Niranjan Thatte

The Gemini NICI Planet-Finding Campaign: Discovery of a Multiple System Orbiting the Young A Star HD?1160

We report the discovery of two low-mass companions to the young A0V star HDxa01160 at projected separations of 81 ± 5xa0AU (HDxa01160xa0B) and 533 ± 25xa0AU (HDxa01160xa0C) by the Gemini NICI Planet-Finding Campaign. Very Large Telescope images of the system taken over a decade for the purpose of using HDxa01160xa0A as a photometric calibrator confirm that both companions are physically associated. By comparing the system to members of young moving groups and open clusters with well-established ages, we estimate an age of 50+50 – 40xa0Myr for HDxa01160 ABC. While the UVW motion of the system does not match any known moving group, the small magnitude of the space velocity is consistent with youth. Near-IR spectroscopy shows HDxa01160xa0C to be an M3.5 ± 0.5 star with an estimated mass of 0.22+0.03 – 0.04 M ☉, while NIR photometry of HDxa01160xa0B suggests a brown dwarf with a mass of 33+12 – 9 M Jup. The very small mass ratio (0.014) between the A and B components of the system is rare for A star binaries, and would represent a planetary-mass companion were HDxa01160xa0A to be slightly less massive than the Sun.

The HARMONI/ELT spectrographs

HARMONI is an Integral Field Spectrograph (IFS) for ESO’s ELT. It has been selected as the first light spec- trograph and will provide the workhorse spectroscopic capabilities for the ELT for many years. HARMONI is currently at the PDR-level and the current design for the HARMONI IFS consists of a number of spaxel scales sampling down to the diffraction limit of the telescope. It uses a field splitter and image slicer to divide the field into 4 sub-units, each providing an input slit to one of four nearly identical spectrographs. All spectrographs will operate at near infrared wavelengths (0.81-2.45 micrometers), sampling different parts of the spectrum with a range of spectral resolving powers (3300, 7000, 18000). In addition, two of the four spectrographs will have a Visible capability (0.5-0.83 micrometers) operating with seeing-limited observations. This proceeding presents an overview of the opto-mechanical design and specifications of the spectrograph units for HARMONI.

HARMONI pre-optics design at PDR

HARMONI is a visible and near-infrared (0.5 to 2.45 μm) integral field spectrograph, providing the E-ELTs core spectroscopic capability, over a range of resolving powers from R (λ/Δλ) ~ 3500 to ~18000. The instrument provides simultaneous spectra of ∼32000 spaxels arranged in a sqrt(2):1 aspect ratio contiguous field. The pre-optics take light entering the science cryostat (from the telescope or calibration system), reformatting and conditioning to be suitable for input for the rest of the instrument. This involves many functions, mainly relaying the light from the telescope focal plane to the integral field unit (IFU) focal plane via a set of interchangeable scale changing optics. The pre-optics also provides components including a focal plane mask wheel, cold pupil masks, spectral order sorting filters, a fast shutter, and a pupil imaging capability to check telescope/instrument pupil alignment. In this paper, we present the optical design of the HARMONI pre-optics at Preliminary Design Review and, in particular, we detail the differences with the previous design and the difficulties salved to the Preliminary Design Review.

System analysis and expected performance of a high-contrast module for HARMONI

HARMONI is a first-light visible and near-IR integral field spectrograph of ESO’s Extremely Large Telescope (ELT) which will sit on top of Cerro Armazones, Chile. A Single Conjugate Adaptive Optics (SCAO) subsystem will provide diffraction-limited spectro-images in a Nyquist-sampled 0.61 x 0.86 arcsec field of view, with a R=3000-20000 spectral resolution. Inside the instrument, a High Contrast Module (HCM) could give HARMONI the ability to spectrally characterize young giant exoplanets (and disks) with flux ratio down to 10−6 as close as 100-200mas from their star. This would be achieved with an apodized pupil coronagraph to attenuate the diffracted light of the star and limit the dynamic range on the detector, and an internal ZELDA wavefront sensor to calibrate non-common path aberrations, assuming that the surface quality of the relay optics of HARMONI satisfy specific requirements. This communication presents (a) the system analysis that was conducted to converge towards these requirement, and the proposed HCM design, (b) an end-to-end simulation tool that has been built to produce realistic datacubes of hour-long observations, and (c) the estimated performance of the HCM, which has been derived by applying differential imaging techniques on the simulated data.

