Optical spectroscopic classification and membership of young M dwarfs in star-forming regions
Monthly Notices of the Royal Astronomical Society 381:3 (2007) 1067-1076
Abstract:
The spectral type is a key parameter in calibrating the temperature which is required to estimate the mass of young stars and brown dwarfs. We describe an approach developed to classify low-mass stars and brown dwarfs in the Trapezium Cluster using red optical spectra, which can be applied to other star-forming regions. The classification uses two methods for greater accuracy: the use of narrow-band spectral indices which rely on the variation of the strength of molecular lines with spectral type and a comparison with other previously classified young, low-mass objects in the Chamaeleon I star-forming region. We have investigated and compared many different molecular indices and have identified a small number of indices which work well for classifying M-type objects in nebular regions. The indices are calibrated for young, pre-main-sequence objects whose spectra are affected by their lower surface gravities compared with those on the main sequence. Spectral types obtained are essentially independent of both reddening and nebular emission lines. Confirmation of candidate young stars and brown dwarfs as bona fide cluster members may be accomplished with moderate resolution spectra in the optical region by an analysis of the strength of the gravity-sensitive Na doublet. It has been established that this feature is much weaker in these very young objects than in field dwarfs. A sodium spectral index is used to estimate the surface gravity and to demonstrate quantitatively the difference between young (1-2 Myr) objects, and dwarf and giant field stars. © 2007 RAS.Silicate absorption in heavily obscured galaxy nuclei
Monthly Notices of the Royal Astronomical Society 375:1 (2007) 99-104
Abstract:
Spectroscopy at 8-13 μm with T-ReCS on Gemini-S is presented for three galaxies with substantial silicate absorption features, NGC 3094, NGC 7172 and NGC 5506. In the galaxies with the deepest absorption bands, the silicate profile towards the nuclei is well represented by the emissivity function derived from the circumstellar emission from the red supergiant, μ Cephei which is also representative of the mid-infrared absorption in the diffuse interstellar medium in the Galaxy. There is spectral structure near 11.2 μm in NGC 3094 which may be due to a component of crystalline silicates. In NGC 5506, the depth of the silicate absorption increases from north to south across the nucleus, suggestive of a dusty structure on scales of tens of parsecs. We discuss the profile of the silicate absorption band towards galaxy nuclei and the relationship between the 9.7-μm silicate and 3.4-μm hydrocarbon absorption bands. © 2007 RAS.Gemini mid-IR polarimetry of NGC 1068: Polarized structures around the nucleus
ASTROPHYSICAL JOURNAL 661:1 (2007) L29-L32
Mid-infrared sub-arcsecond spectroscopy of active galaxy nuclei
REV MEX AST ASTR 29 (2007) 117-119
Abstract:
I present recent spectroscopic observations of nearby AGN obtained at 8-13 mu m with TReCS on Gemini-S. The observations reveal variations in absorption depth on subarcsecond scales in Circinus and NGC 5506 and delineate the regions of ionized gas and PAH emission region in Circinus. Several nearby active galaxies show evidence of resolved circumnuclear dusty structures with sizes of 10s of parsecs. The silicate absorption profile towards the most heavily obscured nuclei are narrower than those in the galaxies with tau((9.7 mu m)) < 3, possibly reflecting larger average grain sizes in the latter resulting from destruction of the smallest grains by the AGN.Spectropolarimetry of the 3.4 μm feature in the diffuse ISM toward the Galactic center quintuplet cluster
Astrophysical Journal 651:1 I (2006) 268-271