| 1. |
- Engler, N., et al.
(författare)
-
The HIP 79977 debris disk in polarized light
- 2017
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 607
-
Tidskriftsartikel (refereegranskat)abstract
- Context. Debris disks are observed around 10 to 20% of FGK main-sequence stars as infrared excess emission. They are important signposts for the presence of colliding planetesimals and therefore provide important information about the evolution of planetary systems. Direct imaging of such disks reveals their geometric structure and constrains their dust-particle properties. Aims. We present observations of the known edge-on debris disk around HIP 79977 (HD 146897) taken with the ZIMPOL differential polarimeter of the SPHERE instrument. We measure the observed polarization signal and investigate the diagnostic potential of such data with model simulations. Methods. SPHERE-ZIMPOL polarimetric data of the 15 Myr-old F star HIP 79977 (Upper Sco, 123 pc) were taken in the Very Broad Band (VBB) filter (lambda(c) = 735 nm, Delta lambda = 290 nm) with a spatial resolution of about 25 mas. Imaging polarimetry efficiently suppresses the residual speckle noise from the AO system and provides a differential signal with relatively small systematic measuring uncertainties. We measure the polarization flux along and perpendicular to the disk spine of the highly inclined disk for projected separations between 0 : 200 (25 AU) and 1 : 600 (200 AU). We perform model calculations for the polarized flux of an optically thin debris disk which are used to determine or constrain the disk parameters of HIP 79977. Results. We measure a polarized flux contrast ratio for the disk of (F-pol) disk/F-* = (5 : 5 +/- 0 : 9) x 10(-4) in the VBB filter. The surface brightness of the polarized flux reaches a maximum of SBmax = 16.2 mag arcsec(-2) at a separation of 0 : 200 -0 : 500 along the disk spine with a maximum surface brightness contrast of 7 : 64 mag arcsec(-2). The polarized flux has a minimum near the star < 0 : 200 because no or only little polarization is produced by forward or backward scattering in the disk section lying in front of or behind the star. The width of the disk perpendicular to the spine shows a systematic increase in FWHM from 0 : 1 (12 AU) to 0 : 3 -0.5, when going from a separation of 0 : 2 to > 1. This can be explained by a radial blow-out of small grains. The data are modelled as a circular dust belt with a well defined disk inclination i = 85(+/- 1 : 5)degrees and a radius between r(0) = 60 and 90 AU. The radial density dependence is described by (r/r(0))alpha with a steep (positive) power law index alpha = 5 inside r(0) and a more shallow (negative) index alpha = -2 : 5 outside r(0). The scattering asymmetry factor lies between g = 0.2 and 0.6 (forward scattering) adopting a scattering-angle dependence for the fractional polarization such as that for Rayleigh scattering. Conclusions. Polarimetric imaging with SPHERE-ZIMPOL of the edge-on debris disk around HIP 79977 provides accurate profiles for the polarized flux. Our data are qualitatively very similar to the case of AU Mic and they confirm that edge-on debris disks have a polarization minimum at a position near the star and a maximum near the projected separation of the main debris belt. The comparison of the polarized flux contrast ratio (F-pol)(disk)/F* with the fractional infrared excess provides strong constraints on the scattering albedo of the dust.
|
|
| 2. |
- Vigan, A., et al.
