Astronomers measure detailed chemical abundances of 158 stars in a nearby dwarf galaxy

This image shows the model of the tidally shredded
Sagittarius dwarf galaxy wrapping around a 3-D representation
of the Milky Way disk (flattened blue spiral). The yellow dot
represents the position of the Sun. Credit: David R. Law/UCLA.

(Phys.org)—An international team of astronomers has performed
detailed measurements of the chemical composition of 158 red
giant stars in the nearby Sagittarius dwarf galaxy. The
study, presented in a paper published July 11 on arXiv.org,
is so far the largest and most chemically extensive
high-resolution survey of this galaxy.

Discovered in 1994, Sagittarius is a nearby, massive,
elliptical loop-shaped satellite galaxy of the Milky Way. The
dwarf is currently merging with our galaxy, resulting in
massive tidal tails that can be found in the Galactic halo.
Therefore, detailed studies of Sagittarius could clarify the
formation of Milky Way’s halo. Due to its proximity (about
88,000 light years away), the in the core of this dwarf galaxy are
excellent targets for high-resolution spectroscopy observations
using ground-based telescopes.

So a team of researchers led by Sten Hasselquist of the New
Mexico State University recently conducted detailed
spectroscopic observations of Sagittarius as part of the Apache
Point Observatory Galactic Evolution Experiment, or APOGEE. The
main goal of this survey is to study over 100,000 red giant
stars across the full range of the galactic bulge, bar, disk,
and halo. APOGEE makes use of a high-resolution near-infrared
spectrograph connected to the Sloan Foundation 2.5m Telescope
at Apache Point Observatory in New Mexico in order to penetrate
the dust that obscures significant fractions of the disk and
bulge of our galaxy.

Hasselquist and colleagues has used APOGEE’s spectrograph to
estimate chemical composition of a large group of stars in
Sagittarius. They managed to measure chemical abundances for
the 16 elements, namely carbon (C), nitrogen (N), oxygen (O),
sodium (Na), magnesium (Mg), aluminium (Al), silicon (Si),
phosphorus (P), potassium (K), calcium (Ca), vanadium (V),
chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co) and
nickel (Ni).

“The Apache Point Observatory Galactic Evolution Experiment
provides the opportunity to measure elemental abundances for C,
N, O, Na, Mg, Al, Si, P, K, Ca, V, Cr, Mn, Fe, Co, and Ni in
vast numbers of stars. We analyze the chemical abundance
patterns of these elements for 158 red giant stars belonging to
the Sagittarius dwarf galaxy (Sgr),” the researchers wrote in
the paper.

These measurements revealed that Sagittarius is deficient, at
various levels, in all the studied chemical-abundance ratios
relative to iron. This indicates that the most recent
generation stars in the dwarf galaxy with metallicity higher
than -0.8 formed from gas that was much less polluted with Type
II supernovae than the gas that formed stars in the Milky Way’s
disk and bulge.

“We find that the Sgr stars with [Fe/H] > -0.8 are deficient
in all elemental abundance ratios (expressed as [X/Fe])
relative to the Milky Way, suggesting that Sgr stars observed
today were formed from gas that was less enriched by Type II
SNe than stars formed in the Milky Way,” the paper reads.

The team also also found that asymptotic giant branch stars are
much larger contributors to the chemical enrichment of
Sagittarius when compared to our Milky Way galaxy.

“We find clear signs of AGB enrichment beginning at [Fe/H] ∼
-0.6 in [(C+N)/Fe], [Na/Fe], and [Al/Fe], and that the
abundance patterns of these elements approach the MW trend in
the most metal-rich Sgr stars,” the authors noted.

Explore further:

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More information: APOGEE Chemical Abundances of the
Sagittarius Dwarf Galaxy, arXiv:1707.03456 [astro-ph.GA]
arxiv.org/abs/1707.03456

Abstract
The Apache Point Observatory Galactic Evolution Experiment
(APOGEE) provides the opportunity to measure elemental
abundances for C, N, O, Na, Mg, Al, Si, P, K, Ca, V, Cr, Mn,
Fe, Co, and Ni in vast numbers of stars. We analyze the
chemical abundance patterns of these elements for 158 red giant
stars belonging to the Sagittarius dwarf galaxy (Sgr). This is
the largest sample of Sgr stars with detailed chemical
abundances and the first time C, N, P, K, V, Cr, Co, and Ni
have been studied at high-resolution in this galaxy. We find
that the Sgr stars with [Fe/H] ≳ -0.8 are deficient in all
elemental abundance ratios (expressed as [X/Fe]) relative to
the Milky Way, suggesting that Sgr stars observed today were
formed from gas that was less enriched by Type II SNe than
stars formed in the Milky Way. By examining the relative
deficiencies of the hydrostatic (O, Na, Mg, and Al) and
explosive (Si, P, K, and Mn) elements, our analysis supports
the argument that previous generations of Sgr stars were formed
with a top-light IMF, one lacking the most massive stars that
would normally pollute the ISM with the hydrostatic elements.
We use a simple chemical evolution model, flexCE to further
backup our claim and conclude that recent stellar generations
of Fornax and the LMC could also have formed according to a
top-light IMF.

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