Observing strategies for LSST optimezed for young stars

The Vera C. Rubin is currently under construction in Cerro Pachón, northern Chile. Equipped with an 8.4-meter primary mirror, the telescope is designed to monitor the entire visible sky for ten years with six filters that cover the entire optical band, along with part of the UV band (the u filter) and the IR band (the y filter). This survey, known as the Legacy Survey of Space and Time (LSST), will be groundbreaking in various fields of astronomy, from the study of transients and variable stars to galactic astronomy and cosmology. The main survey of Rubin LSST is the Wide-Fast-Deep survey, which is designed to monitor 18,000 square degrees of the sky, approximately 40 times the size of the full moon, covering the entire visible sky in just three nights with a mosaic of observations that are 30 seconds deep per filter. On average, each field of the mosaic will be observed 825 times. One of the most innovative components of Rubin LSST is its camera, which is the size of a car and composed of 3.2 billion pixels. The camera will produce about 20 terabytes of data per observing night.


Such an ambitious project requires extensive preliminary work, both technical and scientific. A team from the INAF – Astronomical Observatory of Palermo, which has been involved in this project since 2017, has recently published two works that describe the optimization of the observing strategy for the Vera Rubin Observatory for two scientific cases: the study of variable young stars and a deep survey for young stars in the galactic plane.


The former study is led by the astronomer R. Bonito (INAF – Astronomical Observatory of Palermo), who is also the co-chair of the “Transient and Variable Stars Science Collaboration” of Rubin LSST. This study describes how the cadence of Rubin LSST observations can be optimized to study the variability in young stars, which are only a few million years old and are surrounded by a disk of gas and dust called “protoplanetary disks”. Within a period of typically less than 3 million years, part of the gas in these disks accretes onto the central star with a variable accretion rate of about 10-10 — 10-9 solar masses per year. In addition, the material associated with the inner part of the disk can cause variable absorption of stellar radiation (extinction) due to the inner disk’s inhomogeneities. As shown in this study, Rubin LSST can study these phenomena with a very dense cadence of observations (>100 in a week) of a single star-forming region rich in young stars. The first region chosen for this study is the Carina Nebula, which, thanks to the capabilities of the Vera Rubin Observatory, offers the unique possibility of studying the variability in an enormous sample of young stars (a few tens of thousands). This study, together with the development of 3D models designed for accessibility for visually impaired researchers, is described in the paper  “Young Stellar Objects, Accretion Disks, and Their Variability with Rubin Observatory LSST“, which recently appeared in The Astrophysical Journal Supplement.


The latter paper, led by astronomer L. Prisinzano (INAF – Astronomical Observatory of Palermo), focuses on the capabilities of Rubin LSST in observing the young stellar population with unprecedented depth, meaning the ability to observe at very large distances from the Sun. Most existing studies of the young stellar population in the Galactic plane are limited to a nearby region around the Sun, approximately 6,000 light-years away, or to single young star clusters located at large distances. This is because to study these stars, we must observe through the Galactic plane, which is rich in gas and dust that absorb most of the incident stellar radiation, resulting in high-extinction regions. These high-extinction regions have been excluded from the Wide-Fast-Deep survey, with a high risk of not being observed. However, the study led by L. Prisinzano demonstrates that by adopting the proper observing strategies, it is possible to observe up to 5-6 million young stars up to distances of 30,000-50,000 light-years, something that has never been achieved before. This will not only allow us to observe young stars in the Milky Way at distances from us that we have never reached before, but also to map the nearby Galactic arms and gain a global view of star formation and young star evolution in the Milky Way for the first time. This study is described in the paper, “Rubin LSST observing strategies to maximize volume and uniformity coverage of Star Forming Regions in the Galactic Plane“, reacently appeared on The Astrophysical Journal Supplement.


The figure (click here to view it in its entirety) shows the main results of these studies. The top panel displays a light curve (the temporal variability of stellar luminosity in a given filter) of a young star variable due to the accretion process. The black points show how the variability will be sampled with the observing strategies described in R. Bonito’s paper. The bottom panel shows a map of the entire sky visible by the Vera Rubin Observatory. The color scheme shows the number of young stars that will be observed using the observing strategy described in L. Prisinzano’s paper. The yellow region corresponds to the Galactic plane. To the right, some “high-extinction” regions can also be seen, where the number of observed stars is small. The study demonstrates that the most crowded regions of the Galactic plane are the most difficult to observe, rather than the most extinguished ones.


Mario Giuseppe Guarcello  ( follow mariospiegacose) ( mariospiegacose) ( follow mariospiegacose)

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