Cooling curves and initial models for low-mass white dwarfs (<0.25 M⊙) with helium cores

We present a detailed calculation of the evolution of low-mass (<0.25 M_⊙) helium white dwarfs. These white dwarfs (the optical companions to binary millisecond pulsars) are formed via long-term, low-mass binary evolution. After detachment from the Roche lobe, the hot helium cores have a rather thick hydrogen layer with mass between 0.01 and 0.06 M_⊙. As a result of mixing between the core and outer envelope, the surface hydrogen content ( X _surf) is 0.5–0.35 , depending on the initial value of the heavy element Z and the initial secondary mass. We found that the majority of our computed models experience one or two hydrogen shell flashes. We found that the mass of the helium dwarf in which the hydrogen shell flash occurs depends on the chemical composition. The minimum helium white dwarf mass in which a hydrogen flash takes place is 0.213 M_⊙ ( Z =0.003), 0.198 M_⊙ ( Z =0.01), 0.192 M_⊙ ( Z =0.02) or 0.183 M_⊙ ( Z =0.03). The duration of the flashes (independent of chemical composition) is between a few ×10⁶ and a few ×10⁷ yr. In several flashes the white dwarf radius will increase so much that it forces the model to fill its Roche lobe again. Our calculations show that the cooling history of the helium white dwarf depends dramatically on the thickness of the hydrogen layer. We show that the transition from a cooling white dwarf with a temporarily stable hydrogen-burning shell to a cooling white dwarf in which almost all residual hydrogen is lost in a few thermal flashes (via Roche lobe overflow) occurs between 0.183 and 0.213 M_⊙ (depending on the heavy element value).     

Evolution of a 3-M⊙ star from the main sequence to the ZZ Ceti stage: the role played by element diffusion

The purpose of this paper is to present new full evolutionary calculations for DA white dwarf stars with the major aim of providing a physically sound reference frame for exploring the pulsation properties of the resulting models in future communications. Here, white dwarf evolution is followed in a self-consistent way with the predictions of time-dependent element diffusion and nuclear burning. In addition, full account is taken of the evolutionary stages prior to white dwarf formation. In particular, we follow the evolution of a 3-M_⊙ model from the zero-age main sequence (the adopted metallicity is   Z =0.02)  , all the way from the stages of hydrogen and helium burning in the core up to the thermally pulsing phase. After experiencing 11 thermal pulses, the model is forced to evolve towards its white dwarf configuration by invoking strong mass loss episodes. Further evolution is followed down to the domain of the ZZ Ceti stars on the white dwarf cooling branch.
Emphasis is placed on the evolution of the chemical abundance distribution caused by diffusion processes and the role played by hydrogen burning during the white dwarf evolution. We find that discontinuities in the abundance distribution at the start of the cooling branch are considerably smoothed out by diffusion processes by the time the ZZ Ceti domain is reached. Nuclear burning during the white dwarf stage does not represent a major source of energy, as expected for a progenitor star of initially high metallicity. We also find that thermal diffusion lessens even further the importance of nuclear burning.
Furthermore, the implications of our evolutionary models for the main quantities relevant for adiabatic pulsation analysis are discussed. Interestingly, the shape of the Ledoux term is markedly smoother compared with previous detailed studies of white dwarfs. This is translated into a different behaviour of the Brunt–Väisälä frequency.     

The first magnetic maps of a pre-main-sequence binary star system – HD 155555

We present the first maps of the surface magnetic fields of a pre-main-sequence binary system. Spectropolarimetric observations of the young, 18 Myr, HD 155555 (V824 Ara, G5IV+K0IV) system were obtained at the Anglo-Australian Telescope in 2004 and 2007. Both data sets are analysed using a new binary Zeeman–Doppler imaging (ZDI) code. This allows us to simultaneously model the contribution of each component to the observed circularly polarized spectra. Stellar brightness maps are also produced for HD 155555 and compared to previous Doppler images.
Our radial magnetic maps reveal a complex surface magnetic topology with mixed polarities at all latitudes. We find rings of azimuthal field on both stars, most of which are found to be non-axisymmetric with the stellar rotational axis. We also examine the field strength and the relative fraction of magnetic energy stored in the radial and azimuthal field components at both epochs. A marked weakening of the field strength of the secondary star is observed between the 2004 and 2007 epochs. This is accompanied by an apparent shift in the location of magnetic energy from the azimuthal to radial field. We suggest that this could be indicative of a magnetic activity cycle. We use the radial magnetic maps to extrapolate the coronal field (by assuming a potential field) for each star individually – at present ignoring any possible interaction. The secondary star is found to exhibit an extreme tilt (≈75°) of its large-scale magnetic field to that of its rotation axis for both epochs. The field complexity that is apparent in the surface maps persists out to a significant fraction of the binary separation. Any interaction between the fields of the two stars is therefore likely to be complex also. Modelling this would require a full binary field extrapolation.     

