Liller1

GeMS UNVEILS THE STELLAR

CONTENT OF LILLER 1

The revolutionary instrumentation mounted at Gemini South Observatory on Cerro Pachon in Chile, revealed its extraordinary potential by producing an unprecedented sharp view of one of the most obscured globular clusters in the Galaxy: Liller 1

The spectacular image of Liller 1 obtained with GeMS@GEMINI. This false-color image (with a total size of 95"x95") has been obtained by combining a 2x2 array of images acquired in the near-infrared J and K pass-bands. A zoom of the core region (15"x15") is shown in the inset. North is up, East is on the right.

Image Credit: F. R. Ferraro/E. Dalessandro (Cosmic-Lab - University of Bologna - Italy)

A technical jewel named GeMS (derived from "Gemini Multi-conjugate adaptive optics System"), in combination with the powerful infrared camera Gemini South Adaptive Optics Imager (GSAOI), was finally able to penetrate the dense fog surrounding Liller 1 and to provide astronomers with an unprecedented view of its stars. This has been made possible thanks to the combination of two specific characteristics of GeMS: first, the capability of operating at near-infrared wavelengths (especially in the K pass-band); second, an innovative and revolutionary way to remove the distortions (blurriness) that the Earth's turbulent atmosphere inflicts on astronomical images. To compensate for the degradation effects of the Earth atmosphere, the GeMS system uses three natural guide stars, a constellation of five laser guide stars and multiple deformable mirrors (see www.gemini.edu/node/11925 for a detailed description of the technology used by GeMS). The correction is so good that astronomers are provided with images of unprecedented sharpness. In the best K-band exposures of Liller 1, stellar images have an angular dimension of only 75 milli-arcseconds, just slightly larger than the theoretical limit of Gemini's 8-meter mirror (diffraction limit). This means that GeMS performed an almost perfect correction of the atmospheric distortions.

These images turned out to be comparable in sharpness with those of the Hubble Space Telescope at infrared wavelengths, with two big additional advantages: (1) the access to the K pass-band, which is the most insensitive to dust extinction and which is not available at HST, (2) a much larger collecting area (a mirror of 8m diameter at the Gemini Observatory Telescope in Chile, against a 2m mirror on board the Hubble Space Telescope).

Indeed, the GeMS observations of Liller 1 show that adaptive optics systems mounted on ground-based 8m telescopes can provide space-like clarity images and demonstrate the remarkable potentiality of this instrument in the exploration of the Galactic Bulge.

Liller 1 is a stellar system made of a few hundreds thousand stars, orbiting the Bulge of our Galaxy at a distance of 26.000 light years from the Earth. It is suspected to be the second Milky Way star cluster (after Terzan 5) where stellar collisions occur most frequently, and to harbor a large and still undetected population of Millisecond pulsars. Moreover it is expected to still hide precious information about the Bulge formation processes. In spite of its importance, Liller1 has been poorly studied so far, because it is located in one of the most inaccessible region of our Galaxy, where thick clouds of dust prevent the optical light from emerging (indeed, a huge extinction reaching up to ~10 magnitudes affects optical observations). Only infrared radiation can travel across these clouds and bring us direct information on its stars.

Two images (15"x15") of the central region of Liller 1 acquired in the NIR pass-band with two different ground-based telescopes: the ESO New Technology Telescope (upper panel), and GeMS at the Gemini South Observatory (lower panel). In the latter case, the angular dimension of each stellar image is only 0.075".

Image Credit: F. R. Ferraro/ E. Dalessandro (Cosmic-Lab - University of Bologna, Italy)

A nice color view of the brightness profile of a typical star in the field of Liller 1, obtained with GeMS at the Gemini South Observatory. The profile is incredibly regular and it has an angular dimension of only 0.075".

Image Credit: F. Mauro (University of Conception, Chile)

Astronomers are using these images to measure the magnitude and color (which are closely related to

The (Ks, J-Ks) color-magnitude diagram obtained from the GeMS images. More than 65,000 stars have been measured in Liller 1. The main evolutionary sequences are clearly visible (Red Giant Branch, Red Clump and Main Sequence Turn-Off region).

luminosity and temperature) of each detected source and construct the so-called color-magnitude diagram,where stars define different sequences depending on their evolutionary stage and on the thermonuclear reactions occurring in their interior. The detailed study of this diagram will provide crucial information about the nature of Liller 1, its age and the degree of dynamical evolution suffered by this stellar system. It will possibly also shade new light on the formation of the Galactic Bulge and the Galaxy.

THE PROJECT AT GEMINI

This is the first result of a collaboration between the Cosmic-Lab team led by Francesco R. Ferraro at the University of Bologna (Italy), and the stellar group led by Douglas Geisler at the University of Conception (Chile). The global project is aimed at exploring the properties of a set of globular cluster-like stellar systems orbiting the Galactic Bulge. In addition to Liller 1, four other clusters have been observed with GeMS/GSAOI, namely Terzan 6, NGC 6440, NGC 6569 and NGC 6624. Preliminary results obtained from the analysis of the GeMS images of NGC 6624 are very encouraging: they turned out to be absolutely comparable to HST images, both in depth and in resolution. The obtained (J, J-Ks) and (Ks, J-Ks) color-magnitude diagrams (see below), reaching ~3 magnitudes below the MS-TO show very clearly all the main evolutionary sequences.

(J, J-Ks) and (Ks, J-Ks) color-magnitude diagrams obtained for NGC 6624 from GeMS images. They are able to sample over 3 magnitudes below the Main Sequence Turn-Off (which is marked in the figure, together with the other main evolutionary sequences).

THE COSMIC-LAB PROJECT

This result is part of the Cosmic-Lab project, a 5-year project funded by the European Research Council, and led by Francesco R. Ferraro at the University of Bologna (Italy) specifically designed to probe the complex dynamical phenomena occurring into the congested regions characterizing star clusters. "Thanks to this grant we have formed a team of prepared and motivated (young) researchers. The results we are obtaining represent a major success for the Italian and the European astrophysical community and suggests that we are on the right track not only to open new perspectives to the astrophysical knowledge, but also to provide a fruitful breeding ground for the formation of young scientists with a broad view in Physics and Astrophysics. This project is contributing to create a top-level research centre for stellar Astrophysics at the Bologna University, that will be a reference for young students, not only in Italy, but also in Europe and all over the world", concludes Ferraro.