The Asymmetric Nebula Surrounding the Extreme Red Supergiant Vy Canis Majoris

We present HST/WFPC2 images plus ground-based infrared images and photometry of the very luminous OH/IR star VY Canis Majoris. Our WFPC2 data show a complex distribution of knots and filamentary arcs in the asymmetric reflection nebula around the obscured central star. The reflection arcs may result from multiple, asymmetric ejection episodes due to localized events on VY CMa's surface. Such events probably involve magnetic fields and convection, by analogy with solar activity. Surface photometry indicates that the star may have experienced enhanced mass loss over the past 1000 yr. We also demonstrate that the apparent asymmetry of the nebula results from a combination of high extinction and backscattering by dust grains. Thermal-infrared images reveal a more symmetric distribution, elongated along a nearly east-west direction. VY CMa probably has a flattened disklike distribution of dust with a northeast-southwest polar axis and may be experiencing activity analogous to solar prominences. The presence of an axis of symmetry raises interesting questions for a star the size of Saturn's orbit. Magnetic fields and surface activity may play an important role in VY CMa's mass-loss history.

[1]  C. Jager.  The yellow hypergiants , 1998 .

[2]  C. Townes,et al.  Characteristics of dust shells around 13 late-type stars. , 1994 .

[3]  G. Neugebauer,et al.  Observations of the infrared object, VY Canis Majoris , 1969 .

[4]  E. Ney.  Infrared observations of comet Kohoutek near perihelion , 1974 .

[5]  David F. Buscher,et al.  High-resolution imaging of Betelgeuse and Mira , 1992 .

[6]  A. Cowley.  THE VV CEPHEI STARS , 1969 .

[7]  K. Johnston,et al.  Observations of OH and H 2 O microwave maser emission from VY Canis Majoris. , 1978 .

[8]  Peter G. Tuthill,et al.  Hotspots on late-type supergiants , 1997 .

[9]  F. Low,et al.  Photometric and Spectroscopic Observations of Infrared Stars , 1970 .

[10]  D. Buhl,et al.  Detection of new stellar sources of vibrationally excited silicon monoxide maser emission at 6.95 millimeters , 1975 .

[11]  Han Uitenbroek,et al.  Spatially Resolved Hubble Space Telescope Spectra of the Chromosphere of α Orionis , 1998 .

[12]  N. Smith,et al.  Thermal Infrared Images of the Remarkable Young Nearby Multiple Star HD 98800 , 1999 .

[13]  Kris Davidson,et al.  THE LUMINOUS BLUE VARIABLES: ASTROPHYSICAL GEYSERS , 1994 .

[14]  A. Dupree,et al.  First Image of the Surface of a Star with the Hubble Space Telescope , 1996 .

[15]  C. Lada,et al.  CO observations of a molecular cloud complex associated with the bright rim near VY Canis Majoris , 1978 .

[16]  G. Wallerstein.  THE SPECTRUM OF THE IRREGULAR VARIABLE VY CANIS MAJORIS , 1958 .

[17]  Observatoire de la Cote d'Azur,et al.  The last gasps of VY Canis Majoris: Aperture synthesis and adaptive optics imagery , 1998 .

[18]  Nicholas J. Wooder,et al.  Reality and the speckle imaging of stellar surfaces - II. the asymmetry of Alpha Orionis , 1997 .

[19]  F. Forbes.  The infrared polarization of eta Carinae and VY Canis Majoris , 1971 .

[20]  C. D. Perrine.  SPECTROSCOPIC NOTES ON SOUTHERN CLUSTERS, NEBULÆ AND RED STARS , 1923 .

[21]  J. Yates,et al.  MERLIN observations of water maser proper motions in VY Canis Majoris , 1998 .

[22]  K. Serkowski.  LARGE OPTICAL POLARIZATION OF THE OH EMISSION SOURCE VY CANIS MAJORIS. , 1969 .

[23]  J. Benson,et al.  VLBI observations of the main line OH masers in VY Canis Majoris , 1982 .

[24]  J. F. Mccarthy,et al.  16-39 micron spectroscopy of oxygen-rich stars , 1979 .

[25]  J. Kastner,et al.  The Massive Post--Red Supergiants IRC +10420 and HD 179821 , 1995 .

[26]  J. Benson,et al.  Multibaseline VLBI observations of the 1612 MHz OH masers toward NML Cygni and VY Canis Majoris , 1979 .

[27]  R. Humphreys,et al.  HST and infrared images of the circumstellar environment of the cool hypergiant IRC+10420 , 1997 .

[28]  H. M. Lee,et al.  Optical properties of interstellar graphite and silicate grains , 1984 .

[29]  J. Chapman,et al.  MERLIN observations of the circumstellar envelope of VX Sagittarius , 1986 .

[30]  G. Herbig.  VY CANIS MAJORIS. III. POLARIZATION AND STRUCTURE OF THE NEBULOSITY. , 1972 .

[31]  D. W. Strecker,et al.  SPECTROSCOPIC AND PHOTOMETRIC OBSERVATIONS OF M SUPERGIANTS IN CARINA. , 1972 .

[32]  G. F. Lawrence,et al.  IRC +10420 - A cool hypergiant near the top of the H-R diagram , 1993 .

[33]  F. Gillett,et al.  The spectrum of VY Canis Majoris from 2.9 to 14 microns , 1970 .

[34]  G. Herbig.  VY Canis Majoris. II. Interpretation of the Energy Distribution , 1970 .

[35]  F. Han,et al.  Rapid Fluctuations of Water Maser Emission in VY Canis Majoris , 1998 .

[36]  L. Auer,et al.  The geometry of VY Canis Majoris derived from SiO maser lines , 1976 .

[37]  P. Bowers,et al.  A study of broad-line OH emission sources : OH 231.8+4.2, VY CMa and M1-92. , 1980 .

[38]  R. Gehrz,et al.  0.7- to 23 μm photometric observations of P/Halley 1986 III and six recent bright comets , 1992 .

[39]  R. Humphreys.  Veiling and the presence of circumstellar gas and dust in some infrared stars , 1974 .

[40]  G. Wallerstein.  An interpretation of the apparent orbit of VY CMa AB: the rotating holey dust cloud hypothesis , 1978 .

[41]  Blair D. Savage,et al.  Observed Properties of Interstellar Dust , 1979 .

[42]  Martin Schwarzschild,et al.  On the scale of photospheric convection in red giants and supergiants. , 1975 .

[43]  B. Balick,et al.  Disk Formation by Asymptotic Giant Branch Winds in Dipole Magnetic Fields , 2000, astro-ph/0008129.

[44]  P. Bowers,et al.  Circumstellar envelope structure of late-type stars , 1983 .

[45]  A. H. Barrett,et al.  Discovery of Hydroxyl Radio Emission from Infrared Stars , 1968, Science.

[46]  X. Mao,et al.  SIO ISOTOPIC MASER EMISSION FROM VY CANIS MAJORIS , 1983 .

[47]  M. Rowan-Robinson,et al.  Models for IRAS observations of circumstellar dust shells around late-type stars , 1986 .

[48]  W. C. Danchi,et al.  Temporal Variations of Midinfrared Spectra in Late-Type Stars , 1998, astro-ph/9803027.

[49]  R. Gehrz,et al.  The Infrared Morphology of η Carinae , 1998 .