This article is dedicated to Professor Dr Andrew Charles Gomez, a Consultant Otolaryngology Surgeon from John Hopkins Hospital for his encouragement urging me to complete the last lap of this course in astronomy
by:
lim ju boo BSc PG Dip Nutri MSc (Food QC) MD PhD (Med) FRSPH FRSM
C.Ed Cert Evol. (June, 2019) University of Cambridge
C.Ed Cert Astronomy (December, 2019) University of Oxford
C Ed Cert Forensic Science in Toxicology University of Cambridge
(This paper was written as one of the two assignments required by the University of Oxford in astronomy which the author has successfully submitted for assessment on 24 Nov 2019
Introduction:
Yellowballs are a group of approximately 900 compact, infrared sources identified in the Milky Way Galaxy, and were named by volunteers participating in the Milky Way Project (MWP), a citizen science project that uses GLIMPSE/MIPSGAL images from Spitzer to explore these objects
GLIMPSE (Galactic Legacy Infrared Midplane Extraordinaire) is a survey of the inner Milky Way Galaxy and MIPSGAL is a 278 degree Galactic plane survey conducted through the Multiband Infrared Photometer for Spitzer (MIPS) instrument on the Spitzer Space Telescope.
The GLIMPSE/MIPSGAL images are infrared portrait of dust and stars radiating in the inner Milky Way. More than 800,000 frames from NASA's Spitzer Space Telescope were stitched together to create the full image, capturing more than 50 percent of our entire galaxy (1)
These images shows areas where stars and yellowballs are formed
In a paper by C. R. Kerton, G. Wolf-Chase, K. Arvidsson, C. J. Lintott, R. J. Simpson et al (2) they showed through a combination of catalogue that cross matched with infrared color analysis that yellow balls are a mix of compact star-forming regions, including ultra-compact and compact H II regions, as well as analogous regions for less massive B-type stars.
H II region is a region of interstellar atomic hydrogen that is ionized.
The resulting MWP yellowball catalogue provided a useful complement to the Red MSX Source (RMS) survey as discussed in C. R. Kerton, et al paper.
In their paper, they highlighted regions of massive star formation, but the selection of objects was purely on the basis of their infrared features, and color in Spitzer images that identifies footprints of compact star-forming regions shared across a broad range of luminosities, by inference, and by masses.
The authors discussed the origin of their striking mid-infrared appearance, and proposed that future studies of the yellow ball sample will increase our understanding of how massive and intermediate-mass star-forming regions transition occurs from compact to more extended bubble-like structures.
The Milky Way Project (MWP) is one of a collection of highly prolific online citizen science enterprises in the Zooniverse, developed and maintained by the Citizen Science with the partnership of Lintott et al. 2008 (3), Smith et al. 2011 (4); Fortson et al. 2012. (5)
Spitzer Space Telescope:
The first implementation of the MWP utilized Spitzer GLIMPSE/MIPSGAL images by Benjamin et al. 2003 (6); Mizuno et al. 2008 (7); Churchwell et al. 2009 (8); Carey et al. 2009 (9) was to study star formation over a third of the Galactic plane through the categorization of infrared “bubbles”, which are typical of H II regions and their allied photo dissociation regions (PDRs).
Hydrogen Regions:
The PDRs are conspicuous in the Infrared Array Camera (IRAC) 8 µm band, which traces emission from polycyclic aromatic hydrocarbons (PAHs), while MIPS images of bubble interiors often show 24 µm radiation, which is likely linked with thermal emission from dust grains within H II regions, such as shown by Watson et al. 2009 (10).
The main task of citizen science volunteers is to use an ellipse-drawing tool to mark the sizes, orientations, ellipticities, and thicknesses of H II region/PDR features. The first data expanded the previous bubble catalogs of Churchwell et al. (11, 12) by nearly an order of a magnitude (Simpson et al. 2012 (13).
Additionally, the MWP citizen science classifications have been used as training sets for learning procedure as suggested by Beaumont et al. 2014 (14).
Demonstrating the serendipitous nature of citizen science efforts, volunteers went beyond their assigned role and started tagging and discussing, using the MWP ‘Talk’ interface, compact yellow objects (“yellowballs”) in the GLIMPSE/MIPSGAL images soon after the MWP was opened to the public.
