Article

# Detection of an orbiting gas disk in the Red Rectangle

Authors:
• Observatorio Astronómico Nacional, Spain
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## Abstract

We present interferometric maps of CO emission in the Red Rectangle, a well known protoplanetary nebula. The CO emission is found to arise from a relatively thin equatorial disk, extending about 5\arcsec in the direction perpendicular to the symmetry axis of the optical nebula. The velocity dispersion of the emission clearly increases towards the center, in a pattern significantly coincident with that expected for a Keplerian velocity field. Modeling of the CO maps confirms that the emitting gas is probably rotating around the central star(s), with a Kepler-like velocity distribution (at least in the central regions) that would correspond to a central mass ~ 0.9 Msun. Other possible explanations to the observations are discussed, but are found to be unlikely. Our models also suggest that the density and temperature increase towards the center roughly proportionally to the inverse radius. The asymmetry observed in the line profile and intensity distribution (the red part being stronger) can be explained by self-absorption if, superimposed to the rotation velocity, there is a low radial expansion at a velocity of about 0.4 km s-1, at least in the outer disk regions. This is the first probable detection of a gas disk in Kepler-like rotation around a post-AGB star. Based on observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).

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... The Red Rectangle is the prototype of this class of post-AGB stars and indeed is the first one where a disk in Keplerian rotation was spatially and spectrally resolved with IRAM-NOEMA (formerly known as Plateau de Bure Interferometer) by Bujarrabal et al. [35]. The disk of the Red Rectangle, which has been accurately modeled by these authors, has large dimensions, with an outer radius of about 2000 AU, and surrounds the central binary system, for which a central mass of about 1.5 M was deduced. ...
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... We adopt three different scenarios in order to take into account the filling factor of the narrow source in the IRAM beam of the HCO + transitions: a face-on disk of 5″ diameter, a Gaussian circular distribution with 5″ fwhm (full width at half maximum), and a Gaussian elliptical distribution with 5″ × 1″ fwhm. Using the excitation conditions derived from CO interferometric observations with high spatial resolution by Bujarrabal et al., 119 we derive an upper limit for the column density of 4 × 10 11 cm −2 by using a large velocity gradient (LVG) model with a kinetic temperature for the gas of T = 40 K and a molecular hydrogen density of 6 × 10 6 cm −3 . Given the small source size and the large densities, the absence of HCO + (3−2) emission is providing the most strict constraint on the HCO + column density. ...
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An investigation into the dissociative recombination process for H(13)CO(+) using merged ion-electron beam methods has been performed at the heavy ion storage ring CRYRING, Stockholm, Sweden. We have measured the branching fractions of the different product channels at ~0 eV collision energy to be the following: CO + H 87 ± 2%, OH + C 9 ± 2% and O + CH 4 ± 2%. The formation of electronically excited CO in the dominant reaction channel has also been studied, and we report the following tentative branching fractions for the different CO product electronic states: CO(X (1)Σ(+)) + H: 54 ± 10%, CO(a (3)Π) + H: 23 ± 4% and CO(a' (3)Σ(+)) + H: 23 ± 4%. The absolute cross section between ~2 - 50 000 meV was measured and showed resonance structures between 3 - 15 eV. The cross section was fitted in the energy range relevant to astrophysics, i.e. between 1-300 meV and was found to follow the expression σ = 1.3 ± 0.3 × 10(-16) E(-1.29±0.05) cm(2) and the corresponding thermal rate constant was determined to be k(T) = 2.0 ± 0.4 × 10(-7)(T/300)(-0.79±0.05) cm(3)s(-1). Radio astronomical observations with the IRAM 30 m telescope of HCO(+) towards the Red Rectangle yielded an upper column density limit of 4×10(11) cm(-2) of HCO(+) at the 1σ level in that object, indicating that previous claims that the dissociative recombination of HCO(+) plays an important role in the production of excited CO molecules emitting the observed Cameron-bands in that object are not supported.
... From this, disk sizes (outer radii) of a few 100-1000 au were inferred (de Ruyter et al. 2006). CO rotational mapping has confirmed the Keplerian rotation in a sample of systems (Bujarrabal et al. 2013a) and succeeded to resolve the large scale gas disks and their kinematics in several systems (Bujarrabal et al. 2003(Bujarrabal et al. , 2013b(Bujarrabal et al. , 2015(Bujarrabal et al. , 2016(Bujarrabal et al. , 2017(Bujarrabal et al. , 2018. Only one similar disk around the central star of the Red Rectangle (HD 44179) could be imaged in scattered light with the Hubble Space Telescope (HST) (Osterbart et al. 1997;Cohen et al. 2004). ...
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Circumbinary disks are common around post-asymptotic giant branch (post-AGB) stars with a stellar companion on orbital timescales of a few 100 to few 1000 days. The presence of a disk is usually inferred from the system's spectral energy distribution and confirmed, for a sub-sample, by interferometric observations. We used the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument on the Very Large Telescope to obtain extreme adaptive optics assisted scattered light images of the post-AGB binary system AR Puppis. Data have been obtained in the V, I, and H bands. Our observations have produced the first resolved images of AR Puppis's circumbinary disk and confirm its edge-on orientation. In our high-angular-resolution and high-dynamic-range images we identify several structural components such as a dark mid-plane, the disk surface, and arc-like features. We discuss the nature of these components and use complementary photometric monitoring to relate them to the orbital phase of the binary system. Because the star is completely obscured by the disk at visible wavelengths, we conclude that the long-term photometric variability of the system must be caused by variable scattering, not extinction, of starlight by the disk over the binary orbit. Finally, we discuss how the short disk lifetimes and fast evolution of the host stars compared to the ages at which protoplanetary disks are typically observed make systems like AR Puppis valuable extreme laboratories to study circumstellar disk evolution and constrain the timescale of dust grain growth during the planet formation process. © 2019. The American Astronomical Society. All rights reserved..
