This week on HST

HST Programs: April 13 - April 19, 2015

Selected highlights

GO 13331: Confirmation and characterization of young planetary companions hidden in the HST NICMOS archive

Archival HST NICMOS images of the planetary companions to HR 8799

Planet formation occurs in circumstellar disks around young stars. Most of the gaseous content of those disks dissipates in less than 10 million years, leaving dusty debris disks that are detectable through reflect light at near-infrared and, to a lesser extent, optical wavelengths. The structure of those disks is affected by massive bodies (i.e. planets and asteroids), which, through dynamical interactions and resonances, can produce rings and asymmetries. Analysis of the rangle of morphological structure in these systems provides insight into the distribution of properties of planetary systems; in a few cases, massive planetary companions can be detected directly. Ground-based instruments such as GPI and Sphere, mounted on Gemini and the VLT, are starting to have a significant impact on this subject, but the 2.4-metre HST remains a highly effective means of achieving the high-contrast required for the detection of both disks and candidate companions. To date, while many systems have been observed, only a relatively small number of systems have been imaged successfully at visual or near-infrared wavelengths. Archival HST imaging exists for many nearby stars, and a suite of new reduction techniques have been applied to those data, removing the direct starlight with greater fidelity to enable detection of faint debris disks. The present program builds on those results, using Wide-Field Camera 3 to obtain high-contrast images of six young stars that have been identified as having candidate planetary companions.

GO 13694: UV spectra of the icy Saturnian satellites: Understanding exogenic processes and NH3 in the system

Cassini image of Saturn's satellite, Mimas

The Saturnian planetary system includes seven major satellites with diameters exceeding 300 kilometres,together with 50 or more smaller bodies. The largest satellite, Titan, is bigger than Mercury; the remaining satellites have diameters between ~400 km (Mimas) and ~1,500 km (Iapetus and Rhea). These satellites have been studied extensively with ground- and space-based observatories and, since 2004, through in situ measurements made by the Cassini mission. Those observations have provided clues to the surface composition, specifically the likely presence of ices. However, current estimates remain inconclusive due to relatively poor spectral coverage at wavelengths between 1900 and 3500 Angstroms. the present program aims to remedy this defect by using STIS to obtain spectra covering that wavelength raneg for Mimas, Dione and Rhea. Ammonia is suspected as being a key contributor to the icy surfaces; if so, characteristic absorption features should be evident near 2000 Angstroms.

GO 13847: Determining the Mass of Proxima Centauri through Astrometric Microlensing

An AAO image centred on the nearby red dwarf, Proxima Centauri

Gravitational lensing is a consequence of general relativity. Its effects were originally quantified by Einstein himself in the mid-1920s. In the 1930s, Fritz Zwicky suggested that galaxies could serve as lenses, but lower mass objects can also also lens background sources. Bohdan Paczynski pointed out in the mid-1980s that this offered a means of detecting dark, compact objects that might contribute to the dark-matter halo. Paczcynski's suggestion prompted the inception of several large-scale lensing surveys, notably MACHO, OGLE, EROS and DUO. Those wide-field imaging surveys have target high density starfields towards the Magellanic Clouds and the Galactic Bulge, and have succeeded in identifying numerous lensing events. The duration of each event depends on several factors, including the tangential motion of the lens and its mass. Long-term events are generally associated with a massive lens, but duration alone is not sufficient to characterise the lens since a slow-moving source with low mass can mimic a fast-moving high-mass lens. However, microlensing not only leads to flux amplification, but also to small astrometric motions, caused by the appearance and disappearance of features in the lensed light. Those motions serve as a mass discriminant - higher mass lenses produce larger amplitude motions. The present program aims to capitalise on this fact by measuring the positional deflection of a background stars induced by the close passage of Proxima Centauri, the late-type, low-luminosity M dwarf tertiary companion of Alpha Centauri and the nearest star to the Sun. As a nearby star, Proxima has a well-defined proper motion and parallax, and it will pass close (within 1.5 arcsecnds) to two 18th magnitude stars in May 2015 and June 2015, respectively. The expected signals during the encounters (i.e. the deflection of the background stars) are expected to be approximately 0.5 millarcseconds, and therefore within HST's astrometric capabilities.

GO 13928: HST and Gaia, Light and Distance

HST WFPC2 image of NGC 4639, one of the Cepheid-rich spiral galaxies used to calibrate SNe Ia

The cosmic distance scale and dark energy are two key issues in modern astrophysics, and HST has played a vital role in probing both. On the one hand, HST has been involved in cosmic distance measurements since its inception, largely through the H0 Key Project, which used WFPC2 to identify and photometer Cepheids in 31 spiral galaxies at distances from 60 to 400 Mpc. On the other, HST is the prime instrument for investigating cosmic acceleration by searching for and following Type Ia supernovae at moderate and high redshift. These two cosmological parameters are directly related, and recent years have seen renewed interest in improving the accuracy of H0 with the realization that such measurements, when coupled with the improved constraints from the Cosmic Microwave Background, provide important constraints on cosmic acceleration and the nature of Dark Energy. Previous HST programs have focused on identifying and measuring light curves for cepheids in external galaxies (eg GO 10802 , GO 11570 ) or quantifying the effects of variations in intrinsic stellar parameters, such as metallicity (eg GO 10918 , GO 11297 ). The present SNAP program is part of a suite of HST programs focusing on the Galactic Cepheids that form the foundation for the whole distance ladder. These programs employ a revived version of an old technique to determine both accurate astrometry, hence trigonometric parallaxes and reliable distances, and accurate photometry, hence flux emasurements. The technique is drift-scanning - tracking HST during the observation so that stars form trails on the detector. This mode of observations was available in the early years of HST's operations, and has been revived primarily as a means of obtaining high signal-to-noise grism spectroscolpic data of stars hosting transiting exoplanets. However, the same technique can be used in imaging mode, and the extended trails allow not only multiple measurements of position differences for stars in the field but also extremely high signal-to-noise photometry. The latter is crucial in obtaining direct photometry of tghe local calibrations on the same HST system, the same system that is being used for photometry of Cepheids in the external galaxies that serve as the basis for the distance scale. The present SNAP program includes 67 longer-period Galactic Cepheids.