The discovery of a low-mass, pre-main-sequence stellar association around γ Velorum

We report the serendipitous discovery of a population of low-mass, pre-main-sequence (PMS) stars in the direction of the Wolf–Rayet/O-star binary system γ ²  Vel and the Vela OB2 association. We argue that γ ²  Vel and the low-mass stars are truly associated and approximately coeval, and that both are at distances between 360 and 490 pc, disagreeing at the 2 σ level with the recent Hipparcos parallax of γ ²  Vel, but consistent with older distance estimates. Our results clearly have implications for the physical parameters of the γ ²  Vel system, but also offer an exciting opportunity to investigate the influence of high-mass stars on the mass function and circumstellar disc lifetimes of their lower mass PMS siblings.     

The depletion of carbon by extra mixing in metal-poor giants

There is an apparent dichotomy between the metal-poor  ([Fe/H]≤−2)  yet carbon-normal giants and their carbon-rich counterparts. The former undergo significant depletion of carbon on the red giant branch after they have undergone first dredge-up, whereas the latter do not appear to experience significant depletion. We investigate this in the context that the extra mixing occurs via the thermohaline instability that arises due to the burning of  ³He  . We present the evolution of [C/Fe], [N/Fe] and  ¹²C/¹³C  for three models: a carbon-normal metal-poor star, and two stars that have accreted material from a  1.5 M_⊙  AGB companion, one having received  0.01 M_⊙  of material and the other having received  0.1 M_⊙  . We find the behaviour of the carbon-normal metal-poor stars is well reproduced by this mechanism. In addition, our models also show that the efficiency of carbon-depletion is significantly reduced in carbon-rich stars. This extra-mixing mechanism is able to reproduce the observed properties of both carbon-normal and carbon-rich stars.     

Young star clusters in the Large Magellanic Cloud: NGC 1805 and 1818

We present colour–magnitude diagrams for two rich (≈10⁴ M_⊙) Large Magellanic Cloud star clusters with ages ≈10⁷ yr, constructed from optical and near-infrared data obtained with the Hubble Space Telescope . These data are part of an HST project to study LMC clusters with a range of ages. In this paper we investigate the massive star content of the young clusters, and determine the cluster ages and metallicities, paying particular attention to Be-star and blue-straggler populations and evidence of age spreads. We compare our data with detailed stellar-population simulations to investigate the turn-off structure of ≈25 Myr stellar systems, highlighting the complexity of the blue-straggler phenomenon.     

Do binaries in clusters form in the same way as in the field?

We examine the dynamical destruction of binary systems in star clusters of different densities. We find that at high densities  (10⁴– 10⁵ M_⊙ pc⁻³)  almost all binaries with separations  >10³  au are destroyed after a few crossing times. At low densities [     ], many binaries with separations  >10³  au are destroyed, and no binaries with separations  >10⁴  au survive after a few crossing times. Therefore, the binary separations in clusters can be used as a tracer of the dynamical age and past density of a cluster.
We argue that the central region of the Orion nebula cluster was ∼100 times denser in the past with a half-mass radius of only 0.1–0.2 pc as (i) it is expanding, (ii) it has very few binaries with separations  >10³  au and (iii) it is well mixed and therefore dynamically old.
We also examine the origin of the field binary population. Binaries with separations  <10²  au are not significantly modified in any cluster, therefore at these separations the field reflects the sum of all star formation. Binaries with separations in the range  10²– 10⁴  au are progressively more and more heavily affected by dynamical disruption in increasingly dense clusters. If most star formation is clustered, these binaries must be overproduced relative to the field. Finally, no binary with a separation  >10⁴  au can survive in any cluster and so must be produced by isolated star formation, but only if all isolated star formation produces extremely wide binaries.     

