Researchers determine chemical composition of two extremely metal-poor stars

Top panel: Fe abundances derived from all lines, as a
function of the lower excitation potential, for the adopted
model for SDSS J0826+6125. Lower panel: Fe abundances, as a
function of reduced equivalent widths, for the measured
lines. Credit: Bandyopadhyay et al., 2018.

A group of scientists led by Avrajit Bandyopadhyay of the
Indian Institute of Astrophysics, Bangalore, has conducted
chemical analysis of two bright, extremely metal-poor stars.
The results of the analysis, presented May 6 in a paper
published on arXiv.org, provide important insights into the
nature of these stars, and could help astronomers better
understand their origin.


The stars that are the subject of the study are designated SDSS
J082625.70+612515.10 and SDSS J134144.60+474128.90. The two
stars were identified by the SDSS-MARVELS spectroscopic
pre-survey as extremely metal-poor, with iron to hydrogen
abundance ratios of –3.1 and –3.2 respectively.

Bandyopadhyay’s team observed both stars with the Hanle Echelle
Spectrograph (HESP) on the 2.3-m Himalayan Chandra telescope
(HCT) at the Indian Astronomical Observatory (IAO) in India.
The observational campaign, conducted between November 2015 and
November 2016, allowed the astronomers to obtain essential
information about the chemical composition of the two objects.

“In this paper, the first science results using HESP, we
present a detailed analysis of their chemical abundances,” the
paper reads.

According to the study, SDSS J082625.70+612515.10 (SDSS
J0826+6125 for short), with an apparent magnitude of 11.44 and
an effective temperature of 4,300 K, has a generally consistent with
stars in halo of our Milky Way galaxy. It turns out that this
star is carbon-enhanced and exhibits depletion in nitrogen,
what could be due to internal mixing within the star. Moreover,
the researchers found no lithium in the star’s spectra.

When it comes to light elements, SDSS J0826+6125 has a low
sodium and aluminum content, however, it is abundant in
magnesium. Furthermore, in regards to the star’s abundances of
iron-peak elements, the researchers found that it appears is
relatively rich in cobalt, but poor in chromium, manganese, and
nickel.

SDSS J134144.60+474128.90 (or SDSS J1341+4741) has an apparent
magnitude of 12.38 and an of 5,450 K. The astronomers
found that this star is enhanced in carbon and has a low
abundance of lithium. Moreover, it exhibits low sodium and
aluminum content together with relatively high magnesium
abundance. The observations also detected an over-abundance of
chromium and nickel.

According to the authors of the paper, the analysis of
elemental abundances for SDSS J1341+4741 suggests that it is
the so-called CEMP-no star – a carbon enhanced metal-poor star
with no enhancement in r-process or s-process elements.

“At a given metallicity, CEMP-no stars appear to have larger
abundances of Cr. This might provide important clues to the
nature of the progenitors that contributed to the origin of
carbon,” the researchers wrote about SDSS J1341+4741.

The astronomers also noted that radial velocity variations of
SDSS J0826+6125 and SDSS J1341+4741 strongly suggest that both
stars are members of binary systems.

“The spectra were obtained over a span of 6-24 months, and
indicate that both could be members of binary
systems,” the researchers concluded.

Explore further:

Astronomers identify a mega metal-poor dwarf star

More information: Chemical composition of two bright
extremely metal-poor stars from the SDSS MARVELS pre-survey,
arXiv:1805.02280 [astro-ph.SR] arxiv.org/abs/1805.02280

Abstract
sdsszeroeight (V = 11.4; [Fe/H] = −3.1) and sdssonethree (V =
12.4; [Fe/H] = −3.2) were observed with the SDSS 2.5-m
telescope as part of the SDSS-MARVELS spectroscopic pre-survey,
and were identified as extremely metal-poor (EMP; [Fe/H]

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