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1.
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.  相似文献   

2.
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.  相似文献   

3.
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'.  相似文献   

4.
We model the dynamics of Magellanic Stream with the ram-pressure scenario in the logarithmic and power-law galactic halo models and construct numerically the past orbital history of Magellanic Clouds and Magellanic Stream. The parameters of models include the asymptotic rotation velocity of spiral arms, halo flattening, core radius and rising or falling parameter of rotation curve. We obtain the best-fit parameters of galactic models through the maximum likelihood analysis, comparing the high resolution radial velocity data of HI in Magellanic Stream with that of theoretical models. The initial condition of the Magellanic Clouds is taken from the six different values reported in the literature. We find that oblate and nearly spherical shape halos provide a better fit to the observation than the prolate halos. This conclusion is almost independent of choosing the initial conditions and is valid for both logarithmic and power-law models.  相似文献   

5.
N-body simulations of the Magellanic stream   总被引：1，自引：0，他引：1  
A suite of high-resolution N -body simulations of the Magellanic Clouds–Milky Way system are presented and compared directly with newly available data from the H  i Parkes All-Sky Survey (HIPASS). We show that the interaction between Small Magellanic Clouds (SMC) and Large Magellanic Clouds results in both a spatial and kinematical bifurcation of both the stream and the leading arm. The spatial bifurcation of the stream is readily apparent in the HIPASS data, and the kinematical bifurcation is also tentatively identified. This bifurcation provides strong support for the tidal disruption origin for the Magellanic stream. A fiducial model for the Magellanic Clouds (MCs) is presented upon completion of an extensive parameter survey of the potential orbital configurations of the MCs and the viable initial boundary conditions for the disc of the SMC. The impact of the choice of these critical parameters upon the final configurations of the stream and leading arm is detailed.  相似文献   

6.
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.  相似文献   

7.
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.  相似文献   

8.
We present a radio survey of X-ray sources in the Large and Small Magellanic Clouds with the Australia Telescope Compact Array at 6.3 and 3.5 cm. Specifically, we have observed the fields of five LMC and two SMC supersoft X-ray sources, the X-ray binaries LMC X-1, X-2, X-3 and X-4, the X-ray transient Nova SMC 1992, and the soft gamma-ray repeater SGR 0525-66. None of the targets are detected as point sources at their catalogued positions. In particular, the proposed supersoft jet source RXJ 0513-69 is not detected, placing constraints on its radio luminosity compared to Galactic jet sources. Limits on emission from the black hole candidate systems LMC X-1 and X-3 are consistent with the radio behaviour of persistent Galactic black hole X-ray binaries, and a previous possible radio detection of LMC X-1 is found to be almost certainly a result of nearby field sources. The SNR N49 in the field of SGR 0525-66 is mapped at higher resolution than it has been previously, but there is still no evidence for any enhanced emission or disruption of the SNR at the location of the X-ray source.  相似文献   

9.
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.  相似文献   

10.
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)  相似文献   

11.
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   

12.
The stars that will be detectable in the Magellanic Clouds by the DENIS and 2MASS near infrared surveys are enumerated. All thermally-pulsing AGB stars will be observable in I, J, H and K, along with the top two magnitudes of both the early-AGB and the first giant branch. All carbon stars will be visible, and normal (N type) C stars will be easily distinguished by their large J-K colours. However, it will not be possible to separate faint, warm C stars from K and M stars using the photometry alone. Photometry of AGB stars in clusters will allow an accurate evaluation of the AGB tip luminosities as a function of initial mass. Random phase K magnitudes of LPVs and Cepheids should provide a better measure of the LMC tilt and distortions in the SMC. The K survey should turn up 100 to 150 objects undergoing superwind mass loss, these objects being OH/IR stars and the dust-enshrouded C star equivalents of OH/IR stars. It is shown that crowding should not be a problem even in the LMC bar.  相似文献   

13.
We observed near-Main-Sequence B-stars in the Magellanic Clouds with the 3.6 m telescope and CASPEC at La Silla. We obtained spectra of high resolution and high S/N-ratio. The stars are members of the blue globular clusters NGC 1818 (LMC) and NGC 330 (SMC). The spectra are used for differential abundance analyses using HER as galactic reference star. Apart from CNO the metals are underabundant by about a factor of three and four for the LMC and SMC star, respectively. The CNO pattern is interesting since in both stars oxygen is considerably more abundant than carbon.Based on observations collected at ESO, La Silla and on Calar Alto, Spain and on observations made with the International Ultraviolet Explorer.  相似文献   

14.
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.  相似文献   

15.
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.  相似文献   

16.
Detailed abundances are measured in cool (K type) supergiants in LMC young populous clusters (NGC 2004, NGC 2100 and NGC 1818), and compared to those, recently observed in samples of field supergiants in both Magellanic Clouds. This completes the work already done onthe SMC cluster NGC 330 (Hill et al., 1997b).The analysis of the field supergiants showed no evidence of any abundance dispersion among the stars, compatible with a well mixed gas in thesegalaxies. However, previous determinations of the metallicity ofNGC 330 and NGC 1818 gave values well below the respective field values. This difference is seeked, using a homogeneous method for the field andcluster samples, and the abundance ratios of various elements are examined. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

17.
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)  相似文献   

18.
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.  相似文献   

19.
We discuss the potential for using extragalactic eclipsing binaries with well-determined physical properties as standard candles to improve the extragalactic distance scale. The advent of high quantum efficiency/low noise CCDs has now made it possible to obtain high precision light and radial velocity curves for the more luminous OB-type eclipsing binaries in the Magellanic Clouds with even small to moderate size (1–2m) telescopes. This can lead to the determination of distance moduli (m-M)0 to the LMC and SMC with precisions of about 0m.15 for individual binaries.These distances are essentially free from the assumptions made using other distance indicators.  相似文献   

20.
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.  相似文献   

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