Microlensing of tidal debris on the Magellanic great circle

Increasing evidence suggests that the Galactic halo is lumpy on kpc scales as a result of the accretion of at least a dozen small galaxies [Large and Small Magellanic Clouds (LMC/SMC), Sgr, Fornax, etc.]. Faint stars in such lumpy structures can significantly microlense a background star with an optical depth of 10⁻⁷–10⁻⁶, which is comparable to the observed value to the LMC. The observed microlensing events towards the LMC can be explained by a tidal debris tail from the progenitor of the Magellanic Clouds and Magellanic Stream. The LMC stars can either lense stars in the debris tail a few kpc behind the LMC, or be lensed by stars in the part of the debris tail in front of the LMC. The models are consistent with an elementary particle dominated Galactic halo without massive compact halo objects (MACHOs). They also differ from Sahu's LMC-self-lensing model by predicting a higher optical depth and event rate and lower concentration of events to the LMC centre.     

Tracing the formation history of intermediate-age star clusters in the Small Magellanic Cloud

Colour–magnitude diagrams (CMDs) are presented for the first time for 10 star clusters projected on to the Small Magellanic Cloud (SMC). The photometry was carried out in the Washington system C and T ₁ filters allowing the determination of ages by means of the magnitude difference between the red giant clump and the main-sequence turnoff (MSTO), and metallicities from the red giant branch (RGB) locus. The clusters all have ages in the range 1.5–4 Gyr and metallicities between  −1.3 < [Fe/H] < −0.6  , with respective errors of ∼0.5 Gyr and 0.3 dex. This increases substantially the sample of intermediate-age clusters in the SMC with well-derived parameters. We combine our results with those for other clusters in the literature to derive as large and homogeneous a data base as possible (totalling 26 clusters) in order to study global effects. We find evidence for two peaks in the age distribution of SMC clusters, at ∼6.5 and 2.5 Gyr, in good agreement with previous hints involving smaller samples. The most recent peak occurs at a time that corresponds to a very close encounter between the Large Magellanic Cloud (LMC) and the SMC according to the recent dynamical models of Bekki et al. that they used to explain the enhancement of LMC clusters with this age. It appears cluster formation may have been similarly stimulated in the SMC by this encounter as well. We also find very good agreement between cluster ages and metallicities and the prediction from a bursting model from Pagel and Tautvaišienė with a burst that occurred 3 Gyr ago. These two lines of evidence together favour a bursting cluster formation history as opposed to a continuous one for the SMC.     

J-type carbon stars in the Large Magellanic Cloud

A sample of 1497 carbon stars in the Large Magellanic Cloud (LMC) has been observed in the red part of the spectrum with the 2dF facility on the Anglo-Australian Telescope. Of these, 156 have been identified as J-type (i.e. ¹³C-rich) carbon stars using a technique which provides a clear distinction between J stars and the normal N-type carbon stars that comprise the bulk of the sample, and yields few borderline cases. A simple two-dimensional classification of the spectra, based on their spectral slopes in different wavelength regions, has been constructed and found to be related to the more conventional c and j indices, modified to suit the spectral regions observed. Most of the J stars form a photometric sequence in the   K − ( J − K )  colour–magnitude diagram, parallel to and 0.6 mag fainter than the N-star sequence. A subset of the J stars (about 13 per cent) are brighter than this J-star sequence; most of these are spectroscopically different from the other J stars. The bright J stars have stronger CN bands than the other J stars and are found strongly concentrated in the central regions of the LMC. Most of the rather few stars in common with Hartwick and Cowley's sample of suspected CH stars are J stars. Overall, the proportion of carbon stars identified as J stars is somewhat lower than has been found in the Galaxy. The Na D lines are weaker in the LMC J stars than in either the Galactic J stars or the LMC N stars, and do not seem to depend on temperature.     

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.     

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'.     

Period–colour and amplitude–colour relations in classical Cepheid variables – V. The Small Magellanic Cloud Cepheid models

Period–colour (PC) and amplitude–colour (AC) relations at maximum, mean and minimum light are constructed from a large grid of full amplitude hydrodynamic models of Cepheids with a composition appropriate for the Small Magellanic Cloud (SMC). We compare these theoretical relations with those from observations. The theoretical relations are, in general, in good agreement with their observational counterparts, though there exist some discrepancy for short period  (log [ P ] < 1)  Cepheids. We outline a physical mechanism which can, in principle, be one factor to explain the observed PC/AC relations for the long and short period Cepheids in the Galaxy, Large Magellanic Cloud (LMC) and SMC. Our explanation relies on the hydrogen ionization front (HIF)–photosphere interaction and the way this interaction changes with pulsation period, pulsation phase and metallicity. Since the PC relation is connected with the period–luminosity (PL) relation, it is postulated that such a mechanism can also explain the observed properties of the PL relation in these three galaxies.     

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.     

