Unveiling the near-infrared structure of the massive-young stellar object NGC 3603 IRS 9A with sparse aperture masking and spectroastrometry

摘要

Contemporary theory holds that massive stars gather mass during their initialphases via accreting disk-like structures. However, conclusive evidence fordisks has remained elusive for the most massive young objects. This is mainlydue to significant observational challenges. Incisive studies, even targetingindividual objects, are therefore relevant to the progression of the field. NGC3603 IRS 9A* is a young massive stellar object still surrounded by an envelopeof molecular gas. Previous mid-infrared observations with long-baselineinterferometry provided evidence for a disk of 50 mas diameter at its core.This work aims at a comprehensive study of the physics and morphology of IRS 9Aat near-infrared wavelengths. New sparse aperture masking interferometry datataken with NACO/VLT at Ks and Lp filters were obtained and analysed togetherwith archival CRIRES spectra of the H2 and BrG lines. The calibratedvisibilities recorded at Ks and Lp bands suggest the presence of a partiallyresolved compact object of 30 mas at the core of IRS 9A, together with thepresence of over-resolved flux. The spectroastrometric signal of the H2 lineshows that this spectral feature proceeds from the large scale extendedemission (300 mas) of IRS 9A, while the BrG line appears to be formed at thecore of the object (20 mas). This scenario is consistent with the brightnessdistribution of the source for near- and mid-infrared wavelengths at variousspatial scales. However, our model suffers from remaining inconsistenciesbetween SED modelling and the interferometric data. Moreover, the BrGspectroastrometric signal indicates that the core of IRS 9A exhibits some formof complexity such as asymmetries in the disk. Future high-resolutionobservations are required to confirm the disk/envelope model and to flesh outthe details of the physical form of the inner regions of IRS 9A.