Colour Images:
Colour and compact appearance in the GLIMPSE/MIPSGAL images were used to depict wavelengths of emissions such as blue for 4.5 µm green for 8 µm and red to indicate 24 µm and other representative colour scheme.
All in, a total of 928 yellowballs were identified by MWP participants. Most yellow balls appear in three types of environments, namely as isolated objects in filamentary infrared dark clouds (RDCs), along with bright 24 µm point sources that are typically associated with embedded massive proto-stars (Rathborne et al. 2010 (15); Battersby et al. 2014 (15) which are clustered at the intersection of bipolar bubbles that have been related with outflows from massive proto-cluster.
These are often perpendicular to filamentary IRDCs; and in bubble hierarchies, frequently along the rims of large bubbles, numerous of which have been connected with known H II regions.
The researchers then used the spatial distribution of yellowballs and cross-matched them with existing
MWP images viewed by volunteers:
The catalogues of star-formation tracers, and mid- and far-infrared photometry mentioned in C. R. Kerton at al paper showed that the yellowballs identified in the MWP are a collection of objects tracing a compact, dense phase of massive (O- and B-type) star formation. They mentioned this included a mix of compact and ultra-compact H II regions as well as analogous regions for less massive B stars.
They then examined the origin of the striking mid-infrared appearance of yellowballs and discussed how they fitted into our current picture of massive star formation.
They then discussed their findings on the association of Yellowballs with Red MSX Sources, the method (photometry) used in the survey, the findings and results in sections
Briefly, what they then showed was the distribution of yellowballs in star forming regions by giving their Galactic longitude and latitude along with the distribution of MWP bubbles, and young Red MSX Source (RMS) objects, which have been presented as tracers of Galactic star formation activity (Kendrew et al. 2012 (17); Simpson et al. 2012 (18); Lumsden et al. 2013 (19); Urquhart et al. 2014 (20).
The authors C. R. Kerton and his collaborators then constructed a histogram by dividing the entire range of values into a series of interval and then counted out how many values fall into each interval.
They took the largest bin values and apply offsets so that comparison could be made
Discovery:
What they found was the yellowball distributions have many similarities with the Milky Way Project whereby the bubbles and radio millimeter submillimeter wavelength (RMS) object distributions have mean distribution in Galactic latitude that is slightly below the Galactic mid-plane
In other words, the Galactic longitude and latitude distribution of yellowballs supports the RMS findings in that sources where yellowballs are found is associated with where Galactic star formation is found.
They also looked at the association of Yellowballs with Dense Clumps/Cores
What they did was to use the Virtual Astronomical Observatory (VAO) application topcat (Taylor 2005 (21) to cross-match the list of yellowball positions with the ATLASGAL catalog of compact 870 µm sources
What they found was that 245 (49%) of the 502 yellow balls were found within the Bolocam Galactic Plane Survey (BGPS) which is a 1.1mm continuum survey of 170 square degrees of the galactic plane visible from the northern hemisphere.
The APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) survey encompasses the entire MWP region, and, using the same match distance, the researchers found 524 (56%) of the 928 yellowballs matched with an ATLASGAL compact source. They then listed their comparison in a BGPS and ATLASGAL catalog for identification
They concluded that any contamination from random associations was minimal, and the proximity of yellowballs to regions of dense molecular gas is again consistent with what would be expected for a population of objects associated with star formation activity.
They also looked at the association of Yellow balls with H II Regions by performing a similar analysis to cross-match the yellowball positions with the Wide-field Infrared Survey Explorer (WISE) catalog (Anderson et al. 2014 (22).
They found that 599 (65%) yellowballs have matches with the ATLASGAL, BGPS, WISE-H II region, and RMS catalogs, indicates that the majority of these objects are unambiguously associated with dense molecular clumps and other signposts of star formation.
No yellow balls were associated with any of the evolved star categories in the RMS
Results and Discussion:
In their discussion the authors concluded that the distinct mid-infrared appearance of the yellowballs in the GLIMPSE/MIPSGAL images used in the MWP is not chiefly due to the rough equivalence of the 8 µm and 24 µm fluxes as mentioned in Section 3 of their paper as this was anticipated for all H II regions, but came about because the emission is spatially coincident. This spatial chance they feel is expected in the early stages of the evolution of H II regions/PDRs.
