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1.
The actinium abundance in the atmospheres of red supergiants PMMR23 and PMMR144 in the Small Magellanic Cloud and RM_1-667 in the Large Magellanic Cloud was estimated. The results of spectral observations with the ESO 3.6-m telescope with resolving power R = 30000 were used. Since actinium was not found in the atmospheres of PMMR23 and PMMR144, only the upper limits were set on its abundance: logN(Ac/H) <–15.1 and–15.0, respectively. The estimated abundance of actinium in the atmosphere of RM_1-667 varied with parameters of the atmospheric model from–14.1 to–13.3. The lines of ionized actinium λ 616.475 nm and λ 581.085 nm were used in this analysis.  相似文献   

2.
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   

3.
Excitation of radial pulsations in red supergiants of Magellanic Clouds is investigated using the stellar evolution calculations and the self-consistent solution of the equations of radiation hydrodynamics and turbulent convection. The stars with initial masses 6M ⊙ ≤ M ZAMS ≤ 28M ⊙ and the initial chemical composition X = 0.7, 0.004 ≤ Z ≤ 0.008 are shown to be unstable against fundamental mode oscillations with periods from 17 to 1200 days as they become helium burning red supergiants. The period-luminosity relation slightly depends on the mass loss rate varying with a factor of three, whereas its dependence on the metal abundance is given by δM bol = 0.89δ log Z. In comparison with galactic red supergiants the low metal abundances in red supergiants of Magellanic Clouds are responsible for their higher effective temperatures and substantially narrower ranges of evolutionary radius change during helium burning. Therefore on the period-mass diagram the red supergiants of Magellanic Clouds are located within the strip with width of δ logM ≈ 0.09, so that the uncertainty of mass evaluation of the red supergiant with the known pulsation period is nearly 25%.  相似文献   

4.
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 .  相似文献   

5.
We present the results of the analysis of thorium absorption lines in the spectra of metal-deficient stars: the thorium-rich Galactic halo star HD 221170 and some supergiants in the Magellanic Clouds. For all the analyzed stars, the Th II lines were identified by comparing the synthetic and observed spectra. The Th II 6044.433 Å line and the weaker Th II 6619.943 Å line were identified in the spectra of red supergiants in the Magellanic Clouds.  相似文献   

6.
Chemical evolution of the Magellanic Clouds: analytical models   总被引：1，自引：0，他引：1  
We have extended our analytical chemical evolution modelling ideas for the Galaxy to the Magellanic Clouds. Unlike previous authors (Russell &38; Dopita, Tsujimoto et al. and Pilyugin), we assume neither a steepened initial mass function nor selective galactic winds, since among the α-particle elements only oxygen shows a large deficit relative to iron and a similar deficit is also found in Galactic supergiants. Thus we assume yields and time delays identical to those that we previously assumed for the solar neighbourhood. We include inflow and non-selective galactic winds and consider both smooth and bursting star formation rates, the latter giving a better fit to the age–metallicity relations. We predict essentially solar abundance ratios for primary elements and these seem to fit most of the data within their substantial scatter. Our model for the Large Magellanic Cloud also gives a remarkably good fit to the anomalous Galactic halo stars discovered by Nissen &38; Schuster.   Our models predict current ratios of Type Ia supernova to core-collapse supernova rates enhanced by 50 and 25 per cent respectively relative to the solar neighbourhood, in fair agreement with ratios found by Cappellaro et al. for Sdm–Im relative to Sbc galaxies, but these ratios are sensitive to detailed assumptions about the bursts and a still higher enhancement in the Large Magellanic Cloud has been deduced from X-ray studies of remnants by Hughes et al. The corresponding ratios integrated over time up to the present are slightly below 1, but they exceed 1 if one compares the Magellanic Clouds with the Galaxy at times when it had the corresponding metallicities.  相似文献   

7.
The abundances of two chemical elements of the iron group, viz., Cr and Ti, were determined by their high-resolution spectra for 22 A, F, and G supergiants in the solar neighborhood (within 700 pc). The titanium and chromium abundances were obtained using the Cr II and Ti II lines. The average chromium abundance of log?(Cr) = 5.70 ± 0.13 within the error limit corresponds to the solar abundance of log?⊙(Cr) = 5.64. The average titanium abundance of log?(Ti) = 4.89 ± 0.10 within the error limit is very close to the solar abundance of log?⊙(Ti) = 4.95. The average Cr and Ti abundances may be indicative of the fact that the average metallicity of young closely located stars is identical to that of the Sun.  相似文献   

