The Red Rectangle: Its Shaping Mechanism and its Source of Ultraviolet Photons

The proto-planetary Red Rectangle nebula is powered by HD 44179, a spectroscopic binary (P = 318 d), in which a luminous post-AGB component is the primary source of both luminosity and current mass loss. Here, we present the results of a seven-year, eight-orbit spectroscopic monitoring program of HD 44179, designed to uncover new information about the source of the Lyman/far-ultraviolet continuum in the system as well as the driving mechanism for the bipolar outflow producing the current nebula. Our observations of the H-alpha line profile around the orbital phase of superior conjunction reveal the secondary component to be the origin of the fast (max. v~560$km s$^{-1}$) bipolar outflow in the Red Rectangle. The variation of total H-alpha flux from the central H II region with orbital phase also identifies the secondary or its surroundings as the source of the far-ultraviolet ionizing radiation in the system. The estimated mass of the secondary (~0.94 M$\sun$) and the speed of the outflow suggest that this component is a main sequence star and not a white dwarf, as previously suggested. We identify the source of the Lyman/far-ultraviolet continuum in the system as the hot, inner region (T$_{max} \ge 17,000$K) of an accretion disk surrounding the secondary, fed by Roche lobe overflow from the post-AGB primary at a rate of about$2 - 5\times10^{-5}$M$\sun$yr$^{-1}$. The total luminosity of the accretion disk around the secondary is estimated to be at least 300 L$\sun$, about 5% of the luminosity of the entire system. (abridged)Comment: Accepted for publication in Ap

Imaging the circumstellar envelopes of AGB stars

We report the results of an exploratory program to image the extended circumstellar envelopes of asymptotic giant branch (AGB) stars in dust-scattered galactic light. The goal is to characterize the morphology of the envelopes as a probe of the mass-loss process. The observations consist of short exposures with the VLT and longer exposures with 1-2m telescopes, augmented with archival images from the Hubble Space Telescope. We observed 12 AGB stars and detected the circumstellar envelopes in 7. The detected envelopes have mass loss rates more than about 5 10E-6 solar mass per year, and they can be seen out to distances of about 1 kpc. The observations provide information on the mass loss history on time scales up to about 10,000 years. For the five AGB envelopes in which the circumstellar geometry is well determined by scattered light observations, all except one (OH348.2-19.7) show deviations from spherical symmetry. Two (IRC+10216 and IRC+10011) show roughly spherical envelopes at large radii but asymmetry or bipolarity close to the star; one (AFGL 2514) shows an extended, elliptical envelope, and one (AFGL 3068) shows a spiral pattern. The non-spherical structures are all consistent with the effects of binary interactions. Our observations are in accord with a scenario in which binary companions play a role in shaping planetary nebulae, and show that the circumstellar gas is already partly shaped on the AGB, before evolution to the proto-planetary nebula phase.Comment: Accepted by AA 21 Feb 2006; 18 pages, 14 figs; for high resolution images, contact mauron at graal.univ-montp2.f

Infrared Imaging of GRB 970508

We have observed the field of the gamma-ray burst GRB 970508 at infrared wavelengths (2.2 μm) and have found a variable source coincident with the visible transient thought to be associated with the burst. The source was decaying in brightness with Ks magnitudes of 18.2±0.1, 18.8±0.1, and 19.0±0.3 mag on May 13.25, 16.25, and 20.21 UT, respectively. A 1 σ upper limit of K=21.3 mag was obtained for the brightness of the source on June 14.27 UT. The infrared light curve during this period is consistent with a ~t^(-1.2) power law, similar to the visible light curve. We do not find evidence for extended structure around the burst, as has been claimed for GRB 970228, and we obtain an upper limit of 0.04L_* for the luminosity of an underlying galaxy at the position of the infrared transient

