Harmonizing the RR Lyrae and clump distance scales – stretching the short distance scale to intermediate ranges?

I explore the consequences of making the RR Lyrae and clump giant distance scales consistent in the solar neighbourhood, Galactic bulge and Large Magellanic Cloud (LMC). I employ two major assumptions: (i) that the absolute magnitude–metallicity, M _V (RR)–[Fe/H], relation for RR Lyrae stars is universal, and (ii) that absolute I magnitudes of clump giants, M _I (RC), in Baade's Window are known (e.g. can be inferred from the local Hipparcos -based calibration or theoretical modelling). A comparison between the solar neighbourhood and Baade's Window sets M _V (RR) at [Fe/H]=−1.6 in the range (0.59±0.05, 0.70±0.05), somewhat brighter than the statistical parallax solution. More luminous RR Lyrae stars imply younger globular clusters, which would be in better agreement with the conclusions from the currently favoured stellar evolution and cosmological models. A comparison between Baade's Window and the LMC sets M ^LMC(RC) _I in the range (−0.33±0.09,−0.53±0.09). The distance modulus to the LMC is μ ^LMC∈(18.24±0.08,18.44±0.07). Unlike M ^LMC(RC) _I , this range in μ ^LMC does not depend on the adopted value of the dereddened LMC clump magnitude, I ^LMC(RC)₀. I argue that the currently available information is insufficient to select the correct distance scale with high confidence.     

Period–magnitude relations for M giants in Baade's Window NGC 6522

A large and complete sample of stars with   K < 9.75  in the NGC 6522 Baade's Window towards the Galactic bulge is examined using light curves extracted from MACHO and IJK photometry from DENIS.
The improved statistics resulting from a sample of over 1000 variables allow all four of the sequences A, B, C and D in the   K _S   , log  P diagram of the Large Magellanic Cloud to be seen in the Galactic bulge. The bulge sequences, however, show some differences from those in the Magellanic Clouds, possibly due to the effects of higher metallicity. These sequences may have value as distance indicators.
A new diagram of the frequency of late-type variables at a given amplitude is derived and compared with the old one from Payne-Gaposchkin. The catalogued semiregular variables of the solar neighbourhood are found to be a subset of the total of semiregulars, biased towards large amplitude.     

Asymptotic giant branch variables in Baade's Windows

In this work, a sample of luminous M-type giants in the Baade's Windows towards the inner Galactic bulge is investigated in the near-infrared. The ISOGAL survey at 7 and 15 μm has given information concerning the mass-loss rates of these stars and their variability characteristics have been extracted from the MACHO data base. Most are known to be semi-regular variables (SRVs). Here we discuss how their IJHK _S -region colours depend on period and on the presence or absence of mass loss, using results mainly taken from the DENIS and 2MASS surveys.
In order to compare their colours with solar neighbourhood stars, photometric colours on the DENIS, 2MASS and ESO photometric systems have been synthesized for objects in the spectrophotometric atlas of Lançon & Wood. In addition, they have been used to predict the differences in colour indices when stars with strong molecular bands are observed using different photometric systems.
The SRVs are found to inhabit the upper end of the     , K _S colour–magnitude diagram, lying just below the Miras. High mass-loss rates are associated with high luminosity. The near-infrared colours of the SRVs increase in a general way with period and are the reddest for stars with significant mass loss. The average colours of Mira variables, whose periods start at around 200 d in the bulge, are bluer than those of the semi-regulars at this period, particularly in     , thanks to the association of deep water vapour bands with large amplitude.     

Cuspy dark matter haloes and the Galaxy

The microlensing optical depth to Baade's Window constrains the minimum total mass in baryonic matter within the Solar circle to be greater than ∼     , assuming the inner Galaxy is barred with viewing angle ∼20°. From the kinematics of solar neighbourhood stars, the local surface density of dark matter is ∼     . We construct cuspy haloes normalized to the local dark matter density and calculate the circular-speed curve of the halo in the inner Galaxy. This is added in quadrature to the rotation curve provided by the stellar and ISM discs, together with a bar sufficiently massive so that the baryonic matter in the inner Galaxy reproduces the microlensing optical depth. Such models violate the observational constraint provided by the tangent-velocity data in the inner Galaxy (typically at radii     . The high baryonic contribution required by the microlensing is consistent with implications from hydrodynamical modelling and the pattern speed of the Galactic bar. We conclude that the cuspy haloes favoured by the cold dark matter cosmology (and its variants) are inconsistent with the observational data on the Galaxy.     

