Mid-infrared spectroscopy of SVS13: Silicates, quartz and SiC in a protoplanetary disc

We present $N$-band (8$-$13 $\mu$m) spectroscopic observations of the low-mass, embedded pre-main-sequence close binary system SVS13. Absorption features are clearly detected which are attributable to amorphous silicates, crystalline forsterite, crystalline enstatite and annealed SiO$_{2}$. Most intriguingly, a major component of the dust in the envelope or disc around SVS13 appears to be SiC, required to model adequately both the total intensity and polarisation spectra. Silicon carbide is a species previously detected only in the spectra of C-rich evolved star atmospheres, wherein it is a dust condensate. It has not been unambiguously identified in the interstellar medium, and never before in a molecular cloud, let alone in close proximity to a forming star. Yet pre-Solar grains of SiC have been identified in meteorites, possibly suggesting an interesting parallel between SVS13 and our own Solar-System evolution. The uniqueness of the spectrum suggests that we are either catching SVS13 in a short-lived evolutionary phase and/or that there is something special about SVS13 itself that makes it rare amongst young stars. We speculate on the physical origin of the respective dust species and why they are all simultaneously present toward SVS13. Two scenarios are presented: a disc-instability-induced fragmentation, with subsequent localised heating and orbital evolution firstly annealing initially amorphous silicates and then dispersing their crystalline products throughout a circumstellar disc; and a newly discovered shock-heating mechanism at the interface between the circumstellar and circumbinary discs providing the crystallisation process. One or both of these mechanisms acting on carbon-rich grain material can also feasibly produce the SiC signature

Properties of active galactic star-forming regions probed by imaging spectroscopy with the Fourier transform spectrometer (FTS) onboard AKARI

We investigate the structure of the interstellar medium (ISM) and identify the location of possible embedded excitation sources from far-infrared (FIR) line and mid-infrared continuum emission maps. We carried out imaging spectroscopic observations of four giant Galactic star-forming regions with the Fourier Transform Spectrometer (FTS) onboard AKARI. We obtained [OIII] 88 micron and [CII] 158 micron line intensity maps of all the regions: G3.270-0.101, G333.6-0.2, NGC3603, and M17. For G3.270-0.101, we obtained high-spatial-resolution [OIII] 88 micron line-emission maps and a FIR continuum map for the first time, which imply that [OIII] 88 micron emission identifies the excitation sources more clearly than the radio continuum emission. In G333.6-0.2, we found a local [OIII] 88 micron emission peak, which is indicative of an excitation source. This is supported by the 18 micron continuum emission, which is considered to trace the hot dust distribution. For all regions, the [CII] 158 micron emission is distributed widely as suggested by previous observations of star-forming regions. We conclude that [OIII] 88 micron emission traces the excitation sources more accurately than the radio continuum emission, especially where there is a high density and/or column density gradient. The FIR spectroscopy provides a promising means of understanding the nature of star-forming regions.Comment: 14 pages with 15 figures, accepted for publication in Astronomy and Astrophysic

Resolved 24.5 micron emission from massive young stellar objects

Massive young stellar objects (MYSO) are surrounded by massive dusty envelopes. Our aim is to establish their density structure on scales of ~1000 AU, i.e. a factor 10 increase in angular resolution compared to similar studies performed in the (sub)mm. We have obtained diffraction-limited (0.6") 24.5 micron images of 14 well-known massive star formation regions with Subaru/COMICS. The images reveal the presence of discrete MYSO sources which are resolved on arcsecond scales. For many sources, radiative transfer models are capable of satisfactorily reproducing the observations. They are described by density powerlaw distributions (n(r) ~ r^(-p)) with p = 1.0 +/-0.25. Such distributions are shallower than those found on larger scales probed with single-dish (sub)mm studies. Other sources have density laws that are shallower/steeper than p = 1.0 and there is evidence that these MYSOs are viewed near edge-on or near face-on, respectively. The images also reveal a diffuse component tracing somewhat larger scale structures, particularly visible in the regions S140, AFGL 2136, IRAS 20126+4104, Mon R2, and Cep A. We thus find a flattening of the MYSO envelope density law going from ~10 000 AU down to scales of ~1000 AU. We propose that this may be evidence of rotational support of the envelope (abridged).Comment: 21 pages, accepted for A&

10- m imaging of the bipolar protoplanetary nebula Mz-3

A 10-µm image is presented of the bipolar protoplanetary nebula Mz-3 made at the 3.9-m Anglo-Australian Telescope using the NIMPOL mid-infrared imaging polarimeter. The image shows extended emission from warm (110–130 K) dust both to the north and to the south of the central star, which correlates well with the visible bipolar lobes. The observed surface brightness of this emission is consistent with radiative heating of the dust by both direct stellar illumination and trapped Lyman α photons. Emission in excess of the point-source profile indicates that there is also an extended shell of dust surrounding the central star

Mid- and far-infrared polarimetric studies of the core of OMC-1: the inner field configuration

We present imaging polarimetry of the central 30 arcsec of OMC-1 at 12.5 and 17 μm with arcsecond resolution, together with complementary spectropolarimetry in the ranges 8–13 and 16–22 μ ;m at selected positions, and polarimetry at 800 μm over an approximately 1-arcmin field. The polarization is due to the dichroism of aligned grains in emission in the farinfrared, and predominantly due to absorption in the mid-infrared. The images reveal large variations of polarization fraction and position angle in BNKL, the central region, and these can explain the low fractional polarization observed when this region is unresolved, as in the far-infrared. The mid-infrared polarization indicates that a substantial component of magnetic field is aligned with the plane of the disc-like structures inferred from millimetre-wave studies, and suggests the presence of a toroidal field within the disc

Polarimetric Standard Stars Observed with FORS1 at ESO-VLT

A Paranal Observatory project aims to analyze all the polarimetric standard stars in use at FORS1 to both check they have a constant polarization signal and measure the instrumental polarization. Preliminary results for 3 standards in the FORS1 calibration plans are presented