Principal Components of the Distribution of Molecules in the Orion Molecular Cloud

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Session 61 -- Molecular Clouds, Star Formation
Display presentation, Thursday, 9:20-4:00, Pauley Room

[61.09] Principal Components of the Distribution of Molecules in the Orion Molecular Cloud

H.Ungerechts, E.A.Bergin, P.F.Goldsmith, W.M.Irvine, F.P.Schloerb, and R.L.Snell (FCRAO, University of Massachusetts)

We have completed the observations for our chemical survey of 30 rotational transitions of 21 chemical and isotopic molecular species in the central Orion molecular ridge. The observations were done with Nyquist sampling using the 15~element focal plane array QUARRY on the FCRAO 14~m telescope. Apart from the standard tracers of molecular gas at low and high densities, CO and CS, we mapped important ions and radicals, the isomeric pair HCN and HNC, {CH$_3$OH}, {C$_3$H$_2$}, and multiple transitions of {HC$_3$N} and {CH$_3$CCH} as measures of density and temperature in the dense gas. The maps have 360 pixels on a $0.41'$ grid and cover an area of $4.5'$ by $12'$, extending from the radical region $\approx4'$ north of Orion-KL to the bar $\approx4'$ south.

We employ a principal component (PC) analysis (see, e.g., Deeming, 1964, MNRAS, 127, 35) to visualize the relation between the derived maps of integrated line intensity and to find classes of species that have similar distributions. At the same time we can identify regions in the cloud with similar relative line strengths and therefore presumably similar physical properties and chemical abundances. The main result of our analysis is that a surprisingly small number of principal components, only 3 to 5, is sufficient to represent the major features in about 30 different maps. The first PC is a weighted mean of all line intensities and reflects the overall distribution of molecular gas in the Orion ridge, the next few higher order PCs reveal variations of the relative intensities, in particular in the radical region, around KL, and in the southern bar. The second PC shows the enhancement of {H$^{13}$CO$^+$}, {N$_2$H$^+$}, CN, {C$_2$H}, {C$_3$H$_2$}, and {HN$^{13}$C} in the radical region, and to a lesser degree in the southern bar. Positions around Orion-KL fall onto the extreme opposite end of the second PC characterized by high intensities of {$^{12}$CO}, SO, HCN, {CH$_3$OH}, and {HC$_3$N}. The third PC differentiates between the properties of the radical region and the southern bar.

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