The effect of the 19F(α, p)22Ne reaction rate uncertainty on the yield of fluorine from Wolf–Rayet stars

In the light of recent recalculations of the  ¹⁹F(α, p)²²Ne  reaction rate, we present results of the expected yield of ¹⁹F from Wolf–Rayet (WR) stars. In addition to using the recommended rate, we have computed models using the upper and lower limits for the rate, and hence we constrain the uncertainty in the yield with respect to this reaction. We find a yield of  3.1 × 10⁻⁴ M_⊙  of ¹⁹F with our recommended rate, and a difference of a factor of 2 between the yields computed with the upper and lower limits. In comparison with previous work we find a difference in the yield of a factor of approximately 4, connected with a different choice of mass loss. Model uncertainties must be carefully evaluated in order to obtain a reliable estimate of the yield, together with its uncertainties, of fluorine from WR stars.     

An iterative filter to reconstruct planetary transit signals in the presence of stellar variability

We report the identification, from a photometric, astrometric and spectroscopic study, of a massive white dwarf member of the nearby, approximately solar metallicity, Coma Berenices open star cluster (Melotte 111). We find the optical to near-infrared energy distribution of WD 1216+260 to be entirely consistent with that of an isolated DA and determine the effective temperature and surface gravity of this object to be   T _eff= 15 739⁺¹⁹⁷₋₁₉₆ K  and  log  g = 8.46^+0.03_−0.02  . We set tight limits on the mass of a putative cool companion,   M ≳ 0.036 M_⊙  (spatially unresolved) and   M ≳ 0.034 M_⊙  (spatially resolved and   a ≲ 2500 au  ). Based on the predictions of CO core, thick H layer evolutionary models we determine the mass and cooling time of WD 1216+260 to be   M _WD= 0.90 ± 0.04 M_⊙  and  τ_cool= 363⁺⁴⁶₋₄₁ Myr  , respectively. For an adopted cluster age of  τ= 500 ± 100 Myr  we infer the mass of its progenitor star to be   M _init= 4.77^+5.37_−0.97 M_⊙  . We briefly discuss this result in the context of the form of the stellar initial mass–final mass relation.     

Sakurai's Object: characterizing the near-infrared CO ejecta between 2003 and 2007

We present observations of Sakurai's Object obtained at 1–5 μm between 2003 and 2007. By fitting a radiative transfer model to an echelle spectrum of CO fundamental absorption features around  4.7 μm  , we determine the excitation conditions in the line-forming region. We find  ¹²C/¹³C = 3.5^+2.0_−1.5  , consistent with CO originating in ejecta processed by the very late thermal pulse, rather than in the pre-existing planetary nebula. We demonstrate the existence of  2.2 × 10⁻⁶≤ M _CO≤ 2.7 × 10⁻⁶ M_⊙  of CO ejecta outside the dust, forming a high-velocity wind of  500 ± 80 km s⁻¹  . We find evidence for significant weakening of the CO band and cooling of the dust around the central star between 2003 and 2005. The gas and dust temperatures are implausibly high for stellar radiation to be the sole contributor.     

ASCA spectroscopy of the hard X-ray emission from the colliding wind interaction in γ2 Velorum

We discuss an ASCA observation of the eccentric WC8+O7.5 III binary γ ² Velorum near apastron. The X-ray spectrum is compared with two previous observations obtained when the system was near periastron. All three spectra display a hard-emission component that undergoes strong variability over the orbital cycle. The properties of the hard X-ray emission of γ ² Vel are constrained by taking into account the contribution from contaminating soft X-ray sources in the vicinity of γ ² Vel. We find that the observed variations are in qualitative agreement with the predictions of colliding wind models. We investigate for the first time the effect of uncertainties in the chemical composition of the X-ray emitting plasma on our understanding of the high-energy properties of the wind interaction region. Our results indicate that these uncertainties significantly affect the derived shock temperature and absorption column, but play a smaller role in determining the intrinsic X-ray luminosity of the colliding wind zone. We further find that the intrinsic luminosity from the hard X-ray component in γ ² Vel does not follow the 1/ D distance relation expected from simple models of adiabatic shocks.     

