Deep imaging survey of young, nearby austral stars: VLT/NACO near-infrared Lyot-coronographic observations

Context. High contrast and high angular resolution imaging is the optimal search technique for substellar companions to nearby stars at physical separations larger than typically 10 AU. Two distinct populations of substellar companions, brown dwarfs and planets, can be probed and characterized. As a result, fossile traces of processes of formation and evolution can be revealed by physical and orbital properties, both for individual systems and as an ensemble. Aims. Since November 2002, we have conducted a large, deep imaging, survey of young, nearby associations of the southern hemisphere. Our goal is detection and characterization of substellar companions with projected separations in the range 10–500 AU. We have observed a sample of 88 stars, primarily G to M dwarfs, younger than 100 Myr, and within 100 pc of Earth. Methods. The VLT/NACO adaptive optics instrument of the ESO Paranal Observatory was used to explore the faint circumstellar environment between typically 0.1 and 10". Diffraction-limited observations in H and K_s-band combined with Lyot-coronagraphy enabled us to reach primary star-companion brightness ratios as small as 10^(-6). The existence of planetary mass companions could therefore be probed. We used a standardized observing sequence to precisely measure the position and flux of all detected sources relative to their visual primary star. Repeated observations at several epochs enabled us to discriminate comoving companions from background objects. Results. We report the discovery of 17 new close (0.1–5.0") multiple systems. HIP 108195 AB and C (F1 III-M6), HIP 84642 AB (a~14 AU, K0-M5) and TWA22 AB (a~1.8 AU; M6-M6) are confirmed comoving systems. TWA22 AB is likely to be a rare astrometric calibrator that can be used to test evolutionary model predictions. Among our complete sample, a total of 65 targets were observed with deep coronagraphic imaging. About 240 faint companion candidates were detected around 36 stars. Follow-up observations with VLT or HST for 83% of these stars enabled us to identify a large fraction of background contaminants. Our latest results that pertain to the substellar companions to GSC 08047-00232, AB Pic and 2M1207 (confirmed during this survey and published earlier), are reviewed. Finally, a statistical analysis of our complete set of coronagraphic detection limits enables us to place constraints on the physical and orbital properties of giant planets between typically 20 and 150 AU

Adaptive optics imaging survey of the Tucana-Horologium association

We present the results of an adaptive optics (AO) imaging survey of the common associations of Tucana and Horologium, carried out at the ESO 3.6m telescope with the ADONIS/SHARPII system. Based on our observations of two dozen probable association members, HIP 1910 and HIP 108422 appear to have low-mass stellar companions, while HIP 6856 and GSC 8047-0232 have possible sub-stellar candidate companions. Astrometric measurements, performed in November 2000 and October 2001, indicate that HIP 1910 B likely is bound to its primary, but are inconclusive in the case of the candidate companion to HIP 6856. Additional observations are needed to confirm the HIP 6856 companionship as well as for HIP 108422 and GSC 8047-0232.Comment: 7 pages, 4 figures postscript. A&A accepte

Cloud Atlas: Weak Color Modulations Due to Rotation in the Planetary-mass Companion GU Psc b and 11 Other Brown Dwarfs

Among the greatest challenges in understanding ultracool brown dwarf and exoplanet atmospheres is the evolution of cloud structure as a function of temperature and gravity. In this study, we present the rotational modulations of GU Psc b—a rare mid-T spectral type planetary-mass companion at the end of the L/T spectral type transition. Based on the Hubble Space Telescope/WFC3 1.1–1.67 μm time-series spectra, we observe a quasi-sinusoidal light curve with a peak-to-trough flux variation of 2.7% and a minimum period of 8 h. The rotation-modulated spectral variations are weakly wavelength-dependent, or largely gray between 1.1 and 1.67 μm. The gray modulations indicate that heterogeneous clouds are present in the photosphere of this low-gravity mid-T dwarf. We place the color and brightness variations of GU Psc b in the context of rotational modulations reported for mid-L to late-T dwarfs. Based on these observations, we report a tentative trend: mid-to-late T dwarfs become slightly redder in J − H color with increasing J-band brightness, while L dwarfs become slightly bluer with increasing brightness. If this trend is verified with more T-dwarf samples, it suggests that in addition to the mostly gray modulations, there is a second-order spectral-type dependence on the nature of rotational modulations

A Sensitive Search for Variability in Late L Dwarfs: The Quest for Weather

We have conducted a photometric monitoring program of three field late L brown dwarfs (DENIS-P J0255-4700, 2MASS J0908+5032, and 2MASS J2244+2043) looking for evidence of nonaxisymmetric structure or temporal variability in their photospheres. The observations were performed using Spitzer IRAC 4.5 and 8 μm bandpasses and were designed to cover at least one rotational period of each object; 1 σ rms uncertainties of less than 3 mmag at 4.5 μm and around 9 mmag at 8 μm were achieved. Two out of the three objects studied exhibit some modulation in their light curves at 4.5 μm—but not 8 μm—with periods of 7.4 hr (DENIS 0255) and 4.6 hr (2MA 2244) and peak-to-peak amplitudes of 10 and 8 mmag. Although the lack of detectable 8 μm variation suggests an instrumental origin for the detected variations, the data may nevertheless still be consistent with intrinsic variability, since the shorter wavelength IRAC bandpasses probe more deeply into late L dwarf atmospheres than the longer wavelengths. A cloud feature occupying a small percentage (1%-2%) of the visible hemisphere could account for the observed amplitude of variation. If, instead, the variability is indeed instrumental in origin, then our nonvariable L dwarfs could be either completely covered with clouds or objects whose clouds are smaller and uniformly distributed. Such scenarios would lead to very small photometric variations. Follow-up IRAC photometry at 3.6 and 5.8 μm bandpasses should distinguish between the two cases. In any event, the present observations provide the most sensitive search to date for structure in the photospheres of late L dwarfs at mid-IR wavelengths, and our photometry provides stringent upper limits to the extent to which the photospheres of these transition L dwarfs are structured