The Dawn Gravity Investigation at Vesta and Ceres

The objective of the Dawn gravity investigation is to use high precision X-band Doppler tracking and landmark tracking from optical images to measure the gravity fields of Vesta and Ceres to a half-wavelength surface resolution better than 90-km and 300-km, respectively. Depending on the Doppler tracking assumptions, the gravity field will be determined to somewhere between harmonic degrees 15 and 25 for Vesta and about degree 10 for Ceres. The gravity fields together with shape models determined from Dawn's framing camera constrain models of the interior from the core to the crust. The gravity field is determined jointly with the spin pole location. The second degree harmonics together with assumptions on obliquity or hydrostatic equilibrium may determine the moments of inertia

The Shapes of AGB Envelopes as Probes of Binary Companions

We describe how the large scale geometry of the circumstellar envelopes of asymptotic giant branch stars can be used to probe the presence of unseen stellar companions. A nearby companion modifies the mass loss by gravitationally focusing the wind towards the orbital plane, and thereby determines the shape of the envelope at large distances from the star. Using available simulations, we develop a prescription for the observed shapes of envelopes in terms of the binary parameters, envelope orientation, and type of observation. The prescription provides a tool for the analysis of envelope images at optical, infrared, and millimetre wavelengths, which can be used to constrain the presence of companions in well observed cases. We illustrate this approach by examining the possible role of binary companions in triggering the onset of axi-symmetry in planetary nebula formation. If interaction with the primary leads to axi-symmetry, the spherical halos widely seen around newly formed nebulae set limits on the companion mass. Only low mass objects may orbit close to the primary without observable shaping effects: they remain invisible until the interaction causes a sudden change in the mass loss geometry.Comment: 11 pages, 7 figures, to appear in MNRA

Connecting RS OPh to [some] Type Ia Supernovae

Aims: Recurrent nova systems like RS Oph have been proposed as a possible channel to Type Ia Supernova explosions, based on the high mass of the accreting white dwarf. Additional support to this hypothesis has been recently provided by the detection of circumstellar material around SN2006X and SN2007le, showing a structure compatible with that expected for recurrent nova outbursts.In this paper we investigate the circumstellar environment of RS Oph and its structure, with the aim of establishing a firmer and independent link between this class of objects and Type Ia SN progenitors. Methods: We study the time evolution of CaII, NaI and KI absorption features in RS Oph, before, during, and after the last outburst, using multi-epoch, high-resolution spectroscopy, and applying the same method adopted for SN2006X and SN2007le. Results: A number of components, blue-shifted with respect to the systemic velocity of RS Oph, are detected. In particular, one feature strongly weakens in the first two weeks after the outburst, simultaneously with the disappearance of very narrow P-Cyg profiles overimposed on the much wider nova emission lines of H, He, FeII and other elements. Conclusions: We interpret these facts as the signature of density enhancements in the circumstellar material, suggesting that the recurrent eruptions might indeed create complex structures within the material lost by the donor star. This establishes a strong link between RS Oph and the progenitor system of the Type Ia SN2006X, for which similar features have been detected.Comment: 8 pages, 10 figures. Accepted for publication in Astronomy and Astrophysic

Planets and Axisymmetric Mass Loss

Bipolar planetary nebulae (PNe), as well as extreme elliptical PNe are formed through the influence of a stellar companion. But half of all PN progenitors are not influenced by any stellar companion, and, as I show here, are expected to rotate very slowly on reaching the upper asymptotic giant branch; hence they expect to form spherical PNe, unless they are spun-up. But since most PNe are not spherical, I argue that about 50 percents of AGB stars are spun-up by planets, even planets having a mass as low as 0.01 times the mass of Jupiter, so they form elliptical PNe. The rotation by itself will not deform the AGB wind, but may trigger another process that will lead to axisymmetric mass loss, e.g., weak magnetic activity, as in the cool magnetic spots model. This model also explains the transition from spherical to axisymmetric mass loss on the upper AGB. For such low mass planets to substantially spin-up the stellar envelope, they should enter the envelope when the star reaches the upper AGB. This "fine-tuning" can be avoided if there are several planets on average around each star, as is the case in the solar system, so that one of them is engulfed when the star reaches the upper AGB.Comment: 8 pages, 1 figure. To appear in the proceedings of the conference, "Post-AGB Objects (proto-planetary nebulae) as a Phase of Stellar Evolution", Torun, Poland, July 5-7, 2000, eds. R. Szczerba, R. Tylenda, and S.K. Gorn

