Proto-planetary Nebula Candidates Research Articles

High-resolution Very Large Array observations of 18 MIPSGAL bubbles

We present radio observations of 18 MIPSGAL bubbles performed at 5 GHz (6 cm) with the Karl G. Jansky Very Large Array in configuration B and BnA. The observations were aimed at understanding what kind of information high-resolution and high-sensitivity radio maps can supply on the circumstellar envelopes of different kinds of evolved stars and what their comparison with infrared images with similar resolution can tell us. We found that the 18 bubbles can be grouped into five categories according to their radio morphology. The three bubbles presenting a central point source in the radio images all correspond to luminous blue variable star candidates. Eleven bubbles show an elliptical shape and the total lack of a central object in the radio, and are likely associated with planetary nebulae. Under this assumption we derive their distance, their ionized mass and their distribution on the Galactic plane. We discuss the possibility that the MIPSGAL bubbles catalogue (428 objects) may contain a large fraction of all Galactic planetary nebulae located at a distance between 1.4 kpc and 6.9 kpc and lying in the MIPSGAL field of view. Among the remaining bubbles we identify also a H II region and a proto-planetary nebula candidate.

Study of extremely reddened AGB stars in the Galactic bulge

Context. Extremely reddened AGB stars lose mass at high rates of >10^-5 Msun/yr. This is the very last stage of AGB evolution, in which stars in the mass range 2.0--4.0 Msun (for solar metallicity) should have been converted to C stars already. The extremely reddened AGB stars in the Galactic bulge are however predominantly O-rich, implying that they might be either low-mass stars or stars at the upper end of the AGB mass range. Aims. To determine the mass range of the most reddened AGB stars in the Galactic bulge. Methods. Using Virtual Observatory tools, we constructed spectral energy distributions of a sample of 37 evolved stars in the Galactic bulge with extremely red IRAS colours. We fitted DUSTY models to the observational data to infer the bolometric fluxes. Applying individual corrections for interstellar extinction and adopting a common distance, we determined luminosities and mass-loss rates, and inferred the progenitor mass range from comparisons with AGB evolutionary models. Results. The observed spectral energy distributions are consistent with a classification as reddened AGB stars, except for two stars, which are proto-planetary nebula candidates. For the AGB stars, we found luminosities in the range 3000--30,000 Lsun and mass-loss rates 10^-5--3x10^-4 Msun/yr. The corresponding mass range is 1.1--6.0 Msun assuming solar metallicity. Conclusions. Contrary to the predictions of the evolutionary models, the luminosity distribution is continuous, with many O-rich AGB stars in the mass range in which they should have been converted into C stars already. We suspect that bulge AGB stars have higher than solar metallicity and therefore may avoid the conversion to C-rich. The presence of low-mass stars in the sample shows that their termination of the AGB evolution also occurs during a final phase of very high mass-loss rate, leading to optically thick circumstellar shells.

Nonstationarity of hot post-AGB objects: Variations of the brightness and spectrum of IRAS 01005+7910, IRAS 22023+5249, and IRAS 22495+5134

UBV photometry for two hot protoplanetary nebula candidates, early B supergiants with emission lines in the spectrum, IRAS 01005+7910 and IRAS 22023+5249, as well as for IRAS 22495+5134, the central star of the young compact planetary nebula M 2–54, is presented for the first time. Fast irregular brightness variations of the stars with maximum amplitudes up to and color-brightness correlations have been found: the B - V colors tend to become redder with increasing brightness, while U - B more likely decrease with brightening. The color excesses E(B - V) have been determined. The equivalent widths and absolute intensities of emission lines have been derived from low-resolution spectroscopy. Spectral variability has been revealed in all objects. The parameters of the gaseous nebula have been calculated for M 2–54: N e ∼ 104 cm−3and T e = 7700 ± 200 K. A joint analysis of the photometric and spectroscopic data suggests that stellar wind variations can be one of the causes of the stellar variability. The luminosities and distances of all three objects have been estimated.

