arXiv subjects

L. D. Anderson

Publications and source records attributed to L. D. Anderson.

112 records · Page 7Linked to original sources

arXiv2010

The physical properties of the dust in the RCW 120 HII region as seen by Herschel

Context. RCW 120 is a well-studied, nearby Galactic HII region with ongoing star formation in its surroundings. Previous work has shown that it displays a bubble morphology at mid-infrared wavelengths and has a massive layer of collected neutral material seen at sub-mm wavelengths. Given the well-defined photo-dissociation region (PDR) boundary and collected layer, it is an excellent laboratory to study the "collect and collapse" process of triggered star formation. Using Herschel Space Observatory data at 100, 160, 250, 350, and 500 micron, in combination with Spitzer and APEX-LABOCA data, we can for the first time map the entire spectral energy distribution of an HII region at high angular resolution. Aims. We seek a better understanding of RCW120 and its local environment by analysing its dust temperature distribution. Additionally, we wish to understand how the dust emissivity index, beta, is related to the dust temperature. Methods. We determine dust temperatures in selected regions of the RCW 120 field by fitting their spectral energy distribution (SED), derived using aperture photometry. Additionally, we fit the SED extracted from a grid of positions to create a temperature map. Results. We find a gradient in dust temperature, ranging from >30 K in the interior of RCW 120, to ~20K for the material collected in the PDR, to ~10K toward local infrared dark clouds and cold filaments. Our results suggest that RCW 120 is in the process of destroying the PDR delineating its bubble morphology. The leaked radiation from its interior may influence the creation of the next generation of stars. We find support for an anti-correlation between the fitted temperature and beta, in rough agreement with what has been found previously. The extended wavelength coverage of the Herschel data greatly increases the reliability of this result.

astro-ph.GA↗

arXiv2009

Tracing Multiple Generations of AGN Feedback in the Core of Abell 262

We present new radio and X-ray observations of Abell 262. The X-ray residual image provides the first evidence of an X-ray tunnel in this system while the radio data reveal that the central radio source is more than three times larger than previously known. We find that the well-known cluster-center S-shaped radio source B2 0149+35 is surrounded by extended emission to the east and south-west. The south-western extension is co-spatial with the X-ray tunnel seen in our new Chandra images while the eastern extension shows three clumps of emission with the innermost coincident with a faint X-ray cavity. The outer two eastern radio extensions are coincident with a newly detected X-ray depression. We use the projected separation of the emission regions to estimate a lower limit of tau_rep=28 Myr to the outburst repetition timescale of the central AGN. The total energy input into the cluster over multiple outburst episodes is estimated to be 2.2x 10^{58} ergs, more than an order of magnitude larger than previously thought. The total AGN energy output determined from our new observations shows that the source should be capable of offsetting radiative cooling over several outburst episodes.

astro-ph.GA↗

arXiv2008

Resolution of the Distance Ambiguity for Galactic HII Regions

We resolve the kinematic distance ambiguity for 266 inner Galaxy HII regions out of a sample of 291 using existing HI and 13CO sky surveys. Our sample contains all HII regions with measured radio recombination line (RRL) emission over the extent of the 13CO Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey (18 deg, < l < 55 deg. and |b| < 1) and contains ultra compact, compact, and diffuse HII regions. We use two methods for resolving the distance ambiguity for each HII region: HI emission/absorption (HIEA) and HI self-absorption (HISA). We find that the HIEA and HISA methods can resolve the distance ambiguity for 72% and 87% of our sample, respectively. When projected onto the Galactic plane, this large sample appears to reveal aspects of Galactic structure, with spiral arm-like features at Galactocentric radii of 4.5 and 6 kpc, and a lack of HII regions within 3.5 kpc of the Galactic center. Our HII regions are approximately in the ratio of 2 to 1 for far verses near distances. The ratio of far to near distances for ultra-compact HII regions is 2.2 to 1. Compact HII regions are preferentially at the near distance; their ratio of far to near distances is 1.6 to 1. Diffuse HII regions are preferentially at the far distance; their ratio of far to near distances is 3.8 to 1. This implies that the distinction between ultra compact and compact HII regions is due largely to distance, and that the large angular size of diffuse HII regions is not due solely to proximity to the Sun.

astro-ph↗

arXiv2008

The Molecular Properties of Galactic HII Regions

We derive the molecular properties for a sample of 301 Galactic HII regions including 123 ultra compact (UC), 105 compact, and 73 diffuse nebulae. We analyze all sources within the BU-FCRAO Galactic Ring Survey (GRS) of 13CO emission known to be HII regions based upon the presence of radio continuum and cm-wavelength radio recombination line emission. Unlike all previous large area coverage 13CO surveys, the GRS is fully sampled in angle and yet covers ~75 square degrees of the Inner Galaxy. The angular resolution of the GRS 46" allows us to associate molecular gas with HII regions without ambiguity and to investigate the physical properties of this molecular gas. We find clear CO/HII morphological associations in position and velocity for ~80% of the nebular sample. Compact HII region molecular gas clouds are on average larger than UC clouds: 2.2' compared to 1.7'. Compact and UC HII regions have very similar molecular properties, with ~5K line intensities and ~4 km/s line widths. The diffuse HII region molecular gas has lower line intensities, ~3K, and smaller line widths, ~3.5 km/s. These latter characteristics are similar to those found for quiescent molecular clouds in the GRS. Our sample nebulae thus show evidence for an evolutionary sequence wherein small, dense molecular gas clumps associated with UC HII regions grow into older compact nebulae and finally fragment and dissipate into large, diffuse nebulae.

astro-ph↗