Optical Gravitational Lensing Experiment Cepheids Have Lower Amplitudes in the Small Magellanic Cloud than in the Large Magellanic Cloud

We selected Cepheids from the Optical Gravitational Lensing Experiment database for the Magellanic Clouds in the period range of 101.1相似文献   

The MOA catalogue of eclipsing binary stars in the Small Magellanic Cloud

We present a catalogue of 167 eclipsing binary stars in the Small Magellanic Cloud (SMC) derived from the data base of time-series photometry for 400 000 SMC stars acquired by the Microlensing Observations in Astrophysics (MOA) project during 1997. We print coordinates, ephemerides, magnitudes and light curves for the 35 new detections; similar data and finding charts are available electronically for the whole catalogue. The majority of periods lie within the range 0.4 to 20 d; six systems are possibly eccentric while 14 are probably or certainly so. The majority of the newly identified systems lie in the outer regions of the SMC.     

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)     

Five young star clusters in the outer region of the Small Magellanic Cloud

Colour–magnitude diagrams in the Washington system are presented for the first time for five star clusters projected on to the outer region of the Small Magellanic Cloud (SMC). The clusters are found to have ages in the range 0.1–1.0 Gyr, as derived from the fit of isochrones with   Z = 0.004  . This sample increases substantially the number of young clusters in the outer SMC – particularly in the south-east quadrant – with well-derived parameters. We combine our results with those for other clusters in the literature to derive as large and homogeneous a data base as possible (totalling 49 clusters) in order to study global effects. We find no conclusive evidence for a dispersion in the cluster ages and metallicities as a function of their distance from the galaxy centre, in the SMC outer region. L 114 and 115, although very distant, are very young clusters, lying in the bridge of the SMC and therefore most likely formed during the interaction which formed this feature. We also find very good agreement between the cluster age–metallicity relation (AMR) and the prediction from a bursting model from Pagel & Tautvaišienė with a burst that occurred 3 Gyr ago. Comparing the present cluster AMR with that derived by Harris & Zaritsky for field stars in the main body of the SMC, we find that field stars and clusters underwent similar chemical enrichment histories during approximately the last couple of Gyr, but their chemical evolution was clearly different between 4 and 10 Gyr ago.     

A photometric study of 11 massive stars in the Magellanic Clouds

We present and discuss V BLUW photometry of eleven massive stars in the Magellanic Clouds: the SMC stars AzV121, AzV136 = HD5277 = R10, AzV197, AzV310 = R26 and AzV 369; the LMC stars GV80 = HD32034 = R62, GV91 = HDE 268 819, GV346 = HDE 269661 = R111, GV352 = HDE 269697, GV423 = HDE 269953 = R150 and GV460 = HDE 270111. Only one G0 Ia SMC supergiant is found to be variable, whereas all members of the LMC sample show definite variability. We find that roughly above M /M_⊙ = 25, supergiants become photometrically unstable. The reddening‐independent metal‐index [B –L ] is used to investigate the metallicity of the late‐type supergiants in both galaxies relative to similar supergiants in the solar neighbourhood. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Kinematics of massive stars in the Small Magellanic Cloud

We present radial velocities for 2045 stars in the Small Magellanic Cloud (SMC), obtained from the 2dF survey by Evans et al. The great majority of these stars are of OBA type, tracing the dynamics of the young stellar population. Dividing the sample into ad hoc 'bar' and 'wing' samples (north and south, respectively, of the line:  δ=−77°50'+[4α]'  , where α is the right ascension in minutes of time) we find that the velocities in the SMC bar show a gradient of 26.3 ± 1.6 km s⁻¹ deg⁻¹ at a position angle of 126°± 4°. The derived gradient in the bar is robust to the adopted line of demarcation between the two samples. The largest redshifts are found in the SMC wing, in which the velocity distribution appears distinct from that in the bar, most probably a consequence of the interaction between the Magellanic Clouds that is predicted to have occurred 0.2 Gyr ago. The mean velocity for all stars in the sample is +172.0 ± 0.2 km s⁻¹ (redshifted by ∼20 km s⁻¹ when compared to published results for older populations), with a velocity dispersion of 30 km s⁻¹.     

Constraining the LMC cluster age gap: Washington photometry of NGC 2155 and SL 896 (LW 480)

We carried out Washington system photometry of the intermediate-age Large Magellanic Cloud (LMC) star clusters NGC 2155 and SL 896 (LW 480). We derive ages and metallicities from the T ₁ versus     colour–magnitude diagrams (CMDs). For the first time an age has been obtained for SL 896,     . For NGC 2155 we derive     . The two clusters basically define the lower age limit of the LMC age gap. In particular, NGC 2155 is confirmed as the oldest intermediate-age LMC cluster so far studied. The derived metallicities are     and     for NGC 2155 and SL 896, respectively. We also studied the CMDs of the surrounding fields, which have a dominant turn-off comparable to that of the clusters themselves, and similar metallicity, showing that one is dealing with an intermediate-age disc where clusters and field stars have the same origin. We inserted the present clusters in the LMC and Small Magellanic Cloud (SMC) age–metallicity relations, using a set of homogeneous determinations with the same method as in our previous studies, now totalling 15 LMC clusters and four SMC clusters, together with some additional values from the literature. The LMC and SMC age–metallicity relations appear to be remarkably complementary, since the SMC was actively star-forming during the LMC quiescent age gap epoch.     

