The pattern speed of the OH/IR stars in the Milky Way

We show how the continuity equation can be used to determine pattern speeds in the Milky Way Galaxy (MWG). This method, first discussed by Tremaine & Weinberg in the context of external galaxies, requires projected positions, ( l , b ), and line-of-sight velocities for a spatially complete sample of relaxed tracers. If the local standard of rest (LSR) has a zero velocity in the radial direction ( u _LSR), then the quantity that is measured is  Δ V ≡Ω_p R ₀- V _LSR  , where Ω_p is the pattern speed of the non-axisymmetric feature, R ₀ is the distance of the Sun from the Galactic centre and V _LSR is the tangential motion of the LSR, including the circular velocity. We use simple models to assess the reliability of the method for measuring a single, constant pattern speed of either a bar or spiral in the inner MWG. We then apply the method to the OH/IR stars in the ATCA/VLA OH 1612-MHz survey of Sevenster et al., finding  Δ V =252±41 km s^-1,  if   u _LSR=0  . Assuming further that   R ₀=8 kpc  and   V _LSR=220 km s^-1,  this gives  Ω_p=59±5 km s^-1 kpc^-1  with a possible systematic error of perhaps 10 km s⁻¹ kpc⁻¹. The non-axisymmetric feature for which we measure this pattern speed must be in the disc of the MWG.     

Kinematics of O-B5 giants

To study the kinematics of O-B5 giant stars (luminosity class III), 290 non-Gould belt stars with proper motions taken from the Hipparcos catalogue are used, of which 107 have radial velocities taken from other sources. Semidefinite programming solves for the kinematical parameters and the coefficients of the velocity ellipsoid. The condition that both solutions must yield the same solar velocity is enforced. The results obtained are reasonable: solar velocity of 13.83 ± 0.17 km s⁻¹; Oort's constants, in units of km s⁻¹ kpc⁻¹, A = 16.08 ± 0.72 and   B =−10.74 ± 0.65,  implying a rotational velocity of 228.0 ± 21.4 km s⁻¹ if we take the distance to the Galactic Centre as 8.5 ± 1.1 kpc; velocity dispersions, in units of km s⁻¹, of  σ _x = 32.44 ± 5.04, σ _y = 26.16 ± 2.75, σ _z = 18.71 ± 2.39  with a vertex deviation of         

Inversion of stellar statistics equation for the Galactic bulge

A method based on Lucy's iterative algorithm is developed to invert the equation of stellar statistics for the Galactic bulge and is then applied to the K -band star counts from the Two-Micron Galactic Survey in a number of off-plane regions (10°>| b |>2°, | l |<15°). The top end of the K -band luminosity function is derived and the morphology of the stellar density function is fitted to triaxial ellipsoids, assuming a non-variable luminosity function within the bulge. The results, which have already been outlined by López-Corredoira et al., are shown in this paper with a full explanation of the steps of the inversion: the luminosity function shows a sharp decrease brighter than M _K =−8.0  mag when compared with the disc population; the bulge fits triaxial ellipsoids with the major axis in the Galactic plane at an angle with the line of sight to the Galactic centre of 12° in the first quadrant; the axial ratios are 1:0.54:0.33, and the distance of the Sun from the centre of the triaxial ellipsoid is 7860 pc. The major–minor axial ratio of the ellipsoids is found not to be constant, the best fit to the gradient being K _z =(8.4±1.7)×exp(− t /(2000±920) pc), where t is the distance along the major axis of the ellipsoid in parsecs. However, the interpretation of this is controversial. An eccentricity of the true density-ellipsoid gradient and a population gradient are two possible explanations. The best fit for the stellar density, for 1300 pc< t <3000 pc, is calculated for both cases, assuming an ellipsoidal distribution with constant axial ratios, and when K _z is allowed to vary. From these, the total number of bulge stars is ∼3×10¹⁰ or ∼4×10¹⁰, respectively.     

