C. Sánchez Contreras

A high-resolution line survey of IRC+10216 with Herschel/HIFI. First results: Detection of warm silicon dicarbide (SiC2)

We present the first results of a high-spectral-resolution survey of the carbon-rich evolved star IRC+10216 that was carried out with the HIFI spectrometer onboard Herschel. This survey covers all HIFI bands, with a spectral range from 488 to 1901 GHz. In this letter we focus on the band-1b spectrum, in a spectral range 554.5 − 636.5 GHz, where we identified 130 spectral features with intensities above 0.03 K and a signal–to– noise ratio >5. Detected lines arise from HCN, SiO, SiS, CS, CO, metal-bearing species and, surprisingly, silicon dicarbide (SiC2). We identified 55 SiC2 transitions involving energy levels between 300 and 900 K. By analysing these rotational lines, we conclude that SiC2 is produced in the inner dust formation zone, with an abundance of ∼2×10−7 relative to molecular hydrogen. These SiC2 lines have been observed for the first time in space and have been used to derive an SiC2 rotational temperature of ∼204 K and a source-averaged column density of ∼6.4×1015 cm−2. Furthermore, the high quality of the HIFI data set was used to improve the spectroscopic rotational constants of SiC2.We present the first results of a high-spectral-resolution survey of the carbon-rich evolved star IRC+10216 that was carried out with the HIFI spectrometer onboard Herschel. This survey covers all HIFI bands, with a spectral range from 488 to 1901 GHz. In this letter we focus on the band-1b spectrum, in a spectral range 554.5−636.5 GHz, where we identified 130 spectral features with intensities above 0.03 K and a signal-tonoise ratio >5. Detected lines arise from HCN, SiO, SiS, CS, CO, metal-bearing species and, surprisingly, silicon dicarbide (SiC2). We identified 55 SiC2 transitions involving energy levels between 300 and 900 K. By analysing these rotational lines, we conclude that SiC2 is produced in the inner dust formation zone, with an abundance of ∼2 × 10 −7 relative to molecular hydrogen. These SiC2 lines have been observed for the first time in space and have been used to derive an SiC2 rotational temperature of ∼204 K and a source-averaged column density of ∼6.4 × 10 15 cm −2 . Furthermore, the high quality of the HIFI data set was used to improve the spectroscopic rotational constants of SiC2.

1'' Resolution Mapping of the Molecular Envelope of the Protoplanetary Nebula CRL 618

We present 1 00 resolution interferometric maps of the CO J ¼ 2 1a nd HC3N J ¼ 25 24 emission in the protoplanetary nebula CRL 618 obtained with the millimeter array of the Owens Valley Radio Observatory (OVRO). Our CO data trace with high accuracy the spatio-kinematic structure of different molecular components: (1) the fast, bipolar outflow, which is observed up to axial distances of � 2B 5( � 2:7 ; 10 16 cm) from the nebula center with a maximal deprojected expansion velocity of Vexp � 340 km s � 1 ; (2) the roughly round outer halo, which is slowly expanding at Vexp � 17 km s � 1 ; (3) an extended structure that is elongated in the polar direction � 6 00 (� 8:1 ; 10 16 cm),surroundingtheopticallobes,andthatexpandsatVexp � 22kms � 1 (slowaxialcomponent); and(4)a dense, inner torus-likecore (Rout � 1:6 ; 10 16 cm) expanding atVexpP12kms � 1 . Components (3) and (4) have been revealed for the first time by our high angular resolution data. The geometry, kinematics, density, and temperaturedistributionoftheindividualcomponentsofthemolecularenvelopeofCRL618havebeenconstrained by fitting a spatio-kinematic model to the data. We interpret our results in terms of nebular evolution as follows. In the last k2500 yr, there have been at least two distinct episodes of mass loss at a large scale in the form of a slow wind. The firsttookplace at a rate of ˙

HST observations of the protoplanetary nebula OH 231.8+4.2: The structure of the jets and shocks

