Towards the solution of the λ Bootis problem

Having analysed the accretion-based model of the dust–gas separation, which is regarded to be the most promising for the explanation of the anomalous properties of λ Bootis-type stars, we can conclude that: (i) for any reasonable density profiles of the shell, dust grains appear to be decoupled from the gaseous background within the region where the temperature drops to a value that is less than the condensation temperature of heavy elements such as Mg, Ca, Fe, etc; (ii) most likely, in the shell of λ Bootis-type stars only small dust particles (of less than ≈10⁻⁶ cm in size) can be created; (iii) significant alteration of the initial atmospheric chemical composition can take place in the case when the density in the shell changes as ∼ r ⁻².     

The pulsating λ Bootis star HD 105759

We present spectroscopic and multisite time series photometric observations of the star HD 105759 which confirm that this is a new pulsating λ Bootis star. Our spectroscopy indicates an overall metallicity of [Z] = log Z − log Z_⊙ = −1 and that T _eff = 8000 ± 300 K and log  g  = 4.0 ± 0.2. The photometric data reveal that this star pulsates with at least five frequencies: 8.62, 12.69, 20.15, 21.27 and 23.66 d⁻¹. These frequencies do not all match those found in an earlier single-site study, indicating that the pulsation spectrum changes with time.     

Short-period line profile and light variations in the Be star λ Eridani

We present three seasons of photometric observations and one season of intensive high-dispersion spectroscopic observations of the Be star λ Eridani. We show that only one period,   P =0.70173 d  , is present in the photometry, although there are large light amplitude variations from season to season. We confirm a suspicion that light outbursts repeat at intervals of about 475 d. A total of 348 echelle spectra of the star were obtained over a 2-week observing run. We show that the periodic variations are present in the emission wings of the helium lines, in the emission wings of the H α line and in the absorption cores of H β and H γ . Together with the fact that the periodic variations appear outside the projected rotational velocity limit, this indicates that they are associated with circumstellar material immediately above the photosphere and supports the idea of corotating gas clouds. We present evidence in support of a true rotational period of  2 P =1.40346 d  and suggest that the mass loss in Be stars is caused by centrifugal magnetic acceleration.     

Empirical distributions of galactic λ spin parameters from the Sloan Digital Sky Survey

Using simple dimensional arguments for both spiral and elliptical galaxies, we present formulae to derive an estimate of the halo spin parameter λ for any real galaxy, in terms of common observational parameters. This allows a rough estimate of λ, which we apply to a large volume-limited sample of galaxies taken from the Sloan Digital Sky Survey data base. The large numbers involved (11 597) allow the derivation of reliable λ distributions, as signal adds up significantly in spite of the errors in the inferences for particular galaxies. We find that if the observed distribution of λ is modelled with a lognormal function, as often done for this distribution in dark matter haloes that appear in cosmological simulations, we obtain parameters  λ₀= 0.04 ± 0.005  and  σ_λ= 0.51 ± 0.05  , interestingly consistent with values derived from simulations. For spirals, we find a good correlation between empirical values of λ and visually assigned Hubble types, highlighting the potential of this physical parameter as an objective classification tool.     

Specific angular momentum of disc merger remnants and the λR-parameter

We use two-dimensional kinematic maps of simulated binary disc mergers to investigate the  λ_R  -parameter, which is a luminosity-weighted measure of projected angular momentum per unit mass. This parameter was introduced to subdivide the SAURON sample of early-type galaxies in so-called fast  λ_R > 0.1  and slow rotators  λ_R < 0.1  . Tests on merger remnants reveal that  λ_R  is a robust indicator of the true angular momentum content in elliptical galaxies. We find the same range of  λ_R  values in our merger remnants as in the SAURON galaxies. The merger mass ratio is decisive in transforming fast rotators into slow rotators in a single binary merger, the latter being created mostly in an equal-mass merger. Slow rotators have a  λ_R  which does not vary with projection. The confusion rate with face-on fast rotators is very small. Mergers with a gas component form slow rotators with smaller ellipticities than collisionless merger remnants have, and are in much better agreement with the SAURON slow rotators. Remergers of merger remnants are slow rotators, but tend to have too high ellipticities. Fast rotators maintain the angular momentum content from the progenitor disc galaxy if merger mass ratio is high. Some SAURON galaxies have values of  λ _R   as high as our progenitor disc galaxies.     