Mass models of the Milky Way

A parametrized model of the mass distribution within the Milky Way is fitted to the available observational constraints. The most important single parameter is the ratio of the scalelength R _d* of the stellar disc to R ₀. The disc and bulge dominate v _c( R ) at R ≲ R ₀ only for R _d,*/ R ₀≲0.3. Since the only knowledge we have of the halo derives from studies like the present one, we allow it to contribute to the density at all radii. When allowed this freedom, however, the halo causes changes in assumptions relating to R  ≪  R ₀ to affect profoundly the structure of the best-fitting model at R  ≫  R ₀. For example, changing the disc slightly from an exponential surface-density profile significantly changes the form of v _c( R ) at R  ≫  R ₀, where the disc makes a negligible contribution to v _c. Moreover, minor changes in the constraints can cause the halo to develop a deep hole at its centre that is not physically plausible. These problems call into question the proposition that flat rotation curves arise because galaxies have physically distinct haloes rather than outwards-increasing mass-to-light ratios.   The mass distribution of the Galaxy and the relative importance of its various components will remain very uncertain until more observational data can be used to constrain mass models. Data that constrain the Galactic force field at z ≳ R and at R  >  R ₀ are especially important.     

SDSS absolute magnitudes for thin-disc stars based on trigonometric parallaxes

We present a new luminosity–colour relation based on trigonometric parallaxes for thin-disc main-sequence stars in Sloan Digital Sky Survey (SDSS) photometry. We matched stars from the newly reduced Hipparcos catalogue with the ones taken from Two-Micron All-Sky Survey (2MASS) All-Sky Catalogue of Point Sources, and applied a series of constraints, i.e. relative parallax errors  (σ_π/π≤ 0.05)  , metallicity  (−0.30 ≤[M/H]≤ 0.20 dex)  , age  (0 ≤ t ≤ 10 Gyr)  and surface gravity  (log  g > 4)  , and obtained a sample of thin-disc main-sequence stars. Then, we used our previous transformation equations ( Bilir et al. 2008a ) between SDSS and 2MASS photometries and calibrated the   M_g   absolute magnitudes to the  ( g − r )₀  and  ( r − i )₀  colours. The transformation formulae between 2MASS and SDSS photometries along with the absolute magnitude calibration provide space densities for bright stars which saturate the SDSS magnitudes.     

Could the Galactic disc heating be due to globular cluster impacts?

So far, six mechanisms have been proposed to account for the Galactic disc heating. Of these, the most important appear to be a combination of scattering of stars by molecular clouds and by spiral arms. We study a further mechanism, namely the repeated disc impact of the original Galactic globular cluster population up to the present. We find that globular clusters could have contributed at most a small fraction of the current vertical energy of the disc, as they could heat the whole disc to  σ _z = 5.5 km s⁻¹  (c.f. the observed 18 and 39 km s⁻¹ for the thick and thin discs, respectively). We find that the rate of rise of disc heat (  α= 0.22  in  σ _z ∼ t ^α  with t being time) is close to that found for scattering by molecular clouds.     

Bulk viscosity of mixed nucleon–hyperon–quark matter in neutron stars

We calculate the coefficient of bulk viscosity by considering the non-leptonic weak interactions in the cores of hybrid stars with both hyperons and quarks. We first determine the dependence of the production rate of neutrons on the reaction rate of quarks in the non-leptonic processes, that is,  Γ _n = K _s Γ _s +Γ_Λ+ 2Γ_Σ−  . The conversion rate,   K _s   , in our scenario is a complicated function of baryon number density. We also consider the medium effect of quark matter on bulk viscosity. Using these results, we estimate the limiting rotation of the hybrid stars, which may suppress the r-mode instability more effectively. Hybrid stars should be the candidates for the extremely rapid rotators.     

Spectroscopic solution of the star QX Cas

High-resolution spectroscopic observations around the Hα line of the binary star QX Cas covering the whole orbital period are presented. Our radial velocity solution, the first ever determined, requires an eccentric orbit with the following orbital parameters: eccentricity,   e = 0.22 ± 0.01  ; longitude of periastron,  ω= 45°± 5°  ; semi-amplitudes of the radial velocity curves of the primary and secondary stars,   K ₁ sin  i = 125.8 ± 0.9 km s⁻¹  and   K ₂ sin  i = 144.8 ± 1.1 km s⁻¹  ; gamma velocity,   V ₀= 65.1 ± 0.5 km s⁻¹  ; and mass ratio,   q = 0.869 ± 0.013  . The corresponding lower limits of the masses of the components and their separation are         , and   a sin  i = 31.34 ± 0.48 R_⊙  .     

