On the Age of Galactic Bulge CSPNe: Too Young and Complicated?

: We present preliminary results of our study of a small sample of planetary nebulae in the Galactic Bulge for which high-angular resolution Hubble Space Telescope imaging is available. From this and from archival spectroscopy, we were able to calculate temperatures and luminosities for their central stars. These were then correlated to up-to-date evolutionary tracks found in the literature to help us estimate stellar masses and therefore ages for the central stars. Our current analysis indicates that our sample appears to represent a somewhat mixed population of planetary nebulae central stars, while at least one of the nebulae might have been formed by a more massive progenitor (i.e., M ZAMS ∼ 4 M (cid:12) ).


Introduction
The central stars of planetary nebulae (CSPNe hereafter)-their ages, masses, chemical composition and the associated planetary nebulae (PNe) luminosities-are unique tools in the study of older Galactic populations. To name just a few fields, CSPNe provide crucial feedback for the development of theoretical evolutionary models, for the study of Galactic mass/chemical distributions, and a better understanding of low-to-intermediate-mass binary evolution, as many, especially bipolar PNe are thought to emerge from binary systems. Here, we focus on a number of PNe within the Galactic Bulge for which high-angular resolution imagery from the Hubble Space Telescope (HST) is available, a Galactic region whose overall age is over 8 Gyr and that appears to have a mixed population of stars (e.g., Surot et al. [1]).
We report preliminary results from our study of CSPNe of a small sample of PNe in the Galactic Bulge (GBPNe), observed with the Hubble Space Telescope. These nebulae are compact with an average angular size < 8", i.e., <0.4 pc at 8.5 kpc. We have calculated the stellar photometry in a filter similar to Johnson V, and we have estimated the stellar temperatures and luminosities (Section 2). We give tentative correlations to the latest evolutionary tracks of Miller Bertolami [2], which allow us to estimate progenitor masses and current ages, and identify outliers (Sections 3 and 4).

From Hubble Space Telescope Images to Stellar Temperatures
Observations of 17 planetary nebulae in the Galactic Bulge were obtained with the HST WFPC2 instrument in several filters (proposal ID: 9356, PI: A.A. Zijlstra). Here, we report only results from the F547M filter images (λ eff = 5446Å, ∆λ = 486.6Å, transmission 91.3%), which corresponds roughly to the Johnson V filter, and it is not contaminated by the λλ5007 [O III] nebular line. We extracted the data from the Hubble Legacy Archive and calculated the photometry of each visible CSPN in that filter following the WFPC2 Photometry Cookbook 1 . Where necessary, we corrected the photometry for nebular contamination by subtracting nebular emission within a similar aperture. The final photometry was dereddened for interstellar extinction using the Balmer decrement from the corresponding, available flux-calibrated spectra for this sample from Acker et al. [3].
Stellar temperatures were estimated via the Zanstra method (Zanstra [4]) using the tool PyNeb (Luridiana et al. [5]). A PyNeb recipe calculates the Zanstra temperature from the approximations given by Pottasch [6]. Essentially, the Zanstra temperature is derived from the total stellar ionizing flux, which in turn is defined as the ratio of the nebular recombination line flux (in Hβ) to the stellar continuum flux (in the visual). The method demands that the nebula is of low density, but optically thick in Lyman continuum. Only lower limits to the temperature can be obtained, when the latter condition is not satisfied 2 . PyNeb does not provide error estimates from the derivation of the Zanstra temperature. Our photometry is estimated to have a ∼5% error. We used these errors to roughly estimate upper and lower limits for the Zanstra temperature. The preliminary results are shown in Figure 1. The CSPNe HST photometry (corrected for extinction), the derived Zanstra temperatures and luminosities (see next section) are stated in Table 1.