IRAS 12311−3509: a carbon star with SiC2 emission

The optical spectrum of the carbon star IRAS 12311−3509 is dominated by the Merrill–Sanford emission bands of SiC₂, by absorption and emission in the Swan system of C₂, and by resonance emission lines of neutral metals. The infrared energy distribution is flat from 1 to 60 μm. These observations are interpreted as arising from a star with a cool dusty disc which is edge-on to the observer and obscures direct starlight. The infrared continuum is caused predominantly by absorption of stellar light by dust in the disc and re-emission at longer wavelengths. The optical stellar spectrum is seen by reflection off dusty material which lies out of the plane of the disc, and the molecular and atomic emission arises in the same geometry through resonance fluorescence. The object has similarities to the J-silicate stars, but may have a carbon-rich rather than oxygen-rich disc. A full spectroscopic assignment and discussion of the SiC₂ bands and their intensities are given. Modelling of the rotational contours of the     band yields a rotational temperature of 250 K, indicating very cool gas.     

The initial conditions of isolated star formation – IV. C18O observations and modelling of the pre-stellar core L1689B

We present C¹⁸O observations of the pre-stellar core L1689B, in the J =3→2 and 2→1 rotational transitions, taken at the James Clerk Maxwell Telescope in Hawaii. We use a λ -iteration radiative transfer code to model the data. We adopt a similar form of radial density profile to that which we have found in all pre-stellar cores, with a 'flat' inner profile, steepening towards the edge, but we make the gradient of the 'flat' region a free parameter. We find that the core is close to virial equilibrium, but there is tentative evidence for core contraction. We allow the temperature to vary with a power-law form and find that we can consistently fit all of the CO data with an inverse temperature gradient that is warmer at the edge than at the centre. However, when we combine the CO data with the previously published millimetre data we fail to find a simultaneous fit to both data sets without additionally allowing the CO abundance to decrease towards the centre. This effect has been observed qualitatively many times before, as the CO freezes out on to the dust grains at high densities, but we quantify the effect. Hence we show that the combination of millimetre/submillimetre continuum and spectral line data is a very powerful method of constraining the physical parameters of cores on the verge of forming stars.     

IRAS 22036+5306: an Al2O3 oxide-dominated post-AGB star

Radiative transfer modelling of the Infrared Space Observatory ( ISO ) spectrum of IRAS 22036+5306 has shown that its unusual 11-μm band can be suitably modelled with an alumina-olivine mixture substantially dominated by the former. The results of this work add further credence to recent findings that significant amounts of Al  ₂O₃  dust grains are present in the dust shells of stars near or beyond the tip of the asymptotic giant branch. Indeed, in the case of IRAS 22036+5306, Al  ₂O₃  dominates the dust composition to the extent that it shifts the 9.8-μm band due to amorphous silicates to 11 μm. IRAS 22036+5306 may be an unusual case in that the inner dust torus is maintained at a sufficiently high temperature for Al  ₂O₃  condensation, but not silicate.     

On the photodissociation of H2 by the first stars

The first star formation in the Universe is expected to take place within small protogalaxies, in which the gas is cooled by molecular hydrogen. However, if massive stars form within these protogalaxies, they may suppress further star formation by photodissociating the H₂. We examine the importance of this effect by estimating the time-scale on which significant H₂ is destroyed. We show that photodissociation is significant in the least massive protogalaxies, but becomes less so as the protogalactic mass increases. We also examine the effects of photodissociation on dense clumps of gas within the protogalaxy. We find that while collapse will be inhibited in low-density clumps, denser ones may survive to form stars.     

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.     

Triggered star formation and evolution of T-Tauri stars in and around bright-rimmed clouds

The aim of this paper is to quantitatively testify the ' small-scale sequential star formation ' hypothesis in and around bright-rimmed clouds (BRCs). As a continuation of the recent attempt by Ogura et al., we have carried out   BVI_c   photometry of four more BRC aggregates along with deeper re-observations of two previously observed BRCs. Again, quantitative age gradients are found in almost all the BRCs studied in the present work. Archival Spitzer /Infrared Array Camera data also support this result. The global distribution of near-infrared excess stars in each H  ii region studied here clearly shows evidence that a series of radiation-driven implosion processes proceeded in the past from near the central O star(s) towards the peripheries of the H  ii region. We found that in general weak-line T-Tauri stars (WTTSs) are somewhat older than classical T-Tauri stars (CTTSs). Also the fraction of CTTSs among the T-Tauri stars (TTSs) associated with the BRCs is found to decrease with age. These facts are in accordance with the recent conclusion by Bertout, Siess & Cabrit that CTTSs evolve into WTTSs. It seems that in general the equivalent width of Hα emission in TTSs associated with the BRCs decreases with age. The mass function (MF) of the aggregates associated with the BRCs of the morphological type 'A' seems to follow that found in young open clusters, whereas 'B/C'-type BRCs show significantly steeper MF.     

Supplementary southern standards for UBV(RI)C photometry

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.