Population III stars: hidden or disappeared?

A Population III/Population II transition from massive to normal stars is predicted to occur when the metallicity of the star-forming gas crosses the critical range   Z _cr= 10^−5±1 Z_⊙  . To investigate the cosmic implications of such a process, we use numerical simulations which follow the evolution, metal enrichment and energy deposition of both Population II and Population III stars. We find that: (i) due to inefficient heavy element transport by outflows and slow 'genetic' transmission during hierarchical growth, large fluctuations around the average metallicity arise; as a result, Population III star formation continues down to   z = 2.5  , but at a low peak rate of  10⁻⁵ M_⊙ yr⁻¹ Mpc⁻³  occurring at   z ≈ 6  (about 10⁻⁴ of the Population II one); and (ii) Population III star formation proceeds in an 'inside–out' mode in which formation sites are progressively confined to the periphery of collapsed structures, where the low gas density and correspondingly long free-fall time-scales result in a very inefficient astration. These conclusions strongly encourage deep searches for pristine star formation sites at moderate  (2 < z < 5)  redshifts where metal-free stars are likely to be hidden.     

The kinematics and zero-point of the log P–〈MK〉 relation for Galactic RR Lyrae variables via statistical parallax

We use accurate absolute proper motions and Two-Micron All-Sky Survey   K_s   -band apparent magnitudes for 364 Galactic RR Lyrae variables to determine the kinematical parameters of the Galactic RR Lyrae population and constrain the zero-point of the   K_s   -band period–luminosity relation for these stars via statistical parallax. We find the mean velocities of the halo- and thick-disc RR Lyrae populations in the solar neighbourhood to be  [ U ₀(Halo), V ₀(Halo), W ₀(Halo)]= (−12 ± 10, −217 ± 9, −6 ± 6) km s⁻¹  and  [ U ₀(Disc), V ₀(Disc), W ₀(Disc)]= (−15 ± 7, −44 ± 7, −25 ± 5) km s⁻¹  , respectively, and the corresponding components of the velocity-dispersion ellipsoids,  [σ V_R (Halo), σ V _θ(Halo), σ W (Halo)]= (167 ± 9, 86 ± 6, 78 ± 5) km s⁻¹  and  [σ V_R (Disc), σ V _θ(Disc), σ W (Disc)]= (55 ± 7, 44 ± 6, 30 ± 4) km s⁻¹  , respectively. The fraction of thick-disc stars is estimated at  0.25 ± 0.03  . The corrected infrared period–luminosity relation is     , implying a Large Magellanic Cloud (LMC) distance modulus of  18.27 ± 0.08  and a solar Galactocentric distance of  7.58 ± 0.40 kpc  . Our results suggest no or slightly prograde rotation for the population of halo RR Lyraes in the Milky Way.     

Radial scalelengths of the galactic thin and thick disc with 2MASS data

This paper presents a global analysis of the 2MASS (Two Micron All Sky Survey) data as observed in seven fields at different galactic latitudes in our Galaxy. The data allow the preliminary determination of the scale parameters, which lead to strong constraints on the radial and vertical structure of the galactic thin and thick disc. The interpretation of star counts and colour distributions of stars in the near-infrared with the synthetic stellar population model gives strong evidence that the galactic thin disc density scalelength ( h _R ) is rather short (2.8±0.3 kpc). The galactic thick disc population is revisited in the light of new data. We find the thick disc to have a local density of 3.5±2.0 per cent of the thin disc, exponential scaleheight ( h _z ) of 860±200 pc and exponential scalelength ( h _R ) of 3.7±_0.5^0.8 kpc.     