We present high-resolution images obtained with the WFPC2, on board the HST, of the protoplanetary nebula (PPN) OH 231.8+4.2. H and NII line emission and scattered light in the continuum at 6750 and 7910 A were observed. We also discuss NIR NICMOS images from the HST archive. The images show with high accuracy the shape and excitation state of the shocks developed in the nebula. Our high-resolution images (and data from other works) allow a very detailed and quantitative description of the dierent nebular components and of the physical conditions in them. We interpret specic structures identied in our images using existing models of shock interaction. In the center of the nebula, there is a dense torus- or disk-like condensation continued by an hourglass-like structure, with relatively high densities (10 5 {10 6 cm 3 ) and temperatures (30 K). Inside this torus we have identied the location of the central star, from SiO maser observations. Two shock regions are detected from the optical line emission images, respectively in the north and south lobes. In both regions, a forward and a backward shock are identied. The densities of this hot gas vary between 40 and 250 cm 3 ,w ith the densest clumps being placed in the reverse shocks. The total mass of the shocked hot gas is210 3 M, both lobes showing similar masses in spite of their dierent extents. The relatively collimated jet that impinges on an originally slow shell, so producing the shocks, is identied from the scattered light images and in CO maps. This flow is signicantly denser and cooler than the shocked H regions. Its density decreases with the distance to the star, with typical values10 5 {10 4 cm 3 , and its temperature ranges between about 25 and 8 K. We explain the high H emission of the backward shock assuming that it propagates in a diuse gas component, entrained by the observed collimated flow and sharing its axial movement. The existence of shocks also in the collimated densest flow is suggested by the high abundance of some molecules like HCO + and its structure and kinematics in certain regions, but they are not seen in H emission, probably because of the absence of (well developed) hot components in this dense flow. We think that the exceptionally detailed and quantitative image derived for the wind interaction regions in OH 231.8+4.2 is a challenge to check and improve hydrodynamical models of wind interaction in PPNe.

ALMA observations of the Red Rectangle, a preliminary analysis

We aim to study equatorial disks in rotation and axial outflows in post-AGB objects, as to disclose the formation and shaping mechanisms in planetary nebulae. So far, both disks and outflows had not been observed simultaneously. We have obtained high-quality ALMA observations of 12CO and 13CO J=3-2 and 12CO J=6-5 line emission in the Red Rectangle, the only post-AGB/protoplanetary object in which a disk in rotation has been mapped up to date. These observations provide an unprecedented description of the complex structure of this source. Together with an equatorial disk in rotation, we find a low-velocity outflow that occupies more or less the region placed between the disk and the optical X-shaped nebula. From our observations and preliminary modeling of the data, we confirm the previously known properties of the disk and obtain a first description of the structure, dynamics, and physical conditions of the outflow.

Echelle Long-Slit Optical Spectroscopy of Evolved Stars

We present echelle long-slit optical spectra of a sample of objects evolving off the asymptotic giant branch (AGB), most of them in the preplanetary nebula (PPN) phase, obtained with the ESI and MIKE spectrographs at the 10 m Keck II and 6.5 m Magellan-I telescopes, respectively. The total wavelength range covered with ESI (MIKE) is ~3900-10900 A (~3600-7200 A). In this paper, we focus our analysis mainly on the Hα profiles. Prominent Hα emission is detected in half of the objects, most of which show broad Hα wings (with total widths of up to ~4000 km s^−1). In the majority of the Hα-emission sources, fast, post-AGB winds are revealed by P-Cygni profiles. In ~37% of the objects Hα is observed in absorption. In almost all cases, the absorption profile is partially filled with emission, leading to complex, structured profiles that are interpreted as an indication of incipient post-AGB mass loss. The rest of the objects (~13%) are Hα nondetections. We investigate correlations between the Hα profile and different stellar and envelope parameters. All sources in which Hα is seen mainly in absorption have F-G type central stars, whereas sources with intense Hα emission span a larger range of spectral types from O to G, with a relative maximum around B, and also including very late C types. Shocks may be an important excitation/ionization agent of the close stellar surroundings for objects with late type central stars. Sources with pure emission or P Cygni Hα profiles have larger J − K color excess than objects with Hα mainly in absorption, which suggests the presence of warm dust near the star in the former. The two classes of profile sources also segregate in the IRAS color-color diagram in a way that intense Hα-emitters have dust grains with a larger range of temperatures. Spectral classification of the central stars in our sample is presented. For a subsample (13 objects), the stellar luminosity has been derived from the analysis of the O I 7771-7775 A infrared triplet. The location in the HR diagram of most of these targets, which represent ~30% of the whole sample, is consistent with relatively high final (and, presumably, initial) masses in the range M_f ~ 0.6–0.9 M_☉ (M_i ~ 3–8 M_☉).

