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tests:astropars:challenge1 [2014/10/30 13:13] – created randraetests:astropars:challenge1 [2022/10/24 12:26] (current) – external edit 127.0.0.1
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 ====== Challenge 1: Moving groups ======  ====== 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. 
 +
 +
 +
 +
 +
 +
 +
 +
 +
tests/astropars/challenge1.1414674820.txt.gz · Last modified: 2022/10/24 12:26 (external edit)