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

Deep optical imaging of AGB circumstellar envelopes

We report results of a program to image the extended circumstellar envelopes of asymptotic giant branch (AGB) stars in dust-scattered Galactic light. The goal is to characterize the shapes of the envelopes to probe the mass-loss geometry and the presence of hidden binary companions. The observations consist of deep optical imaging of 22 AGB stars with high mass loss rates: 16 with the ESO 3.5 m NTT telescope, and the remainder with other telescopes. The circumstellar envelopes are detected in 15 objects, with mass loss rates > 2E-6 Msun/year. The surface brightness of the envelopes shows a strong decrease with Galactic radius, which indicates a steep radial gradient in the interstellar radiation field. The envelopes range from circular to elliptical in shape, and we characterize them by the ellipticity (E = major/minor axis) of iso-intensity contours. We find that about 50 percent of the envelopes are close to circular with E 1.2. We interpret the shapes in terms of populations of single stars and binaries whose envelopes are flattened by a companion. The distribution of E is qualitatively consistent with expectations based on population synthesis models of binary AGB stars. We also find that about 50 percent of the sample exhibit small-scale, elongated features in the central regions. We interpret these as the escape of light from the central star through polar holes, which are also likely produced by companions. Our observations of envelope flattening and polar holes point to a hidden population of companions within the circumstellar envelopes of AGB stars. These companions are expected to play an important role in the transition to post-AGB stars and the formation of planetary nebulae.Comment: 19 pages, 13 figures, color pictures in Appendix, accepted by A&

Gas phase atomic metals in the circumstellar envelope of IRC+10216

We report the results of a search for gas phase atomic metals in the circumstellar envelope of the AGB carbon star IRC+10216. The search was made using high resolution (R=50000) optical absorption spectroscopy of a backgound star that probes the envelope on a line of sight 35" from the center. The metal species that we detect in the envelope include NaI, KI, CaI, CaII, CrI, and FeI, with upper limits for AlI, MnI, TiI, TiII, and SrII. The observations are used to determine the metal abundances in the gas phase and the condensation onto grains. The metal depletions range from a factor of 5 for Na to 300 for Ca, with some similarity to the depletion pattern in interstellar clouds. Our results directly constrain the condensation efficiency of metals in a carbon-rich circumstellar envelope and the mix of solid and gas phase metals returned by the star to the ISM. The abundances of the uncondensed metal atoms that we observe are typically larger than the abundances of the metal-bearing molecules detected in the envelope. The metal atoms are therefore the major metal species in the gas phase and likely play a key role in the metal chemistry.Comment: 11 pages, 8 Figures. Accepted by Astronomy and Astrophysic

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

The envelope of IRC+10216 reflecting the galactic light: UBV surface brightness photometry and interpretation

We present and analyse new optical images of the dust envelope surrounding the high mass-loss carbon star IRC+10216. This envelope is seen due to external illumination by galactic light. Intensity profiles and colors of the nebula were obtained in the UBV bandpasses. The data are compared with the results of a radiative transfer model calculating multiple scattering of interstellar field photons by dust grains with a single radius. The data show that the observed radial shape of the nebula, especially its half maximum radius, does not depend on wavelength (within experimental errors), suggesting that grains scatter in the grey regime, etc, etc (this abstract has been shortened)Comment: accepted by A

Model for the hydrogen adsorption on carbon nanostructures

The hydrogen sorption capacity of carbon nanostructures was for several years a very controversial subject. Theoretical models have been published demonstrating a great potential for a large hydrogen sorption capacity of carbon nanostructures. Here we present a simple empirical model where condensation of hydrogen as a monolayer at the surface of nanotubes as well as bulk condensation in the cavity of the tube is assumed. The maximum potential amount of hydrogen absorbed according to the model was calculated to be 2.28×10-3mass % S[m2g-1]=3.0mass % for the adsorption of a monolayer hydrogen at the surface. The condensation of hydrogen in the cavity of the tube leads to a potential absorption for single wall nanotubes starting at 1.5mass % and increasing with the diameter of the tubes. The experimentally measured hydrogen capacity of the nanotube samples correlates with the B.E.T. specific surface area. The slope of the linear relationship is 1.5×10-3mass %/m2g-1. Therefore, the extrapolated maximum discharge capacity of a carbon sample is 2mass %. Furthermore, it can be concluded, that the hydrogen sorption mechanism is related to the surface of the sample, i.e. a surface adsorption proces

