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--- tests:astropars:challenge1 [2014/10/31 11:00] – randrae
+++ tests:astropars:challenge1 [2022/10/24 12:26] (current) – external edit 127.0.0.1
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 ====== Challenge 1: Moving groups ======
 ===== Objectives =====
+  * Test our ability to identify known young moving groups in velocity space as a function of distance and RVS precision.
+  * Investigate Gaia-like data, to quantify the expected errors and reveal new methods for identifying groups with accurate distance information
+===== YOUNG MOVING GROUPS =====
+There are around a dozen young (<100 Myr) sparse moving groups known within the
+solar neighbourhood (d<150 pc). They are the remnants of recent low-density star
+formation events, some of which are associated with the Sco-Cen OB association (Beta
+Pic, TW Hydrae, Eps Cha), the Pleiades (AB Dor) and IC 2391 (Argus).
+We know that these groups are reasonably coherent in velocity space (see below).
+Most of the scatter in this plot is due to uncertain distances, usually found from
+proper motions and the mean group space motion (derived from a handful of early-type
+members in each group observed by Hipparcos). While several of the groups have
+distinct velocities (Argus, Octans, AB Dor), many are found within a “good box”
+(Zuckerman & Song 2004) around the solar peculiar motion.
+{{:tests:uv_groups.png|}}
+Unlike open clusters, association members are very sparse, with their nearest
+stellar neighbours highly unlikely to be group members.
+{{:tests:groupc_xy.png|}}
+{{:tests:groupc_xz.png|}}
+With the high quality Gaia data to first order it is possible to visually identify
+similar over-densities in phase space. To test the effectiveness of RVS velocities,
+we offset several known moving in velocity and increasing distances from the Sun,
+trying to recover the samples in UVWXYZ space. Even at 300 pc it is trivial to find
+over-densities in distance-velocity space (e,g, near U=0 km/s,V=0 km/s).
+{{:tests:groupc_uv_.png|}}
+However, it’s necessary to consider full 3D velocities, as the over-densities are
+not always apparent in some projections:
+{{:tests:groupc_uw_.png|}}
+{{:tests:groupc_uwmemb_.png|}}
+This could obviously be automated with well-known clustering methods.
+*** To do: test on the 500 pc Group D sample ***
+The weak point is the RVS velocities, which have typical errors of at least several
+km/s at these distances and spectral types:
+{{:tests:groupc_rverr.png|}}
+{{:tests:groupc_rv.png|}}
+This has the effect of smearing out the UVW over densities, reducing their contrast
+over the background field stars.
+Young stars are typically several magnitudes above the main sequence and well
+separated from older field stars, as seen in the G-band HR diagram. With accurate
+distances for millions of stars, we can use this property to increase the contrast
+between the groups and the field stars.
+*** GOG does not appear to be respecting the absolute magnitudes  *** Need to
+investigate this properly.
+Also combine with other ancillary catalogues, e.g. GALEX, 2MASS, WISE
+===== NEXT STEPS =====
+  * What about log(g)? Is there sufficient information in the BP/RP spectra to select
+low-gravity objects?
+  * (Re-)defining empirical samples of young stars in Gaia photometric/astrometric system
+Most of the scatter in the CMD and phase space plots above is due to the uncertain
+kinematic distances of members without Hipparcos parallaxes. How these look with
+essentially zero distance error remains to be seen.
+With the mean space motions of known groups nailed down, it will be trivial to
+identify new faint group members from their Gaia proper motions and distances along,
+with confirmatory RVs and youth indicators from ground-based spectroscopy.
+===== FAKE GROUP GENERATOR =====
+Probably very instructive to quickly code up a package to sample “perfect” IMFs and
+isochrones and create fake groups with known velocity dispersions, spatial
+structures (spherical, filaments, sub-structure) and binary properties. These can be
+convolved with realistic Gaia errors and embedded in GUMS samples to test our
+methods for identifying groups.