M. Santander-García

Extended rotating disks around post-AGB stars

Context. There is a group of binary post-AGB stars that show a conspicu ous NIR excess, usually assumed to arise from hot dust in very compact possibly rotating disks. These stars are sur rounded by significantly fainter nebulae than the standard, well studied protoplanetary and planetary nebulae (PPNe, PNe), and, at l east some of them, have significantly lower luminosities. Aims. To identify and study extended rotating disks around these s tars, shedding light on the role of disks in the formation and shaping of planetary nebulae. Methods. We present high-sensitivity mm-wave observations of CO lin es in 24 objects of this type. The resulting CO lines are compared with profiles expected to arise from rotating disks , both from theoretical and observational grounds. We deriv e s mple formulae that allow determining the mass of the CO-emitting gas and estimating its extent; the reliability and uncertai nty of the methods are widely discussed. Results. CO emission is detected in most observed sources and the line profiles show that the emissions very probably come from disks in rotation. We derive typical values of the disk mass b etween 10−3 and 10−2 M⊙, about two orders of magnitude smaller than the (total) masses of standard PPNe. The high-detection rate (upper limits being in fact no t very significant) clearly confirm that the NIR excess of these stars arises from compact disks in rot ation, very probably the inner parts of those found here. Low -velocity outflows are also found in about eight objects, with moderate expansion velocities of ∼ 10 km s−1, to be compared with the velocities of about 100 km s −1 often found instandard PPNe. Except for two sources with complex profiles, the outflo wing gas in our objects represents a minor nebular component. Our simple estimates of the disk typical sizes yields values ∼ 0.5 – 1 arcsec, i.e. between 5 10 and 3 10 cm. Estimates of the linear momenta carried by the outflows, w hich can only be performed in a few well studied objects, also yield moderate values, compared with the line ar momenta that can be released by the stellar radiation pres sure (contrary, again, to the case of the very massive and fast bipolar outflow s in standard PPNe, that are strongly overluminous). The mass and dynamics of nebulae around various classes of post-AGB star s differ very significantly, and we can expect the formation of PNe w ith very different properties.

Discovery of close binary central stars in the planetary nebulae NGC 6326 and NGC 6778

Original article can be found at : http://www.aanda.org/ Copyright The European Southern Observatory

IPHAS and the symbiotic stars. II. New discoveries and a sample of the most common mimics

Context. Knowledge of the total population of symbiotic stars in the Galaxy is important for understanding basic aspects of stellar evolution in interacting binaries and the relevance of this class of objects in the formation of supernovae of type Ia. Aims. In a previous paper, we presented the selection criteria needed to search for symbiotic stars in IPHAS, the INT Hα survey of the Northern Galactic plane. IPHAS gives us the opportunity to make a systematic, complete search for symbiotic stars in a magnitude-limited volume. Methods. Follow-up spectroscopy at different telescopes worldwide of a sample of sixty two symbiotic star candidates is presented. Results. Seven out of nineteen S-type candidates observed spectroscopically are confirmed to be genuine symbiotic stars. The spectral type of their red giant components, as well as reddening and distance, were computed by modelling the spectra. Only one new D-type symbiotic system, out of forty-three candidates observed, was found. This was as expected (see discussion in our paper on the selection criteria). The object shows evidence for a high density outflow expanding at a speed ≥65 km s −1 . Most of the other candidates are lightly reddened classical T Tauri stars and more highly reddened young stellar objects that may be either more massive young stars of HAeBe type or classical Be stars. In addition, a few notable objects have been found, such as three new Wolf-Rayet stars and two relatively high-luminosity evolved massive stars. We also found a helium-rich source, possibly a dense ejecta hiding a WR star, which is surrounded by a large ionized nebula. Conclusions. These spectroscopic data allow us to refine the selection criteria for symbiotic stars in the IPHAS survey and, more generally, to better understand the behaviour of different Hα emitters in the IPHAS and 2MASS colour-colour diagrams.

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.

Menzel 3: dissecting the ant

The structure and kinematics of the bipolar nebula Mz 3 have been investigated by means of HST, CTIO and ESO images and spectra. At least four distinct outflows have been identified which, from the inside to the outside, are the following: a pair of bright bipolar lobes, two opposite highly collimated column-shaped outflows, a conical system of radial structure, and a very dim, previously unnoticed, low-latitude and flattened (ring-like) radial outflow. A simple Hubble-law describes the velocity field of the ballisticaly expanding lobes, columns and rays, suggesting that their shaping has being done at very early stages of evolution, in a sort of eruptive events with increasing degree of collimation and expansion ages ranging from ∼600 for the inner structures to ∼1600 years (per kpc to the nebula) for the largest ones.

