Globular clusters and the Mira period–luminosity relation

A globular cluster distance scale based on Hipparcos parallaxes of subdwarfs has been used to derive estimates of M _K for cluster Miras, including one in the Small Magellanic Cloud (SMC) globular cluster NGC 121. These lead to a zero-point of the Mira infrared period–luminosity (PL) relation, PL( K ), in good agreement with that derived from Hipparcos parallaxes of nearby field Miras. The mean of these two estimates together with data on LMC Miras yields a Large Magellanic Cloud (LMC) distance modulus of     in evident agreement with a metallicity-corrected Cepheid modulus     .
The use of luminous asymptotic giant branch (AGB) stars as extragalactic population indicators is also discussed.     

The Magellanic Clouds: their evolution,structure and composition

Summary The Magellanic Clouds play a fundamental role in a number of fields of astronomical research. Their distances are most relevant to the extragalactic distance scale. Their relative proximity offers exceptional opportunities for detailed studies of their stellar and interstellar content. They serve therefore as testing grounds for modern astrophysical theories, in particular concerning the chemical evolution of stars and galaxies.In this review we will discuss recent attempts to determine accurate distances to the Magellanic Clouds. We will consider their stellar generations as the results of interactions between the Large and the Small Magellanic Cloud as well as between the Clouds and the Galaxy. Recent determinations of the chemical abundances of the various age groups will be presented. The fact that the evolution of the Clouds has been slower than that of our Galaxy gives us the opportunity to study the conditions in slightly metalpoor galaxies. Recent progress in observing techniques has added much to our knowledge about the interstellar medium of the Clouds.The Magellanic System, which comprises the Magellanic Clouds, the Inter-Cloud Region and the Magellanic Stream, will be described. We will in particular consider the complex structure of the Large and the Small Cloud and the kinematics of their populations.     

Magellanic Cloud WC/WO Wolf–Rayet stars – I. Binary frequency and Roche lobe overflow formation

A nearly complete sample of 24 Magellanic Cloud WC/WO subclass Wolf–Rayet stars is studied spectroscopically and photometrically to determine its binary frequency. Theory predicts the Roche lobe overflow produced Wolf–Rayet binary frequency to be 52±14 per cent in the Large Magellanic Cloud and 100 per cent in the Small Magellanic Cloud, not counting non-Roche lobe overflow Wolf–Rayet binaries. Lower ambient metallicity ( Z ) leads to lower opacity, preventing all but the most massive (hence luminous) single stars from reaching the Wolf–Rayet stage. However, theory predicts that Roche lobe overflow even in binaries of modest mass will lead to Wolf–Rayet stars in binaries with periods below approximately 200 d, for initial periods below approximately 1000 d, independent of Z . By examining their absolute continuum magnitudes, radial velocity variations, emission-line equivalent widths and full widths at half-maximum, a WC/WO binary frequency of only 13 per cent, significantly lower than the prediction, is found in the Large Magellanic Cloud. In the unlikely event that all of the cases with a less certain binary status actually turn out to be binary, current theory and observation would agree. (The Small Magellanic Cloud contains only one WC/WO star, which happens to be a binary.) The three WC+O binaries in the Large Magellanic Cloud all have periods well below 1000 d. The large majority of WC/WO stars in such environments apparently can form without the aid of a binary companion. Current evolutionary scenarios appear to have difficulty explaining either the relatively large number of Wolf–Rayet stars in the Magellanic Clouds, or the formation of Wolf–Rayet stars in general.     

Extinction of H ii regions in the Large Magellanic Cloud

The extinction properties of H  ii regions in the Large Magellanic Cloud are investigated using radio continuum data obtained from the Molonglo Observatory Synthesis Telescope, digitized and calibrated Hα data and published Balmer decrement measurements. The resulting extinction–colour excess diagram suggests that (1) most H  ii regions in the Magellanic Clouds have similar extinction properties to the Galactic ones, (2) all imaginable gas/dust configurations are possible, (3) the extinction of some highly reddened H  ii region cores originates externally in cocoon shells.   The puzzle of different extinction–colour excess ratios of Galactic and extragalactic H  ii regions is explained as being due to the different populations of observed samples rather than any intrinsic differences. The extinction of the observed Galactic H  ii regions produced by foreground dust overwhelms the internal extinction, while the situation in the observed extragalactic H  ii regions is just the opposite.     