A rigorous statistical test for the existence of supermetallicity in F, G and K dwarfs

In this paper, the first statistically rigorous test is performed to determine whether there are dwarfs in the solar neighbourhood with [Fe/H] > +0.2 dex. The possibility that no such stars exist is rejected with a confidence limit exceeding 99.994 per cent. If the threshold value is raised to +0.38 dex, the corresponding hypothesis is rejected with a confidence limit of 98.5 per cent. It is tentatively found that this upper limit does not depend on the presence or absence of planets orbiting metal-rich stars. In a result that is unchanged from a previous paper, the upper limit for the known metallicities of giants is found to be about 0.2 dex lower than the limit for dwarfs. Stars that might be observed to improve the upper metallicity limit for dwarfs are listed.     

The incidence of mid-infrared excesses in G and K giants

Using photometric data from the Two-Micron All-Sky Survey (2MASS) and GLIMPSE catalogues, I investigate the incidence of mid-infrared (mid-IR) excesses  (∼10 μm)  in G and K stars of luminosity class III. In order to obtain a large sample size, stars are selected using a near-IR colour–magnitude diagram. Sources which are candidates for showing mid-IR excess are carefully examined and modelled to determined whether they are likely to be G/K giants. It is found that mid-IR excesses are present at a level of  (1.8 ± 0.4) × 10⁻³  . While the origin of these excesses remains uncertain, it is plausible that they arise from debris discs around these stars. I note that the measured incidence is consistent with a scenario in which dust lifetimes in debris discs are determined by Poynting–Robertson drag rather than by collisions.     

The infrared properties of barium stars

The infrared properties of barium stars are studied using published data in the K band and from IRAS . At 12 and 25 μm the emission from barium stars shows no excess over photospheric emission. Thus the claim made by Hakkila that some barium stars show evidence of the presence of warm (∼300  K ) circumstellar material is not supported. The 60-μm properties of barium stars are studied using survival analysis methods, and it is found that very few (3.7 ± 2.6 per cent) barium stars exhibit far-infrared excesses. Furthermore, it is found that the incidence of excess emission at 60 μm is lower in barium stars than for normal G and K giants. This may indicate that the mass-transfer event that is assumed to have taken place in barium stars has removed any cool circumstellar material that may have existed in these systems. Alternatively, it is suggested that the incidence of infrared excesses in normal G and K giants may have been over-estimated as a result of not fully accounting for foreground contamination by interstellar cirrus.     

Chemical evolution of the Magellanic Clouds: analytical models

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.     

The helium abundance and ΔY/ΔZ in lower main-sequence stars

We use nearby K dwarf stars to measure the helium-to-metal enrichment ratio  Δ Y /Δ Z   , a diagnostic of the chemical history of the solar neighbourhood. Our sample of K dwarfs has homogeneously determined effective temperatures, bolometric luminosities and metallicities, allowing us to fit each star to the appropriate stellar isochrone and determine its helium content indirectly. We use a newly computed set of Padova isochrones which cover a wide range of helium and metal content.
Our theoretical isochrones have been checked against a congruous set of main-sequence binaries with accurately measured masses, to discuss and validate their range of applicability. We find that the stellar masses deduced from the isochrones are usually in excellent agreement with empirical measurements. Good agreement is also found with empirical mass-luminosity relations.
Despite fitting the masses of the stars very well, we find that anomalously low helium content (lower than primordial helium) is required to fit the luminosities and temperatures of the metal-poor K dwarfs, while more conventional values of the helium content are derived for the stars around solar metallicity.
We have investigated the effect of diffusion in stellar models and the assumption of local thermodynamic equilibrium (LTE) in deriving metallicities. Neither of these is able to resolve the low-helium problem alone and only marginally if the cumulated effects are included, unless we assume a mixing-length which is strongly decreasing with metallicity. Further work in stellar models is urgently needed.
The helium-to-metal enrichment ratio is found to be  Δ Y /Δ Z = 2.1 ± 0.9  around and above solar metallicity, consistent with previous studies, whereas open problems still remain at the lowest metallicities. Finally, we determine the helium content for a set of planetary host stars.     