(författare)
-
First light of the VLT planet finder SPHERE I. Detection and characterization of the substellar companion GJ 758 B
- 2016
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 587
-
Tidskriftsartikel (refereegranskat)abstract
- GJ 758 B is a brown dwarf companion to a nearby (15.76%) solar-type, metal-rich (M/H = +0.2 dex) main-sequence star (G9V) that was discovered with Subaru/HiCIAO in 2009. From previous studies, it has drawn attention as being the coldest (similar to 600 K) companion ever directly imaged around a neighboring star. We present new high-contrast data obtained during the commissioning of the SPHERE instrument at the Very Large Telescope (VLT). The data was obtained in Y-, J-, H-, and K-s-bands with the dual-band imaging (DBI) mode of IRDIS, thus providing a broad coverage of the full near-infrared (near-IR) range at higher contrast and better spectral sampling than previously reported. In this new set of high-quality data, we report the re-detection of the companion, as well as the first detection of a new candidate closer-in to the star. We use the new eight photometric points for an extended comparison of GJ 758 B with empirical objects and four families of atmospheric models. From comparison to empirical object, we estimate a T8 spectral type, but none of the comparison objects can accurately represent the observed near-IR fluxes of GJ 758 B. From comparison to atmospheric models, we attribute a T-eff = 600 +/- 100 K, but we find that no atmospheric model can adequately fit all the fluxes of GJ 758 B. The lack of exploration of metal enrichment in model grids appears as a major limitation that prevents an accurate estimation of the companion physical parameters. The photometry of the new candidate companion is broadly consistent with L-type objects, but a second epoch with improved photometry is necessary to clarify its status. The new astrometry of GJ 758 B shows a significant proper motion since the last epoch. We use this result to improve the determination of the orbital characteristics using two fitting approaches: Least-Squares Monte Carlo and Markov chain Monte Carlo. We confirm the high-eccentricity of the orbit (peak at 0.5), and find a most likely semi-major axis of 46.05 AU. We also use our imaging data, as well as archival radial velocity data, to reject the possibility that this is a false positive effect created by an unseen, closer-in, companion. Finally, we analyze the sensitivity of our data to additional closer-in companions and reject the possibility of other massive brown dwarf companions down to 4-5 AU.
|
|
| 3. |
- Kasper, M., et al.
(författare)
-
NEAR: : Low-mass Planets in α Cen with VISIR
- 2017
-
Ingår i: The Messenger. ; 169, s. 16-20
-
Tidskriftsartikel (refereegranskat)abstract
- ESO, in collaboration with the Breakthrough Initiatives, is working to modify the Very Large Telescope mid-IR imager (VISIR) to greatly enhance its ability to search for potentially habitable planets around both components of the binary Alpha Centauri, part of the closest stellar system to the Earth. Much of the funding for the NEAR (New Earths in the Alpha Cen Region) project is provided by the Breakthrough Initiatives, and ESO mostly provides staff and observing time. The concept combines adaptive optics using the deformable secondary mirror at Unit Telescope 4, a new annular groove phase mask (AGPM) coronagraph optimised for the most sensitive spectral bandpass in the N-band, and a novel internal chopper system for noise filtering based on a concept for longer wavelengths invented by the microwave pioneer Robert Dicke. The NEAR experiment is relevant to the mid-infrared METIS instrument on the Extremely Large Telescope, as the knowledge gained and proof of concept will be transferable.
|
|
| 4. |
|
|
| 5. |
- Pathak, P., et al.
(författare)
-
High-contrast imaging at ten microns : A search for exoplanets around Eps Indi A, Eps Eri, Tau Ceti, Sirius A, and Sirius B
- 2021
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 652
-
Tidskriftsartikel (refereegranskat)abstract
- Context. The direct imaging of rocky exoplanets is one of the major science goals of upcoming large telescopes. The contrast requirement for imaging such planets is challenging. However, the mid-IR (infrared) regime provides the optimum contrast to directly detect the thermal signatures of exoplanets in our solar neighbourhood.Aims. We aim to exploit novel fast-chopping techniques newly developed for astronomy with the aid of adaptive optics to look for thermal signatures of exoplanets around bright stars in the solar neighbourhood.Methods. We used the upgraded Very Large Telescope Imager and Spectrometer for the mid-InfraRed (VISIR) instrument with high-contrast imaging capability optimised for observations at 10 μm to look for exoplanets around five nearby (d < 4 pc) stars. The instrument provides an improved signal-to-noise ratio (S/N) by a factor of ~4 in the N-band compared to standard VISIR for a given S/N and time.Results. In this work, we achieve a detection sensitivity of sub-mJy, which is sufficient to detect a few Jupiter mass planets in nearby systems. Although no detections are made, we achieve most sensitive limits within <2″ for all the observed targets compared to previous campaigns. For ϵ Indi A and ϵ Eri, we achieve detection limits very close to the giant planets discovered by RV, with the limits on ϵ Indi A being the most sensitive to date. Our non-detection therefore supports an older age for ϵ Indi A. The results presented here are promising for high-contrast imaging and exoplanet detections in the mid-IR regime.
|
|
| 6. |
- Ryde, Nils, et al.