The Companion to the Central Mira Star of the Protoplanetary Nebula OH 231.8+4.2

We present deep optical long-slit spectra of the peculiar protoplanetary nebula (PPN) OH 231.8+4.2 obtained with the 6.5 m Baade Telescope at Las Campanas Observatory (Chile). In addition to the molecular absorption bands characteristic of the M9–10 III star inside OH 231.8+4.2 (QX Pup), we identify lines of the Balmer series in absorption, which do not form in the cool atmospheres of late M-type stars. We also confirm the presence of a blue continuum excess with an intensity that is a factor ~30 larger than that expected for an M9–10 III star. Our results indicate the presence of a source hotter than QX Pup illuminating OH 231.8+4.2 that is likely a mainsequence star with spectral type A. We discuss how the formation and nebular evolution of OH 231.8+4.2 could have been affected by the presence of a binary system in its core.

Submilliarcsecond-resolution mapping of the 43 GHz SiO maser emission in the bipolar post-AGB nebula OH231.8+4.2

We present ∼0.3 milliarcsec-resolution maps of the SiO (v = 2, J = 1-0) maser emission in the bipolar post-AGB nebula OH 231.8+4.2 obtained with the Very Long Baseline Array. These observations have provided for the firsttime 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 ∼10 1 3 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.

Binarity in Cool Asymptotic Giant Branch Stars: A GALEX Search for Ultraviolet Excesses

The search for binarity in AGB stars is of critical importance for our understanding of how planetary nebulae acquire the dazzling variety of aspherical shapes which characterizes this class. However, detecting binary companions in such stars has been severely hampered due to their extreme luminosities and pulsations. We have carried out a small imaging survey of AGB stars in ultraviolet light (using GALEX), where these cool objects are very faint, in order to search for hotter companions. We report the discovery of significant far-ultraviolet excesses toward nine of these stars. The far-ultraviolet excess most likely results either directly from the presence of a hot binary companion or indirectly from a hot accretion disk around the companion.

Sculpting a Pre-Planetary Nebula with a Precessing Jet: IRAS 16342-3814

We have imaged the bipolar pre-planetary nebula IRAS 16342-3814 with the Keck adaptive optics (AO) system in four near-infrared bands in the 1.6-4.7 μm range. The lobes, which showed smoothly varying brightness distributions in previous optical images taken with the Hubble Space Telescope, have a limb-brightened appearance in the AO images, with a remarkable corkscrew structure inscribed on the lobe walls. A well-collimated, precessing jet with a diameter 100 AU and a precession period 50 yr, interacting with ambient circumstellar material, is most likely responsible for the corkscrew structure and the lobes, as indicated by a detailed comparison of our observations with published numerical simulations. The very red colors of the lobes in the near-infrared, coupled with their visibility at optical wavelengths, require that at least half, but not all, of the light of the central star be trapped by a compact circumstellar dust cloud heated to ~600-700 K and reradiated in the infrared. The lobes are thus illuminated both by the infrared light from this dust cloud as well as by the optical light from the central star.

An Icy, Bipolar Proto-Planetary Nebula with Knotty Jets: IRAS 22036+5306

Using the Hubble Space Telescopes Wide Field Planetary Camera 2, we have discovered that the proto-planetary nebula (PPN) IRAS 22036+5306 (I22036) is an extended (57 × 13) bipolar nebula. A dark, (partial) ringlike band defines the nebular waist region and probably results from a tilted, geometrically thick disk that obscures the central star. Knotty, linear structures are seen in each lobe. Very Large Array A configuration observations of OH maser emission in I22036 show the 1667 MHz emission features aligned in a roughly 18 long linear structure along the nebular axis. The sources far-infrared fluxes imply the presence of a massive [~5(D/2 kpc)2 M☉], dusty, cool (Tdust ~ 35-67 K), circumstellar envelope. Strong 3.08 and 11 μm absorption features in the Infrared Space Observatory spectra show the presence of icy silicate grains, which probably reside in the disks shielded outer regions. A low-resolution optical spectrum indicates an F5 (or earlier) spectral type for the post-asymptotic giant branch (AGB) central star and shows Hα and Hβ emission lines. The elongated and highly structured lobes of I22036 are likely to have been produced as a result of the interaction of a collimated, fast wind (CFW) with the AGB progenitors dense, slow wind. Highly collimated jetlike components in the CFW have probably operated at some time during the formation of this PPN.