Identification and characterization of nascent planetary nebulae with OH and H$_2$O masers

TL;DR

This study expands the catalog of maser-emitting nascent planetary nebulae by combining public maser and radio continuum data with targeted ATCA/VLA interferometric observations. By matching maser positions to continuum emission and applying multiwavelength diagnostics, the authors confirm one new PN and identify 12 PN candidates, while also recognizing three evolved stars as water fountains. Multiwavelength analyses including SEDs, infrared morphologies, MSX/WISE colors, and mid-infrared spectroscopy indicate a heterogeneous population with evidence for mixed chemistry and photoionization-dominated ionization in several sources. The work highlights the value of high-precision maser astrometry for tracing the earliest PN stages and jet/outflow activity, and it lays a path for future surveys such as GASKAP-OH to further expand the sample.

Abstract

Stars like the Sun expel their outer layers and form planetary nebulae (PNe) as they evolve into white dwarfs. PNe exhibit diverse morphologies, the origins of which are not fully understood. PNe with OH (OHPNe) and H$_{2}$O (H$_{2}$OPNe) masers are thought to be nascent PNe. However, the number of known OHPNe and H$_{2}$OPNe remains small, and only in eight cases the position of the maser emission has been found to coincide with the PN, using the high astrometric accuracy of interferometric observations. In order to identify more OHPNe and H$_{2}$OPNe, we used public databases and our own ATCA/VLA observations to match the positions of OH and H$_{2}$O masers with known PNe and radio continuum emitters, considering radio continuum emission as a possible tracer of the photoionized gas that characterizes PNe. Here we report the confirmation of positional coincidence of maser emission with one more PN, and 12 PN candidates. Moreover, we have confirmed three evolved stars as `water fountains' (WFs) hosting H$_2$O masers. These WFs are associated with radio continuum emission, but their possible nature as PNe has not yet been confirmed. Although a final characterization of maser-emitting PNe as a group still requires confirmation of more objects, their distribution in the infrared color-color diagrams suggests that they are a heterogeneous group of PNe. In particular, the new OHPN IRAS 07027$-$7934 has been reported to contain a late [WC]-type central star, while the maser emission implies an O-rich envelope. This property is found in only one other known maser-emitting PN, although we found evidence that other confirmed and candidate OHPNe may also have mixed chemistry, since they show emission from polycyclic aromatic hydrocarbons. The new WF IRAS 18443$-$0231 shows radio continuum that is dominated by strong and variable non-thermal emission, as in magnetized outflows.

Figures (13)

• Figure 1: Spectra of OH and H$_{2}$O emission detected in our observations. In IRAS 18443$-$0231 we show an enlarged view to the most blueshifted and redshifted H$_{2}$O masers, which are indicated by a blue and red arrow, respectively.
• Figure 2: Spatial distribution of the H$_{2}$O maser components (crosses) with respect to the position of the radio continuum emission peak (filled circle) at 22.22 GHz. The colorbar represent the LSR velocity of the masers and the symbols sizes are their positional uncertainties. The bottom right panel is a close-up of H$_{2}$O maser components in the central area of IRAS 18443$-$0231 (marked by a black dashed rectangle in the left panel), close to the continuum emission peak.
• Figure 3: SED of the new maser-emitting PN (indicated with an asterisk next to their name) and candidates (rest of the panels). The circles are the photometric values found for each of the new identified sources. The size of the circles is larger than the errors. Red color stands for upper limits.
• Figure 4: MSX color-color diagram, as defined by sev02, including all known and new maser-emitting PNe and candidates, and adopted from cal22. The vertical and horizontal dashed lines separate the diagram into four quadrants, where different types of sources tend to cluster. The filled star and square symbols without source names represent the previously reported maser-emitting PNe and candidates, respectively. These same symbols with source names are the new maser-emitting PNe and candidates, respectively. IRAS 16029$-$5055, IRAS 17375$-$2759 and IRAS 19176+1251 are included as PNe (see Section \ref{['sec:spitzer']}). Symbols with red color represent sources with WF characteristics.
• Figure 5: WISE color–color diagram, similar to the one presented in gom17, including all known and new maser-emitting PNe and candidates, and adopted from cal22. The stars and squares represents the confirmed maser-emitting PNe and candidates, respectively, as presented in Fig. \ref{['fig:msx']}. The red color stands for WF nature of the source. The open circles represent obscured post-AGB candidates ram09ram12. The solid line represents the loci of the colors for blackbody brightness distributions. The tick marks in the blackbody line go from 150 to 350 K at increment steps of 25 K.