This week on HST

HST Programs: March 29 - April 3, 2016

Selected highlights

GO 13646: Understanding the Progenitor Systems, Explosion Mechanisms, and Cosmological Utility of Type Ia Supernovae

Supernova 2015F in NGC 2442; image by Stan Howerton

Supernovae are the most spectacular form of stellar obituary. Since B2FH, the physical processes underlying their eruptive deaths have been known to play a key role in populating the ISM with metals beyond the iron peak. More recently, these celestial explosions have acquired even greater significance through the use of Type Ia supernovae as distance indicators in mapping the `dark energy' acceleration term of cosmic expansion. However, while there are well-established models for the two main types of supernovae (runaway fusion on the surface of a white dwarf in a binary system for Type Ia, or detonation of the core in Type II), some significant uncertainties remain concerning the physical details of the disruption, and, potentially, the overall uniformity of these events. Consequently, there is potential for systematic bias in the distance estimates. The present program aims to address this issue through detailed observations of a small number of relatively nearby Type Ia Sne, using the Space telescope Imagign Spectrograph to provide spectral coverage over the full UV and visual wavelength ergime. The program aims to pick up the supernovae before maximum, and take spectra at regular (~4 day) intervals, providing detailed coverage of the changing abundances within the SNe ejecta. This is a target of opportunity, and the present set of observations are aimed at SN 2015f in NGC 2442/2443, an active spiral galaxy, also known as the "Meathook", lying at a distance of ~20 Mpc. The supernova was discovered at 13th magnitude in mid-March. The first HST observations were made on March 16th, with subsequent observations through April 12th.

GO 14206: A New Threshold of Precision, 30 micro-arcsecond Parallaxes and Beyond

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. Observations have been obtained for more than 20 such stars. The present program aims to refine the distance estimates by obtaining four additional epochs for 9 core Cepheids (Z Sct, DD Cas, VX Per, SZ Cyg, SS CMa, XY Car, S Vul, X Pup and WZ Sgr). These data will improve the precision of the final parallaxes by identifying and eliminating binaries among ther eference stars, providing a longer baselien for proper motion determination, and providing direct overlap with Gaia observations.

GO 14235: Globular Cluster Orbits from HST Proper Motions: Constraining the Formation and Mass of the Milky Way Halo

Hubble image of the metal-poor globular cluster, M15

Globular clusters are members of the Galactic halo population, representing remnants from the first extensive period of star formation in the Milky Way. As such, the properties of the 106 to 107 stellar constituents can provide crucial insight into the earliest stages of galaxy formation. Until recently, conventional wisdom was that these are simple systems, where all the stars formed in a single starburst and, as a consequence, have the same age and metallicity. One of the most surprising disoveries in recent years is the realisation that this simple picture no longer holds. A substantial number of thee systems show complex structure in the colour-magnitude diagram, indicating the presence ofmultiple stellar populations. Current globulars may therefore represent the remnant cores of dwarf galaxy-like systems. Regardless of their origin, the present systems can also serve to map the potential of the Milky Way. Undertaking that analysis demands the determination of three-dimensional motions, requiring that we obtain accurate absolute proper motions for these systems. That is now becoming possible; thanks to Hubble's longevity, a significant number of clusters have imaging data from 10-20 years ago. Coupled with new observations, those data provide the baseline to allow measurement of the tranverse motion, and hence orbit determinations.

GO 14467: A White Dwarf with an Actively Disintegrating Asteroid

Artist's impression of a comet spiralling in to the white dwarf variable, G29-38

The Kepler satellite was designed to identify exoplanet systems through measuring the diminution of light as the planets transit across the face of the host star. Kepler's high precision photomtery has resulted in a deluge of new systems, but has also revealed interesting sources of variability. The present program focuses on one such star, the white dwarf WD 1145+017. This object shows multiple dips in brightness, with the most significant reaching a depth of 40% and recurring with a period close to 4.5 hours. In addition, the stellar spectrum shows prominent absorption lines due to metals (magnesium, aluminium, silicon, calcium, iron and nickel). Such features have now been detected in a significant number of white dwarfs, including several that show significant infrared excess flus, notably G29-38, indicating the presence of circumstellr dust. Those lines are generally attributed to "pollution" of the white dwarf atmospheres. Given that the diffusion time for metals within the atmospheres is short (tens to hundreds of years), the only reasonable means of maintaining such lines in ~20% of the DA population is to envisage continuous accretion from a surrounding debris disk. In the present case, the Kepler observations suggest that we are witnessing the disruption of a larger body, an asteroid, that is generating (much of) the dust. The present program aims to use the Cosmic Origins Spectrograph to search for additional absorption features due to carbon, probe the stellar spectrum for changes during transits and measure the ultraviolet light curve.