共查询到20条相似文献，搜索用时 31 毫秒
1.
Nikolay Tikhonov Olga Galazutdinova Olga Sholukhova Antoniya Valcheva Petko Nedialkov Olga Merkulova 《天文和天体物理学研究(英文版)》2021,(4):186-192
This paper introduces a technique for searching for bright massive stars in galaxies beyond the Local Group.To search for massive stars,we processed the results of stellar photometry from the Hubble Space Telescope(HST) images using the DAOPHOT and DOLPHOT packages.The results of such searches are demonstrated with examples of the galaxies DDO 68,M94 and NGC 1672.In the galaxy DDO 68,the LBV star changes its brightness,and massive stars in M94 can be identified by excess in the Ha band.For the galaxy NGC 1672,we measure the distance for the first time by the TRGB method,which enabled determining the luminosities of the brightest stars,likely hypergiants,in the young star formation region.So far,we have performed stellar photometry on HST images of 320 northern sky galaxies located at a distance less than 12 Mpc.This allowed us to identify 53 galaxies with probable hypergiants.Further photometric and spectral observations of these galaxies are planned to search for massive stars. 相似文献
2.
We present the results of our stellar photometry and spectroscopy for the new Local Group galaxy VV124 (UGC4879) obtainedwith
the 6-m BTAtelescope. The presence of a fewbright supergiants in the galaxy indicates that the current star formation process
is weak. The apparent distribution of stars with different ages in VV 124 does not differ from the analogous distributions
of stars in irregular galaxies, but the ratio of the numbers of young and old stars indicates that VV 124 belongs to the rare
Irr/Sph type of galaxies. The old stars (red giants) form the most extended structure, a thick disk with an exponential decrease
in the star number density to the edge. Definitely, the young population unresolvable in images makes a great contribution
to the background emission from the central galactic regions. The presence of young stars is also confirmed by the [OIII]
emission line visible in the spectra that belongs to extensive diffuse galactic regions. The mean radial velocity of several
components (two bright supergiants, the unresolvable stellar population, and the diffuse gas) is υ
h = −70 ± 15 km s−1 and the velocity with which VV 124 falls into the Local Group is υ
LG = −12 ± 15 km s−1. We confirm the distance to the galaxy (D = 1.1 ± 0.1 Mpc) and the metallicity of red giants ([Fe/H] = −1.37) found by Kopylov et al. (2008). VV 124 is located on
the periphery of the Local Group approximately at the same distance from M31 and our Galaxy and is isolated from other galaxies.
The galaxy LeoA nearest to it is 0.5 Mpc away. 相似文献
3.
L. Bianchi Y. B. Kang B. Efremova D. Thilker P. Hodge P. Massey K. Olsen 《Astrophysics and Space Science》2011,335(1):249-255
The study of the stellar constituents of star-forming sites in a wide variety of conditions yields the key to interpreting
wide-field UV-optical imaging of extended nearby galaxies, and of distant galaxies. We obtained six-band imaging (from far-UV
to I) with HST-WFPC2 of 67 sites of recent star formation in eight Local Group galaxies. HST pointings were selected from
GALEX wide-field FUV imaging, which traces the young stellar populations. The HST observations were optimized to characterize
the hottest, most massive stars in these regions. From the HST photometry, analyzed with stellar model colors, we derived
the physical parameters of the massive stars in each field, and of the extinction by interstellar dust. The HST results are
used to interpret GALEX UV measurements of SF across the entire galaxies. Our comprehensive photometric study at HST resolution
(sub-pc scale in these galaxies) also provides an ideal selection of targets for follow-up spectroscopy with large ground-based
telescopes, and in the UV with HST- or WSO-class telescopes, to clarify the influence of metallicity on the properties and
the evolution of massive stars. 相似文献
4.
Based on archival Hubble Space Telescope ACS/WFC images, we have performed stellar photometry for an isolated pair of interacting galaxies, IC 1727 and NGC 672, and constructed their Hertzsprung-Russell diagrams. The galaxy IC 1727 exhibits a strong asymmetry in the apparent distribution of young stars. The distribution of old stars is smoother and less asymmetric. In the galaxy NGC672, there is no noticeable asymmetry in the distribution of stars of different ages. Based on the TRGB method, we have determined an accurate distance to each galaxy for the first time: D = 7.14 ± 0.10 Mpc for IC 1727 and D = 7.22 ± 0.10 Mpc for NGC 672, confirming that these galaxies are closely spaced. The luminosity functions of red supergiants and peripheral AGB stars in both galaxies have positionally coincident local maxima, suggesting the simultaneous enhancement of star formation in the two galaxies occurred in the intervals 20–30 and 450–700 Myr ago. The results obtained point to an enhancement of star formation processes in the interacting galaxies. 相似文献
5.
