The Implications of Ultra-Faint Dwarf Galaxy Reticulum II on the Common Envelope Jets Supernova r-process Scenario

We show that the common envelope jets supernova (CEJSN) r-process scenario is compatible with very recent observationally determined properties of the stars in the ultra faint dwarf (UFD) galaxy Reticulum II that are strongly enhanced in r-process elements. These new results, like efficient mixing of the r-process elements in the Reticulum II galaxy, have some implications on the CEJSN r-process scenario for UFD galaxies. In particular, the energetic jets efficiently mix with the common envelope ejecta and then with the entire interstellar medium of Reticulum II. The compatibility that we find between the scenario and new observations suggests that the CEJSN r-process scenario supplies a non-negligible fraction of the r-process elements.

The properties of r-process enriched stars in ultrafaint dwarf (UFD) galaxies constrain and challenge these r-process nucleosynthesis scenarios. For example, the metal poor stars of Reticulum II that are highly rprocess enriched (Ji et al. 2016;Roederer et al. 2016) imply short delay from star formation to the r-process event(s). Moreover, it is possible that the low-metalicity r-process stars in the Milky Way Galaxy originated from UFD galaxies that the Galaxy accreted (e.g., Hirai et al. 2022).

THE R-PROCESS IN THE RETICULUM II GALAXY
In very recent studies Ji et al. (2022) and Simon et al. (2022) explore the distribution of r-process nucleosynthesis in the UFD galaxy Reticulum II. We list some of their findings in the first column of Table 2, and the implications of these findings to a potential r-process site in the second column. We point out that the CEJSN rprocess scenario, in which a cold NS accretes mass from the core of a giant star and launches neutron-rich jets, can account for, or at least is compatible with, the properties of r-process nucleosynthesis in Reticulum II, as we list in the third column of Table 2.
Note that in addition to the delay time from star formation to the r-process event, we follow Grichener & Soker (2019b) and define the delay distance D nk = v nk (t 0−rp − t kick ), where t 0−rp is the delay time from star formation to r-process nucleosynthesis, and t kick ≃ 10 − 30 Myr is the time from star formation to the time the system suffers the natal kick. This is the distance from the formation place of the binary system to the r-process event.
The very recent studies by Ji et al. (2022) and Simon et al. (2022) have some new implications to the CEJSN r-process scenario with respect to the UFD galaxy Reticulum II, as we present in the fourth column of Table 2. These implications are not applicable to the r-process in the Milky Way Galaxy, that the CEJSN r-process scenario can account for (Grichener & Soker 2019b;Grichener et al. 2022).

CONCLUSIONS
Our conclusion is that the CEJSN r-process scenario might account for the new findings regarding the rprocess properties of the UFD galaxy Reticulum II. In addition to the already mentioned characteristics of this scenario that account for the general r-process proper-

CCSNe and/or
SNe Ia after the mixing of r-process and before or during the next star forming episode.

CCSNe and even
SNe Ia occur during the postmixing star formation episode.
High r-process enrichment.
5.The r-process event takes place inside the galaxy. Namely, short delay distance D nk .
D nk RDG leaves most r-process isotopes inside this UFD galaxy [GS19b] . Total number of CCSNe in Reticulum II ≈ 180 [Si22] . About third in the first episode of non-enriched stars.
6. One r-process event per ≈ 100 CCSNe in very low-metallicity stars.
The expected NScore merger to CCSN ratio is 1% [Gr23] Acronyms: CEJSN: common envelope jets supernova; ISM: interstellar medium; UFD: ultra-faint dwarf; CCSN: core collapse supernova; SN Ia: type Ia supernova; NS: neutron star. Definitions: D nk : distance from star formation zone to r-process event; RDG: the radius of the UFD galaxy. ties (e.g., Grichener & Soker 2019a), there are new implications that are applicable to the Reticulum II and possibly other UFD galaxies, but not for the Milky Way Galaxy. First, a very efficient mixing of the r-process isotopes must take place in the entire UFD galaxy (implication 2 in Table 2). This is not the case with the Milky Way. We attribute this to the energetic jets of the CEJSN event that first mix the jets with the CEJSN ejecta, and later with the entire interstellar medium (ISM). The explosion energy cannot efficiently mix the ejecta in a large galaxy as the Milky Way.
The required rate of about one event per 100 CCSNe (implication 6 in Table 2) is compatible with new results of the CEJSN rate (Grichener 2023, preprint).
Despite the compatibility of the CEJSN r-process scenario with the properties in Reticulum II and other advantages of this scenario, we do not claim that this is the only r-process site, but rather that it contributes significantly to r-process nucleosynthesis.