This week on HST

HST Programs: January 12 - January 18, 2015

Selected highlights

GO 13390: A Time-Lapse Movie of the Kinematics Across the Carina Nebula with ACS
GO 13395: Constraining the evolutionary state of the hot, massive companion star and the wind-wind collision region in Eta Carinae

Eta Carinae and the Homunculus Nebula as imaged by Hubble

Eta Carinae is one of the brightest and most famous stellar systems in the southern sky. Originally catalogues at fourth magnitude by Edmond Halley in the late 1600s, the star had risen to 2nd magnitude by 1730, fading back to 4th magnitude by the end of the century. In the 1800s the star underwent an even more spectacular outburst, rivalling Sirius at magnitude -0.8 by 1843, but fading below naked eye brightness to below 8th magnitude by 1875. In the 20th century the system brightened to just below 6th magnitude from ~1940 to 1997, and then sharply brightened to 5th magnitude by 1999. Detailed observations from ground and space through the last century have identified the system as a massive binary, with a ~30 solar mass hot supergiant orbitting a ~150 solar mass luminous blue variable. The system is shrouded in extensive clouds of dust and gas produced by mass loss from both stars. Eta Carinae has been studied intensely by Hubble throughout its lifetime, revealing fine detail in the central nebular structure. In particular, the inner structure of the nebula was studied in detail by the Advanced Camera for Surveys in Cycles 13 and 14; program GO 13390 is using the revitalised ACS to obtain new observations and probe how structure has changed over the past decade. Program GO 13395 is using the Space Telescope Imaging Spectrograph to investigate the properties of the stellar winds, particularly the interaction between the winds generated by the two components. At the same time, observations will probe the nature of the hidden secondary star, which is thought to be the prime source of far-UV radiation and X-ray emission.

GO 13677: See Change: Testing time-varying dark energy with z>1 supernovae and their massive cluster hosts

HST/ACS images of a supernova in a galazy at z=1.2

The last few years of the twentieth century saw a revolution in cosmology, with the measurement of the acceleration term in expansion at high redshifts and the identification of dark energy as a major cosmological component. The overall significance of this result has been recognised through the award of the Nobel prize and, most recently, the Fundamental Physics Breakthrough Prize to Perlmutter, Riess and Schmidt and their respective teams. Type Ia supernovae are the prime yardstick for measuring the rate of expansion at moderate and high redshifts. The seminal work in this field was carried out with ground-based telescopes, but Hubble offers almost the only way of obtaining reliable post-maximum photometry of these objects to determine the full shape of the light curve. Many previous HST supernovae programs have concentrated on field galaxies, but applying appropriate corrections for in situ reddening by dust remains an issue in these systems, while the overall SNe detection rates are relatively low at high redshifts. The present program takes a different tack, and aims to minimise the uncertainties by searching for supernovae in massive, high-redshift clusters. The expectation is that the majority of detections lie within dust-poor elliptical galaxies; moreover, supernova rates may be higher. The program will obtain ACS observations of ten of the most massive galaxy clusters lying at redshifts 1.1 < z < 1.75.

GO 13679: Europa's Water Vapor Plumes: Systematically Constraining their Abundance and Variability
GO 13829: The ice plumes of Europa

The HST imaging of a potential water plume around Europa's south pole superimposed on an image of the satellite

Europa is the smallest, and the most intriguing, of the four Galilean satellites of Jupiter. With a diameter of 3139 km, Europa is almost twice the size of Earth's moon and significantly larger than Mercury. In 1957, Gerard Kuiper commented that both infrared spectroscopy and the optical colours and albedo suggested that Jovian satellite II (Europa) is covered "by H2O snow". Images taken by the Voyager space probes in the late 1970s (see left) reveal a smooth surface, with only a handful of craters larger than a few kilometres. These features are consistent with a relatively young, icy surface. Subsequent detailed investigations by the Galileo satellite strongly suggest that a substantial body of liquid water, heated by tidal friction, underlies a 5 to 50 km thick icy crust. The presence of this subterranean (subglacial?) ocean clearly makes Europa one of the two most interesting astrobiology targets in the Solar System. Most recently, analysis of observations taken by the Space Telescope imaging Spectrograph (STIS) on Hubble indicated the presence of an extended cloud of Lyman-alpha emission near the polar regions while Europa was furthest in its orbit from Jupiter, strongly suggesting that Europa's oceans may be vaporising into space.Follow-up observations on two further occasions earlier in 2014 failed to detect any emission, suggesting that the emission is either sporadic or periodic; in the latter case, the emission might be related to the location of Europa within its orbit and the consequent tidal strain imposed by Jupiter. The two programs cited here follow up on these observatons. GO 13679 is conducting a methodical series of observatons designed to image Europa at a progressive series of orbital locations. This program uses STIS to search for H and O auroral emissions at UV wavelengths and will aim to map the distribution of emission at different phases of the Europan orbit.. GO 13829 is using UV imaging and spectroscopy with the ACS Solar Blind Camera is searching for fluorescence, with the majority ofthe observations will be taken while Europa is in eclipse. Those observations will be matched against reference data taken out of eclipse.

GO 13790: Frontier Field Supernova Search

Finding chart for the multiply imaged supernova, SN Refsdal, discovered in November 2014 in cluster MACJ1149

The overwhelming majority of galaxies in the universe are found in clusters. As such, these systems offer an important means of tracing the development of large-scale structure through the history of the universe. Moreover, as intense concentrations of mass, galaxy clusters provide highly efficient gravitational lenses, capable of concentrating and magnifying light from background high redshift galaxies to allow detailed spectropic investigations of star formation in the early universe. Hubble imaging has already revealed lensed arcs and detailed sub-structure within a handful of rich clusters. At the same time, the lensing characteristics provide information on the mass distribution within the lensing cluster. Hubble is currently undertaking deep imaging observations of up to 6 galaxy clusters as part of the Frontier Fields Director's Time program (GO 13495/13496). Those observations have provided a basis for several synergistic programs. The present program is using the Frontier Field observations to search for supernovae at high redshifts, z> 1.5, aiming to set further constraints on dark energy and probing the frequency of supernovae as a function of redshift, the delay time and hence the likely progenitors. Recent observations of the fourth cluster, MACSJ1149.5+2223, resulted in the detection of a particularly unusual object - multiple lensed images of a supernova in a redshift z=1.49 galaxy that is itself multiply lensed. Each of those images results from light following a different path due to the gravitational potential of the foreground cluster and galaxies. The persent program is obtaining follow-up observations to monitor the light-curves of each component, hence determining the time-delay for each light path. Those measured delays can be matched against the predictions of gravitational lensing models. Moreover, since the galaxy itself has multiple images it is possible that future observations may detect images of the supernova in other components. Thus, this supernova can (perhaps even will) be discovered more than once - and might even be predicted.