Simulations predict that large PDRs will form around any initial dust-filled ionized region, and that the maximum size of the PDR, and the time at which the maximum size is obtained, are both relatively insensitive to stellar luminosity (Roger & Dewdney 1992 (23).
They also felt that as the ionized region evolves it is expected to catch-up to the photo-dissociation front resulting in a thin, shocked H I region/PDR surrounding the H II region.
Much of the dust will then be removed from the central portion of the H II region via the action of radiation pressure and stellar winds (Draine 2011 (24), and PAHs will be destroyed within the ionized gas (Giard et al. 1994 (25).
This leads to a clear spatial separation between the F8 emitting region (PAH-rich, PDR) shown as green in the MWP images, and the F24 emitting region (depleted interior hot dust, perhaps resupplied by the erosion of denser clumps in the region (Everett & Churchwell 2010 (26) shown as red in the MWP images in their conclusion.
See my other article on the number of stars in the Universe vs. the amount of sands on Earth here:
References:
1. http://www.spitzer.caltech.edu/images/3341-ssc2008-11a2-GLIMPSE-MIPSGAL-Milky-Way-
2. C. R. Kerton, G. Wolf-Chase, K. Arvidsson, C. J. Lintott, R. J. Simpson. The Milky Way Project: What are Yellowballs? arXiv.org > astro-ph > arXiv:1502.01388v1
3. Lintott, C. J., Schawinski, K., Slosar, A., et al. 2008, MNRAS, 389, 1179
4. Smith, A. M., Lynn, S., Sullivan, M., et al. 2011, MNRAS, 412, 1309
5. Fortson, L., Masters, K., Nichol, R., et al. 2012, in Advances in Machine Learning and Data Mining
for Astronomy, ed. M. J. Way, J. D. Scargle, K. M. Ali, & A. N. Srivastava (CRC Press,
Taylor Francis Group) 213
6. Benjamin, R. A., Churchwell, E., Babler, B. L., et al. 2003, PASP, 115, 953
7. Mizuno, D. R., Carey, S. J., Noriega-Crespo, A., et al. 2008, PASP, 120, 1028
8. Churchwell, E., Babler, B. L., Meade, M. R., et al. 2009, PASP, 121, 213
9. Carey, S. J., Noriega-Crespo, A., Mizuno, D. R., et al. 2009, PASP, 121, 76
10. Watson, C., Corn, T., Churchwell, E. B., et al. 2009, ApJ, 694, 546
11. Churchwell, E., Povich, M. S., Allen, D., et al. 2006, ApJ, 649, 759
12. Churchwell, E., Watson, D. F., Povich, M. S., et al. 2007, ApJ, 670, 428
13. Simpson, R. J., Povich, M. S., Kendrew, S., et al. 2012, MNRAS, 424, 2442
14. Beaumont, C. N., Goodman, A. A., Kendrew, S., Williams, J. P., & Simpson, R. 2014, ApJS, 214, 3
15. Rathborne, J. M., Jackson, J. M., Chambers, E. T., et al. 2010, ApJ, 715, 310
16. Battersby, C., Ginsburg, A., Bally, J., et al. 2014, ApJ, 787, 113
17. Kendrew, S., Simpson, R., Bressert, E., et al. 2012, ApJ, 755, 71
18. Simpson, R. J., Povich, M. S., Kendrew, S., et al. 2012, MNRAS, 424, 2442
19. Lumsden, S. L., Hoare, M. G., Urquhart, J. S., et al. 2013, ApJS, 208 11
20. Urquhart, J. S., Figura, C. C., Moore, T. J. T., et al. 2014, MNRAS, 437, 1791
21. Taylor, M. B. 2005, in ASP Conf. Ser. 347, Astronomical Data Analysis Software and Systems XIV, ed. P. Shopbell, M. Britton, Csen& R. Ebert (San Francisco, CA: ASP) 29
22. Anderson, L. D., Bania, T. M., Balser, D. S., et al. 2014, ApJS, 212, 1
23. Roger, R. S. & Dewdney, P. E. 1992, ApJ, 385, 536
24. Draine, B. T. 2011, ApJ, 732, 100
25. Giard, M., Bernard, J. P., Lacombe, F., Normand, P., & Rouan, D. 1994, A&A, 291, 239
26. Everett, J. E. & Churchwell, E. 2010, ApJ, 713, 592
No comments:
Post a Comment