(författare)
-
Abundances in bulge stars from high-resolution, near-IR spectra I. The CNO elements observed during the science verification of CRIRES at VLT
- 2009
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 496:3, s. 701-U141
-
Tidskriftsartikel (refereegranskat)abstract
- Context. The formation and evolution of the Milky Way bulge is not yet well understood and its classification is ambiguous. Constraints can, however, be obtained by studying the abundances of key elements in bulge stars. Aims. The aim of this study is to determine the chemical evolution of C, N, O, and a few other elements in stars in the Galactic bulge, and to discuss the sensitivities of the derived abundances from molecular lines. Methods. High-resolution, near-infrared spectra in the H band were recorded using the CRIRES spectrometer on the Very Large Telescope. Due to the high and variable visual extinction in the line-of-sight towards the bulge, an analysis in the near-IR is preferred. The C, N, and O abundances can all be determined simultaneously from the numerous molecular lines in the wavelength range observed. Results. The three giant stars in Baade's window presented here are the first bulge stars observed with CRIRES during its science verification observations. We have especially determined the C, N, and O abundances, with uncertainties of less than 0.20 dex, from CO, CN, and OH lines. Since the systematic uncertainties in the derived C, N, and O abundances due to uncertainties in the stellar fundamental parameters, notably T-eff, are significant, a detailed discussion of the sensitivities of the derived abundances is included. We find good agreement between near-IR and optically determined O, Ti, Fe, and Si abundances. Two of our stars show a solar [C+N/Fe], suggesting that these giants have experienced the first dredge-up and that the oxygen abundance should reflect the original abundance of the giants. The two giants fit into the picture, in which there is no significant difference between the oxygen abundance in bulge and thick-disk stars. Our determination of the sulphur abundances is the first for bulge stars. The high [ S/Fe] values for all the stars indicate a high star-formation rate in an early phase of the bulge evolution.
|
|
| 7. |
- Viswanath, Gayathri, et al.
(författare)
-
Constraints on the nearby exoplanet ϵ Indi Ab from deep near- and mid-infrared imaging limits
- 2021
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 651
-
Tidskriftsartikel (refereegranskat)abstract
- The past decade has seen increasing efforts in detecting and characterising exoplanets using high-contrast imaging in the near- and mid-infrared, which is the optimal wavelength domain for studying old, cold planets. In this work, we present deep adaptive optics imaging observations of the nearby Sun-like star E Ind A with the NaCo (L ') and NEAR (10-12.5 microns) instruments at VLT in an attempt to directly detect its planetary companion, whose presence has been indicated from radial velocity (RV) and astrometric trends. We derive brightness limits from the non-detection of the companion with both instruments and interpret the corresponding sensitivity in mass based on both cloudy and cloud-free atmospheric and evolutionary models. For an assumed age of 5 Gyr for the system, we get detectable mass limits as low as 4.4 M-J in NaCo L ' and 8.2 M-J in NEAR bands at 1.5 ' ' from the central star. If the age assumed is 1 Gyr, we reach even lower mass limits of 1.7 M-J in NaCo L ' and 3.5 M-J in NEAR bands at the same separation. However, based on the dynamical mass estimate (3.25 M-J) and ephemerides from astrometry and RV, we find that the non-detection of the planet in these observations puts a constraint of 2 Gyr on the lower age limit of the system. NaCo offers the highest sensitivity to the planetary companion in these observations, but the combination with the NEAR wavelength range adds a considerable degree of robustness against uncertainties in the atmospheric models. This underlines the benefits of including a broad set of wavelengths for the detection and characterisation of exoplanets in direct imaging studies.
|
|
