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A. -M. Lagrange

Publications and source records attributed to A. -M. Lagrange.

205 records · Page 12Linked to original sources
arXiv2003

The infrared L'-band view of the Galactic Center with NAOS-CONICA at VLT

We report on Galactic Center L'-band observations made during NAOS/CONICA Science Verification. Colors of the inner 2" stars reveal an infrared excess of S2, the closest star to the black hole, that could sign the first thermal infrared detection of Sgr A*. A multi-wavelength maximum likelihood analysis has allowed us to eliminate all but two of the candidates for gravitational lensing proposed by Alexander (2001). Our observations of the thin and intersecting structures of the Northern Arm could trace several shocks heating the neighbooring dust rather than a stream of matter in orbit around the central mass as previously thought.

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arXiv2003

NACO Polarimetric Differential Imaging of TW Hya: A Sharp Look at the Closest T Tauri Disk

We present high-contrast imaging data on the disk of the classical T Tauri star TW Hya. The images were obtained through the polarimetric differential imaging technique with the adaptive optics system NACO. Our commissioning data show the presence of polarized disk emission between 0.1" and 1.4" from the star. We derive the first Ks-band radial polarized intensity distribution. We show that the polarized intensity compares well to shorter wavelengths surface brightness observations and confirm the previously reported gradual slope change around 0.8". These results show the potential of the new polarimetric differential imaging technique at 8m-class telescopes to map the inner regions of protoplanetary disks.

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arXiv2003

Direct measurement of the size and shape of the present-day stellar wind of Eta Carinae

We present new high angular resolution observations at near-IR wavelengths of the core of the Luminous Blue Variable Eta Carinae, using NAOS-CONICA at the VLT and VINCI at the VLT Interferometer (VLTI). The latter observations provide spatial information on a scale of 5 milli-arcsec or ~11 AU at the distance of Eta Carinae. The present-day stellar wind of Eta Carinae is resolved on a scale of several stellar radii. Assuming spherical symmetry, we find a mass loss rate of 1.6x10^{-3} M_sun/yr and a wind clumping factor of 0.26. The VLTI data taken at a baseline of 24 meter show that the object is elongated with a de-projected axis ratio of approximately 1.5; the major axis is aligned with that of the large bi-polar nebula that was ejected in the 19th century. The most likely explanation for this observation is a counter-intuitive model in which stellar rotation near the critical velocity causes enhanced mass loss along the rotation axis. This results from the large temperature difference between pole and equator in rapidly rotating stars. Eta Carinae must rotate in excess of 90 per cent of its critical velocity to account for the observed shape. The large outburst may have been shaped in a similar way. Our observations provide strong support for the existence of a theoretically predicted rotational instability, known as the Omega limit.

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arXiv2003

Near-IR coronagraphic imaging of the companion to HR 7672

This article presents coronagraphic images of the low-mass companion to the star HR 7672 observed at the Palomar 200 inch telescope and first detected at Gemini and Keck in the K band by \citet{liu02}. We obtained additional photometry in $J$($1.2μm$), $H$($1.6μm$) and $K_s$ ($2.2μm$) bands to cover the full near-IR domain and hence to further constrain the nature of the companion. A mass estimate of 58-71$ \mathrm{M_J}$ is derived from evolutionary models of very low-mass objects.

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arXiv2002

Closest Star Seen Orbiting the Supermassive Black Hole at the Centre of the Milky Way

Measurements of stellar velocities and variable X-ray emission near the centre of the Milky Way have provided the strongest evidence so far that the dark mass concentrations seen in many galactic nuclei are likely supermassive black holes, but have not yet excluded several alternative configurations. Here we report ten years of high resolution astrometric imaging that allow us to trace two thirds of the orbit of the star currently closest to the compact radio source and massive black hole candidate SgrA*. In particular, we have observed both peri- and apocentre passages. Our observations show that the star is on a bound, highly elliptical Keplerian orbit around SgrA*, with an orbital period of 15.2 years and a peri-centre distance of only 17 light hours. The orbital elements require an enclosed point mass of 3.7+-1.5x10^6 solar masses. The data exclude with high confidence that the central dark mass consists of a cluster of astrophysical objects or massive, degenerate fermions, and strongly constrain the central density structure.

astro-ph↗
arXiv1999

Possible spectral lines from the gaseous Beta Pictoris disk

We present the results of exploratory HST observations to detect the emission lines of ions in the Beta Pic disk with the spectrograph slit placed off the star. Possible emission lines from FeII have been detected at $\sim 10^{-14}$erg cm-2 s-1 at 0.5 arcsec from Beta Pic, which would suggest a total FeII density of > 2.$10^{-2}$ cm-3 at 10 AU. This detection needs confirmation. If real, it requires a large production rate of gas and dust equivalent to the total disruption of ten bodies 30 kilometers in radius per year.

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