The Submillimeter Array



Dying Stars:
Line survey in the 345-GHz band of the peculiar super giant VY CMa

VY CMa is an evolved massive star with a very high mass-loss rate. The dusty material expelled by the cool star formed an extended and complex nebula which has been studied extensively at optical wavelengths. The Submillimeter Array obtained a spectral-line survey of this oxygen-rich red supergiant between 279 and 355 GHz. 220 lines from 19 molecules and their isotopic species were observed, including for the first time the rotational spectra of TiO, TiO2, and AlCl. Many of the species are observed at excited vibrational states. Maps made for each line with a synthesized beam of 0.9 arcsec demonstrate the asymmetric angular distributions of some of the distinct features that comprise the complex morphology observed previously using the Hubble Space Telescope. In addition, the complex spatio-kinematical structure of the nebula at large scales is revealed at an unprecedented detail. Unlike in optical observations, where scattering on dust complicates the analysis of the kinematical structure of the nebula, at sub-millimeter wavelengths the kinematical structure can be studied directly. We have also performed a simple excitation analysis of each molecule seen in multiple transitions which reveals a wide range in excitation temperatures, densities, and abundances in the circumstellar gas. The presented observational material can be used in future studies to gain our understanding of the molecular chemistry in oxygen-dominated circumstellar environments and help us better understand the nature of the peculiar red supergiant, VY CMa.



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Example results of the SMA survey toward VY CMa. Top panel shows contours of molecular emission overlayed on an optical image of the nebula. Bottom panel shows complex kinematics of the nebula traced in molecular lines. The different colors mark different kinematical components. The bottom right panel contains a three-color image from the Hubble Space Telescope. This is an enviroment in which a (core-collapse) supernova explosion will take place in future.

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The top panel present the entire spectrum of VY CMa acquired with the SMA where only the strongest emission lines are recognizable. The bottom panel shows a zoomed-in sample portion of the spectrum, which is marked with a red box in the upper panel. VY CMa is a source of a myriad of emission lines with many different line profiles which reflect a very high chemical and kinematical complexity of gas surrounding this massive dying star. The interferometric maps delivered by SMA for each frequency allow us to study chemical processes that lead to molecule and dust formation.