This week on HST

HST Programs: November 12 - November 18, 2012

Selected highlights

GO 12528: Probing the nature of LBVs in M31 and M33 - Blasts from the Past

GR290, Romano's star, a luminous blue varibale in M33

Luminous Blue Variables (LBVs) are just what they sound like - bright blue stars that show low-level (few tenths of a magnitude), irregular changes in brightness over timescales of years, punctuated by substantial outbursts, when the star brightens by several magnitudes. The prototype is S Doradus, the brightest star in the Large Magellanic Cloud (MV < -10, so brighter than many dwarf galaxies), and there are only around 20 other such stars known, including P Cygni and eta Carinae. They are massive hypergiants, with radii over 100 times that of the Sun and system masses that may exceed 100 MSun. At least some of these systems are binary stars. The outbursts are very likely caused by substantial mass loss events, perhaps stimulated by the binary companion, and can lead to the star's brightness increasing over a hundredfold over a period of 5-10 years. The classic example is Eta Carinae, which was originally catalogued by Edmond Halley as 4th magnitude in 1677, but had brightened by 3-4 magnitudes by 1730, returning to its previous brightness by 1782. There have been a number of subsequent outbursts, and detailed, high-resolution imaging ahs revealed a complex series of nebular shells surrounding the stellar system, indicative of substantial mass-loss events. The present HST program aims to probe the LBV phenomenon through high spatial-resolution spectroscopy of the 6 known LBVs in the nearby spirals, M31 and M33. The goal is to use those spectral data to probe the outburst history of those stars, as traced by the surrounding circumstellar material, and hence shed light on the long term evolution of these systems.

GO 12283/12568: WISP - A Survey of Star Formation Across Cosmic Time

A region of massive star formation

Star formation is the key astrophysical process in determining the overall evolution of galactic systems, the generation of heavy elements, and the overall enrichment of interstellar and intergalactic material. Tracing the overall evolution through a wide redshift range is crucial to understanding how gas and stars evolved to form the galaxies that we see around us now. The present program builds on the ability of HST to carry out parallel observations, using more than one instrument. While the Cosmic Origins Spectrograph is focused on obtaining ultraviolet spectra of unparalleled signal-to-noise, this program uses the near-infrared grisms mounted on the Wide-Field Camera 3 infrared channel to obtain low resolution spectra between 1 and 1.6 microns of randomly-selected nearby fields. The goal is to search for emission lines characteristic of star-forming regions. In particular, these observations are capable of detecting Lyman-alpha emission generated by star formation at redshifts z > 5.6. A total of up to 40 "deep" (4-5 orbit) and 20 "shallow" (2-3 orbit) fields will be targeted in the course of this observing campaign.

GO 12596: In search of a Young Solar Wind

Measurements of the velocity of the solar wind made by NASA satellite missions

The solar wind comprises charged particles, primarily simple protons and electrons, tha are driven off the outer layers of the sun's atmosphere. These particles have typical energies between 1 and 10 keV and register their persence most notably through interactions with planetary magnetic fields and atmospheres, generating auroral displays on Earth and the gas giants and contributing to atmospheric depletion in non-magnetic planets, like Mars. The solar wind also likely played a role in dissipating the Sun's angular momentum, and also likely contributed to the dispersal of the residual debris disk that survived planet formation. It is extremely likely that the solar wind has evolved and declined in strength over the course of the Sun's lifetime. However, there is currently no standard model for that evolution. The present program aims to investigate this issue through observations of 4 nearby, active F and G type stars that are significantly younger than the Sun. The Cosmic Origins Spectrograph will be used to search for narrow Lyman-alpha absorption features due to stellar winds from those stars.

GO 12971: Completing the Empirical White Dwarf Cooling Sequence: Hot White Dwarfs in 47 Tucanae

Hubble image of the globular cluster, 47 Tucanae

Globular clusters are members of the Galactic halo population, which formed during 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. Hubble has conducted a significant number of observing programs targeting these systems, with the majority designed to obtain moderately deep, multicolour imaging data of a range of clusters. Those programs probe evolved stars, on the red giant and horizontal branch, and generally extend no more than a few magnitudes below the main-sequence turnoff. A few clusters, however, have been studied in detail - specifically, the two nearest clusters, NGC 6397, an extremely metal-poor cluster, and M4, a moderately metal-rich systems; Omega Centauri, one of the most massive clusters, perhaps even the remnant core of a dwarf galaxy; and 47 Tucanae, one of the higher metallicity systems, lying in the foreground of the Small Magellanic Cloud. Deep imaging of all four clusters has succeeded in clear detecion of the white dwarf cooling sequence in those clusters, and those data have been used to derive age estimates. The present observation builds on past observations of 47 Tucanae ( GO 11677 ) where WFC3/UVIS and ACS/WFC were used to obtain multi-colour imaging of the cluster. Those observations allowed measurement of the white dwarf sequence and luminosity, extending to temperatures below 10,000K. The areal coverage, hwoever, was only sufficient to reveal a handful of high-termperature (>30,000K) degenerates; consequently, the luminosity function is poorly constrained at those temperatures. The current observations will obtain moderately shallow data over a wider region of the cluster, expanding significantly the white dwarf sample and providing a correspondingly more secure measurement of the luminosity function.