Massive spectroscopic binaries in the Magellanic Clouds

We present results of our ongoing observing program on search and studies of massive stars (O type) in binary systems in our neighbour galaxies, the Magellanic Clouds. Radial velocity orbits are presented for two new binaries, one in the Small Magellanic Cloud and another in the Large Magellanic Cloud.     

The chemical composition of the Small Magellanic Cloud

The Small Magellanic Cloud is a close, irregular galaxy that has experienced a complex star formation history due to the strong interactions occurred both with the Large Magellanic Cloud and the Galaxy. Despite its importance, the chemical composition of its stellar populations older than ∼ 1–2 Gyr is still poorly investigated. I present the first results of a spectroscopic survey of ∼ 200 Small Magellanic Cloud giant stars performed with FLAMES@VLT. The derived metallicity distribution peaks at [Fe/H] ∼ –0.9/–1.0 dex, with a secondary peak at [Fe/H] ∼ –0.6 dex. All these stars show [α /Fe] abundance ratios that are solar or mildly enhanced (∼+0.1 dex). Also, three metal‐poor stars (with [Fe/H] ∼ –2.5 dex and enhanced [α /Fe] ratios compatible with those of the Galactic Halo) have been detected in the outskirts of the SMC: These giants are the most metal‐poor stars discovered so far in the Magellanic Clouds. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Early spectroscopic observations of four extragalactic novae

We report the spectroscopic observations of four extragalactic novae in M31 and in M33, secured at Loiano Observatory, Italy. Nova M31 2009-10b is a luminous nova that needed some days to achieve the peak brightness. We confirm that it is a Fe II nova and we show that it is one of the most luminous novae ever observed in M31, showing an evolution close to that of the bright extragalactic novae M31 2007-11d, LMC 1991 and SN 2010U. The novae M31 2010-07a and M31 2011-07b are standard Fe II novae. Nova M33 2010-07a is a Fe II nova that showed a peculiar rebrightening, during which we observed emission lines with P Cyg profiles: we suggest that it is the first observed nova in M33 that underwent a second mass ejection.     

Abundances and absolute stellar magnitudes for F and G supergiants of Magellanic Clouds

Using high-resolution spectra, we study 31 yellow supergiants of the Large and Small Magellanic Clouds by the method of atmosphere models. Abundance values of 20 chemical elements are determined. It is shown that ??-elements are in a slight excess and neutron-capture elements have an excess up to 0.60 dex. Approbation of a new technique for the determination of absolute stellar magnitudes of late-type supergiants is performed. The technique is based on the use of the spectroscopic criterion, namely, depth-line ratios for iron. Absolute stellar magnitudes of nine supergiants of the Large Magellanic Cloud are calculated using this technique. A value of the distance modulus of the Large Magellanic Cloud is estimated: m ? M = 18.4 ± 0.3 ^m .     

Large Magellanic Cloud Planetary Nebula Morphology: Probing Stellar Populations and Evolution

Planetary nebulae (PNe) in the Large Magellanic Cloud (LMC) offer the unique opportunity to study both the population and evolution of low- and intermediate-mass stars, by means of the morphological type of the nebula. Using observations from our LMC PN morphological survey, and including images available in the Hubble Space Telescope Data Archive and published chemical abundances, we find that asymmetry in PNe is strongly correlated with a younger stellar population, as indicated by the abundance of elements that are unaltered by stellar evolution (Ne, Ar, and S). While similar results have been obtained for Galactic PNe, this is the first demonstration of the relationship for extragalactic PNe. We also examine the relation between morphology and abundance of the products of stellar evolution. We found that asymmetric PNe have higher nitrogen and lower carbon abundances than symmetric PNe. Our two main results are broadly consistent with the predictions of stellar evolution if the progenitors of asymmetric PNe have on average larger masses than the progenitors of symmetric PNe. The results bear on the question of formation mechanisms for asymmetric PNe-specifically, that the genesis of PNe structure should relate strongly to the population type, and by inference the mass, of the progenitor star and less strongly on whether the central star is a member of a close binary system.     