Something about the structure of the Galaxy

We analyse a sample of 507 evolved (OH/IR) stars in the region (10°>ℓ>−45°), (| b |<3°). We derive average ages for subsets of this sample, and use those sets as beacons for the evolution of the Galaxy. In the bulge, the oldest OH/IR stars in the plane are 7.5 Gyr (1.3 M_⊙), and in the disc 2.7 Gyr (2.3 M_⊙). The vertical distribution of almost all AGB stars in the disc is found to be nearly exponential, with scaleheight increasing from 100 pc for stars ≲1 Gyr old to 500 pc for stars ≳5 Gyr old. There may be a small, disjunct population of OH/IR stars. The radial distribution of AGB stars is dictated by the metallicity gradient. Unequivocal morphological evidence is presented for the existence of a central bar, but parameters can be constrained only for a given spatial-density model. Using a variety of indicators, we identify the radii of the inner ultraharmonic (2.5 kpc) and corotation resonance (3.5 kpc). We show that the 3-kpc arm is likely to be an inner ring, as observed in other barred galaxies, by identifying a group of evolved stars that is connected to the 3-kpc H  i filament. Also, using several observed features, we argue that an inner-Lindblad resonance exists, at ∼1–1.5 kpc. The compositions of OH/IR populations within 1 kpc of the Galactic Centre give insight into the bar-driven evolution of the inner regions. We suggest that the bar is ∼8 Gyr old, relatively weak (SAB), and may be in a final stage of its existence.     

Supergiant temperatures and linear radii from near-infrared interferometry

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.     

Absolute parameters of young stars – I. U Ophiuci

We have carried out an investigation of the early-type multiple star U Oph. We have used new high-resolution spectroscopy with the High Efficiency and Resolution Canterbury University Large Echelle Spectrograph (HERCULES) and 1-m McLellan Telescope of the University of Canterbury at Mt John University Observatory and literature-sourced optical and ultraviolet photometry. We applied the local reduction package [HERCULES Reduction Software Package (HRSP)] and other software to the spectroscopic data to find radial velocities. Information limit optimization techniques (ILOT) utilizing physically realistic fitting functions were applied to these data to yield new sets of absolute parameters:   M ₁= 5.13, M ₂= 4.56 (±2 per cent); R ₁= 3.41, R ₂= 3.08 (±1 per cent)  ; for the early-type eclipsing binary that dominates the system. We have combined times-of-minima photometry with other data for the triple system that makes up ADS 10428A, utilizing the wide orbit of Wolf et al. as well as HIPPARCOS astrometry of U Oph. ILOT techniques applied to the astrometric orbit yield a mass of the third star as  0.83 M_⊙  . We estimate an age of the system of around 30–40 Myr, from the isochrones of Bertelli, results given by Vaz, Andersen & Claret, as well as our own tests with an updated version of Paczyński's stellar modelling code. This age and other details are consistent with a possible origin in Gould's Belt. Such information for this, and comparable young multiple star systems, may help to clarify general properties of star formation and the subtle interactions of stars and their environment.     

Metal-rich carbon stars in the Sagittarius dwarf spheroidal galaxy

We present spectroscopic observations from the Spitzer Space Telescope of six carbon-rich asymptotic giant branch (AGB) stars in the Sagittarius dwarf spheroidal galaxy (Sgr dSph) and two foreground Galactic carbon stars. The band strengths of the observed C₂H₂ and SiC features are very similar to those observed in Galactic AGB stars. The metallicities are estimated from an empirical relation between the acetylene optical depth and the strength of the SiC feature. The metallicities are higher than those of the Large Magellanic Cloud, and close to Galactic values. While the high metallicity could imply an age of around 1 Gyr, for the dusty AGB stars, the pulsation periods suggest ages in excess of 2 or 3 Gyr. We fit the spectra of the observed stars using the dusty radiative transfer model and determine their dust mass-loss rates to be in the range  1.0–3.3 × 10⁻⁸ M_⊙ yr⁻¹  . The two Galactic foreground carbon-rich AGB stars are located at the far side of the solar circle, beyond the Galactic Centre. One of these two stars shows the strongest SiC feature in our present Local Group sample.     