8.
Previous models for the chemical evolution of the Magellanic Clouds have assumed either a steepened IMF compared to the solar neighbourhood or preferential expulsion of oxygen and α-particle elements by selective galactic winds. These assumptions were largely motivated by a belief that the O/Fe ratio in the Clouds is substantially lower than in the Galaxy, but the difference appears to have been exaggerated: Galactic supergiants have a similar O/Fe ratio as Cloud supergiants, there is no corresponding effect in Mg and other α-elements and a combination of data from planetary nebulae, H II regions and supernova remnants indicates an O/Fe ratio more or less equal to solar. Consequently new analytical models for the chemical evolution of the Magellanic Clouds have been developed, assuming chemical yields and time delays identical to those we previously assumed for the solar neighbourhood, but assuming (in addition to infall) non-selective galactic winds and burst-like modes of star formation represented by discontinuous variations in the star formation rate per unit gas mass. We find adequate agreement with age-metallicity relations and element:element ratios within their substantial uncertainties, whereas our LMC model turns out to give an excellent fit to the anomalous Galactic halo stars discovered by Nissen and Schuster (1997). It also gives an enhanced SNIa/SNII ratio compared to the solar neighbourhood, due to the assumption that the SFR has declined in the past 1 to 2 Gyr. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

9.
We describe quantitatively the neutral hydrogen (HI) and dust content of the interstellar medium (ISM) in the Small Magellanic Cloud (SMC),using the spatial power spectrum. The velocity modification of the HI density power spectrum is investigated and discussed.  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 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   

12.
Colour–magnitude diagrams are presented for the first time for L32, L38, K28 (L43), K44 (L68) and L116, which are clusters projected on to the outer parts of 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 turn-off, and metallicities from the red giant branch locus. The clusters have ages in the range 2–6 Gyr , and metallicities in the range −1.65<[Fe/H]<−1.10, increasing the sample of intermediate-age clusters in the SMC. L116, the outermost cluster projected on to the SMC, is a foreground cluster, and somewhat closer to us than the Large Magellanic Cloud. Our results, combined with those for other clusters in the literature, show epochs of sudden chemical enrichment in the age–metallicity plane, which favour a bursting star formation history as opposed to a continuous one for the SMC.  相似文献   

13.
Despite considerable observational information on infrared absorption and emission spectra of interstellar matter, together with extensive laboratory data on the spectra of possible constituents, there are presently few firm identifications of individual chemical components. Simple chemical compounds including H2O, CO and CO2 are detected in dust in dark interstellar clouds, but other molecules such as ethane, acetylene and benzene do not appear to be present with appreciable abundance. Nevertheless, IR spectra show that hydrocarbons are aubiquitous component of interstellar matter. The nature of these materials and their relation to specific molecular components is discussed in this paper. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

14.
The local correlation between far-infrared (FIR) emission and radio-continuum (RC) emission for the Small Magellanic Cloud (SMC) is investigated over scales from 3 kpc to 0.01 kpc. Here, we report good FIR/RC correlation down to ～15 pc. The reciprocal slope of the FIR/RC emission correlation (RC/FIR) in the SMC is shown to be greatest in the most active star forming regions with a power law slope of ～1.14 indicating that the RC emission increases faster than the FIR emission. The slope of the other regions and the SMC are much flatter and in the range of 0.63–0.85. The slopes tend to follow the thermal fractions of the regions which range from 0.5 to 0.95. The thermal fraction of the RC emission alone can provide the expected FIR/RC correlation. The results are consistent with a common source for ultraviolet (UV) photons heating dust and Cosmic Ray electrons (CRe?s) diffusing away from the star forming regions. Since the CRe?s appear to escape the SMC so readily, the results here may not provide support for coupling between the local gas density and the magnetic field intensity.  相似文献   

15.
This paper is the next one in the series of our works aimed at determining the atmospheric parameters and chemical composition of southern-hemisphere Cepheids. We present the results of our study for six bright Cepheids: V Cen, V737 Cen, BB Sgr, W Sgr, X Sgr, and Y Sgr. We have analyzed 14 high-resolution spectra taken with the 1.9-m telescope at the South African Astronomical Observatory. In addition to determining the chemical composition and atmospheric parameters, we point out and discuss several features in the spectra of individual Cepheids. In particular,we have detected emission in the cores of the Hβ and Hα lines forWSgr near its maximum light, while X Sgr shows the splitting of metal absorption lines into individual components without any change of the hydrogen lines. These peculiarities can be explained by different manifestations of shock waves in the Cepheid atmospheres and by the presence of circumstellar envelopes around X Sgr and W Sgr. The chemical composition has been estimated for V737 Cen, BB Sgr, and X Sgr for the first time. On the whole, our abundance estimates for α-elements, iron-peak elements, and r- and s-process elements are close to the solar ones for all objects, carbon is underabundant, the oxygen abundance is nearly solar, the “odd” elements, Na and Al, are overabundant (except X Sgr), magnesium is underabundant for V Cen and X Sgr and overabundant for the remaining objects. Such a chemical composition is typical of yellow supergiants after the first dredge-up. Keywords: Cepheids, spectra, atmospheric parameters, chemical composition.  相似文献   