The deepest Hubble Space Telescope far-ultraviolet observations in the Large Magellanic Cloud

We present a new analysis of the deepest pure-ultraviolet (UV) observations with the highest angular resolution ever performed. A set of 12 exposures with the Hubble Space Telescope ( HST ) WFPC2 and F160BW filter obtained in parallel observing mode, which cover ∼12 arcmin² in the Large Magellanic Cloud (LMC), north of the bar and in the 'general field' region of the LMC, contain stars with far-UV monochromatic magnitudes as faint as 22 mag. The 198 detected UV sources represent an accumulated exposure of  ≥ 2 × 10⁴ s  and reveal stars as faint as   m _UV≃ 20 mag  . We combine these observations with deep UBVI charge-coupled device (CCD) imaging of the same region reaching as faint as   V ≃ 26 mag  , and reselect probable optical counterparts for the UV sources. After a two-stage search-and-analysis process, we detect robust counterparts for 129 stars. These are mostly upper main-sequence stars, from early B to early A spectral classes, with several F stars. We point out the lack of blue supergiants, which could have been easily detected in our survey. We measure a foreground extinction   E ( B − V ) ≃ 0.08 mag  by Galactic dust and a surface density of star formation rate twice the average Galactic value. These observations indicate that relatively recent star formation took place even off the bar of the LMC.     

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 physical parameters of Markarian 501 during flaring activity

We analyse an N -body simulation of the Small Magellanic Cloud (SMC), that of Gardiner & Noguchi, to determine its microlensing statistics. We find that the optical depth owing to self-lensing in the simulation is low, 0.4×10⁻⁷, but still consistent (at the 90 per cent level) with that observed by the EROS and MACHO collaborations. This low optical depth is due to the relatively small line-of-sight thickness of the SMC produced in the simulation. The proper motions and time-scales of the simulation are consistent with those observed assuming a standard mass function for stars in the SMC. The time-scale distribution from the standard mass function generates a significant fraction of short time-scale events: future self-lensing events towards the SMC may have the same time-scales as events observed towards the Large Magellanic Cloud (LMC). Although some debris was stripped from the SMC during its collision with the LMC about 2×10⁸ yr ago, the optical depth of the LMC owing to this debris is low, a few ×10⁻⁹, and thus cannot explain the measured optical depth towards the LMC.     

A theoretical analysis of the systematic errors in the red clump distance to the Large Magellanic Cloud (LMC)

We present a detailed analysis of the uncertainty on the theoretical population corrections to the Large Magellanic Cloud (LMC) red clump (RC) absolute magnitude, by employing a population synthesis algorithm to simulate theoretically the photometric and spectroscopic properties of RC stars, under various assumptions concerning the LMC star formation rate (SFR) and age–metallicity relationship (AMR). A comparison of the outcome of our simulations with observations of evolved low- to intermediate-mass stars in the LMC allows one to select the combinations of SFR and AMR that bracket the real LMC star formation history, and to estimate the systematic error on the associated RC population corrections.
The most accurate estimate of the LMC distance modulus from the RC method (adopting the OGLE-II reddening maps for the LMC) is obtained from the K -band magnitude, and provides  ( m − M )_0,LMC= 18.47 ± 0.01(random)^+0.05_−0.06(systematic)  . Distances obtained from the I band, or from the multicolour RC technique which determines at the same time reddening and distance, both agree (albeit with a slightly larger error bar) with this value.     

FLAIR observations of planetary nebulae in the Large Magellanic Cloud

This paper presents new observations of 97 planetary nebulae in the Large Magellanic Cloud (LMC) obtained using the FLAIR system on the UK 1.2-m Schmidt Telescope. These nebulae are mostly at the fainter end of the known population, and about 75 per cent have not been observed before in spectroscopic mode. Radial velocities have been measured using cross-correlation techniques, and represent an increase of 66 per cent in the sample of LMC planetary nebulae with known radial velocities. The major line ratios are given, and are analysed in conjunction with published data. One-quarter of the faint nebulae are Type I objects with very strong [N II ] and [S II ] lines; most of the other faint nebulae have low density, low excitation and relatively strong [S II ] lines.     

The contribution of pulsars to diffuse gamma-rays in the Large Magellanic Cloud

We study the contribution of young pulsars, with characteristic ages of less than 10⁶ yr, to the diffuse γ-ray emission from the Large Magellanic Cloud (LMC). Based on the outer gap model for γ-ray emission proposed by Zhang & Cheng and pulsar properties in the LMC given by Hartmann, Brown & Schnepf, we simulate the properties of the young pulsars in the LMC. We show that γ-rays produced by the pulsars in the LMC may make an important contribution to the diffuse γ-rays in the LMC, especially in the high-energy range. We calculate the γ-ray energy spectrum of the pulsars in the LMC and show that the γ-ray component contributed by the pulsars to the diffuse γ-rays in the high-energy range (above ∼1 GeV) becomes dominant. We expect that none of the young pulsars should be detectable as an individual point source of γ-ray emission by EGRET. We also expect that pulsar contribution above ∼1 GeV in the SMC is very important.