A Brief History of Star Formation and Chemical Enrichment in the Bulge of the Milky way

Observations of the stellar content of the bulge of the Milky Way can provide critical guidelines for the interpretation of observations of distant galaxies, in particular for understanding their stellar content and evolution. In this brief overview I will first highlight some recent work directed towards measuring the history of star formation and the chemical composition of the central few parsecs of the Galaxy. These observations point to an episodic history of star formation in the central region with several bursts having occurred over the past few 100 Myr (e.g. Blum et al., 1996b). High resolution spectroscopic observations by Ramírez et al. (1998) of luminous M stars in this region yield a near solar value for [Fe/H] from direct measurements of iron lines. Then I will present some results from an ongoing program by my colleagues and myself the objective of which is the delineation of the star formation and chemical enrichment histories of the central 100 parsecs of the Galaxy, the `inner bulge'. From new photometric data we have concluded that there is a small increase in mean [Fe/H] from Baade's Window to the Galactic Center and deduce a near solar value for stars in the central region. For radial distances greater than 1^° from the Galactic Center we fail to find a measurable population of stars that are significantly younger than those in Baade's Window. Within 1^° we find a number of luminous M giants that most likely are the result of a star formation episode not more than one or two Gyr ago. This revised version was published online in July 2006 with corrections to the Cover Date.     

Semiregular variables in the solar neighbourhood

Period–luminosity sequences have been shown to exist among the semiregular variables (SRVs) in the Magellanic Clouds, the bulge of the Milky Way galaxy and elsewhere. Using modern-period and revised Hipparcos parallax data, this paper demonstrates that they also appear among the M giant SRVs of the solar neighbourhood. Their distribution in the   K , log  P   diagram resembles that of Bulge stars more closely than those in the Magellanic Clouds. The prevalence of mass-loss among local M-type SRVs and its dependence on period and spectral subtype are also discussed.   K −[12]  , a measure of circumstellar dust emission, increases clearly with V amplitude, M giant subtype and  log  P   .     

Surface abundances of light elements for a large sample of early B-type stars – IV. The magnesium abundance in 52 stars – a test of metallicity

From high-resolution spectra a non-local thermodynamic equilibrium analysis of the Mg  ii 4481.2-Å  feature is implemented for 52 early and medium local B stars on the main sequence (MS). The influence of the neighbouring line Al  iii 4479.9-Å  is considered. The magnesium abundance is determined; it is found that  log ɛ(Mg) = 7.67 ± 0.21  on average. It is shown that uncertainties in the microturbulent parameter V_t are the main source of errors in  log ɛ(Mg)  . When using 36 stars with the most reliable V_t values derived from O  ii and N  ii lines, we obtain the mean abundance  log ɛ(Mg) = 7.59 ± 0.15  . The latter value is precisely confirmed for several hot B stars from an analysis of the Mg  ii 7877-Å  weak line. The derived abundance  log ɛ(Mg) = 7.59 ± 0.15  is in excellent agreement with the solar magnesium abundance  log ɛ_⊙ (Mg) = 7.55 ± 0.02  , as well as with the proto-Sun abundance  log ɛ _ps (Mg) = 7.62 ± 0.02  . Thus, it is confirmed that the Sun and the B-type MS stars in our neighbourhood have the same metallicity.     