Binary brown dwarfs in the Galactic halo?

Microlensing events towards the Large Magellanic Cloud imply that a sizeable fraction of dark matter is in the form of MACHOs (Massive Astrophysical Compact Halo Objects), presumably located in the halo of the Galaxy. Within the present uncertainties, brown dwarfs are viable candidates for MACHOs. Various considerations strongly suggest that a large number of MACHOs should actually consist of binary brown dwarfs. Yet this circumstance appears to be in flat contradiction with the fact that MACHOs have been detected as unresolved objects so far. We show that such an apparent paradox does not exist within a model in which MACHOs are clumped into dark clusters along with cold molecular clouds, since dynamical friction on these clouds makes binary brown dwarfs very close. Moreover, we argue that future microlensing experiments with more accurate photometric observations can resolve binary brown dwarfs.     

On the intrinsic bias in detecting caustic crossings between Galactic halo and self-lensing events in the Magellanic Clouds

In this paper, we investigate the intrinsic bias in detecting caustic crossings between the Galactic halo and self-lensing gravitational microlensing events in the Magellanic Clouds. For this, we determine the region for optimal caustic-crossing detection in the parameter space of the physical binary separations, ℓ, and the total binary lens mass, M , and find that the optimal regions for both populations of events are similar to each other. In particular, if the Galactic halo is composed of lenses with the claimed average mass of 〈 M 〉∼0.5 M_⊙, the optimal binary separation range of Galactic halo events of 3.5 au≲ℓ≲14 au matches well with that of a Magellanic Cloud self-lensing event caused by a binary lens with a total mass of M ∼1 M_⊙; well within the mass range of the most probable lens population of stars in the Magellanic Clouds. Therefore, our computation implies that if the binary fractions and the distributions of binary separations of the two populations of lenses are not significantly different from each other, there is no strong detection bias against Galactic halo caustic-crossing events.     

Suspected cool R Coronae Borealis stars in the Magellanic Clouds

Six stars out of a sample of ∼2300 carbon stars in the Magellanic Clouds have been identified as having strong C₂ bands but CN bands that are very weak or absent. It is argued that five of these are likely to be R Coronae Borealis (RCB) stars on the basis of their spectral characteristics and peculiar colours. Most are variables and the Large Magellanic Cloud (LMC) members have extreme radial velocities that are more like the planetary nebula population than the carbon stars. This sample consists of four LMC members (only one of them previously recognized as an RCB star), one Small Magellanic Cloud (SMC) member (the first RCB star reported in the SMC) and one foreground Galactic star.     

Effects of gravitational microlensing of binary and multiple stars

The observation of microlensing events towards the Large Magellanic Cloud and Galactic Bulge discovers a new population of our Galaxy which is that of dark bodies with masses of the order of 0.1M . Astronomy has now a unique ability: one can use the microlensing as a space telescope with extremely high angular resolution. Here we discuss the opportunity of the application of this ability to observation of close binaries. Stars in a binary system move around their barycenter and as a result the apparent motion is modulated by binary motion. The light curve of this microlensing event becomes nonsymmetrical. If a binary has two stars with different spectral types, one can expect significant variation of color during microlensing effect. Accurate light curves for some typical binaries have been calculated and are presented here. The total fraction of binaries in our Galaxy is around 50%. Therefore one can expect half of the microlensing events to have nonsymmetrical and wavelength depending light curves which would indicate that background star is binary. Our opinion is that the absence of these light curves are due to some selection effect. This leads to an underestimation of the density of the dark body population in our Galaxy by a factor of about two.     

