Proto-planetary Nebula Candidates Research Articles

Observations of Water Masers Associated with the Proto–Planetary Nebula Candidate IRAS 19296+2227

We present the first very long baseline interferometric observations of the 616–523, 22 GHz H2O maser emission associated with a proto–planetary nebula candidate. We obtained three epochs of observation of the water masers associated with IRAS 19296+2227, using the Very Long Baseline Array, operated by the National Radio Astronomy Observatory. These observations show that the water masers are distributed over a 50 mas arclike structure covering a limited velocity range of ~9 km s-1. The masers most likely reside in an asymptotic giant branch–like circumstellar shell, although we cannot rule out the possibility that they lie in a shocked region. Over the 40 days spanning the epochs of observation, we have measured an error-weighted average proper motion of 6.16 ± 1.32 km s-1, indicating that the masers are not in a high-velocity outflow.

A Mid‐infrared Imaging Survey of Proto–Planetary Nebula Candidates

We present the data from a mid-infrared (MIR) imaging survey of 66 proto-planetary nebula candidates using two MIR cameras (MIRAC2 and Berkcam) at the NASA Infrared Telescope Facility and the United Kingdom Infrared Telescope. The goal of this survey is to determine the size, flux, and morphology of the MIR emission regions, which sample the inner regions of the circumstellar dust shells of proto-planetary nebulae. We imaged these proto-planetary nebulae with narrowband filters(Δλ/λ~10%) at wavelengths of notable dust features. With our typical angular resolution of 1'', we resolve 17 sources, find 48 objects unresolved, and do not detect one source. For several sources we checked optical and infrared associations and positions of the sources. In table format, we list the size and flux measurements for all of the detected objects and show figures of all of the resolved sources. The proto-planetary nebula candidate sample includes, in addition to the predominant proto-planetary nebulae, extreme asymptotic giant branch stars, young planetary nebulae, a supergiant, and a luminous blue variable. We find that dust shells that are cooler (T~150 K) and brighter in the infrared are more easily resolved. Eleven of the seventeen resolved sources are extended and fall into one of two types of MIR morphological classes: core/elliptical or toroidal. Core/elliptical structures show unresolved cores with lower surface brightness elliptical nebulae. Toroidal structures show limb-brightened peaks suggesting equatorial density enhancements. We argue that core/ellipticals have denser dust shells than toroidals.

Infrared Space ObservatoryObservations of IRAS 16594−4656: A New Proto–Planetary Nebula with a Strong 21 Micron Dust Feature

Based on ISO short-wavelength spectrograph (SWS) + long-wavelength spectrograph (LWS) observations, IRAS 16594-4656 is identified as a new member of the rare class of C-rich proto-planetary nebulae with the 21 ?m dust feature in emission. In addition, the major polycyclic aromatic hydrocarbon (PAH) emission bands at 3.3, 6.2, 7.7, 8.6, and 11.3 ?m are detected, together with unusually strong features at 12.6 and 13.4 ?m, very rarely observed, attributed to PAH species with a high degree of hydrogenation. The shapes and relative intensities of the various features found are consistent with a mixed population of transient decomposition products of large hydrogenated amorphous carbonaceous (HAC) grains, consisting of fullerenes with different degrees of hydrogenation, which we suggest to be the main carriers of the strong 21 ?m feature, and a combination of small and large partially dehydrogenated cationic PAH molecules excited by the visible radiation coming from the central star. Additional features corresponding to crystalline silicates (mostly pyroxenes) are also tentatively detected, together with a possible very broad unidentified emission feature centered around 30 ?m, previously reported as appearing in combination with the 21 ?m feature in other proto-planetary nebulae candidates. The combination of features corresponding to O-rich and C-rich dust grains, if real, would suggest a recent change to a C-rich chemistry in the outer envelope of IRAS 16594-4656. With the help of optical ground-based spectroscopy and recently obtained Hubble Space Telescope (HST) images, we identify IRAS 16594-4656 as a new transition object between the asymptotic giant branch (post-AGB) and the planetary nebula (PN) stage. The ground-based optical spectrum shows only the Balmer lines in emission and a faint, probably shock-excited, [O I] emission over a very red continuum where strong, probably circumstellar, diffuse interstellar absorption bands (DIBs) at 5780 and 6281-6284 ? are clearly seen. HST WFPC2 continuum images reveal the presence of a bright central star surrounded by a bipolar reflection nebula with a multiple-axis morphology and a maximum size of ~5''?11''.

The chemical composition of the protoplanetary nebula candidate HD 179821

The chemical composition of the protoplanetary nebula candidate HD 179821

Optical spectroscopy of carbon-rich proto-planetary nebulae

We are engaged in a program of optical spectroscopy of proto-planetary nebulae (PPN). The objects were initially selected from the IRAS database on the basis of their strong infrared excesses, indicating dust temperature of 150 – 250 K. Spectra have been obtrained at medium resolution for the purpose of obtaining the spectral type, luminosity class, and to search for chemical peculiarities resulting from the post-AGB nature of the objects. We found our PPN candidates to typically display F-G supergiant spectra.

