Abstract
We present new near-infrared Gemini Planet Imager (GPI) spectroscopy of HD 206893 B, a substellar companion orbiting within the debris disk of its F5V star. The
J
,
H
,
K1
, and
K2
spectra ...from GPI demonstrate the extraordinarily red colors of the object, confirming it as the reddest substellar object observed to date. The significant flux increase throughout the infrared presents a challenging atmosphere to model with existing grids. Best-fit values vary from 1200 to 1800 K for effective temperature and from 3.0 to 5.0 for log(
g
), depending on which individual wavelength band is fit and which model suite is applied. The extreme redness of the companion can be partially reconciled by invoking a high-altitude layer of submicron dust particles, similar to dereddening approaches applied to the peculiar red field L dwarf population. However, reconciling the HD 206893 B spectra with even those of the reddest low-gravity L dwarf spectra still requires the contribution of additional atmospheric dust, potentially due to the debris disk environment in which the companion resides. Orbit fitting from 4 yr of astrometric monitoring is consistent with a ∼30 yr period, an orbital inclination of 147°, and a semimajor axis of 10 au, well within the estimated disk inner radius of ∼50 au. As one of a very few substellar companions imaged interior to a circumstellar disk, the properties of this system offer important dynamical constraints on companion–disk interaction and provide a benchmark for substellar and planetary atmospheric study.
Aims. We study the radio emission mechanism of the FR-I AGN NGC 5128 (Centaurus A). Methods. We determine the centimeter and millimeter continuum spectrum of the whole Centaurus A radio source and ...measure at frequencies between 86 GHz (3.5 mm) and 345 GHz (0.85 mm) the continuum emission from the active radio galaxy nucleus at various times between 1989 and 2005. Results. The integral radio source spectrum becomes steeper at frequencies above 5 GHz, where the spectral index changes from $\alpha_{\rm low}$ = -0.70 to $\alpha_{\rm high}$ = -0.82. The SW outer lobe has a steeper spectrum than the NE middle and outer lobes (α = -1.0 vs. -0.6). Millimeter emission from the core of Centaurus A is variable, a variability that correlates appreciably better with the 20-200 keV X-ray variability than with 2-10 keV variability. Conclusions. In its quiescent state, the core has a spectral index α = -0.3, which steepens when the core brightens. The variability appears to be mostly associated with the inner nuclear jet components that have been detected in VLBI measurements. The densest nuclear components are optically thick below 45-80 GHz.
ABSTRACT
Spatially resolved images of debris discs are necessary to determine disc morphological properties and the scattering phase function (SPF) thatantifies the brightness of scattered light as a ...function of phase angle. Current high-contrast imaging instruments have successfully resolved several dozens of debris discs around other stars, but few studies have investigated trends in the scattered-light, resolved population of debris discs in a uniform and consistent manner. We have combined Karhunen-Loeve Image Projection (KLIP) with radiative-transfer disc forward modelling in order to obtain the highest-quality image reductions and constrain disc morphological properties of eight debris discs imaged by the Gemini Planet Imager at H-band with a consistent and uniformly applied approach. In describing the scattering properties of our models, we assume a common SPF informed from solar system dust scattering measurements and apply it to all systems. We identify a diverse range of dust density properties among the sample, including critical radius, radial width, and vertical width. We also identify radially narrow and vertically extended discs that may have resulted from substellar companion perturbations, along with a tentative positive trend in disc eccentricity with relative disc width. We also find that using a common SPF can achieve reasonable model fits for discs that are axisymmetric and asymmetric when fitting models to each side of the disc independently, suggesting that scattering behaviour from debris discs may be similar to Solar system dust.
Aims. HD 42527A is one of the most studied Herbig Ae/Be stars with a transitional disk, as it has the largest imaged gap in any protoplanetary disk: the gas is cleared from 30 to 90 AU. The HD 142527 ...system is also unique in that it has a stellar companion with a small mass compared to the mass of the primary star. This factor of ≈20 in mass ratio between the two objects makes this binary system different from any other YSO. The HD 142527 system could therefore provide a valuable test bed for understanding the impact of a lower mass companion on disk structure. This low-mass stellar object may be responsible for both the gap and dust trapping observed by ALMA at longer distances. Methods. We observed this system with the NACO and GPI instruments using the aperture masking technique. Aperture masking is ideal for providing high dynamic range even at very small angular separations. We present the spectral energy distribution (SED) for HD 142527A and B. Brightness of the companion is now known from the R band up to the M′ band. We also followed the orbital motion of HD 142527B over a period of more than two years. Results. The SED of the companion is compatible with a T = 3000 ± 100 K object in addition to a 1700 K blackbody environment (likely a circum-secondary disk). From evolution models, we find that it is compatible with an object of mass 0.13 ± 0.03 M⊙, radius 0.90 ± 0.15 R⊙, and age 1.0+1.0-0.75 Myr. This age is significantly younger than the age previously estimated for HD 142527A. Computations to constrain the orbital parameters found a semimajor axis of 140+120-70 mas, an eccentricity of 0.5 ± 0.2, an inclination of 125 ± 15 degrees, and a position angle of the right ascending node of −5 ± 40 degrees. Inclination and position angle of the ascending node are in agreement with an orbit coplanar with the inner disk, not coplanar with the outer disk. Despite its high eccentricity, it is unlikely that HD 142527B is responsible for truncating the inner edge of the outer disk.