Modelling the components of binaries in the Hyades: the dependence of the mixing-length parameter on stellar mass

We present our findings based on a detailed analysis of the binaries of the Hyades, in which the masses of the components are well known. We fit the models of the components of a binary system to observations so as to give the observed total V and B − V of that system and the observed slope of the main sequence in the corresponding parts. According to our findings, there is a very definite relationship between the mixing-length parameter and the stellar mass. The fitting formula for this relationship can be given as  α= 9.19( M /M_⊙− 0.74)^0.053− 6.65  , which is valid for stellar masses greater than  0.77 M_⊙  . While no strict information is gathered for the chemical composition of the cluster, as a result of degeneracy in the colour–magnitude diagram, by adopting   Z = 0.033  and using models for the components of 70 Tau and θ² Tau we find the hydrogen abundance to be   X = 0.676  and the age to be 670 Myr. If we assume that   Z = 0.024  , then   X = 0.718  and the age is 720 Myr. Our findings concerning the mixing-length parameter are valid for both sets of the solution. For both components of the active binary system V818 Tau, the differences between radii of the models with   Z = 0.024  and the observed radii are only about 4 per cent. More generally, the effective temperatures of the models of low-mass stars in the binary systems studied are in good agreement with those determined by spectroscopic methods.     

Expected planets in globular clusters

We argue that all transient searches for planets in globular clusters have a very low detection probability. Planets of low-metallicity stars typically do not reside at small orbital separations. The dependence of planetary system properties on metallicity is clearly seen when the quantity   I _e ≡ M _p[ a (1 − e )]²  is considered;   M _p, a   and e are the planet mass, semimajor axis and eccentricity, respectively. In high-metallicity systems, there is a concentration of systems at high and low values of I _e , with a low-populated gap near   I _e ∼ 0.3 M _J au²  , where M _J is Jupiter's mass. In low-metallicity systems, the concentration is only at the higher range of I _e , with a tail to low values of I _e . Therefore, it is still possible that planets exist around main-sequence stars in globular clusters, although at small numbers because of the low metallicity, and at orbital periods of ≳10 d. We discuss the implications of our conclusions on the role that companions can play in the evolution of their parent stars in globular clusters, for example, influencing the distribution of horizontal branch stars on the Hertzsprung–Russell diagram of some globular clusters, and in forming low-mass white dwarfs.     

The carbon Cepheid RT Trianguli Australis: additional evidence of triple-α and CNO cycling

We have used echelle spectra of resolving power 35 000 to derive chemical abundances and the ¹²C/¹³C ratio in the 1.9-d carbon Cepheid RT TrA and the Cepheid U TrA, employed as a comparison star. We confirm that RT TrA is very metal-rich with [Fe/H]=+0.4. In addition, C and N are substantially in excess, and a small deficiency in O is present. We interpret these anomalies as resulting from the appearance on the stellar surface of material enriched in ¹²C by the 3- α process, followed by CNO cycling to convert ¹²C to ¹³C and ¹⁴N. In addition, some ¹⁶O has been processed to ¹⁴N. The partial processing of ¹⁶O to ¹⁴N indicates that substantial ¹⁷O may be present. Proton capture seems to have enhanced ²³Na from the Ne isotopes.     

What is the nature of RX J0720.4–3125?

RX J0720.4–3125 has recently been identified as a pulsating soft X-ray source in the ROSAT all-sky survey with a period of 8.391 s. Its spectrum is well characterized by a blackbody with a temperature of 8 × 10⁵ K. We propose that the radiation from this object is thermal emission from a cooling neutron star. For this blackbody temperature we can obtain a robust estimate of the object's age of ∼ 3 × 10⁵ yr, yielding a polar field ∼ 10¹⁴ G for magnetic dipole spin-down and a value of P compatible with current observations.     