Task Shifting for Scale-up of HIV Care: Evaluation of Nurse-Centered Antiretroviral Treatment at Rural Health Centers in Rwanda

Fabienne Shumbusho and colleagues evaluate a task-shifting model of nurse-centered antiretroviral treatment prescribing in rural primary health centers in Rwanda and find that nurses can effectively and safely prescribe ART when given adequate training, mentoring, and support

The Anatomy of Star Formation in NGC 300

The Spitzer Space Telescope was used to study the mid-infrared to far-infrared properties of NGC 300, and to compare dust emission to Halpha to elucidate the heating of the ISM and the star formation cycle at scales < 100 pc. The new data allow us to discern clear differences in the spatial distribution of 8 micron dust emission with respect to 24 micron dust and to HII regions traced by the Halpha light. The 8 micron emission highlights the rims of HII regions, and the 24 micron emission is more strongly peaked in star forming regions than at 8 microns. We confirm the existence and approximate amplitude of interstellar dust emission at 4.5 microns, detected statistically in Infrared Space Observatory (ISO) data, and conclude it arises in star forming regions. When averaging over regions larger than ~ 1 kpc, the ratio of Halpha to Aromatic Feature emission in NGC 300 is consistent with the values observed in disks of spiral galaxies. The mid-to-far-infrared spectral energy distribution of dust emission is generally consistent with pre-Spitzer models.Comment: to appear in the ApJS Spitzer special issue (September 2004

The LAOG-Planet Imaging Surveys

With the development of high contrast imaging techniques and infrared detectors, vast efforts have been devoted during the past decade to detect and characterize lighter, cooler and closer companions to nearby stars, and ultimately image new planetary systems. Complementary to other observing techniques (radial velocity, transit, micro-lensing, pulsar-timing), this approach has opened a new astrophysical window to study the physical properties and the formation mechanisms of brown dwarfs and planets. I here will briefly present the observing challenge, the different observing techniques, strategies and samples of current exoplanet imaging searches that have been selected in the context of the LAOG-Planet Imaging Surveys. I will finally describe the most recent results that led to the discovery of giant planets probably formed like the ones of our solar system, offering exciting and attractive perspectives for the future generation of deep imaging instruments.Comment: 6 pages, 5 figures, Invited talk of "Exoplanets and disks: their formation and diversity" conference, 9-12 March 200

Giant Planet Companion to 2MASSW J1207334-393254

We report new VLT/NACO imaging observations of the young, nearby brown dwarf 2MASSW J1207334-393254 and its suggested planetary mass companion (2M1207 b). Three epochs of VLT/NACO measurements obtained over nearly one year show that the planetary mass companion candidate shares the same proper motion and, with a high confidence level, is not a stationary background object. This result confirms the status of 2M1207 b as of planetary mass (5 times the mass of Jupiter) and the first image of a planetary mass companion in a different system than our own. This discovery offers new perspectives for our understanding of chemical and physical properties of planetary mass objects as well as their mechanisms of formation.Comment: 4 pages, 2 figures. accepted in Astronomy & Astrophysic

In-flight performance and calibration of the Infrared Array Camera (IRAC) for the Spitzer Space Telescope

The Infrared Array Camera (IRAC) is one of three focal plane instruments on board the Spitzer Space Telescope. IRAC is a four-channel camera that obtains simultaneous broad-band images at 3.6, 4.5, 5.8, and 8.0 μm in two nearly adjacent fields of view. We summarize here the in-flight scientific, technical, and operational performance of IRAC

Deep search for companions to probable young brown dwarfs. VLT/NACO adaptive optics imaging using IR wavefront sensing

Aims. We have obtained high contrast images of four nearby, faint, and very low mass objects 2MASS J04351455-1414468, SDSS J044337.61+000205.1, 2MASS J06085283-2753583 and 2MASS J06524851-5741376 (hereafter 2MASS0435-14, SDSS0443+00, 2MASS0608-27 and 2MASS0652-57), identified in the field as probable isolated young brown dwarfs. Our goal was to search for binary companions down to the planetary mass regime. Methods. We used the NAOS-CONICA adaptive optics instrument (NACO) and its unique capability to sense the wavefront in the near-infrared to acquire sharp images of the four systems in K_s, with a field of view of 28" x 28". Additional J and L' imaging and follow-up observations at a second epoch were obtained for 2MASS0652-57. Results. With a typical contrast ΔK_s = 4.0−7.0 mag, our observations are sensitive down to the planetary mass regime considering a minimum age of 10 to 120 Myr for these systems. No additional point sources are detected in the environment of 2MASS0435-14, SDSS0443+00 and 2MASS0608-27 between 0.1−12" (i.e. about 2 to 250 AU at 20 pc). 2MASS0652-57 is resolved as a ~230 mas binary. Follow-up observations reject a background contaminate, resolve the orbital motion of the pair, and confirm with high confidence that the system is physically bound. The J, K_s and L′ photometry suggest a q ~ 0.7−0.8 mass ratio binary with a probable semi-major axis of 5−6 AU. Among the four systems, 2MASS0652-57 is probably the less constrained in terms of age determination. Further analysis would be necessary to confirm its youth. It would then be interesting to determine its orbital and physical properties to derive the system’s dynamical mass and to test evolutionary model predictions