This week on HST

HST Programs: July 3 - July 9, 2017

Selected highlights

GO 14618:Ultraviolet Flashers in M87: Rapidly Recurring Novae as SNIa Progenitors

The giant elliptical, M87

Novae are generally believed to originate in binary systems with a white dwarf paired with a non-degenerate companion. The orbital separation of the components is sufficiently small that the non-degenerate star fills its Roche lobe, leading to accretion onto the surface of the white dwarf. That material accumulates until a runaway nuclear reaction occurs, leading to the nova outburst. Typcially, the system survives to undergo subsequent outbursts. In some cases, the mass loss is sufficiently high that the time between outbursts is a measured in years rather centuries. These systems are known as recurrent novae; the companion is generally a subgiant or a red giant star, rather than the main sequence star in classical novae. One of the most remarkable such systems is M31 2008-12a, originally discovered in Deecmber 2008. Since that date, this system has undergone no less than eight outbursts. This indicates a very high accretion rate. Moreover, models suggest that typically 30% of the accreted material is retained after each outburst. Thus, the mass of the white dwarf is likely growing significantly with each outburst. This system is therefore a strong candidate as a progenitor of a supernova, triggered when the white dwarf mass exceeds the Chandrasekhar limit. The present program aims to use HST observations to monitor the giant elliptical galaxy, M87, and search for counterparts in that system. The observations are being taken using the F275W and F606W filters on the WFC3 UVIS camera, and with a 5-day cadence. These observations should be capable of detecting novae a faint as 27th magnitude.

GO 14634: HST-Juno synergistic approach of Jupiter's magnetosphere and ultraviolet aurorae

Hubble ultraviolet image of auroral activity near Jupiter's north magnetic pole

Planetary aurorae are stimulated by the influx of charged particles from the Sun, which travel along magnetic field lines and funnel into the atmosphere near the magnetic poles. Aurorae therefore require that a planet has both a substantial atmosphere and a magnetic field. They are a common phenomenon on Earth, sometimes visible at magnetic latitudes more than 40 degrees from the pole, and have also been seen on Jupiter, Saturn, Uranus and Neptune. Jovian auroral activity is also affected by the Galilean satellites, which generate electric currents that can produce bright auroral spots (see figure), and, in some cases, have their own auroral storms. Hubble has monitored activity for more than two decades now, with a dedicated campaign during the International Heliosphysical Year (2007/2008) using the Advanced Camera for Surveys Solar Blind Channel. In 2011, NASA launched the JUpiter Near-polar Orbiter (JUNO) which will provide the first in situ measurements of Jupiter's magnetic field and polar magnetosphere since the Galileo satellite which actively monitored Jupiter from 1995 to 2003. Juno arrived at Jupiter on July 4th 2016 and entered a 53-day orbit. Hubble is obtaining a series of far-UV images using the Space Telescope Imaging Spectrograph (STIS). Those data complement the on-board instrumentation by providing global maps of auroral activity.

GO 14738: A farultraviolet study of the globular clusters in NGC 3115

The lenticular galaxy, NGC 3115

Old stellar populations are dominated by stars that are as cool or cooler than the Sun. As such, only small fraction of the total flux is emitted at ultraviolet wavelengths. However, that fraction, while small, shows significant variations from system to system. The primary source of UV light in these systems is the population of horizontal branch stars - stars that have completed evolution to the tip of the red giant branch, triggering the helium flash and central helium burning. Observations of galactic globular clusters show a wide range of HB morphologies, with some systems having relatively stubby branches that reach surface temperatures of ~10,000K or less, while others extend much further into bluer colours and much higher surface temperatures. Similar population changes may be present in external systems and, inded, have been detected around the elliptical galaxy, M87. The present program aims to survey the numerous globular clusters surrounding the relatively nearby lenticular galaxy, NGC 3115. The ACS Solar Blind Camera will be used to obtain UV imaging at far-UV wavelengths to determine the distribution of properties of these systems

GO 14922: Probing the Nature of Dark Matter with Individual Stars Highly Magnified by a Galaxy Cluster

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

Throughout Cycles 21, 22 and 23, Hubble conducted deep imaging observations of six galaxy clusters as the Frontier Fields Director's Time program. Those observations have provided a basis for several synergistic programs. In particular, the observations enabled a 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. In 2014, 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. Dubbed Supernova Refsdahl, after the gravitational lensing pioneer, the original detections were followed over the course of their fading. But, more spectacularly, models of the cluster potential and the consequent light paths led to a prediction that the supernova should appear in one of the other lensed images of the parent galaxy in late 2015. The supernova was not present in observations obtained on November 14, 2015, but was detected in the December 11 observations, thus representing the first time that a supernova has been "predicted" successfully.Further observations are being obtained to monitor the light curve. Subsequent observations have revealed a new variable object within the cluster, approximately 5 arcsec from Refsdahl. Dubbed "Icarus", models strongly suggest that the source originates from a single early-type star at edshift z~1.49 that is crossing a gravitational caustic, leading to an brightness amplfication of somehwere between 105 and 106. Subsequent observations, tracking the brightness evolution of Icarus, have revealed two new sources - "Chucky" and "Perdix". This behaviour may be providing insight on the nature of dark amtter, perhaps pointing to a granular structure that can produce successive microlensing events from a background star cluster. The present program will continue observations of MACS 1149 to track these events and monitor for new occurences.