VLT/NACO near-infrared imaging and spectroscopy of N88A in the SMC

Abstract

Aims: We present near-infrared imaging and spectroscopic high spatial resolution observations of the SMC region N88 containing the bright, excited, extincted and compact H II region N88A of the size of about 1 pc. Methods: To investigate its stellar content and reddening, N88 was observed using spectroscopy and imagery in the JHKs- and L-band at a spatial resolution of ∼0.1-0.3″, using the VLT UT4 equipped with the NAOS adaptive optics system. In an attempt to establish if the origin of the infra-red (IR) excess is due to bright nebulosity, circumstellar material and/or local dust, we used Ks vs. J - K color-magnitude (CM) and JHK color-color (CC) diagrams, as well as L′ imagery. Results: Our IR-data reveal in the N88 area an IR-excess fraction of ≥30 per cent of the detected stars as well as an unprecedently detailed morphology of N88A. It consists of an embedded cluster of ∼3.5″ (∼1 pc) in diameter of at least thirteen resolved stars superposed with an unusual bright continuum centered on a very bright star. The four brightest stars in this cluster lie red-ward of H - K ≥ 0.45 mag and could be classified as young stellar object (YSO) candidates. Four other probable YSO candidates are also detected in N88 along a north-south bow-shaped thin H2 filament at ∼7″ east of the young central bright star. This star, which we assume to be the main exciting source, could also be complex. At 0.2″ east of this star, a heavily embedded core is detected in the L′band. This core with L′ ∼ 14 mag and L′ - K ≥ 4.5 mag could be a massive class I protostar candidate. The 2.12 μm H2 image of N88A resembles a shell of a diameter of ∼3″ (∼0.9 pc) centered on the bright star. This shell consists of three bright components, of which the brightest one superposes the ionization front. The line ratios of H2 2-1 S(1) and 1-0 S(0) relative to 1-0 S(1), as well as the presence of high v lines, are indicative of photodissociation regions, rather than shocks. © 2010 ESO.