To study the spatial distribution of galaxies lying between the Virgo cluster and the Local Group, a search was made for probable nearby galaxies. Using the method of brightest stars and of blue and red supergiants made it possible to determine the distances to 24 galaxies, among which six relatively nearby galaxies were identified. The results of the distance determinations showed that the maximum in the number distribution of galaxies lies at 17.0 Mpc, which we take as the distance to the Virgo cluster. Using the difference between the distance moduli of two clusters of galaxies, in Virgo and Coma Berenices, from literature sources and the velocity of the latter cluster, we determined the Hubble constant to be H
0 = 77 ± 7 km·sec–1·Mpc–1. 相似文献
6.
Gustavo A. Lanfranchi Francesca Matteucci Gabriele Cescutti 《Monthly notices of the Royal Astronomical Society》2006,365(2):477-488
By means of a detailed chemical evolution model, we follow the evolution of barium (Ba) and europium (Eu) in four Local Group Dwarf Spheroidal (dSph) galaxies, in order to set constraints on the nucleosynthesis of these elements and on the evolution of this type of galaxies compared with the Milky Way. The model, which is able to reproduce several observed abundance ratios and the present-day total mass and gas mass content of these galaxies, adopts up-to-date nucleosynthesis and takes into account the role played by supernovae (SNe) of different types (II, Ia) allowing us to follow in detail the evolution of several chemical elements (H, D, He, C, N, O, Mg, Si, S, Ca, Fe, Ba and Eu). By assuming that Ba is a neutron-capture element produced in low-mass asymptotic giant branch stars by s-process but also in massive stars (in the mass range 10–30 M⊙ ) by r-process, during the explosive event of SNe of Type II, and that Eu is a pure r-process element synthesized in massive stars also in the range of masses 10–30 M⊙ , we are able to reproduce the observed [Ba/Fe] and [Eu/Fe] as functions of [Fe/H] in all four galaxies studied. We confirm also the important role played by the very low star formation (SF) efficiencies (ν= 0.005–0.5 Gyr−1 ) and by the intense galactic winds (6–13 times the star formation rate) in the evolution of these galaxies. These low SF efficiencies (compared to the one for the Milky Way disc) adopted for the dSph galaxies are the main reason for the differences between the trends of [Ba/Fe] and [Eu/Fe] predicted and observed in these galaxies and in the metal-poor stars of our Galaxy. Finally, we provide predictions for Sagittarius galaxy for which data of only two stars are available. 相似文献
7.
Eline Tolstoy 《Astrophysics and Space Science》1999,265(1-4):199-206
What can we learn from the somewhat arduous study of old stellar populations in nearby galaxies? Unless the nearby universe is subtly anomalous, it should contain a relatively normal selection of galaxies whose histories are representative of field galaxies in general throughout the Universe. We can therefore take advantage of our ability to resolve local galaxies into individual stars to directly, and accurately, measure star formation histories. The star formation histories are determined from numerical models, based on stellar evolution tracks, of colour-magnitude diagrams. The most accurate information on star formation rates extending back to the earliest epoches can be obtained from the structure of the main sequence. However, the oldest main sequence turnoffs are very faint, and it is often necessary to use the brighter, more evolved, populations to infer the star formation history at older times. A complete star formation history can be compared with the spectroscopic properties of galaxies seen over a large range of lookback times in redshift surveys. There is considerable evidence that the faint blue galaxies seen in large numbers in cosmological surveys are the progenitors of the late-type irregular galaxies seen in copious numbers in the Local Group, and beyond. We consider how the `Madau-diagram', the star formation history of the Universe, would look if the Local Group were to be considered representative of the Universe as a whole. 相似文献
8.
R. Michael Rich 《Astrophysics and Space Science》1999,265(1-4):453-460
This review considers implications regarding galaxy formationand evolution that can be drawn from study of the ages,abundances,
and kinematics of stellar populations in the LocalUniverse. The wide abundance range in the Galactic bulge and in thehalo
of M31 is consistent with chemical evolution in a starburstwith wind outflow. We question the notionthat the Galactic halo
population is assembled fromdisrupted dwarf spheroidal galaxies, based on the presenceof metal rich stars in Local Group halos.The
alpha-element enhancements of bulge giants are those expected from a system forming on a timescale of less than 1 Gyr. Direct
measurement of the star formationhistory from turnoff photometry in Local Group galaxiesis not in complete agreement with
the universal star formation rateinferred from high redshift studies. We argue that the properties of the Local Universe constrain
galaxyformation theory just as strongly as the findings gleaned fromhigh redshift galaxies.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
9.