CSPNe Luminosities
Frew et al. [7] estimated distances for some of the nebulae in our sample using a surface brightness-radius relation, however we find that the surface areas used in that work (which were derived from digitized photographic plates in Hα) are unsurprisingly over-estimated for some of these compact GBPNe, compared to the Hα images from the HST. Furthermore, given the lack of Gaia distances (Gaia Collaboration [8]) for the majority of our sample 3 , we adopt a distance of 8.5 ± 2.5 kpc for the Galactic Bulge and subsequently for all these PNe in our sample. To calculate the luminosities, we use the method described in Zijlstra & Pottasch [9], where L * = 150 × L Hβ . This method is an analogous to the Zanstra method, and it therefore stands only for PNe that are optically thick to the ionizing radiation. For the Hβ fluxes (dereddened), we used the results of Gesicki et al. [10]. Figure 1 shows a preliminary Hertzsprung-Russell diagram of our sample with the latest evolutionary tracks of Miller Bertolami [2]. These evolutionary tracks are a result of the most up-to-date study on post-AGB stellar evolution models, which found that post-AGB stars are brighter and can evolve faster as opposed to the results of previous works. Here, we compare our results to the Solar metallicity models of Miller Bertolami [2] from which we interpolate stellar masses and evolutionary timescales.

Results
All seventeen (17) CSPNe appear to correlate well with the evolutionary tracks, indicating progenitor masses between 1 and 4 M . Except for one CSPN (see below), all others are still on the post-AGB track. The Zanstra method requires that the nebula is optically thick to ionizing photons. The fact that the Hβ-derived luminosities are in accordance with the tracks suggests that flux loss due to leakage is minor 4 .
Two outliers can be identified in Figure 1, one that has log L ≥ 4.1 and another that appears to be descending the white dwarf cooling track. The most luminous one is M 1-31, a bright, multipolar 3 A few CSPNe were found in the Gaia Data Release 2, however either the distances are highly unreliable or Gaia has detected nebular features. Therefore, we draw caution to the current Gaia detections for such distant sources, as we have noticed that in some cases, where the nebula is less than 3" in size, Gaia astrometry indicates that the detected "source" is either a nebular feature or a nearby star. This discrepancy could only be found whilst comparing current Gaia astrometry to high-angular resolution images such as the ones here. 4 Nevertheless, 4 CSPNe in our sample are positioned slightly below the 1 M track. Unless their distances are larger than 10 kpc, the discrepancy in their positions might result from an erroneous assumption that these nebulae are optically thick to ionizing radiation. nebula, whose central star can be easily distinguished (left panel, Figure 2). It appears that this star is consistent with a 4 M track. If we assume this evolutionary track as an upper limit, then this star is the youngest in the present sample (<200 Myr). The most evolved one is the faint nebula H 2-15 (right panel, Figure 2). In this case, we presume that the faint central star is located near the geometric center of the nebular waist (yellow cross, Figure 2) and the final photometry might be contaminated by nebular emission. A comparison with deep-imaging from the SuperCosmos Hα Survey (Parker et al. [11]) suggests that this nebula is in fact bipolar, while HST imaging recovered only the central waist. Given how faint the star is and taking into account that its distance is not known (cf. Section 2.2), its location in the HR diagram is less certain.

Discussion and Future Work
This work shows only preliminary results of 17 CSPNe from our sample 5 . We find that all but one of the CSPNe are still on the post-AGB track, suggesting that these are young PNe (age < 5000 years), with a progenitor mass range of 1-4 M . The clustering around the 1.5-2 M tracks ( Figure 1) may indicate that these compact PNe derive from a younger stellar population in the Galactic Bulge, as proposed in Gesicki et al. [10]. One nebula in our sample may have formed from a more massive star (∼4 M ) suggesting a CSPN age of <200 Myr, and thus much younger than the presumed age of the Galactic Bulge. Only one CSPN appears to be tracing the white dwarf cooling track and therefore this would be the oldest member of our sample.
We acknowledge that hydrogen Zanstra temperatures might not be the most accurate method in estimating CSPN temperatures compared to, for example, the helium Zanstra method (Kaler & Jacoby [12]). However, none of the PNe in our sample show the He II 4686Å emission line in the nebular spectra indicative of higher degree of ionization. In the absence of spectroscopic data for these central stars 6 , the use of the Zanstra method for atomic hydrogen can only be a rough indicator of stellar temperature.
In the future, we aim at calculating synthetic photometry from TMAP 7 non-LTE model atmospheres of H-rich white dwarfs, and compare them to our findings in an effort to get some 5 The final sample will consist 40 CSPNe for which we have HST data. 6 Nebular spectra are available in the literature, however these are of low spectral resolution and perhaps not centered on the CSPNe. better estimates of temperature and surface gravity ranges. Moreover, we intend to use the distances of Frew et al. [7] after re-calibrating them for the surface areas measured from the HST images.