The Nature of Post-AGB Candidates: A Search for Post-AGB Stars


Luis F. Miranda (1) and Claudio B. Pereira (2)

(1) Instituto de Astrofísica de Andalucía, CSIC, Granada, Spain
(2) Observatorio Nacional, Rio de Janeiro, Brasil



Introduction.

Post-Asymptotic Giant Branch (AGB) stars are luminous objects of low/intermediate main-sequence mass (M < 8 M) in the last stages of their evolution. They are the descendants of AGB stars, evolve in a fast track to hotter effective temperature but are not hot enough yet to photoionize the circumstellar envelope and to have entered the planetary nebula (PN) phase.

In the last years, observations and theoretical models have demonstrated that AGB evolution is more complex than previously though. The strong mass loss during AGB will eject the stellar atmosphere in time scales of 104 - 105 yr forming an expanding, spherical envelope around the stellar nucleus. At some time at the very end of AGB or during the post-AGB stage, the geometry of mass ejection should drastically change: the typical spherical geometry of the AGB envelopes is lost and most PNe present axis-symmetric (bipolar and elliptical) morphologies. It is still a matter of debate when the mass loss mode abruptly changes and which mechanisms are involved. In addition, dredge-up processes in the AGB phase will determine the chemical properties of the objects. During the AGB evolution, the H-shell is periodically interrupted by a He-shell flash; contact may be induced between the H-poor, He- and C-rich intershell so that the H-rich shell becomes enriched in nuclear ashes. If efficient, the envelope chemical composition can reach a C/O ratio larger than 1, which has fundamental impact on the ensuing chemical evolution.

Post-AGB stars may bear witness on these processes. The morphology of the envelopes around post-AGB stars may provide important clues on the time when the mass loss geometry changes and on the possible physical phenomena involved in that change. In addition, the geometry of post-AGB envelopes will determine the geometric/kinematic evolution of the object, which finally leads and should be linked to the morphologies observed in PNe. The chemical composition of a post-AGB star should reflect the initial composition of the object modulated by the dredge-up processes from the stellar interior. Because post-AGB stars are mostly of spectral types F-G, molecular veiling is negligible and the abundance of many chemical species, from CNO to heavy s-process elements, can be obtained from atomic line spectra. Thus, chemical studies of post-AGB stars are important not only to track the evolutionary nature of the object itself but also to constraint chemical evolutionary models. It is worth noting that the diversity in chemical composition of post-AGB stars is larger than anticipated.

The duration of the post-AGB phase strongly depends on the initial mass. Although for the less massive objects (that also are probably the faintest ones) the transition time may be as large as 30.000-40.000 yr, more massive objects may last in the post-AGB phase only a few decades/centuries. In these circumstances, identification of post-AGB stars is difficult. The IRAS satellite was successful in enabling systematic identifications of post-AGB candidates. The IRAS [12]-[25] versus [25]-[60] colour-colour diagram and the J-K versus H-K diagram (see Figure 1, Preite-Martínez 1988; García-Lario et al 1997) have been widely used to identify possible objects in the post-AGB phase. However, these diagrams alone do not allow us to distinguish between post-AGB stars and other kind of objects (Fig.1). In order to determine the true nature of the candidates, other observations are necessary (e.g., Suárez 2004).


Figure 1: IRAS two-colour diagram with the position of several objects including planetary nebulae, late-AGB and post-AGB stars, young stellar objects and galaxies (adapted from Garcia-Lario et al. 1997)


Observations.

We have started a program to obtain optical long-slit spectra of post-AGB candidates with CAFOS at the 2.2 m telescope on Calar Alto Observatory. The spectra, covering the range 3500-9000 Å, allow us to carry out a broad investigation of the absorption/emission characteristics of the candidate stars in order to identify their nature and, in the case of identified post-AGB stars, to select a suitable sample for future detailed studies. The spectroscopic information is complemented with images taken in narrow-band filters (Hα, [NII] and [OIII]) at the 1.5 m telescope on the Observatorio de Sierra Nevada in order to study the morphology of the objects.

Results.