Eline Tolstoy 《Astrophysics and Space Science》2003,284(2):579-588
The smallest dwarf galaxies are the most straight forward objects in which to study star formation processes on a galactic
scale. They are typically single cell star forming entities, and as small potentials in orbit around a much larger one they
are unlikely to accrete much (if any) extraneous matter during their lifetime (either intergalactic gas, or galaxies) because
they will typically lose the competition with the much larger galaxy. We can utilise observations of stars of a range of ages
to measure star formation and enrichment histories back to the earliest epochs. The most ancient objects we have ever observed
in the Universe are stars found in and around our Galaxy. Their proximity allows us to extract from their properties detailed
information about the time in the early Universe into which they were born. A currently fashionable conjecture is that the
earliest star formation in the Universe occurred in the smallest dwarf galaxy sized objects. Here I will review some recent
observational highlights in the study of dwarf galaxies in the Local Group and the implications for understanding galaxy formation
and evolution.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
10.
It is well known that galaxies accumulating large quantities of gas undergo violent bursts of star formation. This is believed to be due to tidal interactions of galaxies leading to the infall of gas into their central regions. Bursts of star formation in this scenario are transitory phenomena and can be induced only by external sources.However, in some cases there is no direct evidence of tidal interactions in starburst galaxies.We discuss another possibility of bursting phenomena in galaxies connected with nonlinear feedback processes in mass-exchange between components of star-forming region. We consider a three-component model including cold clouds, warm gas and massive stars and take into account the delay processes in the transformation of hot gas ejected by massive stars and evaporated from cold phase, into the warm phase. Self-regulating mechanism of phase transition of small clouds into warm gas due to heating radiation of massive stars is also taken into account.The analysis of stability of the system shows that it could be unstable even in case of a small efficiency in the birth of massive stars. The evolution of unstable nonlinear perturbations leads to the development of self-sustained nonlinear oscillations of star formation. 相似文献
11.
Based on archival Hubble Space Telescope ACS/WFC images, we have performed stellar photometry for more than 0.6 million stars
in the interacting galaxies NGC 5194 and NGC 5195 of the M51 system. Stars of various ages have been identified on the constructed
Hertzsprung-Russell diagram: blue and red supergiants, AGB stars, and red giants. The distance to M51 has been measured from
the position of the tip of the red giant branch, D = 9.9 ± 0.7 Mpc. We have determined the change in the metallicity of red supergiants along the galactic radius in NGC 5194.
Despite the gravitational interaction, the distribution of stars in NGC 5194 does not differ from that in isolated galaxies.
The asymmetric stellar structures of NGC5195 (the so-called “feathers”) formed through the interaction of two galaxies have
been found to consist mostly of AGB stars. 相似文献
12.
M. Mapelli E. Ripamonti G. Battaglia E. Tolstoy M. J. Irwin B. Moore S. Sigurdsson 《Monthly notices of the Royal Astronomical Society》2009,396(3):1771-1782
The existence of blue straggler stars (BSSs) in dwarf spheroidal galaxies (dSphs) is still an open question. In fact, many BSS candidates have been observed in the Local Group dSphs, but it is unclear whether they are real BSSs or young stars. Shedding light on the nature of these BSS candidates is crucial in order to understand the star formation history of dSphs. In this paper, we consider BSS candidates in Sculptor and Fornax. In Fornax, there are strong hints that the BSS population is contaminated by young stars, whereas in Sculptor there is no clear evidence of recent star formation. We derive the radial and luminosity distribution of BSS candidates from wide field imaging data extending beyond the nominal tidal radius of these galaxies. The observations are compared with the radial distribution of BSSs expected from dynamical simulations. In Sculptor, the radial distribution of BSS candidates is consistent with that of red horizontal branch (RHB) stars and is in agreement with theoretical expectations for BSSs generated via mass transfer in binaries. On the contrary, in Fornax, the radial distribution of BSS candidates is more concentrated than that of all the considered stellar populations. This result supports the hypothesis that most of BSS candidates in Fornax are young stars, and this is consistent with previous studies. 相似文献
13.
We present the study of the star formation histories (SFHs) of a sample of Local Group dwarf galaxies (LGDGs), via the analysis
and modelling, with the means of an evolutionary stellar population synthesis, of their colour-magnitude (CM) diagrams. It
appears that important parameters to describe the SFHs are star formation rate (SFR) and duration of star formation. We find
a possible correlation between the mass and the SFRs. The correlation might be the origin of the observed luminosity-metallicity
relation in the LGDGs. A well-defined correlation between the durations of star formation and the distance from M31 or the
Galaxy indicates that the current early-type dwarf galaxies should have been transformed from late-type by strong tidal force
of the massive galaxies.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
14.
