Multiple stellar populations at less evolved stages: detection of chemical variations among main-sequence dwarfs in NGC 1978. (arXiv:2010.15447v1 [astro-ph.SR])

Multiple stellar populations at less evolved stages: detection of chemical variations among main-sequence dwarfs in NGC 1978. (arXiv:2010.15447v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Li_C/0/1/0/all/0/1">Chengyuan Li</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tang_B/0/1/0/all/0/1">Baitian Tang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Milone_A/0/1/0/all/0/1">Antonino P. Milone</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Grijs_R/0/1/0/all/0/1">Richard de Grijs</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hong_J/0/1/0/all/0/1">Jongsuk Hong</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Yang_Y/0/1/0/all/0/1">Yujiao Yang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wang_Y/0/1/0/all/0/1">Yue Wang</a>

Multiple stellar populations (MPs) with different chemical compositions are
not exclusive features of old GCs (older than 10 Gyr). Indeed, recent studies
reveal that younger clusters ($sim$2–6 Gyr-old) in the Magellanic Clouds also
exhibit star-to-star chemical variations among evolved stars. However, whether
MPs are present among less evolved dwarfs of these intermediate-age clusters is
still unclear. In this work, we search for chemical variations among GK-type
dwarfs in the $sim$2 Gyr-old cluster NGC 1978, which is the youngest cluster
with MPs. We exploit deep ultraviolet and visual observations from the Hubble
Space Telescope to constrain the nitrogen (N) and oxygen (O) variations among
MS stars. To do this, we compare appropriate photometric diagrams that are
sensitive to N and O with synthetic diagrams of simple stellar populations and
MPs. We conclude that the G- and K-type MS stars in NGC,1978 host MPs. Our
statistical analysis shows that the fraction of N-rich stars ranges from
$sim$40% to $sim$80%, depending on the detailed distributions of nitrogen
and oxygen.

Multiple stellar populations (MPs) with different chemical compositions are
not exclusive features of old GCs (older than 10 Gyr). Indeed, recent studies
reveal that younger clusters ($sim$2–6 Gyr-old) in the Magellanic Clouds also
exhibit star-to-star chemical variations among evolved stars. However, whether
MPs are present among less evolved dwarfs of these intermediate-age clusters is
still unclear. In this work, we search for chemical variations among GK-type
dwarfs in the $sim$2 Gyr-old cluster NGC 1978, which is the youngest cluster
with MPs. We exploit deep ultraviolet and visual observations from the Hubble
Space Telescope to constrain the nitrogen (N) and oxygen (O) variations among
MS stars. To do this, we compare appropriate photometric diagrams that are
sensitive to N and O with synthetic diagrams of simple stellar populations and
MPs. We conclude that the G- and K-type MS stars in NGC,1978 host MPs. Our
statistical analysis shows that the fraction of N-rich stars ranges from
$sim$40% to $sim$80%, depending on the detailed distributions of nitrogen
and oxygen.

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