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J/A+A/595/A22    Nine new open clusters within 500pc from the Sun (Roser+, 2016)

Nine new open clusters within 500 pc from the Sun. Roser S., Schilbach E., Goldman B. <Astron. Astrophys., 595, A22 (2016)> =2016A&A...595A..22R (SIMBAD/NED BibCode)
ADC_Keywords: Clusters, open ; Proper motions ; Photometry Keywords: catalogs - surveys - proper motions Abstract: One of the results of the Milky Way Star Clusters (MWSC) survey was the detection of a slight under-density of old (ca. 1Gyr) clusters within the nearest kilo-parsec from the Sun. This under-density may be due to an ineffectiveness in the detection of larger structures with lower surface brightness. We report on our attempts to reveal such clusters. We derived proper motions from a combination of Tycho-2 with URAT1, and obtained a mean precision of about 1.4mas/yr per co-ordinate for 1.3 million stars north of -20° declination. We cut the sky into narrow proper motion slices and searched for spatial over-densities of stars in each slice. We then examined stars from over-densities in optical and near-infrared colour-magnitude diagrams to determine if they are compatible with isochrones of a cluster. We estimated the field star contamination using our data and the Besancon Galactic model. We detected nine hitherto unknown open clusters in the vicinity of the Sun with ages between 70Myr and 1Gyr, and distances between 200 and 500pc. Description: We used URAT1 (Zacharias et al., 2015, Cat. I/329) to improve the Tycho-2 proper motions and to test what proper motions, which are more precise than those of Tycho-2 (Hog et al., 2000, Cat. I/259), can do for open cluster studies. URAT1 contains 228 million objects down to about R=18.5 mag, north of about -20° declination. For the bulk of the Tycho-2 stars, URAT1 gives positions at a mean epoch around 2013.5 and an accuracy level of about 20mas per co-ordinate. We cross-matched URAT1 with Tycho-2 (the original data set tyc2.dat from CDS), and obtained new proper motions via a least-squares adjustment as described, for example in PPMXL (Roeser et al., 2010, Cat. I/317). To avoid formally ultra-precise astrometry for a small number of stars, we chose a 10mas floor for the precision of a URAT1 position. The newly detected clusterings are located in the solar neighbourhood at distances below 500pc from the Sun. The candidates RSG1 to RSG8 are very probably genuine physical groups. Membership and astrophysical parameters could be determined sufficiently well. Nevertheless, accurate parallaxes of at least several reliable cluster stars could improve the quality of parameter determination. A definite age cannot be derived for RSG9; this critically depends on the secure membership status of the two brightest stars. Table 1 summarises the astrophysical parameters of the newly found objects. File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file table1.dat 83 10 New open clusters
Byte-by-byte Description of file: table1.dat
Bytes Format Units Label Explanations
1- 5 A5 --- Name Cluster name (RSGN, RSG9y RSG9o) 7- 8 I2 --- Nmenb Number of probable members 10- 15 F6.2 deg GLON Galactic longitude 17- 21 F5.2 deg GLAT Galactic latitude 23- 28 F6.2 deg RAdeg Cluster center right ascension (J2000.0) 30- 34 F5.2 deg DEdeg Cluster center declination (J2000.0) 36- 40 F5.2 mas/yr pmRA Average proper motion in RA*cos(DE) 42- 45 F4.2 mas/yr e_pmRA Standard error in pmRA 47- 52 F6.2 mas/yr pmDE Average proper motion in DE 54- 57 F4.2 mas/yr e_pmDE Standard error in pmDE 59- 61 I3 pc Dist Distance 63- 66 F4.2 [yr] logt Logarithm of age 68- 72 F5.1 pc Z Distance from the Galactic Plane 74- 78 F5.3 mag E(B-V) Colour-excess 80- 83 F4.2 mag MOD Apparent distance modulus
Acknowledgements: Siegfried Roeser, roeser(at)
(End) Patricia Vannier [CDS] 02-Mar-2017
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