The G-dwarf problem in the solar neighbourhood: a Lagrangian picture

Recent data on the empirical metallicity distribution of G dwarfs in the disk solar neighbourhood are fitted in two different ways. We use an extended Poisson distribution in the limit where the probability of star formation is small, and a Gauss distribution in the limit where a large number of physical variables is required to determine stellar metal abundance. Both are found to reproduce the data at the same (acceptable) extent, with a slight preference for the former. The emprirical, differential metallicity distribution of G dwarfs in the disk solar neighbourhood is compared with its theoretical counterpart, in the picture of a closed, comoving model of chemical evolution. The limits of the currently used infall models are discussed and a scenario of galactic formation and evolution is presented. The Galactic history is thought as made of two main phases: contraction (which produces the extended component) and equilibrium (which gives the disk). In this view, the stars observed within the solar cylinder did not necessarily arise from the primordial gas which later collapsed into the disk solar neighbourhood. It is found that the G-dwarf problem is strongly alleviated, with the possible exception of the low-metallicity and high-metallicity tail of the distribution. The best choice of parameters implies: (i) a metal yield in the contraction phase which is larger by a factor of about 5 with respect to the equilibrium phase; (ii) a model halo mass fraction of about 0.3; (iii) a model disk mass fraction of about 0.6. It provides additional support to the idea of a generalized Schmidt star formation law, which is different in different phases of evolution. The model, cumulative, G-dwarf metallicity distribution in the disk solar neighbourhood is found to predict too may low-metallicity stars with respect to its empirical counterpart, related to a Poissonian or Gaussian fit. The main resons for the occurrence of a G-dwarf problem are discussed. Finally, a stochastic process of star formation, related to a Poisson distribution, is briefly outlined.     

Metallicity effects on the chromospheric activity–age relation for late-type dwarfs

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.     

A revision of the solar neighbourhood metallicity distribution

We present a revised metallicity distribution of dwarfs in the solar neighbourhood. This distribution is centred on solar metallicity. We show that previous metallicity distributions, selected on the basis of spectral type, are biased against stars with solar metallicity or higher. A selection of G-dwarf stars is inherently biased against metal-rich stars and is not representative of the solar neighbourhood metallicity distribution. Using a sample selected on colour, we obtain a distribution where approximately half the stars in the solar neighbourhood have metallicities higher than [Fe/H]=0 . The percentage of mid-metal-poor stars ([Fe/H]<−0.5) is approximately 4 per cent, in agreement with present estimates of the thick disc.
In order to have a metallicity distribution comparable to chemical evolution model predictions, we convert the star fraction to mass fraction, and show that another bias against metal-rich stars affects dwarf metallicity distributions, due to the colour (or spectral type) limits of the samples. Reconsidering the corrections resulting from the increasing thickness of the stellar disc with age, we show that the simple closed-box model with no instantaneous recycling approximation gives a reasonable fit to the observed distribution. Comparisons with the age–metallicity relation and abundance ratios suggest that the simple closed-box model may be a viable model of the chemical evolution of the Galaxy at solar radius.     

LAMOST-Kepler视场中富锂巨星表面转动研究

Kepler卫星提供的长时序、高精度的光度观测和郭守敬望远镜(LAMOST)提供的大规模光谱观测为研究恒星表面转动周期与富锂巨星锂丰度关系提供了良好的数据.将LAMOST搜寻到的富锂巨星与Kepler观测交叉,获得了619颗共同源,研究了其中295颗有良好观测数据的富锂巨星的表面转动.在205颗有星震学参数的恒星中提取出14颗恒星的转动周期,其中氦核燃烧星(HeB) 11颗,红巨星支(RGB) 2颗, 1颗演化阶段未确定.本样本中的极富锂巨星(A(Li) 3.3 dex)皆为HeB;对于90颗没有星震学参数的样本因而没有依靠星震学手段确定演化阶段的恒星中,有22颗提取出了自转周期.前者的自转探测率为6.8%,显著高于之前工作中大样本巨星2.08%的探测率.同时,此研究首次从自转周期的角度确认了恒星转动与巨星锂增丰存在相关性,在增丰程度较弱时,自转周期分布比较弥散;强锂增丰的星倾向于快速转动.富锂巨星与极富锂巨星在转动速度随锂丰度的演化上展现了两个序列,在转动-锂丰度图上的A(Li)≈3.3 dex处产生第2个下降序列,或许暗示了两者在形成机制上的不同.极富锂巨星的样本中,随巨星锂增丰程度增强,恒星转速加快.这种相关性为由转动引起的额外混合作为富锂巨星形成的机制提供了支持.     

High-resolution spectra of very low-mass stars

We present the results of high-resolution (1–0.4 Å) optical spectroscopy of a sample of very low-mass stars. These data are used to examine the kinematics of the stars at the bottom of the hydrogen-burning main sequence. No evidence is found for a significant difference between the kinematics of the stars in our sample with I  −  K  > 3.5 ( M _bol ≳ 12.8) and those of more massive M dwarfs ( M _bol ≈ 7–10). A spectral atlas at high (0.4-Å) resolution for M8–M9+ stars is provided, and the equivalent widths of Cs  I , Rb  I and Hα lines present in our spectra are examined. We analyse our data to search for the presence of rapid rotation, and find that the brown dwarf LP 944-20 is a member of the class of 'inactive, rapid rotators'. Such objects seem to be common at and below the hydrogen-burning main sequence. It seems that in low-mass/low-temperature dwarf objects either the mechanism that heats the chromosphere, or the mechanism that generates magnetic fields, is greatly suppressed.     

