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The Nebula Around the Post-AGB Star 89 Herculis

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The Nebula Around the Post-AGB Star 89 Herculis A&A 468, L45–L48 (2007) Astronomy DOI: 10.1051/0004-6361:20066969 & c ESO 2007 Astrophysics Extended baselines for the IRAM Plateau de Bure interferometer: First results Special feature Letter to the Editor The nebula around the post-AGB star 89 Herculis V. Bujarrabal1,H.VanWinckel2,R.Neri3,J.Alcolea4, A. Castro-Carrizo3,andP.Deroo2 1 Observatorio Astronómico Nacional (OAN-IGN), Apartado 112, 28803 Alcalá de Henares, Spain e-mail: [email protected] 2 Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium e-mail: [Pieter.Deroo;Hans.VanWinckel]@ster.kuleuven.be 3 Institut de Radio Astronomie Millimétrique (IRAM), 300 rue de la Piscine, 38406 St. Martin d’Hères, France e-mail: [ccarrizo;neri]@iram.fr 4 Observatorio Astronómico Nacional (OAN-IGN), C/ Alfonso XII 3, 28014 Madrid, Spain e-mail: [email protected] Received 19 December 2006 / Accepted 24 February 2007 ABSTRACT Aims. We aim to study the structure of the nebula around the post-AGB, binary star 89 Her. The presence of a rotating disk around this star had been proposed but not been yet confirmed by observations. Methods. We present high-resolution PdBI maps of CO J = 2−1and1−0. Properties of the nebula are directly derived from the data and model fitting. We also present N-band interferometric data on the extent of the hot dust emission, obtained with the VLTI. Results. Two nebular components are found: (a) an extended hour-glass-like structure, with expansion velocities of ∼7kms−1 and a −3 total mass ∼3 × 10 M, and (b) an unresolved very compact component, smaller than ∼0. 4 and with a low total velocity dispersion of ∼5kms−1. We cannot determine the velocity field in the compact component, but we argue that it can hardly be in expansion, since this would require too recent and too sudden an ejection of mass. On the other hand, assuming that this component is a Keplerian disk, we derive disk properties that are compatible with expectations for such a structure; in particular, the size of the rotating gas disk should be very similar to the extent of the hot dust component from our VLTI data. Assuming that the equator of the extended nebula coincides with the binary orbital plane, we provide new results on the companion star mass and orbit. Key words. stars: AGB and post-AGB – stars: winds, outflows – radio lines: stars – stars: individual: 89 Her 1. Introduction and, in a review paper of de Ruyter et al. (2006), fifty one ob- jects are listed. These features are systematically found to be Planetary and protoplanetary nebulae (PNe, PPNe) very often associated to binarity, which would explain the excess of angu- show axisymmetric shapes and fast axial expansion, which are lar momentum needed to form Keplerian disks from previously thought to be due to shock interaction between the very col- ejected gas. limated post-AGB jets and the slow and isotropic AGB wind. The existence of rotating disks around post-AGB stars is Theoretical calculations show that accretion from rotating disks therefore a basic question to understand the post-AGB evolution, onto the post-AGB star or a companion can explain the axial jets in particular, the shape and dynamics of PPNe. (see Soker 2002; Frank & Blackman 2004, etc.) and provide the Disks or tori of molecular gas around post-AGB stars are high energy and momentum required to explain the PPN dynam- ics (Bujarrabal et al. 2001). On the other hand, the presence of commonly detected as the central part of PPNe, although in gen- eral they are not observed to rotate, but to be in expansion, a large amount of orbiting material has been invoked to explain like the rest of the nebula. It has been suggested (e.g. Fong some remarkable observational properties: 1) the peculiar abun- dances sometimes found in the atmospheres of post-AGB stars, et al. 2006) that wide oblate structures found around a few AGB stars could be in rotation, but the evidence is very controversial where refractory elements are deficient, presumably because of (Josselin et al. 2000; Hirano et al. 2004, etc). Direct detection of reaccretion of material in which grains have been efficiently formed (removing such elements from the gas) and expelled a Keplerian velocity field has been obtained for only one PPN till now: the Red Rectangle (Bujarrabal et al. 2005), a well-known (Van Winckel 2003); and 2) the detection of a NIR excess due to PPN surrounding a binary star. dust at temperatures close to that of sublimation, which should correspond to