Search

Context. Our knowledge of the inner structure of embedded massive young stellar objects is still quite limited. Thus, it is difficult to decide to what extent the mass accumulation onto forming massive stars differs from the process of low-mass star formation.

The characterisation of the stellar populations of ultra-compact Hii(UCHii) regions is one of the key means of understanding the formation and evolution of massive stars. Adaptive Optics (AO) assisted near-infrared (NIR) observations provide sufficient resolution and sensitivity to detect such populations at moderate extinction values. We present NIR high-resolution observations of G61.48+0.09, a morphologically complex UCHiiregion with two components, whose accessible stellar content in the NIR  has been widely studied before. A polarimetric map in the K'band, as well as H2(1–0) S(1) and Brγnarrow-band images of the region have been obtained with the AO system ALFA at the Calar Alto Observatory's 3.5 m telescope. We also present high-resolution imaging of the same region in the L'band with NAOS+CONICA at the VLT (UT4). The study of the nebular scattered light points to an internal-illumination model for the eastern component (B2). This model is confirmed by the detection of a point-like source in the L'-band data. In the western component (B1) the observed NIR colours of star 82 are consistent with spectral types BIa or early OV, while star 83 is reproduced by a B0V. Therefore, we identify these objects as the main detected ionising sources. However, in terms of illumination, our polarimetric map indicates that these two stars are not the dominating illuminators of the reflection nebula. Illumination from multiple sources (possibly still undetected) seems to influence the polarisation pattern for this component. Geometric considerations in the calculation of the energetics of this region, combined with the knowledge of the detected stellar content, also points to the presence of other ionising sources. Our study reinforces the hypothesis that there is a champagne flow towards the south-west part of the region.

We present optical and NIR (near infrared) follow up observations of the GRB 001109from 1 to 300 days after the burst. No transient emission was found at these wavelengths within this GRB's (Gamma Ray Burst) 50´´ radius BeppoSAX error box. Strong limits (3σ) are set with: $R\ga 21$, 10.2 h after the GRB; $I\ga 23$, 11.4 h after the GRB; $H\ga 20.7$, 9.9 h after the GRB; and $K_{\rm S}\ga 20$, 9.6 h after the GRB. We discuss whether the radio source found in the GRB's error box ([CITE]) might be related to the afterglow. We also present a multiwavelength study of a reddened starburst galaxy, found coincident with the potential radio and the X-ray afterglow. We show that our strong Iband upper limit makes of the GRB 001109the darkest one localised by the BeppoSAX's NFI (Narrow Field Instrument), and it is one of the most constraining upper limits on GRB afterglows to date. Further to it, the implications of these observations in the context of dark GRBs are considered.

The BL Lacertae object AO 0235+16is well known for its extreme optical and radio variability. New optical and radio data have been collected in the last four years by a wide international collaboration, which confirm the intense activity of this source: on the long term, overall variations of $5\rm mag$in the Rband and up to a factor 18 in the radio fluxes were detected, while short-term variability up to $0.5\rm mag$in a few hours and $1.3\rm mag$in one day was observed in the optical band. The optical data also include the results of the Whole Earth Blazar Telescope (WEBT) first-light campaign organized in November 1997, involving a dozen optical observatories. The optical spectrum is observed to basically steepen when the source gets fainter. We have investigated the existence of typical variability time scales and of possible correlations between the optical and radio emissions by means of visual inspection and Discrete Correlation Function (DCF) analysis. On the long term, the autocorrelation function of the optical data shows a double-peaked maximum at 4100-4200 days (11.2-11.5 years), while a double-peaked maximum at 3900-4200 days (10.7-11.5 years) is visible in the radio autocorrelation functions. The existence of this similar characteristic time scale of variability in the two bands is by itself an indication of optical-radio correlation. A further analysis by means of Discrete Fourier Transform (DFT) technique and folded light curves reveals that the major radio outbursts repeat quasi-regularly with a periodicity of ~5.7 years, i.e. half the above time scale. This period is also in agreement with the occurrence of some of the major optical outbursts, but not all of them. Visual inspection and DCF analysis of the optical and radio light curves then reveal that in some cases optical outbursts seem to be simultaneous with radio ones, but in other cases they lead the radio events. Moreover, a deep inspection of the radio light curves suggests that in at least two occasions (the 1992-1993 and 1998 outbursts) flux variations at the higher frequencies may have led those at the lower ones.

We present the first near-infrared images of the two massive southern star-forming regions G11.11-0.40 and G341.21-0.21, obtained with adaptive optics at sub-arcsecond resolution. Together with conventional narrow-band near-infrared images and polarization maps, mid-infrared data, millimetre and radio data, we were able to determine the morphological structure and to constrain the stellar content of the regions. In both cases the molecular cloud cores have masses of about 500 ${{M}_{\odot}}$and contain clusters of OB stars. This supports the view that massive stars do not form in isolation. In the case of G11.11-0.40 we could identify the central O star responsible for the ionization of the ultracompact Hiiregion. Our results for G341.21-0.21 indicate that this source is a precursor of an ultracompact Hiiregion.

Aims.We present simulations of the perfomances of the future SPHERE IFS instrument designed for imaging extrasolar planets in the near infrared (Y, J, and Hbands).

The formation scenario for massive stars is still under discussion. To further constrain current theories, it is vital to spatially resolve the structures from which material accretes onto massive young stellar objects (MYSOs). Due to the small angular extent of MYSOs, one needs to overcome the limitations of conventional thermal infrared imaging, regarding spatial resolution, in order to get observational access to the inner structure of these objects. We employed mid-infrared interferometry, using the MIDI instrument on the ESO/VLTI, to investigate the Kleinmann-Wright Object, a massive young stellar object previously identified as a Herbig Be star precursor. Dispersed visibility curves in the N-band (8–13 μm) have been obtained at 5 interferometric baselines. We show that the mid-infrared emission region is resolved. A qualitative analysis of the data indicates a non-rotationally symmetric structure, e.g. the projection of an inclined disk. We employed extensive radiative transfer simulations based on spectral energy distribution fitting. Since SED-only fitting usually yields degenerate results, we first employed a statistical analysis of the parameters provided by the radiative transfer models. In addition, we compared the ten best-fitting self-consistent models to the interferometric observations. Our analysis of the Kleinmann-Wright Object suggests the existence of a circumstellar disk of 0.1 M⊙at an intermediate inclination of 76°, while an additional dusty envelope is not necessary for fitting the data. Furthermore, we demonstrate that the combination of IR interferometry with radiative transfer simulations has the potential to resolve ambiguities arising from the analysis of spectral energy distributions alone.