The power spectrum of flux-limited X-ray galaxy cluster surveys

We compute the redshift space power spectrum of two X-ray cluster samples: the X-ray Brightest Abell Cluster Sample (XBACS) and the Brightest Cluster Sample (BCS) using the method developed by Feldman, Kaiser & Peacock. The power spectra derived for these samples are in agreement with determinations of other optical and X-ray cluster samples. For XBACS we find the largest power spectrum amplitude expected, given the high richness of this sample ( R ≥2) . In the range 0.05< k <0.4  h  Mpc⁻¹ the power spectrum shows a power-law behaviour P ( k )∝ k ⁿ with an index n ≃−1.2 . In a similar range, 0.04< k <0.3  h  Mpc⁻¹ , the BCS power spectrum has a smaller amplitude with index n ≃−1.0 . We do not find significant evidence for a peak at k ≃0.05  h  Mpc⁻¹ , suggesting that claims such of feature detections in some cluster samples could rely on artificial inhomogeneities of the data. We compare our results with power spectrum predictions derived by Moscardini et al. within current cosmological models (LCDM and OCDM). For XBACS we find that both models underestimate the amplitude of the power spectrum but for BCS there is reasonably good agreement at k ≳0.03  h  Mpc⁻¹ for both models.     

RR Pic (1925): a Chandra X-ray view

We present the Chandra ACIS-S3 data of the old classical nova RR Pic (1925). The source has a count rate of 0.067 ± 0.002 count s⁻¹ in the 0.3–5.0 keV energy range. We detect the orbital period of the underlying binary system in the X-ray wavelengths. We also find that the neutral hydrogen column density differs for orbital minimum and orbital maximum spectra with values  0.25^+0.23_−0.18× 10²²  and  0.64^+0.13_−0.14× 10²² cm⁻²  at 3σ confidence level. The X-ray spectrum of RR Pic can be represented by a composite model of bremsstrahlung with a photoelectric absorption, two absorption lines centered around 1.1–1.4 keV and five Gaussian lines centered at emission lines around 0.3–1.1 keV corresponding to various transitions of S, N, O, C, Ne and Fe. The bremsstrahlung temperature derived from the fits ranges from 0.99 to 1.60 keV and the unabsorbed X-ray flux is found to be  2.5^+0.4_−1.2× 10⁻¹³ erg  cm⁻² s⁻¹  in the 0.3–5.0 keV range with a luminosity of 1.1 ± 0.2  10³¹ erg s⁻¹  at 600 pc. We also detect excess emission in the spectrum possibly originating from the reverse shock in the ejecta. A fit with a cooling flow plasma emission model shows enhanced abundances of He, C, N, O and Ne in the X-ray emitting region indicating existence of diffusive mixing.     

Does Collinder 236 host a Cepheid calibrator?

Photoelectric UBV photometry and star counts are presented for the previously unstudied open cluster Collinder 236, supplemented by observations for stars near the Cepheid WZ Car. Collinder 236 is typical of groups associated with Cepheids, with an evolutionary age of  (3.2 ± 1.1) × 10⁷ yr  , but it is  1944 ± 71  pc distant, only half the predicted distance to WZ Car. The cluster is reddened by   E ( B − V ) ≃ 0.26  , and has nuclear and coronal radii of   r _n≃ 2 arcmin (1.1 pc)  and   R _c≃ 8 arcmin (4.5 pc)  , respectively. The Cepheid is not a member of Collinder 236 on the basis of location beyond the cluster tidal radius and implied distance, but its space reddening can be established as   E ( B − V ) = 0.268 ± 0.006  s.e. from five adjacent stars. Period changes in WZ Car studied with the aid of archival data are revised. The period of WZ Car is increasing, its rate of  +8.27 ± 0.19 s yr⁻¹  being consistent with a third crossing of the instability strip.     

