluster, and we extend earlier detections of the main sequence in the Arches cluster to Minitial < 10 M⊙. We estimate that the Arches cluster has an initial mass function slope which is greater than the Salpeter value. Given their stellar content, the Galactic Center clusters provide both the best nearby examples of super star clusters and the best nearby locale in which to investigate WR phenomena in starburst galaxies and galactic nuclei. We discuss the content of the Galactic Center clusters, with a particular emphasis on how they compare to other massive clusters of the local group. We expect that many of the massive stars in the Galactic Center will soon evolve to become WR stars, and eventually become supernovae at a rate of ∼ 1 per 20 000 years for the next several Myr. We note that our preliminary N-body simulations suggest that such dense clusters are short-lived in the strong tidal field of the Galactic Center, consistent with the fact that no older dense clusters are seen in the central 50 pc. This implies a star formation rate of 5(10−3) M⊙ yr−1 in the Galactic Center.