So far we have obtained spectra of 12 post-AGB candidates which are being analyzed. Narrow band images have been obtained for 30 objects, 12 of which show extended emission. In the following we will present the results obtained for PM1-322 and preliminary results for PM1-253.

PM1-322 was not previously observed spectroscopically. The spectrum of the object (Figure 2) is typical of a PN (see Pereira & Miranda 2005 for details). The most interesting aspect of the spectrum is the relative strength of the [OIII]4363 emission line relative to Hβ with an [OIII]4363/Hβ intensity ratio of 1.20. This ratio indicates high electron densities of log(Ne) > 6. Similar densities are found in very young PN and in D and D'-type symbiotic stars.




Figure 2: Flux calibrated, optical spectrum of PM1-322.


In order to investigate in more detail the nature of PM1-322 we have analyzed the position of this object in the I([OIII]4363)/I(Hγ) versus I[OIII]5007/I(Hβ) diagram that separates quite well PNe from symbiotic stars (Gutiérrez-Moreno et al 1995). In this diagram (Figure 3), PM1-322 is located in the region occupied by symbiotic stars. However, the spectrum of PM1-322 does not show TiO bands, absorption lines and increasing of the continuum toward the red, which could be indicative for the presence of a cool companion. In addition, some symbiotic stars may present several iron emission lines, which is not the case of PM1-322. Therefore, taking into account that other young high density PNe also occupy the same region as symbiotic stars (Fig.3), PM1-322 may be identified with a young PN. Nevertheless, it should be mentioned that in some symbiotic stars the optical spectrum does not show absorption lines or TiO bands and the presence of a (very weak) cool companion can only be established in the infrared. In addition, other young PNe in the diagram (Hen2-57, Hen2-25 and IC4997) are bipolar. The images of PM1-322 (not shown here) show an stellar object without traces of internal structure at a spatial resolution of 1.8 arcsec. Therefore, although there is strong evidence for PM1-322 being a young high density PN, the possibility of a symbiotic nature cannot be totally ruled out.




Figure 3: Position of different objects in the I([OIII]4363)/I(Hγ) versus I([OIII]5007)/I(Hβ) diagram. Open squares: planetary nebulae; filled squares: D- and D'- type symbiotic stars; filled circles: S-type symbiotic stars; open circle: Hen 2-57; asterisk: PM1-322; star: IC4997; cross: Hen 2-25 (see Pereira & Miranda 1995 for references about the objects).


PM1-253 was included in the sample because, during the selection of the objects, we did not realize that PM1-253 was already demonstrated to be a bipolar PN by van den Steene et al.(1996). Nevertheless, our observations allow us a more detailed description of the internal structure and spectral characteristics of this interesting object (Miranda & Pereira 2005).




Figure 3: Grey-scale and contour representation of the Hα, [NII] and [OIII] images of PM1-253.


Figure 4 shows the Hα, [NII] and [OIII] images of PM1-253. The nebula presents a prominent bipolar structure, mainly defined by two emission maxima, embedded in faint elliptical emission. The central star is clearly visible in Hα and [OIII] but extremely faint in [NII]. The major nebular axis is oriented at position angle (PA) -8 degrees, while the two emission maxima are oriented at PA -15 degrees and -20 degrees in Hα ([OIII]) and [NII], respectively, suggesting some point-symmetry in this PN. PM1-253 shows a low excitation emission spectrum (Figure 5). The [NII] emission lines are very strong as compared to Halpha (I([NII]6548+6583)/Hα about 1.24) whereas the [OIII] lines present a rather moderate intensity (I([OIII]4959+5007)/Hβ about 1.3). From the spectrum we obtain a mean electron density of 2500 cm-3 and a mean electron temperature of 11.000-13.000 K. Both the morphology and spectral characteristics make PM1-253 an interesting PN that deserved further studies.




Figure 5: Normalized optical spectrum of PM1-253.


The rest of the observed sample includes other PNe, pre-main sequence stars and some objects the spectrum of which should be analyzed in more detail for an accurate classification. Although post-AGB stars are difficult to find, their identification and detailed analysis are a key piece in our knowledge of this rapid evolutionary phase and on the AGB to PN evolution, a matter which is far from clear.

References