On the origin of double main-sequence turn-offs in star clusters of the Magellanic Clouds

Recent observational studies of intermediate-age star clusters (SCs) in the Large Magellanic Cloud (LMC) have reported that a significant number of these objects show double main-sequence turn-offs (DMSTOs) in their colour-magnitude diagrams (CMDs). One plausible explanation for the origin of these DMSTOs is that the SCs are composed of two different stellar populations with age differences of ∼300 Myr. Based on analytical methods and numerical simulations, we explore a new scenario in which SCs interact and merge with star-forming giant molecular clouds (GMCs) to form new composite SCs with two distinct component populations. In this new scenario, the possible age differences between the two different stellar populations responsible for the DMSTOs are due largely to secondary star formation within GMCs interacting and merging with already-existing SCs in the LMC disc. The total gas masses being converted into new stars (i.e. the second generation of stars) during GMC-SC interaction and merging can be comparable to or larger than the masses of the original SCs (i.e. the first generation of stars) in this scenario. Our simulations show that the spatial distributions of new stars in composite SCs formed from GMC-SC merging are more compact than those of stars initially in the SCs. We discuss both advantages and disadvantages of the new scenario in explaining fundamental properties of SCs with DMSTOs in the LMC and in the Small Magellanic Cloud (SMC). We also discuss the merits of various alternative scenarios for the origin of the DMSTOs.     

The Large Magellanic Cloud and the distance scale

The Magellanic Clouds, especially the Large Magellanic Cloud, are places where multiple distance indicators can be compared with each other in a straight-forward manner at considerable precision. We here review the distances derived from Cepheids, Red Variables, RR Lyraes, Red Clump Stars and Eclipsing Binaries, and show that the results from these distance indicators generally agree to within their errors, and the distance modulus to the Large Magellanic Cloud appears to be defined to ±3% with a mean value (m−M)₀=18.48 mag, corresponding to 49.7 kpc. The utility of the Magellanic Clouds in constructing and testing the distance scale will remain as we move into the era of Gaia.     

The Brave New World of Binary Star Studies

In this paper we discuss some of the new and exciting developments in the study of binary stars. Recent technological advances (such as CCDs) now make it possible (even easy) to study faint, astrophysically important binaries that in the past could only be done with large 4 + meter class telescopes. Also, the panoramic nature of CCDs (and the use of mosaics), permit large numbers of stars to be imaged and studied. At this conference, most of the observational material discussed was secured typically with smaller aperture 0.5 – 2 m telescopes. Excellent examples are the discovery of over 10⁴ new ～13 – 20 mag eclipsing (and interacting) binaries now found in nearby galaxies from the EROS, OGLE, MACHO and DIRECT programs. As briefly discussed here, and in more detail in several papers in this volume, a small fraction of these extragalactic eclipsing binaries are now serving as “standard candles” to secure accurate distances to the Magellanic Clouds, as well as to M31 and M33. Moreover, the discovery of increasingly larger numbers of eclipsing binaries has stimulated the development of automatic methods for reducing and analyzing the light curves of thousands of systems. In the near future, hundreds of thousands (possibly millions) of additional systems are expected to be discovered by Pan-STARRS, the Large Synoptic Survey Telescopes (LSST), and later by GAIA. Over the last decade, new classes of binary systems have also been found which contain Jupiter-size planets and binaries containing pulsating stars. Some examples of these important binaries are discussed. Also discussed are the increasing numbers (now eight) of eclipsing binary planet–star systems that have been found from high precision photometry. These systems are very important since the radii and masses of the hosted planets can be directly measured. Moreover, from the upcoming COROT and KEPLER missions hundreds of additional transiting planet-star systems are expected to be found. All in all, we hope in this paper to highlight some of the current developments and new directions in the “Brave New World” of binary star studies.     