... Part of the low-velocity molecular emission in PPNs may also arise in a compact rotating disk around the star; these disks have been proposed to be the main agents for the launch and collimation of pAGB winds (Soker 2002;Frank & Blackman 2004, and references therein). To date, the presence of a compact disk in slow Keplerian rotation has been confirmed in one PPN, the Red Rectangle (Bujarrabal et al. 2003). ...
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... In most astrophysical objects, however, from AGNs to young stellar objects, the accretion disk blows collimated winds, or jets, but not biconical flows. The large cold disk around the binary stellar system HD 44179 at the center of the Red Rectangle (e.g., Jura & Kahane 1999; Dominik et al. 2003; Bujarrabal et al. 2003) is not expected to blow a wind energetic enough to form the bipolar structure of the Red Rectangle. (2) The simulations of Icke (2003) do not reproduce the exact structure of the Red Rectangle. ...
Article
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... In contrast to most pPNe, these objects show narrow CO emission profiles consistent with rotating disks [6]. The prototype of this class of post-AGB objects is the Red Rectangle, which was the first source in which a Keplerian disk was directly confirmed and spatially resolved with interferometric CO emission observations [7]. Modelling of these data enabled a detailed characterization of the structure and kinematics of the disk, as well as an estimate of the mass of the central binary system of ∼1.5 M . ...
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Article
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We also present the optical detection of the circumbinary disk of the RR in the light of the BL. The total intensities from the two optical band emissions (BL and ERE) when summed over the nebula are of comparable magnitude. Their spatial distributions with respect to the embedded illumination sources lead us to suggest that they may be attributed to different ionization stages of the same family of carriers. Article Full-text available HD 44179 is the binary, post-asymptotic giant branch, central star of the Red Rectangle Nebula. Echelle spectra of the star have been obtained over a wavelength range from about 3800 to 10000 Å at a resolving power R = 38,000. A maximum S/N of 850 was achieved near 6800 Å. Fifty-seven identified atomic or ionic emission lines of 12 elements are detected in the star's spectrum, along with 76 emission lines of CH, CH+, or CN. Three other CN lines are also present in absorption. Fewer than 30 of these 136 lines apparently have been previously reported, and the newly detected species include N II, Mg I, S II, K I, Fe I, Fe II, Rb I, Ba II, CH, and CN. On the basis of their shapes and widths, the line profiles of the various species can be classified into three groups: narrow, broad, or double-peaked. The emission apparently originates in an unusual, compact H II region and in associated neutral gas, both concentrated primarily within the small, dusty torus that optically obscures the central star. The detection of the Ba II emission from the gas may support the hypothesis of Waelkens et al. that HD 44179 is destined to become a barium star. Article This paper reports the results of a small imaging survey of eight evolved stars including two AGB stars (IRC +10216 and Mira), five proto-planetary nebula (PPN) candidates (AFGL 2688, IRAS 22272+5435, HD 161796, 89 Her, and HD 179821), and a planetary nebula (PN, NGC 7027). We present high-resolution 12CO J = 1 → 0 maps of their full molecular envelopes made by combining BIMA Millimeter Array and NRAO 12 m telescope observations. For the PPNe and PN, the neutral molecular envelopes are compared with images taken at optical, near-IR, and mid-IR wavelengths. Drawing from the literature, we augmented our BIMA survey sample to 38 well-studied sources with CO emission maps. We classified this sample of sources based on the kinematics and morphologies of the CO emission into three types: spherical/elliptical/shell sources, disk sources, and structured outflow sources. Confirming previous studies, we find strong evidence for the photodissociation of the molecular envelope as an object evolves from the AGB to PN stages. While the spherical AGB stars follow theoretical expectations for mass-loss rate versus envelope size, the post-AGB structured outflow sources have significantly higher mass-loss rates than expected probably because of their recent superwinds. We find evidence that the structured outflows are clearly younger than the AGB wind. The disk sources have little correlation between mass-loss rate and envelope size because their properties are determined more by the properties of the central stars and disk evolution than by the mass-loss rate history that shapes the spherical and structured-outflow sources. Article Molecular line emission in the mm- and submm-wave domains is found to be a very useful tool to study planetary nebulae (PNe). Molecular lines, particularly the low-$J$transitions of carbon monoxide, are known to probe most of the nebular material in young PNe and protoplanetary nebulae (PPNe). Many quantitative results on these objects have been so obtained, including general structure, total mass and density distribution, kinetic temperatures, velocity fields, etc. In more evolved PNe, however, the molecular line intensity (or even detection) is quite uncertain, due to photodissociation. Molecular lines often trace just certain parts of these nebulae, like equatorial tori or axial clumps. Other molecules are in general more affected by photodissociation than CO, but photo- or shock-induced chemistry produces high abundances of radicals and large molecules in some intermediate-evolution objects. The presence of rotating disks in some objects is finally discussed. Article We investigate the circumstellar dust properties of the oxygen-rich bipolar proto-planetary nebula IRAS 18276-1431 by means of two-dimensional radiative transfer simulations of the circumstellar dust shell. The model geometry is assumed to have a torus and an envelope. The parameters of the dust and the dust shell are constrained by comparing the SED and NIR intensity and polarisation data with the models. The polarisation in the envelope reaches 50 -- 60 % and is nearly constant in the H and K_S bands in the observations. This weak wavelength dependence of the polarisation can be reproduced with a grain size distribution function for the torus: 0.05 micron <= a with n(a)=a^{-(p=5.5)}exp(-a/{a_c=0.3 micron}). The power index p is significantly steeper than that for interstellar dust. Similar results have also been found in some other PPNs and suggest that mechanisms that grind