Citizen Scientists Map Massive Star Formation Throughout the Milky Way
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Citizen Scientists Map Massive Star Formation Throughout the Milky Way www.milkywayproject.org Matthew S. Povich California State Polytechnic University, Pomona Milky Way Project Zookeeper and “Science Guru” To be a professional astronomer... What the public thinks we do: What we actually do: Percival Lowell and the Maran “Canals”: An Historical Anecdote on the Perils of “By-Eye” Astronomy Maran “canals” as drawn by Lowell It seems a thousand pities that all those magnificent theories of Lowell was particularly human habitation, canal construction, planetary crystallisation, and interested in the canals of the like are based upon lines which our experiments compel us to Mars, as drawn by Italian declare non-existent; but with the planet Mars still left, and the astronomer Giovanni imagination unimpaired, there remains hope that a new theory no Schiaparelli, director of the less attractive may yet be developed, and on a basis more solid than MilanPercival Lowell (1855-1916) Observatory, during the opposition of 1877. “mere seeming.” (wikipedia) —J. E. Evans & E.W. Maunder Monthly Notices of the Royal Astronomical Society 19033 • The original Galaxy Zoo was launched in 2007. • The data: One million galaxies imaged by the robotic Sloan Digital Sky Survey Telescope. • The response: >50 million classifications by 150,000 people in the first year! • The outcome: 50 articles (and counting) in professional astronomy journals, and the Zooniverse was born... Galaxy classificaon—A simple problem, difficult for computers Normal spiral galaxy Red spiral galaxy Ellipcal galaxy The human eye–brain combinaon is I know my sll the best paern-recognion system Mommy’s in the known universe! voice and face! 5 2008: Hanny van Arkel discovers a “Voorwerp” Interested citizens who are NOT professional scientists can make discoveries reported in leading journals. www.zooniverse.org Transcribe ancient Greek and Lan texts from Papyri unearthed near Oxyrhynchus, Egypt. Help recover worldwide weather observations made by Royal Navy ships around the time of World War I. Help decide the final desnaon for the New Horizons spacecra. Hunt for planets orbing other stars. Find and measure star-forming nebulae throughout our Milky Way Galaxy. Count and measure craters and boulder fields on the Moon. Classify galaxies in Hubble Space Telescope archive images. Spot explosions on the Sun and track their progress across space to Earth. Match the best simulaons of colliding galaxies to images. Hunt for exploding stars in the newest astronomical images. 11 Most stars form in massive clusters.* *So says the prevailing wisdom, anyway—but how massive is “massive”? Hubble Space Telescope view of the Trapezium — heart of the Orion Nebula Cluster High-mass stars like A low-mass star like the Sun… the Trapezium stars… H II region theory 101: Strömgren spheres Strömgren (1939): H II region expansion under thermal pressure RS0 = (3Q0/[4πnH2aB])1/3 (Spitzer 1978): is the Strömgren radius. R (t) This is a sharp boundary S between ionized and = [1 + (7/4)(cs2t/RS0)]4/7 neutral gas. (cs2 ~ 10 km/s is the sound speed in the ionized gas). (Q0 is ionizing photon rate, nH is H gas density, aB is Case B recombination coefficient.) Generally, stars earlier than B3 V emit sufficient Q0 to produce detectible Galactic radio H II regions. Wind-blown bubbles Outer shock—“snowplow” Photoionized shell Contact discontinuity Hot! >107 K; X-rays Wind shock Castor et al. (1975), Weaver et al. (1977), McKee et al. (1984), Koo & McKee (1992), Capriotti & Kozminski (2001), Freyer Not to scale! et al. (2003), Harper-Clark & Murray (2009) Strömgren spheres Wind-blown bubbles Complications: • Ambient medium is never uniform— “Do not underestimate the clumpiness of the ISM!” • Multiple stars often contribute to ionization. • Ambient medium generally in motion with respect to ionizing star(s). • Dust! (And magnetic fields.) • Radiation pressure (see Krumholz & Matzner 2009, Draine 2011). No. 2, 2008 IR DUST BUBBLES 1345 Chandra diffuse soft X-rays Contours: VLA 20 cm (0.5–2 keV) continuum IRAC 5.8 µm Diamonds: OB stars MSX 21.3 µm Spitzer “false-color” IRAC 4.5 µm • stars Fig. 8.—N49 slice at latitude b 0:23, with 20 cm (solid line, magnified [+shocked/ionized gas] 106 times), 24 m(dotted line), and¼À 8 m(dashed line, magnified 5 times). Note that there is no central peak at 24 m, as there is in N10 and N21. IRAC 8.0 µm • PAHs [+hot dust] 24 m/20 cm dip appears to be the central wind-evacuated cavity MIPS 24 µm • warm dust expected around early-O stars. 