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1.
We present angular diameters for 42 Luminosity Class (LC) I stars and 32 LC II stars that have been interferometrically determined with the Palomar Testbed Interferometer. Derived values of radius and effective temperature are established for these objects, and an empirical calibration of these parameters for supergiants will be presented as a function of spectral type and colours. For the effective temperature versus  ( V − K )0  colour, we find an empirical calibration with a median deviation of  Δ T = 70 K  in the range of  0.7 < ( V − K )0 < 5.1  for LC I stars; for LC II, the median deviation is  Δ T = 120 K  from  0.4 < ( V − K )0 < 4.3  . Effective temperature as a function of spectral type is also calibrated from these data, but shows significantly more scatter than the T EFF versus  ( V − K )0  relationship. No deviation of T EFF versus spectral type is seen for these high-luminosity objects relative to LC II giants. Directly determined diameters range up to  400 R⊙  , though are limited by poor distance determinations, which dominate the error estimates. These temperature and radii measures reflect a direct calibration of these parameters for supergiants from empirical means.  相似文献   

2.
We present a new Cepheid reddening and effective temperature scale based on the uvby photometry published in the first paper of this series. Using all available information about the companion stars in Cepheids with bright blue secondaries, we remove their light from the observed light and colour curves. The resulting corrections are as large as 0.05–0.15 mag in several cases for different colour indices. A new photometric approach based on the ( b  −  y ) versus ( B  − V ) two-colour diagram is tested with three other previous calibrations taken from the literature. Two uvby relations in earlier studies turn out to be the most reliable and consistent, and so they are used in deriving colour excesses. We determine systematically higher reddenings for Cepheids with a significant secondary light correction. The dereddened Stro¨mgren colours are calibrated in terms of T eff and log  g using the most recent synthetic colour grids. Our temperature scale is very close to that of Kraft, which is supported by other recent temperature determinations using the infrared flux method or Geneva photometry. The photometric gravities fit some of the earlier theoretical and observational (mainly spectroscopic) results very well.  相似文献   

3.
The purpose of this work is to explore the evolution of helium-core white dwarf stars in a self-consistent way with the predictions of detailed non-grey model atmospheres and element diffusion. To this end, we consider helium-core white dwarf models with stellar masses of 0.406, 0.360, 0.327, 0.292, 0.242, 0.196 and 0.169 M⊙ and follow their evolution from the end of mass-loss episodes, during their pre-white dwarf evolution, down to very low surface luminosities.
We find that when the effective temperature decreases below 4000 K, the emergent spectrum of these stars becomes bluer within time-scales of astrophysical interest. In particular, we analyse the evolution of our models in the colour–colour and in the colour–magnitude diagrams and find that helium-core white dwarfs with masses ranging from ∼0.18 to 0.3 M⊙ can reach the turn-off in their colours and become blue again within cooling times much less than 15 Gyr and then remain brighter than M V ≈16.5 . In view of these results, many low-mass helium white dwarfs could have had enough time to evolve to the domain of collision-induced absorption from molecular hydrogen, showing blue colours.  相似文献   

4.
We present new determinations of bolometric corrections and effective temperature scales as a function of infrared optical colours, using a large data base of photometric observations of about 6500 Population II giants in Galactic globular clusters (GGCs), covering a wide range in metallicity (−2.0 < [Fe/H] < 0.0).   New relations for BC K versus ( V  −  K ) , ( J  −  K ) and BC V versus ( B  −  V ), ( V  −  I ), ( V  −  J ), and new calibrations for T eff, using both an empirical relation and model atmospheres, are provided.   Moreover, an empirical relation to derive the R parameter of the infrared flux method as a function of the stellar temperature is also presented.  相似文献   

5.
We show that there is a relationship between the age excess, defined as the difference between the stellar isochrone and chromospheric ages, and the metallicity as measured by the index [Fe/H] for late-type dwarfs. The chromospheric age tends to be lower than the isochrone age for metal-poor stars, and the opposite occurs for metal-rich objects. We suggest that this could be an effect of neglecting the metallicity dependence of the calibrated chromospheric emission–age relation. We propose a correction to account for this dependence. We also investigate the metallicity distributions of these stars, and show that there are distinct trends according to the chromospheric activity level. Inactive stars have a metallicity distribution which resembles the metallicity distribution of solar neighbourhood stars, while active stars appear to be concentrated in an activity strip on the log  R 'HK × [Fe/H] diagram. We provide some explanations for these trends, and show that the chromospheric emission–age relation probably has different slopes on the two sides of the Vaughan–Preston gap.  相似文献   