Time resolved spectroscopy of BD+46 442: gas streams and jet creation in a newly discovered evolved binary with a disk

Previous studies have shown that many post-AGB stars with dusty disks are associated with single-lined binary stars. To verify the binarity hypothesis on a larger sample, we started a high-resolution spectral monitoring of about 40 field giants, whose binarity was suspected based on either a light curve, an infrared excess, or a peculiar chemical composition. Here we report on the discovery of the periodic RV variations in BD+46 442, a high-latitude F giant with a disk. We interpret the variations due to the motion around a faint companion, and deduce the following orbital parameters: Porb = 140.77 d, e = 0.083, asini=0.31 AU. We find it to be a moderately metal-poor star ([M/H]=-0.7) without a strong depletion pattern in the photospheric abundances. Interestingly, many lines show periodic changes with the orbital phase: Halpha switches between a double-peak emission and a PCyg-like profiles, while strong metal lines appear split during the maximum redshift. Similar effects are likely visible in the spectra of other post-AGB binaries, but their regularity is not always realized due to sporadic observations. We propose that these features result from an ongoing mass transfer from the evolved giant to the companion. In particular, the blue-shifted absorption in Halpha, which occurs only at superior conjunction, may result from a jet originating in the accretion disk around the companion and seen in absorption towards the luminous primary.Comment: 16 pages, accepted in A&

Low-ionization structures in planetary nebulae: confronting models with observations

Around 50 PNe are presently known to possess "small-scale" low-ionization structures (LISs). We consider here jets, jet-like, symmetrical and non-symmetrical LISs and present a detailed comparison of the existing model predictions with the observational morphological and kinematical properties. We find that nebulae with LISs appear indistinctly spread among all morphological classes of PNe, indicating that the processes leading to the formation of LISs are not necessarily related to those responsible for the asphericity of the large-scale morphological components of PNe. We show that both the observed velocities and locations of most non-symmetrical LISs can be reasonably well reproduced assuming either fossil condensations originated in the AGB wind or in-situ instabilities. The jet models proposed to date (HD and MHD interacting winds or accretion-disk collimated winds) appear unable to account simultaneously for the kinematical ages and the angle between the jet and the symmetry axes of the nebulae. The linear increase in velocity observed in several jets favors MHD confinement compared to pure HD interacting wind models. On the other hand, we find that the formation of jet-like systems characterized by relatively low expansion velocities cannot be explained by any of the existing models. Finally, the knots which appear in symmetrical and opposite pairs of low velocity could be understood as the survival of fossil (symmetrical) condensations formed during the AGB phase or as structures that have experienced substantial slowing down by the ambient medium.Comment: 21 pages including 5 tables. To appear in ApJ 547, Jan 2001. Also available at http://www.iac.es/publicaciones/preprints.htm

Heterogeneous mass distribution of the rubble-pile asteroid (101955) Bennu

The gravity field of a small body provides insight into its internal mass distribution. We used two approaches to measure the gravity field of the rubble-pile asteroid (101955) Bennu: (i) tracking and modeling the spacecraft in orbit about the asteroid and (ii) tracking and modeling pebble-sized particles naturally ejected from Bennu’s surface into sustained orbits. These approaches yield statistically consistent results up to degree and order 3, with the particle-based field being statistically significant up to degree and order 9. Comparisons with a constant-density shape model show that Bennu has a heterogeneous mass distribution. These deviations can be modeled with lower densities at Bennu’s equatorial bulge and center. The lower-density equator is consistent with recent migration and redistribution of material. The lower-density center is consistent with a past period of rapid rotation, either from a previous Yarkovsky-O’Keefe-Radzievskii-Paddack cycle or arising during Bennu’s accretion following the disruption of its parent body