Interaction of the Large Magellanic Cloud with the halo population and the companions of the Galaxy

Numerical simulations have been performed in order to study the gravitational interaction between the outlying population of the Galaxy and the Large Magellanic Cloud (LMC). The following effects have been noted: (1) halo objects are partially removed from the orbital distance of the LMC such that a local minimum is created in the radial density profile; (2) a large number of halo objects go into escape orbits. This possibility makes the determination of the mass of the Galaxy by virial theorem arguments unreliable; (3) the orbit of the LMC decays faster than what one would expect on the basis of the classical dynamical friction calculation. The excess friction derives from three-body effects, which are not accounted for in the classical dynamical friction. Finally, the fates of globular clusters and dwarf galaxies near the Galaxy and the LMC are discussed.     

Atomic hydrogen shells in the Messier 82 starburst

We report on a search for atomic hydrogen holes and shells in the nearby starburst galaxy M82, using high angular resolution (∼1.3 arcsec) VLA H  i absorption observations. From this study, we have detected four H  i shells in the central kiloparsec of M82. The sizes of these shells (∼30–50 pc) are smaller than those of the majority of shells observed in the Large Magellanic Cloud, although the M82 shells have higher expansion velocities (∼30 km s⁻¹) and typical kinetic energies of  10⁵¹–10⁵² erg  . Because our observations were made in absorption, strong selection effects are present which hinder the detection of shells that could be present outside, or behind, the extended radio continuum associated with the starburst. Nevertheless, our detection of four shells in M82 actually represents a higher density of shells per unit area compared with the Large Magellanic Cloud.
We also discuss the gas dynamics in the central kiloparsec of M82, and discuss the velocity structure of gas in a barred potential and in wind-driven shells. We conclude that in M82 the observed gas dynamics are most likely a superposition of both effects.     

Radio planetary nebulae in the Magellanic Clouds

We report the extragalactic radio-continuum detection of 15 planetary nebulae (PNe) in the Magellanic Clouds (MCs) from recent Australia Telescope Compact Array+Parkes mosaic surveys. These detections were supplemented by new and high-resolution radio, optical and infrared observations which helped to resolve the true nature of the objects. Four of the PNe are located in the Small Magellanic Cloud (SMC) and 11 are located in the Large Magellanic Cloud (LMC). Based on Galactic PNe the expected radio flux densities at the distance of the LMC/SMC are up to ∼2.5 and ∼2.0 mJy at 1.4 GHz, respectively. We find that one of our new radio PNe in the SMC has a flux density of 5.1 mJy at 1.4 GHz, several times higher than expected. We suggest that the most luminous radio PN in the SMC (N S68) may represent the upper limit to radio-peak luminosity because it is approximately three times more luminous than NGC 7027, the most luminous known Galactic PN. We note that the optical diameters of these 15 Magellanic Clouds (MCs) PNe vary from very small (∼0.08 pc or 0.32 arcsec; SMP L47) to very large (∼1 pc or 4 arcsec; SMP L83). Their flux densities peak at different frequencies, suggesting that they may be in different stages of evolution. We briefly discuss mechanisms that may explain their unusually high radio-continuum flux densities. We argue that these detections may help solve the 'missing mass problem' in PNe whose central stars were originally  1–8 M_⊙  . We explore the possible link between ionized haloes ejected by the central stars in their late evolution and extended radio emission. Because of their higher than expected flux densities, we tentatively call this PNe (sub)sample –'Super PNe'.     

Calibrating the Cepheid Period-Luminosity relation from the near-infrared surface brightness technique

We have applied the near-infrared surface-brightness method to 111 Cepheids in the Milky Way and in the Large and the Small Magellanic Clouds determining distances and luminosities for the individual stars. We find that the K-band Period-Luminosity (PL-)relations for Milky Way and Large Magellanic Cloud Cepheids are almost identical, whereas the zero point of the Wesenheit relation depends significantly on metallicity, metal poor Cepheids being fainter.