On the reliability of C iv λ1549 as an abundance indicator for high-redshift star-forming galaxies

We reconsider the use of the equivalent width of C  iv  λ1549, EW(C  iv ), as an indicator of the oxygen abundance in star-forming galaxies, as proposed by Heckman et al. for nearby starbursts. We refine the local calibration of EW(C  iv ) versus log (O/H) by using a restricted wavelength window which minimizes blending with interstellar absorption lines. When applied to the stellar component only of the complex C  iv  λ1549 features in two high-redshift galaxies with good quality spectra,  MS 1512−cB58 ( z = 2.7268)  and  Q1307−BM1163 ( z = 1.4105)  , the local calibration gives values of the oxygen abundance which are in good agreement with other metallicity determinations based on nebular emission and interstellar absorption lines. Our main conclusion is that for this method to give reliable results at high redshifts, it should only be used on data of sufficiently high spectral resolution  ( R ≳ 1000)  for stellar and interstellar C  iv components to be clearly separated. Oxygen abundances will be systematically overestimated if the local calibration is applied to spectra of high-redshift galaxies obtained with the low resolving powers  ( R ≃ 200–300)  of many current wide-field surveys. It will also be necessary to understand better the causes of the scatter in the local relation, before we can be confident of inferences from it at high z .     

Prediction for the He iλ10830 Å absorption wing in the coming event of η Carinae

We propose an explanation for the puzzling appearance of a wide blue absorption wing in the He  i  λ10830 Å  P Cygni profile of the massive binary star η Car several months before periastron passage. Our basic assumption is that the colliding winds region is responsible for the blue wing absorption. By fitting observations, we find that the maximum outflow velocity of this absorbing material is  ∼2300 km s⁻¹  . We also assume that the secondary star is towards the observer at periastron passage. With a toy model, we achieve two significant results. (1) We show that the semimajor axis orientation we use can account for the appearance and evolution of the wide blue wing under our basic assumption. (2) We predict that the Doppler shift (the edge of the absorption profile) will reach a maximum 0–3 weeks before periastron passage, and not necessarily exactly at periastron passage or after periastron passage.     

Beware λ-truncation! Sample truncation and bias in luminosity calibration using trigonometric parallaxes

The common practice in luminosity calibration of sample truncation according to relative parallax error λ can lead to bias with indirect methods such as reduced parallaxes as well as with direct methods. This bias is not cancelled by the Lutz–Kelker corrections and in fact can be either negative or positive. Making the selection stricter can actually lead to a larger absolute amount of bias and lower accuracy in certain cases.
The degree to which this bias is present depends upon whether the sample is more nearly specified by the relative parallax error or by the limiting apparent magnitude when both limits formally apply; when the latter limit dominates it is absent. The difference between the means for the two extreme cases is what is customarily termed the Malmquist bias. However, it is not truly bias, but rather what we call here an offset .
For a sample to be effectively magnitude-limited, there is a lower bound imposed on the mean absolute magnitude which depends on the limiting magnitude. If a wide-ranging luminosity relation such as the Wilson–Bappu relation is to be calibrated, some portion of the relation may be magnitude-limited and the rest not. In that case there will be offsets between the different parts of the relation, including the transition region between the two extremes, as well as bias outside the magnitude-limited part.
Another, less common, practice is truncation according to weight, specifically with the reduced parallax method. Such truncation can also bias the calibration with one variant of the method. Indeed, the weighting scheme used with that variant introduces bias even without truncation.
For calibration it is probably best to use a general maximum likelihood method such as the grid method with a magnitude-limited sample and no limit on relative parallax error. The Malmquist shift could then be applied to obtain an estimate of the volume-limited mean.     