Spectroscopic and photometric observations of the eclipsing star UV Leo

High-resolution spectroscopic observations around the Hα line and BVRI photometry of the eclipsing short-period RS CVn star UV Leo are presented. The simultaneous light-curve solution and radial velocity-curve solution led to the following values of the global parameters of the binary: temperatures   T ₁= 6000 ± 100 K  and   T ₂= 5970 ± 20 K  ; masses   M ₁= 0.976 ± 0.067 M_⊙  and   M ₂= 0.931 ± 0.052 M_⊙  ; separation   a = 3.716 ± 0.048 R_⊙  ; orbital inclination     ; radii   R ₁= 1.115 ± 0.052 R_⊙  and   R ₂= 1.078 ± 0.051 R_⊙  ; equatorial velocities   V ₁= 98.8 ± 2.3 km s⁻¹  and   V ₂= 89.6 ± 2.7 km s⁻¹  . These results lead to the conclusion that the two components of UV Leo are slightly oversized for their masses and lie within the main-sequence band on the mass–radius diagram, close to the isochrone 9 × 10¹⁰ yr.     

The density distribution of the Galactic stellar halo as traced by blue horizontal branch stars

Data of blue horizontal branch (BHB) stars and RR Lyrae variable stars from the literature are combined with unpublished observations of BHB stars in five fields. A flattened power law is used to model the spatial distribution of the horizontal branch stars. Completeness of the data sample and contamination by blue stragglers and metal-rich main-sequence A stars are considered, and taken into account. Using a maximum-likelihood method, the following best-fitting parameters are obtained: a power-law index α=−3.2±0.3 and an axial ratio of q =0.52±0.11 for the isodensity surfaces. From the fit a value for the local density of BHB stars of ρ₀=26⁺²⁰₋₁₁ kpc⁻³ is found. The values of the three parameters are in complete agreement with recent determinations by other authors.     

Baryonic conversion tree: the global assembly of stars and dark matter in galaxies from the Sloan Digital Sky Survey

Using the spectroscopic sample of the Sloan Digital Sky Survey Data Release 1 (SDSS DR1), we measure how gas was transformed into stars as a function of time and stellar mass: the baryonic conversion tree (BCT). There is a clear correlation between early star formation activity and present-day stellar mass: the more massive galaxies have formed approximately 80 per cent of their stars at   z > 1  , while for the less massive ones the value is only approximately 20 per cent. By comparing the BCT with the dark matter merger tree, we find indications that star formation efficiency at   z > 1  had to be approximately a factor of two higher than today (∼10 per cent) in galaxies with present-day stellar mass larger than  2 × 10¹¹ M_⊙  , if this early star formation occurred in the main progenitor. Therefore, the λ cold dark matter (LCDM) paradigm can accommodate a large number of red objects. On the other hand, in galaxies with present-day stellar mass less than  10¹¹ M_⊙  , efficient star formation seems to have been triggered at   z ∼ 0.2  . We show that there is a characteristic mass  ( M _*∼ 10¹⁰ M_⊙)  for feedback efficiency (or lack of star formation). For galaxies with masses lower than this, feedback (or star formation suppression) is very efficient while for higher masses it is not. The BCT, determined here for the first time, should be an important observable with which to confront theoretical models of galaxy formation.     

Seismological studies of ZZ Ceti stars – II. Application to the ZZ Ceti class

We used the detected pulsation modes and adiabatic pulsation models to do seismology of the class of ZZ Ceti stars and measure the H layer mass for 83 stars. We found the surface hydrogen layer to be within the range  10^−9.5≥ M _H/ M _*≥ 10⁻⁴  , with an average of   M _H/ M _*= 10^−6.3  , which is thinner than the predicted value of   M _H/ M _*= 10⁻⁴  , indicating that the stars lose more mass during their evolution than previously expected. These results are preliminary and do not include the possible effects of realistic C/O profiles on the fits.     

The minimum mass ratio of W UMa-type binary systems

When the total angular momentum of a binary system   J _tot= J _orb+ J _spin  is at a certain critical (minimum) value, a tidal instability occurs which eventually forces the stars to merge into a single, rapidly rotating object. The instability occurs when   J _orb= 3 J _spin  , which in the case of contact binaries corresponds to a minimum mass ratio   q _min≈  0.071–0.078. The minimum mass ratio is obtained under the assumption that stellar radii are fixed and independent. This is not the case with contact binaries where, according to the Roche model, we have   R ₂= R ₂( R ₁, a , q )  . By finding a new criterion for contact binaries, which arises from  d J _tot= 0  , and assuming   k ²₁≠ k ²₂  for the component's dimensionless gyration radii, a theoretical lower limit   q _min= 0.094–0.109  for overcontact degree   f = 0–1  is obtained.