H 0 from an orientation-unbiased sample of Sunyaev–Zel'dovich and X-ray clusters

We have observed the Sunyaev–Zel'dovich (SZ) effect in a sample of five moderate-redshift clusters with the Ryle Telescope, and used them in conjunction with X-ray imaging and spectral data from ROSAT and ASCA to measure the Hubble constant. This sample was chosen with a strict X-ray flux limit using both the Bright Cluster Sample and the Northern ROSAT All-Sky Survey (RASS) cluster catalogues to be well above the surface brightness limit of the RASS, and hence to be unbiased with respect to the orientation of the cluster. This controls a major potential systematic effect in the SZ/X-ray method of measuring H ₀. Taking the weighted geometric mean of the results and including the main sources of error, namely the noise in the SZ measurement, the uncertainty in the X-ray temperatures and the unknown ellipticity and substructure of the clusters, we find   H ₀= 59⁺¹⁰₋₉ (random)⁺⁸₋₇(systematic) km s⁻¹ Mpc⁻¹  assuming a standard cold dark matter model with  Ω_M= 1.0, Ω_Λ= 0.0  or   H ₀= 66⁺¹¹₋₁₀ ⁺⁹₋₈ km  s⁻¹ Mpc⁻¹  if  Ω_M= 0.3, Ω_Λ= 0.7  .     

High-resolution infrared spectra of NGC 6342 and 6528: two moderately reddened bulge globular clusters

Using the Near Infrared Spectrometer (NIRSPEC) spectrograph at Keck II, we have obtained infrared (IR) echelle spectra covering the range  1.5 –1.8 μm  for the moderately reddened bulge globular clusters NGC 6342 and 6528, finding  [Fe/H]=−0.60  and −0.17 dex, respectively. We measure an average α-enhancement of ≈+0.33 dex in both clusters, consistent with previous measurements on other metal-rich bulge clusters, and favouring the scenario of a rapid bulge formation and chemical enrichment. We also measure very low ¹²C/¹³C isotopic ratios (≈5 in NGC 6342 and ≈8 in NGC 6528), suggesting that extra-mixing mechanisms resulting from cool bottom processing are at work during evolution along the red giant branch (RGB).     

Magnetic hydrogen atmosphere models and the neutron star RX J1856.5–3754

RX J1856.5−3754 is one of the brightest nearby isolated neutron stars (INSs), and considerable observational resources have been devoted to it. However, current models are unable to satisfactorily explain the data. We show that our latest models of a thin, magnetic, partially ionized hydrogen atmosphere on top of a condensed surface can fit the entire spectrum, from X-rays to optical, of RX J1856.5−3754, within the uncertainties. In our simplest model, the best-fitting parameters are an interstellar column density   N _H≈ 1 × 10²⁰ cm⁻²  and an emitting area with   R ^∞≈ 17 km  (assuming a distance to RX J1856.5−3754 of 140 pc), temperature   T ^∞≈ 4.3 × 10⁵ K  , gravitational redshift   z _g ∼ 0.22  , atmospheric hydrogen column   y _H≈ 1 g cm⁻²  , and magnetic field   B ≈ (3–4) × 10¹² G  ; the values for the temperature and magnetic field indicate an effective average over the surface. We also calculate a more realistic model, which accounts for magnetic field and temperature variations over the NS surface as well as general relativistic effects, to determine pulsations; we find that there exist viewing geometries that produce pulsations near the currently observed limits. The origin of the thin atmospheres required to fit the data is an important question, and we briefly discuss mechanisms for producing these atmospheres. Our model thus represents the most self-consistent picture to date for explaining all the observations of RX J1856.5−3754.     

Dynamical parallax of σ Ori AB: mass, distance and age

The massive OB-type binary σ Ori AB is in the centre of the very young σ Orionis cluster. I have computed the most probable distances and masses of the binary for several ages using a dynamical parallax-like method. It incorporates the BVRIH -band apparent magnitudes of both components, precise orbital parameters, interstellar extinction and a widely used grid of stellar models from the literature, Kepler's third law and a  χ²  minimization. The derived distance is  334⁺²⁵₋₂₂ pc  for an age of 3 ± 2 Ma; larger ages and distances are unlikely. The masses of the primary and the secondary lie on the approximate intervals  16–20 and 10–12 M_⊙  , respectively. I also discuss the possibility of σ Ori AB being a triple system at ∼ 385 pc. These results will help to constrain the properties of young stars and substellar objects in the σ Orionis cluster.