Searching Heavily Obscured Post-AGB Stars and Planetary Nebulae I. IRAS Candidates with 2MASS PSC Counterparts

The transition from the Asymptotic Giant Branch (AGB) to the planetary nebula (PN) phase is critical in the shaping of PNe. It is suggested that the most asymmetric PNe are the descendant of massive AGB stars. Since these AGB stars are believed to evolve into heavily obscured post-AGB stars and PNe, the compilation of a sample of bona fide obscured post-AGB stars and PNe is important to help understand the formation of asymmetric PNe. We have used 2MASS, Spitzer GLIMPSE, MSX, and IRAS data in search of the near-IR counterparts of a sample of 165 presumably obscured IRAS post-AGB and PN candidates, and DSS red images to identify the optical counterparts among the objects detected in the near-IR. The IR spectral energy distributions (SEDs) in the wavelength range from 1 to 100 microns of the sources with unambiguous near-IR counterparts have been analyzed using appropriate colour-colour diagrams. We have identified the near-IR counterparts of 119 sources out of the 165 IRAS post-AGB and PN candidates in our sample. The improved astrometric coordinates of these sources have allowed us to find optical counterparts for 59 of them, yielding a reduced sample of 60 optically obscured post-AGB star and PN candidates. Among the 119 sources with near-IR counterparts, only 80 have unambiguous identifications in the 2MASS Point Source Catalogue.

💡 Research Summary

The transition from the asymptotic giant branch (AGB) to the planetary nebula (PN) phase is a short‑lived but pivotal stage that determines the eventual morphology of planetary nebulae. In particular, highly asymmetric PNe are thought to descend from massive AGB progenitors that evolve through a heavily dust‑enshrouded post‑AGB phase. To investigate this evolutionary link, the authors assembled a sample of 165 IRAS sources that were previously identified as likely obscured post‑AGB stars or young PNe. Their primary goal was to locate reliable near‑infrared (NIR) counterparts for these objects, improve their astrometric positions, and assess how many remain optically invisible despite being detectable in the infrared.

The methodology combined several large‑scale surveys: the Two‑Micron All‑Sky Survey (2MASS) Point Source Catalogue (PSC), the Spitzer GLIMPSE survey, the Midcourse Space Experiment (MSX) catalogue, and optical Digitized Sky Survey (DSS) red plates. Starting from the IRAS coordinates, the authors performed a systematic cross‑match within a 3‑arcsecond radius against the 2MASS PSC. When multiple 2MASS sources fell within this radius, they examined J, H, and K magnitudes, colours, and the shape of the spectral energy distribution (SED) to select the most plausible counterpart. For IRAS objects lacking a clear 2MASS match, the higher‑resolution GLIMPSE and MSX images were inspected to identify a unique infrared source that could be associated with the IRAS position.

Through this multi‑wavelength approach, near‑infrared counterparts were found for 119 of the 165 IRAS candidates. Of these, 80 have unambiguous entries in the 2MASS PSC, while the remaining 39 are identified only via GLIMPSE or MSX detections. The refined coordinates enabled a subsequent search on DSS red images, revealing optical counterparts for 59 of the 119 NIR‑identified sources. Consequently, a reduced sample of 60 objects was defined as “optically obscured” – they are invisible or extremely faint in the optical but clearly present in the infrared.

The authors then constructed SEDs spanning 1–100 µm for the 119 NIR‑identified objects and placed them on several colour–colour diagrams (e.g., J–H vs. H–K, IRAS