16.
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   

17.
A new PL-relation (10) — Figure 2 for the Cepheids in the Galaxy, the Magellanic Clouds and M31 has been constructed. On deriving this relation both the period-radius (3) and period-colour relations (7), (8) and (9) are essentially used. The PC-relation (7) determined after the colours of 88 galactic cepheids (Table I), which are obtained from the colour-spectrum relation (6) — Figure 1, common for Cepheids and non-variable supergiants, are used also for the M31 Cepheids, whereas (8) and (9) are for the Large and Small MC Cepheids, respectively, all three PC-relations having a common slope. The comparison of the relations (8) and (9) with (7) shows that the LMC and SMC Cepheids are bluer than the galactic ones with 0m.04 and 0m.19, respectively, probably because of their metal-poor abundance. The places of thes-Cepheids in Figure 2 show that these Cepheids possess a dissimilar PL-relation with a different slope. The reason for such a difference is that thes-Cepheids are first harmonic pulsators. The distance moduli of the three galaxies under discussion are obtained from the PL-relation (10). The colour-coefficient of period-colour-luminosity-relation is briefly discussed. The general conclusion based on a comparison of the PL-relation in the present paper with those by other authors (Table V) is that our PL-relation differs in the zero-point by less than 0m.2; therefore, the manner of constructing the PL-relation by means of PR and PC-relations is reasonable and useful.  相似文献   

18.
Places of formation have been derived for a sample of 23 Cepheids with well-determined atmospheric abundances in an attempt to study the chemical inhomogeneities in the local interstellar medium. The abundance data available for the sample Cepheids is compiled and critically reviewed to adopt the most reliable esimates. We find that the most conspicuous irregularity in the metallicity is exhibited by stars that are born in the local arm or in the interarm region. We propose a scenario to explain these local variations in terms of supernova-induced star formation in interstellar gas enriched by massive stars formed in the density wave.  相似文献   

19.
We have determined the atmospheric abundances of selected Cepheids in order to study the large-scale chemical inhomogeneities across the galactic disk. The classical Cepheids were selected as probes to study the variation of metallicity in the galactic disk, because of their high intrinsic luminosity, small age and the existence of period-luminosity and period-age relationships. High dispersion spectra of programme stars WZ Sgr, X Sgr, ? Gem, T Mon and S V Mon were obtained using the 102-cm reflector of Kavalur Observatory. The atmospheric abundances were determined by theoretically synthesizing the selected portions of the stellar spectrum and comparing with the observed spectra. In order to compute the theoretical spectrum, the formal solution of the equation of radiative transfer was numerically evaluated with the simplifying assumptions of local thermodynamical equilibrium, plane-parallel geometry and hydrostatic equilibrium. These assumptions are reasonably good for the metallic lines of F-G supergiants and hence the observations were confined to the phases where Cepheids behave like nonvariable F-G supergiants. The atmospheric abundances of iron-peak elements, Fe, Cr, Ti, Ca and heavier s-process elements Y, Ba, La, Ce, Sm were obtained by synthesizing a selected spectral region in the range 4330 Å — 4650 Å. We derive a radial abundance gradient for iron \(\frac{{d(Fe/H)}}{{dr_{gc} }} = - 0.056 \pm 0.08\) for the region of galactic disk between 6.7 and 10.9 kpc from the galactic centre (assuming rgc = 8.5 kpc for the Sun). This value agrees with the one obtained from the general sample of Cepheids for which spectroscopic abundances are available, and also with the existing photometric determinations, but is shallower than the one derived by Luck (1982). Abundances of the elements derived in the present investigation do not show any significant correlation with atomic number. Also the abundance ratio of s-process elements does not show any correlation with Fe. This lack of correlation for disk population stars shows the inadequacy of simple models of galactic chemical evolution and favours the infall models. Alternately, the evolution of [s/Fe] may be determined by the ratio of intermediate-mass stars (which contribute s-process nuclei) to high-mass stars (which contribute Fe peak nuclei). Thus the different behaviour of halo and disk population may indicate a difference in the mass spectrum of star formation.  相似文献   

20.
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   

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