6.
Surface photometry of 18 Virgo cluster dwarf elliptical (dE) and dwarf lenticular (dS0) galaxies, made by Gavazzi et al. in the H band (1.65 μm) and in the B band (0.44 μm), shows that the ratio of the effective radii of these stellar systems in the B and H bands,   r e B / r e H   , ranges between 0.7 and 2.2. In particular, dwarf ellipticals and lenticulars with a red total colour index   B - H   (i.e. with  3.2< B - H <4)  have equal effective radii in these two passbands. By contrast, blue (i.e. with  2.5< B - H <3.1)  dEs and dS0s have B -band effective radii about 50 per cent larger than the H -band ones, on average. Consistently, strong negative gradients in   B - H   along the galactocentric radius are found to be associated with blue total colours. This trend is not found in a sample of 29 giant E and S0 galaxies of the Coma cluster with analogous data available in the literature. These early-type giants span a broad range in    r e B / r e H    (0.2–2.2)  , with a mean   r e B / r e H ∼1.1  , but a narrow range in (red) colour  (3.3< B - H <4.2)  . In these stellar systems, colour gradients are usually interpreted as arising either from age/metallicity gradients along the radial coordinate or from dust attenuation, whatever the total colour of the system is. Assuming each of these three distinct interpretations of the origin of colour gradients, we discuss the origin of the association of strong negative colour gradients with blue colours found in the early-type dwarfs under study, in relation with current scenarios of formation and evolution of dE and dS0 galaxies.  相似文献   

7.
The Edinburgh–Cape Blue Object Survey is an ongoing project to identify and analyse a large sample of hot stars selected initially on the basis of photographic colours (down to a magnitude limit     over the entire high-Galactic-latitude southern sky, and then studied with broad-band UBV photometry and medium-resolution spectroscopy. Due to unavoidable errors in the initial candidate selection, stars that are likely metal-deficient dwarfs and giants of the halo and thick-disc populations are inadvertently included, yet are of interest in their own right. In this paper we discuss a total of 206 candidate metal-deficient dwarfs, subgiants, giants, and horizontal-branch stars with photoelectric colours redder than     and with available spectroscopy. Radial velocities, accurate to ∼10–15 km s−1, are presented for all of these stars. Spectroscopic metallicity estimates for these stars are obtained using a recently recalibrated relation between Ca  ii K-line strength and     colour. The identification of metal-poor stars from this colour-selection technique is remarkably efficient, and competitive with previous survey methods. An additional sample of 186 EC stars with photoelectric colours in the range     composed primarily of field horizontal-branch stars and other, higher gravity, A- and B-type stars, is also analysed. Estimates of the physical parameters T eff, log  g , and [Fe/H] are obtained for cooler members of this subsample, and a number of candidate RR Lyrae variables are identified.  相似文献   

8.
This paper presents a catalogue and the method of determining averaged quadratic effective magnetic fields  〈 B e〉  for 1212 main-sequence and giant stars, and 11 white dwarf stars. The catalogue includes stars that are members of several open clusters. We have compiled measurements of the longitudinal magnetic field for those stars, which were scattered in the existing literature. A new parameter, magnetization (MA), has been defined, and we present values of MA for stars of various spectral classes. Our sample includes a subset of 610 chemically peculiar early-type stars. We confirm the conclusion of our previous study that the number distribution of all chemically peculiar stars versus the averaged magnetic field strength is described by a decreasing exponential function. Relations of this type also hold for stars of all the analysed subclasses of chemical peculiarity. Magnetization tends to correlate with the effective temperature only at high MA, for He-weak and He-rich stars.  相似文献   