stable structures (i.e. in Keplerian rotation) close 89 Her (HR 6685) was first studied by Bidelman (1951), who to the star. Such SEDs are rather common among post-AGB stars noted the presence of a presumably massive supergiant at high Galactic latitude. There is now, however, general consensus that Based on observations carried out with the IRAM Plateau de Bure 89 Her is a low-mass object in its post-AGB phase of evolution. Interferometer, as well as on observations of the Belgian Guaranteed It is one of the few post-AGB stars with a significant Hipparcos / 2.3 time on VISA (ESO). IRAM is supported by INSU CNRS (France), parallax, placing it at 10.6 kpc from the Sun. Interestingly, 89 Her MPG (Germany) and IGN (Spain). is a binary system that shows both a conspicuous NIR excess, Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20066969 L46 V. Bujarrabal et al.: The nebula around the post-AGB star 89 Herculis Fig. 1. Channel maps of the 12CO J = 2−1(upper panels)andJ = 1−0 lines (lower panels) from 89 Her. The first contours are, respectively, 15 and 30 mJy; the contours are separated by a factor 2 and the negative contours (at −15 and −30 mJy, respectively) are indicated by dashed lines. The LSR velocity in km s−1 for the center of each channel is indicated in the upper left corner. The J2000 coordinates for the reference position, the cross in the maps, are RA 15:55:25.19 and Dec +26:03:00.0. The black ellipses in the last panels indicate the beam half-intensity sizes. due to hot dust (Waters et al. 1993; de Ruyter et al. 2006), and The line emission was resampled to an effective velocity a far-IR flux indicating the presence of large grains (Shenton resolution of 1 km s−1 yielding a one-sigma sensitivity level et al. 1995). Its CO lines show a peculiarly sharp profile very of ∼9.3mJy/beam for the 12CO(1−0) and of ∼15.3mJy/beam similar to the one detected in the Red Rectangle (Bujarrabal et al. for the 12CO(2−1) line. The continuum sensitivities were es- 2001). Previous interferometric maps of the CO emission from timated from the line-free emission regions of the bands to 89 Her (Alcolea & Bujarrabal 1995; Fong et al. 2006) have not be ∼0.3mJy/beam at 2.6 mm and ∼0.5mJy/beam at 1.3 mm. yielded conclusive results, due to the small size and low intensity Continuum emission was well-detected at 2.6 mm and 1.3 mm of the source. A roughly rounded structure was found, showing a but remained unresolved, yielding flux densities of 2.7 mJy central core plus an extended clumpy halo, with no clear velocity and 9.2 mJy, respectively, compatible with thermal dust emis- pattern. sion. The size of the continuum source is smaller than 0. 3. 2. Observations 2.1. VLTI data We report on high-resolution interferometry with the IRAM We also performed N-band interferometric observations using / Plateau de Bure Interferometer of the 12CO(1−0) and 12CO(2−1) the two-beam VLTI MIDI instrument. Three set-ups were used lines and continuum emission from 89 Her. Observations were in April 2006 during Belgian guaranteed time on the auxil- carried out under excellent atmospheric conditions (pwv = iary telescopes (VISA). For one of the observations, the base- = 0.5−2 mm, seeing 0. 1−0. 4) with the six-element array in the line of 16 m (projected angle 78 degrees) was used, while D configuration (November 11, 2005) and in the newly extended the other two were conducted on a larger baseline of 32 m A configuration (January 19 and 25, 2006). The cross-correlator with similar projected angles (80, 77 degrees). The visibili- was set up to cover the lines with 20 and 40 MHz bands each and ties were calibrated using observations of unresolved calibra- the continuum emission with two 320 MHz bands. The flux cal- tion stars (HD 168454 and HD 123139), which were observed in ibration was referred to MWC349 and 3C273, while phase and strict concatenation with the science target. These observations amplitude calibrations were done using 1751+288. were performed in the context of a large program to systemati- Data were calibrated in the antenna-based manner using the cally study the circumstellar discs around binary post-AGB stars. GILDAS1 software package. Velocity maps were produced with With the 1.8 m auxiliary telescopes, the sensitivity of MIDI al- natural weights at 2.6 mm in order to analyze the extended lowed fringe detection on a correlated flux larger than 20 Jy µ molecular distribution and by applying an inverse Gaussian ta- at 12 m. For the details on the data reduction, we refer the per at 1.3 mm to analyze in detail the compact emission in reader to Deroo et al.
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