Aims. Spectroscopic, polarimetric, and high spectral resolution interferometric data covering the period 1995–2011 are analyzed to document the transition into a new phase of circumstellar disk ...activity in the classical Be-shell star 48 Lib. The objective is to use this broad data set to additionally test disk oscillations as the basic underlying dynamical process. Methods. The long-term disk evolution is described using the V/R ratio of the violet and red emission components of Hα and Brγ, radial velocities and profiles of He i and optical metal shell lines, as well as multi-band BVRI polarimetry. Single-epoch broad-band and high-resolution interferometric visibilities and phases are discussed with respect to a classical disk model and the given baseline orientations. Results. Spectroscopic signatures of disk asymmetries in 48 Lib vanished in the late nineties but recovered some time between 2004 and 2007, as shown by a new large-amplitude and long-duration V/R cycle. Variations in the radial velocity and line profile of conventional shell lines correlate with the V/R behavior. They are shared by narrow absorption cores superimposed on otherwise seemingly photospheric He i lines, which may form in high-density gas at the inner disk close to the photosphere. Large radial velocity variations continued also during the V/R-quiet years, suggesting that V/R may not always be a good indicator of global density waves in the disk. The comparison of the polarization after the recovery of the V/R activity shows a slight increase, while the polarization angle has been constant for more than 20 years, placing tight limits on any 3-D precession or warping of the disk. The broad H-band interferometry gives a disk diameter of (1.72 ± 0.2) mas (equivalent to 15 stellar radii), position angle of the disk (50 ± 9)° and a relatively low disk flattening of 1.66 ± 0.3. Within the errors the same disk position angle is derived from polarimetric observations and from photocenter shifts across Brγ. The high-resolution interferometric visibility and phase profiles show a double or even multiple-component structure. A preliminary estimate based on the size of the Brγ emitting region indicates a large diameter for the disk (tens of stellar radii). Overall, no serious contradiction between the observations and the disk-oscillation model could be construed.
Context.Optical long-baseline interferometry is moving a crucial step forward with the advent of general-user scientific instruments that equip large aperture and hectometric baseline facilities, ...such as the Very Large Telescope Interferometer (VLTI). Aims.AMBER is one of the VLTI instruments that combines up to three beams with low, moderate and high spectral resolutions in order to provide milli-arcsecond spatial resolution for compact astrophysical sources in the near-infrared wavelength domain. Its main specifications are based on three key programs on young stellar objects, active galactic nuclei central regions, masses, and spectra of hot extra-solar planets. Methods.These key science goals led to scientific specifications, which were used to propose and then validate the instrument concept. AMBER uses single-mode fibers to filter the entrance signal and to reach highly accurate, multiaxial three-beam combination, yielding three baselines and a closure phase, three spectral dispersive elements, and specific self-calibration procedures. Results.The AMBER measurements yield spectrally dispersed calibrated visibilities, color-differential complex visibilities, and a closure phase allows astronomers to contemplate rudimentary imaging and highly accurate visibility and phase differential measurements. AMBER was installed in 2004 at the Paranal Observatory. We describe here the present implementation of the instrument in the configuration with which the astronomical community can access it. Conclusions.After two years of commissioning tests and preliminary observations, AMBER has produced its first refereed publications, allowing assessment of its scientific potential.
Aims.In this paper, we present an innovative data reduction method for single-mode interferometry. It has been specifically developed for the AMBER instrument, the three-beam combiner of the Very ...Large Telescope Interferometer, but it can be derived for any single-mode interferometer. Methods.The algorithm is based on a direct modelling of the fringes in the detector plane. As such, it requires a preliminary calibration of the instrument in order to obtain the calibration matrix that builds the linear relationship between the interferogram and the interferometric observable, which is the complex visibility. Once the calibration procedure has been performed, the signal processing appears to be a classical least-square determination of a linear inverse problem. From the estimated complex visibility, we derive the squared visibility, the closure phase, and the spectral differential phase. Results.The data reduction procedures have been gathered into the so-called amdlib software, now available for the community, and are presented in this paper. Furthermore, each step in this original algorithm is illustrated and discussed from various on-sky observations conducted with the VLTI, with a focus on the control of the data quality and the effective execution of the data reduction procedures. We point out the present limited performances of the instrument due to VLTI instrumental vibrations which are difficult to calibrate.
This paper presents a study of the atmospheric refraction and its effect on the light coupling efficiency in an instrument using single-mode optical fibres. We show the analytical approach which ...allowed us to assess the need to correct the refraction in J and H bands while observing with an 8-m Unit Telescope. We then developed numerical simulations to go further in calculations. The hypotheses on the instrumental characteristics are those of AMBER (Astronomical Multi BEam combineR), the near-infrared focal beam combiner of the Very Large Telescope Interferometric mode, but most of the conclusions can be generalized to other single-mode instruments. We used the software package caos to take into account the atmospheric turbulence effect after correction by the European Southern Observatory system Multi-Application Curvature Adaptive Optics. The optomechanical study and design of the system correcting the atmospheric refraction on AMBER is then detailed. We showed that the atmospheric refraction becomes predominant over the atmospheric turbulence for some zenith angles z and spectral conditions: for z larger than 30° in J band for example. The study of the optical system showed that it allows to achieve the required instrumental performance in terms of throughput in J and H bands. First observations in J band of a bright star, α Cir star, at more than 30° from zenith clearly showed the gain to control the atmospheric refraction in a single-mode instrument, and validated the operating law.
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