down large particles may also have acted when the dust particles formed. The spectral opacity index beta is found to be 0.6+/-0.5 from the millimeter fluxes. This low value indicates the presence of large dust grains in the torus. We discuss two possible dust models for the torus. One has a size distribution function of 1.0 micron <= a <= a_max=5,000.0 micron with n(a)=a^{-(p=2.5)} and the other is 1.0 micron <= a <= a_max=10,000.0 micron with n(a)=a^{-(p=3.5)}. The former has beta of 0.633, but we are not able to find reasonable geometry parameters to fit the SED in the infrared. The latter has beta of 1.12, but reproduces the SED better over a wide wavelength range. With this dust model, the geometric parameters are estimated as follows: the inner and outer radii are 30 AU and 1000 AU and the torus mass is 3.0 M_sun. Assuming an expansion velocity of 15 kms^{-1}, the torus formation time and mass-loss rate are found to be \sim300 yrs and \sim10^{-2}M_sun yr^{-1} respectively. Article Aims: We combined bispectrum speckle interferometry, adaptive optics (AO) imaging polarimetry, and radiative transfer modeling of polarized light to derive various physical properties of the proto-planetary nebula Frosty Leo. Methods: We performed bispectrum K'-band speckle interferometry and H- and K-band imaging polarimetry of Frosty Leo using the ESO 3.6 m telescope and the AO-equipped CIAO instrument of the 8 m Subaru telescope, respectively. Two-dimensional radiative transfer modeling was carried out in order to obtain a quantitative interpretation of our observations. Results: Our diffraction-limited speckle image shows distinct hourglass-shaped, point-symmetric bipolar lobes, an equatorial dust lane, and complex clumpy structures in the lobes. Our polarimetric data display a centro-symmetric polarization vector pattern with P~30-50% in the bipolar lobes and a polarization disk between them. The polarization images also reveal an elongated region with low polarization along a position angle of -45°. The observations suggest that this region has a low dust density and was carved out by a jet-like outflow. Our radiative transfer modeling can simultaneously explain the observed spectral energy distribution, the intensity distribution of the hourglass-shaped lobes, and our polarization images if we use two grain species with sizes of 0.005 Article Full-text available In this 14th edition of ApXX,1 we bring you the Sun (§ 2) and Stars (§ 4), the Moon and Planets (§ 3), a truly binary pulsar (§ 5), a kinematic apology (§ 6), the whole universe (§§ 7 and 8), reconsideration of old settled (§ 9) and unsettled (§ 10) issues, and some things that happen only on Earth, some indeed only in these reviews (§§ 10 and 11). Article High spatial and spectral resolution spectroscopy of the OH/IR supergiant VY CMa and its circumstellar ejecta reveals evidence for high mass loss events from localized regions on the star occurring over the past 1000 yr. The reflected absorption lines and the extremely strong K I emission lines show a complex pattern of velocities in the ejecta. We show that the large, dusty northwest arc, expanding at ~50 km s-1 with respect to the embedded star, is kinematically distinct from the surrounding nebulosity and was ejected about 400 yr ago. Other large, more filamentary loops were probably expelled as much as 800-1000 yr ago, whereas knots and small arcs close to the star resulted from more recent events 100-200 yr ago. The more diffuse, uniformly distributed gas and dust is surprisingly stationary, with little or no velocity relative to the star. This is not what we would expect for the circumstellar material from an evolved red supergiant with a long history of mass loss. We therefore suggest that the high mass loss rate for VY CMa is a measure of the mass carried out by these specific ejections accompanied by streams or flows of gas through low-density regions in the dust envelope. VY CMa may thus be our most extreme example of stellar activity, but our results also bring into question the evolutionary state of this famous star. In a separate appendix, we discuss the origin of the very strong K I and other rare emission lines in its spectrum. Article It is firmly established since more than 20 years, that Planetary Nebulae (PNe) are the result of the evolution of the Circumstellar Envelopes (CEs) around intermediate-mass stars in the Asymptotic Giant Branch (AGB), once these stars leave the AGB and become much hotter [16, 18]. However we still have problems in explaining the origin and formation of non spherical PNe, also known as asymmetric PNe, since yet they must be the remnants of the isotropic AGB CEs; how this metamorphosis can happen still is a matter of debate. In order to solve the mystery, in the past 15 yr, a growing attention has been focused on the so called pre-Planetary Nebulae (pPNe). These intermediate stage sources are nebulae around post-AGB stars not hot enough to ionize them. This is not a minor problem since in the last few years we have realized that most PNe and almost all pPNe are not spherically symmetric but strongly bipolar and/or elongated. Article Context. The Red Rectangle proto-planetary nebula (HD 44179) is known for a number of rather narrow emission features superimposed on a broad extended red emission (ERE) covering the 5000-7500 Å regime. The origin of these emission features is unknown. Aims: The aim of the present work is to search for potential carriers by combining new observational and laboratory data. This also allows to interpret spectral emission features in terms of actual physical conditions like temperature and density constraints and to trace chemical processes in the outflows of the Red Rectangle. Methods: Observational spectra have been obtained with the EMMI-NTT at offsets of 3'', 6'', 7'', 11'', 16'' and 20'' distance to the central star HD 44179. The spectra are compared to the outcome of a time-gated laser induced fluorescence laboratory study of an expanding acetylene plasma using a special supersonic pin-hole discharge source. With this set-up the hydrocarbon chemistry in the Red Rectangle nebula is simulated under laboratory controlled conditions. The plasma source has the unique feature to generate electronically and vibrationally excited species at low rotational temperatures. The comparison is facilitated by a simple model for fluorescent emission in the nebula. Results: Two of the astronomically observed narrow emission bands can be assigned as originating from unresolved rovibronic progressions within the d3Pi_g --> a3Pi_u Swan system of the C2 radical. The band appearance corresponds to a rotational temperature between 200 and 1000 K. The emission is driven by absorption in the C2 Phillips bands followed by intersystem crossing