Red giant stellar collisions in the Galactic Centre

We show that collisions with stellar-mass black holes can partially explain the absence of bright giant stars in the Galactic Centre, first noted by Genzel et al. We show that the missing objects are low-mass giants and asymptotic giant branch stars in the range  1–3 M_⊙  . Using detailed stellar evolution calculations, we find that to prevent these objects from evolving to become visible in the depleted K bands, we require that they suffer collisions on the red giant branch, and we calculate the fractional envelope mass losses required. Using a combination of smoothed particle hydrodynamic calculations, restricted three-body analysis and Monte Carlo simulations, we compute the expected collision rates between giants and black holes, and between giants and main-sequence stars in the Galactic Centre. We show that collisions can plausibly explain the missing giants in the  10.5 < K < 12  band. However, depleting the brighter  ( K < 10.5)  objects out to the required radius would require a large population of black hole impactors which would in turn deplete the  10.5 < K < 12  giants in a region much larger than is observed. We conclude that collisions with stellar-mass black holes cannot account for the depletion of the very brightest giants, and we use our results to place limits on the population of stellar-mass black holes in the Galactic Centre.     

Late-type members of young stellar kinematic groups – I. Single stars

This is the first paper of a series aimed at studying the properties of late-type members of young stellar kinematic groups. We concentrate our study on classical young moving groups such as the Local Association (Pleiades moving group,     , IC 2391 supercluster (35 Myr), Ursa Major group (Sirius supercluster, 300 Myr), and Hyades supercluster (600 Myr), as well as on recently identified groups such as the Castor moving group (200 Myr). In this paper we compile a preliminary list of single late-type possible members of some of these young stellar kinematic groups. Stars are selected from previously established members of stellar kinematic groups based on photometric and kinematic properties as well as from candidates based on other criteria such as their level of chromospheric activity, rotation rate and lithium abundance. Precise measurements of proper motions and parallaxes taken from the Hipparcos Catalogue, as well as from the Tycho-2 Catalogue, and published radial velocity measurements are used to calculate the Galactic space motions ( U , V , W ) and to apply Eggen's kinematic criteria in order to determine the membership of the selected stars to the different groups. Additional criteria using age-dating methods for late-type stars will be applied in forthcoming papers of this series. A further study of the list of stars compiled here could lead to a better understanding of the chromospheric activity and their age evolution, as well as of the star formation history in the solar neighbourhood. In addition, these stars are also potential search targets for direct imaging detection of substellar companions.     

Long Secondary Periods in variable red giants

We present a study of a sample of Large Magellanic Cloud red giants exhibiting Long Secondary Periods (LSPs). We use radial velocities obtained from VLT spectral observations and MACHO and OGLE light curves to examine properties of the stars and to evaluate models for the cause of LSPs. This sample is much larger than the combined previous studies of Hinkle et al. and Wood, Olivier & Kawaler.
Binary and pulsation models have enjoyed much support in recent years. Assuming stellar pulsation, we calculate from the velocity curves that the typical fractional radius change over an LSP cycle is greater than 30 per cent. This should lead to large changes in T _eff that are not observed. Also, the small light amplitude of these stars seems inconsistent with the radius amplitude. We conclude that pulsation is not a likely explanation for the LSPs. The main alternative, physical movement of the star – binary motion – also has severe problems. If the velocity variations are due to binary motion, the distribution of the angle of periastron in our large sample of stars has a probability of  1.4 × 10⁻³  that it comes from randomly aligned binary orbits. In addition, we calculate a typical companion mass of  0.09 M_⊙  . Less than 1 per cent of low-mass main-sequence stars have companions near this mass  (0.06–0.12 M_⊙)  whereas ∼25–50 per cent of low-mass red giants end up with LSPs. We are unable to find a suitable model for the LSPs and conclude by listing their known properties.