1cmPhysical properties of circumnuclear ionising clusters S. Zamora *1,2, Ángeles I. Díaz,1,2 1Departamento de Física Teórica, Universidad Autónoma de Madrid (UAM), Spain 2Centro de Investigación Avanzada en Física Fundamental (CIAFF), Spain *sandra.zamora@uam.es Abstract Nuclear rings are generally formed because gas is accumulated in orbital resonances, due to a non-axisymmetric gravitational potential related to a barred structure. However, NGC7742 does not exhibit this latter feature, thus being an ideal candidate for a minor merger event. In this work, we analyze 88 HII circumnuclear starforming regions (CNSFRs) of the nuclear ring, from MUSE integral-field spectroscopy data. We expect to test if this is compatible with the idea of all regions having formed at the same time and if a minor merger event is consistent with observations. Observations and analysis NGC7742 was observed with MUSE (Multi-UnitSpectroscopic Explorer, Bacon et al. (2010)). This face-on galaxy is classified as an SA(r)b galaxy. It has a weakly active nucleus and its morphology is dominated by a nuclear ring. Figure 1. Maps for the observed line and continuum fluxes, Av extinction and EW(Hα). Our HII regions segregation is based in HIIEXPLORER (Sánchez et al., 2012) with some additional requirements. We have a total of 88 regions inside and 158 outside of the ring. We have extracted spectra of each region and we have measured: Hβ, Hα, [OIII]λλ4959,5007, [NII]λλ6548,84, [SII]λλ6716,31 and [SII]λ9069 strong lines (S/N>3); and [SIII]λ6312, [OII]λλ7320,30 weak lines (S/N>1). Also, we have calcu- lated integrate fluxes of Sloan Digital Sky Survey(SDSS) filters of each spectrum. Figure 2. Contours of HII regions over the Hα observed map. Characteristics and chemical abundances In general, although the electron density shows similar distributions in the inner and outer HII region, regions within the ring seem to be more diffuse, showing lower filling factors, and therefore lower ionised hydrogen masses, than the regions outside the ring. Figure 3. Funcional parameters of inner and outer ring regions. Direct abundances have been ob- tained for 38 ring HII regions. For the rest of the regions, we have used the recent calibration of the S23 pa- rameter presented in Díaz & Zamora (2022). Total sulphur abundances are between 0.25 and 2.40 times the solar value. The two Ionic species present, S+ and S++, contribute ap- proximately 50% each to the total abundance. Figure 4. Distribution of the total sulphur abundances for the ring HII regions. Ionising cluster properties Figure 5. Ionising and photometric masses. Assuming single stellar population (SSP) PopStar models (Mollá et al., 2009) and the Salpeter IMF we can derive ionis- ing and photometric masses. We have obtained values between 1.22·104 - 5.93·105 M⊙and 2.95·104 - 1.10·106 M⊙respectively. There is an intrinsic difference between photometric and ionizing masses of the clusters in this study, with the first being more than twice the former. Figure 6. Equivalent width and the (R-I) color. Using the equivalent width and the (R-I) color we have estimate a narrow range of ages ∼6.2 Ma. However, a blue and a red excess can be identified in the ring, which suggests the presence of ionizing and non- ionizing composite stellar populations. The underlying disk stellar populations has been subtracted, and the age of the ionising clus- ters is ∼4.7 Ma. Conclusions We can conclude that young stars in the ring of NGC7742 have been formed, almost simultaneously, and they have an age younger than 7.5 Ma. In most cases an underlying stellar population can be identified, besides that of the galaxy disk, pointing to a composite population in the ring. Additionally, regions within the ring show very similar characteristics and abundances among them. This scenario is compatible with the idea of all regions having formed at the same time and if a minor merger event. Bacon R., et al., 2010. p. 773508, doi:10.1117/12.856027 Díaz