GeminiFocus July 2017 | Page 15

Abhijith Rajan (School of Earth and Space Exploration, Arizona State University), led the international team that observed 51 Eri b using GPI spectroscopy (Figure 1) as part of the Gemini Planet Imager Exoplanet Sur- vey (GPIES), combined with mid-infrared photometry at the W.M. Keck Observatory. These data were used to determine that the planet — a young, cool object between 2–10 Jupiter masses — is redder than brown dwarfs seen elsewhere. The enhanced red- dening may be the result of clouds forming as the planet transitions from a partially- to partly-cloudy atmosphere, with lower mean surface temperatures. If true, 51 Eri b ap- pears to be one of the only directly imaged planets that is consistent with the cold-start scenario, resulting in a low temperature, low luminosity planet. The full results have been accepted for pub- lication in The Astronomical Journal. A pre- print is available here. Gemini South Joins HST in Joint Proper Motion Study Tobias Fritz (University of Virginia) and col- leagues used the wide-field Gemini Multi- conjugate adaptive optics System (GeMS) at Gemini South, combined with the Gemini South Adaptive Optics Imager (GSAOI), to study the proper motion of stars in the Ga- lactic halo globular cluster known as Pyxis. These data, together with those from the Hubble Space Telescope, allowed the team to set a lower limit for the Milky Way’s mass of 950 million Suns. This value is consistent with most, but not all, previous determina- tions. GeMS/GSAOI was crucial to the study, be- cause traditional ground-based telescopes are seeing limited and need a time baseline of more than 15 years for the types of mea- surements required in this survey. On the other hand, GeMS/GSAOI has better spatial resolution and can complete the project in five years — about the same time required for HST. Using GeMS/GSAOI, the team mea- sured absolute proper motions of Pyxis to a resolution of 0.08 arcsecond (Figure 2), and combined these data with those from ar- chival HST images, with a resolution of ~ 0.1 arcsecond. Lying at a distance of some 130,000 light years, Pyxis is one of the most distant ex- amples of a globular cluster. It is also about 2 billion years younger than other globular clusters with the same metallicity. Together, these characteristics imply that Pyxis was likely formed in a massive dwarf galaxy that the Milky Way then cannibalized. Thus, Pyxis may have an extragalactic origin. One mystery, however, is that the orbits of other known massive dwarf galaxies are inconsis- tent with the orbit of Pyxis, which is derived from the new proper motion measurements. The research is part of a much larger ef- fort now underway to study the proper motion of several substructures across the Milky Way’s halo. It is also part of a Large and Long pro- gram at Gemini that July 2017 GeminiFocus Figure 2. Left: GMOS-South image of the Pyxis field, with the center of the cluster marked with a red star. Right: A zoom of the Pyxis area with GeMS- GSAOI. The field-of- view of GMOS is 5 x 5 arcminutes; GeMS is 85 x 85 arcseconds. 13