from the singlet to the triplet state and pumping in the Swan bands. Conclusions: These observations imply an active (photo)chemistry in the ejecta of the Red Rectangle. Based on observations collected at the European Southern Observatory, Chile. Program ID: 080.C-0814(A). Article Full-text available The close binary system NN Serpentis must have gone through a common envelope phase before the formation of its white dwarf. During this phase, a substantial amount of mass was lost from the envelope. The recently detected orbits of circumbinary planets were suggested to be inconsistent with planet formation before the mass loss. We explore whether new planets may have formed from the ejecta of the common envelope, and derive the expected planetary mass as a function of radius. We employ the model of \citet{Kashi11} to estimate the amount of mass that is retained during the ejection event, and infer the properties of the resulting disk from the conservation of mass and angular momentum. The resulting planetary masses are estimated from models with and without radiative feedback. We show that the observed planetary masses can be reproduced for appropriate model parameters. Photoheating can stabilize the disks in the interior, potentially explaining the observed planetary orbits on scales of a few AU. We compare the expected mass scale of planets for 11 additional systems with observational results and find hints for two populations, one consistent with planet formation from the ejecta of common envelopes, and a separate population that may have formed earlier. The formation of the observed planets from the ejecta of common envelopes seems feasible. The model proposed here can be tested through refined observations of additional post-common envelope systems. While it appears observationally challenging to distinguish between the accretion on pre-existing planets and their growth from new fragments, it may be possible to further constrain the properties of the protoplanetary disk through additional observations of current planetary candidates and post-common envelope binary systems. Article To understand the evolution of planetary systems, it is important to investigate planets in highly evolved stellar systems, and to explore the implications of their observed properties with respect to potential formation scenarios. Observations suggest the presence of giant planets in post-common-envelope binaries (PCEBs). A particularly well-studied system with planetary masses of 1.7 M_J and 7.0 M_J is NN Ser. We show here that a pure first-generation scenario where the planets form before the common envelope (CE) phase and the orbits evolve due to the changes in the gravitational potential is inconsistent with the current data. We propose a second-generation scenario where the planets are formed from the material that is ejected during the CE, which may naturally explain the observed planetary masses. In addition, hybrid scenarios where the planets form before the CE and evolve due to the accretion of the ejected gas appear as a realistic possibility. Article We present new optical broad-band (UBVRI) aperture polarimetric observations of 53 post-asymptotic giant branch (AGB) stars selected to exhibit a large near-infrared excess. 24 out of the 53 stars (45% of our sample) are presented for the first time. A statistical analysis shows four distinctive groups of polarized post-AGB stars: unpolarized or very lowly polarized (degree of polarization or DoP < 1%), lowly polarized (1% < DoP < 4%), moderately polarized (4% < DoP < 8%) and highly polarized (DoP > 8%). 23 out of the 53 (66%) belong to the first group, 10 (19%) to the second, five (9%) to the third and only three (6%) to the last group. Approximately, 34% of our sample was found to be unpolarized objects, which is close to the percentage of round planetary nebulae. On average, the low and moderate groups show a wavelength-dependent polarization that increases towards shorter wavelength, implying an intrinsic origin of the polarization, which signifies a Rayleigh-like scattering spectrum typical for non-symmetrical envelopes composed principally of small dust grains. The moderately polarized stars exhibit higher K-W3 and W1-W3 colour indices compared with the group of lowly polarized stars suggesting a possible relation between DoP and mass-loss rate. Moreover, they are found to be systematically colder (redder in B-V), which may be associated with the condensation process close to these stars that results in higher degree of polarization. We also provide evidence that multiple scattering in optically thin polar outflows is the mechanism that gives high DoP in post-AGB stars with a bipolar or multi-polar envelopes. Article Full-text available We present the first mid-IR long baseline interferometric observations of the circumstellar matter around binary post-AGB stars. Two objects, SX Cen and HD 52961, were observed using the VLTI/MIDI instrument during Science Demonstration Time. Both objects are known binaries for which a stable circumbinary disc is proposed to explain the SED characteristics. This is corroborated by our N-band spectrum showing a crystallinity fraction of more than 50 % for both objects, pointing to a stable environment where dust processing can occur. Surprisingly, the dust surrounding SX Cen is not resolved in the interferometric observations providing an upper limit of 11 mas (or 18 AU at the distance of this object) on the diameter of the dust emission. This confirms the very compact nature of its circumstellar environment. The dust emission around HD 52961 originates from a very small but resolved region, estimated to be ~ 35 mas at 8 micron and ~ 55 mas at 13 micron. These results confirm the disc interpretation of the SED of both stars. In HD 52961, the dust is not homogeneous in its chemical composition: the crystallinity is clearly concentrated in the hotter inner region. Whether this is a result of the formation process of the disc, or due to annealing during the long storage time in the disc is not clear. Comment: 12 pages, 10 figures, accepted for publication in A & A Article Full-text available We present ~ 0.3 milliarcsec-resolution maps of the SiO (v=2, J=1-0) maser emission in the bipolar post-AGB nebula OH231.8+4.2 obtained with the Very Long Baseline Array. These observations have provided for the first time the structure and kinematics of the close stellar environment in a proto-Planetary Nebula. Our observations reveal the SiO maser emission arising in several bright spots of less than ~ 1013 cm in size forming a structure elongated in the direction perpendicular to the symmetry axis of the nebula. Such a distribution is consistent with an equatorial torus with a radius of ~ 6 AU around the central star. A