Molecular Envelopes of Planetary Nebulae and Proto-Planetary Nebulae

As stars evolve up the asymptotic giant branch (AGB), they begin to lose mass at a high rate, and in the process they create extended circumstellar molecular envelopes. Since the transition from AGB to planetary nebula stages is of the order of 1000 yr, the remnant of such molecular envelopes should still be observable in pro to-planetary nebulae (PPN) and planetary nebulae (PN). Recent ground-based survey of cool IRAS sources have discovered ~30 candidates of PPN (Kwok 1992). These sources show the characteristic “double-peak” energy distribution. The cool component is due to the remnant of the AGB dust envelope, and the hot component represents the reddened photosphere. The fact that the two components are clearly separated suggests that the dust envelope is well detached from the photosphere. Radiative transfer model fits to the spectral energy distributions of PPN suggest a typical separation of ~1 arc sec between the dust envelope and the photosphere, and such “hole-in-the-middle” structure can be mapped by millimeter interferometry in CO.

Line-profiles of F supergiant stars as candidates of proto-planetary nebulae

Line-profiles of F supergiant stars as candidates of proto-planetary nebulae

Line-Profiles of F Supergiant Stars as Candidates of Proto-Planetary Nebulae

In this report the results of spectroscopic observations on selected F supergiant stars as the candidates of proto-planetary nebulae (PPNs) are presented. Volk and Kwok(1989) selected nearly one hundred new candidates of PPNs based upon the scenario on the evolution of them. They summarized four classes of stars which should be the PPNs and are described in various papers. These are (i) High galactic latitude supergiants, (ii) Nonvariable OH/IR stars, (iii) Low color temperature infrared objects, and (iv) R. CrB. In order to examine the extended envelope of the PPNs we have started our project to obtain the Hα profile which should give us some clue to the structure of the envelope. We are mainly concerned with F supergiant stars. Our sample objects are mostly classified as pulsating or semiregular variables.

The Molecular Features in the Optical Spectra of the Proto-Planetary Nebulae

An AGB star evolves into the proto-planetary nebula(PPN) phase, when the stellar surface temperature is hotter than 5000K(Schonberner, 1983). But the remnant of AGB dust/gas envelope is still thick in the early stages of PPN evolution. The observed optical spectrum of PPN is a spectrum of central star(T∗ hotter than 5000K, the spectral type earlier than G5) overlaped with molecular features formed by molecules in the envelope. The features are changing during the envelope turns to optical thin. Based on our systematic observations of IRAS selected PPN candidates(Hu et al. 1990) we found that molecular features in the spectra evolve as following: 1.When object just exposes from dust shell, the column density of molecules in the envelope is very high. The molecular absorption bands are dominant in the spectra, which are resemble to the M supergiant. There are 6 objects from our 62 PPN candidates sample in this stage.2.When column density of molecules decreases, the spectra mainly show F-(or G-)type supergiant features from photosphere and partly also from envelope (especially the neutral atomic lines). But we still can detect the molecular bands such as C2, CN in the spectra of carbon-rich objects(3 objects in our sample, IRAS23304+6147, 20000+3239 and 19477+2401), TiO of oxygenrich object(one object, IRAS20406+2953), and ZrO and LaO of S-type transit object(one object, IRAS19454+2920).3.The evelope becomes optical thin and molecules absorb photons from central star then radiate by fluorescence process. In the spectrum apear the molecular emission bands resemble to the cometary spectra. A well known object in which spectrum C2 Swan bands in emission is CRL 2688(Crampton et al. 1975). We found another object, IRAS14429-4539, in the sample showing similar features.