9.
We present integrated JHK S Two-Micron All-Sky Survey photometry and a compilation of integrated-light optical photoelectric measurements for 84 star clusters in the Magellanic Clouds. These clusters range in age from ≈200 Myr to >10 Gyr, and have [Fe/H] values from −2.2 to −0.1 dex. We find a spread in the intrinsic colours of clusters with similar ages and metallicities, at least some of which is due to stochastic fluctuations in the number of bright stars residing in low-mass clusters. We use 54 clusters with the most-reliable age and metallicity estimates as test particles to evaluate the performance of four widely used simple stellar population models in the optical/near-infrared (near-IR) colour–colour space. All models reproduce the reddening-corrected colours of the old (≥10 Gyr) globular clusters quite well, but model performance varies at younger ages. In order to account for the effects of stochastic fluctuations in individual clusters, we provide composite   B − V , B − J , V − J , V − K S  and   J − K S  colours for Magellanic Cloud clusters in several different age intervals. The accumulated masses for most composite clusters are higher than that needed to keep luminosity variations due to stochastic fluctuations below the 10 per cent level. The colours of the composite clusters are clearly distinct in optical–near-IR colour–colour space for the following intervals of age: >10 Gyr, 2–9 Gyr, 1–2 Gyr, and 200 Myr−1 Gyr. This suggests that a combination of optical plus near-IR colours can be used to differentiate clusters of different age and metallicity.  相似文献   

10.
Near-infrared spectra for a sample of 31 elliptical galaxies in the Coma cluster are obtained. The galaxies are selected to be ellipticals (no lenticulars), with a large spatial distribution, covering both the core and outskirts of the cluster (i.e. corresponding to regions with large density contrasts). COsp (2.3-μm) absorption indices, measuring the contribution from intermediate-age red giant and supergiant stars to the near-infrared light of the ellipticals, are then estimated.
It is found that the strength of COsp features in elliptical galaxies increases from the core ( r <02) to the outskirts ( r >02) of the Coma cluster. Using the Mg2 strengths, it is shown that the observed effect is not caused by metallicity and is mostly caused by the presence of a younger population (giant and supergiant stars) in ellipticals in the outskirts (low-density region) of the cluster.
Using the COsp features, the origin of the scatter on the near-infrared Fundamental Plane (the relation between the effective diameter, effective surface brightness and velocity dispersion) of elliptical galaxies is studied. Correcting this relation for contributions from the red giant and supergiant stars, the rms scatter reduces from 0.077 to 0.073 dex. Although measurable, the contribution from these intermediate-age stars to the scatter on the near-infrared Fundamental Plane of ellipticals is only marginal.
A relation is found between the COsp and V − K colours of ellipticals, corresponding to a slope of 0.036±0.016, significantly shallower than that from the Mg2–( V − K ) relation. This is studied using stellar synthesis models.  相似文献   

11.
We present wide-area UBRI photometry for globular clusters around the Leo group galaxy NGC 3379. Globular cluster candidates are selected from their B -band magnitudes and their  ( U − B ) o   versus  ( B − I ) o   colours. A colour–colour selection region was defined from photometry of the Milky Way and M31 globular cluster systems. We detect 133 globular cluster candidates, which supports previous claims of a low specific frequency for NGC 3379.
The Milky Way and M31 reveal blue and red subpopulations, with  ( U − B ) o   and  ( B − I ) o   colours indicating mean metallicities similar to those expected based on previous spectroscopic work. The stellar population models of Maraston and Brocato et al. are consistent with both subpopulations being old, and with metallicities of  [Fe/H]∼−1.5  and −0.6 for the blue and red subpopulations, respectively. The models of Worthey do not reproduce the  ( U − B ) o   colours of the red (metal-rich) subpopulation for any modelled age.
For NGC 3379 we detect a blue subpopulation with similar colours, and presumably age/metallicity, to that of the Milky Way and M31 globular cluster systems. The red subpopulation is less well defined, perhaps due to increased photometric errors, but indicates a mean metallicity of [Fe/H]∼−0.6.  相似文献   

12.
The absolute visual magnitudes, MV , of A–M stars are based on calculated Hipparcos trigonometric parallaxes. The sample used consists of 30 986 unreddened and reddened A–M stars in luminosity classes Ia, Iab, Ib, II, III, IV and V. The colour excesses of the reddened stars were calculated using the mean colour indices, according to the SIMBAD data base and the intrinsic B − V values calibrated for the given spectral types and luminosity classes by Schmidt-Kaler. The values of the total-to-selective extinction,   RV = AV / E ( B − V )  , for all the reddened stars were calculated from previously published near-infrared photometric measurements. The calculated visual magnitudes, MV , of A–M stars compare with the earlier determinations of Schmidt-Kaler. The mean absolute magnitudes published by Schmidt-Kaler are generally brighter (except for the stars in luminosity classes V and IV) than those determined in this paper.  相似文献   