complex velocity gradient is found along the torus, which suggests rotation and infall of material towards the star. The rotation and infalling velocities deduced are of the same order and range between ~ 7 and ~ 10 km s-1. From our data, we estimate the mass of the SiO torus and the central star, as well as a stringent upper limit to the present stellar mass-loss rate. Based on observations with the Very Large Baseline Array of the The National Radio Astronomy Observatory (NRAO). NRAO is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. Article Full-text available We report high signal-to-noise ratio radio CO emission from the circumstellar envelope around the Red Rectangle. The CO (2-1) line displays a narrow central spike with an FWHM ˜ 2 km s-1, which is much narrower than the circumstellar CO lines typically found around most mass-losing red giants. In addition, instead of the sharp edges expected for a spherical envelope, the lines show extended blueshifted and redshifted wings, with a full width at zero intensity of 12 km s-1, consistent with a bipolar distribution. The circumstellar CO traces the center of mass of the system rather than the binary motion of HD 44179, the optically visible star in the Red Rectangle. There appears to have been significant de-acceleration of the gas ejected from the mass-losing star; the observed CO emission may result from a bipolar outflow from a disk that is viewed edge-on. We suggest that much of the material around the Red Rectangle resides in a long-lived configuration, such as a gravitationally bound disk. Because the material may dwell for a long time near the binary system, the resulting substantial processing and evolution may explain why the circumstellar matter is so different around the Red Rectangle compared to that around AFGL 2688, another post-main-sequence bipolar nebula. We speculate that large carbon particles similar to the grains up to 20 microns in diameter found in the solar system might grow around the Red Rectangle. Article Full-text available We present high-resolution ^13^CO J=1-0 maps of the protoplanetary nebula M 1-92, Minkowski's Footprint, obtained with the IRAM interferometer at Plateau de Bure. Previous ^12^CO J=1-0 maps and single dish observations of the J=1-0, J=2-1, and J=3-2 transitions in both isotopic substitutions are also discussed. The cartography confirms the axial symmetry and complex structure already found from ^12^CO data. The gas velocity presents a dominant axial component that increases in absolute value from the center, up to a (deprojected) velocity of 70km/s. Most of the observed emission is at velocities clearly above the expected AGB expansion kinematics. This fact, together with a remarkable continuity found along the axis in the structure and velocity of the nebula, is interpreted as showing that the present CO nebula has been shaped by momentum transport from the fast post-AGB flow to the rest of the (AGB) envelope, probably by means of a bow-like shock. The comparison of the different transitions is used to deduce the physical conditions in the molecular gas. Most of the observed (post-shock) material is found to have a low temperature of about 15K, indicating that the cooling processes are efficient after the passage of the shock. Typical densities of 3x10^4^-2x10^5^cm^-3^ are deduced, corresponding to a total gas mass of about 1Msun_. This high value shows that most of the nebular material is molecular and probed by the CO observations. Therefore, our maps effectively represent the disruption of the AGB envelope by the passage of a shock, allowing a description of the physical conditions present in the nebula during this evolutionary phase. Article Full-text available We have studied the CO emission from protoplanetary nebulae (PPNe). Our sample is composed of 37 objects and includes, we think, all well identified PPNe detected in CO, together with the two yellow hypergiants emitting in CO and one young PN. We present a summary of the existing CO data, including accurate new observations of the$^{12}$CO and$^{13}$CO$J=1$-0 and$J=2$-1 lines in 16 objects. We identify in the nebulae a slowly expanding shell (represented in the spectra by a central core) and a fast outflow (corresponding to the line wings), that in the well studied PPNe is known to be bipolar. Excluding poor data, we end up with a sample of 32 sources (including the 16 observed by us); fast flows are detected in 28 of these nebulae, being absent in only 4. We present a method to estimate from these data the mass, "scalar" momentum and kinetic energy of the different components of the molecular outflows. We argue that the uncertainties of our method can hardly lead to significant overestimates of these parameters, although underestimates may be present in not well studied objects. The total nebular mass is often as high as ~1$M_{\odot}$, and the mass-loss rate, that (presumably during the last stages of the AGB phase) originated the nebula, had typical values ~10$^{-4}M_{\odot}$yr$^{-1}$. The momentum corresponding to this mass ejection process in most studied nebulae is accurately coincident with the maximum momentum that radiation pressure, acting through absorption by dust grains, is able to supply (under expected conditions). We estimate that this high-efficiency process lasts about 1000-10 000 yr, after which the star has ejected a good fraction of its mass and the AGB phase ends. On the other hand, the fast molecular outflows, that have probably been accelerated by shock interaction with axial post-AGB jets, carry a significant fraction of the nebular mass, with a very high momentum (in most cases between 10$^{37}$and 10$^{40}$g cm s$^{-1}$) and very high kinetic energy (usually between 10$^{44}$and 10$^{47}\$ erg). In general, yellow hypergiants and post-AGB objects with low initial mass show nebular masses and momenta that are, respectively, higher and lower than these values. We compare the momenta of the fast outflows with those that can be supplied by radiation pressure, taking into account the expected short acceleration times and some effects that can increase the momentum transfer. We find that in about 80% of PPNe, the fast molecular flows have too high momenta to be powered by radiation pressure. In some cases the momentum of the outflow is ~1000 larger than that carried by radiation pressure; such high factors are difficult to explain even under exceptional conditions. Wind interaction is the basic phenomenon in the PN shaping from the former AGB envelopes; we conclude that this interaction systematically takes place along a dominant direction and that this process is not powered by radiation pressure. Due to the lack of theoretical studies, the possible momentum source remains a matter of speculation.