Hα Profiles of Selected Candidates for Proto-Planetary Nebulae

The high-dispersion Hα line profiles of the candidate stars for proto-planetary nebulae(PPNs) are presented. Samples include mostly F supergiant stars. Volk and Kwok(1989) summarized four classes of stars which had been described in various literature as PPNs. They selected nearly one hundred new candidates of PPNs due to IRAS data. Our goal is to make clear the mass-losing phenomena and to establish the adequate models for the explaination of observed data from such stars. F supergiant stars also attract our attention to the spectroscopic variability in connection to their extended, rarefied and therefore complicated atmosphere. Observations were made at the Okayama Astrophysical Observatory with the 188-cm reflector and its coudé spectrograph. We used the intensified Reticon as the detector which has one dimensional array of 1024 pixels. The size of one pixel is 25 μm and corresponds to 0.13 Åand 5.9 km/sec at Hα. Our stellar samples are HD 46703, HD 56126, HD 112374, R CrB, HD 161796, 89 Her, IRAS 18095+2704, AFGL 2343, HD 187885, and UU Her. We obtained the Hα profiles and found a couple of absorption lines like the FeIλ6569, FeIIλ6516, and CIλ6587 among them. An express report and a short discussion about a part of these samples was already presented by Tamura and Takeuti(1991). Subsequent reports appeared and will appear in the articles of Tamura, Takeuti, and Zalewski(1992); Tamura and Takeuti (1992). We show various kinds of Hα profiles which consist of absorption and emission components and show clear evidence of activity in the extended atmosphere. We classify them into (i)P Cyg type profile, e.g. 89 Her, (ii)inverse P Cyg type profile, e.g. HD 187885, (iii)profiles with the central reversal at the bottom of absorption, e.g. HD 46703, (iv)profile with shoulder emission components, e.g. HD 56126, (v) asymmetric absorption profile, e.g. AFGL 2343. Concerning HD 161796 the Hα profile seems to have only an absorption component. It should be attributed to time variations, because the profile observed by Luck et al.(1990) is similar to (iv). In addition to a wide variety of the Hα profiles we have noticed their time variations ranging over different time spans, month, day, even in several ten minutes. Arellano Ferro(1985) noticed the change of Hα profiles of luminous yellow supergiants in various time scales too. We obtained radial velocities and compared with the expected values by the galactic rotation. We can recognize our sample stars are far from the galactic kinematics of disk members.

Near-Infrared Spectroscopy of Proto-Planetary Nebulae

Sixteen candidates for proto-planetary nebulae have been observed with low-resolution infrared spectroscopy in the H and K bands, and 6 in the L band, using the United Kingdom Infrared Telescope. In the H band, the objects show hydrogen Brackett lines in absorption. In the K band, absorption bands (del v=2) of CO were observed, and in three cases the CO bands are in emission. The CO spectrum of IRAS 22272+5435 was found to change from emission to absorption over a three-month interval. This CO emission can be interpreted as an indication of some recent episodes of mass loss in these objects. Four of the objects were found to possess an emission feature at 3.3 um, usually associated with PAHs, and two of these show an unusually strong 3.4 um emission feature (Geballe, Tielens, Kwok, & Hrivnak 1992, ApJ, 387, L89).

Proto-Planetary Nebulae

The progress in the search for proto-planetary nebulae (PPN) in the last 5 years is reviewed. An observational definition of PPN is developed and a list of current PPN candidates is given. The optical, infrared, and radio properties of PPN are summarized and compared with theoretical models.

Optical Spectroscopy of Six Carbon-Rich Proto-Planetary Nebulae

We are engaged in a program of optical spectroscopy of proto–planetary nebulae (PPN). The objects were initially selected from the IRAS database on the basis of their strong infrared excesses, indicating dust temperature of 150–250 K. Spectra have been obtrained at medium resolution for the purpose of obtaining the spectral type, luminosity class, and to search for chemical peculiarities resulting from the post–AGB nature of the objects. We found our PPN candidates to typically display F–G supergiant spectra.

Misidentified OH/IR Stars

AbstractPlots of periods against (25μm – 12μm) colour of OH/IR stars suggest that four IRAS sources, that have been associated with known OH/IR stars, either provide anomalous identifications, or are weak rather than regular variables. It is argued here that only one of these cases is an association error. OH15.7+0.8 and OH37.1–0.8 are the new proto–planetary nebula candidates.

A study of several F and G supergiant-like stars with infrared excesses as candidates for proto-planetary nebulae

Ground-based observations have been obtained for eight F and G supergiant-like stars showing large IR excesses. The combination of ground-based and IRAS data shows that these objects have dual-peak energy distributions, with comparable amounts of energy emitted in the visible and the IR. The IR-emitting cool dust shells are likely to represent the remnants of ejecta from an earlier phase of evolution. It is suggested that these eight objects are similar to IRAS 18095 + 2704 and are intermediate-mass stars in a post-AGB phase of evolution. Model fittings to the 0.4-100 micron energy distribution of these objects suggest that they left the AGB within the last 1000 yr.

Evolution of protoplanetary nebulae

Model spectra of protoplanetary nebulae have been calculated assuming that mass loss on the asymptotic giant branch terminates when the hydrogen envelope is almost totally depleted. The models predict spectral characteristics of protoplanetary nebulae which can be used to search for new candidates of protoplanetary nebulae in the IRAS Low Resolution Spectra Catalog. Three candidates (IRAS 10216-5916, IRAS 18095 + 2704, and IRAS 20004 + 2955) were identified in this way. Using the infrared colors of recently proposed candidates of protoplanetary nebulae as a guide, a region of the color-color diagram is suggested as the most likely area to contain protoplanetary nebulae. A search of the IRAS Point Source Catalog results in about 100 new candidates of protoplanetary nebulae. Twenty candidates are found to be associated with bright stars. Further ground-based observations are needed to confirm their nature. 45 refs.