13.
Effective temperatures T eff, surface gravities log  g and interstellar extinctions A V are found for 107 B stars. Distances d of the stars, which are based on the derived T eff, log  g and A V values, show good agreement with those obtained from the Hipparcos parallaxes. Comparing the T eff and log  g values with evolutionary computations, we infer masses, radii, luminosities, ages and relative ages of the stars. Empirical relations between the T eff and log  g parameters, on the one hand, and the photometric indices Q , [ c 1] and β , on the other hand, are constructed; these relations give a fast method for the T eff and log  g estimation of early and medium B stars. Inclusion of the infrared J , H and K colours into the T eff, log  g and A V determination shows that (i) the T eff and log  g parameters are altered only slightly; (ii) the A V value is rather sensitive to these colours, so an accuracy better than 0.05 mag in the JHK data is necessary for precise A V evaluation.  相似文献   

14.
We present a homogeneous set of stellar atmospheric parameters ( T eff, log  g , [Fe/H]) for a sample of about 700 field and cluster stars which constitute a new stellar library in the near-IR developed for stellar population synthesis in this spectral region ( λ 8350–9020) . Having compiled the available atmospheric data in the literature for field stars, we have found systematic deviations between the atmospheric parameters from different bibliographic references. The Soubiran, Katz & Cayrel sample of stars with very well determined fundamental parameters has been taken as our standard reference system, and other papers have been calibrated and bootstrapped against it. The obtained transformations are provided in this paper. Once most of the data sets were on the same system, final parameters were derived by performing error weighted means. Atmospheric parameters for cluster stars have also been revised and updated according to recent metallicity scales and colour–temperature relations.  相似文献   

15.
We present UBV ( RI )C photometry for 80 southern red and blue stars for use as additional standards. The data are tied to the Johnson UBV and Cousins ( RI )C systems and extend the range of the available stars for colour equation determination, especially in ( U − B ) for blue stars and ( V − R ) and ( V − I ) for red stars. Comparisons with published data are made and particularly good agreement is found with Bessell for the red (Gliese) stars.  相似文献   

16.
We derive an empirical effective temperature and bolometric luminosity calibration for G and K dwarfs, by applying our own implementation of the Infrared Flux Method to multiband photometry. Our study is based on 104 stars for which we have excellent   BV ( RI )C JHK S  photometry, excellent parallaxes and good metallicities.
Colours computed from the most recent synthetic libraries (ATLAS9 and MARCS) are found to be in good agreement with the empirical colours in the optical bands, but some discrepancies still remain in the infrared. Synthetic and empirical bolometric corrections also show fair agreement.
A careful comparison to temperatures, luminosities and angular diameters obtained with other methods in the literature shows that systematic effects still exist in the calibrations at the level of a few per cent. Our Infrared Flux Method temperature scale is 100-K hotter than recent analogous determinations in the literature, but is in agreement with spectroscopically calibrated temperature scales and fits well the colours of the Sun. Our angular diameters are typically 3 per cent smaller when compared to other (indirect) determinations of angular diameter for such stars, but are consistent with the limb-darkening corrected predictions of the latest 3D model atmospheres and also with the results of asteroseismology.
Very tight empirical relations are derived for bolometric luminosity, effective temperature and angular diameter from photometric indices.
We find that much of the discrepancy with other temperature scales and the uncertainties in the infrared synthetic colours arise from the uncertainties in the use of Vega as the flux calibrator. Angular diameter measurements for a well-chosen set of G and K dwarfs would go a long way to addressing this problem.  相似文献   