Article
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The structure and dynamics of the molecular envelope of M2-56 proplanetary nebula (PPN) were analyzed. High-resolution maps of the emission of 12CO J = 2-1 and J = 1-0 in M2-56 were presented. Maps showed a bipolar, molecular nebula that extended ∼28″ along the symmetry axis. The nebula was found to be composed of two contiguous, incomplete shells located along the symmetry axis which had an inclination of ∼17° with respect to the plane of the sky. It was concluded that the momentum won by the molecular gas in the post-asymptotic giant branch (AGB) phase was much higher than the maximum momentum that the stellar radiation could carry in a typical PPN life.
Article
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We present the Spectral Energy Distribution of HR 4049 based on literature data and new continuum measurements at 850 mum. The SED shows variable absorption in the UV, and a large IR excess, both caused by circumstellar dust. The shape of the IR excess from 1 mum all the way down to 850 mum can be nearly perfectly fitted with a single blackbody function at T approximate to1150 K or alternatively with a sum of blackbodies in a narrow temperature range. The energy emitted in this IR continuum radiation is about one-third of the stellar luminosity. We show that this blackbody radiation must be due to the presence of a circumbinary disk with a large height. This disk must also be gas-rich, in agreement with the observations of molecular bands in the ISO-SWS spectrum. We present two possible scenario's for explaining the shape and the intensity of the IR excess. The first scenario involves large grains (a greater than or equal to 1 mm) that each radiate like a blackbody. The second scenario argues that the blackbody radiation is due to a very optically thick circumbinary disk. We investigate if such a disk would indeed produce blackbody radiation by presenting results from radiative transfer calculations. We further quantify the properties of such a disk and its stability in the framework of (hydro)dynamics, grain settling, radiation pressure and grain drift. The virtues and shortcomings of both models for the origin of the IR blackbody are discussed by contrasting them with other observations and assessing them in the framework of (binary) (post-)AGB evolution.
Article
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We present new near-infrared (2.1-3.3 mum) images of the Red Rectangle with unprecedented diffraction-limited angular resolutions of 46-68 mas; 4 times higher than that of the Hubble space telescope and almost a factor of two improvement over the previous 6 m SAO telecope speckle images presented by Men'shchikov et al. (1998). The new images, which were reconstructed from Keck telescope speckle data using the bispectrum speckle interferometry method, clearly show two bright lobes above and below the optically thick dark lane obscuring the central binary. X-shaped spikes, thought to trace the surface of a biconical flow, change the intensity distribution of the bright lobes, making them appear broadened or with an east-west double-peak in images with the highest resolution. The striking biconical appearance of the Red Rectangle is preserved on scales from 50 mas to 10 and from the visible (red) to at least 10 mum, implying that large grains of at least several microns in size dominate scattering. The new images supplement previous 76 mas resolution speckle reconstructions at shorter wavelengths of 0.6-0.8 mum (Osterbart et al. 1997) and 0.7-2.2 mum (Men'shchikov et al. 1998), allowing a more detailed analysis of the famous bipolar nebula. The intensity distribution of the images is inconsistent with a at disk geometry frequently used to model the bipolar nebulae. Instead, a geometrically thick torus-like density distribution with bipolar conical cavities is preferred. The extent of the bright lobes indicates that the dense torus has a diameter of greater than or similar to 100 AU, for an assumed distance of 330 pc. This torus may be the outer reaches of a flared thick disk tapering inwards to the central star, however such a density enhancement on the midplane is not strictly required to explain the narrow dark lane obscuring the central stars.
Article
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New diffraction-limited speckle images of the Red Rectangle in the wavelength range 2.1-3.3 mum with angular resolutions of 44-68 mas (Tuthill et al. 2002) and previous speckle images at 0.7-2.2 mum (Osterbart et al. 1997; Men'shchikov et al. 1998) revealed well-resolved bright bipolar outflow lobes and long X- shaped spikes originating deep inside the outflow cavities. This set of high-resolution images stimulated us to reanalyze all infrared observations of the Red Rectangle using our two- dimensional radiative transfer code. The high-resolution images imply a geometrically and optically thick torus-like density distribution with bipolar conical cavities and are inconsistent with the flat disk geometry frequently used to visualize bipolar nebulae. The new detailed modeling, together with estimates of the interstellar extinction in the direction of the Red Rectangle enabled us to more accurately determine one of the key parameters, the distance D approximate to 710 pc with model uncertainties of 70 pc, which is twice as far as the commonly used estimate of 330 pc. The central binary is surrounded by a compact, massive (M approximate to 1.2 M-.), very dense dusty torus with hydrogen densities reaching n(H) approximate to2.5 x 10(12) cm(-3) (dust-to-gas mass ratio rho(d)/rho approximate to 0.01). The model implies that most of the dust mass in the dense torus is in very large particles and, on scales of more than an arcsecond, the polar outflow regions are denser than the surrounding medium. The bright component of the spectroscopic binary HD44179 is a post-AGB star with mass M-star approximate to 0.57 M-star, luminosity L- lozenge approximate to 6000 L-., and effective temperature T 7750 K. Based on the orbital elements of the binary, we identify its invisible component with a helium white dwarf with M-WD approximate to 0.35 M-., L-WD similar to 100 L-., and T-WD similar to 6 x 10(4) K. The hot white dwarf ionizes the low- density bipolar outflow cavities inside the dense torus, producing a small HII region observed at radio wavelengths. We propose an evolutionary scenario for the formation of the Red Rectangle nebula, in which the binary initially had 2.3 and 1.9 M-. components at a separation of similar to130 R-.. The nebula was formed in the ejection of a common envelope after Roche lobe overflow by the present post-AGB star.
