* Astronomy

Blobrana · L

RE: VY Canis Majoris

Blobrana · L

Title: Large dust grains in the wind of VY Canis Majoris
Author: P. Scicluna, R. Siebenmorgen, R. Wesson, J.A.D.L Blommaert, M. Kasper, N.V. Voshchinnikov, S. Wolf

Massive stars live short lives, losing large amounts of mass through their stellar wind. Their mass is a key factor determining how and when they explode as supernovae, enriching the interstellar medium with heavy elements and dust. During the red supergiant phase, mass-loss rates increase prodigiously, but the driving mechanism has proven elusive. Here we present high-contrast optical polarimetric-imaging observations of the extreme red supergiant VY Canis Majoris and its clumpy, dusty, mass-loss envelope, using the new extreme-adaptive-optics instrument SPHERE at the VLT. These observations allow us to make the first direct and unambiguous detection of submicron dust grains in the ejecta; we derive an average grain radius ~ 0.5 µm, 50 times larger than in the diffuse ISM, large enough to receive significant radiation pressure by photon scattering. We find evidence for varying grain sizes throughout the ejecta, highlighting the dynamical nature of the envelope. Grains with 0.5 µm sizes are likely to reach a safe distance from the eventual explosion of VY Canis Majoris; hence it may inject upwards of 10^-2 solar masses of dust into the ISM.

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Blobrana · L

The first known recorded observation of VY Canis Majoris is in the star catalogue of Jérôme Lalande, on 7 March 1801
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Blobrana · L

Title: Adaptive Optics Imaging of VY Canis Majoris at 2 - 5 micron with LBT/LMIRCam
Authors: Dinesh P. Shenoy, Terry J. Jones, Roberta M. Humphreys, Massimo Marengo, Jarron M. Leisenring, Matthew J. Nelson, John C. Wilson, Michael F. Skrutskie, Philip M. Hinz, William F. Hoffmann, Vanessa Bailey, Andrew Skemer, Timothy Rodigas, Vidhya Vaitheeswaran

We present adaptive optics images of the extreme red supergiant VY Canis Majoris in the Ks, L' and M bands (2.15 to 4.8 micron) made with LMIRCam on the Large Binocular Telescope (LBT). The peculiar "Southwest Clump" previously imaged from 1 to 2.2 micron appears prominently in all three filters. We rule out thermal emission as a source of its brightness, which we attribute to scattering alone. We model its brightness as optically thick scattering from silicate dust grains using typical size distributions. We find a lower limit mass of approximately 5E-03 solar masses in this single feature. The presence of the Clump as a distinct feature with no apparent counterpart on the other side of the star is suggestive of an ejection event from a localized region of the star and is consistent with VY CMa's history of asymmetric high mass loss events.

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Astronomers detect titanium oxide and titanium dioxide around the giant star VY Canis Majoris

An international team of astronomers, including researchers from the Max Planck Institute for Radio Astronomy and from the University of Cologne, successfully identified two titanium oxides in the extended atmosphere around a giant star. The object VY Canis Major is one of the largest stars in the known universe and close to the end of its life. The detection was made using telescope arrays in the USA and in France.
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Blobrana · L

Title: Pure rotational spectra of TiO and TiO_2 in VY Canis Majoris
Authors: T. Kaminski, C. A. Gottlieb, K. M. Menten, N. A. Patel, K. H. Young, S. Brunken, H. S. P. Muller, M. C. McCarthy, J. M. Winters, L. Decin

We report the first detection of pure rotational transitions of TiO and TiO_2 at (sub-)millimetre wavelengths towards the red supergiant VY CMa. A rotational temperature, T_rot, of about 250 K was derived for TiO_2. Although T_rot was not well constrained for TiO, it is likely somewhat higher than that of TiO_2. The detection of the Ti oxides confirms that they are formed in the circumstellar envelopes of cool oxygen-rich stars and may be the "seeds" of inorganic-dust formation, but alternative explanations for our observation of TiO and TiO_2 in the cooler regions of the envelope cannot be ruled out at this time. The observations suggest that a significant fraction of the oxides is not converted to dust, but instead remains in the gas phase throughout the outflow.

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Blobrana · L

Title: Aluminium oxide in the optical spectrum of VY Canis Majoris
Authors: T. Kaminski, M. R. Schmidt, K. M. Menten

We report the first identification of the optical bands of the B-X system of AlO in the red supergiant VY CMa. In addition to TiO, VO, ScO, and YO, which were recognised in the optical spectrum of the star long time ago, AlO is another refractory molecule which displays strong emission bands in this peculiar star. Simulating the bands of AlO, we derive a rotational temperature of the circumstellar gas of Trot=700K. By resolving individual rotational components of the bands, we derive the kinematical characteristics of the gas, finding that the emission is centred at the stellar radial velocity and its intrinsic width is 13.5 km/s (full width at half maximum). It is the narrowest emission among all (thermal) features observed in VY CMa so far. The temperature and line widths suggest that the emission arises in gas located within ~20 stellar radii, where the outflow is still being accelerated. This result contradicts equilibrium-chemistry models which predict substantial AlO abundances only to within a few stellar radii. We argue that non-equilibrium models involving propagation of shocks are needed to explain the observations.

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Blobrana · L

Planets and Stars - Size Comparison HD

Blobrana · L

Title: Distance and Kinematics of the Red Hypergiant VY CMa: VLBA and VLA Astrometry
Authors: B. Zhang, M. J. Reid, K. M. Menten, X. W. Zheng

We report astrometric results of phase-referencing VLBI observations of 43 GHz SiO maser emission toward the red hypergiant VY Canis Majoris using the Very Long Baseline Array (VLBA). We measured a trigonometric parallax of 0.83 ±0.08 mas, corresponding to a distance of 1.20^{+0.13}_{-0.10} kpc. Compared to previous studies, the spatial distribution of SiO masers has changed dramatically, while its total extent remains similar. The internal motions of the maser spots are up to 1.4 mas yr^-1, corresponding to 8 kms, and show a tendency for expansion. After modelling the expansion of maser spots, we derived an absolute proper motion for the central star of \mux\ = -2.8 ±0.2 and \muy\ = 2.6 ±0.2 \masy\ eastward and northward, respectively. Based on the maser distribution from the VLBA observations, and the relative position between the radio photosphere and the SiO maser emission at 43 GHz from the complementary Very Large Array (VLA) observations, we estimate the absolute position of VY CMa at mean epoch 2006.53 to be
a\ = 07h 22m 58s.3259 ±0s.0007, \dec\ = -25° 46 03".063 ±0".010. The position and proper motion of VY CMa from the VLBA observations differ significantly with values measured by the Hipparcos satellite. These discrepancies are most likely associated with inhomogeneities and dust scattering the optical light in the circumstellar envelope. The absolute proper motion measured with VLBA suggests that VY CMa may be drifting out of the giant molecular cloud to the east of it.

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Blobrana · L

Title: Snooping around the big dog: VY CMa as seen with Herschel/HIFI
Authors: E. De Beck, L. Decin, K. M. Menten, A. Marston, D. Teyssier, the HIFISTARS team

In the framework of the HIFISTARS guaranteed time key programme, we measured more than 70 molecular emission lines with high signal-to-noise ratio towards VY CMa using the high-resolution HIFI spectrometer on board the Herschel satellite. The kinematic information obtained from the measured water lines supports the hypothesis of multiple outflow components. The observed high-intensity maser lines give no indication for strong polarisation.

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