4. ANALYSIS Bubble N49 We proposeText the following picture for the IR bubbles: ionized gas with a hot dust component is surrounded by a PDR contain- ing swept-up interstellar gas, PAHs, and dust. The ionized gas is traced by 20 cm free-free emission, and the hot dust within the H ii region is bright at 24 m via thermal continuum emission. The IR bubbles are enclosed by a shell of 8memissiondominated by PAH emission features in IRAC bands 3.6, 5.8, and 8.0 m. The inner face of the 8 m shell defines the PAH destruction radius from the central ionizing star(s). In the following sections, we determine the PAH destruction radii and PDR shell thick- Triggered massivenesses based on 5.8 m/4.5 m and 8.0 m/4.5 mTownsley et al. (2003) flux density ratios. star formaon? Povich et al. (2007) C. Watson, Povich et al. 4.1. PAH Destruction Povich et al. (2007) argued that ratios of IRAC bands that con- tain strong PAH emission features (8.0 and 5.8 m bands) to the 4.5 m band (which contains no PAH feature) can be used to de- Evere & Churchwell (2010): Dusttermine must be the PAHconnuously replenished destruction radius and define the extentwithin N49. of PDRs around hot stars. This technique was applied by Povich Fig. 7.—N49 24 m(red ), 8Draine (2011): 20 cm shell in N49 shaped by m(green), 4.5 m(blue), and 20 cm (con- radiaon pressure and winds. tours, bottom panel). The white dashed line in the top panel indicates the loca- et al. (2007) to derive both the PAH destruction region in M17 tion of the cross-cut in Fig. 8. and the extent of its PDR because the 8.0, 5.8, and 3.6 m IRAC bands all contain PAH bands, whereas band ratios involving the 4.5 m PAH-free band should be especially sensitive to regions of 5:7 0:6 kpc (Churchwell et al. 2006). At 1.4 GHz it has an containing PAHs. They supported their interpretation of these ra- integratedÆ flux density of 2.8 Jy and an angular radius out to the tios by showing that the 5.8 m/3.6 m ratio does not delineate background of 1.50 ( 2.5 pc; Helfand et al. 2006). To maintain the PDR boundaries. They also presented IRS spectra that proved 48 1 its ionization, 7:8 ; 10 ionizing photons sÀ are necessary, equiv- the disappearance of PAH features within the M17 H ii region. alent to a single O6 V star (MSH05). The radius to the inner face Povich et al. (2007) were unable to use the 8.0 m images of of the 8 m shell is 1.20 (2.0 pc), and out to the background level M17 because the detector was saturated over large regions. We it is 1.70 (2.3 pc). have applied this technique to N10, N21, and N49. The quanti- N49 has a double-shell structure, the outer traced by 8m emis- tative ratios are different from those toward M17 because M17 sion and the inner traced by 24 m and 20 cm emission (see is a much more luminous region, but the principle is the same. Fig. 7). As in N10 and N21, 8memissionenclosesboththe24m Since N10, N21, and N49 do not saturate the 8.0 m detector, we and 20 cm emission. The transition between the 8 m emission are able to use this band in our analysis as well. ring and the 24 m and 20 cm emission ring can be clearly seen Figs. 9–11 show false-color images of the 5.8 m/4.5 m and in the slice at constant latitude in Figure 8. The 20 cm and 24 m 8.0 m/4.5 m band ratios, with accompanying longitude or emission are coincident, and both have a central cavity. The latitude cuts (averaged over 20 pixels) for all three bubbles. We The “Bubble Search” in the Milky Way Project was inspired by the work of undergraduates (including a freshman) at the University of Wisconsin Churchwell et al. (2006, 2007) Bubble Classification and Measurement BUBBLINGMethodology GALACTIC DISK 767 Fig. 1.—(a) Illustration of a bubble ( N4) where all four examiners agree on the extent, thickness, and eccentricity. There was disagreement on the orientation of the major axis, but this1 was or because 2 theprimary bubble is almost (student) circular. (b) Illustration classifiers, of a bubble (S175) plus where the 2-3 examiners secondary had substantial disagreements on the size, thickness, and eccentricity. The disagreement was caused mainly by the bright emission to the left of the bubble. In both panels, the annular ellipses fitted by examiner 1 are shown in green.classifiers The white, cyan, working and yellow arrows with show therepresentative extent of the major and minor sub-sample. axes of the inner and outer ellipses as measured independently by the other three examiners. The horizontal scale bars represent 20. stars, whereas the 8.0 memissionisfaintorabsentinsidethe tively. The W77 theory suggests that it may not be easy to distin- bubbles, is bright along the shell that defines the bubbles, and guish between possible static versus expanding bubbles by shell generally extends well beyond the bubble shell boundary.