Article
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A strong infrared source detected in the AFCRL sky survey is confirmed, and is identified with the binary star HD 44179, embedded in a peculiar nebula. UBVRI and broad-band photometry between 2.2 and 27 microns are combined with blue, red, and near-infrared spectra, polarimetry and spectrophotometry of the star, and a range of direct and image-tube photographs of the nebula, to suggest a composite model of the system. In this model, the infrared radiation derives from thermal emission by dust grains contained in a disklike geometry about the central object, which appears to be of spectral type B9-A0 III and which may be in pre-main-sequence evolution. Two infrared emission features are found, peaking at 8.7 and 11.3 microns, the latter corresponding to the feature seen in the spectrum of the planetary nebula NGC 7027. The complex nebular structure is discussed on the basis of photographs through narrow-band continuum and emission-line filters. The polarization data support the suggestion of a disk containing some large particles. No radio continuum emission is detected.
Article
Full-text available
New diffraction-limited speckle images of the Red Rectangle in the wavelength range 2.1--3.3 microns with angular resolutions of 44--68 mas and previous speckle images at 0.7--2.2 microns revealed well-resolved bright bipolar outflow lobes and long X-shaped spikes originating deep inside the outflow cavities. This set of high-resolution images stimulated us to reanalyze all infrared observations of the Red Rectangle using our two-dimensional radiative transfer code. The new detailed modeling, together with estimates of the interstellar extinction in the direction of the Red Rectangle enabled us to more accurately determine one of the key parameters, the distance D=710 pc with model uncertainties of 70 pc, which is twice as far as the commonly used estimate of 330 pc. The central binary is surrounded by a compact, massive (M=1.2 Msun), very dense dusty torus with hydrogen densities reaching n_H=2.5x10^12 cm^-3 (dust-to-gas mass ratio rho_d/rho~0.01). The bright component of the spectroscopic binary HD 44179 is a post-AGB star with mass M*=0.57 Msun, luminosity L*=6000 Lsun, and effective temperature T*=7750 K. Based on the orbital elements of the binary, we identify its invisible component with a helium white dwarf with Mwd~0.35 Msun, Lwd~100 Lsun, and Twd~6x10^4 K. The hot white dwarf ionizes the low-density bipolar outflow cavities inside the dense torus, producing a small HII region observed at radio wavelengths. We propose an evolutionary scenario for the formation of the Red Rectangle nebula, in which the binary initially had 2.3 and 1.9 Msun components at a separation of 130 Rsun. The nebula was formed in the ejection of a common envelope after Roche lobe overflow by the present post-AGB star. Comment: 20 pages, 10 figures, accepted by Astronomy and Astrophysics, also available at http://www.mpifr-bonn.mpg.de/div/ir-interferometry/publications.html
Article
A theoretical model of the circumstellar envelope which surrounds a OH-IR star is developed. The circumstellar gas is ejected by radiation pressure which acts on dust grains that condense in the atmosphere of the central star. The dust grains transfer momentum to the gas by collisions with the gas molecules. These collisions are the dominant source of heat input to the circumstellar gas. The major sources of cooling are the emission of radiation by H_2O molecules and adiabatic expansion. The gas temperature decreases from T ≈ 2 x 10^3 K near the stellar surface at r ≈ 6 x 10^(13) cm, to T ≈ 8 x 10^2 K at r = 10^(15) cm and to T ≈ 10^2 K at r = 10^(16) cm. The OH molecule abundance in the circumstellar envelope is controlled by chemical exchange reactions and by the dissociation of H^2O molecules. The reaction OH + H_2 ↔ H_2O + H + 0.69 eV, which has an activation energy of 0.3 eV, rapidly converts OH molecules into H_2O molecules in the warm (T ≳ 5 x 10^2 K) inner (r ≾ 2 x 10^(15) cm) region of the circumstellar envelope. Beyond r ≈ 2 x 10^(15) cm, T is so low that the exchange reaction is very slow and the mean lifetime of an OH molecule is greater than the expansion time scale for the circumstellar envelope. In the outer region of the circumstellar envelope, OH molecules are produced from the photodissociation of H_2O molecules by the interstellar ultraviolet radiation and from the dissociation of H_2O molecules by collisions with dust grains. These processes are capable of producing OH number densities greater than 1 cm^(-3) at r ≈ 10^(16) cm. The predicted values of the gas temperature, T, and the OH abundance, n_(OH), depend upon the rate of mass loss from the central star, Ф. The results quoted above are based on a calculation with Ф = 3 x 10^(-5) M_☉ yr^(-1). In general, T varies inversely and n_(OH) varies directly with Ф.
