Manganese spread in Ursa Minor as a proof of sub-classes of type Ia supernovae

Abstract

Context. Recently, new sub-classes of Type Ia supernovae (SNe Ia) were discovered, including SNe Iax. The suggested progenitors of SNe Iax are relatively massive, possibly hybrid C+O+Ne white dwarfs, which can cause white dwarf winds at low metallicities. There is another class that can potentially occur at low or zero metallicities; sub-Chandrasekhar mass explosions in single and/or double degenerate systems of standard C+O white dwarfs. These explosions have different nucleosynthesis yields compared to the normal, Chandrasekhar mass explosions. Aims. We test these SN Ia channels using their characteristic chemical signatures. Methods. The two sub-classes of SNe Ia are expected to be rarer than normal SNe Ia and do not affect the chemical evolution in the solar neighbourhood; however, because of the shorter delay time and/or weaker metallicity dependence, they could influence the evolution of metal-poor systems. Therefore, we have included both in our stochastic chemical evolution model for the dwarf spheroidal galaxy Ursa Minor. Results. The model predicts a butterfly-shape spread in [Mn/Fe] in the interstellar medium at low metallicity and -at the same time -a decrease of [α/Fe] ratios at lower [Fe/H] than in the solar neighbourhood, both of which are consistent with the observed abundances in stars of Ursa Minor. Conclusions. The surprising agreement between our models and available observations provides a strong indication of the origins of these new sub-classes of SNe Ia. This outcome requires confirmation by future abundance measurements of manganese in stars of other satellite galaxies of our Milky Way. It will be vital for this project to measure not the most extreme metal-poor tail, as more commonly happens, but the opposite; the metal-rich end of dwarf spheroidals.

Observational constraints on the origin of the elements

“…However, our sample is not large enough to draw conclusions on the real astrophysical spread in the disc or halo. Such a spread has been proposed by Cescutti & Kobayashi (2017) as a consequence of multiple types of SNe Ia.…”

Evidence for Sub-Chandrasekhar Type Ia Supernovae from Stellar Abundances in Dwarf Galaxies∗

“…So far, we have discounted Type Iax SNe. Kobayashi et al (2015) and Cescutti & Kobayashi (2017) both explored the effect of Type Iax SNe on the chemical evolution of dSphs. They used measurements of α and Fe-peak elements from Venn et al (2012), North et al (2012), Ural et al (2015), and others.…”

Manganese Indicates a Transition from Sub- to Near-Chandrasekhar Type Ia Supernovae in Dwarf Galaxies*

“…North et al (2012) On the other hand, Kobayashi et al (2015) used a different sample to suggest that high Mn abundances point to dense Type Ia SNe, and that a special class of near-M Ch "Type Iax" SNe are needed to produce enough Mn to match observations. Cescutti & Kobayashi (2017) made a similar argument for a combination of "normal" and "Iax" SNe using Mn abundances for N = 20 stars in the dSph Ursa Minor. In both studies, the observations are too incomplete to draw any significant conclusions.…”

“…We return to this point in Section 5.2, where we discuss the potential role of SFH in driving [Mn/Fe]. Cescutti & Kobayashi (2017) Kobayashi (2017) suggested that this distribution might be characteristic of a stochastic chemical evolution model with two channels: a sub-M Ch channel and a near-M Ch channel with relatively weak deflagrations (a "Type Iax" SN channel). We do not directly compare their results with ours, since their chemical evolution model was tuned to match the metallicity distribution function of Ursa Minor.…”