SALT reveals the barium central star of the planetary nebula Hen 2-39

Classical barium stars are binary systems which consist of a late-type giant enriched in carbon and slow neutron capture (s-process) elements and an evolved white dwarf (WD) that is invisible at optical wavelengths. The youngest observed barium stars are surrounded by planetary nebulae (PNe), ejected soon after the wind accretion of polluted material when the WD was in its preceeding asymptotic giant branch (AGB) phase. Such systems are rare but powerful laboratories for studying AGB nucleosynthesis as we can measure the chemical abundances of both the polluted star and the nebula ejected by the polluter. Here we present evidence for a barium star in the PN Hen 2-39. The polluted giant is very similar to that found in WeBo 1. It is a cool (Teff=4250 +/- 150 K) giant enhanced in carbon ([C/H]=0.42 +/- 0.02 dex) and barium ([Ba/Fe]=1.50 +/- 0.25 dex). A spectral type of C-R3 C_24 nominally places Hen 2-39 amongst the peculiar early R-type carbon stars, however the barium enhancement and likely binary status mean that it is more likely to be a barium star with similar properties, rather than a true member of this class. An AGB star model of initial mass 1.8 Msun and a relatively large carbon pocket size can reproduce the observed abundances well, provided mass is transferred in a highly conservative way from the AGB star to the polluted star (e.g. wind Roche-lobe overflow). The nebula exhibits an apparent ring morphology in keeping with the other PNe around barium stars (WeBo 1 and A 70) and shows a high degree of ionization implying the presence of an invisible hot pre-WD companion that will require confirmation with UV observations. In contrast to A 70, the nebular chemical abundance pattern is consistent with non-Type I PNe, in keeping with the trend found from nebular s-process studies that non-Type I PNe are more likely to be s-process enhanced. (abridged)

Further ALMA observations and detailed modeling of the Red Rectangle

AIMS We aim to study the rotating and expanding gas in the Red Rectangle, which is a well known object that recently left the asymptotic giant branch (AGB) phase. We analyze the properties of both components and the relation between them. Rotating disks have been very elusive in post-AGB nebulae, in which gas is almost always found to be in expansion. METHODS We present new high-quality ALMA observations of C17O J=6-5 and H13CN J=4-3 line emission and results from a new reduction of already published 13CO J=3-2 data. A detailed model fitting of all the molecular line data, including previous maps and single-dish observations of lines of CO, CII, and CI, was performed using a sophisticated code that includes an accurate nonlocal treatment of radiative transfer in 2D. These observations (of low- and high-opacity lines requiring various degrees of excitation) and the corresponding modeling allowed us to deepen the analysis of the nebular properties. We also stress the uncertainties, particularly in the determination of the boundaries of the CO-rich gas and some properties of the outflow. RESULTS We confirm the presence of a rotating equatorial disk and an outflow, which is mainly formed of gas leaving the disk. The mass of the disk is ~ 0.01 M⊙, and that of the CO-rich outflow is around ten times smaller. High temperatures of ≳ 100 K are derived for most components. From comparison of the mass values, we roughly estimate the lifetime of the rotating disk, which is found to be of about 10000 yr. Taking data of a few other post-AGB composite nebulae into account, we find that the lifetimes of disks around post-AGB stars typically range between 5000 and more than 20000 yr. The angular momentum of the disk is found to be high, ~ 9 M⊙ AU km s-1, which is comparable to that of the stellar system at present. Our observations of H13CN show a particularly wide velocity dispersion and indicate that this molecule is only abundant in the inner Keplerian disk, at ≲ 60 AU from the stellar system. We suggest that HCN is formed in a dense photodissociation region (PDR) due to the UV excess known to be produced by the stellar system, following chemical mechanisms that are well established for interstellar medium PDRs and disks orbiting young stars. We further suggest that this UV excess could lead to an efficient formation and excitation of PAHs and other C-bearing macromolecules, whose emission is very intense in the optical counterpart.

