J/AJ/125/197 CN abundances of the 47 Tuc main sequence (Harbeck+, 2003)
CN abundance variations on the main sequence of 47 Tucanae.
Harbeck D., Smith G.H., Grebel E.K.
<Astron. J., 125, 197-207 (2003)>
=2003AJ....125..197H 2003AJ....125..197H
ADC_Keywords: Clusters, globular ; Photometry, UBV ; Abundances, peculiar
Keywords: globular clusters: general - stars: evolution - stars: formation -
globular clusters: individual (47 Tucanae) - stars: abundances
Abstract:
We report on a deep spectroscopic survey for star-to-star CN
variations along the main sequence (MS) of the globular cluster 47 Tuc
with ESO's Very Large Telescope. We find a significant bimodal
distribution in the S(3839) index for main-sequence stars in the mass
range ∼0.85 to 0.65M☉, or from the main-sequence turnoff down to
∼2.5 mag below the main-sequence turnoff. An anticorrelation of CN and
CH is evident on the MS. The result is discussed in the context of the
ability of faint MS stars to alter their surface composition through
internal evolutionary effects. We argue against internal stellar
evolution as the only origin for the abundance spread in 47 Tuc; an
external origin such as pollution seems to be more likely.
Description:
Candidate stars in 47 Tuc for spectroscopy with the Very Large
Telescope (VLT) of the European Southern Observatory (ESO) at Cerro
Paranal, Chile, were chosen from CCD images of the cluster. In 2000
September we observed 47 Tuc with the MPG/ESO 2.2m telescope at La
Silla, Chile, using the Wide Field Imager with Johnson B and V
filters.
Spectroscopic observations were carried out in service mode between
2001 July 18 and 2001 July 26, with he Focal Reducer and Low Reducer
and Low Dispersion Spectrograph 2 (FORS2) at the ESO VLT.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 73 111 Data for Program Stars
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See also:
J/PASP/99/739 : BV photometry in 4 fields of 47 Tuc (Hesser+, 1987)
J/A+AS/106/161 : (V,I) photometry in NGC 6356 and 47 Tuc (Bica+ 1994)
J/AJ/127/1588 : 47 Tuc main-sequence star abundances (Briley+, 2004)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 4 I4 --- HSG Stellar identification number,
Cl* NGC 104 HSG NNNN in Simbad
6- 7 I2 h RAh Hour of right ascension (J2000)
9- 10 I2 min RAm Minute of right ascension (J2000)
12- 16 F5.2 s RAs Second of right ascension (J2000)
18 A1 --- DE- Sign of the declination (J2000)
19- 20 I2 deg DEd Degree of declination (J2000)
22- 23 I2 arcmin DEm Arcminute of declination (J2000)
25- 29 F5.2 arcsec DEs Arcsecond of declination (J2000)
31- 36 F6.3 mag Vmag Johnson V-band magnitude
38- 42 F5.3 mag B-V Johnson B-V color
44- 49 F6.3 mag S3839 The δS(3839) spectral index (1)
51- 56 F6.3 mag S4300 The δS(4300) spectral index (1)
58- 63 F6.3 mag CH4300 The CH(4300) spectral index (1)
65- 70 F6.3 mag HK The HK spectral index (1)
72 I1 --- Qflg [0/2] Quality flag, 0=reliable (2)
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Note (1): Spectral indices:
S3839 = -2.5log[F(3861-3884)/F(3894-3910)];
S4142 = -2.5log[F(4120-4216)/(0.4*F(4055-4080) + 0.6*F(4240-4280))];
CH4300 = -2.5log[F(4285-4315)/(0.5*F(4240-4280) + 0.5*F(4390-4460))];
HK = -2.5log[F(3910-4020)/F(4020-4130)].
Where F(3861-3884) for example is the summed spectral flux (in our
case ADU counts) from 3861Å to 3884Å. Each index quantifies the
absorption strength of a molecular band or atomic lines relative to a
nearby pseudocontinuum. For stronger band absorption the index values
increase.
Note (2): Quality flags:
0 = Star for which spectra show clean CN absorption bands with few
noise imperfections and for which a reasonably confident CN
classification was felt to be obtainable.
1 = Star for which we could not judge a unique CN classification by
eye, especially for a few faint stars near to the center of
47 Tuc. Such instances tend to occur when spectra have a low S/N
or do not show consistent trends in the three main absorption
features at 3860Å, 3875Å, or 3889Å of the blue CN band.
The star may be blended with others (as suspected by its
proximity to the cluster center)
2 = Star for which the data reduction produced poor S/N spectra. This
may probably be due to a misalignment of the slits.
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History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Patricia Vannier [CDS] 17-May-2004