Photometric variability of five candidates for protoplanetary nebulae

We analyze long-term UBV observations and ASAS-3 photometry for five candidates for protoplanetary nebulae—F and G supergiants with infrared excesses at highG alactic latitudes—V340 Ser, IRAS 05113+1347, V552 Pup, V448 Lac, and RV Col. These stars exhibit quasi-periodic multifrequency light variations caused by pulsations with characteristic time scales from 83 to 139 days, depending on the stellar temperature. Cooler stars undergo variations with larger amplitudes and periods. The variations at close frequencies with a period ratio of 1.03–1.09 are responsible for the amplitude modulation revealed for most program stars.

Pulsations and long-term photometric variability of three candidates for protoplanetary nebulae

We present new photometric data and analysis of the long-duration UBV photoelectric observations for three candidates to protoplanetary objects - F-supergiants with IR-excesses located at large galactic latitudes, IRAS 18095+2704, IRAS 19386+0155, and IRAS 19500-1709. All three stars have revealed quasiperiodic low-amplitude variabilities caused by pulsations observed against the long-term trends of brightnesses. For IRAS 18095+2704=V887 Her we have found a pulsation period of 109 days and a linear trend of brightness under the constant colours if being averaged over the year timescale. The light curve of IRAS 19386+0155=V1648 Aql over 2000-2008 can be approximated by a wave with a main period of 102 days which is modulated by close frequency, with a period of 98 days, that results in brightness oscillations with a variable amplitude. V1648 Aql has also shown synchronous reddening together with a persistent rise of brightness in the V-band. IRAS 19500-1709=V5112 Sgr experiences irregular pulsations with the periods of 39 and 47 days. The long-term component of the variability of V5112 Sgr may be related to the binary character of this star.

Results of infrared photometry and a model for the dust envelope of the protoplanetary nebula candidate V1027 Cyg

Results of JHKLM photometry for the protoplanetary nebula candidate V1027 Cyg obtained in 1991–2008 are reported. In all bands, the brightness variations did not exceed 0.2m. Estimated linear trends demonstrate no significant changes in the mean brightness and color indices of the object, with the possible exception of the L-M color index, which showed a small decrease. A search for possible periodicities in the brightness variations yielded the most probable period of 237d. A model for a spherically-symmetric dust shell has been calculated based on the photometric results supplemented with data on the mid- and far-infrared fluxes. The estimated mass-loss rate of the star is 1.3 × 10−5M⊙/year.

The magnetic field of the proto-planetary nebula candidate IRAS 19296+2227

Context. Magnetic fields are thought to be one of the possible mechanis ms responsible for shaping the generally spherical outflow of evolved stars into often aspherical planetary nebulae. However, direct measurements of magnetic fields during the tr ansition to the planetary nebula phase are rare. Aims. The aim of this project is to expand the number of magnetic fiel d measurements of stars in the (proto-)planetary nebula phase and find if the magnetic field strength is su ffi cient to affect the stellar outflow. Methods. We used Very Long Baseline Array observations to measure the circular polarization due to the Zeeman splitting of 22 GHz H2O masers in the envelope of the proto-planetary nebula candidate star IRAS 19296+2227 and the planetary nebula K3-35. Results. A strong magnetic field of B|| = −135± 28 is detected in the H2O maser region of the proto-planetary nebula candidate IRAS 19296+2227. The H2O masers of K3-35 are too weak to detect circular polarization although we do present the measurements of weak linear polarization in those masers. Conclusions. The field measured in the masers of IRAS 19296+2227 is dynamically important and, if it is representative o f the large scale field, is an important factor in driving the stellar mas s loss and shaping the stellar outflow.

Full polarization VLBA maps of a proto-planetary nebula

AbstractThe morphological evolution of stars from the asymptotic giant branch (AGB) to the planetary nebula (PN) phases presents an intriguing problem in stellar research. Planetary nebulae show bright-rims, well-defined shell-like structures and a large proportion of them are axisymmetric. In contrast their progenitor AGB stars are largely spherically symmetric (Sahai, 2004). Studies of proto-planetary nebulae, which are objects in transition between these two phases of stellar evolution, offer insight into the mechanisms which are responsible for the onset of axisymmetry. This poster presents the first polarization VLBI observations of the 1612 MHz OH maser emission from the proto-planetary nebula candidate OH O.9+1.3. The morphology, kinematics and polarization properties of these masers are discussed.

Evolution of the Circumstellar Molecular Envelope. I. A BIMA CO Survey of Evolved Stars

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.

