This week on HST

HST Programs: August 10 - August 16, 2015

Selected highlights

GO 13650: The MUSCLES Treasury Survey: Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems

SOHO image of an extremely strong solar flare

M dwarfs - at least, the subset of M dwarfs known as flare stars - are renowned for possessing extremely active chromospheres and coronae. Their discovery as highly variable objects happened largely by chance. Willem Luyten had noticed in 1924 that certain M dwarfs showed spectroscopic variability, with the occasional appearance of emission lines, while in the early 1940s van Maanen commented that two late-type dwarfs, Gl 412B (WX UMa) and Gl 285 (YZ CMi), had brightened by over a magnitude on a handful of parallax plates. The crucial observations came in 1948, when E.F Carpenter noticed that the fainter component of a wide binary system had brightened by more than 3 magnitudes in a matter of minutes. In the succeeding 50 years, these stars have been subjected to extensive observations, particularly at optical and X-ray wavelengths, and the underlying physical processes are relatively well understood. However, most attention has focused on the more active flare stars, and we still have a relatively uncertain grasp on the flare frequency among less active stars. This issue has acquired increased importance with the realisation that somewhere between 10 and 50% of M dwarfs host planetary systems. As the most populous stars in the Galaxy, this also makes M dwarfs the premier planet hosts. The habitable zones in those systems lie much closer to the parent star, and planets are correspondingly vulnerable to detrimental effects from enhanced UV radiation, particularly short-wavelength UV-C. This proposal uses the Cosmic Origins Spectrograph and the Space Telescope Imaging Spectrograph to obtain UV and optical spectra (1150-5700 Angstroms) of nearby M dwarfs, providing a broad sampling of the range of activity levels among these low mass dwarfs.

GO 13772: Joint Chandra and HST Monitoring of the Crab Nebula

The Crab Nebula

Messier 1, the Crab Nebula, provides astronomy with one of its iconic images. The remnant of a bright supernova observed in 1054 by Arabian and Chinese astronomers, the Crab was first recorded in 1731 by the English astronomer, John Bevis, thirt-seven years before Messier compiled his catalogue of non-comets. The energy source for the gaseous emission is the neutron star that lies in the centre of nebulosity, and was one of the first pulsars to be identified. The Crab is also a source of high energy emission, including radiation at X-ray and gamma ray wavelengths. Overall, this system plays a crucial role in aiding our understanding of post-supernova evolutionary processes. However, there are still some notable undertainties in the detailed processes within even this system. In particular, in September of 2010 the Crab surprised the astronomial community by producing a powerful flare at gamma-ray wavelengths that persisted for 4 days (see GO 12381 ).A second flare of similar magnitude occurred in May, 2011. Observations taken during the 2010 flare by HST and Chandra provided insight into the effects of the flare, but analysis was hampered by the absence of a comparison set of pre-outburst images of comparable resolution and depth. The present program builds on a Cycle 19 program, and aims to address that issue through coordinated monitoring of the Crab at X-ray and optical wavelengths. Tne Advanced Camera for Surveys on HST will be used to take images in the F550M filter at 4 epochs, with the observations timed to be within 10 days of X-ray images taken by Chandra using the AXAF CCD Imaging Spectrometer. These data will establish a reference set should a further flare occur.

GO 13658: Farewell to the Voayagers: Measuring the Local ISM in the Immediate Path of the Two Voyager Spacecraft

The Voyager spacecraft

The Voyager spacecraft were launched in 1977 as NASA's first reconnaissance mission of the outer solar system. Taking advantage of gravitational assists provided by planetary alignments, Voyager 2, launched in August 1977, conducted fly-bys of Jupiter (July 1979), Saturn (August 1981), Uranus (january 1986) and Neptune (August 1989); Voyager 1, launched in September 1977, only conducted fly-bys of Jupiter (March 1979) and Saturn/Titan (November 1980), but it achieved substantially higher velocities than Voyager 1. Both probes are now headed into the outermost regions of the solar system, with Voyager 2 currently at 108 AU and Voyager 1 at ~132 AU. In August 2012, Voyager 1 crossed the heliopause at ~122 AU, the boundary between the region dominated by the solar wind and outer space. Voyager 2 will reach similar distances in the next few years. Both spacecraft continue to transmit information regarding the local environment - the heliopause transition was marked by a substantial decline in the detection of solar wind particles. The present HST proposal aims to provide complementary information to these is situ measurments by using the Space Telscope Imaging Spectrograph to obtain spectra of 4 stars close to the Voyager sight-lines, 2 for each Voyager. Those spectra will be used to search for absorption features, probing the composition and density of the lcoal interstellar medium.

GO 13783: Thermal evolution of old neutron stars

Artist's impression of a neutron star

Neutron stars are extremely compact (~10 km diameter) massive (~1.4 to 2 solar mass) remnants of high mass (> 7 solar mass) stars. Formed during the gravitational core-collapse of Type Ib, Ic and II supernovae, they have initial temperatures exceeding 1011 Kelvon, but cool very rapidly with time. Their existence was originally proposed by Zwicky and Baade in the 1930s, but they remained unobserved until the late-1960s, when Hewish and Bell identified a pulsing radio source in the Crab nebula as a neutron star. (Hewish and Okoye had previously identified "an unusual source of high radio brightness temperature" within the crab.) Neutron stars have strong magnetic fields and rapid rotation, leading to radiation beaming along the magnetic poles; the observed pulses result from the beams sweeping across the terrestrial line of sight. Numerous pulsars are currently known, but direct observations of the neutron star's "photosphere" are much rarer. At their birth, neutron stars are extremely hot, and the initial thermal evolution can be traced by monitoring the base level X-ray flux (ie photospheric, not beamed, radiation). That method is only available for neutron stars younger than a few Myrs; older systems have cooled to the extent that the peak flux is at UV wavelengths, but those systems are also extremely faint. To date only one system has been detected at far-UV wavelengths by HST. The present program aims to use the ACS Solar Blind Camera to obtain far-UV imaging of three pulsars with ages between 17 Myrs and ~6 Gyrs, with the aim of further constraining the cooling timescales.