Gaia and new star clusters Gaia and new star clusters Gaia and new star clusters Designation of star clusters • IAU: – C aa bb ± ccd – aah bbm ± cco.d, Coordinates (1950.0) • Catalogues: – IC, M(essier), NGC, and OCL • „Discoverer“, surveys and „special names“ – Basel, Bochum, Lynga, Melotte, Stock, Trumpler and much more • Pleiades: C 0344+239, M45, Melotte 22 Classification of open clusters • Trumpler, 1930, Lick Observatory Bulletin, 420, 154, three criteria 1. Degree of Concentration 2. Range of Brightness 3. Number of Stars in the Cluster • Janes & Adler, 1982, ApJS, 49, 425: definition of a so-called richness class • Open clusters can also be classified on the basis of color-magnitude diagrams Trumplers classification • Degree of Concentration – I … Detached clusters with strong central concentration – II … Detached clusters with little central concentration – III … Detached cluster with no noticeable concentration – IV … Clusters not well detached, but has a strong field concentration Trumplers classification • Range of Brightness – 1 … Most of the cluster stars are nearly the same apparent brightness – 2 … A medium range of brightness between the stars in the cluster – 3 … Cluster is composed of bright and faint stars Trumplers classification • Number of Stars in the Cluster – p … Poor clusters with less than 50 stars – m … Medium rich cluster with 50 to 100 stars – r … Rich clusters with over 100 stars • Open clusters with any type of nebulosity are denoted with an "n" at the end of the classification. • Richness Class (Janes & Adler) – 1 … Less than 25 stars – 2 … Between 25 and 50 stars – 3 … Between 50 and 100 stars – 4 … Between 100 and 250 stars – 5 … More than 250 stars • How “good” can the number of members be established? Diameters of open clusters • How could we determine the diameter of a star cluster? 1. The determination, for example inspection by eye, should be no problem. Be careful, most open clusters show no real concentration 2. Count the number of stars (members) in concentric rings around the cluster center 3. If the derived distribution is not symmetric => go to 1. and shift the coordinates of the center • This procedure could be easily done via a computer program Pietrukowicz et al., 2006, MNRAS, 365, 110 III 2 m II 2 m Galactic field Ferreira et al., 2019, MNRAS, 483, 5508 Gaia data Diameters of open clusters van den Bergh, 2006, AJ, 131, 1559 No correlation with the age Janes et al., 1988, AJ, 95, 771 all young Galactic Distribution +- 20 degree Galactic latitude Kharchenko et al., 2013, A&A, 558, A53 Blue dots: OCLs Red triangles: GCs Spiral arms Projection on the Galactic plane The spiral arms of the Milky Way Cantat-Gaudin et al., 2018, A&A, 618, A93 Gaia has not changed our picture of the Milky Way The local motion of the stars Oh et al., 2017, AJ, 153, 257 4 555 groups Systematics? The local motion of the stars Oh et al., 2017, AJ, 153, 257 4 555 groups What is the smallest number of members of a star cluster? Classification of Globular Clusters: Shapley H. & Sawyer H.B., 1927, Harvard College Observatory Bulletin No. 849, pp.11-14 Definition - Radii • Core Radius: Distance at which the apparent surface luminosity has dropped by half • Half-Light Radius: Distance from the core within which half the total luminosity from the cluster is received • Half-Mass Radius: The radius from the core that contains half the total mass • Tidal Radius: Distance from the center at which the external gravitation of the galaxy has more influence over the stars in the cluster than does the cluster itself Important observables • Single stars: „all“ we can think of • Star clusters 1. Hertzsprung-Russell-diagram 2. Kinematic data 3. Integrated spectra 4. Integrated colors 5. Polarimetric measurements From the Pre-Main Sequence to Globular clusters NGC 104: d = 4500 pc, D = 31‘, [Fe/H] = -0.76 dex, t = 12 Gyr Harris, 2000, Space Telescope Science Institute Symposium Series, Vol. 14, p. 78 Hyades log t = 8.90 d = 45 pc [Fe/H] = +0.17 dex Width of Main Sequence about 1.8 mag in MV NO observational error A typical example before Gaia One typical example from the literature: Piatti et al., 2006, MNRAS, 367, 599: First estimates of the fundamental parameters of the relatively bright Galactic open cluster NGC 5288 CCD BVI Photometry, 1 Pixel = 0.4´´, 13.6x13.6´ field, No other observations available for this open cluster 15 688 stars in the complete field Different „main sequences“ due to fore- and background populations Klein Wassink, 1927, Publications of the Kapteyn Astronomical Laboratory Groningen, Vol. 41, 1 Praesepe d = 190 pc Field stars A typical example after Gaia One typical example from the literature: Castro-Ginard et al., 2018, A&A, 618, A59: A new method for unveiling open clusters in Gaia. New nearby open clusters confirmed by DR2 Gaia DR2 G, BP, and RP photometry Full astrometrical data set 330 pc 440 pc Sanner et al., 2001, A&A, 369, 511 (Hipparcos and Tycho data) The proper motion for „distant“ star clusters is almost zero. Only field stars with large proper motions can be sorted out. These are almost only foreground stars. Field stars Field stars + Members Hole et al., 2009, AJ, 138, 159: NGC 6819, one of the „best“ cases, more than three measurements for each star, 6571 radial velocities for 1207 stars, 3.5 meter telescope Observed sample Hole et al., 2009, AJ, 138, 159 Field stars + Members Hole et al., 2009, AJ, 138, 159 members non-members Integrated properties • Integrated spectra and colors • Especially interesting for distant and extragalactic star clusters • “Think small” • Pleiades: 2 o 1’ Bica & Alloin, 1986, A&A, 162, 21 Age Integrated spectra Z Same age Lata et al., 2002, A&A, 388, 158 Integrated colors “HRD diagram”