J/ApJ/632/894 Blue stragglers, HB and turnoff stars in 4 GC (De Marco+, 2005)
A spectroscopic analysis of blue stragglers, horizontal branch stars, and
turnoff stars in four globular clusters.
De Marco O., Shara M.M., Zurek D., Ouellette J.A., Lanz T., Saffer R.A.,
Sepinsky J.F.
<Astrophys. J., 632, 894-919 (2005)>
=2005ApJ...632..894D 2005ApJ...632..894D (SIMBAD/NED BibCode)
ADC_Keywords: Clusters, globular ; Stars, horizontal branch ; Stars, blue ;
Photometry, HST ; Stars, masses
Keywords: stars: blue stragglers -
galaxy: globular clusters: individual: 47 Tucanae -
galaxy: globular clusters: individual: M3 -
galaxy: globular clusters: individual: NGC 6397 -
galaxy: globular clusters: individual: NGC 6752 -
methods: data analysis - methods: numerical -
stars: fundamental parameters - techniques: spectroscopic
Abstract:
We present a spectroscopic analysis of HST STIS and FOS low- and
intermediate-resolution spectroscopy of 55 stars in four globular
clusters (47 Tucanae, M3, NGC 6752, and NGC 6397). Stars hotter than
Teff=5750K and with a signal-to-noise ratio larger than 15 were
analyzed with non-local thermodynamic equilibrium model atmospheres,
and values for their effective temperatures and gravities were
obtained. Using photometric fluxes, we also obtained radii,
luminosities, and spectroscopic masses. Twenty-four stars in our
sample are blue stragglers (BSs). Their photometric colors and
magnitudes place these BSs above and redward of the clusters' zero-age
main sequence: this is consistent with the gravities we find for these
stars, which are lower than zero-age main-sequence gravities. A
comparison with stellar evolutionary tracks shows that almost all of
our BSs are in the Hertzsprung gap. This is contrary to theory,
because of the short timescale expected for stars in this evolutionary
phase. The mean BS mass is 1.04M☉for 14 nonvariable stars, or
1.07M☉counting all 24 BSs in our sample. For the nonvariable
stars the mean BS masses for individual clusters are 1.73, 1.01, 0.95,
and 0.72M☉for NGC 6397, NGC 6752, 47 Tuc, and M3, respectively.
Adding the variable stars (which improves the statistics but increases
the uncertainty), the mean masses become 1.27, 1.05, 0.99, and
0.99M☉, respectively. Although there is considerable scatter,
the BS spectroscopic masses correlate with both effective temperature
and brightness of the stars, as expected. The mean nonvariable turnoff
star mass (0.58M☉) is significantly below the values determined
for the BSs and below the main-sequence turnoff mass. The mean
nonvariable horizontal-branch (HB) star mass is higher than expected
(0.79M☉). In particular, several HB stars have masses well above
the main-sequence turnoff mass. Some of these HB stars are suspected
of actually being BSs, since most of them reside at ambiguous
locations on the CMD, making them prone to misclassification. Values
and limits to the stellar rotation rates (vsini) are imposed by
fitting weak metal lines, the Ca II K line wings, or the helium lines
for the hotter stars. Five BSs with reasonably constrained rotations
show average and median vsini values of 109 and 100km/s, respectively,
suggesting v∼160km/s. At least some GC BSs are very rapid rotators,
but this information cannot yet constrain their origin as stellar
collision or binary mergers because of the lack of clear theoretical
predictions. Six extreme HB stars have rotation rates vsini between 50
and 200km/s, which are high for these stars and might indicate a
binary origin. De Marco et al. found that four BSs and two HB stars in
our sample have Balmer jumps that are too large for the effective
temperatures implied by the slopes of their Paschen continua. Two
additional HB stars are now identified in the current study as having
the same feature. For these stars, the presence of a disk of partly
ionized material is suspected, although high stellar rotation rates
could also partly explain the data.
Description:
Spectra of stars in the GCs 47 Tuc, M3, and NGC 6752 were acquired
through HST proposal GO-8226 (PI: Shara) using the Space Telescope
Imaging Spectrograph (STIS) between 1999 April and 2000 May, with a
few frames acquired in 2001 March.
