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J/A+A/493/71        UBV photometry of Shorlin 1                (Carraro+, 2009)

Searching for spiral features in the outer Galactic disk. The field towards WR38 and WR38a. Carraro G.., Costa E. <Astron. Astrophys. 493, 71 (2009)> =2009A&A...493...71C
ADC_Keywords: Clusters, open ; Photometry, CCD Keywords: Galaxy: general - Galaxy: structure - Galaxy: stellar content Abstract: The detailed spiral structure of the outer Galactic disk remains poorly constrained, and for several line of sights in the Galaxy we depend on model extrapolations. One of these regions is the fourth Galactic quadrant, between Vela and Carina (270°≤l≤300°), where, apart from the conspicuous Carina branch of the Carina Sagittarius arm, no spiral arms have been detected so far in the optical beyond l∼270°. By means of deep UBVI photometry, we search for spiral features in known low absorption windows. Although observationally demanding, U photometry constitutes a powerful tool for detecting and characterizing distant aggregates of young stars, and allows firmer distance estimates to be derived. We study a direction close to the tangent (l∼290°) of the Carina arm, in an attempt to detect optical spiral tracers far beyond the Carina branch, where radio observations and model predictions appear to indicate the presence of an extension of the Perseus and Norma-Cygnus spiral arms in the fourth quadrant. Along this line of sight, we detect three distinct groups of young stars. Two of them, at distances of ∼2.5 and ∼6.0kpc, belong to the Carina spiral arm, which is traversed twice in this particular direction. Interestingly, the latter is detected for the first time. The third group, at a distance of ∼12.7kpc, is probably related to the Perseus arm which lies beyond the Carina arm, and constitutes the first optical detection of this arm in the fourth Galactic quadrant. The position of this feature is consistent with both HI observations and model predictions. We also present evidence that this extremely distant group, formerly assumed to be a star cluster (Shorlin 1), is a diffuse, young population, typically found in spiral galaxies. In addition, our data-set does not support, as claimed in the literature, the possible presence of the Monoceros Ring in this direction. This study highlights how multicolor optical studies can be effective in probing the spiral structure of the outer Galactic disk. More fields need to be studied in this region of the Galaxy to constrain the spiral structure in the fourth Galactic quadrant more accurately, in particular, the shape and extent of the Perseus arm, and, possibly, to detect the even more distant Norma-Cygnus arm. Description: Photometric data in the UBV pass-bands of 7425 stars in the field of the two WR stars WR38 and WR38a. File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file cluster.dat 104 7425 UBV photometry of Shorlin 1
Byte-by-byte Description of file: cluster.dat
Bytes Format Units Label Explanations
1- 4 I4 --- ID [1/7425] Sequential number, X sorted 6- 7 I2 h RAh Right ascension (J2000.0) 9- 10 I2 min RAm Right ascension (J2000.0) 12- 15 F4.1 s RAs Right ascension (J2000.0) 17 A1 --- DE- Declination sign (J2000) 18- 19 I2 deg DEd Declination (J2000) 21- 22 I2 arcmin DEm Declination (J2000) 24- 27 F4.1 arcsec DEs Declination (J2000) 29- 36 F8.3 pix Xpos X coordinate 38- 45 F8.3 pix Ypos Y coordinate 47- 52 F6.3 mag Vmag V magnitude 54- 58 F5.3 mag e_Vmag V magnitude error 60- 65 F6.3 mag U-B (U-B) colour index 67- 71 F5.3 mag e_U-B (U-B) colour index error 73- 78 F6.3 mag B-V (B-V) colour index 80- 84 F5.3 mag e_B-V (B-V) colour index error 86- 91 F6.3 --- chi2 chi squared parameter 93- 98 F6.3 --- sharp IRAF sharpness parameter (best around 0.6) 100-104 A5 --- Name Name (for RW38 and RW38a)
Acknowledgements: Giovanni Carraro, gcarraro(at)eso.org
(End) Giovanni Carraro [ESO, Chile], Patricia Vannier [CDS] 07-Nov-2008
The document above follows the rules of the Standard Description for Astronomical Catalogues.From this documentation it is possible to generate f77 program to load files into arrays or line by line

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