Searching for nascent planetary nebulae: OHPNe candidates in the SPLASH survey

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.The evolution of asymptotic giant branch stars from the spherical symmetry into the diverse shapes of planetary nebulae (PNe) is a topic of intensive research. Young PNe provide a unique opportunity to characterize the onset of this transitional phase. In particular, OH maser-emitting PNe (OHPNe) are considered nascent PNe. In fact, only six OHPNe have been confirmed to date. In order to identify and characterize more OHPNe, we processed the unpublished continuum data of the interferometric follow-up of the Southern Parkes Large-Area Survey in Hydroxyl (SPLASH). We then matched the interferometric positions of OH maser and radio continuum emission, considering the latter as a possible tracer of free–free emission from photoionized gas, characteristic of PNe. We report eight objects with a positive coincidence, four of which are classified as candidate OHPNe here for the first time (IRAS 16372–4808, IRAS 17494–2645, IRAS 18019–2216, and OH 341.6811+00.2634). Available evidence strongly indicates that they are evolved stars, while the comparison with confirmed OHPNe indicates that they are likely to be PNe. Their final confirmation as bona fide PNe, however, requires optical/infrared spectroscopy. The obtained spectral indices of the radio continuum emission (between ≃0.4–1.3) are consistent with partially optically thick free–free emission from photoionized gas. Also, they cluster in the same region of a WISE colour–colour diagram as that of the confirmed OHPNe (⁠9.5≲[3.4]−[22]≲13.5⁠, and 4.0≲[4.6]−[12]≲7.0⁠), thus this diagram could help to identify more OHPNe candidates in the future. © The Author(s) 2022. Published by Oxford University Press on behalf of Royal Astronomical SocietyRC, JFG, and LFM acknowledge support from grant PID2020-114461GB-I00 and the ‘Center of Excellence Severo Ochoa’ award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709), funded by MCIN/ AEI /10.13039/501100011033. RC is also supported by the predoctoral grant PRE2018-085518, funded by MCIN/AEI/ 10.13039/501100011033 and by ESF Investing in your future. This work is also partially supported by grants R18-RT-3082 (IAA4SKA) and P20-00880, funded by the Economic Transformation, Industry, Knowledge and Universities Council of the Regional Government of Andalusia and the European Regional Development Fund from the European Union. LU acknowledges support from the University of Guanajuato (Mexico) grant ID CIIC 164/2022. HI has been supported by JSPS KAKENHI (25610043, JP16H02167). H.-H.Q. is partially supported by the Special Funding for Advanced Users, budgeted and administrated by Center for Astronomical Mega-Science, Chinese Academy of Sciences (CAMS-CAS), CAS ‘Light of West China’ Program and the National Natural Science Foundation of China (grant No. 11903038).Peer reviewe