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Register a new userOptical Polarimetric and Near-Infrared Photometric Study of the RCW95 Galactic HII Region
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Jaime Vargas-Gonz\\'alez
Publication date
2017
fields
Physics
and research's language is
English
Authors
Jaime Vargas-Gonzalez
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We carried out an optical polarimetric study in the direction of the RCW95 star forming region in order to probe the sky-projected magnetic field structure by using the distribution of linear polarization segments which seem to be well aligned with the more extended cloud component. A mean polarization angle of $theta=49.8^opm7.7^o$ was derived. Through the spectral dependence analysis of polarization it was possible to obtain the total-to-selective extinction ratio ($R_V$) by fitting the Serkowski function, resulting in a mean value of $R_V=2.93pm0.47$. The foreground polarization component was estimated and is in agreement with previous studies in this direction of the Galaxy. Further, near-infrared images from Vista Variables in the Via Lactea (VVV) survey were collected to improve the study of the stellar population associated with the HII region. The Automated Stellar Cluster Analysis (ASteCA) algorithm was employed to derive structural parameters for two clusters in the region, and a set of PAdova and TRieste Stellar Evolution Code (PARSEC) isochrones was superimposed on the decontaminated colour-magnitude diagrams (CMDs) to estimate an age of about 3 Myr for both clusters. Finally, from the near-infrared photometry study combined with spectra obtained with the Ohio State Infrared Imager and Spectrometer (OSIRIS) mounted at the Southern Astrophysics Research Telescope (SOAR) we derived the spectral classification of the main ionizing sources in the clusters associated with IRAS 15408$-$5356 and IRAS 15412$-$5359, both objects classified as O4 V stars.
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Triggered star formation around HII regions could be an important process. The Galactic HII region RCW 79 is a prototypical object for triggered high-mass star formation. We take advantage of Herschel data from the surveys HOBYS, Evolution of Interstellar Dust, and Hi-Gal to extract compact sources in this region, complemented with archival 2MASS, Spitzer, and WISE data to determine the physical parameters of the sources (e.g., envelope mass, dust temperature, and luminosity) by fitting the spectral energy distribution. We obtained a sample of 50 compact sources, 96% of which are situated in the ionization-compressed layer of cold and dense gas that is characterized by the column density PDF with a double-peaked lognormal distribution. The 50 sources have sizes of 0.1-0.4 pc with a typical value of 0.2 pc, temperatures of 11-26 K, envelope masses of 6-760 $M_odot$, densities of 0.1-44 $times$ $10^5$ cm$^{-3}$, and luminosities of 19-12712 $L_odot$. The sources are classified into 16 class 0, 19 intermediate, and 15 class I objects. Their distribution follows the evolutionary tracks in the diagram of bolometric luminosity versus envelope mass (Lbol-Menv) well. A mass threshold of 140 $M_odot$, determined from the Lbol-Menv diagram, yields 12 candidate massive dense cores that may form high-mass stars. The core formation efficiency (CFE) for the 8 massive condensations shows an increasing trend of the CFE with density. This suggests that the denser the condensation, the higher the fraction of its mass transformation into dense cores, as previously observed in other high-mass star-forming regions.
The expansion of HII regions can trigger the formation of stars. An overdensity of young stellar objects (YSOs) is observed at the edges of HII regions but the mechanisms that give rise to this phenomenon are not clearly identified. Moreover, it is difficult to establish a causal link between HII-region expansion and the star formation observed at the edges of these regions. A clear age gradient observed in the spatial distribution of young sources in the surrounding might be a strong argument in favor of triggering. We have observed the Galactic HII region RCW120 with herschel PACS and SPIRE photometers at 70, 100, 160, 250, 350 and 500$mu$m. We produced temperature and H$_2$ column density maps and use the getsources algorithm to detect compact sources and measure their fluxes at herschel wavelengths. We have complemented these fluxes with existing infrared data. Fitting their spectral energy distributions (SEDs) with a modified blackbody model, we derived their envelope dust temperature and envelope mass. We computed their bolometric luminosities and discuss their evolutionary stages. The herschel data, with their unique sampling of the far infrared domain, have allowed us to characterize the properties of compact sources observed towards RCW120 for the first time. We have also been able to determine the envelope temperature, envelope mass and evolutionary stage of these sources. Using these properties we have shown that the density of the condensations that host star formation is a key parameter of the star-formation history, irrespective of their projected distance to the ionizing stars.
The Galactic HII region luminosity function (LF) is an important metric for understanding global star formation properties of the Milky Way, but only a few studies have been done and all use relatively small numbers of HII regions. We use a sample of 797 first Galactic quadrant HII regions compiled from the WISE Catalog of Galactic HII Regions to examine the form of the LF at multiple infrared and radio wavelengths. Our sample is statistically complete for all regions powered by single stars of type O9.5V and earlier. We fit the LF at each wavelength with single and double power laws. Averaging the results from all wavelengths, the mean of the best-fit single power law index is $langlealpharangle=-1.75,pm,0.01$. The mean best-fit double power law indices are $langlealpha_1rangle=-1.40,pm,0.03$ and $langlealpha_2rangle=-2.33,pm,0.04$. We conclude that neither a single nor a double power law is strongly favored over the other. The LFs show some variation when we separate the HII region sample into subsets by heliocentric distance, physical size, Galactocentric radius, and location relative to the spiral arms, but blending individual HII regions into larger complexes does not change the value of the power law indices of the best-fit LF models. The consistency of the power law indices across multiple wavelengths suggests that the LF is independent of wavelength. This implies that infrared and radio tracers can be employed in place of H$alpha$.
The origin of the Galactic center diffuse X-ray emission (GCDX) is still under intense investigation. We have found a clear excess in a longitudinal GCDX profile over a stellar number density profile in the nuclear bulge region, suggesting a significant contribution of diffuse, interstellar hot plasma to the GCDX. We have estimated that contributions of an old stellar population to the GCDX are about 50 % and 20 % in the nuclear stellar disk and nuclear star cluster, respectively. Our near-infrared polarimetric observations show that the GCDX region is permeated by a large scale, toroidal magnetic field. Together with observed magnetic field strengths in nearly energy equipartition, the interstellar hot plasma could be confined by the toroidal magnetic field.
We report near-infrared photometric measurements of 35 Herbig Ae/Be candidate stars obtained with direct imaging and aperture photometry. Observations were made through the broadband J, H, and K filters, with each source imaged in at least one of the wavebands. We achieved subarcsecond angular resolution for all observations, providing us with the opportunity to search for close binary candidates and extended structure. The imaging revealed five newly identified binary candidates and one previously resolved T Tauri binary among the target sources with separations of <~2.5. Separate photometry is provided for each of the binary candidate stars. We detect one extended source that has been identified as a protoplanetary nebula. Comparing our magnitudes to past measurements yields significant differences for some sources, possibly indicating photometric variability. H-band finding charts for all of our sources are provided to aid follow-up high-resolution imaging.
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