HST FOS (Faint Object Spectrograph) spectroscopy of six BSs and one HB
star in NGC 6397 (proposal GO-6697; PI: De Marchi) was acquired on
1996 October 5.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 92 55 Observations
table4.dat 96 55 Results
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 9 A9 --- Frame STIS/FOS Frame designation
11- 21 A11 --- Star Star ID (NGC NNNN-NN)
23- 24 I2 h RAh Right ascension (J2000.0) (1)
26- 27 I2 min RAm Right ascension (J2000.0)
29- 33 F5.2 s RAs Right ascension (J2000.0)
35 A1 --- DE- Declination sign (J2000.0) (1)
36- 37 I2 deg DEd Declination (J2000.0)
39- 40 I2 arcmin DEm Declination (J2000.0)
42- 46 F5.2 arcsec DEs Declination (J2000.0)
48- 51 I4 pix Xpix Pixel image (1)
53- 56 I4 pix Ypix Pixel image (1)
58- 60 I3 --- Slit ?=- Pixel slit, y-pixel position of the
spectrum on the CCD image
62- 66 F5.2 arcsec R Radial distance from cluster center
68- 73 F6.3 mag F336W ?=- Observed WFPC2 F336W magnitude (2)
75- 79 F5.2 mag F439W ?=- Observed WFPC2 F439W magnitude (2)
81- 85 F5.2 mag F555W ?=- Observed WFPC2 F555W magnitude (2)
87- 92 F6.3 mag F814W ?=- Observed WFPC2 F814W magnitude (2)
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Note (1): Right ascension, declination, and pixel positions of the stars
analyzed were determined with the cursor on the images presented in
Figs. 9-12. The uncertainties are 0.01s, 0.01", and 1-pixel.
Note (2): Observed WFPC2 photometric magnitudes determined from PSF photometry.
The best-fitting synthetic spectra for nonvariable stars were scaled to these
F555W magnitudes in the way described in Sect. 4.
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Byte-by-byte Description of file: table4.dat
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Bytes Format Units Label Explanations
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1- 11 A11 --- Star Star ID (NGC NNNN-NN)
12 A1 --- Flag [f] Flag on Star (1)
14- 15 A2 --- Notes Variability note (1)
17- 19 A3 --- CMD CMD Type
21- 24 F4.2 [cm/s2] logg Surface gravity
26- 29 F4.2 [cm/s2] e_logg rms uncertainty on logg
32- 36 I5 K Teff Effective temperature
38- 40 I3 K e_Teff rms uncertainty on Teff
42- 44 F3.1 Rsun Rad Radius
46- 48 F3.1 Rsun e_Rad rms uncertainty on Rad
50- 53 F4.1 Lsun Lum Luminosity
55- 58 F4.1 Lsun e_Lum rms uncertainty on Lum
60- 63 F4.2 Msun Mass Mass
65- 68 F4.2 Msun b_Mass Lower value of Mass interval
69 A1 --- --- [-]
70- 73 F4.2 Msun B_Mass Upper value of Mass interval
75- 78 F4.2 --- f Scaling factor (2)
80- 84 F5.2 mag Dm F555WSP-F555Wp difference
86- 90 F5.2 mag d(B-V) ?=- (F439WSP-F555WSP)-(F439Wp-F555Wp)
difference
92- 96 F5.2 mag d(U-V) ?=- (F3369WSP-F555WSP)-(F336P-F555WP)
difference
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Note (1): Notes as follows:
v = indicates stars that are suspected of variability (Sect. 4). Radii,
luminosities, and masses of these stars have been determined by scaling
the models to the spectroscopy times 0.80, not the photometry, as is the
case for the rest of the sample (see Sect. 7.2).
":" = a single colon indicates stars suspected of low-level blending based on
the F555W magnitude photometry/spectrophotometry comparison (Sect. 7.2)
"::" = a double colon indicates stars suspected of moderate blending or where
the magnitude and color photometry/spectrophotometry comparison is
outside the random error range; these stars have been given a larger
temperature error (Sect. 7.2).
f = This star's parameters were derived from the Paschen continuum and Balmer
lines, but for these values the Balmer continuum of the data is
fainter than that of the model. Stars with this characteristic are
discussed in Sect. 10.
Note (2): Scaling factors are derived from a ratio of the spectrophotometric
(SP) and photometric (P; Table 2) values: 10( F555WSP-F555WP)/2.5.
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History:
From electronic version of the journal
(End) Patricia Vannier [CDS] 05-Jan-2016