Modeling the physical and excitation conditions of the molecular envelope of NGC 7027

Context. The link between the shaping of bipolar planetary nebulae and the mass ejection activity of their central stars is still poorly understood. Appropriately characterizing the evolution of shells ejected during the late stages of stellar evolution and the interaction between these shells is fundamental to gain insight into the mechanism of nebular shaping. It must include the study of the molecular emission, which tracks the mass-loss history during the late asymptotic giant branch (AGB) and post-AGB stages, when the nebula is being actively shaped. Aims. The Heterodyne Instrument for the Far Infrared (HIFI) aboard Herschel is an invaluable tool because it opens a new window (most of the sub-mm and far-infrared range is only accessible from space) from which to probe warm molecular gas (∼50–1000 K). This paper presents a radiative transfer, spatio-kinematic modeling of the molecular envelope of the young planetary nebula NGC 7027 in several high- and low-J 12 CO and 13 CO transitions observed by Herschel/HIFI and the IRAM 30-m radio telescope, and discusses the structure and dynamics of the molecular envelope. Methods. We developed a code that, used along with the SHAPE software, implements spatio-kinematic modeling with accurate non-LTE calculations of line excitation and radiative transfer in molecular species. We used this code to build a relatively simple “Russian doll” model to account for the physical and excitation conditions of the molecular envelope of NGC 7027. Results. The model nebula consists of four nested, mildly bipolar shells plus a pair of high-velocity blobs. The innermost shell is the thinnest and shows a significant increase in physical conditions (temperature, density, abundance and velocity) compared to the adjacent shell. This is a clear indication of a shock front in the system, which may have played a role in shaping the nebula. Each of the high-velocity blobs is divided into two sections with considerably different physical conditions. The striking presence of H2 Oi n NGC 7027, a C-rich nebula, is likely due to photo-induced chemistry from the hot central star, although formation of water by shocks cannot be ruled out. The computed molecular mass of the nebula is 1.3 M� , compatible with that derived from previous works.

IPHAS and the symbiotic stars - III. New discoveries and their IR spectral energy distributions

Context. The IPHAS Hα survey provides a rich database to search for emission-line sources in the northern Galactic plane. Aims. We are systematically searching for symbiotic stars in the Milky Way using IPHAS. Our final goal, a complete census of this class of objects in the Galaxy, is a fundamental figure for discussing their overall properties and relevance to other classes of stars. Methods. Candidate symbiotic stars were selected using a refined combination of IPHAS and 2MASS photometric colours. Optical spectroscopy, together with the analysis of their spectral energy distribution in the IR, were obtained to confirm their nature and determine their main properties. Results. Five new symbiotic stars have been confirmed from spectroscopy at the 10.4 m GTC telescope. In one case, confirming the presence of a red giant star required near infrared spectroscopy. In another case, its symbiotic nature was adopted based on the strong similarity of its optical spectrum and spectral energy distribution to those of other genuinely symbiotic stars. The spectral energy distribution of the two S-types found is well fitted by red-giant model atmospheres up to 22 μm without evidence of IR excesses due to dust. In contrast, the three D-types mostly show emission from hot dust with a temperature around 1000 K. We also present the spectroscopic and photometric monitoring of the symbiotic star IPHASJ190832.31+051226.6 that was originally discovered in outburst, and it has now returned to a lower luminosity status. The spectra of thirteen other sources, all classified as young stellar objects except for a new compact planetary nebula, are also presented. Conclusions. The refinement of our discovery method, the completion of the IPHAS survey and photometric calibration, and the start of the twin survey in the south, VPHAS+, provide excellent perspectives for completing a reliable census of symbiotic stars in the Galaxy in the next few years.

HST and VLT observations of the symbiotic star Hen 2–147 - Its nebular dynamics, its Mira variable and its distance

Aims. We investigate the dynamics of the nebula around the symbiotic star Hen 2-147, determine its expansion parallax, and compare it with the distance obtained via the period-luminosity relation for its Mira variable. Methods. A combination of multi-epoch HST images and VLT integral field high-resolution spectroscopy is used to study the nebular dynamics both along the line of sight and in the plane of the sky. These observations allow us to build a 3D spatio-kinematical model of the nebula, which, together with the measurement of its apparent expansion in the plane of the sky over a period of 3 years, provides the expansion parallax for the nebula. Additionally, SAAO near-infrared photometry obtained over 25 years is used to determine the Mira pulsation period and derive an independent distance estimation via the period-luminosity relationship for Mira variables. Results. The geometry of the nebula is found to be that of a knotty annulus of ionized gas inclined to the plane of sky and expanding with a velocity of ∼90 km s -1 . A straightforward application of the expansion parallax method provides a distance of 1.5 ± 0.4 kpc, which is a factor of two lower than the distance of 3.0 ± 0.4 kpc obtained from the period-luminosity relationship for the Mira (which has a pulsation period of 373 days). The discrepancy is removed if, instead of expanding matter, we are observing the expansion of a shock front in the plane of the sky. This shock interpretation is further supported by the broadening of the nebular emission lines.