Article
We have mapped the ^12^CO J=1-0 emission in the bipolar planetary nebula M2-9 using the IRAM interferometer. From the maps we were able to investigate in detail the morphology and the kinematics of the molecular gas. The data are best explained by assuming that the molecular gas is concentrated in an expanding, clumpy torus. The torus, which surrounds the nucleus of M2-9, has a mean diameter of about 6". Its symmetry axis is tilted by 17° with respect to the plane of the sky. The de-projected expansion velocity is 7km/s, and its kinematical age is about 2100years assuming a distance of 1kpc. The lower limit for the total mass of the molecular gas is estimated to be 9x10^-3^Msun_, i.e. at least comparable to the ionized mass in the nebula.
Article
A non-spherical dust shell radiative simulation, aiming both at analysing the existing data on evolved stars and at preparing the interpretation of the scheduled high angular resolution imaging, is described. The calculated radiative equilibrium derives from a Monte Carlo method. The exact solution of the transfer is thus computed for different wavelengths. Results are obtained for different modellings with peculiar axisymmetric dust density distributions. The broad band spectra, the brigthness distribution of the shell as a fonction of wavelength and view angle and the corresponding visibility maps, display typical observable properties. In a first attempt the spectrum and the visible image of the Red Rectangle are reproduced with a flared disc.
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Near-infrared, 0.1 arcsec resolution images of the bipolar nebulae Frosty Leo and Red Rectangle have been obtained with an adaptive optics system developed at the University of Hawaii. In both cases evidence is found supporting a binary star formation mechanism for the nebulae.
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Using a detailed radiative transfer analysis, combined with an energy balance equation for the gas, we have performed extensive modelling of circumstellar CO radio line emission from a large sample of optically bright carbon stars. We determine some of the basic parameters that characterize circumstellar envelopes (CSEs), e.g., the stellar mass loss rate, the gas expansion velocity, and the kinetic temperature structure of the gas. The derived mass loss rates span almost four orders of magnitude, from 5E-9 up to 2E-5 solar masses per year, with the median mass loss rate being 3E-7 solar masses per year. We estimate that the estimated mass loss rates are typically accurate to 50% within the adopted circumstellar model. The physical conditions prevailing in the CSEs vary considerably over such a large range of mass loss rates. Among other things, it appears that the dust-to-gas mass ratio and/or the dust properties change with the mass loss rate. We find that the mass loss rate and the gas expansion velocity are well correlated, and that both of them clearly depend on the pulsational period and (with larger scatter) the stellar luminosity. Moreover, the mass loss rate correlates weakly with the stellar effective temperature, in the sense that the cooler stars tend to have higher mass loss rates, but there seems to be no correlation with the stellar C/O-ratio. We conclude that the mass loss rate increases with increased regular pulsation and/or luminosity, and that the expansion velocity increases as an effect of increasing mass loss rate (for low mass loss rates) and luminosity.
Article
We investigate the development of bipolar outflows during the early post-AGB evolution. A sample of 10 OH/IR stars with irregular OH spectra and unusually large expansion velocities is observed at high angular resolution. The sample includes bipolar nebulae (e.g., OH231.8+4.2), bright post-AGB stars (HD 101584) and reflection nebulae (e.g., Roberts 22). The IRAS colour–colour diagram separates the sample into different types of objects. One group may contain the immediate progenitors to the (few) extreme bipolar planetary nebulae. Two objects show colours and chemistry very similar to the planetary nebulae with late IR-[WC] stars. One object is a confirmed close binary. A model is presented consisting of an outer AGB wind which is swept up by a faster post-AGB wind, with either the AGB or post-AGB wind being non-spherically symmetric. The interface of the two winds is shown to exhibit a linear relation between velocity and distance from the star, giving the impression of an accelerating outflow. The OH data confirm the predicted linear velocity gradients, and also reveal torus-like, uniformly expanding components. All sources are discussed in detail using optical/HST images where available. ISO data for Roberts 22 reveal a chemical dichotomy, with both crystalline silicates and PAH features being present. IRAS 16342-3814 shows a dense torus with mass 0.1 M⊙ and density of 108 cm−3; HST data show four point-like sources located symmetrically around the nebula, near the outer edge of the dense torus. Lifetimes for the bipolar OH/IR stars are shown to be in excess of 104 yr, longer than normal post-AGB time-scales. This suggests that the toruses or discs are near-stationary. We suggest that accretion from such a disc slows down the post-AGB evolution. Such a process could explain the link between the long-lived bipolar nebular geometry and the retarded star.
Article
We derive hydrostatic, radiative equilibrium models for passive disks surrounding T Tauri stars. Each disk is encased by an optically thin layer of superheated dust grains. This layer re-emits directly to space about half the stellar energy it absorbs. The other half is emitted inward and regulates the interior temperature of the disk. The heated disk flares. As a consequence, it absorbs more stellar radiation, especially at large radii, than a flat disk would. The portion of the spectral energy distribution contributed by the disk is fairly flat throughout the thermal infrared. At fixed frequency, the contribution from the surface layer exceeds that from the interior by about a factor 3 and is emitted at more than an order of magnitude greater radius. Spectral features from dust grains in the superheated layer appear in emission if the disk is viewed nearly face-on. Comment: 29 LaTeX pages w/ 10 eps. figures, aaspp4.sty, final version with few minor stylistic alterations and 1 content change (section 4.1.1 on GM Aur and non-zero inclination)