A Detailed Analysis of the Dust Formation Zone of IRC+10216 Derived from Mid-IR Bands of C2H2 and HCN

A spectral survey of IRC+10216 has been carried out in the range 11 to 14 um with a spectral resolution of about 4 km s^-1. We have identified a forest of lines in six bands of C2H2 involving the vibrational states from the ground to 3nu5 and in two bands of HCN, involving the vibrational states from the ground up to 2nu2. Some of these transitions are observed also in H13CCH and H13CN. We have estimated the kinetic, vibrational, and rotational temperatures, and the abundances and column densities of C2H2 and HCN between 1 and 300 R* (1.5E16 cm) by fitting about 300 of these ro-vibrational lines. The envelope can be divided into three regions with approximate boundaries at 0.019 arcsec (the stellar photosphere), 0.1 arcsec (the inner dust formation zone), and 0.4 arcsec (outer dust formation zone). Most of the lines might require a large microturbulence broadening. The derived abundances of C2H2 and HCN increase by factors of 10 and 4, respectively, from the innermost envelope outwards. The derived column densities for both C2H2 and HCN are 1.6E19 cm^-2. Vibrational states up to 3000 K above ground are populated, suggesting pumping by near-infrared radiation from the star and innermost envelope. Low rotational levels can be considered under LTE while those with J>20-30 are not thermalized. A few lines require special analysis to deal with effects like overlap with lines of other molecules.Comment: 8 pages, 16 figures, 2 machine-readable tables, accepted in the Astrophysical Journa

Filaments as Possible Signatures of Magnetic Field Structure in Planetary Nebulae

We draw attention to the extreme filamentary structures seen in high-resolution optical images of certain planetary nebulae. We determine the physical properties of the filaments in the nebulae IC 418, NGC 3132, and NGC 6537, and based on their large length-to-width ratios, longitudinal coherence, and morphology, we suggest that they may be signatures of the underlying magnetic field. The fields needed for the coherence of the filaments are probably consistent with those measured in the precursor circumstellar envelopes. The filaments suggest that magnetic fields in planetary nebulae may have a localized and thread-like geometry.Comment: 26 pages with 7 figures. To be published in PASP. For full resolution images see http://physics.nyu.edu/~pjh

Optical Signatures of Circumstellar Interaction in Type IIP Supernovae

We propose new diagnostics for circumstellar interaction in Type IIP supernovae by the detection of high velocity (HV) absorption features in Halpha and He I 10830 A lines during the photospheric stage. To demonstrate the method, we compute the ionization and excitation of H and He in supernova ejecta taking into account time-dependent effects and X-ray irradiation. We find that the interaction with a typical red supergiant wind should result in the enhanced excitation of the outer layers of unshocked ejecta and the emergence of corresponding HV absorption, i.e. a depression in the blue absorption wing of Halpha and a pronounced absorption of He I 10830 A at a radial velocity of about -10,000 km/s. We identify HV absorption in Halpha and He I 10830 A lines of SN 1999em and in Halpha of SN 2004dj as being due to this effect. The derived mass loss rate is close to 10^{-6} Msun/yr for both supernovae, assuming a wind velocity 10 km/s. We argue that, in addition to the HV absorption formed in the unshocked ejecta, spectra of SN 2004dj and SN 1999em show a HV notch feature that is formed in the cool dense shell (CDS) modified by the Rayleigh-Taylor instability. The CDS results from both shock breakout and radiative cooling of gas that has passed through the reverse shock wave. The notch becomes dominant in the HV absorption during the late photospheric phase, ~60 d. The wind density deduced from the velocity of the CDS is consistent with the wind density found from the HV absorption produced by unshocked ejecta.Comment: 38 pages, 12 figures, ApJ, in pres

Сіверські князі Наримунтовичі

У статті досліджується генеалогія та діяльність сіверських Наримунтовичів – однієї з гілок литовсько-руської династії Гедиміновичів, представники якої наприкінці XIV – початку XV ст. володіли кількома удільними князівствами у Сіверській землі.В статье исследуется генеалогия и деятельность северских Наримунтовичей – одной из ветвей литовско-русской династии Гедиминовичей, представители которой в конце XIV – начале XV вв. владели несколькими удельными княжествами в Северской земле.The article deals with genealogy and activity of the seversky Narimuntoviches as one of branches of the Lithuanian-Russian dynasty of the Gediminoviches, representatives of which at the end XIV – beginning of XV centuries owned some specific principalities in the Seversky land