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 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:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
cluster.dat 104 7425 UBV photometry of Shorlin 1
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Byte-by-byte Description of file: cluster.dat
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Bytes Format Units Label Explanations
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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)
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Acknowledgements:
Giovanni Carraro, gcarraro(at)eso.org
(End) Giovanni Carraro [ESO, Chile], Patricia Vannier [CDS] 07-Nov-2008