COJ= 2–1 and 4–3 Observations of Proto–planetary Nebulae: Time‐variable Mass Loss

Observations made with the Heinrich Hertz Telescope of CO millimeter and submillimeter emission toward a sample of 22 proto-planetary nebula (PPN) candidates resulted in detections of 12 sources in the CO J = 2-1 line. Of these 12, seven sources were also detected in the J = 4-3 line. These 4-3 transitions are the highest yet observed in all but one of these PPNs. Statistical equilibrium/radiative transfer models were calculated for the CO emission in the circumstellar envelopes (CSEs), assuming various power-law density distributions. These models were compared with the intensity and profile shape of the observed spectra. For the region of the CSE probed by CO emission, the density laws must be steeper than inverse squared and are consistent with power laws between ρ ∝ r-3 and r-4. These radial density distributions imply that the mass loss was not constant but increased during the last part of the asymptotic giant branch (AGB) phase. Mass-loss rates at the end of the AGB for the three best-constrained sources are found to be 7.7 × 10-5 M☉ yr-1 (IRAS 22272+5435), 2.3 × 10-5 M☉ yr-1 (IRAS 07134+1005), and 1.3 × 10-5 M☉ yr-1 (IRAS 17436+5003) for the case of ρ ∝ r-3. These time-varying mass-loss rates can be integrated to calculate the enclosed envelope masses ejected in the past ~10,000 yr. The ejected envelope masses close to the star lie in the range 0.02-0.30 M☉; these values are consistent with theoretical models, which indicate that <20% of the stellar mass loss occurs in the last 10,000 years of the AGB. These results are in contrast to some recent dust studies based on infrared emission, however, in which much higher envelope masses are determined. The density laws, mass-loss rates, and enclosed envelope masses that we derive furnish important constraints for evolutionary models of stars in the late AGB and during the transition to the planetary nebula phase.

Burst of strongly polarized maser emission at 1612 MHz in the proto-planetary nebula OH17.7–2.0

We report the discovery of a burst of highly polarized OH maser emission at 1612 MHz in the proto-planetary nebula candidate OH17.7-2.0. The total flux density of two red-shifted features at 72.8 and 73.3 km s -1 increased nearly threefold over a period of ∼430 days. This burst was not associated with any systematic changes of the other 1612 MHz maser features as well as of the 1665 and 1667 MHz masers. The burst was characterized by a linear growth of left-hand circularly polarized emission accompanied by considerable linearly polarized emission. During the burst, the degrees of circular and linear polarization increased up to -80% and 15% respectively. A relation found between the line width and the peak flux density of the two bursting features implies a saturated amplification. Among several possible causes of the burst, a local propagation effect due to Zeeman overlap initiated by evolutionary changes in the shell appears most plausible.

Polarimetric observations of OH masers in proto-planetary nebulae

The 1612 and 1667 MHz OH maser lines have been measured in all four Stokes parameters in 47 proto-planetary nebula (PPN) candidates. Out of 42 objects detected, 40 and 34 are 1612 and 1667 MHz emitters, respectively. The spectral extent of the 1667 MHz line overshoots that of the 1612 MHz line in about 80% of the targets. 52% and 26% of the 1612 and 1667 MHz sources, respectively, show linear polarization in at least some features. Circular polarization is more frequent, occurring in 78% and 32% of sources of the respective OH lines. The percentage polarization is usually small (<15%) reaching up to 50-80% in a few sources. Features of linearly polarized emission are usually weak (0.5-4 Jy) and narrow (0.3-0.5 km s -1 ). The strength of the magnetic field inferred from likely Zeeman pairs in two sources of a few mG is consistent with values reported elsewhere for those classes of objects. An upper limit of the electron density in the envelope of OH17.7-2.0 derived from the difference in the position angle of polarization vectors for the two OH lines is about 1 cm -3 . Distinct profiles of polarization position angle at 1612 and 1667 MHz are seen in about one third of the sources and strongly suggest that the envelopes are permeated by structured magnetic fields. The geometry of the magnetic field is implicated as an important cause of the depolarization found in some PPN candidates. For the subset of targets which show axisymmetric shells in the optical or radio images we found a dominance of magnetic field components which are orthogonal to the long axis of the nebulae. This finding supports the hypothesis that such bipolar lobes are shaped by the magnetic field.