依传统的局部混合长理论进行了对流包层模型计算,在黄红巨星和超巨星对流区的顶部出现超声速对流的情况,这与局部对流理论本身是不自恰的．文章详细分析了恒星大气超声速对流的成因,并按照星族Ⅰ恒星的演化轨迹,在赫罗图上给出了出现超声速对流的区域．其结果是。在演化过程中, (1)低质量星基本不出现超声速对流; (2)中等质量星的超声速对流主要出现在3．6相似文献
15.
Georges Meynet 《Astrophysics and Space Science》2002,279(1-2):183-186
Stellar models with axial rotation and very low metallicity Z, typical of the early star generations have been computed. These
models account for the effects of rotation in the same way as previous grids which successfully reproduce the He- and N-enrichments
observed at the surface of massive stars, and also the high numbers of red supergiants observed in low Z galaxies (Meynet and Maeder, 2000; Maeder and Meynet, 2001). At very low metallicity (Z = 0.00001, i.e. one two thousandth of the solar metallicity),these models naturally lead to the production of primary nitrogen.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
16.
Simple theoretical arguments indicate that cooled interstellar gas in bright elliptical galaxies forms into a young stellar population having a bottom-heavy but optically luminous initial mass function extending to approximately 2 M middle dot in circle. When the colors and spectral features of this young population are combined with those of the underlying old stellar population, the apparent ages are significantly reduced, similar to the relatively young apparent ages observed in many elliptical galaxies. Galactic mergers are not required to resupply young stars. The sensitivity of continuous star formation to LB and LX&solm0;LB is likely to account for the observed spread in apparent ages among elliptical galaxies. Local star formation is accompanied by enhanced stellar Hbeta equivalent widths, stronger optical emission lines, more thermal X-ray emission, and lower apparent temperatures in the hot gas. The young stars should cause M&solm0;L to vary with galactic radius, perturbing the fundamental plane of the old stars alone. 相似文献
17.
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
18.
邓李才 《紫金山天文台台刊》2003,(2)
大质量恒星由于其高光度、短寿命和质量损失 ,对星系的积分光谱能量分布和重元素增丰起主导作用 ,从而在研究星系的形成和演化上具有特殊的意义。特别是随着天文设备的长足进展 ,我们可以回溯宇宙演化的历史 ,得到形成初期时星系的观测性质。那时 ,大质量恒星主导星系的辐射性质 ,这更加突出了对大质量恒星进一步了解的迫切性。但是大质量恒星的演化性质相对中小质量恒星而言 ,有很多不确定性。本文通过对比现有恒星模型与实测结果 ,对现有大质量恒星演化理论中存在的几个与对流和质量损失相关的问题进行了评述 ,并对从理论上 ,特别是通过数字模拟方法对这些问题进行诊断提出了展望。 相似文献
19.
Olga K. Sil’chenko 《Astrophysics and Space Science》2011,335(1):171-176
I review the origin of UV-radiation in galaxies of different morphological types. UV-excess in spectra of massive elliptical
galaxies which have predominantly old stellar populations is traditionally explained by the contribution of low-mass stars
at very late, poorly known stages of evolution—by so called ‘AGB-manqué’ stars or by the population of extended horizontal
branch. However recent results from the GALEX survey of a large sample of nearby ellipticals have also demonstrated probable
traces of recent star formation in a third of all ellipticals observed. In spiral galaxies extended UV-disks have been discovered
by the GALEX; they are certainly illuminated by the current star formation, but what has provoked star formation in the areas
of very low gas density, beyond the distribution of older stars, is a puzzle yet. The UV-spectra of starburst galaxies or
starforming galactic nuclei are characterized by weak emission lines, if any, quite dissimilar to their optical spectra. 相似文献
20.
Stephen J. Smartt 《Astrophysics and Space Science》2002,281(1-2):187-190
The masses and the evolutionary states of the progenitors of core-collapse supernovae are not well constrained by direct observations.
Stellar evolution theory generally predicts that massive stars with initial masses less than about 30M⊙ should undergo core-collapse when they are cool M-type supergiants. However the only two detections of a SN progenitor before
explosion are SN1987A and SN1993J, and neither of these was an M-type supergiant. Attempting to identify the progenitors of
supernovae is a difficult task, as precisely predicting the time of explosion of a massive star is impossible for obvious
reasons. There are several different types of supernovae which have different spectral and photometric evolution, and how
exactly these are related to the evolutionary states of the progenitor stars is not currently known. I will describe a novel
project which may allow the direct identification of core-collapse supernovae progenitors on pre-explosion images of resolved,
nearby galaxies. This project is now possible with the excellent image archives maintained by several facilities and will
be enhanced by the new initiatives to create Virtual Observatories, the earliest of which (astrovirtel) is already producing results.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献