Astronomers discover a post-asymptotic giant branch star with unusual evolutionary scenario

October 17, 2017 by Tomasz Nowakowski, Phys.org report

Spectral energy distribution for J005252. The black symbols represent the dereddened photometry. The black line represents the best-fitting scaled Kurucz model atmosphere. Credit: Kamath et al., 2017.
(Phys.org)—Astronomers have detected a new metal-poor, luminous post-asymptotic giant branch star in the Small Magellanic Cloud. The newly found object, designated J005252.87-722842.9, appears to have undergone an unusual evolution process. The finding was presented October 12 in a paper published on the arXiv pre-print server.

J005252.87-722842.9 (J005252 for short) was first spotted in 2014 as part of an extensive low-resolution spectroscopic survey. The object was initially identified as post- (post-AGB) candidate with an observed luminosity of about 8,000 solar luminosities.

Post-AGB are believed to be on their way to the planetary nebula stage as they have evolved from the asymptotic giant branch as the result of a strong dusty mass loss removing most of the stellar envelope. However, the post-AGB phase is still one of the least understood phases of the evolution of low- and intermediate-mass stars. That is why astronomers are searching for new post-AGB stars that could advance knowledge of this mysterious phase of stellar evolution.

According to a new study conducted by a team of researchers led by Devika Kamath of the Catholic University of Leuven in Belgium, J005252 is a post-asymptotic giant branch star, which could be distinguished among other ordinary post-AGB by its unusual path of evolution. This star appears to have failed the so-called third dredge-up—a period during the process of  when a surface convection zone extends down to the layers where material has undergone nuclear fusion. This period occurs after a star enters the AGB and a flash occurs along a helium-burning shell.

Third dredge-up enriches the stellar photosphere with products of internal nucleosynthesis such as carbon, nitrogen, oxygen and heavy elements produced by the slow neutron capture process known as the s-process. However, when it comes to J005252, the research by Kamath's team shows no signs of such enrichment.

Using the UVES echelle spectrograph at the Paranal Observatory in Chile, the researchers have conducted a detailed chemical abundance study of J005252 that provides important hints about the of this star. "Through a systematic abundance study, using high-resolution optical spectra from UVES, we found that this likely post-AGB object shows an intriguing photospheric composition with no confirmed carbon enhancement nor enrichment of s-process elements," the paper reads.

The authors assume that the peculiar chemistry of J005252 could be explained by some mechanism that results in an AGB life without dredge-up episodes. They also take into account the possibility that this star could be a product of a merger scenario. However, the results of their spectroscopic observations show no signs of strong rotation that is characteristic for mergers.

Therefore, in order to finally explain this issue, the researchers plan further observations of J005252 that would reveal the abundance of other elements such as nitrogen.

"Our current spectral region does not cover usable N lines. We aim to obtain spectra covering the N lines to be able to determine if whether the N abundance can give us hints for solving the nucleosynthetic mystery of J005252," the astronomers concluded.

Explore further: Astronomers reveal insights into the nature of a distant ultraviolet-bright star

More information: D. Kamath et al. Discovery of a Metal-poor, Luminous Post-AGB Star that Failed the Third Dredge-up, The Astrophysical Journal (2017). DOI: 10.3847/1538-4357/836/1/15 . On Arxiv: https://arxiv.org/abs/1710.04368

Abstract
Post-asymptotic giant branch (post-AGB) stars are known to be chemically diverse. In this paper we present the first observational evidence of a star that has failed the third dredge-up (TDU). J005252.87-722842.9 is a A-type (Teff = 8250 ± 250K) luminous (8200 ± 700 L⊙), metal-poor ([Fe/H] = −1.18± 0.10), low-mass (Minitial ≈ 1.5 − 2.0 M⊙) post-AGB star in the Small Magellanic Cloud. Through a systematic abundance study, using high-resolution optical spectra from UVES, we found that this likely post-AGB object shows an intriguing photospheric composition with no confirmed carbon-enhancement (upper limit of [C/Fe] < 0.50) nor enrichment of s-process elements. We derived an oxygen abundance of [O/Fe] = 0.29 ± 0.1. For Fe and O, we took into account the effects of non-local thermodynamic equilibrium (NLTE). We could not derive an upper limit for the nitrogen abundance as there are no useful nitrogen lines within our spectral coverage. The chemical pattern displayed by this object has not been observed in single or binary post-AGBs. Based on its derived stellar parameters and inferred evolutionary state, single star nucleosynthesis models predict that this star should have undergone TDU episodes while on the AGB and be carbon-enriched. However, our observations are in contrast with these predictions. We identify two possible Galactic analogues which are likely to be post-AGB stars, but the lack of accurate distances (hence luminosities) to these objects does not allow us to confirm their post-AGB status. If they have low luminosities then they are likely to be dusty post-RGB stars. The discovery of J005252.87-722842.9 reveals a new stellar evolutionary channel whereby a star evolves without any third dredge-up episodes.

Related Stories

Jupiter-mass planet orbiting giant star discovered

August 10, 2017

(Phys.org)—An international team of astronomers has discovered a Jupiter-mass alien world circling a giant star known as HD 208897. The newly detected exoplanet was found as a result of high-precision radial velocity measurements. ...

Young lithium-rich giant star discovered

December 4, 2015

(Phys.org)—An Argentinian team of astronomers from the Astronomical Observatory of Córdoba has announced the finding of a new exceptional young lithium-rich giant star, designated KIC 9821622. Using the data obtained with ...

Rare extreme helium star identified by astronomers

June 20, 2017

(Phys.org)—Astronomers have identified another rare example of an extreme helium star. The star, designated GALEX J184559.8−413827 (or J1845−4138 for short), was initially classified as a faint helium-rich "hot subdwarf," ...

Recommended for you

The powerful meteor that no one saw (except satellites)

March 19, 2019

At precisely 11:48 am on December 18, 2018, a large space rock heading straight for Earth at a speed of 19 miles per second exploded into a vast ball of fire as it entered the atmosphere, 15.9 miles above the Bering Sea.

Revealing the rules behind virus scaffold construction

March 19, 2019

A team of researchers including Northwestern Engineering faculty has expanded the understanding of how virus shells self-assemble, an important step toward developing techniques that use viruses as vehicles to deliver targeted ...

OSIRIS-REx reveals asteroid Bennu has big surprises

March 19, 2019

A NASA spacecraft that will return a sample of a near-Earth asteroid named Bennu to Earth in 2023 made the first-ever close-up observations of particle plumes erupting from an asteroid's surface. Bennu also revealed itself ...

Nanoscale Lamb wave-driven motors in nonliquid environments

March 19, 2019

Light driven movement is challenging in nonliquid environments as micro-sized objects can experience strong dry adhesion to contact surfaces and resist movement. In a recent study, Jinsheng Lu and co-workers at the College ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.