Building the HARMONI engineering model

HARMONI (High Angular Resolution MOnolithic Integral field spectrograph)1 is a planned first-light integral field spectrograph for the Extremely Large Telescope. The spectrograph sub-system is being designed, developed, and built by the University of Oxford. The project has just completed the Preliminary Design Review (PDR), with all major systems having nearly reached a final conceptual design. As part of the overall prototyping and assembly, integration, and testing (AIT) of the HARMONI spectrograph, we will be building a full-scale engineering model of the spectrograph. This will include all of the moving and mechanical systems, but without optics. Its main purpose is to confirm the AIT tasks before the availability of the optics, and the system will be tested at HARMONI cryogenic temperatures. By the time of the construction of the engineering model, all of the individual modules and mechanisms of the spectrograph will have been prototyped and cryogenically tested. The lessons learned from the engineering model will then be fed back into the overall design of the spectrograph modules ahead of their development.

The pre-optics mechanism prototypes for HARMONI

HARMONI is a visible and near-infrared (0.47 to 2.45 μm) integral field spectrograph, providing the ELTs core spectroscopic capability at first light. A pre-optics subsystem provides four selectable spatial pixel scales, in addition to other beam conditioning functions such as shutter and pupil masks. For the validation of the mechanisms in charge of these functions (fast shutter and the plane mask wheel) we have planned some prototypes to test the design solutions. The focal plane mask wheel sits in the input focus of the cryostat. It provides 16 user-selectable positions for masks (28x40 mm) used in observation. The key driver for this mechanism is the high repeatability (±2.5 μm) required, equivalent to ~1mas in the input focal plane. The IAC has previously designed, manufactured, tested and put in operation cryogenic wheels with high repeatability; however, the challenge of obtaining a wheel with such repeatability requires testing new concepts of detent positioning systems. The shutter allows for exposures shorter than the minimum read time of the near-IR detectors and is needed for any CCD observations with the visible cameras. A dual shutter design is needed to achieve the necessary open/close times (<20 ms), but this also provides some redundancy and a graceful failure mode for this critical device. To mitigate risks on the proper behaviour of a fast cryogenics shutter a prototype based on a simple concept has been manufactured. We present the design and results for the performed cryogenic tests of a mask wheel and a shutter prototypes that we have developed.

Analysis and mitigation of pupil discontinuities on adaptive optics performance

As already noticed in other telescopes, the presence of large telescope spiders and of a segmented deformable mirror in an Adaptive Optics system leads to pupil fragmentation and may create phase discontinuities. On the ELT telescope, a typical effect is the differential piston, where all disconnected areas of the pupil create their own piston, unseen locally but drastically degrading the final image quality. The poor sensitivity of the Pyramid WFS to differential piston will lead to these modes been badly seen and therefore badly controlled by the adaptive optics (AO) loop. In close loop operation, differential pistons between segments will start to appear and settle around integer values of the average sensing wavelength. These additional differential pistons are artificially injected by the adaptive optics control loop but do not have any real physical origin, contrary to the Low Wind Effect. In an attempt to reduce the impact of unwanted differential pistons that are injected by the AO loop, we propose a novel approach based on the pair-wise coupling of the actuators sitting on the edges of the deformable mirror segments. In this paper, we present the correction principle, its performance in nominal seeing condition, and its robustness relative to changing seeing conditions, wind speed and natural guide star magnitude. We show that the edge actuator coupling is a simple and robust solution and that the additional quadratic error relative to the reference case (i.e. no spiders) is of only 40 nm RMS, well within the requirements for HARMONI.

Star Formation in NGC 4038/4039

We performed near infrared integral field spectroscopy of several star clusters and the nuclei of the prototypical merger NGC 4038/39 (The Antennae). Near infrared (NIR) images covered both galaxies. The collision of the two gas-rich spiral galaxies has triggered a starburst obvious from a large number of young star clusters. ISO data suggest that the most active star formation occurs in the region where the two galaxy disks overlap. A significant fraction of the total bolometric luminosity of the system is produced there. Since this region is heavily extincted in the optical, the investigation was made in the NIR. Using Br-y emission and CO absorption features as age indicators, we derive the ages and mass estimates of the star clusters from a comparison with stellar synthesis models. Extinction is calculated from the Br-y/Ra ratio. The young, bright star clusters have ages ranging from 4-12 Myrs, while the nuclear starbursts are much older (50-80 Myrs). The overlap region hosts most of the youngest star clusters below -8 Myrs, while the northwestern region is dominated by star clusters in the age range between 8 and 12 Myrs. Several regions, including the northern nucleus, show evidence for sequential star formation on small spatial scales «100pc).