[Paper Review] The Low Excitation Planetary Nebulae HuDo 1 and HuBi 1 and their [WC10] Central Stars

M. Peña|Redalyc (Universidad Autónoma del Estado de México)|Jun 17, 2005

Stellar, planetary, and galactic studies3 citations

TL;DR

This study analyzes the low-excitation planetary nebulae HuDo 1 and HuBi 1 and their [WC10] central stars using high- and low-resolution spectroscopy. It confirms both stars as [WC10] type, finds HuBi 1 to be a low-density, He-rich nebula around a faint, low-mass, slowly-evolving star, and provides evidence that [WC10] stars span a wide range of intrinsic luminosities and masses despite similar effective temperatures.

ABSTRACT

Low- and high-resolution spectra of the planetary nebulae HuDo 1 and HuBi 1, around [WC-late] stars, are analyzed. The objects belong to the galactic disk, with heliocentric radial velocities of -12 km/s (HuDo 1) and 57 km/s (HuBi 1). C(Hbeta) reddening values are of 2.04 for HuDo 1 and 1.22 for HuBi 1. Plasma line ratios are used to estimate physical conditions and abundances. We find log (O/H)+12 = 8.43 and 8.57, and N/O = 0.2 and 0.1 for HuDo 1 and HuBi 1 respectively. HuBi 1 is the only PN excited by a very late [WC] star showing intense nebular HeI recombination lines. From the stellar lines we derive a [WC 10] type for both stars, although HuBi 1 central star should be slightly hotter for providing a large amount of He^0 ionizing photons. Nebular and stellar parameters of HuDo 1 and HuBi 1 are compared with those of other [WC 10] objects, concluding that the stars of HuDo 1 and HuBi 1 should be intrinsically fainter. In particular HuBi 1 seems a low-density evolved nebula around a low-mass slowly-evolving star.

Motivation & Objective

• To determine the spectral type and physical properties of the central stars in the low-excitation planetary nebulae HuDo 1 and HuBi 1.
• To assess the nebular physical conditions, including electron density, temperature, and elemental abundances, in these systems.
• To investigate the evolutionary status of [WC10] central stars by comparing nebular and stellar parameters with other known [WC10] objects.
• To test whether HuBi 1, with its low-density and high-diameter nebula, might be a born-again planetary nebula or a product of low-mass, slowly-evolving post-AGB stars.

Proposed method

• High- and low-resolution spectroscopy of HuDo 1 and HuBi 1 were obtained at the Observatorio Astronómico Nacional in Mexico.
• Stellar spectral classification was performed using recent quantitative criteria, confirming [WC10] type for both central stars.
• Nebular line ratios were used to derive electron density, temperature, and ionic abundances, with extinction corrected using C(Hβ) values.
• Abundance analysis included determination of O/H, N/O, and He/H ratios, with special attention to He I recombination lines in HuBi 1.
• Stellar luminosity and mass were estimated by comparing observed fluxes with model atmospheres and assuming a distance of 6.8 kpc for HuBi 1.
• A comparison was made with other [WC10] planetary nebulae to assess evolutionary trends and intrinsic luminosity dispersion.

Experimental results

Research questions

• RQ1 What is the true spectral type of the central stars in HuDo 1 and HuBi 1 using modern quantitative classification criteria?
• RQ2 Why does HuBi 1 exhibit intense He I recombination lines, indicating a He-rich nebula, while other [WC10] nebulae do not?
• RQ3 How do the nebular physical conditions—especially electron density and dynamical age—compare with expectations from the [WC]-type vs. nebular density relation?
• RQ4 Why are the central stars of HuDo 1 and HuBi 1 intrinsically fainter than other [WC10] stars despite similar effective temperatures?
• RQ5 Is HuBi 1 a born-again planetary nebula or a product of a low-mass, slowly-evolving post-AGB star?

Key findings

• Both HuDo 1 and HuBi 1 central stars are classified as [WC10] (peculiar), confirming their status as among the coolest known [WC] stars.
• HuBi 1 exhibits strong He I recombination lines, indicating a He-rich nebula, and is the only [WC10] nebula known to show such features.
• The nebular O/H abundance is log(O/H)+12 = 8.57 for HuBi 1 and 8.43 for HuDo 1, typical of galactic disk planetary nebulae.
• The N/O ratio is 0.1 for HuBi 1 and 0.2 for HuDo 1, slightly below typical values and consistent with solar-like abundance patterns.
• HuBi 1 has a low electron density of ~10^3 cm^-3, placing it below the typical [WC]-type vs. density relation, suggesting it is a low-mass, slowly-evolving post-AGB system.
• The central star of HuBi 1 is intrinsically fainter than other [WC10] stars, implying a lower mass and supporting the idea that [WC10] stars span a wide range of post-AGB masses.