2006A&A...455..943M


C.D.S. - SIMBAD4 rel 1.7 - 2021.02.26CET17:50:56

2006A&A...455..943M - Astronomy and Astrophysics, volume 455, 943-952 (2006/9-1)

Observations of the hot horizontal-branch stars in the metal-rich bulge globular cluster NGC 6388. Indications of helium enrichment and a lesson in crowded field spectroscopy.

MOEHLER S. and SWEIGART A.V.

Abstract (from CDS):

The metal-rich bulge globular cluster NGC 6388 shows a distinct blue horizontal-branch tail in its colour-magnitude diagram (Rich et al., 1997ApJ...484L..25R) and is thus a strong case of the well-known 2nd Parameter Problem. In addition, its horizontal branch (HB) shows an upward tilt toward bluer colours, which cannot be explained by canonical evolutionary models. Several noncanonical scenarios have been proposed to explain these puzzling observations. In order to test the predictions of these scenarios, we have obtained medium resolution spectra to determine the atmospheric parameters of a sample of the blue HB stars in NGC 6388. Using the medium resolution spectra, we determine effective temperatures, surface gravities and helium abundances by fitting the observed Balmer and helium lines with appropriate theoretical stellar spectra. As we know the distance to the cluster, we can verify our results by determining masses for the stars. During the data reduction we took special care in subtracting the background, which is dominated by the overlapping spectra of cool stars. The cool blue tail stars in our sample with effective temperatures Teff≃10000K have lower than canonical surface gravities, suggesting that these stars are, on average, ≃0.4mag brighter than canonical HB stars in agreement with the observed upward slope of the HB in NGC 6388. Moreover, the mean mass of these stars agrees well with theoretical predictions. In contrast, the hot blue tail stars in our sample with Teff≥12000K show significantly lower surface gravities than predicted by any scenario, which can reproduce the photometric observations. Their masses are also too low by about a factor of 2 compared to theoretical predictions. The physical parameters of the blue HB stars near 10000K support the helium pollution scenario. The low gravities and masses of the hot blue tail stars, however, are probably caused by problems with the data reduction, most likely due to remaining background light in the spectra, which would affect the fainter hot blue tail stars much more strongly than the brighter cool blue tail stars. Our study of the hot blue tail stars in NGC 6388 illustrates the obstacles which are encountered when attempting to determine the atmospheric parameters of hot HB stars in very crowded fields using ground-based observations. We discuss these obstacles and offer possible solutions for future projects.

Abstract Copyright:

Journal keyword(s): stars: horizontal-branch - stars: evolution - techniques: spectroscopic - Galaxy: bulge - globular clusters: individual: NGC 6388

Status at CDS : Objects in title, abstract, text, figures, and all or part of small table(s) being processed in SIMBAD.

CDS comments: Table 1: Cl* NGC 6388 PKD NNNNN, see 2002A&A...391..945P.

Simbad objects: 21

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Number of rows : 21

N Identifier Otype ICRS (J2000)
RA
ICRS (J2000)
DEC
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2021
#notes
1 NGC 2808 GlC 09 12 03.10 -64 51 48.6   7.77 5.69     ~ 1276 0
2 NGC 5139 GlC 13 26 47.28 -47 28 46.1   6.12 5.33     ~ 3071 0
3 M 3 GlC 13 42 11.62 +28 22 38.2     6.39     ~ 2298 0
4 LP 916-15 PM* 15 38 59.6477173130 -28 35 36.970418054   12.19 11.62     A 107 0
5 M 13 GlC 16 41 41.634 +36 27 40.75     5.8     ~ 2036 0
6 Cl* NGC 6388 PKD 1775 * 17 36 14.921 -44 43 08.87   19.103 18.811     ~ 1 0
7 Cl* NGC 6388 PKD 7567 * 17 36 16.614 -44 44 59.74   19.225 18.882     ~ 1 0
8 NGC 6388 GlC 17 36 17.461 -44 44 08.34   7.40       ~ 827 1
9 Cl* NGC 6388 PKD 7104 * 17 36 17.879 -44 45 05.02   19.54 19.310     ~ 1 0
10 Cl* NGC 6388 PKD 6239 * 17 36 19.813 -44 43 05.88   19.391 19.147     ~ 1 0
11 Cl* NGC 6388 PKD 396 * 17 36 21.160 -44 44 24.42   17.686 17.383     ~ 1 0
12 Cl* NGC 6388 PKD 5235 * 17 36 21.301 -44 45 14.05   17.454 17.113     ~ 1 0
13 Cl* NGC 6388 WF 2-42 * 17 36 21.842 -44 42 55.70   17.77 17.41     ~ 2 0
14 Cl* NGC 6388 PKD 2483 * 17 36 21.915 -44 44 08.02   17.394 16.927     ~ 1 0
15 Cl* NGC 6388 PKD 2612 * 17 36 22.560 -44 44 01.93   19.901 19.622     ~ 1 0
16 Cl* NGC 6388 PKD 4113 * 17 36 22.884 -44 44 24.63   17.321 16.776     ~ 1 0
17 Cl* NGC 6388 PKD 2897 * 17 36 23.334 -44 43 56.46   19.099 18.835     ~ 1 0
18 Cl* NGC 6388 PKD 1283 * 17 36 23.899 -44 44 58.45   18.92 18.690     ~ 1 0
19 Cl* NGC 6388 PKD 1233 * 17 36 24.565 -44 43 15.21   17.295 16.867     ~ 1 0
20 NGC 6441 GlC 17 50 13.06 -37 03 05.2   9.26 8.00     ~ 788 0
21 NGC 6752 GlC 19 10 52.11 -59 59 04.4   6.96 6.28     ~ 1835 0

    Equat.    Gal    SGal    Ecl

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2021.02.26-17:50:56

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