The kinematics of water masers in the stellar molecular outflow source, IRAS 19134+2131

Using the Very Large Array (VLA) and the Very Long Baseline Array (VLBA), we have observed water maser emission in the proto-planetary nebula candidate IRAS 19134+2131, in which the water maser spectrum has two groups of emission features separated in radial velocity by ~100 km s -1 . The blue-shifted and red-shifted clusters of maser features are clearly separated spatially by ~150 mas, indicative of a fast collimated flow. However, not all of the maser features are aligned along the axis of the flow, as is seen in the similar high-velocity water maser source, W43A. Comparing the VLA and VLBA maps of the water maser source, we find 4 maser features that were active for 2 years. Using only VLBA data, we identified proper motions for 8 maser features. The full 3D outflow velocity is estimated to be ~130 km s -1 , indicating that the dynamical age of the flow is only ~50 yr. On the basis of the relative positions with respect to the nearby extragalactic reference source, J1925+2106, we also obtain a secular motion of IRAS 19134+2131of $\mu_{l}=-4.6\pm0.7$ mas yr -1 along the Galactic plane toward the Galactic centre. This indicates a “far distance ( ≥ 16 kpc) for IRAS 19134+2131 if the Galactic rotation curve remains flat at 220 km s -1 .

Spectroscopic observations of two proto planetary-nebulae candidates: SS73 56 &amp; Hen 2-79

We report new spectroscopic observations of two proto planetary-nebulae candidates: SS73 56 and Hen 2-79 between 3200 A and 8000 A. From the measurements of several forbidden and permitted emission lines of single ionized iron, we derived the color excess and the excitation temperature. We discuss the nature of the two objects based on data obtained. The iron spectrum closely resembles Car but the absence or the weakness of (Nii)5754 A compared to Car reduces the possibility of being a LBV object. We also compare the two objects with the peculiar emission-line object SS73 11 that was recently analyzed by us.

Near-Infrared Photometric Survey of Proto–Planetary Nebula Candidates

We present JHK' photometric measurements of 78 objects mostly consisting of proto?planetary nebula candidates. Photometric magnitudes are determined by means of imaging and aperture photometry. Unlike the observations with a photometer with a fixed-sized beam, the method of imaging photometry permits accurate derivation of photometric values, because the target sources can be correctly identified and confusion with neighboring sources can be easily avoided. Of the 78 sources observed, we report nearly 10 cases in which the source seems to have been misidentified or confused by nearby bright sources. We also present nearly two dozen cases in which the source seems to have indicated a variability that prompts a follow-up monitoring. There are also a few sources that show previously unreported extendedness. In addition, we present H-band finding charts of the target sources.

MERLIN polarimetry of the OH masers in OH17.7-2.0

We report the first results of our study of magnetic fields in protoplanetary nebula candidates. Such fields may have important implications for the currently accepted Generalized Interacting Stellar Winds path to planetary nebula formation. We present the first images of the 1612 and 1667 MHz full polarization OH maser emission from the protoplanetary nebula, OH17.7–2.0, which were observed with MERLIN, at an angular resolution of 0.2 arcsec. We have detected one fully-resolved Zeeman σ-component pair at each observing frequency, from which we measure magnetic field strengths of B=+4.6 mG (1612 MHz) and B=+2.5 mG (1667 MHz). This is the first time a magnetic field strength has been measured in a protoplanetary nebula. The observations suggest a large-scale, regular magnetic field structure consistent with that of a stretched dipole field. When considered with near-infrared dust imaging polarimetry data, OH17.7–2.0 is shown to be a prolate spheroid, inclined at 20° to the sky plane, with the polar (major axis) outflow at a position angle of 20° in the plane of the sky and the denser equatorial axis at a position angle of between 120° in the same plane. The masers are shown to be contained in, and aligned with, this equatorial axis. There is additional 1612 MHz OH maser emission that can be associated with the interaction region between the fast wind from the post-AGB star and the remnant AGB shell. These data show that the stellar magnetic field can produce the necessary latitude-dependent mass loss on the AGB that is required in addition to the Generalized Interacting Stellar Winds models to produce ellipsoidal and bipolar planetary nebulae.

