Abundance anomalies in pre-main-sequence stars: Stellar evolution models with mass loss

The effects of atomic diffusion on internal and surface abundances of A and F pre-main-sequence stars with mass loss are studied in order to determine at what age the effects materialize, as well as to further understand the processes at play in HAeBe and young ApBp stars. Self-consistent stellar evolution models of 1.5 to 2.8Msun with atomic diffusion (including radiative accelerations) for all species within the OPAL opacity database were computed and compared to observations of HAeBe stars. Atomic diffusion in the presence of weak mass loss can explain the observed abundance anomalies of pre-main-sequence stars, as well as the presence of binary systems with metal rich primaries and chemically normal secondaries such as V380 Ori and HD72106. This is in contrast to turbulence models which do not allow for abundance anomalies to develop on the pre-main-sequence. The age at which anomalies can appear depends on stellar mass. For A and F stars, the effects of atomic diffusion can modify both the internal and surface abundances before the onset of the MS. The appearance of important surface abundance anomalies on the pre-main-sequence does not require mass loss, though the mass loss rate affects their amplitude. Observational tests are suggested to decipher the effects of mass loss from those of turbulent mixing. If abundance anomalies are confirmed in pre-main-sequence stars they would severely limit the role of turbulence in these stars.Comment: 9 pages, 6 figures, accepeted for publicatio

The kinematics of the most oxygen-poor planetary nebula PN G135.9+55.9

PN G135.9+55.9 is a compact, high excitation nebula that has been identified recently as the most oxygen-poor halo planetary nebula. Given its very peculiar characteristics and potential implications in the realms of stellar and Galactic evolution, additional data are needed to firmly establish its true nature and evolutionary history. Here we present the first long-slit, high spectral resolution observations of this object in the lines of H$\alpha$ and He II 4686. The position-velocity data are shown to be compatible with the interpretation of PN G135.9+55.9 being a halo planetary nebula. In both emission lines, we find the same two velocity components that characterize the kinematics as that of an expanding elliptical envelope. The kinematics is consistent with a prolate ellipsoidal model with axis ratio about 2:1, a radially decreasing emissivity distribution, a velocity distribution that is radial, and an expansion velocity of 30 km/s for the bulk of the material. To fit the observed line profiles, this model requires an asymmetric matter distribution, with the blue-shifted emission considerably stronger than the red-shifted emission. We find that the widths of the two velocity components are substantially wider than those expected due to thermal motions, but kinematic structure in the projected area covered by the slit appears to be sufficient to explain the line widths. The present data also rule out the possible presence of an accretion disk in the system that could have been responsible for a fraction of the H$\alpha$ flux, further supporting the planetary nebula nature of PN G135.9+55.9.Comment: accepted by Astronomy & Astrophysic

An object that defies stereotypes : X-ray observations of SBS 1150+599A - the binary nucleus of PN G135.9+55.9

We present X-ray observations of the close binary nucleus of the planetary nebula (PN) PN G135.9+55.9 obtained with the XMM satellite. The nebula is the most oxygen-poor PN known to date and is located in the Galactic halo. It is known to harbor a close binary nucleus of which only one component can be observed in optical-UV range. New X-ray observations show that the invisible component is a very hot compact star. This finding allows us to reconstruct the immediate past of the object and predict its future. The parameters of the binary components we determine strongly suggest that the precursor was a symbiotic supersoft X-ray source that finished its life by Roche lobe overflow. PN G135.9+55.9 is an excellent candidate for a future type Ia supernova

Sirius A: turbulence or mass loss?

Context. Abundance anomalies observed in a fraction of A and B stars of both Pop I and II are apparently related to internal particle transport. Aims. Using available constraints from Sirius A, we wish to determine how well evolutionary models including atomic diffusion can explain observed abundance anomalies when either turbulence or mass loss is used as the main competitor to atomic diffusion. Methods. Complete stellar evolution models, including the effects of atomic diffusion and radiative accelerations, have been computed from the zero age main-sequence of 2.1M\odot stars for metallicities of Z0 = 0.01 \pm 0.001 and shown to agree with the observed parameters of Sirius A. Surface abundances were predicted for three values of the mass loss rate and for four values of the mixed surface zone. Results. A mixed mass of ~ 10^-6 M\odot or a mass loss rate of 10^-13 M\odot/yr were determined through comparison with observations. Of the 17 abundances determined observationally which are included in our calculations, up to 15 can be predicted within 2 sigmas and 3 of the 4 determined upper limits are compatible. Conclusions. While the abundance anomalies can be reproduced slightly better using turbulence as the process competing with atomic diffusion, mass loss probably ought to be preferred since the mass loss rate required to fit abundance anomalies is compatible with the observationally determined rate. A mass loss rate within a factor of 2 of 10^-13 M\odot/yr is preferred. This restricts the range of the directly observed mass loss rate.Comment: Accepted by A&A, 25/07/201

The Evolution of the Kinematics of Nebular Shells in Planetary Nebulae in the Milky Way Bulge

We study the line widths in the [\ion{O}{3}]$\lambda$5007 and H$\alpha$ lines for two groups of planetary nebulae in the Milky Way bulge based upon spectroscopy obtained at the Observatorio Astron\'omico Nacional in the Sierra San Pedro M\'artir (OAN-SPM) using the Manchester Echelle Spectrograph. The first sample includes objects early in their evolution, having high H$\beta$ luminosities, but [\ion{O}{3}]$\lambda 5007/\mathrm H\beta < 3$. The second sample comprises objects late in their evolution, with \ion{He}{2} $\lambda 4686/\mathrm H\beta > 0.5$. These planetary nebulae represent evolutionary phases preceeding and following those of the objects studied by Richer et al. (2008). Our sample of planetary nebulae with weak [\ion{O}{3}]$\lambda$5007 has a line width distribution similar to that of the expansion velocities of the envelopes of AGB stars, and shifted to systematically lower values as compared to the less evolved objects studied by Richer et al. (2008). The sample with strong \ion{He}{2} $\lambda 4686$ has a line width distribution indistinguishable from that of the more evolved objects from Richer et al. (2008), but a distribution in angular size that is systematically larger and so they are clearly more evolved. These data and those of Richer et al. (2008) form a homogeneous sample from a single Galactic population of planetary nebulae, from the earliest evolutionary stages until the cessation of nuclear burning in the central star. They confirm the long-standing predictions of hydrodynamical models of planetary nebulae, where the kinematics of the nebular shell are driven by the evolution of the central star.Comment: accepted for publication in the Astrophysical Journa

The white dwarf cooling sequence in the old open cluster NGC 188

We develop the white dwarf luminosity function (LF) of the old open cluster NGC 188 in order to determine a lower limit to the age of the cluster by using the faint end of the cooling sequence. To produce an extensive sequence of the cooling white dwarfs we imaged four contiguous HST-WFPC2 fields in the center of the cluster in the F555W and F814W filters. After imposing selection criteria on the detected objects we found a white dwarf cooling sequence (down to V ~26.5) including 28 candidate white dwarfs in the cluster. The exposures are not deep enough to reach the end of this sequence, but the results of our analysis allow us to establish a lower limit to the age of the cluster independently of the isochrone fit to the cluster turnoff. The most ancient white dwarfs found are ~4 Gyr old, an age that is set solely by the photometric limit of our data. Classical methods provide an estimate of \~7 Gyr (Sarajedini et al., 1999).Comment: 6 pages, 5 figures, accepted on Astronomy and Astrophysic