17.
Elemental abundances in late-type stars are of interest in several ways: they determine the location of the stars in the HR diagram and therefore their ages, as well as the atmospheric structure in their middle and upper photospheres. Especially in the case of chromospherically active late-type stars the question arises to what degree the upper photosphere is influenced by the nearby chromosphere. Analysing S/N ∼ 200 and Δλ/λ ∼ 20 000 data, we found a mean metallicity index [M/H] = −0.2 for programme K and M field stars based on an analysis of spectra in the region 5500–9000 Å. We also found that the Ca  I 6162-Å transition is a potential surface gravity indicator for K-type stars. For the chromospheric activity interval 4.4 < log  F Mg II  < 6.6 we did not find any chromospheric activity impact on photospheric and upper photospheric transitions. With the derived metallicity, we confirmed the Li abundance from our previous paper and thus its dependence on the Mg  II chromospheric activity index. The nature of the spectrum for the active M-type star Gl 896A is explained by pure rotation of 14 km s−1. As far as the lithium–rotation relation is concerned, the spectrum of Gl 517 is rotationally broadened as well, by 12 km s−1, and the Li abundance is the second highest in our sample of stars. However, there is no link between very high Li abundance, 2.2 dex, in the K dwarf star Gl 5 and stellar rotation.  相似文献   

18.
We have identified 317 stars included in the Hipparcos astrometric catalogue that have parallaxes measured to a precision of better than 15 per cent, and the location of which in the ( M V ,( B − V ) T ) diagram implies a metallicity comparable to or less than that of the intermediate-abundance globular cluster M5. We have undertaken an extensive literature search to locate Strömgren, Johnson/Cousins and Walraven photometry for over 120 stars. In addition, we present new UBV ( RI )C photometry of 201 of these candidate halo stars, together with similar data for a further 14 known metal-poor subdwarfs. These observations provide the first extensive data set of R C I C photometry of metal-poor, main-sequence stars with well-determined trigonometric parallaxes. Finally, we have obtained intermediate-resolution optical spectroscopy of 175 stars.
47 stars still lack sufficient supplementary observations for population classification; however, we are able to estimate abundances for 270 stars, or over 80 per cent of the sample. The overwhelming majority have near-solar abundance, with their inclusion in the present sample stemming from errors in the colours listed in the Hipparcos catalogue. Only 44 stars show consistent evidence of abundances below [Fe/H]=−1.0 . Nine are additions to the small sample of metal-poor subdwarfs with accurate photometry. We consider briefly the implication of these results for cluster main-sequence fitting.  相似文献   

19.
We have performed deep imaging of a diverse sample of 26 low surface brightness galaxies (LSBGs) in the optical and the near-infrared. Using stellar population synthesis models, we find that it is possible to place constraints on the ratio of young to old stars (which we parametrize in terms of the average age of the galaxy), as well as the metallicity of the galaxy, using optical and near-infrared colours. LSBGs have a wide range of morphologies and stellar populations, ranging from older, high-metallicity earlier types to much younger and lower-metallicity late-type galaxies. Despite this wide range of star formation histories, we find that colour gradients are common in LSBGs. These are most naturally interpreted as gradients in mean stellar age, with the outer regions of LSBGs having lower ages than their inner regions. In an attempt to understand what drives the differences in LSBG stellar populations, we compare LSBG average ages and metallicities with their physical parameters. Strong correlations are seen between an LSBG's star formation history and its K -band surface brightness, K -band absolute magnitude and gas fraction. These correlations are consistent with a scenario in which the star formation history of an LSBG primarily correlates with its surface density and its metallicity correlates with both its mass and its surface density.  相似文献   

20.
This paper is based on 2MASS photometry (J H Ks magnitudes) of 1172 Be stars. The observed mean intrinsic colours have been derived with aid of two‐colour diagrams for Be stars of luminosity classes Ie‐IIe, IIIe and IVe‐Ve. The obtained results are the first determinations of their intrinsic colours in the astronomical literature. The smoothed infrared colours are compared with those obtained for “normal” B stars. Several two‐colour diagrams and plots of observed and smoothed intrinsic colour versus spectral type of luminosity classes Ie‐IIe, IIIe and IVe‐Ve are presented. Generally the determined infrared intrinsic colours of Be stars (V – J)0, (V – H)0, and (V – Ks)o differ substantially from those of “normal” B stars. It is found that the intrinsic colours of B stars are generally bluer than Be stars of corresponding spectral type and luminosity class. The mean absolute visual magnitude Mv of 528 Be stars for luminosity classes Iae, Ibe‐Iabe, IIe, IIIe and IVe‐Ve is derived from HIPPARCOS parallaxes. The Mv calibration is compared with the existing ones. The Be stars are generally brighter than “normal” B stars of corresponding spectral types. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

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