Magnetic Field in the Proto-Planetary Nebula OH17.7–2.0

We have used radio observations of OH masers in proto-planetary nebula (PPN) candidates to probe the magnetic field structure of these objects. Here we present the first results of our study, concerning the PPN OH17.7–2.0.

IRAS 01005+7910, A High Galactic Latitude Post-AGB Star

Hu et al (1993) have selected a sample of proto-planetary nebula candidates based on the IRAS color-color diagram. IRAS 01005+7910 is one object of this sample. We have observed this object photometrically (Table 1) and spectroscopically (Fig. 1). From its spectral type of B2I and optical color of B - V = 0.23, we can derive the reddening as E(B - V) = 0.39 and interstellar/circumstellar absorption Av = 1.20. If it is a normal B-type supergiant, the distance module will be m - M = 16.05 and distance d = 16.2 kpc. Due to the galactic latitude b = 16.6, it should be located at about 4.6 kpcabove the galactic plane. This does not fit with normal B-type supergiant and we considered that it is a post-AGB star located at high galactic latitude. Recently Hrivnak et al (2000) found that this object shows carbon-rich features in the infrared. A paper on observations, reduction and discussions of this object has been submitted to the Chinese Journal of Astronomy and Astrophysics.

Discovery of spectral variation in the optical counterpart of IRAS 01005+7910

We present a study of the high-resolution spectroscopic data for the proto-planetary nebula candidate IRAS 01005+7910. For the first time a careful spectral line identification is carried out, and a significant variability of the optical spectrum is detected. We found absorption lines of C II/III, N II, O II, Al III, Si III, and Mg II (&#x03BB 4481 &#x00C5), as well as emission lines of Si II and [Fe II]. Both absorption and emission components are present in the Balmer lines, Na I resonance D 1,2 lines, He I, and Fe III lines. The He I line profiles vary from straight to inverse P Cyg-type on a timescale of days to months. The resonance Na I lines show 5 absorption components at a resolution of R = 60000. Additionally, the Na I D 2 line exhibits a variable emission component with a width comparable to that of the Balmer line emission components. Using the model atmospheres method within the LTE-approximation, the effective temperature ( T eff ~ 21500 K), the metallicity [Fe/H]&#x2A00 = - 0.31, and the ratio C/O > 1 is reported. Finally, we suggest that IRAS 01005+7910 is a carbon-rich post-AGB star with a luminosity log( L / L &#x2A00) = 3.6 at a distance about 3 kpc.

AHubble Space TelescopeSnapshot Survey of Proto–Planetary Nebula Candidates: Two Types of Axisymmetric Reflection Nebulosities

We report the results from an optical imaging survey of proto-planetary nebula candidates using the Hubble Space Telescope (HST). The goals of the survey were to image low surface brightness optical reflection nebulosities around proto-planetary nebulae and to investigate the distribution of the circumstellar dust, which scatters the star light from the central post-asymptotic giant branch star and creates the optical reflection nebulosities. We exploited the high resolving power and wide dynamic range of HST and detected nebulosities in 21 of 27 sources. The reduced and deconvolved images are presented along with photometric and geometric measurements. All detected reflection nebulosities show elongation, and the nebula morphology bifurcates depending on the degree of the central star obscuration. The star-obvious low-level-elongated (SOLE) nebulae show a bright central star embedded in a faint, extended nebulosity, whereas the dust-prominent longitudinally extended (DUPLEX) nebulae have remarkable bipolar structure with a completely or partially obscured central star. The intrinsic axisymmetry of these proto-planetary nebula reflection nebulosities demonstrates that the axisymmetry frequently found in planetary nebulae predates the proto-planetary nebula phase, confirming previous independent results. We suggest that axisymmetry in proto-planetary nebulae is created by an equatorially enhanced superwind at the end of the asymptotic giant branch phase. We discuss that the apparent morphological dichotomy is caused by a difference in the optical thickness of the circumstellar dust/gas shell with a differing equator-to-pole density contrast. Moreover, we show that SOLE and DUPLEX nebulae are physically distinct types of proto-planetary nebulae, with a suggestion that higher mass progenitor AGB stars are more likely to become DUPLEX proto-planetary nebulae.