Environment and mass dependencies of galactic λ spin parameter: cosmological simulations and observed galaxies compared

We use a sample of galaxies from the Sloan Digital Sky Survey (SDSS) to search for correlations between the λ spin parameter and the environment and mass of galaxies. In order to calculate the total value of λ for each observed galaxy, we employed a simple model of the dynamical structure of the galaxies, which allows a rough estimate of the value of λ using only readily obtainable observables from the luminous galaxies. Use of a large volume-limited sample (upwards of 11 000) allows reliable inferences of mean values and dispersions of λ distributions. We find, in agreement with some N -body cosmological simulations, no significant dependence of λ on the environmental density of the galaxies. For the case of mass, our results show a marked correlation with λ, in the sense that low-mass galaxies present both higher mean values of λ and associated dispersions, than high-mass galaxies. These results provide interesting constrain on the mechanisms of galaxy formation and acquisition of angular momentum, a valuable test for cosmological models.     

The λ=6 cm VLBI polarization structure of nine BL Lacertae objects

VLBI total intensity ( I ) and linear polarization ( P ) images at λ =6 cm have been obtained for nine radio-bright BL Lacertae objects. These are the first VLBI P images for these sources, and, in a number of cases, the first I images as well. They confirm the previously noted tendency for the jet magnetic fields of BL Lacertae objects to be transverse to the local jet direction, but also provide new evidence that a sizeable minority of BL Lacertae objects have VLBI jet components with longitudinal magnetic fields. In addition, two sources have VLBI jet components in which the direction of the electric vector χ bears no obvious relation to the apparent local jet direction; the origin of these arbitrary χ offsets is unclear. A new tentative superluminal speed of β =6.3  h ⁻¹ has been determined for 0828+493; tentative speeds for two knots in 1418+546 are β =4.3 and 2.5  h ⁻¹. This work is part of an ongoing programme to determine the VLBI I and P structure of all 34 sources in the 1-Jy sample of northern BL Lacertae objects defined by Kühr & Schmidt.     

Observations of flat-spectrum radio sources at λ850 μm from the James Clerk Maxwell Telescope – I. 1997 April to 2000 April

Calibrated data for 65 flat-spectrum extragalactic radio sources are presented at a wavelength of 850 μm, covering a three-year period from 1997 April. The data, obtained from the James Clerk Maxwell Telescope using the SCUBA camera in pointing mode, were analysed using an automated pipeline process based on the Observatory Reduction and Acquisition Control–Data Reduction ( orac–dr ) system. This paper describes the techniques used to analyse and calibrate the data, and presents the data base of results along with a representative sample of the better-sampled light curves.     

VLBI polarization images of eight compact active galactic nuclei at λ = 1.3 cm

Global VLBI images in intensity and linear polarization at     of the BL Lacertae objects 0235+164, 0300+470, 0735+178, 0954+658, 1803+784, 1823+568, and BL Lac and OVV quasar 3C 279 are presented and analysed. These are the highest resolution polarization images currently available for several of these sources. On the whole, the tendencies observed at longer centimetre wavelengths are also exhibited in these 1.3-cm images. When core polarization is detected, the corresponding polarization angles χ tend to lie either parallel or perpendicular to the direction of the inner jet. The core degrees of polarization are usually comparable to those typical at 6 cm, but somewhat lower in several cases, suggesting that the higher resolution data are better separating the contributions of intrinsically weakly polarized cores and highly polarized emerging knots. The observed χ vectors in the jets of 0735+178, 0954+658, 1823+568, BL Lac and 3C 279 are aligned with the jet, implying the presence of transverse magnetic fields. In 0300+470 and BL Lac, there are regions where the direction of χ seems to be neither parallel nor perpendicular to the jet; it is not clear whether this is due to genuine obliquity of the magnetic field structure or to inadequate knowledge of the local flow direction.