PHR~J1724-3859: A Bipolar Planetary Nebula in Open Cluster Trumpler 25

TL;DR

This study identifies PHR J1724-3859 as a planetary nebula physically associated with open cluster Trumpler 25, established through precise radial velocities, reddening, and distance congruence derived from multi-telescope spectroscopy and deep imaging. It derives a CSPN Teff of about 250 kK, a PN kinematic age of roughly 23 kyr, and progenitor/final masses of ~5.12 and ~0.95 M⊙, respectively, placing this OC–PN in the intermediate initial-mass regime of the IFMR. The work reinforces a consistent OC–PN phenotype (Type I chemistry, bipolar morphology, hot CSPN) and provides a meaningful data point to test and refine IFMR models, highlighting slight systematic offsets relative to some WD-based relations and aligning better with recent population-synthesis IFMR results. Overall, the results strengthen the case for OC–PNe as powerful probes of late stellar evolution and the connection between cluster properties and PN progenitors while informing stellar evolution timescales and nucleosynthesis contributions to the Galaxy.

Abstract

Planetary nebulae (PNe) studies are essential for understanding late stellar evolution of low-to-intermediate mass stars. PNe in open clusters (OC) are rare but valuable since their study directly links their properties to those of their progenitors, something that cannot be achieved for field PNe. Here, we report the identification of one more OC-PN association to add to the small sample of, now, five pairs. The physical properties of the host cluster, PN and its central star (CSPN) have been explored using high and intermediate-resolution spectral and deep photometric data. The close agreement of the radial velocities of the PN and host cluster, together with concordance of reddening and distance, show that the PN PHR~J1724-3859 is highly likely to be physically associated with the OC Trumpler~25. Deep photometric data allowed clear identification of the CSPN. We find a CSPN effective temperature of around 250 kK and a nebular kinematic age of 23 kyrs, both at the extreme end, like the other members of this small class. The progenitor and final CSPN masses have been estimated to be 5.12$_{-0.15}^{+0.16}$ $M_\odot$ and 0.95$\pm$0.12 $M_\odot$ respectively. These latest results agree with the emerging trend for our other OC-PNe, falling below, but approximately parallel to, the latest initial-to-final-mass relation estimates derived from cluster white dwarfs and has important implications for stellar evolution models. All OC-PNe also possess some other common properties (e.g. all are Type-I PNe and bipolars) to be explored in future studies.

Figures (9)

• Figure 1: A 3-colour (R is $H\alpha$, G is short-red and B is blue) SHS image, $15\times15$ arcminutes in size, of the open cluster Trumpler 25 and PN PHR J1724-3859. North East is to top-left. The image is centred on the faint bipolar PN, clearly identified just 90 arcseconds North of the cluster's reported equatorial coordinate centre of RA:17h24m30.70s and
• Figure 2: A quotient image from the SHS $\rm H\alpha$ divided by the accompanying broad-band 'short-red' image of the PN PHR J1724-3859. The selected PN spectroscopic sampling points for HRS (in blue), GHOST (in red) and sky GHOST (in light blue), together with the FORS2 slits (continuous and dashed magenta lines for the blue and red grisms, respectively) are indicated. The size and shape of the point markings represent the respective HRS and GHOST fields of view which are very similar.
• Figure 3: The Trumpler 25 CMD constructed from FORS2 photometric b-high and v-high data of stars with cluster membership probability of 100 $\%$2023AA...675A..68V. The green and brown lines are the theoretical isochrones produced from the cluster literature mean parameters and the ASteCa code, respectively. Although they both present a good fit, the ASteCa code presents a slightly better fit to our data. The data are not extinction corrected, with the interstellar extinction being incorporated in the fitted isochrones. The light red squares and blue circle depict the only blue stars within the nebula field. These were not part of the original cluster CMD that was used for the isochrone fit since they are too faint to be detected with Gaia. Their colour uncertainties (ranging from 0.15 to 0.22) do not affect their identification as evolved stars. The blue circle shows the location on the CMD of the blue star that is closest to the nebula apparent centre that we identify as the true CSPN. The yellow triangle (whose position in the plot is shown by the yellow arrow) indicates the location of the star that has been previously falsely identified as the CSPN by 2021AA...656A..51G. It is evident that this star is not lying bluewards the MS (even if considering its colour uncertainty of 0.02) and towards fainter magnitudes as it would be expected for an evolved star as a CSPN.
• Figure 4: The HRS and GHOST reduced 1-D spectral data showing the [Nii]$\lambda6548,6584\AA$ doublet and H$\alpha$ line for HRS pointings ha-hd and GHOST pointings ga-gf. The [Nii]$\lambda6548,6584\AA$ lines present a prominent split in the spectra of pointings hb, hc, gc, gd and gf, while the same is true for the H$\alpha$ line in the spectrum of pointing gd.
• Figure 5: The FORS2 blue arm (left panel) and red arm (right panel) 1-D spectra. Modest sky line over-subtraction is evident, especially in the blue arm. The high excitation He II $\lambda4686\AA$ emission line and $H\beta$, [Oiii]$\lambda4959,5007\AA$, [Nii] $\lambda6548,6563\AA$, $H\alpha$ and [Sii] $\lambda6716,6731\AA$ lines are clearly identified.